多晶MAPbBr_(3)薄膜中束缚载流子热发射复合过程北大核心CSCD

有机-无机卤化铅钙钛矿(organic inorganic lead halide perovskite,OLHP)半导体材料内部的陷阱是影响OLHP的光电性能的重要因素。为了理解多晶的甲胺溴基钙钛矿((Methylammonium)PbBr_(3),MAPbBr_(3))薄膜中陷阱对光生载流子复合的影响,本文采用了时间分辨微波光电导(time resolved microwave conductivity,TRMC)技术探究了多晶MAPbBr_(3)薄膜的光生载流子复合动力学过程。实验测量结果表明多晶MAPbBr_(3)薄膜的载流子复合过程包括自由载流子复合与束缚载流子的热发射复合两部分。其中,与束缚载流子热发射复合相关的能级远离连续带,且对应的能级深度约为0.6 eV,分布宽度约为89.2 meV。本文同时利用变激发波长TRMC对比实验,分析浅束缚光生电子与导带光生电子复合过程的差异。相比于导带上的电子,实验结果表明浅束缚电子跃迁到深束缚能级的概率更大。     

Molecular engineering to improve carrier lifetimes for organic photovoltaic devices with thick active layers

The morphology of the bulk heterojunction absorber layer in an organic photovoltaic (OPV) device has a profound effect on the electrical properties and efficiency of the device. Previous work has consistently demonstrated that the solubilizing side-chains of the donor material affect these properties and device performance in a non-trivial way. Here, using Time-Resolved Microwave Conductivity (TRMC), we show by direct measurements of carrier lifetimes that the choice of side chains can also make a substantial difference in photocarrier dynamics. We have previously demonstrated a correlation between peak photoconductance measured by TRMC and device efficiencies; here, we demonstrate that TRMC photocarrier dynamics have an important bearing on device performance in a case study of devices made from donor materials with linear vs. branched side-chains and with variable active layer thicknesses. We use Grazing-Incidence Wide Angle X-ray Scattering to elucidate the cause of the different carrier lifetimes as a function of different aggregation behavior in the polymers. Ultimately, the results help establish TRMC as a technique for screening OPV donor materials whose devices maintain performance in thick active layers (>250 nm) designed to improve light harvesting, film reproducibility, and ease of processing.     

Origin of Reduced Bimolecular Recombination in Blends of Conjugated Polymers and Fullerenes

Bimolecular charge carrier recombination in blends of a conjugated copolymer based on a thiophene and quinoxaline (TQ1) with a fullerene derivative ((6,6)‐phenyl‐C₇₁‐butyric acidmethyl ester, PC₇₁BM) is studied by two complementary techniques. TRMC (time‐resolved microwave conductance) monitors the conductance of photogenerated mobile charge carriers locally on a timescale of nanoseconds, while using photo‐CELIV (charge extraction by linearly increasing voltage) charge carrier dynamics are monitored on a macroscopic scale and over tens of microseconds. Despite these significant differences in the length and time scales, both techniques show a reduced Langevin recombination with a prefactor ζ close to 0.05. For TQ1:PC₇₁BM blends, the ζ value is independent of temperature. On comparing TRMC data with electroluminescence measurements it is concluded that the encounter complex and the charge transfer state have very similar energetic properties. The ζ value for annealed poly(3‐hexylthiophene) (P3HT):(6,6)‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) is approximately 10^?4, while for blend systems containing an amorphous polymer ζ values are close to 1. These large differences can be related to the extent of charge delocalization of opposite charges in an encounter complex. Insight is provided into factors governing the bimolecular recombination process, which forms a major loss mechanism limiting the efficiency of polymer solar cells.     

The Locus of Free Charge‐Carrier Generation in Solution‐Cast Zn1–xMgxO/Poly(3‐hexylthiophene) Bilayers for Photovoltaic Applications

