Photoinduced Absorption Spectra Research Articles

Doping effect on the light absorption and photoluminescence of Ge/Si quantum dots in the infrared spectral range

The results of comprehensive studies of near-infrared photoluminescence and mid-infrared equilibrium and photoinduced absorption spectra in structures with Ge/Si quantum dots with different doping levels at different optical pumping intensities and different temperatures are presented. Obtained dependences of interband photoluminescence spectra on temperature and optical pumping intensity are explained by the change in the intensities of direct and indirect in real space electron-hole recombination in Ge/Si quantum dots. The spectra of equilibrium and photoinduced absorption of polarized mid-infrared radiation demonstrate peaks associated with transitions of holes from the ground and excited states to continuum states above quantum dots and with optical transitions from the ground states to excited ones.

Photoinduced Absorption Spectra of Donor–Acceptor Systems Based on Cobalt(II) and Manganese(III) Phthalocyanine Complexes with Femtosecond Time Resolution

Photoinduced Absorption Spectra of Donor–Acceptor Systems Based on Cobalt(II) and Manganese(III) Phthalocyanine Complexes with Femtosecond Time Resolution

Disentangling Light- and Temperature-Induced Thermal Effects in Colloidal Au Nanoparticles.

We present temperature-dependent (from room temperature to 80 °C) absorption spectra of Au/SiO2 core–shell nanoparticles (NPs) (core diameter: ∼25 nm) in water in the range from 1.5 to 4.5 eV, which spans the localized surface plasmon resonance (LSPR) and the interband transitions. A decrease in absorption with temperature over the entire spectral range is observed, which is more prominent at the LSPR. These changes are well reproduced by theoretical calculations of the absorption spectra, based on the experimentally measured temperature-dependent real (ε1) and imaginary (ε2) parts of the dielectric constant of Au NPs and of the surrounding medium. In addition, we model the photoinduced response of the NPs over the entire spectral range. The experimental and theoretical results of the thermal heating and the simulations of the photoinduced heating are compared with the ultrafast photoinduced transient absorption (TA) spectra upon excitation of the LSPR. These show that while the latter is a reliable monitor of heating of the NP and its environment, the interband region mildly responds to heating but predominantly to the population evolution of charge carriers.

In Situ Investigation of Charge Performance in Anatase TiO2 Powder for Methane Conversion by Vis-NIR Spectroscopy.

The intrinsic behavior of photogenerated charges and reactions with chemicals are key for a photocatalytic process. To observe these basic steps is of great importance. Here we present a reliable and robust system to monitor these basic steps in powder photocatalysts, and more importantly to elucidate the key issue in photocatalytic methane conversion over the benchmark catalyst TiO2. Under constant excitation, the absorption signal across the NIR region was demonstrated to be dominated by photoexcited electrons, the absorption of photoexcited holes increases toward shorter wavelengths in the visible region, and the overall shapes of the photoinduced absorption spectra obtained using the system demonstrated in the present work are consistent with widely accepted transient absorption results. Next, in situ measurements provide direct experimental evidence that the initial step of methane activation over TiO2 involves oxidation by photoexcited holes. It is calculated that 90 ± 6% of photoexcited electrons are scavenged by O2 (in dry air), 61 ± 9% of photoexcited holes are scavenged by methane (10% in argon), and a similar amount of photoexcited electrons can be scavenged by O2 even when the O2 concentration is reduced by a factor of 10. The present results suggest that O2 is much more easily activated in comparison to methane over anatase TiO2, which rationalizes the much higher methane/O2 ratio frequently used in practice in comparison to that required stoichiometrically for photocatalytic production of value-added chemicals via methane oxidation with oxygen. In addition, methanol (a preferable product of methane oxidation) is much more readily oxidized than methane over anatase TiO2.

Studies of photoexcitations in polymer/non-fullerene blend for high-efficiency organic solar cells

