Abstract:
A flat organic photodetector has a structured first electrode that forms several sub-electrodes, a second electrode, at least one first organic layer, and a second organic layer. The organic layers are situated between the two electrodes and are structured in conformity with the structuring of the first electrode, so that the two organic layers are subdivided into multiple active regions respectively corresponding to the sub-electrodes of the first electrode. An x-ray detector has such a flat organic photodetector and an x-ray absorbing layer applied thereon.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a laminar organic photodetector, an organic x-ray flat panel detector, method for production of a laminar organic photodetector, and a method for production of an organic x-ray flat panel detector. 
     2. Description of the Prior Art 
     With a laminar photodetector, light striking the photodetector is transduced into electrical signals that can be converted into an image data set with a suitable evaluation device. Laminar organic photodetectors, namely photodetectors with a photoactive layer made from an organic semiconductor material, represent an interesting alternative to photodetectors with a photoactive layer made from an inorganic material. 
     The production of an x-ray flat panel detector with an organic photodetector can be relatively cost-effective, it is the object of an x-ray flat panel detector to transduce an x-ray radiation penetrating through an examination subject (and thereby attenuated) into corresponding electrical signals that can then in turn be converted into an x-ray image data set with an evaluation device. The x-ray image associated with the x-ray image data set can be visualized with a viewing apparatus. Such an x-ray flat panel detector is known from United States Patent Application Publication No. 2003/0025084 A1, for example. 
     In particular, large-area photodetectors as are sometimes used for x-ray flat panel detectors, can be produced relatively cost-effectively because the organic layers of the organic photodetector can be applied over a large area with relatively cost-effective methods such as, for example, with rotation coating (spin coating), scraping or printing techniques. 
       FIG. 1  shows in section an example of a laminar organic photodetector PD 1  for explanation of the general problem associated therewith. 
     The photodetector PD 1  shown in section in  FIG. 1  has a number of layers  1  through  7  attached to one another. The known photodetector PD 1  has a laminar substrate  1  in which transistors (not shown in  FIG. 1 ) are embedded in a matrix configuration. Each of the individual transistors is associated with one of the pixels of the image to be acquired with the photodetector PD 1 . 
     A passivation layer  2  is applied on the substrate  1 , on which passivation layer  2  is structured in turn (for example by means of a lithography process) a laminar and structured electrode  3  that is shown in section in plan view in  FIG. 2 . For example, the structured electrode  3  is formed of gold, platinum, palladium, silver or indium-tin oxide. 
     As can be seen from  FIG. 2 , the electrode  3  is structured like a matrix and has a number of sub-electrodes  3   a  through  3   r  that are electrically insulated from one another. Each of the sub-electrodes  3   a  through  3   r  is electrically connected with one of the respective transistors of the substrate  1 . Each of the sub-electrodes  3   a  through  3   r  is therefore respectively associated with one of the pixels of the image to be acquired with the photodetector PD 1 . 
     An organic hole transport layer  4  (for example made from PEDOT:PSS) is applied over the area of the laminar and structured electrode  3 . A photoactive layer  5  (made from an organic semiconductor material, for example poly-3-hexylthiophene/PCBM) is in turn applied over the area of the laminar organic hole transport layer  4 . 
     The laminar organic photoactive layer  5  connects to an unstructured, at least semi-transparent laminar electrode  6 . The laminar electrode  6  is, for example, a thin metal layer made from calcium or silver. In order to protect the photodetector PD 1  from damage and degradation due to oxygen and moisture, a protective layer  7  is finally applied on the electrode  6 . The protective layer is formed, for example, of glass, an optimally transparent polymer, or a multi-layer system made from organic polymers and inorganic barrier layers such as Al 2 O 3  or Si 3 N 4 . 
     If an image is to be acquired with the photodetector PD 1 , the light distribution associated with the image thus penetrates the protective layer  7  and the at least semi-transparent electrode  6  and is transduced into electrical signals by the photoactive layer  5  in connection with the hole transport layer  4  and the two electrodes  6  and  3 , which electrical signals are read out with the transistors of the substrate  1 . The read signals are in turn relayed to an evaluation device (not shown in  FIGS. 1 and 2  but known to those skilled in the art) and are processed into an image data set. The image data set can then be visualized as an image with a viewing apparatus (likewise not shown in  FIGS. 1 and 2 ). 
