# Innovative Approaches to Semi-Transparent Perovskite Solar Cells
## 2 Perovskite Photoactive Layers for ST-PSCs
The ST-PSC with a thickness in the range of 100-400 nm achieved a PCE between 4.2% and 15.4%, with an AVT in the range of 52.4-20.8%. The devices exhibited a stability of 85% for 1000 h under continuous illumination [28]. A thin layer of perovskite is the most common and comfortable approach for obtaining transparency in ST-PSCs. The AVT of the perovskite layer is changed by varying the layer thickness. More visible light can pass through a thinner perovskite layer, which relies on the absorption coefficient as well. The trapping states found at the conduction band minimum (CBM) and valence band maximum (VBM) in polycrystalline perovskite structures are responsible for absorption coefficient changes [29]. In addition, a smooth, uniform and pinhole-free surface is necessary to attain a high transparency, which leads to increased V\({}_{\text{OC}}\) and shunt resistance [30]. Various techniques have been employed to develop thin perovskite absorbers, such as spin coating with a low concentration of precursor solution and different spinning speeds, vacuum evaporation and vacuum-assisted techniques, additive engineering, thermal-pressed recrystallizing, etc. [31; 32; 33]. The most common and easy approach is to manage the thickness of a perovskite thin layer through spin coating with a different rotational speed with a low-concentration precursor solution. [PERSON] et al. varied the perovskite film thickness between 140 and 240 nm, optimizing spin coating conditions [34]. As the perovskite thickness decreases, the transmittance increases in the visible region of 500-800 nm, and the corresponding AVT is tuned between 37.5 and 13%, as observed in Figure 2a. In addition, the color properties were also modified based on the color coordinates and film thicknesses, but additional optimization was required to obtain a neutral-colored ST-PSC. The optimization of spinning speed was demonstrated by [PERSON] et al. They fabricated various thicknesses of perovskite layers by varying the spinning speed [35]. They demonstrated that the films prepared with 3 wt% of polyvinyl pyrrolidone (PVP) show enhanced PCE, stability and AVT, and an ST-PSC with 3 wt% of PVP shows a PCE of 5.36% and 34% AVT (Figure 2b,c). In addition, they showed that the device color can be changed to red, yellow, and green through the spin coating of gold fragments (inset of Figure 2c). In order to obtain a thin layer of perovskite without pinholes, vacuum evaporation techniques have been applied. This method was utilized by [PERSON] et al. to develop a pinhole-free perovskite thin layer of all-inorganic CsPbBr\({}_{3}\) by using the dual-source  
Figure 1: (**a**) Photographic images and (**b**) transmittance spectra of ST-PSCs for perovskite films with different concentrations. (**c**) UV-Visible absorption spectra and transmission spectra for device with FaPbBr\({}_{3}\). (**d**) J-V curves with different concentrations. (**e**) Photograph of perovskite films’ perovskite compositions. (**f**) Dependence of LUE values on perovskite film thickness. Reprinted with permission from Refs. [25; 27; 28]. vacuum co-evaporation (DVSE) technique (Figure 2d) [36]. In this technique, they co-evaporated PbBr\({}_{2}\) and CsBr, varying the molar ratio and evaporation rate using a thermal evaporator. The controlled slow evaporation rate led to the formation of bigger grains and pinhole-free morphology and achieved a PCE of 5.98% with an AVT of 90.9% in the wavelength range of 530-800 nm (Figure 2e,f). This DVSE approach was further modified to obtain a uniform and continuous pinhole-free perovskite structure with high transmittance, for which the technique of simultaneously using spinning and vacuuming was applied. [PERSON] et al. fabricated fine, smooth perovskite films using a short spinning and vacuum drying (SSVD) process and increased visual transparency by limiting optical scattering at the perovskite layer [37].