Unlocking the Full Potential of Electron‐Acceptor Molecules for Efficient and Stable Hole Injection

Abstract

Understanding the hole‐injection mechanism and improving the hole‐injection property are of pivotal importance in the future development of organic optoelectronic devices. Electron‐acceptor molecules with high electron affinity (EA) are widely used in electronic applications, such as hole injection and p‐doping. Although p‐doping has generally been studied in terms of matching the ionization energy (IE) of organic semiconductors with the EA of acceptor molecules, little is known about the effect of the EA of acceptor molecules on the hole‐injection property. In this work, the hole‐injection mechanism in devices is completely clarified, and a strategy to optimize the hole‐injection property of the acceptor molecule is developed. Efficient and stable hole injection is found to be possible even into materials with IEs as high as 5.8 eV by controlling the charged state of an acceptor molecule with an EA of about 5.0 eV. This excellent hole‐injection property enables direct hole injection into an emitting layer, realizing a pure blue organic light‐emitting diode with an extraordinarily low turn‐on voltage of 2.67 V, a power efficiency of 29 lm W−1, an external quantum efficiency of 28% and a Commission Internationale de l'Eclairage y coordinate of less than 0.10.

Neoteric Advances in Oxygen Bridged Triaryl Boron‐based Delayed Fluorescent Materials for Organic Light Emitting Diodes

“…These results highlight that the B‐based MR core can be a potential acceptor for devising efficient TADF emitters. On the other hand there are some host type materials also reported based on DOBNA core, which showed reasonably good performances [81–84] …”

“…On the other hand there are some host type materials also reported based on DOBNA core, which showed reasonably good performances. [81][82][83][84]…”

Highly Efficient and Spectrum‐Tunable Hybrid Tandem White Organic Light‐Emitting Diodes Achieved by Strategic Exciton Management based on Ultrathin Emitting Layer Structures

“…[ 19 ] As shown in Figure S2a, Supporting Information, the electron acceptor HAT‐CN (low LUMO energy level of −4.8 eV) with strong electron‐withdrawing property will lead HAT‐CN to easily accept electrons from TAPC (HOMO energy level of −5.5 eV), thus generating a large amount of charge. [ 37 ] It is well known that the charge transfer complex is usually formed by means of electron transfer from the HOMO of host material to the LUMO of dopant material, which generates an additional free charge. [ 38 ] In this case, the HAT‐CN (0.3 wt%):TAPC blended film will generate additional free holes and electrons.…”

Energy-Level Alignment Governs Doping-Related Fermi-Level Shifts in Polymer Films

“…There, one could argue that solid-state effects should not be taken into account, as charge-transfer processes intrinsically take place at the molecular level. Thus, it was suggested that energy levels taken from cyclic voltammetry (CV) or energy resolved-electrochemical impedance spectroscopy (ER-EIS) are most appropriate for discussing CPX or IPA formation. ,, In contrast, the energy-level alignment in heterostructures takes place on a nonlocal, macroscopic scale, and the relevant IEs and EAs are routinely taken from ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission (IPES), respectively. , Notably, these thin-film IE and EA values are not material parameters but depend strongly on the film thickness and the intermolecular organization in the thin films. , As an example, for P3HT, reported thin-film IEs range from around 4 to 5 eV, ,, and for the EA of F4TCNQ, values of 5.24 and 5.08 eV have been reported. Also, it has been put forward that the EA of dopants embedded in a host matrix differs from that in a pristine dopant film. , Furthermore, an intrinsic energetic disorder of organic thin films broadens the density of states (DOS) of molecular energy levels leading to typical standard deviations (σ) of the Gaussian HOMO DOS of 250 meV in UPS measurements. ,, For the determination of IE and EA, the onsets of the HOMO- and LUMO-derived peaks are usually used as they contribute to charge transport and exchange processes (commonly referred to as “mobility edge”) .…”

“…16,17,30 In contrast, the energy-level alignment in heterostructures takes place on a nonlocal, macroscopic scale, and the relevant IEs and EAs are routinely taken from ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission (IPES), respectively. 31,32 Notably, these thin-film IE and EA values are not material parameters but depend strongly on the film thickness 33 and the intermolecular organization in the thin films. 34,35 As an example, for P3HT, reported thin-film IEs range from around 4 to 5 eV, 23,36,37 and for the EA of F4TCNQ, values of 5.24 38 and 5.08 eV 13 have been reported.…”