Correlating three-dimensional morphology with function in PBDB-T:IT-M non-fullerene organic solar cells

In this work, the correlation between three‐dimensional morphology and device performance of PBDB‐T:IT‐M non‐fullerene organic solar cells is investigated. We found that a PBDB‐T‐rich top layer formed when the PBDB‐T:IT‐M film is cast on PEDOT:PSS, indicating a vertical component distribution that will hinder electron transport toward the cathode in a conventional device. This PBDB‐T‐rich top layer remained upon low‐temperature annealing at 80 °C, but disappeared when the annealing temperature is raised, resulting in an optimum annealing temperature of 160 °C for conventional devices as the removal of this polymer‐rich layer facilitates electron transport toward the top cathode layer. The PBDB‐T‐rich layer is also found in the surface region of the PBDB‐T:IT‐M film cast on a TiO2 substrate, but in this case it remained after thermal annealing at 80 or 160 °C, leading to a favorable vertical stratification for efficient charge collection in inverted devices. Although thermal annealing can enhance the crystallinity of PBDB‐T:IT‐M blend and lead to improved charge mobility, the correlation length of the PBDB‐T component increased excessively under annealing at 160 °C. Further, the packing of IT‐M clusters became loose upon high temperature annealing, an effect we believe results in more diffuse interfaces with PBDB‐T that result in reduced charge separation efficiency, consequently reducing the short‐circuit current in the inverted devices.