Title of Invention

AN IMPROVED ROOM TEMPERATURE CHEMICAL VAPOUR DEPOSITION PROCESS FOR THE DEPOSITION OF POLY (PHENYLENE-VINYLENE) IN THE MANUFACTURE OF ORGANIC BASED ELECTRONIC DEVICES

Abstract

An improved process for the manufacture of organic semiconducting polymer Poly (phenylenevinylene) for use in electronic devices such as light emitting diodes, thin film transistors etc., whereby it would be possible to obtain such semiconducting polymer on a wide variety of substrates and favour cost effective manufacture of semi conducting polymers. In particular, the invention relates to manufacture of the semiconducting polymer Poly(phenylenevinylene) at room temperature using ultraviolet light. Importantly, the improved process can be carried out at lower temperature and will thus favour obtaining of the semi conducting polymer from cost effective sources and enable manufacture of such cost effective polymers. Moreover the improved process would favour use of roll to roll processing in the manufacturing of abovesaid semiconducting polymers and further enable band-gap tuning of Poly (phenylenevinylene) via photoinduced dehydrohalogenation. Additionally the improved process is adapted to pattern Poly (phenylenevinylene) using a lift-off process.

Full Text

THE PATENTS ACT, 1970 (39 OF 1970) COMPLETE SPECIFICATION (See Section 10) The following specification particularly describes the* nature of the invents the manner in which it is to be performed. The present invention relates to an improved process for the manufacture of thin films of Poly (phenylenevinylene) comprising the step of irradiating the precursor polymer with ultraviolet light to thereby form conjugated, semiconducting polymer Poly (phenylenevinylene) for use in electronic devices such as Light Emitting Diodes, Thin Film Transistors etc., whereby it would be possible to obtain such semiconducting polymer on a wide variety of substrates and favour cost effective manufacture of semiconducting polymers. In particular, the invention relates to manufacture of the semiconducting polymer Poly (phenylenevinylene) at room temperature using ultraviolet light. Background and Prior Art: Among the large palette of organic compounds which effectively transport charge, conjugated polymers have attracted considerable attention in the last ten years because these materials fluoresce efficiently in the visible range of the spectrum. This property has enabled the fabrication of devices such as Light Emitting Diodes and Thin Film Transistors in which the thin semiconducting layer is made from a conjugated material, e.g., Poly(phenylenevinylene) or an oligothiophene. [F. Gamier, G. Horowitz, X. Z. Peng and D. Fishon, Adv. Mater. 2, 592 (1990) ; J. H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. McKay, R. H., Friend, P. N. Burns and R. B. Holmes, Nature, 341, 539 (1990)]. These polymers combine a number of advantageous features such as good mechanical properties, ease and diversity of synthesis and the potential for molecular engineering. The intimate chemical structure of the molecular units involved in the construction of the material and the spatial organization of these macromolecular assemblies are some of the important structural parameters which allows a priori tuning of devices such as those mentioned above. A typical process to make thin films of these polymers consists of spin coating a solution of the Precursor Polymer on a substrate as a thin film by evaporating the solvent and thermally converting it to the final semiconducting form by heating it to about 200°C. For instance, Poly(phenylenevinylene) films are conventionally made by processing a sulfonium-based polyelectrolyte polymer precursor in water or methanol, and thermally converting the film to the fully conjugated polymer. The inherent problem associated with this method is, undesirable side reactions with the processing solvents which can result in the formation of 2 carbonyl and hydroxy groups at the vinyl linkages of the polymer backbone. Carbonyl defects are incorporated into the Poly(phenylenevinylene) film by residual oxygen during the thermal process. These defects adversely affect the electronic properties of the polymer by altering its conjugation length and also act as non-radiative centres. [B. R. Hsieh, H. Antoniadis, M. A. Abkowitz and M, Stolka, Polym. Prepr. 