In Situ Film Stress Measurements Capture Increased Photostability in Additive Engineered Wide Bandgap Perovskites

Metal halide perovskites are the fastest growing contender in the next-generation solar cell materials race because of their attractive optoelectronic properties and low-cost solution processed fabrication. Their strongly absorbing nature, long diffusion length, high carrier mobility, and tunable bandgap properties make them a fitting top cell candidate in tandem with silicon technologies. Currently, the record for a monolithic perovskite-silicon tandem is 33.9% and has the potential to reach up to 40%. However, long-term environmental stability continues to stand in the way of commercialization.

Perovskite absorber layers are susceptible to mechanical instabilities, which manifest in the form of biaxial tensile stresses on the thin film that induce driving forces for damage. These stresses scale up with the active area and can result in failure through cracking or delamination at the device scale. To quantify stress, we use various techniques including the following: X-ray diffraction, laser-based curvature, and a custom-built system that records in-situ capacitance corresponding to changes in film curvature with rapid time-resolution (1 millisecond) and can record values in response to a controlled external stimulus such as light (or heat or moisture). All these capabilities along with hyperspectral photoluminescence under controlled external stimuli have enabled us to track both phase segregation and degradation using stress measurements of the perovskite film.

In our recent work, we demonstrated the ability to control film stresses in MAPbI₃ perovskites to a desirable compressive value using biopolymers such as gellan gum and corn starch. Our deposition is performed as a one-step, quench-free process using blade coating in open-air to help realize larger area tandem-ready configurations. The additives assisted with a balanced nucleation and growth regime as well as passivation of grain boundary defects to lower the intrinsic stresses, also showing an improved film stability during thermal cycling from -40 to 85°C.

Our goal in this work to apply this concept to mixed halide perovskites such as MAPb(I_0.8Br_0.2)₃ and other state-of-the-art wide-bandgap perovskite compositions which are additionally vulnerable to light-induced halide segregation (LIHS). In this work, we use corn starch and polyvinylpyrrolidone biopolymers which we hypothesize passivate halide vacancies and limit LIHS. We show a 20 MPa change in film stresses when mixed halide samples are exposed to 1-sun illumination the in-situ capacitance measurement which was alsao confirmed by XRD sin²Ψ. This change is partially reversible and decreases significantly to 5 MPa with the inclusion of polymer additives, while hyperspectral photoluminescence mapping of the perovskite sample also shows the presence of a single phase throughout these films.

We have shown a correlation for the first time between LIHS and a mechanical-based mechanism in wide bandgap perovskites based on corresponding changes in film stress. By tracking the changes in-situ as well as on larger substrates, we believe that our platform is a tool to validate both the mechanical and photo-stability of our samples. We are also able to control both the reversible and irreversible LIHS with the help of polymer additives. There is evidence of a new mechanism suggesting stress generation due to halide segregation. In current work, we are studying LIHS for Cs_0.22(FA_0.8MA_0.2)_0.78Pb(I_0.83Br_0.14Cl_0.03)₃ triple halide compositions as a function of process parameters and the MACl additive to establish a link between the rate of change in stress, phase segregation and spatial mapping of the optical properties of the perovskite film.

M. Ahmad et al., “Tuning Film Stresses for Open-Air Processing of Stable Metal Halide Perovskites,” ACS Appl Mater Interfaces, vol. 15, no. 44, pp. 51117–51125, Nov. 2023, doi: 10.1021/acsami.3c11151.