Electro-physical interpretation of the degradation of the fill factor of silicon heterojunction solar cells due to incomplete hole collection at the a-Si: H/c-Si thermionic emission barrier

An electro-physical interpretation for the degradation of the Fill Factor in p⁺/n silicon heterojunction solar cells (SHJ) due to incomplete hole collection at the thermionic emission barrier at the amorphous/crystalline silicon (a-Si:H/c-Si) hetero-interface is proposed supported by results of AFORS-HET device simulations. Under illumination, reflected holes at the thermionic barrier pile up at the hetero-interface which strengthens the dipole with the negative dopant ions in the doped a-Si:H(p⁺) layer and enhances the electric field passing through the a-Si:H layer. Such an enhanced electric field sweeps back the free holes spilling over in the intrinsic a-Si:H(i) layer from the a-Si:H(p⁺) layer considerably depleting the double a-Si:H layer and enhancing its resistance and the overall cell series resistance. Therefore, the degradation due to incomplete hole collection at the thermionic emission barrier under illumination can be assimilated to the effect of a series resistance does not affect the cell open circuit voltage but degrades only its fill factor. The resistance enhancement is found to be bias-dependent and to increase with decreasing the doping level in a-Si:H(p⁺). Predictions of the proposed model for different hole reflection probability at the barrier and for different thicknesses of the intrinsic a-Si:H(i) layer agree perfectly with the results of simulations.