The main limiting factors of multijunction solar cells operating under ultra‐high concentration (>1000 suns) are examined by means of 2D physically based numerical modelling. The validation of the model is carried out by fitting calibrated light concentration measurements. Because the series resistance is the most important constraint in the electrical performance of the solar cell under ultra‐high irradiance, it is analysed and quantified detailing different contributions such as: (i) the electrical properties of the emitter; (ii) window layer of the top cell; and (iii) the band discontinuities formed at heterojunctions. We found the role of window layer to be important at very high concentrations (above 700 suns), while at ultra‐high concentrations, (above 1000 suns) a gain in efficiency (~ 1% absolute) can be obtained by a proper structural design of the window layer. In the case of the heterojunctions included in the multijunction solar cell, the impact of a high‐band offset can be mitigated by increasing the doping level density thus favouring the tunnelling effect. Moreover, the influence of different recombination mechanisms and high‐injection effects at ultra‐high irradiance is discussed. Finally, an optimisation of the complete solar cell taking into account the ohmic contacts to work under ultra‐high irradiances (from 1000 to 5000 suns) is presented as well as the implications on the use of ultra‐high irradiance in different multijunction solar cell architectures. Copyright © 2016 John Wiley & Sons, Ltd. A 2D numerical model based on drift‐diffusion equations and quantum models that take into account tunnel junctions, heterojunctions and typical recombination mechanisms has been developed and validated to analyse the limiting factors on the semiconductor structure of multijunction solar cells operating at ultra‐high concentration. The contribution to series resistance from top cell window and emitter layers is examined and optimised for solar cell performance operating at ultra‐high irradiance. The role of top cell window layer has been found to be more important for concentrations above 700 suns where an efficiency gain up to 1% can be achieved. No signs of high injection effects were detected nor Auger‐limiting process that may affect the performance of the cells up to a concentration level of 5000 suns. The potential barriers at heterojunctions were analysed, and no important losses have been found for literature band offsets. However, when a high potential barrier is present, its impact can be mitigated by increasing the doping level favouring the tunnelling of carriers through the potential barriers. The resulting optimised structure suggests that, by using an appropriate design, it is possible to reach high efficiencies (above 42%) in a conventional GaInP/GaAs/Ge multijunction structure. Finally, the implications on the use of other multijunction structures such as inverted metamorphic, wafer bonded and dilute nitrides are described in order to explore the potential of these cells at ultra‐high irradiance.
Limiting factors on the semiconductor structure of III–V multijunction solar cells for ultra‐high concentration (1000–5000 suns)
Progress in photovoltaics ; 24 , 10
2016
Article (Journal)
English
BKL: | 53.36 Energiedirektumwandler, elektrische Energiespeicher |
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