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While the performance enhancement witnessed in the field of perovskite solar cells over the recent years has been impressive, a detailed analysis of various loss mechanisms is required for further performance optimization. Here, we address the same through numerical simulations of optical and electrical characteristics. We quantify various losses like optical losses (5%–6%), recombination losses (3%–4%), and resistive losses against the Auger recombination induced practical efficiency limits. Through this, we identify schemes to reduce these losses and hence lead to an increase in efficiency. In addition, we find that the optimum thickness of the perovskite (with material parameters comparable to MAPbI3-xClx and Eg = 1.55 eV) for solar cell fabrication is around 300 nm (comparable to the well-established value); however, the same could be as large as 900 nm for a trap free perovskite (∼ms as the minority carrier Shockley-Read-Hall recombination lifetime). The analyses also enable us to provide the design charts that could lead to >25% efficient perovskite solar cells on the planar structure. [ABSTRACT FROM AUTHOR]

Because the expansion of single Shockley stacking faults (1SSFs) is an important problem for the viability of SiC bipolar devices, there is a need to suppress it during device operation. The expansion mechanism, however, is still unclear. Therefore, the method to suppress the expansion has never been established. An important factor for the expansion could be carrier recombination in 1SSFs because the expansion has only been observed during bipolar operation or light illumination. In this study, we characterized carrier recombination by observing the photoluminescence from 1SSFs and partial dislocations (PDs). The luminescence from 1SSFs and PDs showed a fast decay component compared with that from the band edge. This result indicates that the carrier recombination in 1SSFs and at PDs was faster than that in regions without 1SSFs in 4H-SiC. In addition, because of the slower recombination at Si-core PDs compared with that in 1SSFs and at C-core PDs, the velocity of 1SSF expansion would be limited by the carrier recombination at Si-core PDs. The temperature dependence of the decay time implies that the recombination at the Si-core PD was enhanced on increasing the temperature. [ABSTRACT FROM AUTHOR]

Understanding carrier recombination mechanisms and quantifying recombination dynamics are key to improving the performance of state-of-the-art perovskite solar cells. Here, we present a method to quantify the quality of perovskite thin films using a combination of steady-state and transient photoluminescence measurements. The combined experimental datasets are fitted using a single, general recombination model, from which detailed trap and recombination parameters can be extracted, and the accuracy of the fitted values is estimated. This approach expands the application of photoluminescence measurements to include quantitative evaluation of the most relevant defect characteristics, including trap density, energy level, and carrier capture coefficients. We apply this approach to compare perovskite films of the widely studied methyl-ammonium lead iodide (MAPbI3) with the high performance quadruple-cation, mixed-halide composition Cs0.07Rb0.03(FA0.85MA0.15)0.9Pb(I0.85Br0.15)3. Our quantitative analysis of trap properties in these perovskite films suggests that the superior performance of the quadruple cation films may be due to a greatly reduced electron capture coefficient, rather than a significant reduction in the trap density. [ABSTRACT FROM AUTHOR]

The effect of the magnetic field on radiative and non-radiative mechanisms of charge carriers in GaAs/AlGaAs quantum wells (QWs) is investigated via quasi-simultaneous magneto-photoluminescence (PL) and magneto-surface photo-voltage (SPV) spectroscopy. At low-temperature, the luminescence intensity of ultra-low disordered GaAs/AlGaAs QWs generally increases under strong magnetic perturbation. Even at relatively high-temperature (100K), the magnetic field driven enhancement of PL intensity is observed for thick QWs. On the other hand, it is found that the PL intensity of narrow QWs gradually decreases under a strong magnetic field at 100 K. The magnetic field driven enhancement (suppression) of radiative recombination efficiency for wide (narrow) QWs is investigated by considering the oscillator strength, thermal effects, and carrier re-distribution in energy states. Also, the charge carriers which escape from narrow QWs or are captured by interface defects are probed via magneto-SPV measurements. Radiative recombination and thermionic emission of charge carriers, investigated by magneto-PL and magneto-SPV spectroscopy, provide a clear guideline of the critical QW width which would be essential for magnetic field driven hightemperature operation of advanced emission based-devices. [ABSTRACT FROM AUTHOR]

We have studied the correlation between the photoconductivity and the dark resistivity of single ZnO microwires. We found that as-grown microwires with higher dark resistivities have higher photoconductivities. However, when the microwires are thermal treated in vacuum, this correlation is inverted. We have also analyzed the behavior of photoconductivity on protonated as-grown samples. We discuss the origin of these behaviors in terms of the interplay of oxygen and zinc vacancies and their complexes acting as recombination or trapping centers. [ABSTRACT FROM AUTHOR]

The determination of minority-carrier lifetimes and surface recombination velocities is essential for the development of semiconductor technologies such as solar cells. The recent development of two-photon time-resolved microscopy allows for better measurements of bulk and subsurface interfaces properties. Here, we analyze the diffusion problem related to this optical technique. Our three-dimensional treatment enables us to separate lifetime (recombination) from transport effects (diffusion) in the photoluminescence intensity. It also allows us to consider surface recombination occurring at a variety of geometries: a single plane (representing an isolated exposed or buried interface), a two parallel planes (representing two inequivalent interfaces), and a spherical surface (representing the enclosing surface of a grain boundary). We provide fully analytical results and scalings directly amenable to data fitting and apply those to experimental data collected on heteroepitaxial CdTe/ZnTe/Si. [ABSTRACT FROM AUTHOR]

Based on quantum efficiency and time-resolved electroluminescence measurements, the effects of carrier localization and quantum-confined Stark effect (QCSE) on carrier transport and recombination dynamics of Ga- and N-polar InGaN/GaN light-emitting diodes (LEDs) are reported. The N-polar LED exhibits shorter ns-scale response, rising, delay, and recombination times than the Ga-polar one does. Stronger carrier localization and the combined effects of suppressed QCSE and electric field and lower potential barrier acting upon the forward bias in an N-polar LED provide the advantages of more efficient carrier relaxation and faster carrier recombination. By optimizing growth conditions to enhance the radiative recombination, the advantages of more efficient carrier relaxation and faster carrier recombination in a competitive performance N-polar LED can be realized for applications of high-speed flash LEDs. The research results provide important information for carrier transport and recombination dynamics of an N-polar InGaN/GaN LED. [ABSTRACT FROM AUTHOR]

