BScThesis_Fartunov

8/8/2019 BScThesis_Fartunov

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Jacobs University Bremen

School of Engineering and Science

Organic Schottky Diodes on PET-foil

BSc thesis in Physics

As part of the course

200322 Guided Research Physics

By

Ivan Fartunov

Bremen, 7th

May 2010

Supervisor: Prof. Dr. Veit Wagner

2nd Reader: Prof. Dr. Ulrich Kleinekathfer

Advisor: Marlis Ortel


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Contents

1. Introduction and Motivation ................................................................................................ 32. Theoretical Background........................................................................................................ 42.1. Schottky Contact .................................................................................................................... 4

2.2. Current Voltage Characteristics ............................................................................................ 5

2.3. Vissenberg Matters Model and Space Charge Limited .................................................... 5

2.4. Doping .................................................................................................................................... 7

3. Materials and Procedures .................................................................................................... 83.1. Electrodes and Substrate ........................................................................................................ 8

3.1.1. Substrate ............................................................................................................................ 83.1.2. Contact materials .............................................................................................................. 83.1.3. Sputter coating and Lithography ...................................................................................... 93.2. Organic Semiconductors ........................................................................................................ 9

3.2.1. n-type Organic Semiconductors ....................................................................................... 93.2.2. p-type Organic Semiconductors ..................................................................................... 103.2.3. Preparation and Deposition of P3HT ............................................................................. 103.3. Doping deposition techniques .............................................................................................. 12

3.4. Data Collection ..................................................................................................................... 12

4. Analysis of Experimental Results ...................................................................................... 144.1. Structures Cu/P3HT/Au and Au/P3HT/Cu.......................................................................... 14

4.2. Lithography patterned vs. shadow-mask patterned gold contacts .................................... 174.3. Doped samples ..................................................................................................................... 20

5. Conclusion and Outlook...................................................................................................... 26


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1. Introduction and MotivationIn 1977 the first conducting polymers were reported

1

. Since then organic polymers were subjectof extensive research. This discovery earned A.J. Heeger, A.G. MacDiamid and H. Shirakawa a

Nobel Prize in chemistry in year 20002. Organic electronics cannot reach the high performance

of modern silicon-crystalline based technologies3 but there are applications like radio frequency

identification (RFID) where not such a high performance is necessary and the low cost of

polymer semiconductor devices makes them preferable. Other potential applications are

electronic paper and displays4.

A major advantage of organic semiconductors is that they are solution-processable, which allows

working at room temperature and atmospheric pressure using spin-coating5, stamping

6and

printing7

techniques (Figure 1). In addition polymer semiconductor devices can be produced on

thin, lightweight and flexible substrates. This can create a whole new market of low cost flexible

electronic applications.

Significant research is focusing on improving the characteristics of organic semiconductor

devices. For RFID high frequency applications high on-currents are required. This thesis aims at

investigating ways of improving the characteristics of wet-processed organic Schottky diodes.

The widely examined organic p-type semiconductor poly(3-hexylthiophene) also known as

P3HT was used for the experiments. The key characteristics of the produced devices were on-off

current density ratios, charge carrier mobilities, depletion layer thickness and electron acceptor

concentration. In order to investigate those characteristics J V and C V measurements wereconducted and analyzed. First different structures and contact deposition techniques areexamined. After an optimal preparation procedure was determined further ways of increasing

diode performance were examined. The aim was to improve charge carrier injection, and from

there on-currents, through the introduction of a dopant. Different techniques of contact treatment

with an electron acceptor F4TCNQ were applied and the results were analyzed.

.

Figure 1: Printed RFID tags at PolyIC


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2. Theoretical Background2.1. Schottky Contact

The Schottky contact is a rectifying Metal-Semiconductor contact. The name originates from

the Schottky potential barrier formed on the interface between two materials with different work

functions. As the work functions of the metal and the semiconductor are different theformed junction has a built in voltage . Figure 28 represents the band gap diagram of aSchottky contact between a metal and a p-type semiconductor at thermal equilibrium. When the

contact is operating in forward bias the metal work function gets elevated and the charge

transport is eased.