     The image is constructed of a number of pixels. Each of the sub-electrodes  3   a  through  3   r  of the structured laminar electrode  3  or each transistor of the substrate  1  that is connected with a corresponding sub-electrode is associated with one of these pixels. 
     The two organic layers  4  and  5  have a relatively high conductivity and therefore a relatively high quantum efficiency in a range from 60% to 85%. However, since the two organic layers  4  and  5  are applied unstructured over the area of on the structured electrode  3  and the two organic layers have a relatively high transverse conductivity (i.e. a conductivity parallel to their area dimensions), it leads to a relatively large crosstalk of the electrical signals destined for the respective sub-electrodes  3   a  through  3   r  or their associated transistors of the substrate  1 . A limited spatial resolution of the image acquired with the photodetector PD 1  is the consequence. 
     SUMMARY OF THE INVENTION 
     An object of the invention to provide a laminar organic photodetector and an x-ray flat panel detector with an organic photodetector such that the spatial resolution of the image acquired with the photodetector or of the x-ray image acquired with the x-ray flat panel detector is improved. It is a further object of the invention to specify suitable methods for production of such a laminar organic photodetector or, respectively, x-ray flat panel detector. 
     This object is achieved in accordance with the present invention by a laminar organic photodetector having a structured first electrode having a number of sub-electrodes, a second electrode, at least one first organic layer and a second organic layer, wherein the two organic layers are arranged between the two electrodes and are structured corresponding to the first electrode such that the two organic layers are sub-divided into a number of active regions associated with the individual sub-electrodes of the first electrode. It is the basic idea for the present inventive photodetector that the two organic layers are not applied unstructured on the structured electrode (whose sub-electrodes are respectively associated with one pixel of the image to be acquired with the printing device), as is typical according to the prior art. Instead, the two organic layers are likewise structured corresponding to the structured electrode. Due to the structuring of the organic layers, an active region of the organic layers is associated with each of the sub-electrodes of the first electrode, meaning that not only one sub-electrode of the first electrode but also an active region of the organic layers is associated with each pixel of the image acquired with the photodetector. This meets requirements that the individual active regions associated with the pixels of the photodetector have optimally little influence. Crosstalk within the organic layers of the signals associated with the image to be acquired can thus at least be significantly reduced, so the spatial resolution of the acquired image is improved. 
     The two organic layers are a photoactive layer and a hole transport layer made from organic materials. Suitable organic materials for the photoactive layer are, for example, poly-3-hexylthiophene (P3HT) or general poly-3-alhylthiophene (P3AT) and PPVs as hole-transporting materials or PCBM as electron-transporting materials. Further organic materials are, among others, CuPc/PTCBI, ZNPC/C60, conjugated polymer components or fullerene components. PEDOT:PSS is also a suitable organic material for the hole transport layer, for example. 
     According to one variant of the inventive photodetector, the active regions of the organic layers are separated from one another by trenches. The trenches can advantageously be inserted into the organic layers with a laser. For production-related reasons it has proven to be advantageous when, according to preferred variants of the inventive organic photodetector, the trenches of the two organic layers have a width smaller than 50 μm and/or a width greater than 5 μm. 
     According to a preferred embodiment of the inventive laminar organic photodetector, an additional material divides the active regions of the two organic layers from one another. The additional material is, for example, a photoresist that is initially applied to the first electrode in the production of the photodetector and, for example, exhibits an overhanging structure. Given an overhanging structure, regions of the webs of the wall structure that are further removed from the first electrode overhang the active regions, similar to a mushroom. Overhanging structures are, for example, known from the technology for organic LEDs and known as “mushrooms” (mushrooms). The two organic layers can subsequently be applied on the first electrode provided with the photoresist, such that the individual active regions of the two organic layers are separated by the photoresist. The photoresist structure preferably has a width smaller than 20 μm and/or a width greater than 2 μm. 
     According to a preferred embodiment of the inventive laminar organic photodetector, the additional material has a property of repelling the organic materials of the organic layers. Examples of such a material are a fluorinated photoresist or a photoresist on which a fluorinated plasma is applied. In the production of the photodetector the photoresist is applied at the points on the first electrode at which the individual active regions of the organic layers should be separated from one another. Due to the poor wettability of, for example, the fluorinated photoresist, the two organic layers are interrupted at the points at which the fluorinated photoresist is applied, wherein the organic layers are structured. The individual webs of the wall structure preferably have a relatively flat angle of approximately 3° to 30° relative to the first electrode. It is thereby possible to vacuum deposit or sputter the two electrodes onto the organic layers. Moreover, requirements that the two electrodes do not tear are met due to the relatively flat angle. 
