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M7 Electroluminescence of Polymers

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M7 Electroluminescence of Polymers Universität Potsdam Institute of Physics and Astronomy Advanced Physics Lab Course March 2020 M7 ELECTROLUMINESCENCE OF POLYMERS I. INTRODUCTION The recombination of holes and electrons in a luminescent material can produce light. This emitted light is referred to as electroluminescence (EL) and was discovered in organic single crystals by Pope, Magnante, and Kallmann in 1963.[1] EL from conjugated polymers was first reported by Burroughes et al.[2] The polymer used was poly(p-phenylenevinylene) (PPV). Since then, a variety of other polymers has been investigated. Organic EL devices have applications in a wide field ranging from multi-color displays and optical information processing to lighting. Polymers have the advantage over inorganic and monomolecular materials in the ease with which thin, structurally robust and large area films can be perpared from solutions. Using printing techniques, patterned structures can be produced easily. Even flexible displays can be produced because of the good mechanical properties of polymers. In this lab course, basic optical and electrical properties of conjugated polymers will be investigated. Advanced Lab Course: Electroluminescence of Polymers 2 EXPERIMENTAL TASKS Measure the absorption spectra of your polymers (thin films spin coated onto glass substrates). Characterize the setup used for luminescence measurements. Identify possible sources of error and collect data necessary for their correction. Measure the photoluminescence emission spectra for the polymer films, using suitable excitation wavelengths. Measure the photoluminescence excitation spectra for the polymer films, using suitable detection wavelengths. Measure the current through the OLEDs and the spectral radiant intensity of electroluminescence as a function of applied voltage (the current-radiance-voltage characteristics). Measure the electroluminescence emission spectrum at a suitable voltage or current. Measure the radiance of an OLED as a function of viewing angle. Determine the radiometric and luminous efficiencies of the OLEDs. Summarize your results. CONTENTS I. INTRODUCTION ................................................................................................................................................... 1 EXPERIMENTAL TASKS ................................................................................................................................................. 2 CONTENTS .......................................................................................................................................................................... 2 II. FUNDAMENTALS .................................................................................................................................................. 3 II.1 CONJUGATED POLYMERS ....................................................................................................................................................... 3 II.2 LIGHT EMISSION BY CONJUGATED POLYMERS ................................................................................................................... 3 II.3 LIGHT-EMITTING DIODES ..................................................................................................................................................... 6 II.4 POLYMERS FOR LIGHT-EMITTING DIODES ...................................................................................................................... 12 III. EXPERIMENT EXECUTION AND ANALYSIS............................................................................................... 14 III.1 GENERAL REMARKS ....................................................................................................................................................... 14 III.2 ABSORPTION SPECTRA .................................................................................................................................................. 14 III.3 LUMINESCENCE SPECTRA .............................................................................................................................................. 15 III.4 ELECTROLUMINESCENCE .............................................................................................................................................. 16 III.5 SUMMARY OF EXPERIMENT .......................................................................................................................................... 18 IV. INSTRUMENTATION AND POSSIBLE SOURCES OF ERROR ................................................................ 19 IV.1 CORRECTION OF SPECTRA ............................................................................................................................................ 19 IV.2 INNER FILTER EFFECTS ................................................................................................................................................ 20 V. REFERENCES AND FURTHER READING .................................................................................................... 21 VI. RADIOMETRIC AND PHOTOMETRIC QUANTITIES ............................................................................... 22 Advanced Lab Course: Electroluminescence of Polymers 3 II. FUNDAMENTALS II.1 CONJUGATED POLYMERS Organic electronics can be divided into two major classes of materials. One of them is the class of small organic molecules. Electroluminescence was first observed in crystals of the small molecule anthracene in 1961.[1] However, these materials are usually not soluble and have to be deposited by thermal evaporation. Hence the production of small molecular electronics needs expensive technology comparable to inorganic electronics. The second class of materials used are polymers. While most polymers are electrical insulators, for electronics applications, materials with semiconducting properties are needed. Conductivity needs the presence of free, mobile charges. In polymers, these can be provided by a conjugated 휋 electron system. This conjugated system is often depicted as an alternating chain of single and double bonds between the carbon atoms. However, quantum chemical calculations show that this picture is not correct, the electrons forming the double bonds are delocalized over the main chain. In a thin solid film, many chains are in close contact with each other (allowing orbital overlap between the chains), so that excess charges can travel over macroscopic distances in an electrical field. Table II.1 shows a selection of polymers which have been studied with respect to their semiconducting properties. While the polymers shown in Table II.1 possess a conjugated backbone, that is not generally necessary. Other polymers have been used which are built from an saturated backbone bearing conjugated side groups. The conjugated system can be easily disturbed by either conformational defects (e.g., kinks) or chemical impurities in the material. In addition, for entropic reasons it is very unlikely for a polymer chain to be completely extended. At the resulting geometric distortions, the conjugated system is interrupted. As a result, a polymer chain is not one single conjugated system but a chain of conjugated segments of different length. The segments are often called “chromophores” because they determine the optical and electronic properties of the materials. A polymer chain can be regarded as a chain of chromophores of different lengths and thus different properties. The length distribution can be regarded as being Gaussian. II.2 LIGHT EMISSION BY CONJUGATED POLYMERS Absorption of energy by atoms, molecules or condensed matter will result in the generation of excited states, i.e. it increases the potential energy of the electrons in the substance rather than its heat. If the excited state decays under emission of visible light, that emission is called luminescence. Luminescence is observed from many inorganic and organic substances, the luminophores, and can be induced by various physical processes. Two possible excitation mechanisms relevant for this lab course are the absorption of photons, which leads to photoluminescence, or the recombination of injected charges, which is called electroluminescence. Luminescence spectra are determined by the properties of the material. In contrast, the thermal generation of light by heat radiation is called incandescence. It is mainly determined by the material’s temperature. The excited state is called an exciton, a coulombically bound electron-hole pair. In inorganic (crystalline) semiconductors, the binding energy of an exciton is on the order of or even below the thermal energy at room temperature (about 0.025 eV), such that the exciton will dissociate into free charges. In organic materials, the exciton is much stronger bound (about 0.5 eV) and will not dissociate easily. The generated excitons on polymer chromophores thus resemble the excited states of small molecules. Thus, absorption and (photo-)luminescence can be described in a molecular picture. To understand the optical properties of a thin polymer film, one needs to keep in mind that such films comprise a distribution of chromophores in close contact. Advanced Lab Course: Electroluminescence of Polymers 4 Table II.1: Polymers which have been extensively studied for EL [3] PPV poly(p-phenylene vinylene) n O MEH-PPV poly[2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene] n O OR1 PPE n poly[2,5-dialkoxy-1,4-phenylene ethynylene] R O 2 N PPy n poly(p-pyridine) n PF poly(9,9-dialkylfluorene) R R R P3AT poly(3-alkylthiophene)
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