Photoconductivity in Materials Research

Photoconductivity is the incremental change in the electrical conductivity of a semiconductor or insulator upon illumination. The behavior of photoconductivity with photon energy, light intensity and temperature, and its time evolution and frequency dependence, can reveal a great deal about carrier generation, transport and recombination processes. Many of these processes now have a sound theoretical basis and so it is possible, with due caution, to use photoconductivity as a diagnostic tool in the study of new electronic materials and devices. This chapter describes the main steady-state and transient photoconductivity techniques applied in the investigation of semiconductors whose performance is limited by the presence of localized electronic states. These materials tend to be disordered, and possess low carrier mobilities and short free-carrier lifetimes in comparison with crystalline silicon. They are often prepared as thin films, and are of interest for large-area applications, for example in solar cells, display backplane transistors, photoemissive devices such as organic light-emitting diodes (OLEDs) and medical imagers. However, examples of where these techniques have been useful in the study of defective crystalline semiconductors are also given. The approach followed here is by way of an introduction to the techniques, the physics supporting them, and their applications, it being understood that readers requiring more detailed information will consult the references provided.