Optical Modulation of the Charge Injection in an Organic Field-Effect Transistor Based on Photochromic Self-Assembled-Monolayer-Functionalized Electrodes

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Optical Modulation of the Charge Injection in an Organic Field-Effect Transistor Based on Photochromic Self-Assembled-Monolayer-Functionalized Electrodes

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Núria Crivillers , Emanuele Orgiu , Federica Reinders , Marcel Mayor , * and Paolo Samorì *
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In the last decades a great amount of effort has been devoted to the fabrication of organic fi eld-effect transistors (OFET) featuring high performance as key elements for new organic-based logic applications. The endeavor has been focused on the improvement of each of the main contributing components of the devices. This includes design and synthesis of new semi-conductors (p- and n-type), which show improved air stability, self-assembly behavior, and electrical characteristics, and, more recently, the development of novel high-performance gate die-lectrics and novel processing techniques. [ 1 ] In addition to the optimization of the device architecture, the engineering of the interface semiconductor–dielectric [ 2 ] and semiconductor–metallic contacts have attracted much interest due to the importance of the interfacial morphology and the hole–electron injection barrier on the device performance. In particular, self-assembled monolayers (SAMs) are widely used to tune the wet-tability and work function of the metal–organic junctions. [ 3 ] In contrast to the extensively employed chemisorbed alkane- and arene-thiol SAMs, the use of SAMs based on molecules that respond to external stimuli has still not been explored. Among responsive systems, azobenzenes are known to undergo revers-ible photoinduced isomerization between trans and cis forms, which can exhibit different optical and electrical properties. [ 4 ] Such unique photoisomerization process in SAMs based on azobenzene derivatives has been exploited for applications, such as the generation of light-triggered dynamic surfaces showing reversibly switchable wettability [ 5 ] and controlled DNA delivery, [ 6 ] and are an interesting platform to study switchable electronic characteristics at the nanoscale. [ 7 ] Although most photochromic molecules exhibit a poor switching capacity (i.e., yield) at the ensemble level when chemisorbed at surfaces, we recently found that the terminally thiol-functionalized biphenyl

© 2011 WILEY-VCH Verlag GmAdv. Mater. 2011, 23, 1447–1452

DOI: 10.1002/adma.201003736

Dr. N. Crivillers , Dr. E. Orgiu , Prof. P. Samorì Nanochemistry LaboratoryISIS – CNRS 7006, Université de Strasbourg8 allée Gaspard Monge, 67000 Strasbourg, France E-mail: [email protected] F. Reinders, Prof. M. Mayor Department of ChemistryUniversity of Basel19 St. Johannsring, 4056 Basel, SwitzerlandE-mail: [email protected] Prof. M. Mayor Institute for NanotechnologyKarlsruhe Institute of Technology (KIT)P.O. Box 3640, 76021 Karlsruhe, Germany

azobenzene (AZO, Scheme 1 ) [ 8 a] forms highly ordered and tightly packed SAMs on Au(111), which display isomerization yields exceeding 96%. The unexpectedly effi cient isomeriza-tion reaction in the monolayer most likely emerges from the light-induced collective isomerization over entire crystalline domains. [ 8 b]

Here such a SAM is exploited once it is chemisorbed on the Au source and drain electrodes of an OFET to optically modu-late for the fi rst time the charge injection at the electrode–semiconductor interface.

As a test bed OFET devices were fabricated in a bottom-gate bottom-contact confi guration using an air-stable n-type system, i.e., N , N ’-1H,1H-perfl uorobutyl dicyanoperylenecarboxydiimide (PDIF-CN 2 ), [ 9 ] as organic semiconductor (SC). By employing such a geometry, the illumination has to be performed from the top of the device, implying that the SC can act as a light-fi ltering and absorbing layer. To overcome this issue, two critical aspects had to be considered when choosing the ideal semiconductor. First, the absorption spectra of the SC should not overlap with the characteristic absorption bands of the azobenzene derivative and second, the alteration of the electrical properties of the SC during light exposure (UV and visible) should be minimized. The spectrum of the trans azobenzene isomer shows an intense π – π ∗ transition band around 365 nm and a weaker (forbidden) n– π ∗ band around 450 nm, whereas the spectrum of the cis isomer is characterized by a more intense n– π ∗ transition band around 450 nm. [ 10 ] Upon illumination of a trans SAM with UV light the isomerization from trans to cis occurs. The back cis to trans isomerization can be triggered thermally because the trans form is thermodynamically more stable or by irradiation into the n– π ∗ band of the cis form using visible light. Although most of the reported semiconductors used for OFETs show a broad absorption spectrum with wavelengths, λ , between 350 and 600 nm, [ 11 ] PDIF-CN 2 thin fi lms display only minor absorp-tion features between 350 and 410 nm and an intense broad band with an absorption maximum at ≈ 550 nm. [ 12 ] Due to these unique optical properties, PDIF-CN 2 is an ideal semiconductor in an AZO-SAM-controlled FET.

