Trap healing and ultralow-noise Hall effect at the surface ......“Trap healing and ultra low-noise Hall effect at the surface of organic semiconductors” B. Lee, Y. Chen, D. Fu,

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Supplementary Information. Sept. 6, 2013

“Trap healing and ultra low-noise Hall effect at the surface of organic semiconductors”

B. Lee, Y. Chen, D. Fu, H. T. Yi, K. Czelen, H. Najafov, V. Podzorov* ([email protected])

1. DFT hybrid functional calculations of charge density distribution and optimized

geometry of PFPE (the case of n = m = 2).

Calculations of the charge density distribution, total dipole moment and optimized

geometry of PFPE have been carried out using a B3LYP/6-31G(d) basis set DFT hybrid

functional with Gaussian-09 software for the polymer repeat indexes n = m = 2. The results of

these calculations are shown in Fig. S1. These calculations show that there are pockets of strong

charge polarization within the PFPE macromolecule, leading to distinct local dipole moments on

the polymer chain and resulting in the net dipole moment of Ptotal = 0.4047 Debye (for n = m = 2).

The most polarized regions correspond to clusters of atoms.

According to the calculated charge with Mulliken charge analysis, each F atom in this

cluster carries about -0.25e, C in CF3 carries +1e, C in the middle has +0.4e, and O holds -0.525e

(accepts electrons from the two adjacent C atoms). Thus, the group that has the greatest local

dipole moment is CF-CF3. In addition, this dipole will have the largest component normal to the

sample’s surface, assuming that PFPE chains lie flat at this surface. The C-F bond affects the

strength of the local and total dipole moment more significantly than C-O. In addition, the

optimized geometry calculations show that the CF-CF3 groups partially linearize PFPE chain by

keeping it from folding on itself. On the contrary, ongoing calculations for similar structures but

without these side groups (such as that of PFTG) show a considerable folding, which results in

partial canceling of the local dipole moments.

Trap healing and ultralow-noise Hall effect at the surface of organic semiconductors

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Figure S1. DFT hybrid functional calculations of the charge density distribution and optimized

geometry of PFPE. An optimized geometry (top) and the charge density distribution (bottom) in PFPE

molecule with n = m = 2. The table on the right gives the calculated charge at every atom.

These calculations show that the dipole moments associated with PFPE polymer structure

must play an essential role in the observed effects of charge accumulation at the

semiconductor/PFPE interface. However, they also suggest that a more complicated picture

must be considered for each individual polymer under investigation. Indeed, more atoms in the

vicinity of each functional group might contribute to the local dipole at that position, as well as

to the net dipole moment of the polymer, as exemplified by the polar C-O bond within the PFPE

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backbone. In addition, functional side groups, such as CF-CF3 might affect the polymer chain

conformation and its resultant shape, which can also have an impact on the interaction of this

polymer with the surface of organic semiconductors. More computational studies, including

PFTG and FC-70 polymers, as well as PFPE with more repeat units, are under way.

2. Effect of non-conjugated fluoropolymers on the surface conductivity of rubrene.

Figure S2. Effect of different fluoropolymers on the surface conductivity of rubrene. Surface

conductivity of pristine rubrene, σ(t), monitored as a function of time, as the following non-conjugated

fluoropolymers are deposited at the crystal surface: perfluoropolyether PFPE (black), perfluorotetraglyme

PFTG (red), and Fluorinert FC-70 (blue). The polymers are dropped at (a,b) facet of the crystals at t = 0.

While PFPE induces a significant surface conductivity in the majority of p-type organic

semiconductors we have investigated, a few other fluorinated oligomers with somewhat similar

molecular structure have much less prominent or even a detrimental effect on σ. Figure S2

shows the effect of PFPE, perfluorotetraglyme (PFTG), and Fluorinert (FC-70) on surface

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Figure S1. DFT hybrid functional calculations of the charge density distribution and optimized

geometry of PFPE. An optimized geometry (top) and the charge density distribution (bottom) in PFPE

molecule with n = m = 2. The table on the right gives the calculated charge at every atom.

These calculations show that the dipole moments associated with PFPE polymer structure

must play an essential role in the observed effects of charge accumulation at the

semiconductor/PFPE interface. However, they also suggest that a more complicated picture

must be considered for each individual polymer under investigation. Indeed, more atoms in the

vicinity of each functional group might contribute to the local dipole at that position, as well as

to the net dipole moment of the polymer, as exemplified by the polar C-O bond within the PFPE

3

backbone. In addition, functional side groups, such as CF-CF3 might affect the polymer chain

conformation and its resultant shape, which can also have an impact on the interaction of this

polymer with the surface of organic semiconductors. More computational studies, including

PFTG and FC-70 polymers, as well as PFPE with more repeat units, are under way.

2. Effect of non-conjugated fluoropolymers on the surface conductivity of rubrene.

Figure S2. Effect of different fluoropolymers on the surface conductivity of rubrene. Surface

conductivity of pristine rubrene, σ(t), monitored as a function of time, as the following non-conjugated

fluoropolymers are deposited at the crystal surface: perfluoropolyether PFPE (black), perfluorotetraglyme

PFTG (red), and Fluorinert FC-70 (blue). The polymers are dropped at (a,b) facet of the crystals at t = 0.

While PFPE induces a significant surface conductivity in the majority of p-type organic

semiconductors we have investigated, a few other fluorinated oligomers with somewhat similar

molecular structure have much less prominent or even a detrimental effect on σ. Figure S2

shows the effect of PFPE, perfluorotetraglyme (PFTG), and Fluorinert (FC-70) on surface




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conductivity of pristine rubrene. The inset shows the molecular formulas of all these compounds.

FC-70 has three identical fluoroalkyl branches attached to a central nitrogen atom. As a result of

high symmetry, there is no permanent dipole on this molecule, which explains the minor effect

of this polymer on σ of rubrene. On the other hand, both PFPE and PFTG have outstanding -CF3

groups (end -O-CF3 groups in both molecules, and side -C-CF3 groups in PFPE) whose net

dipole is not counterbalanced by other fluorine atoms. Such dipole induces charge carriers at the

surface of rubrene crystals. Given the similarity in the backbone structure of PFPE and PFTG,

however, the drastic difference in their effects strongly suggests that the side -CF3 group in PFPE

is mostly responsible for the conductivity induced at the crystal/PFPE interface.

3. Effect of thermal annealing on conductivity of rubrene-PFPE samples.

A very interesting behavior of rubrene-PFPE system is the slow dynamics of surface

conductivity, σ, observed after the initial jump at the moment of PFPE application (Fig. 1 of the

main text). This slow dynamics is very reproducible: in most of the samples, σ slowly increases

for hours or even days after PFPE deposition. This instability however does not prevent reliable

measurements of intrinsic transport properties, such as conductivity anisotropy or Hall effect,

because this dynamics is rather slow compared to the time needed for these measurements. It is

interesting that the rate of this dynamics can be drastically increased by annealing samples at an

elevated temperature.

In order to demonstrate this convincingly, we have performed the following additional

experiment. First, PFPE was applied to the surface of a pristine rubrene crystal at room

temperature, while monitoring the surface conductivity (Fig. S3). After the initial spike at t 3

min, the conductivity starts to increase slowly (5 < t < 30 min: this is the same kind of slow

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dynamics as seen in Fig. 1 of the main text). At t 30 min, a heater was turned on, and the

sample’s temperature was increased to ~ 100 oC (measured by a thermocouple attached near the

crystal) and maintained for about 25 min, after which the heater was turned off at t 55 min, and

the sample was allowed to gradually cool down to room temperature. We have also performed

exactly the same procedure using a control pristine rubrene crystal without PFPE. The data for

the control sample are shown in the inset. Both samples have the same type of contacts, and both

measurements were performed in a sealed sample chamber filled with an inert gas (UHP Argon)

in order to prevent any interaction of the samples with air.

Figure S3. Effect of thermal annealing on conductivity of rubrene-PFPE samples. Main panel:

surface conductivity of a rubrene single crystal functionalized with PFPE and monitored as a function of

time, σ(t), while the sample is annealed at 100 oC (t = 30-55 min) and subsequently cooled to room

temperature (t > 80 min). The moments of PFPE application, turning the heater ON and OFF are marked.

The inset: a control experiment using a bare (unfunctionalized) rubrene crystal performed at exactly the

same conditions. Both devices have the same type of graphite contacts, L ~ W ~ 1-2 mm, measurements

are performed at V = 1 V. Large irreversible increase of conductivity after the annealing suggests that

thermal reorganizations of PFPE polymer chains and their built-in dipole moments are responsible for the





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conductivity of pristine rubrene. The inset shows the molecular formulas of all these compounds.

FC-70 has three identical fluoroalkyl branches attached to a central nitrogen atom. As a result of

high symmetry, there is no permanent dipole on this molecule, which explains the minor effect

of this polymer on σ of rubrene. On the other hand, both PFPE and PFTG have outstanding -CF3

groups (end -O-CF3 groups in both molecules, and side -C-CF3 groups in PFPE) whose net

dipole is not counterbalanced by other fluorine atoms. Such dipole induces charge carriers at the

surface of rubrene crystals. Given the similarity in the backbone structure of PFPE and PFTG,

however, the drastic difference in their effects strongly suggests that the side -CF3 group in PFPE

is mostly responsible for the conductivity induced at the crystal/PFPE interface.

3. Effect of thermal annealing on conductivity of rubrene-PFPE samples.

A very interesting behavior of rubrene-PFPE system is the slow dynamics of surface

conductivity, σ, observed after the initial jump at the moment of PFPE application (Fig. 1 of the

main text). This slow dynamics is very reproducible: in most of the samples, σ slowly increases

for hours or even days after PFPE deposition. This instability however does not prevent reliable

measurements of intrinsic transport properties, such as conductivity anisotropy or Hall effect,

because this dynamics is rather slow compared to the time needed for these measurements. It is

interesting that the rate of this dynamics can be drastically increased by annealing samples at an

elevated temperature.

In order to demonstrate this convincingly, we have performed the following additional

experiment. First, PFPE was applied to the surface of a pristine rubrene crystal at room

temperature, while monitoring the surface conductivity (Fig. S3). After the initial spike at t 3

min, the conductivity starts to increase slowly (5 < t < 30 min: this is the same kind of slow

5

dynamics as seen in Fig. 1 of the main text). At t 30 min, a heater was turned on, and the

sample’s temperature was increased to ~ 100 oC (measured by a thermocouple attached near the

crystal) and maintained for about 25 min, after which the heater was turned off at t 55 min, and

the sample was allowed to gradually cool down to room temperature. We have also performed

exactly the same procedure using a control pristine rubrene crystal without PFPE. The data for

the control sample are shown in the inset. Both samples have the same type of contacts, and both

measurements were performed in a sealed sample chamber filled with an inert gas (UHP Argon)

in order to prevent any interaction of the samples with air.

Figure S3. Effect of thermal annealing on conductivity of rubrene-PFPE samples. Main panel:

surface conductivity of a rubrene single crystal functionalized with PFPE and monitored as a function of

time, σ(t), while the sample is annealed at 100 oC (t = 30-55 min) and subsequently cooled to room

temperature (t > 80 min). The moments of PFPE application, turning the heater ON and OFF are marked.

The inset: a control experiment using a bare (unfunctionalized) rubrene crystal performed at exactly the

same conditions. Both devices have the same type of graphite contacts, L ~ W ~ 1-2 mm, measurements

are performed at V = 1 V. Large irreversible increase of conductivity after the annealing suggests that

thermal reorganizations of PFPE polymer chains and their built-in dipole moments are responsible for the





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formation of conducting state at the rubrene/PFPE interface and the observed slow dynamics of σ. These

thermally activated rearrangements must occur at the interface in such a way that more dipole induced

charges are created in the crystal.

The data in Fig. S3 clearly show that in the case of rubrene-PFPE samples there is an

annealing effect: keeping these samples at an elevated temperature for a period of time

irreversibly increases their room-temperature conductivity by a factor of ~ 3. A similar increase

would have also occurred at room temperature, but much longer time would be needed: a room-

temperature annealing (compare with Fig. 1 of the main text). The control experiment with

unfunctionalized crystal, showing no annealing effect, indicates that neither the crystal itself, nor

the contacts are responsible for the annealing effect. This is not surprising, because our crystals

and contacts are already optimized, and the devices are not contact limited; thus, no benefits of

annealing are expected for pristine samples. In the rubrene-PFPE structure, the only material

that could undergo considerable changes during annealing is the PFPE polymer, which is a

viscous liquid at room temperature.