     The two electrodes are an anode/cathode pair. Depending on whether the first electrode is a cathode or an anode, the first organic layer is either the hole transport layer or the photoactive layer. 
     In order to protect the inventive organic photodetector from contamination, damage or degradation, according to an embodiment of the photodetector a laminar protective layer is applied on the second electrode. 
     Since the use of organic photodetectors is particularly interesting for x-ray flat panel detectors, it is provided in particular to use the inventive organic photodetector as an x-ray flat panel detector. Such an inventive x-ray flat panel detector comprises a layer absorbing x-rays, which layer is applied on the inventive laminar organic photodetector. A suitable layer absorbing x-rays (which layer is also designated as a scintillator) comprises cesium iodide, for example. 
     A further object of the invention is achieved by a method for production of a laminar organic photodetector that exhibits a structured first electrode having a number of sub-electrodes, a second electrode, a first organic layer and a second organic layer; wherein the two organic layers are arranged between the two electrodes and are structured corresponding to the first electrode; such that the two organic layers are sub-divided into a number of active regions associated with the individual sub-electrodes of the first electrode; including the following steps: 
     application of an intermediate material on the structured first electrode, wherein the intermediate material exhibits a wall structure corresponding to the structure of the two organic layers, 
     laminar application of the first organic layer on the first laminar electrode, 
     laminar application of the second organic layer on the first organic layer and 
     laminar application of the second electrode on the second organic layer. 
     One difficulty in the structuring of the organic layers is that the organic layers are not damaged in the structuring process. Based on the inventive method it is proposed to produce the inventive organic photodetector layer by layer. The structured first electrode is initially produced as is already typical, for example. The intermediate material (that, according to a preferred variant of the inventive method, is a photoresist) is subsequently applied on the structured first electrode, in particular by means of a lithographic process. Due to the wall structure the two organic layers should be separated into the active regions upon subsequent application. 
     According to an advantageous variant of the inventive method, the wall structure is an overhanging structure. Methods for production of an overhanging structure for a photoresist structure are known in the production of organic LEDs, for example. 
     The two organic layers (which are the photoactive layer and the organic hole transport layer) are subsequently applied in succession on the structured first electrode on which the wall structure is applied. The two organic layers are thereby sub-divided by the overhanging wall structure into a number of active regions independent of one another. The wall structure is preferably executed such that active regions are separated from one another by at least 2 μm and at maximum 20 μm. 
     Finally, the second electrode is applied on the two organic layers. The second electrode can additionally be coated with an optimally transparent protective layer, for example made from glass or an optimally transparent synthetic. 
     According to a further preferred embodiment of the inventive method, the intermediate material has a property of repelling the organic materials of the organic layers. An example of such a photoresist is a fluorinated photoresist. Due to the repellent property of the photoresist, upon application of the organic layers on the first electrode these are poorly wetted, wherein the structuring of the organic layers arises. 
     The second object of the invention is also achieved by a method for production of a laminar organic photodetector comprising the following method steps: 
     laminar application of a first organic layer on a structured first electrode that, due to the structuring, comprises a number of sub-electrodes, 
     laminar application of a second organic layer on the first organic layer, 
     insertion of a trench structure into the two organic layers with a laser, whereby the trench structure corresponds to the structure of the first electrode and 
     laminar application of a second electrode on the second organic layer. 
     After the two organic layers have been applied on the structured first electrode, the trenches (which preferably have a width smaller than 50 μm and/or greater than 5 μm) are thus inventively lazed into the organic layers (laser patterning process). In particular if a laser with light in the visible or near-UV range is used, a damage of the organic layers due to the lasers is thus not to be expected. 
     In order to protect the photodetector, the second electrode can additionally be coated with an optimally transparent protective layer (for example made from glass or an optimally transparent synthetic). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a sectional view of a portion of an organic photodetector. 
         FIG. 2  is a plan view of the portion of the photodetector shown in  FIG. 1 . 
         FIGS. 3 through 6  respectively show stages in the production of the organic photodetector in accordance with the present invention. 
         FIGS. 7 through 10  respectively show stages in the production of a further embodiment of an organic photodetector in accordance with the present invention. 
