Hybrid zinc oxide conjugated polymer bulk heterojunction solar cells

Bulk heterojunction photovoltaic devices based on blends of a conjugated polymer poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as electron donor and crystalline ZnO nanoparticles (nc-ZnO) as electron acceptor have been studied. Composite nc-ZnO:MDMO-PPV films were cast from a common solvent mixture. Time-resolved pump-probe spectroscopy revealed that a photoinduced electron transfer from MDMO-PPV to nc-ZnO occurs in these blends on a sub-picosecond time scale and produces a long-lived (milliseconds) charge-separated state. The photovoltaic effect in devices, made by sandwiching the active nc-ZnO:MDMO-PPV layer between charge-selective electrodes, has been studied as a function of the ZnO concentration and the thickness of the layer. We also investigated changing the degree and type of mixing of the two components through the use of a surfactant for ZnO and by altering the size and shape of the nc-ZnO particles. Optimized devices have an estimated AM1.5 performance of 1.6% with incident photon to current conversion efficiencies up to 50%. Photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy have been used to gain insight in the morphology of these blends.     

Transport and recombination dynamics studies of polymer/fullerene based solar cells

We have studied the electron/hole transport and recombination dynamics in blends of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene], (MDMO-PPV) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) at room temperature, as a function of laser excitation density and PCBM concentration. The experimental results of these studies indicate the important role played by hole-trap states in MDMO-PPV. Electron and hole transport are not balanced within the blend. PCBM is a less disordered material than MDMO-PPV and electron transport dominates the response of the solar cell device.     

Observation of the missing mode effect in a poly-phenylenevinylene derivative: effect of solvent, chain packing, and composition

Optical emission spectra of poly[2-methoxy-5-[3('),7(')-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) in dilute solutions exhibit a vibronic progression interval (～1225?cm(-1)) that does not correspond to any ground state vibrational mode frequency. This phenomenon is assigned as the missing mode effect (MIME) in which five key displaced polymer backbone vibrational modes in the range of 800-1600?cm(-1) contribute to the MIME interval. Emission spectra are calculated by analytically solving the time-dependent Schro?dinger equation using estimates of mode-specific vibrational displacements determined independently from preresonance Raman intensities. Emission spectra of MDMO-PPV thin films and nanoparticles are measured and lineshapes show an increase of the MIME frequency to ～1340?cm(-1) in addition to changes in vibronic intensity distributions and energies. Composite blend thin films consisting of MDMO-PPV and a fullerene derivative (1:1 w/w) exhibit a substantially larger MIME interval (～1450?cm(-1)) that arises from an increase in polymer chain planarity. This structural change is most apparent from large decreases of the excited state displacement of an out-of-plane C-H bending mode (961?cm(-1)) that becomes forbidden in the planar structure.     

Photo- and thermo-oxidation of poly(p-phenylene-vinylene) and phenylene-vinylene oligomer

This paper reports a study of the photo- and thermo-degradation of poly(p-phenylene-vinylene)-type (PPVs) materials, including a five-ring n-octyloxy substituted phenylene-vinylene oligomer (Ooct-OPV5) and poly(p-phenylene-vinylene) (PPV). The modifications in the chemical structure of the thin films submitted to UV-visible light irradiation and thermal oxidation were analysed with infrared spectroscopy, and the oxidation products were identified by derivatisation reactions. Results showed that the photochemical and thermal behaviour of the Ooct-OPV5 oligomer was similar to that of MDMO-PPV (poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene), which is a polymer used in organic solar cells. Additionally, the identification of the products resulting from the oxidation of the vinylene bonds was simpler in Ooct-OPV5 and PPV compared to MDMO-PPV In contrast, the oxidation mechanisms of PPV, which has no ether substituent, and MDMO-PPV are not identical; the disappearance of the double bonds in PPV does not involve the formation of aromatic ketones. It was also shown that the absence of ether substituents does not decrease the rate of photo-oxidation of PPV compared to MDMO-PPV. Finally, as the same mechanisms proposed for Ooct-OPV5 and PPV occur under both photo- and thermo-oxidative conditions of ageing, this confirms that singlet oxygen does not play a decisive role in the photo-oxidation of PPVs.     

Towards optically responsive smart materials: electronic interactions between polymeric semiconductor and photochromic molecule

Electronic interactions between π-conjugated poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and photochromic 6-nitro-1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline] (SP) were studied using the optical spectroscopy. The observed fluorescence quenching of MDMO-PPV by the open merocyanine form of the SP was explained as a photoinduced charge transfer. This opens the possibility to reversibly switch between low and high photoconductive states which can be used for the construction of electro-optical bi-stable devices or sensors.     

