Time-resolved Photoluminescence Lifetime Research Articles

Electrically tunable non-radiative lifetime in WS2/WSe2 heterostructures.

Van der Waals heterostructures based on transition metal dichalcogenides (TMDs) have emerged as excellent candidates for next-generation optoelectronics and valleytronics, due to their fascinating physical properties. The understanding and active control of the relaxation dynamics of heterostructures play a crucial role in device design and optimization. Here, we investigate the back-gate modulation of exciton dynamics in a WS2/WSe2 heterostructure by combining time-resolved photoluminescence (TRPL) and transient absorption spectroscopy (TAS) at cryogenic temperatures. We find that the non-radiative relaxation lifetimes of photocarriers in heterostructures can be electrically controlled for samples with different twist-angles, whereas such lifetime tuning is not present in standalone monolayers. We attribute such an observation to doping-controlled competition between interlayer and intralayer recombination pathways in high-quality WS2/WSe2 samples. The simultaneous measurement of TRPL and TAS lifetimes within the same sample provides additional insight into the influence of coexisting excitons and background carriers on the photo-response, and points to the potential of tailoring light-matter interactions in TMD heterostructures.

Characterizing the Ligand Shell Morphology of PEG-Coated ZnO Nanocrystals Using FRET Spectroscopy.

Poly(ethylene glycol) (PEG) ligands can inhibit proteins and other biomolecules from adhering to underlying surfaces, making them excellent surface ligands for nanocrystal (NC)-based drug carriers. Quantifying the PEG ligand shell morphology is important because its structure determines the permeability of biomolecules through the shell to the NC surface. However, few in situ analytical tools can reveal whether the PEG ligands form either an impenetrable barrier or a porous coating surrounding the NC. Here, we present a Förster resonance energy transfer (FRET) spectroscopy-based approach that can assess the permeability of molecules through PEG-coated ZnO NCs. In this approach, ZnO NCs serve as FRET donors, and freely diffusing molecules in the bulk solution are FRET acceptors. We synthesized a series of variable chain length PEG-silane-coated ZnO NCs such that the longest chain length ligands far exceed the Förster radius (R0), where the energy transfer (EnT) efficiency is 50%. We quantified the EnT efficiency as a function of the ligand chain length using time-resolved photoluminescence lifetime (TRPL) spectroscopy within the framework of FRET theory. Unexpectedly, the longest PEG-silane ligand showed equivalent EnT efficiency as that of bare, hydroxyl-passivated ZnO NCs. These results indicate that the "rigid shell" model fails and the PEG ligand shell morphology is more likely porous or in a patchy "mushroom state", consistent with transmission electron microscopy data. While the spectroscopic measurements and data analysis procedures discussed herein cannot directly visualize the ligand shell morphology in real space, the in situ spectroscopy approach can provide researchers with valuable information regarding the permeability of species through the ligand shell under practical biological conditions.

Natural luminescent carbon nanoparticles extracted from coconut water

The synthesis methods of carbon nanoparticles (CNPs) are of utmost importance to the commercialization of CNPs as they need to have an efficient cost-to-gain. In this paper, we report on a facile method for extracting CNPs out of a renewable and green source of coconut water without the need for any time or energy consuming synthesis procedure. Namely, the CNPs were essentially extracted by filtration of coconut water using a dialysis bag to remove long-chain carbohydrates and bulk contaminants. The obtained CNPs were characterized using several techniques including high-resolution transmission electron microscope (HRTEM), photoluminescence (PL) spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), zeta potential analyzer, time-resolved photoluminescence lifetime measurement, and X-ray diffraction (XRD). It was found that the obtained CNPs have an average lateral size of 4.5 nm and interestingly, show UV and blue light emissions when excited optically by deep UV light. The latter indicates the potential application of these CNPs in optoelectronics. XPS data shows a C/O ratio of 65%/28% which is an expected result for the surface bonds of a CNPs and is in agreement with the FTIR data which demonstrates a dominance of carbon and oxygen-based function groups. The CNPs also demonstrate a long fluorescence lifetime of 5.3 ns which is relatively longer than that in literature.

