In this work, we have synthesized two new quinoxaline derivatives: 2,3-bis(n-octylthiomethyl)-5,8-dibromoquinoxaline (QS) and 2,3-bis[(5-octylthio)thiophen-2-yl]-5,8-dibromoquinoxaline (QTS); in addition, three new donor–acceptor (D–A) copolymers: poly{4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′]-dithiophene-alt-2,3-bis(n-octylthiomethyl)quinoxaline} (PBDTQS), poly{4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′]-dithiophene-alt-2,3-bis[(5-octylthio)thiophen-2-yl]quinoxaline} (PBDTQTS), and poly{2,3-bis[(5-octylthio)thiophen-2-yl]quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl} (PTQTS) were designed from the view of extending the length of conjugated side chain and reducing the steric hindrance of building blocks. Replacing the carbon atom in the side chain of polymer PBDTQS with a thiophene ring could increase the conjugation length and improve the absorption in the visible region of the copolymer PBDTQTS and PTQTS. Furthermore, polymer PTQTS exhibited a more planar backbone and increased intermolecular π-stacking compared to PBDTQTS, which was because the thiophene unit in PTQTS had smaller size and less steric hindrance than those of benzo[1,2-b:4,5-b′]-dithiophene unit in PBDTQTS. For the optimized polymer solar cell of PTQTS:PC61BM, PCE of 3.73% with Voc of 0.76 V, Jsc of 9.41 mA cm−2 and FF of 52.33% under an AM 1.5 G solar simulator with an intensity of 100 mW cm−2 was achieved, which was the best performance among the three copolymers. The results implied that the PTQTS with thiophene as the donor unit and QTS as the acceptor unit in the main chain would be a promising donor candidate in the application of polymer solar cells.

•High-quality CH3NH3PbI3 perovskite films with a mirror-like surface were prepared.•The fill factor of the perovskite solar cells approached 80.52%.•The high-quality of the perovskite films greatly improves cell stability.The quality and stability of perovskite films are critical for solar cells using this technology. Here, we fabricate a high-quality CH3NH3PbI3 perovskite film by introducing small amounts of N-methyl-2-pyrrolidone into a mixture of γ-butyrolactone and dimethylsulphoxide. The perovskite film consists of compact grains (average sizes of ∼57 nm) with unclear boundaries which show a mirror-like surface with the root mean square roughness of only 2.39 nm. The power conversion efficiency of 11.77% is obtained with the fill factor as high as 80.52% under one sun illumination (100 mW cm−2). Furthermore, the stability of the perovskite solar cells is greatly improved by forming compact perovskite films with excellent surfaces. The results indicate that high-quality perovskite films with compact grains are a promising choice for high-stable, efficient perovskite solar cells.