Charge Transfer State Absorption Research Articles

Thermal activation induced charge transfer state absorption redshift realizes strong anti-thermal quenching in Pr3+-activated phosphor.

In our work, a totally anomalous thermal quenching phenomenon of red-shifted and enhanced charge transfer state (CTS) absorption is found for the first time in LiTaO3:xPr3+ phosphors. The crystal structure, luminescent properties and the mechanism of abnormal thermal quenching were investigated in detail.

Enhancing sub-bandgap external quantum efficiency by photomultiplication for narrowband organic near-infrared photodetectors

Detection of electromagnetic signals for applications such as health, product quality monitoring or astronomy requires highly responsive and wavelength selective devices. Photomultiplication-type organic photodetectors have been shown to achieve high quantum efficiencies mainly in the visible range. Much less research has been focused on realizing near-infrared narrowband devices. Here, we demonstrate fully vacuum-processed narrow- and broadband photomultiplication-type organic photodetectors. Devices are based on enhanced hole injection leading to a maximum external quantum efficiency of almost 2000% at −10 V for the broadband device. The photomultiplicative effect is also observed in the charge-transfer state absorption region. By making use of an optical cavity device architecture, we enhance the charge-transfer response and demonstrate a wavelength tunable narrowband photomultiplication-type organic photodetector with external quantum efficiencies superior to those of pin-devices. The presented concept can further improve the performance of photodetectors based on the absorption of charge-transfer states, which were so far limited by the low external quantum efficiency provided by these devices.

Amplified spontaneous emission in thin films of quasi-2D BA3MA3Pb5Br16 lead halide perovskites.

Quasi-2D (two-dimensional) hybrid perovskites are emerging as a new class of materials with high photoluminescence yield and improved stability compared to their three-dimensional (3D) counterparts. Nevertheless, despite their outstanding emission properties, few studies have been reported on amplified spontaneous emission (ASE) and a thorough understanding of the photophysics of these layered materials is still lacking. In this work, we investigate the ASE properties of multilayered quasi-2D BA3MA3Pb5Br16 films through the dependence of the photoluminescence on temperature and provide a novel insight into the emission processes of quasi-2D lead bromide perovskites. We demonstrate that the PL and ASE properties are strongly affected by the presence, above 190 K, of a minor fraction of the high temperature (HT) phase. This phase dominates the PL spectra at low excitation density and strongly affects the ASE properties. In particular, ASE is only present between 13 K and 230 K, and, at higher temperatures, it is suppressed by absorption of charge transfer states of the HT phase. Our results improve the understanding of the difficulties to obtain ASE at room temperature from these quasi-2D materials and are expected to guide possible materials improvement in order to exploit their excellent emission properties also for the realization of low threshold optically pumped lasers.

Effect of Ba2+ doping on the photoluminescence of YVO4: Eu3+ phosphor and first principles calculations

In this work, YVO4: Eu3+ phosphors with and without Ba2+ doping (having the formula of Y1-x-y (EuxBay)VO4) were prepared by the high-temperature solid-state reaction method, and their photoluminescence and relevant physical mechanism were studied. A large enhancement in the photoluminescence of the YVO4: Eu3+ phosphors by Ba2+ doping has been observed. When the Ba2+ doping concentration is at 3 at%, the YVO4: Eu3+ phosphors have the strongest photoluminescence. It has been found that the Ba2+ doping extends the luminescence decay lifetimes of YVO4: Eu3+. In addition, the Ba2+ doping also makes the charge transfer state absorption band of VO43− blue shifted and widens the 5D0→7F2 emission of Eu3+. The underlying mechanism for these results has been investigated using the first principles calculations. It has been shown that the Ba2+ doping increases the bandgap value of YVO4 and causes the energy level of the Eu-d orbit to split, and the individual atomic orbitals move toward higher energy after Ba2+ doping. A higher energy transfer efficiency from VO43− to Eu3+ has been predicted by the first principles calculations. The theoretical calculation results explain the photoluminescence enhancement of YVO4: Eu3+ by Ba2+ doping, and the mechanism has been revealed.

Near infrared organic photodetectors based on enhanced charge transfer state absorption by photonic architectures

Solution processed organic photodetectors with a nanostructured active layer in the shape of a photonic crystal exhibit an improved NIR response, below the band gap of the active layer materials, that can be tuned by varying the lattice parameter.

