Photoinduced Absorption Research Articles

Photoluminescence in Organometal Halide Perovskites: Free Carrier Versus Exciton

The photoexcitation dynamics in CH3NH3PbI3 thin films has been studied by nanosecond time-resolved photoluminescence (PL) and transient photoinduced absorption. Under excitation intensity from ∼0.1 to ∼100 μJ/cm2, free carrier is the dominant photoexcitation species. However, in the intermediate excitation intensity from ∼1 to ∼10 μJ/cm2, a small fraction of photoexcitations are excitons, which, however, are responsible for PL within this excitation intensity range. The branching ratio between excitons and free carriers at 2 μJ/cm2 was estimated to be 17%. Furthermore, the spectral narrowing PL was found at the low temperature after the formation of excitons, indicating that laser action could be due to excitonic emission. Our findings provide fundamental insights into the optical properties of organometal halide perovskites with direct implications for optoelectronics.

Multi-particle effects in far-IR optical transmission spectra of Ge/Si quantum dots

Photoinduced and equilibrium optical transmission was studied in undoped and δ-doped Ge/Si quantum dot structures in the far-infrared spectral range. Multi-particle excitations were observed in the absorption spectra of the structures with different doping levels. Photoinduced absorption spectra demonstrate the thermal distribution of the photoexcited holes over the single-particle states of the quantum dot ensemble in contrast with the equilibrium absorption spectra.

Defect-related optical absorption bands in CdSiP_2 crystals

When used as optical parametric oscillators, CdSiP2 crystals generate tunable output in the mid-infrared. Their performance, however, is often limited by unwanted optical absorption bands that overlap the pump wavelengths. A broad defect-related optical absorption band peaking near 800 nm, with a shoulder near 1 µm, can be photoinduced at room temperature in many CdSiP2 crystals. This absorption band is efficiently produced with 633 nm laser light and decays with a lifetime of ∼0.5 s after removal of the excitation light. The 800 nm band is accompanied by a less intense absorption band peaking near 1.90 µm. Data from eight CdSiP2 crystals grown at different times show that the singly ionized silicon vacancy (VSi−) is responsible for the photoinduced absorption bands. Electron paramagnetic resonance (EPR) is used to identify and directly monitor these silicon vacancies.

Photo-induced absorption spectra of a poly(p-phenylenevinylene) polymer with fluorinated double bonds

We report on the optical and photo-physical properties of a fluorinated poly(p-phenylenevinylene) derivative, namely poly(2-methoxy-5-[(2-ethylhexyl)oxy]-1,4-phenylenedifluorovinylene) (MEH-PPDFV) and we compare them with those of the well-known non fluorinated reference system, poly(2-methoxy-5-[(2-ethylhexyl)oxy]-1,4-phenylenevinylene) (MEH-PPV) synthesized with the same procedure.Remarkable blue-shifts of absorption and emission spectra of MEH-PPDFV with respect to MEH-PPV are observed, due to the larger distortion from planarity of the fluorinated species. In addition, the blue-shift for optical transitions in absorption spectra is more pronounced than in emission ones, since the molecule conformation in the excited states is more planar than in the ground one.Microsecond photo-induced absorption spectra of MEH-PPDFV reveal three bands possessing spectral properties and/or kinetics different from those observed for MEH-PPV. The assignment of these transitions is provided in terms of free and trapped polarons as well of triplet excitons. Free-polarons have an intrinsic decay constant of 3 μs, while the trapping dynamics has a longer kinetics of 40 μs. The triplet exciton, instead, shows a monoexponetial decay behavior of constant about 10 μs.

Electrostatic Field Influence on Luminescence Features of Cadmium Sulphide Quantum Dots in Silica Matrix

The effect of electrostatic field on optical properties of silicate nanocomposite with cadmium sulfide quantum dots was investigated. It was found that the electrostatic field causes quantum dot orientation along the field force lines leading to changes in the polarized components of the luminescence spectrum. The influence of field force line direction on photoinduced absorption characteristics arised from λ = 405.9nm laser radiation exposure to nanocomposite was shown.

