Making silver a stronger n-dopant than cesium via in situ coordination reaction for organic electronics

Abstract

N-doping is an effective way to increase the electron conductivity of organic semiconductors and achieve ohmic cathode contacts in organic electronics. To avoid the use of difficult-to-handle highly reactive n-dopants, air-stable precursors are widely used nowadays, which could decompose to release reactive species in a subtractive way though always with unwanted and even harmful byproducts during processing. Here, we show that air-stable metals, such as copper, silver and gold, could release free electrons readily in the presence of chelating ligands, as the irreversible coordination reaction between metal ions and the ligands would push the equilibrium between metals and metal ions to the forward direction. By using a well-designed multi-functional electron transport material with a strong nucleophilic quality, 4,7-dimethoxy-1,10-phenanthroline (p-MeO-Phen), silver could function as an n-dopant stronger than cesium and could be used to fabricate organic light-emitting diodes with higher performance than the cesium-doped control device.

Image-force effects on energy level alignment at electron transport material/cathode interfaces

“…25,26 This phenomenon is well known when active metals are evaporated later or together with the ETMs. [27][28][29] Though ultraviolet photoelectron spectroscopy (UPS), gap states between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are detected and also consistent with the DFT calculation result. [28][29][30] However, this only occurs when the molecules are deposited on clean reactive metal surfaces with low WF, such as BCP on Ag, Al, Ca, Mg.…”

High‐Performance Quantum‐Dot Light‐Emitting Diodes Using NiOx Hole‐Injection Layers with a High and Stable Work Function

“…The current efficiency of this device reaches a maximum of 19.4 cd A −1 at a brightness of 3400 cd m −2 and maintains to be above 18.5 cd A −1 at a brightness of 10 000 cd m −2 . Remarkably, the driving voltages for the luminance of the device reaching 1000 cd m −2 and 10 000 cd m −2 are 2.1 and 3.3 V, respectively, both of which are the lowest for red solution‐processed LEDs and red vacuum‐deposited OLEDs (see Table S2, Supporting Information) . A histogram of 45 devices shows an average current efficiency of 17.4 cd A −1 (see Figure S12, Supporting Information) with a relative standard deviation of 12%.…”

Understanding coordination reaction for producing stable electrode with various low work functions

“…The development of stable n-dopants has been hindered by the energy-level matching between the organic semiconductor and the dopant, since the stability is compromised by the lower IP. On the other hand, improvement of the electron injection/collection efficiency utilising the electron exchange between the nitrogen atom of organic compounds and inorganic materials has also been intensively studied [23][24][25][26][27][28][29][30] . Promising organic compounds include amine-containing molecules such as polyethyleneimine (PEI), which can tune the surface WF of about 1 eV by utilising both the ET from PEI to the electrode surface and the orientation of the molecular dipole of PEI 23 .…”