Triplet Energy Transfer from PbS(Se) Nanocrystals to Rubrene: the Relationship between the Upconversion Quantum Yield and Size

Photon upconversion has attracted enormous attention due to its wide range of applications in biological imaging, photocatalysis, and especially photovoltaics. Here, the effect of quantum confinement on the efficiency of Dexter energy transfer from PbS and PbSe nanocrystals (NCs) to a rubrene acceptor is studied. A series of experiments exploring the relationship between NC size and the upconversion quantum yield (QY) in this hybrid platform show that energy transfer occurs in the Marcus normal regime. By decreasing the NC diameter from 3.5 to 2.9 nm for PbS and from 3.2 to 2.5 nm for PbSe, the relative upconversion QY is enhanced about 700 and 250‐fold respectively. In addition, the dynamic Stern–Volmer constant (K_SV) for the quenching of PbSe NCs by rubrene increases approximately fivefold with a decrease in NC diameter from 3.2 to 2.5 nm to a value of 200 m ^?1. This work shows that high quality, well‐passivated, small NCs are critical for efficient triplet energy transfer to molecular acceptors.     

Nanostructured Polymers Enable Stable and Efficient Low-Power Photon Upconversion

Photon upconversion based on sensitized triplet–triplet annihilation (sTTA-UC) is a wavelength-shifting technique with potential use in actuators, sensing, and solar technologies. In sTTA-UC, the upconverted photons are the result of radiative recombination of high-energy singlets, which are created through the fusion of metastable triplets of two annihilator/emitter molecules. The emitter triplets are populated via energy transfer (ET) from a low-energy absorbing light-harvester/sensitizer. The process is highly efficient at low powers in solution but becomes relatively ineffective in solid matrices since the limited molecular mobility precludes bimolecular interactions. The realization of efficient solid-state upconverters that exhibit long-term stability and are compatible with industrial fabrication processes is an open challenge. Here, nanophase-separated polymer systems synthesized under ambient conditions that contain the upconverting dyes in liquid nanodomains is reported. The nanostructured polymers show an excellent optical quality, an outstanding upconversion efficiency of up to ≈23%, and excellent stability in air, with only negligible performance losses over a period of three months. Moreover, the dyes’ confinement in nanosized domains <50 nm results in an increased effective local density of chromophores that enables hopping-assisted ET and TTA and confers to the upconversion process peculiar kinetics that enhances the material performance at low powers.     

Band Gap Engineering Improves the Efficiency of Double Quantum Dot Upconversion Nanocrystals

Solution‐processed core/multishell semiconductor quantum dots (QDs) could be tailored to facilitate the carrier separation, promotion, and recombination mechanisms necessary to implement photon upconversion. In contrast to other upconversion schemes, upconverting QDs combine the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. Nevertheless, their upconversion quantum yield (UCQY) is fairly low. Here, design rules are uncovered that enable to significantly enhance the performance of double QD upconversion systems, and these findings are leveraged to fabricate upconverting QDs with increased photon upconversion efficiency and reduced saturation intensities under pulsed excitation. The role of the intra‐QD band alignment is exemplified by comparing the upconversion process in PbS/CdS/ZnSe QDs with that of PbS/CdS/CdSe ones with variable CdSe shell thicknesses. It is shown that electron delocalization into the shell leads to a longer‐lived intermediate state in the QDs, facilitating further absorption of photons, and enhancing the upconversion process. The performance of these upconversion QDs under pulsed excitation versus continuous pumping is also compared; the reasons for the significant differences between these two regimes are discussed. The results show how one can overcome some of the limitations of previous upconverting QDs, with potential applications in biophotonics and infrared detection.     

连续波与脉冲激光器在三重态-三重态湮灭上转换中的应用（特邀）

三重态?三重态湮灭上转换是一种新型光子上转换技术,具有使用连续波光源激发、上转换波长灵活可调和上转换量子效率高等优点。在该上转换过程中,三重态光敏剂吸收激发光、发生系间窜越并作为三重态能量给体,通过三重态能量转移过程,敏化能量受体,处于三重态的受体分子发生三重态–三重态湮灭,产生能发生高效荧光过程的单重激发态（即上转换发光）,从而将低能量的光子转换为高能量的光子,这为提高太阳能电池的光电转换效率或光催化的效率等,提供了一种新的途径。实验中需要选用合适的激光器激发上转换体系产生上转换发光,并研究其具体光物理过程。如选用连续波二极管泵浦固体激光器（DPSSL）作为光源,激发光敏剂/受体体系进行上转换实验,可方便地研究激光功率密度对上转换发光效率的影响。此外,为了研究上转换发光的动力学过程,使用光学参量振荡器（OPO）可调谐纳秒脉冲激光器激发上转换体系,可以研究光敏剂的三重态寿命、分子间能量转移以及三重态–三重态湮灭等过程的动力学特征。文中介绍了在三重态–三重态湮灭上转换实验中连续波和脉冲激光器的使用方法。     

