THIN MICROCRYSTALLINE SILICON (μc-Si:H) DOPED LAYERS GROWN BY LAYER-BY-LAYER PECVDMADE FOR SOLAR CELLS WITH HIGH GROWTH RATE I-LAYERS

摘要

Microcrystalline silicon (μc-Si:H) p- and n-type layers have been developed at high temperatures byLayer-by-Layer (LbL) deposition. The LbL deposition consists of alternating boron or phosphorous dopedamorphous silicon depositions and hydrogen plasma treatments by Very High Frequency Chemical Vapor Deposition(VHF PECVD). The layers are developed to be resistant to the temperature and hydrogen flux of a micro- ofpolycrystalline i-layer grown at a high deposition rate in a p-i-n or n-i-p solar cell device.It is concluded that the LbL method is suitable to produce device quality μc-Si:H p- and n-type doped layers in atemperature range from 250 to 400 °C. This is not possible by the standard continuous (CW) PECVD employinghighly hydrogen diluted silane gas, where the addition of dopants reduces the crystallinity. An optimum effectivethickness per deposition cycle (total thickness divided by the number of cycles) of 1.5 nm/cycle is needed forcrystallization. This optimal thickness is independent of dopants and deposition temperature. A minimum thicknessof the first layer is needed for sustaining growth in the LbL process. The doped layers grown by LbL are smootherthan reference samples grown by CW (observed from the Cross-Sectional Transmission Electron Microscopy (XTEM)images). The doping efficiencies in our LbL deposited layers are fundamentally higher than those in CWdeposition (for p layers a doping efficiency of 39% in case of LbL, compared to 1% for CW). The properties of thebest high-temperature doped layers are: LbL p-type μc-Si:H (Ts = 350 °C, d = 29 nm) activation energy = 0.11 eVand dark conductivity = 0.1 ?-1cm-1; LbL n-type μc-Si:H (Ts = 400 °C, d = 31 nm) activation energy = 0.056 eV anddark conductivity = 2.7 ?-1cm-1.Test solar cell devices have been deposited using HWCVD and VHF PECVD deposited μc-Si:H i-layers on top ofthe high temperature (400 °C) LbL μc-Si:H n-type doped layer in an n-i-p configuration on a stainless steel substratewithout back reflector. A high open circuit voltage of 0.65 V and a fill factor of 0.7, show the high doping efficiencyand crystallinity of the n-type doped layer and the resistance of the layer to the high flux of atomic hydrogen duringthe HWCVD deposition. Moreover, the high temperature LbL n-layer withstands the conditions of the high substratetemperature (430 °C) and hydrogen flux when depositing high growth rate (1.6 nm/s) HWCVD a-Si:H i-layer.