Miniature silicon solar cells for High Efficiency Tandem Cells

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 photoconductance 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.