Sponge-like Si-SiO2 nanocomposite as photovoltaic absorber - influence of composition of the SiOx precursor

Absorber layers consisting of nanostructured Si are candidates to improve the effciency of thin film Si solar cells. Si-SiO2 nanocomposites with sponge-like Si embedded in SiO2 are promising materials as they exhibit a widened band gap and maintain the electrical interconnectivity. These structures can be formed upon annealing of SiOx films (x<1), which leads to spinodal phase separation into a percolacated network of Si nanowires embedded in SiO2. This can be accompanied by crystallization of the silicon. The influence of the composition of the precursor SiO2 on the evolving sponge-like nanostructure is investigated. SiOx layers have been grown by reactive sputter deposition. SiOx layers with compositions between x=0 and x=1.2 have been studied. The transformation of SiOx into Si-SiO2 nanocomposites has been performed by scanning a diode laser line source. Dwell times in the ms range and power densities of the red laser light of about 103 W/cm2 have been investigated. while thin a-Si films show crystallization under our annealing conditions, oxygen-rich films with Si structures smaller than 2 nm do not crystallize. Our results demonstrate that the composition of the precursor material is of crucial importance to obtain a Si-SiO2 nano sponge-like material suitable as photovoltaic absorber.