Impact of Different Thermal Annealing Sequences on Nonfullerene Acceptor-Based Organic Solar Cells

Thermal annealing is one of the most commonly applied post-processing techniques to improve organic solar cell (OSC) performance. In this work, organic solar cells based on PBDB-T:IT-2F bulk heterojunctions were subjected to thermal annealing at different stages of the fabrication process to investigate their impact on the photovoltaic parameters. Various electro-optical measurement techniques were used to reveal the charge recombination dynamics and the charge extraction process. According to our findings, both a single-step postprocessing thermal annealing procedure and a two-step thermal annealing procedure improved the device fill factor (FF) and, thus, the power conversion efficiency (PCE), with the latter leading to a more substantial improvement. Additionally, the impact of thermal annealing on the hole transport layer (HTL) of MoOx was also investigated. The annealing altered the MoO_x’s work function, resulting in a larger internal electric field, thereby facilitating hole extraction, as demonstrated by transient photocurrent (TPC) measurements. Devices subjected to a two-step annealing procedure exhibited a faster carrier extraction rate, leading to improved FFs.