Cathodoluminescence study of h– and c–GaN single crystals grown by a Na or K flux

Luminescence properties of hexagonal (h-) and cubic (c-) GaN freestanding single crystals were studied by means of cathodoluminescence spectroscopy. The h-GaN crystals of about 0.2–2 mm in dimension were synthesized at 750 °C by the reaction of Ga and N2 in a Na flux, while c-GaN crystals of about 0.3 mm or less in a K flux. The h-GaN showed rather strong band edge emission at room temperature compared with the crystal grown by using NaN3 as a nitrogen source. At 20 K, the band edge emission of h-GaN was split into four peaks. The main energy peak position was 3.478 eV, which was assigned as the A-free exciton emission. The energy position of the main peak of c-GaN was 3.268 eV. Assuming the binding energies of excitons in h- and c-GaN as 25 and 26 meV, respectively, the energy difference of bandgap between h-and c-GaN is estimated to be 209 meV. Since these crystals are free from strain from the substrates, the peak energies are reliable for the intrinsic GaN crystals. The full widths at half maximum of the emission of c-GaN were much broader than those of h-GaN, suggesting that the cubic phase is much defective compared with the hexagonal one.     

Exciton management by co-doping of blue triplet emitter as a lifetime improving method of blue thermally activated delayed fluorescent devices

Co-doping of a blue phosphorescent emitter in a thermally activated delayed fluorescent (TADF) emitter based emitting layer was developed as an approach to extend the lifetime of blue TADF devices by managing excitons and polarons in the emitting layer. The blue phosphorescent emitter was doped at a very low doping concentration below 1 wt% to suppress triplet-triplet and triplet-polaron quenching effect in the TADF emitting layer. The doping of the blue phosphorescent emitter led to great extension of the lifetime of the TADF devices by hole trapping effect of the blue triplet emitter which widened exciton formation zone in the TADF emitting layer. More than twice extension of the operational lifetime of the device was demonstrated by the co-doping approach irrespective of the doping concentration of the TADF emitter in the emitting layer.     

Efficient small molecule-based bulk heterojunction photovoltaic cells with reduced exciton quenching in fullerene

Most highly efficient small molecule-based bulk heterojunction (BHJ) photovoltaic cells contain a large concentration of fullerene in their blend active layers. However, the excitons generated in fullerene can seriously quench at the surface of the commonly used MoO₃ buffer layer, becoming a key limitation to the photovoltaic performance of these cells. In this study, we’ve investigated various anode buffer layers in the thermally evaporated tetraphenyldibenzoperiflanthene (DBP) and C₇₀-based BHJ cells with high C₇₀ concentration. It’s been found that obviously enhanced power conversion efficiency (PCE) of up to 6.26% can be obtained in DBP and C₇₀-based BHJ cells via simply replacing the MoO₃ buffer by poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS), which is also a commonly used anode buffer material in polymer-based BHJ cells. Photoluminescence spectra results have confirmed the suppression of exciton quenching at the anode interface by inserting this PEDOT: PSS buffer. Moreover, after adding a C₇₀ interlayer for better electron extraction and the further suppression of exciton quenching, the DBP and C₇₀-based M-i-n photovoltaic cells show a remarkable PCE of 7.04% under illumination with 100 mW/cm², AM 1.5G simulated solar light.     

States confined in the barriers of type-III HgTe/CdTe superlattices

We present a theoretical study of states quasi-localized in CdTe barriers of HgTe/CdTe superlattices. We show that the quasi-localization of both electrons and holes will lead to strong Coulomb interaction, and thus to the formation of excitons. It is further demonstrated that such quasi-localized states, including excitons, exhibit confinement effect similar to those of loclaized states in quantum wells.     

Triplet exciton diffusion and delayed interfacial charge separation in a Tio2/PdTPPC bilayer: Monte Carlo simulations

Nanosecond photoexcitation of a bilayer of smooth anatase TiO₂ coated with palladium tetrakis(4-carboxyphenyl)porphyrin (PdTPPC) results in a delayed, after-pulse growth of the conductivity over many microseconds as monitored by time-resolved microwave conductivity. This phenomenon is attributed to the slow diffusion of PdTPPC triplet excitons followed by electron injection into the TiO₂ conduction band. The temporal form and intensity dependence of this process have been simulated by Monte Carlo (MC) calculations of exciton diffusion and exciton–exciton annihilation. Good agreement between the experimental results and MC simulations are obtained using a triplet exciton diffusion coefficient D_E=8×10⁻¹¹ m²/s, exciton lifetime τ_E≥10 μs, effective triplet–triplet annihilation distance R_a=1.5 nm and interfacial electron injection efficiency φ_inj=0.44.     

