Optimizing the energy level alignment for achieving record-breaking efficiency in hot exciton deep red OLEDs

Area of Science:

• Materials Science
• Organic Electronics
• Photophysics

Background:

• The "energy gap law" hinders efficient light emission in deep red (DR) and near-infrared (NIR) regions due to quenching processes.
• Hot exciton emitters offer a potential solution by enabling rapid radiative decay, but require specific energy level configurations.
• Achieving narrow energy gaps between singlet (S₁) and triplet (T₂) excited states, with T₂ slightly above S₁, is crucial for hot exciton functionality.

Purpose of the Study:

• To design and synthesize novel hot exciton emitters for efficient DR and NIR emission.
• To investigate the relationship between molecular design, energy level alignment, and photophysical properties.
• To demonstrate the performance of these emitters in non-doped deep red organic light-emitting diodes (DR-OLEDs).

Main Methods:

• Synthesis of two proof-of-concept hot exciton emitters, αT-IPD and βT-IPD, by coupling specific electron donors (αTPA, βTPA) with an electron acceptor (IPD).
• Characterization of photophysical properties, including energy gaps (ΔE_S and ΔE_T), photoluminescence quantum yields (PLQYs), and aggregation-induced emission (AIE) characteristics.
• Fabrication and testing of non-doped DR-OLED devices using the synthesized emitters.

Main Results:

• Both αT-IPD and βT-IPD emitters exhibited narrow ΔE_S with T₂ slightly exceeding S₁, and large ΔE_T.
• The emitters displayed aggregation-induced emission (AIE) and favorable intermolecular interactions in the crystalline state.
• Remarkable PLQYs of 68.5% (αT-IPD) and 73.5% (βT-IPD) were achieved in non-doped films.
• A non-doped DR-OLED based on βT-IPD achieved a maximum external quantum efficiency (EQE) of 15.5% at 667 nm.

Conclusions:

• The designed αT-IPD and βT-IPD molecules effectively function as hot exciton emitters.
• The narrow ΔE_S and specific molecular packing contribute to high PLQYs and efficient DR emission.
• The achieved 15.5% EQE represents a significant advancement for hot exciton DR-OLEDs, overcoming the energy gap law limitations.