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In this study the geometrical optimization of monolithically integrated solar cells into serially connected solar modules is reported. Based on the experimental determination of electrodes′ sheet and intermittent contact resistances, the overall series resistance of individual solar cells and interconnected solar modules is calculated. Taking a constant photocurrent generation density into account, the total Joule respectively resistive power losses are determined by a self-consistent simulation according to the 1-diode model. This method allows optimization of the solar module geometry depending on the material system applied. As an example, polymer solar modules based on ITO-electrodes and ITO-free electrodes were optimized with respect to structuring dimensions.

We applied complementary imaging methods to investigate processing failures of roll-to-roll solution processed polymer solar modules based on polymer:fullerene bulk heterojunctions. For investigation of processing deficiencies in solar modules we employed dark lock-in thermography (DLIT), electroluminescence (ELI) and photoluminescence/reflection imaging (PLI/RI) complemented by optical imaging (OI). The combination of all high resolution images allowed us to allocate the origin of processing errors to a specific deposition process, i.e. the insufficient coverage of an electrode interlayer. The investigation can be divided into a fast DLIT overview of the module and a successive more detailed analysis of the suspicious region by all imaging methods.

Fully roll-to-roll processed polymer solar cell modules were prepared, characterized, and laminated. Cell modules were cut from the roll and matched pairs were selected, one module with exposed cut edges, the other laminated again with the same materials and adhesive sealing fully around the cut edges. The edge sealing rim was 10 mm wide. Cell modules were characterized by periodic measurements of IV curves over extended periods in a variety of conditions, as well as by a variety of spatial imaging techniques. Data show significant stability benefits of the edge sealing process. The results of the imaging experiments show that the ingress of atmospheric reactants from the edges leads to degradation. In the case of edge sealed devices the same effects are observed but significantly slowed down. In particular, the fast nonlinear degradation is eliminated.

We show that laterally resolved luminescence detection is a highly versatile measurement technique for the characterization of polymer solar cells and modules. Besides lock-in thermography also luminescence imaging is highly suitable for quality control of processing steps, especially the control of homogeneous layer deposition and the proper lateral function of polymer solar cells and modules, by identification of local defects. Furthermore the application of luminescence imaging allows discrimination between active layer and organic/electrode-interface degradation in stability experiments. By correlation with photovoltaic parameters important conclusions can be drawn with respect to the specific degradation mechanism. For quantitative interpretation of such electroluminescence images, we propose an equivalent circuit model in which local solar cells are interconnected by resistors representing the sheet resistance of the transparent electrode. In combination with the laterally resolved measurement of electroluminescence, the application of this model allows calculation of local photovoltaic parameters and quantification of the lateral current and voltage distribution.

We apply spectral ellipsometry (SE) to analyze the in-depth morphology of a polymer/fullerene film. Observed fullerene concentration gradients within the film are investigated with respect to temporal changes. The changes within the fractional composition of the polymer/fullerene film are resolved and can be correlated to the formation of fullerene segregations near the surface.

The changes in the blend morphology of the thin films accelerate when thermally annealed at increased temperatures. Tapping-mode atomic force microscopy (AFM) measurements provide deeper insight into the nano- and micro-meter scale of the phase separation observable on the surface of the film. Furthermore, to prove the coarsening of phase separation on various length scales, optical microscopy, UV–Vis, photoluminescence and measurements were conducted. Of special interest is tracking down the formation of fullerene aggregates and correlation of their growth in time. We demonstrate ways to detect characteristic stages of phase separation directly using simple optical measurements.

An electron spin resonance (ESR) X-band study of regio-regular poly(3-hexylthiophene) (RR-P3HT) and RR-P3HT/PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) composites blended with a low concentration (∼0.1–1%) of single wall carbon nanotubes (SWCNT) is presented. The substantial line-width broadening of polaron and PCBM anion radical ESR spectra was registered after SWCNT incorporation. The possibility of an electron spin exchange interaction between SWCNT conductive electrons and polymer polarons in the RR-P3HT:SWCNT blend without light illumination and between SWCNT conductive electrons and polymer polarons and PCBM anion radical electrons in the RR-P3HT:PCBM:SWCNT blend under light illumination is discussed. In addition, the unusually significant RR-P3HT film morphology (dominant crystalline phase orientation) is substantially altered, in particular a rotation of up to 90° of the P3HT crystalline phase relative to the film plane, due to SWCNT annexation in the polymer, is registered by the ESR spectra external magnetic field angular dependences.

