Organic light-emitting diodes (OLEDs) draw intensive research interest for displays and lighting owing to advantages including high contrast, wide color gamut, fast response, light weight and mechanical flexibility. For thirty years, researchers have strived to develop high-performance OLEDs with high brightness, color purity, efficiency and stability. Advances in novel emitters and functional materials greatly elevate OLED electroluminescence (EL), delivering maximum brightness above 10 5 cd m −2 and peak external quantum efficiency (EQE) over 35%. Nevertheless, intrinsic instability of organic materials restricts long-term device stability. Weak chemical bonds trigger organic decomposition under electrical bias; decomposition products form non-radiative recombination sites, luminescence quenchers and deep charge traps, causing device instability and degradation.
OLEDs prepared via vacuum evaporation or solution processing consist of amorphous organic thin films and metal electrodes. Disordered molecular stacking in amorphous films generates pinholes for water and oxygen permeation. Moisture and oxygen induce organic oxidation and structural variation, producing dark spots and accelerating device deterioration. Encapsulation has been recognized as one of the most straightforward and effective approaches to restricting the infiltration of moisture and oxygen. Several advanced encapsulation techniques have been developed to reduce the influence of moisture and oxygen on the overall EL performance of OLEDs. However, moisture and oxygen penetration could not be completely prevented by any external encapsulation; the inherent stability as well as the ability to impede moisture and oxygen penetration of the inner functional layers is an important supplement to the operational stability of the OLEDs.
Organic single crystals (SCs), with long-range periodic ordering and superior charge-transport properties, have recently been demonstrated as a superior charge-transporting layer to realize high-performance OLEDs. Different from amorphous layers, the SCs possess outstanding properties from the aspect of inherent material stability, including thermal, morphological, and electrochemical stabilities. Furthermore, their dense molecular packing of SCs with strong intermolecular π-π interactions is prone to restrict moisture and oxygen penetration. Therefore, the SCs may be a promising candidate to improve device operational stability by serving as an intrinsically stable functional layer in company with the internal encapsulation ability within the OLED devices.
Recently, a research team led by Jilin University employed a high-quality organic single crystal (SC) of 1,4-bis (4-methylstyryl) benzene (BSB-Me) as both a stable hole-transporting layer (HTL) and an internal barrier layer. Benefiting from dense ordered molecular packing and strong intermolecular interactions, the BSB-Me SC exhibits superior thermal, morphological, and electrochemical stability, outperforming mainstream HTL materials such as NPB and TAPC. It shows a glass-transition temperature of 119 °C and a decomposition temperature of 280 °C, retaining its morphological integrity after 120 hours of ambient exposure and seven days of storage in air, with no detectable molecular degradation or bond cleavage under prolonged electrical stress.
The BSB-Me SC also demonstrates exceptional barrier properties. It achieves an ultra-low water vapor transmission rate (WVTR) of 3.58×10 −4 g m −2 day −1 , a value comparable to that of 50 nm ALD-deposited Al 2 O 3 barrier films. First-principles calculations, time-of-flight secondary-ion mass spectrometry (TOF-SIMS) depth profiling, and energy-dispersive X-ray spectroscopy (EDS) elemental mapping collectively confirm its robust capability to block water and oxygen permeation, thereby enabling effective internal encapsulation for OLED devices.
With an optimized 500 nm-thick BSB-Me SC HTL, full-color SC-OLEDs achieve outstanding electroluminescence performance. Red, green, and blue devices reach peak external quantum efficiencies (EQEs) of 13.78%, 12.51%, and 12.14%, respectively, with the red SC-OLED establishing a new EQE record among single-crystal OLEDs. The devices also exhibit low efficiency roll-off, excellent batch-to-batch reproducibility, and suppressed Joule heating losses during operation.
Notably, this technology addresses the long-standing industry challenge of short blue OLED lifetimes. The optimized blue SC-OLED attains an LT 75 lifetime of 1,209 hours at an initial luminance of 1000 cd cm –2 and exceeds 453,000 hours at 100 cd cm –2 —a more than 30-fold improvement over conventional amorphous blue OLEDs. The dual-action mechanism, combining stable charge transport with built-in barrier protection, fundamentally curtails device degradation and energy dissipation.
This study breaks the efficiency-stability trade-off of traditional OLEDs. The BSB-Me SC dual-functional design integrates intrinsic material stability and internal encapsulation, offering a promising pathway for next-generation high-efficiency, ultra-long-lifetime OLED display and lighting technologies.
Science Bulletin
Experimental study