TECHNOLOGYSOLAR CELLS
Figure 3 a) Daily incident energy density (blue), energy harvest efficiency (red), b) device energy output (black) and mean current match ratio (orange) for a
simulated device under an hourly varying spectrum specific to the South West US
peak delivery times. Doubling up on quantum wells Looking to the future, we have started to investigate the advantages of MQWs in the top and the middle cells of a triple-junction device. One motivation behind this effort, which has been pursued in partnership with the Imperial College London QPV group that first developed the QW cell, is to optimise the absorption profiles of the cells. In a conventional triple-junction device, the absorption edges of both the top and middle cell are at shorter wavelengths than ideal.
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Our modeling indicates that efficiencies well in excess of 40 percent are possible with MQW- based cells that shift the absorption edge to longer wavelengths (see Figure 4). These calculations – which are based on a concentration factor of 500 and have been plotted for the specific choice of the solar spectrum used in the standard test condition – show how optimising the absorption edge of our top and middle cells can yield overall efficiencies up to 45 percent. To illustrate what is feasible for a dual MQW device, a rectangular area
bounded by a green dotted line is included in this plot and it can be seen that absorption edges reaching the 45 percent efficiency contour are possible. As the solar spectrum changes by day and during the year, it shifts these contours that map out the conversion efficiency. The large rectangular area shows that the quantum wells add tremendous flexibility in the choice of absorption edges for both cells, indicating that it should be possible to design for maximum energy harvest over a very wide range of spectral conditions.
Figure 4. The colored areas separated by black numbers show how the efficiency of a dual-MQW triple junction cell at 500x concentration varies with the absorption edge of the top cell and the middle cell. The green broken-line rectangle represents the variation in absorption edge possible for the dual-MQW cell. The black broken-line arrow represents the absorption edge variation possible for the competitor metamorphic cell. In all cases the recombination loss in the top and middle cell is assumed to be radiative.
The primary rival to our dual-MQW triple-junction cell is the metamorphic triple-junction cell grown on a relaxed buffer layer, which is often referred to as a virtual substrate. With this design, the absorption edges of the top and middle cells can be extended to longer wavelengths. However, these adjustments cannot be made independently – the tuning of absorption that is possible with the metamorphic design is shown by the black arrow in Figure 4. This arrow can, in principle, be extended to higher efficiency contours. However, such a move pays the penalty of greater relaxation in the buffer, leading to more residual dislocations and ultimately an increase in efficiency loss. One assumption in these calculations is that the main loss mechanism in the top MQW cell is radiative recombination, which is known to dominate the loss in the middle cell. An intriguing possibility arises if this also happens in the top cell. In that case, the majority of the photons created by radiative recombination in the top cell will be emitted from the bottom of the quantum well, and most of these will then be re-absorbed in the middle cell, boosting its current output.
To understand how this affects the overall performance of our triple junction device, we have repeated the calculations for this scenario. Results are plotted in Figure 4, using blue contours and blue numbers. These results indicate that radiative recombination of photons from the top cell that are subsequently absorbed by the middle cell has a major benefit - a widening of the sweet spot for very efficient operation. This means that the
www.solar-pv-management.com Issue III 2011
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