The annealing effect speaks in favor of the proposed mechanism, according to which

thermally activated structural rearrangements of PFPE polymer chains near the crystal-polymer

interface occur in such a way that more weakly bound charge-dipole states are formed at the

interface to minimize the total energy of the system, thus increasing the density of induced

charges and σ. It is likely that these rearrangements involve both re-organization of the polymer

chains and rotation of individual built-in (CF)+-(CF3)

- dipoles toward the surface of rubrene in

order to create a greater number of dipole-induced holes in the crystal. Figure S4 schematically

depicts this process: initially, right after PFPE deposition, a random distribution of PFPE chains

and dipoles results in only a few holes induced in rubrene (top); after the polymer relaxation

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occurs, considerably more weakly bound dipole-charge states are formed near the interface due

to the rearrangements of the PFPE chains (bottom). Large horizontal ovals represent holes

delocalized over a number of lattice sites (mobile carriers); small circles are holes localized on

individual sites (deeply trapped charges).

Figure S4. Cartoon of the interface between an organic crystal and PFPE polymer. The conductivity

induced at the crystal-PFPE interface originates from the dipole-induced mobile charges in the crystal.

Local permanent dipoles on PFPE chains (represented by the arrows) induce positive polarons (+) in the

semiconductor (red), provided that the dipoles are located close enough to the interface and oriented

correctly (for hole accumulation, the dipoles must be oriented away from the semiconductor). Initially,

right after the interface is formed (t = 0, upper panel), only a few holes are induced. With time (t > 0,

lower panel), facilitated by thermally activated reorganization of the polymer, more dipole-charge states

are formed at the interface, lowering the total electrostatic energy of the system. For delocalized holes

(horizontal ovals), the dipole-charge interaction is weak, allowing the charge to remain mobile. For holes

temporarily localized on a single lattice site due to, for example, shallow traps (small circles), the

electrostatic interaction with nearby PFPE dipoles is expected to be much stronger, thus resulting in

further localization of such holes.





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formation of conducting state at the rubrene/PFPE interface and the observed slow dynamics of σ. These

thermally activated rearrangements must occur at the interface in such a way that more dipole induced

charges are created in the crystal.

The data in Fig. S3 clearly show that in the case of rubrene-PFPE samples there is an

annealing effect: keeping these samples at an elevated temperature for a period of time

irreversibly increases their room-temperature conductivity by a factor of ~ 3. A similar increase

would have also occurred at room temperature, but much longer time would be needed: a room-

temperature annealing (compare with Fig. 1 of the main text). The control experiment with

unfunctionalized crystal, showing no annealing effect, indicates that neither the crystal itself, nor

the contacts are responsible for the annealing effect. This is not surprising, because our crystals

and contacts are already optimized, and the devices are not contact limited; thus, no benefits of

annealing are expected for pristine samples. In the rubrene-PFPE structure, the only material

that could undergo considerable changes during annealing is the PFPE polymer, which is a

viscous liquid at room temperature.

The annealing effect speaks in favor of the proposed mechanism, according to which

thermally activated structural rearrangements of PFPE polymer chains near the crystal-polymer

interface occur in such a way that more weakly bound charge-dipole states are formed at the

interface to minimize the total energy of the system, thus increasing the density of induced

charges and σ. It is likely that these rearrangements involve both re-organization of the polymer

chains and rotation of individual built-in (CF)+-(CF3)

- dipoles toward the surface of rubrene in

order to create a greater number of dipole-induced holes in the crystal. Figure S4 schematically

depicts this process: initially, right after PFPE deposition, a random distribution of PFPE chains

and dipoles results in only a few holes induced in rubrene (top); after the polymer relaxation

7

occurs, considerably more weakly bound dipole-charge states are formed near the interface due

to the rearrangements of the PFPE chains (bottom). Large horizontal ovals represent holes

delocalized over a number of lattice sites (mobile carriers); small circles are holes localized on

individual sites (deeply trapped charges).

Figure S4. Cartoon of the interface between an organic crystal and PFPE polymer. The conductivity

induced at the crystal-PFPE interface originates from the dipole-induced mobile charges in the crystal.

Local permanent dipoles on PFPE chains (represented by the arrows) induce positive polarons (+) in the

semiconductor (red), provided that the dipoles are located close enough to the interface and oriented

correctly (for hole accumulation, the dipoles must be oriented away from the semiconductor). Initially,

right after the interface is formed (t = 0, upper panel), only a few holes are induced. With time (t > 0,

lower panel), facilitated by thermally activated reorganization of the polymer, more dipole-charge states

are formed at the interface, lowering the total electrostatic energy of the system. For delocalized holes

(horizontal ovals), the dipole-charge interaction is weak, allowing the charge to remain mobile. For holes

temporarily localized on a single lattice site due to, for example, shallow traps (small circles), the

electrostatic interaction with nearby PFPE dipoles is expected to be much stronger, thus resulting in

further localization of such holes.





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4. Measurements of anisotropic surface conductivity induced by PFPE in organic crystals.

For devices used in the anisotropy studies, a 35 nm-thick silver film was thermally

evaporated on chosen single crystals through a shadow mask that consisted of two gold wires

with a diameter of 25 µm crossed at 90o (Fig. S5). This resulted in two perpendicular channels

with the channel length L = 25 µm oriented along a and b crystal axes (in the case of rubrene).

Electrical connection was made by attaching gold wires to the silver contact pads with a

conductive silver paint.

Figure S5. Device geometry and electrical measurement setup for the studies of conduction

anisotropy in organic single crystals. Grey-colored area represents a silver film thermally evaporated

through a shadow mask; red-colored area is the exposed surface of a crystal (channel area). L = 25 µm;

W1 and W2 are determined by crystal dimensions.

A constant voltage V was applied to one of the contacts using a Keithley 2400 Source-

Meter, and the current was simultaneously extracted from the two separate channels, labeled I1

and I2 in Fig. S5, using two Keithley 6514 electrometers. I1 = (W1/L)σbV and I2 = (W2/L)σaV,

where σa and σb are conductivity along the crystalline axis a and b, respectively (case of rubrene).

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The anisotropy in transport, characterized by σb/σa, can thus be determined as σb/σa =

(I1W2)/(I2W1). The advantage of this technique is that conductivity in a and b directions and their

ratio can be measured simultaneously, which is important when the overall conductivity is

changing with time – the situation frequently encountered in dynamic doping processes.

It is interesting to note that in the case of rubrene, a rather significant initial anisotropy

(σb/σa ~ 3) already existed before the application of PFPE (Fig. 2(a) in the main text), which

indicates the high purity (low trap density) of as-grown rubrene crystals. On the contrary, due to

a relatively high density of traps, tetracene crystals usually show a negligible initial anisotropy

(Fig. 2(b) in the main text). In addition, anisotropy in surface conductivity of tetracene treated

with PFPE can be reproducibly observed only in freshly grown crystals after the material has

been purified at least 3 to 4 times.

5. Vacuum-lamination method of OFET fabrication.

To sequentially assemble OFETs using the same single crystal several times, we

employed a novel method of OFET fabrication recently developed in our group [1]. Each time, a

fresh 2.5 µm-thick Mylar film is laminated by vacuum on a chosen crystal in its pristine or

treated forms (Fig. S6). The treatment used in this study included, e.g., photo-oxidation and

PFPE-functionalization. The ultra-thin polymeric Mylar films are very flexible, conformable

and have a very smooth surface. Applying a suction using a small diaphragm pump forces the

membrane to collapse against the surface of the crystal with prefabricated contacts, forming a

high-quality crystal-dielectric interface. A silver paint is then used to paint a top gate electrode.

By breaking the vacuum, one can carefully detach the Mylar film without damaging or

contaminating the crystal’s surface.





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4. Measurements of anisotropic surface conductivity induced by PFPE in organic crystals.

For devices used in the anisotropy studies, a 35 nm-thick silver film was thermally

evaporated on chosen single crystals through a shadow mask that consisted of two gold wires

with a diameter of 25 µm crossed at 90o (Fig. S5). This resulted in two perpendicular channels

with the channel length L = 25 µm oriented along a and b crystal axes (in the case of rubrene).

Electrical connection was made by attaching gold wires to the silver contact pads with a

conductive silver paint.

Figure S5. Device geometry and electrical measurement setup for the studies of conduction

anisotropy in organic single crystals. Grey-colored area represents a silver film thermally evaporated

through a shadow mask; red-colored area is the exposed surface of a crystal (channel area). L = 25 µm;

W1 and W2 are determined by crystal dimensions.

A constant voltage V was applied to one of the contacts using a Keithley 2400 Source-

Meter, and the current was simultaneously extracted from the two separate channels, labeled I1

and I2 in Fig. S5, using two Keithley 6514 electrometers. I1 = (W1/L)σbV and I2 = (W2/L)σaV,

where σa and σb are conductivity along the crystalline axis a and b, respectively (case of rubrene).

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The anisotropy in transport, characterized by σb/σa, can thus be determined as σb/σa =

(I1W2)/(I2W1). The advantage of this technique is that conductivity in a and b directions and their

ratio can be measured simultaneously, which is important when the overall conductivity is

changing with time – the situation frequently encountered in dynamic doping processes.

It is interesting to note that in the case of rubrene, a rather significant initial anisotropy

(σb/σa ~ 3) already existed before the application of PFPE (Fig. 2(a) in the main text), which

indicates the high purity (low trap density) of as-grown rubrene crystals. On the contrary, due to

a relatively high density of traps, tetracene crystals usually show a negligible initial anisotropy

(Fig. 2(b) in the main text). In addition, anisotropy in surface conductivity of tetracene treated

with PFPE can be reproducibly observed only in freshly grown crystals after the material has

been purified at least 3 to 4 times.

5. Vacuum-lamination method of OFET fabrication.

To sequentially assemble OFETs using the same single crystal several times, we

employed a novel method of OFET fabrication recently developed in our group [1]. Each time, a

fresh 2.5 µm-thick Mylar film is laminated by vacuum on a chosen crystal in its pristine or

treated forms (Fig. S6). The treatment used in this study included, e.g., photo-oxidation and

PFPE-functionalization. The ultra-thin polymeric Mylar films are very flexible, conformable

and have a very smooth surface. Applying a suction using a small diaphragm pump forces the

membrane to collapse against the surface of the crystal with prefabricated contacts, forming a

high-quality crystal-dielectric interface. A silver paint is then used to paint a top gate electrode.

By breaking the vacuum, one can carefully detach the Mylar film without damaging or

contaminating the crystal’s surface.





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Figure S6. Illustration of vacuum lamination method of single-crystal OFET fabrication. For

details, see ref. [1].

6. Comparison between PFPE and ionic liquids.

Electrostatic gating using ionic liquids (electrolytes) has been widely used in OFETs to

achieve high carrier density and low-voltage operation [2,3]. Given a similar fluidic nature of

ionic liquids and PFPE, it is natural to ask whether the accumulation channel induced by PFPE

might also be related to an electrolytic activity of this material. In the following, we show that

even though both PFPE and ionic liquids can induce mobile charges at the surface of a

semiconductor, PFPE does not have any mobile ions and does not show any electrolytic activity,

and thus PFPE and ionic liquids are essentially different systems, each having its own unique

properties and mechanisms of charge induction.

We first demonstrate that unlike ionic liquids, PFPE does not contain mobile ions. For

this purpose, we have measured and compared dc and transient conductivity of PFPE and an ion

gel. dc electrical conductivity of PFPE has been tested in different contact configurations using a

sensitive low-noise measurement system based on Keithley electrometers equipped with shielded

triax cables and an external grounded metal sample enclosure. The sensitivity of our current

measurements in routine experiments is better than pA. For these measurements, contacts in

different geometries were evaporated through a shadow mask on pre-cleaned glass slides, wired

to the sample holder, and PFPE polymer was drop cast between the contacts. We could only

11

register ~ pA currents (similar to the noise floor of our setup), when voltages up to ±210 V were

applied across the contacts submerged in PFPE. Thus, the dc electrical conductivity of PFPE is

less than 10-14

S.

Figure S7. Transient response of ion gel and PFPE to a step voltage. The inset in (a) is the

schematics of our measurement setup. Details of the setup and measurement are described in the text.

Next, we have performed the measurements of transient current response of an ion gel (as

a sample for comparison) and PFPE to an abrupt voltage step. The measurement setup is

depicted in the inset in Fig. S7(a). The ion gel used in this test was formed by gelation of a

copolymer, poly(vinylidene fluoride-co-hexafluoropropylene), in an ionic liquid 1-ethyl-3-

methylimidazolium bis(trifluoromethylsulfonyl) amide. More details of the ion gel can be found





10

Figure S6. Illustration of vacuum lamination method of single-crystal OFET fabrication. For

details, see ref. [1].