Tuning the Bandgap Character of Quantum‐Confined Si–Sn Alloyed Nanocrystals

Nanocrystals in the regime between molecules and bulk give rise to unique electronic properties. Here, a thorough study focusing on quantum‐confined nanocrystals (NCs) is provided. At the level of density functional theory an approximate (quasi) band structure which addresses both the molecular and bulk aspects of finite‐sized NCs is calculated. In particular, how band‐like features emerge with increasing particle diameter is shown. The quasiband structure is used to discuss technological‐relevant direct bandgap NCs. It is found that ultrasmall Sn NCs have a direct bandgap in their at‐nanoscale‐stable α‐phase and for high enough Sn concentration (≈41%) alloyed Si–Sn NCs transition from indirect to direct bandgap semiconductors. The calculations strongly support recent experiments suggesting a direct bandgap for these systems. For a quantitative comparison many‐body GW + Bethe–Salpeter equation (BSE) calculations are performed. The predicted optical gaps are close to the experimental data and the calculated absorbance spectra compare well with the corresponding measurements.     

Ultrahigh Efficiency Fluorescent Single and Bi‐Layer Organic Light Emitting Diodes: The Key Role of Triplet Fusion

A new family of anthracene core, highly fluorescent emitters is synthesized which include diphenylamine hole transport end groups. Using a very simple one or two layer organic light emitting diode (OLED) structure, devices without outcoupling achieve an external quantum efficiency of 6% and photonic efficiencies of 20 cd/A. The theoretical maximum efficiency of such devices should not exceed 3.55%. Detailed photophysical characterization shows that for these anthracene based emitters 2T₁≤T_n and so in this special case, triplet fusion can achieve a singlet production yield of 0.5. Indeed, delayed electroluminescence measurements show that triplet fusion contributes 59% of all singlets produced in these devices. This demonstrates that when triplet fusion becomes very efficient, fluorescent OLEDs even with very simple structures can approach an internal singlet production yield close to the theoretical absolute maximum of 62.5% and rival phosphorescent‐based OLEDs with the added advantage of much improved stability.     

Near‐Field Energy Transfer Using Nanoemitters For Optoelectronics

Effective utilization of excitation energy in nanoemitters requires control of exciton flow at the nanoscale. This can be readily achieved by exploiting near‐field nonradiative energy transfer mechanisms such as dipole‐dipole coupling (i.e., Förster resonance energy transfer) and simultaneous two‐way electron transfer via exchange interaction (i.e., Dexter energy transfer). In this feature article, we review nonradiative energy transfer processes between emerging nanoemitters and exciton scavengers. To this end, we highlight the potential of colloidal semiconductor nanocrystals, organic semiconductors, and two‐dimensional materials as efficient exciton scavengers for light harvesting and generation in optoelectronic applications. We present and discuss unprecedented exciton transfer in nanoemitter–nanostructured semiconductor composites enabled by strong light–matter interactions. We elucidate remarkably strong nonradiative energy transfer in self‐assembling atomically flat colloidal nanoplatelets. In addition, we underscore the promise of organic semiconductor–nanocrystal hybrids for spin‐triplet exciton harvesting via Dexter energy transfer. These efficient exciton transferring hybrids will empower desired optoelectronic properties such as long‐range exciton diffusion, ultrafast multiexciton harvesting, and efficient photon upconversion, leading to the development of excitonic optoelectronic devices such as exciton‐driven light‐emitting diodes, lasers, and photodetectors.     

Efficient Broadband Triplet–Triplet Annihilation‐Assisted Photon Upconversion at Subsolar Irradiance in Fully Organic Systems

The latest trend in solar cell technology is to develop photon managing processes that adapt the solar emission to the spectral range at which the devices show the largest intrinsic efficiency. Triplet–triplet annihilation‐assisted photon upconversion (sTTA‐UC) is currently the most promising process to blue‐shift sub‐bandgap photons at solar irradiance, even if the narrow absorption band of the employed chromophores limits its application. In this work, we demonstrate how to obtain broadband sTTA‐UC at sub‐solar irradiance, by enhancing the system's light‐harvesting ability by way of an ad‐hoc synthesized family of chromophores with complementary absorption properties. The overall absorptance is boosted, thus doubling the number of upconverted photons and significantly reducing the irradiance required to achieve the maximum upconversion yield. An outstanding yield of ≈10% is obtained under broadband air mass (AM) 1.5 conditions, which allows a DSSC device to operate by exploiting exclusively sub‐bandgap photons.     

