Solar cells based on dye-sensitized mesoporous films of TiO₂ are low cost alternatives to inorganic semiconductor devices. Solar energy conversion efficiencies of up to 10% have been achieved with such films when used in conjunction with liquid electrolytes. Practical advantage may be gained by the replacement of the liquid electrolyte with a solid charge transport material and various concepts have been proposed in literature to realize such solid-state dye-sensitized heterojunctions. Recently high incident photon to electric current conversion efficiencies have been achieved with a cell consisting of a dye-derivatized mesoporous TiO₂ film contacted by a new organic hole conductor. Photoinduced charge carrier generation shows to be very efficient in such devices, while interfacial charge recombination during charge collection can be revealed as the major loss mechanism. Surface treatments with pyridine derivatives proved to significantly improve the energy conversion efficiency of the device.

Industrially viable assembly techniques have been established for dye solar cells to be used in low light (indoor) and outdoors applications. Stability behavior under thermal stress, UV and visible light irradiation is investigated, in particular in view of real outdoors conditions. Solid-state dye solar cells were also assembled, where an organic p-type semiconductor material replaces the currently employed liquid electrolyte.

Photovoltaic devices, both Schottky-type and pn- heterojunction devices, using several novel organic materials were fabricated and their performance examined. The materials studied include electrochemically doped non- conjugated polymers containing pendant (pi) -electron systems, oligothiophenes with well-defined structures, titanyl phthalocyanine, and amorphous molecular materials. Structure-property relationship and the effect of the materials' morphology on the performance are discussed.

The phase behavior of three N-alkyl-substituted perylene diimide derivatives is examined by differential scanning calorimetry and Polarized Optical Microscopy. The occurrence of multiple phase transitions indicates several crystalline and several liquid crystalline phases. The liquid crystalline phases display high structural ordering in all three dimensions: smectic layers are formed and within these smectic layers an additional ordering in columns is observed. Molecular modeling confirms this result and substantiates smectic ordering with interdigitating alkyl chains that determine the distance between the smectic layers. The ordering in columns is favored by (pi) -(pi) interactions between the cofacially oriented perylene molecules and by the elliptic shape of the molecule. Finally, intermolecular dipole-dipole interactions between the carbonyl groups of the imide moieties cause the perylene molecules to orient on average with a slight rotation between neighboring molecules within a columnar stack. Following the determination of the electronic transition dipole moment, this orientation, which still involves substantial (pi) -(pi) interactions, could be confirmed by UV/vis spectroscopy of perylene aggregates. In combination with the high electron mobility--at least 0.1 cm²V^-1s^-1 in the liquid crystalline phases, and >= 0.2 cm²V^-1s^-1 for the crystalline phases--these materials are very promising materials for use as electron conductors in e.g., organic solar cells.

In order to improve the photogeneration efficiency and charge transport in polymer photovoltaic cells, we orient diode like molecules inside a polymeric monolayer. Previous results gave experimental evidence of the induction of a rectification behavior as well as an increase in the charge mobility through polar orientation. We present here the first experimental realization where the photovoltaic conversion efficiency is increased by two orders of magnitude in a semiconducting polymer blend.

Polymer photovoltaic cells and photodetectors have passed their infancy and become mature technologies. The energy conversion efficiency of polymer photovoltaic cells have been improved to over 4.1% (500 nm, 10 mW/cm²). Such high efficiency polymer photovoltaic cells are promising for many applications including e-papers, e-books and smart- windows. The development of polymer photodetectors is even faster. The performance parameters have been improved to the level meeting all specifications for practical applications. The polymer photodetectors are of high photosensitivity (approximately 0.2 - 0.3 A/Watt in visible and UV), low dark current (0.1 - 1 nA/cm²), large dynamic range (> 8 orders of magnitude), linear intensity dependence, low noise level and fast response time (to nanosecond time domain). These devices show long shelf and operation lives. The advantages of low manufacturing cost, large detection area, and easy hybridization and integration with other electronic or optical components make the polymer photodetectors promising for a variety of applications including chemical/biomedical analysis, full-color digital image sensing and high energy radiation detection.

The main factors inhibiting higher conversion efficiencies in plain polymer layer sandwich photovoltaic devices are the low exciton dissociation efficiency and the low carrier mobilities in the polymer. We consider two different blend approaches for increasing these qualities. NiO (or LiNiO) hole transporting nanoparticles are blended into the photoactive polymer MEH-DOO-PPV in an attempt to increase hole mobility across the device. Improvements to device performance were not significant at these blend concentrations. Devices made using blends of hole and electron transporting polymers M3EH-PPV and CN-ether-PPV showed increased dissociation efficiency and gave power conversion efficiencies of up to 0.6% with stable electrodes.

Gas effusion measurements show that ZnPc layers absorb considerable quantities of oxygen and water if exposed to ambient air. Bulk concentrations of O₂ and H₂O reach levels of (1.1 +/- 0.3)*10²⁰ and (1.7 +/- 0.4)*10²⁰ molecules per cm³ respectively. O₂ is observed to maintain its molecular form when incorporated into the ZnPc matrix. At room temperature (296 K), the oxygen molecules are free to move diffusively in the layer with a diffusion coefficient of (3.0 +/- 0.4)*10^-8 cm²/s. Electrical analysis of ZnPc layers in controlled gas atmospheres show that O₂ establishes a p-type doping in the bulk. The space charge density increases linearly with the external oxygen pressure. In ambient conditions, the O₂ concentration equals (1.6 +/- 0.2)*10¹⁶ ions per cm³, which corresponds with a thermal activation energy of (0.23 +/- 0.05) eV for ionizing O₂. In contrast to O₂, H₂O does not affect the space charge density of the depletion layer. The performance of p-n type solar cells incorporating a ZnPc layer was investigated under different oxygen pressures. It was possible to raise the efficiency of the cell when applying higher O₂ pressures. The short-circuit current increased by a factor of 1.5 when raising the oxygen pressure threefold from atmosphere pressure under AM1.5 illumination. This can be attributed to a more efficient hole collection through the organic layer resulting from a higher internal electrical field strength. However, the exciton diffusion length was observed to decrease approximately 25% when raising the external oxygen pressure from 0.02 to 0.47 bar. We conclude that the exciton diffusion length in organic solar cells is limited by the ionic impurity density, which on the other hand is required for the effective collection of charge carriers.

