Theoretical Study On Interface Characteristics Of A-Si:H/c-Si Heterojunctions

Heterojunction solar cells of a-Si:H and c-Si are promising candidates for low cost photovoltaic applications and highly efficient solar cells, because it combines the merits of c-Si solar cells and the a-Si:H production techniques.Two key interface parameters that affect the properties of heterojunctions are of interface states and the band offsets. By integrating the equation of the electron current density at the c-Si depletion region under illumination and at open circuit, the expression of theoretical open-circuit voltage of a-Si:H/c-Si heterojunction solar cells is developed. Deduced from the expression of the open-circuit voltage, at very low values of D_it, the effective interface recombination velocity S_p （S_n） is very low, and then the effect of the interface states can be neglected; at high values of D_it, the increase of D_it results in the increas of S_p （S_n） and the decreases of the excess carrier density, and then the open-circuit voltage VOC and the fill factor FF decreases with D_it increasing, and hence the conversion efficiency. At very low values of D_it, VOC of （p） a-Si:H/（n） c-Si heterojunction solar cells obtained on n type c-Si wafers is higher than that on p type c-Si wafers, but their properties is more sensitive to D_it at high values of D_it. As D_it increases, the BSF effect drops down. The intrinsic a-Si:H layer not only passivates the c-Si interface, but also plays the role of counter diffusion that it can hinder the photongenerated majority carriers from diffusing into the emitter to recombine, and improve the short-circuit current.Base on the developed expression of the diffusion capacitance C_D, C_D decreases with D_it increasing since interface states act as recombination centers to decrease the excess carrier density in c-Si. C_D increases with D_it decreasing till 1010 cm-2eV-1. The measurement is sensitive to D_it down to 1010 cm-2eV-1. Accordingly, interface states can be characterized directly by the theoretical diffusion capacitance.The band offsets affect S_p （S_n） and the carrier thransport, and then the I-V characteristics. At non-negligible interface states, the effect of interface states on I-V characteristics of （p） a-Si:H/（n） c-Si heterojunctions under illumination gradually eliminate with the valence band offsetsΔEV increasing atΔEV≤0.5 eV. As a results, opportune band offsets can improve I-V characteristics of a-Si:H/c-Si heterojunctions under illumination. I-Vcharacteristics of silicon heterojunction solar cells under illumination are different with the band gap of the window layer. The short-circuit current of silicon heterojunction solar cells increases with the band gap of the window layer, E_g2, increasing. At non-negligible interface states, since the effects of E_g2 on I-V characteristics operate through the band offsets’role, with E_g2 increasing, S_p （S_n） decreases which results in the open-circuit voltage VOC increases. At E_g2 < 1.9 eV, since the carriers’transport are not limited by the interface barrier, the fill factor increases with E_g2 increasing; at E_g2≥1.9 eV, since the barrier at the heterojunction interface hinders the collection of photogenerated holes, which results in the decrease of the fill factor, and then the conversion efficiency. The optimum efficiency is obtained at E_g2 = 1.7-1.8 eV for silicon heterojunction solar cells. The optimum window layer material is assumed to correspond to a-Si:H. At low work temperatures, low impurity concentrations in the a-Si:H layer, high valence band offsets or high interface defect densities, the barrier at the amorphous/crystalline interface hinders the collection of photogenerated holes. This leads to an enhanced recombination inside the c-Si depletion region and the c-Si interface which causes a significant current loss. S-shaped J–V characteristics occur. The BSF effect of a-Si:H on （n） a-Si:H/（p） c-Si heterojunctions is bad since the valence band barrier limits the hole transport. From the simulated results, it was predicted that the thin film silicon capable of producing the optimal BSF effect is the microcrystalline silicon with band gap of 1.6 eV.Due to a strong inversion layer at the c-Si interface or non-negligible interface states, there may be errors in the determination of the band offsets of a-Si:H/c-Si heterojunctions estimated from the usual capacitance–voltage （C-V） method. At negligible interface states, by taking account of the charge effect of the strong inversion layer, the theoretical capacitance of （n+） a-Si:H/（p） c-Si heterojunctions in the high frequency including the effect of a strong inversion layer is developed, and a modification to the apparent diffusion potential is established. Deduced from the expression of the capacitance, errors depend on the minority carrier density at the c-Si interface. AtΔE_C > 0.05 eV, there are obvious errors for （n+） a-Si:H/（p） c-Si heterojunctions, and errors increase with the conduction band offset increasing. However, the band offsets can be determined more precisely by the modification to the apparent diffusion potential. At low interface states, deduced from the modified expression of V_int*, the conduction band offset between a-Si:H and c-Si is obtainedΔE_C = 0.19 eV.