Engineering Stress in Perovskite Solar Cells to Improve Stability

Abstract

developed low-cost active PV material. [1][2][3][4][5] To fulfill this promise, perovskites must first overcome the chemi cal [6][7][8] and thermomechanical instability [9] that has long been observed in them. Recent efforts have demonstrated progress in improving the chemical stability of perovskite devices intrinsically by tuning cation composition [10,11] and extrinsically by encapsulation. [12][13][14][15] However, the perovskite field has demonstrated insouciance toward thermomechanical stability, even though it is essential for the commercialization of photovoltaic cells or lightemitting diodes. While some promising results have recently been achieved in scalability with power conversion efficiencies (PCE) of 12% for 100 cm 2 perovskite devices, [16] the thermomechanical instability of perovskites remains a significant challenge to producing module-scale perovskite solar cells [17] with operational lifetimes comparable to c-Si and CdTe. [18] In particular, stresses are generated in perovskite films during processing and magnified in service by environmental effects such as thermal cycling, resulting in the formation of defects and propagation of fracture and delamination. [19] Additionally, cracks that develop in the film are a source of accelerated degradation for the transport of gases, moisture, and other environmental species. Unfortunately, perovskite layers are exceptionally fragile and susceptible to delamination as measured by their fracture energy [20,21] -less robust than organic photovoltaics (OPVs) by an order of magnitude [22] and c-Si or copper indium gallium diselenide (CIGS) solar cells by two orders of magnitude. [9] Despite the significance of film stresses for device stability, the origin and magnitude of stresses in perovskite films have been largely overlooked. In addition to causing fracture, stress accelerates the rate of photochemical degradation in many materials-such as fuel cell membranes, [23] laser diodes, [24] and encapsulants [25] -and a recent report shows that perovskite are no exception. [26] Perovskites can accumulate residual stresses during processing through several pathways. For example, in solar cell devices, perovskites have much higher coefficients of thermal expansion (CTE) than the other device layers and the glass substrate. [27] When perovskites are annealed after deposition and subsequently cooled back to room temperature, a lower-CTE substrate constrains the An overlooked factor affecting stability: the residual stresses in perovskite films, which are tensile and can exceed 50 MPa in magnitude, a value high enough to deform copper, is reported. These stresses provide a significant driving force for fracture. Films are shown to be more unstable under tensile stress-and conversely more stable under compressive stresswhen exposed to heat or humidity. Increasing the formation temperature of perovskite films directly correlates with larger residual stresses, a result of the high thermal expansion coefficient of perovskites. Specifically, this tensile stress forms u...

The balance between efficiency, stability and environmental impacts in perovskite solar cells: a review

“…tin vacancies) and therefore reduce the background dopant density and improve charge extraction [118]. An important aspect that has not deserved much attention so far is the residual stress in the structure that may lead to structural instabilities, the residual stresses (tensile) in perovskite films can exceed 50 MPa in magnitude, and simple recommendations such as a bath conversion method to form the perovskite film at room temperature leads to low stress values that are unaffected by further annealing [119]. Therefore, the transition at 330 K and the reduction of stress can be seen as an opportunity for self-healing of the structures.…”

Role of Water and Defects in Photo‐Oxidative Degradation of Methylammonium Lead Iodide Perovskite

“…Thus, concerns have recently been turned to the long‐term operational stability of perovskite solar cells in ambient conditions . Unfortunately, organic–inorganic halide perovskites (e.g., MAPbI 3 ) are very sensitive to many factors, such as heat, moisture, light, oxygen, electric‐field, and mechanical stress . Understanding the degradation induced by these factors at the atomic level will lay important foundation for the development of proper protection schemes to realize the long‐term stability of perovskite‐based solar cells.…”

“…[10][11][12][13] Unfortunately, organic-inorganic halide perovskites (e.g., MAPbI 3 ) are very sensitive to many factors, such as heat, [14][15][16] moisture, [17,18] light, [19,20] oxygen, [21,22] electric-field, [23] and mechanical stress. [24,25] Understanding the degradation induced by these factors at the atomic level will lay important foundation for the development of proper protection schemes to realize the long-term stability of perovskite-based solar cells.…”

Beneficial impact of materials with reduced dimensionality on the stability of perovskite-based photovoltaics

“…Long-term stability of solar cells under realistic operational conditions becomes the chief concern for researchers, as perovskite technology closes the gap between realized progress and prerequisites set by the industry. Materials with low dimensionality display remarkable potential to aid in overcoming obstacles related to device stability, release the strain-induced phenomena and simplify fabrication routes [68,69]. First, it is vital to develop complete and predictive comprehension of the complex interplay mechanism between 2D materials and perovskite absorber apart from improved stability, as inclusion of additional constituents into light harvesting system exhibits adverse effect on PCE.…”