Degradation of Ge subcells by thermal load during the growth of multijunction solar cells

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Germanium solar cells are used as bottom subcells in many multijunction solar cell designs. The question remains whether the thermal load originated by the growth of the upper layers of the multijunction solar cell structure affects the Ge subcell performance. Here, we report and analyze the performance degradation of the Ge subcell due to such thermal load in lattice-matched GaInP/Ga(In)As/Ge triple-junction solar cells. Specifically, we have detected a quantum efficiency loss in the wavelength region corresponding to the emitter layer (which accounts for up to 20% loss in equivalent JSC) and up to 55 mV loss in VOC of the Ge subcell as compared with analogous devices grown as single-junction Ge solar cells on the same type of substrates. We prove experimentally that there is no direct correlation between the loss in VOC and the doping level of the base. Our simulations show that both the JSC and VOC losses are consistent with a degradation of the minority carrier properties at the emitter, in particular at the initial nanometers of the emitter next to the emitter/window heterointerface. In addition, we also rule out the gradual emitter profile shape as the origin of the degradation observed. Our findings underscore the potential to obtain higher efficiencies in Ge-based multijunction solar cells if strategies to mitigate the impact of the thermal load are taken into consideration.


  • Enrique Barrigón
  • Mario Ochoa
  • Ivan García
  • Laura Barrutia
  • Carlos Algora
  • Ignacio Rey-Stolle
External organisations
  • Technical University of Madrid
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Energy Systems


  • Ge solar cells, multijunction solar cells, thermal degradation, thermal load, EU Horizon 2020, NEXTNANOCELLS, Grant 656208
Original languageEnglish
Pages (from-to)102-111
Number of pages10
JournalProgress in Photovoltaics: Research and Applications
Issue number2
StatePublished - 2018 Feb 1
Publication categoryResearch

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