Electrophotoluminescence Research Articles

A novel InGaN-based bidirectional wavelength converter and optical amplifier based on hot electron-induced blue light-emitting device

In this work, the functioning of the Top-Hat Hot-Electron Light Emission and Lasing in a Semiconductor Heterostructure (TH-HELLISH) structure, which is based on InGaN/GaN multiple quantum wells, is investigated to employ it as an optical amplifier and wavelength converter. The heterojunction structure is formed by placing four InGaN quantum wells in the n-type GaN region between the n- and p-type GaN layers. The device with a TH-HELLISH structure was grown on a sapphire (Al2O3) substrate by Metal-Organic Chemical Vapor Deposition (MOCVD) with an indium ratio of 0.16. The TH-HELLISH device was fabricated in Top-Hat HELLISH (THH) geometry with four independent n- and p-channels contacts to create a non-isotropic potential distribution in the heterojunction. The electro-photoluminescence (E-PL) technique was used to obtain light amplification and the converted wavelength. A 370 nm excitation light is absorbed by the TH-HELLISH device, subsequently leading to the emission at 440 ± 1 nm, which fulfills the role of a wavelength converter. It was also observed that TH-HELLISH operates as an optical amplifier with a 1.6-times increase in emission at 370 nm excitation. When the device is illuminated with a wavelength of 370 nm, it is monitored that the threshold electric field decreased from 0.25 kV/cm to 0.18 kV/cm, accompanied by a corresponding decrease in the threshold voltage from 12.5 V to 9.0 V. In addition, the TH-HELLISH device is found to be operated independently of the polarity of the applied voltage, and the absorption and emission regions of the device can be switched by changing the polarity of the applied voltage, therefore behaving as a bi-directional wavelength converter and optical amplifier.

Photoluminescence of P3HT:PCBM bulk heterojunction thin films and effect of external electric field

AbstractSteady‐state and time‐resolved photoluminescence (PL) and electrophotoluminescence (E‐PL) have been recorded for bulk heterojunction samples of thin films of regio‐regular poly(3‐hexylthiophene) (P3HT) and methyl [6,6]‐phenyl‐C61‐butanoate (PCBM). The PL spectra of the bulk heterojunction samples showed a monotonic decrease of PL quantum yield with increased PCBM content in the film. The PL decay was measured with both a femtosecond up‐conversion technique and a time‐correlated single‐photon‐counting technique. The mechanism of PL quenching deviated from a simple Stern‐Volmer equation, explained in terms of an apparent static quenching with a sphere‐of‐action model and dynamic quenching. With increased content of PCBM, the static quenching played a dominant role. Upon application of an electric field, the E‐PL spectra of both P3HT and bulk heterojunction samples showed an enhanced quantum yield and incremented lifetime of PL of P3HT. The long‐range electron transfer from P3HT to PCBM was retarded upon application of an electric field; the field‐induced change increased with increasing PCBM content.

Electro-Photoluminescence Color Change for Deformable Visual Encryption.

Although structural coloring and photoluminescence (PL) have been investigated for radiation-responsive color change, electroluminescence (EL) has not been used for the radiation-responsive system. An electro-photoluminescence (EPL) color change is presented here. The phosphors in the alternating current electroluminescence (ACEL) act simultaneously as electro-luminophores and photo-luminophores. The EPL chromaticity is systematically investigated depending on the ACEL frequency and UV intensity. It is found that the PL variation depending on UV intensity is the mechanism of the EPL color change. It is revealed that EL and PL can be controlled independently in the low electric field so that the EPL chromaticity can be adjusted by a linear combination of the EL color and the PL color. The EPL color-changing device is used as a deformable visual encryption system and a soft skin for a soft robotic rover, imitating the concealment and signaling functions in nature.

