Wanyi Nie1 Hsinhan Tsai1 2 Jean-Christophe Blancon1 Jacky Even3 Pulickel Ajayan2 Muhammad Alam4 Mercouri Kanatzidis5 Aditya Mohite1

1, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
2, Rice University, Houston, Texas, United States
3, Centre National de la Recherche Scientifique (CNRS), Rennes, , France
4, Purdue University, West Lafayette, Indiana, United States
5, Northwestern University, Evanston, Illinois, United States

The structure of a material, light and electrical field are fundamental ingredients for any opto-electronic semiconducting device. In state-of-the-art semiconductors like Silicon and Gallium Arsenide, the crystal structural parameters such as bond length, crystallinity, vacancy and strains are invariant to external stimuli such as electromagnetic radiation and electric fields. However, in sharp contrast, hybrid perovskites exhibit a strong propensity for undergoing structural modifications with light and electric fields. While this raise challenge for elucidating the exact mechanisms of device operation but also offer now opportunity to discover new functionalities. Therefore, a basic principle on the interplay between structure, light and electrical field with in-situ correlated measurement is critical for understanding optoelectronic transport and determine the design principles for operation of perovskite based devices.
In my talk, using correlated in-situ structure and transport measurements, I will focus on understanding these complex effects arising from the interaction between structure, light and field during perovskite cell operation in both 3D and 2D systems. Briefly, in a 3D perovskite system, we discover that continuous light illumination leads to a uniform lattice expansion in hybrid perovskite thin-films, which is critical for obtaining high-efficiency photovoltaic devices. Measurements reveal that light-induced lattice expansion significantly benefits the performance of a mixed-cation pure-halide planar device, boosting the power conversion efficiency from 18.5% to 20.5%. This is a direct consequence of lattice strain relaxation and increase in the crystallite size that dramatically suppresses the interface non-radiative recombination, resulting in enhanced photovoltage and photocurrent collection near low field. In 2D solution-processed quantum wells, on the other hand, optical field generates coulomb bound electron-hole pairs due to the unique low dimension structure of the materials. Furthermore, the multi-stacked quantum wells present potential barriers that block the efficient separation of electron-hole pairs. Both of those properties are unfavorable for photovoltaic cell operation. Here we elucidate the critical role of field-assisted charge carrier separation that overcomes these bottlenecks leading to the efficient photocurrent collection.
As a summary, our studies demonstrate the key factors that should be accounted for in the design of optoelectronic devices using hybrid perovskite materials in both 3D and 2D systems.