An extraordinary cyclohexylmethyl side chain dominating polymeric donor packing patterns and energy levels for efficient non-fullerene polymer solar cells

Abstract

Benefiting from the tremendous efforts devoted to small molecule electron acceptors (SMAs), great progress has been made in non-fullerene polymer solar cells (NF-PSCs). Yet compared with the structurally diverse SMAs, much less attention has been paid to polymeric donors, which are also extremely important components dominating the development of NF-PSCs. A material design concept for obtaining an easily achieved phase separated morphology between the polymeric donor and SMA is presented. The key concept lies in optimizing the polymer crystallinity, by inserting a newly developed cyclohexylmethyl side chain, thus improving the polymer:SMA miscibility, and simultaneously, decreasing the energy levels, and strengthening the overall packing of the polymers in their film states. Once the cyclohexylmethyl chains are introduced, PTAZ–CH and PTAZ–CH–S exhibit a higher absorption coefficient and deeper highest occupied molecular orbital (HOMO) energy levels, in comparison with the reference polymer. This methodology of using a cyclic side chain to control the polymer's crystallinity has the added benefits of forming optimal polymer:SMA morphologies without any post-treatments like thermal- or solvent-annealing, making such low energy consuming and environmentally friendly PSC fabrication available. Impressively, more than doubled power conversion efficiencies (PCEs) are obtained employing ordinary ITIC and IT-M as the electron accepting partners.