Fabrication Processes Of Thin Films For A-Si:H/c-Si Heterojunction Solar Cells And Their Optimizations

With development of PV industry and market, high efficiency and low cost become major objectives of PV research. Amorphous hydrogenated silicon/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells have advantages of high efficiency and low process temperature, hence potentially low cost. The success of Panasonic in this type of solar cells in industrial scale, as marked by their23%efficiency, proves its feasibility. However, the technical know-how in fabrication of the cell has been kept strictly closed, and therefore the researches on this type of cells become a hot topic in the world. The present study in a-Si:H/c-Si heterojunction solar cells will focus on fabrication processes of the passivation layers, doped layers and transparent conducted thin films, and their optimizations. Here are the research contents and results in the following.Firstly, a-Si:H films are deposited by means of PECVD, discussing different deposition process parameters how affect performances of a-Si:H film, the passivation efficiency are characterized by minority carrier lifetime for passivated wafers. It turned out that:as thickness of intrinsic films increased, wafer minority carrier lifetime ascended firstly, then descended; minority carrier lifetime ascended firstly as temperature increasing, reaching200℃, tended to descend; minority carrier lifetime ascended firstly then descended with radio power enhancing; as deposited pressure increased, minority carrier lifetime also ascended firstly then tended to descend; hydrogen dilution can improve minority carrier lifetime, but when hydrogen dilution was too high, minority carrier lifetime was reduced on the contrary. The highest minority carrier lifetime was obtained using the optimized parameters, which was849μs, and the surface recombination velocities fell to lcm/s. Compared to a-Si:H passivated film, hydrogenated amorphous silicon oxide layers (a-SiOx:H) performed more stable passivation efficiency, pressure ranging15Pa to45Pa, the wafer surface recombination velocity was inferior to10cm/s, when pressure was35Pa, the wafer surface recombination velocity was only3cm/s, and Implied Voc reached over730mV. Secondly, based on a-Si:H film deposition, phosphorane and borane were regarded as doped gas, were used to deposit doped amorphous silicon layers. Experimental results showed that:the bulk resistance of p style amorphous silicon layer reached as low as5.8Ω·cm when radio power was chosen as50W and pressure as20Pa; by contrast, the bulk resistance of n type amorphous silicon layer was only0.12Ω·cm while power was10W and pressure was50Pa.Thirdly, ITO layer, which is one of transparent conductive oxide films for heterojunction solar cells, optimizing its deposition process parameters and improving electrical properties of ITO layers via magnetron sputtering technology. Results showed that:ITO layer with excellent electrical properties was gained, whose bulk resistivity was around3.7x10-4Ω·cm when argon oxygen ratio was3/10000, sputtering power was70W and substrate temperature was250℃. What’more, contact of ITO layer and each layer was analyzed quatitatively using electrochemical workstation in the paper, and conclusion can be made that contacts of between ITO layers and aluminium electrode, ITO and doped amorphous silicon layers were all good ohmic contact.