Determining interfacial energy levels between the crystalline emitting layer and the amorphous electron transport layer: UPS-assisted efficiency optimization in crystalline OLEDs

Abstract

In crystalline organic light-emitting diodes (C-OLEDs), the interfacial energy level matching between the amorphous electron transport layer (ETL) and the crystalline emitting layer (EML) is of vital importance for the charge injection efficiency and device performance. In traditional studies, the bulk highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels of a single material are often determined using cyclic voltammetry (CV), and these values are employed to indicate the materials’ energy levels in OLEDs. However, this method neglects key factors such as interface energy level bending or offset, which could greatly influence crystalline material interfaces. In this study, taking blue-light 2FPPICz:BPPI C-OLED as the model system and selecting five typical electron transport materials including TmPyPB, TPBI, and Bphen as the amorphous ETL, the key differences in interfacial energy level characterization are revealed through multi-dimensional methods. By comparing the bulk energy levels of the amorphous ETL determined using CV and the interfacial energy levels of the crystalline EML/amorphous ETL measured using ultraviolet photoelectron spectroscopy (UPS), it is found that there is a significant difference between the molecular energy levels measured by CV and the actual interfacial energy levels in C-OLEDs, leading to an obvious deviation between the actual external quantum efficiency (EQE) of the device and the expected value. For example, CV shows that the bulk LUMO and HOMO of Bphen match with that of the EML, but UPS measures an interfacial energy level mismatch, resulting in a maximum external quantum efficiency of only 1.70% for the device, which is much lower than the theoretical expectation. In contrast, although CV shows a large energy difference between the bulk LUMO of TPBI and the EML, UPS measures well-matched interfacial energy levels with only a 0.09 eV energy difference, achieving a high EQE of 5.98%. The research results indicate that interfacial energy level analysis based on UPS can precisely describe the interface physics, guide energy level optimization and improve the luminous efficiency of C-OLEDs. This work proves a necessary characterization paradigm for interface engineering of organic crystalline optoelectronic devices, especially crystalline OLEDs.