Electron Recombination Centers Research Articles

Influence of Quenching on the Opto-Electronic Properties of F:SnO2 Layers.

For many opto-electronic applications, F:SnO2 materials must benefit from high transparency, high conductivity, and high mechanical strength even after quenching. The purpose of this study was to investigate the influence of quenching on the opto-electronic properties of the F:SnO2 layers synthesized at high temperature on SixCyO-coated soda-lime glass by atmospheric chemical vapor deposition. The morphology, structure, and composition of the layers were studied before and after quenching in air- and oxygen-rich atmospheres at 670 °C. The free carrier concentration was reduced by oxygen vacancy (VO) passivation, as well as by F and Na diffusion, with all effects scaling up with quenching time in air. The transmittance also decreased with quenching time as Na impurities acted as absorption and electron recombination centers. In an oxygen-rich atmosphere, the VO passivation was even more emphasized, with however a moderate contribution to conductivity loss. The F:SnO2 layer microstructure and composition were rather fringed through high-temperature deposition. The almost invariable free carrier concentration and transmittance of the F:SnO2 samples quenched in O2 versus air were related to a moderation in Na diffusion. For long quenching times (>20 min) in air, Na and F diffusion prevailed explaining the conductivity drop.

Influence of Perovskite Grain Size and TiO2 Surface States to the Performance of Perovskite Solar Cell

Introduction Currently, perovskite solar cells (PSCs) have received great curiosity from solar cell field, due to rapid improvement in their photoelectric conversion efficiencies (PCEs) from 9% to over 22% [1]. PSCs can be constructed with either invert or regular structure. In invert type, organic materials are employed as the hole transporting layer (HTL) and electron transporting layer (ETL). The PSCs with invert structure has reached PCE 20.15% [2]. The PSCs with regular structure typically employs both mesoporous and planar inorganic material of TiO2 to serve as ETL because the affinity of perovskite layers with TiO2 is generally lower than that with organic ETL. When considering the long stability, inorganic material would be better than organic material as ETL. The current best performing PSCs with mesoporous TiO2 as ETL perform PCE >22% because mesoporous TiO2 helps to support perovskite layer acting as a scaffold and make better connection with perovskite layer[1]. On the other hand, PSCs with planar TiO2 also shows PCE about 20.2%, which is lower than with mesoporous TiO2 but provides a more straightforward platform for fundamental investigations [3]. However, in planar structure, the quality of each layers is an important factor that affect overall cell performance, especially ETL [4]. However, a mechanism depending on ETL on cell performances should be investigated in detail. For ETL, various deposition methods are explored, such as spin coating, atomic layer deposition and sputtering, especially spray-coating has a great potential for the large-scale production due to the low cost and capability to obtain thin films in large areas for industrial application. The polycrystalline has grain boundaries. When having polycrystalline Si with more grain boundaries, Si solar cell shows lower performance because grain boundaries act as recombination centers for electrons and holes and become scattering centers of free carriers [5]. CuInSe2 (CIS) solar cell showed PCE about 20% without special grain boundaries passivation [6]. Similar to CIS solar cell, perovskite grain boundaries seem to have small effect on photoelectronic properties because even high efficient PSCs have smaller grain size less than 1 um. Therefore, the relationship between grain size and performance of PSCs have not been clarified clearly. Here, Kim et al. showed planar PSCs of FTO/compact TiO2/MAPbI3/spiro-OMeTAD/Au. They obtained dense MAPbI3 layers with different grain sizes ranging from ~100 to ~500 nm. With increasing grain size of perovskites, all the photovoltaic parameters were improved. As a result, the best performance was observed for the largest perovskite grain size (~500 nm) with PCE of 19.4% [7]. In this paper, we investigated the relationship between PSC performance and grain size distribution on the top of planar TiO2 by spray pyrolysis technique to construct more defined structure. SEM image from top view and cross section were observed and used to identify the grain size distribution. The surface roughness of TiO2 measured by AFM used for investigation of the relationship with perovskite grain size. These findings indicated that wide variations in the device efficiency also influence by perovskite grain size distribution in planar heterojunction structure. Experimental An around 80-100 nm thick compact-TiO2 layer was deposited on ITO by spray pyrolysis at 350, 450 and 550°C to investigate the of spray temperature (Ts) effect on compact-TiO2 surface state. Also, ethanol, propanol and 1-butanol were used as a precursor solvent with different boiling temperature (Tb) to find their effects on compact-TiO2 surface state. Next, samples were annealed at different temperatures (400, 450, 500 or 600°C) for 1 hr to study an effect of annealing temperature (Ta) as shown in Table 1. The device structure consists of; ITO/compact-TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au with active area 0.5 cm2. Results and discussion In this work, planar PSCs were assembled with different ETL fabrication condition for comparison. The uniform TiO2 film without pin hole were successfully deposited by using 1-butanol as a solvent. However, the wide range variations of PCE values were observed even at the same fabrication condition. So, grain size distribution was measured and the results indicated that higher device efficiencies at the same ETL fabrication condition resulted from larger perovskite grain size distribution. The obtained dense MAPbI3 layers showed different grain sizes ranging from ~100 to ~300 nm. In addition, the relationship between roughness of TiO2 and perovskite grain size showed that the larger grain sizes were from lower roughness of TiO2 film. Therefore, higher PCEs were strongly achieved from larger perovskite grain size on planar TiO2. Figure 1

