Radiative Recombination Lifetime Research Articles

Improving performance of Cs-based perovskite light-emitting diodes by dual additives consisting of polar polymer and n-type small molecule

Abstract Organic-inorganic halide perovskites were emerging as promising light emitters due to the high photoluminescence quantum efficiency (PLQE), narrow color spectrum, and facile bandgap tunability. However, the morphological issues of this class of materials remain as a bottleneck in generating high-performance light-emitting devices (LED). In this work, we describe a facile and cost-effective method to improve the morphology of an all-inorganic-perovskite emissive layer by using a dual-additive blending strategy. A polar polymer, polyethylene oxide (PEO), and an n-type small molecule, TPBi, are blended with CsPbBr3 to provide synchronous effect in passivizing the surface defects and reducing the exciton quenching, as evidenced by the formation of smaller uniform grains and improved radiative recombination lifetime. Consequently, the optimized CsPbBr3/PEO/TPBi LED reaches maximum luminance, current efficiency, and external quantum efficiency of 6807 cd m−2 (at 6 V), 0.86 cd A−1 (at 5.5 V) and 0.25% (at 5.5 V), far surpassing the performance of the control CsPbBr3 device. On this basis, we further demonstrated a proof-of-concept LED device with touch-responsivity by using a flexible composite electrode. The merit of dual-additive strategy has been clearly manifested and will facilitate the future development of cost-effective solution processable perovskite LEDs.

Radiative and non radiative recombinations study in the novel nanocomposites BiVO4/3DOM-TiO2, ZnO/3DOM-TiO2 and BiVO4/3DOM-ZnO: Application to the photocatalysis

A theoretical investigation of optical proprieties of three nanostructures such as BiOV4/3DOM TiO2, ZnO/3DOM TiO2 and BiOV4/3DOM ZnO has been presented and discussed. The study based on a one band effective mass approximation model was developed to calculate the charge carrier energies on various parameters of nanostructures including their size. We computed the optical properties for both spherical BiVO4 and ZnO nanoparticles incorporated in the three dimensionally ordered macroporous inverse opal TiO2 nanocomposites (3DOM-TiO2). At first and by solving a three dimension Schrödinger equation we calculated the dependence of charge carriers energies on the size of nanoparticles. Then we investigated the variation of the excitonic energy of BiOV4/3DOM-TiO2, ZnO/3DOM-TiO2 and BiOV4/3DOM-ZnO as function of the radius of these nanostructures into taken account the effects of dielectric and coulomb interaction between charge carriers. We showed that the radiative recombination lifetime is enhanced with decreasing BiVO4 and ZnO quantum dots (QDs) size. It was found that as the size of the dot is reduced the confinement of charge carriers and the optical gap are increased. The optical gap of the ZnO/3DOM-TiO2 is larger than both BiOV4/3DOM-TiO2 and BiOV4/3DOM-ZnO especially for small radius. It was also found that radiative life time in the BiOV4/3DOM-TiO2, ZnO/3DOM-TiO2 and BiOV4/3DOM-ZnO increased by increasing the radius of these nanostructures. In addition, we have also showed that, the radiative lifetime in the ZnO/3DOM-TiO2 nanostructure is less than both BiOV4/3DOM-TiO2 and BiOV4/3DOM-ZnO nanostructures. Finally, we have studied and discussed the Auger process in BiOV4/3DOM-TiO2, ZnO/3DOM-TiO2 and BiOV4/3DOM-ZnO nanostructures. At first, we have showed that the auger process is important when the radiuses of nanostructures are less than 2 nm. Then we have also showed that this process is more important for ZnO/3DOM-TiO2 compared to the other nanostructures.

Spectroscopic Characterization and Mechanistic Studies on Visible Light Photoredox Carbon–Carbon Bond Formation by Bis(arylimino)acenaphthene Copper Photosensitizers

Currently, the most popular molecular photosensitizers used for synthetic organic chemistry and energy applications are still the noble-metal-based ruthenium and iridium complexes that usually require expensive metal and ligand precursors. In contrast, bis(arylimino)acenaphthene (Ar-BIAN) is an established redox noninnocent π-accepting ligand that is easily assembled in one condensation step from affordable and commercially available precursors. Herein, we have developed a series of Ar-BIAN CuI complexes as visible-light-harvesting photosensitizers. Notably, one of these panchromatic, homoleptic Ar-BIAN CuI complexes exhibits a radiative recombination lifetime that is longer than diffusion-controlled reactions, as observed by time-correlated single-photon counting spectroscopy. Ar-BIAN CuI facilitates visible-light-promoted atom-transfer radical addition reactions via carbon–carbon bond formation with CBr3 radicals in good yields of up to 75%. Steady-state and transient absorption spectroscopic measuremen...