6. Comparison between PFPE and ionic liquids.

Electrostatic gating using ionic liquids (electrolytes) has been widely used in OFETs to

achieve high carrier density and low-voltage operation [2,3]. Given a similar fluidic nature of

ionic liquids and PFPE, it is natural to ask whether the accumulation channel induced by PFPE

might also be related to an electrolytic activity of this material. In the following, we show that

even though both PFPE and ionic liquids can induce mobile charges at the surface of a

semiconductor, PFPE does not have any mobile ions and does not show any electrolytic activity,

and thus PFPE and ionic liquids are essentially different systems, each having its own unique

properties and mechanisms of charge induction.

We first demonstrate that unlike ionic liquids, PFPE does not contain mobile ions. For

this purpose, we have measured and compared dc and transient conductivity of PFPE and an ion

gel. dc electrical conductivity of PFPE has been tested in different contact configurations using a

sensitive low-noise measurement system based on Keithley electrometers equipped with shielded

triax cables and an external grounded metal sample enclosure. The sensitivity of our current

measurements in routine experiments is better than pA. For these measurements, contacts in

different geometries were evaporated through a shadow mask on pre-cleaned glass slides, wired

to the sample holder, and PFPE polymer was drop cast between the contacts. We could only

11

register ~ pA currents (similar to the noise floor of our setup), when voltages up to ±210 V were

applied across the contacts submerged in PFPE. Thus, the dc electrical conductivity of PFPE is

less than 10-14

S.

Figure S7. Transient response of ion gel and PFPE to a step voltage. The inset in (a) is the

schematics of our measurement setup. Details of the setup and measurement are described in the text.

Next, we have performed the measurements of transient current response of an ion gel (as

a sample for comparison) and PFPE to an abrupt voltage step. The measurement setup is

depicted in the inset in Fig. S7(a). The ion gel used in this test was formed by gelation of a

copolymer, poly(vinylidene fluoride-co-hexafluoropropylene), in an ionic liquid 1-ethyl-3-

methylimidazolium bis(trifluoromethylsulfonyl) amide. More details of the ion gel can be found





12

in refs. [4,5]. The sample was prepared either by laminating a piece of ion gel or by depositing a

drop of PFPE on an insulating glass substrate with pre-fabricated contacts. The sample was

connected to an oscilloscope set in an external trigger mode which would register a transient

signal, if a sufficiently large current flows across the load resistor, Z. A step voltage ∆V was

then applied to the sample. Fig. S7(a) shows the transient current detected by the oscilloscope

when a ∆V = 0.1 V was applied to an ion gel sample. In this case, the step voltage caused the

mobile ions in the ion gel (initially at equilibrium and distributed uniformly) to move, which

produced the transient current. Eventually, as the ion gel became fully polarized, no further ion

motion was possible and the transient current disappeared. In this case, the signal was large and

easily detectible even though a very small voltage step was applied. Integrating the current over

the time span gives the total amount of charge transferred through the circuit, ~ 78 nC. An

approximate value of capacitance can thus be calculated as 78 nC/0.1 V = 0.78 µF, which is

consistent with the values reported in the literature for ion gels [4,5]. As shown in Fig. S6(b),

even with a much smaller voltage ∆V = 1 mV, the transient current was still detectable. The

charge transferred in this case was about 0.8 nC, giving a similar capacitance, 0.8 µF. As

opposed to the behavior of the ion gel, no signal could be detected at all in PFPE samples even

with a much higher voltage ∆V = 100 V (Fig. S7(c)). If one takes the 0.8 nC measured in (b) as

the detection limit of charge transfer of the measurement setup, the upper bound for the

capacitance of PFPE sample is only 0.8 nC/100 V = 8 pF, which means that its capacitance is

essentially similar to the pair of contacts with air or vacuum between them. All these results

prove that PFPE is highly insulating, and there are no mobile ions in it.

Due to such an essential difference, the mechanisms of charge induction by PFPE and

ionic liquids are fundamentally different. In the case of an ion gel, this is realized by applying an

13

external gate voltage which drives mobile ions and thus polarizes the ion gel. Solvated ions with

the polarity of the gate voltage aggregate at the semiconductor/ion gel interface and induce

charge carriers of the opposite sign in the semiconductor. In this case, an external gate voltage is

essential. On the contrary, charge accumulation at the semiconductor/PFPE interface is achieved

by local permanent dipoles in PFPE structure (the polar (CF)+-(CF3)

- groups). There is no

external gate voltage needed. However, in a FET-like structure like those shown in Fig. 3 of the

main text and Fig. S8(a), PFPE and an external gate electric field co-exist and both induce charge

carriers in the accumulation channel. The situation thus becomes more complicated than in the

case of just a semiconductor/PFPE interface without any external fields, and a further elaboration

will be necessary. In the following, we present a proper model for understanding charge

induction in FETs with an interfacial PFPE and also compare it to the case of ion gel FETs.

Figure S8. FETs with different gate dielectric layer: (a) Mylar and PFPE, (b) ion gel. C1 and C2 are

capacitance of the Mylar film and the PFPE layer, respectively. Thicknesses of these layers are shown

not to scale. The dipoles in (a) (white ovals) represent the permanent dipoles of PFPE associated with the

polar (CF)+-(CF3)

- groups. C is the capacitance of an electrolyte or ion gel in (b). Semiconductor is

shown in red.

Fig. S8(a) shows the schematic structure of FETs used in Fig. 3 of the main text. A

Mylar film (2.5 µm-thick) is vacuum laminated on top of a rubrene single crystal with pre-





12

in refs. [4,5]. The sample was prepared either by laminating a piece of ion gel or by depositing a

drop of PFPE on an insulating glass substrate with pre-fabricated contacts. The sample was

connected to an oscilloscope set in an external trigger mode which would register a transient

signal, if a sufficiently large current flows across the load resistor, Z. A step voltage ∆V was

then applied to the sample. Fig. S7(a) shows the transient current detected by the oscilloscope

when a ∆V = 0.1 V was applied to an ion gel sample. In this case, the step voltage caused the

mobile ions in the ion gel (initially at equilibrium and distributed uniformly) to move, which

produced the transient current. Eventually, as the ion gel became fully polarized, no further ion

motion was possible and the transient current disappeared. In this case, the signal was large and

easily detectible even though a very small voltage step was applied. Integrating the current over

the time span gives the total amount of charge transferred through the circuit, ~ 78 nC. An

approximate value of capacitance can thus be calculated as 78 nC/0.1 V = 0.78 µF, which is

consistent with the values reported in the literature for ion gels [4,5]. As shown in Fig. S6(b),

even with a much smaller voltage ∆V = 1 mV, the transient current was still detectable. The

charge transferred in this case was about 0.8 nC, giving a similar capacitance, 0.8 µF. As

opposed to the behavior of the ion gel, no signal could be detected at all in PFPE samples even

with a much higher voltage ∆V = 100 V (Fig. S7(c)). If one takes the 0.8 nC measured in (b) as

the detection limit of charge transfer of the measurement setup, the upper bound for the

capacitance of PFPE sample is only 0.8 nC/100 V = 8 pF, which means that its capacitance is

essentially similar to the pair of contacts with air or vacuum between them. All these results

prove that PFPE is highly insulating, and there are no mobile ions in it.

Due to such an essential difference, the mechanisms of charge induction by PFPE and

ionic liquids are fundamentally different. In the case of an ion gel, this is realized by applying an

13

external gate voltage which drives mobile ions and thus polarizes the ion gel. Solvated ions with

the polarity of the gate voltage aggregate at the semiconductor/ion gel interface and induce

charge carriers of the opposite sign in the semiconductor. In this case, an external gate voltage is

essential. On the contrary, charge accumulation at the semiconductor/PFPE interface is achieved

by local permanent dipoles in PFPE structure (the polar (CF)+-(CF3)

- groups). There is no

external gate voltage needed. However, in a FET-like structure like those shown in Fig. 3 of the

main text and Fig. S8(a), PFPE and an external gate electric field co-exist and both induce charge

carriers in the accumulation channel. The situation thus becomes more complicated than in the

case of just a semiconductor/PFPE interface without any external fields, and a further elaboration

will be necessary. In the following, we present a proper model for understanding charge

induction in FETs with an interfacial PFPE and also compare it to the case of ion gel FETs.

Figure S8. FETs with different gate dielectric layer: (a) Mylar and PFPE, (b) ion gel. C1 and C2 are

capacitance of the Mylar film and the PFPE layer, respectively. Thicknesses of these layers are shown

not to scale. The dipoles in (a) (white ovals) represent the permanent dipoles of PFPE associated with the

polar (CF)+-(CF3)

- groups. C is the capacitance of an electrolyte or ion gel in (b). Semiconductor is

shown in red.

Fig. S8(a) shows the schematic structure of FETs used in Fig. 3 of the main text. A

Mylar film (2.5 µm-thick) is vacuum laminated on top of a rubrene single crystal with pre-





14

fabricated source and drain contacts and pre-deposited liquid drop of PFPE. Vacuum lamination

uniformly presses the Mylar film against the crystal and squeezes out the excess of PFPE,

producing an extremely thin (<< 1 µm) interfacial PFPE layer. Very small thickness of PFPE

layer produced in such a way has been verified by independent measurements of capacitance of

Mylar/metal/PFPE/metal/glass and metal/Mylar/PFPE/metal/glass structures (see the discussion

below). Here, we want to emphasize that since PFPE does not contain mobile ions, the bulk of

the PFPE layer in the FET structure shown in Fig. S8(a) simply serves as a part of the gate

dielectric, and its contribution to the overall capacitance of the structure is negligible because of

its small thickness. Thus, the only significant role of PFPE in this structure is to induce charge

carriers at the semiconductor/PFPE interface via the mechanism based on the local PFPE dipole

moments interacting with the semiconductor. Charge induction by an external gate voltage and

by PFPE molecules at the interface should be considered separately.

Capacitance of the bulk PFPE layer is C2 = εPFPEε0/d, where εPFPE and ε0 are permittivity

of PFPE and vacuum, d is the thickness of the PFPE layer, respectively. The total capacitance of

the structure in Fig. S8(a) is C1C2/(C1+C2). We experimentally determined C2 in the following

way. We first measured C1 by vacuum laminating a Mylar film (without PFPE) directly onto a

metallized glass slide, then laminated the same Mylar film onto the same metallized glass slide

but with pre-deposited PFPE and measured the total capacitance again, which should be

C1C2/(C1+C2). We found no significant difference between the two cases (< 3% variation),

meaning that C2 must be much larger than C1, which is expected because of a very small

thickness of PFPE layer. Indeed, since PFPE does not have mobile ions and cannot contribute a

large bulk capacitance like an ion gel, a large C2 can be only associated with a small d. In other

words, the PFPE layer is much thinner than the Mylar film, and its contribution to the total gate-

15

channel capacitance of the FET in Fig. S8(a) is negligible. This validates the mobility extraction

in Fig. 3 of the main text, where the same gate-channel capacitance was used for all the devices,

with and without PFPE. Since the contribution of PFPE as a gate dielectric is negligible, its

mere role in the operation of these FETs is contributing to the charge accumulation at the

semiconductor/PFPE interface via the dipolar effect described above. The amount of this

contribution can be estimated by comparing the difference between the two transfer curves in Fig.

3(a) of the main text (red and black symbols). At a given gate voltage deep in the accumulation

regime (VG < -20 V), the device with PFPE has conductivity higher by ∆σ ~ 40 nS,

corresponding to an additional amount of charges ∆n ~ 7 × 1010

cm-2

.

Charge induction in an ion-gel FET is depicted in Fig. S8(b). Due to its mobile ions, ion

gel becomes highly polarized under an external gate voltage and an electric double layer forms at

the ion gel/semiconductor interface. This mechanism is of course different from that in (a),

where charge induction occurs through two independent ways: the dipolar effect of PFPE

described in this work and the conventional gate electric field-induced accumulation channel.

