Improvements in Electroluminescent Polymers
Introduction: Electroluminescent Polymers
Polymers (i.e. plastics) have revolutionized the way society lives. They exhibit excellent physical and mechanical properties coupled with an ease of processing that have allowed for their use in a multitude of applications. Typical polymers exhibit insulating characteristics, which in many applications is a desirable trait. However, in the past three decades much effort has been focused on creating a semiconductive polymer in the hope being able to leverage the advantages of common polymers.
In arriving at a semiconductive polymer, the majority of attention has been centered on conjugated polymers. Conjugated polymers have been found to have the necessary metallic and electrical characteristics required of a semiconductor. In comparison with a normal polymer, conjugated polymers possess a delocalized pi-electron system along the polymer backbone. It is the delocalized pi-electron system that allows the polymer to support negative and positive charge carriers along the chain.
Figure 1: Formation of pi-electron cloud due to Pz orbital overlap1
The first use of conjugated polymers was as conductors in applications varying from battery electrodes to long-term stable polymer capacitors. However, in the late nineteen-eighties a group headed by Prof. Richard Friend of Cambridge University, UK discovered a new application for these polymers, namely as an electroluminescent device. His work showed that the semiconductive conjugated polymer poly (p- phenylenevinlene) (PPV) showed electroluminescent characteristics if an appropriate choice of contact layers was made. EL polymers promise to allow the creation of usable displays at much lower costs than possible with current flat-panel display technologies (e.g. LCD).
Figure 2: Polymer LED device configuration2
Although huge gains have been made in reliability, efficiency, and usability of EL polymers since their discovery, the entire area of EL polymer research is still in its infancy and some of the problems yet to be overcome include:
Improving Processing and Efficiency Characteristics via New Materials
Article #1 "Soluble Phenyl-Substituted PPVs" by H. Spreitzer et al.
One of the major drawbacks of first generation PPVs was the high-temperature processing required. The necessity of the high-temperature treatment laid root with in the insolubility of conjugated PPV in an organic solvent (e.g. toluene). Because of PPVs insolubility, the original manufacturing process suggested by Burroughes relied on a soluble precursor polymer that was spin-coated onto the device substrate and then heat-treated to form the final conjugated PPV. The ability to use a spin coating as a manufacturing process is a major advantage over normal semiconductors that usually rely on epitaxal growth. However, the high temperatures involved in the heat treatment phase of the soluble polymer precursor precluded the use of many substrates such as flexible polymers. The search was on for a soluble conjugated polymer.
In the years succeeding since the suggestion of using PPV as an EL polymer, a few derivatives of PPV have shown promise as soluble conjugated polymers. One of the tricks used in making a PPV soluble is to artificially decrease the molecular weight. However, reducing the molecular weight has the side effect of deteriorating the mechanical stability of the molecule. In the work done by Spreitzer et al. a new series of polymer compounds based on alkoxy-substituted 2-phenyl PPV were created that showed good solubility in the conjugated state, good film-forming properties and excellent electroluminescence properties while still at large molecular weight. Three monomers were chosen as the basis for six polymers consisting of various proportions of base monomers. The six test polymer compositions were compared against poly[2-methoxy-5(3,7dimethyloctyloxy)1,4-phenylenvinylen] (OC1C10), which has the same characteristics as the well-known soluble conjugated EL polymer poly(2-methyoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV).
The various polymer combinations were tested in two polymer LED device configurations, either as a single layer (glass/indium tin oxide(ITO)/polymer/cathode) or as a double layer device (glass/ITO/buffer layer/polymer/cathode). The buffer layer in the double layer device consisted of another conductive polymer, either polyaniline (PANI) or poly(3-4-ethylenedioxthiophene) (PEDOT). Although the single layer devices exhibited good EL properties, the double layer devices increased the reproducibility of results due to a minimization of variations in the ITO by the buffer layer.
Whether by accident or through judicious choosing of monomers, the experimental polymers exhibited luminance and power efficiencies unheard of in the EL polymer community. The baseline polymer, OC1C10, had a luminance efficiency of 2.1 cd/A and a power efficiency of 2.5 lm/W at 100 cd/m2. In comparison, the experimental polymer 1c consisting of 2% of monomer 2, 49% of monomer 3, and 49% of monomer 4 had a luminance efficiency of 14.8 cd/A and power efficiency of 16.1 lm/W at 100 cd/ m2. These high efficiency figures compare favorably to values of 20 lm/W for ordinary light bulbs.
