Passivation Of Silicon Solar Cells By Anodic Oxidation Method

Passivation is one of the important reasons that effect the efficiency of solar cells, The passivation method commonly used in the industrial production is high temperature thermally grown silicon dioxide and a plasma enhanced chemical vapor deposition (PECVD) of silicon nitride (SiNx). But the lattice mismatch in SiNx/Si surface cause the higher interface defect density and decrease the effective minority carrier lifetime. Reasons for the high cost, thermal silica didn’t obtain a wide range of applications. The silica also can grow by electrochemical methods, this process is low-temperature process and easily grown on a rough surface, etc., so how to improve the interface characteristics of electrochemically grown SiO2/Si is particularly important.In this paper, silicon dioxide passivation layer was grown by electrochemical method using nitric acid solution on the p-type silicon. Surface morphology observed by SEM photo. High-frequency capacitance voltage (C-V) and surface photovoltaic (SPV) technology were used to analysis the SiO2/Si interface characteristics and the photo-generated minority carriers recombination process in the different growth and annealing conditions. It was found that a low voltage (10V), high concentration solution (3mol/L), short time(20minutes) and high-temperature annealing (500℃) can obtain better SiO2/Si interface characteristics and can slow down the photo induced minority carrier recombination velocity. Analysis found that: interface state and charge controlled by tunneling current, the rate of oxidation, the content of water in the solution and annealing temperature. The tunneling current can easily produce negatively charged center and tunneling current also can generate a new interface state. The oxidation rate controlled by the water content in the solution, high oxidation rate will lead to non-uniform oxide film thus more interface states generated. After annealing in lower temperature, positive charge reduced and interface defect state does not changed, thereby speeding up the photo generated minority carrier recombination. Both surface states and positive fixed charges decreased when the annealing temperature arrives at500℃, but the surface states were decreased significantly. Then minority carriers’ recombination velocity was decreased.