Derck Schlettwein

Controlled p-doping of pigment layers by cosublimation: Basic mechanisms and implications for their use in organic photovoltaic cells

Abstract We present a systematic study on doping of vanadyl- and zinc-pathalocyanine by a fully fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic films by cosublimation of matrix and dopant. The films are characterized in situ by temperature dependent Seebeck and conductivity measurements. We observe a drastic increase of conductivity and a corresponding shift of the Fermi level towards the valence states with increasing dopant concentration. We thus conclude that doping has the potential of both reducing the series resistance and increasing the photovoltage of organic solar cells. As a first step to exploit this potential, we present two different ways of preparing diodes with rectification ratios in excess of 10 4 using doped phthalocyanines. By adding an undoped interlayer between the contact and the doped layer, we have produced diodes which work already in the strict absence of oxygen and are stable in air. To increase the efficiency of charge carrier generation in photovoltaic cells, we need to use photoactive donor–acceptor-heterojunctions. We present here first examples of pn- and pin-type heterojunctions combining p-doped and nominally undoped layers.

Investigations of n/p-junction photovoltaic cells of perylenetetracarboxylic acid diimides and phthalocyanines

The preparation of organic photovoltaic cells in the configuration ITO/n-conductor/p-conductor/Au obtained by vapour deposition of the organic compounds is described in detail. As n-conductors perylenetetracarboxylic acid diimides (e.g., dimethyl derivative) and as p-conductors phthalocyanines (e.g., zinc phthalocyanine) were used. The film thicknesses of the layers were varied between 10 and 250 nm. The following investigations were carried out: reproducibility of cell preparation, influence of purity of the organic materials, the type of ITO substrate, deposition rate, substrate temperature and thickness of the organic films. Photovoltaic parameters were determined as follows: short-circuit currents ⩽ 3.5 mA cm–2, open-circuit voltages ⩽ 685 mV, fill-factors ⩽ 0.52, photovoltaic efficiencies < 0.43% under white light of 100 mW cm–2. The cells exhibit a high series resistance of around 70 Ω cm2 and a low shunt resistance of around 670 Ω cm2. Under 1 mW cm–2 illumination the efficiency increased to 1.6%. In a cell of ITO/n-conductor (10 nm)/p-conductor (50 nm)/Au quantum yields up to 80% were obtained. Results of stability measurements show a 50% decrease in the short-circuit current over time. The results are discussed in detail considering the positions of energy levels in the n- and p-conducting materials.

Improved photoelectrochemical performance of electrodeposited ZnO/EosinY hybrid thin films by dye re-adsorption

Dye desorption and re-adsorption post treatments on electrochemically self-assembled nanoporous ZnO/eosinY hybrid thin films lead to a large improvement of the dye-sensitized photoelectrochemical performance, achieving an incident photon to current conversion efficiency up to 90%.

Selective electroacatalysis for CO2 reduction in the aqueous phase using cobalt phthalocyanine/poly-4-vinylpyridine modified electrodes

Abstract The electrocatalytic reduction of CO 2 in aqueous electrolytes by a cobalt phthalocyanine (CoPc)/poly-4-vinylpyridine (PVP) membrane coated on a graphite electrode was studied. Enhanced catalysis for CO 2 electroreduction at more positive potential and of greater efficiency was achieved with the present system than with the system using pure CoPc as catalyst, whereas both coatings were found to catalyze proton reduction as well. The electrochemical behavior of the PVP-bound CoPc was examined at different pH, taking the normally adsorbed CoPc as reference. The greatly enhanced and sustained catalytic activity of the CoPc/PVP membrane for CO 2 electroreduction in water is discussed in terms of electron donation of the pyridine group to the central cobalt by an axial coordination and a proton-exchanging property of the PVP and the increased local concentration of CO 2 in the PVP layer owing to their hydrophobic and/or acid-base interaction. A reaction scheme for the electrocatalytic reduction of CO 2 by CoPc is proposed.

