Time‐Resolved Electroluminescence Study for the Effect of Charge Traps on the Luminescence Properties of Organic Light‐Emitting Diodes

Abstract

The electroluminescence characteristics of pristine and degraded organic light‐emitting diode (OLED) devices are studied using the time‐resolved electroluminescence (TREL) technique. It is found that materials degradation results in notable changes in the temporal profile of TREL curve, which involves a shorter onset time, a longer rise time, and a longer decay time of electroluminescence. It is found that these temporal characteristics are affected by trapped charges formed during the OLED operation. More detailed analysis reveals that there are two types of charge traps. The longer decay time is found to be due to the excitons produced by weakly bound charges trapped in the organic layer that are released by thermal energy even without applying the voltage pulse. In contrast, the shorter onset time of electroluminescence is due to the excitons from strongly bound charges that can be released only when a voltage pulse is applied. It is demonstrated that TREL can be used to investigate the underlying mechanisms of OLED degradation. The presence of different types of charge traps found in this study may prove useful for more elaborate design of OLED devices toward enhanced durability against degradation and higher duty cycle of device operation.

Improved transient electroluminescence technique based on time-correlated single-photon counting technology to evaluate organic mobility

“…Figure 2b shows a typical transient EL profile measured by TCSPC technique with a pulse length, t P , of 2 µs and amplitude of 7 V. Three regions are clearly observed in Fig. 2b, which are consistent with the analog detection results [28]. In region I, external pulse is applied and charge carriers are injected from the electrodes.…”

Quantitative Determination of Charge Accumulation and Recombination in Operational Quantum Dots Light Emitting Diodes via Time-Resolved Electroluminescence Spectroscopy

“…The rising time and falling time of the pulse are within 0.02 μs, which is far less than the response time of QLED devices. Based on the previous understanding into the operation of a LED device, we first extracted the features to describe TREL and TRC curves, including delay, rising, balance, and decay . We then analyzed the operation mechanism of QLED in the delay, rising, balance, and decay stages.…”

“…Based on the above understanding into TRC spectra, we further investigate the stages of delay, rising, and decay in TREL spectra. Delay time ( T d ) refers to the time difference pulsed voltage and transit luminescence, which can be explained to the unbalanced carrier injection or capacity charging in the previous works. − To clarify the key factors in determining the delay time, we varied the charging process by introducing a series resistance. As shown in Figure a,b, the delay time of a typical QLED can be alternated by controlling the capacitor charging using different values of a rheostat (100, 200, and 300 Ω).…”

“…Based on the previous understanding into the operation of a LED device, we first extracted the features to describe TREL and TRC curves, including delay, rising, balance, and decay. 15 We then analyzed the operation mechanism of QLED in the delay, rising, balance, and decay stages. Typically, the operation of a QLED device can be described as four periods.…”

“…Efforts have been made to explain the physical processes related to these stages. For the delay stage, it was normally explained by the unbalanced carrier injection, − while the works by Shen et al and Pommerehne et al attributed such a phenomenon to the capacity charging. Keating et al proposed that both capacity charging and unbalanced carrier injection contribute to delay time .…”