Modification of Optical Properties of CdS-silicate Nanocomposites by Thermal Annealing

Temperature influence on optical and structural properties of nanocomposite structures based on cadmium sulfide quantum dots in silicate matrix was investigated. The mechanism of occurrence of reversible laser-induced absorption is proposed. It was found that annealing of nanocomposite up to 120°C leads to substantial change of its absorption and luminescence spectra caused by transformation of quantum dot crystal lattice from amorphous to sphalerite. The nanocomposite with the sphalerite lattice QDs does not possess the effect of the photo-induced absorption.

Excited-states spectroscopies and its magnetic field effect of π-conjugated polymer-fullerene blends with below-gap excitation

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long-lived polarons in blend of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly (3-hexylthiophene) (RR-P3HT) with below-gap excitation. Compared to above-gap excitation, the polarons exhibited thermally activated and multi-trap limited bimolecular recombination too, but with narrower distribution profile in polaron states in millisecond time regime. However, magnetophoto-induced absorption (MPA) of polarons presented much weaker magnetic field effect in below-gap excitation (∼0.5mT) than in above-gap excitation (order of ∼10mT), although the underline mechanism is same as spin mixing of polaron pairs by the hyperfine interaction at small field region.

Influence of fullerene-based acceptor materials on the performance of indacenodithiophene-cored small molecule bulk heterojunction organic solar cells

A model system is presented to investigate the influence of fullerene-based acceptor materials on the performance of small molecule organic solar cells (SM-OSCs). Two indacenodithiophene-(IDT)-cored small molecules and one diketopyrrolopyrrole-(DPP)-cored molecule are used as donor materials, and two most widely applicable fullerene-based acceptors, [6,6]-phenyl-C61(71)-butyric acid methyl esters (PC61(71)BM) are used as the acceptor material, respectively. The charge generation, transport, and recombination behaviors were studied under various irradiation intensities, and these parameters are relevant to device performance. The transient photocurrent and transient photovoltage operation are utilized to directly compare the photo-induced absorption, charge generation, and charge recombination properties of the corresponding blend films. Our results indicate that different fullerene derivatives not only affect the device performance with their different light absorption abilities but also have roles in charge generation and charge recombination, which greatly helps to understand how to select a better fullerene-based acceptor for the high-performance SM-OSCs.

Revealing the spin-vibronic coupling mechanism of thermally activated delayed fluorescence.

Knowing the underlying photophysics of thermally activated delayed fluorescence (TADF) allows proper design of high efficiency organic light-emitting diodes. We have proposed a model to describe reverse intersystem crossing (rISC) in donor–acceptor charge transfer molecules, where spin–orbit coupling between singlet and triplet states is mediated by one of the local triplet states of the donor (or acceptor). This second order, vibronically coupled mechanism describes the basic photophysics of TADF. Through a series of measurements, whereby the energy ordering of the charge transfer (CT) excited states and the local triplet are tuned in and out of resonance, we show that TADF reaches a maximum at the resonance point, substantiating our model of rISC. Moreover, using photoinduced absorption, we show how the populations of both singlet and triplet CT states and the local triplet state change in and out of resonance. Our vibronic coupling rISC model is used to predict this behaviour and describes how rISC and TADF are affected by external perturbation.

Characterization of oxidized carbon materials with photoinduced absorption response

An efficient application of fast remote diagnostics for carbon material (CM) bulk particles was demonstrated. Porous layers of CM particles with different oxidation levels were characterized by self-action of picosecond laser pulses at 1064 nm. Nitrogen adsorption, Boehm titration, and thermal analysis of the oxidized CMs revealed diverse specific surface area \(S_\mathrm {BET}\), reasonable surface acidity, and high concentration of surface oxygen-containing groups. Dense CM porous layers showed a monotonous reduction of the absorptive nonlinear optical (NLO) response efficiency versus the oxidation level with characteristic magnitude Im(\(\chi _\mathrm{C}^{(3)})\sim 10^{-10}\) esu for the carbon particles fraction. The obtained Im(\(\chi _\mathrm{C}^{(3)})/S_\mathrm {BET}\) ratio remains approximately constant, which indicates the certain proportion between the absorptive NLO response efficiency and the specific surface area. We suggest to use Im(\(\chi _\mathrm{C}^{(3)})\) as a figure of merit for carbons subjected to the oxidation—the route to enhance the CM surface reactivity.

