Abstract: A method of forming a photovoltaic device includes a plurality of quantum wells and a plurality of barriers. The quantum wells and barriers are disposed on an underlying layer. The barriers alternate with the quantum wells. One of the plurality of quantum wells and the plurality of barriers is comprised of tensile strained layers and the other of the plurality of quantum wells and the plurality of barriers is comprised of compressively strained layers. The tensile and compressively strained layers have elastic properties. The method includes selecting compositions and thicknesses of the barriers and quantum wells taking into account the elastic properties such that each period of one tensile strained layer and one compressively strained layer exerts substantially no shear force on a neighboring structure; providing the underlying layer; and forming the quantum sells and barriers on the underlying layer according to the derived compositions and thicknesses.

Abstract: A photovoltaic system includes a photovoltaic device that includes a lower photovoltaic cell fabricated from semiconductor material having a first bandgap, and having first electrical contacts for extraction of current from the lower cell; an electrically insulating layer monolithically fabricated on the lower photovoltaic cell; and an upper photovoltaic cell monolithically fabricated on the electrically insulating layer from semiconductor material having a second bandgap larger than the first bandgap, and having second electrical contacts for extraction of current from the upper cell. The photovoltaic system also includes one or more first photon sources operable to supply photons to the photovoltaic device, the photons having wavelengths in a first wavelength range associated primarily with the first bandgap.

Abstract: A photovoltaic cell to convert low energy photons is described, consisting of a p-i-n diode with a strain-balanced multi-quantum-well system incorporated in the intrinsic region. The bandgap of the quantum wells is lower than that of the lattice-matched material, while the barriers have a much higher bandgap. Hence the absorption can be extended to longer wavelengths, while maintaining a low dark current as a result of the higher barriers. This leads to greatly improved conversion efficiencies, particularly for low energy photons from low temperature sources. This can be achieved by strain-balancing the quantum wells and barriers, where each individual layer is below the critical thickness and the strain is compensated by quantum wells and barriers being strained in opposite directions minimizing the stress. The absorption can be further extended to longer wavelengths by introducing a strain-relaxed layer (virtual substrate) between the substrate and the active cell.