# A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells
## 1 Introduction  
Increasing global demand for energy and continued reductions in fossil fuel-based energy sources call for the greater use of alternative renewable energy sources. Among the possible renewable sources of energy, solar energy is the most promising, as it can be converted into electrical energy and reach almost every part of the world. Commercially available solar cells are based on silicon, GaAs (gallium arsenide), CdTe (cadmium telluride) and CIGS (copper indium gallium (di)selenide) and have sufficiently established efficiencies to meet global energy demand. However, efforts have been made to reduce the cost [1] of these existing technologies by replacing them with cheaper alternative photovoltaic devices such as perovskite solar cells (PSCs). Currently, the photo-conversion efficiency (PCE) of PSCs has soared beyond 20% in less than five years of laboratory research [2]. [PERSON], et al. first introduced organic-inorganic hybrid perovskite absorber layer in dye sensitized solar cells (DSSCs) configuration with an efficiency of only 3.81% [3]. The poor stability and efficiency of liquid electrolyte-based structures were attributed to iodine-based redox processes. The efficiency increased to 10% when the liquid electrolyte was replaced with a solid-state hole transport layer of spiro-OMeTAD and led to stability improvements as well [4]. Currently, for PSCs, a planar heterojunction structure (introduced by [PERSON], et al. [4]) is widely used, and consists of a solid perovskite layer with electron and hole selective contacts. The planar heterojunction structure can be fabricated at a low temperature (<150 \({}^{\circ}\)C), while the mesoporous TiO\({}_{2}\) scaffold structure requires high temperature (<400 \({}^{\circ}\)C), making the former structure superior to the latter structure in terms of cost [5]. The projected theoretical maximum efficiency of solar cells made of this type of device structures is higher than 30%, which would surpass the practical efficiency of \(\sim\)25% for silicon solar cells [6]. The promising performance of perovskite solar cells has been attributed to extraordinary material properties, including high absorption coefficient, long charge carrier diffusion length, low exciton binding energy, and tunable bandgap [7; 8; 9; 10; 11; 12]. However, despite achieving lab-scale device efficiency comparable to that of commercially available solar cells, PSCs retain critical issues regarding stability. Standard PV modules available on the market are typically warrantied to retain their initial efficiency for 20-25 years [13]. Since perovskite solar cells are prone to degradation when exposed to air, UV light, thermal stress (heat), light soaking, electric fields, and many other factors [14; 15; 16], they cannot currently achieve such a market requirement. Hence, extensive research has recently focused on the study of degradation mechanisms to improve perovskite solar device stability. Stability improvements could be achieved by device encapsulation, adding UV filters, and suppressing trap states for degradation caused by air, UV light, and electric fields, respectively [17; 18; 19; 20], but degradation due to thermal stress is considered inevitable since it is difficult to avoid the temperature rise of the solar cells during operation. The effective operational temperature can range from \(-40\)\({}^{\circ}\)C to \(+85\)\({}^{\circ}\)C [21], so the standard heat stability test is commonly run within this temperature window. In this review paper, we aim to present a survey of the existing literature on the thermal stability of the most commonly used methyl ammonium halide perovskite solar cells. We will first focus on the evolution of the structural stability of perovskite materials, followed by the impact of thermal stress on perovskite solar devices. Perovskite solar cells consist of many layers and this study specifically highlights the intrinsic degradation mechanisms of the perovskite absorber layer, including chemical, morphological, and optical degradation.