Origin of passivation in hole-selective transition metal oxides for crystalline silicon heterojunction solar cells

Transition metal oxides (TMOs) have recently demonstrated to be a good alternative to boron/phosphorous doped layers in crystalline silicon heterojunction solar cells. In this work, the interface between n-type cSi (n-Si) and three thermally evaporated TMOs (MoO3, WO3, and V2O5) was investigated by transmission electron microscopy, secondary ion-mass, and x-ray photoelectron spectroscopy. For the oxides studied, surface passivation of nSi was attributed to an ultrathin (1.9-2.8 nm) SiOx similar to 1.5 interlayer formed by chemical reaction, leaving oxygendeficient species (MoO, WO2, and VO2) as byproducts. Carrier selectivity was also inferred from the inversion layer induced on the nSi surface, a result of Fermi level alignment between two materials with dissimilar electrochemical potentials (work function difference Delta phi >= 1 eV). Therefore, the holeselective and passivating functionality of these TMOs, in addition to their ambient temperature processing, could prove an effective means to lower the cost and simplify solar cell processing.