Abstract
The energy gap law (EG-law) and aggregation quenching are the main limitations to overcome in the design of near-infrared (NIR) organic emitters. Here, we achieve unprecedented results by synergistically addressing both of these limitations. First, we propose porphyrin oligomers with increasing length to attenuate the effects of the EG -law by suppressing the non-radiative rate growth, and to increase the radiative rate via enhancement of the oscillator strength. Second, we design side chains to suppress aggregation quenching. We find that the logarithmic rate of variation in the non-radiative rate vs. EG is suppressed by an order of magnitude with respect to previous studies, and we complement this breakthrough by demonstrating organic light-emitting diodes with an average external quantum efficiency of ~1.1%, which is very promising for a heavy-metal-free 850 nm emitter. We also present a novel quantitative model of the internal quantum efficiency for active layers supporting triplet-to-singlet conversion. These results provide a general strategy for designing high-luminance NIR emitters.
Highlights
Near-infrared (NIR) emitters are attracting significant interest for integration in a variety of applications, spanning from photodynamic therapy[1] to security and defense[2]
We note that a “roll-off” of the efficiency with increasing current occurred in other host-blend systems and characterized early phosphorescent organic lightemitting diodes (OLEDs) in particular but has mostly been addressed by appropriate device and materials engineering, and we expect similar advances to be possible for the systems we present here
The basic photophysics and material design breakthrough has been confirmed by incorporating an F8BT:lP6(THS) blend into OLEDs, with which we demonstrated an average external quantum efficiency (EQE) of 1.1% and a maximum EQE of 3.8% at a peak wavelength of 850 nm
Summary
Near-infrared (NIR) emitters are attracting significant interest for integration in a variety of applications, spanning from photodynamic therapy[1] to security and defense[2]. With respect to inorganic emitters, organic NIR emitters offer the possibility to further extend the range of applications thanks to their mechanical flexibility, conformability, and biocompatibility. Most of the recent research on NIR organic lightemitting diodes (OLEDs)[5] has focused on rare-earth and transition metal complexes[6,7], small molecules[8,9], conjugated polymers, and their combinations[9,10,11,12,13,14]. The emission efficiency of organic emitters in the NIR is hindered by some intrinsic limitations. The extended conjugation length needed to achieve a sufficiently small energy gap (EG) dictates a very planar molecular conformation, which in turn favors the formation of poorly emissive H-type aggregates[15]. Undesired intermolecular interactions can be suppressed in conjugated systems by diluting the chromophores in solid solutions[13,14,16,17,18,19] or via molecular design[20], including threading into cyclodextrin rings to form conjugated polyrotaxanes[21,22,23]
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