Elizabeth Andreoli de Oliveira (USP)
Leni Akceruld (UFPR, LACTEC)
Marco Cremona (PUC-Rio)
FIRST-PRINCIPLES ANALYSIS OF OPTOELECTRONIC PROPERTIES IN SEMICONDUCTING POLYMERS: THE ROLE OF SOLID-STATE PACKING
G. Bussi, A. Ferretti, A. Ruini, E. Molinari, INFM, Modena, Italy, and M.J. Caldas, Instituto de Física da USP, SP, Brazil
Ordered films of organic conjugated polymers offer an ideal scenario for the study of electronic and excitonic confinement, being composed of quasi-one-dimensional systems arranged in a three-dimensional crystalline environment. Sofar, experiments are usually interpreted in terms of a single-chain model, therefore neglecting any effect of solid-state arrangement and interchain interaction. We investigate the effect of solid-state chain packing on both optical and transport properties for a prototype polymer, poly-para-phenylenevynilene (PPV), in the two different crystalline packings most often found in real films, herring-bone (HB) and pi-stack. We start from a density-functional theory calculation, for the crystalline system. The optical behaviour is then explored through an ab-initio scheme based on the Bethe-Salpeter equation, that allows us to include electron-hole interaction. We find that the details of crystalline arrangement can alter dramatically the optical properties, leading to a rich excitonic structure involving direct (with electron and hole on the same chain) and charge transfer excitons. Indeed, in the case of HB PPV, the optical inactivity of the lowest component crucially quenches the photoluminescence efficiency. Transport properties have been investigated by converting the band-structure results to a tight-binding formulation. This approach provides a first-principles determination of the transfer integrals, which are found to be a crucial quantity to assess the impact of crystalline aggregation on conduction properties; in particular, we obtain a clear difference in the effect of HB versus pi-stacking.Our results suggest that control of interchain interaction and solid-state packing can be a tunable parameter for the design of efficient optoelectronic devices.
Self-assembled monolayers as interfaces of molecular optoelectronic devices
F. A. Nüesch, Laboratory of Optoelectronics of Molecular Materials, Institute of Materials, EPFL, 1015 Lausanne, Switzerland
Optoelectronic and electronic devices such as light emitting diodes, photodiodes or transistors are traditionally made from inorganic materials such as silicon or gallium arsenide due to their unparalleled performance. Only recently, the commercialisation of organic light emitting devices has swept away the fear that organic materials would lack the stability required for device manufacture and reliability. As a matter of fact, polymer transistors can now exhibit charge carrier mobilities of the same order of magnitude as amorphous silicon.
To achieve such high performances, it is necessary to control the growth of the organic films and to study the electric contact to the device electrode. It appears that the interface between electrode and organic film is crucial for charge injection into organic materials. In this work, a strategy is developed, which utilises self-assembled monolayers attached to the device electrodes. Different grafting methods are elaborated and discussed. In particular, the role played by the electrical dipole, conjugation and polarizability of the grafted molecules is elucidated.
SUPRAMOLECULAR SYSTEMS FOR MOLECULAR ELECTRONICS AND OPTICAL DEVICES.
O.L. Malta, Departamento de Química Fundamental-CCEN-UFPE. Cidade Universitária, Recife-PE, 50670-901, Brazil.
Supramolecular chemistry provides the way to obtain nano-objects that present high functionality, based on the assembly of molecular units. In these nano-objects, or supramolecular systems, the bonding between the molecular units is rather weak and has a low covalent character. Modeling and manipulating these systems become facilitated by the low energy involved in breaking and forming bonds between the molecular units, making them adequate and efficient molecular complexes to be used in nanodevices. They are of particular interest in molecular electronics and in the design of high performance optical materials based on coordination compounds of transition metal and lanthanide ions. In this thirty minutes talk we will present and discuss, through a few examples, some of the basic aspects of the role played by supramolecular systems in these two subjects, with emphasis on the molecular shuttle switch and on lanthanide complexes for electroluminescent materials.
Financial support: CNPq, RENAMI, IMMC
FINE CONTROL OF PHOTOLUMINESCENCE IN NANOSTRUCTURED LAYER-BY-LAYER FILMS OF PPV
Osvaldo N. Oliveira Jr.,Valtencir Zucolotto, Ângelo D. Faceto, Felipe R. Santos, Francisco E. G. Guimarães, IFSC, Universidade de São Paulo, USP, CP 369, 13560-970, São Carlos, SP, Brazil IFSC/USP, São Carlos/SP, Brazil.
