Controlling stress and strain in perovskite solar cells to enhance efficiency and stability
Abstract
We plan to fabricate exceptionally stable metal halide perovskite tandem solar cells with greater than 30% efficiency on flexible substrates. These tandems could be gamechangers for a wide variety of military applications that require lightweight and flexible solar cells at affordable costs. We are leveraging other research grants that have the primary goal of developing the high and low bandgap semiconductors along with the appropriate contacts for achieving high efficiency. In this project we will develop a deeper understanding of how mechanical stress in perovskite semiconductors affects their long-term stability in solar cells. We will also understand the factors that determine the density of halide vacancies and their mobility in perovskite films since the diffusion of halides plays a major role in degradation. We are testing the hypothesis that mechanical strain might influence the halogen vacancy density. The thermal expansion coefficient of metal halide perovskites is approximately 10 times higher than that of glass or silicon. With most of the perovskite deposition methods, the perovskite crystallizes and attaches to the substrate at temperatures in the range of 120-160°C. As the films cool back to room temperature, they would like to shrink but are unable to do so because they are attached to a substrate with a much lower thermal expansion coefficient. Consequently > 50 MPa of residual tensile stress exists in most perovskite films. Studies have shown that perovskite films degrade faster at elevated temperatures and under light due to this tensile stress. One set of goals will be to reduce the tensile stress and possibly even create compressive stress in perovskite films by making the films on substrates with thermal expansion coefficients that are closer to those of perovskites, searching for perovskites with lower thermal expansion coefficients, developing methods for growing the perovskites at lower temperatures and other techniques described in the proposal. We have preliminary data which shows that the degradation rate of perovskite films does not increase exponentially with temperature because the tensile stress is relieved as the temperature increases. We plan to measure the rate of degradation as a function of temperature for a wide variety of perovskites and develop models that will make it possible to forecast the long-term stability of perovskites after performing accelerated lifetime testing at elevated temperatures. Another goal is to understand precisely why tensile stress influences the rate of thermal and photodegradation. We will measure the density of halide vacancies as a function of stress in the film to see if the stress influences the degradation process by increasing the rate of halide diffusion. We will also measure the rate of diffusion in perovskite films and see how it depends on the grain size, stress, composition, and the presence of additives at grain boundaries
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 20, 2020
- Source ID
- N000142012573
Entities
People
- Michael McGehee
Organizations
- Office of Naval Research
- Regents of the University of Colorado
- United States Navy