# Innovative Approaches to Semi-Transparent Perovskite Solar Cells
## 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. The representative micro- and nanostructured ST-PSCs are summarized in Table 3.  
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. [PERSON] et al. first demonstrated the concept of microstructured ST-PSCs defined as \"islands\" via the partial dewetting of the substrate (Figure 5a) [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 (Figure 5b) [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, [PERSON] 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\({}_{\text{OC}}\), owing to the immediate contact between the ETL and HTL [121]. [PERSON] et al. controlled a perovskite crystal's domain size and microstructure using a metal oxide honeycomb structure via colloidal monolayer lithography to overcome this issue [42]. Holes in the honeycomb structure were filled with perovskite, which had a fully controllable domain size and tunable film thickness. The honeycomb zone was primarily transparent; however, the perovskite crystals inside it absorbed enough light. This approach produced ST-PSCs with a PCE of 9.5% and an AVT of roughly 37% in the active layer, and it enhanced the V\({}_{\text{OC}}\) and fill factor. However, this approach requires additional work to optimize pore size and suppress light scattering from rough perovskite surfaces to achieve maximum efficiency. In terms of further improvements, [PERSON] et al. fabricated ST-PSCs with 30 wt% of perovskite solution, a 600 nm polystyrene scaffold and 8 min of plasma etching, achieving a PCE of 10.3% and AVT of 38% on the same line (Figure 5c,d) [122]. To fabricate patterned and microstructured perovskite films, they used microsphere lithography with a template of a SiO\({}_{2}\) honeycomb scaffold, derived from a combination of air-water interface self-assembly and oxygen plasma etching. Oxygen plasma etching allowed the precise removal of polystyrene layer-by-layer, which enabled accurate size control of the polystyrene core. These approaches show the feasibility of ST-PSCs, which can be adjusted to a near-neutral color. Figure 5: (**a**) Schematic representation of cells fabricated with dewetted region in planar perovskite heterojunction solar cells (**b**) FESEM and digital images of ST-PSC. (**c**) Schematic representation for the fabrication process for SiO\({}_{2}\) HC structure patterned perovskite layer. (**d**) Transmittance spectra for these active layers of perovskite films. Reprinted with permission from refs. [20; 122].