Two-dimensional numerical optimization study of the rear contact geometry of high-efficiency silicon solar cells

Abstract

Under one-sun illumination, the highest energy conversion efficiencies of silicon solar cells are presently obtained with bifacially contacted n+p cells, where contact to the p-type substrate is made via small openings in the rear passivating oxide. In this work, a state-of-the-art 2-dimensional (2D) semiconductor device simulator is applied to these devices in order to investigate the effects arising from the rear metallization scheme. The impact of various cell parameters [i.e., substrate resistivity, rear surface recombination model (flatband or surface band bending conditions), positive oxide charge density, capture cross section ratio] on the cell’s current-voltage (I-V) characteristics and the optimum rear contact spacing is investigated. The highly nonideal I-V curves of rear point-contacted high-efficiency silicon solar cells made at the University of New South Wales (UNSW) are modeled with a high degree of accuracy. This is achieved by properly accounting for the complex recombination behavior at the rear oxidized surface. The 2D simulations show that the nonideal I-V curves result from the unequal capture cross sections of electrons and holes at the rear Si-SiO2 interface and the surface band bending induced by positive oxide charges and metal/silicon work function differences. For the UNSW cells, optimum one-sun efficiency is obtained on 2 Ω cm substrates and rear contact spacings of 0.2–0.3 mm. The 2D simulations presented in this work clearly confirm the experimental findings and reveal the physical mechanisms which favor this particular contact design.