The photoconductivity of solution‐cast Zn_1–xMg_xO (x=0‐0.4) and poly(3‐hexylthiophene) (P3HT) thin films, and Zn_1‐xMg_xO/P3HT bilayers is investigated using Time‐Resolved Microwave Conductivity (TRMC) with the aim of determining the locus of free charge carrier generation in the bilayer system. The photoconductivity of Zn_1–xMg_xO thin films, under illumination with 300 nm laser pulses, is limited by the formation of stable excitons and by scattering of the carriers at grain boundaries. The electron mobility in Zn_1–xMg_xO films decreases exponentially with Mg concentration, up to x=0.4. In agreement with previous work, free carriers are observed in the P3HT film under illumination with 500 nm pulses in the absence of an acceptor. Under illumination with 500 nm pulses, where only the polymer absorbs, the TRMC signal for the Zn_1–xMg_xO/P3HT bilayers for x≥0.2 is the same as that of pure P3HT, indicating that free carrier generation in these bilayers occurs predominately by exciton dissociation in the polymer bulk, and not at the interface between the polymer and the solution‐cast oxide. At lower Mg concentrations (x<0.2) the TRMC signal increases with decreasing x following the dependence of the electron mobility in the oxide but its light intensity dependence remains consistent with free carrier generation in the polymer bulk. To explain these results and previously published photovoltaic device data (Adv. Funct. Mater. 2007 , 17, 264) we propose that free carrier generation in the bilayers predominantly occurs in the bulk of P3HT, and is followed by electron injection to the oxide to yield photocurrent in photovoltaic cells. The dependence of the TRMC signal of the bilayers on Mg concentration is explained in terms of the yield for free carrier generation in the polymer and the relative contributions of electrons in the oxide and holes in the polymer.     

Understanding Local and Macroscopic Electron Mobilities in the Fullerene Network of Conjugated Polymer‐based Solar Cells: Time‐Resolved Microwave Conductivity and Theory

The efficiency of bulk heterojunction (BHJ) organic photovoltaics is sensitive to the morphology of the fullerene network that transports electrons through the device. This sensitivity makes it difficult to distinguish the contrasting roles of local electron mobility (how easily electrons can transfer between neighboring fullerene molecules) and macroscopic electron mobility (how well‐connected is the fullerene network on device length scales) in solar cell performance. In this work, a combination of density functional theory (DFT) calculations, flash‐photolysis time‐resolved microwave conductivity (TRMC) experiments, and space‐charge‐limit current (SCLC) mobility estimates are used to examine the roles of local and macroscopic electron mobility in conjugated polymer/fullerene BHJ photovoltaics. The local mobility of different pentaaryl fullerene derivatives (so‐called ‘shuttlecock’ molecules) is similar, so that differences in solar cell efficiency and SCLC mobilities result directly from the different propensities of these molecules to self‐assemble on macroscopic length scales. These experiments and calculations also demonstrate that the local mobility of phenyl‐C₆₀ butyl methyl ester (PCBM) is an order of magnitude higher than that of other fullerene derivatives, explaining why PCBM has been the acceptor of choice for conjugated polymer BHJ devices even though it does not form an optimal macroscopic network. The DFT calculations indicate that PCBM's superior local mobility comes from the near‐spherical nature of its molecular orbitals, which allow strong electronic coupling between adjacent molecules. In combination, DFT and TRMC techniques provide a tool for screening new fullerene derivatives for good local mobility when designing new molecules that can improve on the macroscopic electron mobility offered by PCBM.     

FeCl3‐Based Few‐Layer Graphene Intercalation Compounds: Single Linear Dispersion Electronic Band Structure and Strong Charge Transfer Doping

Graphene has attracted much attention since its first discovery in 2004. Various approaches have been proposed to control its physical and electronic properties. Here, it is reported that graphene‐based intercalation is an efficient method to modify the electronic properties of few‐layer graphene (FLG). FeCl₃ intercalated FLGs are successfully prepared by the two‐zone vapor transport method. This is the first report on full intercalation for graphene samples. The features of the Raman G peak of such FLG intercalation compounds (FLGIC) are in good agreement with their full intercalation structures. The FLGICs present single Lorentzian 2D peaks, similar to that of single‐layer graphene, indicating the loss of electronic coupling between adjacent graphene layers. First principle calculations further reveal that the band structure of FLGIC is similar to single‐layer graphene but with a strong doping effect due to the charge transfer from graphene to FeCl₃. The successful fabrication of FLGIC opens a new way to modify properties of FLG for fundamental studies and future applications.     

Effect of Co Doping on the Properties of ZnO Bulk Samples

In this study, Co-doped Zn_1?x Co _x O (x = 0.04, 0.06, 0.08, and 0.10, where x is the concentration of guest atom cobalt in the ZnO lattice) bulk samples were examined to determine the effect of doping on electronic, structural, magnetic, and optical properties. The samples were prepared using a standard high-temperature solid-state reaction technique. All samples were characterized by x-ray diffraction (XRD), magnetization measurements using vibrating-sample magnetometry (VSM), optical absorption spectroscopy, and scanning electron microscopy (SEM) with energy-dispersive x-ray spectroscopy (EDAX). The electron densities derived from the maximum-entropy method (MEM) show an improvement of electronic charge densities with Co impurity addition. The magnetic measurements at room temperature show variations in ferromagnetic behavior with respect to Co addition. The optical study shows a decrease in energy gap of ZnO with increasing cobalt content.     

