Analysis of interrelationship between efficiency and charge transport properties of green TADF organic light-emitting diodes with mixed host by impedance spectroscopy

Abstract

Many studies have analyzed the mobility and injection characteristics of charge carriers for organic light-emitting diodes (OLEDs) using hole-only and electron-only devices (HODs and EODs). However, the charge dynamics of a single type of carrier cannot fully represent the characteristics of light-emitting devices such as charge balance, recombination, and efficiency. In this report, we applied the impedance spectroscopy (IS) measurement to operating devices with light-emission. A series of thermally activated delayed fluorescence (TADF) OLEDs with a single or mixed host were fabricated and their electro-optical properties were comprehensively investigated. 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) and 2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) were used for the single host and a mixture of CBP and TPBi (1:1 ratio) was used for the mixed host; 2,4,5,6-Tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN) was used as an emitter. In all the OLEDs, the injection and transporting property of holes and electrons were successfully discriminated from the Bode plots of the modulus. Besides, the recombination properties depending on the type of hosts were also thoroughly understood from capacitance ( C )-voltage ( V ), Re(z)-frequency and Cole-Cole plots. The modulus Bode plot revealed that the voltage of the maximum efficiency coincided with the voltage at which the low frequency (electron) signal merges with the high frequency (hole) signal in the mixed host system. • Charge injection and transporting properties in single OLED were investigated by applying impedance spectroscopy measurements. • TADF OLEDs with n-, p-type and mixed host were successfully correlated with charge dynamics in terms of recombination process and efficiency. • Modulus Bode plot revealed the interrelationship between max efficiency and low (electron)/high (hole) frequency signals.