Time-resolved Photoluminescence Measurements Research Articles

Improved Modulation Bandwidth of Blue Mini-LEDs by Atomic-Layer Deposition Sidewall Passivation

By adopting atomic layer deposition (ALD) sidewall passivation, the electroluminescence (EL) and communication performances of mini-light emitting diodes (mini-LEDs) of different sizes were improved. In particular, the most significant improvement in the electroluminescence properties of the external quantum efficiency (EQE) (a 7.2% increase), leakage current, and the communication properties of the modulation bandwidth (a 20% increase) transmission rate and bit error rate (BER) was found in the smallest mini-LEDs (80 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}\,\,\times $ </tex-math></inline-formula> 120 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> ). According to the results of time-resolved photoluminescence (TRPL) measurements, the carrier lifetime of the samples can be affected by both the size and ALD sidewall passivation. In addition, the effects of ALD sidewall passivation for visible-light communication (VLC) were demonstrated.

An intermediate model for fitting triplet–triplet annihilation in phosphorescent organic light emitting diode materials

Triplet–triplet annihilation (TTA) is one of the primary contributors to efficiency roll-off and permanent material degradation in phosphorescent organic light-emitting diodes. The two limiting case models typically used to quantify this quenching mechanism are multi-step Dexter and single-step Förster, which, respectively, assume ideal Fickian diffusion or perfect trapping of triplet excitons. For device-relevant guest doping levels (typically 5–12 vol. %), both significant diffusion of excitons and trapping due to spatial and energetic disorder exist, so neither conventional model fits experimental data well. We develop and validate an intermediate TTA model, which is a weighted average of the limiting cases of pure radiative decay (no TTA) and multi-step Dexter based TTA that returns an effective TTA rate constant and a parameter quantifying the portion of well-isolated excitons. Kinetic Monte-Carlo simulations and time-resolved photoluminescence measurements of an archetype host–guest system demonstrate that our intermediate model provides significantly improved fits with more realistic physical values, is more robust to variations in experimental conditions, and provides an analysis framework for the effects of trapping on TTA.

Efficient interfacial charge transfer of 2D/2D CoP/CdS heterojunction for extremely boosted photocatalytic H2 evolution performance

Constructing 2D/2D heterojunction photocatalysts has attracted great attentions due to their inherent advantages such as larger interfacial contact areas, short transfer distance of charges and abundant reaction active sites. Herein, 2D/2D CoP/CdS heterojunctions were successfully fabricated and employed in photocatalytic H2 evolution using lactic acid as sacrificial reagents. The multiple characteristic techniques were adopted to investigate the crystalline phases, morphologies, optical properties and textual structures of heterojunctions. It was found that integrating 2D CoP nanosheets as cocatalysts with 2D CdS nanosheets by Co–S chemical bonds would significantly boost the photocatalytic H2 evolution performances, and the 7 wt% 2D/2D CoP/CdS heterojunction possessed the maximal H2 evolution rate of 92.54 mmol g−1 h−1, approximately 31 times higher than that of bare 2D CdS nanosheets. Photoelectrochemical, steady photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements indicated that there existed an effective charge separation and migration over 2D/2D CoP/CdS heterojunction, which then markedly lengthened the photoinduced electrons average lifetimes, retarded the recombination of charge carriers, and caused the dramatically boosted photocatalytic H2 evolution activity. Moreover, the density functional theory (DFT) calculation further corroborated that the efficient charge transfer occurred at the interfaces of CoP/CdS heterojunction. This present research puts forward a promising strategy to engineer the 2D/2D heterojunction photocatalysts endowed with an appealing photocatalytic H2 evolution performance.

