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Balanced Ambipolar Organic Field-Effect Transistors by Polymer Preaggregation Lukasz Janasz, Adam Luczak, Tomasz Marszalek, Bertrand Dupont, Jaroslaw Jung, Jacek Ulanski, and Wojciech Pisula ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 1, 2017

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ACS Applied Materials & Interfaces

Balanced Ambipolar Organic Field-Effect Transistors by Polymer Preaggregation Lukasz Janasz,1 Adam Luczak,1 Tomasz Marszalek,2,3 Bertrand G. R. Dupont,1 Jaroslaw Jung,1 Jacek Ulanski,*,1 Wojciech Pisula*1,4 1. Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland 2. Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany 3. InnovationLab, Speyererstr. 4, 69115 Heidelberg, Germany 4. Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Keywords: Organic field-effect transistors, ambipolar charge carrier transport, heterojunction films, poly(3-hexylothipohene), [6,6]-phenyl-C61-butyric acid methyl ester Abstract: Ambipolar organic field-effect transistors based on heterojunction active films still suffer from their unbalanced transport of electrons and holes. This problem is related to an uncontrolled phase separation between the donor and acceptor organic semiconductors in the thin film. In this work, we have developed a concept to improve the phase separation in heterojunction transistors to enhance their ambipolar performance. The concept is based on a pre-aggregation of the donor polymer, in this case poly(3-hexylthiophene) (P3HT), before

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solution mixing with the small molecular weight acceptor, phenyl-C61-butyric acid methyl ester (PCBM). The resulting heterojunction transistor morphology consists of self-assembled P3HT fibers embedded in a PCBM matrix ensuring balanced mobilities reaching 0.01 cm2/Vs for both holes and electrons. These are the highest mobility values reported so far for ambipolar OFETs based on P3HT/PCBM blends. The pre-aggregation of the conjugated polymer before fabricating binary blends can be regarded as a general concept for a wider range of semiconducting systems applicable in organic electronic devices.

Introduction Ongoing development in the field of organic electronics, despite the rapid progress, still faces fundamental challenges, which need to be overcome in order to turn organic-based electronic devices commercially available.1-4 One issues is related to the fabrication of stable and reproducible ambipolar organic field-effect transistors (OFETs) with balanced hole and electron transport.5-7 Ambipolar OFETs are highly desirable for the realization of complementary inverters which are a vital element of any logic circuits.8-9 Among various concepts for ambipolar transistors, composite active films obtained from two solution processable n-type and p-type semiconductors attract attention due to simplicity and low cost of the deposition process. Wet techniques are promising alternatives to the evaporation process which requires an expensive vacuum-based technology.10 Blending two organic compounds with opposite electron affinity as donor and acceptor semiconductors is a well-known concept for the fabrication of organic heterojunction photovoltaic devices.11-13 The heterojunction film consisting of the binary blend can be deposited in just one-step in contrary to devices based on multilayer structures. However, control over the film structure and morphology is challenging, especially when the film consists of two materials with different physico-chemical properties. Charge transport properties of the film strongly depend 2 ACS Paragon Plus Environment

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on microstructure (e.g. sizes of aggregates), crystal structure and surface arrangement of the molecules (e.g. edge- or face-on orientation).14 Therefore, control over the microstructure is a vital aspect for ambipolar OFETs with balanced and efficient hole and electron transport. One of the most popular small molecular semiconductor applied as acceptor in ambipolar blends, is a fullerene derivative - phenyl-C61-butyric acid methyl ester (PCBM) due to its electron transporting properties and good solubility in various organic solvents. Poly(3hexylthiophene) (P3HT) is commonly used as donor together with PCBM as acceptor for the formation of the heterojunction blend. Alignment of energy bands of these two semiconductors (P3HT: HOMO ~ -4.9 eV, LUMO ~ -3.0 eV; PCBM: HOMO ~ -6.1 eV, LUMO ~ -3.7 eV) and facile solution processing turn the P3HT/PCBM system as one of the most studied heterojunction blends in organic electronics.15-16 The hole and electron transport in such P3HT/PCBM blends depends on annealing temperature,