Recent investigations have shown that an anthracene containing poly( p -arylene-ethynylene)-alt-poly( p -arylene-vinylene) statistical copolymer consisting of a well defined conjugated backbone, along which linear and branched alkoxy side chains are attached in a random manner, yields, compared to its counterparts with regular side chain substitution, an improved performance in polymer [6,6]:-phenyl-C 61 -butyric acid methyl ester (PCBM) bulk-heterojunction solar cells. The microscopic origin for the improved power conversion efficiency ( η ≈ 3.8%) of the statistical copolymer – which is the best in its material class – has not been resolved. We conducted grazing incidence wide-angle x-ray scattering investigations in order to correlate the nanomorphology of the active layers to the photovoltaic performance of the device. A comparison of the results obtained for the statistical copolymer to those obtained for the corresponding regular copolymers shows that the improved performance of the former may be attributed to a combination of the following structural characteristics: 1. well, ordered stacked domains that promote backbone planarization, 2. partly face-on alignment of domains relative to the electrodes for an improved active layer-electrode charge transfer, and 3. a more isotropic domain orientation throughout the active layer that ensures that the backbone alignment direction has components perpendicular and parallel to the electrodes in order to compromise between light absorption and efficient intra-chain charge transport. The regular copolymers exhibiting inferior performance lack either sufficient stacking order or face-on alignment of the domains. None of them shows isotropic domain orientation.

The influence of the polymer/fullerene blend ratio on the morphological properties of organic solar cells is investigated. Spectroscopic ellipsometry is applied as a tool for analyzing fullerene domains and the influence on polymer crystallinity within the film. Furthermore, the measurements are correlated with a percolation limit of as-cast films at around 40 wt% fullerene content.

Polymer/fullerene bulk heterojunction solar cells consisting of PDDTP:PCBM blends have been investigated and optimized. The structural properties of the photoactive layers were studied by GID-XRD, which revealed a pronounced response of photovoltaic parameters on the polymer crystallinity. In addition to P3HT, another donor polymer was found that develops crystalline domains upon annealing. We found a clear correlation between annealing time, power conversion efficiency, and polymer crystallinity. The PDDTP crystallization ability is a promising property. This is a first step toward the synthesis of low-bandgap polymers, which likewise show the ability to form crystalline domains and may therefore lead to a substantial increase in efficiency.

This article presents the lock-in thermography measurement technique applied to encapsulated organic heterojunction solar cells, built from poly(3-hexylthiophene) (P3HT) and 1-(3-methaoxycarbonyl)propyl-1-phenyl[6,6]C61 (PCBM). The realizable temperature resolution grants the possibility to visualize even very weak thermal losses in solar cells. The thermal behavior of the cells is demonstrated for different applied voltages. Especially, parallel and serial resistances can be spatially resolved. In order to explain the nonuniform behavior of the serial resistance a simplified replacement circuit of the organic cell is created, according to the Kirchhoff laws. The matrix of series and parallel resistivities allows the simulation of the currents flowing vertically through the semiconducting layer. Bias dependent simulations reveal that the considerable difference of the sheet resistances between the indium-tin-oxide (ITO) and aluminum contacts is responsible for the experimentally observed inhomogeneous temperature profiles of the organic solar cells.

This article reports on the synthesis, characterization and properties of a series of anthracene−containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene)s (PPE−PPV) copolymers with general constitutional unit (Ph−C≡C−Anthr−C≡C−Ph−CH═CH−Ph−CH═CH−)n denoted AnE-PV. Solely linear (AnE-PVaa, -ad, -ae) and solely branched (AnE-PVbb) as well as mixed linear and branched (AnE-PVab, -ac, -ba, -cc) alkoxy side chains were grafted to the backbone in order to tune the π−π-stacking ability of the materials. It has been possible to establish a correlation between π−π-stacking ability, absorptive behavior, charge carrier mobility, solar cell active layer nanoscale morphology and resulting photovoltaic performance. Solar cells energy conversion efficiencies between 0.34% and 3.14% were achieved. The best performance was achieved from AnE-PVab showing both stacking ability and highest π−π-stacking distance of 0.386 nm as compared to 0.380 nm for the others. Poorer performance resulted from polymers with no ...

The performance of polymer solar cells (PSC) strongly depends on the 3D morphological organization of the donor and acceptor compounds within the bulk heterojunction active layer. The technique of electron tomography is a powerful tool for studying 3D morphology of the layers composed of poly(3-hexylthiophene) (P3HT) and a fullerene derivative ([6,6]-phenyl-C61-butyric acid methyl ester; PCBM), especially to quantify the amount and distribution of fibrillar P3HT nanocrystals throughout the volume of the active layer. In this study, electron tomography is used to characterize P3HT/PCBM layers with different blend compositions, both before and after thermal annealing. The power conversion efficiency of the corresponding PSCs is strongly dependent on the overall crystallinity of P3HT and the way P3HT crystals are distributed throughout the thickness of the active layer.

