Hopping process of bound excitons under an energy gradient

We report on the mechanism of hopping for bound excitons under an energy gradient. By means of a Monte-Carlo simulation, we show that this mechanism explains the movement of bound excitons observed experimentally. We show that the speed of the excitons decreases quickly with temperature. Thanks to an effective medium approximation, we deduce an analytical model to estimate the average speed at T = 0K. Finally, we compare our simulations results to the speed observed in bent ZnO wires and find a good agreement between theory and experiments. (C) 2014 AIP Publishing LLC

Function Follows Form: Correlation between the Growth and Local Emission of Perovskite Structures and the Performance of Solar Cells

Understanding the relationship between the growth and local emission of hybrid perovskite structures and the performance of the devices based on them demands attention. This study investigates the local structural and emission features of CH3NH3PbI3, CH3NH3PbBr3, and CH(NH2) 2PbBr(3) perovskite films deposited under different yet optimized conditions using X-ray scattering and cathodoluminescence spectroscopy, respectively. X-ray scattering shows that a CH3NH3PbI3 film involving spin coating of CH3NH3I instead of dipping is composed of perovskite structures exhibiting a preferred orientation with [202] direction perpendicular to the surface plane. The device based on the CH3NH3PbI3 film composed of oriented crystals yields a relatively higher photovoltage. In the case of CH3NH3PbBr3, while the crystallinity decreases when the HBr solution is used in a single-step method, the photovoltage enhancement from 1.1 to 1.46 V seems largely stemming from the morphological improvements, i.e., a better connection between the crystallites due to a higher nucleation density. Furthermore, a high photovoltage of 1.47 V obtained from CH(NH2)(2)PbBr3 devices could be attributed to the formation of perovskite films displaying uniform cathodoluminescence emission. The comparative analysis of the local structural, morphological, and emission characteristics of the different perovskite films supports the higher photovoltage yielded by the relatively better performing devices

Biexcitonic molecules survive excitons at the Mott transition

When the carrier density is increased in a semiconductor, according to the predictions of Sir Nevil Mott, a transition should occur from an insulating state consisting of a gas of excitons to a conductive electron-hole plasma. This crossover, usually referred to as the Mott transition, is driven by the mutual effects of phase-space filling and Coulomb screening because of the presence of other charges nearby. It drastically affects the optical and electrical characteristics of semiconductors and may, for example, drive the transition from a polariton laser to a vertical cavity surface-emitting laser. Usually, the possible existence of excitonic molecules (or biexcitons) is neglected in the understanding of the Mott transition because the biexciton is supposed to be less robust against screening effects. Here, against common beliefs, we observe that the biexciton in a GaN quantum well is more stable towards the Mott transition than the exciton

Exciton Drift in Semiconductors under Uniform Strain Gradients: Application to Bent ZnO Microwires

Optimizing the electronic structures and carrier dynamics in semiconductors at atomic scale is an essential issue for innovative device applications. Besides the traditional chemical doping and the use of homo/heterostructures, elastic strain has been proposed as a promising possibility. Here, we report on the direct observation of the dynamics of exciton transport in a ZnO microwire under pure elastic bending deformation, by using cathodoluminescence with high temporal, spatial, and energy resolutions. We demonstrate that excitons can be effectively drifted by the strain gradient in inhomogeneous strain fields. Our observations are well reproduced by a drift-diffusion model taking into account the strain gradient and allow us to deduce an exciton mobility of 1400 +/- 100 cm(2)/(eV s) in the ZnO wire. These results propose a way to tune the exciton dynamics in semiconductors and imply the possible role of strain gradient in optoelectronic and sensing nano/microdevices

Photovoltaic and Amplified Spontaneous Emission Studies of High-Quality Formamidinium Lead Bromide Perovskite Films

This study demonstrates the formation of extremely smooth and uniform formamidinium lead bromide (CH(NH2)(2)PbBr3 = FAPbBr(3)) films using an optimum mixture of dimethyl sulfoxide and N,N-dimethylformamide solvents. Surface morphology and phase purity of the FAPbBr(3) films are thoroughly examined by field emission scanning electron microscopy and powder X-ray diffraction, respectively. To unravel the photophysical properties of these films, systematic investigation based on time-integrated and time-dependent photoluminescence studies are carried out which, respectively, bring out relatively lower nonradiative recombination rates and long lasting photogenerated charge carriers in FAPbBr(3) perovskite films. The devices based on FTO/TiO2/FAPbBr(3)/spiro-OMeTAD/Au show highly reproducible open-circuit voltage (V-oc) of 1.42 V, a record for FAPbBr(3)-based perovskite solar cells. V-oc as a function of illumination intensity indicates that the contacts are very selective and higher V-oc values are expected to be achieved when the quality of the FAPbBr(3) film is further improved. Overall, the devices based on these films reveal appreciable power conversion efficiency of 7% under standard illumination conditions with negligible hysteresis. Finally, the amplified spontaneous emission (ASE) behavior explored in a cavity-free configuration for FAPbBr(3) perovskite films shows a sharp ASE threshold at a fluence of 190 mu J cm(-2) with high quantum efficiency further confirming the high quality of the films

Molecular Origin of the Asymmetric Photoluminescence Spectra of CsPbBr3 at Low Temperature