17

blend ratio,18 solvent,19 or

light exposure.20 Generally, the aim is to fabricate an interpenetrating networks morphology with individual and continuous transport pathways for holes and electrons. Other binary systems were also successfully introduced in ambipolar OFETs such as P3HT / perylene enabling control over the phase separation,21 or uniaxially oriented Tips-pentacene / naphthalene structures.22 In this work, we present a concept of improving the ambipolar balance between hole and electron transport in two-component blends. In the binary model system of P3HT and PCBM the ambipolar OFET behavior is enhanced by introducing a fibrillar microstructure of the polymer that facilitates the transport of charge carriers of both signs. The fibrillar microstructure is achieved by preaggregation of the polymer in solution before blending with PCBM leading to charge carrier mobilities of 0.01 cm2/Vs for both holes and electrons. Typically, ambipolar OFETs based on blends of these compounds exhibit rather poor balanced charge carrier mobilities of approx. 10-4 cm2/Vs.17, 20, 23-24 So far, mobilities in the



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order of 0.01 cm2/Vs were achieved only for a multilayer structure which were obtained by connecting two separate P3HT and PCBM films together.25 Such film fabrication is more complex than simple spin coating of the binary blend as presented in this work.

Experimental Materials P3HT with Mw = 94.1 kg/mol, polydispersity index PDI = 1.90 and regioregularity (RR) = 95.5 was purchased from Ossila. Phenyl-C61-butyric acid methyl ester 99.5% (research grade) was purchased from Sigma-Aldrich. Chlorobenzene and toluene (both HPLC grade) were purchased from Sigma-Aldrich. All compounds and solvents were used as received. Fabrication and measurement of OFETs OFETs based on bulk heterojunction blends of P3HT and PCBM were prepared in bottom gate, top contact configuration on silicon substrates with 300 nm thick silicon dioxide layer. Firstly, the substrates were cleansed in an ultrasonic bath of acetone and isopropanol for 15 min each. Afterwards, the substrates were treated with oxygen plasma for 5 min and functionalized with HMDS (hexamethyldisilazane) by vapor for 4 h. These surface modified substrates were then transferred to a glove-box system and from this point the OFET fabrication and characterization took place under nitrogen atmosphere. Solutions of P3HT were prepared in toluene at a concentration of 6.0 mg/ml (dissolved at 80oC for 2h.) which were aggregated in dark conditions at room temperature for 24 h. PCBM was dissolved in toluene at a concentration of 20.0 mg/ml. In the next step, 0.5 ml of preaggregated P3HT solution was added to the PCBM solution. The final solution consisted of 3 mg of P3HT, 20 mg of PCBM and 1.5 ml of toluene resulting in a ratio of P3HT:PCBM ~1:7. This mixture was spin-coated on the silicon substrates into films which were subsequently annealed at 75 °C and at 120 °C in vacuum for 2h. In the case of films with non-aggregated P3HT, the solution was spin-coated directly after removing from the hot plate. Gold source and drain 4 ACS Paragon Plus Environment

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electrodes were evaporated on the deposited films in high vacuum (~10-6 mPa). Each sample consisted of 20 OFETs with 30 µm channel length and 1 mm width. Transistors were measured by using a Keithley 2634B source meter. Mobilities were calculated from the transfer characteristics in the saturation regime, using the following formula: =

( − )

where Id is the drain current, W the channel width, L the channel length, Ci the capacitance of gate dielectric, Vg the gate voltage and Vth the threshold voltage. UV/Vis spectroscopy The UV/Vis spectroscopy measurements were performed using a Cary UV/Vis/Nir 5000 spectrophotometer from Varian and quartz glass cuvettes of 1 mm thickness. GIWAXS measurements GIWAXS experiments were performed by means of a solid anode X-ray tube (Siemens Kristalloflex X-ray source, copper anode X-ray tube operated at 30kV and 20mA), Osmic confocal MaxFlux optics, X-ray beam with pinhole collimation and a MAR345 image plate detector. The samples were prepared as thin films following the same procedure as was applied for the OFET fabrication. Atomic Force Microscopy Veeco Dimension 3100 Atomic Force Microscope was used to investigate the microstructure and to determine the thickness of the blend films. All images were obtained in the tapping mode with Olympus silicon cantilevers at 320 kHz resonance frequency. The film thickness was determined by the NanoScope Analysis software by plotting the step high images that represent the average cross sections. Each presented value is an average derived from eight measurements of different areas of the samples. The uncertainty was estimated as standard deviation from the measurements.



