ICMCTF2009 Session C2-1: Optical Thin Films for Active Devices and Microstystems
Wednesday, April 29, 2009 8:00 AM in Room Royal Palm 4-6
C2-1-1 Conjugated Polymer Based Organic Solar Cells: State of the Art and Future Challenges
G. Dennier, C.J. Brabec, N. Drolet (Konarka Technologies Inc.)
Low-cost photovoltaic cells, which are light and flexible, could open up many new applications for solar cells, from self-powered electronic newspapers to self sufficient buildings. Among the various thin film techniques, solution processed organic solar cells have the highest potential for a true low cost technology since production requires only low temperature solution coating with low resolution. Such photovoltaic elements based on donor acceptor composites, being manufactured by printing and coating techniques from reel to reel, are not only scientifically interesting but highly attractive from a cost standpoint. One of the most critical issues to ensure efficient charge separation is the optimization of the interface between donor and acceptor phases, that is, the nanostructure of the composite film. By mixing the two components, an interpenetrating network of donor and acceptor materials is created inducing a 3 dimensional “bulk” interface photovoltaic cells. Morphology studies of these active blends revealed that the solvent used to prepare them plays a critical role for the quality of the “bulk heterojunction”. But several other approaches have recently been employed to enhance the structure of the composite and control its nanoscopic arrangement. Although the absorption of the conjugated polymers available now do not perfectly fit the solar emission spectrum, efficiencies higher than 6 % have been reported, paving the way to the production of low-cost, flexible, light-weight solar cells.
C2-1-3 Electrochromic Performance of PECVD-Synthesized WO@sub x@C@sub y@ Thin Films on Flexible PET/ITO Substrates for Flexible Electrochromic Devices
Y.-S. Lin (Feng Chia University, Taiwan)
Electrochromic performance of WO@sub x@C@sub y@ films deposited onto 60 Ω/□ flexible PET (polyethylene terephthalate)/ITO (indium tin oxide) substrates by low temperature plasma-enhanced chemical vapor deposition (PECVD) was investigated. It was proven that extraordinary electrochromic performance was provided when the precursor [tungsten carbonyl, W(CO)6] vapor, carried by argon gas, was mixed with air gas and synthesized by radio frequency (r.f.) power at room temperature (23@super o@C). Cyclic voltammetry switching measurements found that only low driving voltages from -1 V to 1 V were needed to provide reversible Li+ ion intercalation and deintercalation. The light modulation with up to 62.3 % of transmittance variance at a wavelength of 650 nm was obtained for 150 cycles of Li+ intercalation and deintercalation in a 0.1 M LiClO@sub 4@-PC electrolyte.
C2-1-5 Large, Room-Temperature Magnetoresistance in Organic Light-Emitting Diodes
M. Wohlgenannt, T.D. Nguyen, J. Rybicki, Y. Sheng (University of Iowa)
We report on the extensive characterization of a recently discovered large, room-temperature magnetoresistive effect in organic semiconductor thin film devices@footnote 1@@footnote2@@footnote 3@. The discovery of this effect came as a surprise, since it has generally been believed that large magnetoresistance at room-temperature requires the presence of ferromagnetic materials. To the best of our knowledge, the mechanism causing this magnetoresistive effect is not currently known with certainty. However, it has been established that it is caused by interactions between paramagnetic species, in particular by the effect of the hyperfine interaction on the spin-selection rules of these interactions. However, the specific nature of the interaction and the involved paramagnetic species is heavily debated in the community. Three different models have been proposed: (i) the electron-hole pair model@footnote 4@, (ii) the triplet exciton-polaron model@footnote 5@, and (iii) the bipolaron model@footnote 6@. We introduce these models and discuss the bipolaron model in more detail . Experiments that can distinguish between these three directions are therefore needed. We report on several different approaches to such experiments including measurements in unipolar and bipolar organic devices, and spectroscopic experiments that measure the density of singlet and triplet excitons@footnote 7@.@paragraph@@footnote 1@T. L. Francis, O. Mermer, G. Veeraraghavan, and M. Wohlgenannt, New J. Phys. 6, 185 (2004). @footnote 2@O. Mermer, G. Veeraraghavan, T. Francis, and M. Wohlgenannt, Solid State Commun. 134, 631 (2005). @footnote 3@J. Kalinowski, M. Cocchi, D. Virgili, P. D. Marco, and V. Fattori, Chem. Phys. Lett. 380, 710 (2003). @footnote 4@J. D. Bergeson, V. N. Prigodin, D. M. Lincoln, and A. J. Epstein, Physical Review Letters 100, 067201 (2008). @footnote 5@P. Desai, P. Shakya, T. Kreouzis, W. P. Gillin, N. A. Morley, and M. R. J. Gibbs, Phys. Rev. B 75, 094423 (2007). @footnote 6@P. A. Bobbert, T. D. Nguyen, F. van Oost, B. Koopmans, and M. Wohlgenannt, Phys. Rev. Lett. 99, 216801 (2007). @footnote 7@T. D. Nguyen, J. Rybicki, Y. Sheng, and M. Wohlgenannt, Phys. Rev. B 77, 035210 (2008).}
C2-1-7 Improved Multi-layer OLED Architecture Using Evolutionary Genetic Algorithm
M. Cremona (PUC-Rio, Brazil); W.Q. Gianini, C. Legnani, K.C. Teixeira (CeDO - Inmetro, Brazil); B. Messer, O.P. Vilela Neto, M.A.C. Pacheco (ICA - DEE - PUC-Rio, Brazil)
Organic light emitting diodes (OLED) constitute a new class of emissive devices, which present high efficiency and low voltage operation, among other advantages over current technology. Multilayer architecture (M-OLED) is generally used to optimize these devices, specially overcoming the suppression of light emission due to the exciton recombination near the metal layers. However, improvement in recombination, transport and charge injection can also be achieved by blending electron and transporting layers into the same one. Graded emissive region devices can provide promising results regarding quantum and power efficiency and brightness, as well. The massive number of possible model configurations, however, suggests that a search algorithm would be more suitable for this matter. In this work, multilayer OLED devices were simulated and fabricated using Genetic Algorithms as evolutionary strategy to improve their efficiency. Genetic Algorithms are stochastic algorithms ba sed on genetic inheritance and Darwinian strife to survival. In our simulations, it was assumed a 50nm width graded region, divided into five equally sized layers. The relative concentrations of the materials within each layer were optimized to obtain the lower V/J@super 0.5@ ratio, where V is the applied voltage and J the current density. The best M-OLED architecture obtained by genetic algorithm presented a V/J@super 0.5@ ratio four times lower than the value reported in the literature. In order to check the experimental validity of the improved results obtained in the simulations, M-OLEDs with different architectures were fabricated by thermal deposition in high vacuum environment. By using a specific deposition system it was possible to produce multilayer OLEDs with highly controlled deposition rate as low as 0.3Å/s, with an accuracy of 2%. The results of the comparison between simulation and experiments carried out in controlled environment and at room temperature ar e presented and discussed.
