The multijunction-based tandem solar cells, which belong to 3rd generation photovoltaics, made by stacking the two or more cells of different bandgap in series, are getting more attention by the research community to reach high efficiency by crossing the so-called Shockley–Queisser limit (S–Q limit) of single junctions. This has become a next target, considering that the conventional high-efficiency crystalline silicon (c-Si) solar cells are already inching towards the S-Q limit η ≈ 29.4% (Eg = 1.1 eV). Recently, a monolithically integrated two-terminal (2T) tandem solar cell based on perovskite (top cell) and c-Si (bottom cell), also called “2T monolithic perovskite/silicon tandem solar cell”, has recorded the highest efficiency η ≈ 33.9%, surpassing the S-Q efficiency limit. However, these 2T monolithic perovskite/silicon tandem cells suffer from efficiency losses due to; (a) the top perovskite cells require a planar surface for the conformal growth, without any defect, instead of a textured Si surface used in the conventional high-efficiency c-Si solar cell, (b) due to the indirect bandgap of c-Si, the c-Si heterojunction cells at the bottom side of tandem with a planar front surface suffer from absorption losses, and to compensate this, the thickness of the c-Si cell has to be increased.
The present work focuses on tackling these issues in c-Si based tandem solar cells and design a high-efficiency thin tandem solar cell by incorporating other alternate absorber materials in the bottom cell such as SiGe quantum dots (QDs) or a planar nanostructured light trapping layer in the intermediate region between the top and the bottom sub-cells. Therefore, the thesis covers two parts; In the first part, the synthesis of QD-sized SiGe alloy nanocrystals (NCs) in a very-high frequency plasma enhanced chemical vapor deposition (VHF PECVD) reactor at different plasma condition is explored. The advantage of these QDs is that compared to bulk c-Si, they have direct bandgap and tunable from 0.7-1.76 eV by tuning the NC size, and, the Ge content in the SiGe alloy. Moreover, the high exciton Bohr radius of SiGe (24.3 nm) compared to Si (~5 nm) results in more pronounced quantum confinement in SiGe NCs and also ease the fabrication of QD-sized particles. One of the fundamental queries is whether an alloyed QD can be formed without segregation. The TEM, SAED, EDS, HAADF-STEM and Raman studies of the SiGe alloy NCs confirm their size, crystallinity, homogeneity of Si and Ge atom distribution in the NC, and the alloy nature. Further, an electrical study of SiGe alloy NCs has been done by scanning tunneling spectroscopy at room temperature to analyze the bandgap of individual NCs. Interestingly, one of the unexpected findings from the analysis is the shallow p-type character of these SiGe alloy QDs despite the fact that no dopants were introduced during the deposition. Besides this, presence of midgap defect states is also observed in some of the samples. These two observations are interesting, considering the “self-purification” nature of QD bulk, as proposed in the literature. Therefore, to get the device quality SiGe alloy NCs, a complete understanding of the behaviour of plasma modes need to be explored to suppress the defects and control the doping nature of SiGe alloy NCs.
In the second part, optical simulations were explored in the FDTD Solver to design a light trapping layer for the planar monolithic perovskite/silicon tandem solar cell, incorporated in the intermediate region between the top and the bottom sub-cell to manage the light in the bottom sub-cell. Firstly, a 2D nanostructured layer, similar to a 2D grating, which is topologically flat but optically rough (TFOR) also called TFOR layer and a DBR (distributed Bragg reflector) was designed and the parameters were optimized by checking the reflectance-transmittance spectrum w.r.t air. Further, the TFOR layer were incorporated in the intermediate region of very-thin (60 μm) SHJ (silicon heterojunction cell) bottom cell based planar monolithic perovskite/silicon tandem solar cell, and the photocurrent density, absorptance, reflectance and electric field were studied in the top and bottom sub-cells to optimize the tandem cell performance. Besides this, the tandem cell performance was also checked by incorporating (TFOR+DBR) combination layer in the intermediate region. The simulated tandem cell shows a good current-match between the top (perovskite) and the bottom (c-Si) with the incorporation of TFOR layer. The present work thus helps to obtain a deeper insight into the delineation of the underlying physical processes of light management and highlights some practical avenues towards the realization of better efficiencies for tandem solar cells.
Publications based on the present work:
1. Md Seraj Uddin, et al., Eur. Phys. J. Appl. Phys. 91, 20801 (2020).
2. Md Seraj Uddin et al, Physica B (Revised manuscript at the Editor).
3. Md Seraj Uddin, et al., Materials Today: Proceedings 39, 1974–1977 (2021).
4. Md Seraj Uddin, et al., J Mater Sci: Mater Electron 34, 1753 (2023).