An asymmetric small-molecule donor enables over 18% efficiency in ternary organic solar cells

Abstract

An asymmetric small molecule donor, TTBT-R, incorporated with 4,5-di(2-ethylhexyl)oxybenzo[2,1-b:3,4-b′]dithiophene as the core, was exploited for high-performance ternary organic solar cells (OSCs), and possesses a complementary absorption band and cascaded energy levels with PM6 and Y6. The ternary blend film (PM6:TTBT-R:Y6 = 0.9 : 0.1 : 1) attains the most optimized morphology with a nanofiber-like structure and small domain size as well as strong face-on molecular packing, which leads to the most balanced charge transport, highest charge mobilities and lowest charge recombination in its device. In addition, TTBT-R possesses a lower-lying HOMO energy level than PM6, which brings about higher V_oc in ternary OSCs. Accordingly, the ternary OSC with 10 wt% TTBT-R achieved a PCE of 18.07% due to the comprehensively improved V_oc (0.863 V), J_sc (27.38 mA cm⁻²), and FF (76.46%) values compared with the PM6:Y6 binary OSC. This work provides not only a small molecule with 4,5-dialkyloxybenzo[2,1-b:3,4-b′]dithiophene as the core but also an asymmetric strategy with a non-fused structure to tune the molecular packing, energy levels and absorption spectrum of small molecules for constructing high-performance ternary OSCs.