Optimization of the Pechini-derived synthesis of rare-earth free aluminum borate phosphors presenting tunable white emission

The present study is dedicated to the development of aluminum borate luminescent powders which appear as promising and more environmental-friendly than conventional phosphors. The different steps of the luminescent powder synthesis, in particular the precursors (inorganic and organic) ratios and calcination conditions, are adjusted by studying the resulting optical features to get a broader and warmer white emission. As shown in previous works, the photoluminescence emission originates from polycyclic aromatic hydrocarbons (PAH) trapped in the inorganic aluminum borate matrix. Morphological and structural properties of the particles remain otherwise identical for every optimized parameter. The aluminum borate powder synthesized in these optimized conditions exhibits a wide and intense emission band under near-UV excitation. These structural and optical properties combined with time-resolved photoluminescence measurements demonstrate that the adjustments of the synthetic process allow the formation and trapping of more and different emitting centers. The modulation of the excitation wavelength (from 305 to 405 nm) leads to a tunable photoluminescence emission characterized by a large band lying between 400 and 700 nm. This feature, associated with the use of secure and abundant precursors makes aluminum borate powders very interesting phosphors for optical applications.

Experimental and modeling analysis of photocarrier dynamics in CZTSSe using temperature-dependent photoluminescence

CZTSSe shows great potential in photovoltaic applications. However, the device efficiency is limited by the high density detrimental defects in the absorber. Therefore, investigating the influence of defects on absorber properties is essential. In this study, temperature-dependent steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements were performed on a CZTSSe film. Four emission components involving defect-associated transitions were obtained by analyzing the PL spectra. Beyond the traditional exponential analysis, a diffusion-recombination model was applied to simulate the TRPL curves, by which the key parameters describing the carrier recombination and diffusion were extracted. The model includes defect-mediated and band-involved recombination rates, surface/interface recombination velocities and their corresponding temperature dependences. The results indicate that the increasing Shockley–Read–Hall recombination rate is the dominant factor that leads to the enhanced PL quenching with increasing temperature. Our results help to further explain the key factors that influence the optoelectronic properties of CZTSSe.

Ultra-small size (1–20 μ m) blue and green micro-LEDs fabricated by laser direct writing lithography

Ultra-small micro-LEDs are essential for next-generation display technology. However, micro-LEDs below 5 μm have been seldom reported. In this work, we demonstrate InGaN-based blue and green micro-LEDs from 1 to 20 μm by using laser direct writing lithography. The 1-μm blue micro-LEDs show a peak external quantum efficiency of 13.02%, which is 9.57% for green ones. By characterizing the size-dependent external quantum efficiency and simply assuming that this variety is dominantly determined by the dry-etching induced dead zone, we deduce that the dead zone sizes of carrier injection at the edge of chips are 0.18 and 0.15 μm in blue and green ones, respectively. A time-resolved photoluminescence measurement also shows that carrier lifetime reduction at the edge of blue ones is more serious than that of green ones, reflecting the easier carrier lateral diffusion in the former than the latter. These results exhibit the ability of laser direct writing lithography on micro-LED fabrication and also provide a reference for predicting the limit of their chip size scaling-down.

(Invited) Characterization of Semiconductor Crystals Based on Omnidirectional Photoluminescence (ODPL) Spectroscopy