Thermally stable indoloacridine type host material for high efficiency blue phosphorescent organic light-emitting diodes

High triplet energy materials derived from carbazole or α-carboline modified indoloacridine were synthesized and device characteristics of blue triplet emitter doped devices were investigated. The indoloacridine derived host materials showed a high triplet energy above 2.80 eV and a high glass transition temperature over 170 °C due to rigid nature of the molecular structure. The indoloacridine based host materials could approach high external quantum efficiency above 20% in blue phosphorescent organic light-emitting diodes.     

Chloride and Indium‐Chloride‐Complex Inorganic Ligands for Efficient Stabilization of Nanocrystals in Solution and Doping of Nanocrystal Solids

Here, the surface functionalization of CdSe and CdSe/CdS core/shell nanocrystals (NCs) with compact chloride and indium‐chloride‐complex ligands is reported. The ligands provide not only short interparticle distances but additionally control doping and passivation of surface trap states, leading to enhanced electronic coupling in NC‐based arrays. The solids based on these NCs show an excellent electronic transport behavior after heat treatment at the relatively low temperature of 190 °C. Indeed, the indium‐chlorido‐capped 4.5 nm CdSe NC based thin‐film field‐effect transistor reaches a saturation mobility of μ = 4.1 cm² (V s)^?1 accompanied by a low hysteresis, while retaining the typical features of strongly quantum confined semiconductor NCs. The capping with chloride ions preserves the high photoluminescence quantum yield ( ≈ 66%) of CdSe/CdS core/shell NCs even when the CdS shell is relatively thin (six monolayers). The simplicity of the chemical incorporation of chlorine and indium species via solution ligand exchange, the efficient electronic passivation of the NC surface, as well as their high stability as dispersions make these materials especially attractive for wide‐area solution‐processable fabrication of NC‐based devices.     

Energy Upconversion via Triplet Fusion in Super Yellow PPV Films Doped with Palladium Tetraphenyltetrabenzoporphyrin: a Comprehensive Investigation of Exciton Dynamics

One of the key issues concerning the development of efficient polymer solar cell technology is the lack of viable materials which absorb in the near‐infrared (NIR) region. This could be resolved by up‐converting energy from the NIR into visible using triplet fusion (TF) with an additional layer that is fabricated separately from the solar cell and deposited on top. Theoretically a maximum upconversion (UC) via TF efficiency of 50% could be obtained. Here, it is demonstrated that in a film of commercially available poly(para‐phenylene vinylene) copolymer “super yellow” (SY) doped with 4% palladium(meso‐tetraphenyl‐tetrabenzoporphyrin) (PdTPBP) sensitizer, an UC efficiency of 6% can be achieved. By using femtosecond and nanosecond spectroscopies it is shown that the main UC efficiency loss mechanism is due to triplet quenching in PdTPBP aggregates. The PdTPBP intersystem crossing rate constant is determined to be 1.8 × 10¹¹ s^?1 and the triplet energy transfer rate constant from PdTPBP to SY to be 10⁹ s^?1. Quenching in PdTPBP aggregates can account for a triplet concentration loss in the range of 76‐99%. As such, preventing sensitizer aggregation in NIR‐to‐visible upconverting films is crucial and may lead to substantial increase of UC efficiencies in films.     

Preparation‐Condition Dependence of Hybrid SiO2‐Coated CdTe Nanocrystals with Intense and Tunable Photoluminescence

When aqueously prepared CdTe nanocrystals (NCs) are coated with a SiO₂ shell containing Cd ions and a sulfur source, they show a drastic increase in photoluminescence (PL) efficiency with a significant red shift and spectral narrowing after reflux. This is ascribed to the creation of a hybrid structure characterized by the formation of CdS‐like clusters in the vicinity of the NCs in the SiO₂ shell. Since these clusters are close to the NCs, their effective size increases to reduce the quantum size effect. The dependences of the PL properties on the preparation conditions are systematically investigated. The PL efficiency increases from 28% to 80% in the best case with a red shift of 80 nm. The PL behaviors differ from those of normal CdTe NCs and include less temperature quenching and longer PL lifetime. The SiO₂ coating enables bioconjugation with IgG without deterioration of PL efficiency, making hybrid NCs amenable for bioapplication.     