We studied the photoexcitations properties in the blend of PBDB-T-SF, a donor π-conjugated copolymer, and ITIC-2F, a non-fullerene acceptor molecule, which is used as an active layer in high-performance organic photovoltaic (OPV) solar cells. We used several steady-state spectroscopies such as photoinduced absorption (PIA), photoluminescence (PL), magneto-photoinduced absorption (MPA), and magneto-PL in the pristine and blend films. The PIA spectra of the pristine copolymer and acceptor films contain a photoinduced absorption (PA) band due to triplet excitons, which is confirmed by the MPA(B) response, whereas the PIA spectrum of the copolymer/molecule blend contains several PA bands due to polaron pairs at the copolymer chain/molecule interfaces. Interestingly, when exciting the PIA spectrum of the blend at two different photon energies, namely, ℏω≈2.39 eV that preferentially excites the copolymer chains and ℏω≈1.58 eV that excites only the acceptor molecules, we found that the PIA spectra are the same. This shows that charge photogeneration is possible when exciting either the copolymer chains or the acceptor molecules, which might explain the high power conversion efficiency of OPV cells based on this blend. We also found from the MPA(B) responses of the pristine copolymer and acceptor films and their blend that the back reaction of the photogenerated polaron pairs into the copolymer forming triplet excitons is absent, further explaining the high power conversion efficiency of this blend as the active layer in OPV solar cells.

Unusual Hole Transfer Dynamics of the NiO Layer in Methylammonium Lead Tri-iodide Absorber Solar Cells.

Nickel oxides (NiO) as hole transport layers (HTLs) in inverted-type perovskite solar cells (PSCs) have been widely studied mainly because of their high stability under illumination. Increases in the power conversion efficiency (PCE) with NiO HTLs have been presented in numerous reports, although the photoluminescence (PL) quenching behavior does not coincide with the PCE increase. The dynamics of the charge carrier transport between the NiO HTLs and the organic-inorganic halide perovskite absorbers is not clearly understood yet and quite unusual, in contrast to organic/polymerics HTLs. We deposited NiO HTLs with precisely controlled thicknesses by atomic layer deposition (ALD) and studied their photovoltaic performances and hole transfer characteristics. Ground state bleaching (GSB) recovery was observed by ultrafast transient absorption spectroscopy (TAS), which suggested that backward hole injection occurred between the perovskites and NiO HTLs, so that the uncommon PL behaviors can be clearly explained. Backward hole injection from the NiO HTL to the perovskite absorber originated from their similar valence band (VB) energy positions. The thickness increase of the NiO HTLs induced VB sharing, which caused a red-shift of the photoinduced hole absorption spectrum in near-infrared (NIR) femtosecond TAS and a decrease in the PL intensity. Our studies on inorganic metal oxide transport layers, NiO in this work, with a thickness dependence and the comparison with organic layers provide a better understanding of the interfacial carrier dynamics in PSCs.

Photosensitivity of composite erbium-doped phosphorosilicate optical fibres to 193-nm laser radiation

Results of studying photoinduced changes of a refractive index in the core of a composite erbium-doped phosphorosilicate optical fibre under pulsed UV irradiation at a wavelength of 193 nm and the annealing of these changes are presented and analysed. It is shown that the preliminary saturation of a composite optical fibre by molecular hydrogen substantially increases the photosensitivity. Analysis of thermal anneal kinetics revealed fundamental differences in the photosensitivity mechanisms between the optical fibres saturated with hydrogen and not subjected to hydrogen treatment. Initial and photoinduced absorption spectra of the phosphorosilicate glass are investigated. It is found that in the UV range, the induced absorption is mainly determined by a contribution from the intense absorption band of photoinduced phosphorus electron centres PO2 with a maximum at 4.7 eV (λ = 260 nm).

Radiation Hardness of Nd:GdVO4 Laser Crystal under γ-Ray Irradiation

The radiation hardness of neodymium-doped gadolinium vanadate crystal (Nd:GdVO4) has been investigated. Crystal with sizes of 4×4×8 mm doped to a level of 0.5% Nd3+ and (for comparison) undoped GdVO4 crystal were γ-irradiated by a 60Co source to an absorbed dose of about 106 to 107 rad. The photoinduced absorption spectra of the crystals have been studied. Intense lasing with a differential efficiency up to about 25% was obtained in an irradiated Nd:GdVO4 crystal. It is shown that irradiation of a crystal up to a dose of 107 rad does not reduce significantly its laser efficiency.

A Wide-Band Femtosecond Pump–Probe Spectrometer Based on a Laser with an Active Medium Based on Chromium-Doped Forsterite Crystal

An instrument for recording photoinduced-absorption spectra using the pump–probe method with femtosecond time resolution is described, in which a regenerative amplifier based on a chromium-doped forsterite crystal with a fundamental harmonic of 1240 nm is used as the laser system. Two optical schemes with pumping at a wavelength of 620 nm and a wavelength of 413 nm that are described in detail allow measurements in the spectral range of 280–1000 nm. The time resolution of the system is 250 fs.