Revisiting radiative deep-level transitions in CuGaSe2 by photoluminescence

Recent defect calculations suggest that the open circuit voltage of CuGaSe2 solar cells can be limited by deep intrinsic electron traps by GaCu antisites and their complexes with Cu-vacancies. To gain experimental evidence, two radiative defect transitions at 1.10 eV and 1.24 eV are characterized by steady-state photoluminescence on epitaxial-grown CuGaSe2 thin films. Cu-rich samples are studied, since they show highest crystal quality, exciton luminescence, and no potential fluctuations. Variations of the laser intensity and temperature dependent measurements suggest that emission occurs from two deep donor-like levels into the same shallow acceptor. At 10 K, power-law exponents of 1 (low excitation regime) and 1/2 (high excitation regime) are observed identically for both transitions. The theory and a fitting function for the double power law is derived. It is concluded that the acceptor becomes saturated by excess carriers which changes the exponent of all transitions. Activation energies determined from the temperature quenching depend on the excitation level and show unexpected values of 600 meV and higher. The thermal activation of non-radiative processes can explain the distortion of the ionization energies. Both the deep levels play a major role as radiative and non-radiative recombination centers for electrons and can be detrimental for photovoltaic applications.

First principles calculations of electronic and optical properties of Mo and C co-doped anatase TiO2

Using the first principles calculations, the electronic and optical properties of C, Mo and Mo-C-doped anatase TiO2 are studied. For the Mo mono-doped TiO2, the band gap reduces little, and the largest perturbation occurs at the CBM of TiO2. C mono-doping suppresses the effective band gap, but the partially occupied subbands in the gap probably also serve as the recombination centers for electrons and holes. Therefore, the Mo-C co-doping is investigated for the charge compensation consideration. We discuss six doped configurations and find that the total energy of the system is increased with increasing distance of C and Mo. It is found that co-doped configurations with C nearest to Mo possess the lowest total energy. Then, we focus on discussing three possible Mo-C adjacent co-doped configurations. The subbands mainly induced by C-2p states in the band gap become fully occupied because the Mo atom contributes sufficient electrons to C anion for compensation. At the same time, the effective band gap is narrowed about 0.9 eV and the perturbation at the CBM occurred in Mo mono-doped TiO2 disappears, which means the band edges of doped system still straddle the redox potentials of water. Furthermore, the optical properties of the compensated Mo-C adjacent co-doped TiO2 and pure TiO2 are calculated. The optical absorption edges of the Mo-C co-doped TiO2 shift towards the visible light region.