33, 414 (1992); R. W. Lenz, C. Han, J. Stenger-Smith and F. E. Karasz, J. Polym. Sci.; Part A: Polym. Chemistry, 26, 3241 (1988) ; F. Papadimitrakopoulos, K. Konstandinidis, T. M. Miller, R. Opila, E. A. Chandross and M. E. Galvin, Chem. Mater., 6, 1563 (1994) ; U. Murase, T. Ohnishi, T. Noguchi and M. Hirooka, Polymer Comm., 25, 327 (1984)] An alternate process to make thin films is to use the technique of Chemical Vapour Deposition (CVD). This technique eliminates the use of solvents as a possible source of contamination. To make thin films of Poly(phenylenevinylene) using this technique, a vapour of chlorinated p-xylene i.e. cc.oc"-p-dichloroxylene is pyrolylized at low pressure of O.ITorr to form chlorinated xylylene which polymerizes on surfaces having a temperature lower than 90°C, referred to as Precursor Polymer. The Precursor Polymer is heated to about 200°C to eliminate HCI along its backbone to form the conjugated structure of Poly(phenylenevinylene). [E. G. J. Staring, D. Brawn, G. L. J. A. Rikken, R. J. C. E. Demandt, Y. A. A. R.; Kessener, M. Bauwmans, D. Broer, Synth. Met. 67, 711 (1994); K. M. Vaeth and K. F. Jensen. Advanced Materials, 9, 490 (1997)]. Both the methods, described above, put constraints on the choice of substrates for depositing the thin films of Poly(phenylenevinylene) as the substrates have to be thermally stable up to about 200°C. This requirement excludes the use of cheap plastic substrates and roll to roll deposition for high throughput in a manufacturing environment. Object of the Invention: It is thus the basic object of the present invention to provide an improved process for manufacture of organic semiconducting polymer Poly(phenylenevinylene) especially for use in electronic devices, which would avoid the above discussed 3 problems and limitations of the known art of manufacture of such semiconducting polymers. Another object of the present invention is directed to an improved process for manufacture of the organic semiconducting polymer Poly (phenylenevinylene), which can be carried out at lower temperature and will thus favour obtaining of the semiconducting polymer from cost effective sources and enable manufacture of such cost effective polymers. Yet further object of the present invention is directed to an improved process of manufacture of Poly(phenylenevinylene), which would favour use of roll to roll processing in the manufacturing of above said semiconducting polymers. Yet further object is directed to provide electronic devices, involving use of Poly(phenylenevinylene), at cost-effective rates. Yet further object of the invention is directed to provide an improved process of manufacture of Poly(phenylenevinylene), which would favour band-gap tuning of Poly(phenylenevinylene) via photoinduced dehydrohalogenation. Yet another object of the present invention is directed to provide an improved process of manufacture of Poly(phenylenevinylene) adapted to pattern Poly(phenylenevinylene) using a lift¬off process. Summary of the Invention : An improved room temperature deposition for manufacturing thin films of Poly(phenylenevinylene) comprising the step of pyrolizing at 600DC a vapour of a,a p-dichloroxylene at a pressure of 0.1 Torr - to form a precursor polymer of a specified thickness on surfaces of substrates having a temperature lower than 90°C; and irradiating the precursor polymer, thus obtained, with ultraviolet light at room temperature to eliminate HCI along its backbone to form the conjugated semi conducting polymer Poly (phenylenevinylene). In accordance with a preferred aspect the process of manufacture of the film of Poly(phenylenevinylene) comprise : 4 a) providing the Precursor Polymer comprising of pyrolizing a vapour of x,cc"-p-dichloroxylene of a specified thickness at a pressure preferably of O.ITorr to form chlorinated xylylene which polymerizes on substrates having a temperature lower than 90°C ; b) irradiating the Precursor Polymer, thus obtained, with ultraviolet light at room temperature to eliminate HCI along its backbone to form the conjugated, semiconducting polymer Poly(phenylenevinylene). The reaction scheme for the synthesis of Poly(phenylenevinylene) by the process of the invention is represented hereunder: Dichloro-p-xylene 700C, O.ITorr Argas Precursor Polymer ultraviolet light ci 5 In the above process it is possible to use a variety of substrates for deposition. These include optical glass, quartz, mica, mylar, polypropylene and polyvinylchloride and polyester based substrates. In accordance with a further aspect of the present invention the above disclosed process of the invention provides for controlling the conjugation length of the polymer by irradiating the Precursor Polymer for different periods. This enables band-gap control of the Polymer. In the fabrication of Light Emitting Diodes, this process can be used to decide the wavelength of emission (colour) of the Light Emitting Diodes. The details of the invention, its objects and advantages are explained hereunder in greater