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Results and discussion Aggregation of P3HT As mentioned in the Introduction, ambipolar OFETs based on bulk P3HT:PCBM blends exhibit rather poor mobility of both holes and electrons. The standard preparation procedure of the transistors was also applied in this study for comparison. The film processing consists of mixing the two semiconductors in solution in a ratio of 1:1 at a concentration of 5 mg/ml in toluene for both compounds. This procedure was reported to exhibit so far the most symmetric electron and hole transport in OFETs based on P3HT/PCBM blends.17,

20, 26

Devices prepared in this way show indeed also in this study a symmetric ambipolar behavior with mobilities of µh = 9.0 x 10-4 cm2/Vs and µe = 8.0 x 10-4 cm2/Vs for holes and electrons, respectively (Figure 1). The same films were also annealed at 120 °C for the same time (Figure S1). After this procedure the device turned in a unipolar behavior with solely a hole mobility up to 0.08 cm2/Vs, which is one order magnitude higher, than in the case of films annealed at 75 °C. After annealing at 120 °C, no electron transport was observed (Figure S1). The rise of the hole mobility may be related to improved inter-chain transport induced by crystallization of the polymer at high temperature.26 The drop of the electron transport was the result of PCBM clustering and therefore disruption of the electron percolation paths at the dielectric / semiconductor interface.17 In order to improve the transport properties of the blend, the polymer was aggregated prior to mixing with PCBM. P3HT is known to self-assemble into elongated, fibril aggregates when the solution is either ultrasonicated,27 UV-irradiated,28 mixed with poor solvent29 or aged for a specific time.30 These aggregates withstand the spin-coating process and form, under suitable conditions, a net of dense fibrils which improve the charge carrier transport in the film.30, 31 In this report, aggregates of P3HT were created in toluene by aging the solution for 24 h. Figure 2 shows AFM images of films spin-coated from pristine (non-aggregated) and 6 ACS Paragon Plus Environment

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aged (aggregated) P3HT solution. The film cast from pristine solution (Figure 2a) exhibited a uniform nodular structure which is typical for spin-coated P3HT. In the case of aged P3HT solution, elongated fibril assemblies were clearly evident in the AFM image (Figure 2b). Such fibril structure may improve the hole mobility in transistors around two orders of magnitude from approx. 10-3 cm2/Vs to approx. 0.1 cm2/Vs.31 This enhancement is related to pronounced self-assembly of the polymer chains within the fibrils. Chain folding of P3HT inside the fibrils leads to improved π-π stacking with increased coherence length.31 In this work, OFETs prepared from aggregated P3HT exhibited a hole mobility above 0.02 cm2/Vs. This value is lower than for highly aggregated films with very dense fibril packing. We attribute the decrease to low thickness of the P3HT films obtained from solutions with only a concentration of 1.5 mg/ml (the same concentration as for the P3HT:PCBM blends described further). In the next step, aggregated P3HT was mixed with the pristine PCBM solution resulting in a mass ratio of ~1:7 P3HT:PCBM. This ratio exhibited in the case of aged P3HT the most balanced electron and hole transport with the highest mobilities (see next section). When for instance a P3HT:PCBM ratio of 1:1 was applied (like in the case of pristine P3HT shown in Figure 1) only a p-type transport was observed due to the dominating role of P3HT fibrils on the overall transport. Small change in the blend ratio to P3HT:PCBM 1:5 resulted in much lower electron mobility (µe = 2 x 10-3 cm2/Vs, µh = 1 x 10-2 cm2/Vs) in comparison to the balanced ambipolar transport of the 1:7 blend (Figure S2). Similarly, the blend with higher amount of PCBM (P3HT:PCBM 1:10) showed a lower hole mobility (µe = 0.9 x 10-2 cm2/Vs, µh = 1.4 x 10-3 cm2/Vs) (Figure S3).

