TECHNOLOGYSOLAR CELLS
Figure 1. The blue histogram shows efficiency measurements at 200x concentration of 171 cells of standard triple junction design from one of eight wafers grown on a multi-wafer production reactor. The red histogram shows measurements of cells on a similar wafer with 50 quantum wells grown in the middle cell.
cells with QWs for several years, beginning with the incorporation of these layers into single and dual junction cells. Incorporating wells into the cell increases device efficiency, because it enables the cell to absorb light at longer wavelengths. It is only possible to benefit from this if the introduction of multiple quantum wells (MQWs) does not degrade the material quality of the entire cell.
We meet this criterion by strain-balancing our structure. In these QW cells of high material quality, the only loss mechanism for current carriers generated by the incoming sunlight is radiative recombination back to photons, a process that cannot be prevented from happening. But we do not waste these photons. Instead, we confine a significant proportion of them within the device – so that they can be re-absorbed and generate electricity – by incorporating a reflector into the structure that is similar to the type used in vertical- cavity lasers.
Recently, we have been developing triple-junction cells with QWs in the middle junction. This structure gives us a major advantage over competitors, because our engineers can optimise the absorption edge of the middle cell during growth. Thanks to this, the absorption in the middle cell can be tuned to match the spectrum of sunlight at different times of day or year, and consequently optimise the cell for a given concentrator, at a given geographic location, for maximum electrical energy harvest over the year.
Optimising short wavelength performance CPV systems rarely operate at their full potential at shorter wavelengths due to relatively high optical attenuation in this spectral range. Mornings and
evenings, atmospheric pollution and turbidity, and the concentrator optics themselves all serve to reduce the energy available for the top two cells in triple-junction devices. However, by adding quantum wells and redesigning the top cell absorption appropriately, it is possible to increase the spectral envelope from which the top two cells harvest energy by about 5 percent, according to calculations that presented at the 5th World Conference on Photovoltaic Energy Conversion in Valencia last September. These calculations employed a variety of atmospheric data sources, which were used to generate the solar spectra for each hour of the year in locations deemed favourable for the first CPV installations. The solar spectra were used to model the annual energy harvest of a traditional triple-junction device and another featuring MQWs. Efficiency gains stemming from the introduction of the QWs varied from 3.5 percent to more than 5 percent, with the greatest benefit coming in areas where the spectrum contains more red-light than the ASTM reference spectrum, such as Solar Village in Saudi Arabia (see Figure 2).
Even higher gains are possible by modifying the band-edge of the middle cell and the transparencies of the top two sub-cells so that they are all tuned to the incident spectra. This results in roughly another 1 percent increase in energy harvest. Deciding on the best design for a particular location requires a full-year analysis of the ratio of short circuit currents for the top and middle cells. The photocurrents from these cells must be closely matched to achieve the highest energy harvest efficiencies (see Figure 3). Interestingly, the highest daily energy output coincides with the peak in incident energy (blue and black lines in Figure 3). So, to extract as much electrical energy as possible from our cells, we are tailoring device design to maximize the energy harvest efficiency at the same time of the day and year as the peak in irradiance. This principle is also being applied to boost revenue generation by tuning cells to perform at their best during utility
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Figure 2. The
simulated increase in power output of a MQW triple-junction device over a bulk triple-junction device under the ASTM G173- 03 reference spectrum, and the increase in energy harvest
calculated using hourly spectra specific to La Parguera in Puerto Rico, South West US, Gujarat in India and Solar Village in Saudi Arabia
www.solar-pv-management.com Issue III 2011
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