7. Recovery of the “gauge effect” damage in rubrene OFETs by PFPE functionalization.

“Gauge effect” is a well known degradation phenomenon observed when a

semiconductor, e.g. rubrene single crystal, is placed in a high-vacuum chamber with an operating

high-vacuum gauge. It has been shown that high-vacuum gauges (and resistively heated

filaments or evaporation boats) generate electrically neutral free radicals that land on exposed

surfaces of the sample and create hole traps, including deep and shallow trap states [6]. For

instance, if a PDMS vacuum-gap OFET is placed in a high-vacuum chamber, the exposed

accumulation channel of the device will interact with species generated by a high-vacuum gauge,





14

fabricated source and drain contacts and pre-deposited liquid drop of PFPE. Vacuum lamination

uniformly presses the Mylar film against the crystal and squeezes out the excess of PFPE,

producing an extremely thin (<< 1 µm) interfacial PFPE layer. Very small thickness of PFPE

layer produced in such a way has been verified by independent measurements of capacitance of

Mylar/metal/PFPE/metal/glass and metal/Mylar/PFPE/metal/glass structures (see the discussion

below). Here, we want to emphasize that since PFPE does not contain mobile ions, the bulk of

the PFPE layer in the FET structure shown in Fig. S8(a) simply serves as a part of the gate

dielectric, and its contribution to the overall capacitance of the structure is negligible because of

its small thickness. Thus, the only significant role of PFPE in this structure is to induce charge

carriers at the semiconductor/PFPE interface via the mechanism based on the local PFPE dipole

moments interacting with the semiconductor. Charge induction by an external gate voltage and

by PFPE molecules at the interface should be considered separately.

Capacitance of the bulk PFPE layer is C2 = εPFPEε0/d, where εPFPE and ε0 are permittivity

of PFPE and vacuum, d is the thickness of the PFPE layer, respectively. The total capacitance of

the structure in Fig. S8(a) is C1C2/(C1+C2). We experimentally determined C2 in the following

way. We first measured C1 by vacuum laminating a Mylar film (without PFPE) directly onto a

metallized glass slide, then laminated the same Mylar film onto the same metallized glass slide

but with pre-deposited PFPE and measured the total capacitance again, which should be

C1C2/(C1+C2). We found no significant difference between the two cases (< 3% variation),

meaning that C2 must be much larger than C1, which is expected because of a very small

thickness of PFPE layer. Indeed, since PFPE does not have mobile ions and cannot contribute a

large bulk capacitance like an ion gel, a large C2 can be only associated with a small d. In other

words, the PFPE layer is much thinner than the Mylar film, and its contribution to the total gate-

15

channel capacitance of the FET in Fig. S8(a) is negligible. This validates the mobility extraction

in Fig. 3 of the main text, where the same gate-channel capacitance was used for all the devices,

with and without PFPE. Since the contribution of PFPE as a gate dielectric is negligible, its

mere role in the operation of these FETs is contributing to the charge accumulation at the

semiconductor/PFPE interface via the dipolar effect described above. The amount of this

contribution can be estimated by comparing the difference between the two transfer curves in Fig.

3(a) of the main text (red and black symbols). At a given gate voltage deep in the accumulation

regime (VG < -20 V), the device with PFPE has conductivity higher by ∆σ ~ 40 nS,

corresponding to an additional amount of charges ∆n ~ 7 × 1010

cm-2

.

Charge induction in an ion-gel FET is depicted in Fig. S8(b). Due to its mobile ions, ion

gel becomes highly polarized under an external gate voltage and an electric double layer forms at

the ion gel/semiconductor interface. This mechanism is of course different from that in (a),

where charge induction occurs through two independent ways: the dipolar effect of PFPE

described in this work and the conventional gate electric field-induced accumulation channel.

7. Recovery of the “gauge effect” damage in rubrene OFETs by PFPE functionalization.

“Gauge effect” is a well known degradation phenomenon observed when a

semiconductor, e.g. rubrene single crystal, is placed in a high-vacuum chamber with an operating

high-vacuum gauge. It has been shown that high-vacuum gauges (and resistively heated

filaments or evaporation boats) generate electrically neutral free radicals that land on exposed

surfaces of the sample and create hole traps, including deep and shallow trap states [6]. For

instance, if a PDMS vacuum-gap OFET is placed in a high-vacuum chamber, the exposed

accumulation channel of the device will interact with species generated by a high-vacuum gauge,





16

resulting in a drastic deterioration of the field-effect mobility and an increase of the threshold

voltage that can be detected in in-situ measurements [6].

Here, we have applied PFPE to the surface of rubrene crystals intentionally degraded by

the gauge effect and observed a complete recovery of the intrinsic properties of rubrene. In this

experiment, we first fabricated a high-performance OFET using a pristine rubrene single crystal

and a vacuum-laminated Mylar as a gate dielectric (for details on vacuum lamination method see

ref. [1]). This resulted in a device with an outstanding linear transconductance, intrinsic mobility

of ~ 5.5 cm2V

-1s

-1 and nearly zero threshold/onset voltage (black solid circles, Fig. S9). We then

delaminated the Mylar and exposed the crystal surface to a high-vacuum gage in a vacuum

chamber (at 10-7

Torr for 10 min). Then, another Mylar lamination was performed on the same

crystal right after removing it from the chamber, which revealed that a typical gauge effect took

place: the OFET’s mobility decreased noticeably, and the threshold voltage increased (blue open

circles, Fig. S9). After this, we immediately delaminated the Mylar and applied a very thin layer

of PFPE to the crystal surface. Transconductance was measured again several hours later and the

result clearly showed that the intrinsic (pristine) behavior of rubrene was recovered (red solid

squares, Fig. S9).

We note that usually a waiting period of several hours after application of PFPE, during

which samples should be left unperturbed, is necessary for observing a full recovery to the

pristine state. Attempts to obtain time dependent transconductance characteristics by performing

multiple sequential FET measurements as the PFPE functionalization occurs usually lead to only

a partial recovery. This happens because repeated Mylar laminations apparently perturb the

PFPE functionalization, although the details of this process are unclear at this moment. Similarly,

a full recovery of intrinsic properties in OFETs based on photo-oxidized crystals (Fig. 3(b))

17

requires a few-hour waiting period between PFPE application to the crystal surface and Mylar

lamination for FET measurements. We also note that vacuum lamination technique allowed us

to use the same crystal for all three measurements, eliminating possible variations of crystal

properties from sample to sample.

Figure S9. Recovery of pristine transport characteristics of rubrene after damage by gauge effect.

Transfer characteristics, ISD(VG), of a rubrene single-crystal OFET (formed by the vacuum lamination

technique [1]) sequentially measured in the following order: (1) on pristine crystal (black solid circles),

(2) the same crystal after exposing its surface to an operating high-vacuum gauge (blue open circles), and

(3) the same crystal (degraded by the gauge effect) after a thin layer of PFPE was applied (red solid

squares). Vacuum-lamination OFET method with a detachable 2.5 µm-thick Mylar gate dielectric was

used to perform all three measurements on the same single crystal. The source-drain voltage was VSD = 10

V. The data clearly demonstrate a trap healing effect of PFPE.

This additional experiment shows once again that PFPE has a remarkable trap healing

ability when applied to trap-dominated samples. It confirms our conclusion that the effect





16

resulting in a drastic deterioration of the field-effect mobility and an increase of the threshold

voltage that can be detected in in-situ measurements [6].

Here, we have applied PFPE to the surface of rubrene crystals intentionally degraded by

the gauge effect and observed a complete recovery of the intrinsic properties of rubrene. In this

experiment, we first fabricated a high-performance OFET using a pristine rubrene single crystal

and a vacuum-laminated Mylar as a gate dielectric (for details on vacuum lamination method see

ref. [1]). This resulted in a device with an outstanding linear transconductance, intrinsic mobility

of ~ 5.5 cm2V

-1s

-1 and nearly zero threshold/onset voltage (black solid circles, Fig. S9). We then

delaminated the Mylar and exposed the crystal surface to a high-vacuum gage in a vacuum

chamber (at 10-7

Torr for 10 min). Then, another Mylar lamination was performed on the same

crystal right after removing it from the chamber, which revealed that a typical gauge effect took

place: the OFET’s mobility decreased noticeably, and the threshold voltage increased (blue open

circles, Fig. S9). After this, we immediately delaminated the Mylar and applied a very thin layer

of PFPE to the crystal surface. Transconductance was measured again several hours later and the

result clearly showed that the intrinsic (pristine) behavior of rubrene was recovered (red solid

squares, Fig. S9).

We note that usually a waiting period of several hours after application of PFPE, during

which samples should be left unperturbed, is necessary for observing a full recovery to the

pristine state. Attempts to obtain time dependent transconductance characteristics by performing

multiple sequential FET measurements as the PFPE functionalization occurs usually lead to only

a partial recovery. This happens because repeated Mylar laminations apparently perturb the

PFPE functionalization, although the details of this process are unclear at this moment. Similarly,

a full recovery of intrinsic properties in OFETs based on photo-oxidized crystals (Fig. 3(b))

17

requires a few-hour waiting period between PFPE application to the crystal surface and Mylar

lamination for FET measurements. We also note that vacuum lamination technique allowed us

to use the same crystal for all three measurements, eliminating possible variations of crystal

properties from sample to sample.

Figure S9. Recovery of pristine transport characteristics of rubrene after damage by gauge effect.

Transfer characteristics, ISD(VG), of a rubrene single-crystal OFET (formed by the vacuum lamination

technique [1]) sequentially measured in the following order: (1) on pristine crystal (black solid circles),

(2) the same crystal after exposing its surface to an operating high-vacuum gauge (blue open circles), and

(3) the same crystal (degraded by the gauge effect) after a thin layer of PFPE was applied (red solid

squares). Vacuum-lamination OFET method with a detachable 2.5 µm-thick Mylar gate dielectric was

used to perform all three measurements on the same single crystal. The source-drain voltage was VSD = 10

V. The data clearly demonstrate a trap healing effect of PFPE.

This additional experiment shows once again that PFPE has a remarkable trap healing

ability when applied to trap-dominated samples. It confirms our conclusion that the effect





18

discovered in this work is not just another doping method. Indeed, a straightforward increase of

the carrier density by, for example, application of a greater negative VG in the state labeled “after

gauge effect” (blue open circles, Fig. S9) would not change the slope of transconductance, which

is already in the linear regime at VG < -30V. Also, a conventional chemical doping of the surface

would not increase the slope either; it would simply shift the entire transconductance curve to the

right (for p-type doping) and is even likely to decrease the slope (mobility), because chemical

dopants usually introduce additional traps and scattering centers. Thus, this experiment

emphasizes an important novel aspect of the two-dimensional conducting state that we have

observed at the interface between an organic semiconductor and PFPE polymer: shallow traps

are electronically passivated. It is also worth noting that in this experiment traps have been

generated using a completely different method (compared to photo-oxidation used in Fig. 3 of

the main text), thus showing that the exact microscopic nature of trap states and the type of

species creating them do not matter for the trap healing effect of PFPE.

Another important detail shown in the inset in Fig. S9 is a positive shift of the device’s

threshold voltage that occurs as a consequence of PFPE functionalization. Even though the trap-

healing effect is described in this work as a conversion of shallow traps to deep traps, we should

keep in mind that the resultant deep traps are filled with holes interacting with PFPE dipoles.

Therefore, we do not expect to see a shift of the threshold voltage to the left (negative) in p-type

OFETs with an interfacial PFPE layer, as would be the case in p-type OFETs with additional

empty deep traps. On the contrary, the opposite should be observed similarly to the cases of a

chemical hole doping in FETs. Indeed, this is exactly what is observed in our experiment (Figs.

3 S9); in all these cases, the devices with PFPE have the threshold shifted to the right.

19

8. Trap healing effect of PFPE in photoconductivity measurements.

Studies of spectrally resolved photoconductivity and polarization dependence of

photocurrent were shown to be very useful for characterization of surface traps in organic single

crystals. For instance, photocurrent excitation spectroscopy can be used to distinguish pristine

organic semiconductors (that is, trap-free crystals) from trap-dominated ones [ 7 ]. Photo-

conductivity of organic crystals, σPC, typically exhibits periodic modulations (the so-called

“oscillations”) when polarization of a linearly polarized photoexcitation is rotated with respect to

the crystal axes [7]. These oscillations are due to a strong dependence of the absorption

coefficient α (and the absorption length, α-1) on polarization angle, . In addition, an enhanced

exciton diffusion and the surface nature of photoconductivity generation in certain molecular

crystals also play an important role [8].

For example, in pristine (trap-free) rubrene crystals, photocurrent modulations with

have the character of the so-called “normal oscillations”: that is, σPS() resembles a sinusoidal

function with maxima for polarization along the b axis of the crystal ( = 90o + πn, n = 0, 1, 2…)

and minima – for polarization along the a axis ( = 180o + πn, n = 0, 1, 2…). In a trap-

dominated crystal, σPC() will exhibit the so-called “anti-oscillations”: the minima will occur at

polarization along the b axis. There could also be intermediate states, when the samples are still

pure, but there is a moderate density of traps at the surface. In such a case, σPC() may still have

mainly the “normal” character, but the maxima will be “eaten up” (a dip develops on top of each

maximum). These interesting effects in polarization dependence of photocurrent in rubrene have

been recently discovered and interpreted [7].