The fabrication of the photoswichable transistor was accom-plished as follows. First, the functionalization of the electrodes was achieved by immersing the SiO 2 substrates exposing pre-patterned interdigitated gold source and drain electrodes in a 0.1 m M solution of the AZO compound in chloroform for 48 h in the dark to ensure the trans AZO-SAM formation. Imme-diately after, the semiconductor was deposited following two different approaches. In the fi rst approach a thin fi lm was spin-coated from a 1 mg mL − 1 PDIF-CN 2 solution in toluene and annealed at 50 ° C for 30 min in the glove box, whereas in the

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Scheme 1 . Schematic representation of the device structure showing the reversible isomeri-zation reaction (trans–cis) that takes place at the interface between the semiconductor and AZO-functionalized electrodes.

latter, PDIF-CN 2 fi bers (formed by solvent-induced precipita-tion (SIP)) were drop-cast in air on the azobenzene-derivatized electrodes and then transferred into an inert atmosphere. All the electrical characterizations were performed in a glove box and in the dark.

The output and transfer characteristics of the fi lm PDIF-CN 2 /azobenzene-SAM-based OFET obtained on a device with a channel width ( W ) of 10 mm, a channel length ( L ) of 10 μ m, and a fi lm thickness of 8–10 nm, are displayed in Figure 1 a,b. All output curves showed a good fi eld-effect response. To trigger the isomerization cycles, the trans OFET (being the ini-tial state of the AZO-SAM within the transistor) was exposed to UV light (365 nm) for 90 min, then the reversible thermal back reaction was achieved by keeping the sample in the dark for 24 h. The photoinduced cis-to-trans isomerization (with vis-ible light) could not be carried out due to the observed increase of the source-drain current for a PDIF-CN 2 -based transistor when it was exposed to white light, which was attributed to the common phenomenon of photoexcitation of the semiconductor, i.e., the photogeneration of charge carriers [ 13 ] (see Supporting Information, Figure S1).

The cis - SAM-based device exhibits an increase of the max-imum source-drain current ( I D,max ), both in the output and transfer characteristics of the transistor (Figure 1 a,b). Figure 1 b shows that in the linear regime (source-drain voltage ( V D ) = 20 V) the I D increases 20% upon isomerization. Such an increase is also accompanied by a decrease of the threshold voltage ( V TH ) and Δ V TH(trans-cis) of 9.2 V and 7.1 V in the linear and saturation regimes, respectively. This behavior provides unambiguous evi-dence for the better charge injection capacity in the cis-based device, which can be explained by the smaller thickness of the SAM, i.e., a decrease in the tunneling barrier thickness from trans to cis. This is in full agreement with previous current–voltage ( I – V ) characterizations of the AZO-SAM when incor-porated in two terminal junctions, as studied by conducting atomic force microscopy (C-AFM) [ 14 ] and Hg-drop-based measurements, [ 15 ] demonstrating that the switching effect of the transport properties relies on the difference in the tun-neling barrier thickness. [ 7 ]

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Signifi cantly, the optical modulation process was found to be reversible, as proven by the recovery of I D and V TH measured after each cycle (Figure 1 c,d). Up to four reversible cycles were carried out without signifi cant lost in effi ciency of the switch, i.e., fatigue.

To demonstrate that the charge injec-tion modulation was indeed caused by the isomerization of the SAM at the interface of the semiconductor and gold electrodes, and not as a result of an alteration of the prop-erties of the pure semiconducting fi lm upon exposure to the UV light, two separate con-trol experiments were carried out. A PDIF-CN 2 spin-coated fi lm on bare gold electrodes and a PDIF-CN 2 spin-coated fi lm on gold electrodes coated with an undecanethiol SAM were irradiated with UV light for 90 min. The electrical characterization performed on both transistor types did not reveal any change

