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

Abstract

A trade-off between open-circuit voltage ( V oc ) and short-circuit current density ( J sc ) 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 V oc and J sc , 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 V oc . Absorption property of asymmetric ICCl-IDTT-BOACl is superior to two symmetric SMAs, which is conducive to improving J sc . 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 V oc 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 V oc - J sc 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 J sc - V oc trade-off of PSCs to improve photovoltaic performance. Two symmetric SMAs ( IDTT-BOA and IDTT-BOACl ) and an asymmetric SMA ( ICCl-IDTT-BOACl ) with the same central donor unit (IDTT) and different end groups (BOA, BOACl, and ICCl) were developed to balance the V oc - J sc trade-off of PSCs. Results demonstrated that IDTT-BOA and IDTT-BOACl -based on PSCs exhibited rather high V oc s exceeding 1.16 V but poor J sc s, leading to mediocre PCEs; whereas a significantly improved PCE of 7.83% was achieved for ICCl-IDTT-BOACl -based PSCs. Moreover, the device efficiency can be further improved up to 10.20% by changing donor due to the balanced trade-off between V oc (0.99 V) and J sc (18.37 mA cm −2 ). • Two symmetric acceptors IDTT-BOA and IDTT-BOACl were synthesized with BOA and BOACl as end units, respectively. • An asymmetric acceptor ICCl-IDTT-BOACl was further gained by end-capping the different end units of BOACl and ICCl. • PSCs based on IDTT-BOA and IDTT-BOACl showed mediocre PCEs but very high V oc s over 1.16 V. • ICCl-IDTT-BOACl -based PSCs showed an enhanced PCE of 10.20% due to the balanced V oc - J sc trade-off.