Fabrication of anodic photocurrent generation systems by use of 6-O-dihydrophytylcellulose as a matrix or a scaffold of porphyrins

Langmuir-Blodgett (LB) films containing porphyrin molecules were fabricated by use of 6-O-dihydrophytylcellulose (DHPC) toward anodic photocurrent generation systems. To suppress the porphyrin aggregation, two different approaches were applied: (1) mixing a low-molecular-weight porphyrin having a diterpenoid carbon skeleton (DPor) with DHPC as a matrix (matrix fabrication) and (2) bonding porphyrin molecules to the hydroxyl groups of DHPC covalently, converting into 6-O-dihydrophytyl-2,3-di-O-[p-(10,15,20-triphenyl-5-porphyrinyl)-benzoyl]cellulose as a scaffold (scaffold fabrication). The structure and film properties of the monolayers and the LB films were investigated by the surface pressure (π)–area (A) isotherm measurements, atomic force microscopy, UV–Vis spectroscopy, and absorption dichroism measurements. The porphyrin aggregation in the LB film could be well suppressed only by the scaffold fabrication, leading to the improvement of the photocurrent quantum yields. The efficient photocurrent performance can be demonstrated by the isolation and the parallel orientation of porphyrin moieties due to the cellulose rigid scaffold. This paper was the subject of the Best Poster Award of the 235th edition of the ACS National Meeting, Cellulose and Renewable Materials.     

MONOLAYER STUDIES OF 5-(4-CARBOXYPHENYL)-10,15,20-TRITOLYL-PORPHYRIN–I. OPTICAL STUDIES OF FILMS AT THE AIR-WATER INTERFACE and OF FILMS TRANSFERRED ONTO SOLID SUBSTRATES†

Abstract— The title compound forms well-behaved monomolecular films at the air-water interface. The surface pressure-molecular area isotherms change with the pH of the subphase in a manner which suggests that the carboxylic acid group acts as the hydrophylic portion of the molecule with a pK_a of –7.3. In compressed monolayers the porphyrin ring appears to be oriented so that the plane of the ring is perpendicular to the surface. Spectroscopic studies of single monolayer films transferred to quartz slides using the Langmuir-Blodgett technique indicate that three distinct species are present in the films, with the amount of each phase depending on the pH of the subphase. One species, present at low pH, is assigned as a monomer on the basis of its optical and fluorescence spectra and its fluorescence lifetime. At low pH this monomer species co-exists with another non-fluorescent aggregated species. For films formed on subphases with pH > 7.3, these two species are converted to a single, weakly fluorescent species which exhibits an unusual absorption spectrum. We postulate that this third species is a constrained aggregate but rule out the possibility of a face-to-face dimer on spectroscopic grounds. For films at the air-water interface specular reflection, indicative of a smooth, highly absorbant film, is observed from monolayers of the title compound. Visual examination of this phenomenon proved to be very useful in assessing the completeness of spreading and the collapse point. Under certain conditions a distinct macroscopic structure is observed in the monolayer film. This structure is interpreted as evidence for the presence of two or more two-dimensional crystalline phases. Support for this view comes from previous measurements of specular reflection on single crystals of tetraphenylporphyrin. There is no indication that variations in the macroscopic structure of the films have any significant effect on the microscopic properties such as the surface pressure-area isotherms.     

含不同链长的三个卟啉LB膜修饰电极的光电响应研究

利用LB (Langmuir-Blodgett)技术将含不同链长的卟啉化合物(C4Py, C6Py和C8Py)单层膜转移到ITO (indium-tin oxide)导电玻璃上, 发现其具有良好的光电转换性质. 卟啉化合物修饰后的紫外吸收光谱与光电流工作谱重叠, 表明卟啉化合物起到了敏化光电流产生的效果; 而且电子给体、电子受体和偏压对其敏化效果的实验结果表明: 光诱导电子转移是产生光电响应的主要原因. 而且, 这三个卟啉化合物的光电响应性质与碳链长度相关, 其中含有六个碳链的C6Py表现出最佳的光电转化效果.     

