Role of crown ether in the perovskite precursor for doctor-bladed perovskite solar cells: investigation by liquid-phase scanning electron microscopy

Deposition of a large-area metal halide perovskite (MHP) film for perovskite solar cells (PSCs) under ambient conditions is highly required for their commercial applications. In this study, we fabricate mixed-cation and mixed-halide perovskite films by doctor-blade coating with a nontoxic solvent system under ambient conditions (a relative humidity of 40-45%). A crystallization controlling agent 18-crown-6 (18C6) is further added to the multiple-component MHP to improve the film quality. Liquid-phase scanning electron microscopy (L-SEM) is conducted to investigate the effects of 18C6 on the MHP precursor and the doctor-bladed MHP film. The L-SEM image of the pristine MHP precursor reveals self-assembly of micelles, while the addition of 18C6 effectively suppresses the size of the MHP-based micelles in the MHP precursor. Strong coordination between 18C6 and metal cations (such as Cs⁺ and Pb²⁺) prevents the MHP-based micelles from segregation, reducing the micellar size from the μm level (without 18C6) to the sub-μm level (with 18C6). This effect could result in uniform-sized micelles in the precursor and a reduced crystal growth rate to achieve a high-quality MHP film with preferred crystallinity, an enhanced MHP domain size, and a smooth surface. The agent 18C6 in PSCs significantly improves the power conversion efficiency (PCE) from 7.8% (without 18C6) to 14.7% (with 18C6).