电子—电子相互作用对电致发光高分子极化特征的影响

用同时计入电子－电子相互作用,外电场及电子－晶格相互作用的理论模型研究分析了电致发光高分子的极化特征,发现在位电子相互作用和相邻格点间电子的相互作用明显增加高分子中双激子态的反问极化。本文还探讨了反向极化这悠闲的的物理现象的意义可能的应用。     

电场对ZnSe/ZnS应变层超晶格的影响

对加电场的ZnSe/ZnS应变层超晶格(SLSs)进行了光致发光测量,并观察到发光猝灭和峰向较低能侧的移动。这些现象与ZnSe阱中由电场引起的电子——空穴分离有关。作者用ZnSe/ZnS SLSs已成功地得到了光调制器,并证明了电场造成的吸收改变会起到光开关的作用。此外,得到了作为ZnSe/ZnS SLSs二次谐波的0.5235和0.436μm波长的相干光。     

极性半导体量子点中双极化子LO声子平均数的温度依赖性

采用Huybrechts线性组合算符法和Lee-Low-Pines变分方法研究了极性半导体量子点中双极化子性质的温度依赖性,推导出了量子点中双极化子的LO声子平均数的表达式。数值计算结果表明,双极化子的LO声子平均数随两电子间相对距离的增大或温度的升高而减小,随电子-LO声子耦合强度的增加而增大;两电子间的相对距离、电子-LO声子耦合强度和温度是影响双极化子束缚态稳定性的重要因素。     

极性晶体中的激子在磁场中的性质

本文采用Haga研究极化子、顾世洧研究激子时采用的方法讨论极性晶体中的激子在磁场中的性质.在忽略激子于反冲效应中发射的不同波矢的虚声子之间的相互作用的近似下,导出了激子的自陷能、电子-空穴有效作用势以及激子-声子- 磁场三者之间的耦合能.我们发现激子的自陷能与晶体的电子-空穴质量比有关,但是对于任意电子-空穴质量比,激子都是自陷的.我们还发现,由于晶格振动对激子运动的影响,使得激子在磁场中的抗磁性能移(diamagnetic shift)有所减弱.我们采用变分法对处于磁场中的激子-声子系统的基态能量进行了计算,并与Behnke的工作进行了比较.结果表明:在α<2的情况下,即使采用零级近似的有效势,已经能得到令人满意的结果.     

改善半导体聚合物的电致发光量子效率

某些共轭聚合物具有发光性能。有望作为发光二极管之类应用，其性能主要受最大可能的理论电致发光效率的限制。[1,2]。若最低能量激发态强束缚子处于单线态或三线态下的电子－空穴对），则电致发光的理论上限仅为相应光致发光量子效率的25％：即一个处于π＊－带内的电子和一个处于π-带的空穴（或缺少电子态）或形成一种自旋多重性为3的三线态，或一种自旋多重性为1的单线态。只有单线态才能辐射跃迁的而发光。如果电子－空穴对束缚能足够弱的话，电致发光和光致发光的最大量子效率之比理论上可接近于1。在本文，我们将报导，若在共轭聚合物内混入电子传输材料，则可改善电子注入特性，所得到的聚合物发光二极管的电致委光效率和光致发光效率之比可接近50％。若假它它们处于最低激发态，这个值得显然高于强束缚单线态和三线态激子的理论极限值。我们的这个结果意味着，这种激子的束缚能很弱，或者是形成单线态的几率可能要比形成三线态的几率高得多。     

光激发下的C60

用电子一晶格耦合的紧束缚模型研究了光激发下的C_(60)分子。结果表明当电子由最高占据态激发到最低空态时,晶格发生弛豫,新激发态是环状双极化子,它有下述特点:键结构由基态时只有2种键长的二聚化变为有7种不同键长的层状,与基态相比,键长变化如环状极化子,赤道附近变化最大,二极处近乎不变;有2个能级进入能隙,成为自陷束缚态,它们定域在赤道附近;对称性由基态时I_h群降为D_(5d)群;预期会伴随出现非线性激子和4条核磁共振谱线。     

