Photoconductive device comprising an organic polyimide film

Abstract: A polyimide compound having a carbazole skeleton in a repeating unit thereof, for solar cell or other photo-electric transducer applications. This polyimide compound preferably comprises a repeating unit given by the following chemical formula: where R¹ denotes a tetravalent organic group having 5 to 13 carbons. This polyimide compound may be produced by a dehydrocyclization of a polymer having the repeating unit of the general formula where R² is a bivalent organic group of 5 to 13 carbons.

Description: The present invention relates to a photoconductive device which can be used as a photoelectro-transducing device in a solar cell or photosensor. Solar cells and photosensors used for photoelectro-transducing devices in which light energy is converted directly to electric energy. The development of improved photoelectro-transducing materials is significant in relation to the use of solar energy and in relation to the development of light receiving devices for opto-electronics. Practical forms of photoelectro-transducing devices which have become available up to now employ inorganic semiconductors such as single crystal or amorphous silicon. In contrast, photoelectro-transducing devices based on organic materials are still in the research stage, but are attractive because these devices have the advantages characteristic of organic materials such as light weight, ease of element production, ease of mass production and inexpensiveness. There is therefore a requirement for solar cells and photo sensors based on organic materials. Known organic photoelectro-transducing materials include colouring matters such as chlorophyll, squarilium compounds, phthalocyanines, merocyanines etc, and electro chemically doped semiconductive polymers such as poly(N-vinylcarbozole),poly[4-(N,N-diphenylamino)-phenylmethyl methacrylate] and the like. Organic thin films of these materials are being used in the development of novel photoelectro-transducing devices. In making a practical device based on an organic thin film, it is naturally a requirement that the photoelectro-transducing efficiency should be high. It is also required that the device produced should be stable to heat and that the material should be capable of being worked into the required shaped for the device. For heat stability, resistance to temperatures of at least 200°C is required, and preferably 300°C or above. Heat resistance is closely related to the melting point, the decomposition temperature or the glass transition temperature (Tg) of the material. These temperatures will determine the heat resistance temperature. For example, merocyanine pigments which are photoconductive organic materials of low molecular weight have a heat resistance temperature of approximately 160°C at most. Depending on their molecular weight, semiconductive polymers which are poly(N-vinylcarbazole compounds) have a heat resistance temperature of about 150°C. Polymethylmethacrylate which is a general purpose plastics material has a glass transition temperature of about 100°C, whereas that for polyvinyl chloride is approximately 70°C and that for polystyrene approximately 100°C. In contrast, phthalocyanines have greater heat stability, and some of them exhibit a decomposition temperature of 200°C or above. However, phthalocyanines are significantly inferior in workability and do not readily provide flat uniform thin films, which limit their field of application. Accordingly it is difficult to obtain a melting point, decomposition temperature or Tg of 300°C or above using ordinary organic low molecular weight compounds or organic polymeric compounds, and it is difficult to raise the heat resistance temperature without impairing the workability of the thin film. Therefore when developing a photoelectro-transducing device using an organic thin film, improvement of heat resistance or other environmental stability and thin film workability are important in addition to the improvement of photoelectro-transducing efficiency. Nevertheless, the above mentioned technical difficulties have impeded the production of an organic photoelectro-conductive device with good electroconductive characteristics. In technical fields other than photoelectro-transducing, various heat resistant resins are known, amongst which are polyamide resins which exhibit excellent heat resistance. These polyamide resins have been commercialised only for uses requiring heat resistance, wear resistance, chemical resistance etc and the use of a polyamide for a photoconductive material is still in the research stage. Polyamides having a porphyrin skeleton have also been