# Innovative Approaches to Semi-Transparent Perovskite Solar Cells
## 7 Conclusions and Perspectives  
ST-PSCs are one of the most promising technologies for achieving sustainable and carbon-free energy. In addition, ST-PSCs are suitable for various applications, such as TSCs and BIPV, due to their excellent optoelectronic properties and PCE. Therefore, ST-PSCs have attracted a lot of attention in a wide range of applications, such as windows, curtain walls, canopy, railings, shading, etc. However, ST-PSCs have a challenge due to the compromise between efficiency and light transmittance. To overcome these challenges, many studies are underway, including band-gap tuning, transparent electrodes, and creating island-shaped microstructures. Here, a brief overview of recent progress on ST-PSCs is described, and the essential requirements for realizing ST-PSCs are discussed. The structure and performance of the representative ST-PSCs and each component material are summarized in the tables for each section. For the realization of ST-PSCs, adding personal views on future development directions, there are some promising directions for ST-PSCs and challenges to be solved. Regarding perovskite absorbance, more effort should be made to develop stable new perovskite with an adjustable band-gap. In addition, along with a deposition technique that can accurately adjust the thickness of the perovskite layer, a design or structure that can control transmittance without performance degradation should be developed. The perovskite layer, with an optically enhanced microstructure or nanostructure, could be one of the promising directions for ST-PSCs. For TEs, various materials are currently being studied for transparent electrodes, and there is the possibility of development in each direction. However, because TEs can determine efficiency and transmittance as well as stability, the following factors should be considered when developing TEs: TEs should be excellent in conductivity, transparency, chemical stability and cost-effectiveness, and the energy level of TEs should be well aligned with the other layers to minimize the barrier for the transport of charges. The feasibility of ST-PSCs and their future prospects will be brighter. PSCs are one of the most favorable technologies for addressing the climate crisis. PSC power generation capacity is expected to grow above 21.9 TW to achieve carbon freedom by 2050. TSCs, which combine two or more photovoltaic systems with different band-gap regions in series or parallel, have a huge possibility of beating the Shockley-Queisser limit of 33.7% with a band-gap of 1.34 eV under one-sun illumination for a single-junction solar cell [170]. The theoretical PCE of TSC is calculated as ~46% for two junctions (2J), ~50% for three junctions (3J) and 65% or more for an infinite number of junctions, taking into consideration both solar irradiance and electroluminescence by assuming 100% radiative emission of other cells in TSCs. ST-PSCs have a high potential for use as the top cells of TSCs with c-Si, CIGS and narrow-band-gap PSCs due to their adjustable band-gap with good transparency. Silicon solar cells and PSC-based tandem cells have been reported to have an efficiency of close to 30%. In particular, considering that the building sector currently consumes 40% of the planet's energy and that the figure will be about double to triple by 2050, this increase in photovoltaic (PV) energy is expected to be more evident in the building sector. The application of ST-PSCs to BIPV can provide an extended opportunity for the use of solar-based energy. Based on ST-PSCs, BIPV can replace traditional building envelopes, such as windows, curtain walls, shading, etc. Expanding the total power output of the module per unit of area is the most obvious method to continuously lower the overall price of installed PV power generation and expand the prevalence of PV. Therefore, the development of high-performance ST-PSCs not only in tandem but also for BIPV is expected to become more active.