Drop In Quantum Efficiency Research Articles

Realization of Quasi-Omnidirectional Solar Cells with Superior Electrical Performance by All-Solution-Processed Si Nanopyramids.

Large‐scale (156 mm × 156 mm) quasi‐omnidirectional solar cells are successfully realized and featured by keeping high cell performance over broad incident angles (θ), via employing Si nanopyramids (SiNPs) as surface texture. SiNPs are produced by the proposed metal‐assisted alkaline etching method, which is an all‐solution‐processed method and highly simple together with cost‐effective. Interestingly, compared to the conventional Si micropyramids (SiMPs)‐textured solar cells, the SiNPs‐textured solar cells possess lower carrier recombination and thus superior electrical performances, showing notable distinctions from other Si nanostructures‐textured solar cells. Furthermore, SiNPs‐textured solar cells have very little drop of quantum efficiency with increasing θ, demonstrating the quasi‐omnidirectional characteristic. As an overall result, both the SiNPs‐textured homojunction and heterojunction solar cells possess higher daily electric energy production with a maximum relative enhancement approaching 2.5%, when compared to their SiMPs‐textured counterparts. The quasi‐omnidirectional solar cell opens a new opportunity for photovoltaics to produce more electric energy with a low cost.

Quantum efficiency modeling of thin film solar cells under biased conditions with a case study of CZTSSe solar cells

We have modeled the quantum efficiency of thin film solar cell devices with a case study of kesterite solar cells under a set of biased conditions with SCAPS device simulator. This study reveals that the carrier collection with deep acceptor type defect distribution above and below 0.3eV from the midgap near the heterojunction interface is pronounced at particular wavelength regions, (500–550nm) at particular biased conditions, (0.2–0.25V). Specifically, these acceptor type defect states influence kesterite spectral responses of red and infrared light wavelength regions in quantum efficiency caused by affected space charge region width toward the back contact. As a result, the intensified drop of quantum efficiency occurs at ¿550nm at forward bias (0.25V) with a peak spectral response near 520nm to 530nm. This numerical model elucidates the probable loss mechanism to account for a deficit in open circuit voltage by taking into account of voltage and light bias models.

Radiation tolerance characterization of dual band InAs/GaSb type-II strain-layer superlattice pBp detectors using 63 MeV protons

The radiation tolerance characterization of dual band InAs/GaSb type-II strain-layer superlattice pBp detectors of varying size using 63 MeV proton irradiation is presented. The detectors' mid-wave infrared performance degraded with increasing proton fluence ΦP up to 3.75 × 1012 cm−2 or, equivalently, a total ionizing dose = 500 kRad (Si). At this ΦP, an ∼31% drop in quantum efficiency η, ∼2 order increase in dark current density JD, and consequently, >1 order drop in calculated detectivity D* were observed. Proton damage factors were determined for η and D*. Arrhenius-analysis of temperature-dependent JD measurements reflected significant changes in the activation energies following irradiation.

The Origin of Spectral Effects in Photosynthesis

The non-linearity of photosynthetic light curves and the real reasons leading to the appearance of the "red drop of quantum efficiency" and "enhancement" (Emerson's second) effects are presented. On the contrary to the generally accepted interpretation that these effects are induced from the participation of the two different photochemical systems, it is shown that they are simply a direct consequence of the initial non-linearity of the photosynthetic light curves.

Microscopic Spatial Distribution of Bound Excitons in High-Quality ZnO

The surface morphology of the ZnO layers is dominated by a distinct hexagonal domain structure. While the laterally integrated cathodoluminescence spectrum shows intense and narrow I8 luminescence, a distinct emission line at spectral position of I0/I1 emerges in the local spectra taken at domain boundaries. In contrast, no I0/I1 emission is found inside the domains. Monochromatic images further evidence the selective incorporation of impurities at the grain boundaries of domains. Monochromatic images of the I8 peak wavelength directly visualize the strain relaxation across the domains towards their very center, where a drop in quantum efficiency indicates enhanced defect concentration.

Microscopic spatial distribution of bound excitons in high-quality ZnO

A direct correlation of structural and optical properties in ZnO epitaxial layers has been achieved on a microscopic scale using highly spatially and spectrally resolved cathodoluminescence. The surface morphology of the high-quality ZnO layers is characterized by a distinct hexagonal domain structure. While the laterally averaged cathodoluminescence spectrum shows intense and narrow I 8 luminescence, a distinct emission line at the spectral position of I 0/I 1 emerges in the local spectra taken at domain boundaries. In contrast, no I 0/I 1 emission is found inside the domains. Monochromatic images further evidence the selective incorporation of impurities at the grain boundaries of domains. Monochromatic images of the I 8 peak wavelength directly visualize the strain relaxation across the domains towards their very center, where a drop in quantum efficiency indicates enhanced defect concentration.

