OPTIMUM BANDGAP CALCULATIONS FOR A 4-TERMINAL DOUBLE TANDEM III-V CONCENTRATOR SOLAR CELL STRUCTURE G.M.M.W. Bissels, J.J. Schermer, E.J. Haverkamp, P. Mulder, G.J. Bauhuis Applied Materials Science, Institute for Molecules and Materials, Radboud University Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands Phone: +31 (0)24 3653060, e-mail: G.Bissels@science.ru.nl ABSTRACT: A 4-terminal double tandem III-V concentrator solar cell, consisting of two mechanically stacked monolithic tandem cells, can achieve efficiencies of more than 40 %. Calculations were performed to determine the optimum bandgap combination for the InGaAsP subcells of this design, for an irradiance range of 1 to 1000 suns. Furthermore, the addition of Al to the compound that composes subcell 1 and allowing a certain degree of lattice mismatch were considered as methods to extend the bandgap ranges, in order to increase the efficiency. However, in a first order approximation the implementations of these methods seem to be difficult or to induce little effect. Keywords: Multi-junction Solar Cell, Concentrator Cells, Modelling 1 INTRODUCTION Anthropogenic CO 2 emissions are largely responsible for global warming and its associated damaging effects [1]. A sufficient reduction of this emission requires a dramatic decrease in the use of fossil fuels. One of the most promising long term solutions is to use solar cells to utilize solar energy. The reason that solar cells have not yet been implemented on a large scale is the fact that the cost of the electricity they generate is several factors higher than the prevailing electricity cost. One way to significantly reduce the price of electricity generated by solar cells is to incorporate high efficiency cells into concentrator systems. For this reason, the department of Applied Materials Science (AMS) of the Radboud University Nijmegen has the long-term research strategy to develop concentrator solar cells with efficiencies of more than 40 %. Based on its theoretical maximum efficiency, on possibilities to actually grow the crystalline structures and on the specific capabilities available at the AMS department, a 4-terminal double tandem solar cell structure is considered to be a promising configuration to achieve this goal. In the present study, the optimum bandgap combination for the subcells of the above mentioned structure was calculated for a series of irradiances ranging from 1 to 1000 suns, under the AM1.5D spectrum. Furthermore, two methods of extending the bandgap ranges of its subcells are discussed, since this can increase the efficiency of the configuration somewhat. These are the allowance of a certain degree of lattice mismatch, and the integration of Al to the compound that composes the top subcell. 2 THEORY Many configurations are conceivable to design a high efficiency multi-junction III-V solar cell. One can choose to incorporate any number of junctions in the stack, the bandgap of each subcell can be varied, and there is the choice between monolithic and mechanical stacking. In theory, the efficiency of a solar cell stack increases with each junction added, but the efficiency gained gets smaller with each additional junction. Moreover, each junction that is added to a mechanical stack will require an additional growth run and associated processing, and will add an extra set of terminals to the stack. In practice, each added subcell will also suffer from increasing shadowing losses and will make the structure more prone to mechanical impairment. These drawbacks do not accompany the addition of a junction to a monolithic stack. However, in this case the disadvantages are the fact that the potential improvement of the efficiency, by increasing the number of junctions, is limited due to current- and lattice matching restrictions. Also, the tunnel junctions required in this case have losses associated with them. In practise, therefore, the optimum number of stacked pn-junctions is limited for both mechanical and monolithic stacking methods. This explains the fact that the efficiency record for a multi-junction solar cell is currently held by a solar cell containing only three junctions [2]. An excellent way to achieve a high number of junctions, while largely circumventing the restrictions of both stacking methods, is by combining both forms of stacking in a 4-terminal quadruple-junction solar cell structure, referred to as a double tandem structure, as depicted in figure 1. Figure 1: Schematic representation of the double tandem structure. The envisioned double tandem structure consists of a thin-film semi-transparent monolithic tandem which is released from its GaAs substrate using the epitaxial lift- off (ELO) technique [3, 4], and mechanically stacked on top of a second monolithic tandem cell which is grown on an InP substrate. Recently Griggs et al. also performed calculations on such a structure [5]. Using lattice matched InGaAsP compounds as the materials for the junctions, the top and bottom monolithic tandems can cover the large bandgap ranges of 1.894 to 1.430 eV and