# Innovative Approaches to Semi-Transparent Perovskite Solar Cells
## 4 Stability of ST-PSCs  
The stability of ST-PSCs is one of the major bottlenecks for their commercialization. Hence, studies focusing on the stability of ST-PSCs should be carried out in parallel. The heart of solar cells is the active layer, i.e., the perovskite layer, so protecting the perovskite layer from any possible damage is of prime importance. Several efforts have been made to improve efficiency along with stability. However, from a commercialization point  
Figure 4: (**a**) Schematic representation of fabricated ST-PSC with graphene as the top electrode. (**b**) Transmittance spectra for the corresponding devices with different thicknesses (Inset: Digital images of cells prepared with different thicknesses). (**c**) Transmittance spectra for cells with different thicknesses (Inset: Schematics of device using PEDOT:PSS and digital images of films with different transparencies). (**d**) Interrelation between PCE and AVT for various electrodes used in ST-PSCs: TCO [64; 66; 67; 112; 113]; Metal [40; 41; 74; 114; 115; 116; 117]; Carbon [118; 119]; PEDOT:PSS [109; 111; 120]. Reprinted with permission from Refs. [97; 109]. of view, a lot of studies need to be carried out to enhance stability without compromising other parameters, such as transmittance and efficiency. Composition engineering is the most effective and easily accessible way to improve the stability of perovskite solar cells. For example, in perovskite composed of halide ions (Br and I) and cations (MA and FA), their composition ratio greatly affects phase stability. Increasing the Br concentration could improve the transmittance, but it trades off with stability. The phase instability issue is observed when the Br content is increased. Hence, to address this, the Br content was optimized, and the FA cation was partially substituted by a Cs cation. Furthermore, this method was utilized by various researchers to balance the Cs, FA, MA, I and Br combinations and to obtain highly stable devices with improved efficiency and AVT [23; 24]. The other approach to improve stability and transmittance simultaneously is the additive engineering method [45]. This method has been utilized by various researchers to improve perovskite precursor quality as well as stability. For instance, some bifunctional group additives enhance the device's stability and transmittance along with the passivation of defects, contributing to improved efficiency as well. Some researchers have blended perovskite with polymers to obtain smooth, pinhole-free films for improved performance and transmittance. In a recent approach, ultra-thin perovskite layers were fabricated using the thermal-pressed recrystallizing (TPR) technique. This method demonstrates improved performance, transmittance and stability. The next approach utilizes improvements in the transparent electrodes, which demonstrate a crucial role in ST-PSCs, as the structure of ST-PSCs lies in between two transparent electrodes. Moreover, the main role of TEs is to transmit light and provide an appropriate electric field to collect and transfer the charge. Hence, transparency, conductivity, robustness, cost-effectiveness, energy-level alignment and, most importantly, the mechanical and chemical stability of TEs are of utmost importance for obtaining efficient and stable ST-PSCs. Usually, for bottom electrodes, TCOs such as FTO, ITO and AZO can be utilized. However, the top electrodes are difficult to fabricate, as the perovskite beneath is prone to degradation due to harsh conditions and treatments. Therefore, proper deposition techniques are required for obtaining transparent, conducting and optically transparent electrodes. To avoid damage to the perovskite layer, various methods are adopted, and some of them are low-temperature atomic layer deposition; the insertion of a buffer layer; reducing the sputtering power; and coating with robust materials, such as inorganic MoOx, AZO nanoparticles, thin-metal layers (Ag, Au), ITO nanoparticles, etc., on the perovskite layer before depositing TCO.