         FIGS. 11 through 14  respectively show stages in the production of another embodiment of an organic photodetector in accordance with the present invention. 
         FIG. 15  illustrates an x-ray flat panel detector in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The production of an inventive photodetector PD 2  is illustrated with  FIGS. 3 through 6 : 
       FIG. 3  shows a portion of substrate  31  with an applied passivation layer  32 . The substrate  31  comprises a number of transistors (not shown in detail in Figures) arranged like a matrix that, in the case of the present exemplary embodiment, are thin film transistors (TFTs). In principle the substrate  31  can also embody other transistors such as in particular organic field effect transistors. 
     A structured laminar electrode  33  (that, in the case of the present exemplary embodiment, is produced from gold) is applied on the passivation layer  32 . The electrode  33  is structured like a matrix, wherein it is sub-divided into a number of sub-electrodes  33   a  through  33   h  electrically insulated from one another. Each of the sub-electrodes  33   a  through  33   h  is electrically connected with one of the transistors of the substrate  31  and is thus respectively associated with a pixel of an image to be acquired with the photodetector PD 2 . 
     A hole transport layer  34  made of an organic material is subsequently areally applied on the structured electrode  33 , for example by rotation coating (spin coating), scraping or printing techniques. In the case of the present exemplary embodiment PEDOT:PSS is used as an organic material. A photoactive layer  35  made from an organic semiconductor material (P3HT/PCBM in the case of the present exemplary embodiment) is subsequently applied on the hole transport layer  34 , as is illustrated in  FIG. 4 . 
     Before the photoactive layer  35  is now provided with a laminar electrode  36 , the two organic layers (i.e. the photoactive layer  35  and the hole transport layer  34 ) are structured with a laser (not shown in Figures), as this is shown in  FIG. 5 . 
     In the case of the present exemplary embodiment, trenches  37  with a width from 5 μm up to a maximum of 50 μm are lazed into the two organic layers  34  and  35  with the laser. The trenches  37  are placed and are deep enough such that the hole transport layer  34  and the photoactive layer  35  are sub-divided into active regions  35   a  through  35   h  separated from one another, of which respectively one active region  35   a  through  35   h  interacts with respectively one sub-electrode  33   a  through  33   h  of the electrode  33 , but is not electrically connected with adjacent sub-electrodes  33   a  through  33   h . In order to not damage the two organic layers  34  and  35 , in the case of the present exemplary embodiment a laser that emits light in the visible or near-UV range is used. 
     The unstructured laminar electrode  36 , which in the present exemplary embodiment is provided with a protective layer  38  made from glass, is subsequently applied on the structured photoactive layer  35 . This is shown in  FIG. 6 . 
       FIGS. 7 through 10  illustrate an alternative method for production of an inventive photodetector PD 3 . If it is not specified otherwise in the following, components of the photodetector PD 3  shown in  FIGS. 7 through 10  which are largely structurally and functionally identical with components of the photodetector PD 2  shown in  FIGS. 3 through 6  are provided with the same reference characters. 
     In contrast to the production methods illustrated in  FIGS. 3 through 6 , in the alternative production method a photoresist is initially applied on the structured electrode  33 , which photoresist is structured like a matrix by means of a lithographic process (known, for example, in the production of organic LEDs or inorganic components). The arising photoresist structure  71  is shown in detail in  FIG. 7 . 
     As can be seen from  FIG. 7 , in the case of the present exemplary embodiment the photoresist structure  71  is an overhanging structure in which photoresist is applied in the regions in which the individual sub-electrodes  33   a  through  33   h  are electrically insulated from one another and the active regions of the individual sub-electrodes  33   a  through  33   h  are essentially free of photoresist. Given an overhanging structure, regions of the photoresist structure  71  that are further removed from the electrode  22  overhang the sub-electrodes  33   a - 33   h . In the case of the present exemplary embodiment, the overhanging photoresist structure  71  has a width from 2 μm to a maximum of 20 μm. 
     A hole transport layer  74  made from an organic material is subsequently applied on the structured electrode  33  provided with the photoresist structure  71 . In the exemplary embodiment, PEDOT:PSS is used as an organic material. The electrode  33  provided with the hole transport layer  74  is shown in  FIG. 8 . 
     A photoactive layer  75  made from an organic semiconductor material (P3HT/PCBM in the case of the present exemplary embodiment) is subsequently applied on the hole transport layer  74 , as is illustrated in  FIG. 9 . 