Effect of an Ohmic back contact on the stability of Cu(In,Ga)Se2-based flexible bifacial solar cells

In this work, the influence of the Ohmic-contact behavior at the absorber/transparent back-contact interface on the stability of flexible bifacial Cu(In,Ga)Se2 (CIGSe) solar cells is investigated. In the case of the CIGSe/In2O3:SnO2 (ITO) interface, the Ohmic contact is maintained via the introduction of trap-assisted recombination at the CIGSe surface during the ITO deposition. Post-annealed CIGSe/ITO interfaces are studied via time-resolved photoluminescence (TRPL). It is found that the TRPL lifetime of all the samples investigated drastically decreases after the ITO deposition owing to sputtering damage, whereas the TRPL lifetime tends to increase after heat treatment at 160 °C. This increase is attributed to the partial recovery of the sputtering damage during annealing; the increase is larger in samples with less severe sputtering damage than in samples with more severe sputtering damage. Flexible bifacial solar cells with Ohmic-like contact at the CIGSe/ITO interface show superior performance and long-term stability compared with those with non-Ohmic contact at the interface, which may be correlated with the alteration of the metastability during the ITO deposition and/or partial recovery of the sputtering damage. The best flexible bifacial CIGSe solar cells with Ohmic-like back contact show stable performance for over 70 days with efficiencies of 11.1% and 3.0% for the frontside and backside illuminations, respectively.

Comparison of Triethylgallium and Trimethylgallium Precursors for GaInP Nanowire Growth

Nanowire (NW) arrays containing a top segment of GaxIn1–xP are investigated, comparing NWs grown using two different Ga precursors, trimethylgallium (TMGa) and triethylgallium (TEGa). TMGa is the precursor commonly used for the particle‐assisted vapor–liquid–solid (VLS) growth of GaxIn1–xP NWs. However, it shows inefficient pyrolysis at typical NW growth conditions. The use of the alternative precursor TEGa is investigated by making a direct comparison between NWs grown using TEGa and TMGa at otherwise identical growth conditions. Growth rates, resulting NW materials composition, and time‐resolved photoluminescence (TRPL) lifetimes are investigated. With increasing Ga content of the NWs, the TRPL lifetimes decrease, indicating trap states that are associated with GaP. Somewhat longer TRPL lifetimes for the samples grown using TEGa indicate a lower concentration of deep trap states. For doped NWs, it is found that the strong effect of the p‐type dopant diethylzinc (DEZn) on the NW composition, observed for GaxIn1–xP NWs grown using TMGa, is absent when using TEGa.

Interfacial engineering and down-conversion of ultraviolet light for efficient perovskite solar cells

The addition of a tiny amount of fluorescent organic material N, N′-di (naphtha-1-yl)-N, N′-diphenyl-benzidine (NPB) into polymeric hole transport layer (HTL) is shown to yield a significant enhancement in down-conversion of ultraviolet (UV) light into visible light. As a result, NPB incorporated HTL based devices exhibit enhancement in power conversion efficiency (PCE) of 18.2% compared to 15.1% for pristine devices. This work demonstrated that fluorescent down-conversion of UV light based on doped HTLs is a novel and effective approach for enhanced performance of perovskite solar cells (PSCs). In addition, the incorporated fluorescent material NPB provides a favorable energetic alignment at the interface for low voltage loss. These results are further supported by time-resolved photoluminescence (TRPL) measurements that are performed on perovskite layers deposited on top of NPB doped Poly (triaryl amine) (PTAA), which shows an improvement in lifetime of the charge carriers with increasing concentration of NPB. An increase in the lifetime of the charge carriers is observed on the perovskite layer that is deposited on top of 2.5 wt% NPB doped PTAA layer. The lifetime enhancement is due to additional charge carriers that are generated from the conversion of high energy photons to low energy photons. These additional charge carriers that are generated in HTL occupy the charge transport states after filling the trap states in the perovskite layer and hence the lifetime of the charge carriers increases. The TRPL lifetime decreases with further increase in the NPB concentration more than 2.5 wt% due to efficient charge carrier extraction.

