*4.1.1 PVSK layer on flat substrates*

Under PVSK processed on flat substrate substrates two techniques are discussed; GQ and solution spin-coating. GQ method was developed at the same time with the

#### **Figure 3.**

*a and b: (Left). Schematic of a perovskite-silicon tandem solar cell, together with the absorption spectrum of both perovskite.*

solvent quenching method. Instead of using anti-solvent, in the GQ method, a flow of nitrogen gas was employed to facilitate the evaporation of precursor solution during one-step coating. By annealing, smooth films with densely packed grains are attained. Recently, many studies related to efficient multi-cation and multi-anion PVSK solar cells have been reported. Such a method has been widely used in fabricating PVSK layer in tandem devices. Adopting the GQ method, in 2020, McGehee and his team reported a 1.67 eV wide-band gap PVSK that consisted of triple-halide alloys of chlorine, bromine, and iodine. In addition the realizing the PVSK, the top cell showed improvement in carrier lifetime and charge-carrier mobility comparing to controlled ones. The reason for improvement was attributed to the enhanced solubility of chlorine by replacing iodine with bromine to shrink the lattice parameter. Of great importance, light-induced phase segregation in PVSK films was significantly suppressed. The conversion efficiency of 27% with an area of 1 cm<sup>2</sup> , for this tandem cell was achieved in the laboratory. The cells had an improved stability with less than 4% degradation after 1000 h of MPP operation at around 60°C.

In the solution spin coating technique, a lot of work has also been done in the fabrication of PVSK films. By using this method, in 2016, a team led by Rech fabricated monolithic tandem cells with 18% conversion efficiency [43]. In 2019, Chen et al. combine two additives, MACl and MAH2PO2 in PVSK precursor, which significantly improve the morphology of the wide bandgap (1.74–1.70 eV) PVSK films, resulting in a high tandem *V*OC of 1.80 V and improved conversion 25.4%. In 2020, Shin and his team used the solution spin coating method to develop stable PVSK solar cells with a band gap of about 1.7 eV and conversion efficiency of 20.7%. The fabricated solar cells were tested and found to be stable under extreme conditions. Those cells fabricated could retain nearly 80% of their initial efficiency after 1000 h under continuous illumination. In controlling both structural and electrical properties of the PVSK, anion engineering for materials is undertaken i.e. phenethylammonium (PEA) based 2D additives is found to be important. Under this method high efficiency of 26.7% in a monolithic 2T wide gap PVSK/Si tandem solar cell was realized by combining spectral responses of the top and bottom diodes.

#### *4.1.2 PVSK layer on textured c-Si*

*Combination of two-step deposition method*: Currently, a single-side texturing arrangement of monolithic PVSK/Si tandem devices is predominantly common. Texturing the back side is done to enhance light trapping properties of the solar cell. In comparison with a double-side polished c-Si device, that has their front surface flatpolished in order to be conformable with the solution based PVSK manufacturing process; light trapping property in such an arrangement is not perfect. Consequently, there is a need to build high conversion efficient tandem cells by using double-side textured c-Si approach. This technique has been used by Ballif and his team to come up with a two-step deposition method, where sequential co-evaporation and spincoating processes are applied. This resulted into conformal PVSK absorber layers on the micrometer-sized pyramids of textured monocrystalline Si. This type of arrangement resulted in a high current density of 19.5 mA cm<sup>2</sup> . The team achieved a conversion efficiency of 25.2% after texturing of the c-Si bottom cell in the micrometer range pyramids. This process reduced the primary reflection loss, enhancing light trapping properties in device.

*PVSK on textured c-Si*: Solution processed PVSK on textured c-Si has several limitations. These include uncovered Si peaks, shunt paths, and poor charge collection in

#### *Solar Solutions for the Future DOI: http://dx.doi.org/10.5772/intechopen.105006*

films with variable thickness, etc. The covering of pyramidal peaks by PVSK of good quality has been reported by a team led by Sargent [44]. The improvement of drift and diffusion of photo-generated carriers in these films enhanced charge collection. In this approach, they used PVSK of wide-bandgap solar cells with a bottom cell of pyramidal-textured c-Si. This approach resulted into improvement of depletion width, in the PVSK and enhancement of carrier collection. In addition to increasing the carrier diffusion length, they used a passivator on the PVSK rough surfaces. And this passivation suppresses the undesired phase segregation. These attributes resulted into PVSK/c-Si cells achieving a conversion efficiency of 25.7% and good thermal stability at 85°C and MPP tracking at 40°C. Blade-coated PVSK on textured silicon with pyramids less than 1 μm in height has been reported by Huang–s group in 2020. Similarly a conformal hole transport layer and perovskite layer that fully covers the textured silicon solar cell were fabricated using nitrogen-assisted blading process. This perovskite/silicon tandem device achieved a conversion efficiency of 26% on textured silicon [45].

In conclusion, several deposition techniques for PVSK film have been advanced and widely researched on. A tandem cell of good quality of polycrystalline films can also be fabricated on both flat and textured substrates. In PVSK/Si tandem solar cells, there are no technical difficulties for flat c-Si. But for textured c-Si, it still appears to be challenging to get conformal films with uniform thickness via sequential coevaporation and spin-coating methods, in particular for textured monocrystalline Si with large micrometer-sized pyramids. This calls for more investigation and development of more convenient and efficient deposition methods for PVSK films on the textured substrate.