investigated, but they have a heat resistance temperature of approximately 200°C which is insufficient for the present purposes. An article by Biswas et al, Eur. Polym. J., Vol 18, pages 945 to 948 (1982) discloses the synthesis of polypyromellitimides from carbazole and N-benzyl carbazole by a two-step procedure involving ring-opening polyaddition with pyromellitic dyanhydride and subsequent thermal cyclodehydration. The present invention provides a photoconductive device having the features of claim 1 of the accompanying claims. How the invention can be put into effect will now be described by way of example only, with reference to the accompanying drawings: BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A and Fig. 1B show the infrared absorption spectra for identifying the polyimide having a carbazole skeleton derived in Example 1, wherein Fig. 1A indicates the spectrum of the polyamic acid before imidation, and Fig. 1B indicates the spectrum after the imidation. Fig. 2 is a schematic illustration of a photoconductive device of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT The inventors of the present invention, as the result of comprehensive investigation to introduce a photoconductive organic moiety into a polymer to achieve satisfactory heat resistance and environmental stability, have found that a combination of an imide linkage and a carbazole unit gives a superior organic photoconductive polymer. The present invention provides a polyimide type compound (hereinafter referred to as "the polyimide") which comprises a polymer having a carbazole skeleton and an imide linkage in the repeating unit, and which has electroconductivity. With this compound, a device can be provided which has superior environmental stability. The carbazole skeleton may form a complex such as a charge-transfer complex with an electron-accepting organic compound, or may have a substituent such as an alkyl group. At least one imide linkage should be present in the repeating unit. Preferably the polyimide is a polymer having the repeating unit represented by the general formula (1) below: wherein R 1 is a tetravalent organic group having 5 to 13 carbons. The repeating unit may be other than that represented by the general formula (1), including polyamideimides, polyimides, and the like having an amide linkage, an ether linkage, thioether linkage, and etc. which have a carbazole moiety introduced therein. The group R 1 in the general formula (1) is a tatravalent organic group having 5 to 13 carbons, including those having aromatic rings such as and the like, cyclic hydrocarbon groups such as acyclic hydrocarbon groups such as hexamethylenediamine, among which cyclic hydrocarbons are preferable. If the polyimide is constituted of the compound with the carbon number outside the range specified above, the heat resistance will be somewhat impaired. The group having an aromatic ring is more preferable for improvement of the heat resistance. Particularly preferable are the groups below: The polyimide constituted of the repeating unit of such a structure have a decomposition temperature exceeding 300°C, thereby exhibiting superior heat resistance. As described above, the polyimides containing an imide linkage in the repeating unit are superior in environmental resistance such as heat resistance, solvent resistance, and the like. The stability is ascribable to the skeleton structure of the polyimide which restricts the degree of freedom of the molecular chain movement to raise the melting point, thereby giving high heat resistance. Since carbazole is a fused ring type hydrocarbon, its structure is plane, so that the degree of freedom of movement of the molecules of the polyimide is not increased by introduction of a carbazole ring, and is kept at the same level as of ordinary polyimides. Therefore the introduction of carbazole rings into the main chain of the polyimide does not impair the heat resistance of the polyimide. Additionally, the introduction of a carbazole ring does not impair the superior environmental stability of common polyimides such as solvent resistance. The polyimide containing the carbazole skeleton exhibits characteristics in both a polyimide compound and a carbazole skeleton. The above-described polyimide used in the present invention and having a carbazole skeleton in the repeating unit is obtainable in principle by dehydrating a polyamic acid or a polyamic acid derivative to cause ring closure. In a typical example, diaminocarbazole as a starting material is reacted with a tetracarboxylic anhydride such as pyromellitic anhydride to cause polyaddition to form a polyamic acid as a precursor of the polyimide, and the polyamic acid is