Spectroelectrochemical investigations on the reduction of thin films of hexadecafluorophthalocyaninatozinc (F16PcZn)

Redox potentials were determined for solutions and thin films of hexadecafluorophthalocyaninatozinc (F16PcZn). A value of −0.6 V versus SCE was determined for the first reduction to the radical anion of F16PcZn− in N,N′-dimethylformamide (DMF) and −0.9 V versus SCE for the second reduction to F16PcZn2−. Both potentials were shifted about 0.4 V towards positive potentials when compared to the unsubstituted phthalocyaninatozinc (PcZn) caused by the stabilization of the π-system by the electron-withdrawing fluorine atoms in the ligand. Vapor-deposited thin films of F16PcZn on indiumtinoxide (ITO) were reduced in contact with aqueous electrolytes of pH 5.5 to avoid H+-reduction in acidic regimes. A chemically reversible reduction accompanied by cation intercalation was found. The kinetics of the reaction were studied in detail by cyclic voltammetry under variation of the intercalating ionic species, the film thickness and sweep rate. From the charge uptake as well as from the dependence of peak current densities on the square root of the sweep rate it was found that the reaction rate is limited by the diffusion of intercalating cations. Optical absorption spectra were collected in situ. An irreversible structural change was observed during initial reduction, also in accordance with a strongly irreversible initial CV before the reversible behavior was obtained. The presence of the dianionic F16PcZn2− was detected even under conditions of an average charge uptake of less than 1 electron/molecule. From the analysis of peak current densities according to the Randles–Sevcik equation as well as the observed charge flow dependent on film thickness and chronoamperometric characterization of the reaction a diffusion constant Di for K+ in F16PcZn in the range from 1.6×10−12 to 8.0×10−12 cm2 V−1 s−1 was calculated.

Photochemical stability of various porphyrins in solution and as thin film electrodes

The stabilities of various porphin analogue compounds such as unsubstituted and substituted phthalocyanines, tetraphenylporphyrin, naphthalocyanine and tetraazaporphyrin have been investigated. The compounds were dissolved either in organic solvents or water. Electronic spectra in the visible region were recorded after the samples were kept either under inert gas or air in the dark and under illumination. The rate of decay of absorption is explained by a decomposition reaction which is of first-order kinetics with respect to the porphyrin concentration. Illumination, and the presence of oxygen, enhance the rate of decomposition. In general, an increase of the size of the ligand and an increase of the electron donating ability of substituents further enhance the rate of decomposition. Central metal ions with open shell arrangements of electrons favour the photodecomposition of the molecules. Thin solid films of phthalocyanines obtained by vapour deposition were investigated for their stability in contact with either air or an aqueous electrolyte. The influence of a cathodic bias potential was studied in the dark and under illumination. When compared to the dissolved molecules the crystalline solids turned out to be much more stable although, decomposition could be detected in a long-term photoelectrochemical experiment. Decomposition products were determined and appropriate mechanisms are discussed.

Influence of surface reactions and ionization gradients on junction properties of F16PcZn

Compared to unsubstituted phthalocyaninatozinc(ii) (PcZn), electron withdrawing fluorine atoms in hexadecafluorophthalocyaninatozinc(ii) (F16PcZn) cause a stabilization of the frontier orbitals of about 1.6 eV. This is concluded from photoelectron spectroscopy (UPS) at thin films on Au surfaces. From experiments at thin films [physical vapor deposition (PVD)] of PcZn deposited on top of F16PcZn under UHV conditions it is seen that a closed film of PcZn is formed at least within 5 nm average film thickness, that thermodynamic equilibrium between the films is achieved by charge transfer in redox reactions at the interface which, however, do not lead to a macroscopic space-charge layer. To study electrical device properties thin films of F16PcZn and PcZn were prepared in a range between 90 nm and 240 nm. Changes in electrical properties of ITO, Au|F16PcZn|metal (metal=In, Au) and ITO|F16PcZn|PcZn|Au devices have been studied in the dark and under illumination. Results of current–voltage characteristics and short-circuit photocurrent spectra of devices as prepared and measured under high vacuum (HV, 10–5–10–6 mbar) and after exposure to air are presented. In vacuum symmetrical I(U) characteristics were found for ITO|F16PcZn|Au devices. After exposure to air a decrease in dark conductivity, unsymmetrical I(U) characteristics and a considerable photovoltage (UOC) was measured under illumination. The magnitude of UOC as well as its direction can be clearly correlated with the exposure to atmosphere. This observation leads to a discussion based on a local asymmetry in O2 content as caused by slow diffusion into F16PcZn. O2 would lead to a decrease in the local majority carrier density as typically expected for organic n-type conductors. Rectification found in F16PcZn|In devices can be explained by a chemical reaction between the distinct electron acceptor F16PcZn and In as a donor. Photocurrent action spectra of devices with different thicknesses of F16PcZn layers in ITO|F16PcZn|PcZn|Au revealed detailed information about the site of charge carrier generation (photoactive area). The junction properties are discussed in detail based on the frontier orbital positions of PcZn and F16PcZn, and the work functions of the corresponding electrode materials.

