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Among the three A–D–A-type nonfullerene acceptors, MQ7-i has improved molecular planarity, compact packing motifs, and the largest charge transfer integrals and the highest electron mobilities thereby yielding the highest efficiency of 16.23%.

Ternary polymer solar cells (PSCs) with three active layer components (two donors with one acceptor, or two acceptors with one donor) have been demonstrated as an emerging strategy to improve the power conversion efficiencies (PCEs). So far, most of the third components utilized in the ternary PSCs are relatively complex molecules or polymers which are expensive due to complicated multi-step syntheses. Here, a simple small molecule of 4,7-bis(5-bromothiophen-2-yl)-5,6-difluorobenzo[c][1,2,5]thiadiazole (BTF), is first used as a second acceptor material to fabricate efficient ternary PSCs. The incorporation of BTF into a binary active layer based on a wide-bandgap polymer of J71 and a low-bandgap acceptor of ITIC, results in ternary PSCs with the best PCE of 12.35%, which is higher than the control J71:ITIC-based binary device with the best PCE of 10.79%. This significantly enhanced PCE results from the simultaneously improved short-circuit current density, open-circuit voltage, and fill factor in the ternary PSCs, and they are in part attributed to the increased light-harvesting with the BTF incorporation and in part attributed to the more balanced charge transportation induced by the improved polymer crystallinity. To determine the generality of BTF as a second acceptor in ternary PSCs, another benchmark binary active layer of PM6:Y6 has been incorporated with 10 wt% BTF, leading to a best-performance device with an outstanding PCE of 16.53% which is the highest among all ternary PSCs, to the best of our knowledge. Our work provides a simple strategy to efficiently boost the performance of ternary PSCs by using an easily available, low-cost acceptor of BTF.

Wide-bandgap (WBG) polymers exhibit strong absorption bands in the short wavelength region, and they can offer complementary absorption when blended with their low-bandgap (LBG) counterparts in multi-junction, ternary-blend, or non-fullerene polymer solar cells (PSCs) thereby further enhancing power conversion efficiencies (PCEs) of organic solar cells. In this work, two WBG polymers (PSS2 and PSS3) based on ladder-type dithienocyclopentadibenzothiophene (DTCD) are designed and synthesized successfully. PSS2 and PSS3 possess relatively deep-lying highest occupied molecular orbital (HOMO) energy levels of −5.36 and −5.46 eV with optical bandgaps of 2.01 and 1.92 eV, respectively. In combination with a non-fullerene acceptor (EH-IDTBR), PSS2 exhibited a power conversion efficiency (PCE) of 9.46% with a short-circuit current (JSC) of 15.70 mA cm−2, a fill factor (FF) of 58.3% and a VOC of 1.033 V. The 9.46% PCE is the highest among all copolymers based on ladder-type multi-ring-fused aromatics. With EH-IDTBR, PSS3 showed an inferior PCE of 4.74% with an enlarged VOC of 1.138 V. Furthermore, the best-performing PSS2-based device showed excellent stability with a PCE of 9.03% after storage for 120 days, which is 95% of its original value. Our results indicate that the ladder-type DTCD is a promising building block for designing WBG semiconductors for photovoltaic applications.

Ternary conjugated D–A copolymers are designed and synthesized by using indenothiophene and benzodithiophene as the weak and the strong donors, respectively, and fluorinated benzodithiazole as the strong acceptor. The best performance copolymer delivers a high PCE of 9.08%, which is a record efficiency for ternary conjugated D–A copolymers reported to date.

Five wide bandgap conjugated polymers based on diindenocarbazole (DIC) and dithienylbenzothiadiazole (DTBT) alternating units have been designed and prepared to investigate the effects of side chains on the photovoltaic performance of the polymers. All the polymers are soluble in common organic solvents and they show similar optical bandgaps of around 2.0 eV as well as deep-lying highest occupied molecular orbital (HOMO) energy levels (below −5.48 eV). Bulk heterojunction (BHJ) polymer solar cells (PSCs) using phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor material were fabricated. The side chains on the polymer backbone have a strong impact on the film morphology of the polymer:PC71BM blends. A phase separation with a relatively larger domain size was found for the polymers with longer side chains, while those with relatively shorter alkyl chains could form uniform films featuring a phase separation with a smaller domain size. Finally, PSCs based on PC1BT6:PC71BM exhibited an outstanding power conversion efficiency of 7.34% with a high open-circuit voltage (Voc) of 0.95 V. Our results demonstrate that the judicious design of side-chains is effective in improving the photovoltaic performance of DIC-based polymers. With a larger bandgap than P3HT but an improved power conversion efficiency (PCE) with a large Voc, this type of DIC-based polymer should be a promising bottom layer material for tandem solar cells.