Photoelectronic properties of disordered semiconductors at constant photocurrent

In disordered semiconductors, electronic transport - and with it the practical use of the material - depends highly on the density and distribution of defect levels in the semiconducting bandgap. For that reason, much effort has been devoted in the past to measuring and quantifying those defect distributions. Traditionally, optical absorption measurements have played a major role in this endeavour, but the spectrum of photoconducting techniques also offers a whole range of opportunities to detect and investigate the presence of defect levels in the band gap of amorphous semiconductors. They have the advantage over customary optical absorption measurements that they do not only reflect the optical absorption event but also additional transport and recombination aspects that may show a specific sensitivity to some of the defect centres in the material. One of those photoconducting techniques is the constant photocurrent method (CPM) that is used in this thesis. It was originally developed for the accurate measurement of the optical absorption in the subbandgap region of hydrogenated amorphous silicon (a-Si:H) where classic transmission-reflection measurements offer insufficient sensitivity. However, this method can give valuable information on material properties of a wide range of disordered semiconductors. In this thesis, the main focus is on the application of the constant photocurrent method to hydrogenated amorphous silicon, the chalcogenide glass amorphous selenium and amorphous conjugated polymers. These amorphous semiconductors have a wide variety of practical applications, including solar cells, X-ray imaging and thin film transistors. In chapter I, a brief description of the properties of inorganic amorphous semiconductors such as hydrogenated amorphous silicon and amorphous selenium is given. The transport mechanism and free carrier generation mechanism for these materials is discussed. An important parameter in the description of the carrier generation process is the quantum yield. This is the probability that an absorbed photon leads to the creation of a free electron and a free hole. For amorphous silicon the quantum yield approximates unity, while in chalcogenide glasses the quantum yield is often very small and dependent on the photon energy and the applied electric field. Steady-state and transient photoconductivity experiments are discussed in the next chapter. More specifically, the application of experiments involving the temperature and light intensity dependence of the photoconductivity, the constant photocurrent method and time-of-flight experiments are discussed. Attention is given to the theoretical background (i.e. Simmons-Taylor theory) used in the interpretation of the photoconductivity experiments. In chapter III, amorphous organic semiconductors are discussed. Although there are substantial differences in the transport and carrier generation mechanisms of these materials with respect to inorganic amorphous semiconductors, it will be argued that the Simmons-Taylor theory is still applicable to these materials. In chapter IV, CPM measurements of expanding-thermal-plasma-deposited hydrogenated amorphous silicon are discussed. From the CPM measurements it is concluded that the fact that these materials have good transport properties in terms of hole and electron mobility does not imply a low defect density. CPM measurements on amorphous selenium are discussed in chapter V. Because of the energy dependence of the quantum yield in these materials, CPM experiments lead to the product of the absorption coefficient and the quantum yield, which hampers the interpretation of the measurements. Although no direct information about the density of states can be obtained from the CPM measurements, the measurements do lead to indirect information about the nature of the defect states. In section VI, the CPM method is applied to conjugated polymers and blends of polymers and fullerenes. Because of the spectral dependency of the quantum yield for the unblended polymers, extra measurements of charge carrier sweepouts are made to scale the CPM results. The features in the CPM spectra of the blends cannot be explained as a mere superposition of the spectra of the constituents. This is taken as an indication that there is an interaction between the polymer and the fullerene which causes changes to the absorption spectrum.status: publishe