A homologous series of six novel oligothiophene–naphthalene diimide-based oligomer semiconductors with a donor–acceptor architecture, NDI-nTH (n = 1, 2, 3, 4) and NDI-nT (n = 2, 3), was synthesized and used to explore a set of criteria for the design of non-fullerene electron acceptor materials for organic solar cells. Thin films of the oligomer semiconductors had optical band gaps that varied from 2.1 eV in NDI-1TH and 1.6 eV in NDI-3TH to 1.4 eV in NDI-4TH, demonstrating good potential for light harvesting and exciton generation. The LUMO energy levels of the oligomer semiconductors were similar (ca. −4.0 eV), but the HOMO levels varied from −5.5 eV in NDI-3TH and NDI-4TH to −6.1 eV in NDI-1TH, showing that suitable energy band offsets necessary for efficient photoinduced charge transfer could be achieved with current donor polymers. Single-crystal X-ray structures of NDI-3TH and NDI-4TH showed a slipped face-to-face π-stacking with short intermolecular distances (0.321–0.326 nm), which enabled facile self-assembly of single-crystalline nanowires from solution. Spin coated thin films of NDI-nTH and NDI-nT were mostly crystalline and had field-effect electron mobilities of up to (2–9) × 10–4 cm2/(V s). Bulk heterojunction solar cells incorporating one of the n-type oligomer semiconductors as the electron acceptor and poly(3-hexylthiophene) as the electron donor showed a power conversion efficiency of 1.5% with an open circuit voltage of 0.82 V and a bicontinuous nanoscale morphology.

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Device performance and photoinduced charge transfer are studied in donor/acceptor blends of the oxidation‐resistant conjugated polymer poly[(4,8‐bis(2‐hexyldecyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene)‐2,6‐diyl‐alt‐(2,5‐bis(3‐dodecylthiophen‐2‐yl)benzo[1,2‐d;4,5‐d′]bisthiazole)] (PBTHDDT) with the following fullerene acceptors: [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM); [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM); and the indene‐C60 bis‐adduct IC60BA). Power conversion efficiency improves from 1.52% in IC60BA‐based solar cells to 3.75% in PC71BM‐based devices. Photoinduced absorption (PIA) of the PBTHDDT:fullerene blends suggests that exciting the donor polymer leads to long‐lived positive polarons on the polymer and negative polarons on the fullerene in all three polymer fullerene blends. Selective excitation of the fullerene in PC71BM or PC61BM blends also generates long‐lived polarons. In contrast, no discernible PIA features are observed when selectively exciting the fullerene in a PBTHDDT/IC60BA blend. A relatively small driving force of ca. 70 meV appears to sustain charge separation via photoinduced hole transfer from photoexcited PC61BM to the polymer. The decreased driving force for photoinduced hole transfer in the IC60BA blend effectively turns off hole transfer from IC60BA excitons to the host polymer, even while electron transfer from the polymer to the IC60BA remains active. Suppressed hole transfer from fullerene excitons is a potentially important consideration for materials design and device engineering of organic solar cells.

π-Conjugated polymer semiconductors are of broad interest for applications in organic electronics and optoelectronics including field-effect transistors and photovoltaic cells. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Poly(3-alkylthiophene)s 1 and derivatives 2,3 are among the most studied organic semiconductors for these applications. Because few conjugated polymer semiconductors combine high carrier mobility with good oxidative stability, various approaches have been explored to improve oxidative stability while retaining high performance in devices. [2][3][4][5] Incorporation of aromatic or aromatic heterocyclic rings into a polythiophene backbone by copolymerization represents a successful strategy, leading to polymer semiconductors with enhanced oxidative stability and higher carrier mobilities compared to the parent poly-(3-alkylthiophene)s. [3][4][5] Aromatic heterocyclic rings such as thiazolothiazole, 4a-c 5,5 0 -bithiazole, 4d thienopyrazine, 5a and thieno-thiophenes 3 exemplify this approach, which is successful largely by enhancing intermolecular π-stacking interactions and long-range 3-D or 2-D order in the solid state. [1][2][3][4][5]12 Benzobisthiazole and benzobisoxazole polymers and small molecules are well-known to exhibit efficient π-stacking and strong intermolecular interactions in the solid state, 7 leading to high-temperature resistance with glass transition temperatures that can exceed 300-400°C and relatively high electron affinity. 8-10 However, polybenzobisthiazoles and polybenzobisoxazoles have mainly been previously explored as electron transport materials in organic light-emitting diodes and as nonlinear optical materials. 9,10 Earlier limited studies of a benzobisthiazole polymer as an n-channel semiconductor in field-effect transistors observed a low mobility of electrons, 11a requiring a high electron affinity polymer 11b in a blend to achieve electron injection. Recently, thin film transistors from benzobisthiazole small molecules were reported to exhibit high field-effect mobilities of holes and electrons. 12 We report herein the synthesis and characterization of a new organic solvent soluble benzobisthiazole-thiophene copolymer based on alternating benzobisthiazole and oligo-3-octylthiophene units in the backbone, poly(benzobisthiazole-alt-3-octylquarterthiophene) (PBTOT). The incorporation of benzobisthiazole into a regioregular poly(3-alkylthiophene) is designed to improve oxidative stability by increasing the ionization potential (IP), thermal stability, interchain interactions and thus enhance the charge transport properties of the polymers. The new copolymer semiconductor indeed shows an increased ionization potential and robust thermal and air stability. The highly crystalline PBTOT thin films exhibit a field-effect carrier mobility of up to 0.01 cm 2 /(V s) and bulk heterojunction solar cells made from PBTOT have a power conversion efficiency of 2.1% under 100 mW/cm 2 AM1.5 sunlight in ambient air. The fieldeffect carrier mobility and photovoltaic properties o...

A novel heptacyclic bisindoloquinoline-based organic semiconductor has been synthesized, characterized, and used to fabricate single-crystal field-effect transistors. A synthetic route was developed for the synthesis of heptacyclic bis(indolo{1,2-a})quinoline via an intramolecular cyclization of anthrazoline derivatives. Single-crystal X-ray structure revealed that the seven fused rings of bis(indolo{1,2-a})quinoline are relatively coplanar and lead to a slipped face-to-face π-stacking with the shortest intermolecular spacing of 3.3 Å. Single-crystal field-effect transistors based on the bis(indolo{1,2-a})quinoline had carrier mobility as high as 1.0 cm2/V·s with on/off ratios greater than 104.