Nonfullerene acceptors (NFAs) have contributed significantly to the progress of organic solar cells (OSCs). However, most NFAs feature a large fused-ring backbone, which usually requires a tedious multiple-step synthesis, and are not applicable to commercial applications. An alternative strategy is to develop nonfused NFAs, which possess synthetic simplicity and facile tunability in optoelectronic properties and solid-state microstructures. In this work, we report two nonfused NFAs, BTCIC and BTCIC-4Cl, based on an A–D–A′–D–A architecture, which possess the same electron-deficient benzothiadiazole central core but different electron-withdrawing terminal groups. The optical properties, energy levels, and molecular crystallinities were finely tuned by changing the terminal groups. Moreover, a decent power conversion efficiency of 9.3 and 10.5% has been achieved by BTCIC and BTCIC-4Cl, respectively, by blending them with an appropriate polymer donor. These results demonstrate the potential of A–D–A′–D–A type nonfused NFAs for high-performance OSCs. Further development of nonfused NFAs will be very fruitful by employing appropriate building blocks and via side-chain optimizations.

A donor polymer based on 3-cyanothiophene, a structurally simple unit, is synthesized for organic solar cells, which exhibited prominent power conversion efficiency and excellent batch-to-batch reproducibility in a wide molecular weight range.

High-efficiency organic solar cells (OSCs) largely rely on polymer donors. Herein, we report a new building block BNT and a relevant polymer PBNT-BDD featuring B-N covalent bond for application in OSCs. The BNT unit is synthesized in only 3 steps, leading to the facile synthesis of PBNT-BDD. When blended with a nonfullerene acceptor Y6-BO, PBNT-BDD afforded a power conversion efficiency (PCE) of 16.1 % in an OSC, comparable to the benzo[1,2b:4,5-b']dithiophene (BDT)-based counterpart. The nonradiative recombination energy loss of 0.19 eV was afforded by PBNT-BDD. PBNT-BDD also exhibited weak crystallinity and appropriate miscibility with Y6-BO, benefitting of morphological stability. The singlet-triplet gap (DE ST) of PBNT-BDD is as low as 0.15 eV, which is much lower than those of common organic semiconductors (! 0.6 eV). As a result, the triplet state of PBNT-BDD is higher than the charge transfer (CT) state, which would suppress the recombination via triplet state effectively.

The development of nonfullerene acceptors has brought polymer solar cells into a new era. Maximizing the performance of nonfullerene solar cells needs appropriate polymer donors that match with the acceptors in both electrical and morphological properties. So far, the design rationales for polymer donors are mainly borrowed from fullerene‐based solar cells, which are not necessarily applicable to nonfullerene solar cells. In this work, the influence of side chain length of polymer donors based on a set of random terpolymers PTAZ‐TPD10‐Cn on the device performance of polymer solar cells is investigated with three different acceptor materials, i.e., a fullerene acceptor [70]PCBM, a polymer acceptor N2200, and a fused‐ring molecular acceptor ITIC. Shortening the side chains of polymer donors improves the device performance of [70]PCBM‐based devices, but deteriorates the N2200‐ and ITIC‐based devices. Morphology studies unveil that the miscibility between donor and acceptor in blend films depends on the side chain length of polymer donors. Upon shortening the side chains of the polymer donors, the miscibility between the donor and acceptor increases for the [70]PCBM‐based blends, but decreases for the N2200‐ and ITIC‐based blends. These findings provide new guidelines for the development of polymer donors to match with emerging nonfullerene acceptors.

The power conversion efficiencies (PCEs) of OSCs have reached 19% as the innovation of state-of-the-art nonfullerene acceptors. [2] On the contrary, the development of polymer donors lags much behind. Up to date, the polymer donors are limited to a handful of polymers and, more specifically, thiophene-substituted benzo [1,2-b:4,5-b']dithiophene (BDTT) polymers. [3] The side-chain conjugated substituents on BDTT could extend backbone conjugation and electron delocalization, enhance interchain π-π interaction, and improve charge transport of the polymers. Moreover, the absorption spectra, energy levels, and crystallinity of BDTTbased polymer donors can be finely tuned by both main-chain and side-chain engineering. [4] As a result, these kinds of polymers, such as PM6 and D18 (Figure 1a), [5] perform well in both fullerene and nonfullerene OSCs. [3a,4] However, BDTT-based polymers usually suffer from lengthy synthesis and tedious purification processes, which inevitably result in a high production cost and bring difficulties for the large scale synthesis. [6] Therefore, high-performance polymer donors with simple structure and facile synthesis are urgently needed to drive the further development of OSCs. [7] Among various polymer donors, poly(3-hexylthiophene) (P3HT) is one of the most extensively studied polymers due to its simple structure, low production cost, good self-assembly property, and high charge transport ability. [8] However, the PCEs of P3HT-based OSCs were restrained at about 10% due to the high-lying HOMO level of P3HT and excessively mixed active layer morphology. [9] An alternative strategy is incorporating electron-deficient units into the polymer main chain to form oligothiophene-based donor-acceptor polymers. This kind of polymer employs oligothiophene as a donor unit and thus the properties of the polymers can be adjusted by both the oligothiophene unit and acceptor unit. Moreover, these kinds of polymers can be synthesized more easily than BDTTbased donor-acceptor polymers. [10] A few state-of-the-art oligothiophene-based polymer donors such as PffBT4T-2OD and PNTT [11] (Figure 1a) have been developed, which exhibited good crystallinity and high hole mobility. Moreover, this kind of polymers usually exhibits temperature-dependent aggregation properties, which have provided valuable opportunities to regulate The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have increased rapidly owing to the development of non-fullerene acceptors (NFAs). However, the development of polymer donors lags behind significantly. Currently, the polymer donors are dominated by a handful of thiophenesubstituted benzo[1,2-b:4,5-b']dithiophene (BDTT) polymers, which suffer from lengthy synthesis and high production cost. Compared with BDTT-based polymers, oligothiophene-based donor-acceptor polymers feature much easier synthesis, which were the prevailing polymer donors in fullerene-based OSCs, but almost disappeared in non-fullerene OSCs. Herein, two oligothiophene-based donor-acceptor pol...