Wanghua Chen

Nanophotonics for ultrathin crystalline silicon photovoltaics: When photons (actually) meet electrons

Nanopatterning has recently demonstrated to be an efficient method for boosting the light absorption of thin films (< 50 μm) of crystalline silicon. However, convincing solar cell results are still missing. The goal of the European project PhotoNVoltaics is to investigate the impacts that nanopatterns have on thin crystalline silicon solar cells and to identify the conditions for their efficient integration. The present contribution presents the main findings of the consortium so far. Optical modeling and optimization of nanopatterned thin-film c-Si cells have indicated a few trends regarding the design of the optimum pattern for 1-40 μm thin foils: merged inverted nanopyramids, with their progressive profile seem to provide the best combination of antireflective and light trapping properties, together with negligible surface damage and best coating template for subsequent process steps. But despite the high Jsc enhancement that these nanopatterns are expected to bring, and despite the higher efficiency of these nanophotonic structures for 1-2 μm-thin foils, the absolute Jsc values indicate that thicker foils will have to be preferred if direct competition with wafer-based and thin-film technologies is concerned. An ideal structure is taking shape as an IBC heterojunction cell, with a frontside nanopattern and a thickness of 40 μm.

Low temperature epitaxial growth of boron-doped silicon thin films

Low temperature (175°C) plasma-enhanced chemical vapor deposition (PECVD) is investigated as an alternative way to form p-n junctions for solar cells production. Compared to standard diffusion, PECVD deposition below 200°C ensures a lower thermal budget and the formation of a sharper doping profile. In this work, boron-doped epitaxial silicon films were grown by PECVD on (100) n-type Si substrates to form the emitter. We focus on the correlation between hydrogen incorporation and the structural and electrical properties of the boron-doped layers to assess their quality in view of the realization of p-n junctions. Using X-ray diffraction and electrochemical capacitance voltage, we observe that there is a strong correlation between hydrogen release (upon annealing the samples) and the activation of boron dopants in the epitaxial film. Interestingly, annealing at 300°C for 10 minutes is enough to activate boron in the emitter layers.Low temperature (175°C) plasma-enhanced chemical vapor deposition (PECVD) is investigated as an alternative way to form p-n junctions for solar cells production. Compared to standard diffusion, PECVD deposition below 200°C ensures a lower thermal budget and the formation of a sharper doping profile. In this work, boron-doped epitaxial silicon films were grown by PECVD on (100) n-type Si substrates to form the emitter. We focus on the correlation between hydrogen incorporation and the structural and electrical properties of the boron-doped layers to assess their quality in view of the realization of p-n junctions. Using X-ray diffraction and electrochemical capacitance voltage, we observe that there is a strong correlation between hydrogen release (upon annealing the samples) and the activation of boron dopants in the epitaxial film. Interestingly, annealing at 300°C for 10 minutes is enough to activate boron in the emitter layers.