Rupali Jadhav1,2, Stefan Turk3, Florian Kuhnlenz1, Vera Cimrova4, Silke Rathgeber5, Daniel A. M. Egbe*,2,3, and Harald Hoppe**,1 1 Institute of Physics, Ilmenau University of Technology, Weimarer Str. 32, 98693 Ilmenau, Germany 2 Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University Linz, Altenbergerstr. 69, 4040 Linz, Austria 3 Institute for Print and Media Technology, Chemnitz University of Technology, Reichenhainer Str. 70, 09126 Chemnitz, Germany 4 Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovský Sq. 2, 162 06 Prague 6, Czech Republic 5 Johannes Gutenberg-University Mainz, Staudingerweg 7, 55128 Mainz, Germany

We investigated the structural properties of anthracene containing poly(p-phenylene−ethynylene)-alt-poly(p-phenylene−vinylene) polymers with general constitutional unit: (−Ph−C≡C−Anthr−C≡C−Ph−CH═CH−Ph−CH═CH−)n by means of wide-angle X-ray scattering and fluorescence spectroscopy. Interchain interactions were systematically modified by decorating the conjugated polymer backbone with linear or branched or combinations of linear and branched alkoxy side chains. Special emphasis is taken on an evaluation method for fiber scattering spectra that allows the deduction of important structural details of polymer materials with limited degree of order. These include positional correlations along the backbones, interlayer and π−π stacking distances between different chains as well as a quantification of the degree of order in terms of number of lattice planes per domain. The polymers with all-linear side chains attached close to the anthracenylene−ethynylene unit (AnE) show a layered structure. In contrast attachmen...

In order to study the influence of the organic solar cell device layout on the photovoltaic parameters, we systematically varied its geometry. By knowledge of all sheet resistances in the device, we were able to correlate the series resistance with the geometry of the device using a simple model for its calculation. Deviations between experiment and calculation could be related with the solar cell geometry and understood by postulating curved transport ways of the current within the largely resistive ITO-layer. Thus, a further refinement of the calculation is required in order to minimize the deviation between calculation and experiment. Short solar cell lengths and ITO-bridges yield minimal series resistance and best conversion efficiency.

Polymer solar cell modules based on the standard polymer–fullerene system of to-date, P3HT–PCBM, have been prepared and characterized. We have observed a loss of only 20% when up-scaling the active area of the solar cell by a factor somewhat larger than 10. An average solar cell efficiency of 3% and a module efficiency of 1.9% for three serially interconnected solar cells of 5.4 cm 2 each are reported. The route for further optimization of module performance is discussed based on analyzing the existing loss factors within this design.

We carried out a detailed optimization of P3HT:PCBM polymer solar cells by variation of blending ratio, film thickness and annealing conditions. From our studies it became evident that the film thickness and the fullerene concentration are mutually dependent parameters, what the overall performance concerns. In detail, we revealed a clear relationship between film thickness, PCBM concentration and the blend film morphology. We varied the PCBM concentration in our polymer solar cells between 25% and 50%, and found the best results for 33.3% of PCBM. In this case, the optimum between competing processes like effective charge carrier separation and percolation path establishment was realized. In thicker films, the growth of PCBM aggregates via phase separation leads to formation of percolation paths and therefore improves the photocurrent. In contrast, for thinner films a high PCBM concentration is favourable to achieve optimal efficiencies.

The preparation of 27 different derivatives of C60 and C70 fullerenes possessing various aryl (heteroaryl) and/or alkyl groups that are appended to the fullerene cage via a cyclopropane moiety and their use in bulk heterojunction polymer solar cells is reported. It is shown that even slight variations in the molecular structure of a compound can cause a significant change in its physical properties, in particular its solubility in organic solvents. Furthermore, the solubility of a fullerene derivative strongly affects the morphology of its composite with poly(3-hexylthiophene), which is commonly used as active material in bulk heterojunction organic solar cells. As a consequence, the solar cell parameters strongly depend on the structure and the properties of the fullerene-based material. The power conversion efficiencies for solar cells comprising these fullerene derivatives range from negligibly low (0.02%) to considerably high (4.1%) values. The analysis of extensive sets of experimental data reveals a general dependence of all solar cell parameters on the solubility of the fullerene derivative used as acceptor component in the photoactive layer of an organic solar cell. It is concluded that the best material combinations are those where donor and acceptor components are of similar and sufficiently high solubility in the solvent used for the deposition of the active layer.

