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 ^[1][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 (CaTiO₃); in general, these compounds are represented by the chemical formula ABX₃, 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 ^[2][3][4][5][6,7,8,9]. In addition to being highly efficient, the advantage of perovskite solar cells is their 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’ ^[6][7][8][9][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 ^[10][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 ^[11][12][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. The CRI value range lies between 0 and 100, revealing the ability of ST-PSCs to transmit light with the actual color of the observed light. Higher CRI values refer to a high color rendering capacity, whereas lower values refer to a lower rendering capacity. CCT accuracy stems from the color space standard developed in 1931 known as the CIE xy chromaticity diagram ^[13][17].

Depending on the combination between PCE and optical properties, ST-PSCs could be extended to various technologies, such as TSC and BIPV. Furthermore, their wide-ranging applications, including windows, curtain walls, canopies, balustrades and shading, have drawn a lot of interest ^[14][15][18,19]. However, ST-PSCs still do not have high-efficiency characteristics due to various factors that cause tradeoffs between efficiency and light transmittance. For example, fabricating ST-PSCs requires a compromise in film thickness. This leads to drawbacks such as V_OC and J_SC reductions, in turn reducing the device’s efficiency. In addition, the fabrication of thin perovskite films makes it difficult and intricate to scale-up productions; perovskite thin films can easily make pinholes, which can be a major factor in reducing efficiency and stability. Numerous techniques have been developed to concurrently increase solar cells’ transparency and performance in order to circumvent these limitations, e.g., partially covering perovskites and creating island-shaped structures, fabricating ultra-thin perovskites for restraining light absorption, making discontinuous perovskites or utilizing wide band-gap perovskite (WBG) for enhancing transparency ^[16][20]. However, the trade-off between high transmittance and efficiency still makes the development of ST-PSCs difficult. These issues highlight the need to explore current development progress and derive potential requirements and strategies for ST-PSC development.

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. For example, in MAPbI_3−xBr_x (0 ≤ x ≤ 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 ^[17][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 ^[18][19][22,23]. To emphasize the phase instability problem, researchers of another study partly replaced the FA cation in the FAPbI₃ structure with a Cs cation, and the content of Br was optimized, resulting in stable FA_0.83Cs_0.17PbBr_xI_3−x ^[20][24]. The phase instability was completely eliminated in the iodine-to-bromide compositional range of FA_0.83Cs_0.17Pb(I_0.6Br_0.4)₃, which showed a band-gap of around 1.75 eV with high crystallinity. The FA_0.83Cs_0.17PbBr_xI_3−x was applied to the ST-PSC, achieving up to 15.1% PCE and 12.5% stabilized PCE. 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 ^[21][29]. In addition, a smooth, uniform and pinhole-free surface is necessary to attain a high transparency, which leads to increased V_OC and shunt resistance ^[22][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. ^[23][24][25][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. Jen et al. varied the perovskite film thickness between 140 and 240 nm, optimizing spin coating conditions ^[26][34].  Taking into consideration improving transmittance and stability simultaneously, another effective approach is additive engineering. For instance, Zhang et al. introduced the bifunctional additive 1-propyl-[4,4′-bipyridin]-1-ium iodide (BiPy-I) in a perovskite precursor and improved not only efficiency and stability but also transmittance ^[27][45]. The large pyridine group in the additive enhances the device’s transparency, and the iodine anions passivate the defects due to the loss of iodine. Consequently, ST-PSCs with an 11.74% PCE and 23% AVT were obtained.

