# Innovative Approaches to Semi-Transparent Perovskite Solar Cells
## 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 [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 2, transparent conductive oxides (TCOs), including fluorine-doped tin oxide (FTO), [49; 50], indium tin oxide (ITO) [51], indium-doped zinc oxide [52; 53; 54] and aluminum-doped zinc oxides [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 \(\Omega\)/sq), effective charge collection and long-term stability [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. To solve the problems caused by the high-temperature etching of a perovskite layer accompanied by a deposition process such as ITO and FTO, various top electrode deposition methods capable of reducing damage to the photoactive layer, e.g., low-temperature atomic layer deposition or the insertion of a buffer layer, have been developed. Reducing sputtering power is one of the most efficient methods to protect the perovskite layer from damage [50; 61]. For example, [PERSON] et al. used the magnetron sputtering method for TCOdeposition as the top TE and controlled sputtering power to guard the perovskite layer against damage [56]. In addition, to prevent damage to perovskite, an approach was attempted to coat various robust materials, such as inorganic MoO\({}_{x}\), AZO nanoparticles, thin-metal layers (Ag and Au), ITO nanoparticles, etc., on the perovskite layer prior to TCO deposition. In previous reports, inorganic MoO\({}_{x}\) have been used as a buffer layer between TCO and perovskite [62; 52; 63]. ST-PSCs with the device structure of ITO-coated glass/PTAA or NiO\({}_{x}\)/CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\)/PCBM/ AZO/ITO were fabricated by [PERSON] et al., who made a spin-coated PCBM film and buffer layer (AZO) under sputter-deposited TCO. The AZO layer shielded the PCBM and perovskite films from ITO sputter deposition and eliminated the extraction barrier between PC\({}_{60}\)BM and ITO. As a result, an ST-PSC with a maximum PCE of 12.5% and AVT above 11% was obtained and showed stable performance for up to 4000 h under one-sun illumination [64]. Another problem encountered while using TCO is that conductivity and transmittance are in a trade-off relationship. In general, to increase conductivity, the thickness of the TCO must be increased, but such an increase in thickness leads to a decrease in transmittance. Therefore, increasing conductivity while maintaining transmittance is challenging, and various methods are being studied. For instance, doping has been investigated for the last few years to improve the conductivity of ITO [50]. [PERSON] et al. used Ce-doped In\({}_{2}\)O\({}_{3}\) (ICO) as the transparent top electrode to fabricate ST-PSC. The fabricated film had an average of 83.5% transmittance in the 400-1800 nm range, good mobility (51.6 cm\({}^{2}\)/(V-s)) and low resistivity (5.74 \(\times\) 10\({}^{-4}\)\(\Omega\) cm), demonstrating its appropriateness for ST-PSCs. [65] In another approach, [PERSON] et al. reported high-performing ST-PSCs made with zinc-doped indium tin oxide (IZTO) as the transparent top electrode, made via linear-facing target sputtering without any added thermal treatment.