However, it should be noted that the baseline polymer and the experimental polymer 1c did not emit the same wavelength of light. The baseline polymer emitted at 596nm wavelength (orange), while the experimental emitted at 540nm (yellow-green). Altering the amount of monomer 2 in the experimental polymers allowed for changes in the emitted wavelength, but also dramatically changed the efficiency characteristics. Some insight was provided as to how changing the ratio of monomer 2 shifted color emissions. The argument given was dependent on the assumption that the bandgap of defined oligomeric sequences in monomer 2 was smaller than that of 3 or 4. Increasing the amount of monomer 2 caused a larger percentage of emissions to occur from its longer oligomeric sequences, thus changing the overall emission characteristics.
Some key concerns left unanswered included longevity of the experimental EL polymers and an explanation of what criteria was used in selecting the monomers. Besides bettering efficiency characteristics, improvements in longevity are necessary if EL polymer devices are to compete with commodity LED devices. Underlying the various issues is still the necessity of furthering the fundamental understanding of EL polymers. So far it seems that rough guesses in compound selection have provided the majority of EL polymer progress rather than a well-defined methodology. Like all areas of material science and chemistry dealing with complex organic compounds, researchers in EL polymers would benefit from better chemical modeling at the molecular level.
Article #2: "Improved Quantum Efficiency for Electroluminiscence in Semiconducting Polymers" by Y. Cao et al.
While Spreitzers group used a new polymer compound to improve efficiency characteristics, Caos group used the well-known OC1C10-PPV as the base polymer and then added a secondary compound to improve light efficiency. The main theoretical hurdle in improving efficiency is the assumption that if the lowest-energy states are strongly bound excitons (electron-hole pairs in either single or triplet spin states), then the upper quantum electroluminiscence limit is only 25% of the quantum photoluminescence limit. However, if the exciton binding energy is somehow reduced, then the ratio of the maximum quantum efficiency for electroluminescence and photoluminescence can theoretically approach unity.
The research presented by Cao showed that adding a electron transport material to PPV allowed the achievement of a ~50% (electroluminescence:photoluminescence) ratio through improved injection of electrons. The electron transport material added to the PPV base was (2-(4-biphenyl)-5-(4-tert-butylphenyl)1,3,4-oxidiazole, which is better known as Bu-PBD. The Bu-PBD compound was tested at different concentration levels in the PPV against an unaltered PPV polymer. Shown in Figure 3 are the results from these two different test cases. The triangles represent photoluminescence efficiencies, whereby the solid triangles correspond to PPV with Bu-PBD and clear triangles without. Likewise, the circles represent electroluminescence efficiencies, whereby solid circles correspond to PPV with Bu-PBD and clear triangles without. The trend is evident that PPV with Bu-PBD exhibits better electroluminescence efficiencies as temperature increases. Meanwhile, photoluminescence efficiencies stay relatively stable regardless of temperature.
Figure 3: Comparing OC1C10-PPV With and Without Bu-PBD5
Although the specific mechanism by which Bu-PBD increases PPVs efficiency is still a hotly debated issue, some intuition into the mechanism can be gained through a band diagram argument.
Figure 4: Band Diagrams for Polymer without Bu-PBD
Figure 5: Band Diagrams for Polymer with Bu-PBD
As can shown in Figure 4, polymer semiconductors exhibit simple band diagrams. The band bending in the biased case leads to a constant linear change between the metal and ITO energy levels. The addition of Bu-PBD, as shown in Figure 5, creates an electron and hole barrier at the interface between it and the polymer. This electron and hole barrier allows for efficient recombination since in the non-Bu-PBD case the electrons and holes sometimes traverse the polymer layer without recombining, thus causing degradation in efficiency.
It should be noted the band diagrams shown here are merely an intuitive approach to how Bu-PBD improves efficiency. In reality, there is no Bu-PBD and polymer stacked structure because both compounds are mixed together before creating a device. Hence, the Bu-PBD and polymer layers should be shown as a mixture of both without a strict delineation between them. Exactly here is where the conundrum surfaces as how Bu-PBD improves efficiency in the mixture of it and PPV. In a mixed state how can an electron and hole barrier be created? No definite answer exists, but what is definite that Bu-PBD increases electroluminsence efficiency of common OC1C10-PPV to new levels.
1: Cambridge Display Technology. CDT Technology Backgrounder. Online posting http://www.cdtltd.co.uk/TechnologyBackgrounder.html (26 April 1999)
2: H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Bums, and A.B. Holmes. Nature. 1990.347.539
3: H. Spreitzer, H. Becker, E. Kluge, W. Kreuder, H. Schenk, R. Demandt, and H. Schoo. Advanced Materials. 1998.10.1340
4: R.S. Visser. Philips Journal of Research. 1998.51.471
5: Y.Cao. I.D. Parker, G. Yu, C. Zhang, and A.J. Heeger. Nature. 1999.397.414 Febuary 4, 1999