Development of four-terminal mechanically-stacked solar cells utilising hydrogenated amorphous silicon oxide (a-SiO_x:H) and crystalline silicon cells

Abstract

A tremendous amount of research in improving the efficiency of the single junction crystalline silicon (c-Si) based solar cells has brought its efficiency (26.6%) close to the theoretical maximum achievable conversion
efficiency (29.43%) [1–3]. Thus to further improve the efficiency, new avenues of reducing losses need to be opened up. One of the major loss, spectral mismatch loss, can be reduced by utilising tandem structures. Still, there are shortcomings associated with the conventional tandem device structure.
This thesis describes how higher efficiencies can be achieved by utilising four terminal mechanically stacked structures. This structure utilises two solar cells, which are electrically isolated but optically connected. Isolating the cells electrically negates the problem of current matching which is present
in conventional tandem cells. This structure also removes constraints such as lattice matching for the two cells. Hence many different types of four terminal cells can be developed.
In this work, high bandgap hydrogenated amorphous silicon oxide (a-SiO፱:H) thin film top cell was fabricated using plasma-enhanced chemical vapor deposition (PECVD). Three different types of c-Si cells namely, poly-Si, interdigitated back contact (IBC) and silicon heterojunction (SHJ (Hybrid)) were
utilised as the bottom cells. This is the first instance where an a-SiO፱:H based cell has been used with c-Si cells in a four terminal application.
Before actual fabrication, theoretical calculations using two parameters called as Response 4T and Spectral Response 4T were made to determine the optimal configuration as well as the efficiency enhancement for the four-terminal device. From this theoretical calculation, an efficiency gain of almost 4% can be obtained when an a-SiOx:H top cell is used with a poly-Si cell in four terminal
configuration. Gain between 1% and 2% can be realised by utilising four terminal topology for IBC and SHJ (Hybrid) bottom cells. These gains in efficiency are in comparison to the efficiency of the bottom cell alone.
A first of its kind- bifacial a-SiO፱:H cell with the efficiency of 6.60% (V_oc:0.97 V, J_sc:10.31 mA/cm² and fill factor:0.66) was developed. Using this cell as the top cell four terminal devices were fabricated. A gain in efficiency of 0.46% was obtained for the four terminal device based on the poly-Si bottom cell.
Further analysis of the four terminal devices using GenPro4 simulation tool was also performed. The analysis pointed to the reflection due to the top cell substrate glass as the limiting factor in improving the efficiency of the four terminal cell further. Parasitic absorption in the carrier selective layers of the
thin film cell was another contributor to the losses. Through this work, the potential of four terminal device concept was analysed and actual development of the device was carried out. From the work done, future steps in improving the efficiency of the four terminal device were also deduced.
Present work positively brought hope for further improvement in efficiency of crystalline silicon solar cells.