All‐Inorganic CsPbBr3 Nanowire Based Plasmonic Lasers

AbstractPlasmonic nanolaser holds great potential in breaking down the diffraction limit of conventional optics to the deep sub‐wavelength regime and in ultrafast lasing dynamics. However, plasmonic laser devices are constrained in practical applications due to their high cost and high thresholds. All‐inorganic cesium lead halide perovskites are promising solutions for their excellent optical gain properties and high emission efficiency. In this work, high‐quality single‐crystalline CsPbBr3 perovskite nanowires (NWs) are synthesized by chemical vapor deposition method. The plasmonic lasing is achieved from the CsPbBr3 nanowire based plasmonic devices with lasing threshold down to ≈6.5 µJ cm−2 at room temperature. The highly polarized emission parallel to nanowire axis and polarization‐sensitive pump response confirm the plasmonic characteristic in these devices. Furthermore, time‐resolved photoluminescence study suggests that the radiative recombination lifetime of CsPbBr3 NW is shortened by a factor of ≈6.14 due to Purcell effect. The lasing threshold of plasmonic device increases along with the nanowire length, indicating greater potential in small size and integration in plasmonic device than its photonic counterparts. The results not only provide a solution to fabricate low‐cost nanowire based plasmonic lasers, but also advocate the prospect of all‐inorganic perovskite nanowires as promising candidates in plasmonic‐based devices.

Improving Electron Extraction Ability and Device Stability of Perovskite Solar Cells Using a Compatible PCBM/AZO Electron Transporting Bilayer.

Due to the low temperature fabrication process and reduced hysteresis effect, inverted p-i-n structured perovskite solar cells (PSCs) with the PEDOT:PSS as the hole transporting layer and PCBM as the electron transporting layer have attracted considerable attention. However, the energy barrier at the interface between the PCBM layer and the metal electrode, which is due to an energy level mismatch, limits the electron extraction ability. In this work, an inorganic aluminum-doped zinc oxide (AZO) interlayer is inserted between the PCBM layer and the metal electrode so that electrons can be collected efficiently by the electrode. It is shown that with the help of the PCBM/AZO bilayer, the power conversion efficiency of PSCs is significantly improved, with negligible hysteresis and improved device stability. The UPS measurement shows that the AZO interlayer can effectively decrease the energy offset between PCBM and the metal electrode. The steady state photoluminescence, time-resolved photoluminescence, transient photocurrent, and transient photovoltage measurements show that the PSCs with the AZO interlayer have a longer radiative carrier recombination lifetime and more efficient charge extraction efficiency. Moreover, the introduction of the AZO interlayer could protect the underlying perovskite, and thus, greatly improve device stability.

Enhanced hot electron lifetimes in quantum wells with inhibited phonon coupling

Hot electrons established by the absorption of high-energy photons typically thermalize on a picosecond time scale in a semiconductor, dissipating energy via various phonon-mediated relaxation pathways. Here it is shown that a strong hot carrier distribution can be produced using a type-II quantum well structure. In such systems it is shown that the dominant hot carrier thermalization process is limited by the radiative recombination lifetime of electrons with reduced wavefunction overlap with holes. It is proposed that the subsequent reabsorption of acoustic and optical phonons is facilitated by a mismatch in phonon dispersions at the InAs-AlAsSb interface and serves to further stabilize hot electrons in this system. This lengthens the time scale for thermalization to nanoseconds and results in a hot electron distribution with a temperature of 490 K for a quantum well structure under steady-state illumination at room temperature.

