A two-step strategy to clarify the roles of a solution processed PFN interfacial layer in highly efficient polymer solar cells

Incorporation of a solution processed PFN interfacial layer has been demonstrated as an efficient method to improve the performance of polymer solar cells (PSCs). The champion power conversion efficiency (PCE) of PSCs with PTB7-Th:PC71BM as the active layer was increased from 6.13% to 7.72% by directly spin-coating PFN methanol solution on the surface of active layers or to 8.50% for the active layer with 4 min PFN methanol solution soaking. The champion PCE of PSCs was further increased to 8.69% for the active layers with a two-step treatment, 4 min methanol soaking and then directly spin-coating PFN methanol solution. A 12.6% PCE improvement was obtained by using the two-step strategy compared with directly spin-coating PFN methanol solution on the active layers. Methanol soaking of the active layers plays the key role in forming the more ideal vertical phase separation for efficient exciton dissociation, charge carrier transport and collection. An ultrathin PFN interfacial dipole layer can be obtained by directly spin-coating PFN methanol solution. The two-step strategy may provide a simple and effective method to finely optimize the phase separation and obtain an ultrathin PFN interfacial dipole layer for the performance improvement of PSCs.