C2-1-8 The Effects of Post Annealing on Electrochromic Nb-Doped WO@sub 3@ Films
J.L Huang, C.K. Wang (National Cheng Kung University, Taiwan); S.-C. Wang (Southern Taiwan University, Taiwan); D.R. Sahu (National Cheng Kung University, Taiwan)
The Nb-doped WO@sub 3@ films are deposited by e-beam co-evaporation method using ceramic WO@sub 3@ targets and Nb slugs. The films are analyzed by GIXRD, UV/Visible spectrophotometer, electrochemical cyclic voltammetry, XPS and nanoscratch tests. The as-prepared Nb-doped WO@sub 3@ film is amorphous structure and low transmission in optical visible region. The XPS results indicate that the valence state of W atoms is +4 rich and that of Nb atoms is +5. The electrochromic behaviors are studied via cyclic voltammetry method for 1000 cycles in Li@super +@ organic electrolyte show that the enclosing area of I-V curve decreases after 1000 cycles. The optical modulation between coloration and bleach states is also small. The low electrochemical reaction current and low optical modulation is ascribed to the under-stoichiometry of as-prepared Nb-doped WO@sub 3@ film and the low cycling stability is related to worse film adhesion. The electrochromic properties of Nb-doped WO@sub 3@ films can be improved by post annealing treatment at 100-300 @super o@C in oxygen atmosphere. The annealing electrochromic films are transparent in optical visible region, and the optical modulation between coloration/belach states is also improved. There is an increase in electrochemical reaction current, as compared to as-prepared films, after annealing. But the reaction current decreases with increasing of annealing temperature. This is related to Nb-doped WO@sub 3@ which is more stoichiometric and the partial crystallization of film after annealing. On the other hand, the higher annealing temperature of Nb-doped WO@sub 3@ films possess better cycling stability and higher film adhesion.
C2-1-9 Suppressing Coherent Thermal Transport of Photons by Stacking Multi-layer Photonic Crystals
W. T. Lau, J.-T. Shen, S. Fan (Stanford University)
We will show that in a multi-layer photonic crystal composed of a lossless dielectric and vacuum, where all heat is coherently transported by photons, the presence of photonic band gaps would suppress the thermal conductance below the corresponding vacuum value. Furthermore, the conductance can be driven to extremely low values by stacking several crystals of different layer thicknesses, such that there are minimal overlappings of the photonic bands, and large impedance mismatches between different crystals.
C2-1-10 Effect of Power Density on ITO Thin Films by Facing Targets Sputtering
Y.J. Kim, S.B. Jin, S.I. Kim, Y.S. Choi, I.S. Choi, J.G. Han (Center for Advanced Plasma Surface Technology, Korea)
High quality indium tin oxide (ITO) films with a thickness of 450 nm were deposited on glass substrates at low temperatures using a Facing Targets Sputtering (FTS) system. The crystallinity of the ITO films was investigated using high resolution X-ray diffraction (HRXRD) and field emission scanning electron microscopy (FESEM). The ITO films deposited at a sputter power density < 3@footnote 1@ were amorphous, whereas they were polycrystalline at power density@footnote 2@ 3@footnote 1@. This indicates that the crystallinity of ITO films is strongly affected by the power density. The surface morphology of the films was observed by atomic force microscopy (AFM). The resistivity (ρ), carrier concentration (n), and mobility (μ) of the films were measured using Hall-effect. The resistivity of the ITO films depended on the power density. The specific resistivity value reaches a minimum value of 4.26 ⅹ@footnote 3@@footnote 4@ at a power density of 3@footnote 1@. As the power density increases, the substituted Sn ions in the lattice can produce a free electron carrier to increase the electron carrier concentrations from 2.0 ⅹ@footnote 5@ to 4.6 ⅹ@footnote 5@@footnote 6@. However, ionized impurity scattering centers can simultaneously trap the free electron carriers and decrease the carrier mobility from 48 to 21@footnote 7@. The optical properties of the films were evaluated using UV-VIS-NIR spectrophotometry. A maximum transmittance of approximately 81.2 % was achieved in the visible spectral region at a power density of 1@footnote 1@. The transmittance decreased with increasing power density due to the rough surface morphology caused by the increased optical absorption and surface scattering. @paragraph@@super 1@W/cm, @super 2@ ≥, @super 3@10@super -4@, @super 4@Ωcm, @super 5@10@super 20@, @super 6@cm@super -3@, @super 7@cm@super 2@V@super -1@s@super -1@.