Excitation correlation photoluminescence (ECPL) measurements are often analyzed in the approximation of a cross correlation of charge carrier populations generated by the two delayed pulses. In semiconductors, this approach is valid for a linear non-radiative recombination path, but not for a non-linear recombination rate as in the general Shockley-Read-Hall recombination scenario. Here, the evolution of the ECPL signal was studied for deep trap recombination following Shockley-Read-Hall statistics. Analytic solutions can be obtained for a fast minority trapping regime and steady state recombination. For the steady state case, our results show that the quadratic radiative term plays only a minor role, and that the shape of the measured signal is mostly determined by the non-linearity of the recombination itself. We find that measurements with unbalanced intense pump and probe pulses can directly provide information about the dominant non-radiative recombination mechanism. The signal traces follow the charge carrier concentrations, despite the complex origins of the signal, thus showing that ECPL can be applied to study charge carrier dynamics in semiconductors without requiring elaborate calculations. The model is compared with measurements on a reference sample with alternating layers of InGaAs/InAlAs that were additionally cross-checked with time resolved optical pump terahertz probe measurements and found to be in excellent agreement. [ABSTRACT FROM AUTHOR]

Indium sulfide thin films deposited via thermal evaporation from compound source material have been successfully utilized as a cadmium free buffer layer for Cu(In,Ga)Se2 based solar cells. However, high efficiencies are only reached after an additional annealing step. In this work, the annealing behavior of Cu(In,Ga)(S,Se)2 based indium sulfide buffered solar cells is compared to the annealing behavior of similar cells, which were submitted to wet chemical treatments partly containing cadmium ions. Upon annealing a significant improvement of the initial solar cell characteristics is observed for the untreated cell and is related to the increase of activation energy for the carrier recombination process and a decrease of the ideality factor within the one diode model. It is shown here that this improvement can also be achieved by wet treatments of the absorber prior to buffer layer deposition. Upon annealing these treated cells still gain in collection length but lose open circuit voltage, which is explained here within a model including a highly p-doped absorber surface layer and supported by simulations showing that a decrease in doping density of such a surface layer would lead to the observed effects. [ABSTRACT FROM AUTHOR]

We investigate a recombination active grown-in defect limiting the bulk lifetime (Τbulk) of high quality float-zone (FZ) p-type silicon wafers. After annealing the samples at temperatures between 80°C and 400°C, Τbulk was found to increase from ~500 μs to ~1.5 ms. By isochronal annealing the p-type samples between 80°C and 400°C for 30 min, the annihilation energy (Eann) of the defect was determined to be 0.3

We have employed state-of-the-art cross-correlation noise spectroscopy (CCNS) to study carrier dynamics in silicon heterojunction solar cells (SHJ SCs). These cells were composed of a light absorbing n-doped monocrystalline silicon wafer contacted by passivating layers of i-a-Si:H and doped a-Si:H selective contact layers. Using CCNS, we are able to resolve and characterize four separate noise contributions: (1) shot noise with Fano factor close to unity due to holes tunneling through the np-junction, (2) a 1/f term connected to local potential fluctuations of charges trapped in a-Si:H defects, (3) generation-recombination noise with a time constant between 30 and 50 μs and attributed to recombination of holes at the interface between the ITO and n-a-Si:H window layer, and (4) a low-frequency generation-recombination term observed below 100 K which we assign to thermal emission over the ITO/ni-a-Si:H interface barrier. These results not only indicate that CCNS is capable of reveling otherwise undetectable relaxation process in SHJ SCs and other multi-layer devices, but also that the technique has a spatial selectivity allowing for the identification of the layer or interface where these processes are taking place. [ABSTRACT FROM AUTHOR]

A theoretical description for recombination kinetics of charge carriers in a disordered system with a broad energy distribution of localized states (DOS) is suggested. This kinetics is governed by the exchange of carriers between transport states and traps. Concentration transients in systems with Gaussian DOS, typical for organic semiconductors, appear much steeper than those obtained for systems with exponential DOS. This difference in recombination kinetics is caused by the difference in thermalization kinetics for these two types of the DOS functions. The comparison of the recombination transients for mobile and trapped carriers in exponential and Gaussian DOS might help to distinguish between these two possible shapes of the DOS using experimental data for transient photoconductivity and photoabsorption. [ABSTRACT FROM AUTHOR]

Carrier recombination at the metal contacts is a major obstacle in the development of high-performance crystalline silicon homojunction solar cells. To address this issue, we insert thin intrinsic hydrogenated amorphous silicon [a-Si:H(i)] passivating films between the dopant-diffused silicon surface and aluminum contacts. We find that with increasing a-Si:H(i) interlayer thickness (from 0 to 16nm) the recombination loss at metal-contacted phosphorus (n+) and boron (p+) diffused surfaces decreases by factors of ~25 and ~10, respectively. Conversely, the contact resistivity increases in both cases before saturating to still acceptable values of ~ 50 mΩ cm² for n+ and ~100 mΩ cm² for p+ surfaces. Carrier transport towards the contacts likely occurs by a combination of carrier tunneling and aluminum spiking through the a-Si:H(i) layer, as supported by scanning transmission electron microscopy-energy dispersive x-ray maps. We explain the superior contact selectivity obtained on n+ surfaces by more favorable band offsets and capture cross section ratios of recombination centers at the c-Si/a-Si:H(i) interface. [ABSTRACT FROM AUTHOR]

