A Versatile Low Bandgap Polymer for Air‐Stable, High‐Mobility Field‐Effect Transistors and Efficient Polymer Solar Cells

A new low bandgap polymer POD2T-DTBT has been synthesized and found to give excellent performances in organic thin-film transistor (OTFT) and organic photovoltaic (OPV) applications. OTFTs based on POD2T-DTBT have achieved good hole carrier mobilities of 0.2 cm2V−1s−1 and OPVs based on blends of POD2T-DTBT and PC71BM have demonstrated promising power conversion efficiencies of 6.26%. Polymer-based organic thin-film transistors (OTFTs) and organic photovoltaics (OPVs) have attracted much interest in recent years due to their solution processability and mechanical flexibility, which potentially allow them to be manufactured using low-cost, high-throughput processes such as roll-to-roll printing and inkjet printing.1 In recent years, the development of novel materials for these applications has contributed to significant improvements in device performance. In the field of OTFTs, the development of polythiophene derivatives such as poly(3,3′′′-dialkyl-quaterthiophene) (PQT) and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) has resulted in OTFTs with hole mobilities of between 0.2 and 0.6 cm2V−1s−1.2, 3 The development of low bandgap polymer donors has meanwhile helped to lift solar cell power conversion efficiencies (PCEs) to above 6%.4-6 However, there have been few reports of polymers that perform well in both OTFTs and OPVs. The development of such versatile polymers could be beneficial for applications which incorporate both types of devices. Although the high hole mobility of polythiophenes is an attractive property for OPV applications, their relatively large bandgap of around 1.9 to 2 eV7 means that they are unable to absorb and harvest photons above 650 nm. The incorporation of acceptor moieties into the polythiophene backbone allows the bandgap to be reduced and can thereby improve solar cell performance. A commonly used acceptor moiety in D–A polymers is 2,1,3-benzothiadiazole, which has been copolymerized with donor moieties such as fluorene,8 carbazole,4, 9 and cyclopentadithiophene10, 11 to produce low bandgap polymers and associated solar cells with PCEs up to 6.1%. Copolymers of thiophene and benzothiadiazole have previously been studied in OPVs, with the number of thiophene units in the repeat unit ranging from three to eight. Despite the lowered bandgap and improved absorption properties of the polymers, the PCEs of the resultant devices have been relatively low, ranging from 0.13% to 2.23%.12-15 In this work, we report on the synthesis and chara-cterization of poly{(benzo-2,1,3-thiadiazol-4,7-diyl)-alt-(3′,4′′di (2-octyldodecyl)-2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5′′′-diyl)} (POD2T-DTBT), which is a candidate for high performance OTFTs and OPVs. Field effect transistors made from POD2T-DTBT are shown here to exhibit hole carrier mobilities of 0.13–0.20 cm2 V−1s −1 and when applied in solar cells, blends of POD2T-DTBT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) have yielded promising PCEs of 5.83–6.26%. The chemical structure and synthetic route of POD2T-DTBT is shown in Scheme 1. The polymer structure incorporates benzothiadiazole and dialkylquaterthiophene, with long, branched 2-octyldodecyl alkyl groups being used to ensure good solubility of the polymer even at high molecular weight. POD2T-DTBT was synthesized via Stille co-polymerization from 4,7-bis(2-trimethylstannylthien-5-yl)-2,1,3-benzothiadiazole (1) and 5,5’-dibromo-4,4’-bis(2-octyldodecyl)-2,2’-bithiophene (2). The crude polymer was purified by Soxhlet extraction with methanol, ethyl acetate, and chlorobenzene. The chlorobenzene fraction was concentrated and precipitated into methanol to give the polymer in 72% yield. The polymer was determined to have a number averaged molecular weight (Mn) of 35 kDa using high-temperature gel permeation chromatography (HT-GPC) in 