Network of Percolated Si Filaments in SiO2: A Nanocomposite-absorber for Thin-film PV Cells

Si-based thin film PV cells suffer from a rather low efficiency. This leads to a relatively small market share, although their module prices are comparably low. Here, we present a novel nanostructured Si-based thin film PV cell absorber, which has the potential to increase the efficiency substantially without increasing the module costs.
Spinodal decomposition of metastable SiO into Si and SiO2 is a promising synthesis process of nanostructured Si absorbers for 3rd generation thin-film solar cells. Under appropriate conditions of SiO deposition and subsequent heat treatment, self-organization of a network of percolated Si filaments embedded in SiO2 has been achieved. The SiO layers have been produced by different techniques, sputtering, CVD and e-beam evaporation. Spinodal decomposition has been activated by Rapid Thermal Processing (RTP, several seconds), very Rapid Thermal Processing (vRTP, dwell time tens of msec), and laser annealing. If, after phase separation, the volume fraction of elemental Si exceeds ~30%, then Si forms a percolated network of Si filaments, which will be proven by the aid of Energy-Filtered Transmission Electron Microscopy (EFTEM) images. The diameters of the filaments are in the range of 2…5 nm and can be tuned by the thermal treatment. Due to the small diameters, the band gap is dominated by the quantum size effect. As the wire diameters coarsens with time of heat treatment like t^1/3, and because the Si bandgap opens for nm-structures by quantum confinement, a band gap engineering for PV cell optimization becomes feasible.
This is in excellent agreement with large-scale simulations on the network formation using our 3D kinetic lattice Monte-Carlo program. Electronic band structure calculations of such nanostructured silicon will be presented too. It will be shown that up-scaling of the nanocomposite fabrication as described above to the industrial scale is feasible with available technologies.
The work performed within the project “RainbowEnergy” is supported by the German ministry BMBF and the Turkish funding agency TÜBITAK