# Innovative Approaches to Semi-Transparent Perovskite Solar Cells
## 1 Introduction  
Global energy demands are increasing every day. To fulfill them, fossil fuels are being used at an advancing rate, leading to the vanishing of natural sources as well as to environmental pollution issues. Solar cells, solid-state electrical devices that convert the energy of light directly into electricity via the photovoltaic effect, are one of the promising technologies made to address these issues. There are several types of solar cells, which are used to convert sunlight into electrical energy: monocrystalline silicon solar cells, polycrystalline silicon solar cells, thin-film solar cells, organic solar cells and hybrid solar cells. Among them, the crystalline silicon (c-Si) solar cell has more than 90% of the market share [1,2]. Germanium-based solar cells use germanium as the semiconductor material instead of silicon; they are more efficient at converting sunlight into electricity, making them suitable for space applications due to their higher temperature tolerance and radiation resistance [3,4]. However, their higher cost limits their use in terrestrial applications. Perovskite solar cells (PSCs) have drawn a lot of consideration due to rapid increases in power conversion efficiency in a short period, and their current efficiency has reached up to 25.7%, which is equivalent to that of silicon solar cells [5]. The PSC is a PV solar cell that uses organic-inorganic hybrid perovskite (OIHP) as a photoactive layer. OIHP has the same crystal structure as that of the mineral calcium titanium oxide (CaTiO\({}_{3}\)); in general, these compounds are represented by the chemical formula ABX\({}_{3}\), where 'A' represents the monovalent cation (e.g., methyl-ammonium, formammidinium or cesium), 'B' denotes a rare divalent metal (Pb\({}^{-}\) or Sn\({}^{-}\)), and 'X' is the halide anion (I\({}^{-}\), Br\({}^{-}\), or Cl\({}^{-}\)) that bonds to both [6, 7, 8, 9]. In addition to being highly efficient, the advantage of perovskite solar cells istheir easily tunable band-gap. Their band-gap can be easily modified to achieve broad band-gap perovskite for semi-transparent applications by changing the components' locations at the 'A' site and the concentration of halide elements at 'X' [10; 11; 12; 13]. In addition, the thermochromic behavior of perovskite, i.e., the characteristic of changing color according to temperature, enables various color implementations [14]. These unique features not only create translucent perovskite layers of various colors but also provide extensions to smart windows, tandem solar cells (TSC) and building-integrated photovoltaics (BIPV). The semi-transparent perovskite solar cell (ST-PSC) is a form that best utilizes the characteristics of the PSC. Unlike general PSCs, the ST-PSC is characterized by transmitting a significant amount of visible light while converting solar energy. The main performance factors of ST-PSCs are the average visible light transmittance (AVT), usually referred to as the average value of the transmittance lying in the wavelength range observable to human eyes that is between 370 and 740 nm [15; 16], along with their power conversion efficiency (PCE) and applications due to their optical properties. Evaluating the optical properties of the materials to be used in ST-PSCs is essential for developing efficient ST-PSCs. The main optical properties include the AVT, human luminosity factor, color rendering index (CRI), corresponding color temperature (CCT) and transparency color perception. An average AVT of 20 to 30% is required for window applications. The second important parameter is the human luminosity factor, as human eyes differ from a spectrometer. Human eyes are sensitive to green light but less sensitive to blue and red wavelengths. Hence, harvesting the maximum number of photons in these regions could be one way to achieve high transparency with efficiency. CRI refers to the capability of the light source to render the precise object color compared with that of the reference light source.