Trapping carrier transport

A very important progress has been made recently on the theoretical background of the analysis of the transport problem also (2). Although it has been made clear that multi-trapping process is determining the carrier transport in the polymer, not only the nature of the trap but also the trap depth and the trap population are not definitely known even in pure poly-N-vinylcarbazole. The present report concerns with this problem. 

Various methods are proposed (5, a—g), (6). Among these, the methods in which monomolecular recombination or bimolecular recombination of the carriers are assumed could not be used in our case, because the carrier transport in poly-N-vinylcarbazole is known to be the multi-trapping process of the hole carrier. The values of the trap depthaE of 5°C peak by these several methods are summarized in Tab. 1. Values are widely scattered and it seemed that this is due to the approximations involved in the method of analysis. Our value is calculated by the... 

Traditionally, charge-carrier transport in pure and doped a-Se is considered within the framework of the multiple-trapping model [17], and the density-of-state distribution in this material was determined from the temperature dependence of the drift mobility and from xerographic residual measurements [18] and posttransient photocurrent analysis. 

The filled upper and free lower orbitals in PVC belong to the carbazole groups. So one can consider PVC as a disordered organic medium. The side chromophore groups play the role of traps for hopping charge carrier transport. 

Energy levels 166 Carrier generation 167 Carrier transport 168 Carrier trapping 177... 

The polymer exhibits very low dark currents (o60° = 3 x 10 19 ohm-1 cm-1). The photocurrent was reported to be proportional to the applied voltage and light intensity but its magnitude was far inferior to that of PVK. The poor photoconductivity is attributed to the high concentration of exdmer forming sites acting as exciton traps and also to poor transport characteristics. The carrier transport is expected to be slower since the ionization potential of the polymer is higher (7.88 eV) than that of PVK (7.43 eV). 

One of principal causes of increase of P-conductivity in MF can be magnetosensitive non-equilibrium processes connected with charges carriers transport or change of intensity of capture (or release) by traps of electrons and holes. High times of increase and decrease of P-conductivity confirm the given assumption, indicating the contribution of defect structure to P-conductivity of C6o single crystal in MF. 

Researchers have used various models to explain the charge carrier transport mechanism in organic semiconductors. Two models have been used frequently, (i) the trapping model, which assumes a certain distribution of traps in the energy space and (ii) the field dependent mobility model, which assumes an exponential dependence of mobility on square root of electric field. 

V. Kumar, S.C. Jain, A.K. Kapoor, W. Geens, T. Aernouts, J. Poortmans, R. Mertens, Carrier transport in conducting polymers with field dependent trap occupancy,./. Appl. Phys. 92 (2002) 7325-7329. 

Nanocrystalline systems display a number of unusual features that are not fully understood at present. In particular, further work is needed to clarify the relationship between carrier transport, trapping, inter-particle tunnelling and electron-electrolyte interactions in three dimensional nan-oporous systems. The photocurrent response of nanocrystalline electrodes is nonlinear, and the measured properties such as electron lifetime and diffusion coefficient are intensity dependent quantities. Intensity dependent trap occupation may provide an explanation for this behaviour, and methods for distinguishing between trapped and mobile electrons, for example optically, are needed. Most models of electron transport make a priori assumptions that diffusion dominates because the internal electric fields are small. However, field assisted electron transport may also contribute to the measured photocurrent response, and this question needs to be addressed in future work. 

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