Easy to fabricate, both sides poly-Si passivating contact bottom cell for Perovskite/Silicon tandem devices – EPoBoC

The conservation of resources in the production of highly efficient Si bottom cells (use of cast mono silicon, avoidance of critical resources such as Indium) requires adjustments to the design and process flow, as well as to critical individual processes for manufacturing these solar cells. The use of cast mono silicon offers the opportunity to utilize a very CO2-efficient raw material. However, the initially lower quality of this material compared to standard Cz must be enhanced to the required level by adoptions in the cell processing, such as gettering through the poly-Si layers. Simultaneously, the necessary process adjustments must not compromise the quality of the individual layers; instead, the quality of the individual processes (e.g., passivation of poly-Si layers) must be further improved. In the area of interface layers between silicon and perovskite cell, compatible layers that do not require Indium need to be developed. To achieve this, In-free TCO layers as well as a poly-Si-based tunnel diode are being developed. Ultimately, the entire processing for the bottom solar cell must enable industrial implementation.

By project's end, the aim is to have developed a silicon sub-cell optimized for use in a perovskite/Silicon tandem cell, combining highest efficiency with the efficient use of resources and a lean process chain. Focusing on TOPCon technology, the current standard technology in the field of silicon single-junction solar cells, the TOPCon2 sub-cell can directly benefit from current market developments and offers the opportunity for a swift transition to production as a direct evolution of TOPCon single-junction solar cells. The use of an alternative silicon raw material (quasi-mono silicon) in the sub-cell enables not only CO2 savings but also positions the involved industrial partner at the forefront of technology, both in material manufacturing and its integration into processes.

With the focus on tandem solar cells, EPoBoC project aims to increase the efficiency of PV and will thus contribute to the promotion of renewable energy generation in the long term, thereby helping to protect the environment and the climate
To this end, EPoBoC project aim to address challenges linked to the use of low-CO2 base material (cast mono silicon), its implementation in double-sided Tunnel Oxide Passivated Contact structure (TOPCon2) to provide sub-cell compatible tandem cells technologies as well as development of Indium-free recombination contacts as interlayer in-between the silicon bottom cell and the perovskite top cell (either In-free TCOs or poly-Si based tunnel-junction).
Upon completion of the project, proof of simplified concept, sustainable ( low CO2-efficiency, resources saving and elimination of critical materials) and industrially feasible should be prove by the implementation of highly efficient double-sided Tunnel Oxide Passivated Contact sub-cell (TOPCon2) in a 2-terminal perovskite/Si tandem solar cell full device.

Upcoming results from the project will be available to the scientific, political and industrial specialist public by participating in the relevant scientific conferences (presentation of the results) and by publishing them in journals

To continue to follow the long-term learning curve of ca. 0.5%absolute per year for industrial silicon solar cells, innovation is needed to allow for conversion efficiencies beyond its physical limit of 29.4%. Multi-junction solar cells that encompass silicon and perovskite subcells offer a very promising option to continue the needed efficiency increase for industrial solar cells for the next decades. The scope of this project thus is the development of a highly efficient but simple and resource-saving Si
subcell for a 2-terminal perovskite/Si tandem solar cell. The subcell to be developed within this project encompasses the recombination contact as well as the hole contact layer, which represents the transition from the Si to the perovskite cell. The Si subcell utilizes both sides passivating contacts based on poly-Si/SiOx, which have a passivation performance such that it allows for the highest efficiencies. In addition to the increase in efficiency and the resulting cost benefits, the topic of sustainability will also be directly addressed in the project. “Cast mono” silicon wafers, which are a low CO2 silicon base material will be investigated as an alternative for the standard Czochralski (Cz) silicon wafers for the fabrication of the silicon bottom cell. Furthermore, particular attention will be given to the application of contact layers such as the recombination junction and the hole transport layer, that do not contain rare elements such as Indium.