Development of conductive nc-SiOx thin films for high-efficiency 2-terminal Si heterojunction-perovskite tandem solar cells

Photovoltaics (PV) is the fastest growing electrical energy generation source in the world and has indeed a “bright” future. The cumulative installed capacity of PV in the world has already surpassed 600 GW by 2019. Crystalline-silicon (c-Si) wafer-based solar cells have always been the photovoltaics industry workhorse, and in 2019, they accounted for 95% of the total module production. From around 23% record energy-conversion efficiency in 1993, to almost 27% in 2019, c-Si solar cells in the lab today are getting very close to their theoretical limit [1].

To go beyond the fundamental single-junction limit, a tandem device architecture, which employs 2 or more absorber materials of complementary bandgaps in a stack, must be adopted to reduce thermalisation losses and use the solar spectrum more effectively. A wide bandgap perovskite top cell above a c-Si bottom cell is one of the emerging tandem solar cell configurations that has attracted plenty of recent interest, due to its potential of exceeding 30% while combining the attractive properties of perovskites with the well-understood technologies for c-Si. The best monolithic implementation of this material combination was very recently achieved by OxfordPV with an efficiency of 29.52% [2].

Imec is also involved in the development of such 2 terminal tandem cells with a Si heterojunction (SHJ) bottom cell and a perovskite top cell. An important consideration in such cells is the opto-electrical coupling between the two sub-cells. Typically, a thin TCO is used the recombination layer to ensure good electrical connection between the sub-cells, allowing efficient charge transport between the electron contact of the SHJ bottom cell and the hole transport layer of the perovskite top cell. The electron contact of the SHJ bottom cell comprises of a stack of intrinsic and n-doped amorphous silicon (a-Si) with a total thickness in the order of 20 nm. In order to improve the optical coupling between the sub-cells, a thick n-doped nanocrystalline silicon oxide layer (nc-SiO_x), with a better suited refractive index than a-Si or nc-Si, with an optimal thickness of 90-100 nm should be implemented. This would be the focus of the Master thesis, with the following main tasks, which will be performed in the cleanroom lab (at imec Leuven)

1. Deposition of intrinsic and doped nc-SiO_x films using plasma-enhanced chemical vapour deposition (PECVD).
2. Characterisation of the deposited layers structurally, optically and electrically using different characterisation methods such as XRD, Raman, ellipsometry and reflectance. The passivation properties of the developed films will also be assessed using carrier lifetime measurements.
3. Evaluation of the charge transport through the developed material stack using contact resistivity measurements.
4. Implementation of the developed layers, in the first instance, in a SHJ single-junction solar cells to assess the performance of the developed nc-SiO_x layer(s) at device level.