Improved silicon oxide/polysilicon passivated contacts for high efficiency solar cells via optimized tunnel layer annealing

Abstract

It is well known that the quality of contact passivation can greatly influence the efficiency of Si solar cells. In this work, the impact of annealing a wet-chemically formed silicon oxide (SiOx) tunnel layer (prior to poly-Si capping) to form SiOx/n+ poly-Si passivated contacts for high efficiency solar cells is investigated. The effect of different annealing temperatures (ranging from 425 to 900 °C) and ambients (air and forming gas), on the overall performance of the resulting SiOx/n+ poly-Si passivated contacts are studied. The efficiency potential of these SiOx/n+ poly-Si contacts is calculated from the measured values of the recombination current density (J0,contact), and the effective contact resistivity (ρcontact), according to the generalized Brendel's model. The results are compared to the reference case, i.e. non-annealed SiOx tunnel layer with poly-Si deposition. The lifetime, the fixed charge and also the SiOx tunnel layer thickness increase upon annealing at higher temperatures. The sample annealed in forming gas at 900 °C shows an increase in field effect passivation (Qtot = 1 × 1012 cm−2) as well as in chemical passivation (Dit = 3 × 1011 eV−1 cm−2), along with reduced ρcontact, indicating pinhole formation upon annealing at 900 °C. The best SiOx/n+ poly-Si passivated contact achieved J0,contact = 4.2 fA cm−2, ρcontact = 0.27 Ω cm−2 and a corresponding maximum ideal efficiency potential of 27.9%. This is 1.2% absolute higher as compared to the reference case, when no annealing of the tunnel layer is performed.