Microscopic processes of Rubrene-doped devices with various exciplexes as hosts

Abstract

Recently, exciplex-based organic light-emitting diodes (OLEDs) have become hot topics in organic optoelectronics in terms of improving luminescent efficiency by controlling the energy transfer of excitons. This is because the formation of an exciplex can result in a small energy level difference, Δ E ST between singlet and triplet exciplex states, and with the assistance of external thermal energy, nonradiative triplet exciplex states can transform into radiative singlet ones through the so-called reverse intersystem crossing (RISC) process. Moreover, this RISC process will enhance the luminescence of OLEDs based on host–guest systems through energy transfer from exciplexes to dopants. The energy-transfer capability of OLEDs will affect the number of polaron pairs and exciplexes in host–guest systems. Indeed, it is important to find an effective probing technique for simply studying these microscopic processes. Many literature reports have demonstrated that organic magneto-electroluminescence (MEL) traces could be used for exploring and understanding the formation of and interactions between polaron pairs, excitons, and/or exciplexes. This is because MELs exhibit sensitive fingerprint responses to intersystem crossing (ISC), high-level reverse intersystem crossing (HL-RISC), singlet exciton fission (SF), and triplet exciton fusion (TF). In this study, four different OLEDs with various exciplex hosts have been fabricated, and their MEL curves have been measured at different currents and temperatures. Four exciplexes with different triplet exciton energies were used as host materials, and Rubrene was used as a fluorescent dopant. To study the exciton energy transfer and luminescence mechanism in such doping systems, we fully analyzed the emission spectra of the host materials, absorption spectrum of the guest material, and triplet exciton energy of the host and guest materials. The experimental results show that when the combined energy of the exciplex’s triplet excitons (EX3) is lower than that of the second-order triplet excitons (T2,Rub) of the Rubrene dopant, the MEL curve is dominated by the B -mediated ISC from host polaron pairs (PP1→PP3). Otherwise, the MEL curves are composed of both low-field and high-field components. The low-field components of MEL are governed by the B -mediated HL-RISC (T2,Rub→S1,Rub) process of the Rubrene molecules, while the high-field components of MEL are the result of the TF process between the triplet excitons of Rubrene (T1,Rub+T1,Rub→S1,Rub+S0,Rub) at high current densities or by the SF process of singlet excitons (S1,Rub+S0,Rub→T1,Rub+T1,Rub) at low current densities. In addition, these microprocesses will be markedly affected by the operational temperature and bias-current of the devices, because the Rubrene triplet excitons have long lifetimes (T1,Rub and T2,Rub) at low temperatures, and large quantities of Rubrene excitons are produced at high current densities. Moreover, the energy level difference between the single and triplet excitons of the host and guest will regulate the energy transfer from the former to the latter, i.e., the combined energy from the singlet excitons of the host being higher than that of the guest is the requirement of efficient Forster energy transfer where the smaller is the triplet exciton energy difference between the host and guest, the stronger is the Dexter energy transfer. Consequently, the energy-transfer mechanism will affect the utilization rate of the triplet excitons by adjusting the quantity and longevity of the singlet or triplet excitons on the Rubrene molecules, which ultimately affects the luminous intensity of the device. This work is helpful for further understanding the microscopic mechanisms of Rubrene-based devices and provides a theoretical reference for enhancing their luminescent efficiencies.