Dependence of resonance energy transfer on exciton dimensionality

We investigate the dependence of resonance energy transfer from Wannier-Mott excitons to an organic overlayer on exciton dimensionality. We exploit the excitonic potential disorder in a single quantum well to tune the balance between localized and free excitons by scaling the Boltzmann distribution of excitons through temperature. Theoretical calculations predict the experimentally observed temperature dependence of resonance energy transfer and allow us to quantify the contribution of localized and free excitons. We show that free excitons can undergo resonance energy transfer with an order of magnitude higher rate compared to localized excitons, emphasizing the potential of hybrid optoelectronic devices utilizing resonance energy transfer as a means to overcome charge transfer related limitations.     

Ir(ppy)3掺杂PVK体系发光特性的研究

对常温下磷光染料Ir(ppy)3掺杂PVK薄膜的光致发光(PL)和电致发光(EL)特性进行了研究。器件结构为ITO/PEDOT:PSS/PVK:Ir(ppy)3/BCP/Alq3/Al。实验发现随磷光材料掺杂浓度的不同,器件的发光性能发生变化。当浓度适宜时,主体材料PVK的发光很弱,主要为Ir(ppy)3的磷光发射。通过L-I-V特性曲线的比较,掺杂浓度为5%的光电性能最好,说明器件在掺杂浓度为5%时效果最佳。     

Ir(PPY)3掺杂PVK的电致发光机理

近几年来发展起来的电致磷光(electrophosphorescence)是有机发光二极管(OLED)研究的新生长点。对电致磷光发光机理的研究随即得到了人们普遍的关注。比较了不同正向偏压条件下Ir(PPY)₃掺杂聚乙烯基咔唑(PVK)的光致发光(PL)和电致发光(EL)光谱。研究结果显示在电场和注入电流的共同作用下,PL光谱中基质PVK发光的相对强度并没有发生显著的变化。电场或注入载流子不会影响PVK向Ir(PPY)₃的能量传递。磷光掺杂聚合物EL主要是由于载流子在掺杂磷光分子上的直接复合,而不是由基质向磷光掺杂分子的能量传递。     

Nongeminate recombination in organic bulk heterojunction solar cells: Contribution of exciton–polaron interaction

A new model to explain nongeminate recombination in organic bulk heterojunction solar cells is presented. We suggest that the annihilation of excitons on charge carriers at the interface between donor and acceptor phases competes with the bimolecular recombination of Coulombically bound electron–hole pairs. The exciton–polaron interaction gives visible contribution to the reduction of Langevin recombination. An analytical formula, which describes the reduction prefactor, has been derived. We demonstrate that exciton–charge carrier interactions cause an increase of the recombination order. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excitonic processes at organic heterojunctions

Understanding excitonic processes at organic heterojunctions is crucial for development of organic semiconductor devices. This article reviews recent research on excitonic physics that involve intermolecular charge transfer (CT) excitons, and progress on understanding relationships between various interface energy levels and key parameters governing various competing interface excitonic processes. These interface excitonic processes include radiative exciplex emission, nonradiative recombination, Auger electron emission, and CT exciton dissociation. This article also reviews various device applications involving interface CT excitons, such as organic light-emitting diodes (OLEDs), organic photovoltaic cells, organic rectifying diodes, and ultralow-voltage Auger OLEDs.     

The excitonic insulator route through a dynamical phase transition induced by an optical pulse

We consider a dynamical phase transition induced by a short optical pulse in a system prone to thermodynamical instability. We address the case of pumping to excitons whose density contributes directly to the order parameter. To describe both thermodynamic and dynamic effects on equal footing, we adopt a view of the excitonic insulator for the phase transition and suggest a formation of the Bose condensate for the pumped excitons. The work is motivated by experiments in donor–acceptor organic compounds with a neutral- ionic phase transition coupled to the spontaneous lattice dimerization and to charge transfer excitons. The double nature of the ensemble of excitons leads to an intricate time evolution, in particular, to macroscopic quantum oscillations from the interference between the Bose condensate of excitons and the ground state of the excitonic insulator. The coupling of excitons and the order parameter also leads to self-trapping of their wave function, akin to self-focusing in optics. The locally enhanced density of excitons can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The system is stratified in domains that evolve through dynamical phase transitions and sequences of merging. The new circumstances in experiments and theory bring to life, once again, some remarkable inventions made by L.V. Keldysh.     