Direct evidence for selective impurity incorporation at the crystal domain boundaries in epitaxial ZnO layers

A direct correlation of structural properties with the spatial distribution of bound exciton luminescence in ZnO epitaxial layers has been achieved on a microscopic scale using highly spatially and spectrally resolved cathodoluminescence. The morphology of the high quality ZnO layer is characterized by a distinct domain structure. While the laterally integrated cathodoluminescence spectrum shows narrow (full width at half maximum <3meV) I8 luminescence, a pronounced emission line at I0∕I1 emerges in the local spectra taken at domain boundaries. In complete contrast, no I0∕I1 emission is found inside the domains. Monochromatic images further evidence the selective incorporation of impurities at the grain boundaries of domains. Micro mappings of the I8 peak wavelength directly visualize the strain relaxation across the domains toward their very center, where a drop in quantum efficiency indicates enhanced defect concentration.

Effect of oxygen adsorption on the efficiency of magnesium photocathodes

The quantum efficiency (QE) of photocathodes is key to their being used as electron sources in electron beam lithography and inspection. We have built and are using equipment for measuring quantum efficiency, weight gain, and residual gas pressure. During exposure to oxygen at 10−10–10−8 Torr, we observed an initial rise of between five- and eightfold in quantum efficiency to 0.2% followed by a decrease in QE and saturation of the oxygen uptake (weight gain). At lower oxygen pressures, the cathode exhibits a higher peak QE and larger total weight gain. The peak quantum efficiency can be kept stable to within 1% over 24 h if the ambient pressure is 2×10−10 Torr. The drop in quantum efficiency upon further exposure to oxygen at pressures >10−9 Torr suggests the growth of an oxide layer that reduces emission and the presence of a loosely bound adlayer of molecular oxygen during oxygen exposure.

Quantum efficiency of silicon photodiodes in the near-infrared spectral range.

The quantum efficiency of silicon photodiodes and factors that might be responsible for the drop in quantum efficiency in the near-infrared spectral range were analyzed. It was shown that poor reflectivity from the rear surface of the die could account for a decrease in Si photodiode quantum efficiency in near-infrared spectral range by more than 20%. The photodiode quantum efficiency was modeled with an appropriate representation for the carrier-collection efficiency dependence on the die penetration depth. A corrected analytical expression for calculating the photodiode quantum efficiency is given. Some methods to improve the quantum efficiency of silicon photodiodes in near-infrared spectral range are discussed.

On the use of CsK2Sb photocathodes in RF linacs

The use of CsK2Sb photocathodes in an RF linear accelerator is investigated. Several fabrication techniques for this type of cathodes are reviewed. Measurements are performed on the drop in quantum efficiency due to the use of the cathode in the linac at several wavelengths in the visible. A better lifetime has been found for shorter wavelengths. Finally, the effect of high energy laser pulses on the quantum efficiency in the green is demonstrated.

Drift leakage current in AlGaInP quantum-well lasers

The temperature dependence of threshold current and quantum efficiency for Ga/sub x/In/sub 1-x/P (x=0.4, 0.6; lambda =680, 633 nm) single 80-AA quantum-well lasers is compared and analyzed using a model for the electron leakage current. This model fits the experimental data well, correctly describing the rapid increase in threshold and drop in quantum efficiency as temperature increases. Also, it indicates that the drift (rather than diffusion) component of the electron leakage current, is dominant, because of the poor p-type conductivity in AlGaInP.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Photoconduction Mechanism of Amorphous Selenium: Effect of Surface Recombination

The drop in quantum efficiency of the photo-carrier generation in amorphous selenium at low applied electric fields (<5×105 V/m) is explained theoretically, using Onsager's theory and taking account of surface recombination.

Picosecond exciton annihilation in photosynthetic systems

We have previously reported measurements of fluorescence lifetimes for both photosynthetic systems and chlorophyll solutions using a streak camera technique and picosecond excitation. Our lifetime measurements on antenna systems, however, as well as earlier results obtained using a picosecond resolution optical gate, indicate somewhat shorter decay times than measured with previous techniques. Our present experimental investigations of a possible intensity-dependent effect as the cause of these differences are prompted by the recent results of Mauzerall. He has shown that the quantum efficiency for the fluorescence emission from Chlorella decreases at higher pumping intensities for 7-ns duration excitation pulses, and he has interpreted the decrease in terms of a multitrap model of the photosynthetic unit and exciton-exciton collisions. In order to assess the respective roles that singlet and triplet excitons play in the kinetics of these interactions, it is desirable to repeat these measurements using a much shorter excitation pulse and under conditions comparable to the previously mentioned picosecond experiments. In this letter we report results for the quantum efficiency of Chlorella as a function of intensity, but with picosecond excitation. A single 20 ps pulse has been selected from a mode-locked laser pulse train for these measurements. Results show amore » drop of quantum efficiency with intensity in agreement with results of Mauzerall. Previous picosecond fluorescence lifetime measurements must be reinterpreted in view of this nonlinear optical effect.« less