Kakumachi Shunta, Nakanotani Hajime, Nagasaki Yuto, Adachi Chihaya
Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
ACS Appl Mater Interfaces. 2024 Jun 19;16(24):31392-31398. doi: 10.1021/acsami.4c02821. Epub 2024 Jun 6.
The spontaneous orientation polarization (SOP) of a permanent dipole moment of the molecule induces a giant surface potential (GSP) in an organic semiconductor film, and GSP is expected to be a crucial parameter for understanding the operational mechanism of organic light-emitting diodes (OLEDs). This study demonstrates that the voltage-dependent migration of a carrier recombination zone induced by a polar electron transporting layer (ETL) having a positive SOP causes a decline in the overall performance of the OLED in triplet-triplet upconversion (TTU) based on OLEDs. Specifically, the TTU efficiency in an OLED with 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) as the ETL decreased by 20% due to the reduction of electrically generated triplet exciton density. This decrease resulted in a lower external electroluminescence (EL) quantum efficiency (EQE) of 5.4% at 1 mA cm, while the OLED with a nonpolar ETL resulted in an EQE of around 8.1% at 1 mA cm. We confirmed a shift in the recombination zone from the current density dependence of the EL spectra in the OLEDs. Our results indicate that the fixed carrier recombination zone near a hole transport layer and an emitting layer (HTL/EML) strongly enhanced the TTU process, while the polar EML/ETL interface induced the migration of the recombination zone depending on voltage, resulting in the decrease of triplet exciton density.
分子永久偶极矩的自发取向极化（SOP）会在有机半导体薄膜中诱导出巨大表面电势（GSP），并且GSP有望成为理解有机发光二极管（OLED）工作机制的关键参数。本研究表明，具有正SOP的极性电子传输层（ETL）所诱导的载流子复合区的电压依赖性迁移会导致基于OLED的三重态-三重态上转换（TTU）中OLED的整体性能下降。具体而言，以2,2',2″-(1,3,5-苯三嗪基)-三(1-苯基-1-H-苯并咪唑)（TPBi）作为ETL的OLED中的TTU效率由于电产生的三重态激子密度降低而下降了20%。这种下降导致在1 mA cm²时外部电致发光（EL）量子效率（EQE）降低至5.4%，而具有非极性ETL的OLED在1 mA cm²时的EQE约为8.1%。我们通过OLED中EL光谱的电流密度依赖性证实了复合区的移动。我们的结果表明，空穴传输层和发光层（HTL/EML）附近固定的载流子复合区强烈增强了TTU过程,而极性EML/ETL界面会导致复合区根据电压发生迁移，从而导致三重态激子密度降低。
