Charge Generation and Recombination Characteristics in D18:PY-IT All-Polymer Solar Cells

To gain insight into the characteristics of charge generation and recombination in all-polymer solar cells (all-PSCs), a binary all-PSCs photovoltaic system, D18:PY-IT, was chosen as the research subject to understand the intrinsic relationship among the morphology, photoelectric conversion, and device performance. For ease of comparison studies, the morphology of D18:PY-IT was treated with and without thermal annealing. The morphology (including phase scale and phase separation) of D18:PY-IT was refined after thermal annealing. Additionally, the polymer cation and anion were characterized with spectroelectrochemistry (SEC) to assist identification of the transient charged species. According to SEC and transient absorption spectroscopy, the photoelectric conversion process produced two distinct forms of D18• + polarons: delocalized polaron (DP, absorbing over 600–950 nm) and localized polaron (LP, 750–1100 nm). The findings revealed two charge loss pathways in unannealed and annealed D18:PY-IT solar cells; in detail, the former was mostly governed by geminate recombination, while the latter was determined by nongeminate recombination. Furthermore, the annealed all-PSCs had a more fibrous-like structure, which facilitated charge transfer and charge transport. As a result, the photoelectric conversion efficiency of the annealed all-PSCs had been improved.