Ramesh K. Kasim

Poly(alkyl thiophene-3-carboxylates). Synthesis, properties and electroluminescence studies of polythiophenes containing a carbonyl group directly attached to the ring

Improved synthetic methodology for poly(3-hexyl- and 3-octyloxycarbonylthiophene-2,5-diyl) (1a and 1b) is reported. n =6700 and 9400 ( w / n =2.5 and 3.2), λ max for fluorescence emission=600 and 610 nm and λ max for electroluminescence=600 and 615 nm, for 1a and 1b respectively. The 1 H NMR spectra required that pentads be considered to explain the spectra. That is the four nearest neighbours to a given ring influence the 1 H NMR spectrum. Electroluminescence efficiencies of 0.016% and 0.018% were observed for devices made from 1a and 1b, respectively. A bilayer device of ITO/poly(3-octylthiophene)/1b/Al emitted at 646 nm, the same wavelength where poly(3-octylthiophene) itself emits. The efficiency was low but was an order of magnitude greater than for poly(3-octylthiophene) itself. Regioregular (HH-TT) poly(4,4′-bis(hexyl- and octyloxycarbonyl)[2,2′-bithiophene]-5,5′-diyl) (3a and 3b) were also prepared via the Ullmann reaction and n =7900 and 11000 respectively. Films of 3a and 3b were yellow in color and showed λ max =377 and 381 nm respectively, about 55-80 nm blue shifted compared with 1a and 1b. This is due to the large rotational barrier in the HH dyads which reduces the effective conjugation length in 3a and 3b. 3a and 3b showed bright fluorescence and electroluminescence with emission of yellow light. Electroluminescence efficiencies were 8.5×10 –3 % and 4.7×10 –3 %, respectively.

Investigation of device failure mechanisms in polymer light-emitting diodes

Abstract A polymer light-emitting diode (LED) of configuration —Indium-Tin-Oxide (ITO)/polymer/metal was used for the study of failure mechanisms using optical microscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Better lifetimes were observed for LEDs that were operated at lower voltages. At constant voltage, poly(octylthiophene-3-carboxylate) (POT-3-C) diodes exhibited a sharp decay followed by a gradual decrease with time in total current passing through the device and corresponding absolute light intensity. With the application of voltage, metal electrode surface was observed to lose reflectivity and become dull, leading to increased surface roughness of the film. At high electric fields, bright spots appeared which eventually turned dark and transparent-like. XPS studies of failed devices indicated that oxidation of both metal electrode and polymer film occurred. SEM micrographs of failed devices revealed discontinuities in the metal electrode and polymer film. Post-failure surface composition maps of ITO showed destroyed ITO film exposing bare silicon oxide. Similar failure patterns were observed in the failure of poly(p-phenylene vinylene) (PPV) LEDs under high electric fields.

Poly(alkyl thiophene-3-carboxylates). Synthesis, properties and electroluminescence studies of a polythiophene with a carbonyl group attached to the ring

Abstract Improved synthetic methodology for poly(hexyl and octyl thiophene-3-carboxylates) (la and lb) is reported. n = 6700 and 9400 (M w /M n = 2.5 and 3.2), λ max for fluorescence emission = 600 and 610 nm and for electroluminescence = 600 and 615 nm, for la and lb respectively. The 1 H NMR spectra showed more aromatic peaks than can be explained using triads, and it is suggested that pentads, involving longer range influences, must be considered to explain the spectra. Electroluminescence efficiencies of ca. 0.02% were observed for devices made from la and b. A bilayer device of ITO/poly(3-octylthiophene)/lb/Al emitted at 646 nm, where poly(3-octylthiophene) emits. The efficiency was low but was an order of magnitude greater than for poly(3-octylthiophene) itself.