PHOTOCONDUCTIVITY AND PHOTOVOLTAIC EFFECTS IN LANGMUIR-BLODGETT FILMS OF CHLOROPHYLL-a

The DC photoconductivity and photovoltaic effect in Langmuir films of Chlorophyll- a (Chi a ) of precisely controlled thickness formed between Al and Au electrodes have been extensively investigated. The dark conductivity and dark voltage are almost abolished in a N₂ atmosphere. The action spectrum of the photocurrent closely resembles the monolayer absorption spectrum of Chl indicating that the primary event in photoconduction is the generation of singlet excited states. Thicknessdependence studies on the photoconductivity indicate that carrier generation is a surface process and that the mean diffusion length of the excited state is approximately 20 nm. In short (˜1 s) light exposures the photocurrent always increases linearly with both the applied voltage and the light intensity. In continuous light the current-voltage characteristics are highly non-linear and the photocurrent shows a square-root intensity dependence at high intensities and small applied potentials. These results are interpreted in terms of second-order recombination and charge-trapping processes at high carrier densities. The photovoltage usually shows a logarithmic intensity dependence at high intensities and its maximum value in thick films is 800 mV with the Al electrode acquiring a negative polarity. This behaviour, together with some observations on the asymmetry of photocurrent-voltage characteristics and the effect of substituting an aqueous top electrode for Au, further suggest that the photoactive surface is a p-n junction between the Chl and Al₂O₃ layers.     

Electrochemical deposition of cadmium sulfide from DMSO solution

Semiconducting films of CdS are produced by the cathodic deposition of Cd on inert electrodes in the presence of dissolved S₈ in dimethylsulfoxide solution. Linear potential scans and potential pulse measurements show that the first layers are formed by underpotential deposits of Cd reacting with S₈ from the solution. With increasing coverage of the surface the mechanism changes to a mixed reduction of Cd and a reduction of S₈ to sulfide, which forms the deposit with Cd²⁺ from the solution. Photoeffects are observed with deposits greater than three or four equivalent monolayers of CdS, demonstrating the n-type character of the deposits. The photocurrent spectrum of films in the micrometer range differs considerably from that of single-crystal CdS. Films produced in this way need further treatment before they could be useful as low-cost electrodes for solar cells.     

PHOTOVOLTAIC PROPERTIES OF MESO-TETRAARYLPORPHYRINS

Abstract— The photovoltaic properties of meso -tetraphenylporphyrin and its derivatives with various para -substituents, meso-tetrakis(2,4-dimethoxyphenyl)porphyrin and meso -tetrakis (2-fluorenyl)porphyrin have been investigated. The forward dark current-voltage characteristics of Al/porphyrin/Ag cells are attributed to the MIS Schottky barriers consisting of Al/Al₂O₃/porphyrin. The barrier parameters such as the apparent diffusion potential V ₀ the barrier width w₀ and the density of ionized impurity N are estimated by using the capacitor discharge method. The action spectra of photocurrents closely follow the optical absorption spectra of the porphyrin films. The photocurrents vary as i _pα I ^γ, where I is the incident light intensity, and the Sight exponents y range between 0.83 and 1.0. The sublinear difference from unity could be related to the exponential distribution of hole traps in films of the porphyrins. No obvious correlation between the photocurrent and the fluorescence quantum yields of the sublimed porphyrin films is found. The electron donating substituents such as OCH₃ and CH₃ strikingly increase the photocurrents. The photocurrent quantum yields correlate exponentially with the first ring oxidation potentials of the porphyrins and also with the substituent constants for the Hammett linear free-energy relationship. The current quantum yields estimated for the porphyrins studied, range in the order of 10^-4–10^-2 with power conversion efficiencies 10^-7–10^-3.     

SYNTHESIS, STRUCTURE AND ELECTRICAL PROPERTIES OF POLY(PHENYLENE VINYLENE) FILM DOPED WITH FERRIC CHLORIDE

Poly(phenylene vinylene) (PPV) film was synthesized via a soluble precursor poly-mer. Strong fluorescence at 500-600nm was observed in both precursor and PPV film.Room-temperature conductivity of PPV film doped with FeCl_3 depends on the eliminationtemperature, the concentration of FeCl_3 and doping time. The maximum conductivity ofdoped PPV at room-temperature can reach about 40 S·cm~(-1). The temperature depen-dence of conductivity was controlled by 1D-VRH (1 Dimension Variable Range Hopping)model with T_0 value of 3.9×10~3 K. Non-Ohmic conductivity resulting from Schottky effectwas observed and the value of converted voltage from Ohmic region into non-Ohmic regionat the current-voltage characteristic was found to be dependent upon the work function ofelectrodes.     