Electronic and optical properties of C and Nb co-doped anatase TiO2

The electronic and optical properties of C, C–Nb and C–2Nb doped anatase TiO2 are studied using the first principles calculations. For the C-doped TiO2, three subbands in the band gap are generated due to the C-2p states, which suppress the effective band gap for electron transfer, while the gap states are partially occupied and probably serve as the recombination centers for electrons and holes. Therefore, the C–2Nb co-doping is investigated for the charge compensation consideration. Two possible co-doping configurations are comprehensively studied and compared. The three subbands in the band gap of the C mono-doped TiO2 become fully occupied because the two Nb atoms contribute sufficient electrons for compensation. Furthermore, the optical properties of the co-doped TiO2 and pure TiO2 are calculated. It is found that the optical absorption of the C–2Nb co-doped TiO2 extends its coverage over the visible light region.

Thermoluminescence glow curve deconvolution of LiF:Mg,Cu,Si with more realistic kinetic models

Thermoluminescence (TL) glow curves of LiF:Mg,Cu,Si were deconvoluted with the introduction of enhanced physical model which envisages that both electrons and holes, produced by ionization radiation and trapped at the respective traps, can be thermally released into the conduction and the valence band, respectively and the holes may also radiatively recombine with electrons at the electron recombination centers. The model is more generalized than the ordinary trap interaction model which only permits the traffic of electrons through the conduction band. An effective numerical analysis method was developed to calculate the glow curve to be compatible with the measured curves. The validity of the numerical method was verified through artificially generated TL glow curves for a wide range of trap parameters. In order to identify TL kinetics of LiF:Mg,Cu,Si with higher accuracy, its glow curves were deconvoluted for two more generalized models, namely, the Schön–Klasens model and the Chen–Pagonis–Lawless model as well as the ordinary trap interactive model. The parameters in the more generalized multi-trap multi-recombination center (MTMR) model were found to be consistent with the quasi-static approximation(QSA) method.

High conversion efficiency of pristine TiO2 nanotube arrays based dye-sensitized solar cells

We prepared highly-ordered titanium dioxide nanotube arrays (TNAs) by anodizing Ti foils in F−-containing electrolytes. The crystalline nature and morphology of the TNAs were studied using X-ray diffraction patterns and scanning electron microscopy. We found the morphology of TNAs affects the light-to-electricity conversion efficiency (η) of dye-sensitized solar cells (DSSCs). The efficiency of DSSCs reached 5.95% under the condition of light illuminated from the counter electrode. The high efficiency of TNA-based DSSCs was attributed to the neat top surface of TNAs, which allows more dye molecule loading on the surface of the TiO2 nanotubes, and fewer electron recombination centers and a low interface resistance of integrated TNAs.

Ab initiostudy on band-gap narrowing in SrTiO3with Nb-C-Nb codoping

To improve the photocatalyst efficiency of SrTiO${}_{3}$ for the production of hydrogen and oxygen through water splitting, the band structure needs to be modified, so that the visible light can be absorbed and the band positions remain straddling the hydrogen and oxygen evolution potentials of water. Our first-principle calculations show that Nb-C-Nb codoping could be a candidate method for this purpose. The doping of C atoms may shift three subbands from the valence band to the band gap because the energies of C $2p$ states are higher than those of O $2p$ states; however, the $p$-type doping of C atoms in SrTiO${}_{3}$ is difficult, meanwhile the subbands which are partially occupied may act as recombination centers for electrons and holes. By introducing two Nb atoms on both sides of a doped C atom, these shortages can be surmounted. The subbands become fully occupied because the doped Nb atoms release more valence electrons than the replaced Ti atoms do. The calculation of defect formation enthalpy indicates that the codoping of Nb-C-Nb motifs are energetically favorable in O-rich and Ti-poor environments.

Dye-sensitized solar cells with modified TiO2 surface chemical states: The role of Ti3+

Dye-sensitized solar cells (DSSCs) with TiO2 electrodes, which were modified to have different surface chemical states, were fabricated. The DSSCs had an ultra-flat TiO2 electrode, on which only a mono-layer dye was attached. The cell I-V measurement showed that Ti3+ ion had a dramatic effect on cell performance. The efficiency of a DSSC was shown to have a strong correlation with the concentration of the Ti3+ surface state, it decreased almost linearly with increased Ti3+ concentration. The oxygen vacancy-Ti3+ defect served as electron recombination center and decreased both the open-circuit voltage and the fill factor.