Anisotropic Electric Field Effect on the Photoluminescence of CH3NH3PbI3 Perovskite Sandwiched between Conducting and Insulating Films

Photoluminescence (PL) of a nanocrystalline film of methylammonium lead iodide perovskite (MAPbI3) sandwiched between an electrode of a fluorine-doped tin oxide (FTO) layer and an insulating film of poly(methyl methacrylate) is found to increase and decrease significantly with the application of an external electric field (Fext), depending on the direction of the applied field, based on the measurements of electrophotoluminescence (E-PL) spectra, i.e., field-induced change in PL spectra. The field-induced change in PL intensity is confirmed to originate from the field-induced change in the number of free carriers which induce radiative recombination, based on temporally resolved E-PL measurements. We propose that an internal field (Fint) exists even without application of Fext. The anisotropic behavior of the effect of Fext on PL is interpreted in terms of a synergy effect of Fint and Fext; both fields are additive with the applied field direction from Ag to FTO electrode (positive direction) or subtracti...

Stark Spectroscopy of Absorption and Emission of Indoline Sensitizers: A Correlation with the Performance of Photovoltaic Cells

Electric field effects on photoexcitation dynamics and electronic properties of highly efficient indoline sensitizers, DN488, D205, and DN182, embedded in PMMA films have been examined by using electroabsorption (E-A) and electrophotoluminescence (E-PL) spectroscopic techniques and time-resolved photoluminescence (PL) decay measurements in the presence of electric fields. Photovoltaic performances have been also measured for devices constructed using these sensitizers. Then, field-induced quenching of PL and field-induced change in PL decay profile were observed, and it was found that these field effects, which depend on the sensitizers investigated herein, are well correlated with the trend of power conversion efficiencies of the corresponding photovoltaic cells. Electric dipole moment and molecular polarizability of these sensitizers both in the ground state (S0) and in the excited state have been calculated at the level of B3LYP/6-31G(d), and the differences of these physical parameters between S0 and ...

Electric Field Effects on Photoluminescence of CdSe Nanoparticles in a PMMA Film

External electric field effects on spectra and decay of photoluminescence (PL) as well as on absorption spectra were measured for CdSe nanoparticles in a poly(methyl methacrylate) (PMMA) film. Electrophotoluminescence (E-PL) spectra as well as electroabsorption spectra show a remarkable Stark shift which depends on the particle size, indicating a large electric dipole moment in the first exciton state. The E-PL spectra also show that PL of CdSe is quenched by application of electric fields, and the magnitude of the field-induced quenching becomes larger with increasing size. The PL decay profiles observed in the absence and presence of electric field show that the field-induced quenching of PL mainly originates from the field-induced decrease in population of the emitting state prepared through the relaxation from the photoexcited state.

Field-Induced Fluorescence Quenching and Enhancement of Porphyrin Sensitizers on TiO2 Films and in PMMA Films

Three highly efficient porphyrin sensitizers—YD2, YD2-oC8, and YD30, either sensitized on TiO2 films or embedded in PMMA films—were investigated using electrophotoluminescence (E-PL) spectra. Under both thin-film conditions, on application of an external electric field we observed the quenching of fluorescence of push–pull porphyrins (YD2 and YD2-oC8) and a slightly enhanced fluorescence of the reference porphyrin without an electron donor group (YD30). A nonfluorescent state with charge separation (CS) is proposed to be involved in both YD2 and YD2-oC8 systems so that the electron injection becomes accelerated in the presence of a strong electric field. In contrast, the retardation of the nonradiative process not involving a CS state was the reason for the field-induced enhancement of fluorescence of YD30. The extent of fluorescence quenching of YD2-oC8 was greater than that of YD2 on TiO2 films, indicating that the ortho-substituted long alkoxyl chains play a key role to accelerate the consecutive electron injection involving the CS state. Our E-PL results indicate that a field-induced variation of fluorescent intensity is related to the efficiency of conversion of solar energy and that further improvement of the performance of devices containing push–pull porphyrin dyes is achievable under an applied electric field.

