Balancing the Voc-Jsc trade-off in polymer solar cells based on 2-(benzoxazol-2-yl)-acetonitrile end-capped small-molecule acceptors through asymmetry and halogenation of end groups

A trade-off between open-circuit voltage (Voc) and short-circuit current density (Jsc) is commonly confronted for most polymer solar cells (PSCs). In this study, we employ 2-acetonitrile-benzoxazole (BOA) and its chlorinated counterpart 2-(5-chlorobenzo[d]oxazol-2-yl)acetonitrile (BOACl) as new end groups to design and synthesize two symmetric acceptor-donor-acceptor (A-D-A) small-molecule acceptors (SMAs), IDTT-BOA and IDTT-BOACl, based on indacenodithieno[3,2-b]thiophene (IDTT) central donor unit. Then, to balance the trade-off between Voc and Jsc, an asymmetric SMA ICCl-IDTT-BOACl is developed by end-capping the different chlorinated A units of BOACl and 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (ICCl) on the both sides of IDTT. The results show that IDTT-BOA and IDTT-BOACl exhibit a higher-lying LUMO energy level than that of ICCl-IDTT-BOACl, which is beneficial to obtain a larger Voc. Absorption property of asymmetric ICCl-IDTT-BOACl is superior to two symmetric SMAs, which is conducive to improving Jsc. Using PBDB-T as the donor, IDTT-BOA and IDTT-BOACl-based PSCs deliver a mediocre PCE of 2.16 and 3.45%, but with an extremely high Voc of 1.19 and 1.16 V, respectively. In contrast, a significantly improved PCE of 7.83% is obtained for ICC1-IDTT-BOACl-based PSCs due to the balanced Voc-Jsc trade-off. Encouragingly, the efficiency of ICCl-IDTT-BOACl-based PSCs can be further improved up to 10.20% by replacing PBDB-T with PM6. The enhanced device efficiency of ICCl-IDTT-BOACl-based PSC relative to IDTT-BOA and IDTT-BOACl mainly attributes to larger and more balanced carrier mobility, more efficient charge transport and exciton dissociation, and less bimolecular recombination as well as superior film morphology and compatibility. This work demonstrates that the synergistic effect of asymmetry and halogenation in end groups of A-D-A acceptors could control the Jsc-Voc trade-off of PSCs to improve photovoltaic performance.