1,2,4-trichlorobenzene. Synthesis of the co-monomers and POD2T-DTBT. LTMP: lithium tetramethylpiperidide; THF: tetrahydrofuran; PdCl2(PhCN)2: bis(benzonitrile)palladium (II) chloride; DMSO: dimethyl sulfoxide, Pd2(dba)3: tris(dibenzylideneacetone)dipalladium(0), P(o-tol)3: tri(o-tolyl)phosphine. The absorption spectra for POD2T-DTBT in dichlorobenzene solution and thin film are shown in Figure S1 (Supporting Information). The optical bandgap was calculated to be 1.59 eV from the absorption edge of 780 nm. In solution, the peak at around 716 nm decreased with increasing temperature, implying that it is associated with aggregate absorption. From cyclic voltammetry measurements (Figure S2, Supporting Information), the HOMO of PODBT-DTBT was determined to be –5.18 eV. Under the same conditions, we measured the HOMO level of P3HT to be -5.00 eV. The lower lying HOMO level of POD2T-DTBT is expected to impart better air stability than P3HT. Bottom-gate, top-contact field effect transistors were fabricated from POD2T-DTBT to investigate its charge transport properties. The 42 nm active layer was spin-coated from a dichlorobenzene solution under ambient conditions and annealed at 200 °C for 30 min. The output and transfer characteristics are shown in Figure 1a. The devices showed hole carrier mobilities of 0.13–0.20 cm2V−1s−1 with on/off ratios in the order of 106–107. The devices showed no deterioration in hole mobility and on/off ratio (Figure 1b) even after being stored without encapsulation in air at 56% relative humidity for over five months, demonstrating the excellent stability of the polymer. I–V characteristics of a bottom-gate OTFT device made from POD2T-DTBT. (a) output curves at different gate voltages and device structure (shown inset) (b) transfer curves in the saturated regime for as-prepared and aged devices. Solar cells with a structure of ITO/PEDOT:PSS/POD2T-DTBT:PC71BM (1:1)/Al were fabricated and characterized under glovebox conditions. A 45 nm PEDOT:PSS layer was spin-coated onto an indium tin oxide (ITO)-coated glass substrate. A 110 nm active layer was then spin-coated from dichlorobenzene solution for 40 s. The wet film was then subjected to solvent annealing by placing it in a covered Petri dish for 6 h. The Al electrodes were evaporated onto the film through a shadow mask, followed by post-annealing at 100 °C for 10 min. The active area of each pixel was 0.09 cm2. The devices were determined to have PCEs of between 5.83% to 6.26% under simulated AM 1.5G illumination at 100 mW cm−2. The best device (Figure 2a) gave a Jsc of 12.3 mA cm−2, Voc of 0.722 V and fill factor of 70.5%. The incident-photon to current conversion efficiency (IPCE) of a typical device is shown in Figure 2b along with the UV-Vis absorption spectrum of the POD2T-DTBT:PC71BM blend. The IPCE spectrum peaks at 59% at around 500 nm and indicates that the active layer is photosensitive to beyond 760 nm, thereby enabling it to harvest light across the entire visible spectrum. Cells with larger active areas are currently being investigated and have shown promising results thus far. (a) J–V characteristics for a POD2T-DTBT:PC71BM solar cell (b) UV–vis spectrum of POD2T-DTBT:PC71BM blend and IPCE (c) TEM image of a 1:1 POD2T-DTBT:PC71BM blend film. The scale bar represents 200 nm. The morphology of the active layer was investigated by transmission electron microscopy (Figure 2c). The film was found to contain many long fibres which were around 20–25 nm in width, and are considered to correspond to the polymer phase; as well as dark colored domains of around 25–60 nm that are believed to correspond to the PC71BM phase. The fibres could also be observed in phase images obtained from atomic force microscopy (Figure S3, Supporting Information). The intimately mixed and well-ordered domains within the matrix, together with the high hole carrier mobility of the material, are likely to enable efficient charge separation