Localization of electronic excitations at molecule-sized semiconductor defects often precedes non-radiative (NR) decay, and it is known that many molecules undergo NR decay via conical intersection. Herein, we report the direct simulation of fast and efficient NR decay via a conical intersection at a known semiconductor defect. It is suggested that this silicon epoxide defect may selectively quench photoluminescence (PL) in small silicon nanocrystals (band gap > ∼2.8 eV), and thus influence both the observed PL yield and PL energy of oxidized silicon nanocrystals. [ABSTRACT FROM AUTHOR]

Multiple exciton recombination (MER) and multiple exciton generation (MEG) are two of the main processes for assessing the usefulness of quantum dots (QDs) in photovoltaic devices. Recent experiments, however, have shown that a firm understanding of both processes is far from well established. By performing surface-dependent measurements on colloidal CdSe QDs, we find that surface-induced charge trapping processes lead to false MER and MEG signals resulting in an inaccurate measurement of these processes. Our results show that surface-induced processes create a significant contribution to the observed discrepancies in both MER and MEG experiments. Spectral signatures in the transient absorption signals reveal the physical origin of these false signals. [ABSTRACT FROM AUTHOR]

We present an advanced analytical model which applies to light beam induced current contrast profiles to determine reliably the effective surface recombination velocities (Seff) of grain boundaries (GBs) and diffusion lengths (Ldiff) in the grains, in cases where a GB is close to the studied one or when Ldiff of the neighboring grain differs. We introduce additionally a new method for a very accurate determination of the plateau value of the investigated linescan and make use of simultaneously fitting GB profiles measured at various laser wavelengths both aiming at increasing the accuracy of the Ldiff determination. Through several special case investigations, the various applications and limitations of the model are demonstrated. We discuss the influence of the electrical parameters of the semiconductor on the various zones of the profile as well as the influence of measurement technique parameters on the experimental profile and point out the need of an accurately determined small laser beam radius to ensure a reliable extraction of Seff. We discuss the occurring discrepancy between fit and measured data and show that it gives hints about particular material features and the reliability of the extracted parameters. We finally point out the possibility of determining Ldiff in small grains. This model thus allows more realistic GB situations to be investigated. [ABSTRACT FROM AUTHOR]

The dominant recombination processes controlling the carrier lifetime in n-type 4H–SiC epitaxial layers grown with low concentrations of the Z1/2 defect (the dominant bulk lifetime killer), where Z1/2 no longer determines the lifetime, have been investigated by studying the variation in the carrier lifetime with temperature. The temperature dependent lifetimes were obtained primarily by low-injection photoluminescence decay for several low-Z1/2 epilayers over a wide temperature range. The results were fitted to simulations of the temperature dependent recombination rate, where bulk, surface and interface recombination was considered. No significant contribution from other bulk defects was observed, and upper limits to the bulk recombination rate were found to be consistent with the low Z1/2 concentrations measured in these materials. There was also no significant contribution from carrier capture at the epilayer/substrate interface, which is consistent with behavior expected at low injection for low-doped epilayers grown on n+ substrates. Corresponding high-injection measurements exhibited very different behavior, consistent with the surface/interface under flat-band conditions. Consequently, it is concluded that for low-Z1/2 materials, control of the carrier lifetime has not been transferred from Z1/2 to another bulk defect, but is instead dominated by surface and interface recombination. Simulations suggest that further enhancement of the total lifetime under the high injection conditions of a device structure would require very thick epilayers, effectively passivated surface and interface recombination and a further reduction in the remaining Z1/2 concentrations. The temperature dependence of the low-injection carrier lifetime was also found to provide a method to estimate the surface band bending and the surface defect density. [ABSTRACT FROM AUTHOR]

The microscopic states and performance of organic solar cell are investigated theoretically to explore the effect of the carrier mobility. With Ohmic contacts between the semiconductor and the metal electrodes there are two origins of carriers in the semiconductor: the photocarriers generated by photon absorption and the dark carriers diffused from the electrodes. The power efficiency of the solar cell is limited by the recombination of a carrier with either the photocarrier or a dark carrier. Near the short-circuit condition the photocarrier recombination in the semiconductor bulk decreases as the mobility increases. Near the open-circuit condition the dark carrier recombination increases with the mobility. These two opposite effects balance with one another, resulting in an optimal mobility about 10-2 cm2/V s which gives the highest power conversion efficiency. The balance of the electron and hole mobilities are not necessary to maintain the optimal efficiency also because of the balance of the photocarrier and dark carrier recombination. The efficiency remains about the same as one carrier mobility is fixed at 10-2 cm2/V s while the other one varies from 10-1 to 10-3 cm2/V s. For solar cell with a Schottky barrier between the semiconductor and the metal electrode there is no dark carrier recombination. The efficiency therefore always increases with the mobility. [ABSTRACT FROM AUTHOR]

This work reports on measurements of the Auger recombination coefficients in silicon wafers with pump-probe thermoreflectance techniques operating at two different excitation rates: 250 kHz (low repetition rate) and 80 MHz (high repetition rate). The different excitation frequencies give rise to different thermoreflectance signals in the Si samples, which is ascribed to the excited number density in the conduction band. In the low repetition rate case, the excited carriers recombine via Auger processes before the next pump excitation is absorbed. However, in the high repetition rate case, the rate in which the pump excitations are absorbed at the sample surface is higher than the Auger recombination rate, indicating that the excited carrier densities in the high repetition rate experiments are much higher than in the low repetition rate measurements even though the pump fluences are comparable. This is ascribed to pulse accumulation in the high repetition rate measurements, and is quantified with rate equation and thermoreflectance models fit to the experimental data. Comparing the data taken at the two different excitation modulations gives insight into the excited carrier density when recombination rate are on the same order as excitation frequencies. [ABSTRACT FROM AUTHOR]

