# Innovative Approaches to Semi-Transparent Perovskite Solar Cells
## 2 Perovskite Photoactive Layers for ST-PSCs  
A trade-off occurs between efficiency and light transmittance, and it is important in ST-PSC to find a compromise between high efficiency and transmittance. Interestingly, the transparency of the perovskite layer can be altered by varying the halide element, whereby the increased band-gap can transmit more light in the visible region through the perovskite layer. Representative ST-PSCs with perovskites of different compositions and thicknesses are listed in Table 1. For example, in MAPbI\({}_{3-x}\)Br\({}_{x}\) (0 \(\leq\) x \(\leq\) 3), the perovskite layer has a constant thickness (300 nm), and the average visible transmittance (AVT) of the perovskite layer increases from 10% to 24% as the bromide content increases from x = 0 to x = 1.5 [21]. This is because the band-gap of perovskites containing more bromides increases, and the wavelength of the light absorption spectrum shifts to blue. The composition of perovskite with iodine only appears to be dark brown, which changes to red with an increase in bromide content and turns to clear yellow when the bromide content is more than 80%, whereby the band-gap is changed from 1.5 eV to 2.3 eV with the ratio of halide ions (I and Br) and cations (MA and FA). Moreover, from the viewpoint of stability, the phase separation issue that occurs when the Br concentration is high should be considered [22; 23]. To emphasize the phase instability problem, researchers of another study partly replaced the FA cation in the FAPbI\({}_{3}\) structure with a Cs cation, and the content of Br was optimized, resulting in stable FA\({}_{0.83}\)Cs\({}_{0.17}\)PbBr\({}_{x}\)I\({}_{3-x}\)[24]. The phase instability was completely eliminated in the iodine-to-bromide compositional range of FA\({}_{0.83}\)Cs\({}_{0.17}\)Pb(\({}_{0.6}\)Br\({}_{0.4}\))\({}_{3}\), which showed a band-gap of around 1.75 eV with high crystallinity. The FA\({}_{0.83}\)Cs\({}_{0.17}\)PbBr\({}_{x}\)I\({}_{3-x}\) was applied to the ST-PSC, achieving up to 15.1% PCE and 12.5% stabilized PCE. On a similar note, in recent reports, [PERSON] et al. tried to overcome the problem between efficiency and transmittance by utilizing a wide band-gap (WBG) perovskite absorber with a composition of Cs\({}_{0.2}\)FA\({}_{0.8}\)Pb(\({}_{0.6}\)Br\({}_{0.4}\))\({}_{3}\)[25]. The perovskite with different concentrations varying from 0.2 to 1 M were prepared with a balance between iodine and bromine concentrations to maintain a balance between AVT and efficiency (Figure 1a,b). With interfacial engineering and defect passivation, for ST-PSCs, they obtained a high PCE of 14.40% along with an AVT of 38%. In another approach, [PERSON] et al. prepared triple cation perovskite (Cs\({}_{0.05}\)FA\({}_{0.8}\)MA\({}_{0.15}\)PbI\({}_{2.55}\)Br\({}_{0.45}\)) with a band-gap of 1.63 eV, resulting in a steady-state PCE of 18.2% and an average NIR transmittance (ANT) of 75% for ST-PSCs [26]. Furthermore, they achieved an efficiency of 25% through a combination of narrow band-gap perovskite solar cells and all-perovskite tandem cells. In another method, wide band-gap (\(\sim\)2.3 eV) FAPbBr\({}_{3}\) films were fabricated by [PERSON] et al. with different concentrations of precursor solution from 1.2 M to 0.8 M and 0.4 M, represented as C1, C2 and C3, respectively (Figure 1c and the inset of Figure 1c) [27]. The results demonstrate that the C1, C2 and C3 films showed a PCE of 5.71, 3.25 and 1.86 with high V\({}_{\text{OC}}\), and the corresponding AVTs were 35.6, 42.5 and 49.2%, respectively (Figure 1d). These ST-PSCs with high V\({}_{\text{OC}}\) and optical clarity are useful for wearable lenses and automotive windscreen applications. Similarly, in a recent approach, [PERSON] et al. studied the effect of perovskite compositions with Cs, FA, MA, I and Br combinations. They fabricated perovskite with band-gaps of 1.73 and 1.82 eV and reported ST-PSCs with improved efficiency and stability (Figure 1e,f).