Xu Xu-Rong

Characteristics of Solid State Cathodoluminescence of PPV

Based on our previous discovery [Chem. Phys. Lett. 325 (2000) 420] of the solid-state cathodoluminescence from organic luminescent materials in inorganic/organic heterojunction, we study characteristics of this new kind of electric-field-induced luminescence by means of examining its oscillogram. We prepared three devices with different structures in which PPV was used as luminescent layer, and SiO2 was used as accelerating layer. The experimental results might be understood only by means of the existence of solid-state cathodoluminescence. This new kind of luminescence makes it possible to produce new type of flat panel display.

Performance improvement of MEH-PPV:PCBM solar cells using bathocuproine and bathophenanthroline as the buffer layers

In this work, bathocuproine (BCP) and bathophenanthroline (Bphen), commonly used in small-molecule organic solar cells (OSCs), are adopted as the bufier layers to improve the performance of the polymer solar cells (PSCs) based on poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction. By inserting BCP or Bphen between the active layer and the top cathode, all the performance parameters are dramatically improved. The power conversion e‐ciency is increased by about 70% and 120% with 5-nm BCP and 12-nm Bphen layers, respectively, when compared with that of the devices without any bufier layer. The performance enhancement is attributed to BCP or Bphen (i) increasing the optical fleld, and hence the absorption in the active layer, (ii) efiectively blocking the excitons generated in MEH-PPV from quenching at organic/aluminum (Al) interface due to the large band-gap of BCP or Bphen, which results in a signiflcant reduction in series resistance (Rs), and (iii) preventing damage to the active layer during the metal deposition. Compared with the traditional device using LiF as the bufier layer, the BCP-based devices show a comparable e‐ciency, while the Bphen-based devices show a much larger e‐ciency. This is due to the higher electron mobility in Bphen than that in BCP, which facilitates the electron transport and extraction through the bufier layer to the cathode.

Optical absorption studies on the trapping states of CaS:Eu,Sm

The optical absorption spectra of CaS:Eu,Sm at several concentrations of Eu (from 0 to 0.05 mol%) were examined to clarify the electron trapping mechanism for its photo-stimulated luminescence. Corresponding to the electron and hole trapping states, the difference between the absorption spectra after and before excitation was observed. The substructure of the infrared band of the absorption difference is eclipsed by the 4f-4f absorption of Sm3+ ions in accordance with the photo-stimulation spectrum. In comparison of Sm3+ with La3+ in Eu co-doped CaS, it was inferred that the Sm3+ ions are responsible for the storability of the traps instead of the direct capture of electrons as the previous investigators thought. Our results demonstrate that the Sm3+ ions induce defects in their neighbourhood which act as the trapping centres but the Sm3+ ions do not form Sm2+ ions by trapping electrons.

The studies on the upconversion mechanism of ZBLAN: Er3+, Yb3+

Abstract ZBLAN glass is an interesting host for rare earth doped up-conversion materials due to a lot of advantages. In this paper, we studied the up-conversion mechanism of ZBLAN co-doped with Er 3+ and Yb 3+ ions. The energy transfer between Yb 3+ and Er 3+ was described.

Influence of Concentration of Vanadium in Zinc Oxide on Structural and Optical Properties with Lower Concentration

ZnO films doped with different vanadium concentrations are deposited onto glass substrates by dc reactive magnetron sputtering using a zinc target doped with vanadium. The vanadium concentrations are examined by energy dispersive spectroscopy (EDS) and the charge state of vanadium in ZnO thin films is characterized by x-ray photoelectron spectroscopy. The results of x-ray diffraction (XRD) show that all the films have a wurtzite structure and grow mainly in the c-axis orientation. The grain size and residual stress in the deposited films are estimated by fitting the XRD results. The optical properties of the films are studied by measuring the transmittance. The optical constants (refractive index and extinction coefficient) and the film thickness are obtained by fitting the transmittance. All the results are discussed in relation with the doping of the vanadium.

Thickness dependence of surface morphology and charge carrier mobility in organic field-effect transistors

With the aim of understanding the relationships between organic small molecule field-effect transistors (FETs) and organic conjugated polymer FETs, we investigate the thickness dependence of surface morphology and charge carrier mobility in pentacene and regioregular poly (3-hexylthiophene) (RR-P3HT) field-effect transistors. On the basis of the results of surface morphologies and electrical properties, we presume that the charge carrier mobility is largely related to the morphology of the organic active layer. We observe that the change trends of the surface morphologies (average size and average roughness) of pentacene and RR-P3HT thin films are mutually opposite, as the thickness of the organic layer increases. Further, we demonstrate that the change trends of the field-effect mobilities of pentacene and RR-P3HT FETs are also opposite to each other, as the thickness of the organic layer increases within its limit.

Temperature effect on the photoluminescence intensity and Eu2+ excited state lifetime in EuGa2S4 and EuGa2S4:Er3+

The photoluminescence (PL) spectra and Eu2+ excited state lifetime of EuGa2S4 and EuGa2S4:Er3+ have been studied in the range 78–500 K. The spectra show a band at 545 nm, due to the 4f65d → 4f7(8S7/2) transition. With increasing temperature, the full width at half maximum Γ(T) of the PL band of EuGa2S4 and EuGa2S4:Er3+ crystals increases from 0.15 to 0.22 and from 0.13 to 0.19 eV, respectively. Over the entire temperature range studied, Γ(T) is a linear function of T1/2. The 545-nm emission intensity and Eu2+ excited state lifetime in EuGa2S4 and EuGa2S4:Er3+ vary exponentially with temperature. The luminescence quenching energies evaluated from the Arrhenius plots of I(103/T) and τ(103/T) coincide (0.10 eV) within the error of determination.

Electric Field-Induced Quenching of Photoluminescence from Ir(PPY)3-Doped PVK

We study the photoluminescence (PL) and the PL dynamics of Ir(PPY)3-doped poly(N-vinylcarbazol) (PVK) under the modulation of an electric field. The results show that the electric-field-induced quenching of PL from Ir(PPY)3-doped PVK mainly comes from the dissociation of excitons in the chains of PVK. There is no significant difference in the excited state lifetime of Ir(PPY)3 to be observed under the different applied negative biases. Our experiments demonstrate that the excitons attached to the molecules of Ir(PPY)3 are very stable.

Enhanced organic electroluminescence (OEL) by integration with cathodoluminescence-like (cl-like) emission ☆

Using the secondary properties of inorganic material such as electron multiplication and acceleration, we discovered the cathodoluminescence-like (CL-like) emission. Now, using heterojunction and brightness waveform we investigated the temporary properties of the two peaks in CL-like emission. Further we found the identity of phase relation of luminescence relative to sinusoidal voltage for two different unsymmetrical devices and revealed the existence of CL-like excitation and the mutual enhancement of CL-like emission and organic electroluminescence. These processes are mutually compensatory additive and one strengthening the others. Choosing appropriate material and structural parameters might control the luminescence properties.

Transient demonstration of exciton behaviours in solid state cathodoluminescence under different driving voltage

In the solid state cathodoluminescence (SSCL), organic materials were excited by hot electrons accelerated in silicon oxide (SiO2) layer under alternating current (AC). In this paper exciton behaviours were analysed by using transient spectra under different driving voltages. The threshold voltages of SSCL and exciton ionization were obtained from the transient spectra. The recombination radiation occurred when the driving voltage went beyond the threshold voltage of exciton ionization. From the transient spectrum of two kinds of luminescence (exciton emission and recombination radiation), it was demonstrated that recombination radiation should benefit from the exciton ionization.