detail in relation to the non-limiting exemplary illustrations : EXAMPLE EXAMPLE I The process in accordance with the invention was carried out as discussed hereunder: Apparatus used: A quartz tube is used as the processing chamber. The tube is preferably evacuated to low pressure with a vacuum pump. Carrier Argon gas is adapted to flow through the chamber. The quartz tube is provided with three heaters which can independently be heated and allows different parts of the quartz tube to be maintained at different temperatures. Samples are mounted downstream on a holder whose temperature can be changed from 0°C to 300°C. The substrates used for deposition were optical glass, quartz, mica, mylar, polypropylene, polyvinyl chloride and polyester based materials. The Process : cc.x"-p-dichloroxylene (Aldrich 98%) was used as the starting material without further purification. A vapour of the starting material was pyrolylized at O.ITorr at 6 600-700°C in the quartz tube to form chlorinated xylylene which although stable in gas phase, condenses and polymerizes on surfaces having a temperature lower than 90°C to form the Precursor Polymer. The temperature of the substrates in the deposition chamber is maintained below 90°C by water-cooling. A heat shield is used between the heaters and the substrates to reduce heat transfer by radiative heating from the heaters. The thickness of the film is monitored in situ using a laser interferometric technique. The Precursor Polymer is thus synthesized using this CVD process. Conversion to Poly(phenylenevinylene): To obtain Poly(phenylenevinylene), the substrates (on which the Precursor Polymer was deposited) were transferred to an Argon-filled quartz tube. The samples were irradiated with ultraviolet light (of wavelength 256.4nm) in flowing Argon to enable the conversion to Poly(phenylenevinylene). For a 1000A film the time for conversion is typically 20min for ultraviolet source of intensity 0.8mW placed at a distance of approximately 10cms. EXAMPLE II Under this example the conventional thermal route was followed to convert the Precursor Polymer to Poly(phenylenevinylene). This was done to enable us to compare the properties of Poly(phenylenevinylene) so obtained with the Poly(phenylenevinylene) obtained using the ultraviolet light route to convert the Precursor Polymer at room temperature. In the thermal route the substrate used was quartz. The Precursor Polymer was heated in Argon at O.ITorr pressure at 200°C for 20 minutes in the CVD apparatus itself. Characterization of the films obtained by the process of the invention : Optical absorption spectrum and photoluminescence spectrum were used to study the Precursor Polymer and its conversion to Poly(phenylenevinylene). The optical absorption was measured using a Specord 100B spectrophotometer and the luminescence was measured using a Spex fluorimeter. The results obtained are discussed in relation to accompanying figures wherein 7 Fig. 1 illustrates absorption spectra of Precursor Polymer and Poly(Phenylenevinylene) obtained after heating the Precursor Polymer at 200°C for 20 min. Fig. 2 illustrates PL spectrum of Poly(Phenylenevinylene) obtained by heating the Precursor Polymer to 200°C. Fig. 3 illustrates absorption spectra of precursor polymer and Poly(Phenylenevinylene) obtained after irradiating the Precursor Polymer with uv light for 20 mts. Fig. 4 illustrates PL spectrum of Poly(Phenylenevinylene) obtained by irradiating the Precursor Polymer with uv light for 20 mts. Fig. 5 illustrates absorption spectra of Poly(Phenylenevinylene) on plastic substrates obtained by irradiating the Precursor Polymer with uv light for 20 mts. Fig. 6 illustrates PL spectrum of Poly(Phenylenevinylene) obtained by conversion of Precursor Polymer by shining uv light on Mylar as a substrate. Fig. 7 illustrates absorption spectra of precursor polymer after exposure to uv light for different time intervals using light source at reduced intensity. Fig. 8 illustrates absorption spectra of precursor polymer heated at different temperatures. Fig. 9 illustrates PL spectra after irradiation of precursor polymer with uv light for two different time intervals. As illustrated in accompanying Figure 1, the same illustrates the absorption spectrum of the precursor polymer film after being thermally converted to form Poly(phenylenevinylene). It is found that the absorption at 380nm is the signature for the formation of Poly(phenyienevinylene). Figure 2 shows the luminescence spectrum of the converted Poly(phenylenevinylene) sample. In contrast, the Precursor Polymer does not exhibit any fluorescence. Thus, optical absorption and fluorescence are good probes to measure the