DYNAMIC ELECTRICAL RESPONSE AT THE NANOSCALE IN METAL HALIDE PEROVSKITES

Abstract

Photovoltaic (PV) technology holds a promise that can change the energy dynamics globally. Next-generation materials, with the focus on two factors: high efficiency and economic feasibility, are being extensively explored to enable the widespread implementation of solar cells. Metal halide perovskites (MHPs) provide the ideal combination of both these pressing factors, and also fall under thin-film technology making their applications increasingly ubiquitous. In little over a decade, perovskites have reached a power conversion efficiencies of > 25%. Despite the prodigious advancements in efficiency, device and material instabilities have precluded their commercialization. While the origin of instabilities is multifaceted, instability under environmental factors (light, humidity, oxygen, temperature) is a central hub. Therefore, efforts are being directed toward understanding the behavior of photovoltaic properties under environmental conditions. Investigation at the material level is necessary to develop optimization strategies. My dissertation focuses on the electrical dynamics at length scales of grains and grain boundaries in MHP thin films.

In the first part of my thesis, I present a comprehensive electrical analysis by probing surface voltage and photocurrent on Cs-containing dual-cation and Rb-containing quad-cation perovskite thin films. I measure surface voltage response using Kelvin probe force microscopy (KPFM) and map photocurrent via photoconductive atomic force microscopy (pc-AFM) under an inert environment. The Dark KPFM voltage maps indicate upward band bending at the grain boundaries for both chemical compositions. Using an illumination cycle (OFF-ON-OFF), I find a 55% larger post-illumination residual voltage drop in quad-cation perovskite. Photocurrent maps reveal highly photo-active grain boundaries in the quad-cation, while photo-inactivity is observed at grain boundaries in dual-cation perovskite. With the integrated knowledge about the upward band bending from KPFM and the electrical nature of the grain boundaries in the two chemical compositions, I infer defect passivation at the grain boundaries due to Rb+ cations and defect-assisted recombination at the grain boundaries of dual-cation perovskites. The highly conductive grain boundary network seen in quad-cation perovskite increases the overall photocurrent by 50%. The second part of my thesis demonstrates, for the first time, the ability of in situ humidity-dependent KPFM measurements to capture localized moisture-induced electrical dynamics in MHPs. I perform a controlled humidity cycle from 5 - 65% rH and back down from 65 - 5% rH. I observe an enhanced voltage response up to 45% relative humidity and an electrical failure at 65% rH. I capture a self-recovery value of over 90% post-humidity cycle and a recovery value of 99% 24 hours post-humidity cycle. Using XPS and PL before and after the humidity cycle, I confirm moisture-induced structural and chemical changes at the surface of the perovskite which are interconnected to the unstable electrical behavior seen during the humidity cycle. My comprehensive analytical approach on KPFM, and pc-AFM together with my in situ results showcase powerful methods for perovskite stability investigations.