The low statistical probability factor of the rubrene emitter (f = 15.5 ± 3%) was determined to be the prime TTA-UC efficiency-limiting factor.

Enhanced TTA-UC performance of t-butyl-rubrene films as compared to that of unsubstituted-rubrene films is achieved due to suppressed singlet fission and non-radiative triplet quenching.

Rubrene is the most widely used triplet-triplet annihilation (TTA) emitter for NIR-to-vis photon upconversion (UC), however, strong singlet fission (SF) in the solid films quenches its emission and hampers practical...

Low-power NIR-to-vis upconversion (UC) of incoherent excitation mediated by triplet-triplet annihilation (TTA) has a variety of promising applications, i.e. in photovoltaics, however these are strongly hampered by low UC efficiency...

Remote epitaxy via graphene has recently attracted significant attention, since it provides the possibility to lift-off the grown epitaxial layer, reuse the substrate, and produce flexible devices. However, extensive research is still necessary to fully understand the III-nitride formation on the van der Waals surface of a two-dimensional material and utilize remote epitaxy to its full potential. In this work, the growth of a GaN epilayer using a GaN/sapphire template covered with monolayer graphene is presented. Metalorganic vapor phase epitaxy is chosen to fabricate both the template and the nitride epilayer on top as a cost-effective approach toward GaN homoepitaxy. One-step and multi-step growth temperature protocols are demonstrated while paying particular attention to the graphene interface. GaN seed formation on graphene is analyzed to identify remote epitaxy. Crystalline quality improvement of the epilayer by adjusting the growth parameters is further discussed to provide useful insights into GaN growth on a GaN/sapphire template via monolayer graphene.

Organic single crystals offering numerous advantages due to their long‐range molecular order are considered a promising gain medium for realization of electrically‐pumped organic lasers provided their amplified spontaneous emission (ASE) threshold is sufficiently low. Unfortunately, ASE thresholds of such crystals are typically more than one order of magnitude higher (in the range of tens of kW cm−2) as compared to those of amorphous neat/doped films. Here, this issue is addressed by rationally designing bifluorene‐based compounds to express weak intermolecular coupling in the crystalline phase. The twisted molecular backbone as well as out‐of‐plane twisted dimethyl moieties attached at the fluorene end‐groups additionally benefit with enhanced electron–vibronic coupling resulting in a large Stokes shift (0.5 eV) and reduced reabsorption of emission. Endowed with such features the bifluorenes exhibit similarly high radiative decay rates (≈1.5 × 109 s−1) when doped in polystyrene matrix at low concentrations and in sublimation‐grown single crystals. The high radiative rates accompanied by excellent waveguiding properties, favorable orientation of transition dipole moments as well as non‐overlapping excited‐state absorption and gain regions enable achieving extremely low ASE thresholds (≈700 W cm−2) in the bifluorene single crystals. The achieved low threshold encourages employment of rationally designed molecules in organic crystals for lasing applications.

Singlet fission is detrimental to NIR-to-vis photon upconversion in the solid rubrene (Rub) films, as it diminishes photoluminescence efficiency. Previous studies have shown that thermally activated triplet energy transport drives singlet fission with nearly 100% efficiency in closely packed Rub crystals. Here, we examine triplet separation and recombination as a function of intermolecular distance in the crystalline films of Rub and the t -butyl substituted rubrene ( t BRub) derivative. The increased intermolecular distance and altered molecular packing in t BRub films cause suppressed singlet dissociation into free triplets due to slower triplet energy transport. It was found that the formation of correlated triplet pairs 1 (TT) and partial triplet separation 1 (T···T) occurs in both Rub and t BRub films despite differences in intermolecular coupling. Under weak intermolecular coupling as in t BRub, geminate triplet annihilation of 1 (T···T) outcompetes dissociation into free triplets, resulting in emission from the 1 (TT) state. Essentially, increasing intermolecular distance up to a certain point (a sweet spot) is a good strategy for suppressing singlet fission and retaining triplet–triplet annihilation properties.

Triplet-triplet annihilation (TTA) mediated NIR-to-vis photon upconversion (UC) operating under incoherent and low power excitation is increasingly attractive for various applications including photovoltaics. Unfortunately, efficient TTA emitters/annihilators for this spectral...