Figure 2: Schottky Junction for p-type semiconductor8

On the figure , , , are the elementary electric charge, the maximum valence-bandenergy, the minimum conduction-band energy and the Fermi energy. is the depletion width.This model was developed for inorganic crystalline semiconductors. Some models suited for

organic amorphous semiconductors are discussed in section 2.3. Figure 3 represents the basic

structure of a p-type semiconductor Schottky diode where one can see the p-type semiconductor

between the top and the bottom contact, the latter being grounded.


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Figure 3: Schottky Diode (p-type semiconductor)

2.2. Current voltage characteristics

One of the main ways to describe a diode is the relation between the current density in the diode

and the voltage causing this current density. For a Schottky diode this relation is given byEquation 1

9:

(1)where is the Boltzmann constant, is the temperature and is the elementary charge. Theideality factor

is usually greater than 1 and describes the barrier changes due to image forces.

The saturation current is given by Equation 210: (2)

Where is the effective Richardson constant ( ) and is theSchottky barrier height (built in voltage).

2.3. Vissenberg Matters model and Space Charge Limited Current

In semiconductors with crystalline structure the charge transport is explained by solid statephysics and in particular the band theory. In organic semiconductors with amorphous structure

the states of the charge carriers are localized and the charge transport is described by hopping

of the carriers between those states9,11

. The transport is described as phonon assisted by Miller

and Abrahams12

. According to the Mott13

variable range hopping model the localized states are

to be found in the entire band gap. The Theory of Space Charge Limited Current (SCLC) which

was also described in the Mott-Gurney Law14 provide an alternative model for J V curve thataccounts for those phenomena and given in Equation 3.


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(3)Where and are respectively the permittivity of the semiconductor and that of free space and is the distance between the electrodes. This model assumes that the mobility is independenton the charge carrier density. According to the model the potential distribution between thecontacts is as described by Equation 4.

(4)

From which the electric field and the charge carrier density can be derived.

An improved model introduced by Vissenberg and Matters deals with the distribution of the

localized states within the semiconductor15,16

. The density of states as a function of theenergy is given by Equation 5:

exp (5)where is the total density of states and is a Gaussian distribution. The dependence isvisualized on Figure 4

17.

Figure 4: Density of states as a function of energy

The interpretation of the figure is that with increasing voltage the charge carrier density in the

vicinity of the Fermi energy increases. In the model which was initially developed for transistors

and later adapted for diodes the mobility depends on the charge carrier density . The relationis given by Equation 6:

(6)


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where is the reference charge carrier density. In this thesis was the used value.The exponent term

called disorder parameter is related to the

distribution of density of states by Equation 7:

(7)where is the Boltzmann constant and is the temperature indegrees Kelvin. The model used to fit the J V curves in theexperiments described here was programmed in the iva02

analysis software. It combined the SCLC and the charge carrier

density dependence of the mobility. The effect of the disorder

parameter on the space distributions inside the device isillustrated in Figure 5, where is the potential, is theelectric field and is the charge carrier density. It can beobserved that for the charge carrier injection is highercompared to the case when .

2.4. Doping

One limiting factor on the performance of organic electronic

devices is directly related to the properties of the interface

between the semiconductor and the contacts

18,19

. One effectivemethod to improve the hole injection in the semiconductor, and

from there the devices performance, is by lowering the hole

injection barrier. This is obtained by treating the contacts with an electron acceptor. In addition

to lowering the hole injection barrier, such doping might result

in acceptor molecule penetration in the P3HT which will result

in p-doping the polymer itself20. One of the best such materials

available is 2,3,4,6-tetrafluoro-7,7,8,8-

tetracyanoquinodimethane or F4TCNQ21,22

. The chemical structure of the material is given in

Figure 620:

Figure 6: Chemical structure of F4TCNQ20

Figure 5: Significance of the

disorder parameter


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This material is widely used as a dopant in OFETs and OLEDs. In the experiments described in

this thesis the gold contact was treated with F4TCNQ.

The depletion layer inside of a diode can be seen as an insulator and thus an assumed parallel

plate capacitor is formed within the device. By measuring this capacitance the depletion width can be determined according to Equation 810:

(8)where is the area of the junction. That dependence is used to derive the electron acceptorconcentration from the relation between the capacitance and the applied voltage .Equation 9

23gives that derivation:

(9)

where is the elementary charge.

3. Materials and Procedures3.1. Electrodes and Substrate

3.1.1. Substrate

In order to easily process the diode structures a microscopic glass 5x5 cm was used as a base.

