(508a) Thermomechanical Reliability of Halide Perovskite Semiconductors

Organic-inorganic metal halide perovskite semiconductors have demonstrated real promise as a next-generation thin film photovoltaic technology based on unprecedented gains in device efficiencies and electronic tunability; however, the lack of operational stability remains a key drawback. 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.

Adhesive or cohesive failure that leads to device degradation and lack of operation may result from either residual or applied mechanical or environmental stresses introduced during device processing, handling, packaging, and operation. Mechanical stresses provide the driving force for damage and can vary markedly with device application. The mechanical driving force for failure processes like film cracking or interface debonding can be quantified in terms of a strain energy release rate, G (J/m²)—which is a function of the aforementioned mechanical stresses and the film thickness and modulus—and defined by the following relation: G = Zσ²h/E

where Z is a dimensionless value based on the geometry-dependent cracking configuration, σ is the tensile film stress (σ > 0 is defined as tensile and σ < 0 is defined as compressive), h is the film thickness, and E is the elastic modulus of the film. Fracture occurs when G meets or exceeds the critical value of the strain energy release rate for a material, G_c (G ≥ G_c), where G_c is also known as fracture energy since it involves the energy needed to cause bond breakage—related to bond type and density—and by other energy dissipation processes such as molecular deformation and flow. Measuring G_c does not require any information about the mechanical properties or stresses in the thin film layers, which are often unknown^[15,16] . G_c is a key metric of thermomechanical reliability, yet it is not a well-understood quantity.

Regarding thermomechanical reliability, we have studied the G_c of perovskite PV devices processed from solution using a variety of perovskite compositions, device architectures, and charge transport layers. We discovered that perovskites are the most mechanically fragile—along with being the most operationally unstable—class of PV devices ever tested in comparison to organic and silicon PV (Figure 1). We modeled the film stress that develops in perovskites and found that the large difference in thermal expansion coefficient between the perovskite and substrate accounts for the lattice strain that develops during processing, as film stress was linearly correlated with annealing temperature.

Since the mechanical driving force for damage scales as σ² (Figure 1, inset) the fragility of perovskite materials accelerates degradation processes. In the absence of environmental stresses, crack growth through device layers does not occur below G_c. Real-world operational conditions are far from equilibrium, however, and solar cells are especially likely to endure environments where conditions are subject to rapid change. For example, the synergistic effect of UV photon energy and flux along with operational conditions of elevated heat and humidity have been shown to affect the kinetics of interface debonding in organic PV materials that are more operationally stable and mechanically robust than perovskites. The fundamental connection between material degradation and mechanical/environmental stressors, together with their mechanistic origins, remains largely uncharacterized in perovskite solar cells. This work focuses on discussing the effects of mechanical and environmental stress on perovskite reliability. The goal of this work is to discuss the key mechanisms that link mechanical properties with operational stability and design criteria for device reliability to reduce rates of degradation. Various thin-film metrologies are used to characterize mechanical properties that comprise both well-established techniques—such as double cantilever beam fracture testing, with newly developed and custom-built equipment—such as in situ film stress measurements.