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 was fabricated using Alq<sub>3</sub>/HAT-CN as an interconnect layer. Its photovoltaic properties and exciton regulation mechanism were investigated. The results show that the luminance (11189.86 cd/m<sup>2</sup>) and efficiency (13.85 cd/A) of the tandem OLED reached 2.7 times that of the single EL unit OLED (luminance and efficiency of 4007.14 cd/m<sup>2</sup> and 5.00 cd/A, respectively) at a current density of 80 mA/cm<sup>2</sup>. This proves that Alq<sub>3</sub>/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 exciton (T<sub>1</sub>), which favours charg the 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 Alq<sub>3</sub>/HAT-CN device than in other connected layer devices. Triplet-charge annihilation (TQA) is weak, resulting in a relative increase in the concentration of T<sub>1</sub>, which is not involved in TQA. This suppresses the ISC and leads to a minimal increase in the MEL. As the current strength increases, the T<sub>1</sub> concentration increases, causing TQA toincrease and ISC to decrease. Since TQA is related to charge and T<sub>1</sub> concentration, lowering the temperature decreases the carrier mobility in the device, resulting in a relative decrease in charge concentration and a weakening of TQA. Lowering the temperature decreases the quenching of thermal phonons and increases the concentration of T<sub>1</sub> while extending its lifetime, resulting in enhanced triplet-triplet annihilation (TTA). At low temperatures, the high magnetic field shape of the MEL changes from slowly increasing to rapidly decrease. Therefore, the concentration of T<sub>1</sub> 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 important for the understanding of the luminescence mechanism of small molecule tandem devices and investigating the investigation of the mechanism for improving their photovoltaic performance.