Crystalline silicon solar cells with doped epitaxial silicon films obtained at low-temperature by PECVD

In spite of dramatic improvements in thin film technologies, crystalline silicon (c-Si) solar cells are still leading the photovoltaic market thanks to continuous progress. In this field, one innovative way is the development of thin silicon epitaxial films (epi-Si) on (100) c-Si wafers to fabricate the emitter and/or back surface field of solar cells. Among the possible techniques, we used radio-frequency plasma enhanced chemical vapor deposition (rf-PECVD) for it is a low thermal budget, scalable, wide-spread process in photovoltaics and it is well-suited to make solar cells on thin c-Si wafers (< 150 μm) without too much stress induced. In this paper, we report the obtaining of n-type epi-Si layers and p-type epi-Si layers by adding a proper amount of phosphine (PH₃), diborane (B₂H₆) or trimethylboron (TMB) to the gas phase as well as appropriately adjusting the deposition temperature, pressure and silane gas flow. For n-type epi-Si layers, we managed to obtain degenerate levels of doping while maintaining the epitaxial growth quality of Si films whereas for p-type we obtained highly doped epi-Si lalers with hole concentration spreading from 2.10¹⁸ cm^-3 to 6.10¹⁹ cm^-3 (at 300 K). Another noticeable result is that the deposition temperature dependence of the boron-doped epi-Si films changes according to the doping gas used (TMB or B₂H₆). This behavior has to be opposed to intrinsic epi-Si for which the higher the temperature the better the epitaxy. We also pay a particular attention to reproducibility of doping levels for n-type and p-type epi-Si layers. Finally, we fabricated c-Si solar cells with p-type epi-Si emitter (entirely epitaxial) and a-Si:H back contact (intrinsic and n-type). In this work we compare our previous results based on laser-cutting and screen-printing for which we achieved an efficiency of 14.2 % [1] with a new fully in-house process in which only SF₆ etching is used to define the solar cells and evaporated silver contacts are used for the front grid leading to a solar cell efficiency of 13.4 %.