3. Transparent Electrodes for ST-PSCs

Transparent electrodes (TEs) are indispensable components that implement ST-PSCs. The basic requirements for TEs are determined by several factors, such as conductivity, chemical stability, cost-effectiveness and, more importantly, good transparency along with well-aligned energy levels with the other layers to minimize the barrier to the transport of charges ^[28][29][47,48]. This is because it is the main role of TE to transmit light and provide an appropriate electric field to collect and transfer the charge. As shown in Table 12, transparent conductive oxides (TCOs), including fluorine-doped tin oxide (FTO), ^[30][31][49,50], indium tin oxide (ITO) ^[32][51], indium-doped zinc oxide ^[33][34][35][52,53,54] and aluminum-doped zinc oxides ^[36][37][55,56] are usually used as bottom electrodes for ST-PSCs as well as opaque solar cells. These TEs have shown outstanding transparency, low sheet resistance (Rs = 5 to 20 Ω/sq), effective charge collection and long-term stability ^{[14][37][38][39][40][41]}[18,56,57,58,59,60]. Nevertheless, it is not easy to use these TEs as the top electrodes of the ST-PSC. This is because perovskite is easily damaged during the deposition of the top electrode. Therefore, it is essential to develop a proper deposition method for the top TE to implement high-performance ST-PSCs as well as their excellent electrical and optical properties.

Category	Device Structure	AVT (%)	PCE (%)	Ref.
Transparent conductive oxide-based TEs	FTO/ZnO/PCBM/CH	3	NH	3	PbI	3	/Spiro-OMeTAD/MoO	3	/H-doped In	2	O	3	-	14.1	[31]	[50]
	ITO/PEDOT:PSS/perovskite/PC	60	BM/AZO/ITO	-	12.3	[37]	[56]
	ITO/PTAA/CH	3	NH	3	PbI	3	/PCBM/AZO/ITO	12.08	13.68	[42]	[64]
	ITO/NiO/perovskite/PCBM/ZnO/IZTO	33.9	8.31	[43]	[66]
	ITO/ZnO/PTB7-Th:IEICO 4F/MoO	3	/Ag/ITO	36.2	8.1	[44]	[112]
	ITO/ZnO/PM6:Y6:PC	71	BM/MoO	3	/Ag/ITO	28.6	10.2	[44]	[112]
	ITO/NiOx/PSS/FAPbBr	0.43	Cl	0.57	/PC	61	BM/ZnO-NPs/LS-ITO/M-PEDOT:PSS/PTB7-Th:6TIC-4F/ZnONPs/ITO	52.91	10.55	[45]	[113]
	ITO/NiOx/PSS/Perovskite/PCBM/BCP/IO:GT	21.9	17.9	[46]	[67]
Metal-based TEs	ITO/PEDOT:PSS/CH	3	NH	3	PbI	3	/C	60	/BCP/Ag/MoO	3	7.1	13.49	[47]	[74]
	ITO/ZnO/PM6:N3/MoO	3	/Ag/MoO	3	28.94	10	[48]	[172]
	ITO/SnO	2	/FAPbI	3	/spiro-OMeTAD/MoO	3	/Ag/WO	3	12.18	15.33	[49]	[114]
	ITO/NiO/Cs	0.175	FA	0.825	Pb(I	0.875	Br	0.125	)	3	/C	60	/Ag/C	60	-	5.1	[50]	[173]
	ITO/PEDOT:PSS/PTB7-Th:IEICO-4F/PFN-Br/Ag/PCs	29.5	10.83	[51]	[115]
	ITO/PEDOT:PSS/PTB7-Th:ITVfIC/PDINO/Ag	26.4	8.21	[52]	[116]
	ITO/PEDOT:PSS/perovskite/ALD-ZnO/AgNW/ALD-Al	2	O	3	25.5	10.8	[53]	[41]
	ITO/PEDOT:PSS/CH	3	NH	3	PbI	3−x	Cl	x	/PC	60	BM/ZnO NP/AgNWs	28.4	8.49	[54]	[40]
	FTO/TO	2	/CH	3	NH	3	PbI	3	/spiro-OMeTAD/AgNWs–Au	-	11.1	[55]	[89]
	ITO/ZnO/PM6:Y6/PEDOT:PSS/AgNW	23	9.79	[56]	[117]
Carbon-material-based TEs	FTO/TiO	2	/CH	3	NH	3	PbI	3−x	Cl	x	/spiro-OMeTAD/PEDOT:PSS/graphene	-	6.13	[57]	[97]
	PEN/graphene/PEDOT:PSS/ZnO/PDTPDFBT:PC	70	BM/MoO	3	/graphene	54	3.8	[58]	[118]
	Graphene/PEDOT:PSS/ZnO/PTB7:PC	71	BM/PEDOT:PSS/graphene	40	3.4	[59]	[119]
	ITO/ZnO/PTB7:PC	71	BM/MoO	3	/HNO	3	-CNTs	-	3.7	[60]	[174]
	ITO/SnO	2	/MaPbI	3	/CNT/MoO	3	/Spiro-OMeTAD/Au	-	17.3	[61]	[104]
PEDOT:PSS-based TEs	FTO/TiO	2	/CH	3	NH	3	PbI	3	/Spiro-OMeTAD/PEDOT:PSS	7.3	10.1	[62]	[109]
	ITO/PEDOT:PSS/FAMAPbI	3−x	Br	x	/PCBM/PEDOT:PSS:CFE/PDMS	30.6	12.5	[63]	[111]
	ITO/ZnO/PEIE/P3HT:PCBM/PEDOT:PSS	51.2	2	[64]	[120]

