High-Mobility n‑Type Conjugated Polymers Based on Electron-Deficient Tetraazabenzodifluoranthene Diimide for Organic Electronics

High-mobility p-type and ambipolar conjugated polymers have been widely reported. However, high-mobility n-type conjugated polymers are still rare. Herein we present poly­(tetraazabenzodifluoranthene diimide)­s, PBFI-T and PBFI-BT, which exhibit a novel two-dimensional (2D) π-conjugation along the main chain and in the lateral direction, leading to high-mobility unipolar n-channel transport in field-effect transistors. The n-type polymers exhibit electron mobilities of up to 0.30 cm²/(V s), which is among the highest values for unipolar n-type conjugated polymers. Complementary inverters incorporating n-channel PBFI-T transistors produced nearly perfect switching characteristics with a high gain of 107

n‑Type Naphthalene Diimide–Biselenophene Copolymer for All-Polymer Bulk Heterojunction Solar Cells

A new solution processable n-type polymer semiconductor is synthesized and characterized for use as an electron acceptor material in all-polymer bulk heterojunction solar cells. The new crystalline copolymer, poly­(naphthalene diimide-<i>alt</i>-biselenophene) (PNDIBS), has a high field-effect electron mobility (0.07 cm²/(V s)) and broad visible-near-infrared absorption band with an optical band gap of 1.4 eV. All-polymer bulk heterojunction solar cells comprised of PNDIBS acceptor and poly­(3-hexylthiophene) donor have a photovoltaic power conversion efficiency of 0.9%. The external quantum efficiency spectrum of the all-polymer solar cells shows that about 19% of the photocurrent comes from the near-infrared (700–900 nm) light harvesting by the new n-type polymer semiconductor

High Mobility Thiazole–Diketopyrrolopyrrole Copolymer Semiconductors for High Performance Field-Effect Transistors and Photovoltaic Devices

New donor–acceptor copolymers incorporating both a strong electron-accepting diketopyrrolopyrrole unit and a weak electron-deficient thiazolothiazole or benzobisthiazole moiety were synthesized, characterized, and found to exhibit very high charge carrier mobility. Stille coupling copolymerization gave copolymers having moderate number-average molecular weights of 17.0–18.5 kDa with polydispersities of 3.3–4.0 and optical band gaps of 1.22–1.38 eV. High performance p-channel field-effect transistors were obtained using the thiazolothiazole-linked copolymers, PDPTT and PDPTTOx, giving hole mobilities of 0.5 and 1.2 cm²/(V s), respectively, with on/off current ratios of 10⁵ to 10⁶. In contrast, the benzobisthiazole-linked copolymer PDPBT had a substantially lower field-effect mobility of holes (0.005 cm²/(V s)) due to its amorphous solid state morphology. Bulk heterojunction solar cells fabricated by using one of the thiazolothiazole-linked copolymer, PDPTT, as electron donor and PC₇₁BM acceptor show a power conversion efficiency of 3.4% under 100 mW/cm² AM1.5 irradiation in air

Design of New Electron Acceptor Materials for Organic Photovoltaics: Synthesis, Electron Transport, Photophysics, and Photovoltaic Properties of Oligothiophene-Functionalized Naphthalene Diimides

A homologous series of six novel oligothiophene–naphthalene diimide-based oligomer semiconductors with a donor–acceptor architecture, NDI-<i>n</i>TH (<i>n</i> = 1, 2, 3, 4) and NDI-<i>n</i>T (<i>n</i> = 2, 3), was synthesized and used to explore a set of criteria for the design of <i>non-fullerene</i> 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-<i>n</i>TH and NDI-<i>n</i>T 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

Design of New Electron Acceptor Materials for Organic Photovoltaics: Synthesis, Electron Transport, Photophysics, and Photovoltaic Properties of Oligothiophene-Functionalized Naphthalene Diimides

A homologous series of six novel oligothiophene–naphthalene diimide-based oligomer semiconductors with a donor–acceptor architecture, NDI-<i>n</i>TH (<i>n</i> = 1, 2, 3, 4) and NDI-<i>n</i>T (<i>n</i> = 2, 3), was synthesized and used to explore a set of criteria for the design of <i>non-fullerene</i> 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-<i>n</i>TH and NDI-<i>n</i>T 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

Beyond Fullerenes: Design of Nonfullerene Acceptors for Efficient Organic Photovoltaics

New electron-acceptor materials are long sought to overcome the small photovoltage, high-cost, poor photochemical stability, and other limitations of fullerene-based organic photovoltaics. However, all known nonfullerene acceptors have so far shown inferior photovoltaic properties compared to fullerene benchmark [6,6]-phenyl-C₆₀-butyric acid methyl ester (PC₆₀BM), and there are as yet no established design principles for realizing improved materials. Herein we report a design strategy that has produced a novel multichromophoric, large size, nonplanar three-dimensional (3D) organic molecule, DBFI-T, whose π-conjugated framework occupies space comparable to an aggregate of 9 [C₆₀]-fullerene molecules. Comparative studies of DBFI-T with its planar monomeric analogue (BFI-P2) and PC₆₀BM in bulk heterojunction (BHJ) solar cells, by using a common thiazolothiazole-dithienosilole copolymer donor (PSEHTT), showed that DBFI-T has superior charge photogeneration and photovoltaic properties; PSEHTT:DBFI-T solar cells combined a high short-circuit current (10.14 mA/cm²) with a high open-circuit voltage (0.86 V) to give a power conversion efficiency of 5.0%. The external quantum efficiency spectrum of PSEHTT:DBFI-T devices had peaks of 60–65% in the 380–620 nm range, demonstrating that both hole transfer from photoexcited DBFI-T to PSEHTT and electron transfer from photoexcited PSEHTT to DBFI-T contribute substantially to charge photogeneration. The superior charge photogeneration and electron-accepting properties of DBFI-T were further confirmed by independent Xenon-flash time-resolved microwave conductivity measurements, which correctly predict the relative magnitudes of the conversion efficiencies of the BHJ solar cells: PSEHTT:DBFI-T > PSEHTT:PC₆₀BM > PSEHTT:BFI-P2. The results demonstrate that the large size, multichromophoric, nonplanar 3D molecular design is a promising approach to more efficient organic photovoltaic materials