Mechanical Reliability of Next-Generation Photovoltaics (PV) and Batteries

Mechanical properties are often overlooked but excellent predictors for device resilience to environmental stressors that accelerate the evolution of internal defects and cause delamination and device failure in layered structures such as PV devices and batteries. Our goal is to study the fundamental connection between material degradation and mechanical/environmental stressors, together with their mechanistic origins.

Control of Ion Migration in Halide Perovskites

Ion migration accelerates defect evolution and degradation in perovskites. Although a plausible explanation for the stability challenges in halide perovskites, there is a lack of mechanistic understanding of the specific reactions and changes in material chemistry that occur in the perovskite from this phenomenon. By identifying the relevant reaction pathways, strategies will be designed to effectively mitigate or counter the effects that will subsequently lead to new scientific principles for improving stability in perovskites. Our goal is to study how dynamic processes such as photo-oxidation, carrier accumulation, chemical composition, and lattice strain influences ion migration.

Robust and Lightweight Thin-Film Solid State Batteries

A compelling opportunity for higher energy density batteries is solid-state electrolytes (SSEs), which offer a host of advantages over the liquid electrolytes that dominate the market today: they are leak-proof, energy-dense, flame-resistant, contain no toxic organic solvents, and can charge faster. A challenge to the commercialization of solid-state batteries is the development of a stable SSE that can support the film stresses that develop from significant expansion during cycling and can be processed with low-cost manufacturing processes. The objective of this work is to two-fold: to improve the thermomechanical reliability of SSEs and to subsequently produce safe, durable, and high-specific energy solid state batteries with a robust thin film SSE. The overarching questions that will be investigated are the mechanical properties (fracture behavior and film stress) that develop in SSEs for understanding of chemomechanical degradation modes and thin-film processing of ceramic-based SSEs.

Scaling/Manufacturing of Thin Film Energy Materials, Devices, and Modules

Atmospheric pressure or open-air plasmas can be directly integrated with in-line production. We use a blown arc discharge system that ionizes gas as it passes through the high-voltage region—which generates an arc with the grounded wall of the plasma system—and the resulting discharge is blown out of a nozzle and directed onto the substrate. The uniqueness of the plasma system is the combination of energy sources which are generated: reactive species (ions, radicals, metastables, and photons) are produced in combination with convective heat to rapidly transfer energy to enable ultrafast precursor conversion. Furthermore, the precursor (either in liquid or vapor form) can be directly injected into the afterglow of the plasma to provide additional energy for precursor dissociation or fragmentation. We have focused on using spray coating as a technique for liquid precursor delivery of numerous types of materials. We have shown that stoichiometry, composition, growth rate, density, and defectivity can be controlled through this process for chemistries such as silica, titania, tin oxide, and perovskites without additional annealing to form high-quality, scalable device layers. There is also a clear path for transparent conducting oxides, semiconducting charge transport layers, battery electrode materials, and even tandem module fabrication.

Design of Robust Perovskite/Silicon Tandem PV Devices

Monolithic perovskite/c-Si tandems are now approaching 30% efficiency. However, the most important challenge facing the perovskite PV research community is a lack of operational stability. Although there has been significant progress in recent years on accelerated stability tests and consensus protocols, perovskites must reach the 25+ year lifetimes of c-Si to be a viable tandem top cell. Furthermore, there is little stability data for perovskite/c-Si tandems and a resulting lack of insight into tandem-specific degradation modes. This goal of this project is to design perovskites for reliability by addressing the key photo- and thermomechanical effects that accelerate degradation in tandem architectures. These are the lack of photostability of higher bandgap perovskite compositions for optimized light harvesting on c-Si, the increase in film stress due to the large coefficient of thermal expansion (CTE) mismatch between perovskites and c-Si, and the lack of robust contact layers with good interfacial charge transport and adhesion.

In-Operando Measurements of Perovskite Film Stress and Efficiency

We recently developed a metrology to measure in-situ stress of thin films in a controlled environment while subjected to thermal, UV, and/or solar exposures using capacitance measurements of a cantilever, the deflection of which is mapped to a film stress. We showed that film stress can be precisely calculated by mapping this deflection if the elastic properties of the substrate and film/substrate thicknesses are known with a modified Stoney equation. The use of various perovskite chemistries with a range of thermal expansion and luminescent properties will be used to examine changes in stability when subjected to various combinations of environmental stressors—resulting in the diffusion of layers, migration of mobile ions, and egress of perovskite components—with the goal of determining how the evolution of film stress is affected by material dynamics. This will enable the first in operando measurements of device performance and film stress.