Facile Exfoliation of the Perovskite Thin Film for Visualizing the Buried Interfaces in Perovskite Solar Cells

The morphology and structure of buried interfaces play decisive roles in determining the efficiency and stability of perovskite solar cells (PSCs). However, challenges remain in directly visualizing the buried interfaces by existing nondestructive techniques. Here, we have developed a facile technique to integrally exfoliate the perovskite film (up to an area of 25 cm2) from both flexible and rigid glass substrates by introducing a cross-linkable polymer capping layer on the devices. This simple exfoliation approach enables comprehensive characterization of the influences of bottom electrode materials on the perovskite buried interfaces. This method has also been extended to fabricate the cross-section samples for flexible solar cells. Our results show that the roughness and wettability of substrate materials significantly influence the effectiveness and efficiency of film exfoliation, which could pose great impacts on the reliability in understanding the interfacial properties. We also evaluated the residuals on both sides of the substrate and the perovskite bottom, discovering that a clear and complete perovskite separation can be obtained for SnO2/perovskite interfaces. However, cross-contamination occuring at poly[bis (4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA)/perovskite interfaces considerably restricts the reliable examination of the buried interfaces. Poly[(9,9-bis(3′-((N,Ndimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] dibromide as an interlayer between PTAA and perovskite was found to be favorable in obtaining clean and residual-free exfoliated interfaces. The developed exfoliation technique provides a facile and nondestructive way for directly visualizing the buried interfaces in PSCs, and this work will generate some insights in understanding the buried interfaces in optoelectronic devices.