Miniature Silicon Solar Cells for High Efficiency Tandem Cells Ngwe Soe Zin and Andrew Blakers and Vernie Everett The Australian National University, Canberra ACT 0200, Australia Email: soe.zin@anu.edu.au Abstract— In this paper, a discussion is made of the design of silicon cells to be used in a six-junction tandem solar cell structure as part of the Very High Efficiency Solar Cell (VHESC) program. Minority carrier recombination at surfaces and in the volume, internal quantum efficiency, resistance losses, free carrier parasitic absorption, optical reflection, light trapping, and light absorption must be traded off against each other. Modelling was used to analyse the various parameters and produce estimates of short circuit current, fill factor and open- circuit voltage of the cell. In addition, quasi-steady-state phtotoconductance measurements to analyse carrier recombination and emitter saturation current (Joe) as well as to predict the open-circuit voltage of solar cell is presented. For metallisation of such small solar cells, alternate methods of making contact such as light-induced plating and electrolyte plating in addition to evaporating metal on the contacts were explored and employed. Numerical resistive loss modelling was made to calculate the optimum metal thickness achieved by light- induced and electroplating to minimise resistive losses. Experiments were conducted to determine the proper plating rate by light-induced and electrolyte plating. Cells were fabricated by standard silicon processing techniques followed by testing of IV curves using current-voltage flash-tester to achieve the target efficiency. Keywords-PC1D, QSSPC, Light-Induced Plating, RIE. I. INTRODUCTION A major objective for photovoltaic conversion is to develop high efficiency solar cells. Researchers at ANU have been conducting research on miniature silicon solar cells to be used in conjunction with six-junction tandem solar cells. In six- junction tandem solar cell as shown in fig 1.1, individual solar cells will be arranged so that each solar cell absorbs the appropriate slice of the solar spectrum. The eventual goal of six-junction tandem solar cell package is to achieve the combined efficiency of >50% under 20suns illumination. Silicon is one of the cells in the tandem structure, and absorbs energy of 1.42 – 1.1 eV. The role of the silicon cell is to convert 7% of the light incident on the tandem structure into electricity. Other cells in the stack contribute the balance of the electricity. Key design parameters for the silicon cells are that it should have dimensions of 2.5x8 2 mm and it needs to transfer light energy of less than 1.1eV to the underlying solar cells. Fig 1.1 Band Gap for Six-Junction Tandem Stack II. CELL STRUCTURE AND DESIGN The external dimensions of the silicon solar cell have to be 2.5mm in width and 8mm in length. Cells were fabricated using 450m thick <100> p-type float-zone 1cm wafers. The cells have an active n-type emitter region on both front and back of cell with the dimension of 6.5 x 2.5 2 mm . Metal contacts are made to both sides of the cell. The n-contact to the external world is on sunward side and the P-contact is on the back of cell. Metal contacts are designed with a spacing of 1.9mm in the lateral direction and 5.5 mm in the lengthwise direction as shown in the figure 1. Multiple cells are processed simultaneously on each wafer until the metallization step is completed, at which time they are diced out of the wafer to form individual solar cells. Fig.2.1 Top view and 3D view of Silicon solar cell This research was, in part, funded by the U.S. Government. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the U.S. Government.