Photoconductivity experimental techniques

Photoconductivity in a solid is defined as an increase of conductivity caused by radiation. The phenomenon of photoconductivity involves the processes of absorption of radiation, photogeneration of charge carriers, their separation, diffusion and drift in an applied electric field, their temporary immobilization at sites known as trapping rites, release from traps and finally their recombination. The phenomenological relationships covering all these processes were primarily developed in connection with the study of crystalline covalent solids which dominated the early scientific literature on photoconductivity. Concurrent with the basic understanding of the phenomena was the development of several experimental techniques to study the fundamental processes and the specific identity of the defects and impurities that control these processes. 

In the first part of this chapter we review some basic concepts of photoconductivity which are followed by a renew of some experimental techniques and how these have been applied to characterize some of the well known polymeric systems such as poly(N-vinyl carbazole) (PVK) and the charge transfer complex of PVK and 2,4,7,trinitro-9-fluorenone (TNF). The second part of this chapter is a review of the extensive original and patent literature on a variety of photoconducting polymers. 

There have also been several novel combinations of ESR with other experimental techniques. Such studies as spin-dependent photoluminescence, spin-dependent photoconductivity, and spin-dependent transient transport provide important information concerning the influence of spin statistics on... 

For the characterization of the novel photoconductive compounds two different experimental techniques have been used. Some measurements were made by BASF AG using a steady-state method. These experiments allow a quick characterization, but in contrast to the time-of-flight (TOF) method no statements about transient photocurrents and carrier mobilities are possible. The TOF measurements were carried out in the group of D. Haarer, Universitat Bayreuth. 

We studied photoexcitations in such polymers in a broad time interval from femtoseconds to milliseconds and spectral range from 0.1 to 2.4 eV. However, in this chapter we review only our continuous wave (cw) studies, where the photoexcitations are generated in quasisteady-state conditions. The main experimental technique described herein is photomodulation (PM), which gives information complementary to that obtained by photoluminescence (PL), which is limited to radiative processes, or photoconductivity (PC), which is sensitive to high mobility photocarriers. The PM method, in contrast, is sensitive to nonequilibrium excitations in all states. 

Traditionally, charge transfer mechanisms have been studied by such methods as conductivity, the Hall effect, and thermoelectric effect. Details of these applications may be found in Experimental Methods of Physics, Vol. 6, Pt. b (12), the article on ionic conductivity by Lidiard (70), and in many of the original papers quoted. More recently, techniques such as electron spin resonance (13), dielectric loss and pulsed photoconductivity methods (5—8) have been used to study semiconduction in organic materials. 

It is a transient photoconductivity technique, where the time evolution of the current induced in an external circuit by photogenerated charges moving through the semiconductor under the action of an electric field is monitored. A schematic of the ToF experimental arrangement is shown in Fig. 5.2. 

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