How The Dynamics of Attachment to The Substrate Influence Stress in Metal Halide Perovskites

Metal halide perovskites, hereon referred to perovskites for simplicity, are a promising class of semiconductors eagerly researched for their use in photovoltaic and optoelectronic applications. Despite low fabrication cost and superb initial efficiencies, improvements to the operational stability of perovskites would aid extensive deployment in both terrestrial and space applications. Mechanical stress is an important, but often misunderstood factor impacting chemical and structural degradation as well as reliability during extreme temperature cycling. Tension has been linked to accelerated formation of undesirable PbI₂ which has been attributed to decreased activation energy for ion migration.^[1,2] In addition, the brittle nature of polycrystalline metal halide perovskite films leaves them susceptible to fracture and delamination.^[3] Therefore, it is crucial to accurately identify and reduce sources of tension that arise from film formation processes and in response to environmental exposure.<br/><br/>The formation of perovskite thin films using solution processing methods usually includes a thermal annealing step to fully convert the film from intermediate to perovskite phases. The field has primarily focused on thermal strain originating from an order of magnitude difference in coefficient of thermal expansion (CTE) between perovskite and substrates used for photovoltaic applications, such as silicon and glass.^[2] Consequently, tensile stress is predicted to develop in the perovskite layer while cooling back to room temperature due to the film’s constraint to the substrate and inability to fully contract.<br/><br/>Here, we show that a simple application of the CTE mismatch equation inaccurately predicts residual stress in perovskite films. For example, despite similar CTEs, there is a 60 MPa stress difference between narrow bandgap “SnPb Perovskite” (Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃) and popularized “Triple Cation Perovskite” (Cs_0.05MA_0.16FA_0.79Pb(I_0.83Br_0.17)₃). We probe the cause of residual stress variation in metal halide perovskite with a combination of <i>in situ</i> absorbance and substrate curvature measurements during the spin coating, anneal, and cooldown procedures. This reveals that the degree of partial attachment prior to the anneal can reduce residual tension following cooldown. Additionally, we demonstrate the dynamic nature of stress in perovskite with evidence of tensile stress relaxation which is accelerated by the presence of moisture and oxygen. In turn, we propose a new framework to understand the relationship between stress and degradation as films with tension are driven to uptake moisture and oxygen to release stress. Finally, we present a critical perspective on stress engineering strategies based on high CTE buffer or interlayers which are claimed to modify thermal stress in the perovskite.^[4] We show that these strategies are not grounded in thin-film mechanics as the stress in the perovskite in response to temperature changes is dominated by the thickest, stiffest layer, such as glass.<br/><br/>[1] J. Zhao, Y. Deng, H. Wei, X. Zheng, Z. Yu, Y. Shao, J. E. Shield, J. Huang, <i>Sci. Adv.</i> <b>2017</b>, <i>3</i>, eaao5616.<br/>[2] N. Rolston, K. A. Bush, A. D. Printz, A. Gold-Parker, Y. Ding, M. F. Toney, M. D. McGehee, R. H. Dauskardt, <i>Adv. Energy Mater.</i> <b>2018</b>, <i>8</i>, 1802139.<br/>[3] C. Ramirez, S. K. Yadavalli, H. F. Garces, Y. Zhou, N. P. Padture, <i>Scr. Mater.</i> <b>2018</b>, <i>150</i>, 36.<br/>[4] D.-J. Xue, Y. Hou, S.-C. Liu, M. Wei, B. Chen, Z. Huang, Z. Li, B. Sun, A. H. Proppe, Y. Dong, M. I. Saidaminov, S. O. Kelley, J.-S. Hu, E. H. Sargent, <i>Nat. Commun.</i> <b>2020</b>, <i>11</i>, 1514.