二维强耦合磁双极化子的自旋极化基态性质

基于Lee-Low-Pines变分法研究了抛物量子点中二维强耦合磁双极化子的自旋极化基态性质,推导出磁双极化子的基态能量E0和声子平均数珡N随量子点受限强度ω0、介电常数比η、电子-声子耦合强度α和磁场的回旋频率ωc的变化规律。结果表明,磁双极化子的声子平均数珡N随α、ω0和ωc的增加而增大;磁双极化子的基态能量由两电子的单粒子能量Ee、两电子间库伦相互作用能Ec、电子自旋与磁场相互作用能ES、电子-声子相互作用能Ee-ph四部分组成;电子-声子相互作用能Ee-ph总是小于零,其绝对值随α、ω0和ωc的增加而增大,它是束缚态磁双极化子形成的有力因素;限定势和电子之间的库伦排斥能的存在不利于束缚态磁双极化子的形成;电子自旋-磁场相互作用的效应ES导致基态能级的"精细结构"。     

无序系统的次级光散射

本文试从无序系统的发光及喇曼散射中的几个问题出发,阐述次级光发射中的无序效应。Wolford研究了三元系GaAs_(1-x)P_x:N中束缚激子的发光,发现随着组份x值的变小,电-声子耦合增强,激子的半径变小。张新夷等人对三元系Ga_xIn_(1-x)P:N的束缚激子发光的研究也得到了同样的结论。在三元化合物中,束缚激子趋于局域,激子中电子和空穴的交换能增大。     

聚合物链中光致极化反转的动力学研究

近年来超快过程研究进展迅速，现分辨率已达到１ｆｓ，激发态的寿命是纳秒，因此有可能用超快过程方法研究激发态的响应过程和动力学演化激发态具有基态所不具有的新的物理性质，因此近年来光激发引起的新现象研究已成为物理、化学和材料等交叉学科领域中的一个新的生长点． 本工作用含时薛定谔方程和分子动力学方法研究了在外加电场作用下，具有一个额外电子和空穴的聚合物链（即自陷束缚单激子态）在吸收一个光子，变成具有两个额外电子和空穴的聚合物链（即自陷束缚双激子态）时，聚合物链中键结构和电荷密度分布的动力学演化过程．发现…     

有机半导体CBP双极传输特性研究

有机电致磷光器件典 型双极性主体材料4,4′-N,N′-二咔唑联苯(CBP,4,4′-N,N′-dicarbazolylbiphenyl)中空穴和电子迁移率大小相近,但究竟空 穴传输快还是电子传输快是一 个有争议的问题。为了比较CBP中空穴和电子传输的相对快慢,本文以CBP为激子产生层,制 备了多层结构 的发光器件。在CBP层中插入一层超薄磷光发光材料苯基吡啶铱(Ir(ppy)₃,fac-tris( 2-phenylpyridine)iridium)并 改变插入位置得到了不同颜色的发光器件,通过对不同器件发光性能的比较,研究了CBP的 双极传输特性。结果表明,在CBP中电子和空穴主要在靠近阳极一侧复合产生 激子,从而表明CBP中电子的迁移率大于空穴的迁移率。     

Dynamic recombination of polaron pair with impurity in conjugated polymers

The dynamic collision of an on-chain same charged polaron pair with an off-chain counterion trapped in an impurity molecule, under the influence of an external electric field, has been investigated using a nonadiabatic evolution method. We demonstrate that the bipolaron, the exciton and the trion are the main products in such a reaction, and that the trion can be formed by both the direct recombination of a positive polaron pair with a counterion and the secondary recombination of the formed bipolaron with either a trapped polaron, a charged defect, or an impurity of the opposite sign, which is consistent with the experimental evidence qualitatively. Meanwhile, our results show that the strength of the applied electric field and the impurity potential play important roles in the polaron pair-impurity collision. These results are expected to be helpful to understand the electroluminescence and conductivity in conjugated polymers.     

