Ambipolar Small-Molecule:Polymer Blend Semiconductors for

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Ambipolar Small-Molecule:Polymer Blend Semiconductors for Solution-Processable Organic Field-Effect Transistors Minji Kang, Hansu Hwang, Won-Tae Park, Dongyoon Khim, Jun-Seok Yeo, Yunseul Kim, Yeon-Ju Kim, Yong-Young Noh, and Dong-Yu Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12328 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on January 2, 2017

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ACS Applied Materials & Interfaces

Ambipolar Small-Molecule:Polymer Blend Semiconductors for Solution-Processable Organic Field-Effect Transistors

Minji Kang,a,† Hansu Hwang,a,† Won-Tae Park,b Dongyoon Khim,c Jun-Seok Yeo,a Yunseul Kim,a Yeon-Ju Kim,a Yong-Young Noh,b,* and Dong-Yu Kima,*

a

Heeger Center for Advanced Materials, School of Materials Science and Engineering,

Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea. b

Department of Energy and Materials Engineering, Dongguk University, 30 Pildong-ro, 1-gil,

Jung-gu, Seoul 04620, Republic of Korea. c

Department of Physics, Blackett Laboratory, Imperial College London, London, SW7 2AZ,

UK

KEYWORDS Organic field-effect transistors, ambipolar semiconductors, blend organic semiconductors, small molecule, quinoids, vertical phase separation

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ABSTRACT We report on the fabrication of an organic thin-film semiconductor formed using a blend solution of soluble ambipolar small molecules and an insulating polymer binder that exhibits vertical phase separation and uniform film crystallinity. The semiconductor thin films are produced in a single step from a mixture containing a small molecular semiconductor, namely, quinoidal biselenophene (QBS) and a binder polymer, namely, poly(2-vinylnaphthalene) (PVN). Organic field-effect transistors (OFETs) based on QBS/PVN blend semiconductor are then assembled using top-gate/bottom-contact device configuration, which achieve almost four times higher mobility than the neat QBS semiconductor. Depth profile via secondary ion mass spectrometry and atomic force microscopy images indicate that the QBS domains in the films made from the blend are evenly distributed with a smooth morphology at the bottom of the PVN layer. Bias stress test and variable-temperature measurements on QBS-based OFETs reveal that the QBS/PVN blend semiconductor remarkably reduces the number of trap sites at the gate dielectric/semiconductor interface and the activation energy in the transistor channel. This work provides a one-step solution processing technique, which makes use of soluble ambipolar small molecules to form a thin-film semiconductor for application in highperformance OFETs.

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INTRODUCTION Solution-processable organic semiconductors (OSCs) active layer for the organic fieldeffect transistors (OFETs) has been studied extensively over the last two decades owing to their potential to realize cost effective, lightweight, and flexible devices, e.g., displays, sensor arrays, integrated circuits by various printing processes.1,2 Practical applications require OFETs to possess high charge-carrier mobility and operational stability. Therefore, considerable efforts have not only been devoted to the development of new OSCs, but also to the optimization of morphology and molecular organization in the existing organic semiconductor films, for efficient charge transport.3,4 Among them, controlling thin film morphology is most widely studied because of its remarkable influences on the device performance.5-7 Although small molecule OSCs can easily provide a well-ordered film when compared to conjugated polymers, it is difficult to obtain a film of uniform morphology on an untreated substrate using solution-processes because of the strong dewetting nature of the low viscosity small molecule solution.8 To achieve superior small molecule films with uniform morphology over a large area, the semiconducting small molecules were blended with insulating or conjugated binder polymers in the same organic solvent.9,10 To achieve facilitating charge-carrier transport in the blended semiconductor films, the small molecular semiconductors should be well organized in polymer matrix. The vertically phase-separated morphology of the small molecules and the binder polymers are considered ideal, since the semiconducting molecules can then packed tightly together without experiencing any interference from the heterogeneous binding polymers.11 The vertical phase 3



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separation between two components is influenced by various solution-processing conditions, such as solvent evaporation time, solution concentration, miscibility of the components, and interaction energy between the substrate and the components.11 Structure of the binding polymer can also play a crucial role in determining the formation and position of the polycrystalline small molecules in

the phase-separated film.11-14 Recently, several groups

have reported, that by carefully controlling the above-mentioned experimental parameters, vertical phase separation in the semiconducting small molecule/polymer blend films was achieved, which significantly improved device performance of OFETs. A vertical phaseseparated

structure

of

an

organic

semiconducting

blend,

consisting

of

2,7-

dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) and polystyrene, led to a considerably high, field-effect mobility of up to 43 cm2 V-1 s-1, owing to the formation of highly aligned C8-BTBT films formed using an off-center spin coating method.15 Solutionprocessable