Doping effects of Co2+ ions on ZnO nanorods and their photocatalytic properties

A series of Zn1−xCoxO nanorods with dopant content ranging fromx = 0.00 to 0.10 was prepared by a wet chemical method. AllZn1−xCoxO samples were investigated by x-ray diffraction, transmission electron microscopy, energy-dispersionx-ray line mapping analysis, and UV–visible absorption spectroscopy. It was found thatCo2+ ions were homogeneouslysubstituted for Zn2+ ions in ZnO nanorods. Rhodamine B degradation was used as a probe reaction to evaluate the effectof Co2+ doping on ZnO nanorods and photocatalytic performance under UV light and visible light irradiation.Co2+ ions acted as the trapping or recombination centers for electrons and holes, leading to areduction in photodegradation efficiency under UV light illumination. Alternatively,Co2+ ions enhanced the optical absorption and produced the photoinduced carriersunder visible illumination in terms of two charge transfer transitions involvingCo2+ ions.Consequently, Co2+ ions substituted in the lattice of ZnO nanorods significantly improved the visible lightphotocatalytic activity.

Duplicitous thermoluminescence peak associated with a thermal release of electrons and holes from trapping states

A thermoluminescence (TL) glow peak may result from a transition of electrons from traps into the conduction band, followed by a recombination with holes trapped in a luminescence center. Another possibility is that holes trapped in a hole trap are thermally released into the valence band and recombine with electrons in an electron recombination center. A series of glow peaks emitted from a given sample may include peaks of both kinds. In some cases, peaks may be identified as being of one kind or the other, say, by using thermally stimulated electron emission (TSEE), which can take place when the free carriers are electrons. In the present work, we demonstrate by the use of simulation that two peaks may result from one electron and one hole trapping states and a single hole recombination center. The first TL peak is observed when thermally stimulated electrons recombine with holes in the center. The TL peak is terminated when the holes in the center are exhausted. At higher temperatures, holes from a hole trap are released into the valence band and then captured by the hole center, thus this center is replenished. More electrons from the electron trap are thermally released now and recombine with the newly arrived holes in centers. A second TL peak may be observed which carries some information concerning the hole trap. It is thus demonstrated that some of the usual methods for distinguishing between electron and hole traps can lead to incorrect conclusions. It is possible for a hole trap, for example, to induce an increase in electron recombination in such a way that it produces a peak that looks nearly identical to TL from an electron trap. This simulation may bring about a new look at TL peaks occurring in materials used in TL dosimetry and dating. A new interpretation may also be given to “Auger” TSEE associated with the thermal release of electrons from the surface of a material, which indirectly results from the thermal release of holes from traps. The performance of some methods for evaluating the activation energies and the significance of the results in the present situation are discussed.

Photoluminescence and thermoluminescence of titanium ions in sapphire crystals

Several titanium species (Ti 3+, Ti 4+, Ti 4+-V Al clusters) were identified in sapphire samples from their optical spectra. In highly reduced samples, we observed only Ti 3+ centres while oxidized samples contain Ti 3+ and Ti 4+ ions as well. The formation of Ti 4+ ions occurs simultaneously with the formation of aluminium-vacancies which are charge compensators for Ti 4+ ions and act as hole traps in charge exchange process. We could observe locally and non-locally compensated Ti 4+ as well. It is shown that Ti 3+ is a deep hole trap while Ti 4+ is a deep electron trap. In the TL process they act as recombination centers for electrons and holes, respectively. However, in TL we observed only the emission of locally compensated Ti 4+, probably because of spatial correlation.