Synthesis and Electroluminescence Study of a Novel Copolymer:Poly(Phenylene Vinylene-Co-Quinoline Vinylene)

High-efficiency polymer light-emitting diodes (LEDs) are often fabricated using multilayered structures with separate carrier transport and light emission layers. Recently, we reported on the synthesis and electroluminescence (EL) characteristics of poly(2,6-quinoline vinylene) (PQV) and its potential for use as an electron transport layer in poly(phenylene vinylene) (PPV) LEDs. To take advantage of the high emission efficiency of PPV and electron accepting ability of PQV, a copolymer of PPV and PQV, poly(phenylene vinylene-co-quinoline vinylene) (PPVQV) was synthesized via the precursor polymer route and converted to the conjugated form by thermal elimination. When used as the emissive layer with Indium-Tin Oxide (ITO) and aluminum as positive and negative electrodes respectively, PPVQV emitted blue light at an onset electric field of 1.05x 10 6 V/cm and emission efficiency of 0.08%. Improved efficiencies of the order of 0.15% were obtained when blends of copolymer with PPV were used in conjunction with PPV in a multi-layered structure. Along with copolymer chemical characterization data, results from EL studies on single and multi-layered devices are discussed. We also report on a simple and costeffective chemical deposition of silver for the negative electrode in polymer LEDs.

Synthesis and characterization of poly(2,6-quinoline vinylene)

Abstract A synthetic route for a new conjugated polymer, poly(2,6-quinoline vinylene) ( PQV ) is presented. The UV-Vis absorption spectrum of polymer showed λ max at 430 nm and the fluorescence maximum was observed at 515 nm by using 430 nm excitation wavelength. Polymer light-emitting diodes (LEDs), constructed with ITO/PQV/A1 configuration, showed broad emission with λ max at 530 nm.

Forward/reverse dc bias or ac electroluminescence in poly(p-phenylene vinylene) prepared via sulfonium-salt and xanthate routes

Abstract In light-emitting diodes (LEDs) of standard configuration — Indium-Tin-Oxide (ITO)/polymer/metal, we observed light emission under ac or forward/reverse dc bias, using poly(p-phenylene vinylenes) prepared via the sulfonium salt (S-PPV) or xanthate (X-PPV) routes as active electroluminescent layers. S-PPV devices exhibited approximately symmetrical current-voltage characteristics under forward and reverse dc bias with the same onset voltage. The electroluminescence (EL) spectra either under a dc forward/reverse bias or ac were identical. X-PPV showed EL under reverse bias at higher voltage than in forward bias. Symmetrical current-voltage characteristics in SPPV devices is attributed to ionic impurities present in the polymer.

Synthesis and properties of regioregular poly(dialkyl [2,2'-bithiophene]-5,5'-diyl-4,4'-dicarboxylates)

Abstract Regioregular (HH-TT) polythiophenes with electron withdrawing ester groups attached directly to the ring, namely poly-dihexyl and dioctyl [2,2′-bithiophene]-5,5′-diyl-4,4′-dicarboxylate) ( 2a and 2b ) have been prepared via the Ullmann reaction of 3a and 3b with copper in DMF, Number average molecular weights (GPC/polystyrene standards) of 7,900 and 11,000 respectively were obtained. Cast films of 2a and 2b were yellow in color and showed UV-vis absorption at 377 and 381 nm respectively, about 55–80 nm blue shifted compared with regiorandom poly(alkyl thiophene-3-carboxylates). Quantum mechanical calculations showed that the head-to-head dyads induce regular interruption of the coplanarity of the polymer backbone. Both polymers showed bright fluorescence and electroluminescence with emission of yellow light. External electroluminescence efficiencies were 8.5 and 4.7 × 10 −3 %, respectively.

Poly(3-alkylthiophenes): Optimising conductivity as a function of regioregularity, dopant and casting solvent

Thiophene and 3-methylthiophene as casting solvents are shown to improve the conductivity of cast films of regioregular and regiorandom poly(3-octylthiophenes). The trend is observed from same batch samples using different dopants.