Ultra-thin metal films also serve as an effective approach for TEs, owing to their superior conductivity and transmittance ^{[26][47][65][66][67][68][69][70][71]}[34,39,68,69,70,71,72,73,74]. Similar to other TEs, it is important to carefully manage the coating thickness to produce appropriate conductivity while maintaining high transmittance for optimal performance, even in thin metal films. Accordingly, a deposition technique using a wet or seed layer for adjusting the thickness of a metal film has been developed; organic and inorganic materials with considerable surface energy, such as polymers ^[68][72][70,75], small molecules ^{[47][53][69][70][71][73][74]}[41,71,72,73,74,76,77] and metal oxides ^{[22][73][75][76]}[30,76,78,79], have been used for the wet layer and the seed layer. Etgar et al. used the wet deposition method, which created perovskite grids with different dimensions. The transparency of the perovskite film was attained by controlling the perovskite solution concentration and mesh openings. As a result, ST-PSC showed a transparency between 20 to 70% and achieved a PCE of 5% at 20% transparency ^[66][68].

Ultra-thin metal films also serve as an effective approach for TEs, owing to their superior conductivity and transmittance ^{[26][47][65][66][67][68][69][70][71]}[34,39,68,69,70,71,72,73,74]. Similar to other TEs, it is important to carefully manage the coating thickness to produce appropriate conductivity while maintaining high transmittance for optimal performance, even in thin metal films. Accordingly, a deposition technique using a wet or seed layer for adjusting the thickness of a metal film has been developed; organic and inorganic materials with considerable surface energy, such as polymers ^[68][72][70,75], small molecules ^{[47][53][69][70][71][73][74]}[41,71,72,73,74,76,77] and metal oxides ^{[22][73][75][76]}[30,76,78,79], have been used for the wet layer and the seed layer.