Mechanisms of incorporation of an antimony impurity into cadmium telluride crystals

The results of electrical studies of CdTe crystals grown by the Bridgman-Stockbarger method and doped with Sb impurity to concentrations of 1017–3×1019 cm−3 were considered. An analysis of the temperature dependences of the Hall coefficient, the charge-carrier mobility, and photoconductivity under the intrinsic-absorption excitation for various portions of the ingots made it possible to conclude that SbTe and SbCd centers and SbTeSbCd associations are introduced upon doping CdTe crystals with Sb impurity. The hole conduction in the doped crystals is controlled by A3 (SbTe) acceptors whose concentration is no higher than 5×1016 cm−3 and is much lower than the actual Sb concentration. The ionization energy of the A3 acceptors is 0.28±0.01 eV. Under nonequilibrium conditions, these acceptors act as attachment centers for holes (at high temperatures) and as slow-rate recombination centers for electrons (at low temperatures).

Photoconductivity of CVD diamond under bandgap and subbandgap irradiations

We systematically studied the photoconductivity of nominally intrinsic diamond films grown by two CVD methods. In the 200–275 nm wavelength range covering the bandgap energy (5.5 eV or 225 nm), the measured photocurrent showed characteristic behavior that can be quantitatively related to a fast recombination of electrons and effective trapping of holes, mainly by trap states near the valence band edge. In addition to photoconductivity, thermoelectric emission spectroscopy and thermally stimulated current measurements were made. From the results, the density and location of two distinctive trap levels within the energy bandgap was estimated. The results are not only self-consistent, but also agree with other authors' findings, such as a hole-dominated current, without or under bandgap illumination, a Fermi level close to the valence band edge, an electron recombination center in the middle of the bandgap, and a shallow hole-trap state having an extremely high density of ∼ 1019 cm−3. Based on these data, we suggest a bandgap and trap-state model for intrinsic CVD diamond. The details of this model and the characteristic properties of the defects/traps are consistent with experimental results and theoretical findings made by other authors covering a large diversity of areas, such as photoluminescence and cathodoluminescence, photoabsorption, carrier lifetimes and mobilities, electron paramagnetic resonance, cold-cathode electron emission, and photoconductive current switching.

Photoconductivity of a-Si:H as a Function of Doping, Temperature and Photocarrier Generation Rates Between 1013 and 1028cm-3s-1

AbstractThe steady state photoconductivity up of n-type, p-type and intrinsic a-Si:H has been studied up to photocarrier generation rates of G=5×1027cm-3s-1. In the 20ppm B2H6/SiH4 p-type sample photoconduction switches from holes at low G to electrons at high G. The electron photoconduction at high G is increased by n-type and decreased by p-type doping. This is explained by the charge state of the dominant electron recombination centers. The σp(G) curves of doped and intrinsic a-Si:H merge at the highest G used.

Neutron-treated, ultrafast, photoconductor detectors

We have investigated homogeneous, photoconductive semiconductors as very fast radiation detectors. We irradiated GaAs, Cr-doped GaAs, and Fe-doped InP crystals with 14 MeV neutrons to produce lattice defects that act as fast recombination centers for electrons and holes. Using short-pulse lasers and 17 MeV linear-accelerator electrons and bremsstrahlung x rays, we measured the temporal response and sensitivity of these photoconductors as functions of fluence ranging from 1012 to 1016 neutrons/cm2. The carrier lifetime and mobility decrease monotonically as the neutron fluence increases, resulting in faster detector response at the expense of sensitivity. A resolving time of less than 30 ps (full width at half maximum) was measured for the above photoconductors irradiated with ∼1015 neutrons/cm2.

Effects of dislocation density on the properties of liquid phase epitaxial GaAs

A study has been made of the effect of dislocation density on the properties of p-type GaAs (Ge-doped, p = 1 × 1019 cm−3) grown homoepitaxially on GaAs and heteroepitaxially on GaP. It has been found that dislocation densities > 5 × 106 cm−2 reduce the minority-carrier diffusion length in the p-type material and, consistent with this reduction, degrade the junction electroluminescence and p-layer photoluminescence. At high dislocation densities (> 5 × 106 cm−2), it is also found that subgrains form in the GaAs epitaxial layers grown either heteroepitaxially on GaP or homoepitaxially on GaAs. It is surmised that the dislocations polygonize during growth to form subgrain boundaries rather than form due to nucleation at the substrate. Finally, the results indicate that the electron diffusion length is limited to the average dislocation spacing with the dislocations acting as nonradiative recombination centers for electrons.