Photophysical and Photoelectric Properties of 2—{[4—（N—Hexadecyl—N—methylamino）phenyl]methylene}—propanedinitrile

2-{[4-(N-Hexadecyl-N-methylamino)phenyl]methylene}-propanedinitrile(HMAPN) with typical donor-π-acceptor(D-π-A）structure was synthesized.It could be easily assembled into stable films by LB technique.The photophysical properties of HMAPN were investigated in solution and on LB films.The photoelectric properties of HMAPN were examined and the anodic photocurrent of the ITO electrode modified by the monolayer LB film of HMAPN was measured as 835 nA/cm^2 under the white light of 218.2 mW/cm^2 without bias voltage.The effects of light intensity,bias voltage on the photocurrent were discussed.The possible mechanism of the photocurrent formation was given.     

Synthesis and Characterization of a Sol–Gel Derived Ureasilicate Hybrid Organic-Inorganic Matrix Containing CdS Colloidal Particles

The electrical properties of sol–gel-derived films can be tailored by embedding conductive particles of ruthenium dioxide or carbon black in an insulating amorphous SiO₂ silica matrix. The preparation process included an acid hydrolysis of tetraethoxysilane and methyltrimethoxysilane. Then alcohol solutions of ruthenium chloride or carbon black were added. Films of filler concentration up to 60 vol.% were prepared by dip coating and then dried and heat-treated at various temperatures up to 600_°C. The D.C. resistance of the films can be varied within the range of 10⁹ to 10^–2 cm. A non-linear dependence on filler composition in the films was observed for both systems, which is explained by a modified percolation theory. A percolation threshold of 5.5 vol.% for SiO₂-RuO₂ or 50 vol.% for SiO₂-C films, whereby the resistance drastically decreases, was determined. Moreover the temperature dependency of resistance and the current-voltage characteristics of the films can also be explained by this geometric model.     

Electrocatalytic activity of a new nanostructured polymeric tetraruthenated porphyrin film for nitrite detection

The preparation of the monomer and thin films of a new polymeric tetraruthenated porphyrin material and their characterization by spectroscopic and electrochemical techniques, are described. This material is one of the most active electrocatalyst for the oxidation of nitrite ions to nitrate, exhibiting a heterogeneous cross-exchange rate constant (k_f=(6.2±0.1×10⁴) M^-1 s^-1) 30 times higher than that previously described for the electrostatic assembled porphyrin films. The polymeric films were obtained by electropolymerization of the corresponding molecular films, previously prepared by dip-coating. This strategy leads to an increase in the efficiency of the reticulation process while minimizing the amount of monomer necessary for the preparation of the modified electrodes. The conductivity of the thin films close to the E_1/2 of the Ru(III/II) redox pair is very good, decreasing rapidly as the applied potential departs from it, as expected for a redox polymer. The conductivity decreases when the surface concentration becomes higher than 1.2×10^-8 mol cm⁻² also, reflecting a higher impedance for electrolyte diffusion inside the polymeric material. The high electrocatalytic activity associated with the high conductivity make this new nanostructured material suitable for sensor applications.     

Engineering Chemically Exfoliated Large‐Area Two‐Dimensional MoS2 Nanolayers with Porphyrins for Improved Light Harvesting

