Photoconductive phenomena

From the general discussion of the use of nonlinear properties to investigate dynamic phenomena it is clear that the field modulation techniques is but one example of a broader class of methods for the study of fast processes. A drawback, particular to electric field modulation, is the prohibitive heat dissipation in conducting systems. However, any forcing parameter imposing a conductance modulation could be used in principle as, for example, in the study of the dynamics of photoconductive phenomena. 

Photovoltaic and photoconductive phenomena for various types of CT complexes between saturated polymers and dopant molecules, heterojunctions between polymers and organic and inorganic photoconductors were also investigated in the last few years [86-92]. The quantum efficiency of the energy conversion of 10-3% was obtained for such systems and output power density of 3 x 102 mV cm-2. The mobilities of the heterogeneous polymer systems with despersed inorganic photoconductors reach the value — 10 3-10-4 m2 V 1s 1. 

Within the scope of this review it is not possible to give a comprehensive overview over the research activities concerning photoconductive phenomena of phthalocyaninato and porphyrinato complexes. Thus studies on phthalocyanin-atometal complexes as well as bridged phthalocyaninato systems relating photoconductivity to molecular structure will be sununarized. 

Another disadvantageous phenomenon in TFTs is the photoconductivity of a-Si H [626]. Electrons and holes are photogenerated and recombine at the back surface (gate insulator). The photocurrent reduces the on/off ratio of the TFT. Illumination, however, cannot always be avoided, e.g., in active matrix displays. A way of circumventing this is to make the a-Si H as thin as possible. 

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. 

An atom or molecule that approaches the surface of a solid always experiences a net attractive potential ). As a result there is a finite probability that it is trapped on the surface and the phenomenon that we call adsorption occurs. Under the usual environmental conditions (about one atmosphere and 300 K and in the presence of oxygen, nitrogen, water vapor and assorted hydrocarbons) all solid surfaces are covered with a monolayer of adsorbate and the build-up of multiple adsorbate layers is often detectable. The constant presence of the adsorbate layer influences all the chemical, mechanical and electronic surface properties. Adhesion, lubrication, the onset of chemical corrosion or photoconductivity are just a few of the many macroscopic surface processes that are controlled by the various properties of a monolayer of adsorbates. 

Melnick has suggested that even the fast photoconductive rise and decay is a surface phenomenon, involving the same reactions and model as the slow responses. Again he shows agreement between the theoretical predictions and the experimental results. 

The presence of a photoconductivity peak at 610 nm at the threshold of the absorption spectrum (curve 4) is a common phenomenon in inorganic semiconductors and is explained by competition between surface and volume recombination processes of the charge carriers. The optical activation energy determined from the spectral photoconductivity threshold is equal to 1.82 + 0.02 eV. The thresholds of the photoelectromotive force and the absorption spectra are likewise in agreement with this value. It is remarkable that the same value has been found for the activation energy of the dark conductivity in this polymer... 

If, instead of thermal excitation, a photon of light excites an electron from the valence band to the conduction band, the same situation of electron and hole carriers obtains, and one observes the phenomenon of photoconductivity, useful in photocells and similar devices. 

The onset of photoconductivity is determined from the intersection point of the x-axis (dark current) and the extrapolated line from the linear part of the conductivity. All spectra show a pronounced onset of photoconductivity at 2.40 eV (Na S), 2.25 eV (K S), 2.15 eV (Na Se), and 2.10 eV (K Se), respectively, which is found to be in remarkable agreement with the optical gap. This proves that the photoconductivity is a bulk property instead of a surface conduction phenomenon. 

This means that studies of the conductivity alone are insufficient to obtain a complete understanding of the mechanism of charge transfer. Other techniques such as photoconductivity, thermoelectric effect and mobility measurements are required for a deeper understanding of the phenomenon. 

Photoelectrochemical behavior of metal phthalocyanine solid films (p-type photoconductors) have been studied at both metal (93,94,95,96) and semiconductor (97,98) electrodes. Copper phthalocyanine vacuum-deposited on a Sn02 OTE (97) displayed photocurrents with signs depending on the thickness of film as well as the electrode potential. Besides anodic photocurrents due to normal dye sensitization phenomenon on an n-type semiconductor, enhanced cathodic photocurrents were observed with thicker films due to a bulk effect (p-type photoconductivity) of the dye layer. Meier et al. (9j>) studied the cathodic photocurrent behavior of various metal phthalocyanines on platinum electrodes where the dye layer acted as a typical p-type organic semiconductor. 

As explained in the introduction, the polysilanes (and related polygermanes and poly-stannanes) are different from all other high polymers, in that they exhibit sigma-electron delocalization. This phenomenon leads to special physical properties strong electronic absorption, conductivity, photoconductivity, photosensitivity, and so on, which are crucial for many of the technological applications of polysilanes. Other polymers, such as polyacetylene and polythiophene, display electron delocalization, but in these materials the delocalization involves pi-electrons. 

Photoconductivity in a solid is defined as an increase of conductivity caused by radiation. The phenomenon of photoconductivity involves the processes of absorption of radiation, photogeneration of charge carriers, their separation, diffusion and drift in an applied electric field, their temporary immobilization at sites known as trapping rites, release from traps and finally their recombination. The phenomenological relationships covering all these processes were primarily developed in connection with the study of crystalline covalent solids which dominated the early scientific literature on photoconductivity. Concurrent with the basic understanding of the phenomena was the development of several experimental techniques to study the fundamental processes and the specific identity of the defects and impurities that control these processes. 

The phenomenon of photoconductivity, that of nitro compounds was recently reviewed by Jarosiewicz (25). 

Szyehlihski [29] described the same phenomenon and found the it fluctice of the solvent, for example the photoconductivity, is prominent in ethyl ether or... 

Photoconductivity is based on the conversion of light to electricity. The reverse phenomenon, electroluminescence, is based on the conversion of electricity to light. Electroluminescence is useful for flat-panel display and 11-VI semiconductors such as ZnS are employed for this purpose [132], The current trend is toward the development of polymeric electroluminescent material for their processing flexibility [133,134]. It has already been demonstrated that properly doped semiconductor nanoclusters such as ZnIMn1 IS emits light efficiently [135], With the demonstration of photoconductivity [101 103] these nanocluster-doped polymers can become interesting candidates of electroluminescent materials. No experimental work has been performed yet. 

The phenomenon of excitonlc energy transport in polymer films has been studied actively for the past decade (2 ). This photophysl-cal process is relevant to photodegradation and photoconductivity in polymers, but in this contribution we wish to emphasize the potential application of polymer films as photon "harvesters" with subsequent transfer of energy to a reaction center, analogous to the so-called "antenna effect" in chloroplasts. 

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