Single junction binary and ternary polymer solar cells-based D–A structured copolymer with low lying HOMO energy level and two nonfullerene acceptors

Abstract

A donor–acceptor (D–A) conjugated copolymer denoted as P(DTB-BDD) consisting of benzodithiophenedione (BDD) as a strong acceptor and dithienobenzene (DTB) as a weak donor was prepared, and its optical and electrochemical properties were analyzed. P(DTB-BDD) exhibited a deeper highest occupied molecular orbital and an optical bandgap of 1.74 eV. Polymer solar cells (PSCs) were fabricated through blending P(DTB-BDD) with two non-fullerene acceptors, i.e., narrow bandgap Y6 and medium bandgap DBTBT-IC. The power conversion efficiencies (PCEs) of the PSCs using the bulk heterojunction active layer based on P(DTB-BDD):DBTBT-IC and P(DTB-BDD):Y6 were 13.16% and 12.62%, respectively. The larger open circuit voltage of the DBTBT-IC based PSCs as compared with that of the Y6 counterpart was due to the up-shifted LUMO energy level of DBTBT-IC, and the high short circuit current for the Y6 based PSCs may be associated with the extended absorption profile of Y6. Taking advantage of the high open circuit voltage and short circuit current of the devices based on DBTBT-IC and Y6, the ternary PSCs were prepared by optimizing the weight ratios between two acceptors and maintaining a constant amount of P(DTB-BDD); the resulting ternary PSCs based on P(DTB-BDD) : DBTBT-IC : Y6 (1.0 : 0.2 : 1.0) showed an improved PCE of 16.32%, which is greater than those for the binary PSCs. The enrichment in the PCE of the ternary device may be concomitant with the effective exploitation of excitons via energy transfer from DBTBT-IC to Y6 and increased the D/A interfacial area for more effective charge transfer. The open circuit voltage of the PSC based on ternary lies in between that for the Y6 and DBTBT-IC-based PSCs, demonstrating the formation of an alloy for the two acceptors.