High-performance near-infrared OLEDs maximized at 925 nm and 1022 nm through interfacial energy transfer

The development of efficient near-infrared (NIR) organic fluorescent dyes is crucial for advancements in organic photovoltaics (OPVs) and deep-tissue bioimaging. However, the emission efficiency of most NIR dyes is low, hindered by the emission energy gap law (EEGL), which dictates that as the emission energy gap decreases, non-radiative deactivation increases. One key approach to overcome EEGL is to reduce the internal reorganization energy of the molecule by creating ordered molecular assemblies, increasing exciton delocalization and uniformly distributing vibrational displacements. Previous work by the authors used self-assembling heavy metal Pt(II) complexes to reduce the energy gap to the NIR region via metal-metal-to-ligand charge transfer (MMLCT), suppressing exciton-vibration coupling. This yielded NIR OLEDs with 1000 nm emission and 4.2% external quantum efficiency (EQE). This study aims to develop high-performance fluorescence NIR OLEDs beyond the existing record by exploring efficient interfacial energy transfer between a self-assembled Pt(II) complex layer and a NIR dye layer. Current fluorescence NIR OLEDs of >900 nm achieve only low EQEs (0.15-0.33%), necessitating innovative strategies to improve both EQE and brightness. The theoretical maximum EQE for OLEDs is around 20%, limited by photoluminescent efficiency and charge balance; however, out-coupling efficiency is highly dependent on the device structure and materials. The research employs a transfer printing technique to overcome challenges in using vapor deposition for NIR dyes and avoid the destruction of self-assembled layers during conventional spin-coating.

Existing literature highlights the challenges associated with achieving high-performance NIR OLEDs. The emission energy gap law (EEGL) significantly limits the efficiency of NIR dyes, with most exhibiting very low photoluminescence quantum yields (PLQY). While some NIR dyes show enhanced fluorescence upon aggregation, the PLQY is usually insufficient for OLED fabrication. Recent progress has been made using V-shaped electron donor-acceptor-donor dyes with face-on molecular packing, yielding NIR OLEDs in the 900-1000 nm range with EQEs of 0.15-0.33%. Studies on QD-LEDs and PeLEDs have shown EQEs above 25% without out-coupling enhancement, highlighting the potential for improvements in OLED design and materials.

This study utilizes two self-assembled molecular layers: an energy-donating layer of Pt(fprpz)₂ and an energy-accepting layer containing NIR dyes, such as BTP-eC9. Direct spin-coating of BTP-eC9 onto Pt(fprpz)₂ was unsuccessful due to the disruption of the self-assembled structure. Therefore, a transfer printing technique was employed to transfer a solid-state BTP-eC9 layer onto the Pt(fprpz)₂ layer, maintaining the integrity of both layers and enabling efficient interfacial energy transfer. Time-resolved photoluminescence (TrPL), steady-state PL, and PLQY measurements were performed to analyze the properties of the single and bilayer emitters. The impact of spin coating versus stamping on the morphology and arrangement of the Pt(fprpz)₂ film was analyzed via PL mapping, atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), and grazing-incidence wide-angle X-ray scattering (GIWAXS). The interfacial energy transfer mechanism was investigated using picosecond-transient absorption spectra (ps-TAS), analyzing charge separation dynamics at interfaces. Space charge-limited current (SCLC) techniques were used to analyze charge mobilities, and simulations using Setfos software aided in optimizing the device architecture. OLEDs were fabricated using thermal evaporation and spin-coating, with varying structures to optimize performance. Electroluminescence (EL) spectra, luminance-voltage-current density curves, and EQE were measured to evaluate the OLED performance.

The transfer printing method proved crucial for maintaining the self-assembled structure and achieving efficient energy transfer. TrPL analysis revealed a significant reduction in Pt(fprpz)₂ emission lifetime in the stamped bilayer, indicating efficient energy transfer to BTP-eC9. The energy transfer ratio was calculated to be 68.8%. GIWAXS and NEXAFS analyses confirmed the impact of the stamping method on the molecular packing and crystalline structure, with the stamped film exhibiting a smaller d-spacing in BTP-eC9, improving its face-on orientation and PLQY. ps-TAS measurements showed suppressed charge separation in the stamped bilayer compared to the spin-coated bilayer, highlighting the advantages of the stamping method in maintaining interface quality. SCLC measurements determined hole and electron mobilities. Setfos simulations guided optimization of the OLED architecture, revealing that decreasing the Pt(fprpz)₂ layer thickness suppressed Pt(fprpz)₂ emission and increased BTP-eC9 emission. The optimized sandwiched structure (Pt(fprpz)₂ (5 nm)/BTP-eC9/Pt(fprpz)₂ (5 nm)) achieved a record high radiance of 39.97 W sr⁻¹ m⁻² and EQE of 2.24% (1.94 ± 0.18%) at 925 nm emission. The method was also successful with other NIR dyes, such as BTPV-eC9, demonstrating its applicability for generating hyperfluorescent OLEDs in the NIR region.

The findings demonstrate a significant advancement in NIR OLED technology. The use of interfacial energy transfer, combined with the stamping method, offers a highly effective strategy for creating bright, efficient NIR OLEDs. The optimized sandwiched architecture provides flexibility in choosing NIR dyes without significant modifications to the device structure. The results highlight the importance of meticulous control over the molecular packing and interface quality for enhancing OLED performance. The successful demonstration of this method with different NIR dyes suggests its broad applicability for future NIR OLED development. The design criteria for effective interfacial energy transfer devices are also identified: appropriate energy level alignment, strong photoluminescence of the donor, high absorption of the acceptor, and optimized device structure for efficient exciton recombination near the interface.

This research successfully demonstrates a novel approach for fabricating high-performance NIR OLEDs based on interfacial energy transfer. The optimized sandwiched structure, utilizing the stamping method, achieved record-high radiance and EQE at 925 nm emission. The method's versatility makes it readily adaptable for different NIR dyes, promising further advancements in the field. Future research could explore new NIR dyes with improved properties and further optimize device architecture for even higher efficiency and broader applications.

The study focuses on specific NIR dyes and device architectures. The generalizability of the findings to other NIR dyes and device structures requires further investigation. While the stamping method improves the performance significantly, it adds an extra step to the device fabrication process. The long-term stability of these devices under operational conditions needs further evaluation. The ps-TAS measurements were performed with optical pumping, so direct investigation using electric pumping would provide additional insights into carrier behaviors.