Feng Zhu

Sensitizing fluorescent materials enables fast and efficient exciton utilization, providing an effective approach to improving the performance of organic light‐emitting diodes (OLEDs) beyond new material design. However, conventional OLEDs usually adopt amorphous structures, which limit efficient photon output under low driving voltages. In this work, two fabrication methods for sensitization‐based fluorescent OLEDs using crystalline host matrix (CHM) are proposed. By leveraging the high carrier mobility of CHM and the efficient exciton collection of the hot exciton material PyPO, two sensitized crystalline organic light‐emitting diodes (via hot exciton sensitizer type and exciton nanoaggregates type) exhibit excellent electroluminescence, with maximum external quantum efficiencies of 9.12% and 7.03%, respectively. Both OLEDs exhibit fast turn‐on behavior, with rapid increases in luminance and current density under the same driving voltage. This leads to improved photon output and reduced Joule heat loss ratio from series resistance. Integrating sensitization strategies into crystalline materials provides a novel pathway toward low‐power, high‐brightness OLEDs.

Combining crystalline materials with emissive materials featuring high exciton utilization efficiency represents an important strategy for developing high‐photon‐output organic light‐emitting diodes (OLEDs). In this work, a novel hot exciton material, PCC, is reported. A non‐doped, amorphous deep‐blue OLED based on PCC exhibits an electroluminescent (EL) emission peak at 436 nm and achieves a maximum external quantum efficiency (EQE) of 13.77%. This is one of the highest performance levels reported for emitters based on the “hot exciton” mechanism. PCC is further embedded as nanoaggregates (NA) into a crystalline host matrix (CHM), serving as a sensitizer for blue dopant (D), enabling efficient exciton utilization. This approach enabled the fabrication of a high‐performance blue crystalline OLED (C‐OLED) with an EQE approaching 10%. Benefiting from the high mobility of CHM, the device exhibited a rapid increase in luminance and current density. Compared with previously reported high‐performance amorphous OLEDs (A‐OLEDs, CIE _y ≤ 0.20), the C‐OLED delivered enhanced photon output along with a significantly lower ratio of Joule heat loss due to series resistance. These findings provide new insights into device architecture design for high‐efficiency blue OLEDs.

Organic light‐emitting diode (OLED) urgently needs to improve brightness to meet the demands from diverse light scenes in intelligent era. Owing to their high mobility and superior thermal stability, crystalline organic semiconductors possess intrinsic advantages than present amorphous thin‐film route to break through the bottleneck in OLED field and create high brightness devices. In this work, a deep‐blue crystalline thin‐film OLED (C‐OLED) is reported, which can achieve high brightness at low driving voltage (4817 cd m ⁻² @ 4 V). By integrating band‐like crystalline emitting layer and controllable p/n doping, C‐OLEDs demonstrate low turn‐on voltage, low series‐resistance joule‐heat loss ratio, and high power efficiency. This technological advancement, built on weak epitaxy growth (WEG) method and crystalline organic solid‐solutions (OSS) thin films, further validates the potential of the crystalline thin‐film approach in optimizing OLED performance, and provides a new engineering solution for the development of high‐performance organic light‐emitting devices.

Crystalline organic semiconductor‐based organic light‐emitting diodes (C‐OLEDs) have drawn significant attention in the organic optoelectronics field in the past years, owing to their highly ordered molecular arrangements and excellent carrier transport performance. Among them, crystalline host matrix (CHM) materials represented by 2‐(4‐(9H‐carbazol‐9‐yl)phenyl)‐1‐(3,5‐difluorophenyl)‐1H‐phenanthro[9,10‐d]imidazole (2FPPICz) have become ideal hosts for constructing high‐performance C‐OLEDs due to evident advantages, such as their unique weak epitaxial growth (WEG) characteristics (enabling large‐area uniform film formation), wide bandgap (2.23–5.53 eV), and precisely controllable host–guest molecular arrangements. This review focuses on 2FPPICz crystalline host matrix‐based C‐OLED systems and systematically discusses their recent progress. Their physical structure designs of the light‐emitting layer are specifically highlighted, including i) doping, ii) solid‐solution, and iii) nanoaggregate structures. Additionally, this review explores the unique advantages of C‐OLEDs and discusses their current challenges and future prospects. It is believed that this review will provide valuable references for understanding the luminescence mechanisms of C‐OLEDs. It will also help in formulating strategies to develop high‐performance OLEDs.