In the additional experiment below, we have tested the effect of PFPE functionalization

on photoconductivity of a trap-dominated rubrene crystal. Typical σPC() dependence of a trap





18

discovered in this work is not just another doping method. Indeed, a straightforward increase of

the carrier density by, for example, application of a greater negative VG in the state labeled “after

gauge effect” (blue open circles, Fig. S9) would not change the slope of transconductance, which

is already in the linear regime at VG < -30V. Also, a conventional chemical doping of the surface

would not increase the slope either; it would simply shift the entire transconductance curve to the

right (for p-type doping) and is even likely to decrease the slope (mobility), because chemical

dopants usually introduce additional traps and scattering centers. Thus, this experiment

emphasizes an important novel aspect of the two-dimensional conducting state that we have

observed at the interface between an organic semiconductor and PFPE polymer: shallow traps

are electronically passivated. It is also worth noting that in this experiment traps have been

generated using a completely different method (compared to photo-oxidation used in Fig. 3 of

the main text), thus showing that the exact microscopic nature of trap states and the type of

species creating them do not matter for the trap healing effect of PFPE.

Another important detail shown in the inset in Fig. S9 is a positive shift of the device’s

threshold voltage that occurs as a consequence of PFPE functionalization. Even though the trap-

healing effect is described in this work as a conversion of shallow traps to deep traps, we should

keep in mind that the resultant deep traps are filled with holes interacting with PFPE dipoles.

Therefore, we do not expect to see a shift of the threshold voltage to the left (negative) in p-type

OFETs with an interfacial PFPE layer, as would be the case in p-type OFETs with additional

empty deep traps. On the contrary, the opposite should be observed similarly to the cases of a

chemical hole doping in FETs. Indeed, this is exactly what is observed in our experiment (Figs.

3 S9); in all these cases, the devices with PFPE have the threshold shifted to the right.

19

8. Trap healing effect of PFPE in photoconductivity measurements.

Studies of spectrally resolved photoconductivity and polarization dependence of

photocurrent were shown to be very useful for characterization of surface traps in organic single

crystals. For instance, photocurrent excitation spectroscopy can be used to distinguish pristine

organic semiconductors (that is, trap-free crystals) from trap-dominated ones [ 7 ]. Photo-

conductivity of organic crystals, σPC, typically exhibits periodic modulations (the so-called

“oscillations”) when polarization of a linearly polarized photoexcitation is rotated with respect to

the crystal axes [7]. These oscillations are due to a strong dependence of the absorption

coefficient α (and the absorption length, α-1) on polarization angle, . In addition, an enhanced

exciton diffusion and the surface nature of photoconductivity generation in certain molecular

crystals also play an important role [8].

For example, in pristine (trap-free) rubrene crystals, photocurrent modulations with

have the character of the so-called “normal oscillations”: that is, σPS() resembles a sinusoidal

function with maxima for polarization along the b axis of the crystal ( = 90o + πn, n = 0, 1, 2…)

and minima – for polarization along the a axis ( = 180o + πn, n = 0, 1, 2…). In a trap-

dominated crystal, σPC() will exhibit the so-called “anti-oscillations”: the minima will occur at

polarization along the b axis. There could also be intermediate states, when the samples are still

pure, but there is a moderate density of traps at the surface. In such a case, σPC() may still have

mainly the “normal” character, but the maxima will be “eaten up” (a dip develops on top of each

maximum). These interesting effects in polarization dependence of photocurrent in rubrene have

been recently discovered and interpreted [7].

In the additional experiment below, we have tested the effect of PFPE functionalization

on photoconductivity of a trap-dominated rubrene crystal. Typical σPC() dependence of a trap





20

dominated crystal (with traps at the surface) is shown in Fig. S10 (left panel). The geometry of

the experiment is the same as that used in [7]. For strongly absorbed excitation, the photocurrent

oscillations clearly show a “dip” developing at the maxima for polarization along b. This is a

sign that the crystal has traps at the surface (understanding of this has been developed in a prior

publication [7]). Remarkably, we have observed that functionalization of such a crystal with

PFPE recovers the normal character of photocurrent oscillations (right panel). Indeed, after

coating the crystal with PFPE, σPC() dependences become converted to pure “normal

oscillations”, which is the behavior corresponding to the intrinsic state of a crystal, as if traps

were eliminated. It is worth noting that not only had the character of the photocurrent

oscillations qualitatively changed (from “anti” to “normal” oscillations), but also the absolute

value of photoconductivity has notably increased. The latter is consistent with an increased

surface mobility of the photo-generated carriers due to the trap healing at the surface.

These additional data again show that, from the perspective of surface photoconductivity,

PFPE passivates electronic traps at the surface of trap-dominated organic semiconductors, thus

helping to reveal the intrinsic photoconductive properties of initially trap-dominated samples.

We also note that since the traps leading to “anti-oscillations” in photoconductivity are

distributed over the entire surface of the crystal, and the photoconductivity in this case is not

contact-limited, the trap-healing by PFPE must occur through the entire surface of the crystal,

rather than being a mere contact effect.

21

Figure S10. Effect of PFPE on photoconductivity of rubrene crystals. Left panel: polarization

dependence of photoconductivity, σPC(), measured in a (moderately) trap-dominated rubrene crystal at

several excitation wavelengths, λexc (indicated). The appearance of “anti-oscillations” is clearly visible

(red, green and blue curves). Right panel: σPC() measured after this crystal has been coated with PFPE.

Note that the photocurrent oscillations became “normal”, and the absolute value of σPC is increased.

These data suggest that surface traps were eliminated via PFPE functionalization.

9. Hall effect measurement in rubrene crystals functionalized with PFPE.

In typical Hall measurements, a constant excitation source-drain current, ISD, was applied

to the device under study, and the longitudinal four probe voltage, V4p, as well as the Hall voltage,

VH, were recorded simultaneously, as the magnetic field, B, was varied. In an ideal Hall





20

dominated crystal (with traps at the surface) is shown in Fig. S10 (left panel). The geometry of

the experiment is the same as that used in [7]. For strongly absorbed excitation, the photocurrent

oscillations clearly show a “dip” developing at the maxima for polarization along b. This is a

sign that the crystal has traps at the surface (understanding of this has been developed in a prior

publication [7]). Remarkably, we have observed that functionalization of such a crystal with

PFPE recovers the normal character of photocurrent oscillations (right panel). Indeed, after

coating the crystal with PFPE, σPC() dependences become converted to pure “normal

oscillations”, which is the behavior corresponding to the intrinsic state of a crystal, as if traps

were eliminated. It is worth noting that not only had the character of the photocurrent

oscillations qualitatively changed (from “anti” to “normal” oscillations), but also the absolute

value of photoconductivity has notably increased. The latter is consistent with an increased

surface mobility of the photo-generated carriers due to the trap healing at the surface.

These additional data again show that, from the perspective of surface photoconductivity,

PFPE passivates electronic traps at the surface of trap-dominated organic semiconductors, thus

helping to reveal the intrinsic photoconductive properties of initially trap-dominated samples.

We also note that since the traps leading to “anti-oscillations” in photoconductivity are

distributed over the entire surface of the crystal, and the photoconductivity in this case is not

contact-limited, the trap-healing by PFPE must occur through the entire surface of the crystal,

rather than being a mere contact effect.

21

Figure S10. Effect of PFPE on photoconductivity of rubrene crystals. Left panel: polarization

dependence of photoconductivity, σPC(), measured in a (moderately) trap-dominated rubrene crystal at

several excitation wavelengths, λexc (indicated). The appearance of “anti-oscillations” is clearly visible

(red, green and blue curves). Right panel: σPC() measured after this crystal has been coated with PFPE.

Note that the photocurrent oscillations became “normal”, and the absolute value of σPC is increased.

These data suggest that surface traps were eliminated via PFPE functionalization.

9. Hall effect measurement in rubrene crystals functionalized with PFPE.

In typical Hall measurements, a constant excitation source-drain current, ISD, was applied

to the device under study, and the longitudinal four probe voltage, V4p, as well as the Hall voltage,

VH, were recorded simultaneously, as the magnetic field, B, was varied. In an ideal Hall





22

geometry, where the Hall voltage leads are well aligned, the offset in VH signal should be nearly

zero. In reality, however, a small misalignment of Hall voltage leads is unavoidable, which

results in a non-zero background in the Hall signal. This background is independent of the

magnetic field and typically exhibits a drift with time.

Figure S11. Hall effect measurements in rubrene crystal functionalized with PFPE. (a) Raw data of

Hall voltage, VH (symbols), recorded as the magnetic field, B (solid line), is swept. The linear

background is shown by the dashed line. (b) Hall mobility, µ (open squares), and carrier density, n (solid

squares), as a function of the excitation current, ISD.

23

In the case of rubrene single crystals functionalized with PFPE, as well as in rubrene

OFETs, the drift is usually linear (an example is shown in Fig. S11(a)), and it can thus be easily

subtracted, provided that the correct sign reversal of the Hall voltage is observed in a magnetic

field of alternating polarity. The linear background and the ultra-low noise level presented in

this work enable us to determine the Hall signal with high accuracy, even under such conditions

as extremely small excitation currents and low magnetic fields, at which Hall effect

measurements in organic semiconductors were not possible before. Figure S11(b) shows the

carrier mobility, µ, and carrier concentration, n, extracted from these Hall measurements and

plotted as a function of the excitation current. Only minor variations in µ and n were observed as

the current was varied in a range over two orders of magnitude, which further confirms the

precision of this method.

10. High-precision Hall effect measurements in small magnetic fields.

The intermolecular π-π interactions in small-molecule organic semiconductors could be

sufficiently strong to result in carrier delocalization over a number of lattice sites, leading to a

band-like charge carrier transport in certain systems. However, weak van der Waals interactions

between the molecules still limit the charge carrier mobilities to rather small intrinsic values of µ

~ 1 - 20 cm2V

-1s

-1, which typically results in rather weak Hall signals in molecular crystals (see,

e.g., ref. [2] of the main text and references therein). In addition, electrical noise that originates

from multiple trap and release processes is usually significant in organic semiconductors. Noise

can easily mask weak Hall signal, and, therefore, in order to detect a Hall effect very high

magnetic fields (several Teslas) were always necessary. Such high fields are normally obtained

by operating a liquid-He cooled superconducting magnet, making these experiments expensive,





22

geometry, where the Hall voltage leads are well aligned, the offset in VH signal should be nearly

zero. In reality, however, a small misalignment of Hall voltage leads is unavoidable, which

results in a non-zero background in the Hall signal. This background is independent of the

magnetic field and typically exhibits a drift with time.

Figure S11. Hall effect measurements in rubrene crystal functionalized with PFPE. (a) Raw data of

Hall voltage, VH (symbols), recorded as the magnetic field, B (solid line), is swept. The linear

background is shown by the dashed line. (b) Hall mobility, µ (open squares), and carrier density, n (solid

squares), as a function of the excitation current, ISD.

23

In the case of rubrene single crystals functionalized with PFPE, as well as in rubrene

OFETs, the drift is usually linear (an example is shown in Fig. S11(a)), and it can thus be easily

subtracted, provided that the correct sign reversal of the Hall voltage is observed in a magnetic

field of alternating polarity. The linear background and the ultra-low noise level presented in

this work enable us to determine the Hall signal with high accuracy, even under such conditions

as extremely small excitation currents and low magnetic fields, at which Hall effect

measurements in organic semiconductors were not possible before. Figure S11(b) shows the

carrier mobility, µ, and carrier concentration, n, extracted from these Hall measurements and

plotted as a function of the excitation current. Only minor variations in µ and n were observed as

the current was varied in a range over two orders of magnitude, which further confirms the

precision of this method.

10. High-precision Hall effect measurements in small magnetic fields.

The intermolecular π-π interactions in small-molecule organic semiconductors could be

sufficiently strong to result in carrier delocalization over a number of lattice sites, leading to a

band-like charge carrier transport in certain systems. However, weak van der Waals interactions

between the molecules still limit the charge carrier mobilities to rather small intrinsic values of µ

~ 1 - 20 cm2V

-1s

-1, which typically results in rather weak Hall signals in molecular crystals (see,

e.g., ref. [2] of the main text and references therein). In addition, electrical noise that originates

from multiple trap and release processes is usually significant in organic semiconductors. Noise

can easily mask weak Hall signal, and, therefore, in order to detect a Hall effect very high

magnetic fields (several Teslas) were always necessary. Such high fields are normally obtained

by operating a liquid-He cooled superconducting magnet, making these experiments expensive,





24

time consuming and difficult to perform. However, even with the help of high magnetic field,

good quality Hall effect measurements in organic semiconductors are extremely rare. For this

reason, Hall effect in organic semiconductors has only been demonstrated in a handful of cases

(see, e.g., ref. [2] of the main text and references therein).