UV-Vis spectroscopy and GIWAXS measurements



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UV-Vis spectroscopy was used to clarify if the P3HT aggregates were still preserved in the blend solution after mixing with PCBM in comparison to the pure aged solution (Figure 3). In the case of the aged P3HT solution two additional maxima at approx. 560 nm and 615 nm appeared which correspond to the 0-1 (intrachain) and 0-0 (interchain) transitions (Figure 3a).32-34 The presence of these maxima serves as a proof for increased P3HT aggregation.34 The blend solution with aged P3HT and pristine PCBM revealed the same maxima at approx. 560 nm and 615 nm as footprint for the aggregated polymer (Figure 3b). This result indicated that local polymer order attributed to the P3HT aggregation was still preserved after mixing with the PCBM solution. GIWAXS studies of blends with pristine and aggregated P3HT (Figure 3c) confirmed a considerable improvement in polymer order after polymer aging. Lack of any reflection for the blend with pristine P3HT implied serious polymer disorder. For the blend film with aggregated P3HT the distinct scattering peak attributed to an interlayer distance of 1.57 nm was characteristic for well-ordered polymer chains possibly edge-on arranged on the surface.35 The structural study clearly proved a significant increase in P3HT order and phase separation in the blend film after polymer aging.

Ambipolar Charge Carrier Transport OFETs with a layer made of aggregated P3HT and non-aggregated PCBM exhibited symmetric ambipolar transport with average mobilities of holes µh = 0.008 cm2/Vs and electrons µe = 0.011 cm2/Vs (Figure 4). These values are only slightly lower than in the case of OFETs with layers made of pure P3HT (µh = 0.024 cm2/Vs, Figure S4a), or pure PCBM (µe= 0.027 cm2/Vs, Figure S4b). In the positive voltage regime the transistors revealed a dominating n-type transport above Vgs = 20 V (Figure 4a), while the crossover point from electron to hole dominating transport was observed in the negative voltage regime below -20


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V (Figure 4b). The symmetric form of the characteristics indicated a well-balanced electron and hole charge transport through the active film. The mobility was one order of magnitude higher than for the film deposited from the pristine 1:1 P3HT/PCBM mixture. To our knowledge, these are so far the highest reported balanced ambipolar mobilities for OFETs based on P3HT/PCBM blends. To understand the origin of the increase in hole and electron mobility, the film topography was further investigated by using tapping-mode AFM. The height images of films containing aggregated P3HT showed only ‘traces’ of fibrils (Figure 5a). Interestingly, a dense net of fibril-like structures became obvious just in the phase images under ‘hard tapping’ scanning conditions (Figure 5b). Due to a high degree of entanglement of the densely packed structures the fibril length over several microns could be estimated only from these phase images (Figure 5b), while the fibril width was approx. 40 nm (Figure 5c). Taking the observed morphology into account, the reason of increased hole and electron mobility may be addressed to the phase separation between P3HT and PCBM. The improvement of the hole transport is directly related to the P3HT fibrils and to an increased πorbital overlap of the polymer chains within these structures providing good percolation paths between source and drain electrodes. For the films obtained from pristine P3HT and PCBM solution both compounds mixed well forming a uniform and featureless film (Figure 6). It has been reported that the PCBM miscibility is highly dependent on the degree of P3HT crystallinity in the blend.35 PCBM can interpenetrate only the amorphous regions of P3HT. A morphology of a finely dispersed P3HT/PCBM phase is unfavorable for the charge separation and transport which was thoroughly studied for OPV devices.35 P3HT regions with higher degree of crystallinity do not allow PCBM to fully dissolve the polymer and therefore favor phase separation and aggregation also of the acceptor molecules.15,

36

A phase–separated

morphology is partially achieved by thermal annealing of the film which improves typically



