CH_3NH_3 Formed by Electron Injection at Heterojunction Inducing Peculiar Properties of CH_3NH_3PbI_3 Material

The effect of formed CH_3NH_3 at the heterojunction on properties of CH_3NH_3PbI_3 material is investigated based on experiment and theoretical calculation. Our calculation results show that the giant dielectric constant, anomalous hysteresis and long-lasting polarization for CH_3NH_3PbI_3 originate from the formed CH_3NH_3 at the heterojunction. It is found that the induced weak EPS by the reorientation of CH_3NH_3 sub-group along the built-in electric field enables us to effectively increase the ordering of entire lead-halide framework. In addition, the heterojunction has an advantage of channel separation between carrier transport and electron diffusion. These properties of the heterojunction are the main origin of the high efficiency of CH_3NH_3PbI_3 solar cells.     

Fluorescence Enhancement of Metal-Capped Perovskite CH_3NH_3PbI_3 Thin Films

We fabricate nano-structural metal films to improve photoluminescence of perovskite films. When the perovskite film is placed on an ammonia-treated alumina film, stronger photoluminescence is found due to local field enhancement effects. In addition, the oxide spacer layer between the metal(e.g.,Al, Ag and Au) substrate and the perovskite film plays an important role. The simulations and experiments imply that the enhancement is related to surface plasmons of nano-structural metals.     

Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH_3NH_3PbI_3

The organic-inorganic hybrid perovskite CH_3NH_3PbI_3 has attracted significant interest for its high performance in converting solar light into electrical power with an efficiency exceeding 20%. Unfortunately, chemical stability is one major challenge in the development of CH_3NH_3PbI_3 solar cells. It was commonly assumed that moisture or oxygen in the environment causes the poor stability of hybrid halide perovskites, however, here we show from the first-principles calculations that the room-temperature tetragonal phase of CH_3NH_3PbI_3 is thermodynamically unstable with respect to the phase separation into CH_3NH_3I + PbI_2, i.e., the disproportionation is exothermic,independent of the humidity or oxygen in the atmosphere. When the structure is distorted to the low-temperature orthorhombic phase, the energetic cost of separation increases, but remains small. Contributions from vibrational and configurational entropy at room temperature have been considered, but the instability of CH_3NH_3PbI_3 is unchanged. When I is replaced by Br or Cl, Pb by Sn, or the organic cation CH_3NH_3 by inorganic Cs, the perovskites become more stable and do not phase-separate spontaneously. Our study highlights that the poor chemical stability is intrinsic to CH_3NH_3PbI_3 and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells.     

Importance of PbI_2 morphology in two-step deposition of CH_3NH_3PbI_3 for high-performance perovskite solar cells

Controlling the morphology of the perovskite film is an effective way to improve the photoelectric conversion efficiency of solar cell devices. In this work, we study the influence of the crystallization condition on PbI_2 morphology and the performances of resulting perovskite solar cells. The PbI_2 morphologies and coverage rates under different formation conditions such as solvent effect, slow crystallization at room temperature and substrate-preheating, are found to be of crucial importance for preparing high-quality perovskite. The generation of loosely packed disk-like PbI_2 film with interpenetrating nanopores promotes the penetration of methyl ammonium iodide(MAI), leading to a better crystallinity of the perovskite film, and a best repeatable power conversion efficiency of 11.59% is achieved when methyl ammonium lead triiodide(CH_3NH_3PbI_3, MAPbI_3) is employed. In addition, an excellent device is also obtained with an efficiency of more than 93% to remain after working for 43 days.     

