Lifetime Distribution Of Photoluminescence Research Articles

Controlling Kinetic Quenching Depth Toward High‐Performance and Photo‐Stable Organic Solar Cells Printed from a Non‐Halogenated Solvent

AbstractThe investigation of drying dynamics and kinetic quenching depth related degradation of high‐performance photoactive materials with scalable coating techniques demands significant research attention. Herein, film formation kinetics regulated crystallinity, preferential orientation and vertical phase separation of the active layers is revealed, which then further affects exciton diffusion, dissociation, charge‐transport and recombination processes. By suppressing over aggregation/crystallization in slow drying process and overcoming quenching of disordered liquid phase in fast‐dried films, the optimized PM6:BTP‐eC9‐based organic solar cells obtain power conversion efficiencies up to 16.81%. In addition, photoluminescence lifetime distribution is found to be an alternative probe for kinetic quenching depth that governs the degradation rate. This study provides valuable insights into the control of thin film formation kinetics during scalable processing and develops an effective way to associate the kinetic quenching depth with morphological degradation for mechanistic understanding of long‐term stability.

Exploring the Role of Surface States in Emissive Carbon Nanodots: Analysis at Single-Particle Level.

Fluorescent carbon nanodots (CDs) have been highlighted as promising semiconducting materials due to their outstanding chemical and optical properties. However, the intrinsic heterogeneity of CDs has impeded a clear understanding of the mechanisms behind their photophysical properties. In this study, as-prepared CDs are fractionated via chromatography to reduce their structural and chemical heterogeneity and analyzed through ensemble and single-particle spectroscopies. Many single particles reveal fluorescence intensity fluctuations between two or more discrete levels with bi-exponential decays. While the intrinsic τ1 components are uniform among single particles, the τ2 components from molecule-like emissions spans a wider range of lifetimes, reflecting the inhomogeneity of the surface states. Furthermore, it is concluded that the relative population and chemical states of surface functional groups in CDs have a significant impact on emissive states, brightness, blinking, stability, and lifetime distribution of photoluminescence.

Time-resolved spectroscopy of the ensembled photoluminescence of nitrogen- and boron/nitrogen-doped carbon dots.

Carbon dots (CDs) have potential applications in various fields such as energy, catalysis, and bioimaging due to their strong and tuneable photoluminescence (PL), low toxicity, and robust chemical inertness. Although several PL mechanisms have been proposed, the origin of PL in CDs is still in debate because of the ensembled nature of the heterogeneous luminophores present in the CDs. To unravel the origin of PL in CDs, we performed time-resolved spectroscopy on two types of CDs: nitrogen-doped (N-CD) and boron-nitrogen co-doped (BN-CD). The PL decays were fitted by stretched exponential functions to estimate the distribution of the decay kinetics in the CDs, which have different PL lifetime distributions. Both CDs displayed main, blue emission decaying in 15 ns, which originates from the dominant molecular state. The analysis of the non-exponential PL decay using stretched exponential fits revealed that the functional surface luminophores are of less variety but of more environmental heterogeneity and have much lower populations in BN-CD than in N-CD.

Quasi-continuum photoluminescence: Unusual broad spectral and temporal characteristics found in defective surfaces of silica and other materials

We previously reported a novel photoluminescence (PL) with a distribution of fast decay times in fused silica surface flaws that is correlated with damage propensity by high fluence lasers. The source of the PL was not attributable to any known silica point defect. Due to its broad spectral and temporal features, we here give this PL the name quasi-continuum PL (QC-PL) and describe the features of QC-PL in more detail. The primary features of QC-PL include broad excitation and emission spectra, a broad distribution of PL lifetimes from 20 ps to 5 ns, continuous shifts in PL lifetime distributions with respect to emission wavelength, and a propensity to photo-bleach and photo-brighten. We found similar PL characteristics in surface flaws of other optical materials, including CaF2, DKDP, and quartz. Based on the commonality of the features in different optical materials and the proximity of QC-PL to surfaces, we suggest that these properties arise from interactions associated with high densities of defects, rather than a distribution over a large number of types of defects and is likely found in a wide variety of structures from nano-scale composites to bulk structures as well as in both broad and narrow band materials from dielectrics to semiconductors.