Using a dual inducing layer is a novel strategy for growing high-quality crystalline thin films via weak epitaxy growth. This work shows that a designed dual inducing layer can address critical hole injection challenges in crystalline OLEDs.

Using ultraviolet photoelectron spectroscopy to determine interfacial energy levels between the crystalline light-emitting layer and the amorphous electron transport layer the efficiency of crystalline organic light-emitting diodes can be improved.

Embedding hot exciton nanoaggregates with TTA dopants into an emitting layer crystalline host matrix is effective for fabricating high-efficiency crystalline OLEDs.

Crystalline organic semiconductors, recognized for their highly ordered structures and high carrier mobility, have emerged as a focal point in the field of high-performance optoelectronic devices. Nevertheless, the intrinsic unipolar properties, characterized by imbalanced hole and electron transport capabilities, have continuously represented a significant challenge in the advancement of high-performance crystalline thin-film organic light-emitting diodes (C-OLEDs). Here, a bipolar solid-solution thin film with a maintained crystal structure has been fabricated using 2-(4-(9H-carbazol-9-yl)phenyl)-1(3,5-difluorophenyl)-1H-phenanthro [9,10-d]imidazole (2FPPICz) and 4-(1-(3,5-difluorophenyl)-1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)-N,N-diphenylaniline (2Fn) via a weak epitaxial growth (WEG) process, exhibiting nearly equivalent hole and electron mobilities (10^-2-10^-1 cm² V^-1 s^-1). We have demonstrated a blue C-OLED that achieves high efficiency while maintaining low-efficiency roll-off, employing the bipolar solid-solution thin film as the crystalline host matrix and 4,4'-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl (DPAVBi) as the doped emitter. This device achieves a maximum external quantum efficiency (EQE) of 4.6% with Commission Internationale de L'Eclairage (CIE) coordinates lying around (0.15, 0.22). Among crystalline light-emitting devices utilizing carrier-balanced transport, this EQE stands as one of the highest recorded values. Notable advancements include enhanced photon emission capabilities, optimized driving voltage (4.0 V @ 1000 cd m^-2), and a lower series resistance Joule heat loss ratio (11.8% @ 1000 cd m^-2). This research marks a significant stride in modulating the inherent electrical properties of crystalline host materials, offering a novel and efficacious strategy for the advancement of high-performance crystalline OLEDs.

Crystalline host matrix (CHM)‐nanoaggregates (NA) structure combines the advantages of a crystalline host matrix and high‐performance luminescent materials. This structure emerges as a highly effective strategy for achieving efficient blue organic light‐emitting diodes (OLEDs). This approach can tackle the current challenge of achieving low voltage and high brightness in amorphous OLEDs. In this work, triplet‐triplet annihilation (TTA) material NA are embedded into CHM and sensitize the fluorescent dopant (D) BD1. As a result, a CHM‐TTANA‐D structure device is created, demonstrating an outstanding external quantum efficiency (EQE) of 8.46%. Notably, the efficiency roll‐off is mitigated. The device maintains an EQE of ≈7.50% at 1000 cd m ^{− 2} , which is the highest values among blue crystalline OLEDs. The device leverages the benefits of CHM and exhibits swift turn‐on and a rapid increase in brightness and current density. This leads to a significant improvement in blue photon output and a reduced series‐resistance Joule heat loss ratio. The findings underscore the promising nature of the CHM‐TTANA‐D structure as a favorable design paradigm for high‐performance crystalline thin‐film OLEDs (C‐OLEDs).