Si nanowire networks embedded in SiO2 formed by spinodal decomposition of SiO – a novel absorber material for 3rd generation solar cells

Large-scale patterning by spontaneous self-structuring during spinodal decomposition of metastable SiO is a very promising synthesis process of novel nanostructured Si absorbers for 3rd generation thin-film solar cells [1]. 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) and very Rapid Thermal Processing (vRTP, dwell time tens of msec). When the volume fraction of Si exceeds ~30% after the phase separation SiOx-->0.5SiO2+(1-0.5x)Si, then Si forms a nanowire network. Energy-Filtered Transmission Electron Microscopy (EFTEM) studies show that nanowires have diameters of a few nanometers with a narrow distribution. This is in excellent agreement with large-scale simulations based on bit-coded kinetic Monte-Carlo. There is a considerable Si band gap widening due to quantum confinement in the nanowire network. As the wire diameter coarsens with time of heat treatment like d~t0.33, the band gap of the Si nanosponge can be optimized for solar cell application. Using an atomistic pseudopotential method, the band gap of sponges have been studied. Finally it will be shown that up-scaling of the nanotechnology described above to large-scale PV cell production is under way by industrial partners.