Unraveling the role of crystallization kinetics for fibrillar morphology optimization in all-polymer solar cells

Abstract

All-polymer solar cells (all-PSCs) offer superior mechanical durability and operational stability but still lag behind small-molecule-acceptor-based counterparts in efficiency due to challenges in controlling fibrillar morphology in active layers. Achieving such structures requires precise regulation of polymer ordering and phase separation, which remains poorly understood. Here, we address this knowledge gap by employing in situ grazing incidence wide angle X-ray scattering (GIWAXS) and grazing incidence small angle X-ray scattering (GISAXS) to track the morphological evolutions of slot-die-coated PBTA-BO:N2200 blends. By adding solvent additives, diphenyl ether (DPE) and 1,8-diiodooctane (DIO), we studied how the crystallization kinetics during solution drying affect the phase separation developments and the final thin film morphology. In particular, DIO, with its ultrahigh boiling point and poor polymer solubility, slows polymer crystallization, enhances local ordering, and constrains large-scale chain diffusion. Such a controlled process favors the growth of high-quality crystallites within a uniform, intermixed fibrillar network, in contrast to the coarse, fragmented aggregates observed in as-cast films. As a result, DIO-processed all-PSC devices exhibited enhanced charge mobility, optimized exciton dissociation, and reduced trap states, leading to significantly improved power conversion efficiencies. Our study establishes crystallization kinetics as a key lever for fibrillar morphology optimization, providing a feasible pathway toward high-efficiency all-PSCs.