The evolution of microscopic process mediated by current and temperature in Rubrene-based organic light emitting diodes

Abstract

The magnetic field effects (MFEs) of organic light-emitting diodes (OLEDs) is usually used as a fingerprint response for the microscopic mechanism of excited states in organic semiconductor. In this work, this sensitive method is employed to investigate the evolution of microscopic mechanism with the currents and temperatures in rubrene-based OLEDs. The experimental results of room temperature show that singlet fission (SF), which is rarely dependent on the bias currents, is the primary process in the device at small injection currents; at moderate injection currents, triplet fusion (SF) process gradually occurs besides the SF process; at large injection currents, the inter-system crossing (ISC) process appears except the process of TF and SF. When the temperature is reduced from ambient temperature down to 20 K, the intensity of SF process decreases, but both TF and ISC process become stronger. The temperature and current dependences of ISC in this rubrene-based OLEDs is contrary to those reported from conventional devices without SF process. We combine the rate constants of micro processes with the current- and temperature-dependent lifetime of excited states to well explain the experimental results. Our work is beneficial to a profound understanding of the microscopic mechanisms for OLEDs.