Layer-by-layer (LBL) films are exploited to control the photoluminescence (PL) of poly(p-phenylenevinylene) (PPV) in an unprecedented way. This was achieved by controlling the Förster energy transfer between PPV layers (donors) and layers of a commercial azodye, Brilliant Yellow (BY) (acceptors). Energy transfer was controlled by inserting spacing layers of inert polymers between PPV and BY layers and by photoaligning the BY molecules via trans-cis-trans isomerization with a linearly polarized laser. The intensity of the polarized PL could be varied almost continuously by changing the time of photoalignment. PL was quenched in PPV/BY films with no spacing layers, but increased with the number of spacing layers. Further increase in PL was obtained by photoaligning the BY molecules perpendicularly to the PPV molecules that adopt a preferential orientation in the LBL films. This occurs because Förster transfer is directional, dipole-dependent, being maximum for BY molecules aligned parallel to the PPV molecules. PL polarized parallel to the excitation direction increases up to 4 times the initial value. The precise control of emission offered by the LBL films may be used in smart devices, where the luminescence properties of PPV are combined with the birefringence characteristics of azobenzene-containing materials. Acknowledgment: FAPESP and CNPq.
PHOTOPHYSICS OF SEMICONDUCTING POLYMERS STUDIED BY ULTRAFAST SPECTROSCOPY
P. B. Miranda, Instituto de Física de São Carlos, Universidade de São Paulo, C.P. 369, São Carlos – SP, CEP 13560-970
Semiconducting polymers form a relatively new class of materials with important emerging applications in electronics and optoelectronics. The current state of polymer electronics was made possible by more than two decades of intense investigation, from which much has been learned about their electronic structure, elementary excitations and the role played by polymer morphology and impurities. Although considerable progress has been made in recent years towards applications of luminescent conjugated polymers, a few outstanding issues on their photophysics have profound implications for device development and optimization. In this talk I will review some of these open problems and how they have been investigated by time-resolved spectroscopy in the ultrafast timescale (from a few fs to hundreds of ps). Emphasis will be given to recent studies of excitation energy transfer, charge-carrier photogeneration and formation of triplet states. Both homogeneous polymers and guest-host systems will be considered.
R. Valaski, R. L. Lessmann, L. S. Roman, I. A. Hümmelgen DF/UFPR, C. P. 19044, 81531-990 Curitiba PR, Brazil, R. M. Q. Mello, L. Micaroni, DQ/UFPR, C. P. 19081, 81531–990 Curitiba PR, Brazil.
We investigate the photovoltaic potential of SPAN/PMT (SPAN: sulfonated polyaniline; PMT: poly(3-methylthiophene)) bilayers galvanostaticaly deposited onto tin oxide films (TO), the transparent electrode. TO films are produced by chemical vapour depositon method. Electrochemical polymerization has some advantages like easy thickness control, doping e and dedoping processes. Besides, the production of bilayer polymer devices using chemicall methods may be difficult due to constraints imposed by solvent compatibility limitations. SPAN films present high transparence in the visible range of spectra and ohmic contacts with metal and TO electrodes. PMT films have a high absorption coeficient in the visible range of the spectrum and have good stability at environmental condictions. Our devices were made in the TO/SPAN/PMT/Al sandwich structure, with differents thickness of PMT films, whereas the SPAN film thickness was mainted constant, 120 nm. The measurements were made with illumination through TO and Al electrodes. These devices presented an increase of efficiency with the increase of PMT films thickness till 171 nm. After this value the efficience started to decrease. The best results were obtained with a PMT thickness of 171 nm and illumination through TO electrode: incident photon to collected electron efficiency of 18 %, power conversion efficiency of 1.2 %, open circuit voltage of 0.71 V, short-circuit current density of 0.16 A/m2 and fill-factor of 31%. (at monochromatic radiation of = 580 nm and I = 2.3 W/m2, for all data).
