Improving the exciton dissociation of polymer/fullerene interfaces with a minimal loading amount of energy cascading molecular dopant

In this work, a novel star-shaped conjugated small molecule, 2,2′,2′′-((benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene-2,5,8-triyltris(3-(2-ethylhexyl)thiophene-5,2-diyl))tris(methanylylidene))trimalono-nitrile (BTTCN), with a benzotrithiophene donor unit as the core structure and the electron-withdrawing dicyano groups as the end-cappers, was designed and synthesized. It was used as the third component to construct ternary polymer solar cells (PSCs) based on a typical polymer/fullerene binary host (PTB7-Th:PC₇₁BM). The results manifested that BTTCN served as an energy cascading interlayer to improve the exciton dissociation and transport of the active layer by doping with only a minimal amount of BTTCN that aided to construct a cascade energy level alignment. In contrast to the binary blends, the as-prepared PSCs with 3 wt% BTTCN dopant showed enhanced short-circuit current densities (J_SC), which resulted in an enhanced power conversion efficiency (PCE) from 8.72% to 9.43%.