Imaging Buried Interfaces of Metal-Halide Perovskites Towards Flexible Photovoltaics

Photovoltaics made with 3D organic-inorganic halide perovskite (3D-OIHP) thin films lead a burgeoning field of research into flexible solar cells due to their demonstrated ability to enable flexible and lightweight designs while maintaining competitive power-conversion efficiency and stability. Despite their promise, 3D-OIHP thin film-based flexible devices suffer from inherent defects originating from manufacturing and delamination of their interfaces due to wear and tear from common usage that decrease their performance over time. Recent efforts in interfacial engineering have brought about significant improvements in solar cell reliability, but techniques for the large-scale characterization of defects and delamination between functional layers of OIHP devices have yet to be explored. Standard techniques for imaging the delamination between material interfaces, such as cross-sectional FIB/SEM, have limited throughput and necessitate the deposition of additional material and destructive processing, rendering measured samples inoperable while providing only site-specific information. Here, we demonstrate a vibrational-pump visible-probe microscopy technique to optically probe the presence of buried interfaces in flexible MAPbI₃ thin films and multilayer structures. By exciting the N-H stretching mode present in the methylammonium cations with a mid-infrared pump and probing the transient response of the perovskite’s temperature-dependent bandgap in the visible region, we exploit the localized heat transfer towards the substrate to image delamination between the buried interfaces. This technique allows for the nondestructive imaging of buried interfaces and can provide insight into the effects of interfacial contact on device performance, serving as a useful tool for the interfacial engineering and eventual commercialization of high-performance 3D-OIHP-based flexible photovoltaics and optoelectronic devices.