Abstract
A method to determine the doping induced charge carrier profiles in lightly and moderately doped organic semiconductor thin films is presented. The theory of the method of Charge Extraction by a Linearly Increasing Voltage technique in the doping-induced capacitive regime (doping-CELIV) is extended to the case with non-uniform doping profiles and the analytical description is verified with drift-diffusion simulations. The method is demonstrated experimentally on evaporated organic small-molecule thin films with a controlled doping profile, and solution-processed thin films where the non-uniform doping profile is unintentional, probably induced during the deposition process, and a priori unknown. Furthermore, the method offers a possibility of directly probing charge-density distributions at interfaces between highly doped and lightly doped or undoped layers.
Highlights
Organic semiconducting materials hold great potential for use in future electronic devices such as solar cells, transistors, and thermoelectric generators
The concentration of free charges, Nfree, in a doped semiconductor layer is related to the depletion-region width w and permittivity apnodteentisiatlhderoelpemUeanstNarfryeech=arg2e 9 .0CUo/(mewm2o),nwthecehren iqius etshetoremlaetaivseurdeietlheectcroicnccoenntsrtaatnito,n 0oifs the free vacuum carriers includes ultraviolet photoelectron spectroscopy (UPS) and impedance spectroscopy, while recently a way of determining the free-carrier concentration from conductivity and Seebeck measurements was presented[12, 18,19,20,21]
We recently demonstrated that the Charge Extraction by a Linearly Increasing Voltage technique in the doping-induced capacitive regime can be used to determine the built-in voltage and carrier concentration in sandwich-type diode devices[22]
Summary
Applying the voltage pulse in reverse bias (or with a blocking cathode), so that charge carriers are extracted at the anode, the transient conduction current Jc(x, t) will be vanishingly small in the depleted region. During sufficiently slow ramp-up voltage pulses (tpulse ≫ tmax), the charge carriers maintain equilibrium under steady-state conditions, and p(x, t) will not significantly deviate from its quasi-equilibrium density p(x)[22]. Under these conditions, the electric field at the anode becomes time-independent E(d,t) = Ean (assuming w < d); integrating Eq (6) yields.
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