Migration of an exciton in organic polymers driven by a nonuniform internal electric field

Migration of excitons is of vital importance for the photovoltaic process in polymer-based donor/acceptor (D/A) photovoltaic system. Due to the existence of some trapping charges as well as the mismatch of energy levels at the D/A heterointerface, there will exist a nonuniform internal electric field, which may induce the migration or dissociation of an exciton or charge transfer state generated near the interface. In this work, by choosing the nonuniform internal electric field as a linear gradient form, we theoretically simulate the migration dynamics of an exciton in a polymer. It is found that the exciton will be polarized under the electric field. Especially, the polarized positive and negative charges in the exciton lie in different electric fields, which will induce a net force driving the exciton to migrate. Effects of some factors, such as the electron-phonon (e-ph) coupling, on the exciton migration are analyzed. This research reveals a new exciton migration mechanism in polymer-based D/A photovoltaic system, which might be timely for further understanding their photovoltaic process, and thus provide a new strategy to optimize the photovoltaic efficiency of the devices.     

Charge Photogeneration in Non-Fullerene Organic Solar Cells: Influence of Excess Energy and Electrostatic Interactions

In organic solar cells, photogenerated singlet excitons form charge transfer (CT) complexes, which subsequently split into free charge carriers. Here, the contributions of excess energy and molecular quadrupole moments to the charge separation process are considered. The charge photogeneration in two separate bulk heterojunction systems consisting of the polymer donor PTB7-Th and two non-fullerene acceptors, ITIC and h-ITIC, is investigated. CT state dissociation in these donor–acceptor systems is monitored by charge density decay dynamics obtained from transient absorption experiments. The electric field dependence of charge carrier generation is studied at different excitation energies by time delayed collection field (TDCF) and sensitive steady-state photocurrent measurements. Upon excitation below the optical gap, free charge carrier generation becomes less field dependent with increasing photon energy, which challenges the view of charge photogeneration proceeding through energetically lowest CT states. The average distance between electron–hole pairs at the donor–acceptor interface is determined from empirical fits to the TDCF data. The delocalization of CT states is larger in PTB7-Th:ITIC, the system with larger molecular quadrupole moment, indicating the sizeable effect of the electrostatic potential at the donor–acceptor interface on the dissociation of CT complexes.     

Mechanism for Efficient Photoinduced Charge Separation at Disordered Organic Heterointerfaces

Despite the poor screening of the Coulomb potential in organic semiconductors, excitons can dissociate efficiently into free charges at a donor–acceptor heterojunction, leading to application in organic solar cells. A kinetic Monte Carlo model that explains this high efficiency as a two‐step process is presented. Driven by the band offset between donor and acceptor, one of the carriers first hops across the interface, forming a charge transfer (CT) complex. Since the electron and hole forming the CT complex have typically not relaxed within the disorder‐broadened density of states (DOS), their remaining binding energy can be overcome by further relaxation in the DOS. The model only contains parameters that are determined from independent measurements and predicts dissociation yields in excess of 90% for a prototypical heterojunction. Field, temperature, and band offset dependencies are investigated and found to be in agreement with earlier experiments. Whereas the investigated heterojunctions have substantial energy losses associated with the dissociation process, these results suggest that it is possible to reach high dissociation yields at low energy loss.     

异质谷间转移电子器件工作模式的研究

运用依赖能量的异质界面δ散射势和多能谷有效质量方程计算了 Ga As/Al As异质结构中的能带混合和电子隧穿共振。结合能带混合隧穿共振理论和 Monte Carlo模拟计算 ,编制异质谷间转移电子器件的模拟软件。用该软件模拟了器件的直流伏安特性和射频工作。由模拟结果与测量结果的对比中确定出δ散射势参数和隧穿时间。通过模拟发现了新的弛豫振荡模式。研究了弛豫振荡模的各种振荡特性 ,给出了器件的优化设计     

High-field transport with hot phonons in degenerate semiconductors

The effect of phonons in non-thermodynamic equilibrium (hot-phonons) on steady-state transport in degenerate bulk and two-dimensional (2D) semiconductor structures is studied. The phonon-drift effects are taken to be negligible, and, the formulation is confined to electrons in a single parabolic band interacting with a - population of polar-optic phonons, restricted to interface modes in the 2D case. Hot phonons in bulk semiconductors lead not only to a decrease in the energy relaxation rate, but, also, to a decrease in the mobility as the carrier concentration n_3D is increased. Hot phonons in 2D systems, on the other hand, cause the energy relaxation rate and mobility to go through a minima as the electron concentration n_2D is increased. The electron drift velocity, contrary to the bulk case, is seen to with n_2D for n_2D > 10¹² cm⁻², and reflects the difference in the nature of electron density of states in the two cases.     