4. Stability of ST-PSCs

The stability of ST-PSCs is one of the major bottlenecks for their commercialization. Hence, studies focusing on the stability of ST-PSCs should be carried out in parallel. The heart of solar cells is the active layer, i.e., the perovskite layer, so protecting the perovskite layer from any possible damage is of prime importance. Several efforts have been made to improve efficiency along with stability. However, from a commercialization point of view, a lot of studies need to be carried out to enhance stability without compromising other parameters, such as transmittance and efficiency. Composition engineering is the most effective and easily accessible way to improve the stability of perovskite solar cells. For example, in perovskite composed of halide ions (Br and I) and cations (MA and FA), their composition ratio greatly affects phase stability. Increasing the Br concentration could improve the transmittance, but it trades off with stability. The phase instability issue is observed when the Br content is increased. Hence, to address this, the Br content was optimized, and the FA cation was partially substituted by a Cs cation. The next approach utilizes improvements in the transparent electrodes, which demonstrate a crucial role in ST-PSCs, as the structure of ST-PSCs lies in between two transparent electrodes. Moreover, the main role of TEs is to transmit light and provide an appropriate electric field to collect and transfer the charge. Hence, transparency, conductivity, robustness, cost-effectiveness, energy-level alignment and, most importantly, the mechanical and chemical stability of TEs are of utmost importance for obtaining efficient and stable ST-PSCs. Usually, for bottom electrodes, TCOs such as FTO, ITO and AZO can be utilized. However, the top electrodes are difficult to fabricate, as the perovskite beneath is prone to degradation due to harsh conditions and treatments. Therefore, proper deposition techniques are required for obtaining transparent, conducting and optically transparent electrodes. To avoid damage to the perovskite layer, various methods are adopted, and some of them are low-temperature atomic layer deposition; the insertion of a buffer layer; reducing the sputtering power; and coating with robust materials, such as inorganic MoOx, AZO nanoparticles, thin-metal layers (Ag, Au), ITO nanoparticles, etc., on the perovskite layer before depositing TCO.

5. Device Structure for ST-PSCs

Making ST-PSCs with an ultra-thin perovskite layer is the most favorable and comfortable route to achieve efficient solar cells. However, ultra-thin perovskite films can reduce efficiency and cause pinholes, thereby lowering stability. To resolve this issue and to improve the overall visual transparency without sacrificing the performance of the solar cells, the design and pattern of the perovskite layer, the light distribution and the human luminosity component should be taken into consideration. Microstructure design and patterns have been proposed to increase the transparency of ST-PV using transparent regions. This structure has the option to design a neutral-colored ST-PSC by controlling photon absorption areas, including perovskite layers. Microscale templates and selective dewetting techniques have been utilized to fabricate microstructure designs and patterns for ST-PSCs. Eperon et al. first demonstrated the concept of microstructured ST-PSCs defined as “islands” via the partial dewetting of the substrate ^[77][121]. This interesting property is that the islands were large enough to allow light transmission between two adjacent perovskite islands, but they were tiny enough to appear continuous to the human eye, as seen in the SEM images ^[16][20]. Thus, the transparency of the device was determined according to the surface coverage and film thickness of the perovskite from 0 to 80%, and the overall device showed a neutral color. In addition, Eperon et al. altered the perovskite structure by substituting FAI for MAI, and they increased the efficiency from 4.9% to 7.4%. However, despite their outstanding color neutrality, the performance of these ST-PSC devices was significantly lower than that of continuous thin films. This was mainly due to insufficient perovskite coverage, as well as the loss of V_OC, owing to the immediate contact between the ETL and HTL ^[77][121].  Light scattering caused by undesirable optical scattering from the microstructure of ST-PSC can reduce visual transparency and overall device performance. Therefore, light scattering and human luminosity factors should be considered to improve ST-PSCs. It has been reported that nanostructures can improve transparency by reducing surface roughness and can also stabilize the photo-active cubic phase of perovskite ^{[78][79][80][81]}[123,124,125,126]. To this end, Kwon et al. used anodized aluminum oxide (AAO) as a scaffold layer for vertically aligned 1D-nanostructure-based ST-PSCs. They controlled the transparency of the perovskite layer via AAO pore size and height. The 1D-nanostructure ST-PSC showed a PCE of 9.6% and a 33.4% AVT, and the self-packing perovskite via the AAO layer improved stability under continuous illumination and reduced hysteresis ^[82][127].

