High-efficiency electroluminescence and amplified spontaneous emission from a thermally activated delayed fluorescent near-infrared emitter

Efficient Near‐Infrared Electroluminescence at 840 nm with “Metal‐Free” Small‐Molecule:Polymer Blends

“…To the best of our knowledge, the EQE and radiance we obtained from such a blend are the highest obtained so far from a fluorescent NIR OLED with EL peaked above 800 nm, and, crucially, within the class of devices based on an active layer free from heavy/ toxic metals. [21] These results, together with the possibility to operate such devices up to 200 mA cm −2 while maintaining the EQE above 0.5% and turn-on voltages as low as 1.7 V make them promising candidates for application in wearable, skinmounted, or implantable bioelectronics, in which an active layer free from heavy metals is of crucial importance.…”

“…To the best of our knowledge, such PLQY values are the highest reported so far for a metal-free moiety in the solid state and emitting in this spectral range. [21] In this regard, we also note that although the PL quantum yield of a pure species only depends on the ratio of the radiative to the sum of the radiative and nonradiative decay constants (for monomolecular decay), the case is indeed more complicated in the case of a blend, and the factors influencing the PLQY of the acceptor when selectively exciting the matrix (donor) in the blends, depend on the interplay of the PLQY of both donor and acceptor, and on the efficiency of energy transfer, which also involve spectral overlap between the donor emission and acceptor absorption spectra. In our case, the PLQY of the isolated PIDT-2TPD is 18% but lower for PIDT-TPD (5%).…”

“…The latter exhibits also one of the highest EQE (≈1.5%, Table 2) reported so far in the literature for a red/ NIR emitting metal-free polymer. [21] Notably, the maximum EL efficiency of PIDT-2TPD (see Figure 3c) was measured at a relatively low driving voltage (≈3.5 V) and at a current density of ≈20 mA cm −2 , which corresponds to an optical output of ≈0.5 mW cm −2 . To get further insight into the transport properties of the polymeric host materials, we also fabricated hole-only and electron-only devices, whose current density versus voltage (JV) characteristics are discussed fully in Figure S25 and the related discussion (Supporting Information).…”

“…To the best of our knowledge, such values are the best ever reported in this spectral range from a polymer-based light-emitting diode (PLED) based on a heavy-metal-free, solution-processed active layer not "intentionally" leveraging triplet-assisted emission (e.g., as for thermally activated delayed fluorescence materials). [21] To obtain the BTT* NIR emitter, we re-engineered a previously reported BTT-based monomer featuring just one thiophene unit on either side of BTT, [30] by introducing an additional thiophene on either side of the molecule (Figure 1a and Figure S1 (Supporting Information)), so as to decrease the energy gap and thus further redshift the emission of the dye into the NIR region. In addition, to obtain good miscibility with the host polymer matrices and improve the solubility in organic solvents, such as toluene used in this study, we specifically tailored branched 2-butyloctyl side chains in both the BTT-based acceptor and the thiophene donors.…”

“…[12][13][14][15][16][17][18][19] However, although such hybrid materials afford substantial electroluminescence (EL) external quantum efficiency (EQE) in the NIR, in some cases exceeding 10% [8,10] or even 20% or so, [13] their use of heavy, toxic, and/or costly metals is not ideal for manufacturing, sustainability, environmental impact, and, in perspective, biocompatibility. Furthermore, in such hybrid systems, and in general in materials that leverage triplet excitons to boost the EQE, [20,21] exciton recombination dynamics typically fall in the hundreds of nanoseconds or even in the microsecond (or longer) range, which intrinsically limits the bandwidth when integrated in devices for telecommunications. For Li-Fi applications, [2][3][4] fluorescent molecular and polymeric materials are preferred, given that the typical fluorescence lifetime of these materials is of the order of few nanoseconds or less, thereby ideally allowing data transmission rates up to the Gb s −1 regime.…”