High‐forward‐bias transport mechanism in a‐Si:H/c‐Si heterojunction solar cells

In order to elucidate the transport mechanism in a‐Si:H/c‐Si heterojunction solar cells under high forward bias ( U  > 0.5 V), we conducted temperature‐dependent measurements of current–voltage ( I – V ) curves in the dark and under illumination. ZnO:Al/(p)a‐Si:H/( n )c‐Si/(n + )a‐Si:H cells are compared with inversely doped structures and the impact of thin undoped a‐Si:H buffer layers on charge carrier transport is explored. The solar cell I – V curves are analyzed employing a generalized two‐diode model which allows fitting I – V data for a broad range of samples. The fitting results are complemented with numerical simulations using AFORS‐HET under consideration of microscopic a‐Si:H parameters as determined by constant‐final‐state‐yield photoelectron spectroscopy (CFSYS) to identify possible origins for a systematic increase of the high‐forward‐bias ideality factor along with the open‐circuit voltage ( V oc ). It is further shown that also for a‐Si:H/c‐Si heterojunctions, dark I – V curve fit parameters can unequivocally be linked to V oc under illumination, which may prove helpful for device assessment.