2020 Forgash Scholar
Cation Migration in Mixed Cation Lead Halide Perovskites Thin Films and Solar Cells
Department of Chemistry and Biochemistry
Faculty Advisor: Masaru (Ken) Kuno
Mixed cation and anion APbX3-type lead halide perovskites (A - cation; A=Cs, CH3NH3 [MA], or (NH2)2CH [FA]; X-anion; X=Br or I) have received extensive attention for use as cheap, efficient light harvesters in solar cells. However, issues regarding their long term stability represent an unsolved problems which ultimately prevents their commercialization. While most stability studies focus on external effects such moisture and heat, electric field induced ion migration represent an intrinsic instability of these solar cells, which is relatively unstudied. Ion migration induces current-voltage hysteresis, perovskite phase deterioration and overall attrition of solar cell performance. Most investigations into ion focus on anion (halide) migration and phase segregation. Much less attention has been focused on understanding cation migration. My proposed work will focus on exploring, understanding, and suppressing electric-field induced cation migration in high quality FAxMAyCs1-x-yPbI3 (where x,y, and 1-x-y is final product stoichiometry) perovskite thin films and solar cells.
Bias-induced cation migration in FAxMAyCs1-x-yPbI3 will be studied in both lateral devices (thin films) and ~16-20% efficient solar cells. Cation movement will be probed via a number of material composition analyses. The most important one I will use is a unique super-resolution infrared imaging setup (Infrared Photothermal Heterodyne Imaging, IR-PHI) capable of obtaining high-resolution infrared images and spectra. The instrument was built in the Kuno laboratory. IRPHI will provide information about the local infrared absorption of perovskite absorbers and will ultimately give access to local chemical composition. To test and corroborate obtained results, we will use the following instrumentation: (a) time-of-flight secondary ion mass spectroscopy (TOF-SIMS) via collaboration with the National Energy Renewal Laboratory (NREL), (b) scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDXS), and (c) wavelength dispersive spectroscopy (WDS). Through the subsequent analyses of electric-field induced cation migration using above mentioned techniques we will develop design rules for how to suppress cation migration in these absorbers. This knowledge should lead to increased lifetime of the perovskite solar cells, by overcoming one of their intrinsic instabilities.