and transport, and could help to explain the good performance of the cells. In conclusion, we have demonstrated the design and synthesis of a polymer material POD2T-DTBT. Its excellent solution processability, OTFT hole mobility of up to 0.20 cm2 V−1s−1, and solar cell PCE of up to 6.26% make it a promising candidate for both of these applications together with new potential applications where dual transistor and OPV functions are required on a single substrate. Characterization: 1H and 13C NMR were performed using a Bruker Avance 400 spectrometer. Elemental analysis was carried out using a Flash EA 1112 series elemental analyser from Thermo Electron Corporation. UV-visible spectra were recorded using a Shimadzu model 3101-PC UV-VIS spectrometer. Gel permeation chromatography was carried out at 160 °C using a Polymer Labs PL 220 system using a refractive index detector and 1,2,4-trichlorobenzene as the eluent. The bright field image of the polymer blend film was collected using a Jeol 2100 TEM operated at accelerated voltage of 200 kV. Synthetic procedures: All chemicals were purchased from Aldrich or Pacific Chemical Source and used without further purification. Tetrahydrofuran (THF) was distilled over sodium benzophenone ketyl before use. 4,7-bis(thien-2-yl)-2,1,3-benzothiadiazole 116 and 2-bromo-3-(2-octyldodecyl)thiophene 317 were synthesized using methods adapted from literature. 4,7-bis(2-trimethylstannylthien-5-yl)-2,1,3-benzothiadiazole (2): This compound was synthesized using a method adapted from literature.18 A mechanically stirred solution of 2,2,6,6-tetramethylpiperidine (TMP) (1.24 g, 8.8 mmol) in dry THF (100 mL) under argon was cooled to –78 °C and n-butyllithium (8.6 mmol) was added rapidly. The resulting solution was allowed to warm to room temperature. It was kept at room temperature for 10 min and subsequently cooled to –78 °C. A solution of 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (1.0 g, 3.3 mmol) in dry THF (10 mL) was then added dropwise. The deep purple solution was stirred at –78 ° C for 30 min, following which a 1 M solution of trimethyltin chloride in hexanes (8.8 mL, 8.8 mmol) was added. The reaction mixture was then allowed to warm to room temperature and stirred overnight. Water was added to quench the reaction. Diethyl ether was added, and the mixture was washed 3 times with 0.1 M hydrochloric acid (HCl) to remove the TMP. The solution was then dried with MgSO4. The solvent was removed, and the title compound (1.78 g, 85%) was obtained as orange needles after recrystallisation in ethanol. 1H NMR (400 MHz, CDCl3, δ) δ 8.19 (d, J = 3.6Hz, 2H), 7.88 (s, 2H), 7.30 (d, J = 3.6Hz, 2H), 0.44 (s, 18H). 13C NMR (100 MHz, CDCl3, δ) 152.7, 145.1, 140.2, 136.1, 128.4, 125.9, 125.8, -8.2. 5,5’-Dibromo-4,4’-bis(2-octyldodecyl)-2,2’-bithiophene (4): This compound was synthesized via a palladium-catalyzed homocoupling method described in the literature.19 2-bromo-3-(2-octyldodecyl)thiophene (4.5 g, 10 mmol) was weighed into a Schlenk flask and the flask was purged with nitrogen. Anhydrous dimethylsulfoxide (DMSO) (45 mL) was added to the flask and the mixture was heated to 70 °C. Silver nitrate (3.4 g, 20 mmol), potassium fluoride (1.16 g, 20 mmol) and bis(benzonitrile)palladium (II) chloride (77.8 mg, 2 mol%) were added in one portion, and the resulting mixture was stirred at 70 °C. Two additional portions of AgNO3 and KF (same quantities as above) were added after 3 hours and 6 hours and the mixture was stirred overnight at the same temperature. The mixture was then cooled, filtered through a short silica column, and washed 3 times with water. The title compound (2.42 g, 54%) was obtained as a yellow oil after column chromatography on silica with hexane as the eluent. 1H NMR (400 MHz, CD2Cl2, δ) 6.80 (s, 2H), 2.49 (d, J = 7.2 Hz, 4H), 1.67 (m, 2H), 1.27 (m, 64H), 0.91 – 0.87 (t, 12H). 