Determination of standard redox rate constants of OLED active compounds by electrochemical impedance spectroscopy

A number of commercial organic compounds (m-MTDATA, PBD, CBP, TAPC, NPB, TPBi, etc) as well as several donor-acceptor-donor (D-A-D) compounds were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The compounds were chosen as they are promising components of small-molecule-based high efficiency Thermally Activated Delayed Fluorescence (TADF) emitters in Organic Light Emitting Diodes (OLEDs). Electrochemical impedance spectra of a Pt electrode in CH2Cl2 solutions containing the investigated compound and Bu4NBF4 as the electrolyte were obtained and analyzed by electrochemical electrical circuit methods. Charge transfer resistance, double layer capacitance, Warburg constant and other parameters were determined and represented as a function of the potential. Analysis of charge transfer resistance as a function of potential allowed an estimation of standard redox rate constants for the compounds' oxidation and reduction processes. Two main features concerning the redox reaction rates of OLED-active compounds were revealed: (i) the oxidation and reduction rates of ambipolar compounds, i. e. containing both donor and acceptor parts, were found to be much higher than those of unipolar donor-only and acceptor-only molecules; (ii) the relationship between the oxidation and reduction rate constants was shown to be related to the compounds' conductivity type in the solid state. (c) 2017 The Authors. Published by Elsevier Ltd.