One example of organic electronics is the application of polymer based light emitting devices (PLEDs). PLEDs are very attractive for large area and fine-pixel displays, lighting and signage. The polymers are more amenable to solution processing by printing techniques which are favourable for low cost production in large areas. With phosphorescent emitters like Ir-complexes higher quantum efficiencies were obtained than with fluorescent systems, especially if multilayer stack systems with separated charge transport and emitting layers were applied in the case of small molecules. Polymers exhibit the ability to integrate all the active components like the hole-, electron-transport and phosphorescent molecules in only one layer. Here, the active components of a phosphorescent system – triplet emitter, hole- and electron transport molecules – can be linked as a side group to a polystyrene main chain. By varying the molecular structures of the side groups as well as the composition of the side chains with respect to the triplet emitter, hole- and electron transport structure, and by blending with suitable glass-forming, so-called small molecules, brightness, efficiency and lifetime of the produced OLEDs can be optimized. By choosing the triplet emitter, such as iridium complexes, different emission colors can be specially set. Different substituted triazine molecules were introduced as side chain into a polystyrene backbone and applied as electron transport material in PLED blend systems. The influence of alkyl chain lengths of the performance will be discussed. For an optimized blend system with a green emitting phosphorescent Ir-complex efficiencies of 60 cd/A and an lifetime improvement of 66.000 h @ 1000 cd/m2 were achieved.
Organic solar cells are a favorable alternative to their inorganic counterparts because the functional layers of these devices can be processed with printing or coating on a large scale. In this study, a novel absorber polymer was synthesized, blended with fullerene and deposited with inkjet printing for solar cell applications. A fluorene based terpolymer with dialkyl substituted diphenyl-dithienylbenzopyrazine and triphenylamine units was synthesized by Suzuki polymerization with high molecular weights. The introduction of dialkyl substituted diphenyldithienylbenzopyrazine in the fluorene main chain leads to LUMO-energy level of -3.1 eV and to an open circuit voltage of 0.96 V in solar cells. All the requirements were fulfilled to achieve absorber polymers with high efficiencies including a HOMO energy level which is lower than -5.2 eV, a band gap in the range of 1.3-1.9 eV and a hole mobility in OTFTs greater than 1 x 10-3cm2/Vs. Solar cells with printed layers were compared to those with spin coated films in order to evaluate inkjet printing as a thin film deposition method. Efficiency values of 3.7% were found for devices with inkjet printed layers or spin coated layers when using chlorinated solvents. In order to be able to use inkjet printing on a large scale, hazardous, chlorinated solvents should be avoided when depositing the functional materials. Anisol/tetralin was used as an alternative solvent system. It was found that devices prepared from the chlorine-free system showed only slightly lower efficiencies of 2.7% with respect to the chlorinated system. A coarser phase separation was found with energy filtered transmission electron microscopy plasmon mapping which most likely resulted in the performance differences for the chlorinated and chlorine-free solvent systems. This paper, originally published on 17 October 2013, was replaced with the correct version on 30 January 2014. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
The aim of this work is to develop some new polymer materials with typical n-type semiconducting properties and low reduction peak potentials. Therefore new organo-soluble copolyquinoxalines were used in polymer blends with poly-[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenvinylene] or regioregular poly-(3-hexylthiophene). The photovoltaic properties of these polymer blends depend on the blend morphology. So the short circuit current density can be improved by better solvent mixtures and by temper processes. But nevertheless the photovoltaic properties are low and a stable and well ordered phase morphology is difficult to obtain with polymer blend systems. Therefore another approach to receive more effective full polymer photovoltaic cells was done. Block copolymers consisting of n-type and p-type sequences in one polymer chain were synthesized. As n-type material a quinoline monomer and as p-type material 3-hexylthiophene were used. The synthesis of these new materials is described. The spectroscopic and cyclovoltammetric investigations clearly indicate their block copolymer structure. The study of their phase morphology and photovoltaic properties is in progress.
It is well known of regioregular poly(3-hexylthiophene) (P3HT) to self-assemble on hydrophobic surfaces, because the regular arrangement of its side chains allows an efficient π-stacking of the conjugated backbones. This should be the reason for the very high reported field effect mobilities (0.2 cm2/Vs)1. We present an alternative approach to increase the field effect mobility and the transistor stability by introducing a strong acceptor dopant in the main chain of P3HT while preserving the regioregularity of the 3-hexylthiophene segments in the polymer chain. P3HTs with different contents of acceptor molecules which are fixed linked in the main chain of the polymer, were synthesized using the McCullough Grignard metathesis method. As acceptor unit has been integrated 9-dicyanomethane-fluorene. The introduced dopant amount has been varied in order to obtain an optimum between the processability of the polymers and the resultant transistor performance. The utilized organic field effect transistor (OFET) substrates with SiO2 gate dielectric were always pre-treated with a silylating agent (HMDS) to facilitate the self-organization properties of the polymers by hydrophobization of the SiO2 surface. The optimized doped structures showed higher field effect mobility by one order of magnitude compared to the conventional P3HT, (μFE~9 x 10-3 cm2/V.s for the doped compound vs. μFE~5 x 10-4 cm2/V.s for P3HT), combined with a marked current modulation with ON/OFF ratio of ~ 7 x 103 and a better operational stability of the resultant OFET-devices.
The aim of this work is to develop some new polymer materials with typical n-type semiconducting properties and low reduction peak potentials. Therefore new synthetic routes are presented leading to organo-soluble polyquinolines and polyquinoxalines. The results of the synthesis and characterization of the obtained new organo-soluble polymers are shown. The electrochemical reduction and oxidation behavior of these polymers is studied by cyclovoltammetric measurements. All studied polymers can be reversibly oxidized and reduced. In addition the absorption and luminescence properties are investigated. Polymer/polymer solar cells are prepared from blends of the new polyquinoxalines and poly-[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenvinylen] as active single layer. First results are discussed.
A unusual way is presented to obtain a new class of deep red emitting polymers. Polyurethanes with covalently attached fluorescent dyes were developed. The DCM-dye seems to be a favorable candidate but it has no reactive groups for linking into a polymer structure. DCM can be synthesized by the monofunctional addition of (2,6-dimethyl-4H-pyrane-4-ylidene)-malononitrile with 4-dimethylaminobenzaldehyde. We realized a bifunctional condensation of the pyrane with N-methyl-N-(2-hydroxyethyl)-4-aminobenzaldehyde to enlarge the conjugated system and to shift the emission maximum to more than 650 nm. Simultaneously we introduced two reactive hydroxy terminal groups into the dye molecule. Using this functionality we were able to synthesize several new polyurethanes with covalently attached DCM dye in the main chain. By co-condensation with non-dye molecules like N,N-bis-(2-hydroxyethyl)-aniline or butan-1,4-diole the dye content in the main chain can be varied and the influence of the absorption and emission behavior can be studied. Red emitting device structures were realized and some of the device properties will be given. It will be shown that the stability and the lifetime of the device can be increased by simple structure modification of the polyurethane, e.g. alkylation of the urethane groups or the change of the co-components.