Advanced optical confinement and further improvements for crystalline silicon thin-film solar cells

Every silicon thin-film solar cell concept is dependent on an excellent optical confinement. As well as texturisation and an anti-reflection coating on the front side, the rear-side needs a reflector for the wavelength region exceeding 600 nm to enhance the long-wavelength response of the solar cell. In our Recrystallised Wafer Equivalent (RexWE) [1] the rear-side of the silicon layer is not accessible during the solar cell process. Therefore, several important features have to be implemented via an intermediate layer: it needs to act as a diffusion barrier of sufficiently high electrical conductivity, an excellent optical reflector, and ideally also as a passivation layer for interface defects. We try to satisfy these requirements with a specially designed reflector. It consists of SiC and SiO2 layers with alternating refractive indices and varying characteristics that can be realised by changing the stoichiometry and layer network. These layer-stacks were implemented into RexWE solar cells by a process sequence including thermal annealing, Si seeding layer deposition, recrystallisation and epitaxial Si growth. To surmount the lack of electrical conductivity of the SiO2 layers we drilled holes through the stacks using a laser. We call this process laser-fired rear access (LFA). The best solar cell incorporating the SiC/SiO2 reflector shows a Jsc of 26.3 mA/cm² (with front side plasma texture) which constitutes an enhancement of 4 mA/cm² compared with a single SiC layer. The cell efficiency was thereby increased from 8.8% to 11.1%.