The glass was cleaned with isopropanol (2-propanol) and dried with a N2-pressure gun. Adhesion

foil was mounted on the base. For the adhesion foil to stick better the glass was warmed up. For

the substrate of the device a flexible transparent polymer was used PET-foil (Polyethylene

terephthalate) produced by IPM Freiburg. After applied on the base, the substrate was cleaned

with acetone and isopropanol and dried with N2-pressure gun. Treatment in the ultrasonic bath

was found to not improve the performance of the devices and thus was excluded from theproduction process.

3.1.2. Contact Materials

The metals used for the contacts of the diode were gold (Au) and copper (Cu). The values for the

work functions were given in the literature as 5 eV 24 for gold and 47 eV24 above for copper.The interface between the gold and the semiconductor is described as an ohmic contact

25. The

P3HT was assumed to have its highest occupied molecular orbital (HOMO) at an energy level


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of 5 eV 6. Due to a difference between the work function of copper and the HOMO level ofP3hT a Schottky barrier was formed. The copper was always applied through sputter-coating

with a shadow-mask while for the gold contacts lithography patterned structures with thicknessof 30 nm were also used. The approximate deposition rate of the sputter-coater was 4 nm/s.

3.1.3. Lithography and Sputter coating

For some of the prepared samples PET-foil with gold contacts patterned using lithography was

provided. The principle technology27 of lift-off photo-lithography using positive photo-resist is

described here. A photo-resist is spin-coated on the sample surface. Then an optical mask is

applied and the sample is exposed to light. Following the light exposure is a developing process

during which the part of the photo-resist exposed to light is removed. After the development a

layer of gold is deposited on the sample and it is again exposed to light this time without amask. The process is finished by removing the remaining photo-resist and the gold on top of it,

leaving gold contacts with the shape of the mask used in the initial light exposure. The main

advantage of the lithography process over using a shadow-mask is that the contacts obtained

using the former have sharper edges.

In the cases when the bottom contacts were not already on the PET-foil, they were sputter-coated

using shadow-masks MADRIX ID001. Before sputter-coating the gold, the chamber was

evacuated to a vacuum level of3 mbar. For copper the vacuum used was mbaras, in comparison with gold, copper is less noble. In order to avoid residual air in the chamber,

before the process was started several empty cycles of the pump were completed. After eachempty pump run as well as during the sputter-coating the chamber was flooded with argon

(Ar). A shadow-mask was attached to the sample using magnets in order to avoid metal going

under the mask. The operating current and pressure of the sputter-coater were 45 mA and mbar. Using a shadow-mask is faster and cheaper compared to the alternative technology photo-lithography.

3.2. Organic Semiconductors

Organic semiconductors are carbon based materials showing semiconductor properties. They are

most often divided in two groups, due to differences in the layer preparation: small-molecular

materials and polymers22. The former are usually evaporated in vacuum, the latter are solution

processed. Both p and n-type organic semiconductors are present.

3.2.1. n type Organic Semiconductors

In 1995 Haddon et al.28

for the first time reports n-type behavior in an organic semiconductor. In

general n-type organic semiconductors perfume worse than p-type but active research in the field

gives promising results29

. A major problem is the instability of n-type organic semiconductors


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when exposed to oxygen9. A mixture of n-type PCBM (example on Figure 7) and p-type P3HT

(Figure 8) is one of the most commonly used organic solar cell material.

Figure 7: [6,6]-phenyl C61-butyric acid methylester (PCBM)

3.2.2. p type Organic Semiconductors / P3HT

During the experiments on which this thesis is based the used p-type organic semiconductor was

poly(3-hexylthiophene) also known as P3HT presented on Figure 8. It is one of the most

investigated polymeric semiconductors. P3HT is a thiophene based polymer with alternating

single (-bonds) and double (-bonds) bonds in the carbon structure. In such conjugatedpolymers the extended -orbitals are the source of their semiconductor properties30. In P3HTbased devices the mobility is found to be dependent on the regioregularity of the polymer chains.

The P3HT is described as rr (regioregular) if there are more than 98% head-to-tail chains at the

thiophene ring31.

Figure 8: rr-P3HT chemical structure molecule

3.2.3. Preparation and Deposition of P3HT

For all experiments a 2.5 weight percent solution of P3HT in toluene was used. To ensure that

the semiconductor material will fully dissolve the mixture was heated while being on the

magnetic stirrer. Approximately 3 grams of solution were prepared each time. The

semiconductor was either spin-coated directly on the bottom contacts or on top of the dopant

later on.


