Films photoconductive properties

As noted at the beginning of this chapter, most of the early studies in CD focused on PbS, followed by PbSe, driven by their photoconducting properties. For optimum use as photoconductors, the deposited films were annealed in an oxygen-containing atmosphere. Most of this section will focus on nonannealed films, and annealed films will be treated only very briefly. More details on the annealed as well as as-deposited fihns will be given in Chapter 5. 

We can conclude this section with the insight, gained from this overview of the electrical and photoconductivity properties of these films, that, in spite of the many studies already carreid out, a comprehensive and systematic study of these properties and their correlation with a wide range of deposition parameters is still needed in order to understand what determines these properties. These studies should also include postdeposition treatments— not so much annealing, which has been carried out, but surface treatments (e.g., immersion in triethanolamine), which could show the importance (or lack of it) of the crystal surface condition. 

Specific photoconductive properties of PbS films have already been treated. This section deals with more general aspects of CD PbS (for the most part, also relevant for PbSe) films. 

Photoconductivity of Kapton, (polypyromellitimide where R is oxygen) was investigated in detail [224-234]. Frenkel, Onsager, hopping and other models were used by different authors for explanation of the photoconductive properties. The photoconductivity spectra of Kapton film for various directions of the electric field are presented in Fig. 29 [230]. The high anisotropy depending on... 

Polymers and oligomers with conjugated C -C, N- and -N=N-double and -C=C— triple bonds in the main chain received recently some attention for their photoconductive properties. Although most of these polymers are infusible and intractable, some of them are reported to possess good solubility and film-forming properties and outstanding thermal stability. 

Vinylcarbazole polymers and their CT complexes are widely known for their excellent photoconductive properties [115]. The CT processes in this polymer system have been extensively worked out by the conventional absorption spectroscopic techniques, but the application of the XPS technique to them has been rather limited [116]. For poly(Af-vinylcarbazole) (PVK)/perchlorate complex film synthesized via electrochemical polymerization and oxidation, it was shown that the NIs core-level spectrum exhibits a new high BE component shifted by about + 3.0 eV from the neutral carbazole nitrogen at about 400 eV. This new component was attributed to the positively charged carbazole nitrogen associated with CIO4 anion [117],... 

The formation of photoactive films on metal electrodes is not restricted to inorganic materials. Copper, for example, can be anodised to form polymeric phenylacetylide [33] and acetylide [34] layers that appear to behave as p-type organic semiconductors. The photoconducting properties of the arylethynyl polymers have been known for some time, although the mechanism of photoconductivity is not well understood. It seems probable that charge carriers are created by the annihilation of mobile Frenkel excitons at electron traps such as adsorbed oxygen rather than by direct interband excitation. 

The fabrication of infra-red photodetectors, based on photoconducting properties, has been the first industrial application of chemically deposited films with PbS and PbSe [3] and is still in use for this application. During this time, many candidates for... 

The electric conductivity of p-PODA at room temperature is 10 Scm This corresponds to common insulating polymers in the glassy state. However, above 440 K, ionic conduction is observed. " Electrically conductive PODAs can be obtained by doping with electron donors or electron acceptors. Some varieties of PODA are semiconductors and exhibit photoconductive properties. Therefore, they are used in electrical applications. PODA films exhibit exceptional gas separation properties. ... 

Thin-film semiconductors are another example of such an application. Photoconductive properties of CdSe-based thin films have been explored, with potential solar cell applications in mind (Leatherdale et al., 2(X)0). This may... 

A. Torres-Filho and R. W. Lenz, Electrical, thermal, and photoconductive properties of poly(phenylene vinylene) precursors I. Microwave-induced elimination reactions on sulfonium precursor-films. J. Appl. Polym. Sci. 52 377 (1994). 

Rovkina et al. [222,280] reported on the synthesis of oligoesters having pendant carbazole groups. They were prepared by polycondensation of EPK (32a) or 3-(9-carbazolyl)propane-diol-l,2 (34a) with dicarboxylic acids (adipic, sebacic or phthalic). The carbazole-containing oligoesters had relative molecular masses from 640 to 2640 and exhibited photoconductivity and good film-forming properties. 

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. 

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. 

ELECTRICAL PROPERTIES AND PHOTOCONDUCTIVITY. Electrical resistivity of CD CdS films is commonly studied. Values for this (dark) resistivity vary over many orders of magnitude from one fihn to another, usually for reasons that are not understood. Values as high as 10 O-cm and as low as 15 O-cm have been reported for undoped films (doped films have been reported with still lower resistivities). 

---