Visualization of 3D to quasi 2D conversion of perovskite thin films via in situ photoluminescence measurement: a facile route to design a graded energy landscape.

2D-perovskites are generally more stable than 3D perovskites while charge transport in 2D-perovskites becomes inefficient. On the other hand, the instability of 3D perovskite films under heat, light and environmental conditions makes them inapplicable for practical purposes. Therefore, quasi-2D perovskites could be the optimum solution for stable yet highly efficient devices. Using the post-fabrication treatment method, we have converted methylammonium lead tribromide (MAPbBr 3 ) 3D perovskite films into a quasi 2D-perovskite interfacial layer. In situ photoluminescence measurement during spin coating indicates a rapid conversion of 3D-perovskite into 2D-perovskites. The kinetics of oxygen and moisture diffusion, ion diffusion and electronic charge transport can be estimated from the time dependent PL measurements in the 3D and 2D/3D perovskite samples. 2D terminated perovskite samples show enhanced photoluminescence and improved stability in moisture and UV-irradiation. We also propose that a relatively wide bandgap of 2D-perovskite can give rise to a graded energy landscape at the interface for favorable charge separation. Simulation results reveal that the power conversion efficiency can be improved from 2.83% to 4.02% due to an increase in open-circuit voltage and fill factor in 2D/3D based MAPbBr 3 solar cells without using any electron transport layer.