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A standard procedure for depositing the P3HT layer was used. The sample was placed in the

spin-coater. Each time exactly 2 ml of P3HT solution were deposited using an Eppendorf pipette

and the spin-coater was started. The coating was 90 second with acceleration rate of rpm/s and a maximum rotation speed ofV 5 rpm. Immediately after the coatingwas finished the sample was baked for 60 s at 9 C. The same procedure was used every time inorder to keep the semiconductor layer unchanged.

The effect of the maximum rotation speed on the P3HT layer was examined by altering it from

500 rpm to 3000 rpm while keeping all other parameters of the process constant. The surface

structure of the P3HT layer did not show significant differences in the examined speed range. An

AFM picture was taken of the surface using Naosurf Mobile S AFM system and then analyzed

using Gwyddion

graphics analyzing software. An AFM picture of the surface at the used

rotational speed ofV 5 rpm is shown on Figure 9.

Figure 9: AFM picture of P3HT layer spin-coated at 2500 rpm

The roughness of the surface was characterized by a root mean square value ofR 66 nm.The top surface of the P3HT layer formed the interface with the copper. The thickness of the

P3HT was also measured as a function of the maximum rotation speed. Using a DekTak system,

six or seven measurements were taken from different places on the samples for each rotation

speed. The measurements were taken around the center of the sample as it was visible that

around the edges the layer was thicker. Figure 10 shows the layer thickness in dependence of the

rotation speed. The error bars are determined as the standard deviation of the measurements for

velocity.


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Figure 10: P3HT layer thickness as function of spin-coating speed

It can be clearly seen that the thickness decreases with increasing rotational speeds. The value

obtained for the rotation speed of V 5 rpm used in the diode production was d 745 nm.3.3. Doping Deposition Techniques

The dopant F4TCNQ was applied on the gold contacts using 2 different techniques drop-

casting and spin-coating. For the drop-casting process the sample was placed on the hot plate at

temperature of9C. Then a solution from F4TCNQ in toluene was deposited using anEppendorf pipette. The whole sample area was treated. The second technique used to deposit thedopant was the spin-coating. The parameters used were the same as those for P3HT. Afterwards

the sample was baked. With both techniques after applying the F4TCNQ the sample was washed

with Isopropanol before the semiconductor was spin-coated. As discussed in Koch 2005 21 the

dopant bonds with the gold contact so it will not be washed off during the isopropanol cleaning.

This is done because we aim at obtaining monolayer of F4TCNQ so only the gold contact is

doped. For the spin-coating doping, two different solvents were used for the F4TCHQ solution

toluene and chloroform. This was done in order to determine the effect of the dopant solvent on

the characteristics of the diodes.

3.4. Data Collection

Each sample consisted of 15 structures with 6 or 7 devices per structure. After the devices were

prepared, the glass base was removed. The structures were separated and placed in special plugs.

The plugs were then connected to the STALA measurement system as shown on Figure 11.

250

270

290

310

330

350

0 1000 2000 3000 4000

thickness[n

m]

Rotation speed [rpm]


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4. Analysis of Experimental ResultsAs said in Section 2.3 the iva02 analysis software was used to fit the obtained J V curves. Thedisorder parameter and the mobility were determined form the fit. For the thickness of thesemiconductor layer the value determined in Section 3.2.3. ofd 74 nm was used. As pointedout in Section 2.3. the reference charge carrier density was . The value used forthe permittivity of the semiconductor was . For samples with bottom gold contacts (bothdoped and undoped) a working device is a device with stable J V curve and on off currentdensity ratio above 100. This value was chosen because devices, measured beforehand in the

Wagner group, of the type Au/P3HT/Au where no Schottky barrier was formed had ratios around

70.

4.1. Structures Cu/P3HT/Au and Au/P3HT/Cu

Two different configurations of diodes were produced with the shadow-mask patterned contacts

(using MADRIX ID001). In the first case the gold contacts were on the bottom and in the

second case they were on top. This is represented on Figure 11.

Figure 12: Cu/P3HT/Au structure (left) and Au/P3HT/Cu structure (right)

The diodes patterned with the MADIX ID001 mask had areas of 5 mm, mm, mm, mm, 5 mm and mm. In the data analysis the current density is used insteadof the current. This means the areas are already taken into account.