NMR CHARACTERIZATION OF THE LECTROLUMINESCENT MATERIAL Poly[(9,9-dihexylfluorenyl-2,7-diyl)-co-(1,4-vinylenephenylene)]
L. Akcelrud, Departamento de Química – UFPR, A. Glogauer, Departamento de Química - UFPR , A. R. M. de Oliveira, Departamento de Química - UFPR, R. Ferracin, Instituto de Tecnologia para o Desenvolvimento - LACTEC and A. G. Ferreira Departamento de Química – UFSCar
Electroluminescent polymers are macromolecules that can emit light when an electrical field is imposed, in the same way as conventional inorganic semiconductors or the more recent low molecular weight organic molecules do. Among the additional advantages found with polymers for potential technological applications is the possibility of fabrication of large area displays by means of the usual techniques of thin film processing as dipping or spin coating. The synthesis of these polymers is much more involved than those employed in conventional polymers, however. In the majority of cases the monomers are not available commercially and therefore have to be prepared in a series of steps prior to the polymerization. The characterization of these monomers, as well as of the final product is a most important issue, since a high regularity is a main requirement for the electro-optical performance of the electroluminescent polymer. In this communication we describe briefly the synthesis, and the NMR characterization of poly[(9,9-dihexylfluorenyl-2,7-diyl)-co-(1,4 vinylenephenylene)] which structure is similar to the extensively explored poly(phenylene vinylene) (PPV) with an additional fluorene moiety, which should impart better optical and thermal resistance characteristics.
LACTEC, UFPR, CNPq/ Instituto do Milênio em Materiais Poliméricos – IMMP, CAPES.
exciton formation IN OLEDS as observed by EDMR
G. B. Silva, C. F. O. Graeff, DFM-FFCLRP-USP, Av. Bandeirantes 3900, 14040-901 Ribeirão Preto, Brasil; F. Nüesch, L. Zuppiroli, LOMM-IMX-EPFL, CH-1015 Lausanne, Switzerland.
In this work the injection, transport and recombination properties of tris-8-hydroxynolate-aluminum (Alq3) diodes in different structures, such as ITO/CuPc/-NPD/Alq3/LiF/Al, ITO/BAC10/Alq3/-NPD/Ag and Al/LiF/Alq3/LiF/Al, were investigated by electrically detected magnetic resonance (EDMR). In an EDMR experiment, microwave-induced changes in the conductivity are measured as the sample is subjected to a swept dc magnetic field. The measurements were done using a X-Band (9 GHz) and a K-Band (24 GHz) spectrometer at room temperature. In the case of CuPc/-NPD/Alq3 device, the EDMR signal is quenching, typically 10-5 in amplitude and is composed by two gaussian lines. The predominant line has a linewidht of 16 G and its g-factor decreases from 2.0046 to 2.0041, as the voltage increases, while the second one has a g-factor = 2.0035 and its linewidth increases from 20 to 35 G. The spin dependent process observed in these measurements is assigned to exciton formation due to spin-1/2 particles resonance. In this case the process occurs through two molecules: a cation (Alq+ or NPD+) and an anion (Alq-). Thus, the predominant line arises from cations and the broad gaussian line from anions. The EDMR signatures of cations and of anions are also found at ITO/BAC10/Alq3/-NPD/Ag and at Al/LiF/Alq3/LiF/Al devices, respectively, in which only holes or electrons are injected into the organic layers.
MOLECULAR ORGANIZATION IN LANGMUIR-BLODGETT FILMS OF PPV DERIVATIVES
M. Ferreira, D. T. Balogh, C. C. Olivati,R. M. Faria and O. N. Oliveira Jr., IFSC/USP, 13560-970, São Carlos/SP, Brazil; Ricardo F. Aroca, University of Windsor, N9B 3P4, Windsor/On, Canada; Carlos J.L. Constantino, DFQB/FCT/UNESP, 19060-900, Presidente Prudente/SP, Brazil
The molecular organization of poly[(2-methoxy-5-n-hexyloxy)-p-phenylenevinylene] (OC1OC6-PPV) and poly-[(2-methoxy-5-n-octadecyl)-p- phenylenevinylene (OC1OC18-PPV)in Langmuir-Blodgett (LB) films have been investigated using FTIR spectra recorded for both transmission and reflection modes considering the surface selection rules and some key vibrational modes. The bands at 970 cm-1 (vinylic C-H wagging) and 854 cm-1 (aromatic C-H wagging), whose induced molecular dipoles are perpendicular to the plane containing the aromatic and vinylic groups (in a planar trans form), are stronger in the transmission mode and weaker in the reflection mode. This shows that OC1OC6-PPV molecules must be anchored to the substrate by the side chain with the aromatic ring practically perpendicular to the substrate. This is confirmed by the Raman spectrum in which a distortion of the vinylene group was observed and by surface enhanced fluorescence (SEF) on an LB monolayer deposited onto Ag nanoparticles. On the other hand, the OC1OC18-PPV has presented a random molecular organization in the LB films. Acknowledgements: FAPESP, CNPq, IMMP/MCT.
