Al2O3 and SiNx is used for surface passivation. but which one best for reducing recombination loss?
compare the band gap of materials that you want to use as a passivation layer so choose higher band gap.
The preferred passivation on the p-type silicon is Al2O3; while for n-type Si is SiNx:H. since charged dielectric passivation layers perform best on substrates with opposite charged majority carriers.
The best passivation is achieved when all dangling bonds are saturated with the passivating material. So, Not only the material used but also how this material is deposited on the surface to be passivated influences the effectiveness of the passivation. The effectiveness of a passivation of the surface is measured by the surface recombination velocity. The good passivation is reached when the recombination velocity is below about 10 cm/s.
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The question is general. What kind of silicon material; mono or multicrystalline. Also the type of the substrate p or n. You specify the rear side, what is the cell structure: standard or PERC, HIT or other? For standard cells ( Si-mc p-type generally) Al(BSF) layer give an efficient passivation. Also the presence of a rear emitter affect the passivation way. When we use a SiNx layer or a stack of SiO2/SiNx we obtain an excellent passivation. This can degraded slightly if we use a capped Al(BSF) layer. For PERC solar cells A2O3/SiNx or SiO2/SiNs with a locally Al(BSF) scheme give a very excellent passivation with SRV values lower than 5 cm/sec. For HIT solar cell (n Type monocrystalline silicon) a very thin layer (50 nm) of a-Si:H give the best passivation. Ad all this are affected by the rapid thermal annealing process (RTA). Finally, the choice of the passivated layer for rear silicon side depend on several parameters to reach a lower values of SRV.
Adding to the above answers, The rear side of the solar cell is normally completely metallized to serve as optical reflector and low ohmic electrical accessing of the solar cell. The front side the matellization must be partial to allow the passage of light. The parts which are not metallized must be passivated to reduce the surface recombination velocity.
Passivation can be achieved by materials given in details by the post of Bouhafs.
If want to passivate the rear side of the solar cells you have to make this before metallization. On of the methods is to use back surface field where you need only a very thin layer of heavily doped layer to form low high junction. If the material is p type the layer must be heavily doped P-yype. It could be also heavily doped amorphous Si: H layer.
This layer could be a tunnel able oxide or nitride layer.
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It depends on what type of Si you are using. Generally for p-type Si oxide based material i.e. Al2O3, a-SiOx:H are used for passivation whereas for n-type wafer SiNx and high quality a-Si:H are the best option.
Al2O3 has negative fix charges and best for rear contact where SiNx has positive fix charges which means not good for rear contact
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