O_3 fast and simple treatment-enhanced p-doped in Spiro-MeOTAD for CH_3NH_3I vapor-assisted processed CH_3NH_3PbI_3 perovskite solar cells

We demonstrate a simple and fast post-deposition treatment with high process compatibility on the hole transport material(HTM) Spiro-MeOTAD in vapor-assisted solution processed methylammonium lead triiodide(CH_3NH_3PbI_3)-based solar cells. The prepared Co-doped p-type Spiro-MeOTAD films are treated by O_3 at room temperature for 5 min,10 min, and 20 min, respectively, prior to the deposition of the metal electrodes. Compared with the traditional oxidation of Spiro-MeOTAD films overnight in dry air, our fast O_3 treatment of HTM at room temperature only needs just 10 min,and a relative 40.3% increment in the power conversion efficiency is observed with respect to the result of without-treated perovskite solar cells. This improvement of efficiency is mainly attributed to the obvious increase of the fill factor and short-circuit current density, despite a slight decrease in the open-circuit voltage. Ultraviolet photoelectron spectroscopy(UPS) and Hall effect measurement method are employed in our study to determine the changes of properties after O_3 treatment in HTM. It is found that after the HTM is exposed to O_3, its p-type doping level is enhanced. The enhancement of conductivity and Hall mobility of the film, resulting from the improvement in p-doping level of HTM, leads to better performances of perovskite solar cells. Best power conversion efficiencies(PCEs) of 13.05% and 16.39% are achieved with most properly optimized HTM via CH_3NH_3I vapor-assisted method and traditional single-step method respectively.     

In-plane oriented CH_3NH_3PbI_3 nanowire suppression of the interface electron transfer to PCBM

One-dimensional nanowire is an important candidate for lead-halide perovskite-based photonic detectors and solar cells. Its surface population, diameter, and growth direction, etc., are critical for device performance. In this research,we carried out a detailed study on electron transfer process at the interface of nanowire CH_3 NH_3 PbI_3(N-MAPbI_3)/Phenyl C61 butyric acid methyl-ester synonym(PCBM), as well as the interface of compact CH_3 NH_3 PbI_3(C-MAPbI_3)/PCBM by transient absorption spectroscopy. By comparing the carrier recombination dynamics of N-MAPbI_3, N-MAPbI_3/PCBM,C-MAPbI_3, and C-MAPbI_3/PCBM from picosecond(ps) to hundred nanosecond(ns) time scale, it is demonstrated that electron transfer at N-MAPbI_3/PCBM interface is less efficient than that at C-MAPbI_3/PCBM interface. In addition, electron transfer efficiency at C-MAPbI_3/PCBM interface was found to be excitation density-dependent, and it reduces with photo-generation carrier concentration increasing in a range from 1.0 × 1018 cm~(-3)–4.0 × 1018 cm~(-3). Hot electron transfer,which leads to acceleration of electron transfer between the interfaces, was also visualized as carrier concentration increases from 1.0 × 10~(18) cm~(-3)–2.2 × 10~(18) cm~(-3).     

Improvement of Photoluminescence of Perovskite CH_3NH_3Pb I_3 by Adding Additional CH_3NH_3I during Grinding

The organic-inorganic hybrid perovskite CH_3NH_3PbI_3 has been a good candidate for many optoelectronic applications such as light-emitting diodes due to its unique properties.Optimizing the optical properties of the CH_3NH_3PbI_3 material to improve the device performance is a hot topic.Herein,a new strategy is proposed to enhance the light emission of CH_3NH_3PbI_3 phosphor effectively.By adding the reactant CH_3NH_3I powder in an appropriate proportion and simply grinding,the emission intensity of CH_3NH_3PbI_3 is greatly improved.The advantages of the proposed method are swiftness,simplicity and reproducibility,and no requirement for a complex organic ligand.The mechanism of this phenomenon is revealed by x-ray diffraction,scanning electron microscopy,energy dispersive spectroscopy,photoluminescence,and temperature-dependent photoluminescence.This study offers a unique insight for optimizing the optical properties of halide perovskite materials.     

