An Operando Study on the Photostability of Nonfullerene Organic Solar Cells

Nonfullerene organic solar cells (OSCs) have achieved an impressive power conversion efficiency (PCE) over the past few years, showing a great potential for real applications. However, the study on the photostability and degradation mechanism of nonfullerene OSCs is far behind than that of fullerene-based solar cells, which is crucial for the commercial applications of the technology. Herein, an efficient and stable nonfullerene OSC based on PCE10:rhodanine-benzothiadiazole-coupled indacenodithiophene with branched 2-ethylhexyl side chains (EH-IDT) is fabricated from environmentally benign solvent. The PCE10:EH-IDT solar cell shows a high PCE of 9.17% and a long operational lifetime (T-80) of 2132 h, compared with other two OSCs based on 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']-dithiophene (ITIC) and another fuse ring acceptor with withdrawing units of 1,1-dicyanomethylene-3-indanone and hexyl side chains (IDIC) nonfullerene acceptors, with tested lifetimes of only 221 and 558 h, respectively. As indicated by the Flory-Huggins interaction parameters, ITIC and IDIC have poor miscibility with PCE10, which leads to morphology degradation, suppressed charge generation, increased trap states, and charge recombination in the photoaging test, which accounts for the significant loss of short-circuit current density and fill factor during operation. The improved miscibility of the donor and the acceptor results in a more stable morphology, and the PCE10:EH-IDT solar cells thus achieve an outstanding overall performance that combines high efficiency and superior photostability and paves the way for the potential practical applications of OSCs.