Note: in this version the excitation densities were computed using measured laser profiles instead of those calculated using the diffraction limited formula. AbstractWe compare three representative high performance PV materials: halide perovskite MAPbI3, CdTe, and GaAs, in terms of photoluminescence (PL) efficiency, PL lineshape, carrier diffusion, and surface recombination, over multiple orders of photo-excitation density. An analytic model is used to describe the excitation density dependence of PL intensity and extract the internal PL efficiency and multiple pertinent recombination parameters. A PL imaging technique is used to obtain carrier diffusion length without using a PL quencher, thus, free of unintended influence beyond pure diffusion. Our results show that perovskite samples tend to exhibit lower Shockley-Read-Hall (SRH) recombination rate in both bulk and surface, thus higher PL efficiency than the inorganic counterparts, particularly under low excitation density, even with no or preliminary surface passivation. PL lineshape and diffusion analysis indicate that there is considerable structural disordering in the perovskite materials, and thus photo-generated carriers are not in global thermal equilibrium, which in turn suppresses the nonradiative recombination. This study suggests that relatively low point-defect density, less detrimental surface recombination, and moderate structural disordering contribute to the high PV efficiency in the perovskite. This comparative photovoltaics study provides more insights into the fundamental material science and the search for optimal device designs by learning from different technologies.

CdTe/MgCdTe double heterostructures (DHs) are grown on InSb substrates using molecular beam epitaxy and reveal strong photoluminescence with over double the intensity of a GaAs/AlGaAs DH with an identical layer structure design grown on GaAs. Time-resolved photoluminescence of the CdTe/MgCdTe DH gives a Shockley-Read-Hall recombination lifetime of 86 ns, which is more than one order of magnitude longer than that of typical polycrystalline CdTe films. These findings indicate that monocrystalline CdTe/MgCdTe DHs effectively reduce surface recombination, have limited nonradiative interface recombination, and are promising for solar cells that could reach power conversion efficiencies similar to that of GaAs.

Efficiency enhancement calculations of state-of-the-art solar cells by luminescent layers with spectral shifting, quantum cutting, and quantum tripling function J. Appl. Phys. 114, 084502 (2013); 10.1063/1.4819237 1.5 μm to 0.87 μm optical upconversion using wafer fusion technology

The mean inner potential (MIP) and inelastic mean free path (IMFP) of undoped ZnTe are determined using a combination of off-axis electron holography and convergent beam electron diffraction. The ZnTe MIP is measured to be 13.7±0.6 V, agreeing with previously reported simulations, and the IMFP at 200 keV is determined to be 46±2 nm for a collection angle of 0.75 mrad. Dynamical effects affecting holographic phase imaging as a function of incident beam direction for several common semiconductors are systematically studied and compared using Bloch wave simulations. These simulation results emphasize the need for careful choice of specimen orientation when carrying out quantitative electron holography studies in order to avoid erroneous phase measurements.

Radiative and nonradiative recombination processes in lattice-matched (Cd,Zn)O/(Mg,Zn)O multiquantum wells

The ability to spatially resolve the degree to which extended defects impact carrier diffusion lengths and lifetimes is important for determining upper limits for defect densities in semiconductor devices. We show that a new spatially and temporally resolved photoluminescence (PL) imaging technique can be used to accurately extract carrier lifetime values in the immediate vicinity of dark-line defects in CdTe=MgCdTe double heterostructures. A series of PL images captured during the decay process show that extended defects with a density of 1.4 × 10 5 cm −2 deplete photogenerated charge carriers from the surrounding semiconductor material on a nanosecond time scale. The technique makes it possible to elucidate the interplay between nonradiative carrier recombination and carrier diffusion and reveals that they both combine to degrade the PL intensity over a fractional area that is much larger than the physical size of the defects. Carrier lifetimes are correctly determined from numerical simulations of the decay behavior by taking these two effects into account. Our study demonstrates that it is crucial to measure and account for the influence of local defects in the measurement of carrier lifetime and diffusion, which are key transport parameters for the design and modeling of advanced solar-cell and light-emitting devices.

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Effects of (P, N) dual acceptor doping on band gap and p-type conduction behavior of ZnO filmsEffects of nitrogen doping of ZnO ͑ZnO:N͒ during deposition and after postdeposition annealing have been studied by optical techniques, electronic properties, and the application to metal-semiconductor-metal photodetectors ͑MSM-PDs͒. Films of ZnO, nitrogen doped during rf sputtering, show larger grain size, narrower full width at half maximum, band gap emission shift in photoluminescence, and higher conductivity. Postannealing has been studied using tube furnace and rapid thermal annealing in nitrogen. These annealing methods not only reconstruct the lattice structure but also activate the nitrogen in the film to improve the conductivity of the film. The MSM-PDs having high photo to dark current ratio of 10 4 and responsive ͑R͒ of 1.79 A/W have been fabricated with those films.