Exciton regulation mechanism of Alq₃/HAT-CN tandem electroluminescent devices

Abstract

Tandem organic electroluminescent devices (OLEDs) have attracted widespread attention due to their long lifetime and high current efficiency. In this study, a double-emitting unit tandem OLED is fabricated by using Alq₃/HAT-CN as an interconnect layer. Its photovoltaic properties and exciton regulation mechanism are investigated. The results show that the luminance (11189.86 cd/m²) and efficiency (13.85 cd/A) of the tandem OLED reaches 2.7 times that of the single electroluminescent (EL) unit OLED (luminance and efficiency of 4007.14 cd/m² and 5.00 cd/A, respectively) at a current density of 80 mA/cm². This proves that the Alq₃/HAT-CN is an efficient interconnect layer. At room temperature, the polaron pair undergoes intersystem crossing (ISC) due to hyperfine interaction (HFI) when a magnetic field is applied to the device. This increases the concentration of the triplet excitons (T₁), thus promoting the charge scattering. The result is a rapid increase in the low magnetic field and a slow increase in the high magnetic field of the MEL. When the injection current strength is constant, there is less uncompounded charge in the Alq₃/HAT-CN device than in other connected layer devices. Triplet-charge annihilation (TQA) is weak, resulting in a relative increase in the value of T₁, which is not involved in the TQA. This suppresses the ISC and leads to a minimal increase in the MEL. As the current strength increases, the T₁ value increases, causing TQA to increase and ISC to decrease. Since the TQA is related to charge and T₁ value, lowering the temperature reduces the carrier mobility in the device, resulting in the relative decreasing of charge concentration and the weakening of TQA. Lowering the temperature reduces the quenching of thermal phonons and increases the T₁ value while extending its lifetime, resulting in the enhancement of triplet-triplet annihilation (TTA). At low temperatures, the high magnetic field shape of the MEL changes from slowly increasing to rapidly decreasing. Therefore, the T₁ value can be regulated by varying the current strength and temperature, which further affects the strength of ISC, TQA and TTA, and the luminescence and efficiency of the device can be effectively improved by reducing TQA and ISC. This work is of great significance in understanding the luminescence mechanism of small molecule tandem devices and studying the mechanism for improving their photovoltaic properties.