420nm thick CH_3NH_3PbI_(3-x)Br_x capping layers for efficient TiO_2 nanorod array perovskite solar cells

The rutile TiO_2 nanorod arrays with 240 nm in length, 30 nm in diameter, and 420 μm~(-2) in areal density were prepared by the hydrothermal method to replace the typical 200-300 nm thick mesoporous TiO_2 thin films in perovskite solar cells. The CH_3NH_3PbI_3-xBrx capping layers with different thicknesses were obtained on the TiO_2 nanorod arrays using different concentration PbI_2·DMSO complex precursor solutions in DMF and the photovoltaic performances of the corresponding solar cells were compared. The perovskite solar cells based on 240 nm long TiO_2 nanorod arrays and 420 nm thick CH_3NH_3PbI_(3-x)Br_x capping layers showed the best photoelectric conversion efficiency(PCE) of 15.56%and the average PCE of 14.93 ± 0.63% at the relative humidity of 50%-54% under the illumination of simulated AM 1.5 sunlight(100 mW·cm~(-2)).     

Temperature-dependent photoluminescence on organic-inorganic metal halide perovskite CH_3NH_3PbI_(3-x)Cl_x prepared on ZnO/FTO substrates using a two-step method

Temperature-dependent photoluminescence characteristics of organic-inorganic halide perovskite CH_3NH_3Pb I_(3-x)Cl_x films prepared using a two-step method on ZnO/FTO substrates were investigated. Surface morphology and absorption characteristics of the films were also studied. Scanning electron microscopy revealed large crystals and substrate coverage. The orthorhombic-to-tetragonal phase transition temperature was~140 K. The films' exciton binding energy was 77.6 ± 10.9 meV and the energy of optical phonons was 38.8 ± 2.5 meV. These results suggest that perovskite CH_3NH_3Pb I_(3-x)Cl_x films have excellent optoelectronic characteristics which further suggests their potential usage in perovskitebased optoelectronic devices.     

Spin-Selective Electron Transfer and Charge Recombination in Self-Assembled Porphyrin Naphthalenediimide Dyads

The spin selectivity of electron transfer in a series of metalloporphyrin pyridyl-linked naphthalenediimides (MTPP-Pyr(CH₂) _n NDI, where M = Zn, n = 2, 4, 7, and M = Al(OCOPh), n = 7) is studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal 4-cyano-4′-pentylbiphenyl (5CB). Following pulsed laser excitation, all of the complexes show a narrow antiphase doublet that is assigned to the triplet state of the radical pair MTPP^•+NDI^•−. Initially, the antiphase doublet has an emission/absorption (E/A) polarization pattern characteristic of singlet electron transfer. At later times the polarization inverts to an A/E pattern. The intensity of the late signal depends very strongly on the nature of the metal in the porphyrin. A qualitative model that rationalizes this result is presented. It is proposed that both singlet and triplet electron transfer occur in the dyads and that the differences in the intensity of the polarization are the result of differences in the spin selectivity of intersystem crossing for the different metals. The consequences of this model for magnetic field effects in such systems are briefly discussed.     

Device simulation of lead-free CH_3NH_3SnI_3 perovskite solar cells with high efficiency

The lead-free perovskite solar cells(PSCs) have drawn a great deal of research interest due to the Pb toxicity of the lead halide perovskite.CH_3NH_3SnI_3 is a viable alternative to CH_3NH_3PbX_3,because it has a narrower band gap of 1.3 eV and a wider visible absorption spectrum than the lead halide perovskite.The progress of fabricating tin iodide PSCs with good stability has stimulated the studies of these CH_3NH_3SnI_3 based cells greatly.In the paper,we study the influences of various parameters on the solar cell performance through theoretical analysis and device simulation.It is found in the simulation that the solar cell performance can be improved to some extent by adjusting the doping concentration of the perovskite absorption layer and the electron affinity of the buffer and HTM,while the reduction of the defect density of the perovskite absorption layer significantly improves the cell performance.By further optimizing the parameters of the doping concentration(1.3 × 10~(16) cm~3) and the defect density(1 × 10~(15) cm~3) of perovskite absorption layer,and the electron affinity of buffer(4.0 eV) and HTM(2.6 eV),we finally obtain some encouraging results of the J_(sc) of 31.59 mA/cm~2,V_(oc) of 0.92 V,FF of 79.99%,and PCE of 23.36%.The results show that the lead-free CH_3NH_3SnI_3 PSC is a potential environmentally friendly solar cell with high efficiency.Improving the Sn~(2+) stability and reducing the defect density of CH_3NH_3SnI_3 are key issues for the future research,which can be solved by improving the fabrication and encapsulation process of the cell.     

