Asymmetric Wide-Bandgap Polymers Simultaneously Improve the Open-Circuit Voltage and Short-Circuit Current for Organic Photovoltaics

Abstract

A trade-off between open-circuit voltage (V _OC ) and high short-circuit (J _SC ) becomes one of the most vital problems limiting further improvement in polymer solar cells' (PSCs) efficiency. In this work, two asymmetric polymer donors PBDT-F-2TC and PBDT-SF-2TC are designed and synthesized. When blended with a state-of-the-art acceptor IT-4F with low lowest-unoccupied molecular orbital level, simultaneously high V _OC (up to 0.94 V) and J _SC (up to 20.73 mA cm^-2 ) are obtained for both copolymers. Note that the V _OC value of 0.94 V is the highest value of PSCs based on IT-4F reported so far. The simultaneously improved V _OC and J _SC in resulting devices are discovered from the deep highest-occupied molecular orbital levels (-5.5 to -5.7 eV) and the hyperchromic effect of the polymers, the small driving force, and the small energy loss during the charge transfer, due to the synergistic effect of asymmetric carboxylate unit and fluorine/sulfur atoms. More importantly, thanks to the asymmetric 2TC, both PBDT-F-2TC- and PBDT-SF-2TC-based PSCs can be successfully processed by non-halogenated solvent 1,2,4-trimethylbenzene (TMB) to yield device efficiencies of 10.29% and 10.39%, respectively, which are the maximum values for non-fullerene PSCs fabricated using the eco-friendly solvent TMB.

Keywords: energy loss; non-halogenated solvents; open-circuit voltages; polymer solar cells; short-circuit current.