Figure S12. High-precision Hall effect measurements performed in small magnetic field in

rubrene/PFPE. The main panel shows Hall voltage (red open symbols) measured as the magnetic field

(black solid line) is swept between -0.3 to 0.3 T. The inset compares the same data (red open symbols) to

the result of a separate measurement (blue open circles) performed up to a much greater magnetic field of

±2 T. Both measurements give consistent results. The signal to noise ratio of this Hall effect measurement

is excellent even in small magnetic fields. Geometry of the sample and Hall measurement is conventional

(see the sketch in Fig. 4 of the main text).

The trap-healing effect of PFPE reported in this study may help to dramatically improve

the quality of Hall effect measurements in organic semiconductors. With a significantly

enhanced signal to noise ratio, precise Hall effect measurements at small magnetic fields are now

25

possible. Figure S12 shows an example where Hall effect is clearly demonstrated in a PFPE-

functionalized rubrene crystal at small magnetic fields (< 0.3 Tesla). Such magnetic fields are

well within the reach of typical small-size laboratory electromagnets that do not require any

cryogenic liquids for operation. Even at such low fields, the Hall signal in our samples is well

above the noise. In addition, a comparison between the low-field result and a separate

measurement performed in much higher magnetic fields (up to 2 Tesla) reveals an excellent

consistency, reassuring the precision of both measurements (inset in Fig. S12).

11. Comparison of noise in rubrene FETs and rubrene functionalized with PFPE.

The mechanism of trap healing at organic semiconductor/PFPE interfaces can be further

elucidated by studying the electrical noise in these devices. In this additional experiment we

have measured the noise power spectra of rubrene/PFPE samples and compared them with those

measured in conventional rubrene single-crystal OFETs. An example of such comparison is

given in Fig. S13. The red curve in Fig. S13 is a typical noise spectrum of rubrene FETs. It has

a clear 1/f form. 1/f noise, ubiquitous in electrical conductors, is a manifestation of fluctuations

of the sample’s conductivity. Its power spectrum density commonly takes the form of 1/f α

,

where α is a constant between 0.8 and 1.4 [9,10]. In most semiconductors, the dominant

mechanism of 1/f noise is occupation fluctuations of electronic traps. These traps capture and

release charge carriers randomly and independently, which results in fluctuations of the electrical

conductivity. While 1/f noise usually spans over many decades in frequency, the frequency

range especially important for Hall effect measurements is f > 0.01 Hz (that is, both the relatively

low and high-frequency parts of the spectrum). The low-frequency domain is important, because

the typical time scales involved in these measurements are 1 - 100 s, corresponding to rates of





24

time consuming and difficult to perform. However, even with the help of high magnetic field,

good quality Hall effect measurements in organic semiconductors are extremely rare. For this

reason, Hall effect in organic semiconductors has only been demonstrated in a handful of cases

(see, e.g., ref. [2] of the main text and references therein).

Figure S12. High-precision Hall effect measurements performed in small magnetic field in

rubrene/PFPE. The main panel shows Hall voltage (red open symbols) measured as the magnetic field

(black solid line) is swept between -0.3 to 0.3 T. The inset compares the same data (red open symbols) to

the result of a separate measurement (blue open circles) performed up to a much greater magnetic field of

±2 T. Both measurements give consistent results. The signal to noise ratio of this Hall effect measurement

is excellent even in small magnetic fields. Geometry of the sample and Hall measurement is conventional

(see the sketch in Fig. 4 of the main text).

The trap-healing effect of PFPE reported in this study may help to dramatically improve

the quality of Hall effect measurements in organic semiconductors. With a significantly

enhanced signal to noise ratio, precise Hall effect measurements at small magnetic fields are now

25

possible. Figure S12 shows an example where Hall effect is clearly demonstrated in a PFPE-

functionalized rubrene crystal at small magnetic fields (< 0.3 Tesla). Such magnetic fields are

well within the reach of typical small-size laboratory electromagnets that do not require any

cryogenic liquids for operation. Even at such low fields, the Hall signal in our samples is well

above the noise. In addition, a comparison between the low-field result and a separate

measurement performed in much higher magnetic fields (up to 2 Tesla) reveals an excellent

consistency, reassuring the precision of both measurements (inset in Fig. S12).

11. Comparison of noise in rubrene FETs and rubrene functionalized with PFPE.

The mechanism of trap healing at organic semiconductor/PFPE interfaces can be further

elucidated by studying the electrical noise in these devices. In this additional experiment we

have measured the noise power spectra of rubrene/PFPE samples and compared them with those

measured in conventional rubrene single-crystal OFETs. An example of such comparison is

given in Fig. S13. The red curve in Fig. S13 is a typical noise spectrum of rubrene FETs. It has

a clear 1/f form. 1/f noise, ubiquitous in electrical conductors, is a manifestation of fluctuations

of the sample’s conductivity. Its power spectrum density commonly takes the form of 1/f α

,

where α is a constant between 0.8 and 1.4 [9,10]. In most semiconductors, the dominant

mechanism of 1/f noise is occupation fluctuations of electronic traps. These traps capture and

release charge carriers randomly and independently, which results in fluctuations of the electrical

conductivity. While 1/f noise usually spans over many decades in frequency, the frequency

range especially important for Hall effect measurements is f > 0.01 Hz (that is, both the relatively

low and high-frequency parts of the spectrum). The low-frequency domain is important, because

the typical time scales involved in these measurements are 1 - 100 s, corresponding to rates of





26

data acquisition and magnetic field sweeps. Thus, the presence of slow fluctuations may

compromise the detection of Hall effect which is typically very small in organic semiconductors.

The high-frequency fluctuations are also detrimental, because a small Hall signal can be easily

masked by high-frequency noise, even if slow fluctuations are absent. Therefore, the dominant

contribution to 1/f noise at the frequencies of interest for Hall effect measurements comes from

the traps with characteristic times, τ < 100 s, that is, both shallow and rather deep traps.

Figure S13. Power spectrum density (PSD) of the current noise in a single-crystal rubrene FET

(red) and a rubrene single crystal functionalized with PFPE (black). The FET has parylene-N

polymer gate dielectric with a thickness of 1 µm, corresponding to a gate-channel capacitance Ci = 2.1

nF/cm2. At a gate voltage of -20 V, the FET’s channel is already deep in the accumulation regime with

the carrier density n = 2.6 × 1011

cm-2

. In this regime, the even noisier sub-threshold regime is avoided.

The functionalized device was prepared by coating a rubrene single crystal (with pre-fabricated source

and drain contacts) with PFPE. The inset is a sketch of the measurement setup. A constant voltage is

applied across the device under test (DUT). The current noise in the DUT is converted to a voltage noise

through a load resistor Z, and its PSD is measured by a spectrum analyzer (S.A.). The carrier densities in

both devices during these measurements were similar.

27

Note that typical trapping time on shallow traps relevant to polaronic charge transport in

OFETs is ~ 0.7 - 2 ns, as recently determined by ESR spectroscopy in pentacene OFETs [11].

Such a short trapping time corresponds to very shallow traps with an activation energy ~ 10 mK,

which is smaller than the room-temperature thermal energy (kBT ≈ 30 mK). According to the

band tail model of trap states, an exponential density of states below the mobility edge dictates

that the density of these shallow traps is higher than the density of slower (deeper) traps. Such

traps are thus important for multiple trap and release (MTR) conduction process governing the

operation of trap-dominated semiconductors. These considerations show that trap energies

relevant to the noise in Hall effect measurements must span rather large range of energies in the

band tail, including the energies smaller and greater than kBT.

Our data show that PFPE functionalization passivates such slow traps very efficiently.

This is demonstrated in the above figure as the drastic suppression of the overall noise level.

Integrating over the shown frequency range gives the total suppression factor of the noise power

as roughly 500. While this by itself is impressive, it is important to note that PFPE

functionalization results in so quiet samples that their noise is below the resolution limit of our

instrument: in the frequency range of our measurements, the noise is completely suppressed (Fig.

S13). Numerous studies of 1/f noise show that the 1/f

α dependence of the noise PSD extends to

arbitrarily low frequency without having any intrinsic cut-off other than the limit of the

measurement time span. Therefore, in the current case, even if the noise of the rubrene/PFPE

device rises above the resolution background at frequencies f < 0.1 Hz (Fig. S13), it will still

follow 1/f dependence, and thus its PSD must still remain about 5 orders of magnitude smaller





26

data acquisition and magnetic field sweeps. Thus, the presence of slow fluctuations may

compromise the detection of Hall effect which is typically very small in organic semiconductors.

The high-frequency fluctuations are also detrimental, because a small Hall signal can be easily

masked by high-frequency noise, even if slow fluctuations are absent. Therefore, the dominant

contribution to 1/f noise at the frequencies of interest for Hall effect measurements comes from

the traps with characteristic times, τ < 100 s, that is, both shallow and rather deep traps.

Figure S13. Power spectrum density (PSD) of the current noise in a single-crystal rubrene FET

(red) and a rubrene single crystal functionalized with PFPE (black). The FET has parylene-N

polymer gate dielectric with a thickness of 1 µm, corresponding to a gate-channel capacitance Ci = 2.1

nF/cm2. At a gate voltage of -20 V, the FET’s channel is already deep in the accumulation regime with

the carrier density n = 2.6 × 1011

cm-2

. In this regime, the even noisier sub-threshold regime is avoided.

The functionalized device was prepared by coating a rubrene single crystal (with pre-fabricated source

and drain contacts) with PFPE. The inset is a sketch of the measurement setup. A constant voltage is

applied across the device under test (DUT). The current noise in the DUT is converted to a voltage noise

through a load resistor Z, and its PSD is measured by a spectrum analyzer (S.A.). The carrier densities in

both devices during these measurements were similar.

27

Note that typical trapping time on shallow traps relevant to polaronic charge transport in

OFETs is ~ 0.7 - 2 ns, as recently determined by ESR spectroscopy in pentacene OFETs [11].

Such a short trapping time corresponds to very shallow traps with an activation energy ~ 10 mK,

which is smaller than the room-temperature thermal energy (kBT ≈ 30 mK). According to the

band tail model of trap states, an exponential density of states below the mobility edge dictates

that the density of these shallow traps is higher than the density of slower (deeper) traps. Such

traps are thus important for multiple trap and release (MTR) conduction process governing the

operation of trap-dominated semiconductors. These considerations show that trap energies

relevant to the noise in Hall effect measurements must span rather large range of energies in the

band tail, including the energies smaller and greater than kBT.

Our data show that PFPE functionalization passivates such slow traps very efficiently.

This is demonstrated in the above figure as the drastic suppression of the overall noise level.

Integrating over the shown frequency range gives the total suppression factor of the noise power

as roughly 500. While this by itself is impressive, it is important to note that PFPE

functionalization results in so quiet samples that their noise is below the resolution limit of our

instrument: in the frequency range of our measurements, the noise is completely suppressed (Fig.

S13). Numerous studies of 1/f noise show that the 1/f

α dependence of the noise PSD extends to

arbitrarily low frequency without having any intrinsic cut-off other than the limit of the

measurement time span. Therefore, in the current case, even if the noise of the rubrene/PFPE

device rises above the resolution background at frequencies f < 0.1 Hz (Fig. S13), it will still

follow 1/f dependence, and thus its PSD must still remain about 5 orders of magnitude smaller





28

than that of the conventional rubrene OFET. Therefore, we believe that ultra slow traps that

might have an even greater negative impact on Hall effect measurements are suppressed as well.

As mentioned above, slow traps have a large activation energy. Passivation of a slow

trap must thus involve a change of its activation energy to either a much smaller energy, thus

producing a fast (shallow) trap, or a larger energy, thus effectively producing a permanent (deep)

trap. As noise is concerned, a systematic shift of the activation energy of an assemble of traps to

smaller values equivalently shifts the 1/f noise knee to larger frequencies, which would lead to

enhanced 1/f noise at any given frequency. Since in contrast a suppression of 1/f noise is

observed in this experiment, PFPE functionalization appears to increase the activation energy of

traps and thus effectively passivates them. In other words, PFPE functionalization converts slow

traps to even slower ones.

The change of activation energy of an individual trap cannot be realized with a uniform

electric field, such as that produced in OFETs, which can only shift the potential landscape in the

channel homogeneously. An electric field inhomogeneous on the length scale comparable to the

cross section of the trap must be involved. In addition, the correct polarity of such a field is also

required in order to increase (rather than decrease) the activation energy of a trapped charge.

