Symmetric Dopant-Free Si Solar Cells Enabled by TiO_x Nanolayers: An In-Depth Study on Bipolar Carrier Selectivity

High-efficiency solar cells require two contact structures, engineered for efficient extraction of photogenerated holes and electrons at the respective electrodes. Herein, crystalline Si solar cell featuring hole- and electron-selective passivating contacts composed entirely of a single material, amorphous titanium oxide (TiO_x), without extrinsic doping is demonstrated. The hole/electron selectivity of the TiO_x layers (≈5 nm) is tailored by the oxidation process and the choice of Ti precursor in the atomic layer deposition (ALD). Ex situ and in situ X-ray photoelectron spectroscopy measurements elucidate that the hole-selective TiO_x induces significant band bending in the Si (Φ≈0.7 eV), generating a p-type inversion layer in the n-Si absorber. The electron-selective TiO_x induces a smaller band bending of Φ<0.35 eV. This clarifies that the bipolar carrier selectivity of TiO_x is associated with the different amount of negative fixed charges generated during the ALD process, depending on the choice of Ti precursor and oxidant. In addition, the growth of a hydrogen-containing SiO_y nanolayer (≈1-1.5 nm) at the Si/TiO_x interface during postdeposition oxidation is crucial for providing chemical passivation in both types of TiO_x. These findings pave the way for a deeper understanding of the charge generation mechanism and chemistry at the Si/metal oxide interfaces.