Sequential solvent processing with hole transport materials for improving efficiency of traditionally-structured perovskite solar cells

The solvent treatment of the perovskite surfaces has been popularly applied to enhance the device performance of planar perovskite solar cells (PSCs) and become the most useful method for fully solution-processed devices. However, the simple solvent engineering cannot address all interfacial contact problems in the traditionally-structured perovskite devices, limiting the applicability of solution processing. Here, we introduce a universal solvent engineering technology, termed sequential solvent processing with hole transport materials (HTMs), which can be used to fabricate high efficient traditionally-structured PSCs. Our new approach induces inter-diffusion between the perovskite and HTM layers, which improves their interfacial contacts, by dripping a chlorobenzene solution of the HTM onto a perovskite precursor before the perovskite crystallization is complete, thereby enabling the HTM to penetrate into the perovskite layer. Furthermore, the approach enhances the quality of the perovskite crystals, improves interfacial energy band alignment, and densifies the interface, which are advantageous for carrier extraction from the perovskite layer to the HTM. Our simple and effective strategy achieves power conversion efficiency (PCE) of 18.39% with 22.6% increment, compared to the control device (PCE = 15.00%). Overall, this approach is a significant improvement over the existing fabrication methods and proves to be an importantly new method for future research on PSCs.