Analysis and optimization approach for the doped amorphous layers of silicon heterojunction solar cells
Comparison of the open-circuit voltage (external voltage V(oc,ext)) determined by Suns-V(oc) measurements with the implied voltage V(oc, impl) determined by transient photoconductance decay lifetime measurements can yield a quick and easy analysis of silicon heterojunction (SHJ) solar cells, especially in regard to finding the optimum doping concentration of the emitter layer [or back surface field (BSF)]. A sufficiently high doping concentration of the emitter and BSF is mandatory to extract the internal Fermi-level splitting and thus the internal voltage, at the solar cell contacts. However increasing the concentration of doping gases during the deposition of doped amorphous silicon layers results in a reduction of the interface passivation quality and Voc, impl. The best trade off is realized when the ratio of V(oc,ext) to V(oc,impl) (external/internal V(oc)-ratio zeta) reaches a saturation value near 1 upon increasing the doping concentration. AFORS-HET (Automat FOR Simulation of HETerostructures) simulations resulted in the conclusion that the characteristics of the external/internal Voc-ratio are mainly determined by the active doping concentration (doping minus defect concentration).