D.D.C. Bradley

Light-emitting diodes based on conjugated polymers

CONJUGATED polymers are organic semiconductors, the semiconducting behaviour being associated with the π molecular orbitals delocalized along the polymer chain. Their main advantage over non-polymeric organic semiconductors is the possibility of processing the polymer to form useful and robust structures. The response of the system to electronic excitation is nonlinear—the injection of an electron and a hole on the conjugated chain can lead to a self-localized excited state which can then decay radiatively, suggesting the possibility of using these materials in electroluminescent devices. We demonstrate here that poly(p-phenylene vinylene), prepared by way of a solution-processable precursor, can be used as the active element in a large-area light-emitting diode. The combination of good structural properties of this polymer, its ease of fabrication, and light emission in the green–yellow part of the spectrum with reasonably high efficiency, suggest that the polymer can be used for the development of large-area light-emitting displays.

Electroluminescence in conjugated polymers

Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic specifications for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Poly(p‐phenylenevinylene) light‐emitting diodes: Enhanced electroluminescent efficiency through charge carrier confinement

We have fabricated light‐emitting diodes with poly(p‐phenylenevinylene) as the emissive layer, and with an electron‐transporting layer formed from a solid state dispersion of 2‐(4‐biphenylyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole in poly(methyl methacrylate), placed between this and the negative electrode. These structures show typically a tenfold improvement in efficiency in the low‐voltage regime and an eightfold improvement in the high‐voltage regime over devices without the electron‐transporting layer. Typical efficiencies are about 0.8% photons/electron. We consider that the role of the electron‐transport layer is to confine holes to the emissive layer.

The photovoltaic response in poly(p-phenylene vinylene) thin-film devices

We report measurements of the photovoltaic effect in diode structures formed with thin films (100 nm) of the conjugated polymer poly(p-phenylene vinylene), PPV, sandwiched between electrodes of indium/tin oxide, ITO, and either aluminium, magnesium or calcium. Under illumination incident through the ITO contact, large open-circuit voltages were measured, which saturated at approximately 1.2 V for Al and Mg devices, and approximately 1.7 V for Ca devices. Quantum efficiencies (short-circuit current/incident photon flux) of order 1% were measured at low intensities (0.1 mW cm-2). The spectral response of the photocurrent demonstrates that photon absorption near the electron-collecting electrode optimizes the photocurrent, indicating that device performance is limited by low electron mobilities in the bulk PPV. The photocurrent exhibits a weak temperature dependence, with an activation energy that is a function of the electric field in the polymer. We have used these measurements to estimate an exciton binding energy in PPV of approximately 0.4 eV.

Optical spectroscopy of highly ordered poly(p-phenylene vinylene)

The authors report a study of the photophysical properties of poly(p-phenylene vinylene), PPV, prepared in a way that gives an especially high degree of intrachain order. Optical absorption, photoluminescence, photoinduced absorption, and photoconductivity excitation spectra are presented and compared to data reported for less well ordered PPV. Spectral red shifts, sharpening of spectral lines, and a transfer of oscillator strength into the vibronic ground states of the electronic transitions are observed. Photoinduced absorption due to long-lived charged excitations, previously reported for less ordered PPV, could not be detected in this material. Photoconductivity excitation spectra show a steep rise at the absorption edge with no appreciable offset between the onsets for photoconduction and absorption. A very slow photocurrent component is observed, which the authors associated with the trapping and subsequent thermal release of photocarriers.

Conformational effects in poly(p-phenylene vinylene)s revealed by low-temperature site-selective fluorescence

Low-temperature site-selective fluorescence (SSF) spectroscopy is employed to study morphological effects on the conformation of poly(p-phenylene vinylene) (PPV) and its phenyl-substituted, soluble derivative poly(phenylphenylenevinylene) (PPPV). Samples of PPV prepared as spin-coated thin films and stretch-aligned free-standing films, and samples of PPPV prepared as cast films and as blends with poly(methylmethacrylate) and polycarbonate have been studied. The results that the authors present are considered with the notion that each polymer sample consists of an array of ordered chain segments whose average length reflects the perfection of the local structure. The statistical distribution of the segment lengths is responsible for inhomogeneous broadening of the optical spectra (absorption and emission). The dominant electronic excitation created by photoexcitation across the pi - pi * energy gap is a singlet exciton that can execute a random walk among the chain segments. SSF spectroscopy allows the authors to distinguish the contributions to the apparent fluorescence Stokes shift that arise from energy relaxation through excitation migration (spectral diffusion) and from structural relaxation of the polymer chain (self-localization). The structural contribution to the Stokes shift approaches zero in well aligned PPV and reaches values of up to 500 cm-1 in highly disordered PPPV films. The SSF method also provides a means of assessing the extent of phase separation that occurs in PPPV blends.

High electron affinity polymers for LEDs

Summary form only given. A new series of alternating copolymers based on poly(p-phenylenevinylene) (PPV) was synthesized. The electron affinity of the polymers is substantially increased through the use of electron-withdrawing substituents. Optimum conditions for the condensation polymerization were explored, and polymers with a M/sub n/ in excess of 30,000 were produced. The synthesis is extremely flexible and allows a large variety of conjugated polymers with different emission properties to be synthesized. Cyclic voltammetry was used to estimate the position of the HOMO and LUMO relative to poly(2,5-dialkoxy-1,4-phenylenevinylene) analogues. Fabrication of efficient bilayer polymer LEDs using PPVin conjunction with the new polymers and stable metal negative contacts such as aluminum and gold is reported.

OPTICAL-SPECTRA AND EXCITATIONS IN PHENYLENE VINYLENE OLIGOMERS

Abstract Experimental results of optical absorption spectra, photoluminescence spectra due to singlet exciton emission and photoinduced absorption spectra due to triplet-triplet transitions of triplet excitons for several oligomers of poly( p -phenylene vinylene) are reported. From the red-shift of the peaks with increasing chain length, the effective conjugation length for long-chain precursor-route samples of the polymer are estimated to be about 10 to 17 repeat units. Theoretical studies, using the valence-effective Hamiltonian (VEH) technique, of the effects of ring torsions upon the electronic properties are also reported. Comparison between absorption and luminescence spectra in solution and in the solid state shows a blue-shift for the solution spectra; this is modelled in terms of a greater degree of ring torsion for these oligomers in solution.

Electroluminescence from multilayer conjugated polymer devices: Spatial control of exciton formation and emission

Abstract We have constructed electroluminescent diodes using several layers of conjugated polymers with differing band gaps; these provide a range of different colour light-emitting layers and can be used to control charge injection and transport. Poly(1,4-phenylenevinylene, PPV, and derivatives have been used, with indium/tin oxide as hole-injecting layer and calcium as electron-injecting contact layer. For this selection of materials, we show that the ordering of the polymer layers allows control of the colour of device emission. Emission can be produced in more than one layer.

Optical spectroscopy of triplet excitons and charged excitations in poly (p-phenylenevinylene) light-emitting diodes

Abstract Sub-gap absorption is measured under forward drive conditions in electroluminescent devices fabricated from thin films of poly( p -phenylenevinylene) sandwiched between electrodes of indium/tin oxide and calcium. Quantitative concentrations of excitations are obtained. Absorption seen at 1.37 eV is due to an allowed transition to a higher-lying state of the excitons formed by capture of positive and negative charges in the polymer. Absorption bands at 0.65 and 1.55 eV are assigned to charged bipolarons which do not directly participate in the electroluminescence process.