So-called heterojunction silicon solar cells are generating increasing interest. They differ from conventional crystalline-or multicrystalline-silicon devices in that. instead of forming the emitter/s by thermal-diffusion doping of the absorber, they are deposited as silicon thin films by plasma-enhanced chemical vapour deposition (PECVD) or some other thin-film technique. The approach has important advantages such as good uniformities, low temperatures, excellent thickness control, etc., making the procedure essential for the development of new-generation cells based on ribbon silicon and similar materials. The performance of heterojunction silicon cells is dramatically influenced by the thin-film/ absorber interface. Impurities in the junction, passivation of defects, lattice matching and band structure are absolutely crucial. The present paper describes a number of investigations on the preparation of n-p cells made by depositing n-type thin-film silicon by PECVD onto p-type crystalline-silicon wafers. The work is focused on the dependence of cell performance on the treatments applied to the crystalline- silicon surface prior to thin-film deposition. The results show that a treatment with a hydrogen plasma prior to emitter deposition can lead to important efficiency improvements attributed to defect passivation.