Mobility, charge-carrier magnitude

Electrical conduction and heat transport are closely linked, the connection being described by three thermoelectric coefficients, the Seebeck coefficient (or thermopower), the Peltier coefficient and the Thomson coefficient, all of which have relevance to thermoelectric power generation and refrigeration. In perovskites, the most reported values are for the Seebeck coefficient. The magnitude and sign (+ or -) of the Seebeck coefficient are related to the concentration and type of mobile charge carriers present. For band-like perovskites, the magnitude of the Seebeck coefficient is proportional to the density of states, either in the conduction band, for electron transport, or the valence band for hole transport. 

There are two principle differences which distinguish the electrochemical behavior of semiconductors from that of metal electrodes. First, the by orders of magnitude smaller concentration of mobile charge carriers leads to a different double layer structure and to different kinetic relations. Secondly, the existence of an energy gap in the electronic band structure leads to a distinction between electrode reactions in which the charge carriers of either the conduction band or the valeTice band are involved. 

With a combination of in situ conductivity and potential-step chronocoulometric measurements, Harima et al. [988] studied the enhancement of carrier mobilities in poly(3-methylthiophene). In addition to the obvious effect of the electrochemically induced increase in the concentration of mobile charge carriers, a drastic mobility enhancement of over 4 orders of magnitude, which implies a change from hopping to metallic transport, was concluded. 

Measurements of mobility in PS suffer from the fact that the number of free charge carriers is usually small and very sensitive to illumination, temperature and PS surface condition. Hall measurements of meso PS formed on a highly doped substrate (1018 cm3, bulk electron mobility 310 cm2 V-1 s-1) indicated an electron mobility of 30 cm2 V 1 s 1 and a free electron density of about 1013 cm-3 [Si2]. Values reported for effective mobility of electron and hole space charges in micro PS are about five orders of magnitude smaller (10-3 to 10 4 cm2 V 1 s ) [PelO]. The latter values are much smaller than expected from theoretical investigations of square silicon nanowires [Sa9]. For in-depth information about carrier mobility in PS see [Si6]. 

What is the situation inside the electrode That depends upon whether the electrode is a metal or a semiconductor. What is the most important difference between a metal and a semiconductor Operationally speaking, it is the order of magnitude of the conductivity. Metals have conductivities on the order of about 106 ohm-1 cm-1 and semiconductors, about 102-1(T9 ohm-1 cm"1. These tremendous differences in conductivity reflect predominantly the concentration of free charge carriers. In crystalline solids, the atomic nuclei are relatively fixed, and the charge carriers that drift in response to electric fields are the electrons. So the question is What determines the concentration of mobile electrons One has to take an inside look at electrons in crystalline solids. 

In any discussion of photoconductivity the importance of the ambient atmosphere and surface must be considered. Not only is the magnitude of the photocurrent dependent on the state of the surface but the provision of guard rings may completely alter the spectral response of the photocurrent. This, it has been suggested, is because the mobility of charge carriers is greater over the surface than through the bulk (35). 

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