Photovoltaics: Upconversion Configurations versus Tandem Cells
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Photovoltaics: Upconversion Configurations versus Tandem Cells Joop van Deelen1 1 Solliance/TNO, High Tech Campus, 21, 5656 AE, Eindhoven, The Netherlands.
ABSTRACT For a wide range of bandgaps of solar cell materials, the potential contribution of upconversion materials was calculated and related to various configurations of the solar cell and upconversion layers. Moreover, by comparing these various strategies with the potential of a dual junction tandem cell configuration, a compelling case is made for upconverters. At idealized 100% conversion efficiency, the upconverter with a single junction cell is more efficient than a dual junction tandem cell. It was also found that a single junction cell with an upconverter that is ‘only’ 80% efficient has a similar efficiency as an ideal dual junction cell. This result shows that upconverters are certainly a route worthwhile to pursue, especially because the single junction cells plus upconverters could have more cost reduction potential than dual junction cell configurations. Additionally, it was investigated if an upconverter that uses two different photon energies would create a large surplus in efficiency. For a cell band gap of 1.55 eV a theoretical maximum efficiency (here defined as Voc*Isc) of 54.5% was calculated. Although there is a further increase in efficiency compared to converters with a single conversion energy, very careful bandgap tuning with a tolerance < 0.02 eV is required, which makes this system rather sensitive for material and solar spectrum fluctuations and it is suggested that a simple upconverter material is a more favorable strategy.
INTRODUCTION The envisioned benefit of quantum cutting and upconversion for various opto-electronic devices is tremendous [1-4]. Unfortunately, the experimental state of the art efficiency of upconversion is far from ideal yet [5-9]. In fact, solar cell enhancement due to upconversion is modest to say the least [5,10,11]. For high efficiency III-V cells [12], there has not been a demonstration of a significant enhancement by upconversion [8]. Therefore, the photovoltaics (PV) industry has not invested in this direction so far. However, the potential of upconversion becomes clear if the number of captured photons in a PV system is seen relative to the number of photons in the spectrum as shown in figure 1. In case of CdTe, or another high band gap absorber material, the amount of utilized photons is less than half of what is reasonably available. These photons that are not used should be made available and upconversion could be a suitable way to achieve this. Upconversion for solar cells has a requirement for the minimum energy of the upconverted photons (i.e. at least the bandgap of the solar cell absorber) and this puts a limit on the total amount of photons that are usable. Figure 2 illustrates the bandgap of a solar cell (Eg) and how an upconverter could serve as a mini staircase to come to the bandgap.
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