Page 273 - Organic Electronics in Sensors and Biotechnology
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250    Cha pte r  S i x



                              Light scattering
             ITO
                Transport layer
                            Conduction
               Generation layer
           Contact pad  Gap
                     Organic sensor
           Passivation
                    TFT   Capacitor
                     Substrate
                        (a)                               (b)
          FIGURE 6.29  (a) Cross section of a pixel in an X-ray imaging circuit comprising an
          organic photodiode and an amorphous silicon driver. (b) Photograph of an X-ray
          image obtained with the sensor array. (Reprinted from Ref. 88. Copyright 2002, with
          permission from Elsevier.)



               efficiencies of ideally 40–50% or greater.” In practice, as we have
               shown above, these performance levels are quite achievable using
               devices available today, so organic photodiode X-ray imagers have
               very considerable promise.
                   There have been relatively few reports of OPD-based XRIs, but
               Street and coworkers have reported a prototype 512 × 512 array with
               a pixel size of 100 μm × 100 μm. Their system used a back plane of a-
               Si TFTs, which was coated with a simple continuous layer of the
               organic sensor material (see Fig. 6.29a). The address circuitry for each
               pixel was similar to Fig. 6.28 and comprised an a-Si thin film transis-
               tor, a 0.4 pF storage capacitor, the address lines, and a contact pad to
               the photodiode. The organic photodiode itself was based on a dis-
               crete heterojunction geometry, comprising a 300 nm vacuum-deposited
               layer of benzimidazole perylene (BZP) and a 10 μm blade-coated hole
               transport layer of tetraphenyldiamine (TPD) dispersed in a binder. †
               The pixels were completed with a semitransparent layer of evapo-
               rated Au/Pt. Finally, a GdO S :Tb scintillator screen (which emits at
                                       2 2
               550 nm) was placed on top of the array. Figure 6.29b shows an XRI
               image obtained using the panel; detailed performance characteristics
               were not provided.
                   The detectors in XRI systems experience varying amounts of X-
               ray exposure depending on factors such as the X-ray source intensity,



               † The large thickness of the TPD layer was intended to minimize the dark current
               although, as shown above, with careful fabrication a conventional thin-film
               structure would suffice. The hole mobility in the TPD layer was reported to be
                             .
                      −5
                         2
               about 10  cm /(V  s), which corresponds to a transit time of 3 ms under a 20 V
               reverse bias, which is broadly sufficient for video imaging purposes.
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