Optimisation of Intrinsic a-Si:H Passivation Layers in Crystalline-amorphous Silicon Heterojunction Solar Cells

Heterojunction silicon wafer solar cells using an intrinsic amorphous silicon (a-Si:H) thin-film passivation layer between the crystalline c-Si substrate and the thin-film emitter layer have proven to be a viable device structure for high efficiency. While microcrystalline μc-Si:H is a good candidate for the emitter layer due to its high doping efficiency, intrinsic a-Si:H with its low interfacial defect density, high optical bandgap and good passivation ability makes it the ideal buffer layer to passivate the crystalline silicon interface. In this study, we report the film properties of intrinsic a-Si:H passivation layers deposited using RF (13.56 MHz) PECVD, at different SiH4/H2 gas flow ratios, pressures and temperatures. Trends relating deposition conditions to relevant film characteristics, such as thickness, hydrogen bonding, optical bandgap, and effective carrier lifetime of the samples are discussed. Finally, symmetrical p+/i/c-Si wafer/i/p+ heterojunction lifetime test structures, using 20 nm thick p-doped μc-Si:H emitter layers and 10 nm thick intrinsic a-Si:H layers, were made using the optimised parameters for intrinsic a-Si:H layers (discussed in this paper) and for p-doped μc-Si:H layers (discussed in a companion paper [1]). These yield an effective lifetime of 2.4 ms at an injection level of 1015 cm-3, and an implied Voc of 730 mV.