Functionalized OXIdes for ultra-thin junctions and tunneling contacts: Towards a new crystalline silicon photovoltaic cells GENeration – OXYGENE

This project focuses on new fabrication technologies for passivated contacts silicon solar cells, through a collaboration between french academic and industrial partners. The approaches will be the following:

1/ Develop know-how about passivated contacts for bifacial cells technologies
2/ Propose technological solutions adapted for the fabrication of passsivated contacts solar cells in industrial environments

These goals can be achieved first through scientific dissemination (publications, conférences, patents,...). Then, the processes developped in the Pulsion Solar implanter from IBS (Ion Beam Services) will be highlited to allow its use in existing industrial production lines. These processes can reduce the wet chemical consumption (using implantation) as well as replace Indium-based TCOs by AZO layers, allowing a more ecological and long term viable production of PV cells.

During the first half of the project, a lot of simulation, characterization and solar cells results have been obtained. 1D and then 2D modelling of carrier transport through the «tunnel« oxide has been obtained using a TCAD software (Sentaurus) and compared to our experimental results, showing that direct tunneling was involved, but also that «pinholes« could totally change the results. Characterizations such as KPFM, XPS and I-V-T were done on test structure with different tunnel oxide layers, helping to highlite the crucial role of this layer on transport mechanisms. The Plasma Immersion Ion Implantation (PIII - B2H6 and PH3) technology was applied on cristalline silicon and also polysilicon on oxide stacks, validating the very high quality of the fabricated junctions (Implied Voc at 730mV). AZO/SiNx:H stacks have also been developed, allowing after high temperature treatment (750°C) to obtain effective interface passivation while keeping high electrical conductivity in the AZO layer. A lot of integration tests have been done to select the best cells fabrication processes. Efficiencies above 21% have been obtained on n-type industrial substrates (240cm2) with hybrid structures featuring a rear side passivated contact only (poly-Si/oxide). Up to now the «New Gen« structure featuring both sides passivated contacts is mainly limited by optical losses at the front side in the passivated contact stack.

In the following months, the objectives will be:

- Modelling of full PV structures and advanced characterization of different passivated contacts technologies

- Development of TCOs with improved properties, especially for surface passivation and hydrogenation

- Cells structure optimisation through improved fabrication processes for TCO/polySi/oxide stacks

- The use of industrial équipements for all these process steps

Publications in peer-reviewed journals:

1. doi.org/10.1063/1.5041062
2. doi.org/10.1063/1.5049316
3. doi.org/10.1063/1.5049267
4. doi.org/10.1063/1.5049280
5. doi.org/10.1002/pssa.201800603
6. doi.org/10.1016/j.solmat.2019.109912

10 other communications have been presented in international conferences (3 orals)

The OXIGEN project aims to develop a new crystalline silicon (c-Si) photovoltaic (PV) cell generation, and to obtain = 23% efficiency on large area devices. The studies will focus on the fabrication of ultra-thin junctions and functionalized oxides to reach transparent and passivated contacts using industrial processes. Two technologies will be highlighted in this project, the first one being Plasma Immersion Ion Implantation (PIII) which is ideal to obtain ultra-thin junctions. The second one, based on fast Atomic Layer Deposition (ALD), is developed by the French company Encapsulix and will be used for the fabrication of innovative electrodes allowing both surface passivation and charge carrier collection. This collaboration in the field of functionalized oxides for c-Si PV cells will be great to share high level scientific knowledge and research tools. The project will be coordinated by CEA-LITEN (LHMJ) because most of the process integration will be done at INES facilities. The scientific expertise of four academic labs (INL, LMGP, IMEP LAHC, GEEPS-IPVF) on the thin films/interface/device fabrication, simulation and characterizations will be necessary for all technological improvements of OXYGEN cells structures. All technological and scientific improvements will be done in collaboration with a start-up (ENCAPSULIX), which will offer specific skills in industrial process development.