External Electric Field Effect on Fluorescence Spectra of Pyrene in Solution

Electrophotoluminescence (E-PL) spectra, i.e., plots of the electric-field-induced change in photoluminescence intensity as a function of wavenumber, have been measured for pyrene solution. At high concentrations of pyrene where excimer fluorescence is observed along with the monomer fluorescence emitted from the locally excited state, both excimer fluorescence and monomer fluorescence are enhanced by application of electric fields. The results show that the nonradiative decay process at the excimer emitting state is decelerated by application of electric fields. It is also found that molecular polarizability of pyrene excimer is larger than that of pyrene monomer in the ground state by ~270 ± 90 Å(3), based on the analysis of the Stark shift of the excimer fluorescence.

Stark shifts and exciton dissociation in CdSe nanoparticle grafted conjugated polymer

Electroabsorption (E-A) and electrophotoluminescence (E-PL) responses of polymer films of CdSe quantum dots (QDs) incorporated sulfide-substituted poly(1,4-phenylene vinylene) derivative CdSe-S3PPV were measured. The observed Stark shift both in E-A and E-PL responses is likely to be caused by a substantial contribution of change in molecular polarizability (Δα¯) and change in electric dipole moment (|Δμ|) following photoexcitation. Together with Stark shift, field-induced photoluminescence (PL) quenching and enhancement were observed depending on excitation energy. The quenching of PL of CdSe-S3PPV film is interpreted in terms of an exciton model-a breaking of electron-hole pairs in the presence of electric field.

External Electric Field Effects on Optical Property and Excitation Dynamics of Capped CdS Quantum Dots Embedded in a Polymer Film

Nanocomposites of benzyl mercaptan (BM)-capped cadmium sulfide (CdS) quantum dots (QDs) have been prepared and investigated by using electric field modulation spectroscopy. Applied electric fields on absorption and photoluminescence (PL) spectra as well as PL decays of BM-capped CdS QDs embedded in a poly(methyl methacrylate) (PMMA) film have revealed a field-induced alteration. Electro-photoluminescence (E-PL) exhibits field-induced quenching of photoluminescence in the presence of electric fields. The observed E-PL spectra and field-induced change in decay profile suggest that the PL intensity alternation originates from the electric field effect on emitting state population rather than PL quantum yield, probably as a result of the presence of field-assisted dissociation into hole and electron at the photoexcited state having a charge-separated character. Field-induced change in absorption spectra showed the spectral broadening caused by the stark shifts, indicating a large electric dipole moment in the...

Electric-Field-Induced Enhancement/Quenching of Photoluminescence of π-Conjugated Polymer S3-PPV: Excitation Energy Dependence

The effects of electric field on absorption and photoluminescence (PL) of films of sulfide-substituted PPV derivative S3-PPV, poly[2-(phenyl)-3-(4'-(3,7-dimethyloctyloxy)phenyl)-1,4-phenylenevinylene-co-2-(11'-decyl sulfanylundecanyloxy)-5-methoxy-1,4-phenylene vinylene], were investigated. Electroabsorption (E-A) and electrophotoluminescence (E-PL) responses of S3-PPV show the Stark shifts, indicating a significant alternation in the molecular polarizability (Delta alpha) associated with the optical transitions. Field-induced enhancement or quenching is also observed for PL of S3-PPV, depending on the photoexcitation energy, whereas the shape of the PL spectra is independent of the excitation wavelength. The field effects on the decay profiles of PL indicate that the quenching results from a diminished population of the emitting states on excitation at 300 nm, whereas the PL is enhanced on excitation at 471 nm because the emitting state has an increased lifetime. The efficiency of field-assisted generation of electron-hole pairs produced through excitons monotonically increases with increasing excitation energy, and the nonradiative decay rate in the emitting state is diminished by electric fields in S3-PPV. The photoirradiation of S3-PPV in ambient air resulted in rapid degradation of the polymer film.