The sample for which the copper contacts were on the bottom had approximately 40% working

devices. The best performing devices have given on-off ratios of 15 and the average for the

sample was 7. Cu and Au contacts were deposited by sputter-coating for 3 min using the settings

described in Section 3.1.3. In the case when the gold contact was on the bottom (Au/P3HT/Cu)

the percentage of working devices was the same. This time the gold was sputtered for 2 min and


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the copper for 4 min. The best achieved on-off current density ratio was 7671 and the average for

all working devices was 2814. Representative device from each sample were chosen with on-off

ratio close to the average. Figure 13 shows the J V curves of those representative devices forboth configurations.

Figure 13: J - V curves for diodes with switched contacts

The device with top copper contacts was on (forward bias) for negative voltages, while the

other one was on for positive voltages. This is so because in both cases the Schottky barrier

was formed on the surface between the P3HT and copper electrode. The configuration with

bottom gold contacts and top copper ones (Au/P3HT/Cu) gives a better on-off ratio. This is a

result dominantly due to higher on current density. During the production of the structure with

bottom copper contacts, the latter were exposed to air and got oxidized which is a possibleexplanation for the low on current. The values obtained from the fitting with iva02 are

presented in Table 1:

Mobility Disorder parameter Offset voltage [V]Au top 4 3Cu top 6 7 3

Table 1: Fitting for gold top contacts and copper top contacts

1E-11

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

-10 -5 0 5 10

Currentdensity[

A/mm2]

V [V]

Au top contact

Cu top contact


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The Au/P3HT/Cu (top Cu contacts) had higher mobility as expected from the J V curve. Forthe Au/P3HT/Cu sample the ideality factor is determined. In order to do so the diode equation is

fitted to the J V curve in the region from 5 V to V. The obtained ideality factor is 35. The saturation current density is 5 6 . Those results will be used tocompare this shadow-mask patterned sample with a lithography-patterned one. The fit for the

diode equation is given on Figure 14:

Figure 14: Ideal diode equation fit for shadow-mask patterned diode

The time stability of the two possible configurations discussed in this section was investigated.

For this reason the average on-off current density ratio for the samples was measured over 9 days

for the configuration with top gold contacts (Cu/P3HT/Au) and over 4 days for the one with top

copper contacts (Au/P3HT/Cu). In Figure 14, which shows the time dependence of the on-off

current ratios, day 1 is the day when the sample was produced.

1E-10

1E-09

1E-08

1E-07

1E-06

-0.6 -0.1 0.4

Currentdensity[A/mm2]

V [V]

Shadow - mask patterned top Cu contacts

Values

fit


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Figure 15: Time evolution of on-off ratios

Oxidation of the bottom copper contacts during the production of the sample was probably the

main reason for the low performance of the structure with top gold contacts. The diodes with

copper top contacts, as expected, have decrease in performance with time when exposed to

ambient conditions32. A major reason for the observed degradation is the oxygen doping of the

P3HT which decreased the Schottky barrier and thus increased the off current.

4.2. Lithography patterned vs. shadow-mask patterned gold contacts

After performing better than Cu/P3HT/Au, the shadow-mask patterned Au/P3HT/Cu

configuration (copper contacts on top) was compared with a sample using the same configuration

but with the gold bottom contacts patterned using lithography. The gold contacts patterned by

lithography had both finger structures and capacitors. Figure 16 shows the schematics of a

capacitor contact and a figure structure contact.

Figure 16: Gold bottom lithography patterned contacts (different structures)

1

10

100

1000

10000

0 2 4 6 8 10

On-offratio

time [days]

Top Au

Top Cu

a) Capacitor b) Finger structures


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For this section of the data analysis (Section 4.2) only the finger structures from the lithography

patterned contacts were considered. The reasons for that are discussed in Section 4.3. The best

performing device on shadowmask patterned sample had an on-off current ratio of 7671. Theaverage ratio for the whole sample was 2814. Approximately 41% of the producd devices were

working devices. A comparison between the J V curves for representative devices from eachsample is given on Figure 17.

.

Figure 17: J - V curves for top devices from samples with gold contacts patterned using two

different techniques.