Light-induced degradation of the active layer of polymer-based solar cells

Polymer-based organic solar cells are known to offer a poor stability in real use conditions, and the photodegradation of the active organic layer plays an important role in the reduced lifetime of the devices. This paper focuses on the photodegradation of two conjugated polymers used in organic solar cells, namely poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene (MDMO-PPV) and poly(3-hexyylthiophene) (P3HT), and their blends with [60]PCBM (methano-fullerene[6,6]-phenyl C61-butyric acid methyl ester), a fullerene derivative. MDMO-PPV and P3HT thin films were submitted to photoageing (λ > 300 nm) in the presence and in the absence of oxygen. The mechanisms by which these polymers degrade were elucidated. P3HT, pristine and blended with PCBM, was shown to be much more stable under illumination than MDMO-PPV. The results showed that, if deposited on an inert substrate and well protected from oxygen with a convenient encapsulation, P3HT:PCBM based active layer should be intrinsically stable for several years in use conditions.     

An isoselective and visual inclusion host system using charge-transfer complexes of 3,3′-disubstituted-1,1′-bi-2-naphthol and methylviologen

A charge-transfer (CT) complex, composed of rac-3,3′-dibromo-1,1′-bi-2-naphthol as the electron donor and 1,1′-dimethyl-4,4′-bipyridinium dichloride as the electron acceptor, is formed only by the inclusion of specific guest molecules. The color of this inclusion CT complex is sensitive to the component guest molecules.     

Charge transfer complexes between 4,4′-disubstituted diphenyldiacetylenes: experimental and theoretical study

Novel charge transfer (CT) complexes containing donor and acceptor derivatives of diphenyldiacetylene have been synthesised and characterised. The structure of CT complexes was modelled at the B3LYP/6-31G(d)//B3LYP/6-31G(d) level of theory. It was found that the complex formation is mainly due to dipole–dipole interaction between side groups of diacetylene molecules and there was no significant charge transfer between donor and acceptor in the ground state. On the other hand, optical excitation of CT complexes leads to strong charge transfer from donor to acceptor molecule as followed from the modelling using time-dependent density functional theory (DFT) method. Diacetylene molecules adopt strongly bent configuration in CT complexes which is prohibitive for solid-state topochemical polymerisation of diacetylenes     

Photogeneration and decay of charge carriers in hybrid bulk heterojunctions of ZnO nanoparticles and conjugated polymers

The photogeneration and decay of charge carriers in blend films of ZnO nanoparticles (diameter 5 nm) and poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) or poly(3-hexylthiophene) (P3HT) were studied by means of microwave-photoconductance measurements. Excitation of the polymer in the visible spectrum was found to lead to a transient photoconductance due to dissociation of excitons at the interface between ZnO and the conjugated polymer. From the similar decay kinetics of the photoconductance and the effects of UV illumination, it is concluded that the measured photoconductance is due to electrons in ZnO. Increasing the weight fraction of ZnO in the blend films leads to a higher photoconductance. This is attributed to enhanced formation of mobile electrons by interfacial dissociation of excitons at clusters of ZnO nanoparticles rather than at individual nanoparticles. The dependence of the photoconductance on the weight fraction of ZnO is found to be different for ZnO:MDMO-PPV and ZnO:P3HT blends. This is most likely due to the presence of a smaller number of relatively large ZnO clusters in ZnO:P3HT blends and a shorter exciton diffusion length, as compared with ZnO:MDMO-PPV blends. After exposure of the blend films to UV light, a significant increase in the magnitude and the lifetime of the photoconductance is observed. This is explained in terms of the filling of electron traps in ZnO by UV exposure.     

Molecular Complexes of Crown Ethers, Part 6: Complexes of Cryptand (2,2,2) and Kryptofix 5 with Some Acceptors

The UV-Visible spectra of Cryptand (2,2,2) and Kryptofix 5 as donors, and TCNE (tetracyanoethylene), DDQ (2,3-dichloro-5,6- dicyano-1,4 benzoquinone) and PA (picric acid) as acceptors were studied. Charge transfer (CT) spectra were obtained for these systems. It was found that potassium halides have little effect on the spectra. The major process was due to an electron transfer from the donor to the acceptor. This revealed itself in very high conductivity values for the CT solutions in comparison to that of the donor or the acceptor solutions. The infrared and proton NMR spectra of the complexes indicated a strong interaction between the donor and the acceptor.     