treated chemically or by heating to form the polyimide having the repeating unit of the general formula (1). Alternatively, the derived polyamic acid is mixed with an organic compound having an amino group such as N,N-dimethylhexadecyl amine to form a polyamic acid salt, which is then treated in the same manner as in imidation of the polyamic acid to form the polyimide having the repeating unit of the general formula (1). In still another method, diaminocarbazole is reacted with dialkyl tetracarboxylate such as pyromellitic acid distearyl ester dichloride to cause polyaddition to form a polyamic ester, which is converted to the polyimide in the same manner as above. The polyaddition reaction can be conducted in a known method. In the case where the group R 1 is not a tetravalent organic group, the polyimide having an imide linkage in the repeating unit can be formed by polymerization reaction with diaminocarbazole. In forming a thin film from the above-described polyimide on a substrate, a solution of a polyamic acid which is the precursor of the above-mentioned polyimide is applied on the substrate, and dried, and then subjected to imidation treatment to form a thin film of the polyimide. The polyamic acid solution may be applied on a substrate by any procedure, insofar as a thin film can be formed, including spin coating, spreading, the Langmuir-Blodgett method, and the like. Among the procedures, the Langmuir-Blodgett method (or simply the LB method), which builds up a monomolecular layer one by one, is particularly preferable in view of controllability of the thickness and the smoothness of the film. The film thickness is selected depending on the use of the film, generally in the range of from several tens to several ten-thousands of angstroms. In the imidation by heat treatment, the reaction is conducted at a temperature of from 150°C to 300°C for a predetermined time. The imidation by chemical treatment is conducted by drying the applied coating film and subsequently immersing it in a solution which can dehydrate the film-constituting material to cause ring closure. The imidation method is not limited to these methods, but can be any known technique. In other words, in order to form a thin film of a polyimide having the repeating unit of the general formula (1), a solution of a polyamic acid having the repeating unit represented by the general formula where R 2 is a tetravalent organic group having 5 to 13 carbons is applied on a substrate, and dried, and is imidated. The group R 2 herein corresponds to the aforementioned group R 1 . From the thin film of the polyimide having a carbazole skeleton prepared as above, an element can be prepared which has superior environmental stability such as heat resistance and solvent resistance. In preferred embodiment, the heat stability exceeds 300°C. The thin polyimide film is expected to exhibit a photoelectro-transducing efficiency at the same level as that of a polyvinylcarbazole, and its thin film workability is satisfactory. For forming a pn junction or a pin junction by employing the aforementioned film, the film doped with iodine or the like and the same film dopant-free are built up into a laminated structure. The materials for the substrate useful in the present invention include light-transmissive substances such as glass, quartz, and the like. The electrodes, which are means for applying a voltage or means for outputting the generated electric current, may be of various types, including one pair of an upper electrode and a lower electrode sandwiching the aforementioned film therebetween, or the like. The type of electrode may be suitably selected depending on the use and the shape of the element. The lower electrode may be a vapor-deposited aluminum film of from 30 to 300 nm thick, or the like. The upper electrode may be a vapor-deposited film of ITO or aluminum of 30 nm thick or less, or the like. The electrode may be in a shape of layers, stripes, or dots. The substrate and the electrodes may be prepared by a conventional method for the element of the present invention. The present invention is described in more detail by reference to examples. Example 1 (Synthesis of a polyamic acid by polyaddition of diaminocarbazole with pyromellitic anhydride, and imidation by heating) 0.546 g (0.0028 mol) of 3,6-diaminocarbazole was placed in a 50-ml three-necked flask. Thereto, approximately 2 ml of dried dimethylacetamide (DMAC) was added. The interior of the flask was kept dry by flowing dry nitrogen, and the content was stirred. Subsequently, 0.666 g (0.0031 mol) of solid pyromellitic anhydride was added into the flask, whereby the temperature in the flask rose slightly. Further