parameters (mobility ( μ ), V TH , and I D ) of the
in the electrical device (Figure S2, Supporting Information), thus ruling out a role played by photogenerated charges in the semiconducting material on the increased current within the junction. Moreover the absence of a V TH shift indicates that electron trapping [ 16 ] at the gate dielectric–organic semiconductor interface due to the illumination at 365 nm is irrelevant. In fact, the PDIF-CN 2 fea-tures a very modest absorption in the UV range, thus ruling out a major role of photoexitation of the SC.
In addition to the difference in tunneling resistance between trans and cis AZO-SAMs at the interface, the change in the electrical characteristics upon photoisomerization may in prin-ciple be ascribed to two other aspects, i.e., a variation of the fi lm morphology at the interface and a change in the work function (WF) of the contacts with the electrodes. It has been shown that the hydrophobic–hydrophilic nature of the surface can drastically affect the crystallinity and ordering of the semi-conducting fi lm physisorbed at the interface, thus varying the electrical characteristics of the device. [ 17 ] In our case, trans- and cis - AZO-SAM show different hydrophobic properties, with static water contact angle values of 87.2 ° ± 1.3 ° and 70.6 ° ± 1.9 ° , respectively. Moreover, in the cis form the azo nitrogen atoms with their lone pairs (Ph–N = N–Ph) are more exposed towards the semiconducting material, providing the possibility to form additional interactions between the molecules of both organic layers. Such different wettability can infl uence the interfacial fi lm microstructure. To address this aspect, two additional trans and cis devices were individually prepared. A PDIF-CN 2 fi lm was spin-coated on trans-AZO-SAM-modifi ed electrodes and in parallel the same solution was spin-coated on cis-AZO-SAM-functionalized gold contacts. The trans transistor was obtained as previously described but the incubation time for the SAM formation was reduced to 24 h. The cis based device was prepared by immersion of the patterned gold substrate in an AZO solution that was previously illuminated with UV light for 1 h. During the AZO-SAM formation, the substrate was continuously illuminated to ensure the adsorption of the cis conformer on the electrodes. Scanning tunneling microscopy (STM) studies revealed that, similar to the trans isomer, the cis

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Figure 1 . In situ measured a) output and b) transfer (linear regime, V D = 20 V) characteristics for a PDIF-CN 2 -fi lm-based device with AZO-functionalized electrodes before (trans) and after (cis) UV irradiation. Variation of c) I D,max (linear and saturation regime) and d) V TH (linear and saturation) for con-secutive switching cycles of trans–cis AZO-functionalized electrodes.

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can also chemisorb into thermodynamically stable and highly ordered crystalline fi lms, featuring a packing that is equivalent to that of the trans SAM, within the error bars. [ 8 ] The immer-sion time in the AZO solution was 24 h for both devices and no further annealing treatment was performed after fi lm deposi-tion. In the case of the cis transistor, the substrate was removed from the solution, rinsed, and dried under N 2 avoiding light exposure. The SC fi lm was spin-coated immediately after, fol-lowed by the electrical characterization. Output ( Figure 2 a,b) and transfer (Figure 2 c) characteristics show markedly higher drain currents for the cis-based transistor compared to the corresponding trans, supporting the observed results in the in situ optical modulation of the device. The I D (cis)/ I D (trans) ratio extracted from these measurements was 2.4. Such a sig-nifi cant increase is attributed to an improved morphology of the interface for fi lms separately spin-coated on the two isomer


AZO-SAMs leading to a more pronounced device perform-ance difference. The fi lm surface morphology of both cis and trans transistors was studied using atomic force microscopy (AFM, intermittent contact mode). AFM images showed no sig-nifi cant variation of the fi lm structure on Au, despite a small surface roughness increase, illustrated by its root-mean-square determined on a 1 × 1 μ m 2 were trans 0.57 nm and cis 0.71 nm. However, we can not strictly correlate the observed top layer microstructure with the interfacial one, which is directly involved in the charge injection process. More important than this morphological factor might be the improved switching effi -ciency during the ex situ preparation of the cis SAM, resulting in larger amounts of cis domains and hence in a higher elec-trical transport difference in the FET device.

Interestingly, in the in situ experiment, the higher I D measured for the cis transistor rules out the possibility of the



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Figure 2 . Ex situ measured a,b) output and c) transfer ( V D = 20 V) char-acteristics for a PDIF-CN 2 fi lm based device with a) cis-AZO and b) trans-AZO-functionalized electrodes ( L = 5 μ m).

