Rough and Fine Tuning of Metal Work Function via Chemisorbed Self‐Assembled Monolayers

Abstract

Many chemical and biochemical processes in nature involve charge transport across organic/inorganic interfaces, [1][2][3][4][5][6][7][8] which determines the performance of organic electronic devices. [9][10][11][12][13][14] To achieve efficient charge transport between the electrode and organic molecules, the metal work function has to match the energy level of either the highest occupied or lowest unoccupied molecular orbitals of the molecule, which usually requires adjustments in the metal work function. One of the most common methods used for modification of metal work functions is the adsorption of organic layers. By adsorbing an organic layer onto the electrode surface, one creates a dipole barrier for the charge-carrier transport between the electrode and the organic electronic device. The surface dipole of the organic film depends on the dipole moment of the molecules forming the film, their electrostatic interactions and, for chemisorbed layers, the dipole at the interface due to partial charge transfer between the molecules and the surface. In particular, self-assembled monolayers (SAMs) of thiols are widely used for modification of the work function of gold electrodes. It has been shown that for these systems the charge transfer between the molecules and the gold is under 0.05e and, therefore, the chemical component of the interfacial dipole is negligibly small.[15] The work-function change in these systems is thus mainly determined by the dipole moments of individual molecules, the structure of the monolayer, and dipole depolarization in the SAM. The electrostatic interaction inside SAMs leads to a decrease in the surface dipole of the SAM, compared to that expected from the gas-phase dipoles. [16][17][18] Depolarization effects thus significantly decrease the accessible range of changes in SAM-induced metal work functions.The inevitable presence of defects in the SAMs, such as rotation of the molecules' backbones and changes in the configuration of the tail-group, which is a most common type of gauche defect (Fig. 1a), can also influence the electrostatic interactions in the monolayer. Defects in the monolayers lead to variations in orientation of molecular dipoles in the SAM (Fig. 1a), and, therefore, to local and global variations in the metalwork-function change. Although the nonideal character of SAMs is well-recognized, and confirmed both experimentally and theoretically, [19][20][21] the theoretical studies of depolarization effects in SAMs were until recently still confined to ideal monolayers.Our recent study of HS(CH 2 ) 11 COOH SAMs on gold surfaces showed a strong dependence of SAM depolarization and of the associated metal-work-function change on the structure of the SAM.[22] Here, we report a systematic study of this effect for HS(CH 2 ) n X monolayers, where X ¼ CH 3 , CF 3 , COOH, on Au (111) surfaces, and suggest a general method for rough and fine tuning of metal work functions using SAMs. We identify the polarizability of the polar groups of organic molecules as the main factor dete...

Perfluorinated Ionomer‐Modified Hole‐Injection Layers: Ultrahigh‐Workfunction but Nonohmic Contacts

“…We studied the PEDT:PSSH:PFI system as a model for ultrahigh-workfunction electrodes created through a dipolar dielectric surface layer, an approach that is of widespread current interest. The insights gained here may thus be relevant also to other methods using self-assembled monolayers, [26][27][28] dielectric polyelectrolytes, and amine-functionalized dielectric polymers. [ 29 ] We show here that the ultrahigh workfunction in the PEDT:PSSH:PFI system is generated by a VL shift through the frontier insulating monolayer of the HILs, rather than an electrochemical shift of E F of their hole carriers.…”

QM/MM method for metal–organic interfaces

“…It has been demonstrated experimentally and theoretically that such defects can profoundly affect the electronic properties of SAMs. 103,[124][125][126]129 We note also the importance of the re-orientation of SAM molecules (and their dipoles) inside domains and at their borders for reducing the surface free energy. These effects and functionalization of molecular head-groups can be efficiently studied within the method described earlier.…”

“…22,[24][25][26][27][28]97,103,[123][124][125][126] It has been demonstrated that the intramolecular electrostatic interactions in the monolayer lead to the redistribution of the electron density along the molecules (depolarization effect). This charge redistribution has a profound effect on the electrostatic potential both inside and outside the monolayer and determines the work function of the interface and the interaction of external molecules with the interface.…”

Structural Switching of Self-Assembled Monolayer by External Electric Field

“…One of the main functionalities of the SAM is adjusting the work function of metal electrodes [6][7][8][9]. Since the energy level difference between the HOMO or LUMO of the organic semiconductor and the work function of the electrode affects the electron injection through the metal-organic interfaces, fine tuning of the work function is of importance.…”