Recombination through amphoteric states at the amorphous/crystalline silicon interface: modelling and experiment

The performance of high-efficient crystalline silicon (c-Si) based solar cells is, besides bulk recombination, limited by the recombination losses on both c-Si surfaces. Dangling bonds at the c-Si surface are the defects governing interface recombination irrespective of the overlaying passivation layer (i.e. SiO2, SixNy, a-Si:H). Dangling bonds are also the predominant defects governing recombination in bulk hydrogenated amorphous silicon (a-Si:H). Considering the amphoteric nature of these defects (i.e. different charge states when singly, doubly or not occupied), a closed-form expression exists for recombination in bulk a-Si:H. The application of this dangling bond recombination formalism to c-Si surface dangling bond recombination in the otherwise classical approach allows us to reproduce the measured injectionlevel dependence of the a-Si:H/c-Si interface recombination. Thus we can quantify the respective contribution of the two fundamental mechanisms of surface passivation: the interface defect density and the image charge density in the c-Si induced by the charged defects in the a-Si:H passivation layer. Our experimental data contains a large variety of configurations of c-Si/aSi:H layers. The flat c-Si wafers are of different doping level and doping type (including intrinsic) and the average charge state of the amphoteric recombination centers in the passivating intrinsic a-Si:H layers is varied by microdoping or by capping the passivation layers with doped layers. The good interface passivation properties of a-Si:H layers are exploited in heterojunction solar cells: By a straight-forward development resulting from our passivation studies, we are achieving so far a best VOC of 705mV (with 17.5% efficiency and a FF of 78%) and a best device efficiency of 19% (with a VOC of 680mV and a fill factor (FF) of 82%) on flat 1Ωcm n-type cSi. For example these trade-offs between VOC and FF can partly be linked to the injection-level dependent interface recombination as the VOC corresponds to rather high injection-levels (~5·10cm), whereas the maximum power point (MPP) corresponds to medium injection-levels (~5·10cm).