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the solar cell efficiency.37 In the case of OFETs based on P3HT/PCBM heterojunction film morphology, thermal annealing is also known to increase the charge carrier mobility due to better partial phase separation and accompanied improved molecular order.26 However, the mobility enhancement, achieved in this way, is not as large as by preaggregation of P3HT described in this work. From the fact that fibrillar structures are only visible on the phase diagrams, another interesting hypothesis may be drawn. A similar effect in topography AFM scans was reported for a poly(3-hexylthiophene)-b-poly(styrene) (PS-PHT) copolymer which also self-assembled in a fibrillar fashion.38 The PS-PHT nanowires became visible only under hard tapping conditions during recording of the phase images.38 It was concluded that PS-PHT selfassembled into a core-shell structure. In the case of our system, a similar conclusion can be drawn that the observed microstructure consists of core-shell fibers with the P3HT rigid core wrapped by the PCBM phase. In this configuration of the fibrils, a sufficiently high force of the AFM tip is necessary to penetrate the outer shell to visualize the polythiophene phase inside the structures. However, this hypothesis requires additional investigation to be fully confirmed.

Relation between charge carrier transport and film thickness In order to investigate the influence of film thickness on charge carrier transport and microstructure of the P3HT:PCBM 1:7 blends with aggregated and pristine polymer, a series of OFETs with thicknesses ranging from 20.0 to 70.0 nm was prepared. Figure 7 shows the variation in microstructure with film thickness for aggregated P3HT. In contrary to films where P3HT was aggregated, films from pristine solutions exhibited a uniform nodular structure (Figure 6). Films containing pristine P3HT showed severely higher electron mobility and lower hole mobility (Figure 8a) than 1:1 blends described previously (Figure 1). Although


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the same processing conditions were applied, variation in the ratio of the pristine blend (concentration of PCBM 7-times higher than P3HT) was responsible for vivid changes in both electron and hole mobility. The AFM phase images in Figure 7 revealed a less dense net of fibrils for thinner films simply due to a smaller amount of deposited material. A further reason is the faster spin-coating velocity at which the stronger centrifugal force separated the fibrils further away from each other. The density of the fibers seemed to change more drastically below the film thickness of approx. 40 nm. Below this thickness, the difference in fiber density was not pronounced. It has to be emphasized that these topography phase images only displayed the fiber morphology on the top film surface. The real fiber density including the buried structures in the film might be lower for the sample presented in Figure 7d. This explanation is in agreement with the strong decrease in hole mobility for the 20 nm thick films. In our previous work dedicated to ultrathin films of P3HT fibers, the change in fiber density was gradual with the decrease in film thickness.31 The changes in film microstructure between different thicknesses had a significant influence on the charge carrier transport in the case of blend films containing aggregated P3HT. Figure 8 illustrates the mobility as a function of film thickness for the P3HT/PCBM films with pristine (Figure 8a) and aggregated (Figure 8b) polymer. The hole mobility improved about three orders of magnitude over the whole investigated thickness range due to formation of the P3HT fibrils and induced phase separation described in the previous section. The rise in electron mobility was smaller, but still well pronounced for all thicknesses. Interestingly, the difference in hole and electron mobility increased for thinner films. The mobility for holes decreases stronger with lowering the thickness in comparison to electrons with a small decline from 0.011 cm2/Vs to 7.8 x 10-3 cm2/Vs (Figure 8b). This behavior can be explained in terms of the film microstructure. The AFM height images in Figure 7 exhibited similar film microstructures for the whole thickness range. The phase images provided again more valuable information revealing a clear