THIN FILM OF CARBON NITRIDE AND SILICON NITRIDE AS A BARRIER COATING FOR PLEDs USING LOW TEMPERATURE PLASMA DEPOSITION
A.M. Nardes, E.A.T. Dirani, A.M. Andrade, F.J.Fonseca, EPUSP, Universidade de São Paulo, São Paulo, SP, Brazil; R.Bianchi , IFSC, Universidade de São Paulo, SP, Brazil; FMFT, Pontifícia Universidade Católica de São Paulo, SP, Brazil
Light-emitting diodes based on conjugated polymers (PLEDs) have potentially significant future applications in displays. The stability of devices can be improved by stripping off oxygen and moisture vigorously. Based on that, a PLED must be encapsulated to reduce failure due to moisture or oxygen in the system. Silicon nitride and carbon nitride are widely used in many electronic devices of compound semiconductors (primarily in GaAs and InP) manufacturing for the wireless applications, including devices passivation of metal-semiconductor field effect transistors, high electron mobility transistors (HEMTs), hetero-junction bipolar transistors (HBTs), masking layers, dielectric layers for metal-insulator-metal (MIM) capacitors and encapsulation to protect against the environmental induced degradation or mechanical scratches. In this work a PECVD system was used to deposit silicon and carbon nitrides at low deposition temperatures (around 100ºC) in order to encapsulate PLEDs. Mechanical and optics results concerning the films and electrical characterization concerning the polymeric device are show. Preliminary tests indicate that the film is sufficiently impermeable to moisture and oxygen for application to moisture-sensitive display applications.
Conversion reaction of a polysulfonium precursor into poly
(p-phenylenevinylene) films studied by ellipsometry spectroscopy
R.F. Bianchi, D. Gonçalves, R.M. Faria, Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos - SP, Brazil, E.A. Irene, Department of Chemistry, University of North Carolina, CB #3290, Venable Hall, 27599-3290, Chapel Hill - NC, USA
Poly(p-phenylenevinylene) (PPV) and derivatives have been extensively investigated as active layers in light-emitting devices (LEDs) mainly because of a wide range of emission colors and ease of processing. The precursor route is the most commonly used method of preparing PPV films. Flexible PPV films can be promptly obtained by thermal conversion of a water-soluble precursor, poly-p-xylylidene tetrahydrothiophenium chloride, PTHT. In the present study, our attention focused on the optical properties of PTHT and PPV films on Au-coated substrates as measured by spectroscopic ellipsometry (SE). Fourier transform infrared spectroscopy (FTIR) was used to verify structural differences in the polymer chain such as C=C groups (due to an increase of conjugation) and C=O (due to effects of degradation) groups. Fapesp and Millennium Science Initiative/MCT sponsored this work.
