Electron-blocking NiO/crystalline n-Si heterojunction formed by ALD at 175°C

Silicon heterojunction solar cells have been the subject of growing research interest. Such cells replace the typical p⁺nn⁺ or n⁺pp⁺ structure of standard devices with selective heterojunction contacts, which block one type of carrier while allowing the other to pass freely (Fig. 1) [1-3]. Previously [4], we demonstrated a PEDOT/n-Si/TiO₂ heterojunction cell fabricated below 100°C with no p-n junctions in the Si. However, the organic polymer PEDOT is known to be unstable over long periods of time; furthermore, recent data indicates that the PEDOT/n-Si interface might be a non-ideal minority carrier emitter, leading to a high J₀ and low upper limit to VOC. Therefore, we are currently investigating inorganic electron-blockers on crystalline silicon. Nickel oxide (NiO), because of its large conduction band offset and small valence band offset with silicon (Fig. 2) [5], is a potential candidate for electron-blocking on n-Si. Here, we report atomic layer deposited (ALD) metal/15nm-i-NiO/Si diodes. We find that the NiO film leads to a heterojunction which blocks electrons compared to diodes with the NiO omitted. The characteristics depend on the top metal, indicating that the NiO passivates the Si surface so that the Fermi level is depinned and diodes with a higher Schottky barrier height can be fabricated. Devices with Ag have electron-blocking and hole-transmitting behavior.