Fully Solution‐Processed Conductive Films Based on Colloidal Copper Selenide Nanosheets for Flexible Electronics

A novel colloidal synthesis of copper selenide nanosheets (NSs) with lateral dimensions of up to 3 μm is developed. This material is used for the fabrication of flexible conductive films prepared via simple drop‐casting of the NS dispersions without any additional treatment. The electrical performance of these coatings is benchmarked against copper selenide spherical nanocrystals (SNCs) in order to demonstrate the advantage of 2D morphology of the NSs for flexible electronics. In this contest, Cu_2?xSe SNC films exhibit higher conductivity but lower reproducibility due to the formation of cracks leading to discontinuous films. Furthermore, the electrical properties of the films deposited on different flexible substrates following their bending, stretching and folding are studied. A comparison of Cu_2?xSe SNC and CuSe NS films reveals an increased stability of the CuSe NS films under mechanical stress applied to the samples and their improved long‐term stability in air.     

Impedance Measurements as a Simple Tool to Control the Quality of Conjugated Polymers Designed for Photovoltaic Applications

The problem of batch‐to‐batch variation of electronic properties and purity of conjugated polymers used as electron donor and photon harvesting materials in organic solar cells is addressed. A simple method is developed for rapid analysis of electronic quality of polymer‐based materials. It is shown that appearance of impurities capable of charge trapping changes electrophysical properties of conjugated polymers. In particular, a clear correlation between the effective relaxation time τeff and relative photovoltaic performance (η/η_max) is revealed for samples of poly(3‐hexylthiophene) intentionally polluted with a palladium catalyst. This dependence is also valid for all other investigated samples of conjugated polymers. Therefore, fast impedance measurements at three different frequencies allow one to draw conclusions about the purity of the analyzed polymer sample and even estimate its photovoltaic performance. The developed method might find extensive applications as a simple tool for product quality control in the laboratory and industrial‐scale production of conjugated polymers for electronic applications.     

Thermoelectric Properties of Yb1−x (Er,Lu) x Al3 Solid Solutions

Ytterbium trialuminide (YbAl₃) has one of the largest thermoelectric power factors of known materials below room temperature, making it a material of interest for low-temperature thermoelectric devices. However, the high thermal conductivity, which is due to a combination of a large electronic thermal conductivity and a moderately large lattice thermal conductivity, is detrimental to the figure of merit. Substitution of different atoms on the Yb site was performed in order to assess their ability to favorably alter the electronic structure and/or reduce the lattice thermal conductivity. We have synthesized and studied the thermoelectric properties of the solid solutions of YbAl₃ with ErAl₃ and LuAl₃. Results for electrical conductivity, thermal conductivity, and Seebeck coefficient for several of these solid solutions over the temperature range of 80 K to 300 K are reported. Although most substituted samples are driven toward a metallic state, we find that for some compositions the figure of merit is enhanced relative to pure YbAl₃.     

Impact of Surface Treatment on the Structural and Electronic Properties of Polished CdZnTe Surfaces for Radiation Detectors

We present the effects of surface treatments on the structural and electronic properties of chemomechanically polished Cd_0.9Zn_0.1Te before contact deposition. Specifically, polished CdZnTe (CZT) samples were treated with four distinct chemical etchants: (1) bromine methanol (BM), (2) bromine in lactic acid, (3) bromine in methanol followed by bromine–20% lactic acid in ethylene glycol, and (4) hydrochloric acid (HCl). The surface structure and surface electronic properties were studied with atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). AFM images showed that three of the four etchants significantly altered the surface morphology and structure of CZT. All etchants created smoother surfaces; however, all except HCl also introduced high densities of defects. HCl was found to not affect the surface structure. XPS measurements indicated that a thick, ～3 nm to 4 nm, TeO₂ layer formed about 1 h after etching; hence, it is very important to process devices immediately after etching to prevent oxide formation.     