Analysis of bulk heterojunction organic solar cell blends by solid-state NMR relaxometry and sensitive external quantum efficiency – Impact of polymer side chain variation on nanoscale morphology

Abstract A significant number of organic electronic devices rely on blends of electron-donating and electron-accepting molecules. In bulk heterojunction organic photovoltaics, the nanoscopic phase behavior of the two individual components within the photoactive layer has a major impact on the charge separation and charge transport properties. For polymer:fullerene solar cells, it has been hypothesized that an increased accessibility of the electron-deficient monomer unit in push-pull type low bandgap polymers allows for fullerene ‘docking’. The close proximity of electron donor and acceptor molecules enables more efficient charge transfer, which is beneficial for the device efficiency. With this in mind, we synthesized a series of PBDTTPD [poly(benzodithiophene-thienopyrroledione)] low bandgap copolymers with varying side chains. Solar cells were fabricated for all polymers and the device characteristics were compared. The combination of proton wideline solid-state NMR (ssNMR) relaxometry and sensitive external quantum efficiency (sEQE) measurements was shown to provide essential information on donor-acceptor interactions and phase separation in bulk heterojunction organic photovoltaics. The reduced charge transfer state absorption and the observed phase separation of crystalline PC71BM domains for the polymers containing the most accessible methyl-TPD unit indicate a diminished contact between donor and acceptor, leading to a loss in performance.

Charge Generation via Relaxed Charge-Transfer States in Organic Photovoltaics by an Energy-Disorder-Driven Entropy Gain

In organic photovoltaics, efficient charge generation relies on our ability to convert excitons into free charges. Efficient charge separation from “energetic excitons” has been understood to be governed by delocalization effects promoted by molecular aggregation. A remaining puzzle is, however, the mechanism underlying charge generation via relaxed interfacial charge-transfer (CT) excitons that also exhibit an internal quantum efficiency close to unity. Here, we provide evidence for efficient charge generation via CT state absorption over a temperature range of 50–300 K, despite an intrinsically strong Coulomb binding energy of about 400 meV that cannot be modified by fullerene aggregation. We explain our observation by entropy-driven charge separation, with a key contribution from energy disorder. The energy disorder reduces the charge generation barrier by substantially gaining the entropy as electron–hole distance increases, resulting in efficient CT exciton dissociation. Our results underline an emer...

Revealing the Full Charge Transfer State Absorption Spectrum of Organic Solar Cells

Revealing the Full Charge Transfer State Absorption Spectrum of Organic Solar Cells

Fine structural tuning of diketopyrrolopyrrole-cored donor materials for small molecule-fullerene organic solar cells: A theoretical study

Two novel strategies to tune diketopyrrolopyrrole and its derivatives by lactam-lactim and alkoxy-thioalkoxy exchange were theoretically presented for improving the efficiency of bulk heterojunction solar cells. The structural tunings could synergistically reduce the HOMO level and the energy gap of donors due to the disrupted aromaticity of the linked pyrrole derivatives. Compared with the parent molecule, the new designed donors not only create a more red-shift of absorption spectrum but also show better hole transport rates, larger fill factor, higher open circuit voltage and more favorable solar cell efficiency. Moreover, the arrangements at the interface and the optical properties of the donor-PCBM complexes were computationally investigated to get insight into the absorption of charge transfer states. Consequently, the strategies are judicious approaches to enhance their intrinsic properties of donors and can be used for further improving the performance of other DPP-based molecules in bulk heterojunction solar cells.

Characterizing the Polymer:Fullerene Intermolecular Interactions

Polymer:fullerene solar cells depend heavily on the electronic coupling of the polymer and fullerene molecular species from which they are composed. The intermolecular interaction between the polymer and fullerene tends to be strong in efficient photovoltaic systems, as evidenced by efficient charge transfer processes and by large changes in the energetics of the polymer and fullerene when they are molecularly mixed. Despite the clear presence of these strong intermolecular interactions between the polymer and fullerene, there is not a consensus on the nature of these interactions. In this work, we use a combination of Raman spectroscopy, charge transfer state absorption, and density functional theory calculations to show that the intermolecular interactions do not appear to be caused by ground state charge transfer between the polymer and fullerene. We conclude that these intermolecular interactions are primarily van der Waals in nature.