苯酚钾及对位取代苯酚钾的电离势

在我们以前的研究工作中发现,酚类钾盐是很好的电荷给体,可以和顺丁烯二酸酐、三硝基苯以及醌类等电荷受体生成电荷转移(CT)络合物。表示分子的给电子能力的参数是分子的第一电离势(简称为电离势I_p)。对于苯酚的I_p值,近年来已有文献记载,但苯酚钾     

Charge transfer complex formation between p-chloranil and 1,n-di(9-anthryl)alkanes

Dimer model compounds of polyvinylanthracenes (1,n-di(9-anthryl)alkanes, when n=1-5) were synthesized to model the effects of distance and orientation between anthracene groups in polymeric systems. Charge transfer (CT) complexes of anthracene, 9-methylanthracene and 1,n-di(9-anthryl)alkanes with p-chloranil (p-CHL) have been investigated spectrophotometrically in dichloromethane. The colored products are measured spectrophotometrically at different wavelength depending on the electronic transition between donors and acceptor. The formation constants of the CT complexes were determined by the Benesi-Hildebrand equation. The thermodynamic parameters were calculated by Van't Hoff equation. Stochiometries of the complexes formed between donors and acceptor were defined by the Job's method of the continuous variation and found in 1:1 complexation with donor and acceptor at the maximum absorption bands.     

Charge-transfer complex formation between p-chloranil and 1,n-dicarbazolylalkanes

Dimer model compounds of polyvinylcarbazoles (1,n-di(N-carbazolyl)alkanes, when n=1-5) were synthesized to model the effects of distance and orientation between carbazole groups in polymeric systems. Charge-transfer (CT) complexes of carbazole, N-ethylcarbazole and 1,n-di(N-carbazolyl)alkanes with p-chloranil (p-CHL) have been investigated spectrophotometrically in dichloromethane. The colored products are measured spectrophotometrically at different wavelength depending on the electronic transition between donors and acceptor. The formation constants of the CT complexes were determined by the Benesi-Hildebrand equation. The thermodynamic parameters were calculated by Van't Hoff equation. Stochiometries of the complexes formed between donors and acceptor were defined by the Job's method of the continuous variation and found in 1:1 complexation with donor and acceptor at the maximum absorption bands.     

Phase behavior of PCBM blends with different conjugated polymers

In this work the phase behavior of [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM) blends with different poly(phenylene vinylene) (PPV) samples is investigated by means of standard and modulated temperature differential scanning calorimetry (DSC and MTDSC) and rapid heat-cool calorimetry (RHC). The PPV conjugated polymers include poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene) (MDMO-PPV), High T(g)-PPV which is a copolymer, and poly((2-methoxy-5-phenethoxy)-1,4-phenylene vinylene) (MPE-PPV). Comparisons of these PPV:PCBM blends with regioregular poly(3-hexyl thiophene) (P3HT):PCBM blends are made to see the different component miscibilities among different blends. The occurrence of liquid-liquid phase separation in the molten state of MDMO-PPV:PCBM and High T(g)-PPV:PCBM blends is indicated by the coexistence of double glass transitions for blends with a PCBM weight fraction of around 80 wt%. This is in contrast to the P3HT:PCBM blends where no phase separation is observed. Due to its high cooling rate (about 2000 K min(-1)), RHC proves to be a useful tool to investigate the phase separation in PPV:PCBM blends through the glass transition of these crystallizable blends. P3HT is found to have much higher thermal stability than the PPV samples.     

Charge transfer absorption for pi-conjugated polymers and oligomers mixed with electron acceptors

Pi-conjugated polymers and oligomers show charge transfer (CT) absorption bands when mixed with electron acceptors in chloroform solution. This is attributed to the formation of (ground state) donor-acceptor complexes in solution. By varying the concentration of the donor and acceptor, the extinction coefficient for the CT absorption and the association constant of donor and acceptor are estimated. The spectral position of the CT bands correlates with the electrochemical oxidation potential of the pi-conjugated donor and the reduction potential of the acceptor.     

Synthesis, characterization, spectrophotometric, structural and antimicrobial studies of the newly charge transfer complex of p-phenylenediamine with π acceptor picric acid

Charge transfer complex (CTC) of donor, p-phenylenediamine (PPD) and acceptor, 2,4,6-trinitrophenol (picric acid) has been studied in methanol at room temperature. The CT complex was synthesized and characterized by elemental analysis, FTIR spectra, 1H NMR spectroscopy and electronic absorption spectra which indicate the CT interaction associated with proton migration from the acceptor to the donor followed by hydrogen bonding via N+-H?O-. The thermal stability of CT complex was studied using TGA and DTA analyses techniques. The CT complex was screened for its antifungal activity against Aspergillus niger (Laboratory isolate), Candida albicans (IQA-109) and Penicillium sp. (Laboratory isolate) and antibacterial activity against two Gram-positive bacteria Staphylococcus aureus (MSSA 22) and Bacillus subtilis (ATCC 6051) and two Gram-negative bacteria Escherichia coli (K 12) and Pseudomonas aeruginosa (MTCC 2488). It gives good antimicrobial activity. The stoichiometry of the CT complex was found to be 1:1. The physical parameters of CT complex were evaluated by the Benesi-Hildebrand equation. On the basis of the studies, the structure of CT complex is [(PPDH)+(PA)-], and a general mechanism for its formation is proposed.     