Photophysics of color centers in visible-light-active rutile titania. Evidence of the photoformation and trapping of charge carriers from advanced diffuse reflectance spectroscopy and mass spectrometry

• The photoformation and separation of charge carriers in traps in VLA titania ceramics and titania powder were examined. • Thermoprogrammed annealing (TPA) spectra of Ti 3+ color centers photoinduced in VLA TiO 2 displayed temperature dependencies. • Highest rate of recombination of holes released into the valence band with Ti 3+ centers was due to extra charged centers. • Rate of photodesorption of O 2 occurrence in specific channel to activate VLA TiO 2 by photoexcitation of Ti 3+ centers. We herein report on the photophysics of color centers in visible-light-active (VLA) rutile titania ceramics and titania powder resulting from the photoformation and separation of charge carriers explored by a diffuse reflectance (DR) spectroscopic and kinetic study using a cryostat-type accessory for a Cary 5000 spectrophotometer after exposure of the titanias to UV radiation and Vis-light illumination. The action spectrum of the photoformation of Ti 3+ at 90 K accorded fully with the absorption spectrum of intrinsic defects in the as-synthesized TiO 2 . The photoinduced absorption spectra consisted of a set of individual absorption bands attributable to several different Ti 3+ centers. Analysis of the dependencies of the photoformation of separate centers on the wavelength of illumination and light exposure provided extraction of specific Ti 3+ centers putatively attributed to centers with excess negative charge {2Ti 3+ + V o 2+ } ⟵→ {Ti δ+ + V o 2+ } with 3 > δ > 2 formed at significantly high concentration upon maximal exposure to Vis-light illumination. Thermoprogrammed annealing (TPA) spectra of Ti 3+ color centers photoinduced in VLA TiO 2 displayed temperature dependencies of the rate of detrapping of the photoinduced holes observed optically through annihilation of the Ti 3+ centers. The TPA spectra in the range 90–500 K consisted of a set of first-order peaks corresponding to the traps, whose depths ranged from ∼0.2 eV (peak at 130 K in powder specimen) to 1.06 eV (peak at 455 K in the ceramics). The highest rate of recombination of holes released to the valence band with Ti 3+ centers, an event attributable to the Ti δ+ centers, provided TPA spectra that clearly manifested the existence of shallow traps. We also report mass spectrometric evidence of the photoformation of electrons and holes in VLA TiO 2 under Vis-light illumination through an examination of the photoadsorption of molecular oxygen and the photodesorption of photoadsorbed oxygen from the surface of powdered VLA titania specimens. Moreover, kinetics of the photodesorption of O 2 under orange light illumination, after the photoadsorption of O 2 stimulated by blue light excitation, provided experimental proof of the occurrence of an additional specific channel toward the photoactivation of VLA TiO 2 via the photoexcitation of photoinduced Ti 3+ color centers.

Minority and Majority Charge Carrier Mobility in Cu2ZnSnSe4 revealed by Terahertz Spectroscopy

The mobilities of electrons and holes determine the applicability of any semiconductor, but their individual measurement remains a major challenge. Here, we show that time-resolved terahertz spectroscopy (TRTS) can distinguish the mobilities of minority and majority charge carriers independently of the doping-type and without electrical contacts. To this end, we combine the well-established determination of the sum of electron and hole mobilities from photo-induced THz absorption spectra with mobility-dependent ambipolar modeling of TRTS transients. The method is demonstrated on a polycrystalline Cu2ZnSnSe4 thin film and reveals a minority (electron) mobility of 128 cm2/V-s and a majority (hole) carrier mobility of 7 cm2/V-s in the vertical transport direction relevant for light emitting, photovoltaic and solar water splitting devices. Additionally, the TRTS analysis yields an effective bulk carrier lifetime of 4.4 ns, a surface recombination velocity of 6 * 104 cm/s and a doping concentration of ca. 1016 cm−3, thus offering the potential for contactless screen novel optoelectronic materials.

