Probing the Recombination Process at the Intermixed Interface of a Solution-Processed Organic Light Emitting Diode By Impedance Spectroscopy

Abstract

Solution-processed organic light emitting diode (s-OLED) has garnered considerable research interest due to its cost-effectiveness, improved energy efficiency, and material-saving advantages over conventional vacuum deposition methods in OLED production. Recent studies have shown that the thermal annealing process of multilayered s-OLEDs induces intermixing at the interfaces of the layers and enhances the device performance [1,2]. However, the physical origin of the enhanced s-OLED performance by intermixing at the interfaces is poorly understood. To address this issue, we utilized electrical impedance spectroscopy (IS) to investigate the thermal-induced modifications of charge carrier dynamics in a bilayer s-OLED comprising an electron transporting layer (ETL) and an emission layer (EML). We separately assessed the conductance of the ETL and the charge carrier accumulation at the EML/ETL interface by the characteristic relaxation frequency and the plateau at a low frequency regime (20~100 Hz) extracted from the frequency-dependent capacitance spectra, respectively. The IS analysis revealed that improved charge transport in the ETL by thermal annealing process was mainly responsible for the performance enhancement, rather than intermixing at the EML/ETL interface. Notably, we found that annealing the s-OLEDs above the glass transition temperature (Tg) of the ETL, at which significant intermixing occurs, increased the magnitude of negative capacitance, indicating the increased rate of nonradiative trap-assisted recombination [3]. Accordingly, our IS analysis clarified that extended intermixing at the EML/ETL interface is rather detrimental to the performance of s-OLEDs. This study demonstrates that IS can serve as a powerful tool for accurately analyzing charge carrier dynamics in s-OLEDs by simultaneously probing charge transport, interfacial charge accumulation, and recombination processes, thereby providing valuable insights for optimizing s-OLED performance.[1] ACS Appl. Mater. Interfaces 2015, 7 (37), 20779–20785. https://doi.org/10.1021/acsami.5b05818.[2] Advanced Materials Interfaces 2019, 6 (4), 1801627. https://doi.org/10.1002/admi.201801627.[3] Phys. Rev. Lett. 2018, 120 (11), 116602. https://doi.org/10.1103/PhysRevLett.120.116602. Figure caption. The capacitance at 20 Hz and current efficiency at 100 nit are plotted against the ratio of annealing temperature (T) to glass transition temperature (Tg) of ETL. Figure 1