2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene

(diF-TESADT)

mixed with amorphous insulating polymers exhibited high hole mobility of over 6.7 cm2 V-1 s-1, by obtaining the formation of large coherent crystal domains(millimeter long).16 The small molecule-polymer blend approach has also been applied to high-performance n-type small molecule OSCs.17,18 However, most approaches focus on unipolar semiconductors, which are either n-type or p-type. Although several research groups have reported ambipolar transport in OFETs by simply blending p-type and n-type OSCs,19,20 blend systems with intrinsic ambipolar small molecules and binder polymers have not been reported till now. In this study, we report a method to improve device performance and operational stability of small molecule-based ambipolar OFETs, by using blends of a small molecular quinoidal biselenophene (QBS) and insulating polymer binders. The morphology of solution-processed 4


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QBS film is carefully optimized by selecting a suitable polymer binder, organic solvent, solution concentration and annealing temperature. Best device performance and operational stability was achieved with a bilayered morphology composed of a poly(2-vinylnaphthalene) (PVN)-rich top layer and a QBS-rich bottom layer. Ambipolar OFETs, which used the QBS/PVN bilayer as the active layer, showed an improved bias, air stability and a maximum hole and electron mobility of 0.12 cm2 V-1 s-1 and 0.04 cm2 V-1 s-1, respectively, when compared to the neat QBS semiconductor. EXPERIMENTAL METHODS Materials. Small molecular semiconductor quinoidal biselenophene (QBS) was synthesized following procedures reported in our previous work.21 PαMS (Mw = 4000), PS (MW = 280,000) and PVN (MW = 175,000) were purchased from Sigma Aldrich. Insulating polymer and small molecules blends were prepared by mixing the two components in a 1:1 ratio by weight until completely dissolved in DCB and tetralin to obtain 10 mg mL-1 solutions. The solutions was further stirred for one day, on a hot plate at 100 °C to allow complete mixing. Fabrication of the organic field-effect transistor (OFET) devices. Corning XG glass substrate was used to fabricate the OFET devices in the top-gate/bottom-contact (TG/BC) architecture. Nickel (Ni) adhesion layer (4 nm thick) and Au source/drain electrodes (14 nm thick) were patterned on the substrates by conventional photolithography and lift-off processes. The channel width/length (W/L) was 1.0 mm/20µm. Prepared solutions were spin coated onto a clean glass substrates at 500 rpm for 40 s followed by 2000 rpm for 20s. The semiconductor films were thermally annealed at 150 °C for 30 min. CYTOP dielectric (Asahi Glass) was then deposited onto the semiconductor layer at 2000 rpm for 60s. Finally, an 5



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aluminum (Al) gate electrode (~45 nm thick) was deposited through a metal shadow mask using vapor deposition to complete the OFET devices. Thin film and device characterization. The electrical characteristics of the OFET devices were measured using a Keithley 4200 semiconductor characterization system. Secondary ion mass spectrometry (SIMS) was performed on a CAMECA IMS-6f magnetic sector SIMS, equipped with a Cs+ gun with an impact energy of 15 keV. The beam current was 1 nA and the raster size was 150 µm × 150 µm. An area (Φ) of 30um was analysed and ions

133

Cs12C+,

133

Cs14N+,

133

Cs16O+,

133

Cs28Si+

and 133Cs80Se+ were detected with an electron neutralizer. The height and phase images of the OSC films were measured using a Digital Instruments Multimode atomic force microscopy (AFM). Differential scanning calorimetric (DSC) measurements were acquired using Perkin Elmer DSC 4000.