First-principles hybrid functional study of the electronic structure and charge carrier mobility in perovskite CH_3NH_3SnI_3

We calculate the electronic properties and carrier mobility of perovskite CH_3NH_3SnI_3 as a solar cell absorber by using the hybrid functional method. The calculated result shows that the electron and hole mobilities have anisotropies with a large magnitude of 1.4 × 10~4cm~2·V~(-1)·s~(-1) along the y direction. In view of the huge difference between hole and electron mobilities, the perovskite CH3NH3 Sn I3can be considered as a p-type semiconductor. We also discover a relationship between the effective mass anisotropy and electronic occupation anisotropy. The above results can provide reliable guidance for its experimental applications in electronics and optoelectronics.     

基于分子内电荷转移的多枝状荧光探针光学性质与响应机理分析

本文采用含时密度泛函理论研究了用于检测生物硫醇的荧光探针分子的光学性质.通过计算探针分子Mol.1、Mol.2和Mol.3与半胱氨酸和同型半胱氨酸反应前后的单光子吸收和发射性质,研究了碳碳三键和苯环结构对荧光探针性质的影响.随着给电子体三苯胺结构的逐渐完善和碳碳三键的加入,探针分子的振子强度逐渐增大,展现出了更好的荧光探针性质.同时,研究了不同侧枝数目对探针分子性质的影响,结果表明,相较于单枝分子Z1和三枝分子Mol.3,两个侧枝的探针分子Z2振子强度更大,检测效果更佳.增加了碳碳三键和苯环后的单枝新型探针分子Mol.4,相较于具有三枝结构的探针分子Mol.3,具有良好的探针性质,且结构更为简单.     

First-principles analysis of the structural,electronic, and elastic properties of cubic organic-inorganic perovskite HC(NH_2)_2PbI_3

The structural, electronic, and elastic properties of cubic HC(NH_2)_2PbI_3 perovskite are investigated by density functional theory using the Tkatchenko–Scheffler pairwise dispersion scheme. Our relaxed lattice parameters are in agreement with experimental data. The hydrogen bonding between NH_2 and I ions is found to have a crucial role in FAPbI_3 stability. The first calculated band structure shows that HC(NH_2)_2PbI_3 has a direct bandgap(1.02 eV) at R-point, lower than the bandgap(1.53 eV) of CH_3NH_3PbI_3. The calculated density of states reveals that the strong hybridization of s(Pb)–p(I) orbital in valence band maximum plays an important role in the structural stability. The photo-generated effective electron mass and hole mass at R-point along the R–Γ and R–M directions are estimated to be smaller: m_e~*= 0.06 m0 and m_h~*= 0.08 m0 respectively, which are consistent with the values experimentally observed from long range photocarrier transport. The elastic properties are also investigated for the first time, which shows that HC(NH_2)_2PbI_3 is mechanically stable and ductile and has weaker strength of the average chemical bond. This work sheds light on the understanding of applications of HC(NH_2)_2PbI_3 as the perovskite in a planar-heterojunction solar cell light absorber fabricated on flexible polymer substrates.     

FA离子重定向引起的电荷局域显著抑制了FAPbI3钙钛矿电子-空穴复合：时域从头算研究

本文利用从头算非绝热分子动力学结合时域密度泛函理论模拟,计算表明部分FA(FA=HC[NH₂]₂⁺)阳离子的重定向造成电子和空穴局域在不同位置,减小了非绝热耦合并抑制了原子运动,从而显著抑制了FAPbI₃的非辐射电子-空穴复合. 虽然慢的核运动同时增加了退相干时间,但是减小的非绝热耦合是影响电荷复合的主导因素,将电子-空穴复合的时间尺度延长至数纳秒,比原始的FAPbI₃激发态寿命长约3.9倍,与实验结果相符. 研究厘清了实验报导的FAPbI₃激发态寿命增加的机理,为设计高性能的钙钛矿太阳能电池和光电器件提供了合理的策略.     