Photocatalytic activity of hydrogen production from water over TiO2 with different crystal structures

In this work, the anatase and rutile TiO₂ of single crystal structure are prepared by a facile hydrolysis method. The obtained samples are studied via various characterizations, and compared with different commercial TiO₂ (P25, ST-21, ST-31), which have mixed structures, to investigate the photocatalytic performance from water splitting. The experimental results show that the photocatalytic activity of samples depends on the surface properties and particle sizes. Moreover, compared with the TiO₂ of single crystal structure, P25 and ST-21 mixed crystal structures show superior photocatalytic hydrogen production from water splitting. According to experimental results, the possible photocatalytic mechanism of TiO₂ with mixed crystal structure is proposed.     

Effect of Cooling Conditions on the Microstructure and Thermoelectric Properties of Zn/Si-Codoped InSb

InSb is a good candidate thermoelectric (TE) material owing to its high carrier mobility and narrow band gap around 0.18 eV. However, a high figure of merit (ZT) value has not been achieved with InSb because of its high lattice thermal conductivity (κ _lat). To reduce the κ _lat of InSb, we prepared a ZnIn₁₈SiSb₂₀ alloy by Zn/Si codoping into the In lattice sites of InSb. Polycrystalline samples of ZnIn₁₈SiSb₂₀ were prepared by a solid-state reaction method combined with hot pressing. To investigate the microstructures and TE properties resulting from different cooling conditions, samples were prepared by water quenching or slow cooling after an annealing process. The different cooling conditions led to different ZnIn₁₈SiSb₂₀ microstructures and TE properties. The electrical transport properties showed that both samples exhibited metal-like behavior and p-type conduction. The thermal conductivity values of the quenched and slow-cooled samples at room temperature were 8.7 W m^?1 K^?1 and 11.7 W m^?1 K^?1, respectively. A maximum ZT value of 0.23 was obtained at 723 K for the quenched ZnIn₁₈SiSb₂₀ sample.     

Role of structural order at the P3HT/C60 heterojunction interface

The influence of structural order on the electronic and optical properties of C₆₀/P3HT bulk heterojunctions (BHJs) was studied using first principles DFT and TDDFT methods. The electronic levels alignment between the two phases in the BHJs is mainly controlled by the interfacial dipole moment that shifts the P3HT electronic levels towards higher binding energies with respect to C₆₀ levels. An increasing order translates into an increasing P3HT domains size, for which we considered different stacks of P3HT oligomers. A significant decrease of both the electronic (HOMO_P3HT–LUMO_C60) and the optical (HOMO_P3HT–LUMO_P3HT) band gap is observed with an increasing P3HT domain size. TDDFT approach was used to identify the orbitals involved in the electronic transitions, and to reveal that the reduction of the BHJ optical band gap cannot simply be predicted from the variation of the rrP3HT band gap. The lowest electronic transition in rrP3HT becomes optically forbidden due to the formation of H-aggregates.     

Aligned Thin Films of Discotic Hexabenzocoronenes: Anisotropy in the Optical and Charge Transport Properties

The anisotropy in the optical absorption and photoconductivity of thin layers of mesomorphic derivatives of hexa‐peri‐hexabenzocoronene (HBC) have been investigated for aligned films prepared via three different methods: deposition on friction‐deposited polytetrafluoroethylene (PTFE), zone‐casting (ZC), and Langmuir–Blodgett (LB) multilayer dipping. The ratio of the optical density for light polarized perpendicular to the alignment direction, OD₊, to that for light polarized parallel, OD₌, varies from close to 1.0 up to 12.5 depending on whether the HBC cores are tilted at close to 45° or 90° with respect to the axis of the self‐assembled columnar stacks. For all aligned films the photoconductivity, determined using the electrode‐less flash‐photolysis time‐resolved microwave conductivity technique (FP‐TRMC), was found to be favored in the direction of columnar alignment by up to a factor of 30 for a PTFE‐aligned film. The effect of varying the temperature of the films over a range encompassing the temperature at which the transition from the crystalline solid to the columnar mesophase occurs in the bulk materials has been investigated. High‐temperature annealing increases the optical and conductivity anisotropy for the LB film significantly, but has little effect for the PTFE and the ZC films. The relative efficacy of the different alignment procedures is discussed.     

Li-inserted carbon nanotube Raman scattering

The possibility of inserting lithium into single wall carbon nanotube bundles during the growth process is analyzed in this work by using the Raman technique as probe. The nanotubes were prepared by the arc discharge method by using catalysts prepared by mixing compounds containing lithium and as their counterpart, a similar mixture without this alkali-metal. The two pair of samples studied in this work were obtained with the following catalysts: (i) Li₂CO₃/NiO/CoO and NiO/CoO; or (ii) LiCo_0.5Ni_0.5O₂ and Ni/Co. Raman spectra reveal that the tangential bands profiles of the samples prepared with the catalyst containing lithium is considerably modified in both cases. In the case of the carbon nanotubes obtained using the LiCo_0.5Ni_0.5O₂ catalyst a down shift and severe broadening are observed in addition. Comparison of our results with those published previously for alkali-metals doped single wall carbon nanotubes allowed to conclude that lithium incorporation, actually, occurs during the growth process.     