Broadband down-conversion through the co-contribution of simultaneous energy transfer from Eu3+/2+ to Yb3+ and CTS absorption of Yb3+

The europium (Eu) and ytterbium (Yb) co-doped SiO2–Al2O3–MF2 (M = Ca, Sr) oxyfluoride glasses were prepared by the melting-quenching method, in which the partial reduction of Eu3+ ions during high temperature fabrication in air atmosphere was explained by optical basicity model. In this system, the near-infrared emission of Yb3+ at 974 nm can be observed upon the excitation of both Eu3+ and Eu2+, as well as charge transfer state absorption of Yb3+. The steady and time-resolved luminescence spectra measurements were carried out to further prove the energy transfer from Eu3+/2+ ions to Yb3+. Based on the measured luminescence decay of Eu3+ and Eu2+, the energy transfer efficiency and the quantum efficiency are much higher in the glass containing SrF2 due to the efficient energy transfer from both Eu3+ and Eu2+ to Yb3+.

Role of Charge Transfer States in P3HT-Fullerene Solar Cells

By examining poly(3-hexylthiophene)-fullerene blend solar cells with controlled variations of the active layer using subgap external quantum efficiency combined with intensity dependent quantum efficiency at short circuit, a direct correlation between trap-assisted recombination parameters and the charge transfer state absorption strength is established. Most significantly, by comparing devices with different polymer molecular weight, regioregularity, C60 derivative chemistry, and film drying speed, we find that samples with higher trap-assisted recombination exhibit higher charge generation efficiency. Both findings are in support of the general view that the charge transfer states are involved in both processes. In addition, we find that the traps are full at about 100 mW/cm2, which suggests, together with the correlation with the charge transfer states, that the electrochemical potential difference in the bulk exceeds the open circuit voltage and that the loss of voltage is probably due to recombination associated with charge extraction at the electrodes.

The effect of the size of raw Gd(OH) 3 precipitation on the crystal structure and PL properties of Gd 2O 3:Eu

Our experiments show that cetyltrimethylammonium bromide (CTAB) has an obvious effect on the particle size of the precipitation of Gd(OH) 3. Without CTAB, the particle size of the Gd(OH) 3 precipitation is about 50 nm, whereas with CTAB, the particle size is less than 5 nm. We propose a mechanism where CTAB micelles work as a catalyst for the nucleation process and result in the precipitation of 5 nm particles. These small particles, after heat treatment at 1073 K, did not result in small Gd 2O 3 nanoparticles of the same size, but instead, forming crystals of 30–80 nm size in both cubic and monoclinic phases. When monitoring the photoluminescence (PL) peak at 610 nm, we found that the charge transfer state (CTS) absorption amplitude of Eu–O is reduced in the mixed structures. We speculate that the mixed structures modify the surface properties of the Gd 2O 3:Eu nanoparticles, leading to the reduction of Eu–O CTS absorption.

Luminescent Properties of Eu3 + and Tb3 + Activated Zn3Ta2 O 8

This paper reports a systematic investigation on photoluminescent (PL) properties of and -activated The excitation spectrum of consists of the strong CTS absorption and week f-f transitions of while the excitation spectrum of is composed of the strong d-f transition and weak f-f transitions of The charge transfer state (CTS) band of and the d-f peak of slightly shift to lower energy with increase of dopant contents. exhibits red emission peaks around 610-620 nm, which is related to the transitions of The luminescence of is dominated by green emission peaks at 546 nm, which can be attributed to the transition of For codoped energy transfer process takes place and the resonant mechanism is proposed as the transfer mechanism which can be described as: PL spectra and lifetime measurements support this mechanism. Due to the energy transfer, PL intensity of emission increased about 40%. © 2003 The Electrochemical Society. All rights reserved.

Polarized fs transient absorption of CT states in An-PMDA crystals—Excitonic strings?

Polarized sub-ps transient absorption spectra of anthracene-pyromellitic dianhydride charge-transfer (CT) single crystals reveal four different transient signal contributions. Three of these signals are directly related to the CT state and characterize an ultrafast generation process within 150 fs after photoexcitation. Particular emphasis is put on the different spectroscopic features and the origin of the fourth signal.