The shadow-mask patterned sample shows both higher on and higher off current. A possible

reason is that during patterning using a shadow-mask the edges are not as sharp as withlithography and the actual area is slightly bigger than the one considered in the calculations. The

details however remain to be further investigated.

Using iva02 data analysis the mobility for a representative device from the lithography patterned

samples was determined to be , which was slightly smaller than the valuefrom the shadow-mask patterned sample. The obtained disorder parameter and the offset had

values 4 7 and 3 V.

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

-10 -5 0 5 10


V[V]

shadow-mask

lithography


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The ideality factor and the saturation current for reverse bias of the representative device for the

lithography patterned sample were determined by fitting the diode equation. The fitting is

illustrated in Figure 18. The resulting ideality factor was 3 and the saturation current densitywas 4 . The ideality factor which is used as a measure of the quality of thediodes33 improved, compared to that of the shadow-mask patterned sample, which suggests that

using lithography to pattern contacts is better than using shadow-masks.

Figure 18: Ideal diode fit for a device based on lithography patterned gold contacts

C V Measurements (3 days after the production of the sample) for several devices based onlithography patterned contacts were used together with the equation for capacitance (Equation 8)

in order to determine the depletion layer width as a function of the applied voltage. Figure 19shows the established relation for 3 finger structure devices and one capacitor.

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

-0.6 -0.1 0.4

Currentdensity[A/m

m2]

V[V]

Litography patterned

values

fit


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Figure 19: Depletion layer width as function of applied voltage for undoped sample based on

lithography patterned gold contacts

As expected, for the finger structure diodes, the depletion layer width increased with increasing

voltage. It did not exceed the P3HT layer thickness which means that the obtained results are

plausible. Using Equation 9 the number of electron acceptors was calculated for two ranges:between 0 V and 2 V and between 7.5 V and 9 V. Two different values were obtained. For the

capacitor a constant depletion layer width and from there constant capacitance was observed.

This behavior was very different compared to the finger structures. A major reason is the effect

of the edges. The effect of the shape and area of the structures on their performance is examined

in Section 4.3

The shadow-mask patterned sample showed nearly the same mobility as the lithography

patterned and there was no significant difference in the on-off current ratios. However, the

lithography patterned sample had slightly better ideality factor and much higher percentage

working devices per sample which was the reason why samples based on lithography patterned

contacts were used for further experiments.

4.3. Doped samples

Using the procedures described in Section 3.3. F4TCNQ was applied as dopant on lithography

patterned gold contacts. The performance (on-off current ratios) diverged significantly between

finger structures and capacitors and also between different types of finger structures. Figure 20

illustrates those differences for devices doped through spin-coating toluene solution of F4TCNQ

on the gold contacts. For K50 finger structures the average is based on 6 devices, for K20 on

0

20

40

60

80

100

120

0.00 2.00 4.00 6.00 8.00 10.00

W[nm]

Voltage [V]

3d2_d1

3d2_d2

3d2_d3

3d2_capacitor

NA=5.211017 1/cm3

NA=3.211017 1/cm3


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29 devices, for L10 on 5 and for K10 on 93. For the capacitors the average is based on 25

devices. For the capacitors the range of error is 20% while for the fingers it is less than 10%.

Figure 20: Performance of different structures on a lithography patterned gold contacts doped

with spin-coated toluene solution of F4TCNQ

The differences in performance could be explained by the effects of the edges of the devices.

Also in the case of capacitors because it is a single large structure it would be harder for the

oxygen to dope the P3HT underneath the contact. However, the exact effects remained unclear

and the topic should be further investigated.

Figure 21 gives the schematic view of a doped device.

Figure 21: Basic sketch of doped device

0

10000

20000

30000

40000

50000

Avera

geon-offratios

Capacitors

L10

K10

K20

K50

Contact treated with F4TCNQ

1.99 mm2

0.057 mm2

0.05655 mm2

0.114 mm2

2.285 mm2


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Table 2 summarizes the obtained doped samples that are analyzed in this section.

SolventDeposition

method

Highest on-off current ratios Average on-off current ratios Working

devices [%]capacitors Fingers (K10) capacitors Fingers (K10)

toluene Drop-casted 3700 600 1600 200 24

chloroform Spin-coated 52700 8400 32200 4200 78

toluene Spin-coated 78500 8700 38500 4600 92

Table 2: Doped samples summary

The best performing sample was the one doped with spin-coated toluene solution of F4TCNQ.

