# Innovative Approaches to Semi-Transparent Perovskite Solar Cells
## 3 Transparent Electrodes for ST-PSCs
Moving further, various metal nanostructures, e.g., AgNWs, CuNWs, Ni mesh, Ag mesh, etc., are employed as TEs for ST-PSCs, owing to their excellent electrical and optical properties and their ability to be applied in low-priced and roll-to-roll processes [41; 49; 81; 82; 83; 84; 85; 86]. Among them, AgNW is the most widely used TE material with the easiest application for ST-PSCs. This is because AgNW has considerable advantages as a top TE in ST-PSCs, owing to its excellent electrical and optical properties and simple  
Figure 3: (**a**) Schematic representation of used PSC architecture. Enlarged view of the used top-electrode. SEM image of the used Au film, and of the modified electrode with MoO\({}_{3}\)/Au films. (**b**) Current–voltage characteristics of the fabricated cell with different transparencies. (**c**) Schematic image of devices fabricated with AgNWs composite electrode and ultra-thin Au layer (UTA). (**d**) Transmittance spectra of fabricated devices with different thicknesses. Reprinted with permission from Refs. [30; 81]. production procedure [87]. Using this methodology, [PERSON] et al. constructed an ST-PSC with a structure of ITO/PEDOT:PSS/CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3-}\)xCl\({}_{x}\)/PC\({}_{60}\)BM/ZnO NP/AgNWs and attained a PCE of 8.49% with an AVT of 28.4% [40]. Here, PCBM and ZnO were applied as energy-level modifiers for better transport and energy level alignment. Pores present in the Ag thin film improve transmittance but can act as a factor that degrades the performance of the ST-PSC. The pores provide a pathway for moisture or oxygen to easily contact and react with the perovskite, which leads to the degradation of the perovskite. Therefore, to address this issue, [PERSON] et al. employed an encapsulation layer on the devices to prevent damage caused by oxygen and water [88]. [PERSON] et al. utilized an ultra-thin transparent Au layer (UTA) underneath spin-coated AgNPs to physically separate it from direct contact with the active layer (Figure 3c,d). By doing so, they resolved the stability issues and secured ST-PSCs with an 11% PCE, which were illuminated from the front side [81]. Furthermore, [PERSON] et al. introduced C\({}_{60}\) as a filler in the Ag NWs network; they revealed that C\({}_{60}\) can improve energy level alignment and ohmic contact within the buffer layer and AgNPs, achieving stability and an enhanced PCE of 11.02% [89]. Graphene, a two-dimensional carbon material with robust physical and chemical properties, transparency, conductivity, stability and flexibility, has drawn considerable attention for photovoltaic devices and is also being explored for ST-PSCs [90; 91; 92; 93; 94; 95; 96]. Graphene modified with PEDOT:PSS was investigated by [PERSON] et al. to improve transparency and conductivity and attained a PCE of 12.02% [97]. ST-PSCs were also fabricated with stacked CVD graphene (Figure 4a). The perovskite layer thickness was optimized at 150 nm, and the maximum transmittance reached about 50% at 700 nm (Figure 4b and inset of Figure 4b). The ST-PSC fabricated with stacked CVD graphene achieved a PCE of 5.98% through FTO side illumination [97]. Another carbon form, carbon nanotubes (CNT), are beneficial for creating transparent electrodes in solar cells due to their superior conductivity, flexibility and chemical and mechanical endurance [98; 99; 100]. However, severe fabrication conditions and solvents limit its use as a top electrode in ST-PSCs [101]; the solvent used in processing needs to be favorable for the active and buffer layer in the device [102]. To overcome this issue, a printing technique was used by [PERSON] et al., whereby they utilized carbon grids as top electrodes to produce entirely printable ST-PSCs with a configurable AVT, and the effect of grid spacing was investigated [103]. Grid spacing and PCE have inversely proportional relationships. As grid spacing is reduced, PCE improves and AVT is reduced, and vice versa.