Benzobisthiazole as Weak Donor for Improved Photovoltaic Performance: Microwave Conductivity Technique Assisted Molecular Engineering

New donor–acceptor‐type copolymers comprised of benzobisthiazole (BBTz) as a weak donor rather than acceptor are proposed. This approach can simultaneously lead to deepening the HOMO and LUMO of the polymers with moderate energy offset against fullerene derivatives in bulk heterojunction organic photovoltaics. As a proof‐of‐concept, BBTz‐based random copolymers conjugated with typical electron acceptors: thienopyrroledione (TPD) and benzothiadiazole (BT) based on density functional theory calculations are synthesized. Laser‐flash and Xe‐flash time‐resolved microwave conductivity (TRMC) evaluations of polymer:[6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM) blends are conducted to screen the feasibility of the copolymers, leading to optimization of processing conditions for photovoltaic device application. According to the TMRC results, alternating BBTz‐BT copolymers are designed, exhibiting extended photoabsorption up to ca. 750 nm, deep HOMO (–5.5 to –5.7 eV), good miscibility with PCBM, and inherent crystalline nature. Moreover, the maximized PCE of 3.8%, the top‐class among BBTz‐based polymers reported so far, is realized in an inverted cell using TiO_x and MoO_x as the buffer layers. This study opens up opportunities to create low‐bandgap polymers with deep HOMO, and shows how the device‐less TRMC evaluation is of help for decision‐making on judicious molecular design.     

Effect of compositional dependence on physicochemical properties of Bi2Se3 doped system

Crystalline bulk compositions of Bi₂ (Se_1−xTe_x)₃ system with ( x=0.0–1.00) were prepared using the conventional melting method. The structural properties of bulk samples were studied with the aid of XRD and SEM analysis. Compositional element distribution and elemental ratios were estimated with EDX spectroscopy that attached with SEM. XRD patterns show that the prepared compounds are crystalline materials with single phases of Bi₂Se₃ and Bi₂Te₃ and/or a ternary phase. The grain size calculations were performed using the well-known Scherrer equation. The thermal studies analysis was carried out by using DSC. DSC studies revealed that the prepared samples are stable and none decomposable over the temperature range. Physicochemical properties such as compactness, molar volume and the free volume percentage were calculated for the concerned alloys based on the experimentally calculated densities of each compound. The measured parameters showed a strong dependence on the Te content.     

Properties of the GaAs oxide layer grown by the liquid-phase chemical-enhanced technique on the S-passivated GaAs surface

The properties of GaAs oxide layers prepared by liquid-phase chemical-enhanced oxidation of the (NH₄)₂S_x-passivated GaAs surface are investigated. The initial oxidation rate is suppressed and the refractive indices of oxide layers are lower after S passivation. In accordance with the depth profiles measured by secondary ion mass spectroscopy (SIMS), a model is proposed to illustrate the chemical composition of the oxide layer grown by liquid-phase chemical-enhanced oxidation after S passivation. Oxidation following different surface treatments was studied by analyzing their activation energies. In addition, we find that the leakage current and the breakdown electric field of oxide layers can be improved significantly with the passivation technique.     

Controllable synthesis of Sm2O3 crystallites with the assistance of templates by a hydrothermal–calcination process

Sm₂O₃ crystallites were controllably synthesized by the hydrothermal–calcination process with the assistance of different templates. The phase compositions, morphologies and optical properties of the as-prepared samples were characterized by X-ray diffraction, scanning electron microscope and ultraviolet–visible spectrophotometer. It is investigated that the influence of different templates on the structural and optical properties of Sm₂O₃ samples. Results showed that Sm₂O₃ crystallites with ribbon-like, granular and flake-like morphologies were obtained when NaNO₃, trisodium citrate and polyvinyl-pyrrolidone (PVP) were used as template agents. These templates effectively controlled the microstructure and optical properties of the as-prepared samples. The monoclinic phase Sm₂O₃ crystallites with ribbon-like morphology were prepared by using NaNO₃ as the template agent, which energy gap is only 4.827 eV.