Photoconductors

Quasi-solid state dye-sensitized solar cells (DSCs) have been constructed using a new polymeric ionic fluid as the electrolyte.119 The electrolyte was synthesized by the sol-gel route using MTMSPI+I as the precursor that was made by derivatizing methylimidazolium with triethyoxysilane. Condensation of this material in the presence of formic acid and in the absence of water led to Si-O-Si-O-type polymerization and formation of a polysilsesquioxane-type structure. When this material was mixed with iodine, it served as a redox electrolyte for DSCs. The DSCs made this way are robust and easy to assemble but their efficiency of 3.1% is relatively low. However, possible improvement lies in modification of the organic groups attached to the polysilsesquioxane backbone. 

SiRR -SiRR -SiRR - —h V -SiRR -SiRR. +. SIRR - (2) 

A third process proposed by Michl to be involved in photodegradation is chain cleavage by reductive elimination according to reaction 3  

Copolymers between polysilanes and OH/COH bearing organic polymers have been used for developing procedures in aqueous solutions.47,128 

Photoconductive detectors are based on the increase of electric conductivity in pure or doped semiconductors caused by absorption of light. This increase of the conductivity 

In semiconductors doped with donor or acceptor atoms, optical transitions between discrete levels of these atoms and conduction or valence band are possible. Since these levels are within the band gap, close to the conduction band or valence band, respectively, this extrinsic photoexcitation is already possilbe at low photon energies (Fig.4.77b). Because of these low excitation energies the donor or acceptor levels can easily be ionized by thermal excitation which would decrease the population density and therefore the absorption coefficient. Such extrinsic photoconductors which are sensitive in the infrared up to wavelengths of about 30 ym, have to be operated at low temperatures to prevent thermal ionization. The absorption coef- 

Photoconductive detectors are used as resistors which change their resistance by AR = Rp - Rj from a dark value Rp without illumination to a lower value Rj under illumination. From Fig.4.78, which shows a typical circuit diagram, we obtain with a supply voltage Vp the signal voltage V,  

The optimum signal at a given incident radiation power is achieved for a load resistance of 

The time response of a photoconductive detector is limited by the carrier lifetimes. PbS detectors for instance have time constants between 0.1 - 1 ms while InSb detectors reach a few us. For measurements of fast transient signals photodiodes are more suitable (see Sect.4.5.8). 

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Table 3. Charge-Generation Efficiency of Selected Polymeric Photoconductors ...

Experimental Values of Charge-Generation Efficiencies. In this section the charge-generation efficiencies of many polymeric photoconductors are compared (Table 3). When the experimental data has been fitted to the Onsager model, the initial electron—hole separation distance,... 

Recently photorefractivity in photoconductive polymers has been demonstrated (92—94). The second-order nonlinearity is obtained by poling the polymer doped with a nonlinear chromophore. Such a polymer may or may not be a good photoconductor. Usually sensitizers have to be added to enhance the charge-generation efficiency. The sensitizer function of fuUerene in a photorefractive polymer has been demonstrated (93). 

The noise is expressed as noise density in units of V/(Hz), or integrated over a frequency range and given as volts rms. Typically, photoconductors are characterized by a g-r noise plateau from 10 to 10 Hz. Photovoltaic detectors exhibit similar behavior, but the 1/f knee may be less than 100 Hz and the high frequency noise roU off is deterrnined by the p—n junction impedance—capacitance product or the amplifier (AMP) circuit when operated in a transimpedance mode. Bolometers exhibit an additional noise, associated with thermal conductance. 

Cadmium Sulfide Photoconductor. CdS photoconductive films are prepared by both evaporation of bulk CdS and settHng of fine CdS powder from aqueous or organic suspension foUowed by sintering (60,61). The evaporated CdS is deposited to a thickness from 100 to 600 nm on ceramic substates. The evaporated films are polycrystaUine and are heated to 250°C in oxygen at low pressure to increase photosensitivity. Copper or silver may be diffused into the films to lower the resistivity and reduce contact rectification and noise. The copper acceptor energy level is within 0.1 eV of the valence band edge. Sulfide vacancies produce donor levels and cadmium vacancies produce deep acceptor levels. 

For a weU-designed, weU-made HgCdTe photoconductoi detector (76,77), g-r noise is dominant and may be expressed in terms of a minority carrier densityp and majority carrier density n. Semiconductor noise analysis for the HgCdTe photoconductor yields,... 

At low background flux this gives the temperature dependence of the Z9 shown in Eigure 4. At high flux, the Z9 equation (eq. 37) reduces to equation 12 except for a factor of (2) which is a result of the random recombination process not present in diodes. The scene sensitivity of a scanning photoconductor array infrared camera is ca 0.15°C. 

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