Molybdenum disulfide (MoS₂) is a promising candidate for electronic and optoelectronic applications. However, its application in light harvesting has been limited in part due to crystal defects, often related to small crystallite sizes, which diminish charge separation and transfer. Here we demonstrate a surface‐engineering strategy for 2D MoS₂ to improve its photoelectrochemical properties. Chemically exfoliated large‐area MoS₂ thin films were interfaced with eight molecules from three porphyrin families: zinc(II)‐, gallium(III)‐, iron(III)‐centered, and metal‐free protoporphyrin IX (ZnPP, GaPP, FePP, H₂PP); metal‐free and zinc(II) tetra‐(N‐methyl‐4‐pyridyl)porphyrin (H₂T4, ZnT4); and metal‐free and zinc(II) tetraphenylporphyrin (H₂TPP, ZnTPP). We found that the photocurrents from MoS₂ films under visible‐light illumination are strongly dependent on the interfacial molecules and that the photocurrent enhancement is closely correlated with the highest occupied molecular orbital (HOMO) levels of the porphyrins, which suppress the recombination of electron–hole pairs in the photoexcited MoS₂ films. A maximum tenfold increase was observed for MoS₂ functionalized with ZnPP compared with pristine MoS₂ films, whereas ZnT4‐functionalized MoS₂ demonstrated small increases in photocurrent. The application of bias voltage on MoS₂ films can further promote photocurrent enhancements and control current directions. Our results suggest a facile route to render 2D MoS₂ films useful for potential high‐performance light‐harvesting applications.     

A photoelectrochemical study of passive copper in alkaline solutions

The photoelectrochemical behaviour of copper covered with a passivating Cu₂O layer has been studied in alkaline solution. Cu₂O shows the characteristics of a p-type semiconductor with a band gap of 2.3 eV and a flatband potential of −0.28 V (SHE). Its photocurrent spectrum shows the characteristics of the absorption spectrum of Cu₂O films. Several redox systems have been tested, including a CuO layer of the duplex film formed at sufficiently positive potentials. The cathodic photocurrent leads to a reduction of the CuO overlayer to Cu₂O rather than to a self-reduction of Cu₂O to Cu. For the duplex film a decrease of the band gap and an increase of the flatband potential is found, suggesting a participation of CuO in the generation of photoelectrons.     

Electrochemical synthesis and optical analysis of poly[(2,2′-dithienyl)-5,5′-diylvinylene]

The electrochemical synthesis of poly[(2,2′-dithienyl)-5,5′-diylvinylene tetrafluoroborate] (PDTE/BF₄) has been accomplished and provides coppery-black free-standing films with a room-temperature four-probe conductivity as high as 15 (Ω cm)^?1. The conductivity and morphology of the films were found to be strongly dependent on synthetic conditions. The oxidized form of PDTE is not air stable, as evidenced by a rapid, three order of magnitude drop in the conductivity. Optoelectrochemical experiments demonstrate that bipolarons are the main charge-carrying species in conductive PDTE and allow their evolution to be followed as a function of oxidation.     

基于新型交联聚合物电解质的准固态染料敏化太阳能电池

采用柠檬酸(CA)交联聚乙二醇(oligo-PEG, 平均分子量M_w=200, 400, 1000, 2000), 合成具有可生物降解性能的聚柠檬酸-乙二醇(PCE)交联聚酯, 并以此为基体材料制备得到准固态的三维交联型PCE/LiI/I₂聚合物电解质. 采用红外吸收光谱(IR)、核磁共振氢谱(¹H-NMR)、扫描电镜(SEM)和Raman光谱分别对PCE基体的分子结构、聚合物电解质的微观形貌以及导电离子对的存在形式进行表征; 通过线性扫描伏安法(LSV)研究了聚合物电解质的离子扩散系数、电导率以及电池的输出电流-电压(I-V)性能. 结果表明, PEG的分子量影响PCE基体膜的微观形貌及其吸液性能, 从而影响聚合物电解质的离子导电性能及电池的光电性能: 随着PEG分子量M_w从200, 400, 1000增大到2000, PCE基体膜的结构变得疏松, 吸液率增加, 吸液溶胀后的基体中I-3的跃迁活化能降低, 导致电解质的电导率和电池的短路光电流密度随之增加; 在60 mW·cm^-2的入射光强下, 四种电解质对应电池的光电转化效率依次为3.26%、3.34%、4.26%和4.89%.     

