Atomic-layer-deposited titanium oxide (TiO_x) is examined for the application as electron-selective full-area contact to n-type silicon solar cells. Although the surface passivation quality of TiO_x-passivated n-type silicon wafers is quite poor directly after deposition of the TiO_x, 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 TiO_x during annealing at increased temperature is due to the crystallization of the amorphous TiO_x into the crystalline anatase phase. We implement our optimized ALD-TiO_x 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 S_(rear) 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 TiO_x layer is fully covered with aluminum in the solar cells. This suggests an interaction of Al with the ultrathin TiO_x layer, resulting in an improved thermal stability.
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