Charge Generation\nand Recombination Characteristics\nin D18:PY-IT All-Polymer Solar Cells

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