The performance of optical devices based on nitride semiconductors operating in the ultraviolet and visible wavelength regimes has been remarkably improved. For example, high-efficiency blue InGaN light-emitting diodes (LEDs) with an external quantum efficiency (EQE) of over 80% was achieved [Y. Narukawa, M. Ichikawa1, D. Sanga, M. Sano, and T. Mukai, J. Phys. D: Appl. Phys. 43, 354002 (2010)], and green laser diodes (LDs) was realized [Y. Enya, Y. Yoshizumi, T. Kyono, K. Akita, M. Ueno, M. Adachi, T. Sumitomo, S. Tokuyama, T. Ikegami, K. Katayama and T. Nakamura, Appl. Phys. Express 2, 082101 (2009)]. Recently, deep-ultraviolet AlGaN LEDs and LDs have also become hot topics. The nitride semiconductors have been actively applied not only to optical applications but also to electronic devices such as high-voltage transistors and high-electron mobility transistors based on their high breakdown voltage and saturation electron velocity. The performance of these optical and electronic devices relies on the crystal growth technology, where quality of GaN freestanding substrate is one of the most important issues.In recent years, it has become common for the "high-quality" GaN freestanding crystals to have a threading dislocation (TD) density of 106 cm-2 or less. TD density of 106 cm-2 corresponds to an average distance between TDs of 10 μm, which means that the TDs are sparser than 1 μm, the order of the carrier diffusion length. In other words, the TDs are no longer dominant in recombination processes of minority carriers (although this is a rather broad argument). In this case, the recombination processes, especially nonradiative recombination process, are likely to be dominated by point defects and impurities rather than by the sparse TDs.In fact, a negative correlation between room-temperature (RT) photoluminescence (PL) lifetime for the near-band-edge (NBE) emission and the concentration of VGaVN (a divacancy consisting of Ga and N vacancies) has been pointed out by a combined study of time-resolved PL measurements, positron annihilation spectroscopy, and theoretical calculations [S. F. Chichibu, A. Uedono, K. Kojima, H. Ikeda, K. Fujito, S. Takashima, M. Edo, K. Ueno, and S. Ishibashi, J. Appl. Phys. 123, 161413 (2018)]. Since PL lifetime measured at RT indicates nonradiative recombination lifetime when the internal quantum efficiency (IQE) of the crystal is low, this negative correlation means that the lower concentration of VGaVN induces larger intensity of the NBE emission.Actually, we have observed PL lifetime exceeding 2 ns at RT in a GaN crystal grown by halide vapor phase epitaxy (HVPE) on m-plane GaN grown by the acidic ammonothermal method as a seed crystal [K. Kojima, Y. Tsukada, E. Furukawa, M. Saito, Y. Mikawa, S. Kubo, H. Ikeda, K. Fujito, A. Uedono, and S. F. Chichibu, Appl. Phys. Express 8, 095501 (2015)]. This is an extremely long value for a GaN crystal, which means that the nonradiative recombination lifetime is long, i.e., the IQE of the NBE emission is large. In this case, the validity of the approximation where the PL lifetime at RT is directly read into the nonradiative recombination lifetime becomes an issue.Therefore, in order to compare the quality of GaN freestanding crystals, it is important to measure the IQE as well as the PL lifetime. The IQE value for the NBE emission in semiconductor crystals is a quantity that represents the balance between radiative and nonradiative recombination processes and is directly related to the concentration of point defects and impurities that form deep levels. Therefore, the IQE can be used as an indicator of not only the energy-saving performance of optical devices but also the performance of GaN crystals as a substrate material in electronic devices.In this presentation, the characterization of semiconductors with direct bandgap by using omnidirectional photoluminescence (ODPL) spectroscopy, a technique that can determine the IQE from the EQE without the need for model calculations, will be reviewed. Using GaN, ZnO, and metal halide perovskites as examples, specific procedures and applications of IQE determination will be discussed. In particular, the ability to measure large sample crystals or wafers is one of the major features of the ODPL spectroscopy, and it is expected to be combined with mapping measurements of the entire surface of semiconductor wafers and various types of nonlinear and microscopic spectroscopy.

A novel sulfide phosphor, BaNaAlS3:Eu2+, discovered via particle swarm optimization

A novel quaternary sulfide phosphor, BaNaAlS3:Eu2+, was discovered emitting a cyan light that peaks at around 495 nm with a full width half maximum (FWHM) of 50 nm, and its utility for LED applications was suggested. The exact composition of the phosphor was identified via particle swarm optimization (PSO) in terms of the PL intensity. The PSO-nominated composition resulted in the optimal crystallinity and the highest PL intensity. Density functional theory (DFT) calculations also supported the experimentally measured band gap for the PSO-nominated BaNaAlS3:Eu2+ in a Pnma structure. Neither the typical concentration quenching behavior nor the typical decay-rate dependence on the probe wavelength was detected via time-resolved photoluminescence (TRPL) measurement. The absence of these typical decay traits was evident based on a relatively inactive rate of inter-activator energy transfer compared with that of other well-known Eu2+-activated phosphors.

