Abstract
Interface dipoles formed at an electrolyte/electrode interface have been widely studied and interpreted using the “double dipole step” model, where the dipole vector is determined by the size and/or range of motion of the charged ions. Some electron transport materials (ETMs) with lone pairs of electrons on heteroatoms exhibit a similar interfacial behavior. However, the origin of the dipoles in such materials has not yet been explored in great depth. Herein, we systematically investigate the influence of the lone pair of electrons on the interface dipole through three pyridine derivatives B2–B4PyMPM. Experiments show that different positions of nitrogen atoms in the three materials give rise to different hydrogen bonds and molecular orientations, thereby affecting the areal density and direction of the lone pair of electrons. The interface dipoles of the three materials predicted by the “double dipole step” model are in good agreement with the ultraviolet photoelectron spectroscopy results both in spin-coated and vacuum-deposited films. These findings help to better understand the ETMs/electrode interfacial behaviors and provide new guidelines for the molecular design of the interlayer.
Highlights
Interfacial engineering plays a crucial role in developing efficient and stable organic-basedelectronic devices such as light-emitting diodes and photovoltaic cells.[1−3] In a multilayer stacked device, the organic/electrode interfaces mainly control the injection or extraction of the charge carriers
Incorporating a dipole layer at the electrode interface is a common strategy to achieve Ohmic contacts and minimize the potential barrier. Inorganic salts, such as LiF, CaF2, and Cs2CO3, have been extensively used as a dipole layer and show excellent electron-injection properties.[6−8] considering the high diffusivity of these salts that result in exciton annihilation and poor stability and their limited applications in inverted and flexible devices, several promising organic compounds have been introduced as interlayers that exhibit excellent performance comparable to the inorganic salts, such as small-molecule electron transport materials (ETMs) bathophenanthroline (BPhen), bathocuproine (BCP), polymers polyethyleneimine, poly[(9,9-bis(3′(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)], and some of their polyelectrolyte derivatives.[9−15]
To figure out the origin of the dipole formed at the ETM/ electrode interface, we investigate the influence of the number and orientation of the lone pair of electrons through a series of pyridine derivatives, BPyMPM
Summary
Interfacial engineering plays a crucial role in developing efficient and stable organic-based (opto-)electronic devices such as light-emitting diodes and photovoltaic cells.[1−3] In a multilayer stacked device, the organic/electrode interfaces mainly control the injection or extraction of the charge carriers. Due to mismatch between the lowest unoccupied molecular orbital of organic semiconductors (OSCs) and the work function of stable cathodes, and the generally inferior electron transport properties of OSCs, the organic/cathode interface attracts significant attention.[4,5] Incorporating a dipole layer at the electrode interface is a common strategy to achieve Ohmic contacts and minimize the potential barrier Inorganic salts, such as LiF, CaF2, and Cs2CO3, have been extensively used as a dipole layer and show excellent electron-injection properties.[6−8] considering the high diffusivity of these salts that result in exciton annihilation and poor stability and their limited applications in inverted and flexible devices, several promising organic compounds have been introduced as interlayers that exhibit excellent performance comparable to the inorganic salts, such as small-molecule electron transport materials (ETMs) bathophenanthroline (BPhen), bathocuproine (BCP), polymers polyethyleneimine, poly[(9,9-bis(3′(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)], and some of their polyelectrolyte derivatives.[9−15]. The spectra are collected in partial electron mode with a multichanneltron plate detector with different retard voltages for selected elements
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