Polarons in Narrow Band Gap Polymers Probed over the Entire Infrared Range: A Joint Experimental and Theoretical Investigation.

We investigate the photoinduced absorption (PIA) spectra of the prototypical donor-acceptor polymer [2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT) and its silicon bridged variant Si-PCPDTBT over a spectral range from 0.07 to 1.5 eV. Comparison between time-dependent density functional theory simulations of the electronic and vibrational transitions of singlet excitons, triplet excitons, polarons, and bipolarons with the experimental results proves that the observed features are due to positive polarons delocalized on the polymer chains. We find that the more crystalline Si-bridged variant gives rise to a red-shift in the transition energies, especially in the mid-infrared (MIR) spectral range and furthermore observe that the pristine polymers' responses depend on the excitation energy. Blending with PCBM, on the other hand, leads to excitation-independent PIA spectra. By computing the response properties of molecular aggregates, we show that polarons are delocalized in not only the intra- but also the interchain direction, leading to intermolecular transitions which correspond well to experimental absorption features at the lowest energies.

Recombination dynamics in aerotaxy-grown Zn-doped GaAs nanowires

In this paper we have investigated the dynamics of photo-generated charge carriers in a series of aerotaxy-grown GaAs nanowires (NWs) with different levels of Zn doping. Time-resolved photo-induced luminescence and transient absorption have been employed to investigate radiative (band edge transition) and non-radiative charge recombination processes, respectively. We find that the photo-luminescence (PL) lifetime of intrinsic GaAs NWs is significantly increased after growing an AlGaAs shell over them, indicating that an AlGaAs shell can effectively passivate the surface of aerotaxy-grown GaAs NWs. We observe that PL decay time as well as PL intensity decrease with increasing Zn doping, which can be attributed to thermally activated electron trapping with the trap density increased due to the Zn doping level.

Optical Properties of Heterojunction between Hybrid Halide Perovskite and Charge Transport Materials: Exciplex Emission and Large Polaron

The photoluminescence from exciplex states at lower energy than the bandgap of CH3NH3PbI3 (MAPbI3) was observed in both MAPbI3/Spiro-OMeTAD and MAPbI3/PCBM bilayer films. At the same time, using continuous wave photoinduced absorption (PIA) spectroscopy, we observed the PIA band at 0.14 eV, which was due to the positive or negative polarons in perovskite for MAPbI3/Spiro-OMeTAD and MAPbI3/PCBM film, respectively. The binding energies of both kinds of polarons are about ∼40 meV, with the radius of phonon cloud of about 5.9 nm. Our finding may supply direct spectroscopic evidence for the existence of exciplex states in heterojunction of hybrid halide perovskite and charge transport materials as well as consequent large polarons in perovskite.

Correlation of the Photoinduced Total Transmission with the Degree of Surface Functionalization of Carbon Materials Obtained from Natural Renewable Sources

