Oxygen doping in ZnTe is applied to a junction diode in the aim of utilizing the associated electron states 0.5 eV below the bandedge as an intermediate band for photovoltaic solar cells. The ZnTe:O diodes confirm extended spectral response below the bandedge relative to undoped ZnTe diodes, and demonstrate a 100% increase in short circuit current, 15% decrease in open circuit voltage, and overall 50% increase in power conversion efficiency. Subbandgap excitation at 650 and 1550 nm confirms the response via a two-photon process and illustrates the proposed energy conversion mechanism for an intermediate band solar cell.

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A model for intermediate band solar cells is presented to assess the effect of carrier transport and recombination ͑CTR͒ on the efficiency of these devices. The model includes dependencies of physical parameters including optical absorption, carrier lifetime, and carrier transport on the density of intermediate band electronic states. Simulation results using this model indicate that conversion efficiency degrades when the net carrier recombination lifetime is small ͑range of nanoseconds͒ or when the device length is long relative to carrier drift length. The intermediate band solar cell model provides a method of determining realistic conversion efficiencies based on experimentally measurable input parameters for CTR. The incorporation of CTR provides insight on the dependence of optimal density of states and energetic position of the intermediate band based on carrier lifetime and mobility. The material ZnTeO ͑E G = 2.3 eV, E I = 1.8 eV͒ is used as a numerical example for the intermediate band solar cell model, where conversion efficiency drops from 30.36% to 19.4% for a 10 m long device for a recombination lifetime decrease from 1 s to 5 ns. The optimal impurity concentration is determined to be 10 18 cm −3 for an optical absorption cross section of 10 −14 cm 2. The conversion efficiency of a ZnTe solar cell with a total recombination lifetime of 10 ns is calculated to increase from 14.39% to 26.87% with the incorporation of oxygen.

The integration of III–V and Si multi-junction solar cells as photovoltaic devices has been studied in order to achieve high photovoltaic conversion efficiency. However, large differences in the coefficients of thermal expansion and the lattice parameters of GaAs, Si, and InGaAs have made it difficult to obtain high-efficiency solar cells grown as epilayers on Si and InP substrates. In this paper, two types of devices, including GaInP/GaAs stacked on Si (GaInP/GaAs//Si) and GaInP/GaAs stacked on InGaAs (GaInP/GaAs//InGaAs), are fabricated via mechanical stacking and wire bonding technologies. Mechanically stacked GaInP/GaAs//Si and GaInP/GaAs//InGaAs triple-junction solar cells are prepared via glue bonding. Current-voltage measurements of the two samples are made at room temperature. The short-circuit current densities of the GaInP/GaAs//Si and GaInP/GaAs//InGaAs solar cells are 13.37 and 13.66 mA/cm2, while the open-circuit voltages of these two samples are measured to be 2.71 and 2.52 V, respectively. After bonding the GaInP/GaAs dual-junction with the Si and InGaAs solar cells, the conversion efficiency is relatively improved by 32.6% and 30.9%, respectively, compared to the efficiency of the GaInP/GaAs dual-junction solar cell alone. This study demonstrates the high potential of combining mechanical stacked with wire bonding and ITO films to achieve high conversion efficiency in solar cells with three or more junctions.

In this work, we present the result of nickel (Ni)-based metamaterial perfect absorbers (MPA) with ultra-broadband close-to-one absorbance. The experimental broadband characteristic is significantly improved over the past effort on metamaterial perfect absorbers. An in-depth physical picture and quantitative analysis is presented to reveal the physical origin of its ultrabroadband nature. The key constituent is the cancellation of the reflected wave using ultra-thin, moderate-extinction metallic films. The ultra-thin metal thickness can reduce the reflection as the optical field penetrates through the metallic films. This leads to minimal reflection at each ultra-thin metal layer, and light is penetrating into the Ni/SiO₂ stacking. More intuitively, when the layer thickness is much smaller than the photon wavelength, the layer is essentially invisible to the photons. This results in absorption in the metal thin-film through penetration while there is minimal reflection by the metal film. More importantly, the experimental evidence for omni-directionality and polarization-insensitivity are established for the proposed design. Detailed measurement is conducted. Due to the ultrathin metal layers and the satisfactory tolerance in dielectric thickness, the broadband absorption has minimal degradation at oblique incidence. Such a wide angle, polarization-insensitive, ultra-broadband MPA can be very promising in the future, and the optical physics using sub-skin-depth metal film can also facilitate miniaturized high-performance nano-photonic devices.