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 SiO
2
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 SiO
2
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/SiO
2
reflector shows a J
sc
of 26.3 mA/cm
²
(with front side plasma texture) which constitutes an enhancement of 4 mA/cm
²
compared with a single SiC layer. The cell efficiency was thereby increased from 8.8% to 11.1%.