Alleviating hysteresis and improving efficiency of MA1−yFAyPbI3−xBrx perovskite solar cells by controlling the halide composition

Due to their great potential for application in the future of photovoltaics, perovskite solar cells (PSCs) have attracted much attention recently. Changing the composition in perovskites is important for improving the performance of PSCs. In this work, PSCs with mixed MA/FA and Br/I components are fabricated. By changing the cation/anion composition, the MA0.7FA0.3Pb(I0.72Br0.18)3 PSC shows the negligible I–V hysteresis and the enhanced power conversion efficiency. And the statistics result also shows that devices with proper halide composition have a smaller discreteness in the device performance. We demonstrate that the halide ratio is essential for the formation of high-quality perovskite film with longer radiative carrier recombination lifetime, uniform crystal size, and better surface morphology. It is shown that the halide composition in the mixed PSCs could greatly affect the I–V hysteresis and power conversion efficiency (PCE). A proper Br ratio is important to help the device to alleviate the hysteresis and enhance PCE.

Lighting up silicon nanoparticles with Mie resonances

As one of the most important semiconductors, silicon has been used to fabricate electronic devices, waveguides, detectors, solar cells, etc. However, the indirect bandgap and low quantum efficiency (10−7) hinder the use of silicon for making good emitters. For integrated photonic circuits, silicon-based emitters with sizes in the range of 100−300 nm are highly desirable. Here, we show the use of the electric and magnetic resonances in silicon nanoparticles to enhance the quantum efficiency and demonstrate the white-light emission from silicon nanoparticles with feature sizes of ~200 nm. The magnetic and electric dipole resonances are employed to dramatically increase the relaxation time of hot carriers, while the magnetic and electric quadrupole resonances are utilized to reduce the radiative recombination lifetime of hot carriers. This strategy leads to an enhancement in the quantum efficiency of silicon nanoparticles by nearly five orders of magnitude as compared with bulk silicon, taking the three-photon-induced absorption into account.

(Invited) Design and Modeling of Sigesn Lasers: From Modeling Experiments to Future Device Concepts

We will present modeling of experimentally demonstrated SiGeSn multi-quantum well lasers. In particular, the impact of radiative recombination lifetime reduction in multi-quantum wells as well as temperature dependent L-valley electron concentrations will be shown to yield adequate modeling of these devices. Models will be further extrapolated to yield performance requirements for electrically pumped and room temperature lasing operation.

Radiative Recombination, Carrier Capture at Traps, and Photocurrent Relaxation in PbSnTe:In with a Composition Close to Band Inversion

Based on the notions that PbSnTe:In is a direct-gap semiconductor, the radiative-recombination lifetime is calculated, and the photocurrent relaxation and dependences of the instantaneous electron and hole lifetime are calculated under the assumption that PbSnTe:In is a disordered structure containing capture centers. These calculations explain such experimentally observed peculiarities of PbSnTe:In as a high photosensitivity in a wide wavelength range, pinning of the Fermi level, and long-term photocurrent relaxation. Calculations are also compared with experimental data and the possible parameters of photodetectors are evaluated.

Излучательная рекомбинация, захват носителей заряда на ловушки и релаксация фототока в PbSnTe : In с составом вблизи инверсии зон

AbstractBased on the notions that PbSnTe:In is a direct-gap semiconductor, the radiative-recombination lifetime is calculated, and the photocurrent relaxation and dependences of the instantaneous electron and hole lifetime are calculated under the assumption that PbSnTe:In is a disordered structure containing capture centers. These calculations explain such experimentally observed peculiarities of PbSnTe:In as a high photosensitivity in a wide wavelength range, pinning of the Fermi level, and long-term photocurrent relaxation. Calculations are also compared with experimental data and the possible parameters of photodetectors are evaluated.

Explanation of low efficiency droop in semipolar (202¯1¯) InGaN/GaN LEDs through evaluation of carrier recombination coefficients.

