Deposition of surface passivation layers for silicon heterojunction solar cells by hot-wire CVD
Thin intrinsic amorphous silicon layers acting as surface passivation layers have proven to play an essential role for the performance of silicon heterojunction (SHJ) solar cells. By using hot-wire chemical vapor deposition (HWCVD), intrinsic hydrogenated amorphous silicon layers (a-Si:H) for SHJ solar cells were deposited on both sides of textured n-type semi-square silicon wafers, with sizes of 156 x 156 mm². Amongst the investigated deposition process parameters, substrate temperature and film thickness were identified to have the greatest influence on the passivation properties. Differently prepared passivation layers were characterized by measurements of the minority-carrier lifetimes using the photoconductance method. The best a-Si:H layers were deposited by activating pure silane (SiH4) gas by tungsten wires at pressures around 1.3 Pa and wire temperatures of about 2100 °C. At film thicknesses of approximately 12 nm average minority-carrier lifetimes of 3.5 ms were achieved.