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Interpretation of capacitance spectra and transit times of single carrier space‐charge limited transport in organic layers with field‐dependent mobility

The ac impedance characteristics of a single carrier with space‐charge limited current (SCLC) transport in organic layers with field‐dependent mobility is analyzed, indicating the similarities as well as the differences to the constant mobility case. The model provides capacitance spectra and transit times from different calculation methods, in relation to the electric field distribution in the SCLC regime. It is found that the low frequency capacitance lies in the range 3Cg/4 < Cif < Cg, with respect to the geometric capacitance Cg. An approximated expression for the variation of the transit time with applied bias is derived, in good agreement with exact calculations. Experimental results of capacitance spectra of hole‐only PPV devices indicate the validity of the SCLC model but also show a strong contribution of trapping at low frequencies that overlap the low‐frequency value of the SCLC capacitance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)