Electro-photoluminescence in organics

We report a successful observation of the electro-photoluminescence (EL-PL) in thin, exciplex emitting, organic, solid films. The increasing electric field applied to injecting electrodes tends to reduce the total light emission because of the dissociation of emissive exciplex states and their precursors, and, at the same time, the radiative recombination of injected holes and electrons enhances the output of the electroluminescence (EL), the process that suppresses the total luminescence quenching. Increasing bulk concentration of the charge leads the EL output to exceed the PL component, a negative quenching (enhancement of the total emission) is observed.

Electrophotoluminescence of ZnO film

The electric-field-controlled photoluminescence (PL), i.e., electrophotoluminescence (EPL) of ZnO film has been investigated via a ZnO-based metal-insulator-semiconductor (MIS) structure on a silicon substrate applied with different biases. Compared with the PL of ZnO film in the case where there is no bias on the MIS structure, the positive bias with negative voltage applied on silicon substrate significantly enhances the near-band-edge ultraviolet emission while suppressing the deep-level-related visible emissions, whereas the negative bias hardly changes the PL of ZnO film. The mechanism for EPL of ZnO film is proposed in terms of the electric-field effect on the bending of energy bands of ZnO.

Electric field-induced lateral mobility of photosystem I in the photosynthetic membrane: A study by electrophotoluminescence

Electrophoretic movement of photosystem I (PS I) along the photosynthetic membrane of hypotonically swollen thylakoid vesicles was studied by analyzing the electric field-stimulated delayed luminescence (electrophotoluminescence) emitted from PS I. The electrophoretic mobility was inferred from the differences in electrophotoluminescence (EPL) of the photosynthetic vesicles in presence and absence of trains of low amplitude (<80 V/cm) prepulses of 1 ms duration at 4 ms spacing. The average apparent electric mobility, determined from the time course of EPL increase on one hemisphere or its decrease on the other one, as function of prepulse length and intensity was of the order of 3 . 10(-5) cm(2)V(-1)s(-1). The assymetric distribution of the PS I reached a steady state when the diffusional, electrostatic, and elastic forces balanced the electrophoretic driving force. A lateral diffusion coefficient of approximately 5 . 10(-9) cm(2)s(-1) was found for the PS I complex from the diffusional relaxation after cessation of the electric field pulse train. Experimental conditions such as concentration, temperature, and viscosity of the aqueous solution were not critical for the effect. Between 23 and 150 electron charges per moving particle were estimated from the measured electrophoretic mobility.

Electroselection in the photosynthetic membrane: polarized luminescence induced by an external electric field

Delayed luminescence emission (a general property of photosynthetic systems) is closely related to primary electron flow and energetic events in photosynthesis [ 1,2]. In particular, electrical phenomena at the membrane level have a marked influence on the emission; thus, for example, an artificially induced diffusion electric potential (positive inside) [3] or a change in the dielectric constant of the membrane [2] have been shown to stimulate delayed luminescence. An efficient and kinetically unique method to induce transmembrane potentials in a suspension of membrane-bound vesicles is the application of an external macroscopic electric field [4]. In this way, delayed luminescence can be significantly enhanced (by l-3 orders of magnitude) [5,6], especially in hypotonically extensively swollen particles originating from the chloroplasts (blebs [7]). This phenomenon, termed electrophotoluminescence (EPL) [6] has interesting kinetic features, not yet understood. The role of the electric field is explained [6,8] generally in terms of the primary photosynthetic charge separation in photosystem II, its vectorial nature in the membrane [9] and an additional term in the activation energy for its back reaction introduced by the transmembrane field [8]. The resulting charge recombination produces the chlorophyll excited singlet state, giving rise to luminescence. In order to obtain additional information on the mechanism of EPL production and its relation to membrane topology, one can make use of the directional nature of the external electric field as a triggering agent. Since the electric field induced within the membrane by the external field has a strong angular dependence on the external field direction [6], one

