高效率的有机电致发光器件

有机电致发光器件 (OL EDs)的发光机理包括电子和空穴从电极的注入、激子的形成及复合发光 ,其中 ,空穴和电子的注入平衡是非常重要的。为了平衡载流子的注入以得到高效率和稳定性好的器件 ,人们不仅使用了电子注入更为有效的 L i F/ Al[1] 和 Cs F/ Al[2 ] 等复合电极 ,同时也使用了空穴缓冲层 ,如 S.A.Van Slyke等 [3]在ITO和 NPB之间使用 Cu Pc,使得器件的稳定性得到了明显的提高 ;A.Gyoutoku等[4 ] 用碳膜使器件的半寿命超过 3 5 0 0小时 ;最近 ,Y.Kurosaka等 [5]和 Z.B.Deng[6 ]分别在 ITO和空穴传输层之间插入一薄层 Al…     

Two-carrier transition in radiative recombination of two-dimensional electrons in GaAs/AlGaAs

We observed photoluminescence (PL) and photoluminescence excitation (PLE) spectra due to shake-up processes of recombination of two-dimensional electrons and free excitons in a modulation-doped GaAs quantum well at He temperatures. One of the processes is that when an electron recombines with a hole, another electron is excited from the conduction band in GaAs to that in AlGaAs. The other process is that a hole is excited from an acceptor level or the valence band in GaAs to the valence band in AlGaAs during recombination. The electron process is observed in both PL and PLE spectra while the hole process only in the PL spectra. The excitation-intensity dependence of the peak intensity of hole-excited PL is almost quadratic, indicating three-carrier process in the shake-up process. The band offsets of the conduction and valence bands are estimated to be 220 and 146 meV, respectively.     

Electron heating through a set of random levels in the conduction band of insulators induced by femtosecond laser pulses

We develop a theory describing the heating of electrons in crystalline insulators irradiated by high-intensity laser pulses. In agreement with photoelectron yield versus intensity measurements, we assume that electrons are excited into the conduction band from defect layers and traps. The electron dynamics due to direct inter-branch transitions within the conduction band is simulated by solving of time-dependant Schr?dinger equation. The set of levels for this equation is supposed to be random with a distribution function equal to the density of states in the conduction band. The influence of different parameters on the electron heating efficiency is studied. The theory is applied for diamond; the theoretical spectrum is in qualitative agreement with the experimental observations.     

Gate-dependent orbital magnetic moments in carbon nanotubes

We investigate how the orbital magnetic moments of electron and hole states in a carbon nanotube quantum dot depend on the number of carriers on the dot. Low temperature transport measurements are carried out in a setup where the device can be rotated in an applied magnetic field, thus enabling accurate alignment with the nanotube axis. The field dependence of the level structure is measured by excited state spectroscopy and excellent correspondence with a single-particle calculation is found. In agreement with band structure calculations we find a decrease of the orbital magnetic moment with increasing electron or hole occupation of the dot, with a scale given by the band gap of the nanotube.     

混合发光层有机电致发光器件中的多重成分发射

以等摩尔空穴传输材料TPD和电子传输材料PBD组成结构为ITO/TPD/TPD∶PBD/PBD/Al的混合物发光层有机电致发光(EL)器件,观察到了相对于组成材料的荧光光谱红移的宽发射带。通过比较EL光谱,光致发光光谱及EL光谱分解,表明电致发光中同时包含单体发射、激基复合物和电荷对复合物的发射。激基复合物为TPD的激发态TPD*与PBD的基态相互作用形成TPD*PBD类型的复合物,电荷对复合物是带电荷的空穴传输分子(D+)的空穴和电子传输分子(A-)的电子交叉复合而形成的(D+-A-)*复合物。各激发态在电场作用下呈现不同的形成机理和复合过程,并且单体发射和激发态复合物的比例随电场而变化,导致发射光谱随电场增强而蓝移。该器件的最高亮度和最大外部量子效率分别为240 cd·(cm2)-1和0.49%。有机固态界面激基复合物或电荷对复合物的发射常出现宽的红移发射带,是调节发光颜色的有效手段。     

Deformation-induced excitation of the luminescence centres in coloured alkali halide crystals