Interchain and intrachain triplets in poly(3-thienylene vinylene) derivatives

We are reporting the optical study of two types of poly (3-thienylene vinylene) (PTV) derivatives with linear and bulk side chain configurations. The first one, poly(3-dodecylthienylenevinylene) with controlled regioregularity (PTV-CR) with an alkyl side group, showed a high degree of crystallinity, whereas the second one, imide-PTV with an electron deficient imide side group, is amorphous. Continuous wave-photoinduced absorption revealed different features of the long-lived triplet absorptions, with the geminately generated triplet–triplet pair in imide-PTV bearing more intrachain characteristics with fast recombination kinetics (<10 μs). On the other hand, the isolated triplet in PTV-CR generated by intersystem crossing had more interchain characteristics with slow recombination kinetics (ms). The decay of the triplet–triplet pair in imide-PTV is monomolecular even at a high pump intensity of 240 mW/cm2, whereas the isolated triplets in PTV-CR recombine bimolecularly at a very low pump intensity of 10 mW/cm2. Grazing incidence X-ray diffraction, doping induced absorption, and various dependences of the photoinduced absorption spectrum have corroborated the finding that reduced interchain interaction favors the formation of a correlated triplet–triplet pair, a precursor to two independent triplets formed via fission.

Electron correlation effects and two-photon absorption in diamond-shaped graphene quantum dots

In quasi-1D $\pi$-conjugated polymers such as \emph{trans}-polyacetylene and polyenes, electron correlation effects determine the "reversed" excited state ordering in which the lowest two-photon $2A_{g}$ state lies below the lowest one-photon $1B_{u}$ state. In this work, we present conclusive theoretical evidence of reversed excited state ordering in fairly 2D $\pi$-conjugated systems, namely, diamond-shaped graphene quantum dots (DQDs). Our electron correlated calculations show that DQDs begin to exhibit reversed excited ordering with increasing size, in disagreement with independent-particle picture. This signals the onset of strong correlation effects which renders them nonluminescent. Further, we calculate and analyze the two-photon absorption (TPA) spectra as well as photoinduced absorption (PA) spectra of these systems and find excellent agreement with the available experimental results. Our investigations demonstrate that unlike a strictly 1D system like\emph{trans}-polyacetylene, the non-linear and excited state absorptions in DQDs are highly intricate, with several even parity states responsible for strong absorptions. Our results could play an important role in the design of graphene-based non-linear optical devices.

Long-lived photoinduced absorption in granular molybdenum disulfide thin films

We present results of the differential photoinduced absorption spectra investigation of the molybdenum disulfide nanogranular thin films. The films were produced by means of a sulfurization of pre-deposited by magnetron sputtering thin molybdenum films. It has been shown that photoexcitation with the light quanta with the energy higher than the bandgap leads to a modification of the absorption spectra in the visible range. Possible nature of the photoinduced absorption spectra will be discussed.

Long-lived-correlated triplet-pair state in an imide substituted poly-thienylene vinylene-based π-conjugated polymer

We report on the photophysical properties of a soluble thienylene-vinylene π-conjugated polymer, namely imide poly-thienylene vinylene. Ultrafast pump probe spectroscopy reveals that a broad photoinduced absorption (PA) band is immediately photogenerated along with a narrower PA from the photoinduced singlet excitons. The broad PA is susceptible to magnetic field and thus is assigned to a correlated triple pair state. This feature shows a long lifetime persisting into the μs time domain, in contrast to the singlet exciton PA, which quickly decays on a 10-ps timescale. The steady-state PA spectrum is identical to the transient PA spectrum of the triplet pair state but shows a magnetic field response that is typical to isolated triplet excitons.

Magnetic field-assisted formation of charge transfer complex in π-conjugated polymer and fullerene-blended films

Blended films of poly [2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV)/[6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) were annealed at 70°C under rotational magnetic field (RMF) (∼350 G with 600 RPM) for 10 min. At low temperatures, the films prepared with RMF contained new spectral features. The photoluminescence (PL) spectra of films prepared under RMF had two peaks (1.59 and 1.73 eV), whereas films prepared without RMF had a single PL band at 1.75 eV. Furthermore, the photoinduced absorption (PIA) spectra of films prepared with RMF showed an additional PIA band at 1.15 eV, along with the well-known PIA band at 1.35 eV due to excited state absorption by polarons. We conclude that the spectral features in MEH-PPV/PCBM films were due to charge transfer complexes (CTC), the formation of which is promoted by RMF. Our results suggested that applying proper magnetic field while preparing polymer/fullerene film could improve the formation of CTC. We discuss the relevance to solar cell applications.

