Three terminal tandem HEterojunction on interdigitated back contacts SIlicon Solar cell – THESIS

The methodology consists, on the one hand, in the development and manufacturing of silicon solar cells with interdigital contacts on the back side and, on the other hand, in the deposition of perovskites on the upper surface of the silicon cells, with the management of the interface of the perovskite stack on silicon by implementing a suitable interface layer.
Despite the expertise of the consortium partners and their know-how to separately produce high quality back-contact silicon cells and good quality perovskite layers, there is nevertheless an important problem to be solved, namely the interface between the perovskite stack and the silicon, which requires special care. Thus, part of the project will focus on the study of this interface and will include investigations on the buffer layers. However, there is a risk that the interface may not be able to meet the requirements of strip alignment to, on the one hand, allow the extraction of one type of charge carriers in silicon and, on the other hand, serve as a barrier for the other type of charge carriers as described in a recent patent of one of the project partners and in a recent article by the same partner (see Scientific Productions and Patents). Another challenge is the choice of a silicon passivation on the interface side between the perovskite stack and the IBC sub-cell. This passivation must be compatible with the selectivity of the electronic transport between the perovskite stack and the IBC structure. Several passivations of the silicon at this location of the 3T structure will be studied prior to the deposition of the stack.

The first simulations show that a conversion efficiency above 30% is quite realistic and achievable despite the estimated losses due to the nature of the stacking of the perovskite-silicon interface which must be both selective for the transport of photo-generated carriers, passivating for the silicon surface and compatible with the deposition of the perovskite-silicon structure.
In addition, the first realized structures of the Top sub-cell demonstrate a vertical transport of the photo-generated carriers in the perovskite stack and collected by the backside electrode, as predicted in the simulations. The photovoltaic performances obtained for this sub-cell are indeed very encouraging, even if an optimization of the interface remains to be done.

The stability of the silicon-deposited perovskite structure is a crucial point for maintaining the performance of the three-terminal tandem structure. Encapsulation solutions and compatible anti-reflection layers will be studied by the consortium.
Other passivation solutions for the front side of the silicon IBC structure, compatible with the elaboration of the selective layer and perovskite stacking, will be studied.

Scientifiic production :

1) Z. Djebbour, W. El-Huni, A. Migan Dubois, J-P. Kleider, Bandgap engineered smart three-terminal solar cell: New perspectives towards very high efficiencies in the silicon world, Prog Photovolt Res Appl, First published: 14 January 2019 doi.org/10.1002/pip.3096, hal.archives-ouvertes.fr/hal-02550123

2) D. Sapori et al, Toward Industrialization of Monolithic Perovskite/Silicon-Heterojunction Tandem Solar Cells: Screen-Printing Metallization Development, presented at NIPHO, Sevilla, 2020.

3) K. Ahanogbe, J.P. Connolly, C. Léon, S. Le Gall, M-E. Gueunier-Farret, D. Sapori, S. Berson A-S. Ozanne, Z. Djebbour, J-P. Kleider, Optimisation de cellules solaires tandem à trois terminaux à base de pérovskites, Journées Nationales du PhotoVolataïque Dourdan (91) 3-6 déc. 2019, hal.archives-ouvertes.fr/hal-02344117/

4) K. Ahanogbe, J.P. Connolly, S. Le Gall, M-E. Gueunier-Farret, S. Berson A-S. Ozanne, Z. Djebbour, J-P. Kleider, Récentes études d’optimisation de cellules solaires à trois terminaux à base de pérovskite, Journées Nationales du PhotoVolataïque Dourdan (91) 26-29 jan. 2021

Patent :

patentscope.wipo.int/search/fr/detail.jsf

Today, single junction silicon technology dominates the photovoltaic (PV) market, with more than 90% of market share. However, the power conversion efficiency of silicon solar cells is now close to the theoretical limit. Indeed, the record has been pushed to 26.7 %, which is close to the silicon single junction theoretical limit of approximately 29% when the unavoidable Auger recombination is taken into account.
To increase solar cell efficiency above 30% while keeping the abundant, cheap and stable silicon material as a basis, one solution is to couple silicon with another semiconductor having a larger bandgap in a tandem cell configuration. Currently, silicon based tandem technology follows two paths: the monolithic two terminals tandem (2TT) where the top and the bottom sub-cells are electrically and optically connected, and the four terminals tandem (4TT) where the two sub-cells are electrically independent. However, the 2TT architecture needs to manage photocurrent matching and to optimize the tunnel junction charges transport mechanisms between the top and the bottom sub-cells, while the 4TT device has to deal with issues related to the buried contacts shadowing and access and losses induced by the adhesive interconnection.
The THESIS proposal aims at developing an original 3 terminals tandem solar cell (3TT).
The approach is threefold:

- To propose a new solar cell technology with 3 terminals. This allows us to suppress the constraint of photo-current matching for the two cells constituting the tandem cell. Furthermore, a 3-terminal tandem cell does not need a tunneling junction.
- To facilitate the access to the different contacts of the top and bottom cells without the need for etching and without having to align buried contact grids,
- To combine the advantages of reliable and mastered silicon technology with those of emerging technologies, allowing the creation of a heterojunction stack with the silicon.

This new 3-terminals tandem cell technology we have patented is made possible in an innovative and simple way by using a silicon PV cell with interdigitated back contacts (IBC) on the rear face as a bottom sub-cell and depositing a larger bandgap semiconductor on top of the c-Si surface with a selective band offset barrier (BOB) at the interface in order to form a front heterojunction stack (FHS) realizing a top heterojunction sub-cell. This barrier is chosen so that the heterojunction allows a separation of the operation of the two cells.
In the THESIS project, we propose to focus on the emerging perovskites as the absorber of the p-type FHS. The interface between the perovskite and silicon will be actively studied in the project and will need deep investigations to improve the interface quality and device operation.
We plan to use also p/i a-Si:H stack as the FHS, forming a (p) a-Si:H/ (i) a-Si:H/ (n) c-Si vertical front subcell. Of course, we do not expect the best photovoltaic performances with this subcell due to the limited transport properties of a-Si:H. However, the growth of device quality a-Si:H for the top subcell, and the c-Si IBC technology are already well mastered in the consortium, so this will allow us to fabricate a proof of concept device for this innovative 3TT architecture. This will be a breakthrough in the PV world, since the 3TT architecture has never been demonstrated so far.