We measure the transient photocurrent of APFO3:PCBM bulk heterojunction solar cells illuminated with ns-laser and sub-ms LED light sources. The ratio of the number of collective charges to the number of excited photon (external quantum efficiency, EQE) and the transient photocurrent fall times have been carried out with difference pulse durations and fluences. The EQEs characterized by ns-laser source are shown to obey the bimolecular recombination at high excitation fluences. The increasing of transient photocurrent fall times suggests that the fall times of free charge carriers are effected by deep trap density of state (DoS) and thus the free charge carriers have a sufficient time for bimolecular recombination at short circuit condition. At the same fluences, however, the EQEs characterized by sub-ms LED sources exhibit an excitation fluences independence of EQE. The transient photocurrent fall times with sub-ms LED sources are rather constant when the excitation fluences increases indicating that the deep trap DoS has less effect at short circuit condition for longer pulse duration.
Most efficient polymer solar cells are usually fabricated from toxic organic solvents, such as chloroform, chlorobenzene, or dichlorobenzene (ODCB). Here, we demonstrate a power conversion efficiency of 4.5% in solar cells with a new blue polymer poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) mixed with PC71BM and processed from mixed solvents of toluene and ODCB in a ratio of 9:1. Decreasing the content of ODCB makes device processing more compatible with the environment for large scale production, with 10% reduction of photocurrent compared to devices from pure ODCB under optimized conditions. In addition, less variation of photocurrent is obtained in solar cells processed from mixed solvents than from pure ODCB due to varying nanostructure in the blends, which is also critical for production.
Microstructuring of polymer surfaces on optical spacers allows formation of reflective light traps. Such flexible reflectors can be combined with flexible polymer solar cells. We have demonstrated enhanced absorption using Lambertian and regular light reflectors, demonstrated via luminescence from fluorescent layers. Such light traps are suitable to use in combination with polymer solar cells incorporating transparent electrodes. The possibility to enhance the concentration of excited states and photogenerated charges through light trapping also helps to increase charge carrier mobility.
These experimental results indicate that light confinement may be an alternative approach for boosting the efficiency of thin film conjugated polymer photovoltaics.
Spin-coated thin films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (APFO-3) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are used as the active material in polymer photovoltaic cells. Such blends are known for their tendency to phase separate during film formation. Tuning the morphology of the blend in a controlled way is one possible road towards higher efficiency. We studied the effect of adding chlorobenzene to chloroform-based blend solutions before spin-coating on the conversion efficiency of APFO-3:PCBM photodiodes, and related that to the lateral and vertical morphology of thin films of the blend. The lateral morphology is imaged by atomic force microscopy (AFM) and the vertical compositional profile is obtained by dynamic secondary ion mass spectrometry (SIMS). The profiles reveal compositional variations consisting of multilayers of alternating polymer-rich and PCBM-rich domains in the blend film spin-coated from chloroform. The vertical compositional variations are caused by surface-directed spinodal waves and are frozen in during the rapid evaporation of a highly volatile solvent. With addition of the low-vapour pressure solvent chlorobenzene, a more homogeneous vertical composition is found. The conversion efficiency for solar cells of this blend was found to be optimal for chloroform:chlorobenzene mixtures with a volume-ratio of 80:1. We have also investigated the role of the substrate on the morphology. We found that blend films spin-coated from chloroform solutions on PEDOT:PSS-coated ITO show a similar compositional structure as the films on silicon, and that changing the substrate from silicon to gold only affects the vertical phase separation in a region close to the substrate interface.
