Charge transfer characteristics of organic semiconductors are essential for the understanding and design of organic light-emitting devices (OLEDs) that are generally composed of various stacked organic film layers, with energetic characteristics and interlayer charge transfer kinetics being highly important for understanding their performance [1-3]. Voltammetry is a common method for determining the energetic characteristics of molecules contained in the above films [4]. In particular, cyclic voltammetry (CV) provides information on the formal oxidation potential (E^o’), which is indirectly correlated to the energy level of the highest occupied molecular orbital (HOMO). However, the seemingly easy determination of energetic parameters often leads to misinterpreted voltammetric data and scientific misconduct [5]. Voltammetric methods can be used to estimate various electron-transfer parameters formal oxidation potential (E^o’), HOMO energy level (EHOMO), electron transfer rate constant (k^o’), and oxidation diffusion coefficient (D_o), but careful consideration in a limited condition is required [6].

In this study, voltammetric analyses were utilized to characterize electron transfer in iridium(III) bis(2-phenylpyridinato-N,C^2’)acetylacetonate ((ppy)₂Ir(acac)), a representative green dopant for organic light-emitting devices (OLEDs) (Fig. 1), affording parameters such as E^o’, E_HOMO, k^o’, and D_o. Due to the quasi-reversible electrochemical oxidation of (ppy)₂Ir(acac), the above thermodynamic and even kinetic properties could be easily estimated. CV, chronocoulometry (CC), and the Nicholson method were used, and an in-depth explanation of voltammetric techniques is provided.

¹H and ¹³C NMR spectra were recorded using an AV-300 (Bruker, Germany) NMR spectrometer. Although the investigated Ir(III) complexes were air-stable, all related manipulations were carried out under nitrogen due to the possible oxidation and thermal decomposition of transient intermediates. Electrochemical characterization was performed using a CH Instruments 650B analyzer (CH Instruments, Inc., TX, USA). Individual solutions were characterized using CV and CC to determine the electron transfer characteristics of (ppy)₂Ir(acac).

Prior to the characterization of (ppy)₂Ir(acac), CV measurements were performed for the ferrocene/ferrocenium (Fc^0/+) couple (1.0 mM) in acetonitrile. The formal oxidation potential of Fc was determined as E^o’(Fc^0/+) = 0.090 V vs. Ag/Ag⁺ and was later used as an internal reference for estimating the E_HOMO of (ppy)₂Ir(acac). CV measurements were subsequently performed for a 1.0 mM solution of (ppy)₂Ir(acac), which exhibited quasi-reversible oxidation (Fig. 2) corresponding to the redox conversions of the (ppy)₂Ir(acac)^0/+ couple [9]. The formal oxidation potential of the above couple was determined as E^o’((ppy)₂Ir(acac)^0/+) = 0.444 V (vs. Ag/Ag⁺), being almost equal to the value reported previously [10]. At a scan rate (ν) of 20 mV/s, the (ppy)₂Ir(acac)^0/+ couple showed a peak potential separation (ΔE_pp = E_pa − E_pc) of 77 mV, indicating a stoichiometric number of transferred electrons (namely one) in each process. With increasing ν (from 20 to 500 mV/s), ΔE_pp gradually increased, whereas E^o’((ppy)₂Ir(acac)^0/+) remained constant, and the ratio of anodic and cathodic peak currents (i_pc/i_pa) slightly decreased from unity.

According to Forrest et al. [11], the formal potential of (ppy)₂Ir(acac)^0/+ is related to its E_HOMO:

where E_CV is the relative oxidation potential of (ppy)₂Ir(acac)^0/+ calibrated against the Fc^0/+ couple. The E_CV of (ppy)₂Ir(acac)^0/+ was calculated as 0.354 V (= 0.444 − 0.090), and E_HOMO was estimated as −5.10 eV, being identical to previously reported values determined by ultraviolet photoemission spectroscopic (UPS) measurements [12].

The oxidative diffusion coefficient (D_o) of (ppy)₂Ir(acac) was also determined based on CC measurements. In our approach, a double-step potential was applied to the solution, inducing an instantaneous oxidation of (ppy)₂Ir(acac)in the vicinity of the electrode in the forward step, followed by rapid reduction of the oxidized form to (ppy)₂Ir(acac) in the reverse step. Potentials sufficiently more positive than 0.444 V were applied in the forward step (and vice versa), and almost identical charge (Q) vs. time (t) profiles were obtained at |E_app − E^o’| = 0.591 V (Fig. 3a). The applied overpotential accompanied the changes of the ratio of the concentration of (ppy)₂Ir(acac) and (ppy)₂Ir(acac)⁺ at the surface of electrode. According to the Nernst equation, the [(ppy)₂Ir(acac)]/[(ppy)₂Ir(acac)⁺] ratio at the electrode surface rapidly changed to 1/10¹⁰ under the condition of E_app − E^o’ = 0.591 V, with the diffusion of (ppy)₂Ir(acac) being the only mode of subsequent mass transport. A strictly linear relationship between Q and t^1/2 (y = 1.34 × 10⁻⁵·x, R² = 0.999) was obtained (Fig. 3b), and D_o was calculated using the Anson equation:

where n is the number of electrons, A is the electrode area, and C^* is the bulk concentration of (ppy)₂ Ir(acac).

The A value of glassy carbon was previously measured as 7.64 × 10⁻² cm² using 1.0 mM Fc in acetonitrile (not shown here), and thus, the diffusion coefficient of (ppy)₂Ir(acac) was determined as D_o = 1.35 × 10⁻⁵ cm² /s, being slightly lower than that of Fc under similar conditions (2.4 × 10⁻⁵ cm²/s). CV data pertaining to reversible and quasi-reversible electron transfer not only provides thermodynamic information, but also allows the estimation of kinetic parameters for the redox process. For diagnostic purposes, Nicholson et al. [13] suggested using a dimensionless kinetic parameter Ψ, which is a function of ν, D_o, and k^o’:

The quasi-reversible one-electron oxidation exhibited by (ppy)₂Ir(acac) implies that ΔE_pp is close to 59 mV for slow potential scan rates, increasing concomitantly with the scan rate in the case of fast-scan CV measurements. According to Nicholson’s method, Ψ is correlated with ΔE_pp (Fig. 4a) [14] and can therefore be determined by measuring ΔE_pp values at various scan rates. As shown in Fig. 4b, Ψ values were extracted from CV curves at various scan rates, being linearly dependent on ν^−1/2 (y = 1.80 × 10⁻¹·x, R² = 0.985). Finally, the k^o’ of (ppy)₂Ir(acac) was calculated as 2.68 × 10⁻² cm/s based on eq. (3), being comparable to that of the Fc^0/+ couple (1.91 × 10⁻² cm/s) under similar conditions [15].

Two voltammetric techniques, CV and CC, were used to investigate the redox characteristics of iridium(III) bis(2-phenylpyridinato-N,C^2’)acetylacetonate ((ppy)₂Ir(acac)), a typical green dopant for OLEDs. Although voltammetry is one of the most versatile analytical techniques for the study of electroactive materials, it requires precision, an appropriate level of understanding, and suitable experimental conditions. The quasi-reversible oxidation of (ppy)₂Ir(acac) allowed to estimate the HOMO energy levels, and the electron-transfer parameters such as diffusion coefficients (D_o) and electron transfer rate constants (k^o’) were successfully determined using a combination of CV and CC measurements.