Effect of solvent-induced phase separation on performance of carboxylic indoline-based small-molecule organic solar cells

In this study, we investigate a carboxyl indoline dye named D205 with deep highest occupied molecular orbital energy level of -5.4 eV as donor material together with [6,6] -phenyl C-71 butyric acid methyl ester (PC70BM) as acceptor material in solution-processed bulk heterojunction solar cells. We employ chloroform (CF) and chlorobenzene (CB) as different solvents to investigate the influence of which on the active layer phase separation. The morphologies of D205:PC70BM with CB as solvent seem to be stable as the ratio of PC70BM increasing while it shows obvious change when processed by CF. The root-mean-square (RMS) values processed in CB change from 1.2 nm (D205:PC70BM = 1:2) to 0.8 nm (D205:PC70BM = 1:19). While the RMS values processed in CF change from 1.1 nm (D205:PC70BM = 1:2) to 13 nm (D205:PC70BM = 1:19). This may originate from that the rapid volatilization of CF solvent, leading to the aggregation behavior of the blending layer especially the aggregates of PC70BM. A proper RMS value leads to a better phase separation and thus can prompt balanced holes and electrons transportation. This can eventually benefit the short-circuit current. By regulating the ratio of D205:PC70BM as 1:4 in CB, an optimized power conversion efficiency(PCE) of 3.0% was reached after device annealing at 90 degrees C for 1 min under standard AM 1.5100 mW/cm(2) sunlight.