Efficiency Enhancement of Circularly Polarized Electroluminescence by Eliminating Chiral Inducers

Organic light-emitting diodes (OLEDs) capable of emitting circularly polarized light (CPL) have attracted considerable attention from researchers due to their potential applications, such as 3D display and optical information storage.¹ One of the effective approaches to realize CP-OLEDs is to construct chiral light-emitting layers by chiral induction, where chiral small molecules are doped as chiral inducers into achiral luminescent polymers. Due to the chiral transfer, strong circularly polarized luminescence of the polymer can be achieved. While this method can yield high asymmetry factor g, the presence of chiral inducers within the film can decrease the device performance of the CP-OLEDs.² To address this issue, it may be important to remove the chiral inducers from the emitting polymer after chiral induction is complete.<br/>In this study, we doped the axially chiral small molecule BINOL into thin films of the achiral conjugated polymer F8BT. After the preparation of F8BT/BINOL films, only the CD signal of BINOL was detected and no signal of F8BT was observed. Upon annealing the films, the CD signal associated with BINOL disappeared and a clear CD signal from F8BT appeared, indicating the induction of chiroptical properties in F8BT.³ NMR spectra of the samples before and after annealing confirmed the complete removal of BINOL by sublimation during annealing, resulting in the formation of pure F8BT films. By increasing the annealing temperature to 180 °C and the doping ratio of BINOL to 50 wt%, the CD signal intensity was enhanced and the |g_CD| reached almost 0.1. Moreover, the annealed samples exhibited circularly polarized luminescence (CPL) with the maximum |g_PL| reaching 0.1. The red shift of F8BT in absorption spectra and the fibrous morphology in AFM after annealing indicated that the chiroptical properties of F8BT were caused by intermolecular exciton coupling due to aggregation. The photoluminescence quantum yield (PLQY) of as-prepared F8BT/BINOL films was remarkably low (4.95% for F8BT/R-BINOL film and 8.35% for F8BT/S-BINOL film) but after annealing it increased to that of pure F8BT films (25.70% for F8BT/R-BINOL and 24.20% for F8BT/S-BINOL), suggesting that the removal of chiral inducers by high-temperature sublimation realized both high g factor and PLQY simultaneously.<br/>We applied this thin film to the emitting layer of CP-OLEDs. TFB was used as the hole injection layer and the exciton blocking layer. The device after removing the chiral inducer achieved a maximum current efficiency (CE_max) of 1.46 cd/A and 1.45 cd/A for R-BINOL and S-BINOL samples. The g_EL of -5×10^-3 and 7×10^-3 for R-BINOL and S-BINOL sample were achieved without any optimization of the device structures.<br/>This study demonstrates for the first time a way to realize CP-OLEDs by sublimable chiral inducers to avoid the negative impact of chiral inducers on the device performance.<br/><br/>1) Zhang, D. W.; Li, M.; Chen, C. F. Recent Advances in Circularly Polarized Electroluminescence Based on Organic Light-Emitting Diodes. <i>Chem. Soc. Rev.</i> <b>2020</b>, <i>49</i>, 1331−1343.<br/>2) Yan H, Wade J, Wan L, et al. Enhancing hole carrier injection via low electrochemical doping on circularly polarized polymer light-emitting diodes. <i>J. Mater. Chem. C</i>, <b>2022</b>, 10, 25, 9512-9520.<br/>3) Yuan C, Eguchi K, Murata H. BINOL induces chirality in polyfluorene-based electroluminescent polymers in solid state. <i>Jpn. J. Appl. Phys.</i>, <b>2023</b>, 63, 2, 02SP10.