Photoconductive and Photovoltaic Devices

Organic photoconductivity is generally explained in terms of photogeneration not of free electrons and holes, but rather of bound electron-hole pairs (excitons) and their subsequent dissociation into charge carriers which must then be transported to the electrodes  

For a much more detailed review of the organic photoconductivity, the interested reader is referred to the excellent books by Kao and Hwang [106] and (particularly for xerographic applications) Borsenberger and Weiss [5] and references therein. 

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A photoconductive detector is a semiconductor whose conductivity increases when infrared radiation excites electrons from the valence band to the conduction band. Photovoltaic detectors contain pn junctions, across which an electric field exists. Absorption of infrared radiation creates electrons and holes, which are attracted to opposite sides of the junction and which change the voltage across the junction. Mercury cadmium telluride (Hg,. Cd/Te, 0 < x < 1) is a detector material whose sensitivity to different wavelengths is affected by the stoichiome-try coefficient, x. Photoconductive and photovoltaic devices can be cooled to 77 K (liquid nitrogen temperature) to reduce thermal electric noise by more than an order of magnitude. 

A photomultiplier tube is a sensitive detector of visible and ultraviolet radiation photons cause electrons to be ejected from a metallic cathode. The signal is amplified at each successive dynode on which the photoelectrons impinge. Photodiode arrays and charge coupled devices are solid-state detectors in which photons create electrons and holes in semiconductor materials. Coupled to a polychromator, these devices can record all wavelengths of a spectrum simultaneously, with resolution limited by the number and spacing of detector elements. Common infrared detectors include thermocouples, ferroelectric materials, and photoconductive and photovoltaic devices. 

Kazaoui S, Minami N, Nalini B, Kim Y, Hara K (2005) Near-infrared photoconductive and photovoltaic devices using single-wall carbon nanotubes in conductive polymer films. J Appl Phys 98 084314... 

The reaction of solid porphyrin films with light in the presence of oxygen by producing MgTPP must affect electrical properties, in particular semi conduct on, photoconduction, and photovoltaic properties. We have provided evidence for "photodoping" by light and oxygen, a phenomenon that must be clearly understood if these materials are to have device applications. 

Heterogeneous mixing of fullerenes and fullerene derivatives with Ji-conjugated polymers has been used to produce excellent materials for photovoltaic devices [141], Upon irradiation of fullerene/polymer blends, charge transfer from the polymer to occurs, resulting in efficient photoconductivities. Better behavior of fullerene derivatives than with pristine Cgg has been observed, and attributed to the improved miscibility of the functionalized species. 

Photocurrent — Contribution to the current in a -+ semiconductor due to photon incidence. For both inorganic and organic photoconductors the light intensity II and the photocurrent Jph are related by Iph = l[y where y is characteristic of the photoconducting system. In photovoltaic devices photocurrent denotes the difference between the -> current generated by the device under illumination and the current with the device in the dark, the dark current. See also photoconductivity. 

Figure 4.87 Use of a photodiode a as a photovoltaic device and b as a photoconductive resistor...

The use of materials with different work functions as electrodes in a sandwich structure introduces a built-in electric field in the polymeric layer that can be used in photoconductive tems to produce photovoltaic devices [221]. In these devices, the incident light generates free charge carriers that are transported under the influence of the built-in electric field and may produce an electric current in an external circuit. Earlier PPV-based devices constructed on an ITO/PPV/X structure (X Al, Mg, Ca PPV film... 

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