We report the carrier dynamics and recombination coefficients in single-quantum-well semipolar (202¯1¯) InGaN/GaN light-emitting diodes emitting at 440 nm with 93% peak internal quantum efficiency. The differential carrier lifetime is analyzed for various injection current densities from 5 A/cm2 to 10 kA/cm2, and the corresponding carrier densities are obtained. The coupling of internal quantum efficiency and differential carrier lifetime vs injected carrier density (n) enables the separation of the radiative and nonradiative recombination lifetimes and the extraction of the Shockley-Read-Hall (SRH) nonradiative (A), radiative (B), and Auger (C) recombination coefficients and their n-dependency considering the saturation of the SRH recombination rate and phase-space filling. The results indicate a three to four-fold higher A and a nearly two-fold higher B0 for this semipolar orientation compared to that of c-plane reported using a similar approach [A. David and M. J. Grundmann, Appl. Phys. Lett. 96, 103504 (2010)]. In addition, the carrier density in semipolar (202¯1¯) is found to be lower than the carrier density in c-plane for a given current density, which is important for suppressing efficiency droop. The semipolar LED also shows a two-fold lower C0 compared to c-plane, which is consistent with the lower relative efficiency droop for the semipolar LED (57% vs. 69%). The lower carrier density, higher B0 coefficient, and lower C0 (Auger) coefficient are directly responsible for the high efficiency and low efficiency droop reported in semipolar (202¯1¯) LEDs.

Polychromatic emission from polar-plane-free faceted InGaN quantum wells with high radiative recombination probabilities

We fabricated three-dimensional (3D) InGaN quantum wells (QWs) on semipolar GaN substrates by selective area epitaxy. It was found that polar-plane-free 3D structures can be fabricated with a particular mask pattern on the plane and that InGaN/GaN QWs on these structures exhibit facet-dependent polychromatic emission properties. In addition, time-resolved photoluminescence spectroscopy indicated that the radiative recombination lifetimes of the obtained 3D QWs without the polar (0001) plane are much shorter than those of conventional 3D QWs with the (0001) plane, particularly at long wavelengths.

Efficient Flexible Solar Cell based on Composition-Tailored Hybrid Perovskite.

Organic-inorganic hybrid perovskites (OIHPs) are new photoactive layer candidates for lightweight and flexible solar cells due to their low-temperature process capability; however, the reported efficiency of flexible OIHP devices is far behind those achieved on rigid glass substrates. Here, it is revealed that the limiting factor is the different perovskite film deposition conditions required to form the same film morphology on flexible substrates. An optimized perovskite film composition needs a different precursor ratio, which is found to be essential for the formation of high-quality perovskite films with longer radiative carrier recombination lifetime, smaller density of trap states, reduced precursor residue, and uniform and pin-hole free films. A record efficiency of 18.1% is achieved for the flexible perovskite solar-cell devices made on an indium tin oxide/poly(ethylene terephthalate) substrate via a low temperature (≤100 °C) solution process.

Theoretical and experimental analysis of radiative recombination lifetimes in nonpolar InGaN/GaN quantum dots

AbstractWe present here a combined experimental and theoretical analysis of the radiative recombination lifetime in a‐plane (110) InGaN/GaN quantum dots. The structures have been grown by modified droplet epitaxy and time‐resolved photoluminescence measurements have been performed to gain insight into the radiative lifetimes of these structures. This analysis is complemented by multi‐band calculations. To account for excitonic effects, the theory is coupled with self‐consistent Hartree calculations. Special attention is paid to the impact of the quantum dot size on the results. Our calculations show that the residual built‐in fields in these nonpolar structures are compensated by the attractive Coulomb interaction, leading to the situation that the oscillator strength is almost unaffected by changes in the quantum dot size. Furthermore, our theoretical studies reveal that the radiative lifetimes are one order magnitude lower than values for c‐plane systems of identical size and shape. Our theoretical findings are consistent with experimental results. Also, the calculated lifetimes are comparable in magnitude to the measured values. The majority of the measured dots produce lifetime values of 250–300 ps, highlighting the potential of these nanostructures for future high‐speed single‐photon emitters.

Investigation of the exciton emission lifetime in type-II spherical core/shell semiconductor heteronanostructures

The two-band model effective mass approximation has been adopted to explain the energy spectra in type-I CdSe core-only and type-II CdSe/CdTe core/shell quantum dots (QDs). As optical properties, the emission wavelength, the electron-hole overlap integral and the radiative recombination lifetime have been investigated. The simulated emission spectra are in good agreement with available experimental results for both core-only and core/shell QDs. The radiative recombination lifetime (τrad) has been investigated in different carrier localization regimes and compared to that corresponding to core-only QDs. We have found a sudden increase in τrad at around r1~1.1nm suggesting the transition of the heterostructure from the quasi-type-II to the type-II regime. A monotonic increase in τrad with the core and shell sizes (geometric parameters) was observed. Also found is the possibility of increasing τrad over two orders of magnitude with a suitable change in the geometric parameters. The long radiative lifetime produced by increasing the geometric parameters is found due to spatial separation of the carriers, which makes the type-II core/shell QDs made from large core and shell sizes promising for photovoltaic applications.