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formation of an empty cavity at the SAM–SC interface upon isomerization, possibly driven by a mechanical lifting of the thin fi lm [ 15 ] through the contraction–expansion of the SAM. This leads to an improvement in the charge injection due to the lower resistance of the shorter cis-based SAM. Such observa-tion implies that a molecular reorganization occurs within the SC fi lm at room temperature on a timescale shorter than the measurements. This molecular reorganization is likely to play a role in the observed device fatigue (Figure 1 c). In fact, in addi-tion to the known general problem of fatigue in photochromic mole cules, [ 18 ] in this specifi c case featuring an azobenzene SAMs incorporated in a device confi guration, a few additional limiting factors are important. The most relevant is likely the molecular rearrangement of the semiconductor at the interface. Such a factor may become detrimental for the isomerization yield as a function of time by leading to a more static interface, which is refl ected by a lower effi ciency of the modulation when performing more and more cycles.

Further, a possible WF modulation upon photoisomerization of the SAM was considered, as already reported for asymmetric azobenzene disulfi des monolayers. [ 19 ] Kelvin probe measure-ments showed that the WF of the gold electrodes remains almost unaltered upon chemisorption of the AZO-SAM, giving 5.15 ± 0.02 eV for the trans SAM, with an increase of 0.07 eV for the cis SAM. The bare gold WF was measured as 5.12 ± 0.02 eV. In light of this result, the observed variation of the OFET (trans–cis) characteristics cannot be attributed to the change in the electron injection barrier ( E Fermi(contact) – LUMO (PDIF-CN2) , where LUMO is the energy of the lowest unoccupied mole-cular orbital), i.e., different energetic level alignment due to the presence of the photochromic SAM on the gold contacts, and hence reinforces the difference in tunneling barrier thickness between the cis and trans isomers as a key contribution to the optical modulation of the transistor.

As mentioned above, the SC was chosen because of its optical properties and not because of its electronic character-istics (highest occupied molecular orbital (HOMO): –6.8 eV/LUMO: –4.5 eV [ 20 ] ) with respect to the WF of the azo-function-alized source and drain electrodes. This choice is refl ected in the low charge carrier mobilities obtained in the devices; how-ever, the typical S-shape of the output curves in the linear part, indicative for injection limitation, was not observed in these measurements. The extracted average linear mobilities for the trans and cis transistors (in situ switch) were 4.6 × 10 − 7 ± 0.3 × 10 − 7 cm 2 V − 1 s − 1 and 4.7 × 10 − 7 ± 0.4 × 10 − 7 cm 2 V − 1 s − 1 , respectively, and in the saturation regime they were 8.1 × 10 − 7 ± 0.7 × 10 − 7 cm 2 V − 1 s − 1 and 8.6 × 10 − 7 ± 0.4 × 10 − 7 cm 2 V − 1 s − 1 . Despite not exhibiting a large magnitude change upon photoin-duced isomerization, the difference in mobilities ( μ trans – μ cis ) underscores the better injection in the cis transistor, featuring a higher mobility and following the opposite trend of V TH (Figure S4, Supporting Information). The same behavior was observed for both linear and saturation regimes. These μ values are much lower compared to the recently reported values for PDIF-CN 2 spin-coated fi lms after thermal annealing (110 ° C in vacuum) and surface treatment ( μ e = 0.15 cm 2 V − 1 s − 1 in the saturation regime). [ 21 ]

However, it is well-known that the passivation of the dielec-tric with a specifi c surface treatment is benefi cial for reducing

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Figure 3 . In situ measured transfer characteristics ( V D = 20 V) at different UV irradiation times for an OFET based on PDIF-CN 2 fi bers deposited on AZO-functionalized electrodes. a) High coverage and b) low coverage sample.

the trap density at the dielectric–organic interface and that, together with the change in the surface hydrophobicity, results in an improvement in the performance of the device. [ 22 ] These results prompted us to functionalize the silicon dioxide dielectric with hexamethyldisilazane (HMDS). Such a derivatization was performed after having functionalized the gold electrodes with the AZO-SAM. The OFETs with HMDS treat-ment exhibited a notable improvement of the characteristics, including a saturation-regime mobility four orders of magnitude higher (8.3 × 10 − 3 cm 2 V − 1 s − 1 ) and a shift of the V TH to –3.8 V. However, such a treatment had a negative effect on the switching capacity, with a lower increase (6%) in the drain cur-

Table 1. Charge carrier mobilities and threshold voltage for the fi ber PDIF-CN 2 -based device on AZO-functionalized electrodes before and after exposure to UV light.