We investigate organic polymer/fullerene (P3HT/PCBM) bulk heterojunction solar cells by means of photoinduced absorption spectroscopy (PIA). The dynamic properties of the charge carriers are studied in the temperature range of 5 K to 200 K by recording the PIA signal in the frequency domain. The mobility and the lifetime of the positive polarons are de- termined. The dependence of the short circuit current on the incident light intensity is also investigated. Finally, the properties of annealed and not annealed solar cells are compared. As the most interesting result, the appearance of two polaron species on different time scales is observed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Purpose The aim of this study is to implement augmented reality in real-time image-guided interstitial brachytherapy to allow an intuitive real-time intraoperative orientation. Methods and Materials The developed system consists of a common video projector, two high-resolution charge coupled device cameras, and an off-the-shelf notebook. The projector was used as a scanning device by projecting coded-light patterns to register the patient and superimpose the operating field with planning data and additional information in arbitrary colors. Subsequent movements of the nonfixed patient were detected by means of stereoscopically tracking passive markers attached to the patient. Results In a first clinical study, we evaluated the whole process chain from image acquisition to data projection and determined overall accuracy with 10 patients undergoing implantation. The described method enabled the surgeon to visualize planning data on top of any preoperatively segmented and triangulated surface (skin) with direct line of sight during the operation. Furthermore, the tracking system allowed dynamic adjustment of the data to the patient's current position and therefore eliminated the need for rigid fixation. Because of soft-part displacement, we obtained an average deviation of 1.1 mm by moving the patient, whereas changing the projector's position resulted in an average deviation of 0.9 mm. Mean deviation of all needles of an implant was 1.4 mm (range, 0.3–2.7 mm). Conclusions The developed low-cost augmented-reality system proved to be accurate and feasible in interstitial brachytherapy. The system meets clinical demands and enables intuitive real-time intraoperative orientation and monitoring of needle implantation.

The evolution of nanomorphology within thin solid-state films of poly(3-alkylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3AT:PCBM) blends during the film formation and subsequent thermal annealing is reported. In detail, the influence of the P3AT's alkyl side chain length on the polymer/fullerene phase separation is discussed. Butyl, hexyl, octyl, decyl, and dodecyl side groups are investigated. All of the P3ATs used were regioregular. To elucidate the nanomorphology, atomic force microscopy (AFM), X-ray diffraction, and optical spectroscopy are applied. Furthermore, photovoltaic devices of each of the different P3ATs have been constructed, characterized, and correlated with the nanostructure of the blends. It is proposed that the thermal-annealing step, commonly applied to these P3AT:PCBM blend films, controls two main issues at the same time: a) the crystallization of P3AT and b) the phase separation and diffusion of PCBM. The results show that PCBM diffusion is the main limiting process for reaching high device performances.

This article belongs to the Special Issue Organic Photovoltaics.-- et al., We report on the degradation of organic photovoltaic (OPV) cells in both indoor and outdoor environments. Eight different research groups contributed state of the art OPV cells to be studied at Pomona College. Power conversion efficiency and fill factor were determined from IV curves collected at regular intervals over six to eight months. Similarly prepared devices were measured indoors, outdoors, and after dark storage. Device architectures are compared. Cells kept indoors performed better than outdoors due to the lack of temperature and humidity extremes. Encapsulated cells performed better due to the minimal oxidation. Some devices showed steady aging but many failed catastrophically due to corrosion of electrodes not active device layers. Degradation of cells kept in dark storage was minimal over periods up to one year., We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).

The influence of crystallinity on exciton diffusion and fullerene distribution was investigated by blending amorphous and semicrystalline copolymers. We measured exciton diffusion and fluorescence quenching in such blends by dispersing fullerene molecules into them. We find that the diffusion length is more than two times higher in the semicrystalline copolymer than in the amorphous copolymer. We also find that fullerene preferentially mixes into disordered regions of the polymer film. This shows that relatively small differences in molecular structure are important for exciton diffusion and fullerene distribution.

