Energy level modulation of non-fullerene acceptors enables efficient organic solar cells with small energy loss

Two new non-fullerene (NF) acceptors, namely BDTIT-M and BDTThIT-M, were rationally designed to optimize the energy levels and optical bandgap. BDTIT-M is derived by changing the end-group of NFBDT into slightly weak DCI-M, and BDTThIT-M is obtained by adding two conjugated thiophene side-chains into a ladder-type core of BDTIT-M. By incorporating with the polymer donor PBDB-T, BDTIT-M based organic solar cells (OSCs) deliver a higher PCE of 11.31% compared to that of NFBDT based cells, which is mainly attributed to the increased V_OC and FF. A higher PCE of 12.12% with a small energy loss of ∼0.588 eV is achieved compared with BDTThIT-M based OSCs, benefiting from the elevated LUMO level, narrowed bandgap, and enhanced absorption coefficient and electron mobility of BDTThIT-M compared with BDTIT-M. The combination of a methyl-modified end-group and conjugated side-chain should be an efficient strategy to elevate the LUMO and HOMO levels with different amplitudes for realizing simultaneous improvement in V_OC and J_SC.