The role of solvent vapor annealing in highly efficient air-processed small molecule solar cells
We demonstrate highly-efficient, solution-processed small molecule solar cells with the best power conversion efficiency (PCE) of more than 5%. The active layer consists of a diketopyrrolopyrrole-based donor molecule (DPP(TBFu)2) and a fullerene derivative (PC71BM) that is spin cast and subsequently treated with solvent vapor annealing (SVA) in air. We find not all solvent vapors lead to the best PCE. Solvents of high vapor pressures and medium donor solubilities, such as tetrahydrofuran or carbon disulfide, are most suitable for SVA in the context of organic solar cell application. On the other hand, acceptor solubility plays an insignificant role in such a treatment. An active layer treated with ideal solvent vapors develops desirable phase separation in both lateral and vertical directions, as revealed by AFM, TEM and TEM tomography. The SVA also leads to enhanced hole mobility. We believe the fast SVA treatment performed in air is a viable way to tune the active layer morphology for printed solar cells.