We present a study on the role and optimization of diffraction gratings used as back reflector/scatterer in multiresonant GaAs ultrathin solar cells. We show the influence of parameters variation for the grating and for the pattern on the diffraction efficiencies. With an optimized square pattern, we show a record-high absorption of 92.5% in a 100 nm-thick GaAs absorber. Accounting for parasitic absorption, the estimated short-circuit current is 26.4 mA/cm2. We also discuss routes towards even higher currents by breaking the degeneracy of the modes with non-symmetric structures.
Modeling single junction solar cells composed of III–V semiconductors such as GaAs with the effects of photon recycling yields insight into design and material criteria required for high efficiencies. For a thin-film single junction GaAs cell to reach 28.5% efficiency, simulation results using a recently developed model which accounts for photon recycling indicate that Shockley–Read–Hall (SRH) lifetimes of electrons and holes must be longer than 3 and 1 μs, respectively, in a 2-μm thin active region, and that the native substrate must be removed such that the cell is coupled to a highly reflective rear-side mirror. The model is generalized to account for luminescence coupling in tandem devices, which yields direct insight into the top cell’s nonradiative lifetimes. A heavily current mismatched GaAs/GaAs tandem device is simulated and measured experimentally as a function of concentration between 3 and 100 suns. The luminescence coupling increases from 14% to 33% experimentally, whereas the model requires increasing electron and hole SRH lifetimes to explain these results. This could be an indication of the saturating defects which mediate the SRH process. However, intermediate GaAs layers between the two subcells may also contribute to the luminescence coupling as a function of concentration.
Single junction photovoltaic devices composed of direct bandgap III-V semiconductors such as GaAs can exploit
the effects of photon recycling to achieve record-high open circuit voltages. Modeling such devices yields insight into the design
and material criteria required to achieve high efficiencies. For a GaAs cell to reach 28 % efficiency without a substrate, the
Shockley-Read-Hall (SRH) lifetimes of the electrons and holes must be longer than 3 s and 100 ns respectively in a 2 μm thin
active region coupled to a very high reflective (>99%) rear-side mirror. The model is generalized to account for luminescence
coupling in tandem devices, which yields direct insight into the top cell’s non-radiative lifetimes. A heavily current
mismatched GaAs/GaAs tandem device is simulated and measured experimentally as a function of concentration between 3
and 100 suns. The luminescence coupling increases from 14 % to 33 % experimentally, whereas the model requires an
increasing SRH lifetime for both electrons and holes to explain these experimental results. However, intermediate absorbing
GaAs layers between the two sub-cells may also increasingly contribute to the luminescence coupling as a function of
concentration.
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