Odd–even modulation of electrode work function with self-assembled layer: Interplay of energy barrier and tunneling distance on charge injection in organic light-emitting diodes

In this study, a series of trifluoromethyl-terminated alkanethiols with different chain length (CF3(CH2)nSH, n = 5, 6, 10, 11, 12 and 13), were used to form self-assembled monolayers(SAMs) on silver and gold surfaces. The SAM modulates the metal work function as a result of the different orientation of the terminal dipolar CF3 group due to different chain tilt of the SAM on Au and Ag surfaces, respectively. The SAM-modified Ag and Au were used as the anode in the fabrication of hole-only devices as well as top-emitting organic light-emitting diodes (TOLEDs) with different hole-transporting layers. The correlation between current density/luminous efficiency and the SAM chain length/work function were analyzed to provide information on charge balance. Thus with NPB as the hole-transporting layer, where little difference between the HOMO energy level and the work function of the SAM-modified electrode, the charge injection is modulated by the chain length (tunneling distance-dependent) of the SAM used for modification, decreasing with increasing chain length. With BPAPF as the HTL, where a barrier is present, the charge injection is modulated more by the work function (injection barrier-dependent), which shows an odd–even alternation.

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► Charge injection from an electrode to organic layer can be modulated by SAM modification of the electrode.
► Importance of chain length is manifested where there is little energy barrier between the electrode and the organic layer.
► Effect of dipole modulation is manifested where there is substantial barrier.
► The trend in current efficiency provides information regarding the charge balance in the device.