RESULTS AND DISCUSSIONS We selected small molecular QBS as an organic semiconductor due to its high solubility in common organic solvents. The QBS has quinoidal molecular structure (Figure 1(a)), possesses ambipolar charge transport properties and relatively low band gap due to the efficient delocalization of its molecular orbital.21 In our previous report, top-gate/bottomcontact (TG/BC) QBS-based OFETs showed typical ambipolar characteristics with hole and electron mobilities of 0.05 and 0.02 cm2 V-1 s-1, respectively and complementary-like ambipolar inverters were also demonstrated.21 To improve ambipolar mobility and device

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stability of the QBS-based OFETs, we explored various QBS and polymer binder blends to improve morphology. Depth profiling was performed via secondary ion mass spectrometry (SIMS) to confirm the presence of phase-separated QBS molecules in QBS/PVN blended film, as shown in Figure 1(b). The SIMS analysis indicates the chemical composition with depth profile. The signal from the Se ion was recorded to identify the QBS molecules in the bi-layered blended film. Notably, though the signal from the C ion was detected over the whole region in the blended film, the signal from the Se ion contained in QBS exhibited a lateral gradient, which gradually increased from the top surface to bottom substrate. This result suggests that the QBS-rich phase was segregated at the bottom of the bi-layered blended film towards the substrate.

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Figure 1. (a) Schematic representation of the organic field-effect transistor device configuration using the blended semiconductor in the top-gate/bottom-contact geometry. The chemical structure on the right illustrates the small molecular QBS and the polymer binder PVN. (b) Depth profile measurement by secondary ion mass spectrometry for C, N, O, Si, and Se in the QBS/PVN film, starting from the top surface and terminating at the organic semiconductor/glass interface. (c) Contact angle measurements using a water drop on QBS and on PVN.

The vertical phase separation of the QBS/PVN mixture is thought to occur due to the influence of solidification kinetics and simultaneous enthalpic interactions between the small molecules and the glass substrates.8 Essentially, QBS molecules in the blend solution solidified first and settled at the bottom interface, near the substrate, due to its poor solubility in DCB solvent (7 mg mL-1) when compared to PVN (60 mg mL-1). In addition, the smaller water contact angle on QBS film (~58º, Figure 1(c)) indicates, favorable enthalpic interactions between the hydrophilic QBS molecules and the glass substrate, resulting in the preferential sedimentation of QBS molecules towards the substrate. In contrast, relatively 8


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hydrophobic PVN, which consisted of only non-polar carbon bonds, was isolated at the top surface of the film. The fact that the interaction, depending on surface energy of substrate and polarity of each component, dominantly determined the phase separation behavior was already demonstrated by several groups.3,22,23 Accordingly, differences in the solubility of solutes and used solvent and their preferred interactions with the substrate are crucial to achieve vertical phase separation in the small molecule and polymer mixture.

Figure 2. Transfer (a,d) and output (b,c,e,f) characteristics of QBS OFETs and QBS/PVN OFETs at (a-c) p-channel (Vd = -100 V) and (d-f) n-channel (Vd = 100 V) regions.

Based on SIMS analysis, the influence of neat QBS and QBS/PVN films on the characteristics of ambipolar OFET devices with TG/BC architecture, was investigated. Typical set of transfer and output curves for QBS and QBS/PVN OFETs, were plotted for both hole enhancement (at drain voltage (Vd) of -100 V) and electron enhancement (Vd = 9



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+100 V) operational modes (Figure 2). Table S1 summarizes the measured field-effect mobility (µFET) and threshold voltage (VTh) with the annealing temperature. Saturation µFET was calculated from µ = (L/CiW)·(∂2Id/∂Vg2), where Ci is the dielectric capacitance (Ci = 4.7 nF/cm2), L/W is the channel length/width (L/W = 20 µm/1.0 mm), and Vg is gate voltage (Vg = ±100 V). As shown in Figure 2, plots of Id versus Vg reveal both hole and electron mobilities of 2.31 × 10-2 cm2 V-1 s-1 and 1.60 × 10-2 cm2 V-1 s-1, respectively, for QBS OFETs. Interestingly, the QBS/PVN OFETs exhibit hole and electron µFET of 1.02 × 10-1 cm2 V-1 s-1 and 3.88 × 10-2 cm2 V-1 s-1, respectively and exceed by a factor of 4 when compared to the QBS OFETs. These results strongly suggest that the PVN polymer binder not only induced vertical phase separation, but also improved the electron and hole transport in the QBS semiconductor regions. However, when the annealing conditions (at 150 °C) are optimized, the QBS/PVN blend semiconductor yields higher mobilities compared to neat QBS semiconductor. Figure S1 shows poor OFET performance with thermal annealing at 100 °C. Semiconducting QBS/PVN films annealed at low temperatures are unable to form well organized crystalline structures because QBS requires sufficient thermal annealing for the favored growth of the crystalline domain, as shown in the observation of out-of-plane X-ray diffraction (XRD) pattern (Figure S2).21 Film formation characteristics of the small molecule/polymer blend was predominantly affected by the choice of solvent, polymer binder, and process conditions such as thermal annealing and drying speed of the solvent. QBS molecules were mixed with various polymer binders (polystyrene (PS), poly(α-methylstyrene) (PαMS), and poly(2-vinylnaphthalene (PVN)) in organic solvents such as 1,2,3,4-tetrahydronaphtalene (tetralin) and 1,2dichlorobenzene (DCB). The OFET performance with the choice of polymer binder and 10


