2-DIMENSIONAL NUMERICAL OPTIMIZATION STUDY OF THE REAR CONTACT GEOMETRY OF HIGH-EFFICIENCY SILICON SOLAR-CELLS

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 OMEGA 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.