Deepening Insights into Aggregation Effect of Intermolecular Charge‐Transfer Aggregates for Highly Efficient Near‐Infrared Non‐Doped Organic Light‐Emitting Diodes over 780 nm

Abstract

Though urgently needed, high‐efficiency near‐infrared (NIR) organic light‐emitting diode (OLED) is still rare due to the energy‐gap law. Formation of intermolecular charge‐transfer aggregates (CTA) with nonadiabatic coupling suppression can decelerate non‐radiative decay rates for high‐efficiency NIR‐OLEDs. However, the aggregation effect of CTA is still not fully understood, which limits the rational design of CTA. Herein, two CTA molecules with a same π‐framework but different terminal substituents are developed to unveil the aggregation effect. In highly ordered crystalline states, the terminal substituents substantially affect the molecular packing motifs and intermolecular charge‐transfer states, thus leading to distinct photophysical properties. In comparison, in amorphous states, these two CTA demonstrate similar photophysical behaviors and properties due to their similar molecular packing and intermolecular interactions as evidenced by molecular dynamics simulations. Importantly, the formations of amorphous CTA trigger multifunction improvements such as aggregation‐induced NIR emission, aggregation‐induced thermally activated delayed fluorescence, self‐doping and self‐host features. The non‐doped OLEDs demonstrate NIR emissions centered at 788 and 803 nm, and high maximum external quantum efficiencies of 2.6% and 1.5% with small efficiency roll‐off, respectively. This study provides deeper insight into the aggregation effect of CTA and lays a foundation for the development of high‐efficiency NIR non‐doped OLEDs.

A red thermally activated delayed fluorescence emitter with mitigated efficiency roll-off via a π-stacked multiple donor–acceptor structure

“…One effective strategy to mitigate efficiency roll-off is to decrease Δ E ST and the lifetime of triplet excitons in red/NIR emitters. Hence, it is imperative to meticulously design novel TADF molecules using a variety of strategies, including enhancing the electron-donating and electron-withdrawing abilities, 17,18 incorporating more rigid donors and acceptors, 19 adjusting dihedral angles between donors and acceptors, 20,21 and constructing a wedge-shaped molecular structure. 22 For example, Yang et al revealed a new emitter (TAT-PP) by combining the rigid triazatruxene (TAT) donor and cyano-substituted phenanthropyrazine (PP) acceptor, resulting in a high EQE of approximately 20% at 621 nm, and a low efficiency roll-off ratio of only 8% at 1000 cd m −2 .…”

Introducing Internal Host Component to Thermally Activated Delayed Fluorescence Emitter for Efficient NIR Nondoped Organic Light-Emitting Diodes

“…Organic light-emitting diodes (OLEDs) have been receiving mounting interest due to thermally activated delayed fluorescence (TADF) emitters being an emerging family of emitters for organic light-emitting diodes (OLEDs) that demonstrate the attractive capability of fully harvesting both singlet and triplet excitons free of any noble metals. − In the past decade, significant progress has been made in TADF-OLEDs in the visible region. − In contrast, the development of TADF-OLEDs approaching the near-infrared (NIR) region is significantly lagging behindthere have been only a few reports showing far inferior efficiency, although NIR electroluminescence (EL) permits promising unique applications such as night-vision readable marking, medical therapy, and telecommunication. − The mainstream route is to directly construct electron-donor (D)-electron-acceptor (A)-type TADF emitters that intrinsically emit NIR photons. − For example, Bronstein et al developed a TADF emitter based on a NIR prototypical emitter (APDC-DTPA) by strategically replacing a D group (triphenylamine) with an A group (cyan), affording NIR emission (e.g., peaking at 763 nm in toluene); however, this approach has fallen short in the device efficiency due to the serious energy loss associated with the reduced optical band gap, hampering potential applications . Furthermore, in terms of molecular designs, such candidates of D and A building block pairs that are strong enough for NIR band gaps are very limited, which poses great challenges for developing intrinsically NIR TADF emitters. − …”

Extending rigid electron-deficient skeletons and appending electron-rich units to build high-efficiency red-emitting pyrene-derived TADF materials

“…In general, high photoluminescence quantum yield ( Φ PL ) and exciton utilization efficiency ( Φ EUE ) are simultaneously required to achieve high external quantum efficiency (EQE) in OLEDs according to spin statistics theory. 22–28…”