Charge transfer interactions in some poly[[o-(trimethylsilyl)phenyl] acetylene]-acceptor complexes

Charge transfer (CT) interactions between poly[[o-(trimethylsilyl)phenyl]acetylene] or poly(o-Me₃SiPA) and some electron acceptors were studied by ultraviolet-visible and infrared absorption spectroscopy and by x-ray photoelectron spectroscopy, (XPS). The electron acceptors used included iodine, bromine, o-chloranil, o-bromanil, 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), and tetracyanoethylene (TCNE). Varying degrees of CT interactions were observed in all of the polymer/acceptor complexes studied. The electrical conductivities σ of the organic acceptor complexes exhibited a strong acceptor concentration dependence at low acceptor levels, with the DDQ complex exhibiting the highest σ. The extent of CT and the redistribution of charges resulting from the CT in all the complexes were revealed by XPS. The poly (o-Me₃SiPA)/I₂ complex film lost iodine spontaneously while more than half of the bromine in the poly (o-Me₃SiPA)/Br₂ complex existed as covalently bonded bromine, even at low halogen loading.     

A Novel Polymeric Chemosensor: Dual Colorimetric Detection of Metal Ions Through Click Synthesis

A highly colored polystyrene derivative bearing side chain chromophores composed of dialkylanilino donor and cyano‐based acceptor groups, prepared by atom‐economic click postfunctionalization, displays the dual colorimetric detection behavior of several metal ions based on the specific interactions with different nitrogen atoms. Hard to borderline metal ions, such as Fe³⁺, Fe²⁺, and Sn²⁺, are always recognized by the dialkylanilino nitrogen atom, resulting in a decrease in the charge‐transfer (CT) band intensity of the donor–acceptor chromophores. On the other hand, the recognition site of a soft metal ion of Ag⁺ is the cyano nitrogen atom due to the readily formed multivalent coordination, which produces a bathochromic shift of the CT band.     

Influence of localized excited states on the transition moment directions of charge transfer complex absorptions

The influence of localized excited (LE) states on the spectroscopy of charge transfer (CT) complexes has been examined for a series of complexes formed between methyl-substituted benzene donors and 1,2,4,5-tetracyanobenzene as acceptor in 1,2-dichloroethane and octanenitrile solvents. A molecular orbital model was used to describe the appearance of multiple CT absorption bands that occur in the spectra of these complexes. The influence of LE states in these CT absorptions was explored using time-resolved linear dichroism spectroscopy where the direction of the CT transition moment vector (TMV) was used to probe the magnitude of intensity borrowing. The TMV directions for each of the observed CT transitions within the absorption spectra were determined for several complexes. In some cases, the observed CT transitions were interpreted as being pure CT transitions; in others the observed transitions are influenced significantly by a LE transition. The correlation between the TMV directions and the transition energy suggests that the magnitude of intensity borrowing is influenced not only by the energy difference between the CT and LE transitions but also by the specific character of the transitions under consideration.     

Folding of a Donor‐Containing Ionene by Intercalation with an Acceptor

Cationic ionenes that bear electron‐rich 1,5‐dialkoxynaphthalene (DAN) units within the alkylene segment were allowed to interact with different types of electron‐deficient, acceptor‐containing molecules in an effort to realize intercalation‐induced folding of the ionenes; the collapse of the chains was expected to occur in such a way that the donor and acceptor units become arranged in an alternating fashion. Several acceptor‐bearing molecules were prepared by the derivatization of pyromellitic dianhydride and naphthalene tetracarboxylic dianhydride with two different oligoethylene glycol monomethyl ether monoamines. This yielded acceptor molecules with different water solubility and allowed the examination of solvophobic effects in the folding process. UV/Vis spectroscopic studies were carried out by using a 1:1 mixture of the DAN–ionenes and different acceptor molecules in water/DMSO solvent mixtures. The intensity of the charge‐transfer (CT) band was seen to increase with the water content in the solvent mixture, thereby suggesting that the intercalation is indeed aided by solvophobic effects. The naphthalene diimide (NDI) bearing acceptor molecules consistently formed significantly stronger CT complexes when compared to the pyromellitic diimide (PDI) bearing acceptor molecules, which is a reflection of the stronger π‐stacking tendency of the former. AFM studies of drop‐cast films of different ionene–acceptor combinations revealed that compact folded structures are formed most effectively under conditions in which the strongest CT complex is formed.