Thin films photoconductivity

The main application today for poly(vinyl carbazole) arises out of its photoconductivity and is in electrostatic dry copying machines. The polymer is applied from solution in thin film (10-15 p.m) layers onto a conductive substrate. 

Valyomana, A. G. Sajeev, T. P. 1991. Photoconductivity studies of CuInSe2 thin films prepared by the chemical bath deposition technique. Physica Status Solidi A 127 K113-K116. 

A very common and useful approach to studying the plasma polymerization process is the careful characterization of the polymer films produced. A specific property of the films is then measured as a function of one or more of the plasma parameters and mechanistic explanations are then derived from such a study. Some of the properties of plasma-polymerized thin films which have been measured include electrical conductivity, tunneling phenomena and photoconductivity, capacitance, optical constants, structure (IR absorption and ESCA), surface tension, free radical density (ESR), surface topography and reverse osmosis characteristics. So far relatively few of these measurements were made with the objective of determining mechanisms of plasma polymerization. The motivation in most instances was a specific application of the thin films. Considerable emphasis on correlations between mass spectroscopy in polymerizing plasmas and ESCA on polymer films with plasma polymerization mechanisms will be given later in this chapter based on recent work done in this laboratory. 

Mobility measurements by the TOP methods considered in Chapters 3 and 4 are particularly important, but they cannot give information about the whole spectrum of states in the mobility gap of amorphous chalcogenides. Therefore, in addition to TOP, XTOP, IPTOP, TSC, and TSDC, other complimentary techniques that probe the gap states are needed. Xerographic techniques that were initially developed to characterize properties of electrophotographic (xerographic) receptors [1] seemed to be informative, suitable, and widely applicable for the study of amorphous thin films and photoconductive insulator thin films [2],... 

Thin-film technology is useful in fabricating these devices. CdS-CdSe photoconductive sensors (Boronkay et al., 1978 Komiya et al., 1981) and As-Se-Te photodiodes (Tsukada et al., 1977) were applied to the long linear-image sensor. As-Se-Te photodiodes (Tsukada et al., 1979) and ZnSe - ZnCdTe photodiodes (Terui et al., 1980) were used in the area image sensor. However, there are some problems with these sensors long photoresponse time for the CdS-CdSe sensor, thermal instability, and low yield for the As-Se-Te sensor, and a high-temperature process for the ZnSe-ZnCdTe sensor. 

Where R is the reflectivity and d is the thickness. Very accurate values of R and T are needed when the absorptance, (id, is small. The technique of photothermal deflection spectroscopy (PDS) overcomes this problem by measuring the heat absorbed in the film, which is proportional to ad when ad 1. A laser beam passing just above the surface is deflected by the thermal change in refractive index of a liquid in which the sample is immersed. Another sensitive measurement of ad is from the speetral dependence of the photoconductivity. The constant photocurrent method (CPM) uses a background illumination to ensure that the recombination lifetime does not depend on the photon energy and intensity of the illumination. Both techniques are capable of measuring ad down to values of about 10 and provide a very sensitive measure of the absorption coefficient of thin films. 

Indeed, the fact that the onset of photoconductivity occurs at a higher energy than the absorption edge in the polydiacetylenes, both in single crystal samples and in thin films cast from solution is a clear indication that the photoexcitations generated below 2.3 eV are neutral bound excitons [200]. 

The similarity of the temperature dependence of steady-state photocurrent in thin films to that of the transient photoconductivity in the sub-ns regime implies that, in thin films, carrier sweep-out occurs prior to deep trapping. The weak residual T-dependence above 80 K in the thinnest sample is again similar to that observed in sub-ns time-resolved experiments. In the time domain, this corresponds to the temperature dependent tail characteristic of the transient photocurrent [199]. This weak T-dependence arises from the effect of shallow traps with multiple release and retrapping during carrier sweep-out. Note that the... 

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

Ostroverkhova, O. et al., Bandhke transport in pentacene and functionalized pentacene thin films revealed by subpicosecond transient photoconductivity measurements, Phys. Rev. B, 71, 035204, 2005. 

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