# Innovative Approaches to Semi-Transparent Perovskite Solar Cells
## 6 Applications of ST-PSCs
### 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 [PERSON] et al. They fabricated the device by laminating two different perovskite sub cells of MAPbI\({}_{3}\) and MAPbBr\({}_{3}\). The devices yielded high Voc of \(\sim\)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 [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 MAPbI\({}_{3}\) generated a PCE of 19.08% for 4T devices [152]. [PERSON] and co-workers, via the incorporation of 0.03 mol% Cd\({}^{2+}\) into a perovskite precursor, de-doped Sn-Pb perovskite and optimized the thickness, yielding an efficiency of 20.2% for single-junction solar cells and 23% for 2T-based tandem devices [153]. [PERSON] et al. utilized a comproportionation reaction to decrease the amount of Sn vacancies in the perovskite precursor by reducing Sn\({}^{4+}\) to Sn\({}^{2+}\) by doping Sn into the Sn-Pb mixed perovskite precursor solution. The resulting devices exhibited a PCE of 21.1% and improved fill factor, Jsc and Voc values due to reduced vacancies and an improved charge carrier diffusion length (-3 \(\upmu\)m). They also fabricated a 2T monolithic all-perovskite TSC exhibiting a PCE of 24.8% with high Voc. The devices demonstrated good stability as well, retaining 90% stability even after 463 h of MPP under one-sun illumination (Figure 7c) [154]. In a recent study, [PERSON] et al. investigated two novel solution-processed fullerene derivatives as interlayers for Sn-Pb-based narrow-band-gap PSCs. The interlayer improved charge transportation and decreased non-radiative recombination, delivering an improved PCE of 18.6% with improved Voc and Jsc. They also demonstrated 4T all-perovskite TSCs with a PCE of 24.8%, utilizing an ST wide band-gap of 1.63 eV and an NBG of 1.26 eV [155]. Furthermore, [PERSON] et al. fabricated flexible all-perovskite tandem solar cells with nearly 25% efficiency and with good bending stability. They incorporated a molecular-bridged hole selective contact (HSC) to overcome interfacial recombination and to enhance hole extraction in flexible devices on a low-temperature NiO film. With this strategy, they achieved a PCE of 24.7% and 23.5% for 0.049 cm\({}^{2}\) and 1.05 cm\({}^{2}\), respectively. [PERSON] et al. used a mixture of Pb and Sn to fabricate a Pb-Sn binary 4T all-P-TSC. Upon increasing the Sn concentration from 0 to 75%, the band-gap of MAPb\({}_{1-x}\) Sn\({}_{x}\)I\({}_{3}\) decreased from 1.61 to 1.27 eV [152]. Recently, guanidinium thiocyanate (GuaSCN) was added into a Sn-Pb PVK film by [PERSON] et al. to improve the carrier lifetime. Perovskite with (FASInI\({}_{3}\))\({}_{0.6}\)(MAPbI\({}_{3}\))\({}_{0.4}\), having a 1.25 eV band-gap, demonstrates a carrier lifetime of more than 1 us. In addition, when it is combined with a C8\({}_{0.05}\) FA\({}_{0.8}\)MA\({}_{0.15}\)PbI\({}_{2.55}\)Br\({}_{0.45}\) (Eg = 1.63 eV) front cell, a resultant PCE of 25.4% is obtained for 4T tandem cell [26]. Furthermore, [PERSON] et al. utilized a vacuum-assisted growth method to control the thin-film formation of Sn-Pb binary perovskite with a band-gap of 1.27 eV. They obtained a PCE of 18.2% with increased charge carrier lifetime.