Higher surface coverage Lower surface coverage

Before UV After UV Before UV After UV

μ (cm 2 V − 1 s − 1 ) 2.6 × 10 − 4 3.1 × 10 − 4 1.1 × 10 − 5 1.5 × 10 − 5

V TH 23.4 11.9 11.3 2.0

rent after the fi rst trans → cis transition at the interface and a faster appearance of switching fatigue (Figure S3, Supporting Information). We believe that despite using a mild HMDS deposition procedure followed by a gentle cleaning–annealing treatment to avoid the degradation of the AZO-SAM, it is not possible to exclude a residual contamination from physisorbed silane on the gold electrodes, which would directly interfere with the interfacial parameters modulated with the photoactive SAM. On the other hand, the high improvement in the charge transport through the channel could make the photomodula-tion of the charge injection less noticeable in the fi nal perform-ance of the device. Attempts to perform the HMDS treatment prior to the AZO chemisorption on the electrodes did not lead to any improvement in the device characteristics.

With the aim of increasing the effi ciency of the isomeriza-tion at the interface, a second approach for the preparation of the in situ switchable OFET was taken. The deposition on the AZO-SAM-functionalized electrodes of PDIF-CN 2 fi bers fea-turing a cross section of 1.6 μ m introduces two new aspects to the working principle of the device. The thicker semiconductor architecture decreases the number of photons reaching the SAM, due to the lower transmission through the fi ber. How-ever, for a fi ber sample featuring a low coverage, this drawback is countered by the direct exposure of the AZO-SAM to the incoming UV light in uncovered areas, resulting in a higher isomerization effi ciency; in fact, due to the collective nature of the isomerization, [ 8 ] the conformational change of the AZO molecules is likely to be extended to the AZO molecules beneath the semiconductor layer. Two samples exposing SIP-prepared fi brilar structures with different surface coverage were proc-essed. Transfer characteristics ( V D = 20 V) shown in Figure 3 a,b revealed an increase in I D upon UV irradiation of 30% and 60% for the high and low coverage samples, respectively. This greater photoresponse was also accompanied by a more sig-nifi cant shift of the V TH ( V TH,cis – V TH,trans ) towards zero voltage (see Table 1 ) compared to the fi lm-based device. Unfortunately the back isomerization (cis → trans) was not observed after 24 h. Such a hindered isomerization process can be explained by a mechanical restriction within the system because of the higher thickness of the fi bers compared to the fi lm and its greater crys-talline nature leading to a more stable and static interface. The better molecular packing in the fi brilar structures along the


transport channel was refl ected in the higher mobilities (see Table 1 ). Moreover it also led to a notably large improvement in the switching ratio of the drain current on the sample where there is no fi ltering organic layer between the light source and the AZO-based SAM.

In conclusion, it was demonstrated that light illumination with well-defined wavelengths that trigger the photoisomer-ization of an azobenzene SAM chemisorbed on Au source and drain electrodes can be used to modulate reversibly the charge injection at the interfaces, leading a bifunctional FET. This approach differs from the very recently proposed method by Shen et al. [ 23 ] that rely on the photoswitching functionality on an OFET incorporating a spiropyran deriva-tive in the gate dielectric polymer. In the device presented here the source-drain current through the channel can be gated electrically (through gate control), as in a conventional OFET, and optically (through photochemical control). The photochemical bistable SAM mediates the injection through the variation of the tunneling barrier across the light-responsive SAM. Such a proof of concept is instrumental to the field of organic electronics, which searches for solutions to incorporate new and more functionalities in a device. Cur-rent effort in our labs is focused on improving the cyclability and time response of the switch, as well as testing different device geometries that can better highlight the effect of the injection.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.



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Acknowledgements The authors thank Corinna Raimondo for sharing the results of the contact angle measurements and Dr. Jeffrey Mativetsky for his hints on the SIP procedure to produce PDIF-CN 2 fi bers. This work was supported by the EC Marie-Curie IEF-OPTSUFET (PIEF-GA-2009 – 235967) and ITN-SUPERIOR (PITN-GA-2009 – 238177), FP7 ONE-P large-scale project no. 212311, the NanoSci-E + project SENSORS, and the International Center for Frontier Research in Chemistry (FRC). Financial support by the Swiss Nationals Science Foundation (SNF) and the Swiss Nanoscience Institute (SNI) is gratefully acknowledged. The authors are grateful to Polyera for providing the PDIF-CN 2 (N1100 active ink).

Received: October 11, 2010 Revised: November 18, 2010

Published online: February 9, 2011

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