Electric field perturbed electron tunnelling in nonpolar organic glasses

Isothermoluminescence (ITL) and electrophotoluminescence (EPL) resulting from electron–cation recombination are measured in a 2-methylpentane–methylcyclohexane glass. The ITL is more characteristic of quantum-mechanical tunnelling and the EPL signal is markedly stronger in this new glass than previous measurements in 3-methylpentane. Quantum-mechanical tunnelling theory is used to predict recombination rates of electrons in the potential field of a cation plus an applied field. Numerical integration of the nonhomogeneous kinetic equations resulting from a distribution of cation–electron separations leads to qualitative and quantitative predictions of the EPL signal that are observed experimentally. Fitting of the theory to experiment supports the conclusions that the angular distribution of the photoelectrons about the cations is close to isotropic, that the electrons active in ITL and EPL on the time scale of minutes are separated about 50 Å from their parent cation, and that the trap ionization potential in this nonpolar hydrocarbon glass is in the range of 0.5 to 0.7 eV.

Coupling of Matrix-Trapped Electrons with an External Electric Field: Electrophotoluminescence in a Rigid Organic Solution at 77°K

Electrophotoluminescence (EPL) signals obtained when a photoionized rigid organic solution at 77°K is subjected to an intense electric field are studied from a quantitative point of view. This luminescence appears as a strong enhancement of the isothermal luminescence (ITL) observed under normal conditions. Both EPL and ITL are apparently due to the slow recombination of matrix trapped electrons with those parent positive centers (solute cations) to which they are correlated. Previously, a one-parameter equation has been found which quantitatively relates any EPL decay at fields up to 1×106 V/cm to the ITL decay seen in the same sample. The theoretical approach is in terms of ordinary diffusion theory modified to take explicit account of electron trapping in the solid and how that trapping is altered when an applied perturbation, such as a temperature jump, or an electric field is coupled to the trapped electron. The single parameter is expressed as an exponential in the perturbation (temperature jump or applied field). However, only if the central field of the positive center (solute cation) is considered to assert the dominant role upon the activation of the trapped electrons can the successful EPL–ITL equation be even qualitatively justified. The EPL–ITL equation is successfully extended to a quantitative description of any signal seen after a first field-on period corresponding to either a new ITL, or a new ITL followed by a second EPL obtained when the field is reapplied in the same or in the opposite direction. In addition, it is shown that, after a sufficiently long application of the unidirectional electric field, a situation is reached where removal of the field gives an increase of the light level and subsequent reapplication of the field leads to a field quenching of the recombination luminescence (EPL&amp;lt;ITL). In view of the extended successful testing of the EPL–ITL equation, it is found that a rigorous interpretation of the particular model being employed leads to certain unique statements regarding the distribution and diffusion of electrons in the r, θ, and φ space about the cation. These statements apply only to the 3-methylpentane solid exclusively employed in this work. First it must be concluded that the electrons be radially distributed in the range ∼15 Å &amp;lt; r &amp;lt; 40 Å. Secondly, the electrons must be anisotropically (or sharply) distributed in θ space, and furthermore over periods as long as 15 min, and even in the presence of high fields, they exhibit no diffusion in θ space but only inward diffusion in the radial direction controlled by the central field of the cation. The theory also predicts that photoconductivity in these systems should exhibit Poole's law behavior (deviation from Ohm's law exponential in the field) but that the electron-field coupling parameter must be smaller than that found from the EPL studies. A simultaneous study of Poole's law behavior and the EPL–ITL relationship in one sample fully confirms this. In general it is seen that from sample to sample, in 3-methylpentane at 77°K, the electron-field coupling parameter varies from 0.4–0.8 Å in photoconductivity, while in EPL it varies from 0.9–1.9 Å being highly reproducible for any one preparation.