Understanding the mechanism of PEDOT: PSS modification via solvent on the morphology of perovskite films for efficient solar cells

The properties of hole transport layers (HTL) greatly affect the performance of inverted planar heterojunction perovskite solar cells (PSC). To understand the mechanisms involved, solvent treatments to HTL poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) by DMSO, ethanol, ethanol:10 vol% DMSO, DMF and acetonitrile were performed when fabricating the PSC devices. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results show that the crystal size of CH₃NH₃PbI₃ (MAPbI₃) increased after solvent treatment to PEDOT: PSS. MAPbI₃ film with a uniform crystal size and smooth surface was obtained when being deposited on top of PEDOT: PSS that has been modified with solvent with a strong coordination ability. In the other cases, crystal size homogeneity and surface roughness of the MAPbI₃ film was worse. The improved surface roughness and crystal size homogeneity of the MAPbI₃ film enhanced the short circuit current (J_SC), and therefore the power conversion efficiency (PCE) of PSC. It was found that both the stability of the intermediate phase MAI-DMSO-PbI₂, which is related to the coordination ability of the solvent molecules, and the morphology of PEDOT: PSS film, influenced the morphology of the CH₃NH₃PbI₃ (MAPbI₃) film. Our findings provide the basis for better controlling the morphology of MAPbI₃ films in inverted planar heterojunction PSC.