The present paper reports the deformation-induced excitation of the luminescence centres in coloured alkali halide crystals. The peaks of the mechanoluminescence (ML) in γ-irradiated KCl, KBr, KI, NaCl and LiF crystals lie at 455, 463, 472, 450 and 485 nm, i.e. at 2.71, 2.67, 2.62, 2.75 and 2.56 eV, respectively. From the similarity between the ML spectra and the thermoluminescence (TL) and afterglow spectra, the ML of KCl, KBr, KI, NaCl and LiF crystals can be assigned to the deformation-induced excitation of the halide ions in V₂-centres or any other hole centres. For the deformation-induced excitation of the halide ions in V₂-centres, or in other centres, the following four models may be considered: (i) free electron generation model, (ii) electron–hole recombination model, (iii) dislocation exciton radiative decay model and (iv) dislocation exciton energy transfer model. The dislocation exciton energy transfer model is found to be suitable for the coloured alkali halide crystals. According to the dislocation exciton energy transfer model, during the deformation of solids the moving dislocations capture electrons from the F-centres and then they capture holes from the hole centres and consequently the formation of dislocation excitons takes place. Subsequently, the energy released during the decay of dislocation excitons excites the halide ions of the V₂-centres or any other hole centres and the light emission occurs during the de-excitation of the excited halide ions, which is the characteristic of halide ions. The mechanism of ML in irradiated alkali halide crystals is different from that of the TL in which the electrons released form F-centres due to the thermal vibrations of lattices reach the conduction band and the energy released during the electron–hole recombination excites the halide ions in V₂-centres or in any other hole centres. It is shown that the phenomenon of ML may give important information about the dislocation bands in coloured alkali halide crystals.     

Paramagnetic frenkel defects in irradiated alkali-haloid crystals

In the EPR spectra of γ-irradiated NaF,⁶LiF, and LiF crystals with natural content of isotopes (independent of the impurity composition), the hyperfine structure (HFS) is observed against the background of a broad band. Absorption saturation in the system of defects responsible for the HFS and the broadband occurs at widely different power levels of microwave radiation, and broad band suppression takes place at registration in quadrature. The experimentally measured intensity distribution and the number of EPR lines in the⁶LiF crystal correlate with the calculated data when the spin interaction of an unpaired electron with 14 equaivalent fluorine nuclei is taken into account. A model of major radiation-induced paramagnetic defects in the form of Frenkel pairs, in which one component (the negatively charged quasi-molecule consisting of two halogen atoms) can be responsible for the HFS and the other component (F-center) for the broad band in the EPR spectrum, is proposed.     

Electronic excitations during grazing scattering of hydrogen atoms on KI(001) and LiF(001) surfaces

The energy loss of hydrogen atoms with energies of 400 eV and 1 keV is studied in coincidence with the number of emitted electrons during grazing scattering from atomically clean and flat KI(001) and LiF(001) surfaces. The energy loss spectra for specific numbers of emitted electrons are analyzed in terms of a binary interaction model based on the formation of transient negative ions via local capture of valence band electrons from anion sites. Based on computer simulations we derive for this interaction scenario probabilities for the production of surface excitons, for electron loss to the conduction band of KI, for emission of electrons, and for formation of negative hydrogen ions. The pronounced differences of data obtained for the two surfaces are attributed to the different electronic structures of KI and LiF.     

An algorithm for unified analysis on the thermoluminescence glow curve

An algorithm was developed to integrally handle excitation by radiation, relaxation and luminescence by thermal or optical stimulation in thermoluminescence (TL) and optically stimulated luminescence (OSL) processes. This algorithm reflects the mutual interaction between traps through a conduction band. Electrons and holes are created by radiation in the beginning, and these electrons move to the trap through the conduction band. These holes move to the recombination center through a valence band. The ratio of the electrons allocated to each trap differs with the recombination probability and these values also relevant to the process of luminescence. Accordingly, the glow curve can be interpreted by taking the rate of electron–hole pairs created by ionizing radiation as a unique initial condition. This method differs from the conventional method of interpreting the measured glow curve with the initial electron concentration allocated to each trap at the end of irradiation. A program using the Visual Studio's C# subsystem was made to realize such a developed algorithm. To verify this algorithm it was applied to LiF:Mg,Cu,Si. The TL glow curve was deconvoluted with a model of five traps, one deep trap and one recombination center (RC).     