For the first time, a contactless express method, which is based on the self-action of picosecond range laser pulses at 1064 nm, is used for the characterization of an optically dense porous layer of carbon material (CM) bulk particles obtained from a lignocellulosic source. It is found that the oxidation treatment reduces the Brunauer–Emmett–Teller (SBET) surface area from 9.52 × 105 m2/kg to 2.73 × 105 m2/kg. This reduction occurs due to the destruction of the carbon matrix fraction and to the formation of novel O-containing surface groups. The concentrated 30-mass% HNO3 is found to be the most efficient oxidant giving the highest yield of carboxylic (Cb), anhydridic, lactonic, and phenolic surface functionalities. The concentration of the surface functional groups is determined in a dynamic argon atmosphere by thermogravimetric (TG) analysis and thermoprogrammed desorption coupled with IR (TPD-IR) spectroscopy. The surface acidity defined from data of the Boehm titration shows the acceptable agreement with the data of TG-TPD-IR examination. An enhancement of the surface hydrophilicity allows the use of carbon matrix for the covalent binding of bioligands, amino acids, their residues, and proteins to the oxygen-containing functionalities, such as Cb groups. The observed photoinduced absorption efficiency of the bulk carbon particles Im(X(3)C) ∼ 10−16 m2/W is in the range of that of nanosized carbons. A slight variation of the ratio Im(X(3)C)/SBET within the limits of experimental errors indicates a certain correlation between the absorptive NLO response and the CM specific surface. We suggest to utilize Im(X(3)C) as a quality parameter for carbon materials subjected to the oxidation, which is a typical initial step of the most commonly used functionalization routes for the preparation of biomedical materials.

Greater than 50% inversion in Erbium doped Chalcogenide waveguides.

We report Er-doped Ge-Ga-Se films and waveguides deposited using co-thermal evaporation and patterned with plasma etching. Strong photoluminescence at 1.54 µm with intrinsic lifetime of 1 ms was obtained from deposited films with 1490 nm excitation. Erbium population inversion up to 50% was achieved, with a maximum of ~55% possible at saturation for the first time to the author's knowledge, approaching the theoretical maximum of 65%. Whilst gain was not achieved due to the presence of upconversion pumped photoinduced absorption, this nonetheless represents a further important step towards the realization of future chalcogenide Erbium doped waveguide amplifiers at 1550 nm and in the Mid-infrared.

All-optical controlling based on nonlinear graphene plasmonic waveguides.

We give the effective refractive index of graphene plasmonic waveguides with both linear and nonlinear effects based on the nonlinear cross-phase modulation, and address the effects of photo-induced refractive index change and absorption change. A non-resonant all-optical nonlinear graphene plasmonic switch with an ultra-compact size of 0.25 μm2 is proposed and numerically analyzed based on the dynamics of the photo-induced absorption change. The results show that the all-optical graphene plasmonic switch can realize a broad bandwidth over 5 THz, a potentially very high switching speed and an extinction ratio of 18.14 dB with the electric amplitude of the pump light of 1.5 × 107 V/m at the signal frequency of 28 THz. Our study could provide a possibility for future all-optical highly integrated optical components.

Two-dimensional observation of multicolor multistep photoreaction process by using white light excitation covering entire visible region

Abstract Multicolor multistep photoisomerization process of azobenzene derivative in photostationary state was observed by newly developed Fourier transform two-dimensional (2D) spectroscopic technique. The photostationary state was generated by excitation using white light covering the entire visible region, and the photoreaction was monitored by measuring photoinduced bleaching and absorption signals detected by white light probe. The wavelength participating in excitation process for the observed signal was clarified by modulation frequency marked by passing the pump white light through a scanning tandem Fabry-Perot interferometer, and the 2D amplitude and phase spectra were obtained by Fourier transform of the obtained 2D interferogram. By using the system, we succeeded in observing the multicolor multistep process in photoisomerization of Sudan red 7B [1-(4-[Phenylazo]phenylazo)-2-ethylaminonaphthalene, SR7B] that has two azo groups and shows several configurations. It was concluded from the analysis of obtained 2D amplitude and phase spectra that at least two reaction pathways of sequential photoisomerization reactions exist in the photostationary state of SR7B under the white light irradiation. The interpretation is supported by the results of quantum chemical calculations using density functional theory.