Carrier density distribution in modulation doped GaAs-AlxGa1−xAs quantum well heterostructures

A theoretical model for the distribution of charged carriers at the interface of a doped Al_xGa_1−xAs-undoped GaAs heterostructure is evaluated. Assuming a triangular potential well at the interface, we obtain numerically, curves describing the Fermi energy, depletion width in the alloy, and average separation of impurities and carriers, and the electron density at the interface as functions of alloy layer composition and doping level. The results obtained can be applied to multiple heterointerface quantum well heterostructures and superlattices as well. Carrie concentration profile data on a 51-layer modulation doped QWH grown by MOCVD are presented and are discussed using the results of the model.     

Structural, luminescent, and transport properties of hybrid AlAsSb/InAs/Cd(Mg)Se heterostructures grown by molecular beam epitaxy

The fabrication of new hybrid AlAsSb/InAs/Cd(Mg)Se heterostructures by molecular beam epitaxy and investigation of their structural, luminescent, and transport properties are reported for the first time. These structures show intense luminescence both in the infrared and in the visible regions of the spectrum. This factor, taken together with structural data, indicates a heterointerface of high quality between the III–V and II–VI layers. A theoretical estimate is made of the relative positions of energy bands in the proposed hybrid structures, indicating that the InAs/CdSe interface is a type-II heterojunction, whereas the InAs/Cd_0.85Mg_0.15Se interface is a type-I heterojunction with a large valence band offset ΔE _v≈1.6 eV. The data obtained on the longitudinal electron transport at the InAs/Cd(Mg)Se heterointerface are in good agreement with the theoretical estimate.     

Thermal Misfit Strain Relaxation in Ge/(001)Si Heterostructures

We used x-ray diffraction and transmission electron microscopy to study the mechanism of thermal misfit strain relaxation in epitaxial Ge films grown on Si(001) substrates by the two-step growth technique. Lattice misfit strain associated with Ge/Si(001)Si mismatched epitaxy is largely relieved by a network of Lomer edge misfit dislocations during the first step of growth. However, thermal misfit strain during growth is relieved primarily by interdiffusion at the Si/Ge heterointerface. Two SiGe compositions containing 0.5 at.% and 6.0 at.% Si detected at the interface relieve thermal mismatch strain associated with the two growth steps. A thermodynamic model has been proposed to explain the state of strain in the films.     

The determination of the interface-state density distribution from the capacitance-frequency measurements in Au/n-Si schottky barrier diodes

The Au/n-Si Schottky barrier diodes (SBDs) with 200-μm (sample D200) and 400-μm (sample D400) bulk thicknesses have been fabricated. The ideality factor and the barrier height have been calculated from the forward-bias current-voltage (I-V) characteristics of D200 and D400 SBDs. The energy distribution of the interface states and relaxation time are found from the capacitance-frequency (C-f) characteristics. The density of interface state and relaxation times have a (nearly constant) slow exponential rise with bias in the range of E_c −0.77 and E_c −0.47 eV from the midgap toward the bottom of the conductance band. Furthermore, the energy distribution of the interface states obtained from C-f characteristics has been compared with that obtained from the forward-bias I-V characteristics.     

Capacitance spectroscopy of deep states in InAs/GaAs quantum dot heterostructures

The results of a study of a structure with a single array of InAs quantum dots in a GaAs matrix using capacitance-voltage measurements, deep-level transient spectroscopy (DLTS), photoluminescence spectroscopy, and transmission electron microscopy are reported. Clusters of interacting bistable defects are discovered in GaAs layers grown at low temperature. Controllable and reversible metastable populating of quantum-dot energy states and monoenergetic surface states, which depends on the temperature and conditions of a preliminary isochronal anneal, is observed. This effect is associated with the presence of bistable traps with self-trapped holes. The DLTS measurements reveal variation of the energy for the thermal ionization of holes from surface states of the InAs/GaAs heterointerface and the wetting layer as the reverse bias voltage is increased. It is theorized that these changes are caused by the built-in electric field of a dipole, which can be formed either by wetting-layer holes or by ionized levels located near the heterointerface. Fiz. Tekh. Poluprovodn. 33, 184–193 (February 1999)     

Quasi-Fermi level bending in MODFET's and its effect on FET transfer characteristics