The polar (CF)+-(CF3)

- groups protruding from PFPE backbone may fulfill both conditions, as

required for increasing trap activation energy, when PFPE forms a contact with a molecular

crystal. On the one hand, they produce a local inhomogeneous electric field on a molecular scale.

On the other hand, the unique orientation of such polar groups, as discussed in the main text,

determines the electric field of these local dipoles to be perpendicular to the interface and

pointing away from the semiconductor, thus locally increasing the hole trap activation energy.

29

12. Discriminating channel vs. contact noise sources in rubrene FETs.

Figure S14. Current noise power (integrated from 1 to 10 Hz) in rubrene FETs with different

channel lengths (L). All the FETs were fabricated on the same rubrene single crystal with sequentially

changed L using a vacuum lamination method as described in this section. A 2.5 µm-thick Mylar film

was used as a detachable gate dielectric, resulting in a gate-channel capacitance of 1.1 nF/cm2. In all the

measurements, VG = -40 V was applied, which corresponds to a carrier density n = 2.75 × 1011

cm-2

. The

inset shows the circuit model of noise analysis for the case of a dominant contact noise source. Open

triangles show the actual noise measured in the FETs, with a linear fit (black dashed line) revealing an L-1

scaling behavior and thus suggesting a channel dominated noise. The solid red triangles are the

calculated noise power for the case of a contact dominated noise (the first data point for the device with a

channel length L0 was used as a reference point), showing the L-2

scaling behavior (red dashed curve).

It is believed that there are two common sources of electric noise in semiconductor

devices: one is associated with charge carrier trapping in the semiconductor channel and the

other one is a contact noise due to fluctuations of the contact resistance [9,10]. In the case of

single-crystal rubrene FETs, as shown in Fig. S13, a clear 1/f noise was observed, suggesting that





28

than that of the conventional rubrene OFET. Therefore, we believe that ultra slow traps that

might have an even greater negative impact on Hall effect measurements are suppressed as well.

As mentioned above, slow traps have a large activation energy. Passivation of a slow

trap must thus involve a change of its activation energy to either a much smaller energy, thus

producing a fast (shallow) trap, or a larger energy, thus effectively producing a permanent (deep)

trap. As noise is concerned, a systematic shift of the activation energy of an assemble of traps to

smaller values equivalently shifts the 1/f noise knee to larger frequencies, which would lead to

enhanced 1/f noise at any given frequency. Since in contrast a suppression of 1/f noise is

observed in this experiment, PFPE functionalization appears to increase the activation energy of

traps and thus effectively passivates them. In other words, PFPE functionalization converts slow

traps to even slower ones.

The change of activation energy of an individual trap cannot be realized with a uniform

electric field, such as that produced in OFETs, which can only shift the potential landscape in the

channel homogeneously. An electric field inhomogeneous on the length scale comparable to the

cross section of the trap must be involved. In addition, the correct polarity of such a field is also

required in order to increase (rather than decrease) the activation energy of a trapped charge.

The polar (CF)+-(CF3)

- groups protruding from PFPE backbone may fulfill both conditions, as

required for increasing trap activation energy, when PFPE forms a contact with a molecular

crystal. On the one hand, they produce a local inhomogeneous electric field on a molecular scale.

On the other hand, the unique orientation of such polar groups, as discussed in the main text,

determines the electric field of these local dipoles to be perpendicular to the interface and

pointing away from the semiconductor, thus locally increasing the hole trap activation energy.

29

12. Discriminating channel vs. contact noise sources in rubrene FETs.

Figure S14. Current noise power (integrated from 1 to 10 Hz) in rubrene FETs with different

channel lengths (L). All the FETs were fabricated on the same rubrene single crystal with sequentially

changed L using a vacuum lamination method as described in this section. A 2.5 µm-thick Mylar film

was used as a detachable gate dielectric, resulting in a gate-channel capacitance of 1.1 nF/cm2. In all the

measurements, VG = -40 V was applied, which corresponds to a carrier density n = 2.75 × 1011

cm-2

. The

inset shows the circuit model of noise analysis for the case of a dominant contact noise source. Open

triangles show the actual noise measured in the FETs, with a linear fit (black dashed line) revealing an L-1

scaling behavior and thus suggesting a channel dominated noise. The solid red triangles are the

calculated noise power for the case of a contact dominated noise (the first data point for the device with a

channel length L0 was used as a reference point), showing the L-2

scaling behavior (red dashed curve).

It is believed that there are two common sources of electric noise in semiconductor

devices: one is associated with charge carrier trapping in the semiconductor channel and the

other one is a contact noise due to fluctuations of the contact resistance [9,10]. In the case of

single-crystal rubrene FETs, as shown in Fig. S13, a clear 1/f noise was observed, suggesting that





30

occupation fluctuations of slow traps are responsible for such noise. There is still an important

question of whether a part of this noise comes from contacts. In the following, by studying the

scaling behavior of noise in rubrene FETs, we show that the noise sources are uniformly

distributed along the semiconducting channel in these devices, rather than concentrating near the

contacts. Correspondingly, we conclude that the noise originates mostly from charge traps in the

semiconductor, and the trap-healing effect of PFPE takes place along the entire crystal/PFPE

interface, rather than merely at the contacts.

It is well established that in a macroscopic homogeneous conductor, in which transport is

not limited by contacts (in loose terms, contacts being Ohmic), the spectral density of 1/f noise

scales inversely with the system size, S ∝ L-1

. This is a consequence of uniformly distributed

local independent noise sources (for instance, charge traps in a FET’s accumulation channel) [9].

On the other hand, for systems having some local noise sources distributed inhomogeneously

compared to the sample size (for instance, noisy contacts in a FET), the measured spectral noise

density is expected to have a different scaling behavior. For example, let us consider the case of

a sample having the dominant source of noise at the contacts. This situation is modeled using an

equivalent circuit shown in the inset of Fig. S14 with contact resistance RC in parallel with a

current source i. This current represents a small fluctuating component of the total current due to

the fluctuating contact resistance. R represents the channel resistance and Z is a load resistor,

across which the current noise is measured. It is easy to show that the noise spectral density

measured across Z in this case would be SZ = i2(RC/(RC+R+Z))

2 (in order to arrive to this

expression one should consider the current partition at point A and recall that the output

resistance of the voltage source “V” is very small). Assuming now that the contact resistance

and the load resistance are small compared to the channel resistance (RC << R, and Z << R, as in

31

the case of rubrene FETs and our measurement setup), SZ becomes ~ i2/R

2, which scales

inversely with the square of the channel length, SZ ∝ L-2

(note that we have arrived at this result

because we forced the contacts to be the only source of noise, even though we put RC << R).

Therefore, examining the scaling behavior of noise can differentiate the two different cases.

In this additional experiment, we used a very long rubrene crystal with a pair of painted

graphite source and drain contacts separated by a channel length L0 (L0/W ~ 7). The vacuum

lamination method was employed to make a FET, as detailed in section 4 of the Supplementary

Information. Noise spectral density was then measured using a setup depicted in the inset in Fig.

S13 with the FET in accumulation regime. After that, the laminated Mylar film was carefully

removed from the crystal and the channel length was shortened by extending one of the graphite

contacts into the channel area and thus making L smaller. A new FET was then fabricated on

this shorter channel, and noise spectral density was measured again. This process was repeated

four times, with the channel length each time being L0, 0.43L0, 0.21L0, and 0.07L0, respectively.

The measured noise power as a function of the inverse of the channel length (normalized to L0) is

shown as open triangles in Fig. S14. A linear fit (black dashed curve) of the data plotted on a

log-log scale yields a slope of 1, which means the measured noise scales inversely with the

channel length. On the contrary, if the measured noise were simply created by the contacts, it

should have scaled inversely with the square of the channel length, giving a slope of 2 on a log-

log plot. This situation is shown in Fig. S14 with solid red triangles: we have calculated these

values using the noise measured in the longest device with L0/L = 1 as a reference point).

Clearly, these results demonstrate that the noise in typical rubrene devices is generated by traps

uniformly distributed over the entire channel, rather than by a source located at or near the

contacts.





30

occupation fluctuations of slow traps are responsible for such noise. There is still an important

question of whether a part of this noise comes from contacts. In the following, by studying the

scaling behavior of noise in rubrene FETs, we show that the noise sources are uniformly

distributed along the semiconducting channel in these devices, rather than concentrating near the

contacts. Correspondingly, we conclude that the noise originates mostly from charge traps in the

semiconductor, and the trap-healing effect of PFPE takes place along the entire crystal/PFPE

interface, rather than merely at the contacts.

It is well established that in a macroscopic homogeneous conductor, in which transport is

not limited by contacts (in loose terms, contacts being Ohmic), the spectral density of 1/f noise

scales inversely with the system size, S ∝ L-1

. This is a consequence of uniformly distributed

local independent noise sources (for instance, charge traps in a FET’s accumulation channel) [9].

On the other hand, for systems having some local noise sources distributed inhomogeneously

compared to the sample size (for instance, noisy contacts in a FET), the measured spectral noise

density is expected to have a different scaling behavior. For example, let us consider the case of

a sample having the dominant source of noise at the contacts. This situation is modeled using an

equivalent circuit shown in the inset of Fig. S14 with contact resistance RC in parallel with a

current source i. This current represents a small fluctuating component of the total current due to

the fluctuating contact resistance. R represents the channel resistance and Z is a load resistor,

across which the current noise is measured. It is easy to show that the noise spectral density

measured across Z in this case would be SZ = i2(RC/(RC+R+Z))

2 (in order to arrive to this

expression one should consider the current partition at point A and recall that the output

resistance of the voltage source “V” is very small). Assuming now that the contact resistance

and the load resistance are small compared to the channel resistance (RC << R, and Z << R, as in

31

the case of rubrene FETs and our measurement setup), SZ becomes ~ i2/R

2, which scales

inversely with the square of the channel length, SZ ∝ L-2

(note that we have arrived at this result

because we forced the contacts to be the only source of noise, even though we put RC << R).

Therefore, examining the scaling behavior of noise can differentiate the two different cases.

In this additional experiment, we used a very long rubrene crystal with a pair of painted

graphite source and drain contacts separated by a channel length L0 (L0/W ~ 7). The vacuum

lamination method was employed to make a FET, as detailed in section 4 of the Supplementary

Information. Noise spectral density was then measured using a setup depicted in the inset in Fig.

S13 with the FET in accumulation regime. After that, the laminated Mylar film was carefully

removed from the crystal and the channel length was shortened by extending one of the graphite

contacts into the channel area and thus making L smaller. A new FET was then fabricated on

this shorter channel, and noise spectral density was measured again. This process was repeated

four times, with the channel length each time being L0, 0.43L0, 0.21L0, and 0.07L0, respectively.

The measured noise power as a function of the inverse of the channel length (normalized to L0) is

shown as open triangles in Fig. S14. A linear fit (black dashed curve) of the data plotted on a

log-log scale yields a slope of 1, which means the measured noise scales inversely with the

channel length. On the contrary, if the measured noise were simply created by the contacts, it

should have scaled inversely with the square of the channel length, giving a slope of 2 on a log-

log plot. This situation is shown in Fig. S14 with solid red triangles: we have calculated these

values using the noise measured in the longest device with L0/L = 1 as a reference point).

Clearly, these results demonstrate that the noise in typical rubrene devices is generated by traps

uniformly distributed over the entire channel, rather than by a source located at or near the

contacts.





32

13. Hall effect measurements in tetracene single crystals functionalized with PFPE.

Figure S15. Hall effect measurements in tetracene crystals functionalized with PFPE. (a) Raw data

of the Hall voltage (red open symbols); magnetic field sweeps to ±1 T (solid line); linear fit to VH (dashed

line). (b) The same data after the subtraction of the linear background defined by the dashed line (linear

fit) in (a). Measurement conditions: ISD = 50 nA, room temperature. These data indicate the absence of

Hall effect and thus point to an incoherent hopping as the fundamental charge transport mechanism in

tetracene.

As shown in Fig. 2 of the main text, both rubrene and tetracene single crystals

functionalized with PFPE exhibit an anisotropic intrinsic (i.e., not trap-limited) charge transport.

However, they show fundamentally different behavior in Hall measurements. Whereas PFPE

33

functionalized pristine rubrene always shows a prominent Hall signal in our experiments, no

detectable Hall signal was observed in PFPE functionalized tetracene crystals. An example of

Hall effect measurements in tetracene is given in Fig. S15.