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solvent is summarized in Table S2. The use of PVN and DCB resulted in an ideal vertical phase separation of the QBS molecules and the binder. The inadequately segregated morphology of other blends, with the exception of QBS/PVN, may be due to undesirable thermal annealing process. QBS, when annealed at 150 °C, can form an efficient crystalline structure, as shown in Figure S2 (b), but PS and PαMS have relatively lower glass transition temperature than PVN (Tg of PS, PαMS, PVN was estimated to be 105, 88, and 151 °C, respectively, Figure S3). Sufficiently high temperature above Tg induces segmental chain motion in the polymer. Therefore, We suppose that active molecular motion in the polymer binder not only induced modest, mixed and insufficient phase separation but also prevented gradual segregation of small molecules along the depth of the blended films.24,25 On the other hand, when the thermal properties and processing temperatures of each component in the blend are well matched, distinct vertical phase separation occurs, as seen in case of QBS/PVN blend. Moreover, both QBS and the polymer binder are poorly soluble in tetralin when compared to DCB (solubility of QBS and PVN is 1 mg mL-1 and 10 mg mL-1 in tetralin, respectively), because tetralin is a relatively non-polar in nature. We speculate that the QBS/PVN blended film prepared from DCB as a relatively good solvent has sufficient time to form the favored morphology during solvent evaporation, while, due to poor solubility in tetralin, the miscibility of QBS and PVN greatly decreases, making the blended film disorganized and leading to aggregate formation. Figure S4 shows non-uniform and isolated island morphology of QBS/PS blended films due to a thermal annealing at 150 °C.

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Atomic force microscopy (AFM) images show the surface morphology of neat QBS and QBS/PVN films (Figure 3). In AFM images, significantly different morphology with rootmean-square (RMS) roughness of 2.1 nm (for neat QBS film) and 1.4 nm (for QBS/PVN blended film) were observed. The QBS/PVN blended film showed smooth and continuous morphology caused by distinct vertical phase separation. It revealed that the QBS domains could be evenly distributed at the bottom interface where the conducting channel is located adjacent to the source/drain electrodes of OFETs. Therefore, the enhanced mobility of QBS/PVN OFETs can be attributed to the increase in the grain size leading to reduced grain

boundaries, which act as physical charge traps. Figure 3. AFM images of QBS (a, c) and QBS/PVN (b, d) films spin-coated on glass substrates. (a) and (b) Height images and (c) and (d) phase images. The scan size for all images is 5 µm × 5 µm.

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To check the operational stability, we investigated the device degradation caused by charge trapping at all the interfaces in the OFETs, and the bulk of the semiconductor, the insulating polymer binder and gate dielectric.26 The operational stabilities of QBS-based OFETs were characterized under continuous positive and negative bias stresses. Figure 4 shows the typical bias stress test of Id measured in QBS-based OFETs normalized to the initial ONcurrent values. The normalized Id decreased to 0.61 (for p-channel) and 0.59 (for n-channel) after continuous stress for 500 s at Vg = -60 V and 60 V, in OFETs with neat QBS semiconductor. The current for QBS/PVN OFETs decreased by values of 0.72 in the pchannel regime and the Id in the n-channel regime showed a similar decay value of 0.76, after same bias stress was applied to the neat QBS OFETs.

Figure 4. Bias stress test on p-type (a) and n-type (b) OFETs with neat and blend semiconductors: Normalized source-drain current decay measured as a function of time in QBS-based OFETs.

