Realization and evaluation of diffractive systems on the back side of silicon solar cells

An effective light trapping system is required in silicon solar cells in order to collect a large amount of photons. That is why we focus our investigation on the fabrication and evaluation of two types of optical systems introduced on the back side of solar cells. The aim of these structures is to enhance the light trapping of the long wavelength photons (above 1000 nm). On the one hand, we evaluated a Si/SiO2 linear nanograting; on the other hand, hexagonal nanostructures fabricated with SiO2 nanoparticles and a filling matrix are under investigation. In this paper, we describe the fabrication processes developed for both approaches and we present the solar cell results and characterisation. For the first approach, we show a reflectance reduction on test structures, which occurs at the same wavelength as the increase of absorption induced by the simulated gratings. Moreover, we demonstrate the feasibility of the fabrication of silicon solar cells with the hexagonal nanostructures as a diffractive back reflector. Although no short circuit current increase has been observed due to a poor rear side passivation, a current gain up to 0.3 mA/cm2 is possible in the wavelength range of 1050-1150 nm due to these nanostructures. Finally, we also comment on the advantages and drawbacks of each approach and on the feasibility to introduce these systems in the solar cell process flow.