Sponge-like Si-SiO2 Nanocomposite as Photovoltaic Absorber – Influence of Composition of the SiOx Precursor

Nano structured Si absorber layers are candidates to improve efficiencies of thin film Si solar cells without increasing costs. Si-SiO2 nano sponge-like nanocomposites are promising materials as they exhibit a widened band gap due to quantum confinement and electrical interconnectivity due to percolation of the nanostructured Si. The sponge-like structures can be formed upon annealing of substoichiometric SiOx films (x<1), which leads to spinodal phase separation into a perlocated network of Si nanowires embedded in SiO2, tentatively accompanied by crystallization of the Si.
Here the influence of the precursor composition on the evolving sponge-like nanostructure and on the optical properties is investigated. SiOx layers have been grown by reactive sputter deposition where the composition of SiOx films was controlled by varying the oxygen flow during the deposition and subsequently measured by Rutherford Backscattering Spectroscopy (RBS). SiOx layers with compositions between x=0 and x=1.2 have been addressed. The Si-SiO2 nanocomposites are fabricated using a very rapid thermal processing 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.
Laser treatment of the precursor SiOx layers leads to decomposition into Si and SiO2 thereby forming Si-SiO2 sponge-like structures as observed by energy filtered transmission electron microscopy (EFTEM). While thin a-Si films show crystallization, oxygen rich films with Si structures smaller than 2nm do not show crystallization. The widening of the band gap due to quantum confinement has been confirmed by optical measurements.
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 PV absorber.