Electron-selective atomic-layer-deposited TiOx layers: Impact of post-deposition annealing and implementation into n-type silicon solar cells

Atomic-layer-deposited titanium oxide (TiOx) is examined for the application as electron-selective full-area contact to n−type silicon solar cells. Although the surface passivation quality of TiOx-passivated n−type silicon wafers is quite poor directly after deposition of the TiOx, we demonstrate that annealing in ambient environment at only 250°C reduces the surface recombination velocity to values below 10 cm/s over the entire cell-relevant injection range. By combining lifetime measurements with X-ray diffraction (XRD) characterization we demonstrate that the degradation of the passivation by TiOx during annealing at increased temperature is due to the crystallization of the amorphous TiOx into the crystalline anatase phase. We implement our optimized ALD-TiOx layers as electron-selective full-area rear contacts into n-type silicon solar cells and reach efficiencies up to 20.3% after low-temperature annealing in our first batch. The surface recombination velocity Srear at the cell rear, as extracted from the measured spectral internal quantum efficiency, is (52±20) cm/s. Interestingly, the fabricated solar cells show a much better thermal stability compared to the lifetime test structures, which seems to be a fundamental difference. The main difference of the finished solar cells to our lifetime test structures is that the TiOx layer is fully covered with aluminum in the solar cells. This suggests an interaction of Al with the ultrathin TiOx layer, resulting in an improved thermal stability.