Femtosecond study of the interplay between excitons, trions, and carriers in (Cd,Mn)Te quantum wells

We study the absorption by neutral excitons and positively charged excitons (trions) following a femtosecond, circularly polarized, resonant pump pulse. Three populations are involved: free holes, excitons, and trions, all exhibiting transient spin polarization. In particular, a polarization of the gas of free holes is created by the formation of trions. The evolution of these populations is described, including spin flip and trion formation. We evaluate the contributions of phase space filling and spin-dependent screening. We propose a new explanation of the oscillator strength stealing phenomena observed in doped quantum wells, based on the screening of neutral excitons by charge carriers. We have also found that binding holes into charged excitons excludes them from the interaction with the rest of the system, so that oscillator strength stealing is partially blocked.     

Observation of a donor exciton band in silicon

Far-infrared transmission measurements on P-doped Si at donor concentrations below the metal-insulator transition show absorption bands from donor pairs. We identify the lowest energy band as charge transfer excitations from a donor to its neigbhor, forming excitons (D⁺D^-) in the Mott-Hubbard gap. The results suggest that charge transfer states in complexes dominate the low-energy optical excitations as the insulator approaches the Mott density.     

自旋极化度对GaAs量子阱中吸收饱和效应与载流子复合动力学的影响研究

本文研究了(001) GaAs量子阱薄膜中重空穴激子近共振抽运-探测的载流子自旋弛豫动力学, 发现载流子的自旋极化对传统的线偏振光吸收饱和效应和载流子复合动力学都有影响. 进一步的抽运流依赖的自旋弛豫和复合动力学研究表明, 自旋极化对线偏振光的吸收饱和效应的影响随抽运流降低而变弱. 在低激发流时, 自旋极化对线偏振吸收饱和效应的影响才可忽略. 然而, 又显现出自旋极化对复合动力学的影响. 分析表明复合动力学的自旋极化依赖性起源于重空穴激子形成浓度的自旋极化依赖性. 复合动力学的自旋极化依赖性表明自旋弛豫时间计算中所涉及的复合时间应该使用自旋极化载流子的复合时间. 基于二维质量作用定律的激子浓度计算表明, 库仑屏蔽效应对激子形成的影响在较低激发载流子浓度下可以忽略.     

Excitons and excitonic complexes in GaAs/AlGaAs quantum wells with a low-density quasi-two-dimensional electron and hole channel

The recombination spectra of excitons and excitonic complexes in un-doped GaAs/AlGaAs single quantum wells are investigated. It is shown on the basis of a study of the magnetic-field dependence of the emission spectra and the degree of optical orientation in zero magnetic field and on the basis of electrooptic measurements that not only the density but also the sign of the charge carriers in a well depend strongly on the photoexcitation energy. It is shown on the basis of a comparative analysis of the spin splitting of the recombination lines of free and bound excitons that the recombination line which was attributed earlier to a positively charged exciton corresponds to the recombination of an exciton bound on a neutral acceptor. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 707–713 (10 May 1998)     

Quantized bimolecular auger recombination of excitons in single-walled carbon nanotubes

Auger-like exciton-exciton annihilation in isolated single-walled carbon nanotubes (SWNTs) has been studied by femtosecond transient absorption spectroscopy. We observe a quantization of the Auger recombination process and extract dynamics for 2 and 3 electron-hole pair excited states. We further demonstrate that Auger recombination in SWNTs is a two-particle process involving strongly bound excitons and not a three-particle Auger process involving unbound electrons and holes. We thus provide explicit experimental evidence for one-dimensional discrete excitons in SWNTs.     

Wavelength modulation spectroscopy of epitaxial iron garnet films

Derivative absorption and reflectance spectra are reported for epitaxially grown iron garnet films and compared with derivative absorption results for bulk yttrium gallium garnet dilutely doped with Fe³⁺ Results are briefly discussed in terms of crystal field transitions, charge transfer excitons, and charge transfer bands all involving the iron 3d electrons.     

Spin Physics of Excitons in Colloidal Nanocrystals

We present a review of spin-dependent properties of excitons in semiconductor colloidal nanocrystals. The photoluminescences (PL) properties of neutral and charged excitons (trions) are compared. The mechanisms and the polarization of radiative recombination of a “dark” (spin-forbidden) exciton that determines the low-temperature PL of colloidal nanocrystals are discussed in detail. The radiative recombination of a dark exciton becomes possible as a result of simultaneous flips of the surface spin and electron spin in a dark exciton that leads to admixture of bright exciton states. This recombination mechanism is effective in the case of a disordered state of the spin system and is suppressed if the polaron ferromagnetic state forms. The conditions and various mechanisms of formation of the spin polaron state and possibilities of its experimental detection are discussed. The experimental and theoretical studies of magnetic field-induced circular polarization of PL in ensembles of colloidal nanocrystals are reviewed.     

