Cubic-Like Bimolecular Crystal Evolution and over 12% Efficiency in Halogen-Free Ternary Solar Cells

Abstract

In this study, the solubility properties of a given ternary blend set, with two donors (poly(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl (PTB7-Th) and benzo[1,2-b;4,5-b′]dithiophene-based small molecule (DR3TSBDT)) and one acceptor ([6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)), in a series of solvents are determined, and active material-solvent interactions are used as an aid for finding suitable nonchlorinated solvents to achieve effective ternary organic solar cells (OSCs) based on PTB7-Th:DR3TSBDT:PC71BM. An exceptional power conversion efficiency (PCE) as high as 12.3% (certified 11.94%) is obtained using the developed nonhalogenated processing system. In-depth investigations (morphology, charge mobility, recombination dynamics, and OSC characteristics) uncover the underlying structure-property relationships as a function of the chosen nonhalogenated systems. Another intriguing finding of this study is the formation of a cubic bimolecular crystal structure of PTB7-Th:PC71BM in a nonhalogenated system, which is the first such demonstration in blend films. This sheds light upon the fact that the physical properties of a material applied from different solutions may surpass the variation in the properties between two material having totally different molecular structure. Therefore, this work not only offers important scientific insights into developing highly efficient and eco-friendly OSCs but also improves our understanding of achievable bimolecular crystals with an intercalated structure.