Qing Zhang

The spatio-temporal dynamics of excitons in functional polymer light-emitting diodes reveal that singlet–triplet annihilation (STA) is significantly higher than the often invoked singlet–polaron annihilation (SPA). STA holds back the efficiencies of PLEDs and further realization of organic injection lasers. Additional singlets are found in F8BT that are related to thermally activated delayed fluorescence (TADF), as triplet–triplet annihilation (TTA) alone cannot account for the observed high external quantum efficiencies.

A tandem organic light-emitting diode (OLED) is an organic optoelectronic device that has two or more electroluminescence (EL) units connected electrically in series with unique intermediate connectors within the device. Researchers have studied this new OLED architecture with growing interest and have found that the current efficiency of a tandem OLED containing N EL units (N > 1) should be N times that of a conventional OLED containing only a single EL unit. Therefore, this new architecture is potentially useful for constructing high-efficiency, high-luminance, and long-lifetime OLED displays and organic solid-state lighting sources. In a tandem OLED, the intermediate connector plays a crucial role in determining the effectiveness of the stacked EL units. The interfaces in the connector control the inner charge generation and charge injection into the adjacent EL units. Meanwhile, the transparency and the thickness of the connector affect the light output of the device. Therefore, the intermediate connector should be made to meet both the electrical and optical requirements for achieving optimal performance. Here, recent advances in the research of the tandem OLEDs is discussed, with the main focus on material selection and interface studies in the intermediate connectors, as well as the optical design of the tandem OLEDs.

Recent advances of tandem organic light-emitting diodes (OLEDs) including the material selection, interface engineering, and optical design of numerous intermediate connectors are reviewed. Their interfaces are crucial in determining the driving voltage, efficiencies, and lifetime. The optical transparency, microcavity, light out-coupling, and voltage drop across the intermediate connectors have to be carefully considered for high-performance tandem OLEDs.

A series of pyrimidine-conjugated thermally activated delayed fluorescent (TADF) emitters is developed. Using a highly emissive green pyrimidine conjugate emitter and sophisticated device engineering, the optimized device operates on 2.2 V at 1 cd m⁻² and exhibits a superior performance with an η_ext of over 25%, an η_p of over 110 lm W⁻¹, and exceptionally low efficiency roll-off.

Efficiency roll-off in blue organic light-emitting diodes especially at high brightness still remains a vital issue for which the excitons density-dependent mechanism of host materials takes most responsibility. Additionally, the efficiency roll-off leads to high power consumption and reduces the operating lifetime because higher driving voltage and current are required. Here, by subtly modifying the triphenylamine to oxygen-bridged quasi-planar structure, a novel thermally activated delayed fluorescence type blue host Tri-o-2PO is successfully developed. Efficiency roll-off based on Tri-o-2PO is ultralow with external quantum efficiency (EQE) just dropping by around 2% in the high luminance range from 1000 cd m⁻² to 10 000 cd m⁻². As expected, low turn-on voltage (≈2.9 V) of device is also achieved, which is close to the theory limit value (≈2.62 V). Super-high power efficiency (≈60 lm W⁻¹) and EQE (>22%) are also achieved when utilizing Tri-o-2PO as host. Furthermore, two-color warm-white light with CIE of (0.45, 0.43) and correlated color temperature of 2921 K is also fabricated and a champion EQE of 21% is delivered. These excellent performances prove the strategy of bridging the triphenylamine to reduce ΔE_st is validated and suggest the great potential of this novel skeleton.

A blue thermally activated delayed fluorescence material, Tri-o-2PO, is developed and implemented as the host for organic light-emitting diodes (OLEDs). Efficiency roll-off based on Tri-o-2PO is ultralow with external quantum efficiency (EQE) dropping by only around 2% in the high luminance range (1000 to  10 000 cd m⁻²). Additionally, low turn-on voltages (≈2.9 V) and high power efficiency (≈60 lm W⁻¹) are achieved simutaneously.