CsPbBr3 has received wide attention due to its superior emission yield and better thermal stability compared to other organic-inorganic lead halide perovskites. In this study, through an interplay of theory and experiments, we investigate the molecular origin of the asymmetric low-temperature photoluminescence spectra of CsPbBr3. We conclude that the origin of this phenomenon lies in a local dipole moment (and the induced Stark effect) due to the preferential localization of Cs+ in either of two off-center positions of the empty space between the surrounding PbBr6 octahedra. With increasing temperature, Cs+ ions are gradually occupying positions closer and closer to the center of the cavities. The gradual loss of ordering in the Cs+ position with increasing temperature is the driving force for the formation of tetragonal-like arrangements within the orthorhombic lattice

Optical properties of nearly lattice-matched GaN/(Al,In)N quantum wells

We report a systematic study of the photoluminescence (PL) properties of a series of nearly lattice-matched (LM) GaN/(Al,In)N single quantum well (SQW) samples, with well thickness ranging from 1.5 to 5 nm, grown by metalorganic vapor phase epitaxy. Temperature dependent PL and time-resolved PL measurements reveal similar trends among the studied SQW samples, which also indicate strong localization effects. The observed PL energy behavior, akin to the S-shape, accompanied first by a narrowing and then a broadening of the PL line width with increasing temperature, closely resemble previous observations made on the more established (In,Ga)N/GaN QW system. The similar trends observed in the PL features of those two QW systems imply that the PL properties of LM GaN/(Al, In) N SQW samples are also governed by localized states. The effects of carrier transfer among these localization sites are clearly observed for the 3 nm thick QW, evidenced by an increasing PL intensity in the lower energy spectral window and a concomitant increase in the corresponding PL decay time. Time-resolved data corroborate the picture of strongly localized carriers and also indicate that above a well thickness dependent delocalization temperature carrier distribution across the localized sites reaches thermal equilibrium, as the PL decay times over different spectral regions converge to the same value. Based on the difference between the calculated QW ground state transition energy, obtained using the envelope wave function formalism, and the measured PL energy, a localization energy of at least a few hundreds of meV has been extracted for all of the studied SQW samples. This rather large value also implies that In-related localization effects are more pronounced in the GaN/(Al, In) N system with respect to those in the (In, Ga) N/GaN one for a similar In content. Published by AIP Publishing

Temperature Dependence of Betavoltaic Cell Performance of Diamond pn Junction Diode

International audienceTemperature dependence of the energy conversion efficiency of diamond pn junction betavoltaic cells was evaluated. We fabricated pseudo vertical diamond pn junction diode and characterized its energy conversion efficiency under electron beam irradiation from 5-300 K. The diamond pn junction diode exhibits energy conversion efficiencies of 18-24% at 150-300 K, which is more than twice as high as those of silicon PiN diode. On the other hand, below 100 K, the energy conversion efficiency of the diode significantly drops due to the increase of series resistance of diamond. Above 150K, the diamond pn junction diode presents smaller temperature dependency on energy conversion efficiency than that of silicon diode, which would make diamond pn junction betavoltaic cells, a promising device for energy harvesting in remote sensing devices over a wide temperature range except in the cryogenic region

Cyclopentadithiophene-based hole-transporting material for highly stable perovskite solar cells with stabilized efficiencies approaching 21%

WOS:000563784400025There is an urge to develop new hole-transporting materials (HTMs) for perovskite solar cells (PSCs), which can yield comparable power conversion efficiencies (PCEs) yet mitigate the issue of stability associated with the state-of-the-art HTM Spiro-MeOTAD. Herein, we designed and prepared C-2v-symmetric spiro-configured HTM-1 comprising a central acridine-cyclopentadithiophene core unit flanked with triarylamine moieties. PSCs containing a 40 nm thin HTM-1 layer for hole extraction yielded a stabilized PCE approaching 21% under standard illumination. Owing to its higher hole mobility (mu(h)) at low electric field, an impressive short-circuit current density (J(SC)) of 24.7 mA cm(-2) and a high fill factor (FF) of 0.77 have been achieved. More importantly, HTM-1-based PSCs presented an excellent long-term operational stability under continuous illumination for 400 h and thermal stability at 80 degrees C, which can be ascribed to its high glass transition temperature of 168 degrees C and superior moisture tolerance. Arguably, the confluence of high performance and remarkable stability will lead to the development of technologically interesting new, stable, and efficient PSCs

Color control of nanowire InGaN/GaN light emitting diodes by post-growth treatment

Core/shell InGaN/GaN nanowire light emitting diodes (LEDs) based on vertically standing single nanowires and nanowire arrays were fabricated and extensively characterized. The emission of single wire LEDs with the same conformal contact geometry as the array device exhibits the same broadening as the array LED electroluminescence, which proves an excellent wire-to-wire homogeneity. The electroluminescence spectra present two peaks corresponding to the m-plane InGaN quantum well (blue emission) and to an In-rich region at the m-plane-semipolar plane junction (green emission), in agreement with structural characterizations. Modification of the contact layout and a post-growth plasma treatment enable strongly suppressing the unwanted green electroluminescence while increasing the intensity in the blue spectral range for the same injected electrical power. Electron beam induced current mapping proves the inhibition of the electrical activity of the top part of the nanowire after plasma treatment. Inductively coupled plasma etching of the In-rich region permits one to completely remove the green emission for all injection currents, but loss of intensity in the blue spectral range is observed. Selectively contacting the m-plane and plasma treatment of the top part of the nanowire appear as a viable solution for controlling the color of core/shell nanowire LEDs with an inhomogeneous indium composition