Ternary non-fullerene polymer solar cells with a high crystallinity n-type organic semiconductor as the second acceptor

Abstract

Ternary blend is an effective way to realize high photovoltaic performance of polymer solar cells (PSCs). A highly crystalline n-type organic semiconductor (n-OS) IDIC was introduced into a low crystalline blend of conjugated polymer donor J61 and n-OS acceptor BT-IC. Higher power conversion efficiencies (PCE) of 10.80% were achieved for the ternary PSCs based on J61:IDIC:BT-IC (0.9 : 0.2 : 0.8, w/w), with an improved short-circuit current density (J_sc), fill factor (FF) and unreduced open-circuit voltage (V_oc). The addition of high crystallinity IDIC into the binary J61:BT-IC blend enhanced the coherence length of polymer donor J61 in the blend film, yielding higher hole mobility and achieving higher J_sc and FF. Charge recombination mechanism analysis revealed that the ternary blend based on J61:IDIC:BT-IC exhibited less bimolecular recombination in comparison with the device based on J61:BT-IC. The energy transfer was unveiled from IDIC to BT-IC for the two acceptors based on photoluminescence (PL) and transient absorption measurements. The V_oc of the ternary PSCs with 20% lower-lying the lowest unoccupied molecular orbital (LUMO) IDIC in the acceptors remained the same as the devices based on the host binary blend. This finding may account for the reduced bimolecular recombination and the energy transfer in the ternary devices. This study provides an efficient strategy to obtain a high PCE for ternary PSCs by introducing a second small molecule acceptor with high crystallinity.