Wide bandgap random terpolymers for high efficiency halogen-free solvent processed polymer solar cells

Over the past two decades, bulk heterojunction polymer solar cells (PSCs) have attracted significant attention owing to their potential applications in the mass fabrication of flexible device panels by roll-to-roll printing. To improve the photovoltaic performance of PSCs, much effort has been devoted to the optimization of properties of donor-acceptor (D-A) type polymer donor materials. Until now, the development of high-performance donor polymers is mainly dependent on the design and synthesis of binary polymers with a regular D/A alternating skeleton. Compared to binary polymers, random terpolymers with three different donor or acceptor monomer units possess synergetic effects of their inherent properties, such as optical absorption ability, energy levels, crystallinity, charge mobility, and morphological compatibility with the n-OS acceptors with suitable adjustment of the molar ratio of the three monomers. However, the irregularity in the polymer backbone of the random terpolymers may have an adverse effect on molecular packing, crystallinity, and charge mobility. Therefore, design and synthesis of high-performance terpolymers for PSCs is a challenging task. In this study, a series of wide bandgap random terpolymers PSBTZ-80, PSBTZ-60, and PSBTZ-40 based on alkylthiothienyl substituted benzodithiophene as the donor unit and two weak electron-deficient acceptor units of 5,6-difluorobenzotriazole (FBTz) and thiazolothiazole (TTz) were designed and synthesized for PSC applications. The optical, electrochemical, molecular packing, and photovoltaic properties of the polymers were effectively modulated by varying the FBTz:TTz molar ratio. Therefore, the PSC based on PSBTZ-60 as the donor material and narrow bandgap small molecule 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexyl-phenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']di thiophene) (ITIC) as the acceptor, processed using halogen-free solvents, exhibited high power conversion efficiency (PCE) of 10.3% with high open-circuit voltage (Voc) of 0.91 V, improved short-circuit current density (Jsc) of 18.0 mA·cm⁻², and fill factor (FF) of 62.7%, which are superior to those of PSCs based on binary polymers PSBZ (a PCE of 8.1%, Voc of 0.89 V, Jsc of 14.7 mA·cm⁻², and FF of 61.5%) and PSTZ (a PCE of 8.5%, Voc of 0.96 V, Jsc of 14.9 mA·cm⁻², and FF of 59.1%). These results indicate that random terpolymerization is a simple and practical strategy for the development of high-performance polymer photovoltaic materials.