Cu‐Doped Carbon Dots with Highly Ordered Alignment in Anisotropic Nano‐Space for Improving the Photocatalytic Performance

Extensive research is directed to further prolong the lifetime of charge carriers and to increase the rate of electron transfer for nano‐photoactive catalysts. Especially, spin‐dependent electron‐hole recombination process has gained much attention. However, the direct effect of spin degree on photo‐catalysis performance is largely unexplored, which hinders the advancement of nano‐catalyst design. Here, a model catalyst system is constructed by combing Cu2+, carbon dots (CDs), and layered double hydroxides (LDHs) in order to address the above problems. The results from experimental and theoretical calculations show that: 1) the confined nano‐interlayer of anisotropic LDHs provides Cu‐CDs with highly aligned dipoles and enhanced spin‐orbit coupling, which leads to the forbidden radiative recombination and prolonged exciton lifetime; 2) LDHs as nano‐reactors provide a confined microenvironment for catalysis reaction, which shortens the electron transfer pathway and reduces the exciton loss. Based on these improved properties, the photocatalytic efficiency of Cu‐CDs‐LDHs in photo‐oxidation reaction of 1,4‐dihydro‐2,6‐dimethylpyridine‐3,5‐dicarboxylateis (1,4‐DHP) is 1.54 times higher than pure CDs.

Erratum: “Microwave extraction method of radiative recombination lifetime and photon lifetimes up to 85 °C on 50 Gb/s oxide-vertical cavity surface emitting laser” [J. Appl. Phys. 120, 223103 (2016)

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Effect of electric field on carrier escape mechanisms in quantum dot intermediate band solar cells

Carrier escape and recombination from quantum dot (QD) states reduce the probability of two-step photon absorption (TSPA) by decreasing the available carrier population in the intermediate band (IB). In order to optimize the second photon absorption for future designs of quantum dot embedded intermediate band solar cells, the presented study combined the results of simulations and experiments to quantify the effect of electric field on the barrier height and the carrier escape from the QDs in InAs/GaAs quantum dot solar cells with five-layer QD superlattices. The electric field dependent effective barrier heights for ground state electrons were calculated using eight band k·p theory at short circuit conditions. With an increase in electric field surrounding the QDs from 5 kV/cm to 50 kV/cm, the effective barrier height of the ground state electrons was reduced from 147 meV to 136 meV, respectively. Thus, the increasing electric field not only exponentially enhances the ground state electron tunneling rate (effectively zero at 5 kV/cm and 7.9 × 106 s−1 at 50 kV/cm) but also doubles the thermal escape rate (2.2 × 1011 s−1 at 5 kV/cm and 4.1 × 1011 s−1 at 50 kV/cm). Temperature-dependent external quantum efficiency measurements were performed to verify that the increasing electric field decreases the effective barrier height. Additionally, the electric field dependent radiative lifetimes of the ground state were characterized with time-resolved photoluminescence experiments. This study showed that the increasing electric field extended the radiative recombination lifetime in the ground state of the QDs as a consequence of the reduced wave-function overlap between the electrons and holes. The balance of carrier escape and recombination determines the probability of TSPA.

Microwave extraction method of radiative recombination and photon lifetimes up to 85 °C on 50 Gb/s oxide-vertical cavity surface emitting laser

Optical modulation bandwidth for a semiconductor diode laser is governed by the thermally limited spontaneous radiative recombination lifetime, τrec, photon lifetime, τp, and cavity photon density for stimulated recombination. Thus, temperature dependent recombination lifetime is a critical parameter for the limitation of photonic device operations. Here, we develop a microwave extraction method to accurately determine the radiative recombination and photon lifetimes over a temperature range up to 85 °C through the equivalent circuit modeling based on the measured microwave scattering-parameters. For an 850 nm oxide-vertical cavity surface emitting laser with error free data transmission capability over 50 Gb/s, the extracted lifetimes are τrec = 0.1778 ns and τp = 4.2 ps at 25 °C and τrec = 0.2445 ns and τp = 4.8 ps at 85 °C.