During the last few decades, and in some cases only the last few years, novel thin-film photovoltaic (PV) technologies such as dye-sensitized solar cells (DSSC), organic solar cells (OPV), and, more recently, perovskite-based solar cells (PSC) have been growing in maturity with respect to device performance and device stability. Together with new material systems, novel device architectures have also been introduced. Both parameters will have an effect on the overall device stability. In order to improve the understanding of degradation effects and how they can be prevented, stress testing under different conditions is commonly applied. By careful combination of stress factors and thorough analysis of photovoltaic parameter decaying curves, an understanding of the underlying degradation pathways can be gained. With the help of standardized and accelerated stress tests, as described in the ISOS-protocols, statements concerning application lifetimes can finally be made and compared among different labs. Once a photovoltaic technology has proven long lasting durability, the ultimate barrier for entering the commercial market are the IEC tests, taking a deeper look on overall safety and reliability, not only on durability. Here, the most prominent stress tests are reviewed, discussed and extended with respect to learning the most about photovoltaic device stability., Funded by: German Federal Ministry of Education and Research. Grant Number: 03EK3502.

Novel copolymers consisting of the alternating push–pull comonomers fluorene and thieno[3,4-b]pyrazine/quinoxaline were synthesized by a palladium-catalyzed Suzuki cross-coupling reaction in 60–80% yields. The structure of the deeply colored copolymers was confirmed with 1H and 13C NMR. All the new materials were characterized with spectroscopic and electrochemical methods. Bulk heterojunction organic solar cells based on some of the novel polymers in combination with the well-known fullerene acceptor [6,6]-phenyl C61–butyric acid methyl ester were fabricated, and their photovoltaic parameters were measured. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6952–6961, 2006

Fundamental aspects of the influence of the nanomorphology in phase-separated conjugated polymer/fullerene bulk heterojunction blends are presented. A variety of experimental techniques were combined to resolve the structure of fullerene and polymer domains on the nanometer scale. As predicted theoretically, it is experimentally identified that the phase-separated domain size as well as the percolation of both hole- and electron-conducting phases is crucial to improve the power conversion efficiency of organic solar cell devices. Among the experimental techniques we applied atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), scanning and transmission electron microscopy (SEM, TEM). New insights are presented about the conformation and distribution of the conjugated polymer within the photoactive layer, which has a major impact on the device performance.

Composite films of P3HT/PCBM-(poly[3-2,5-diyl]/[6,6]-phenyl C61 butyric acid methyl ester) are widely used as an active layer in plastic solar cells. We have studied the influence of thermal annealing on nano-structural and optical properties of thin spin-coated P3HT/PCBM-films. Their structural properties were studied by X-ray diffraction in grazing incidence geometry. It was found that the crystallinity of the investigated films is drastically increased upon annealing. Furthermore, the anisotropic dielectric function of such films was determined by spectroscopic ellipsometry. Significant changes were observed both in parallel and perpendicular components of the dielectric function after annealing. These changes were attributed to the formation of crystalline domains upon annealing.

Within the different organic photovoltaic devices the conjugated polymer/fullerene bulk heterojunction approach is one of the foci of today's research interest. These devices are highly dependent on the solid state nanoscale morphology of the two components (donor/acceptor) in the photoactive layer. The need for finely phase separated polymer–fullerene blends is expressed by the limited exciton diffusion length present in organic semiconductors. Typical distances that these photo-excitations can travel within a pristine material are around 10–20 nm. In an efficient bulk heterojunction the scale of phase separation is therefore closely related to the respective exciton diffusion lengths of the two materials involved. Once the excitons reach the donor/acceptor interface, the photoinduced charge transfer results in the charge separation. After the charges have been separated they require percolated pathways to the respective charge extracting electrodes in order to supply an external direct current. Thus also an effective charge transport relies on the development of a suitable nanomorphology i.e. bicontinuous interpenetrating phase structures within these blend films. The present feature article combines and summarizes the experimental findings on this nanomorphology–efficiency relationship.

The electrochemical behaviour of four types of (phenylene ethynylene)-alt-(phenylene vinylene) hybrid polymers, 1, 2, 3, and 4 have been investigated with respect to the influence of the grafted alkoxy side chains. In the case of the fully substituted polymers 2, 3, and 4, the strong insulating nature of longer linear octadecyl or bulky branched 2-ethylhexyl side chains lowers the HOMO levels of the polymers thereby increasing the discrepancy, ΔE g , between the electrochemical, E ec g , and the optical, E opt g , bandgap energies. Thus it is not possible to establish a direct correlation between the open circuit voltage, Voc, of bulk heterojunction solar cell devices of the configuration glass substrate/ITO/PEDOT:PSS/polymer 3:PCBM(1:3, w/w)/ LiF/Al and the HOMO energy levels of polymer 3 solely, as postulated in the literature. The photovoltaic (PV) parameters greatly depend on the grafted side chains.

