The paper reviews first, the recent history of commercial thin film photovoltaic modules in terms of measurements we have performed on their true AM1.5 efficiency (under natural sunlight), and their observed long-term stability properties. Attention is focused on the first generation of single-junction amorphous silicon (a-Si) modules that became available during the 1980s, and we compare their performance with those of late-1990s models fabricated with multi-junction a-Si, CdTe and CuInSe₂ thin films. The efficiency and stability of these modules are compared with corresponding measurements we have performed on the high-efficiency organic solar cells that were recently produced at the Johannes Keppler University of Linz. Finally, we review the economics of grid-connected PV systems in order to provide cost benchmarks for future organic competitors.

The cell structure concepts and materials to build solid-state dye solar cells based on nanocristalline titanium oxide and an organic hole conductor were investigated. The substrate cell is based on a metal foil and a semi-transparent gold window on top of the cell structure and the superstrate cell is deposited on ITO coated polymer foil replacing the traditional conductive glass as transparent substrate. Steel, titanium and polymer foil based cell were assembled. The polymer/ITO cell gave only small currents as the materials are far from optimal in that configuration, but an efficiency of 0.8 % was obtained on a Ti-foil based cell.

Dye sensitized TiO₂ solar cells were assembled using rigid or flexible transparent electrodes (a conductive film deposited on glass or poly(ethylene terephthalate) as substrates and a polymer electrolyte based on (formula available in paper) and poly(epichlorohydrin-co-ethylene oxide). The cells were characterized by current-potential curves and electrochemical impedance spectroscopy under different light intensities. Under 100 mWcm^-2 illumination, the rigid cell exhibited an open circuit potential V_OC=0.75V, a short-circuit photocurrent I_SC=2.5 mAcm^-2 and an efficiency (eta) =0.9%; for the flexible cell, V_OC=0.83V and (eta) and I_SC were almost 10 times smaller. Under illumination, impedance spectra exhibited three semi-circles for the rigid cell. For the flexible cell the time constants were not well defined. In the dark, both systems presented very high impedance. The differences in the efficiency and the impedance spectra of both cells were compared and discussed.

Interfaces are important in all solar cells, but they are especially significant in organic-based photovoltaic (OPV) cells such as organic semiconductor PV cells and dye-sensitized solar cells. In OPV cells, charge generation, charge separation and charge recombination processes often take place at interfaces, while in conventional PV cells these crucial processes occur mainly in the bulk. Interfacial exciton dissociation can lead to photovoltages that exceed the built-in potential of the cell, thus V_oc is not necessarily a measure of the band bending. Photovoltages up to ~1V have been achieved without band bending. Most OPV cells are majority carrier devices, and thus are fundamentally different than conventional p-n junction cells. The interfacial potential induced by the photogeneration of charge carriers may, at high light intensities, overwhelm the equilibrium potential and hinder charge separation. It is often advantageous to increase the exciton-dissociating surface area by structuring the interface, however this also increases the area over which carriers can recombine. In dye-sensitized solar cells, the surface area is so high, and recombination so rapid, that only a single redox couple with ultra-slow kinetics is viable. We describe a method of passivating the interfacial recombination sites in these cells that permits for the first time the use of kinetically fast redox couples and may facilitate the development of solid-state dye cells. Finally, we describe a UV treatment of dye cells that alters the interfacial energetics and dramatically increases the efficiency in some cases.

Published information on polymer composite solar cells was screened to pin down the most critical parameters for a further improvement of these photovoltaic systems towards higher efficiency and stability. Largely irreversible separation of photocharge between the polymer (e.g. PPV) and the fullerene with subsequent percolation of electrons (e.g. to the Al contact) and holes (to the ITO contact), under the symmetry breaking influence of an electric field imposed by the work function difference of the contacts (Al and ITO respectively) accounts for photocurrent generation. The energetics is determined by n-p hetero junction, which is formed between ITO and the polymer/fullerene composite. Positive polarization of the ITO contact leads to the generation of an interfacial potential drop and an increase of the effective ITO work function. This explains the generation of a photovoltage reaching more than 0.8 Volt. Parameters limiting solar cell performance are discussed.

Ultrafast photoinduced absorption by infrared-active vibrational modes (IRAV) is used to detect charged photo- excitations (polarons) in solid films of conjugated luminescent polymers. Experiments, carried out in zero applied electric field, show that polarons are generated within 100 fs with quantum efficiencies of approximately 10%. The ultrafast photoinduced IRAV Absorption, the weak pump-wavelength dependence, and the linear dependence of charge density on pump intensity indicate that both charged polarons and neutral excitons are independently generated even at the earliest times. Measurements of the excitation profile of the transient and steady-state photoconductivity of poly(phenylene vinylene) and its soluble derivatives over a wide spectral range up to h(upsilon) = 6.2 eV indicate an apparent increase in the photoconductivity at h(upsilon) > 3- 4 eV that arises from external currents generated by electron photoemission (PE). After quenching the PE by addition of CO₂+SF₆ (90%:10%) into the sample chamber, the bulk photoconductivity is nearly independent of photon energy in all polymers studied, in a good agreement with the IRAV spectra. The single threshold for photoconductivity is spectrally close to the onset of (pi) - (pi) ^* absorption, behavior that is inconsistent with a large exciton binding energy.