Effects of annealing treatments on the photoluminescence decay properties of Si-rich oxide/SiO2 multilayer films

In this work, Si-rich oxide (SRO)/SiO2 multilayer films have been deposited and the photoluminescence (PL) decay properties of the films with different annealing temperatures are studied. The PL shifts toward low energy with increasing the annealing temperature, and intense PL at around 1.4eV is obtained after annealing at 1100°C. The PL decay curves can be well fitted by a multiexponential PL decay model, and the peak of the PL lifetime distribution band shifts toward longer time with increasing the annealing temperature. Two lifetime distribution bands are obtained after the formation of crystallized Si-QDs, and the proportion of slow component increases from 69.72% to 77.04% after hydrogen passivation. Analyses show that defect states recombination in the SRO layer is the main optical emission mechanism when the annealing temperature is lower than 900°C, and interband transition in the Si-QDs due to quantum confinement effect is the main PL mechanism when the film is annealing at 1100°C.

Recent photoluminescence research on chalcogenide glasses for photonics applications

The optical properties of GeGaSe glasses doped with Er by the addition of Er2S3 have been investigated. Optically uniform glasses have been prepared using stoichiometric compositions with 9–12at.% Ga and doped with 0.5–2at.% Er. The radiative lifetime of the 4I13/2→4I15/2 transition has been estimated to be equal to 1.78ms using the Judd–Ofelt analysis. The photoluminescence lifetime distribution has been investigated in optimized glasses using Quadrature Frequency-Resolved Spectroscopy at room and liquid helium temperatures and at different emission wavelengths. All lifetime distributions were found to be sharp peaks centered at ∼2ms. A radiation diffusion model has been used to understand the discrepancy between measured photoluminescence spectra and those predicted by the McCumber theory. The model predicts a radiative lifetime of the 4I13/2→4I15/2 transition to be around 1.72ms and a much longer non-radiative lifetime. These results assume quasi-uniform distribution of Er3+ ions with negligible concentration-self-quenching and negligible rate of non-radiative relaxation from 4I13/2 to 4I15/2.

Lifetime of defect luminescence in hydrogenated amorphous silicon

We have investigated the dependence of lifetime distribution of photoluminescence (PL) in a-Si:H films on the defect density. In order to analyse the lifetime distribution quantitatively, we estimated characteristic lifetime from the in-phase signal obtained by frequency resolved spectroscopy (FRS). The increase of lifetime of PL due to contribution of emission from triplet excitons trapped at defects is seen at 0.95–1.13 eV. The characteristic lifetime of emission from electron-hole pairs is discussed by considering the probability of tunnelling to non-radiative recombination centres and radii of wave functions of gap states and tail states.

Photoluminescence lifetime distributions of chalcogenide glasses obtained by wide-band frequency resolved spectroscopy

Photoluminescence (PL) lifetime distributions for amorphous arsenic chalcogenides g-As2Se3 and g-As2S3, and amorphous selenium a-Se, are obtained for the first time using a quadrature frequency resolved spectroscopy (QFRS) technique modified for nanosecond resolution. The g-As2S3 and a-Se chalcogenides exhibit double-peak lifetime distributions, whereas the lifetime distribution of g-As2Se3 peaks uniquely at around 10−4 s, which is consistent with earlier results. PL fatigue is found to reduce the intensity of PL but not affect the observed PL lifetimes. A self-trapped exciton model is adopted to explain the experimental results, providing reasonable mechanisms for the two-component lifetimes and associated phenomena. For a-Se, singlet–triplet exchange energy of ≈160 meV is estimated.

Nanosecond QFRS study of photoluminescence in amorphous semiconductors

The paper is concerned with the study of the photoluminescence (PL) lifetime distribution of a-Si : H and a-Ge : H, down to the nanosecond (ns) region using a newly developed dual-phase, double lock-in (DPDL) quadrature frequency-resolved spectroscopy (QFRS). The DPDL-QFRS results show no indication of a peak in the ns region, and the PL lifetime distribution is basically double-peaked with slow (≈ms) and fast (≈μs) components, as reported earlier. The double-peak lifetime, a blue-shift of the fast component spectrum against the slow one, a dependence of the radiative transition rate on the PL emission energy and a fast lifetime (∼μs, this being much longer than singlet-exciton lifetime normally (∼ns)) can be well explained by invoking self-trapped excitons. A singlet–triplet exchange energy ∼40 meV is deduced for a-Si : H. The dependence of the lifetime distribution on the generation rate is discussed on the basis of the non-geminate recombination, where non-geminate pairs prevail over excitons under strong photoexcitation conditions.