We present a combined low-temperature time-resolved cathodoluminescence and photoluminescence study of exciton recombination mechanisms in a 3.8 nm thick a-plane (Al,Ga)N/GaN quantum well (QW). We observe the luminescence from QW excitons and from excitons localized on basal stacking faults (BSFs) crossing the QW plane, forming quantum wires (QWRs) at the intersection. We show that the dynamics of QW excitons is dominated by their capture on QWRs, with characteristic decay times ranging from 50 to 350 ps, depending on whether the local density of BSFs is large or small. We therefore relate the multiexponential behavior generally observed by time-resolved photoluminescence in non-polar (Al,Ga)/GaN QW to the spatial dependence of QW exciton dynamics on the local BSF density. QWR exciton decay time is independent of the local density in BSFs and its temperature evolution exhibits a zero-dimensional behavior below 60 K. We propose that QWR exciton localization along the wire axis is induced by well-width fluctuation, reproducing in a one-dimensional system the localization processes usually observed in QWs. [ABSTRACT FROM AUTHOR]

We show that the recombination lifetimes measured in highly aluminum-doped p-type silicon (Al-p+) regions, fabricated by means of screen printing and firing of aluminum paste, are three orders of magnitude larger than the lifetimes expected from the extrapolation of the lifetime data measured on aluminum-doped Czochralski-grown silicon wafers. Solar cell simulations demonstrate that the lifetime of 130 ns measured in Al-p+ regions with an aluminum doping concentration of ∼2×1018 cm-3 enables solar cell open-circuit voltages of 670 mV and energy conversion efficiencies of 21% on n-type silicon wafers. These results prove that the efficiency potential of screen-printed Al-p+ emitters for the application to rear-junction n-type silicon solar cells is much higher than traditionally assumed. [ABSTRACT FROM AUTHOR]

An analysis of the hydrodynamic motion of the fluid of photoinjected carriers in polar semiconductors is presented. Experiments of time-resolved photoluminescence, which provide relevant insights into the dynamical behavior of heterostructures, are analyzed. We study the propagation and recombination of carriers in semiconductor devices with a large cap layer, where carriers are photoinjected, and a quantum well where they recombine. The movement of the photoinjected, and away from equilibrium, carriers along such cap layer consists, to a good degree of approximation, in an ambipolar diffusivelike one, which decays in time as a result of recombination and the coupling, via Coulomb interaction, with the optical and acoustic plasma waves. The density of the electrons arriving at the interface with the quantum well can be determined; these electrons are transferred through the interface to recombine in the quantum well, and the resulting intensity of the time-resolved luminescence is obtained. Comparison with experimental data shows a good agreement. [ABSTRACT FROM AUTHOR]

The surfaces of solar cells are often textured to increase their capacity to absorb light. This optical benefit is partially offset, however, by an increase in carrier recombination at or near the textured surface. A review of past work shows that the additional recombination invoked by a textured surface varies greatly from one experiment to another. For example, in the most commonly investigated structure—pyramidal textured silicon diffused with phosphorus and passivated with a hydrogenated oxide—recombination ranges from being 1–12 times more than in an equivalently prepared planar {100} surface. Examination of these experiments reveals consistent trends: small increases in recombination occur when the surface is very heavily diffused and dominated by Auger recombination, while larger increases in recombination occur when the surface is lightly diffused and dominated by Shockley–Read–Hall recombination at the surface, making the latter depend critically on surface area and the density of surface states. Comparisons of pyramidal and planar {100} surfaces indicate that when lightly diffused, the difference in recombination is substantially greater than the difference in surface area (1.73) and it is regularly attributed to the pyramid facets having {111} orientations—well known for their higher density of dangling bonds than {100} orientations. This high dangling-bond density makes recombination at pyramidal facets strongly dependent on the passivation scheme, and it is variations in these schemes that led to the wide range of results observed in experimental studies. In addition to surface area and crystal orientation, some experiments suggest a third mechanism that enhances recombination on oxide-passivated pyramids. With capacitance-voltage and photoconductance measurements, we confirm this speculation, showing that oxide-passivated pyramidal textured silicon has a higher density of interface states than can be accounted for by surface area and orientation, and that the additional defects are predominantly acceptorlike when above, or donorlike when below, an energy of 0.3 eV higher than the valence band. [ABSTRACT FROM AUTHOR]

In this paper, we treat diode currents caused by Shockley-Read-Hall recombination at the interface of pn heterojunctions. We are interested in the activation energy of the saturation current. To this end, we discriminate the cases of ¿Ec,v¿0 and ¿Ec,v>0, where ¿Ec,v is the band offset in the minority carrier band of the small band gap heteropartner, as well as the case of Fermi level pinning. Using analytical considerations, we find that the activation energy of the saturation current equals the band gap of the small band gap heteropartner for ¿Ec,v>0 but equals the interface band gap for ¿Ec,v¿0. In the case of Fermi level pinning, the value of the potential barrier of the small band gap minority carrier equals the activation energy. These findings serve to discriminate different cases of interface recombination and to give information about the heterojunction energy band diagram. [ABSTRACT FROM AUTHOR]

We have identified a magnetic resonance spectrum associated with minority carrier lifetime killing defects in device quality 4H SiC through magnetic resonance measurements in bipolar junction transistors using spin dependent recombination (SDR). The SDR spectrum has nine distinguishable lines; it is, within experimental error, essentially isotropic with four distinguishable pairs of side peaks symmetric about the strong center line. The line shape is, within experimental error, independent of bias voltage and recombination current. The large amplitude and spacing of the inner pair of side peaks and three more widely separated pairs of side peaks are not consistent with either a simple silicon or carbon vacancy or a carbon or silicon antisite. This indicates that the lifetime killing defect is not a simple defect but a defect aggregate. The spectrum is consistent with a multidefect cluster with an electron spin S=12. (The observed spectrum has not been reported previously in the magnetic resonance literature on SiC.) A fairly strong argument can be made in terms of a first order model linking the SDR spectrum to a divacancy or possibly a vacancy/antisite pair. The SDR amplitude versus gate voltage is semiquantitatively consistent with a very simple model in which the defect is uniformly distributed within the depletion region of the base/collector junction and is also the dominating recombination center. The large relative amplitude of the SDR response is more nearly consistent with a Kaplan–Solomon–Mott-like model for spin dependent recombination than the Lepine model. [ABSTRACT FROM AUTHOR]