Electron and hole spectra of silicon quantum dots

In the framework of perturbation theory, the first several one-particle energies and wave functions for electrons and holes (six for each) in spherical silicon quantum dots are obtained in the envelope function approximation (kp method). It is shown that the model of an isotropic dispersion relation with the mean reciprocal effective mass is applicable for the ground state of holes in the valence band. Anisotropy of the dispersion relation, which takes place for bulk semiconductors, becomes significant for the electron ground state in the conduction band as well as for all excited (both electron and hole) states.     

Luminescence in x-irradiated CaF2:Co

Photoluminescence of X-irradiated CaF₂:Co single crystals is reported. The emission spectrum shows four peaks at 505, 550, 640 and 685 nm, all of them with an excitation band at 275 nm. The same emission spectrum, plus a band at 280 nm, is found in X-ray excited luminescence measurements. Thermoluminescence of 80 K X-irradiated crystals gives a glow curve with five peaks at 100, 125, 145, 190 and 225 K. The spectral distribution of these glow peaks is similar to that of the X-ray excited luminescence. The 280 nm band is associated with electron—hole recombination. The other four bands are associated with electron transitions among excited states of Co²⁺ produced by recombination of holes and Co⁺-ions created by X-irradiation.     

钻石中硼受主电荷转移引发磷光的直接证据

受主元素硼与不带电荷的替位硼进行电荷重组会导致钻石发出磷光,但Ⅱb型钻石硼含量较高,难以为电荷重组与磷光的相关性提供直接证据。作者针对一粒在常光环境下近无色的钻石,利用DiamondViewTM的深紫外强光源照射样品,使之产生蓝绿色磷光,结合前人研究,推断该类型磷光与含硼元素有关。采集样品红外光谱,在磷光消退前,谱图呈现2 803 cm-1吸收峰,磷光消失后采集的红外谱图不显示2 803 cm-1峰。这一实验中受主硼在紫外光激发下失去电子,成为不带电荷的替位硼原子(B0),B0浓度升高超过红外光谱检测限,红外光谱识别到B0的存在,即引发2 803 cm-1吸收峰。处于激发态的电子返回基态与亚稳态的B0结合过程释放出光子产生磷光。该实验首次直接证实了硼受主与无电荷替位硼之间的电荷重组转移会引发钻石磷光。     

硼氢协同掺杂Ib型金刚石大单晶的高温高压合成与电学性能研究

在压力6.0 GPa和温度1600 K条件下, 利用温度梯度法研究了(111)晶面硼氢协同掺杂Ib型金刚石的合成. 傅里叶红外光谱测试表明: 氢以sp³杂化的形式存在于所合成的金刚石中, 其对应的红外特征吸收峰位分别位于2850 cm^-1和2920 cm^-1处. 此外, 霍尔效应测试结果表明: 所合成的硼氢协同掺杂金刚石具有p型半导体材料特性. 相对于硼掺杂金刚石而言, 由于氢的引入导致硼氢协同掺杂金刚石电导率显著提高. 为了揭示硼氢协同掺杂金刚石电导率提高的原因, 对不同体系进行了第一性原理理论计算, 计算结果表明其与实验结果符合. 该研究对金刚石在半导体领域的应用有重要的现实意义.     

光电容法研究电子辐照GaP中的深能级

本文提出含多个深能级响应的光电容瞬态分析方法:在不考虑各能级之间电子、空穴跃迁的条件下,可出“多指数过程分离法”,将总的瞬态过程分离为各能级上指数型瞬态过程之和.运用这一方法,对lMeV(4×1015cm-2电子辐照GaP LED进行了定态和两种注入条件的瞬态光电容测量,观察到H1、H2、H3三个空穴能级(0.51、0.75、1.15eV)和E1、E2、E3、E4四个电予能级(0.68、0.84、0.89、1.01eV),并得到各能级的光离化截面谱.外量子效率及发射谱测量结果表明;电子辐照引入的深能级(H1-H3,E1-E4)表现出无辐射复合中心的性质.     

