Charge extraction by a linearly increasing voltage of photo-generated carriers: The influence of two mobile carrier types, bimolecular recombination, and series resistance

The charge extraction by a linearly increasing voltage (CELIV) technique is a well-known and commonly used method to characterize charge transport in low-mobility materials. In the original CELIV theory it is assumed that one type of charge carrier is mobile and the other fixed and that recombination during the extraction pulse is negligible. However, this is in general not the case, especially in photo-CELIV where both electrons and holes are generated by light excitation. Moreover, RC effects induced by the series or load resistance of the external circuit are typically assumed to be negligible. In this work, we use drift-diffusion modelling and analytical derivations to show that the standard equations used for calculating the mobility in the moderate conductivity regimes generally leads to errors in the mobility determination in the case when i) two carrier types of similar mobility, ii) recombination, iii) an electric-field-dependent mobility, and iv) RC effects are present in the device. The effect of the external series resistance on the mobility determination becomes of particular importance in devices with relatively large mobilities and/or high carrier concentrations, where the original CELIV theory might give rise to an underestimation of the mobility by several orders of magnitude.