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Diketopyrrolopyrrole derivatives demonstrating moderate NIR-to-Vis TTA-mediated photon upconversion quantum yield (up to 3.8% out of 50%) and statistical probability f (up to 15.6%) are promoted as stable alternatives to a rubrene emitter.

Simplified thermal evaporation of rubrene in an inert atmosphere is demonstrated to alter the morphology of photon upconverting (UC) films, and subsequently ramp their NIR-to-vis UC quantum yield up to a record value of 1.2% (out of maximum 50%).

Triplet and singlet exciton diffusion plays a decisive role in triplet-triplet annihilation (TTA) and singlet fission (SF) processes of rubrene (Rub) films at low excitation power, and therefore has an important implication for TTA-mediated photon upconversion (UC). Although triplet diffusion in crystalline Rub was studied before, there is no quantitative data on diffusion in disordered Rub films most widely employed for NIR-to-Vis UC. The lack of these data hinders the progress of TTA-UC applications relying on a Rub annihilator (emitter). Herein, a time-resolved PL bulk-quenching technique was employed to estimate the exciton diffusion coefficient (

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 TTA-UC applications. Herein, the issue was addressed by decorating the rubrene with sterically demanding 3,5-di-tert-butylphenyl side-moieties at the periphery and the core to result in 40-fold enhancement of the emission quantum yield. Nevertheless, the sterically crowded rubrenes were found to exhibit lower sensitized UC performance compared to the conventional rubrene, which was ascribed to inefficient triplet energy transfer from a sensitizer and poor TTA (for the core-modified rubrene only). By exploiting the distinct feature of rubrenes to simultaneously express both SF and TTA in the solid films, their TTA efficiency was assessed independently from TET in the sensitizer-free films. The results implied a trade-off between suppressed SF and enhanced TTA in the rubrene emitters, which could be addressed via careful selection of the degree of sterical hindrance and linking position of the side-moieties. Thorough analysis of the prompt and delayed fluorescence revealed that the bulky side-moieties at the periphery do not impede TTA, i.e., it is as efficient as that of unsubstituted rubrene, whereas these moieties linked directly to the core suppress TTA dramatically. The current study unveils an advantage of the peripheral linking vs. core linking pattern of rubrene emitters, thereby providing valuable insights for their rational modification towards improved NIR-to-vis UC efficiency in the solid state.

In this work, we report on the fabrication of a GaN/AlGaN waveguiding structure dedicated to modal phase matching, where GaN waveguide has planar polarity inversion. First, we optimized the growth conditions for the AlGaN epilayer. Second, on top of the AlGaN epilayer, we fabricated the waveguiding structure starting with the growth of the Ga-polar GaN epilayer followed by atomic layer deposition (ALD) of an Al $$_{2}$$ O $$_{3}$$ layer, then, continuing with the growth of N-polar GaN epilayer. We tested several layer thicknesses, but with 20 nm we managed to inverse the GaN polarity from Ga to N. To confirm the N-polarity, we etched the GaN epilayer surface in aqueous KOH solution. We performed out-of-plane (0002) and in-plane (11–20) X-ray diffraction and rocking curve measurements to estimate the crystalline quality of the AlGaN epilayer, Ga- and N-polar GaN epilayer. Atomic force microscopy measurement lets us evaluate the epilayer surface morphology and roughness. Optical and scanning electron microscopy inspection revealed characteristic hexagonal N-polar GaN epilayer surface. We used high-resolution transmission electron microscopy to investigate the crystallinity and orientation of the Ga- and N-polar GaN epilayer, also the Al $$_{2}$$ O $$_{3}$$ ALD layer, the interface quality of the waveguide structure.

Low-power near-infrared (NIR)-to-visible (vis) photon upconversion (UC) systems are in demand for biomedical, photovoltaic and photocatalytic applications; however, the practical utilization is still hampered by low UC efficiency. Aiming to identify efficiency-limiting factors, particularly in metallophthalocyanine-sensitized rubrene systems operational in the NIR-vis range, we thoroughly assessed subsequent energy transfer steps in the TTA-mediated UC scheme. A key limiting factor in the optimized UC systems was found to be rubrene's low statistical probability (f = 15.5 ± 3%) to obtain a singlet from two triplets via TTA. The f estimated under the dominance of TTA attained by continuous-wave excitation, i.e. the regime frequently encountered (or desired) in practical applications, was determined to be 4 times lower as compared to that obtained under femtosecond pulsed-laser excitation conditions. The results also demonstrate that the benefit of achieving larger emitter concentrations by introducing solubility increasing alkyl groups into the emitter in anticipation of enhanced triplet energy transfer cannot outcompete the severely reduced statistical probability factor (f = 5.3 ± 1%) of t-butyl-substituted rubrene. The maximum UC quantum yield (ΦUC = 5.6 ± 1.2%) estimated and verified by two independent methods in the optimized Pd-phthalocyanine–rubrene system is among the largest reported for NIR-to-vis UC systems absorbing at >730 nm. ΦUC is defined here as the number of UC photons emitted per number of absorbed ones, implying a theoretical limit of 50% for TTA-mediated UC.

