# Innovative Approaches to Semi-Transparent Perovskite Solar Cells
## 2 Perovskite Photoactive Layers for ST-PSCs
The mean roughness of perovskite was determined to be 6 nm, resulting in suppressed scattering of 2% and high visual transparency, yielding 11.0 and 8.7% PCE from the anterior and posterior incidence, respectively. Recently, [PERSON] et al. demonstrated all-inorganic perovskite with Erbium (Er) passivation, which yielded a PCE of 11.61% [38]. Figure 2: (**a**) UV-Visible spectra with different perovskite film thicknesses (Inset: Photograph of perovskite films with different thicknesses). (**b**) Current–voltage characteristics of the device fabricated with and without PVP. (Inset: Digital photograph of semi-transparent cell.) (**c**) Transmittance spectra for reference device and 3% PVP devices. (Inset: Digital image of various pigment materials coated on Au surface for tuning the color). (**d**) Schematic diagram of the vacuum chamber for the DSVE method. (**e**) Transmittance of the vacuum-evaporated 450 nm thick CsPbBr3 film. (**f**) Current–voltage  
characteristics of the fabricated semi-transparent cell. (**g**) Schematic of device architecture and 2-Hydroxy cellulose molecular structure and a sketch of MAPPl\({}_{3}\). (**h**) Interrelation between PCE and AVT for ST-PSCs with different precursor concentrations [21; 25; 28; 30; 31; 33; 34; 39; 40; 41; 42; 43]. Reprinted with permission from Refs. [34; 35; 36; 44]. Taking into consideration improving transmittance and stability simultaneously, another effective approach is additive engineering. For instance, [PERSON] et al. introduced the bifunctional additive 1-propyl-[4,4\({}^{\prime}\)-bipyridin]-1-ium iodide (BiPy-I) in a perovskite precursor and improved not only efficiency and stability but also transmittance [45]. The large pyridine group in the additive enhances the device's transparency, and the iodine anions passivate the defects due to the loss of iodine. Consequently, ST-PSCs with an 11.74% PCE and 23% AVT were obtained. On the same line, [PERSON] et al. improved the transparency of ST-PSCs by making a blend of perovskite with polymer (Hydroxyethyl cellulose, HEC), as seen in Figure 2g [44]. The ST-PSC with HEC showed an 11.6% PCE with an increased -44% AVT and obtained a maximum light utilization efficiency (LUE) of 2.4%, which indicates that ST-PSCs with HEC are suitable for window applications. Recently, a new approach was established to produce ultra-thin and superior-quality perovskite layers utilizing a thermal-pressed recrystallizing (TPR) technique [46]. In this technique, the top of the precursor layer is covered using a silicon wafer and is tightly wrapped using a Teflon membrane, and then the packed films are transferred into a hot-press autoclave filled with simethicone under a nitrogen atmosphere. The ST-PSC manufactured with the TPR method demonstrated improvement in overall device performance, including transmittance and stability. The devices showed a PCE of 13.45% after TPR treatment, signifying the capability for the development of ST-PSCs for window applications. The compiled graph (Figure 2h) [21; 25; 28; 30; 31; 33; 34; 39; 40; 41; 42; 43] depicts the PCE and AVT graph for an ST-PSC modified with different perovskite precursors. The graph clearly indicates the trade-off relationship between PCE and transmittance. The best PCE obtained with good visible transmittance is still 14.4%, which remains low for practical applications. More work and optimization are required to obtain a high PCE with transmittance for practical applications with ST-PSC.