Optimizing domain size and phase purity in all-polymer solar cells by solution ordered aggregation and confinement effect of the acceptor

Abstract

Domain size, phase purity, and the interpenetrating network within the active layer of all-polymer solar cells (all-PSCs) are crucial for efficient charge generation and carrier transport. However, it is a great challenge to decrease domain size and enhance phase purity simultaneously because of the energetically disfavoring polymer–polymer mixing and chain entanglement. In this work, we manipulated the domain size and phase purity of J51:N2200 blends by promoting their solution ordered aggregation and the confinement of acceptor N2200 to J51 during phase separation. Thus, three solvents, chloroform (CF), mesitylene (Mes), and cyclopentyl methyl ether (CPME), were selected. The solubility of J51 and N2200 in these three solvents decreases and solubility differences between J51 and N2200 increase gradually. Among these three solvents, only in CPME solution does N2200 possess ordered structures, which reduces the nucleation barrier to increase nucleation density and boost the template effect of N2200. During phase separation, the ordered aggregation of N2200 dominates solid–liquid phase separation and has a confinement effect of J51. Thus, the blend films cast from CPME have fine-scale phase separation in contrast to the films derived from CF. In addition, the “memory” effect of ordered aggregations transferred to films can enforce the order of blend films. As a result, the blend film with small domain size (≈21 nm), interpenetrating network structure, and a higher degree of crystallinity was obtained by processing using the green solvent CPME. The improved morphology facilitated the charge-generating process and carrier transport, resulting in a higher short-circuit current (J_sc), fill factor (FF), and power conversion efficiency (PCE).