Electronic and vibrational spectroscopy studies of PffBT4T π-conjugated donor–acceptor copolymer

We used a variety of optical spectroscopies to investigate the charge excitations and correlated infrared (IR)-active and Raman-active vibrations in poly[(difluoro-benzothiadiazol-diyl)-alt-(di(2-octyldodecyl)-quaterthiophen-diyl)], PffBT4T, a π-conjugated donor–acceptor (DA) copolymer, which, when blended with fullerene PCBM molecules, serves as an active layer in high-performance photovoltaic solar cells. The applied optical spectroscopies in films of pristine PffBT4T and PffBT4T/PCBM blend include absorption, photoluminescence, electroabsorption, photoinduced absorption (PA), and resonant Raman scattering. We found that the PffBT4T copolymer chain contains 11 strongly coupled Raman-active vibrational modes, which are renormalized upon photogeneration of charge polarons onto the chain. As the lower energy polaron absorption band overlaps with the renormalized vibrational modes, they appear in the PA spectrum as antiresonance (AR) lines superposed onto the induced polaron absorption band. We show that the Raman scattering, doping induced, and photoinduced AR spectra in PffBT4T are well explained by the amplitude mode model (AMM), where a single vibrational propagator describes the renormalized Raman modes and their related photoinduced AR intensities in detail. Surprisingly, we found that two of the IR-active modes in the pristine copolymer must be included in the AMM propagator for explaining the complete photoinduced AR spectrum. This feature is unique to DA-copolymers and indicates that some intrachain C2v symmetry breaking occurs because of the different electron affinities of the donor and acceptor moieties.

Optical studies of native defects in π-conjugated donor–acceptor copolymers

We used multiple spectroscopies such as photoinduced absorption (PIA), magneto photoinduced absorption, and doping induced absorption for studying native defects in π-conjugated donor–acceptor copolymer chains of benzodithio-phene fluorinated benzotriazole. The PIA spectrum contains characteristic photoinduced absorption bands that are due to polarons and triplet exciton species, whose strengths have different dependencies on the modulation frequency, temperature, and laser excitation, as well as magnetic field response. We found that the native defects in the copolymer chains serve as efficient traps that ionize the photoexcited excitons, thereby generating charge carriers whose characteristic optical properties are similar, but not equal to those of intrachain polarons formed by doping. The native defects density is of the order of 1017 cm−3 indicating that most of the copolymer chains contain native defects upon synthesis; however, this does not preclude their used-for photovoltaic applications.

Revealing Trap States in Lead Sulphide Colloidal Quantum Dots by Photoinduced Absorption Spectroscopy

AbstractDue to their large surface to volume ratio, colloidal quantum dots (CQDs) are often considered to exhibit a significant amount of surface defects. Such defects are one possible source for the formation of in‐gap states (IGS), which can enhance the recombination of excited carriers, i.e., work as electrical traps. These traps are investigated for lead sulphide CQDs of different size, covered with different ligands using a mid‐infrared photoinduced absorption (PIA) technique. The obtained PIA spectra reveal two distinct absorption bands, whose position depends on the particle size, i.e., the electronic confinement in the CQDs. Smaller particles exhibit deeper traps. The chemical nature of the capping ligand does not affect the resulting position other than due to its change in confinement, but better passivating species lead to smaller signals. Furthermore, ligand specific narrow lines observed are superimposed on the broad electronic background of the PIA spectra, which is attributed to Fano resonances caused by the interplay of the narrow molecular vibrations and the continuum of trap states. Mid‐infrared photoinduced absorption represents a valuable tool to unravel distributions of IGS in CQDs and allows for an assessment of the quality of ligand exchanged films. These findings have implications for understanding the performances of CQD‐based (opto‐) electronic devices, such as solar cells, transistors, or quantum dot light emitting diodes, which are limited by frequent carrier trapping events.

Giant Rashba splitting in 2D organic-inorganic halide perovskites measured by transient spectroscopies.

Two-dimensional (2D) layered hybrid organic-inorganic halide perovskite semiconductors form natural "multiple quantum wells" that have strong spin-orbit coupling due to the heavy elements in their building blocks. This may lead to "Rashba splitting" close to the extrema in the electron bands. We have used a plethora of ultrafast transient, nonlinear optical spectroscopies and theoretical calculations to study the primary (excitons) and long-lived (free carriers) photoexcitations in thin films of 2D perovskite, namely, (C6H5C2H4NH3)2PbI4. The density functional theory calculation shows the occurrence of Rashba splitting in the plane perpendicular to the 2D barrier. From the electroabsorption spectrum and photoinduced absorption spectra from excitons and free carriers, we obtain a giant Rashba splitting in this compound, with energy splitting of (40 ± 5) meV and Rashba parameter of (1.6 ± 0.1) eV·Å, which are among the highest Rashba splitting size parameters reported so far. This finding shows that 2D hybrid perovskites have great promise for potential applications in spintronics.