ZnS Shells Enhance Triplet Energy Transfer from CdSe Nanocrystals for Photon Upconversion

Triplet energy transfer (TET) from semiconductor nanocrystals (NCs) to molecules is one of the bottlenecks that limits the efficiency of photon upconversion. While an inorganic shell can enhance the photoluminescence quantum yields (PLQYs), the role of the shell with respect to TET is still not clear. In this work, CdS and ZnS shells with different shell thickness are grown on 2.9 nm diameter CdSe NCs, resulting in nanostructures here that have increased radiative rates compared to the core. TET from these NCs to bound 9-anthracene carboxylic acid is investigated with linear photon upconversion measurements, time-resolved photoluminescence lifetime, and transient absorption spectroscopy. The ZnS shell enhances the photon upconversion QYs 1.6 times from 5.7% to 9.3%, with a concurrent increase of TET efficiency from 6.68% to 12.9% and the net rate of TET from 4.97 × 108 s–1 to 6.67 × 1010 s–1. In contrast, TET is barely observed for CdSe/CdS core–shell NCs. Considering the changes in PLQYs and upconversion...

Mapping charge carrier density in organic thin-film transistors by time-resolved photoluminescence lifetime studies

The device performance of organic transistors is strongly influenced by the charge carrier distribution. A range of factors effect this distribution, including injection barriers at the metal-semiconductor interface, the morphology of the organic film, and charge traps at the dielectric/organic interface or at grain boundaries. In our comprehensive experimental and analytical work we demonstrate a method to characterize the charge carrier density in organic thin-film transistors using time-resolved photoluminescence spectroscopy. We developed a numerical model that describes the electrical and optical responses consistently. We determined the densities of free and trapped holes at the interface between the organic layer and the SiO2 gate dielectric by comparison to electrical measurements. Furthermore by applying fluorescence lifetime imaging microscopy we determine the local charge carrier distribution between source and drain electrodes of the transistor for different biasing conditions. We observe the expected hole density gradient from source to drain electrode.

Time-resolved photoluminescence of Cu(In,Ga)(Se,S)2 thin films and temperature dependent current density-voltage characteristics of their solar cells on surface treatment effect

Influences of the surface treatments of Cu(In,Ga)(Se,S)2 (CIGSSe) thin films, which are KCN, HCl, or thiuorea treatments, were investigated by time-resolved photoluminescence (TRPL) and temperature dependent current density-voltage (J-V) characteristics of their solar cells. It is demonstrated that the KCN treatment optimized under 1 wt% leads to the significant increase in conversion efficiency (η) up to 19.21%. On the other hand, the η of the CIGSSe solar cells is in ranges of 13.70–15.51% and 9.86–10.70% with the HCl treatments (0.3–0.7 mol/L), and thiuorea treatments (0.5–1.5 mol/L), respectively, which are lower than 16.66% that of the reference solar cell without the surface treatment. According to TRPL measurements, the quality of near-surface CIGSSe is improved with the KCN treatment (1 wt%) owing to enhanced TRPL lifetimes, whereas that is deteriorated with the HCl and thiuorea treatments due to decreased TRPL lifetimes. In addition, according to the temperature-dependent J-V measurement, the interface recombination of the CIGSSe solar cell is decreased with the KCN treatment, while that of the CIGSSe solar cells is increased with the HCl and thiuorea treatments. Ultimately, 19.21%-efficient CIGSSe solar cell with the KCN treatment (1 wt%) at room temperature with the increased VOC of 0.692 V was obtained, which is around 15.3% relatively higher η than that of the solar cell without the surface treatment.

Carrier lifetimes in thin-film photovoltaics

The carrier lifetimes in thin-film solar cells are reviewed and discussed. Shockley-Read-Hall recombination is dominant at low carrier density, Auger recombination is dominant under a high injection condition and high carrier density, and surface recombination is dominant under any conditions. Because the surface photovoltage technique is insensitive to the surface condition, it is useful for bulk lifetime measurements. The photoconductance decay technique measures the effective recombination lifetime. The time-resolved photoluminescence technique is very useful for measuring thin-film semiconductor or solar-cell materials lifetime, because the sample is thin, other techniques are not suitable for measuring the lifetime. Many papers have provided time-resolved photoluminescence (TRPL) lifetimes for copper-indium-gallium-selenide (CIGS) and CdTe thin-film solar cell. The TRPL lifetime strongly depends on open-circuit voltage and conversion efficiency; however, the TRPL life time is insensitive to the short-circuit current.

