Al2O3-TiOx as full area passivating contacts for silicon surfaces utilizing oxygen scavenging titanium interlayers

Abstract Full area transparent contacts are the next step in order to enhance the efficiency of silicon based passivated emitter rear cells. Here we present a full area contact based on Al2O3 and reduced TiOx. The thin Al2O3 acts as passivating tunneling layer, whereas the TiOx is used for charge carrier transport. This work provides a way to enhance the conductivity of TiOx by forming gas annealing (FGA) and doping with tantalum. Furthermore, it addresses the native SiOx without chemical treatment. A thin titanium interlayer (Ti-IL) serves this purpose. Current-voltage measurements reveal a contact resistance of 30 Ωcm2 for 50 nm of pure TiOx. Ta-doping combined with a 6 nm thick Ti-IL reduces the contact resistance by two orders of magnitude down to 0.36 Ωcm2. The passivation by an Al2O3/TiOx layer stack leads to an excellent surface recombination velocity (SRV) of 5.6 cm/s with only 3 nm Al2O3 but results in a large contact resistance of about 100 Ωcm2. This contact resistance can be reduced by three orders of magnitude down to 0.25 Ωcm2 including a 10 nm Ti-IL while still providing a sufficient SRV of about 60 cm/s. The absorption coefficient of 6.3×105 cm−1 inhibits the usage as front contact for solar cells. However, it could be used as a backside contact, since the transmission is less important on the backside of a monofacial solar cell.