The photovoltaic technology is pursuing its constant effort for lowering the price of the electricity delivered, notably thanks to the technological innovations. The use of thin-films based solar cells was an important step towards that direction since it enabled to decrease the amount of active material needed. Recently, ultrathin semi-conductor layers have emerged. Due to their limited thickness, those layers are suffering from a weak absorption of the incoming light which degrades the conversion yield of the resulting cells. To tackle this issue, nano-photonic concepts may offer well-suited solutions to handle the light at the wavelength scale. In this context, we propose to pattern the active layer of solar cells as an absorbing photonic crystal (PC). This periodical nano-structure ensures simultaneously an efficient collection of the light at low wavelengths, together with an appropriate method for trapping photons inside the active layer for the wavelengths close to the material bandgap, which in our case consists in hydrogenated amorphous silicon. In the framework of this study, optical simulations were used to optimize the PC parameters so as to provide a significant (+27% in the sole active layer) absorption increase over the whole spectrum considered and guidelines for the fabrication of the patterned cells. The physics principles ruling their optical properties were identified out of an analytical description of the system. Optical measurements carried on the patterned samples confirmed the simulation results and highlighted the robustness of the overall absorption with regards to the angle of incidence of the light and technological imperfections. In addition, opto-electrical simulations revealed that an increase of the conversion yield can be expected, provided that an appropriated surface passivation step is introduced in the fabrication process.
