Heterojunction a-Si/poly-Si solar cells on mullite substrates

Crystalline silicon thin films (thickness < 10 μm) on ceramic substrates provide an alternative approach to bulk silicon based cells, provided the electronic quality of the silicon film is high enough and the junction structure is optimized. In this work, we present the structural and electrical properties of a-Si/poly-Si heterojunction solar cells fabricated on mullite ceramic substrates. The polycrystalline silicon films were formed by high temperature chemical vapour deposition on mullite ceramics coated with spin-on flowable oxides (FOx) serving as intermediate layers. The average size of the grains and their distribution were investigated by optical microscopy and EBSD technique. It is found that more than 60% of the surface of polysilicon films grown on FOx is covered by large grains of 4–12 μm, and that grains are (110) preferentially oriented. Finally, we present the photovoltaic data on test solar cells made on such heterojunction polysilicon based films. An open-circuit voltage of 500 mV was obtained for the polysilicon layers with a heterojunction emitter. This is the highest reported open-circuit voltage for polycrystalline solar cells with a p–n heterojunction on mullite ceramics. It is also a new record for thin-film silicon solar cells deposited by CVD onto ceramic substrates. The limiting factors as well as possible improvements are discussed.