Direct observation of deep electron traps in aged organic light emitting diodes

Despite the importance of the operational stability of organic light emitting diodes (OLEDs) for commercial application, relatively little is known about the intrinsic mechanism of OLED "aging"-gradual loss of electroluminescence efficiency in devices operated for extended periods of time. We have previously found that OLED aging results in the formation of deep hole traps at or near the electron transport layerparallel tohole transport layer interface. We now report that the aging of an archetypal anode parallel toNPBparallel toAlqparallel to cathode OLED device also creates deep electron traps in the vicinity of the recombination zone, which can be selectively populated via short irradiation of an aged device with white light. These traps appear to be weakly emissive, with the estimated luminescence efficiency being 100-150x smaller than the efficiency of Alq. Short irradiation with white light results in the charging of these electron traps, placing the aged OLED device into a state with nearly zero internal charge. The subsequent driving of an electric current results in (i) an "early" electroluminescence pulse, (ii) discharging of the electron traps, and (iii) placing the aged OLED into a state with substantial positive internal charge. Although the chemical nature of these traps remains elusive, the redshifted, broad emission spectrum is consistent with the formation of Alq degradation products as a result of electrical aging. An accumulation of such electron traps can be envisioned to result in the luminance efficiency loss via two principal mechanisms: (i) the direct recombination of a mobile hole and trapped electron yielding a nearly nonemissive excited state, and (ii) long-range energy transfer from the OLED emitter molecule to the unfilled electron trap molecule, yielding the same nearly nonemissive excited state. (C) 2005 American Institute of Physics.