Effect of Crystallinity on Endoergic Singlet Fission in Perylenediimide Single Crystals and Polycrystalline Films

The triplet state energy of bis(3′-aminopentyl)-perylene-(3,4:9,10)-bis(dicarboximide) (C5PDI) in the solid state is 1.1 eV, so that achieving singlet fission (SF) in crystalline films of C5PDI can provide a potential means of delivering triplet excitons to silicon-based solar cells, whose band gap is also 1.1 eV, to enhance their performance by utilizing blue light in the solar spectrum. Here, we use transient absorption spectroscopy and microscopy to assess the effect of solid-state order on SF dynamics by comparing C5PDI single crystals and thin polycrystalline films. The X-ray single-crystal structure of C5PDI shows that it forms π-stacked dimers, wherein the PDIs are twisted ∼51° relative to one another. Formation of the correlated triplet pair state 1(T1T1) in the C5PDI single crystals occurs in τ = 56 ± 4 ps mediated by a mixed state having both excited singlet and charge-transfer character, while in a solvent-vapor-annealed C5PDI polycrystalline thin film, 1(T1T1) formation occurs in τ = 169 ± 6 ps. The quantum yield of the 1(T1T1) state formation in each case is nearly unified, yet the free triplet exciton quantum yield in the single crystals is 70%, while that in the annealed polycrystalline film is only 29%. Steady-state and time-resolved photoluminescence measurements indicate that the disorder in the polycrystalline film hinders free triplet excitons via long-lived excimer trap states at sites with suboptimal electronic coupling. The higher free triplet yield in the single crystal also clearly shows that the high degree of molecular order in the crystal enables competition between triplet annihilation and diffusional escape, which is critical for utilizing the triplet excitons to enhance solar cell performance.

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Improved minority carrier lifetime in p-type GaN by suppressing the non-radiative recombination process

Time-resolved photoluminescence and capacitance-voltage measurement were performed on p-type GaN and InGaN films to study the minority carrier recombination mechanism. The minority carrier lifetime (τ PL) for p-GaN with a Mg concentration of 1.7 × 1019 cm−3 was 46 ps. The non-radiative recombination due to gallium vacancies (V Ga)-related defects is confirmed to dominate the minority carrier transport process. To suppress the formation of V Ga defects, the indium atoms were added into p-GaN. As a consequence, the V Ga-related non-radiative recombination centers were reduced from 8 × 1015 to 5 × 1014 cm−3 and a record long τ PL of 793 ps was obtained for p-In0.035Ga0.95N film.

Improving the Performance of Direct Bonded Five-Junction Solar Cells by Optimization of AlInP Window Layer

It is well-known that the quantum efficiency (QE) of inverted AlGaInP solar cells is less than that of upright ones, and the mechanism has not been well-explained. In this paper, a Si-doped AlInP window layer, compared with an emitter layer, is revealed to be one more important factor that decreases QE. It is noted that the quality of a heavily Si-doped AlInP window layer would decrease and further deteriorate subsequent active layers. An optimization strategy of a Si-doped AlInP window layer is proposed, which proves effective through time-resolved photoluminescence measurements (TRPL) of double heterojunctions. Inverted 2.1 eV AlGaInP solar cells with an improved AlInP window layer are fabricated. A 60 mV Voc increment is achieved with a remarkable enhancement of the fill factor from 0.789 to 0.827. An enhanced QE of 10% to 20% is achieved at short-wavelength and the peak IQE rises from 83.3% to 88.2%, which presents a nearly identical IQE compared with the upright reference. Further optimization in GaAs homojunction sub-cells is performed by introducing an n-GaInP/p-GaAs heterojunction structure, which decreases the recombination loss in the emitter caused by a poor AlInP window layer. The optimized structure significantly improves the Voc of the inverted GaAs-based T-3J solar cells to 3830 mV, boosting the efficiency of SBT five-junction solar cells to 35.61% under AM0 illumination.

Ladder-like Polymer Brushes Containing Conjugated Poly(Propylenedioxythiophene) Chains.