formation of Poly(phenylenevinylene), Figure 3 shows the absorption spectrum of a Precursor Polymer deposited on a quartz substrate before and after exposure to uv light as explained in the earlier section. It is noted that in comparison with figure 1 that after exposure to uv light 8 at room temperature, the absorption spectrum of the polymer has the characteristic absorption of Poly(phenylenevinylene) - establishing the fact that uv irradiation converts the Precursor Polymer to Poly(phenylenevinylene). The luminescence of this sample is shown in figure 4. It is also found that the emission spectrum is similar to that in figure 2. This provides further evidence that under uv illumination, the Precursor Polymer is converted to Poly(phenylenevinylene) at room temperature. Figure 5 shows the absorption spectra of films deposited on various plastic substrates and converted to Poly(phenylenevinylene) at room temperature by shining uv light. Figure 6 shows the luminescence spectrum of Poly(phenylenevinylene) on Mylar. The luminescence spectra of Poly(phenylenevinylene) on plastic substrates such as mica, polypropylene and polyvinylchloride and polyester transparency film are similar to figure 6. It is thus established that uv irradiation of the Precursor Polymer is a low temperature route for making Poly(phenylenevinylene) films. EXAMPLE HI Further studies were carried out to monitor the conversion of the Precursor Polymer to Poly(phenylenevinylene) in real time. These studies were done in vacuum in a special apparatus. The apparatus had quartz windows. The sample holder was aligned with the windows in the apparatus. The temperature of the samples could be changed with an external heater. The whole apparatus was placed inside the spectrophotometer to record absorption. The deuterium lamp (which is built into the spectrometer) was used to expose the sample to uv radiation to convert the Precursor Polymer to Poly(phenylenevinylene). Figure 7 shows the absorption of a Precursor Polymer deposited on a quartz substrate for the as- deposited sample and after exposure to uv light (from the deuterium lamp as explained above) for various exposure times. It is noted as represented in figure 7 that in the early stages of illumination, there are two well-resolved humps at 325nm and 390nm. These correspond to two conjugation lengths in the polymer. As the reaction progresses, the 390nm hump increases in relation to the 325nm hump. It is thus possible to control the conjugation length in Poly(phenylenevinylene) by exposing the Precursor Polymer to ultraviolet light for a desirable length of time. As the Precursor Polymer is converted to 9 Poly(phenylenevinylene), the presence of a clear isobestic point at 243nm is evident. This indicated that the Precursor Polymer converts to Poly(phenylenevinylene) without going through an intermediate state. Figure 8 shows similar data for Poly(phenylenevinylene) obtained by the thermally converted route. EXAMPLE IV In another set of studies, the process of ultraviolet-conversion was interrupted at regular intervals, the sample taken out of jig and Photoluminescence (PL) of the sample measured. The excitation wavelength for the PL measurements was 390nm. Changes in Photoluminescence spectra show a red shift as the reaction progresses towards Poly(phenylenevinylene) formation. These results are given in figure 9. Similar results are obtained when Precursor Polymer is thermally converted to Poly(phenylenevinylene). These results are consistent with our hypothesis that the conjugation length depends on the exposure time to uv light. It is thus possible by way of the improved process of the invention to synthesize the conjugated, conducting polymer Poly(phenylenevinylene) at room temperature (25°C) starting from a commercially available monomer by employing ultraviolet radiation. The process of the invention enabling irradiating the Precursor Polymer with ultraviolet light for different time intervals allows control of the conjugation length and hence the band-gap of the material. This presents the possibility of fabricating Light Emitting Diodes (based on Poly(phenylenevinylene)) whose colour can be tuned over a small range. Since the process of Poly(phenylenevinylene) synthesis is carried out at room temperature the need to go to higher temperatures is eliminated. This method would enable patterning Poly(phenylenevinylene) on substrates. This can be achieved by pre-patterning the substrates using a positive photoresist, depositing the Precursor Polymer, converting the Precursor Polymer to 10 Poly(phenylenevinylene) by shining ultraviolet light and then removing Poly(phenylenevinylene) from the unwanted areas using