Temperature Dependence Of Quantum Efficiency Research Articles

Investigation of the Temperature Dependence of Quantum Efficiency of Multijunction GaN Solar Cells

Recently, nitride based semiconductors have become attractive as high-efficiency electro-optic devices, due to their direct transition, wide bang-gap and high thermal stability. This paper intends to theoretically study the external quantum efficiency of homo-multijunction GaN solar cells by considering the effect of temperature on electron and hole mobilities and consequently on the lifetime of carrier’s through three limiting factors: scattering by ionized impurities, scattering by lattice vibrations and scattering by optical phonons. The influence of Auger recombination on quantum efficiency in addition to two usual types of recombinations: SRH and surface recombination is studied.

Temperature Dependence of Quantum Efficiency of InGaN/GaN Led Structures at High Current Density

Temperature dependence of quantum efficiency of blue LED structures based on multiple InGaN/GaN quantum wells is studied at different forward currents. At high current densities, an increase in the quantum efficiency with increasing temperature is observed. Simulation of the dependences of the quantum efficiency of LED structures on the current showed that, if account is taken of the ballistic and hopping transport of charge carriers in the active region of the structure, the calculated and experimental dependences are in good agreement. A decrease in the thickness of the active region of the structure leads to a weakening of the temperature dependence of the quantum efficiency at high current density.

NIR Detector Nonlinearity and Quantum Efficiency

A study was performed to investigate the experimental conditions and systematic uncertainties that need to be considered in order to precisely characterize quantum efficiency (QE). Measurements were performed on a HAWAII-2RG1.7 μm detector but the methodology of characterization is applicable to other detectors as well and may be useful in characterization of detectors used in future ground and space based surveys. For this study the detector QE as a function of illumination intensity, total integrated signal, and temperature was measured. A 3% relative systematic uncertainty on the measured QE value was achieved at wavelengths longer than 800 nm but the total uncertainty in the determination of absolute QE is dominated by the uncertainty in the conversion gain, which adds an additional 3.4% scale uncertainty. It was found that the measured detector QE depends on illumination intensity and that temperature dependence of QE can, at least in part, be attributed to reciprocity failure. Well-chosen detector bias voltages can reduce integrated signal nonlinearity.

Deep defects in Cu2ZnSnS4 monograin solar cells

Abstract In this report Cu 2 ZnSnS 4 (CZTS) monograin layer (MGL) solar cells were studied using admittance spectroscopy (AS) (frequency range 20Hz-10 MHz) and temperature dependence of quantum efficiency (QE) curves ( T =10K-300K). These studies revealed two deep defect states at E A1 = 120 meV and at E A2 = 167 meV. The first state was present in different CZTS cells while the second state had somewhat different properties in different cells. The temperature dependence of QE curves showed a shift of the long wavelength edge with increasing temperature by about 110 meV towards higher energy. The possible origin of the observed deep defect states is discussed.

Temperature dependence of quantum efficiency in Quantum Dot Infrared Photodetectors

The behavior of quantum efficiency in QDIPs was studied in details with simple InAs/GaAs QDs and DWELL QDs structures. Despite of the large difference of the excited state energy between the two samples, the QE shows similar trends with temperature and bias voltage. The voltage to reach the QE plateau decreases with temperature and the maximum QE decreases with temperature. Considering the repulsive potential from the charge inside the QDs, the effective barrier height and thickness for the photoexcited carrier is much reduced and the QE variation with voltage follows the calculated tunneling probability. Furthermore, the multi-phonon interaction which leads to the relaxation of the excited carrier is shown to be important to the decrease of QE with temperature. The enhanced relaxation rate decreases the maximum QE value at higher temperature.

