# Innovative Approaches to Semi-Transparent Perovskite Solar Cells
## 6 Applications of ST-PSCs
### Building-Integrated Photovoltaic (BIPV) Applications  
PSCs are evaluated as suitable devices for ST-PV, owing to their high efficiency, excellent transmittance and low-cost manufacturing. Neutral-colored ST-PSCs display potential for fulfilling the requirements of power-generated windows on cars, automobiles and buildings. There are various applications in which ST-PSCs are being used in day-to-day life. The application of ST-PSCs to building-integrated photovoltaics (BIPV) can provide an extended opportunity for the usage of solar energy. A semi-transparent BIPV allows for the replacement of conventional building coverings, such as window panes, roof tops, curtain walls, etc., and, at the same time, it reduces energy demand, controls heat loss and can be used for glazing for comfortable daylight (Figure 8) [157; 158]. Several attempts have been made in this regard to develop efficient BIPV systems. Various PV technologies have been reported for BIPV applications, which include amorphous silicon semi-transparent solar cells, Cu(In, Ga)Se\({}_{2}\) (CIGS) semi-transparent solar cells, dye-sensitized cells (DSCs) and perovskite solar cells [159]. Among them, PSCs are the most promising PV technology for semi-transparent BIPV recently. Two approaches are being tried to increase the transparency of PSCs; research on adjusting the thickness and shape of the perovskite layer for improving transmittance is being conducted. To the human eye, the microstructure or nanostructured perovskite layer appears neutrally colored, with little effect on the spectral properties of light transiting through it. [PERSON] et al. conducted real-time research by applying it to buildings in Bari, Italy, to confirm the efficiency of BIPV technology [160]. The overall results concluded that semi-transparent PV-based devices could be a better choice depending on the glazing and shading conditions. For example, the top floor was more effective than the middle floor or the ground floor; this was because the glare and shading of other buildings can reduce the efficiency of the middle floor and the ground floor. In another modification, [PERSON] et al. demonstrated carbon-based ST-PSCs for BIPV applications and achieved an 8.13% PCE  
Figure 8: Application of BIPV into various surfaces of buildings unravelling the potential of solar energy for on-site energy production. Reprinted with permission from Ref. [158]. with an average solar transmission of 30% and an average visible transmission of 20% [161]. [PERSON] et al. developed BIPV using amorphous silica (a-Si) and perovskite [162]. They fabricated ST-PSCs with a 6.64% PCE and 42.4% transparency and compared them with a Si-based BIPV with a 4.80% PCE with 30.1% transparency. These examples confirm that PSC is a suitable candidate for BIPV applications. Lightweight and flexible solar cells can provide scalability to various applications through integration with BIPV technology, which includes smart buildings, windows, facades, etc. BIPV is being studied based on perovskite and lightweight substrate materials, such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). In terms of this modification, [PERSON] et al. fabricated ultralight and flexible ST-PSCs using orthogonal silver nanowires on 1.3 \(\upmu\)m thick polyethylene naphthalate foils and achieved a PCE of 15.18% with a power-per-weight of 29.4 Wg\({}^{-1}\), which is the maximum reported value for lightweight solar cells [163]. For other applications, [PERSON] et al. demonstrated thermochromic solar cells with more than 7% PCE applicable to smart solar windows, in which the transmittance of solar cells changes depending on the temperature [164]. They used the phase transition of CsPbBr\({}_{x}\)3\({}_{-x}\) perovskite according to the temperature. The transparent non-perovskite phase has a visible transparency of 81.7%, whereas the dark perovskite phase is 35.04% visibly transparent. Furthermore, [PERSON] et al.