Blended solvent for tuning vertical phase separation in layer-by-layer processed thick-film organic solar cells

Organic solar cells (OSCs) with a vertically phase-separated active layer are crucial for achieving high power conversion efficiency (PCE). However, obtaining such morphology remains a significant challenge, particularly in thick-film devices. To address this challenge, we develop a blended solvent strategy aimed at regulating the downward penetration of acceptor molecules into the underlying donor layer in layer-by-layer (LBL) solution-processed OSCs. By combining a good solvent with a poor solvent and precisely adjusting the ratio of a fast-volatilizing good solvent (chloroform) to a slow-volatilizing poor solvent (o-xylene), we have finely tuned the spatial distribution of acceptor molecules in the active layer. This has successfully resulted in a vertically phase-separated structure and enhanced crystallinity of the acceptor phase, which is conducive to exciton diffusion, exciton dissociation, and charge transport. We have verified the existence of such morphology through film depth-dependent light absorption spectroscopy and time-of-flight secondary ion mass spectrometry analysis. The PCE of D18/BTP-eC9-4F-based OSCs prepared using this strategy has shown significant improvement, with the PCE of devices with a 100-nanometer-thick active layer increasing from 18.77% to 19.36%. Notably, when employing this strategy to prepare OSCs with a 300-nanometer-thick active layer, an impressive PCE value of 18.06% was achieved, marking it as the highest-performing thick-film binary organic solar cell reported thus far.