     As can be seen from  FIGS. 8 and 9 , the photoresist structure  71  causes the hole transport layer  74  and the photoactive layer  75  to be sub-divided into active regions  75   a  through  75   h  separated from one another, of which respectively one active region  75   a  through  75   h  interacts with respectively one sub-electrode  33   a  through  33   h  of the electrode  33 , however adjacent sub-electrodes are optimally uninfluenced. 
     An unstructured laminar electrode  36  that, in the case of the present exemplary embodiment, is provided with a protective layer  38  made from glass is subsequently applied on the photoactive layer  75 . This electrode  36  is shown in  FIG. 10 . 
     The production of a further flat panel detector PD 4  is shown in  FIGS. 11 through 14 . If it is not specified otherwise in the following, components of the photodetector PD 4  shown in  FIGS. 11 through 14  which are largely structurally and functionally identical with components of the photodetector PD 3  shown in  FIGS. 7 through 10  are provided with the same reference characters. 
     In contrast to the production methods illustrated in  FIGS. 3 through 6 , in the production method shown in  FIGS. 11 through 13  a fluorinated photoresist is initially applied on the structured electrode  33 . The fluorinated photoresist has the property that it is poorly wetted by the organic materials for the photoactive layer and the hole transport layer of the photodetector PD 4 . The fluorinated photoresist is applied at the points that divide the sub-electrodes  33   a  through  33   h  of the first electrode  33 . A wall structure  111  thereby arises that, in the case of the present exemplary embodiment, has a vertical dimension of approximately 0.5 μm to 5 μm. Furthermore, in the case of the present exemplary embodiment the wall structure  111  has a rounded expansion, whereby the angle α is approximately 3° to 30° between the first electrode  33  and the surface of the wall structure  111 . Alternatively, the poor wettability of the photoresist of the wall structure  111  can also be achieved in that the wall structure  111  is initially applied on the electrode  33 , wherein the wall structure  111  has an insulating material that does not necessarily have the property of de-crosslinking organic materials. This wall structure is, for example, subsequently provided with a fluorinated plasma; this in turn has a de-crosslinking property. 
     A hole transport layer  114  made from an organic material is subsequently applied on the structured electrode  33  provided with the wall structure  111 , on which hole transport layer  114  is in turn applied a photoactive layer  115  made from an organic semiconductor material (P3HT/PCBM in the exemplary embodiment). This is illustrated in  FIG. 12 . In the exemplary embodiment, PEDOT:PSS is used as an organic material. The hole transport layer  114  has a vertical dimension of approximately 10 nm to 100 nm and the photoactive layer  115  has a vertical dimension of approximately 100 nm to 1000 nm. 
     In that the photoresist of the wall structure  111  exerts a de-crosslinking effect on the organic materials of the hole transport layer  114  and of the photoactive layer  115 , the hole transport layer  114  and the photoactive layer  115  are sub-divided into active regions associated with sub-electrodes  33   a  through  33   h  of the electrode  33 . 
     An unstructured laminar electrode  116  is subsequently vacuum deposited or sputtered onto the photoactive layer  116 . In the exemplary embodiment, the laminar electrode  116  replicates the surface structure formed by the photoresist, the hole transport layer  114  and the photoactive layer  115 . Via the rounded expansion of the wall structure  111  it is possible to avoid a tearing of the electrode  116 , even given a relatively thin hole transport layer  114  and a relatively thin photoactive layer  115 . The vapor-deposited or sputtered electrode  116  is shown in  FIG. 13 . 
     In order to protect the photodetector PD 4  from, for example, contamination or damage, in the case of the present exemplary embodiment the electrode  116  is provided with a protective layer  38  made from glass. This is shown in  FIG. 14 . 
       FIG. 15  shows an inventive x-ry flat panel detector RPD. The x-ry flat panel detector RPD essentially includes the photodetector PD 2 , PD 3  or PD 4 , an x-ray-absorbing layer SZ (that, in the case of the present exemplary embodiment, comprises cesium iodide and is applied on the protective layer  38  of the photodetector PD 2 , PD 3  or, respectively, PD 4 ) and a housing G that surrounds the photodetector PD 2 , PD 3  or, respectively, PD 4  with applied x-ray-absorbing layer SZ. The housing G is produced from an optimally x-ray-transparent material (for example aluminum) at least on the side facing towards the x-ray-absorbing layer SZ. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.