Solid state NIR-to-visible photon upconversion (UC) mediated by triplet–triplet annihilation (TTA) is necessitated by numerous practical applications. Yet, efficient TTA-UC remains a highly challenging task. In this work palladium phthalocyanine-sensitized NIR-to-vis solid UC films based on a popular rubrene emitter are thoroughly studied with the primary focus on revealing the impact of t-butyl substitution in rubrene on the TTA-UC performance. The solution-processed UC films were additionally doped with a small amount of emissive singlet sink tetraphenyldibenzoperiflanthene (DBP) for collecting upconverted singlets from rubrene and in this way diminishing detrimental singlet fission. Irrespective of the excitation conditions used, t-butyl-substituted rubrene (TBR) was found to exhibit enhanced TTA-UC performance as compared to that of rubrene at an optimal emitter doping of 80 wt% in polystyrene films. Explicitly, in the TTA dominated regime attained at high excitation densities, 4-fold higher UC quantum yield (ΦUC) achieved in TBR-based films was caused by the reduced fluorescence concentration quenching mainly due to suppressed singlet fission. Under low light conditions, i.e. in the regime governed by spontaneous triplet decay, even though triplet exciton diffusion was obstructed in TBR films by t-butyl moieties, the subsequently reduced TTA rate was counterbalanced by both suppressed singlet fission and non-radiative triplet quenching, still ensuring higher ΦUC of these films as compared to those of unsubstituted rubrene films.

We have deposited a series of ALD oxides HfO 2 , ZrO 2 , Ta 2 O 5 , Y 2 O 3 , SiO 2 , Al 2 O 3 within same reactor and prepared as GaN-MOS structure. A comprehensible diverse distribution of flat-band voltage depending on two different deposition temperatures 100-125°C and 250-300°C, and oxidant H 2 O or O 3 was obtained. At zero gate voltage and under unstressed condition we have observed a qualitative feature demonstrating different dependence of the net fixed charge in the oxide or/and at the interface with the deposition temperature, namely, an increase of positive charge for oxidant water oxide and an increase of negative charge for oxidant ozone oxide. Best interface quality, least charge only slightly influenced by the deposition temperature, we have observed for the ZrO 2 /H 2 O (oxide/oxidant). Instead, highest deposition temperature dependence we have observed for the Al 2 O 3 /H 2 O.

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.

Transition metal oxide films HfO2, ZrO2, Ta2O5, TiO2, Nb2O5, and Y2O3, together with Al2O3 and SiO2 for reference, were deposited by atomic layer deposition (ALD) and investigated. For all the films ALD process was performed within the same reactor at two temperature ranges 100–125∘C and 250–300∘C. Both water and ozone were used as oxygen sources if processes were available. Most films are amorphous as-deposited, but HfO2, ZrO2, and Y2O3 can be crystalline, depending on deposition temperature and oxygen source. After deposition and initial characterization, the films were annealed to different temperatures up to 1200∘C, depending on the material, to observe the crystallization process. When crystalline phases were observed, grain sizes have been derived from the X-ray diffractograms and compared with feature sizes in SEM images. RMS analysis revealed a general tendency - an increase of roughness with the appearance of crystallization. The temperature-dependent morphological investigations were supported by breakdown field measurements. Those are also strongly affected when a film becomes crystalline. Most values observed for break down fields agree with literature, some values exceed previously reported numbers.

Deep-blue-emitting benzo[c]fluorene-cored compounds featuring twisted peripheral moieties for suppressed concentration quenching of emission were synthesized and investigated as potential materials for light amplification. This detailed study of the effect of concentration on the spontaneous and stimulated emission, excited-state lifetime and susceptibility to form aggregates obtained for different benzofluorenes, has enabled the understanding of the concentration dependence of the amplified spontaneous emission (ASE) threshold and revealed the optimal concentration for the lowest threshold. The weak concentration quenching accompanied by high fluorescence quantum yield (40%) and radiative decay rate (5 × 10(8) s(-1)) have enabled the attainment of the lowest ASE threshold in the neat amorphous film of benzofluorene bearing dihexylfluorenyl peripheral moieties. Aggregate formation was found to negligibly affect the emission efficiency of the benzofluorene films; however, it drastically increased ASE threshold via the enhanced scattering of directional stimulated emission, and thereby implied the necessity to utilize homogeneous glassy films as the lasing medium. Although the bulky dihexylfluorenyl groups at the periphery ensured the formation of glassy benzofluorene films with the ASE threshold as low as 900 W cm(-2) (under nanosecond excitation), they adversely affected carrier drift mobility, which implied a tradeoff between ASE and charge transport properties for the lasing materials utilized in the neat form. Such a low ASE threshold attained in air is among the lowest reported for solution-processed neat films. The low threshold and enhanced photostability of benzofluorenes against fluorene compounds in air show great potential for benzofluorene-cored molecular glasses as active media for lasing applications.

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.