The photovoltaic effect in organic solar cells is a multistep process involving both bulk and interfacial phenomena. In order to probe the local mechanisms of photocurrent production in organic semiconductors, we studied the influence of light on the junction between the tip of a scanning tunneling microscope (STM) and an organic thin film. We report here on the local through-space I-V and I-(Delta) Z characteristics ((Delta) Z=tip-sample distance) of pentacene (5A), a material with potential applications in photovoltaic solar cells. These systems behave as metal-insulator-semiconductor (MIS) tunnel junctions. The influence of light is investigated by coupling the organic nano-junctions to a laser beam. Monitoring the tip-sample distance (Delta) Z at the angstrom level (i.e. over the 0-20 angstrom range), allows to tune the I-V characteristics of these nanoscale organic tunnel junctions both in the dark and under illumination.

Dye-modified ZnO thin films were prepared by electrochemically induced crystallization from aqueous mixtures of zinc nitrate and water-soluble dyes. A direct crystallization of semiconductor/ dye composites without heat treatment is seen as a significant advantage of this method. Moreover, characterization of these materials has revealed ordered growth of ZnO crystallites as well as formation of ordered dye assemblies, thus characterizing this method as electrochemical self-assembly. The photoelectrochemical properties of these unique ZnO-dye thin film electrodes were investigated in photocurrent transient measurements in the ms-regime and by steady- state voltammetric measurements. Two sets of electrodes are discussed, employing either metal complexes of tetrasulfophthalocyanines (TSPcMt; Mt = Zn, Al, Si) or the xanthene dye Eosin Y. For aggregates of TSPcMt on ZnO, efficient charge-transfer to the electrolyte is found, leading to low surface charging and low surface recombination of photogenerated holes with electrons from the ZnO, at however, rather low injection efficiencies of electrons into the conduction band of ZnO. This efficiency was higher for adsorbed monomers of TSPcMt leading to a considerably higher quantum efficiency of the photocurrent in spite of increased surface charging and recombination of holes. Higher photocurrents were observed for ZnO sensitized with monomers of Eosin Y caused by both, efficient electron transfer from the dye to ZnO as well as hole transfer from the dye to the electrolyte. Not only dye molecules which were directly accessible from the electrolyte, but also those which were enclosed within matrix cavities proved to be photoelectrochemically active.

Recently, there has been increased interest in polymer-based photovoltaic devices due to their promise for the creation of lightweight, flexible, and inexpensive electrical power. WE examined the possibility of using nanoparticles and nanoparticles with tailored interfaces for the creation of hybrid polymer-based devices with enhanced photovoltaic response. Initially, we investigated the incorporation of multi-walled carbon nanotubes (MWNT) in the poly(benzimidazo-benzophenanthroline) ladder (BBL) layer of two-layer poly(p-phenylene vinylene)(PPV)-BBL photovoltaic devices. Subsequently, we explored the possibility of tuning polymer-particle interfaces through the creation of core-shell particles fabricated using electrostatic self- assembly. For the PPV/BBL(MWNT) devices, a doubling of the photocurrent and a drastic reduction in photovoltage with MWNT incorporation is observed for a range of BBL layer thickness values. This behavior is consistent with the MWNTs functioning as a three dimensional extension of the top aluminum electrode. Fabrication studies on core-shell particles demonstrate that the interfacial properties of a variety of particles can be manipulated, shells of up to 10 bilayers can be achieved, and TiO₂ nanoparticles with PPV polymer shells are possible.

In order to elucidate electron migration in dye-sensitised nanoporous anatase TiO₂, time-of-flight short-circuit photocurrents and transient absorption spectra between 500 and 2000 nm have been recorded. It is found that electrons in TiO₂ dominate the transient absorption between 900 and 1100 nm, whereas at wavelengths longer than 1100 nm absorption by electrons in the SnO₂:F substrate prevails. To facilitate a qualitative analysis, the absorption cross-sections of electrons in TiO₂ and SnO₂:F have been measured. Combining transient absorption and photocurrent response data, the time-resolved recombination loss can be determined. When the excitation density is below 33.5 (mu) J/cm₂, on average less than one electron per nanoparticle is injected. Under this condition the IPCE equals unity. When higher excitation densities are applied, more than one electron per nanoparticle is injected, losses become significant, and the IPCE reduces to 40%. The time evolution of the recombination loss reveals that recombination primarily takes place with a few microseconds. One mechanism involves the recombination of electrons with dye cations, occurring in the first 20 ns after laser flash excitation.

