Energy and Charge Transfer in Electroluminescent Polymer/Porphyrin Blends
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ABSTRACT We present a study of the effects of blending electroluminescent polymers with platinum (II) octaethylporphyrin (PtOEP). We find that in the case of polymers which are measured to have HOMO and LUMO levels respectively below and above those of the PtOEP, and which have emission spectra overlapping the PtOEP absorption spectra, energy transfer occurs as expected. We find further evidence, in the form of steady state and time-resolved electroluminescence and photoluminescence measurements, which indicates additional transfer of triplet excitons between polymer and porphyrin. Where the polymers have emission spectra overlapping the absorption spectra of PtOEP, but which are measured to have a HOMO or LUMO level between those of the porphyrin, quenching of the photoluminescence efficiency occurs. We propose this is due to charge separation between the porphyrin and the polymer, and show evidence for this in the form of photoinduced absorption measurements. INTRODUCTION Electroluminescent polymers have been the focus of much research interest since their discovery almost ten years ago 1. The chemistry required to make such polymers has come a
long way, and the spectral range covered by the emission from the materials now covers most of the visible spectrum. The problem of achieving particular colours, for instance for display applications, has led to some investigation into the possibilities of blending electroluminescent materials with other emissive molecules 2. Energy transfer from the polymers to the emissive molecules enables more accurate tuning of the resulting emission from the blend. This work is concerned with the inclusion of the triplet-emitting porphyrin, platinum octaethylporphyrin (PtOEP), which has been used as a molecular dopant in electroluminescent systems 3,4, in blends with several electroluminescent polymers. The effect of the dopant on the blend is evaluated in each case, and we find examples of singlet and triplet energy transfer, and charge transfer, in different systems. We support our predictions for each system with time-resolved and steady state photoluminescence (PL) and electroluminescence (EL) and photoinduced absorption measurements.
EXPERIMENTAL LEDs were fabricated using pre-etched and cut 12mm-square ITO-coated glass substrates with a resistance of approx. 30K2/square. Initial cleaning of the substrates involved submersing them in acetone, followed by isopropanol in an ultrasound bath. This was followed by oxygenplasma treatment in an Alcatel ARF301 plasma asher, with incident power of approx. 270W, and 325 Mat. Res. Soc. Symp. Proc. Vol. 558 ©2000 Materials Research Society
reflected power of less than 5W. All subsequent steps in device fabrication were carried out under a dry nitrogen atmosphere, containing typically less than 5ppm oxygen, and 20ppm water. Solutions were spin-cast onto the substrates at spin speeds of approx. 2000rpm, to give dry films of approximately 100nm after 60 seconds of spinning. The spin casting of the polymer layer(s) was followed by depositio
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