**Molecular Engineering Enhances the Charge Carriers Transport in Wide Band-Gap Polymer Donors Based Polymer Solar Cells**

**Siyang Liu <sup>1</sup> , Shuwang Yi <sup>2</sup> , Peiling Qing <sup>1</sup> , Weijun Li <sup>1</sup> , Bin Gu <sup>1</sup> , Zhicai He <sup>2</sup> and Bin Zhang 1,3,\***


Academic Editors: Tersilla Virgili and Mariacecilia Pasini Received: 10 August 2020; Accepted: 4 September 2020; Published: 8 September 2020

**Abstract:** The novel and appropriate molecular design for polymer donors are playing an important role in realizing high-efficiency and high stable polymer solar cells (PSCs). In this work, four conjugated polymers (PIDT-O, PIDTT-O, PIDT-S and PIDTT-S) with indacenodithiophene (IDT) and indacenodithieno [3,2-b]thiophene (IDTT) as the donor units, and alkoxy-substituted benzoxadiazole and benzothiadiazole derivatives as the acceptor units have been designed and synthesized. Taking advantages of the molecular engineering on polymer backbones, these four polymers showed differently photophysical and photovoltaic properties. They exhibited wide optical bandgaps of 1.88, 1.87, 1.89 and 1.91 eV and quite impressive hole mobilities of 6.01 <sup>×</sup> <sup>10</sup>−<sup>4</sup> , 7.72 <sup>×</sup> <sup>10</sup>−<sup>4</sup> , 1.83 <sup>×</sup> <sup>10</sup>−<sup>3</sup> , and 1.29 <sup>×</sup> <sup>10</sup>−<sup>3</sup> cm<sup>2</sup> V −1 s −1 for PIDT-O, PIDTT-O, PIDT-S and PIDTT-S, respectively. Through the photovoltaic test via using PIDT-O, PIDTT-O, PIDT-S and PIDTT-S as donor materials and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) as acceptor materials, all the PSCs presented the high open circuit voltages (*Vocs*) over 0.85 V, whereas the PIDT-S and PIDTT-S based devices showed higher power conversion efficiencies (PCEs) of 5.09% and 4.43%, respectively. Interestingly, the solvent vapor annealing (SVA) treatment on active layers could improve the fill factors (FFs) extensively for these four polymers. For PIDT-S and PIDTT-S, the SVA process improved the FFs exceeding 71%, and ultimately the PCEs were increased to 6.05%, and 6.12%, respectively. Therefore, this kind of wide band-gap polymers are potentially candidates as efficient electron-donating materials for constructing high-performance PSCs.

**Keywords:** molecular engineering; polymer donors; high hole mobility; polymer solar cells
