Junction formation and interface passivation in homojunction and heterojunction silicon solar cells deposited by VHF PECVD

Homo- and heterojunction silicon solar cells were grown by VHF-PECVD at low temperature. The deposition conditions of the intrinsic layers at the junction interface were varied to obtain epitaxial and amorphous Si buffer layers. The passivating properties of the epitaxial silicon, associated to lower absorption losses compared to amorphous silicon, are investigated. A comparison between amorphous / crystalline heterojunction solar cells and epitaxial c-Si devices shows that in the latter case the larger Jsc partially compensates the inferior passivating properties of the intrinsic epitaxial buffer layer. The Voc of the epitaxial devices is strongly affected by the hydrogen dilution of the gas mixture in the intrinsic buffer layer deposition, and increases up to 613 mV for the higher dilution used. The result is attributed to an improvement of the interface quality for growth conditions as close as possible to equilibrium. The recombination losses in the junction region are also investigated by the saturation current density measurements as a function of temperature. The best cell performance, Voc = 637 mV and 13.7% extrinsic efficiency on planar devices, is obtained in the case of an amorphous i-layer with a p-microcrystalline emitter.