Can interface charge enhance selectivity in tunnel layer passivated contacts? Using negatively charged aluminium oxide capped with dopant free PEDOT or boron doped polysilicon

Abstract

© 2020 Elsevier B.V. Tunnel layer passivated contacts are widely used for selective extraction of electrons or holes in silicon solar cells. We investigate if a tunnel layer with high negative interface charge can further enhance the hole extraction ability (selectivity) of a passivated contact and thus the efficiency of silicon solar cells. High negative-charged, atomic layer deposited AlOx dielectric tunnel layers, capped with either conventional boron doped polysilicon (p+-poly-Si) or low-temperature, dopant-free poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), are investigated in order to form hole selective passivated contacts. Ultra-thin AlOx films in the tunnel regime (0.13–2 nm) are studied, measuring their passivation quality (τeff), fixed charge density (Qtot) and defect density (Dit) prior to capping, and contact recombination density (J0,contact) and effective contact resistivity (ρcontact) after capping with either PEDOT:PSS or p+-poly-Si. The measured J0,contact and ρcontact values are then used to calculate carrier selectivity and efficiency potential if rear-side integrated in a solar cell. Low temperature deposited PEDOT:PSS capped passivated contacts behave differently compared to high temperature deposited p+-poly-Si capped passivated contacts. Yes, negatively charged AlOx tunnel layers can enhance hole extraction capability/selectivity of a passivated contact. For PEDOT:PSS capped passivated contacts, the efficiency potential deploying optimized negatively charged AlOx tunnel layers (1.5 nm, annealed at 425 °C) was measured to be 1.1% higher than conventional reference-moderately charged SiOx tunnel layer (26% as compared to 24.9%). However, this is not observed for p+-poly-Si capped passivated contacts due to boron in-diffusion and associated charge compensation mechanism, a consequence of the high-temperature diffusion process.