Photoconductivity Changes

The reversible light-induced changes in photoconductivity that have been observed in a variety of a-Si H films are also often used to compare the S - W effect in different films. The wide range of photoconductivity changes that have been reported exhibit dynamics during light exposure that are similar to those shown earlier in Fig. 1. However, results such as those shown in Fig. 

Photoconductivity Op is determined by the generation rate of free carriers by light/and their lifetimes r. In the usual case in which photoconductivity is due to electrons, the photoconductivity is given by 

The introduction of these light-induced states affects not only the carrier lifetimes but also the photoconductive response time Tq. Because of the continuous distribution of gap states in a-Si H, there are many more states that can trap photogenerated carriers close to the mobility edges than those that act as recombination centers. The response time depends on the density 

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Work is continuing to correlate the formation of surface oxidation states with changes in the photoconduction of films of PVCa. The relationship between energy transfer and photoconductivity is being investigated. 

Another type of absorption is also possible, i.e., exciton absorption which enriches the crystal in free excitons if the latter annihilate then on the lattice defects, causing a change in the charged state of the defects and leading to the appearance of free carriers in the crystal. In this case photoconduction arises as a secondary effect. 

Quantum detectors are based on semiconductors. The absorption of a photon excites an electron from the valence band into the conduction band. This can be measured either through a change in resistance (photoconductive... 

A turning point in the study of amorphous semiconductors was reached with the discovery that the addition of hydrogen to amorphous silicon could dramatically improve the material s optical and electrical properties. Unlike pure amorphous silicon, which is not photoconductive and cannot be readily doped, hydrogenated amorphous silicon (a-Si H) displays a photoconductive gain of over six orders of magnitude and its dark conductivity can be changed by over ten orders of magnitude by n-type or p-type... 

Figure 2.24 Photoelectric detectors. Photovoltaic detectors measure the flow of electrons displaced by the absorption of radiation. Photoconductive detectors measure the changes in conductivity caused by the absorption of radiation.

The changes in resistivity with annealing of films deposited from selenourea and selenosulphate baths, as well as evaporated films, were compared [71,72]. Although there were small differences between the various films, no major difference was found. Additionally, the resistivity of as-deposited films, deposited from both selenourea and selenosulphate baths, does not change with time over a period of months in air. However, after annealing in air at 350°C when the resistivity increases, there is a gradual decrease in room-temperature resistivity (and also in photoconductivity response) with time [73], These variations were related to formation of PbSeOs and adsorbed oxygen on the surface of the annealed crystals. 

First, we consider the associated changes in the photoelectronic properties of the samples. The spectral characteristics of photoconductivity of the samples display a red shift after irradiation. Such behavior of the photoconductivity is not surprising, because it is in full agreement with the shift in the absorption edge. Additionally, the photoconductivity decreases after photodarkening [2]. The decrease may be attributed to the creation of new defect states or altering the existing localized states. 

The preceding changes in the photocurrent of amorphous chalcogenides may be related to specific changes in electronic gap states, which act as trapping and recombination centers and, therefore, limit the photoconductivity. 

In systems in which the charge-transfer excitation band differs from the action spectrum of photoconductivity, the doping effect may be due to a change of recombination path that results in an enhancement of carrier liefetime (e.g. holes in merocyanines and phthalocyanines). (Details on the mechanism are given in 10,11,74).)... 

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