Tuning Film Stresses for Open-Air Processing of Stable Metal Halide Perovskites

Abstract

Challenges to upscaling metal halide perovskites (MHPs) include mechanical film stresses that accelerate degradation, dominate at the module scale, and can lead to delamination or fracture. In this work, we demonstrate open-air blade coating of single-step coated perovskite as a scalable method to control residual film stress after processing and introduce beneficial compression in the thin film with the use of polymer additives such as gellan gum and corn starch. The optoelectronic properties of MHP films with compression are improved with higher photoluminescence yields. MHP film stability is significantly improved under compression, under humidity, heat, and thermal cycling. By measuring the evolution of film stresses, we demonstrate for the first time that stress relaxation occurs in MHP films with tensile stress that correlates with film degradation. This discovery of a new mechanism underpinning MHP degradation shows that film stress can be used as a parameter to screen MHP devices and modules for quality control before deployment as a design for reliability criterion.

Moisture Uptake Relaxes Stress in Metal Halide Perovskites at the Expense of Stability

“…Due to these characteristics, we are confident that the observed relaxation is not influenced by structural phase transitions or atomic rearrangement processes. Importantly, the relaxation observed here is unique to that reported recently by Ahmad et al in which stress irreversibly relaxed due to the decomposition of MAPbI 3 to nonperovskite phases (i.e., PbI 2 ) . No signs of structural degradation or phase transformations were detected with XRD during air exposure for 4 h (Figure S13).…”

“…Importantly, the relaxation observed here is unique to that reported recently by Ahmad et al in which stress irreversibly relaxed due to the decomposition of MAPbI 3 to nonperovskite phases (i.e., PbI 2 ). 44 No signs of structural degradation or phase transformations were detected with XRD during air exposure for 4 h (Figure S13). To probe the universality of stress and strain relaxation for perovskites in ambient conditions and its mechanism, we explored additional perovskite compositions including MAPbBr 3 and MAPbI 3 (Figure S14, Figure 2).…”

Scalable and Quench-Free Processing of Metal Halide Perovskites in Ambient Conditions

“…For MAPb(I 0.8 Br 0.2 ) 3 devices manufactured with gellan gum, they reported an efficiency of 16.37% with an open-circuit voltage of 1.104, short-circuit current density of 20.36 mA/cm 2 , and fill factor of 72.8% [21]. The addition of gellan gum has also been shown to induce an intrinsic compressive stress within the perovskite film, a property correlated with improved mechanical and environmental stability [22,23].…”

“…Rather than directly controlling solvent evaporation with an antisolvent, air gun, or the use of heat, gellan gum alters fluid dynamics and free energy considerations within the ink to tune the crystallization process by acting as a barrier to excessive homogenous and early heterogeneous nucleation through intermediate interactions and a change in wetting angle thanks to rheological modification. These factors stimulate the radial, spherulitic growth of similarly oriented domains that grow in a space-filling nature that combats the presence of pinholes while possibly collecting defects and impurities, including the additive, along grain boundaries, which may assist in relieving detrimental thermal or intrinsic stresses typically associated with the thermal expansion mismatch between the PVSK layer and substrate without inhibiting optoelectronic properties [12,22].…”