Materials and Surface Chemistry, Chalmers University of Technology, Göteborg: Synthesis of polymers for organic electronics
A new polymer backbone is developed, resulting in a polymer with very low band gap showing unprecedented photovoltaic performance at 1 µm. Preliminary results on another polymer based on a similar backbone shows very good solar cell characteristics. A polymer exhibiting low bandgap (1.4 eV) combined with high Voc in devices have been synthesised. Synthetic routes towards low bandgap polymers with functional side chains are developed.
Biomolecular and organic electronics, IFM, LiU: Devices for organic optoelectronics, printed electronics and nanoelectronics
Polymer solar cells can now generate photocurrent out to 1200 nm, but with a low photovoltage, as must be. Better devices are obtained and the current local record is close to 4% power conversion efficiency. It has been found that the route towards thicker active layers must and can be forsaken, and redirection of efforts to thin solar cells have generated better performance. The combination of optical modelling and experimental optimization indicates that our APFO-3/PCBM blends can be modified to include more polymer, and can be used in thickness of 50-100 nm.
Polymer solar cells without the expensive transparent ITO (indium tin oxide) conductor have been prepared by using micropatterned metal current collectors beneath a PEDOT(PSS) electrode. Microfluidic dispensers are used to form the current collector pattern. Performance is enhanced due to the better electrode, and the cost is reduced, as ITO accounts for a large fraction of materials cost.
Routes towards enhanced optical incoupling of solar light for solar cells is progressing along three routes, viz. plasmon enhanced absorption, microlenses and folded solar cells. All routes appear viable, but the first route appears to give smaller rewards.
Development of wire based electrochemical transistors to form woven logical circuits, a new form of electronic textiles. The simple coating of a textile fibre with PEDOT(PSS) gives an electronic fibre; crossing two fibres with addition of a liquid drop of electrolyte leads to a functional electrochemical transistor. Circuits have been designed, and a first multiplexer has been built.
Progress towards synthesis of biomolecular fibres with coating of electroactive polymer will be combined with molecular multiplexers during year 5.
Chemical Physics, LU: Fast photophysics in polymers and device materials
Extensive time resolved spectroscopy study covering ten decades of time and three orders of magnitude of excitation light intensity, in combination with sophisticated kinetic modelling, established a comprehensive model of excited state dynamics, charge photogeneration and recombination. It was shown that at solar cell operational conditions geminate charge recombination may be a limiting factor for obtaining high photon-to-charge quantum efficiencies. The work suggested that modification of charge carrier mobility and material morphology could be ways to improve material performance. The old problem of excited state torsional planarization of polythiophene polymers was addressed. Using ultrafast spectroscopy on polythiophenes in solution and thin films it could be shown that a polythiophene polymer molecule in solution becomes more planar through a torsional motion occurring on the ~10 ps time scale. The planarization corresponds to extending the conjugation length (size of the spectroscopic unit) by three monomeric units. To include the torsional dynamics is essential for a correct description of spectral evolution and energy transfer in this polymer.
Acreo AB: Printing of organic electronics
Process development of printing process for electrochromic pixels, with statistical process control and extensive process qualification. In line roll-to-roll printing of electrochemical transistors and electronic systems developed. Evaluation of a route to printed solar cells using wrap through electrodes.
Organic Electronics, ITN, LiU, Norrköping: Printed organic electronics
Polyelectrolyte gated polymer field effect transistors
A polyanionic proton conductor is used as gate insulator in an organic field-effect transistor (OFET). Upon operation, protons in the polyelelectrolyte migrate towards the gate and form large electric double layer capacitance (EDLCs) at the polyelectrolyte-gate interface (in order of 10 to 100 μF cm-2). Simultaneously, the depletion of protons at the polyelectrolyte-semiconductor interface effectively allows a p-channel to be created in theorganic semiconductor. This type of transistor displays fast response (< 1 ms) and operates at low voltages (< 1 V). The results are relevant for printed electronics. All-organic active matrix addressed displays based on electrochemical smart pixels made on flexible substrates are reported. Each individual smart pixel device combines an electrochemical transistor with an electrochromic display cell, thus resulting in a lowvoltage operating and robust display technology. The conducting polymer blend poly(3,4-ethylenedioxythiophene) (PEDOT) - poly(styrenesulfonate) (PSS) served as the conducting lines in the active-matrix display, as well as the active material in the electrochemical smart pixels. Different active-matrix display addressing schemes have been investigated and a matrix display fill factor of 65 % was reached. We use a three-terminal electrochemical transistor with an electrochromic display cell architecture, in which an additional layer of PEDOT:PSS was placed on top of the display cell counter electrode. this yields a manufacturing process of only five discrete patterning steps, which in turn promise for that the active matrix addressed displays can be manufactured on paper or plastic substrates in a roll-to-roll production procedure.
ISY, LiU: Electronic design for printed organic electronics and nanoelectronics
Development of improved Spice models for the electrochemical transistor; collaborative efforts with Acreo to design electrochemical transistor circuits for roll-to-roll production; design and simulation of a one-transistor oscillator circuit, and contributions to circuit design for woven logic and for anocircuits.
Theory, ITN, LiU: Functional Organic Optoelectronic Devices
Using recursive Green's function technique we have demonstrated plasmon enhanced optical absorption in polymeric solar cells. In these structures a plasmon is excited at the grated boundary between the metallic substrate and active polymer. We have achieved excellent agreement with the experimental data on these structures. We currently calculatethe absorption spectra for other combinations of metals/active polymers.
In order to provide a proper description of realistic tree-dimensional (3D) plasmonic structures, a dyadic 3D Green's function method has been implemented. The method allows one to calculate electromagnetic wave propagation in a system of dielectric or metallic scatterers of virtually any shape. This tool is currently applied to study of plasmonic structures containing metallic nanoparticle chains and their arrangements in bends and splitters.
Computational Physics, IFM, LiU: Theoretical physics in COE
Development of Monte Carlo modelling of hopping in disordered organic semiconductors, to evaluate a new hypothesis of the mechanism of hysteresis in field effect transistors.
Surface Physics and Chemistry, LiU: Photoelectron spectroscopy of materials and device interfaces
Depending on the synthesis route and additional surface functionalization, the workfunction of polymer electrodes made of PEDOT can be tuned from 3.7 eV to 5.0 eV. The electrical conductivity of PEDOT based layers, even using PSS as counterions, is demonstrated to be sufficient to be used as electrodes in (opto)electronic devices. Low and high workfunction plastic electrodes are used to create the first all-plastic p-LEDs and transparent solar cells fabricated via simple solution processes or cold lamination.
Organic physics, ITN, LiU: Surface spectroscopy
Photoelectron spectroscopy has been used to study vapour-deposited PEDOT (so-called “Danish PEDOT”) to determine the connection between oxidation state – wetting properties – surface composition – deposition/washing procedure. There is a significant difference in the tosylate dopant anion concentration in the films, depending on deposition/washing procedure. A slight loss of tosylate in the surface region of reduced films as compared to pristine (oxidized) films was detected. Photoelectron spectroscopy in combination with DFT calculations have been used to further develop the Integer Charge Transfer model for energy level alignment at weaklyinteracting organic/organic and organic/metal interfaces.
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Last updated: 08/16/07