Using Shockley's diffusion/drift model we calculate the quasi-Fermi level (imref) bending in the depleted AIGaAs barrier layer of GaAs/AIGaAs MODFET's. We show that the assumption of a constant imref from the heterointerface through the barrier layer is not justified when the gate is moderately forward biased. Once the barrier-layer conduction band edge at the gate interface fails below that at the heterointerface, the imref changes both in the vicinity of the heterointerface and at the gate metal. This has important consequences for the MODFET transfer characteristics and necessitates new considerations for the gate control mechanism As a result, sheet electron concentrations in the MODFET channel exceeding the equilibrium concentrations are obtained. Despite the gate being forward biased with voltages close to or larger than the Schottky-barrier height, the gate current is suppressed not only by the barrier at the heterointerface but also by a barrier of about the same height present at the gate metal-semiconductor interface. Experimental results on AIGaAs/GaAs MODFET's are in good quantitative agreement with the theoretical calculations.     

First-principles study on electronic properties and lattice structures of WZ-ZnO/CdS interface

We theoretically investigated the lattice structure, interface bonding energy, optical absorption properties and electronic properties of WZ-ZnO (1 1 2)/CdS (1 1 0) interface from first-principles calculations. The interface lattice mismatch is less than 4.3%. The atomic bond lengths and atomic positions change slightly on the interface after relaxation. The WZ-ZnO (1 1 2)/CdS (1 1 0) interface has bonding energy about −0.61 J/m², suggesting that this interface can exist stably. Through analysis of the density of states, no interface state is found near the Fermi level. In addition, there are orbital hybridizations between different interfacial atoms, and these orbital hybridizations effectively enhance the bonding of Zn and S atoms, Cd and O atoms on the interface. By analysis of difference density charge and Bader charge, we find that electrons on the interface are largely redistributed and charges transport near the Fermi level which strengthen the adhesion of the interface.     

3D structures for UV-NIL template fabrication with grayscale e-beam lithography

The individual steps in fabrication of templates for UV-NIL processes are described. After spin coating a conductive copolymer (ESPACER 300) on top of the resist, insulating substrates have been structured by use of electron beam lithography at 20 keV beam energy. A three-dimensional (3D) pattern has been created in a low contrast positive tone resist PMMA 35k. By RIE in a CHF₃ – O₂ – process, the pattern has been transferred into the quartz substrate. Finally, the 3D structures have been replicated in a UV-NIL process.     

Study of PtGaAs interface states

A capacitance study of a forward biased PtnGaAs junction has been used to determine the energy distribution and relaxation time of the interface states in equilibrium with the semiconductor. Experimental results show that interface states are distributed in two energy bands at given positions, densities and capture cross sections.     

Ultrafast Electron Transfer in Low‐Band Gap Polymer/PbS Nanocrystalline Blend Films

Ultrafast charge transfer dynamics in hybrid blend films of a low band‐gap polymer poly(2,6‐(N‐(1‐octylnonyl)dithieno[3,2‐b:20,30‐d]pyrrole)‐alt‐4,7‐(2,1,3‐benzothiadiazole)) (PDBT) and PbS quantum dots (QDs) are studied by using ultrafast transient transmission spectroscopy. It is observed that the transient bleaching signal arising from excitons of the PDBT displays a much faster recovery, within the time delay of ≈5 ps, in hybrid films than in the neat PDBT film. In contrast, the bleaching signal resulting from the electron filling of the QDs in hybrid films shows an extra rising component during ≈1–5 ps, which is absent in the pristine QDs. These results indicate the ultrafast electron transfer from the lowest unoccupied molecular orbital energy level of the PDBT to the conduction band of the QDs in the time scale of several ps after laser excitation. A transient absorption signal within 1 ps in the hybrid films is also found, indicating the emergence of charge transfer states (CTs). The CTs formed at the interface of the hybrid blend may facilitate the charge separation and transfer. It is estimated that over 80% of the photoexcited electrons in the PDBT may be transferred into the QDs. The transfer efficiencies show a positive correlation with the power conversion efficiencies of the corresponding hybrid solar cells.     

Picosecond photoluminescence measurements of hot carrier relaxation and auger recombination in GaSb

We have measured hot carrier relaxation and recombination of photoexcited carriers in GaSb using picosecond time-resolved photoluminescence. We find that these processes are greatly modified compared to other III–V materials by the conduction band structure which permits a large electron population at the L-minima. Rapid carrier cooling observed at early times is explained by efficient relaxation of electrons within the L-valleys. For carrier densities 10¹⁹cm⁻³ we find that the dominant recombination process at low temperatures (4K) is radiative, but at hihg temperatures Auger recombination becomes dominant. This is manifest in a dramatic reduction of luminescence intensity, faster recombination and very slow energy relaxation.     

