BLACK SI SURFACES FOR HIGH-EFFICIENCY SOLAR CELLS

Crystalline-silicon (c-Si) wafer-based solar cells have always been the photovoltaics industry workhorse, ensuring most of the PV electricity production worldwide. In 2019, they accounted for 95% of the total module production. Based on an abundant and non-toxic material that is very familiar to the semiconductor industry, this technology has progressed at a steady pace. 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. To reach beyond that limit, the most prominent idea is to add a perovskite-based cell on top of the silicon-based cell. Working in tandem, these two solar cells with complementary bandgaps have the potential to extend and optimize the possibilities of sunlight absorption and reach beyond 30% for a limited additional cost, provided the two devices are optimised to function in tandem.

One of the key challenges is to implement a suitable light management approach for the tandem cell. In a typically (mono) c-Si bottom cell, the conventional approach to light management involves random micron-scale pyramid front-side textures and rear-side (scattering) mirrors. However, when used in tandem with a perovskite top cell coated from solution, the micron-sized features on the front make it impossible to achieve a continuous planarised top cell with reasonable thicknesses.

This research topic proposes to go beyond the conventional approach typically used for c-Si cells, by scaling down the dimension of the surface texture to nano-scale features smaller than the wavelength of light. The most well-known of these nano-features is “black Si”, a fine texture typically in the form of needles, cones or holes, named upon the color of the resulting wafer surface [1]. Sub-wavelength features provide broadband antireflective properties, and thus couple-in more infrared photons than standard features, which is advantageous for the bottom cell in tandem operation. Moreover, if the feature heights can be limited to a few hundreds of nm, getting a continuous film of planarised perovskite by solution coating might be more feasible.

Towards this goal, the main tasks of this thesis will be experimental and performed in the cleanroom lab (at imec Leuven):

1. Fabrication of black Si by wet or dry processes (e.g. electrochemical etching, or RIE etching)
2. Characterization of the samples (morphological, optical, electrical and wettability) to assess their potential
3. Coating of black Si by depositing passivating and/or antireflective coatings, or perovskite-top-cell layers