a standard lift off process. The method of the invention is also applicable for obtaining thin film polymer Poly(phenylenevinylene) from precursor polymer obtained from starting monomers containing bromine instead of chlorine and/or starting monomers containing low molecular mass substituents such as alkoxy group on the benzene ring. The process of the invention is further advantageous in terms of its possible industrial application. The ability to carry out the polymerization process at room temperature enables the use of cheap, light, flexible, plastic substrates instead of glass substrates which are fragile and heavier. The process disclosed herein allows easy manufacturability as one can use roll to roll processing for depositing the Precursor Polymer on plastic rolls pre-coated with a transparent conducting coating, suitably textured to optimize the external quantum efficiency. The samples can be converted in an in situ process to Poly(phenylenevinylene) using uv light and then depositing top contacts and a passivating film to protect the sample from atmosphere. Band-gap tuning is possible by irradiating Precursor Polymer with ultraviolet light for different lengths of time. This provides additional flexibility in the design of Light Emitting Diode amplifiers. The process is thus substantially advantageous and improvement over the prior art process in terms of providing ; i) a room temperature route towards synthesis of Poly(phenylenevinylene); use of cheap plastic substrates for deposition of Poly(phenylenevinylene); use of roll to roll processing in a manufacturing process ; construction of Devices for electronic applications based on synthesis of Poly(phenylenevinylene) at room temperature ; band-gap tuning of Poly(pheny\eneviny\ene) via photo"induced dehydrohalogenation ; the capability to pattern Poly(phenylenevinylene) using a lift-off process. WE CLAIM: 1. An improved room temperature deposition process for manufacturing thin films of Poly(phenylenevinylene) comprising the step of pyrolizing at 600°C a vapour of a,a" p-dichloroxylene at a pressure of 0.1 Torr - to form a precursor polymer of a specified thickness on surfaces of substrates having a temperature lower than 90°C; and irradiating the precursor polymer, thus obtained, with ultraviolet light at room temperature to eliminate HCI along its backbone to form the conjugated semi conducting polymer Poly (phenylenevinylene). 2. A process as claimed in claim 1 wherein substrate for deposition is selected from optical glass, quartz, mica, mylar, polypropylene and polyvinylchloride and polyester based substrates. 3. A process as claimed in any one of claims 1 to 2 comprising the step of controlling the conjugated length of the polymer by irradiating the Precursor Polymer for different periods to achieve band-gap control of the polymer. 4. A process as claimed in any one of claims 1 to 3 wherein the irradiation of precursor polymer on substrate is carried out in a quartz tube which is the processing chamber, said tube is preferably evacuated to low pressure with a vacuum pump with carrier Argon gas adapted to flow through the chamber. 5. A process as claimed in any one of claims 1 to 4 wherein said quartz tube is provided with three heaters which can independently be heated and allow different parts of the quartz tube to be maintained at different temperatures. 6. A process as claimed in any one of claims 1 to 5 wherein precursor polymer on substrates are mounted downstream on a holder whose temperature changed from 0°C to 300°C. 7. A process as claimed in any one of claims 1 to 6 wherein cc,oc"-p-dichloroxylene (Aldrich 98%) is used as the starting material without further purification. 8. A process as claimed in any one of claims 1 to 7 wherein a heat shield is used between the heaters and the substrates to reduce heat transfer. 9. A process as claimed in any one of claims 1 to 8 wherein the thickness of the film is monitored in situ using a laser interferometric technique. 10. A process as claimed in any one of claims 1 to 9 wherein to obtain Poly(phenylenevinylene), the precursor polymer on substrate is transferred to an Argon-filled quartz tube and irradiated with ultraviolet light (preferably of wavelength 256.4nm) in flowing Argon to enable the conversion to Poly(phenylenevinylene). 11. An improved process for the manufacture of organic semiconducting polymer Poly(phenylenevinylene) substantially as herein described and illustrated with reference to the foregoing examples and accompanying figures. Dated this 17th day of October, 2001. J^fl^H/j/ Dr. Sanchita Ganguli Of S. Majumdar & Co. Applicant"s Agent

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