Photocurrent spectra and quantum efficiency are investigated in photovoltaic cells with the electron donor material Cu-phthalocyanine (CuPc) together with C60 as electron acceptor material. By a systematic variation of both layer thicknesses in bilayer devices with the main process of photocurrent generation at the donor-acceptor interface we found a significant influence on photocurrent generation due to optical interference effects and determined the optimum layer thicknesses for both materials. The effective dissociation interface is increased by introducing a 1:1 blend of both materials between the pure CuPc and C₆₀ layers. The more extended active volume of exciton dissociation leads to higher quantum efficiencies (IPCE up to approximately equals 22%) of the photovoltaic effect in comparison to the heterojunction bilayer devices.

Solar cells composed of thin layers of titanium dioxide (TiO₂) and zinc phthalocyanine (ZnPc) show a large decay of the photocurrent in ambient atmosphere. This decay is caused by the combined presence of oxygen, light, and an external electric field. A two-step mechanism is proposed which involves the formation of a mobile photodopant arising from the present of molecular oxygen in the layers. In the first step, oxygen radical anions are formed under influence of bandgap illumination of ZnPc. Subsequently these negative charged species drift towards the interface with TiO₂ under the influence of an external electric field. They accumulate and quench (pi) (pi) *-singlet excitons which depresses the photocurrent dramatically. A concentration of more than 5.10¹⁸ cm^-3 ionized oxygen species is found to be present in the ZnPc films under ambient atmosphere and 5-mW 670-nm irradiation, which is two orders of magnitude higher than in the dark.

It has been predicted that nano-phase separated block copolymer systems containing electron rich donor blocks and electron deficient acceptor blocks may facilitate the charge carrier separation and migration in organic photovoltaic devices due to improved morphology in comparison to polymer blend system. This paper presents preliminary data describing the design and synthesis of a novel Donor-Bridge-Acceptor (D-B-A) block copolymer system for potential high efficient organic opto-electronic applications. Specifically, the donor block contains an electron donating alkyloxy derivatized polyphenylenevinylene (PPV), the acceptor block contains an electron withdrawing alkyl-sulfone derivatized polyphenylenevinylene (PPV), and the bridge block contains an electronically neutral non-conjugated aliphatic hydrocarbon chain. The key synthetic strategy includes the synthesis of each individual block first, then couple the blocks together. While the donor block stabilizes and facilitates the transport of the holes, the acceptor block stabilizes and facilitates the transport of the electrons, the bridge block is designed to hinder the probability of electron-hole recombination. Thus, improved charge separation and stability are expected with this system. In addition, charge migration toward electrodes may also be facilitated due to the potential nano-phase separated and highly ordered block copolymer ultra-structure.

Information on doping- and photoinduced phenomena and processes in materials used in organic polymeric photovoltaic systems (plastic solar cells) can be obtained using infrared spectroscopy. The methods and results for the identification of doping- and photoinduced charge carriers, investigation of charge carriers in electrochemical systems, and studies on degradation processes are shown.

Recent developments in the preparation of surfactant-templated mesostructured sol-gel silica materials have extended the morphology from the originally discovered powders, with particle sizes on the order of microns, to mesoporous continuous thin films. These films could find applications in membrane-based separations, selective catalysis and sensors. Particularly, sodium dodecyl sulfate (SDS)- templated sol-gel films formed by the rapid dip-coating sol-gel method possess highly ordered lamellar phase structure. The interest in the potential applications of these films and the introduction of new properties lead to the research of their chemical modifications. The improvement of their photorefractive response requires knowledge of the microscopic processes, such as the charge transport mechanism. The photoconductivity technique provides information about that mechanism, and it allows for measuring the transport parameters. Mesoporous continuous films were prepared by the dip-coating method on glass substrates. The films were doped with SDS, carbazole (SiK) and dispersed red one (DR1) at 1:20:20 molar concentration. Photoconductivity studies were done on them at different illumination wavelengths in order to know the transport mechanism and surfactant influence. The electric field versus current density plot shows a linear behavior, i.e. an ohmic response. The conductivity slope dependence with the polarization time shows a Gaussian behavior. And there is an exponential decay from the absorption coefficient with the accumulated polarization time. Interpretation of these results is presented and the obtained charge transport parameters are reported.

With the development made by Graetzel et al on solar cell, the studies on dye-sensitized nanocrystalline TiO₂ solar cell have been widely investigated. Here we synthesized four cyanine dyes containing carboxyl group. The absorption spectra and fluorescent spectra of the dyes in solution and in adsorbed state on the surface semiconductor were measured. These dyes have high absorption coefficient (~10 ⁵M^-1 cm^-1), which is a major factor that affects the conversion efficiency of the solar cell. These cyanine dyes were used to sensitize nanocrystalline TiO₂, and the photoelectrochemical properties of the electrodes sensitized by these cyanine dyes were measured. From the absorption spectra of the electrode it can be seen that these dyes have been efficiently adsorbed on nanocrystalline TiO₂. The maximum IPCE values of dye CY-1, CY-2, CY-3 and CY-4 were 42%, 51.8%, 75.9% and 22.2% respectively. The results show that cyanine dye derivatives are promising sensitizers for nanocrystalline solar cell.