Photoelectrochemical sensitisation of ZnO–tetrasulfophthalocyaninatozinc composites prepared by electrochemical self-assembly

Electrodes prepared by a one-step electrochemical synthesis of a dye-loaded ZnO thin film from an aqueous mixture of zinc nitrate and a water soluble phthalocyanine, 2,9,16,23-tetrasulfophthalocyaninatozinc(II) (TSPcZn) have been studied. Homogeneously blue transparent particulate thin films were formed. Monomeric or aggregated TSPcZn species could be obtained dependent on the deposition conditions. Visible light absorbed in TSPcZn led to sensitisation of ZnO. Negative shifts of the rest potential and anodic photocurrents were obtained in contact with an iodide-containing electrolyte in accordance with the n-type conduction of ZnO. Time-resolved measurements indicated charging and discharging of surface states, allowing discussion of the alignment of energy levels in the contact as well as of charge transfer and recombination in the cases of monomeric or aggregated TSPcZn. Electrons were injected into ZnO from the first excited singlet state of TSPcZn. Monomeric TSPcZn showed a considerably larger quantum efficiency in sensitisation of ZnO, whereas a larger rate of charge transfer to the electrolyte was derived for the aggregated form.

Electrochemical reduction of substituted cobalt phthalocyanines adsorbed on graphite

Abstract Unsubstituted phthalocyaninatocobalt(II) [PcCo] and the cobalt complexes of the substituted ligands octabutoxyphthalocyanine [Pc(OBu)8Co], octacyanophthalocyanine [Pc(CN)8Co], phthalocyaninetetracarboxylic acid [Pc(COOH)4Co] and tetrapyridotetraazaporphyrin [TPyTAPCo] were absorbed on the basal plane of pyrolytic graphite (BPG) from solutions in organic solvents to form ultrathin films. The films were studied by cyclic voltammetry in the presence of an inert aqueous electrolyte to yield the redox characteristics of the films. The films undergo reduction and reoxidation at the ligand inner π system as well as at the metal centre with a linear dependence of peak currents on scan rate that is typical for surface-confined species. The kinetics of the two reactions are further discussed on the basis of the analysis of peak shapes and charge found in the different waves in the cyclic voltammograms. The redox potentials of both reactions are shifted according to the electron donating or withdrawing character of the ligands periphery as seen by comparison to molecular orbital (MO) calculations and ultraviolet photoelectron spectroscopy (UPS) results. During the reduction step, protons from the electrolyte are interacting with the films and are desorbed during reoxidation, leading to a strong pH dependence of the redox potential. The acid—base properties of the ligands determine a threshold pH above which the potential of the metal-centred first reduction is no longer dependent on pH. The ligand-centred second reduction, however, led to interaction with protons over the whole accessible pH range. Both electrode reactions are discussed in view of chemical interactions of the protons with the various ligand systems.

Charge transport in thin films of molecular semiconductors as investigated by measurements of thermoelectric power and electrical conductivity

Thin films (~ 100 nm) of molecular semiconductors were prepared by physical vapour deposition on quartz substrates equipped with gold electrodes that allowed simultaneous measurement of the electrical conductivity (specific conductivity σ) under an applied electric field and of the thermoelectric power (Seebeck coefficient S) under an applied temperature gradient. The measurements were performed in vacuo and during exposure to molecular oxygen. The thin film samples were chosen as representatives of three characteristic groups of molecular semiconducting materials: an unsubstituted phthalocyanine (phthalo-cyaninatozinc(II) (PcZn)), a modified phthalocyanine with an electron-withdrawing ligand (tetrapyrido(2,3-b;2′3′-g;2″3″-1;2‴3‴-q) tetra-azaporphyrinatozinc(II) (TPyTAPZn)) and a perylene pigment (N,N′-dimethyl-3,4,9,10-perylenetetracar☐ylic acid di-imide (MePTCDI)). The p-type character of PcZn as reported from a number of studies is confirmed whereas the films of TPyTAPZn and MePTCDI showed n-type behaviour. These results are inferred from the sign of S as well as from the influence of oxygen on σ. Both S and σ of the three materials were studied under variation of the film temperature. Activation energies for S and for σ are derived and lead to a discussion of charge carrier generation and charge carrier transport in each film. It is concluded that both a delocalized charge carrier transport as well as a thermally activated hopping mechanism have to be considered to discuss the electrical properties of the samples.