The efficiency of energy conversion and the stability of lifetime of `plastic' photovoltaic cells, based on conjugated polymer/fullerene blends, are the two main issues to be improved for this type of devices. The stability of these PV cells depends potentially on a large number of factors. A brief layer-to-layer overview of these factors is given, with main emphasis on the factors possibly playing a role in the active photovoltaic layer consisting of the interpenetrating network of a conjugated polymer and a fullerene derivative. Complicated sets of photochemical processes can take place in the pure materials and in the donor-acceptor blends, both in the absence and in the presence of oxygen. Especially, photochemical [2+2] cycloaddition and cycloreversion processes have been observed for fullerene derivatives and in certain mixtures containing an oligomer and a fullerene derivative. These and other (photo) chemical processes are very likely to have an influence on the performance of the photovoltaic cell.

We report the fabrication of thin organic layers and photovoltaic devices made from them. Building thin layers of organic materials via the method of ionically self-assembled monolayers provides control over the layer thickness and composition of multilayer structures on a nanometer scale. This allows to accurately dope a photoluminescent host material with energy or charge accepting guests, changing the emissive character of the pure photoluminescent host film to a predominantly non-emissive, charge generating structure. We show that by varying the concentration of the guest Copper phthalocyanine and C₆₀(OH)₂ in poly- (para-phenylene-vinylene) we can measure the energy migration as well as dissociation of the exciton and can determine the lifetime and the diffusion radius of the exciton. Increasing the number of dopands in the host material, the photoluminescence emission spectra shift and decrease in intensity reflecting a decrease in the number of excitons transferring to neighboring chains or conjugation segments. For high dopand concentrations the recombination of excitons only happens on the same chain as the generation. Building a device to achieve the optimal guest/host ratio for optimal exciton dissociation is one important step in the design of high efficiency photovoltaic devices.

A combination of a ruthenium dye-sensitized TiO₂ microporous solar cell, a PMMA gel electrolyte and a WO₃ electrochromic film to produce a new all solid-state photoelectrochromic window is demonstrated here. The photoelectrochromic window has been excellent for chromism and memory characteristics. Even in the presence of radiation, the photoelectrochromic window can be bleached by application of an external voltage after the device has been darkened.

Dye-sensitized hybrid solar cells with a nanoporous TiO₂ layer and different organic hole conducting polymers have been investigated. These hole transport materials (HTM) with low glass temperature (T_g) are based on triphenyldiamines (TPD). To enhance the power conversion efficiency, the HTMs were doped. New low T_g TPD-based polymers were designed and synthesized with long alkoxyl groups as sidechains or in the backbone, respectively, to investigate the influence of the soft alkoxyl chains on the penetration behavior into the nanoporous layer which has been studied by SEM. The effect of the penetration depth and wetting of the dye sensitized TiO₂ layer by HTM on the solar cell efficiencies have been investigated by I-V- characteristics and steady-state measurements. To improve the penetration, the polymers were heated above T_g. The performance of the cell is decreasing probably due to degradation of the dye during thermal treatment. To enhance the conductivity of the hole transport materials, a Li salt has been used and this doped system was compared to a doped standard Gratzel cell with OMeTAD as HTM. Crystallization on the surface could be seen after storing the standard cell for some weeks.

We report here on properties of Metal-Semiconductor-Metal cells based on poly(3-octylthiophene), P3OT. The diodes were fabricated by spin-coating of poly (3-octylthiophene) on an indium-tin oxide coated glass substrate and an aluminum top contact was evaporated onto the film. The optical and electrical characteristics of the diodes were studied. A power efficiency of 10^-4 was obtained at AM 1.5 conditions, while the power efficiency reached its maximum of 6% under illumination at 256 nm at an intensity of 2 (mu) W/cm².

Bulk donor-acceptor heterojunctions between conjugated polymers and fullerene derivatives have been utilized successfully for photovoltaic devices showing monochromatic efficiencies above 1%. The present paper reports the temperature and irradiance dependencies of full-spectrum photovoltaic parameters for such devices. The measurements were performed under real sun conditions and under a solar simulator. The sun provided a light source stable in intensity to within +/- 1% and closely approximating a true AM1.5 spectrum, whereas the simulator enabled the light intensity to be varied in the range 80 - 600 W m^-2. The most interesting feature that was observed for these devices is that above a cell temperature of 20 degree(s)C the positive temperature coefficient observed for the short- circuit current exceeds in magnitude the negative temperature coefficient that was found for the open-circuit voltage. This means that, unlike the situation for conventional PV devices, these cells actually exhibit an increase in efficient with increasing temperature (reaching a value of 0.63% at 40 degree(s)C). We suggest that the observed behavior originates from the temperature dependence of the conductivity of the conjugated polymers-fullerene composite. This hypothesis is confirmed by the irradiance-resolved measurements performed at different cell temperatures. We observed a linear increase in the short-circuit current with light intensity over the whole ranges of irradiances and temperatures but maximum temperature influence is observed at highest light intensity.