13C NMR (100 MHz, CDCl3, δ) δ 142.2, 136.0, 124.9, 108.5, 38.5, 34.3, 33.4, 31.9, 31.9, 30.0, 29.7, 29.6, 29.6, 29.6, 29.3, 29.3, 26.5, 22.7, 22.7, 14.1. POD2T-DTBT: 4,7-bis(2-trimethylstannylthien-5-yl)-2,1,3-benzothiadiazole (140 mg, 0.22 mmol), 5,5’-dibromo-4,4’-di(2-octyldodecyl)-2,2’-bithiophene (198 mg, 0.22 mmol), tris(dibenzylideneacetone)dipalladium(0) (8.2 mg, 4 mol%), and tri(o-tolyl)phosphine (10.8 mg, 16 mol%) were added to a reaction vial equipped with a magnetic stirring bar. The vial was then purged with argon and sealed. Anhydrous chlorobenzene (15 mL) was added through a septum and the reaction mixture was then stirred at 120 °C for 72 h using an oil bath. The reaction mixture was then poured into methanol (200 mL) and HCl (10 mL). The polymer was filtered and subjected to soxhlet extraction with methanol (12 h), ethyl acetate (12 h) and chlorobenzene (12 h). The chlorobenzene fraction was concentrated and precipitated into methanol (200 mL). Following filtration, the purified polymer (165 mg, 72%) was obtained as a black solid. 1H NMR (400 MHz, 1,1,2,2-tetrachloroethane-d4, 70 °C, δ) 8.20 (s, 2H), 7.95 (s, 2H), 7.34 (s, 2H), 7.15 (s, 2H), 2.91 (s, 4H), 1.92 (s, 2H), 1.47 – 1.25 (m, 32H), 0.97 (s, 6H). Mn = 35 kDa, polydispersity = 1.63 Anal. Calcd for C62H92N2S5: C 72.60, H 9.04, N 2.73, S 15.63; found C 72.52, H 8.79, N 2.71, S 15.34. OPV Device Fabrication and Characterization: POD2T-DTBT and PC71BM were dissolved separately in anhydrous 1,2-dichlorobenzene (DCB). The polymer solution was heated on a magnetic hotplate stirrer at 70 °C for 2 h; while the PC71BM solution was stirred at 40 °C for 2 h. The two solutions were then mixed together in a polymer:PC71BM mass ratio of 1:1 and the blend solution was stirred overnight at 40 °C. The final concentration of the polymer in the blend was 10 mg/mL. Devices were prepared on indium tin oxide (ITO) patterned glass substrates. The ITO patterned glass substrates were first sonicated in a detergent (Hellmanex) bath for 30 min, followed by 2 rounds of sonication in de-ionized water for 10 min. This was followed by successive sonication in acetone and isopropanol for 15 min and 20 min, respectively. The cleaning step was concluded by blow-drying the substrates using a nitrogen-gun. The substrates were then placed in an oven at 80 °C for 1 h. The substrates were subjected to an UV ozone treatment for 10 min before a PEDOT:PSS (CLEVIOS P VP Al 4083) layer of 40–45 nm thickness was spin-coated onto the ITO surface. The PEDOT:PSS-coated substrates were then annealed at 120 °C for 10 min. Next, the active layer was spin-coated at 500 rpm for 80 s in an inert glove-box (N2 atmosphere). The wet film was dried slowly in a covered Petri dish. Lastly, 100 nm of aluminium was deposited by thermal evaporation as the cathode followed by post-annealing for 10 min at 100 °C. The active area of the device was 0.09 mm2. The thickness of the film was measured to be 110 nm using a surface profiler (KLA-Tencor P10 surface profiler). Current density-voltage (J-V) measurements were carried out in an inert environment (MBraun glovebox, N2 atmosphere) under 1 Sun (AM1.5G) conditions using a solar simulator (SAN-EI Electric XES-301S 300W Xe Lamp JIS Class AAA) with an intensity of 100 mW cm−2. The incident photon to current conversion efficiency (IPCE) as a function of wavelength was measured with a home-built setup consisting of an Oriel 300-W Xe lamp in combination with an Oriel Cornerstone 130 monochromator and a SRS 810 lock-in amplifier (Stanford Research Systems). The number of photons incident on the device was calculated for each wavelength by using a calibrated Si diode as the reference. Supporting Information is available from the Wiley Online Library or from the author. K.-H. Ong would like to thank A*Star for a research scholarship. The authors would like to thank Dr. Martin van Meurs from the Institute of Chemical Engineering Sciences, Singapore for the HT-GPC measurements. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.