Eliminating Triplet-State Annihilation and Converting Black Triplets into Bright Singlets for Enhancing Light Emission from Thermally Activated Delayed Fluorescence-Based OLEDs Driven by an Elaborately Designed Short Pulse Voltage

Abstract

Thermally activated delayed fluorescence-based organic light-emitting diodes (TADF-OLEDs) have attracted much attention in recent years; yet, it is confronted with the bottleneck of acute efficiency roll-off, which is mainly attributed to the triplet–triplet annihilation (TTA) and singlet–triplet annihilation (STA) induced by the accumulation of long-lived triplets at high current densities. However, our experimental observations herein demonstrate that the accumulation of triplets can also be governed by the duration of the applied voltage rather than solely by the well-accepted large current density. More importantly, we discover that replacing the conventional direct current (DC) driving source with an ingeniously designed short pulse voltage can effectively eliminate TTA and STA while harvesting the accumulated triplets to enhance the device's light emission. That is, an optimized short pulse width can ensure the triplet concentration is below the threshold for the occurrence of TTA and STA, while a suitable pulse interval provides sufficient time for converting black triplets into bright singlets via the reverse intersystem crossing process. Surprisingly, by employing such a simple experimental strategy of the optimized pulse drive, a significant enhancement of more than 90% in the light emission from 585 to 1114 cd m^–2 is achieved compared to that driven by the DC source. Therefore, this work not only clarifies the excited-state dynamic processes within the duration of an applied voltage but also presents a feasible method to suppress TTA and STA, simultaneously harvesting triplets for enhancing light emission in TADF-OLEDs.