The high stability and conductivity of 3,4-disubstituted polythiophenes such as poly(3,4-ethylenedioxythiophene) (PEDOT) make them attractive candidates for commercial applications. However, next-generation nanoelectronic devices require novel macromolecular strategies for the precise synthesis of advanced polymer structures as well as their arrangement. In this report, we present a synthetic route to make ladder-like polymer brushes with poly(3,4-propylenedioxythiophene) (PProDOT)-conjugated chains. The brushes were prepared via a self-templating surface-initiated technique (ST-SIP) that combines the surface-initiated atom transfer radical polymerization (SI-ATRP) of bifunctional ProDOT-based monomers and subsequent oxidative polymerization of the pendant ProDOT groups in the parent brushes. The brushes prepared in this way were characterized by grazing-angle FTIR, XPS spectroscopy, and AFM. Steady-state and time-resolved photoluminescence measurements were used to extract the information about the structure and effective conjugation length of PProDOT-based chains. Stability tests performed in ambient conditions and under exposure to standardized solar light revealed the remarkable stability of the obtained materials.

Ternary Chalcogenide-Based Quantum Dots and Carbon Nanotubes: Establishing a Toolbox for Controlled Formation of Nanocomposites

A general procedure based on electrostatic self-assembly for preparing nanocomposites based on carbon nanotubes (CNTs) and ternary chalcogenide semiconductor nanoparticles is shown. This was achieved by surface functionalization of the single components through well-established protocols, for CNTs, and a transferable general strategy for the nanoparticles. Heterostructures were then synthesized through electrostatic interaction between oppositely charged components. Structural, colloidal, and optical properties were characterized by transmission electron microscopy, X-ray diffraction, infrared spectroscopy, dynamic light scattering, ζ-potential, and absorption- and (time-resolved) photoluminescence measurements. Interestingly, the nanocomposites showed a blue shift in their excitation and emission spectra when compared to the pure nanoparticles but only when analyzed in powder form. Further investigations in the form of density functional theory (DFT) calculations were performed to evaluate the origin of the change in the optical properties.

Evaluation of PbTe and SnTe as ohmic contact layers in CdTe solar cell devices

For solar cells based on CdTe, the choice of a suitable back contact material is limited by CdTe’s deep work function. Here, we explored p-type PbTe and SnTe as ohmic contacts to CdTe. These contact layers were grown on single crystal CdTe substrates by molecular beam epitaxy, and the valence band offset between film and substrate was measured using X-ray photoemission spectroscopy. Polycrystalline device structures were also grown by sublimation to assess performance improvements. Doping was achieved in PbTe by thallium incorporation. Only the highest Tl doped PbTe resulted in a desirable band alignment with the CdTe, forming an electron reflector and no hole barrier. Time-resolved photoluminescence measurements also revealed significant photocarrier lifetime improvements for only the highest doped PbTe. Consequentially, devices incorporating the highest doped PbTe layers showed increased power conversion efficiency, primarily from increased fill factor. Doping of the PbTe was measured via Hall effect with variable magnetic field, which was required due to the formation of an n-type parasitic interface layer. To properly interpret the variable field Hall measurements, we derived an expression for the magnetic field-dependent conductivity tensor of an L-valley semiconductor.

Excitation dependent and time resolved photoluminescence of β-Ga2O3, β-(Ga0.955Al0.045)2O3 and β-(Ga0.91In0.09)2O3 epitaxial layers grown by pulsed laser deposition

We have performed steady state and time-resolved photoluminescence measurements on β-Ga2O3 and β-(Ga0.955Al0.045)2O3 and β-(Ga0.91In0.09)2O3 mixed crystals grown by pulsed laser deposition. The steady state luminescence spectra of all the samples are characterized by transitions in the form of peaks and shoulders. In both Ga2O3 and the alloy oxides there are two transitions in the wavelength ranges of 330–381 nm (ultraviolet) and 486–492 nm (blue). Additionally, in Ga2O3, emissions at 596 nm (yellow) and 734 nm (near infrared) are also observed. Only the yellow transition is observed in (Ga0.955Al0.045)2O3 at 582 nm and neither are observed in (Ga0.91In0.09)2O3. Steady state and time-resolved photoluminescence measurements were also made as a function of excitation intensity. The trend of the variation of integrated and peak intensity, peak position and recombination lifetimes of the transitions with excitation are examined and discussed in the context of the underlying recombination mechanisms.

Cesium Manganese Bromide Nanocrystal Sensitizers for Broadband Vis-to-NIR Downshifting.