Lifetime and intensity of photoluminescence after light induced creation of dangling bonds in a-Si:H

The photoluminescence (PL) fatigue after illumination of pulsed laser light has been studied for a-Si:H films prepared by glow discharge decomposition. The densities of photocreated dangling bonds (DBs) in the a-Si:H films after illumination of sub-bandgap pulsed light were obtained from electron spin resonance (ESR) measurements as a function of illumination time. We have obtained the lifetime resolved PL in microsecond and millisecond regions for a-Si:H films by means of frequency resolved spectroscopy (FRS) before and after the illumination. The decrease in the PL intensity was not so significant as expected from the DB density. We have not found sizable change of lifetime distribution of PL after the illumination. The results suggest inhomogeneity of spatial distribution of photocreated DBs.

Photoluminescence lifetime distribution of a-Si:H and a-Ge:H expanded to nanosecond region using wide-band frequency-resolved spectroscopy

We present the first survey on quadrature frequency-resolved spectroscopy (QFRS) spectra in the nanosecond (ns) region of photoluminescence (PL) of a-Si:H and a-Ge:H. Our QFRS study reveals that there is no evidence of a peak or shoulder for the ns PL lifetime component, and thus the PL lifetime distribution has basically a double-peak structure with two components of μs and ms lifetimes in both materials. The μs component in the lifetime distribution is enhanced at higher PL photon energy, which favors the exciton model. Generation rate ( G) dependence of QFRS spectra shows a common feature, for both a-Si:H and a-Ge:H, where excitonic emission shifts to non-geminate radiative recombination as G increases.

Light-induced Creation of Defects and Lifetime Distribution of Photoluminescence in a-Si:H Based Films

ABSTRACTWe have studied the lifetime distributions of photoluminescence (PL) at 10K after the pulsed excitation for a-Si:H based films. Effects of light-induced creation of defects on the lifetime distributions have been studied. The lifetime distributions of PL in a-Si:H based films have a distinct component at about 10 ns together with a longer lifetime component seen in microsecond region. The PL in a-Si:H decreases in intensity after the illumination of visible light. The decreasing of the nanosecond component is slower than that of microsecond component. The decreasing of the PL intensity and increasing of the defect density have also been observed in a-Si:H after illumination of sub-bandgap light, although the absorption coecient is much smaller than that of visible light. The quenching of PL is discussed with distribution of non-radiative lifetime calculated by assuming random distribution of the DBs.

Lifetime distribution of PL under pulsed excitation in hydrogenated amorphous silicon based films

Nanosecond processes of photoluminescence (PL) in a-Si:H based films have been studied using a tunable pulse laser illuminating samples at temperature of 10 K. The distribution of lifetimes of the PL has a component in the nanosecond region. The nanosecond component becomes largest in a-Si 1− x :N x :H alloys, a-Si:H/a-Si 1− x :N x :H multilayers and band-edge modulated a-Si 1− x :N x :H films. In the case of sub-bandgap excitation the nanosecond component is largest. The results are explained in terms of the singlet exciton model.

Lifetime distribution of photoluminescence in a-Si:H based alloys

The radiative lifetime distributions, G(τ), in a-Si:H based alloys containing Ge, C, N or O have been evaluated. In a-SiGe:H, only the 10 μs component is present even in a specimen having nearly the same optical gap and Urbach energy as in a-Si:H of good quality, whereas, in other alloys, G(τ) is dominated by two lifetime components peaked at specific lifetimes of around 1 ms and 10 μs as in a-Si:H. Those results cannot be reconciled with the usual interpretation of photoluminescence as a recombination between tail-state carriers, but they suggest the presence of localized luminescent centers associated with respective characteristic lifetimes. Based on the difference in G(τ) amongst the alloys, the alloying effect is discussed in relation to the structural nature around the luminescent centers.

Photoluminescence mechanism in hydrogenated amorphous silicon studied by frequency-resolved spectroscopy.