Exciton regeneration at polymeric semiconductor heterojunctions

Control of the band-edge offsets at heterojunctions between organic semiconductors allows efficient operation of either photovoltaic or light-emitting diodes. We investigate systems where the exciton is marginally stable against charge separation and show via E-field-dependent time-resolved photoluminescence spectroscopy that excitons that have undergone charge separation at a heterojunction can be efficiently regenerated. This is because the charge transfer produces a geminate electron-hole pair (separation 2.2-3.1 nm) which may collapse into an exciplex and then endothermically (E(A)=100-200 meV) back transfer towards the exciton.     

Transformation of charge transfer excitons into molecular excitons in molecular organic crystals

Formulae for persec probabilities of the nonradiative and radiative transformation of charge transfer excitons into molecular excitons in molecular organic crystals are derived and discussed. Numerical calculations are performed for solid naphtalene and anthracene.     

Indirect excitons and double electron-hole layers in a wide single GaAs/AlGaAs quantum well in a strong electric field

The recombination spectra of indirect excitons and double electron-hole layers in a wide single quantum well in an electric field are studied. It is found that electrons and holes in the wide well become spatially separated in a sufficiently strong electric field. This leads to a substantial reorganization of the radiative recombination spectrum and to a significant increase in the carrier lifetime. It is shown that the total charge of the electron-hole system can be changed by varying the photoexcitation frequency and the applied electric field, thus passing from the neutral case of indirect excitons to the case of charged double electron-hole layers. The concentration of excess carriers in the well is measured as a function of the electric field strength. The behavior of the excited states of indirect heavy-hole and light-hole excitions is studied for a neutral excitonic system in a strong electric field. It is shown that the electric-field dependences allow the excited states of indirect excitons with a light hole to be distinguished from the excited states with a heavy hole.     

Exciton decay in Cu2O at high density and low temperature: Auger recombination,spin–flip scattering,and molecule formation

In this paper, we examine the relative importance of three density-dependent decay mechanisms for excitons in cuprous oxide (Cu₂O). Rate-equation models including Auger recombination (A), spin–flip scattering (B) and capture into short-lived biexcitons (C) are compared to photoluminescence data for a crystal temperature of 2 K. Process B—converting two orthoexcitons into two paraexcitons by exchanging electrons and holes—leaves the total exciton number unchanged and is inconsistent with the late-time data. In processes A and C, paraexcitons re-generate high energy excitons in agreement with the late-time data; however, the existing theory of Auger recombination seems to eliminate process A, leaving molecule formation as the dominant process.     

Recombination-assisted creation of cation excitons and cross-luminescence quenching in CsCl crystals at high excitation densities

At high excitation densities, recombination-assisted creation of cation excitons, which transfer energy efficiently to the anion sublattice to initiate the luminescence of anion excitons and impurity centers, has been observed in CsCl crystals. At the same time, the creation of cation excitons competes with the electron recombination with cation holes and quenches the cross-luminescence. The intensity ratio of the cross-luminescence to exciton-impurity luminescence is different for crystal irradiation with γ rays and heavy particles.     

间隔层对红绿磷光有机电致发光器件发光性能的影响

制备了结构为ITO/MoO₃(40 nm)/NPB(40 nm)/TCTA(10 nm)/CBP∶GIr1(14%)∶R-4B(2%)(20 nm) /间隔层(3 nm)/ CBP∶GIr1(14%)∶R-4B(2%)(10 nm)/BCP(10 nm)/Alq₃(40 nm)/LiF(1 nm)/Al(100 nm) 的有机电致发光器件,间隔层分别为CBP,TCTA,TPBI和BCP,GIr1和R-4B分别为绿红磷光材料。通过加入不同间隔层来调控载流子和激子在发光层内的分布并研究了其对器件发光性能的影响。研究表明TCTA,TPBI和BCP分别作为间隔层的器件较CBP为间隔层的参考器件,电压为6 V时,电流效率分别高出59%,79%和93%,以BCP为间隔层的器件效率最高达到22.58 cd·A-1;TPBI和BCP为间隔层相对于以TCTA为间隔层的器件,在较高的电流密度下,效率滚降更小。分析原因TCTA间隔层较高的LUMO能级和三线态能量将电子和激子限制在较窄的复合区域,提高了载流子相遇形成激子的概率,在较高电流密度下猝灭也更严重;TPBI和BCP由于具有较高的HOMO能级和电子传输能力,拓宽了激子的复合区域。间隔层引起电子或空穴的累积,形成较高的空间电场,有利于发光层相应载流子的注入与传输。由于发光层掺杂方式为红绿共掺,器件均获得了较好的色坐标稳定性。