Simultaneously achieving both broad absorption and sharp emission in the near-infrared (NIR) is challenging. Coupling of an efficient absorber such as lead halide perovskites to lanthanide emissive species is a promising way to meet the demands for visible-to-NIR spectral conversion. However, lead-based perovskite sensitizers suffer from relatively narrow absorption in the visible range, poor stability, and toxicity. Herein, we introduce a downshifting configuration based on lead-free cesium manganese bromide nanocrystals acting as broad visible absorbers coupled to sharp emission in the NIR-I and NIR-II spectral regions. To achieve this, we synthesized CsMnBr3 and Cs3MnBr5 nanocrystals and attempted to dope them with a series of lanthanides, achieving success only with CsMnBr3. The correlation of the lanthanide emission to the CsMnBr3 visible absorption was confirmed with steady-state excitation spectra and time-resolved photoluminescence measurements, whereas the mechanism of downconversion from the CsMnBr3 matrix to the lanthanides was understood by density functional theory calculations. This study shows that lead-free metal halides with an appropriate phase are effective sensitizers for lanthanides and offer a route to efficient downshifting applications.

Electrically Controlled Wavelength-Tunable Photoluminescence from van der Waals Heterostructures.

Achieving facile control of the wavelength of light emitters is of great significance for many key applications in optoelectronics and photonics, including on-chip interconnection, super-resolution imaging, and optical communication. The Joule heating effect caused by electric current is widely applied in modulating the refractive index of silicon-based waveguides for reconfigurable nanophotonic circuits. Here, by utilizing localized Joule heating in the biased graphene device, we demonstrate electrically controlled wavelength-tunable photoluminescence (PL) from vertical van der Waals heterostructures combined by graphene and two-dimensional transition metal dichalcogenides (2D-TMDCs). By applying a moderate electric field of 6.5 kV·cm-1 to the graphene substrate, the PL wavelength of 2D-TMDCs exhibits a continuous tuning from 662 to 690 nm, corresponding to a bandgap reduction of 76 meV. The electric control is highly reversible during sweeping the bias back and forth. The temperature dependence of Raman and PL spectroscopy reveals that the current-induced local Joule heating effect plays a leading role in reducing the optical direct bandgap of TMDCs. The bias-dependent optical reflectivity and time-resolved photoluminescence measurements show a consistent reduction of the optical band gap of 2D-TMDCs and increased PL lifetimes with the electric field over the heterostructures. Moreover, we demonstrate the consistent device operation from 2D materials grown by chemical vapor deposition, showing great advantages for the scalability.

Extracting Decay-Rate Ratios From Photoluminescence Quantum Efficiency Measurements in Optoelectronic Semiconductors

New materials are needed to achieve cheap, efficient solar panels, LEDs, and other optoelectronic devices. However, key measurements to reveal the fundamental mechanisms dictating device optimization are typically long and expensive. This study shows that the same level of understanding can be achieved far more rapidly by combining measurements of photoluminescence quantum efficiency and time-resolved photoluminescence. These results plus ultraviolet-visible absorption measurements allow device efficiency to be predicted, providing a tool for the rapid screening and optimization of materials, which is of interest to spectroscopists and device fabricators in both academia and industry.

Distance-dependent resonance energy transfer in alkyl-terminated Si nanocrystal solids.

Understanding and controlling the energy transfer between silicon nanocrystals is of significant importance for the design of efficient optoelectronic devices. However, previous studies on silicon nanocrystal energy transfer were limited because of the strict requirements to precisely control the inter-dot distance and to perform all measurements in air-free environments to preclude the effect of ambient oxygen. Here, we systematically investigate the distance-dependent resonance energy transfer in alkyl-terminated silicon nanocrystals for the first time. Silicon nanocrystal solids with inter-dot distances varying from 3 to 5nm are fabricated by varying the length and surface coverage of alkyl ligands in solution-phase and gas-phase functionalized silicon nanocrystals. The inter-dot energy transfer rates are extracted from steady-state and time-resolved photoluminescence measurements, enabling a direct comparison to theoretical predictions. Our results reveal that the distance-dependent energy transfer rates in Si NCs decay faster than predicted by the Förster mechanism, suggesting higher-order multipole interactions.