# Innovative Approaches to Semi-Transparent Perovskite Solar Cells
## 6 Applications of ST-PSCs  
### 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  
Figure 6: (**a**) Nanopillar-structured ST-PSC with different AAO templates and perovskite thicknesses. (**b**) Schematic representation of the device fabricated with Ag nanocubes. (**c**) Current–voltage spectra and transmittance spectra of plasmonic and non-plasmonic PVSCs together with the human luminosity curve. Reprinted with permission from Refs. [127; 128]. 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 [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 [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 [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. As there is no electrical circuit, the tunnelling junction and recombination layer perform a crucial role in charge transport and current losses. The corresponding 4T and 2T tandem solar cells are depicted in Figure 7a. 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 [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 [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 (\(\sim\)1.68 eV) system. [PERSON] et al. delivered the first successful demonstration of a 2T P-TSC, with a certified efficiency of 23.6% for a 1 cm\({}^{2}\) perovskite-Si tandem solar cell. The perovskite upper cell with a band-gap value of 1.63 eV for a particular composition (\(\text{Cs}_{0.17}\text{FA}_{0.83}\text{Pb}(\text{Br}_{0.17}\text{I}_{0.83})\text{s}\)) was immediately placed on a smooth Si heterojunction bottom cell with a surfaceback-side [131]. [PERSON] et al. stacked ST-PSCs on an inter-digitated back contact (IBC) silicon bottom cell mechanically with a 1.73 eV band-gap perovskite absorber to obtain a tandem efficiency of 26.4% for 4T P-TSCs [144]. [PERSON] et al. employed \(\text{Cs}_{0.15}\text{(FA}_{0.83}\text{MA}_{0.17}\text{)}_{0.85}\text{Pb}( \text{I}_{0.8}\text{Br}_{0.2}\text{)}\) with a band-gap of 1.64 eV as the active layer and attained a PCE of 25.4% using it for the top in a 2T P-TSC (Figure 7b) [145].