Impact of Fermi-level dependent defect equilibration on Voc of amorphous/crystalline silicon heterojunction solar cells

We analyze the microscopic mechanisms limiting the open-circuit voltage Voc of high efficiency amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells. The study is based on passivation experiments with undoped a-Si:H layers as well as device-relevant doped/undoped a-Si:H stack structures and extensive layer characterization. We conclude that the density of strained bonds in the a-Si:H and the Fermi level (EF) position at the heterointerface codetermine the passivation potential of a given structure. Thus, the commonly observed deterioration of the undoped a-Si:H/c-Si passivation upon deposition of a doped a-Si:H top layer can be interpreted as defect equilibration in the a-Si:H layer (i.e. the EF-dependent formation of dangling bond defects in a-Si:H from strained Si- Si bonds). Consistently, employing the well-established “defect pool model” which describes the EF-dependent defect formation, the observed recombination velocities of (p/n)a-Si:H/(i)a-Si:H/c-Si stack structures can be quantitatively explained without invoking any fit parameters. The impact of this effect on solar cell Voc is quantified.