Covalently attached self‐assembly of ultrathin films from hydroxy‐containing porphyrin and diazoresin

Ultrathin films from 5,10,15,20‐tetrakis(4‐hydroxyphenyl) porphyrin (HPP) and diazoresin (DR) via a H bonding interaction were fabricated with the self‐assembly technique. Under UV irradiation, the H bonds between the layers will convert to covalent bonds following the decomposition of the diazonium group of DR. The stability of the film against the polar solvent and electrolyte aqueous solution increases a lot because of the formation of the covalent crosslinking structure. Thus, the photoelectric conversion property of DR/HPP film can be measured directly with a normal photoelectric chemical cell with potassium chloride as the electrolyte. The maximum of the anodic photocurrent was measured as 1.7 μ Å for an eight bilayer DR/HPP film deposited on an indium–tin oxide glass electrode. The action spectrum of the photocurrent generation indicated that the HPP contained in the film is responsible for the generation of the observed photocurrent. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3103–3108, 2003     

Comparative study of conventional and hybrid blocking layers for solid-state dye-sensitized solar cells

In hybrid solar cells a blocking layer between the transparent electrode and the mesoporous titanium dioxide is used to prevent short-circuits between the hole-conductor and the front electrode. The conventional approach is to use a compact film of titanium dioxide. This layer has to be of optimum thickness: it has to cover the rough surface of the anode material completely while keeping it as thin as possible since the layer acts as an ohmic resistance itself. A competitive alternative arises when using an amphiphilic diblock copolymer as a functional template to produce thin, hybrid films containing a conducting titanium dioxide network embedded in an insulating ceramic material. These hybrid films can be produced much thinner compared to the conventional approach and, hence, they possess a 32% higher conductivity. The conventional and the hybrid blocking layer are characterized by conductive scanning probe microscopy and macroscopic conductance measurements. Additionally, the functionality of both blocking layers in solid-state dye-sensitized solar cells, as tested with current-voltage measurements, is verified.     

Preparation and Photophysical and Photoelectrochemical Properties of a Covalently Fixed Porphyrin–Chemically Converted Graphene Composite

Chemically converted graphene (CCG) covalently linked with porphyrins has been prepared by a Suzuki coupling reaction between iodophenyl‐functionalized CCG and porphyrin boronic ester. The covalently linked CCG–porphyrin composite was designed to possess a short, rigid phenylene spacer between the porphyrin and the CCG. The composite material formed stable dispersions in DMF and the structure was characterized by spectroscopic, thermal, and microscopic measurements. In steady‐state photoluminescence spectra, the emission from the porphyrin linked to the CCG was quenched strongly relative to that of the porphyrin reference. Fluorescence lifetime and femtosecond transient absorption measurements of the porphyrin‐linked CCG revealed a short‐lived porphyrin singlet excited state (38 ps) without yielding the porphyrin radical cation, thereby substantiating the occurrence of energy transfer from the porphyrin excited state to the CCG and subsequent rapid decay of the CCG excited state to the ground state. Consistently, the photocurrent action spectrum of a photoelectrochemical device with a SnO₂ electrode coated with the porphyrin‐linked CCG exhibited no photocurrent response from the porphyrin absorption. The results obtained here provide deep insight into the interaction between graphenes and π‐conjugated systems in the excited and ground states.     

用紫外光谱和X射线光电子能谱研究金属卟啉自组装膜

金属卟啉化合物由于具有丰富的电子结构和特殊的物理化学性质,因而在光电材料、分子器件和非线性光学等领域具有广阔的应用前景．有机硅可用于提高高分子化合物在氧化物基底表面的附着力;在气相和液相色谱中可作为固定相;在生物传感器中可用于蛋白质的固定．     

Photoconductive UV Detectors Based on ZnO Films Prepared by Sol-Gel Method

Highly c-axis oriented ZnO thin films were deposited on single crystal Si (111) substrates by sol-gel method. The photoconductive UV detectors based on ZnO thin films, being a metal-semiconductor-metal (MSM) structure with interdigital (IDT) configuration, were fabricated by using Au as contact metal. The characteristics of dark and photocurrent of the UV detector, the UV photoresponse of the detector were investigated. The linear current-voltage (I-V) characteristics under both forward and reverse bias exhibit ohmic metal-semiconductor contacts. Under illumination using monochromatic light with a wavelength of 365 nm, photo-generated current arrived at 44.89 μ A at a bias of 6 V. The detector exhibits an evident wide-range spectral responsivity and shows a trend similar to that in photoluminescence (PL) spectrum. PL spectrum of detector exhibits two peaks, one is the near band edge emission, and another is the deep-level emission in the visible region.