Effect of intrachain configuration disorder on the exciton delocalization in π-conjugated polymers

Exciton delocalization in conjugated polymers is investigated in view of its important contribution to the ultrafast charge separation recently focused in polymer solar cells (PSCs). Simulations are performed within the framework of an extended version of the one-dimensional Su-Schrieffer-Heeger (SSH) model with different molecular configuration disorders considered, including transfer integral disorder and on-site energy disorder. To quantitatively describe the exciton delocalization degree, we introduce the exciton binding energy in the numerical calculations. It is found that both the transfer integral and on-site energy disorders can effectively reduce the exciton binding energy, indicating that they should play crucial roles in facilitating the exciton delocalization. Especially, there exists a critical strength for either of the two disorder forms, beyond which only by the internal electric field in PSCs, a photogenerated exciton can be dissociated, consequently contributing to the ultrafast charge separation. In addition, we take the temperature dependent lattice vibration as an example to quantitatively discuss the realization possibility of the critical disorder strength.     

Electron correlation effects on exciton dissociation in the presence of an electric field in polyacetylene

Based on the Su–Schrieffer–Heeger and Pariser–Parr–Pople (SSH + PPP) model and the multiconfigurational time-dependent Hartree-Fock formalism, the dynamics of exciton dissociation in conjugated polymers systems has been simulated using a nonadiabatic molecular dynamics method. Within this approach, the appropriate spin symmetry of the electronic wavefunction is taken into account, which allows us to distinguish between singlet and triplet excited states. The different dynamic dissociation behaviors in the presence of an electric field of singlet excitons (SE) and triplet excitons (TE) due to electron correlation are emphasized. Using numerical simulations, it is found that the dissociation of TE under an applied electric field is more difficult than in the case for SE. The critical dissociation electric field strength for TE is about 3 times higher than that of SE. When the conjugated length of a polymer chain is short, SE and TE can each remain as one entity even under high electric field strengths owing to the confinement effect of the chain ends. In addition, the dependence of exciton dissociation on the Coulomb interaction strength, shielding factor and the conjugated length is discussed.     

Temperature effect on polaron recombination in conjugated polymers

The microcosmic mechanism of electroluminescence in polymer light emitting diodes (PLEDs) is the recombination of the oppositely charged polarons. In previous studies, it has been demonstrated that the temperature-induced irregular lattice vibration may have non-negligible influence on polaron dynamics. Nevertheless, there are few reports about thermal effect on recombination process between polaron pair, although it is very important for the performance of PLEDs. In this paper, we adopt the modified one-dimensional tight-binding model, including to which the thermal random force, and explore the temperature effect on polaron collision driven by electric field with different strengths. The dynamical simulation is performed by using the non-adiabatic evolution method. The results show that under the influence of electric field, the oppositely charged polarons could recombine into either an exciton with one lattice distortion, or the mixed state of polaron pair and exciton with two lattice distortions. It depends on both field strength and temperature. Anyway, after including temperature effect, a significant improvement of exciton yield is obtained. In addition, the new-formed exciton could perform a random walk along the polymer chain driven by the thermal random force when its strength is large enough. If we further increase the temperature, the stability of exciton would become worse.     

The dynamic formation of trions in conjugated polymers

The dynamic formation process of a trion by recombination of an on-chain bipolaron with an off-chain counterion trapped in a radical molecule, under the influence of an external electric field, has been investigated using a nonadiabatic evolution method. We demonstrate that the dynamic formation of the trion depends sensitively on the strength of the applied electric field, the radical potential, and the coupling between the polymer chain and radical. The trion can be formed efficiently via the bipolaron-trapped counterion reaction under the appropriate conditions. Our simulation provides theoretical evidence for the trion formation channel proposed by Kadashchuk et al. The bipolaron-trapped counterion reaction also makes an additional contribution to electroluminescence in conjugated polymers that lies beyond the set of processes involving polaron recombination.     