We report on the initial time-resolved luminescense and nonlinear absorption properties of two polythiophenes 3-substituted with chiral charged amino acid-derivatized substituents, POWT and POMT. The photo-physical characterization yielded quantum efficiency typically in the range 0.01 - 0.1, however, with two-photon absorption cross-section better than or similar to a typical two-photon reference chromophore, such as fluorescein. They were tested as conformational sensitive optical probes for the recording of pH-induced conformational changes of synthetic peptides, proteins and samples of protein amyloid fibrils characteristic of amyloid related diseases. Particularly, the POMT polyelectrolyte with the L-enantiomeric side chains is shown to favor this induction of well defined structure as judged by the circular dichroic signal as well as a stronger enhancement of luminescense for the L-form over the D-form complex. Furthermore, time-resolved fluorescense and two-photon induced fluorescence both also showed a difference in the complexation with the D and L form. This shows that the multi-photon excitation path can be an efficient means for chiral recognition of biomolecular complexes. It is demonstrated how the conjugated polyelectrolyte L-POMT can be used to spectrally image the formation of amyloid fibrils of insulin using both one- and two-photon absorption based fluorescence imaging.
The volume change that the conducting polymer polypyrrole (PPy) undergoes upon electrochemical oxidation and reduction can be used to make microactuators. We have made microactuators based on a PPy/Au bilayer. These actuators have been combined with other micromachined structures to make biomedical microdevices. Using an area of bilayers one can potentially arrest (nerve) fibers. They can also be used to close a micrometer sized cavity with a lid. In addition, we demonstrate a microrobotic arm that may be developed for the manipulation of small particles.
We discuss possible mechanisms of doping induced volume change in conjugated polymers with reference to the case of poly(3,4- ethylenedioxythiophene) (PEDOT). This material has been studied in two forms; as well ordered paracrystalline material and in the form of a water swollen conducting hydrogel. Structural and electrochemical studies are discussed.
Conjugated polymers are currently of extensive interest because they can be doped to become electronically conductive. Actually, they can be repeatedly driven between the conductive and insulating states by electrochemical redox in an electrolyte solution or by chemical redox in a solution or in a gaseous atmosphere. Small volume changes, up to 10%, occur during the transitions. We use the bending bipolymer strips, with a conjugated polymer layer and an inert polymer substrate layer, to sense the volume changes in the responsive conjugated polymer layer.
By combining a processable electronically conductive polymer [poly(3-octylthiophene) (P3OT), polypyrrole (PPy)], a solid polymer electrolyte [(PEO)8LiClO4], and a metal oxide (V2O5), a solid state electrochromic device is constructed. The polymer films are fabricated by spin coating from solution (P3OT) and template polymerization (PPy). The metal oxide is electrochemically doped with Li+ and the electrodes are mounted in a sandwich structure with a thin film of polymer electrolyte in between. As the applied cell potential is changed, the optical absorption of the cell is changed. Owing to the difference in columbic capacity between the different materials, the optical changes of the cell are due to optical changes of the polymer only. This means that, instead of having to adapt often contradictory optical changes in two electrochromic materials for the desired application, one can use a polymer with a proper optical signature, letting the band gap determine the electro-optical behavior of the cell.
The synthesis of highly conducting polyacetylene in the late seventies inaugurated an era of extensive studies of electronic structure and transport in organic polymers. The recent attainment of conductivities in metallic polyacetylene comparable to that of copper is the outcome of major advances in the theoretical understanding and chemical synthesis of these materials. In parallel to this quest for understanding of the electronic properties of "one-dimensional" organic polymers, there has also been an active search for applications of these materials. A few applications are now reaching extensive use, among which should be mentioned polymer secondary batteries exploiting polymer materials as electrodes. Many new techniques for preparing the conductive polymers in forms suited for practical application have been proposed. For application as conductive coatings, anti- static surface treatments and electromagnetic shielding, practical use can be foreseen in the near future. One of the applications that has received considerable study, electrochromic devices utilizing electroactive polymers as active materials, has not reached that stage of maturity yet. In this short review, we present the basic physics of electrochromism in electroactive polymers, also named conductive polymers; point out the requirements for use of electroactive polymers in electrochromic devices (which includes displays, smart windows and electrooptical modulators); and describe some of the materials and processing aspects that are of particular relevance with reference to thin film technology.
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