The transfer curves were also measured for QBS-based OFETs before and after positive bias stress in n-channel regime and negative bias stress in p-channel regime, respectively, (Figure S5). After bias stress test, the neat QBS OFET suffered a strong shift in VTh while the bias stress effect on QBS/PVN OFETs was relatively small. It was confirmed that the VTh shift would be mainly caused by trapped charge carriers in the bulk of semiconductor and 13



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dielectric/semiconductor interface27 and the blend system using PVN reduces the charge trap sites in the bi-layered QBS/PVN film. In addition, air stability of QBS-based OFETs was measured by exposure to air under ~37% humidity, at room temperature. Figure S6 presents the change in the transfer characteristics over a period of two weeks. The neat QBS showed a large drop in saturation mobility after only 1 day, while ambipolar QBS/PVN OFETs showed relatively slow degradation of the field-effect mobility. After 2 weeks, VTh in p-channel region for both QBS and QBS/PVN FETs shifted to positive voltages due to p-doping by oxygen.28 It can be argued that the blend system remarkably reduces the trap states and incorporates encapsulation of the QBS semiconductor without additional processing step.

Figure 5. A plot of the log of the saturation mobility vs. inverse temperature for three types of QBSbased OFETs at (a) p-channel and (b) n-channel regions.

We further confirmed that the continuous formation and large grain size of the QBS/PVN semiconductor film reduced the interface traps and grain boundary scattering. In order to investigate the effect of trap states on ambipolar charge transport, we performed variabletemperature measurements. Figure 5 shows hole and electron saturation mobilities derived from temperature dependence of the transfer characteristics at Vd = ±60 V for the QBS-based 14


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OFETs, as a function of inverse temperature. The saturation mobility on these devices decreases monotonically from 300 K to 100 K, following Arrhenius behavior. Monotonic decrease in the mobility with decreasing temperature are consistent with the presence of distributed trap states in the film (both traps in the semiconductor bulk and at grain boundaries) and thermally activated hopping transport.29 The activation energies determined for the QBS-based semiconductor are also presented in Figure 5. It is clear that the QBS/PVN OFET exhibits lower activation energy (Ea,h = 63.1 meV, Ea,e = 63.7 meV) than QBS OFET (Ea,h = 79.5 meV, Ea,e = 70.4 meV). This indicates that this blend system leads to low density of traps and an improvement of crystalline quality in the QBS/PVN blended film. It would appear that QBS/PVN semiconductor possesses efficient charge transport pathways induced by favorable morphology.

CONCLUSIONS In summary, we investigated the phase separation behavior and the electrical performance of organic thin-film semiconductor by blending soluble ambipolar small molecule with insulating polymer for the first time. The small molecular QBS blended with PVN polymer binder, formed vertically phase separated bilayer structure on relatively hydrophilic glass substrate. In particular, QBS/PVN blended film induced relatively high amount of QBS to settle at bottom of the film due to the combined effect of difference in solubility and enthalpic interaction with the substrate. The blend system contributed to the growing crystalline domains and reduced the grain boundaries, leading to an improvement in ambipolar chargecarrier mobilities and operational stability of the solution-processed OFETs. The field-effect 15



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mobility of QBS/PVN blend semiconductor reached values up to 1.02 × 10-1 cm2 V-1 s-1 for holes and 0.39 × 10-2 cm2 V-1 s-1 for electrons, respectively. The QBS/PVN blend semiconductor also showed enhanced bias and environmental stabilities and this enhancement is attributed to the filling of the trap states by encapsulation effect of PVN polymer binder. Consequently, we demonstrated that by blending ambipolar quinoidal small molecules and insulating polymer binders, significant improvement in device performance for solution-processed OFETs and applications in ambipolar electronics can be achieved. These results provide new guidelines for improving the semiconducting properties of soluble ambipolar small molecules.

ASSOCIATED CONTENT Supporting Information. The summary of field-effect mobility and threshold voltage of QBSbased OFETs, transfer curves of OFETs with thermal annealing at 100 °C, out-of-plane X-ray diffraction patterns for QBS-based films with annealing temperature, DSC plot of polymer binders and QBS, AFM images of QBS/PS films and bias and air stability tests on ambipolar OFETs are available free of charge via the Internet at http://pubs.acs.org

Corresponding author E-mail: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. † These authors contributed equally to this work. 16


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Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research was financially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2015R1A2A1A10054466 and NRF2014R1A2A2A01007159). We thank the Korea Basic Science Institute (KBSI) for AFM measurement.

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