The nanoscale phase separation in polymer/fullerene bulk heterojunction plastic solar cells requires the use of high resolution techniques for imaging. In this study we used a high-resolution transmission electron microscope (HR-TEM) together with selected area electron diffraction (SAED) to visualize the polymer and fullerene distributions and their amorphous or crystalline organization in the film. While pristine polymer films exhibited no crystalline order, the fullerene organized in nanocrystals. Upon annealing of the blend film, the accompanying phase separation reaches the micron level and fullerene phases aggregate to larger single crystals, as could be seen by SAED.

The new developed navigation system HIALS is a High Intensity Approach Light System for image-guided surgery. The surgeon follows the projected approach lights with his ordinary surgical instruments. Thus, tracked instruments are not necessary with HIALS.

Random laser action and amplified spontaneous emission are observed near 425 nm in self-assembled para-sexiphenyl nanofibers following subpicosecond optical pumping. The threshold excitation fluence (photoexcited density) is as low as 0.5 μJ/cm 2 (6×10 16 cm −3 ). The high degree of material crystallinity results in a very large singlet–singlet annihilation rate of ≈10 −7 cm 3 /s. In stationary conditions, assuming a standard singlet-to-triplet density ratio of 0.3 and bimolecular recombination as the only density-dependent loss mechanism, the equivalent current density necessary for lasing threshold is estimated to be as low as 3 kA/cm 2 . The experimental findings suggest that such highly ordered molecular nanoaggregates have great potential as blue-emitting devices for integrated photonic applications.

In this study plastic solar cells based on arylene-ethynylene/arylene-vinylene hybrid polymers in combination with the soluble fullerene PCBM (1-(3-methoxycarbonyl) propyl-1-phenyl [6 6]C61) reaching 2% AM 1.5 solar power conversion efficiency at 80 mW/cm2 are reported. The polymers used are DE105 (poly(-2,5-dioctyloxy-1,4-phenylene-diethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)) and DE142 (poly(2,5-dioctyloxy-1,4-phenylene-ethynylene-9,10-anthracenylene-ethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)), whose main difference lies in the additional anthracene group in the latter one. Comparing results from electrochemical characterizations with IV-measurements reveals a weak dependency of the maximum open circuit voltage on the molecular HOMO level of the polymer used. A coarse grained morphology of the active layers was found responsible for limiting the photocurrent, as shown by AFM measurements.

In this work we report about photoluminescence investigations and the first observation of lasing in highly ordered, crystalline parasexiphenyl (PSP) films grown by hot wall epitaxy on mica substrates. We demonstrate also the fabrication of hot wall epitaxially grown PSP layers for blue light emitting diodes. The electroluminescence (EL) shows two peaks at 425 and 450 nm, which coincide with the corresponding photoluminescence spectra. The electric field required for the onset of the EL in our single layer devices is comparable with that for optimized multilayer devices based on PSP. q 2005 Elsevier Ltd. All rights reserved.

The relation between the nanoscale morphology and associated device properties in conjugated polymer/fullerene bulk-heterojunction “plastic solar cells” is investigated. We perform complementary measurements on solid-state blends of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV) and the soluble fullerene C60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C61 (PCBM), spin-cast from either toluene or chlorobenzene solutions. The characterization of the nanomorphology is carried out via scanning electron microscopy (SEM) and atomic force microscopy (AFM), while solar-cell devices were characterized by means of current–voltage (I–V) and spectral photocurrent measurements. In addition, the morphology is manipulated via annealing, to increase the extent of phase separation in the thin-film blends and to identify the distribution of materials. Photoluminescence measurements confirm the demixing of the materials under thermal treatment. Furthermore the photoluminescence of PCBM clusters with sizes of up to a few hundred nanometers indicates a photocurrent loss in films of the coarser phase-separated blends cast from toluene. For toluene-cast films the scale of phase separation depends strongly on the ratio of MDMO-PPV to PCBM, as well as on the total concentration of the casting solution. Finally we observe small beads of 20–30 nm diameter, attributed to MDMO-PPV, in blend films cast from both toluene and chlorobenzene.

In this work we report about photoluminescence investigations and the fast observation of lasing in highly-ordered, crystalline para-sexiphenyl (PSP) films grown by hot wall epitaxy on mica substrates. We demonstrate also the fabrication of hot wall epitaxially grown PSP layers for blue light emitting diodes. The electroluminescence (EL) shows two peaks at 425 and 450 nm, which coincide with the corresponding photoluminescence spectra. The electric field required for the onset of the EL in our single layer devices is comparable with that for optimized multilayer devices based on PSP.