甲醇锂和乙醇锂拉曼光谱的密度泛函理论研究

前人研究证明在烷氧基锂(ROLi)中锂原子直接和氧原子相连而形成O-Li健,作者依此构造出甲醇锂和乙醇锂的分子构型.目前在实验和理论计算上都尚无对甲醇锂和乙醇锂分子的拉曼光谱的报道.文章用密度泛函珲论中的B3LYP混合泛函联合6-31G(d,p)基函数组进行了这两个分子的结构优化得到其平衡构型.基于此平衡构型计算出甲醇锂和乙醇锂的振动频率和拉曼光谱,对计算所得的振动频率进行了简正坐标分析,根据各频率的势能分布进行了全面的归属指认,旨在为用拉曼光谱技术研究和分析锂离子电池固体电解质膜(SEI膜)组分提供更多的理论依据.     

On the Coupling Between Surface Charge and Hydration in Biomembranes: Experiments with 3-Hydroxyflavone Probes

The newly synthesized 3-hydroxyflavone derivative [2-(4-N,N-diethylaminophenyl)-3-hydroxy-6-chromonyl](N,N-dimethyl-octyl) ammonium bromide (F2) together with already used 4-dimethylamino-3-HF (F) are found to be extremely sensitive to the effects of preferential hydration in model solvent system. This property is explored in the study of phospholipid vesicles of different composition made of neutral, cationic, and anionic lipids. We observe an extremely high level of response of both F and F2 fluorescence emission spectra to the surface charge of the vesicles: the N^* form is strongly favored with less positively charged and more negatively charged membrane surface. The strong red-edge effects, which are almost independent of the lipid composition demonstrate the immobility of the probe environment on the time scale of fluorescence emission and suggest the static nature of hydration effects.     

The Raman Spectrum of The 3-Br Pyridine-Bromine Charge Transfer Complex

The formation of a charge transfer complex between pyridine and halogens or mixed halogens^(1,2) brings about perturbations in the infrared spectra of the pyridine molecule and many halogen complexes show two bands in the low frequency range, which can be attributed to the v _X-X and v _N…Y stretching vibrations. ^(3–5). However, by comparison, very little has been reported on the Raman spectra of such complexes. Klaboe⁽⁶⁾ studied the v _Br-Br Raman frequency of some bromine complexes and more recently the Raman spectra of pyridine complexed with Ni (CN)₄ ⁽⁷⁾ and TiCl₄ ⁽⁸⁾ have been reported. In this work, the Raman spectrum of the 3-Br pyridine-bromine charge transfer complex is investigated.     

钛酸盐ATiO3 钙钛矿的铁电和反铁畸变

钛酸盐钙钛矿氧化物体系的研究对理解具有铁电极化的功能材料具有非常重要的科学意义.本文对ATiO3(A= Ca,Sr,Ba,Pb,Cd)体系的低温相、结构相变、及其对应的位移模式进行综述.我们着重比较了该体系存在的两类不稳定声子模式,即铁电极化模式(Ferroelectric,FE)和反铁畸变模式(Antiferrodistortive,AFD),在不同体系中所起到的作用.我们举例阐述了晶格失配应变、人工构造超晶格和化学掺杂等方法对这两种模式的调控思路.最后我们给出本文的总结讨论及研究展望.     

钛酸盐ATiO3钙钛矿的铁电和反铁畸变

钛酸盐钙钛矿氧化物体系的研究对理解具有铁电极化的功能材料具有非常重要的科学意义.本文对ATiO3(A=Ca,Sr,Ba,Pb,Cd)体系的低温相、结构相变、及其对应的位移模式进行综述.我们着重比较了该体系存在的两类不稳定声子模式,即铁电极化模式(Ferroelectric,FE)和反铁畸变模式(Antiferrodistortive,AFD),在不同体系中所起到的作用.我们举例阐述了晶格失配应变、人工构造超晶格和化学掺杂等方法对这两种模式的调控思路.最后我们给出本文的总结讨论及研究展望.