Light emission of nitride single quantum well light-emitting diodes operating in the range from 395 to 440 nm, grown by metal-organic vapor phase epitaxy, was investigated by means of photoluminescence (PL) and electroluminescence (EL) spectroscopies as functions of temperature and current injection level. The indium content of the active layers was varied via the growth temperature, which ranged from 770 to 810 °C. It was found that samples with higher indium contents (grown at lower temperatures) exhibit larger luminescence full widths at half maximum (FWHMs) and higher EL intensities. The larger FWHM points to a larger amplitude of the potential profile fluctuations, which suggests that these fluctuations may be useful for the increase in the device output power. In agreement with this result, the performed examination by PL measurements shows large thermal stability of the luminescence intensity for the lowest growth temperature sample. On the other hand, a nontrivial dependence of thermal stability on growth temperature suggests the additional involvement of different nonradiative recombination center concentrations in the individual samples, for example, because of dissimilar substrate qualities. [ABSTRACT FROM AUTHOR]

We have investigated the electronic structures and morphologies of two organic–metal–organic junctions, relevant for organic bistable devices, photovoltaics, and light emitting devices. Combined photoemission spectroscopy and scanning probe measurements found that in a 3,4,9,10 perylenetetracarboxylic dianhydride bis-benzimidazole (PTCBI)/Ag/Cu-phthalocyanine (CuPc) junction, Ag undergoes a chemical reaction with PTCBI leading to a discontinuous film of Ag clusters. In contrast, Au forms a continuous metal film in the PTCBI/Au/CuPc junction with no observable chemistry occurring. Photoemission spectroscopy suggests that both heterojunctions exhibit barrierless charge recombination leading to very similar functional behavior when integrated in device structures despite significant chemical and morphological differences. [ABSTRACT FROM AUTHOR]

The mechanism of near-band-edge (NBE) emission from crystalline ZnO (c-ZnO) nanorods grown on c-Si by a catalyst-assisted vapor-liquid-solid method has been investigated by performing temperature-, power-, and time-dependent photoluminescence (PL) measurements at a temperature (T) range of 143–503 K. In contrast to previous reports, we find that the NBE PL is primarily associated with free exciton emission, whereas the contribution of band-to-band and free-to-bound radiative recombinations remains negligible up to the highest T studied. A spectral evolution of the NBE band with T was further analyzed within the framework of a three-parameter model, proposed recently, with the results presented and discussed. Finally, the ratio of excitonic-to-defect luminescence intensity has been observed to change nonmonotonically with T, which is explained based on the difference in the quenching mechanisms of exciton and defect PL. [ABSTRACT FROM AUTHOR]

An analysis of squareness in demagnetization curve of Nd–Fe–B fine-grained alloys prepared by the hydrogenation-disproportionation-desorption-recombination (HDDR) process was carried out in reference to the melt-spun and sintered magnets by measurements of their recoil curves. The recoil curves of the HDDR magnet and the melt-spun magnet suggested that the magnetization process associated with the recoil behavior of the HDDR magnet is dominated by rotation of magnetization, while that of the sintered magnet suggested that domain wall movement is involved. The recoil susceptibilities for recoil curves that returns to nearly demagnetized state were 0.095 and 0.048 T-1 in the isotropic and anisotropic HDDR magnets, respectively, when normalized by saturation magnetization. The normalized susceptibilities could be reproduced quantitatively, assuming that the magnetization recoil process within a grain in the HDDR magnet is described by the coherent rotation of magnetization and a simple model of orientation distribution of the easy axes. It appears that the anisotropic HDDR magnet has a wide distribution of orientation of the easy axes, which should be one of the reasons for poor squareness of the demagnetization curve of the HDDR magnet. [ABSTRACT FROM AUTHOR]

The recombination activity of interstitial chromium (Cri) and pairs of interstitial chromium and substitutional boron (CriBs) in crystalline silicon is studied by combining temperature- and injection-dependent lifetime and deep-level transient spectroscopy measurements on intentionally chromium-contaminated n- and p-type silicon wafers. Cri as well as CriBs pairs are found to be one order of magnitude less recombination active than widely assumed. In the case of Cri, a defect energy level of EC-Et=0.24 eV, an electron capture cross section of σn=2×10-14 cm2, and a hole capture cross section of σp=4×10-15 cm2 are determined. For CriBs pairs, measurements on boron-doped p-type silicon result in Et-EV=0.28 eV, σn=5×10-15 cm2, and σp=1×10-14 cm2. Theoretical calculations using the Shockley–Read–Hall theory show that it depends crucially on the doping concentration whether Cri or CriBs is the more active recombination center. Using a calibration function calculated from the defect parameters determined in this study, lifetime changes measured before and after thermal dissociation of CriBs pairs can be used to determine the interstitial chromium concentration in boron-doped silicon. [ABSTRACT FROM AUTHOR]

The effects of phosphorous gettering and hydrogenation on the minority carrier recombination at crystal defects in directionally solidified multicrystalline silicon are described. Representative industrial wafers, both p- and n-type, and current technologies for the gettering and hydrogenation are used. The main result of this work is a strong link between activation of extended crystal defects (ECDs) by gettering and their passivation by hydrogenation. It is shown that gettering or annealing increases the recombination at active as well as inactive ECDs. Surprisingly, hydrogenation can neutralize this change in activity due to the gettering. However, it neutralizes only part, at most, of the ECD activity already present before the gettering. Therefore, under current industrial processing techniques, these two high-temperature process steps individually give large change but together much less net change of the crystal defect activity. Related phenomena are observed in wafers with strongly varying impurity concentration. Finally, there is little difference in these observations between n- and p-type wafers. [ABSTRACT FROM AUTHOR]