外场对InPBi量子阱中激子结合能的影响

在有效质量近似下运用变分法计算了InAlAs/InPBi/InAlAs量子阱中的激子结合能随阱宽、Al组分、Bi组分的变化情况,分析了外加电场和磁场对激子结合能的影响。结果表明：激子结合能随阱宽增大呈现先增大后减小的趋势;随Al、Bi组分的增大,激子结合能也逐渐增大;外加电场较小时对激子结合能的影响很小,外加电场较大时破坏了激子效应;激子结合能随外加磁场增大呈现单调增大的趋势。计算结果对InAlAs/InPBi/InAlAs量子阱在光电子器件方面的应用有一定指导意义。     

Characteristics of strong-coupling bipolaron qubit in two-dimensional quantum dot in electric field

Based on Lee-Low-Pines (LLP) unitary transformation, this article adopts the variational method of the Pekar type and gets the energy and wave functions of the ground state and the first excited state of strong-coupling bipolaron in two-dimensional quantum dot in electric field, thus constructs a bipolaron qubit. The numerical results represent that the time oscillation period T0 of probability density of the two electrons in qubit decreases with the increasing electric field intensity F and dielectric constant ratio of the medium ; the probability density Q of the two electrons in qubit oscillates periodically with the increasing time t; the probability of electron appearing near the center of the quantum dot is larger, while that appearing away from the center of the quantum dot is much smaller.     

Effects of electric field on the exciton linewidth broadening dueto scattering by free carriers in semiconducting quantum-well structures

The effects of an applied electric field perpendicular to the well layers on the broadening of the exciton linewidth due to scattering by free carriers in semiconducting quantum well (QW) structures is theoretically investigated based on a finite confining potential model. The dependence of the free carrier-exciton linewidth broadening on carrier concentration, temperature, and well width are calculated and discussed for various electric field strengths. It is found that the influence of the electric field on the linewidth broadening is appreciable in wide wells, while in narrow wells little change is shown even in the presence of a strong electric field     

Field-induced optical effect in a five-step asymmetric coupledquantum well with modified potential

A five-step asymmetric coupled quantum well (FACQW) is demonstrated and analyzed for its field-induced optical effect. A strong combined exciton absorption peak is caused by e1hh2 and e2hh1 transitions for the TE mode and e1lh2 and e2lh1 for the TM mode at a small negative applied electric field, while two separated exciton absorption peaks appear at positive applied electric field. The large change of the exciton absorption strength produces large positive refractive index change whose value is larger by over one order of magnitude compared to that in a rectangular quantum well (RQW) when the operation wavelength is away from the absorption edge. A tensile-strained FACQW is employed to improve the polarization dependence. The difference of the refractive index change for TE and TM modes is under 2% while that in the unstrained FACQW is larger than 50%. A modified FACQW structure is also studied for a negative refractive index change. The strong combined exciton absorption peak without a red shift is observed in the measurement of photocurrent spectra of the FACQW at room temperature. The absorption properties of the FACQW under high applied electric field are also observed and analyzed. The measured results are in good agreement with bur numerical analysis. Finally, ridge-waveguide Mach-Zehnder modulators have been fabricated by using the FACQW and RQW structures. The half-wave voltage of the FACQW modulator is as low as 3 V while that of the RQW modulator is 8 V. The result proves that the FACQW structure indeed has larger refractive index change than that of the RQW structure. These properties obtained with the FACQW family have a great potential for application to ultrafast and low-voltage optical modulators and switches     

耦合GaN/AlxGa1-xN量子点的非线性光学性质

在有效质量和偶极矩近似下,考虑了由于压电极化和自发极化所引起的内建电场和量子点的三维约束效应,对纤锌矿对称Al_xGa_(1-x)N/GaN/Al_xGa_(1-x)N/GaN/Al_xGa_(1-x)N圆柱型应变耦合量子点中激子非线性光学性质进行了研究。计算结果表明,内建电场使吸收光谱向低能方向移动,发生红移现象,并且使吸收峰强度大大减小。量子限制效应使光吸收峰强度随着量子点尺寸的减小而增强,并且随着量子点尺寸的减小,吸收光谱发生蓝移现象。