Efficient ternary bulk heterojunction solar cells based on small molecules only

Abstract

Ternary bulk heterojunctions (BHJs) are platforms that can improve the power conversion efficiencies of organic solar cells. In this paper, we report an all-small-molecule ternary BHJ solar cell incorporating [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) and indene-C₆₀ bisadduct (ICBA) as mixed acceptors and the conjugated small molecule (2Z,2′E)-dioctyl 3,3′-(5′′,5′′′′′-(4,8-bis(5-octylthiophen-2-yl)benzo[1,2-b:5,4-b′]dithiophene-2,6-diyl)bis(3,4′,4′′-trioctyl-[2,2′:5′,2′′-terthiophene]-5′′,5-diyl))bis(2-cyanoacrylate) (BDT6T) as a donor. When incorporating a 15% content of ICBA relative to PC₇₁BM, the ternary BHJ solar cell reached a power conversion efficiency of 6.36% with a short-circuit current density (J_SC) of 12.00 mA cm⁻², an open-circuit voltage (V_OC) of 0.93 V, and a fill factor of 0.57. The enhancement in efficiency, relative to that of the binary system, resulted mainly from the increased value of J_SC, attributable to not only the better intermixing of the donor and acceptor that improved charge transfer but also the more suitable morphology for efficient dissociation of excitons and more effective charge extraction. Our results suggest that there is great potential for exceeding the efficiencies of binary solar cells by adding a third component, without sacrificing the simplicity of the fabrication process.