We present a Monte Carlo model of carrier separation and recombination in nanostructured organic photovoltaic (OPV) devices which takes into account all electrostatic interactions, energetic disorder, and polaronic effects. This permits a detailed analysis of the strong morphology dependence of carrier collection efficiency. We find that performance is determined both by the orientation of the heterojunction relative to the external electric field as well as by carrier confinement due to polymer intermixing. The model predicts that an idealized interdigitated structure could achieve overall efficiencies twice as high as blends. The model also reproduces the weakly sublinear intensity dependence of short-circuit photocurrent (ISC) seen in experiment. We show that this is not the result of space-charge effects but of bimolecular recombination. Disconnected islands of polymer in coarser blends result in bimolecular recombination even at low intensities and should therefore be minimized. By including a microscopic description of dark injection, the model can describe the full current-voltage (J-V) characteristics of different OPV structures. We examine the effect of morphology, intensity, mobility, and recombination rate on key parameters such as short-circuit current, open-circuit voltage (VOC), and fill factor (FF). The model reproduces the intensity-dependent contribution to VOC in a bilayer above that of a blend observed in experiment. We find that performance in both bilayers and blends is very sensitive to the recombination rate across the heterojunction. The model also predicts a striking dependence of performance on mobility. Indeed it is shown that a tenfold increase in mobility dramatically improves ISC and FF and doubles the maximum power output in a bilayer device. As well as informing routes for improving device performance, the model also offers an improved microscopic understanding of OPV operation. [ABSTRACT FROM AUTHOR]

Thermophotovoltaic converters based on In0.69Ga0.31As/InAs0.34P0.66 have been fabricated, characterized experimentally, and modeled. Good device performance has been achieved with an open-circuit voltage of 1.46 V, short-circuit current density of 1.06 A/cm2, and a fill factor of 71.3% for a four-junction cell under an optical power density of 3.4 W/cm2. Key material parameters have been extracted from measured device characteristics, providing a detailed quantitative understanding of the dependence of device performance on the electro-optical properties of the InGaAs/InAsP material system. Extracted minority carrier lifetimes of 106 ns in the p-type base and 0.3 ns in the n-type emitter regions were obtained, limited by radiative and Auger recombination, respectively. The recombination velocity for the InGaAs/InAsP interface is found to be below 2000 cm/s. The parameter analysis provides guidance for the design of a high-efficiency monolithically integrated module for use under high illumination intensities. [ABSTRACT FROM AUTHOR]

Postgrowth annealing was carried out to investigate the photoluminescence of zinc oxide (ZnO) nanorods synthesized using a thermal chemical vapor deposition method. The observed change in photoluminescence after the annealing processes strongly suggests that positively charged impurity ions or interstitial Zn ions are the recombination centers for green luminescence observed in the present sample. A model based on the interplay between the band bending at the surface and the migration of positively charged impurity ions or Zn ions was proposed, which satisfactorily explains the observed photoluminescence. [ABSTRACT FROM AUTHOR]

Two-dimensional simulations of ambipolar or double-injection field-effect transistors (FETs) based on organic materials as active layer are presented in this article. These organic FETs are of interest because of the direct recombination accompanied by light emission. When modeling such FETs, one problem is that even advanced existing models based on the Pao-Sah description neglect both the actual contact properties and the recombination process. We present numerical simulations taking account these peculiarities. Current-voltage characteristics are used as reference data, recently obtained on an organic heterostructure ambipolar FET with a bottom Au contact and a Mg top contact [Rost et al., J. Appl. Phys. 95 5872 (2004)]. To investigate the basic ambipolar effects without the additional influences of the special structure, a single-layer model structure is considered in this simulation study. Simulated current-voltage characteristics indeed feature all specific characteristics arising from the ambipolar operation. Their origin becomes clear by inspecting the internal field and concentration profiles. The simulations reveal a complex interplay between the properties of the source and gate contacts, interface charges, the ratio of the electron and hole mobilities, doping, and recombination. Thereby, pronounced ambipolar operation depends sensitively on the combination of the barrier of the electron-injecting contact and the ratio of hole to electron mobilities. Possibilities and limitations of direct parameter extraction based on simple analytical expressions are demonstrated. [ABSTRACT FROM AUTHOR]

A collisional radiative model was developed in order to investigate the influence of dissociative recombination on the Saha–Boltzmann plasma equilibrium. As the dissociative recombination products are not well known, their relative importance was tested through comparison with the distribution of line intensities obtained in a microwave argon discharge produced at atmospheric pressure by a surface wave. It was found that the main dissociation products are the ground state and the 4s levels, the 5p and upper levels playing a negligible role. Because the higher levels are only weakly affected by dissociative recombination, they remain in partial local thermodynamic equilibrium. Therefore, the excitation temperature determined from these levels adequately describes the electron temperature. The model well reproduces experimental measurements of excitation temperature, rotational temperature, electron density, and absolute populations of the excited levels. [ABSTRACT FROM AUTHOR]

The recombination activity of iron-gallium (FeGa) and iron-indium (FeIn) pairs in crystalline silicon is studied by means of injection-dependent carrier lifetime measurements on Fe-implanted, Ga- and In-doped p-type silicon wafers of different resistivities (0.3–15 Ω cm). Compared to FeB pairs, FeGa and FeIn pairs are found to be much more effective recombination centers in p-type silicon. Using Shockley–Read–Hall statistics we determine the energy level Et of the FeGa-related center to be 0.20 eV above the valence-band edge Eν. The strong recombination activity of FeGa is assigned to its large electron-capture cross section σn of 4×10-14 cm2. The hole-capture cross section σp is 2×10-14 cm2. For the FeIn-related recombination center, our measurements show that Et=Eν+0.15 eV, σn=3.5×10-13 cm2, and σp=1.5×10-14 cm2. Strong illumination with white light is found to dissociate both types of pairs. Storage of the samples in the dark leads to a full repairing of FeGa and FeIn pairs. Lifetime changes measured before and after illumination can be used to determine the interstitial iron concentration in Ga- and In-doped silicon using calibration factors determined from the measured defect parameters. [ABSTRACT FROM AUTHOR]

