A study of the use of carbonyl and ester groups in regulating the optical and electrical characteristics of wide bandgap polymers for organic solar cells

Abstract

Alternating weak electron donating and electron withdrawing units can be used to easily achieve wide bandgap (WBG) π-conjugated polymers for non-fullerene polymer solar cells (PSCs), which can match well with low band gap (LBG) non-fullerene acceptors (NFAs) not only in the optical absorption, but also in appropriate energy levels. A wide sunlight absorption of the WBG polymer:LBG-NFA blending film and a deep-lying highest occupied molecular orbital (HOMO) energy level of the polymeric donor are the prerequisites for achieving a high short-circuit current density (J_SC) and open-circuit voltage (V_OC). Therefore, here we design two kinds of polymers by copolymerization of BDT and thiophene units, on which the electron withdrawing ester or carbonyl groups are introduced, namely PTO and PTC, respectively. The carbonyl substituent that has stronger positive electricity than the ester group can decrease the HOMO energy level of the polymer and lead to red-shifted absorption for PTC; however, owing to the larger steric hindrance of the carbonyl group and the cyclohexyl side chain, PTC exhibits weaker aggregation than PTO. Studies have found that the optical, electrochemical, and electrical properties and photovoltaic performances of the polymers are directly affected by the side chain substituent. An overall higher power conversion efficiency (PCE) of 11.83% for PTO is obtained compared to 10.66% for PTC in their bulky heterojunction (BHJ) PSCs. Although the ester group seems to be more efficient in the design of π-conjugated polymers, the carbonyl group which has been relatively less studied on WBG polymers reveals great potential for solar cell application due to the advantages of an improved V_OC and enhanced hole mobility. This work compares in detail the effects of the ester and carbonyl groups on the physicochemical properties of polymers and provides ideas for the design of WBG polymeric donors for BHJ-PSCs.