Features of electron transport in relaxed Si/Si1 − x Ge x transistor heterostructures with a high doping level

The low-temperature electrical and magnetotransport characteristics of partially relaxed Si/Si_{1 ? x} Ge _x heterostructures with an electron conduction channel in an elastically strained nanoscale silicon layer are investigated. It is demonstrated that the electron gas in the system exhibits 2D properties. A dependence of the conductivity along layers in the system on the degree of elastic-stress relaxation in it is observed. To understand the observed regularities, the potential and the electron distribution over the structure layers are calculated in detail for samples with different layer strains and doping levels. For the structure with x = 0.25, the parameters of the potential barrier and characteristics of the quantum well formed in the Si layer are estimated. It is established that the characteristics of the potential formed near interfaces strongly depend on the initial parameters of the system, in particular, on the degree of the plastic relaxation of elastic stresses and on the doping level. The formation of a thin tunneling-transparent barrier near the upper interface can lead to the redistribution of electrons between the 2D and 3D conduction channels in the structure, which ensures the spread of the measured transport characteristics of the samples during the measurements. The interlayer tunneling transitions of carriers from the 2D state in the Si transport channel to the 3D state of the Si_{1 ? x} Ge _x crystal matrix, which are separated by a tunneling-transparent potential barrier near the heterointerface, were observed for the first time during transport in the direction transverse to the layer plane.     

离子注入高分子材料的研究动态及应用

离子注入聚合物(在一定的能量和一定的剂量),可引起聚合物电导率增加几个到十几个数量级.利用这个特性,可制作导电图样,制作连线,制作p型半导体材料和n型半导体材料,制作pn结,制作二维和三维集成电路互连接.离子注入聚合物还使聚合物的颜色加深,光吸收增加,可制作有机太阳能电池.离子注入聚合物还引起聚合物力学、磁学、光学性质发生变化.     

Tailoring Electron‐Transfer Barriers for Zinc Oxide/C60 Fullerene Interfaces

The interfacial electronic structure between oxide thin films and organic semiconductors remains a key parameter for optimum functionality and performance of next‐generation organic/hybrid electronics. By tailoring defect concentrations in transparent conductive ZnO films, we demonstrate the importance of controlling the electron transfer barrier at the interface with organic acceptor molecules such as C₆₀. A combination of electron spectroscopy, density functional theory computations, and device characterization is used to determine band alignment and electron injection barriers. Extensive experimental and first principles calculations reveal the controllable formation of hybridized interface states and charge transfer between shallow donor defects in the oxide layer and the molecular adsorbate. Importantly, it is shown that removal of shallow donor intragap states causes a larger barrier for electron injection. Thus, hybrid interface states constitute an important gateway for nearly barrier‐free charge carrier injection. These findings open new avenues to understand and tailor interfaces between organic semiconductors and transparent oxides, of critical importance for novel optoelectronic devices and applications in energy‐conversion and sensor technologies.     

Photomultiplication-Type Organic Photodetectors with Fast Response Enabled by the Controlled Charge Trapping Dynamics of Quantum Dot Interlayer

A novel photomultiplication (PM)-type organic photodiode (OPD) that responds much faster (109 kHz bandwidth) than conventional PM-type OPDs is demonstrated. This fast response is achieved by introducing quantum dots (QDs) as a PM-inducing interlayer at the interface between the electrode and the photoactive layer. When the device is illuminated, the photogenerated electrons within the photoactive layer are rapidly transferred and trapped in the trap states of the QD interlayer. The electron trapping subsequently leads to charging of the QD and a consequent shift of the QD energy levels, thereby inducing hole injection from the electrode. This PM mechanism is distinct from that of conventional PM-type OPDs, whose PM usually requires a long time to induce hole (or electron) injection because of the slow transport and accumulation of electrons (or holes) within the photoactive layer. Because of its PM mechanism, the proposed QD-interlayer PM-type OPD achieves high bandwidth and high specific detectivity. In addition, it is demonstrated that the response speed of the proposed device is closely related to the charge trapping/detrapping dynamics of the QDs. This work not only offers a new concept in the design of fast-responding PM-type OPDs but also provides comprehensive understanding of the underlying device physics.