Similarly to the undoped sample, time degeneration pattern was observed. In order to avoid sucheffects, a protective PMMA layer should be introduced. All samples compared in this part were

based on lithography patterned bottom contacts. For this reason two representative devices were

chosen from each sample one capacitor and one finger structure diode (K10). Figure 22 shows

the J V curves for the finger structures and the capacitors. An undoped lithography patternedsample is also included in the plot for comparison base.

Figure 22: curves for samples on lithography patterned bottom contactsDuring the drop-casting, rings formed on the edges of each drop. Those rings did not disappear

after washing the sample with isopropanol and remained apparent even after applying the P3HT

layer. It is possible that some of the dopant affected the Schottky contacts in this case. The drop-

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

-10 -5 0 5 10


V [V]

finger structures K10

tol. drop-casted

chlor. spin-coated

undoped

tol. spin-coated

1E-121E-11

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

-10 -5 0 5 10


V [V]

capacitors

tol. drop-casted

chlor.spin-coated

undoped

tol. spin-coated


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casting deposited dopant did not improve the performance of the devices. A possible explanation

is that the process disrupted the P3HT layer and the deposition of the upper contacts.

When the dopant was spin-coated (for both solvents: chloroform and toluene), the performance

of the devices improved. In the case of finger structures the improvement was due to decrease of

the off-current. For the capacitors the improvement in the ratio was driven both by increase in

the on and decrease in the off current densities. The increase in the on-current shows that

doping the samples was successful and it led to improved charge carrier injection. The current

density in the case of capacitors depended also on the change in voltage the J V curves fordecreasing and increasing voltage are different for the same device - Hysteresis.

For each of the three doped samples a K10 finger structure was analyzed using iva02. In order to

obtain a good fitting the number of electron acceptors

was added to the variables of the fit.

Having too many variables returned physically impossible values which made it necessary to

introduce different preset quantities that were obtained from the undoped sample. The disorder

parameter was initially set at . The value was chosen to be close to the resultsobtained for undoped sample. Then the obtained number of electron acceptors ( ) was fixedand the disorder parameter was fitted. The parameters resulting from the analysis are presented

in Table 3.

SolventDeposition

method

Mobility

Offset voltage

[V]

Disorder

parameter

toluene Drop-casted 65 9 933 45chloroform Spin-coated 9 37 43

toluene Spin-coated 39 4 Table 3: iva02 analysis of K10 finger structures from doped samples

The samples where the dopant was spin-coated showed an increase in the mobility which was

coherent with the theoretical expectations. There was a charge in the disorder parameter as well

proving that introducing a dopant affects the ordering of the polymer chains in the P3HT layer.

The sample doped with spin-coated toluene solution of F4TCNQ showed higher average on-off

current ratios and percentage working devices than the other two doped samples. For this reason

it was chosen for further investigation. On Figure 23 the ideality fit for spin-coated toluene

solution of F4TCNQ.


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Figure 23: Ideality fit for device doped through spin-coating of toluene based solution ofF4TCNQ

The obtained saturation current density was

. It was the lowest obtainedvalue in the experiments on which this thesis is based. The obtained ideality factor 3 4 wasslightly higher than the one obtained for the undoped lithography patterned sample.The width of the depletion layer as function of the applied voltage bias was determined for the

sample doped with spin-coated toluene solution of F4TCNQ. Figure 24 illustrates this

dependence.

1E-10

1E-09

1E-08

1E-07

1E-06

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

Currentdensity[A/mm

2]

V[V]

values

fit


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Figure 24: Depletion layer width for a sample doped with spin-coated toluene solution ofF4TCNQ

The electron acceptor concentration was determined for three ranges from 0 V to 1.5 V, from 3.5

V to 5 V and from 7 V to 8.5 V. In the lowest and the highest range the obtained concentrations

were lower than for the undoped equivalent sample. This suggests that the F4TCNQ treatment ofthe Au contact did not result in doping of the P3HT layer. Another possible explanation can be

the fact that the undoped sample was exposed for long time to atmosphere and oxygen doping22

could have altered the properties of the devices. For the finger structure devices a doping profile

can be observed depending on the width of the depletion layer. For the capacitor a constant

depletion lyer width is observed similarly to the undoped sample. The C V measurements forboth undoped and doped sample as well as the J V curves show difference in the behavior ofbetween the finger structures and the capacitors.