Electrical and photovoltaic properties of devices based on PbPc–TiO2 thin films

The electrical, optical and photovoltaic properties of organic–inorganic hybrid devices consisting of Al/TiO₂/PbPc/ITO and Al/PbPc/TiO₂/ITO structures have been investigated through analyzing the current–voltage characteristics, optical absorption and photocurrent action spectra of the devices. The combined presence of oxygen, light and an electric field in the photocurrent decay of Al/TiO₂/PbPc/ITO device have been studied. It is observed that under illumination, the oxygen radical anions and excitons are formed, which subsequently drift towards the interface with TiO₂, where an internal electric field is present. The excitons that reach to the interface are subsequently dissociated into free charge carriers due to the electric field present at the interface. The exciton diffusion length for PbPc calculated from the dependence of luminescence with the PbPc film is about 13 nm. We have also studied the effect of PbPc thickness and hole mobility on the device performance of organic photovoltaic device consisting of PbPc as an optically active layer, TiO₂ as the electron–transporting layer and ITO and Al used as electrodes. We have shown that the power conversion efficiency in the device is primarily limited by the short-exciton diffusion length combined with the low-hole mobility in PbPc layer. The model of charge transport in Al/TiO₂/PbPc/ITO device explained the experimental results where the total current density is a function of injected carriers at electrode–organic semiconductor surface, the leakage current through the organic layer and collected photogenerated current that results from the effective dissociation of excitons.     

CuWO4 as a novel Z-scheme partner to construct TiO2 based stable and efficient heterojunction for photocatalytic hydrogen generation

Synthesis of highly efficient, stable, visible active CuWO₄ nanoparticles through a simple methodology, paves a feasible path for enhancing the efficiency of TiO₂. A novel nanocomposite of CuWO₄ NP loaded TiO₂ NR heterojunction was mounted through a direct Z-scheme mechanism. Optimized composite CWT-3, advances the photocatalytic hydrogen production rates of TiO₂ to 106.7 mmol h^?1 g^?1_cat. CuWO₄ incorporation as OEP compensates inefficiency of WO₃ and other Z-scheme combinations reported so far, on limiting the charge carrier recombination followed by the generation of a greater number of excitons. Specific amounts of catalyst loading, study on the effect of sacrificial reagents, and understanding the effect of the light source, are the three pivotal steps that helped here to hamper the density of overall back reactions. The formation of Z-scheme heterojunction was evidently confirmed on determining the position of CBM and VBM, PL and photoelectrochemical analysis. Recyclability studies further proved the stable and efficient outcomes of CWT-3 for five consecutive cycles. Based on photocatalytic activity, employing BDF by-product glycerol as an optimized sacrificial reagent serves the oxidation demands and triggered 53.26% solar to hydrogen conversion efficiency under natural sunlight irradiation.     

势阱形状对电场下量子阱子带和激子能移的影响

在束缚态近似下,用级数解法求解电场下各种不同形状GaAs/GaAlAs量子阱中电子和空穴子带,进一步采用变分方法得到激子结合能.由此,我们首次得到电场下由抛物阱至方位阱子带和激子峰能移的变化图象.在考虑GaAs/GaAlAs量子阱形状影响的基础上,我们计算所得结果与实验吻合得很好.     

MoS_2激子效应的压电光声研究

本文首先利用光声压电法,研究了MoS_2单晶在540-740nm波段的室温本征吸收边的光声吸收光谱,得到了在2.0eV附近的A、B两个强激子的吸收峰及杂质峰A,与以前报道的用透射光谱法测得的吸收谱及光电流谱的结果一致.     

Band-edge luminescence of deformed hexagonal boron nitride single crystals

We studied deformation effects on band-edge luminescence of pure hexagonal boron nitride single crystals. After simple deformation of the single crystals at room temperature, the band-edge luminescence showed a dramatic change, originating from stacking disorder, in the peak wavelength. It changed from 215 nm in a free exciton band to 227 nm in a bound exciton band. This 227-nm bound exciton luminescence has a fast radiative decay time (below a few ns).