Organic solar cell research has developed during the past 30 years, but especially in the last decade it has attracted scientific and economic interest triggered by a rapid increase in power conversion efficiencies. This was achieved by the introduction of new materials, improved materials engineering, and more sophisticated device structures. Today, solar power conversion efficiencies in excess of 3% have been accomplished with several device concepts. Though efficiencies of these thin-film organicdevices have not yet reached those of their inorganic counterparts (η ≈ 10–20%); the perspective of cheap production (employing, e.g., roll-to-roll processes) drives the development of organic photovoltaic devices further in a dynamic way. The two competitive production techniques used today are either wet solution processing or dry thermal evaporation of the organic constituents. The field of organic solar cells profited well from the development of light-emitting diodes based on similar technologies, which have entered the market recently. We review here the current status of the field of organic solar cells and discuss different production technologies as well as study the important parameters to improve their performance.

We describe a simple organic solar cell structure that allows to study in detail loss mechanisms due to non-ohmic contacts, structural drawbacks and material selection: the MIP-type (metal-intrinsic-p-doped) structure discussed here represents the p-side and the active layer within a PIN type solar cell architecture. We here adapt the bulk hetero-junction concept using blends of zinc-phthalocyanine (ZnPc) and buckminsterfullerene C60. Furthermore, we use doped wide-gap materials as hole transport layers that enable a more sophisticated solar cell development. The samples are characterized by combination of current voltage characteristics, impedance spectroscopy and capacitance–voltage measurements. We present an evolution of MIP-type structures which improves all solar cell parameters; in particular, excessive series resistance and contact problems reducing the fill factor could be almost completely eliminated.

We introduce a p-i-n-type heterojunction architecture for organic solar cells where the active region is sandwiched between two doped wide-gap layers. The term p-i-n means here a layer sequence in the form p-doped layer, intrinsic layer and n-doped layer. The doping is realized by controlled co-evaporation using organic dopants and leads to conductivities of 10-4 to 10-5 S/cm in the p- and n-doped wide-gap layers, respectively. The photoactive layer is formed by a mixture of phthalocyanine zinc (ZnPc) and the fullerene C60 and shows mainly amorphous morphology. As a first step towards p-i-n structures, we show the advantage of using wide-gap layers in M-i-p-type diodes (metal layer–intrinsic layer–p-doped layer). The solar cells exhibit a maximum external quantum efficiency of 40% between 630-nm and 700-nm wavelength. With the help of an optical multilayer model, we optimize the optical properties of the solar cells by placing the active region at the maximum of the optical field distribution. The results of the model are largely confirmed by the experimental findings. For an optically optimized device, we find an internal quantum efficiency of around 82% under short-circuit conditions. Adding a layer of 10-nm thickness of the red material N,N′-dimethylperylene-3,4:9,10-dicarboximide (Me-PTCDI) to the active region, a power-conversion efficiency of 1.9% for a single cell is obtained. Such optically thin cells with high internal quantum efficiency are an important step towards high-efficiency tandem cells. First tandem cells which are not yet optimized already show 2.4% power-conversion efficiency under simulated AM 1.5 illumination of 125 mW/cm2 .

We investigated blends of poly hexylthiophene (P3HT) with copper indium diselenide nanocrystals for photovoltaic applications. Depending on the synthesis, the particles were shielded by different organic surfactants. Different concentrations of these nanoparticles were suspended in the polymer solutions and spin cast onto ITO glass. Morphological studies have been performed by atomic force microscopy and transmission electron microscopy. Films consisting of tri-n-octylphosphine oxide-capped CISe:P3HT show photovoltaic response. The best performance we obtained is an open-circuit voltage of about 1 V and a photocurrent of 0.3 mA/cm2 using a white light illumination intensity of 80 mW/cm2.

We have determined the anisotropic dielectric function of thin spin-coated as well as drop-cast poly(3-octylthiophene)-films from near-normal reflectance and transmittance data and from spectroscopic ellipsometry. The influence of the growth parameters (i.e. spin frequency, concentration of the polymer in solvent, type of solvent) and of the substrate on the anisotropic dielectric function of the films was studied. The determined anisotropic dielectric functions were used to investigate the orientation of the polymer chains within these films. For the spin-coated films we found, that the optical anisotropy increases with increasing spin frequency and decreasing polymer concentration in the solution, indicating that polymer chains become more aligned parallel to the substrate. The drop-cast films were found to be even more anisotropic. In addition, the anisotropy increases considerably if the property of the Si substrate is changed from hydrophilic to hydrophobic (done by HF-treatment). Summing up, all results are discussed in dependence of the film thickness as the decisive parameter for the anisotropy.

