Silicon Hetero Junction Solar Cells by Hot-Wire CVD

We are reporting high performance silicon heterojuncton (SHJ) solar cells fabricated using the hot-wire chemical vapor deposition (HWCVD) technique. On p-type c-Si float-zone wafers, we used an amorphous n/i contact to the top surface and an i/p contact to the back surface to obtain an open circuit voltage (V_oc) of 0.67 V in a 1 cm² cell with an independent confirmed efficiency of 18.2%. This is the best reported p-type SHJ solar cell, at least by HWCVD. On n-type c-Si float-zone wafers, we used an amorphous (p/i) front emitter and an a-Si:H (i/n) back contact to achieve a V_oc of 0.69 V on 1 cm² cell. We found that proper c-Si surface cleaning prior to the amorphous Si deposition and double-heterojunction is a key to the high V_oc. In the heterojunction region, an abrupt interface from c-Si to a-Si:H results in a high V_oc; while incorporating a transition to either microcrystalline or epitaxial Si at the c-Si interface results in a low V_oc. Lifetime measurement shows that the back surface recombination velocity can be reduced to ∼15 cm/s through a-Si:H passivation. Amorphous silicon heterojunction layers on crystalline wafers thus combine low-surface recombination velocity with excellent carrier extraction. The advantages of using HWCVD in comparing with plasma-enhanced CVD are the fast deposition rate and, more important, a wide range of deposition parameters enabling formation of an effective heterojunction with high V_oc. Further, the heterojuction cell processing is entirely below 200°C making it one of the few promising low-stress methods for the manufacturing of next generation ultra-thin Si wafer solar cells.