Thickness insensitive organic solar cells with high figure-of-merit-X enabled by simultaneous D/A interpenetration and stratification

Abstract

Low cost and printing friendly fabrication of organic solar cells (OSCs) require thick-film devices with simply structured photoactive molecules. Thus, achieving high power conversion efficiency (PCE) for non-fused ring acceptor-based devices with high thickness is of great significance. Herein, by transforming traditional blend casting method to emerging sequential deposition (SD) method, D18:A4T-16 active blend exhibits large efficiency improvement from 8.02% to 14.75% in 300 nm thick devices. Systematic morphological and photophysical characterizations showcase the effectiveness of SD processing in achieving sufficient donor/acceptor interpenetration and vertical stratification, which eliminates the dilemma of charge generation/transport in blend casting films. Meanwhile, D18 bottom layer is proven helpful in realizing fast evaporation of postdeposited poor solvent, resulting in naturally thickened active layer with well-regulated crystallization. Furthermore, a new index to emphasize thick-film devices based on nonfused ring acceptors, called figure-of-merit-X (FoM-X), has been defined. The SD processed D18:A4T-16 devices herein, with 300 nm, 500 nm, and 800 nm thicknesses possess leading FoM-X values.