TOPCon-based perovskite/silicon tandemsolar cells : interconnection and system optimization

Engineering notes

Engineering notes will be added by the Power for Solar editorial team.

Chinese explanation / 中文解读

中文解读待补充：本站会优先为光伏效率、钙钛矿太阳能电池、储能技术、太阳能热利用、BIPV、并网技术等高价值论文补充中文说明。

Original abstract

Tandem solar cells are a promising technology to overcome the efficiency limitation of conventional silicon single junction solar cells. In particular, the combination of perovskite top solar cells with established silicon solar cells as bottom cell is studied with great interest worldwide. Thereby, the choice of the bottom cell is pivotal, as it contributes to the total energy output and influences the perovskite formation when used as substrate for the top cell processing. Although silicon heterojunction (SHJ) solar cells are commonly used in research, the presented work focuses on tunnel oxide passivating contacts (TOPCon) bottom cells, as this technology currently dominates industrial production. In this work, TOPCon bottom cells are integrated into perovskite/silicon tandem devices featuring an industrial random pyramids front side texture. Conformal coverage of the textured front side is achieved by depositing the perovskite absorber via a hybrid evaporation/spin-coating process, resulting in the so-called fully-textured solar cell design. The influence of the different lateral conductivity of SHJ- and TOPCon-contact layers on charge carrier diffusion out of the active solar cell area is investigated. Furthermore, an in-depth investigation of various TOPCon2 bottom cell architectures employing p-TOPCon as the hole contact and n-TOPCon as the electron contact is conducted to identify the optimal bottom cell design. In the scope of this thesis, this is a solar cell with a p-TOPCon rear emitter on a non-textured silicon wafer rear side. This combines the advantages of a better surface passivation quality of p-TOPCon on planar surfaces and the higher shunt resistivity of p-TOPCon compared to n-TOPCon. This research enabled the production of TOPCon2 bottom solar cells with remarkably high implied VOCs above 730mV, as well as the demonstration of the first fully-textured tandem solar cell incorporating a TOPCon2 bottom cell with an efficiency of 30.6%. Although ITO was used as the recombination layer for these solar cells to showcase the efficiency potential, indium is not an option for solar cell mass production on terawatt scale. Therefore, the integration of the indium-free alternatives ZTO and AZO is investigated. To this end, the sputter deposition of ZTO and ITO on TOPCon is analysed. Afterwards, the material properties of AZO, ITO and ZTO are compared and their tandem integration is investigated. A central findings of this study is, that TCOs can be sputtered at high power on TOPCon, which enables high throughput and industrial application. Furthermore, ZTO is identified as a promising indium-free recombination TCO which is sputtered in an industrially feasible process without a performance penalty compared to ITO. Polysilicon tunnel junctions consisting of a n-doped TOPCon layer and a second p-doped polysilicon layer on top are investigated as second option for an indium-free recombination junction. A systematic investigation of the process sequence and process parameters for the tunnel junction production provides a fundamental understanding of the process requirements. Subsequently, two process routes for the tunnel junction production are identified which, in addition to low contact resistivity and high substrate surface passivation quality, also enable lean process integration. However, the contact formation between HTL and the p-polysilicon layer of the tunnel junction is a major challenge. Despite systematic investigation of HTL materials, p-TOPCon surface treatments, and p-TOPCon layer properties across several single junction perovskite batches, this challenge could not be fully resolved. Further investigations should focus on a fundamental understanding of the HTL/p-TOPCon contact and comparison to the HTL/TCO contact formation. Finally, proof-of-concept solar cells incorporating a polysilicon tunnel junction are realized by incorporating a Ni/NiOx layer stack between the HTL and p- TOPCon to enhance contact formation. The presented solar cells are strongly limited by the optical transparency of the incorporated Ni/NiOx layer stack, as well as high contact resistivities. Incorporating doped NiOx layers is suggested for future investigations, as a higher doping concentration could eliminate the need for a metallic layer and reduce series resistance, thus resolving both limitations.

Links and sources

Comments