Thin film (∼780 nm and ∼2.5 µm thick) InGaAs/GaAs quantum well and quantum dot-in-well p-i-n single-junction solar cells with various back side reflective and diffractive structures for light trapping have been investigated. The diffractive structures have been optimized for photocurrent generation in the active device using software algorithms. Measurements and rigorous electromagnetic simulations demonstrate that the response of such device structures is significantly influenced by Fabry-Perot interference effects and that the diffractive structures improve light absorption over a broad wavelength range by coupling incident radiation to waveguide modes of the device structures. For Airmass 0 illumination and 100% carrier collection, the simulated short-circuit current density of devices with InxGa1−xAs/GaAs quantum wells with x ≤ 0.3 improves by up to 4.6 mA/cm2 (15%) relative to a GaAs homojunction device. The photocurrent improvement results equally from diffraction of light into thin film modes and from reduction of metal absorption compared to a planar reflective layer.
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