thereto, DMAC was added in the total amount of DMAC of 10 ml. The content of the flask was stirred for one hour at room temperature to prepare a solution of a polyamic acid having a carbazole skeleton in dimethylacetamide. The intrinsic viscosity of the product was 0.5. The DSC thermal analysis of the polymer deposited from the solution showed an endothermal peak of imidation at 130°C, and an exothermal peak of heat decomposition at above 340°C. This solution was applied on a silicon substrate by spin coating, and the solvent was evaporated off. The resulting film was heated at 300°C for one hour to cause imidation. The film thickness was controlled to be 0.5 µm. Fig. 1A shows the infrared spectrum of the polyamic acid before imidation, and Fig. 1B shows the spectrum after the imidation by heat treatment. The absorption peak of 1720 cm -1 is assigned to the imide ring of pyromellitic acid, and the absorption peaks of 1490 cm -1 and 3400 cm -1 are assigned to carbazole. Example 2 (Synthesis of polyamic acid by polyaddition of diaminocarbazole with pyromellitic anhydride, and imidation by chemical treatment) 0.546 g (0.0028 mol) of 3,6-diaminocarbazole was placed in a 50-ml three-necked flask. Thereto, approximately 2 ml of dried dimethylacetamide (DMAC) was added. The interior of the flask was kept dry by flowing dry nitrogen, and the content was stirred. Subsequently, 0.666 g (0.0031 mol) of solid pyromellitic anhydride was added into the flask, whereby the temperature in the flask rose slightly. Further thereto, DMAC was added in the total amount of DMAC of 10 ml. The content of the flask was stirred for one hour at room temperature to prepare a solution of a polyamic acid having a carbazole skeleton in dimethylacetamide. The intrinsic viscosity of the product was 0.5. This solution was applied on a silicon substrate by spin coating. After evaporation of the solvent, the film on the substrate was immersed into a mixed solution consisting of benzene, pyridine, and acetic anhydride at a mixing ratio of 3:1:1 to cause imidation. The imidation proceeded similarly as in Example 1. The film thickness was controlled to be at 0.5 µm. The thermal property was the same as in Example 1. Examples 3 to 8 The polyimides were prepared in the same manner as in Example 1 except that the imidation conditions and the tetracarboxylic anhydride employed were as shown in Table 1. The results are shown in Table 1. The polyimides all exhibited high decomposition temperature and superior heat resistance. Example 9 A device as shown in Fig. 2 was prepared. The polyamic acid solution used in Example 1 was applied by spin coating on a glass substrate 2 having a light-transmissive electrode (ITO) 1 in stripe shape of 1 mm in width. The solvent was evaporated off. The coated film was heated at 300°C for one hour to cause imidation. The thickness of the film 3 was controlled to be 0.5 µm. Further thereon, an aluminum electrode 4 of 1 mm in width was formed in the direction perpendicular to the light transmissive electrode in a thickness of 100 nm by a resistance-heating method (refer to Fig. 2). For measurement of the photoelectric characteristics of the complete device, insulation was provided and the photovoltaic effect and the photoelectric current were observed. After heat treatment of the device further at 300°C for one hour, the measurement of photoelectric characteristics gave the same values as those before the heat treatment, showing superior heat stability. Example 10 A polyamic acid monomolecular built-up film from 78 layers was formed on a glass substrate having a light-transmissive electrode (ITO) in stripe shape of 1 mm in width and treated for hydrophobicity with a silane-coupling agent. The built-up film was heated at 300°C for one hour to cause imidation and to form a photoconductive thin film. The film thickness was 48 nm. The polyamic acid built-up film was formed according to the procedure below. The polyamic acid solution used in Example 1 was diluted with dimethylacetamide (DMAC) to a concentration of 1 × 10 -3 mol/l in terms of the monomer concentration. Thereto N,N-dimethylhexadecylamine was mixed at a ratio of 2 relative to one equivalent of the repeating unit of polyamic acid to prepare a solution of a salt of polyamic acid-N,N-dimethylhexadecylamine. This solution was spread over pure water at temperature of 20°C to form a monomolecular film on the water surface. The surface pressure was raised to 25 mN. With the surface pressure kept constant, the aforementioned glass substrate having the electrode was dipped and lifted in a direction of crossing the water surface at a rate of 5 mm/min. By repeating this operation, a built-up film