6. Applications of ST-PSCs

6.1. Silicon/CIGS–Perovskite Tandem Solar Cells

As one approach to increase the efficiency of solar cells, a method called tandem that simultaneously uses solar cells optimized for each portion of the solar spectrum has been proposed. The tandem configuration is achieved when two or more photovoltaic systems with different band-gap regions are combined in series or parallel, which, when illuminated by 1 sun, has a strong chance of surpassing the Shockley–Queisser limit of 33.7% for a single-junction solar cell with a band-gap of 1.34 eV ^[83][129]. In the tandem solar cell (TSC) with two photovoltaic systems, the upper cell with a wider band-gap absorbs photons with high energy, whereas the bottom side cell with a narrower band-gap absorbs low-energy photons transferred from the upper cell ^[84][130]. Thus, in general, the tandem configuration applies the upper subcell with a wider band-gap to harvest higher energy photons, and bottom cells harvest low energy photons. Based on the device configuration, tandem solar cells are categorized as mechanical/optical four-terminal devices (4T) and monolithic two-terminal (2T) devices ^{[85][86][87][88]}[131,132,133,134]. The 4T cells are mainly designed by mechanically stacking, whereby the upper and bottom-side cells are linked through an electrical circuit, which facilitates independency in the fabrication process for the upper and bottom-side cells. These two cells can also be engineered using an optical splitter with high transmission in the longer-wavelength range and high reflection in the short-wavelength range. While in the 2T monolithic tandem configuration, the two subcells are directly connected via an interconnecting layer to form a tandem architect. ST-PSCs are interesting options for the top cell in TSCs with c-Si, CIGS and narrow band-gap perovskite solar cells because of their tunable band-gaps ^{[85][89][90][91][92][93][94][95][96]}[131,135,136,137,138,139,140,141,142]. When selecting acceptable band-gaps, the NIR transmittance of ST-PSCs is critical for attaining highly efficient perovskite-based tandem solar cells (P-TSCs) for both 4T and 2T devices. The optimum band-gap of upper ST-PSCs for achieving the highest PCE for 4T TSCs is 1.8 eV, and that for 2T ST-PSC is 1.75 eV, established in Shockley–Queisser models due to the current limits ^[97][143]. Compared to 2T TSCs, 4T TSCs are less susceptible to the band-gap combination of bottom cells. Most P-TSCs are configured with crystalline-Si bottom-side cells because the band-gap of Si (1.12 eV) is well matched with the upper PSC (~1.68 eV) system. Bush et al. delivered the first successful demonstration of a 2T P-TSC, with a certified efficiency of 23.6% for a 1 cm² perovskite–Si tandem solar cell. The perovskite upper cell with a band-gap value of 1.63 eV for a particular composition (Cs_0.17FA_0.83Pb(Br_0.17I_0.83)₃) was immediately placed on a smooth Si heterojunction bottom cell with a surfaced back-side ^[85][131].

6.2. Perovskite–Perovskite Tandem Solar Cells

Based on the tunable nature of the band-gap of perovskite materials, the development of perovskite–perovskite tandem solar cells has been widely studied. The first all-perovskite tandem cell with a 2T configuration was demonstrated by Im et al. They fabricated the device by laminating two different perovskite sub cells of MAPbI₃ and MAPbBr₃. The devices yielded high Voc of ~2.25 V, but the fill factor and PCE remained low, with values of 0.56 and 10.4%, respectively, suggesting that tandem cells composed only of WBG perovskite are not desirable ^[98][151]. To address this issue, a mixture of WBGs and narrow band-gaps (NBGs) employing a mixture of Sn-Pb perovskites were utilized for fabricating the devices. It was found that the alloying of Pb and Sn can help to narrow band-gap values, and they could be the ideal candidates for bottom cells and could open the way for highly efficient perovskite–perovskites tandem solar cells. The coupling of a bottom Sn-Pb perovskite cell and semi-transparent MAPbI₃ generated a PCE of 19.08% for 4T devices ^[99][152].

6.3. 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 ^[100][101][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)Se₂ (CIGS) semi-transparent solar cells, dye-sensitized cells (DSCs) and perovskite solar cells ^[102][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.

7. Conclusions

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. 

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 ^[103][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.