In order to reconsider the photoluminescence mechanism in hydrogenated amorphous silicon (a-Si:H), the lifetime distribution of photoluminescence, G(\ensuremath{\tau}), was evaluated over a wide lifetime range between 5.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ s and 8.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$ s using frequency-resolved spectroscopy. The most remarkable finding is that G(\ensuremath{\tau}) is dominated by two kinds of lifetime components characterized by specific peak lifetimes at low temperatures. At 12 K, only the lifetime component peaked at about 1 ms is dominant, whereas another component appears distinctively at around 10 \ensuremath{\mu}s and grows gradually with increasing temperature. Such a discontinuous change of the lifetime from 1 ms to 10 \ensuremath{\mu}s takes place at temperatures lower than 60 K where the photoluminescence intensity is almost constant. Interestingly, the peak lifetimes for both lifetime components are quite insensitive to the Urbach energy or the emission energy as long as the excitation intensity is held low enough to satisfy the condition for geminate-pair recombination. These characteristic features observed in G(\ensuremath{\tau}) do not reconcile with the generally accepted model of tunneling recombination between carriers trapped at the tail states after thermalization. In particular, dominance of the 1-ms lifetime component at 12 K is interpreted following the model as the electron-hole separation enlarges up to a value corresponding to the 1-ms recombination after photogeneration.However, it is hard to understand the absence of the 10-\ensuremath{\mu}s lifetime component at 12 K, since the 10-\ensuremath{\mu}s recombination is expected to take place at much shorter electron-hole separation and is actually observed at elevated temperatures. Because of the several arguments against the generally accepted model and of the coexistence of two lifetime components having specific peak lifetimes, it is more appropriate to consider that the photoluminescence in a-Si:H comes from special localized luminescent centers corresponding to the respective lifetime components. Since the lifetime changes discontinuously from 1 ms to 10 \ensuremath{\mu}s with increasing temperature while the luminescence intensity remains constant at the low temperatures, some correlation is expected to exist between the two kinds of luminescent centers.

Is Thermalization Due to Electronic Self-Trapplng?

ABSTRACTThe interpretation that the red shift of the a-Si:H photoluminescence spectrum is due to carrier hopping is incompatible with Monte-Carlo simulations. We suggest that thermalization is instead the result of slow, gradual, electronic self-trapping. We show how this model reconciles the narrowness of the photoluminescence lifetime distribution with the observed red-shift of the energy spectrum.

Picosecond time-resolved photoluminescence characterization of a-SiC:H films prepared by electron cyclotron resonance plasma

Microwave excited electron cyclotron resonance (ECR) plasmas were used to produce five a-SiC:H films of varying relative carbon content. This was achieved by adjusting the relative hydrogen gas/organic liquid source flow rates and resulted in optical gaps between 2.3eV and 3.3eV. A time-correlated single photon counting technique incorporating a picosecond laser was used to acquire the fast photoluminescence decay emitted at several wavelengths for each sample. A lifetime distribution analysis of the photoluminescence decay reveals the presence of up to four distinct components. These resolved decay times range from 80ps to over 20ns. Each component decay has its own emission spectrum. The blue component has the fastest and the red the slowest decay. We present the variation of photoluminescence lifetime distributions with deposition parameters. We also present continuous wave (cw) and absorption photoluminescence spectra.

The lifetime distribution of photoluminescence in amorphous phosphorus selenides

Frequency-resolved spectroscopy (FRS) has been used to determine the lifetime distribution of the mid-gap photoluminescence (PL) band in phosphorus selenide glasses a-P xSe 1-x, where x = 0.67, 0.75, 0.84, and in c-P 4Se 3. Exceptionally narrow distributions are found, peaking near a frequency of 1kHz in all cases, with widths close to the experimental resolution. The lifetime of the PL band is found to be almost independent of the excitation energy and intensity, up to a certain limit. This result is evidence for geminate recombination in the phosphorus selenides. The similarity between glasses and crystal indicates a common recombination centre, which is believed to be a consequence of the similar microstructure in the two cases.

Luminescence lifetime distributions in μc-Si and glow discharge and sputtered a-Si:H

We present measurements of photoluminescence lifetime distributions in microcrystalline and amorphous silicon. The effects of variations of temperature and excitation density are reported and explored.