was prepared in 78 monomolecular layers. On this substrate, an aluminum electrode of 1 mm in width was formed in the direction perpendicular to the light transmissive electrode in a thickness of 100 nm by a resistance-heating method. For measurement of the photoelectric characteristics of the completed device, insulation was provided and the photovoltaic effect and the photoelectric current were observed at nearly the same level as those of polyvinylcarbazole. After heat treatment of the device at 300°C for one hour, the measurement of the photoelectric characteristics gave the same values as those before the heat treatment, showing superior heat stability. Examples 11 to 16 The polyimides employed were derived by polymerizing diaminocarbazole with the organic compound shown in the column of Acid anhydride in Table 2, and were formed into a thin film on a silicon substrate by the method shown in Table 2. Heat treatment was conducted by heating at 300°C for one hour. The test specimens for measurment of the photoconductive properties were provided with an upper electrode of Al (100 nm) and a lower electrode of ITO (500 nm). The results are shown in Table 2. The resulting polyimide had a high decomposition temperature and deterioration of the photoconductive properties was not observed by further heat treatment at 300°C for one hour. Comparative example A solution of polyvinylcarbazole was applied by spin coating on a glass substrate having a light-transmissive electrode (ITO) in stripe shape of 1 mm in width and having been treated for hydrophobicity with a silane coupling agent. The solvent was evaporated off by heating in an oven at 150°C for one hour. The thickness of the film was approximately 1 µm. Further thereon, an aluminum electrode of 1 mm in width was formed in the direction perpendicular to the light transmissive electrode in a thickness of 100 nm by a resistance-heating method. For measurement of the photoelectric characteristics of the completed device, insulation was provided and the photovoltaic effect and the photoelectric current were observed. The insulation was further treated by heating at 300°C for one hour, By this treatment, the device was lost, and measurement of the photoelectric characteristics could not be conducted. As described above, the photoconductive polyimide compound of the present invention is superior in photoconductivity and heat resistance as well as workability, and enables production of photoelecto-transducing device employing an organic material which was difficult until now. The polyimide compound is prepared by a simple operation. Hence the present invention is greatly usful practically. Furthermore, the photoconductive device comprising the above compound of the present invention enables production of a photoelectro-transducing device extremely superior in heat resistance by using an organic material which has not been readily achievable until now, and the device can be formed in a simple manner. Therefore the present invention is significantly useful from a practical point of view.

Claims: A photoconductive device comprising an organic film held between a pair of electrodes, said organic film comprising a polymer whose repeating units contain a carbazole skeleton and an imide linkage. The device of claim 1, wherein the repeating units are represented by the general formula wherein R 1 represents a tetravalent organic group having 5 to 13 carbon atoms. The device of claim 2, wherein R 1 in the general formula set out in claim 2 is a tetravalent cyclic hydrocarbon group containing 5 to 13 carbon atoms. The device of claim 3, wherein the group R 1 in the formula set out in claim 2 is any of the following groups A device according to any of claims 1 to 4, wherein the organic film is built up from a plurality of monomolecular layers of the polymer. The device of any of claims 1 to 5, wherein the polymer has a decomposition temperature not below 300°C. The device of any of claims 1 to 6, wherein at least one of said pair of electrodes is light-transmissive.

Inventors: KAWADA HARUKI, KAWAGISHI HIDEYUKI, KURIHARA SUOMI, MATSUDA HIROSHI, MORIKAWA YUKO, TAKIMOTO KIYOSHI, YANAGISAWA YOSHIHIRO, KAWADA, HARUKI, KAWAGISHI, HIDEYUKI, KURIHARA, SUOMI, MATSUDA, HIROSHI, MORIKAWA, YUKO, TAKIMOTO, KIYOSHI, YANAGISAWA, YOSHIHIRO, KAWADA, HARUKI, C/O CANON KABUSHIKI KAISHA, KAWAGISHI, HIDEYUKI, C/O CANON KABUSHIKI KAISHA, KURIHARA, SUOMI, C/O CANON KABUSHIKI KAISHA, MATSUDA, HIROSHI, C/O CANON KABUSHIKI KAISHA, MORIKAWA, YUKO, C/O CANON KABUSHIKI KAISHA, TAKIMOTO, KIYOSHI, C/O CANON KABUSHIKI KAISHA, YANAGISAWA, YOSHIHIRO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L51
IPC codes: C08G73, H01L51