Simultaneous Measurement of Minority-Carrier Lifetime in Single-Crystal CdTe Using Three Transient Decay Techniques

Minority-carrier lifetimes have simultaneously been measured on a single-crystal CdTe sample using three transient decay techniques. These measurements are microwave-reflection photoconductive decay (μ-PCD), time-resolved photoluminescence (TRPL), and transient free-carrier absorption (TFCA). The sample is a 0.8-mm-thick single-crystal CdTe sample from JX Nippon Mining & Metals USA, Inc., which is nominally undoped but has a hole concentration of about 2 - 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . Excess carriers are generated using a Nd:YAG laser with 5-ns pulses, and lifetimes are measured at room temperature. Using 532-nm excitation, the decay curves show an initial short-lifetime component, as carriers are generated near the unpassivated front surface. While TRPL shows a short lifetime of ~7 ns, both μ-PCD and TFCA have relatively long single-exponential decays after the initial 100 ns response. These decay times, which are more dominated by the bulk lifetime after the initial surface recombination, are 190 ns for both μ-PCD and TFCA. Simultaneous measurements using twophoton (1064 nm) excitation show bulk-dominated recombination for all three techniques. Lifetimes for both μ-PCD and TFCA are 270 ns, while the TRPL lifetime, which still shows some surfacelimited initial decay, is 160 ns.

Investigation of Cu(In,Ga)Se2 absorber by time-resolved photoluminescence for improvement of its photovoltaic performance

Time-resolved photoluminescence (TRPL) measurement has been performed on Cu(In,Ga)Se2 (CIGS) absorbers. In this contribution, CIGS films on both rigid soda-lime glass and flexible stainless steel (SUS) substrates are fabricated by the so-called “multi-layer precursor method” consisting of Ga–Se/In–Se/Cu–Se stacks. The TRPL lifetime, demonstrating a positive relationship with PL intensity, exhibits a close correlation with all photovoltaic parameters. According to TRPL and sensitive capacitance measurements, the average band-gap energy (Eg) should be in a range of 1.25–1.30eV, giving rise to sufficiently long TRPL lifetimes. Ultimately, CIGS absorber is fabricated with double graded Eg profile with a proper average Eg of 1.27eV and back-surface field of 0.22V/μm, defined as a ratio of the change in Eg divided by the change in a depth range from 1 to 2μm from CIGS surface. This graded Eg profile results in the improvement of the efficiency of the CIGS solar cell on a flexible SUS substrate up to 16.22% without an anti-reflective layer.

Two distinct carrier localization in green light-emitting diodes with InGaN/GaN multiple quantum wells

The effect of carrier localization in InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes is investigated by photoluminescence (PL) and time-resolved PL (TRPL) measurements. PL results show that two peaks obtained by Gaussian fitting both relate to the emission from localized states. By fitting the TRPL lifetimes at various emission energies, two localization depths corresponding to the In-rich regions and quasi-MQWs regions are obtained. Using a model we proposed, we suggest that compositional fluctuations of In content and variation of well width are responsible for carrier localization in In-rich regions and quasi-MQWs regions, respectively.

Comparison of ammonia plasma and AlN passivation by plasma-enhanced atomic layer deposition

Surface passivation of GaAs by ammonia plasma and AlN fabricated by plasma-enhanced atomic layer deposition are compared. It is shown that the deposition temperature can be reduced to 150 °C and effective passivation is still achieved. Samples passivated by AlN fabricated at 150 °C show four times higher photoluminescence intensity and longer time-resolved photoluminescence lifetime than ammonia plasma passivated samples. The passivation effect is shown to last for months. The dependence of charge carrier lifetime and integrated photoluminescence intensity on AlN layer thickness is studied using an exponential model to describe the tunneling probability from the near-surface quantum well to the GaAs surface.

Precision, all-optical measurement of external quantum efficiency in semiconductors

External quantum efficiency of semiconductor photonic devices is directly measured by wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent temperature or power-dependent photoluminescence. Time-resolved photoluminescence lifetime and power-dependent photoluminescence measurements are used to evaluate unprocessed heterostructures for critical performance parameters. The crucial importance of parasitic background absorption is discussed.

Charge transfer dynamics in Cu-doped ZnO nanowires

Time resolved photoluminescence (TRPL) and transient absorption (TA) spectroscopy reveal an ultrafast charge transfer (CT) process, with an electron localization time constant 39±9 ps, between the ZnO host and the Cu dopants in Cu-doped ZnO nanowires. This CT process effectively competes with the ZnO band edge emission, resulting in the quenching of the ZnO UV emission. TRPL measurements show that the UV decay dynamics coincides with the buildup of the Cu-related green emission. TA measurements probing the state-filling of the band edge and defect states provide further support to the CT model where the bleaching dynamics concur with the TRPL lifetimes.