First-principles Band Structures Calculation of Tin-phthalocyanine

SnPc(Tin-phthalocyanine)因在无机/有机二极管等光电结构器件中表现出了很多有趣的特性而备受关注.为了更深地理解载流子的传输特性,利用密度泛函理论,采用广义梯度近似(DFT-GGA),关联函数选择BLYP计算了SnPc的能带结构.从点波函数、能带带宽以及带隙分析了载流子的传输行为. 从前线轨道的带宽以及电子和空穴的有效质量,可以看到电子的传输要比空穴的传输容易两倍左右.而且,当研究费米能级附近的能带时,发现未占有带的带隙总体上要小于占有带的带隙,这表明在考虑声子参与的情况下,电子在带间的跳跃要比空穴容易得多.以上的事实说明SnPc是一种电子传输占主导的材料.     

Relaxation of vibrationally excited reaction products in a crystal lattice

The relaxation of vibrationally excited nitrogen molecules in the matrix of silver azide is studied. The effective rate constants for the interaction of an excited nitrogen molecule with free charge carriers (1.7 × 10^–10 cm³ s–1) and for the generation a hole from the level of the produced defect at the expense of the vibrational energy of the molecule (3 × 10¹⁰ s^–1) are estimated. An associative–dissociative mechanism of the deactivation is proposed, which consists in the capture of an electron onto the level of the produced defect with the subsequent emission of an electron into the conduction band at the expense of the vibrational energy of the excited molecule. The effective rate constants for electron emission from the excited level of a hydrogen-like defect at values of the principal quantum number of 3 and 4 are estimated as 1.8 × 10⁹ and 2.8 × 10⁹ s^–1, respectively. Based on the processes considered, an expression for the probability of chain propagation is obtained.     

Dynamics of exciton formation at the Si(100) c(4 x 2) surface

Carrier recombination at the Si(100) c(4 x 2) surface and the underlying surface electronic structure is unraveled by a combination of two-photon photoemission and many-body perturbation theory: An electron excited to the silicon conduction band by a femtosecond infrared laser pulse scatters within 220 ps to the unoccupied surface band, needs 1.5 ps to jump to the band bottom via emission of optical phonons, and finally relaxes within 5 ps with an excited hole in the occupied surface band to form an exciton living for nanoseconds.     

Investigation of Auger recombination in Ge and Si nanocrystals embedded in SiO2 matrix

We study theoretically the optical properties of embedded Ge and Si nanocrystals (NCs) in wide band-gap matrix and compared the obtained results for both NCs embedded in SiO₂ matrix. We calculate the ground and excited electron and hole levels in both Ge and Si nanocrystals (quantum dots) in a multiband effective mass approximation. We use the envelope function approximation taking into account the elliptic symmetry of the bottom of the conduction band and the complex structure of the top of the valence band in both Si and Ge (NCs). The Auger recombination (AR) in both nanocrystals is thoroughly investigated. The excited electron (EE), excited hole (EH) and biexciton AR types are considered. The Auger recombination (AR) lifetime in both NCs has been estimated and compared.     

Conduction electron-exciton complexes in semiconductor single crystals

We consider the possibility of a bound state being formed from the pairing of an excited electron in the conduction band with an exciton in a semiconductor at low temperatures. The model consists of two levels (the valence and conduction bands) for a simple cubic lattice with periodic boundary conditions and the exciton is intermediate between the Wannier and Frenkel type excitons. The exciton which is discussed consistst of a tightly bound electron from the conduction band and a hole from the valence band on the same lattice site. Electrons and holes are, however, allowed to hop independently between nearest-neighbour lattice sites. The dispersion relations which determine the exciton and the electron-exciton modes are solved numerically. It is found that there are two branches for the coupled mode frequencies. This physical picture is analogous to that for polaritons and magnon-phonon modes in crystals.     

Pulsed photoconductivity in diamond upon quasi-continuous laser excitation at 222 nm at the formation of an electron–hole liquid

An order-of-magnitude enhancement of the pulsed photocurrent in a polycrystalline diamond sample synthesized by chemical vapor deposition is observed under the conditions of formation of an electron–hole liquid. Nonequilibrium charge carriers are excited by laser pulses at a wavelength of 222 nm with FWHM pulse duration of 18 ns and peak intensity above 2.5 MW/cm² upon cooling the sample to 90 K. For peak intensities of laser excitation lower than 1 MW/cm², sample cooling from 300 to 90 K leads to a decrease in pulsed photocurrent by about a factor of 5. The observed increase in pulsed photocurrent is attributed to the formation of the electron–hole liquid.