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difference in fibril density which dropped with lowering the thickness. The PCBM matrix, in which the polythiophene fibrils were embedded, covered the same visible area for all thicknesses. The decrease in electron mobility was less pronounced than in the case of holes because the majority of PCBM created a continuous and uniform matrix in the whole film. It is also worthy to indicate, that PCBM with its LUMO level at ~ -3.7 eV, appears as good candidate for electron transporting component in blends with conjugated polymers, since according to results obtained by Paul Blom and coworkers39 the electron traps in all conjugated polymers have a common origin, most likely related to hydrated oxygen complexes, (H2O)2–O2, centered at an energy of ∼-3.6 eV, i.e. above the LUMO level of PCBM. The thickest films containing aggregated or non-aggregated P3HT were also annealed at 120 °C for 2 h. In the case of blends with pristine P3HT (Figure S5), the hole mobility raised two orders of magnitude up to 1.3 x 10-3 cm2/Vs, but electron mobility dropped severely to 2.4 x 10-4 cm2/Vs. This behavior may be explained in the same manner as for the pristine 1:1 blends. After annealing at high temperature, the 1:7 ratio film with aggregated polymer (Figure S6) showed a decreased hole mobility of µh = 1.0 x 10-3 cm2/Vs and even more decreased electron mobility of µe = 9.5 x 10-5 cm2/Vs. The drop of the electron mobility may be addressed (as in the previous cases) to the clustering of PCBM, while the decline in hole transport might can be explained by disruption of already ordered P3HT fibrillar aggregates.

Summary and Conclusions Organic field-effect transistors consisting of P3HT/PCBM blends revealed a balanced ambipolar performance with mobilities reaching 0.01 cm2/Vs for both holes and electrons. The balanced transport of both types of charge carriers was achieved by aggregating P3HT through solution aging prior to mixing with the PCBM solution. The spin-coated films


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exhibited a dense network of fibrillar P3HT aggregates. These features became visible only in the phase AFM images under “hard tapping” conditions. Due to decreased miscibility of PCBM in the self-assembled P3HT fibers, the resulting films exhibited a high degree of phase-separation. This microstructure significantly improved the transport of holes and electrons leading to a high balanced ambipolar performance. The mobility of 0.01 cm2/Vs for both holes and electrons is, so far, the highest balanced performance reported for blends of P3HT and PCBM. The pre-aggregation of the conjugated polymer before fabricating binary blends can be regarded as a general concept for a wider range of semiconducting systems applicable in organic electronic devices.

Figure 1. Output and transfer characteristics of P3HT/PCBM non-aggregated active film in the a), b) n-type and c), d) p-type operation mode.



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Figure 2. AFM height images of P3HT films spin-coated from a) pristine and b) 24 h aged toluene solution.

Figure 3. UV-Vis spectra of a) P3HT and PCBM and b) blend of P3HT and PCBM before and after P3HT aging in toluene, c) radial integration of the GIWAXS pattern for blend films with pristine and aggregated P3HT (asterisk indicates the P3HT interlayer peak).


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Figure 4. Output and transfer characteristics of OFETs with PCBM and preaggregated P3HT active film in the a),b) n-type and c),d) p-type operation mode.

Figure 5. a) Height and b), c) phase AFM images of films spin-coated from PCBM and preaggregated P3HT solution. In c) the fibril width is marked.



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Figure 6. AFM a) height and b) phase images of P3HT/PCBM film cast from pristine P3HT and PCBM solutions.

Figure 7. AFM phase (left half) and height (right half) images of P3HT/PCBM film cast from solution of PCBM and preaggregated P3HT with thickness of a) 60 nm, b) 40 nm, c) 35 nm and d) 20 nm. 16 ACS Paragon Plus Environment

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Figure 8. Electron and hole mobilities versus thickness of P3HT/PCBM films obtained from solutions a) with non-aggregated P3HT, and b) with aggregated P3HT.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Output characteristics of OFETs with active films of pure compounds and blends.

AUTHOR INFORMATION Corresponding Author * Wojciech Pisula, address: Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany, e-mail: [email protected] * Jacek Ulanski, address: Lodz University of Technology, Faculty of Chemistry, Department of

Molecular

Physics,

Zeromskiego

116,

90-924

Lodz,

Poland,

e-mail:

[email protected]



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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by National Science Centre, Poland through the grant DEC2013/08/M/ST5/00914 and UMO-2015/18/E/ST3/00322 as well as by Foundation for Polish Science through the grant MASTER/MISTRZ 9/2013. REFERENCES (1)

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