Steady-state and time-resolved photoluminescence from relaxed and strained GaN nanowires grown by catalyst-free molecular-beam epitaxy

We report steady-state and time-resolved photoluminescence (TRPL) measurements on individual GaN nanowires (6–20 μm in length, 30–940 nm in diameter) grown by a nitrogen-plasma-assisted, catalyst-free molecular-beam epitaxy on Si(111) and dispersed onto fused quartz substrates. Induced tensile strain for nanowires bonded to fused silica and compressive strain for nanowires coated with atomic-layer-deposition alumina led to redshifts and blueshifts of the dominant steady-state PL emission peak, respectively. Unperturbed nanowires exhibited spectra associated with high-quality, strain-free material. The TRPL lifetimes, which were similar for both relaxed and strained nanowires of similar size, ranged from 200 ps to over 2 ns, compared well with those of low-defect bulk GaN, and depended linearly on nanowire diameter. The diameter-dependent lifetimes yielded a room-temperature surface recombination velocity S of 9×103 cm/s for our silicon-doped GaN nanowires.

Annealing effects on Zn 1−xMg xO/CIGS interfaces characterized by ultraviolet light excited time-resolved photoluminescence

Annealed Zn 1− x Mg x O/Cu(In,Ga)Se 2 (CIGS) interfaces have been characterized by ultraviolet light excited time-resolved photoluminescence (TRPL). The TRPL lifetime of the Zn 1− x Mg x O/CIGS film increased on increasing the annealing temperature to 250 °C, whereas the TRPL lifetime of the CdS/CIGS film had little change by annealing at temperatures lower than 200 °C. This is attributed to the recovery of physical damages by annealing, induced by sputtering of the Zn 1− x Mg x O film. The TRPL lifetime abruptly decreased with annealing at 300 °C. The diffusion of excess Zn from the Zn 1− x Mg x O film into the CIGS interface is clearly observed in secondary ion mass spectroscopy (SIMS) depth profiles. These results indicate that excess Zn at the vicinity of the CIGS surface acts as non-radiative centers at the interface. The TRPL lifetime of the Zn 1− x Mg x O/CIGS film annealed at 250 °C reached values to be comparable to that of the as-deposited CdS/CIGS film. Performance of the Zn 1− x Mg x O/CIGS cells varied with the annealing temperature in the same manner as the TRPL lifetime. The highest efficiency of the Zn 1− x Mg x O/CIGS solar cells was achieved for annealing at 250 °C. The results of the TRPL lifetime on annealing show that the cell efficiency is strongly influenced by the Zn 1− x Mg x O/CIGS interface states related to the damages and diffusion of Zn.

Optical and microstructural studies of atomically flat ultrathin In-rich InGaN∕GaN multiple quantum wells

Optical and microstructural properties of atomically flat ultrathin In-rich (UTIR) InGaN∕GaN multiple quantum well were investigated by means of photoluminescence (PL), time-resolved PL (TRPL), and cathodoluminescence (CL) experiments. The sample exhibits efficient trapping of the photoexcited carriers into quantum wells (QWs) and the effect of internal electric field in the QWs was found negligible by excitation power-dependent PL and TRPL. These phenomena were attributed to the nature of UTIR InGaN QWs, indicating the potential of this system for application in optoelectronic devices. Variation of TRPL lifetime across the PL band and spatially resolved monochromatic CL mapping images strongly suggest that there is micrometer-scale inhomogeneity in effective band gap in UTIR InGaN∕GaN QWs, which is originated from two types of localized areas.

Characterization of Cu(In,Ga)Se2 thin films by time‐resolved photoluminescence

AbstractCu(In,Ga)Se2(CIGS) films were characterized by time‐resolved photoluminescence (TRPL) on performances of solar cells. Open circuit voltage and efficiency of the solar cells increased with increasing the TRPL lifetimes meausred at 77 K and room temperature. The results at 77 K show that the CIGS films with the TRPL lifetime of over 30 ns achieve an efficiency of exceeding 15% of the solar cells. Dependence of the TRPL lifetime on the photon energy was observed on the CIGS film with high efficiency at both temperatures. This dependence is discussed on localization of exited carriers and non‐radiative centres. The minority carrier lifetime which dominates the solar cell performances can be characterized by not only the TRPL lifetime but its dependence on the photon energy. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)