On the possible role of the interfacial inversion layer in the improvement of the performance of hydrogenated amorphous silicon/crystalline silicon heterojunction solar cells [HIT]

The energy band diagram of the hetero-interface between p-type hydrogenated amorphous silicon (a-Si:H(p+)) and n-type crystalline silicon (c-Si(n)) obtained using AFORS-HET one dimensional device simulations reveals that a p+ inversion layer is induced at the hetero-interface, in the c-Si side, with or without the presence of a buffer intrinsic a-Si:H(i) spacer. Such an inversion layer controls the performance of the a-Si:H(p+)/a-Si:H(i)/c-Si(n) HIT cell, because it pushes the p/n junction 20 nm in the c-Si and acts as the cell effective emitter. The formation of the inversion layer is controlled by the valence band offset and by the positioning of the Fermi energy in the energy band-gap. The latter is influenced by the active doping level in the doped a-Si:H(p+) and by the dangling bond defect density in the a-Si:H bulk and at the a-Si:H/c-Si interface. By inserting an intrinsic a-Si:H spacer, the defect density at the interface is strongly reduced, which not only decreases the interface recombination, but also ensures the proper formation of the inversion layer. The study also suggests that significantly reduced band-gap narrowing in the inversion layer emitter contributes to the higher open circuit voltage achieved in the HIT cell compared to c-Si cell with excellent front surface passivation.