Narrow bandgap (1.37-1.46 eV) polymers incorporating a head-to-head linkage containing 3-alkoxy-3'-alkyl-2,2'-bithiophene are synthesized. The head-to-head linkage enables polymers with sufficient solubility and the noncovalent sulfur-oxygen interaction affords polymers with high degree of backbone planarity and film ordering. When integrated into polymer solar cells, the polymers show a promising power conversion efficiency approaching 10%.

Currently, n‐type acceptors in high‐performance all‐polymer solar cells (all‐PSCs) are dominated by imide‐functionalized polymers, which typically show medium bandgap. Herein, a novel narrow‐bandgap polymer, poly(5,6‐dicyano‐2,1,3‐benzothiadiazole‐alt‐indacenodithiophene) (DCNBT‐IDT), based on dicyanobenzothiadiazole without an imide group is reported. The strong electron‐withdrawing cyano functionality enables DCNBT‐IDT with n‐type character and, more importantly, alleviates the steric hindrance associated with typical imide groups. Compared to the benchmark poly(naphthalene diimide‐alt‐bithiophene) (N2200), DCNBT‐IDT shows a narrower bandgap (1.43 eV) with a much higher absorption coefficient (6.15 × 104 cm−1). Such properties are elusive for polymer acceptors to date, eradicating the drawbacks inherited in N2200 and other high‐performance polymer acceptors. When blended with a wide‐bandgap polymer donor, the DCNBT‐IDT‐based all‐PSCs achieve a remarkable power conversion efficiency of 8.32% with a small energy loss of 0.53 eV and a photoresponse of up to 870 nm. Such efficiency greatly outperforms those of N2200 (6.13%) and the naphthalene diimide (NDI)‐based analog NDI‐IDT (2.19%). This work breaks the long‐standing bottlenecks limiting materials innovation of n‐type polymers, which paves a new avenue for developing polymer acceptors with improved optoelectronic properties and heralds a brighter future of all‐PSCs.

Compared to organic solar cells based on narrow bandgap non-fullerene small molecule acceptors, the performance of all-polymer solar cells (all-PSCs) lags much behind due to the lack of highperformance n-type polymers, which should have low-aligned frontier molecular orbital levels and narrow bandgap with broad and intense absorption extended to near-infrared region. Herein, two novel polymer acceptors, DCNBT-TPC and DCNBT-TPIC, are synthesized with an ultra-narrow bandgap (ultra-NBG) of 1.38 and 1.28 eV, respectively. When applied in transistors, both polymers show efficient charge transport with a highest electron mobility of 1.72 cm 2 V -1 s -1 obtained for DCNBT-TPC. Blended with polymer donor PBDTTT-E-T, the resultant all-PSCs based on DCNBT-TPC and DCNBT-TPIC achieve remarkable power conversion efficiencies (PCEs) of 9.26% and 10.22% with short-circuit currents up to 19.44 and 22.52 mA cm -2 , respectively. This is the first example that a PCE of over 10% can be achieved using ultra-NBG polymer acceptor with a photo-response reaching 950 nm in all-PSCs. These results demonstrate that ultra-NBG polymer acceptors, in line with non-fullerene small molecule acceptors, are also available as a highly promising class of electron acceptors for maximizing device performance in all-PSCs.Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))

Building blocks having a high degree of backbone planarity, good solubilizing characteristics, and well-tailored physicochemical properties are highly desirable for constructing high-performance polymer semiconductors. Due to the detrimental steric hindrance created by alkyl chain substituents at the 3-and 3′-positions of bithiophene, "head-to-head" linkage containing 3,3′-dialkyl-2,2′-bithiophenes (BTR) are typically avoided in materials design. Replacing alkyl chains with less steric demanding alkynyl chains should greatly reduce steric hindrance by eliminating two H atoms at the sp-hybridized carbon center. Here we report the synthesis of a novel electron donor unit, 3,3′-dialkynyl-2,2′-bithiophene (BTRy), and its incorporation into conjugated polymer backbones. The alkynyl-functionalized head-to-head bithiophene linkage yields polymers with good solubility without sacrificing backbone planarity; the BTRy-based polymers show a high degree of conjugation with a narrow bandgap of ∼1.6 eV. When incorporated into organic thin-film transistors, the polymers exhibit substantial hole mobility, up to 0.13 cm 2 V −1 s −1 in top-gated transistors. The electron-withdrawing alkynyl substituents lower the frontier molecular orbitals, imbuing the difluorobenzothiadiazole and difluorobenzoxadiazole copolymers with remarkable ambipolarity: electron mobility > 0.05 cm 2 V −1 s −1 and hole mobility ∼0.01 cm 2 V −1 s −1 in bottom-gated transistors. In bulk-heterojunction solar cells, the BTRy-based polymers show promising power conversion efficiencies approaching 8% with very large V oc values of 0.91−1.04 V, due to the weak electron-withdrawing alkynyl substituents. In comparison to the tetrathiophene-based polymer analogues based on the unsubstituted π-spacer design, the BTRy-based polymers have comparable light absorption but with 0.14 V larger open-circuit voltage, translating to enhanced optoelectronic properties for this attractive design strategy. Thus, alkynyl groups are versatile semiconductor substituents, offering good solubility, substantial backbone planarity, optimized optoelectronic properties, and film crystallinity, for materials innovation in organic electronics.

Polythiophenes, built on the electron-rich thiophene unit, typically possess high-lying energy levels of the lowest unoccupied molecular orbitals (LUMOs) and show hole-transporting properties. In this study, we develop a series of n-type polythiophenes, P1–P3, based on head-to-head-linked 3,3′-dialkoxy-4,4′-dicyano-2,2′-bithiophene (BTCNOR) with distinct side chains. The BTCNOR unit shows not only highly planar backbone conformation enabled by the intramolecular noncovalent sulfur–oxygen interaction but also significantly suppressed LUMO level attributed to the cyano-substitution. Hence, all BTCNOR-based polymer semiconductors exhibit low-lying LUMO levels, which are ∼1.0 eV lower than that of regioregular poly­(3-hexylthiophene) (rr-P3HT), a benchmark p-type polymer semiconductor. Consequently, all of the three polymers can enable unipolar n-type transport characteristics in organic thin-film transistors (OTFTs) with low off-currents (I offs) of 10–10–10–11 A and large current on/off ratios (I on/I offs) at the level of 106. Among them, polymer P2 with a 2-ethylhexyl side chain offers the highest film ordering, leading to the best device performance with an excellent electron mobility (μe) of 0.31 cm2 V–1 s–1 in off-center spin-cast OTFTs. To the best of our knowledge, this is the first report of n-type polythiophenes with electron mobility comparable to the hole mobility of the benchmark p-type rr-P3HT and approaching the electron mobility of the most-studied n-type polymer, poly­(naphthalene diimide-alt-bithiophene) (i.e., N2200). The change of charge carrier polarity from p-type (rr-P3HT) to n-type (P2) with comparable mobility demonstrates the obvious effectiveness of our structural modification. Adoption of n-hexadecyl (P1) and 2-butyloctyl (P3) side chains leads to reduced film ordering and results in 1–2 orders of magnitude lower μes, showing the critical role of side chains in optimizing device performance. This study demonstrates the unique structural features of head-to-head linkage containing BTCNOR for constructing high-performance n-type polymers, i.e., the alkoxy chain for backbone conformation locking and providing polymer solubility as well as the strong electron-withdrawing cyano group for lowering LUMO levels and enabling n-type performance. The design strategy of BTCNOR-based polymers provides useful guidelines for developing n-type polythiophenes.