Can interface charge enhance carrier selectivity in tunnel-layer/poly-Si passivated contacts?

This work experimentally determines if a high interface charge in the tunnel layer can enhance the carrier selectivity of a passivated contact and thus the efficiency potential of a solar cell deploying such a contact on its rear-side. Tunnel layers with high interface charge density - (a) LPCVD-SiN_x for high positive charge and (b) ALD-AlO_xfor high negative charge were optimized and deployed in a passivated contact structure (tunnel-layer/doped-poly-Si-capping-layer) to enhance (i) electron or (ii) hole selectivity, respectively. Subsequently, by measuring the total contact induced recombination (J_0,contact) and the effective contact resistivity ( ρcontact), the selectivity and the efficiency potential according to Brendel's model was calculated. For electron selective passivated contacts, the efficiency potential of a solar cell, deploying a positive charged LPCVD-SiN_xtunnel layer was proven to be higher by 1% absolute, as compared to using a conventional SiO_x tunnel layer. However, for hole selective passivated contacts deploying a negative charged ALD-AlO_xtunnel layer, this was not observed, due to boron in-diffusion and associated charge compensation.