In this work we have used atomic force microscopy and X-ray diffraction by synchrotron radiation to investigate the growth kinetics and morphology of para-sexiphenyl layers. The results of our investigations can be summarized as follows: (a) para-sexiphenyl grows on mica epitaxialy; (b) a rearrangement from randomly distributed small para-sexiphenyl islands with compact shape to elongated islands occurs during the growth if the critical island density is reached; (c) with further increase of the growth time the islands become elongated, quickly reaching a fixed asymptotic width while their height remains much smaller than their length and width.

To determine the actual absorption in the photoactive layer of a plastic solar cell, e.g. consisting of blend of poly (2-methoxy5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene )( MDMO-PPV) and a methanofullerene, w6,6x-Phenyl C -butyric acid methyl 61 ester (PCBM), a matrix formalism for the light propagation in this multi-layer system is applied. This calculation results in an upper limit for the incident photon to collected electron (IPCE) conversion efficiency for a given internal quantum efficiency. Comparisons with experimental results as well as conclusions for the optimal layer thickness are drawn. 2003 Elsevier B.V. All rights reserved.

We investigated blends of semiconducting polymers with copper indium diselenide nanocrystals for photovoltaic applications. Depending on the synthesis, the particles are shielded by different amount of organic surfactants. Different concentrations of these nanoparticles were suspended in the polymer solutions and spin cast onto ITO glass. Solar cells were then produced by evaporation of aluminium as the back contact. Optical, electrical and morphological properties of this new prototype of composite solar cells were investigated.

Poly(3-octylthiophene) (P3OT) is a promising candidate for plastic solar cells. We have determined the anisotropic dielectric function (DF) of thin spin-coated and drop-cast P3OT-films deposited on Si and glass substrates from both near-normal reflectance and transmittance spectroscopy and from variable angle spectroscopic ellipsometry. The influence of the deposition parameters (such as spin frequency, concentration of the polymer in solvent, type of solvent, type of substrate) on the anisotropic DF of the film was investigated. The anisotropy of the DF can be related to the orientation of the polymer chains in the film. The optical anisotropy of spin-coated films increases with increasing spin frequency and decreasing polymer concentration in the solvent, indicating that polymer chains become more aligned parallel to the substrate. The drop-cast films were also found to be anisotropic. In addition, the anisotropy increases considerably if the property of the Si substrate is changed from hydrophilic to hydrophobic (by treatment with hydrofluoric acid). With increasing film thickness the optical anisotropy decreases. Summing up, all results are discussed in dependence of the film thickness as the decisive parameter.

The design of novel conjugated polymers suitable for use in plastic solar cells is one of today's challenges aiming towards improved key properties like the increase of photocurrent and open circuit voltage of such devices. In this work we present first results on arylene-ethynylene/arylene-vinylene hybrid polymers 3 (poly(-2,5-dioctyloxy-1,4-phenylene-diethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)) and 5 (poly(2,5-dioctyloxy-1,4-phenylene-ethynylene-9,10-anthracenylene-ethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2′-ethyl)hexyloxy-1,4-phenylene-vinylene)), demonstrating photovoltaic action in combination with the soluble C60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C61 (PCBM). Devices with an active layer thickness of about 100 nm yielded power conversion efficiencies of up to 2% under 100 mW cm−2 AM 1.5 white light illumination. The coarse grained morphology of the active layers was identified as the main limitation for the photocurrent, revealed by AFM measurements. The photovoltaic devices were characterized by current–voltage and spectral photocurrent measurements. The results show that the open circuit voltage is weakly dependent on the HOMO (highest occupied molecular orbital) level of the conjugated polymer used as donor.

In this paper, we report our results on the modeling of the optical properties of the bulk-heterojunction “plastic solar cells”, consisting of a solid-state blend of the conjugated polymer poly-[2-(3,7-dimethyloctyloxy)-5-methyloxy]-para-phenylene-vinylene and the fullerene C 60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C 61 . Upon illuminating these cells with the standard AM 1.5 solar spectrum, the short circuit current can be determined for any given internal quantum efficiency as a function of the active layer thickness. In addition, the depth profiles of photoinduced charge generation rates are calculated. Based on the agreement of this modeling with experimentally determined efficiencies of these solar cells, an internal quantum efficiency of about 80% has been estimated.