Atomic-Layer-Deposited TiO_x Nanolayers Function as Efficient Hole-Selective Passivating Contacts in Silicon Solar Cells

Titanium oxide (TiOx) has recently emerged as an electron-selective passivating contact for solar cell and semiconductor device applications. The mechanism behind this function has been attributed to the lower energy barrier for electrons than holes at the TiOx/semiconductor interface. Here we report an antithetic function of TiOx nanolayers (∼5 nm), which were grown by atomic layer deposition (ALD) on either planar or textured crystalline silicon (Si) without a buffer layer, acting as efficient hole-selective contacts with excellent surface passivation. We demonstrate the proof-of-concept solar cells with power conversion efficiencies above 20% with both n- and p-Si absorbers. We show that the elemental composition in the TiOx/Si interfacial layers (TiOxSiy:H and SiOx:H), which can be manipulated by the ALD process and the post-treatments such as exposure to atomic hydrogen and supply of oxygen during annealing, is a key in the efficient hole extraction and surface passivation. This new hole-selective passivating contact opens opportunities for replacing the widely used heterocontacts and dielectric-passivation layers in various device applications.