Thermal stability improvement of metal oxide-based contacts for silicon heterojunction solar cells

Metal oxides are interesting materials for use as carrier-selective contacts for the fabrication of doping-free silicon solar cells. In particular, MoOx and TiOx have been successfully used as hole and electron selective contacts in silicon solar cells, respectively. However, it is of paramount importance that good thermal stability is achieved in such contacts. In our work, we combined i-a-Si:H/MoOx based hole contacts with electron contacts featuring i-a-Si:H/TiOx/low work function metal (ATOM) to fabricate doping-free cells, termed MolyATOM cells. We found that the thermal stability of the ATOM contact was improved when the i-a-Si:H was annealed (300 °C for 20 min in N2) before depositing TiOx (i.e. pre-TiOx annealing), which reduces the hydrogen content in i-a-Si:H by about 27 %rel, and thereby the H-related degradation of the ATOM contact characteristics. Moreover, it was found that reducing the thickness of the low-work function metal on top of the TiOx enhanced the thermal stability of the ATOM contact. With these adaptations, the MolyATOM cell efficiency was improved by 3.5 %abs, with the highest efficiency of 17.6%. Moreover, the cells show improved thermal stability after the above-mentioned pre-TiOx annealing, which is confirmed by annealing tests at cell level as well as damp-heat tests at module level. The insights of this study could be used to tailor other metal-oxide based electron or hole contacts.