The Shockley-Read-Hall rate equations determine the average carrier transitions via a single-level defect in the band gap of a nondegenerate semiconductor. In the present work the differential rate equations for multiple levels, or localized states systems, are derived from first principles. These multiple level systems comprise the multiple discrete defects system and the coupled or excited states system. The solution for the single-level rate equations, developed recently for transient decay, is represented by an infinite series of monoexponential terms, the frequencies or inverse time constants of which are a linear combination of the fundamental frequencies ω=1/τ. For the multiple localized state solution expressions for the fundamental time constants τ1+k are derived for m states with k=1,2,…,m without an approximation at a given temperature for an excess carrier concentration below nondegenerate doping, arbitrary uniform doping concentration NA,D, defect level concentration Nk, cross section σnk,pk, and energy level Ek. Verification of the set of rate equations for each system is performed by comparing the analysis of the numerical solution for component time constants with the prediction of the theory. The variation of the fundamental time constant τ1 with excess carrier concentration indicates the behavior of minority carrier trapping. [ABSTRACT FROM AUTHOR]

The impact of copper contamination and subsequent hydrogenation on recombination activity and hole-trap levels of misfit dislocations were investigated in p-type Si/Si0.98Ge0.02/Si structures. In the as-grown (noncontaminated) samples, dislocations were found to exhibit very low recombination activity, detectable with the electron-beam-induced current technique only at low temperatures. Deep-level transient spectroscopy revealed a dislocation-related hole-trap level at Et=Ev+0.2 eV. The position of the observed level is close to the theoretically predicted hole-trap state of the intrinsic stacking fault of a dissociated dislocation. Contamination with a low copper concentration [5 (parts per 109) ppb] gave rise to a large increase of the recombination activity of the dislocations and to the appearance of another dislocation-related defect level at Et=Ev+0.32 eV. Hydrogenation of the samples by a treatment with an acid solution and subsequent reverse-bias anneal at 380 K resulted in the evolution of the levels of substitutional copper and its complexes with hydrogen. [ABSTRACT FROM AUTHOR]

The electronic recombination properties of Mg-doped GaN have been investigated by steady state and time-resolved cathodoluminescence (TRCL) in the scanning electron microscope, photocurrent (PC) spectroscopy, and optical beam induced current (OBIC). CL and OBIC maps reveal an inhomogeneous recombination activity in the investigated material. Deep levels giving rise to level-to-band transitions were detected by PC spectroscopy. A large PC quenching observed upon illumination with light of (2.65–2.85) eV is tentatively attributed to metastable traps within the band gap. CL spectra reveal the existence of emission bands centered at 85 K at 3.29, 3.20, 3.15, and 3.01 eV, respectively. Both time-resolved and steady-state CL measurements carried out under different excitation conditions indicate that the 3.15 and 3.01 eV emissions are likely related to donor-acceptor pair transitions. TRCL measurements also reveal different recombination kinetics for these bands and suggest that deep donors are involved in the mechanism responsible for the 3.01 eV emission. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this article, a contactless, all-optical, nondestructive method for separating the minority carrier recombination lifetime and surface recombination velocities in silicon samples at a low injection level is presented. The technique can be described as a pump-probe method in which the excess carrier density is probed by analyzing the free carrier absorption transient following laser pulse excitation that have several wavelengths. An attractive characteristic of the proposed technique is its capability to measure different values of surface recombination velocity on front and back surfaces of the silicon sample. The theoretical foundation of the method is also analyzed. Moreover, numerical simulations which validate the proposed methodology and preliminary experimental results that prove the applicability of scheme are presented. [ABSTRACT FROM AUTHOR]

We report the results of time-integrated and time-resolved photoluminescence (PL) spectroscopy on red-emitting self-organized InAs/Al[sub 0.6]Ga[sub 0.4]As quantum dots with indirect barriers. The time-resolved PL measurements confirm that carriers excited in the Al[sub 0.6]Ga[sub 0.4]As barriers are initially scattered to the X states and are then captured by the dots on a timescale of 45-60 ps, leading to efficient red emission. The dynamics of the PL decay time are consistent with a carrier hopping process between dots, a result also supported by time-integrated PL experiments. [ABSTRACT FROM AUTHOR]

Emission spectra of GaTe single crystals in the range of 1.90-1.38 eV have been analyzed at different temperatures and excitation intensities by photoluminescence, photoluminescence excitation, and selective photoluminescence. A decrease in band gap energy with an increase in temperature was obtained from the redshift of the free exciton recombination peak. The energy of longitudinal optical phonons was found to be 14 ± 1 meV. A value of 1.796 ± 0.001 eV for the band gap at 10 K was determined, and the bound exciton energy was found to be 18 ± 0.3 meV. The activation energy of the thermal quenching of the main recombination peaks and of the ones relating to the ionization energy of impurities and defects was analyzed. The results obtained show the existence of two acceptor levels with ionization energies of 110 ± 5 and 150 ± 5 meV, respectively, and one donor level with an ionization energy of 75 ± 5 meV. The study of chemical composition by inductively coupled plasma-optical emission spectroscopy and x-ray energy dispersion spectroscopy shows the existence of Na, Li, and Si. Sodium and lithium impurities could be associated with acceptor levels at gallium substitutional sites, and silicon ones with a donor level at Ga sites, whose vacancies can also be involved in these electronic levels. [ABSTRACT FROM AUTHOR]