Charge Carriers Motion in P3HT:Capped ZnO Nanoparticles Blend Films; Impact of Capping Agents

<p class="TTPAbstract">Blend of conjugated polymer poly(3-hexylthiophene) or P3HT and Zinc Oxide nanoparticles (ZnO-NP) has been intensively used as active material for high performance hybrid solar cells. However, agglomeration of ZnO-NP hinders efficient charge carrier<span lang="IN">s</span> transfer both from P3HT to ZnO-NP and <span lang="IN">their </span>transport within ZnO-NP which cause to low performance of solar cells. Capping of ZnO-NP is currently applied to avoid this agglomeration effect. In this study, we used three different capping agents to cap ZnO-NP, namely small semiconducting molecules squaraine, 2-naphthalene and insulating polymer polyvinylpyrrolidone. We <span lang="IN">conducted</span> temperature dependence of photoinduced infrared absorption spectroscopy to study charge carriers motion in the P3HT:capped ZnO nanoparticles blend films. The measurement was carried out with light irradiation of 532 nm and temperature ranging from 78 to 300 K. The spectra were analyzed by a bimolecular carrier recombination method with Arrhenius activation energy. Two parallel charge carrier recombination processes are observed, namely polarons recombination along polymer chain (intra-chain) and inter-chain polarons recombination in the P3HT-chains. At low temperatures, polarons recombine along polymer chains (intra-chain) with activation energy between 19-23 eV for all samples. However, the inter-chain polaron motion is influenced by capping agents as shown by a variation in its activation energy ranging from 28 to 58 eV.</p>

Charge Carriers Motion in P3HT:Capped ZnO Nanoparticles Blend Films; Impact of Capping Agents

<p class="TTPAbstract">Blend of conjugated polymer poly(3-hexylthiophene) or P3HT and Zinc Oxide nanoparticles (ZnO-NP) has been intensively used as active material for high performance hybrid solar cells. However, agglomeration of ZnO-NP hinders efficient charge carrier<span lang="IN">s</span> transfer both from P3HT to ZnO-NP and <span lang="IN">their </span>transport within ZnO-NP which cause to low performance of solar cells. Capping of ZnO-NP is currently applied to avoid this agglomeration effect. In this study, we used three different capping agents to cap ZnO-NP, namely small semiconducting molecules squaraine, 2-naphthalene and insulating polymer polyvinylpyrrolidone. We <span lang="IN">conducted</span> temperature dependence of photoinduced infrared absorption spectroscopy to study charge carriers motion in the P3HT:capped ZnO nanoparticles blend films. The measurement was carried out with light irradiation of 532 nm and temperature ranging from 78 to 300 K. The spectra were analyzed by a bimolecular carrier recombination method with Arrhenius activation energy. Two parallel charge carrier recombination processes are observed, namely polarons recombination along polymer chain (intra-chain) and inter-chain polarons recombination in the P3HT-chains. At low temperatures, polarons recombine along polymer chains (intra-chain) with activation energy between 19-23 eV for all samples. However, the inter-chain polaron motion is influenced by capping agents as shown by a variation in its activation energy ranging from 28 to 58 eV.</p>

Photoconvertible Behavior of LSSmOrange Applicable for Single Emission Band Optical Highlighting

Photoswitchable fluorescent proteins are capable of changing their spectral properties upon light irradiation, thus allowing one to follow a chosen subpopulation of molecules in a biological system. Recently, we revealed a photoinduced absorption band shift of LSSmOrange, which was originally engineered to have a large energy gap between excitation and emission bands. Here, we evaluated the performance of LSSmOrange as a fluorescent tracer in living cells. The absorption maximum of LSSmOrange in HeLa cells shifted from 437 nm to 553 nm upon illumination with a 405-, 445-, 458-, or 488-nm laser on a laser-scanning microscope, whereas the emission band remained same (∼570 nm). LSSmOrange behaves as a freely diffusing protein in living cells, enabling the use of the protein as a fluorescence tag for studies of protein dynamics. By targeting LSSmOrange in mitochondria, we observed an exchange of soluble molecules between the matrices upon mitochondrial fusion. Since converted and unconverted LSSmOrange proteins have similar emission spectra, this tracer offers unique possibilities for multicolor imaging. The fluorescence emission from LSSmOrange was spectrally distinguishable from that of eYFP and mRFP, and could be separated completely by applying linear unmixing. Furthermore, by using a femtosecond laser at 850 nm, we showed that a two-photon process could evoke a light-induced red shift of the absorption band of LSSmOrange, providing a strict confinement of the conversion volume in a three-dimensional space.