Page 240 - Organic Electronics in Sensors and Biotechnology
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An Intr oduction to Or ganic Photodetectors     217

               the geometrical capacitance density a value of ~300 pF/mm . This
                                                                    2
               compares with about 30 pF/mm  for a typical silicon photodetector
                                           2
                                                     2
               (Hamamatsu S4797-01) and about 3 pF/mm  for a PIN-type silicon
               photodetector (Hamamatsu S5821). There is some scope for lowering
               the capacitance of OPV devices by using thicker films, but this prob-
               ably allows for a 5- to 10-fold reduction at best (since the use of exces-
               sively thick films frustrates the extraction of charge carriers and tends
               to reduce device efficiencies). Organic photodiodes are therefore rela-
               tively high-capacitance devices, which have important implications
               for their noise characteristics and speed of response.




          6.4 Device Characteristics

               6.4.1 Spectral Response
               The spectral response of a photodetector is normally characterized in
               terms of either its “photosensitivity” or its quantum efficiency, both
               of which are ordinarily measured under short-circuit conditions. The
               photosensitivity S(λ)  at an illumination wavelength λ is defined as the
               ratio of the photocurrent I(λ) to the incident power P(λ):

                                             λ
                                            I()
                                      S() =                         (6.17)
                                        λ
                                             λ
                                            P()
               In situations where the photodiode is illuminated by a broadband
               excitation source, the resultant photocurrent I is given by

                                      λ
                                       max
                                   I =  ∫  P()λ  S()λ  dλ           (6.18)
                                      λ
                                       min
               The quantum efficiency η(λ) is the fraction of incident photons that
               are successfully converted to a photocurrent in the external circuit

                                           I  λ () e
                                              /
                                     ηλ() =                         (6.19)
                                           ℜ  λ ()
               where ℜ(λ) is the rate at which photons of wavelength λ impinge on
               the device. Clearly, ℜ(λ) = P(λ)/(hc/λ). The quantum efficiency and
               the photosensitivity are therefore related by the following identity:

                               /
                                         /
                       ηλ() =  I  λ () e  =  I  λ () e  =  hc I  λ ()  =  hc  S ()  (6.20)
                                                             λ
                             ℜ  λ ()  P  λ () (hc/ λ)  λ e  P(()λ  λ e
                                       /
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