0

20

40

60

80

100

120

0.00 2.00 4.00 6.00 8.00 10.00

W[nm]

V [V]

2a3_d1

2a3_d2

2a3_d3

2a3_capacitor

NA=1.041017 1/cm3

NA=6.1710

17

1/cm

3

NA=8.511016 1/cm3


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5. Conclusion and OutlookRFID and displays need good charge carrier injection properties at the metal/semiconductorsurface in order to achieve high on-currents and on-off current ratios as well as good switching

parameters. Therefore a main goal of this thesis was to investigate the charge carrier injection

properties at a metal/semiconductor interfaces. In order to do that, wet-chemical processed

Schottky diodes were spin-coated from P3HT. Keeping all other parameters constant the

electrodes were modified.

First two different structures of the diode were examined once the bottom contact was Cu and

the top one Au and then the contact materials were swapped. In both cases the contacts were

applied through sputter-coating using a shadow-mask. Between the top and the bottom contact a

P3HT layer was spin-coated. The on-currents, on-off ratios and the nobilities were compared.

The characteristics of the structure with copper bottom contacts were much lower than those of

the one with gold bottom contacts. This phenomenon was attributed to the copper electrode

which was in contact with air for a period of time during the production of the diode. During this

period the oxygen from the air oxidized the contact and created insulating layer. This was the

reason for the overall low currents. The devices with bottom copper contacts showed good

stability over time unlike the ones with gold bottom contacts.

The devices with Au bottom electrodes showed strong time degeneration in the on-off current

density ratios when exposed to ambient conditions for several days. The decrease in the ratios

was caused by increase of the off-current. Those results agree with the theoretical expectations32.

A theoretical explanation is that the oxygen doped

22

the P3HT layer at the rectifying coppercontact. This resulted in decreas of the potential barrier and increase of the off current.

After it was shown that the structure with bottom gold contacts is more suitable for diode

production, the influence of the technique used to pattern the bottom gold contacts was

investigated. Therefore already patterned gold structures on PET-foil from IFM Freiburg were

covered with P3HT and copper was applied on top through sputter-coating using a shadow-mask.

The sample was compared to the one identical structure patterned using shadow-mask. The

mobility was slightly lower at for the lithography patterned devices. Thedisorder parameters were and the on-off current ratios were similar ( 4 7 and 6300for lithography patterned and

7 and 4300 for shadow-mask patterned). The

number working devices per sample was almost double for the lithography patterned sample,

which was the reason for choosing this method for the further experiments.

The last experiments conducted during the thesis examined the effectiveness of treating the gold

contacts with an electron acceptor F4TCNQ. In order to avoid doping the P3HT

semiconductor, a monolayer of the dopant was necessary. The technologies of applying the

dopant as well as the used solvents were varied. At first the F4TCNQ was drop-casted from

toluene solution. A challenge with this technology was obtaining a monolayer because during the

drop-casting rings were formed and they couldnt be removed with isopropanol washing. The


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27

J V measurements showed that the off-current has increased compared to the undoped sample.Thus it was assumed that the copper contact was also doped. A solution was found by changing

the dopant application technology to spin-coating. Two solvents were used with this technology toluene and chloroform. The J V characteristics showed significant improvement. For thediodes including chloroform solution the best on-off current ratio was 52700 and for the toluene

solution 78500. The increase in the ratios was driven both by decrease of the off-current and

increase of the on-current.

The conducted C V measurements showed a doping profile within the semiconductor layer.This was assumed to be due to the influence of the oxygen which may have diffused in the

semiconductor over time and have doped it. To avoid the ageing of the devices and the overall

impact of oxygen the introduction of a protective layer can be investigated in the future. PMMA

or another polymer with low oxygen diffusion rate can be used.

In conclusion the thesis shows that the performance of Schottky diodes can be significantlyimproved. This was achieved by enhancing the charge carrier injection at the

metal/semiconductor interface when the gold electrodes were treated with F4TCNQ.

Acknowledgments

I would like to thank Professor Veit Wagner for the opportunity to work on this project and

Marlis Ortel for valuable help and advice. I would also like to give my gratitude to the rest of the

research group for their support.

I would like to gratefully acknowledge the financial support of PolyIC GmbH & Co KG within

the BMBF project MaDriX.


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