Carrier lifetime is very sensitive to electrically active defects. Apart from detecting the presence of recombination active defects, lifetime measurements allow for a direct identification of defects if the temperature and injection dependence of carrier lifetime is analyzed. In the present work, the Shockley–Read–Hall (SRH) equation is analyzed from a theoretical point of view in order to demonstrate the possibilities and basic restrictions of I njection and t emperature d ependent l ifetime s pectroscopy (IDLS and TDLS), respectively. To show the variety of attainable curve shapes and to evaluate their suitability for a spectroscopic determination of defect parameters, the general impact of the defect and material parameters on the injection and temperature dependence of SRH lifetime is investigated systematically. From this the basic requirements for an unambiguous defect characterization are derived. The experimental applicability of lifetime spectroscopy is demonstrated by TDLS and IDLS measurements performed on an intentionally molybdenum contaminated Si sample using the microwave-detected photoconductance decay technique and the quasi-steady-state photoconductance technique, respectively. The energy level of 0.335 eV below the conduction band determined from TDLS corresponds well with deep level transient spectroscopy-values reported in literature and could also be determined from the corresponding IDLS curve though not unambiguously. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a calculational procedure to obtain the rate of electron–hole recombination, mediated by the Shockley–Read–Hall (SRH) mechanism. Our method uses a combination of first-principles calculations and accurate empirical band structures. First, we use ab initio calculations to identify the point defects, their densities and energy levels in the gap. Then we parametrize the tight-binding interaction between defect and the host atoms in a Green’s function approach to obtain the defect levels as identified by the first-principles calculations. Finally, the resulting tight-binding Hamiltonian is used to obtain the dipole matrix element between the conduction and valence band states, mediated through the defect levels in the gap, in second-order perturbation theory. The states are integrated over the entire Brillioun zone, subject to energy and momentum conservation, to obtain the limiting lifetimes of the carriers. This method is applied to study the minority carrier lifetimes in n-doped InAs. The calculated effective lifetimes that include Auger and SRH recombinations as functions of temperature agree reasonably well with experiment. Our calculation of lifetimes in 3.5×10[sup 16] and 2.0×10[sup 16] cm[sup -3] n-doped InAs indicate that SRH is dominant at low temperatures and that the lifetimes vary between 10[sup -8] and 10[sup -7] s. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have theoretically studied the radiative recombination process between a conduction-band electron and a valence-band hole to analyze the photoluminescence (PL) spectrum. It is shown that due to the characteristics of the energy density of states, the line shape of the PL peak depends strongly on the dimensionality of the system under investigation, thus indicating the importance of the PL line shape in the identification of the corresponding optical transition process. Increasing the quantum confinement of a system from three-dimensional (3D bulk material) to two-dimensional (quantum well), one-dimensional (quantum wire) and zero-dimensional (quantum dot) results in a transition from a highly nonsymmetric PL peak to a symmetric Lorentzian one centered at the optical transition energy. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The electroluminescence (EL) quantum yield (QY) of double- and triple-layer organic electroluminescent diodes based on a N,N'-diphenyl-N,N'bis(3-methylphenyl)-l,l'-biphenyl4,4'-diamine /tris (8-hydroxyquinolinato) aluminum III (Alq[sub 3]) junction has been measured as a function of the electric field and the emitting guest (6,13-diphenylpentacene) concentration in the host Alq[sub 3]. The well-resolved maxima of the QY plots versus applied field for neat and low dopant concentration emitter layers (EMLs) shift strongly toward high fields and disappear at high dopant concentrations. Based on the EL QY data and the measured absolute photoluminescence quantum efficiency of the emitter, the recombination zone width is determined and shown to be a decreasing function of electric field for all of the diodes. The dopant reduces the width of the recombination zone at low dopant concentrations and increases at high dopant concentrations (>0.5 mol %). The results are discussed in terms of a two-step recombination mechanism, assuming disorder-controlled charge carrier mobilities. The dopant concentration effect on the recombination zone width and EL QY can be explained using the disorder formalism that predicts low dopant concentrations create a high degree of positional (off-diagonal) disorder whereas energetic (diagonal) disorder dominates at higher doping levels in the EMLs. The latter makes the recombination zone width as well as EL QY practically field independent. [ABSTRACT FROM AUTHOR]

Analyzes the rise of electroluminescence from bilayer light emitting diodes under space charge limited majority carrier injection. Factors governing the onset of electroluminescence; Time dependencies of electric field within electron-transporting layer; Relationship between recombination current and electroluminescence intensity.

Presents a study on the effects of Shockley-Read-Hall, Auger, radiative, and intrinsic surface base recombination processes in the emitter-base space-charge region on the current gain of Pnp AlGaAs/GaAs heterojunction bipolar transistors. Approach used in the study; Results; Conclusions.

To improve the power conversion efficiency (PCE) and stability of inverted perovskite solar cells (PSCs) prepared in humid air (RH = 55%), a new zinc oxide (ZnO):TIPD composite film is inserted between PCBM layer and Ag electrode for the first time. Compared to the single PCBM layer, the formed PCBM/ZnO:TIPD electron transport layer (ETL) has better coverage on the perovskite layer and more effective charge extraction ability from the perovskite layer. The insertion of ZnO:TIPD film effectively suppress the charge recombination at perovskite/ETL interface. Besides, the PCBM/ZnO:TIPD layer has better ability to prevent moisture from penetrating into the perovskite layer than single PCBM film. For these reasons, the PCE and stability of the PSCs with the PCBM/ZnO:TIPD are significantly improved compared to the PSCs with single PCBM (or ZnO:TIPD) layer. The highest efficiency of the cell with PCBM/ZnO:TIPD ETL reaches (13.7 ± 0.4)% in ambient atmosphere, which is 1.20 times that of the PSCs with PCBM only. • The ZnO:TIPD composite film are fabricated for the first time. • The effect of the ZnO:TIPD film on charge separation and extraction is studied. • The efficiency and stability of the perovskite solar cells are significantly improved. [ABSTRACT FROM AUTHOR]