INFLUENCE OF THE OPTICAL EXCITATION ON THE ELECTRICAL PROPERTIES OF POLYMERIC LIGHT-EMITTING DIODES
R.F. Bianchi, E.L. Queiroz, L. F. Santos, F.T. Reis and R.M. Faria, Instituto de Física de São Carlos / USP, CP 369, 13560-970, São Carlos, SP, Brazil
Since the electroluminescent properties of poly(p-phenylenevinylene) (PPV) were first reported in 1990, this polymer and its derivatives have been used as active materials in light-emitting devices. Although a large number of papers have focused on the electrical properties of PPV, the transport and injection mechanisms involved are still not completely understood. Frequency-dependent conductivity is a powerful technique that can be used to investigate bulk conduction and injection processes in polymeric electronic devices. In this work we have observed the influence of optical excitation on the complex conductivity of a PPV derivative, the poly[(2-methoxy-5-hexyloxy)-p-phenylenevinylene] (MH-PPV), in ITO/MH-PPV/Al and Au/MH-PPV/Au structures. The measurements were carried out at room temperature, under vacuum and illumination intensities varying between 0 and 250 mW/cm2. The results were interpreted using the random free energy barrier model, which is based on a random distribution of energy potential barriers in a disordered system. The fitting parameters have shown that the metal/polymer interfaces act as a light intensity dependent RC circuit in series with the polymer bulk. This work is sponsored by FAPESP, Brazil
AFM STUDIES OF LANGMUIR-SCHAEFER AND LANGMUIR-BLODGETT POLY(5-AMINO–1-NAPHTHOL) FILMS
R.L. Moreira, B.R.A. Neves, C.P.L. Rubinger and L.A. Cury, Depto. de Física, UFMG, C.P. 702, 30123-970 Belo Horizonte, MG, Brazil; C.A. Ferreira and A. Meneguzzi, Depto. de Materiais, UFRGS, 90035-190 Porto Alegre, RS, Brazil
In this work we used atomic force microscopy (AFM) to show that ultrathin Poly(5-amino-1-naphthol) (PAN) films can be transferred onto Si substrates through Langmuir-Schaefer (LS) technique or by combining this technique with the Langmuir-Blodgett (LB) one. Since potential applications of this material are in the fields of molecular electronics and anti-corrosive coatings, well adhered films of controlled thickness are suitable. The AFM tip was used to perform nano-indentation nano-delamination of the polymeric film. By adapting a model for indentation of bulky continuous elastic polymers to our nanometric-scale system, we could estimate the critical interfacial shear strength for the PAN/Si interface. The obtained results indicate that the films are indeed well adhered to the substrate. We also show that multilayer PAN films obtained by an appropriated association of the LS and LB methods were very smooth and uniform, with approximately 5 nm thickness per transferred layer.
ABSORPTION AND EMISSION LINE SHAPE OF POLY(P-PHENYLENE VINILENE) THIN FILMS: AN EXPERIMENTAL AND THEORETICAL STUDY.
M. Veríssimo-Alves, DF, Universidade Federal de São Carlos, CP 676, 13565-905, São Carlos, SP; A. Marletta, FaFis, Universidade Federal de Uberlândia, CP 593, 38400-902, Uberlândia, MG; R. M. Faria, F. E. G. Guimarães, IFSC, Universidade de São Paulo, CP 369, 13660-970, São Carlos, SP
Optical properties of PPV films depend on the processing techniques and chemical synthesis routes. Molecular packing and structural defects can change electron-phonon coupling and reduce the effective conjugation length N of the polymeric chain. Here we present experimental and theoretical investigations on the absorption and emission line forms of PPV films produced by the cast, spin-coating (SC), self-assembly (SA) and Langmuir-Blodgett (LB) techniques.
Absorption spectra are well described by a gaussian distribution of N, with 3 > N > 15. SC, SA and LB films, processed at 110oC, show a shift of the center of the distribution to larger N, and better resolution of emission spectra than films produced by the cast technique. The dependence of the band gap Eg and the dipole moment µ on N are obtained through DFT calculations for isolated PPV chains up to N = 12 using the SIESTA method. Vibrational frequencies and eigenmodes of the oligomers are obtained in order to calculate the dependence of the Huang-Rhys factor on the chain length. Using the theoretical functional forms for these parameters, we fit the absorption spectra and discuss the influence of packing in thin PPV films.
J. A. Freire, Departamento de Física, Universidade Federal do Paraná, Caixa Postal 19044, 81531-990, Curitiba-PR, Brazil.
The mobility of charges in disordered organic materials, measured by time-of-flight experiments, often exhibit an exponential dependence with the square-root of the applied field besides a temperature dependence consistent with a thermally activated process. This is observed in a wide variety of materials and the physics behind the field dependence is still not very clear. It is widely believed that in these materials the transport occurs via thermally activated hops between localized electronic states whose energy is not well defined due to conformational and positional disorder of the molecules. Some attempts to explain the observed field dependence have relied on the charge interaction with the random dipoles present in the material or in the existence of correlations between the molecular orbital energies while assuming a simple Arrhenius form for the hopping mechanism. Here I will discuss the possibility of this dependence be due to the formation of polarons. This will be done in the context of a master equation where the hopping rate between sites takes the form of the small polaron hopping rate of the Holstein model.
HIGHLY DISPERSIVE TIME-OF-FLIGHT SIGNALS IN DISORDERED ORGANIC MATERIALS
J. A. Freire, Departamento de Física, Universidade Federal do Paraná, Caixa Postal 19044, 81531-990, Curitiba-PR, Brazil.