Similarly to the case of rubrene, a linearly drifting background was subtracted from the

signal. The result shown in Fig. S15(b) clearly shows that there is no detectable Hall voltage. If

the intrinsic charge transport in PFPE-functionalized tetracene crystals were band-like, and if one

used the longitudinal carrier mobility, µ ~ 0.4 cm2V

-1s

-1, obtained from the OFET measurements

(Fig. 3(c) of the main text), one would expect to detect a Hall voltage of

=

=≡L

WBV

L

W

en

BV

en

BIV SD

SDSDH µ

σ ~ 1 mV at the maximum used magnetic field B = 1 T

and the measurement conditions used: VSD ~ 16 V, W/L = 1.5. This value is much greater than

the noise floor of our method (Fig. S15). The fact that no Hall voltage was observed in

numerous highly purified tetracene crystals therefore suggests that the intrinsic charge transport

in PFPE-functionalized tetracene is an incoherent hopping, rather than a band-like carrier motion.

The absolute noise amplitude in the tetracene/PFPE sample (Fig. S15) appears to be

greater than that in rubrene/PFPE. However, one should not compare the absolute noise in these

two systems, because (a) the measurement parameters used in these cases (sample size,

excitation currents, total sample resistance, total voltage drop across the sample, etc) are different,

and (b) as we show in this work, these systems operate in different transport regimes (band-like

vs. intrinsic hopping). Noise reduction in tetracene/PFPE samples compared to conventional

tetracene single-crystal OFETs is as dramatic as in the case of rubrene. Further detailed

measurements of normalized noise in both systems are necessary in order to compare them

reliably and possibly reveal signatures of the different intrinsic transport mechanisms.





32

13. Hall effect measurements in tetracene single crystals functionalized with PFPE.

Figure S15. Hall effect measurements in tetracene crystals functionalized with PFPE. (a) Raw data

of the Hall voltage (red open symbols); magnetic field sweeps to ±1 T (solid line); linear fit to VH (dashed

line). (b) The same data after the subtraction of the linear background defined by the dashed line (linear

fit) in (a). Measurement conditions: ISD = 50 nA, room temperature. These data indicate the absence of

Hall effect and thus point to an incoherent hopping as the fundamental charge transport mechanism in

tetracene.

As shown in Fig. 2 of the main text, both rubrene and tetracene single crystals

functionalized with PFPE exhibit an anisotropic intrinsic (i.e., not trap-limited) charge transport.

However, they show fundamentally different behavior in Hall measurements. Whereas PFPE

33

functionalized pristine rubrene always shows a prominent Hall signal in our experiments, no

detectable Hall signal was observed in PFPE functionalized tetracene crystals. An example of

Hall effect measurements in tetracene is given in Fig. S15.

Similarly to the case of rubrene, a linearly drifting background was subtracted from the

signal. The result shown in Fig. S15(b) clearly shows that there is no detectable Hall voltage. If

the intrinsic charge transport in PFPE-functionalized tetracene crystals were band-like, and if one

used the longitudinal carrier mobility, µ ~ 0.4 cm2V

-1s

-1, obtained from the OFET measurements

(Fig. 3(c) of the main text), one would expect to detect a Hall voltage of

=

=≡L

WBV

L

W

en

BV

en

BIV SD

SDSDH µ

σ ~ 1 mV at the maximum used magnetic field B = 1 T

and the measurement conditions used: VSD ~ 16 V, W/L = 1.5. This value is much greater than

the noise floor of our method (Fig. S15). The fact that no Hall voltage was observed in

numerous highly purified tetracene crystals therefore suggests that the intrinsic charge transport

in PFPE-functionalized tetracene is an incoherent hopping, rather than a band-like carrier motion.

The absolute noise amplitude in the tetracene/PFPE sample (Fig. S15) appears to be

greater than that in rubrene/PFPE. However, one should not compare the absolute noise in these

two systems, because (a) the measurement parameters used in these cases (sample size,

excitation currents, total sample resistance, total voltage drop across the sample, etc) are different,

and (b) as we show in this work, these systems operate in different transport regimes (band-like

vs. intrinsic hopping). Noise reduction in tetracene/PFPE samples compared to conventional

tetracene single-crystal OFETs is as dramatic as in the case of rubrene. Further detailed

measurements of normalized noise in both systems are necessary in order to compare them

reliably and possibly reveal signatures of the different intrinsic transport mechanisms.





34

14. Comparison between rubrene/PFPE and rubrene/FTS interfaces.

Self-assembly monolayers of (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTS)

has been found to heavily p-dope organic semiconductors and other carbon based materials,

including graphene [ 12 , 13 ]. In the case of rubrene/FTS system, an increase of surface

conductivity similar to that shown in Fig. 1 was observed. However, no Hall effect could be

measured in the rubrene/FTS system (even in much higher magnetic fields of up to 6 T) because

of an excessive electric noise introduced by FTS monolayer, in sharp contrast to the

rubrene/PFPE system reported here. Such a drastic difference between these systems illustrates

that doping or charge accumulation does not necessarily improve charge transport in terms of

noise and mobility. In fact, in rubrene/FTS system, doping actually increases noise and reduces

the mobility. PFPE functionalization thus represents a rather important case, in which charge

accumulation and suppression of disorder-related noise are achieved simultaneously.

15. Limitations of PFPE functionalization and development of semi-solid PFPE analogs.

Trap-healing effect of PFPE in the form discussed in the main text has certain limitations.

For example, although even in highly disordered samples the charge transport should improve

with PFPE treatment due to a partial healing of traps, we do not expect PFPE treatment to

recover the intrinsic state in systems with trap density greater than the carrier density induced by

PFPE.

As far as potential applications are concerned, the fluidic nature of PFPE makes

fabrication of solid-state devices challenging. However, we have found that the effects of PFPE

functionalization discussed in this work are not limited to solid/liquid interfaces using PFPE oil.

There are commercially available semi-solid forms of PFPE, based on mixtures of PFPE and a

35

gel-like neutral polymer matrix. Such PFPE grease has a similar effect on the surface

conductivity of organic semiconductors, yet it does not flow at room temperature, making device

fabrication much more manageable. We also expect to discover other compounds, not in a liquid

phase, which can produce similar or even better functionalization, based on the same principle.

References (Supplementary):*****

1 H. T. Yi, Y. Chen, K. Czelen and V. Podzorov, “Vacuum Lamination Approach to Fabrication

of High-Performance Single-Crystal Organic Field-Effect Transistors”, Adv. Mater. 23, 5807 (2011).

2 Ono, S., Seki, S., Hirahara, R., Tominari, Y., and Takeya, J. High-mobility, low-power, and fast-

switching organic field-effect transistors with ionic liquids. Appl. Phys. Lett. 92 103313 (2008).

3 Kergoat, L., Herlogsson, L., Braga, D., Piro, B., Pham, M.-C., Crispin, X., Berggren, M., and Horowitz,

G. A water-gate organic field-effect transistor. Adv. Mater. 22 2565-2569 (2010).

4 J. H. Cho, J. Lee, Y. Xia, B. Kim, Y. He, M. J. Renn, T. P. Lodge, and C. D. Frisbie, “Printable ion-gel

gate dielectrics for low-voltage polymer thin-film transistors on plastic”, Nat. Mater. 7, 900-906 (2008).

5 K. H. Lee, M. S. Kang, S. Zhang, Y. Gu, T. P. Lodge and C. D. Frisbie, ““Cut and Stick” Rubbery Ion

Gels as High Capacitance Gate Dielectrics”, Adv. Mater. 24, 4457-4462 (2012).

6 V. Podzorov et al., “Interaction of organic surfaces with active species in the high-vacuum

environment”, Appl. Phys. Lett. 87, 093505 (2005).

7 H. Najafov, D. Mastrogiovanni, E. Garfunkel, L. C. Feldman and V. Podzorov, “Photon-Assisted

Oxygen Diffusion and Oxygen-Related Traps in Organic Semiconductors”, Adv. Mater. 23, 981-985

(2011).





34

14. Comparison between rubrene/PFPE and rubrene/FTS interfaces.

Self-assembly monolayers of (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTS)

has been found to heavily p-dope organic semiconductors and other carbon based materials,

including graphene [ 12 , 13 ]. In the case of rubrene/FTS system, an increase of surface

conductivity similar to that shown in Fig. 1 was observed. However, no Hall effect could be

measured in the rubrene/FTS system (even in much higher magnetic fields of up to 6 T) because

of an excessive electric noise introduced by FTS monolayer, in sharp contrast to the

rubrene/PFPE system reported here. Such a drastic difference between these systems illustrates

that doping or charge accumulation does not necessarily improve charge transport in terms of

noise and mobility. In fact, in rubrene/FTS system, doping actually increases noise and reduces

the mobility. PFPE functionalization thus represents a rather important case, in which charge

accumulation and suppression of disorder-related noise are achieved simultaneously.

15. Limitations of PFPE functionalization and development of semi-solid PFPE analogs.

Trap-healing effect of PFPE in the form discussed in the main text has certain limitations.

For example, although even in highly disordered samples the charge transport should improve

with PFPE treatment due to a partial healing of traps, we do not expect PFPE treatment to

recover the intrinsic state in systems with trap density greater than the carrier density induced by

PFPE.

As far as potential applications are concerned, the fluidic nature of PFPE makes

fabrication of solid-state devices challenging. However, we have found that the effects of PFPE

functionalization discussed in this work are not limited to solid/liquid interfaces using PFPE oil.

There are commercially available semi-solid forms of PFPE, based on mixtures of PFPE and a

35

gel-like neutral polymer matrix. Such PFPE grease has a similar effect on the surface

conductivity of organic semiconductors, yet it does not flow at room temperature, making device

fabrication much more manageable. We also expect to discover other compounds, not in a liquid

phase, which can produce similar or even better functionalization, based on the same principle.

References (Supplementary):*****

1 H. T. Yi, Y. Chen, K. Czelen and V. Podzorov, “Vacuum Lamination Approach to Fabrication

of High-Performance Single-Crystal Organic Field-Effect Transistors”, Adv. Mater. 23, 5807 (2011).

2 Ono, S., Seki, S., Hirahara, R., Tominari, Y., and Takeya, J. High-mobility, low-power, and fast-

switching organic field-effect transistors with ionic liquids. Appl. Phys. Lett. 92 103313 (2008).

3 Kergoat, L., Herlogsson, L., Braga, D., Piro, B., Pham, M.-C., Crispin, X., Berggren, M., and Horowitz,

G. A water-gate organic field-effect transistor. Adv. Mater. 22 2565-2569 (2010).

4 J. H. Cho, J. Lee, Y. Xia, B. Kim, Y. He, M. J. Renn, T. P. Lodge, and C. D. Frisbie, “Printable ion-gel

gate dielectrics for low-voltage polymer thin-film transistors on plastic”, Nat. Mater. 7, 900-906 (2008).

5 K. H. Lee, M. S. Kang, S. Zhang, Y. Gu, T. P. Lodge and C. D. Frisbie, ““Cut and Stick” Rubbery Ion

Gels as High Capacitance Gate Dielectrics”, Adv. Mater. 24, 4457-4462 (2012).

6 V. Podzorov et al., “Interaction of organic surfaces with active species in the high-vacuum

environment”, Appl. Phys. Lett. 87, 093505 (2005).

7 H. Najafov, D. Mastrogiovanni, E. Garfunkel, L. C. Feldman and V. Podzorov, “Photon-Assisted

Oxygen Diffusion and Oxygen-Related Traps in Organic Semiconductors”, Adv. Mater. 23, 981-985

(2011).





36

8 H. Najafov, B. Lee, Q. Zhou, L. C. Feldman and V. Podzorov, “Observation of long-range exciton

diffusion in highly ordered organic semiconductors”, Nature Mater. 9, 938-943 (2010).

9 M. B. Weissman, “1/f noise and other slow, nonexponential kinetics in condensed matter”, Rev. Mod.

Phys. 60, 537 (1988).

10 Sh. Kogan, Electronic Noise and Fluctuations in Solids. Cambridge University Press, Cambridge,

(1998).

11 H. Matsui, T. Hasegawa, Y. Tokura, M. Hiraoka, T. Yamada, “Polaron motional narrowing of electron

spin resonance in organic field-effect transistors”, Phys. Rev. Lett. 100, 126601 (2008).

12 Calhoun, M. F., Sanchez, J., Olaya, D., Gershenson, M. E., and Podzorov, V., Electronic

functionalization of the surface of organic semiconductors with self-assembly monolayers, Nature Mater.

7, 84 (2008).

13 Lee, B., Chen, Y., Duerr, F., Mastrogiovanni, D., Garfunkel, E., Andrei, E. Y., and Podzorov, V.

Modification of electronic properties of graphene with self-assembled monolayers, Nano Lett. 10, 2427-

2432 (2010).





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