Temperature dependence of the photoelectric conversion quantum efficiency of 4H–SiC Schottky UV photodetectors

Ultraviolet Schottky photodetectors based on n-4H–SiC (Nd − Na = 4 × 1015 cm−3) epitaxial layers of high purity have been fabricated. Their spectral sensitivity range is 3.2–5.3 eV peaking at 4.9 eV (quantum efficiency is about ∼0.3 electron/photon), which is close to the bactericidal ultraviolet radiation spectrum. The temperature dependence of the quantum efficiency of 4H–SiC Schottky structure has been investigated to determine the temperature stability and the mechanism of the photoelectric conversion process. At low temperatures (78–175 K) the quantum efficiency increases with increasing temperature for all photon energy values and then tends to saturate. We suppose that some imperfections in the space-charge region act as traps that capture both photoelectrons and photoholes. After some time the trapped electron–hole pairs recombine due to the tunnelling effect. At high temperatures (more than 300 K), the second enhancement region of the quantum efficiency is observed in the photon energy range of 3.2–4.5 eV. It is connected with a phonon contribution to indirect optical transitions between the valence band and the M-point of the conduction band. When the photon energy is close to a direct optical transition threshold this enhancement region disappears. This threshold is estimated to be 4.9 eV. At photon energies more than 5 eV a drastic fall of the quantum efficiency has been observed throughout the temperature interval. We propose that in this case the photoelectrons and photoholes are bound to form hot excitons in the space-charge region due to the Brillouin zone singularity, and do not contribute to the following photoelectroconversion process.

Optical properties of C60 nanocrystals embedded in a SiO matrix

Photoluminescence spectra of (C60)n nanocrystals (1000<n<10 000) in a SiO matrix have been investigated in order to study electronic and vibronic states of mesoscopic phase of C60 crystals. We have found that both vibronic bands, which are derived from the gerade- and ungerade-parity vibrational modes, obviously appear in the spectrum of the C60 nanocrystals at 80 K. Results indicate that Herzberg–Teller transitions become strong with a decrease in the size of the crystal. It has been also found that Herzberg–Teller vibronic bands become relatively dominant with an increase in the temperature compared to Jahn–Teller vibronic bands. We analyzed the data by the temperature dependence of quantum efficiency, and found that the thermal activation energy for nonradiative relaxation can be classified into two groups; 190 and 90 meV for the Herzberg– and Jahn–Teller vibronic bands, respectively. This indicates that two kinds of electronic origins coexist in the C60 nanocrystals at a temperature above 80 K.

Temperature dependence of the short-wave quantum efficiency of GaAs and GaP surface-barrier photosensors

A study has been made of the temperature dependence of the quantum efficiency of photoelectric conversion in GaP and GaAs surface-barrier structures in the region of direct optical transitions. The temperature interval is 120–360 K. Experiments show that the short-wave quantum efficiency γ of the structures increases with temperature T. For higher photon energies the temperature increase is weaker and at high temperatures the γ= f( T) dependence approaches saturation. It is concluded that the increase of the quantum efficiency stems from a temperature-induced increase in the internal quantum yield of the photoelectric effect, not from the effect of temperature on the loss of non-equilibrium charge carriers.

Specific solvent effects of hydroxylic solvents on the emission properties of ruthenium(ii)tris(2,2?-bipyridyl) chloride

The excited state of Ru(II)[bpy]3 (2+) dissolved in hydroxylic solvents is subject to specific solvent effects, which were hitherto not understood on a quantitative basis. We determined the solvent effects of linear monovalent alcohols on the energy gap law of internal conversion with the help of lifetime and intensity measurements. An on-line method for measurement of the temperature dependence of quantum efficiencies was introduced. A modified Franck-Condon analysis of emission spectra by taking into account the shape of a Morse potential of the involved states was applied to compute excited-state energies.

Thermal and non-radiative transitions of electrons at imperfections in ionic crystals

The theory of non-radiative and thermal transitions of electrons at imperfections in ionic crystals is elaborated. The results obtained are used to confirm Mott's and Gurney's theories on the detachment process in two steps of electrons from F centres during the absorption of light in the F band and to explain theoretically the temperature dependence of quantum efficiency. Good agreement with experiment was obtained. The possibility of the luminescence of alkali halide crystals containing F centres at sufficiently low temperatures was also determined.