High triplet energy Al complex as a host material for blue phosphorescent organic light-emitting diodes

An Al complex, tris((2-(pyrazol-1-yl)pyridin-3-yl)oxy)aluminum (Al(pypy)₃), was synthesized as a high triplet energy host material for blue phosphorescent organic light-emitting diodes. A high triplet energy ligand, 2-(1H-pyrazol-1-yl)pyridin-3-ol, was coordinated to the Al to develop the high triplet energy host material derived from Al. The Al(pypy)₃ host showed a high triplet energy of 2.86 eV for efficient energy transfer to blue triplet emitter. A maximum quantum efficiency of 20.5% was achieved in blue device using the Al(pypy)₃ host material.     

Core/Shell Perovskite Nanocrystals: Synthesis of Highly Efficient and Environmentally Stable FAPbBr3/CsPbBr3 for LED Applications

Lead halide perovskite nanocrystals (PeNCs) are promising materials for applications in optoelectronics. However, their environmental instability remains to be addressed to enable their advancement into industry. Here the development of a novel synthesis method is reported for monodispersed PeNCs coated with all inorganic shell of cesium lead bromide (CsPbBr₃) grown epitaxially on the surface of formamidinium lead bromide (FAPbBr₃) NCs. The formed FAPbBr₃/CsPbBr₃ NCs have photoluminescence in the visible range 460–560 nm with narrow emission linewidth (20 nm) and high optical quantum yield, photoluminescence quantum yield (PLQY) up to 93%. The core/shell perovskites have enhanced optical stability under ambient conditions (70 d) and under ultraviolet radiation (50 h). The enhanced properties are attributed to overgrowth of FAPbBr₃ with all‐inorganic CsPbBr₃ shell, which acts as a protective layer and enables effective passivation of the surface defects. The use of these green‐emitting core/shell FAPbBr₃/CsPbBr₃ NCs is demonstrated in light‐emitting diodes (LEDs) and significant enhancement of their performance is achieved compared to core only FAPbBr₃‐LEDs. The maximum current efficiency observed in core/shell NC LED is 19.75 cd A^‐1 and the external quantum efficiency of 8.1%, which are approximately four times and approximately eight times higher, respectively, compared to core‐only devices.     

Low‐Threshold Organic Semiconductor Lasers with the Aid of Phosphorescent Ir(III) Complexes as Triplet Sensitizers

Organic semiconductor lasers (OSLs) have emerged as particularly challenging. One of the major issues preventing the successful realization of lasing from organic emitters under electrically pumped conditions is the inevitable population of triplet excitons. Herein, a novel concept is presented to construct triplet–singlet guest–host gain systems with incorporating iridium complexes as the triplet sensitizers and a fluorescent conjugated polymer as the gain media to achieve light amplification. The direct triplet–singlet energy transfer process is confirmed by photoluminescence excitation spectra, photoinduced absorption spectroscopy, and fluorescence transients of the blend samples. Successful light amplification with a threefold lower amplified spontaneous emission threshold and much better lasing performance is demonstrated for the resulting triplet–singlet guest–host system as compared with the corresponding gain system without triplet sensitizers. Moreover, under electrically driven conditions, the fluorescent organic light‐emitting diodes (OLEDs) based on the triplet–singlet guest–host systems with “triplet sensitizers” exhibit enhanced electrical performance relative to those without. The work suggests an effective general methodology to utilize both the singlet and triplet excitons to contribute to the light amplification with excellent electrical performance in OLEDs, opening prospects toward attempting electrically pumped OSLs.     

Strategies Toward Efficient Blue Perovskite Light-Emitting Diodes

Substantial progress has been made in blue perovskite light-emitting diodes (PeLEDs). In this review, the strategies for high-performance blue PeLEDs are described, and the main focus is on the optimization of the optical and electrical properties of perovskites. In detail, the fundamental device working principles are first elucidated, followed by a systematical discussion of the key issues for achieving high-quality perovskite nanocrystals (NCs) and quasi-2D perovskites. These involve ligand optimization and metal doping in enhancing the carrier transport and reducing the traps of perovskite NCs, as well as the perovskite phase modulation and defect passivation in improving energy transfer and emission efficiency of quasi-2D perovskites. The strategies for efficient 3D mixed-halide perovskite and lead-free perovskite blue LEDs are then briefly introduced. After that, other strategies, including effective charge transport layer, efficient perovskite emission system, and effective device architecture for high light outcoupling efficiency, are further discussed to boost the blue PeLED performances. Meanwhile, the testing standard of blue PeLED lifetime is suggested to enable the direct comparisons of the device operational stability. Finally, challenges and future directions for blue PeLEDs are addressed.     

Triplet Transfer Mediates Triplet Pair Separation during Singlet Fission in 6,13‐Bis(triisopropylsilylethynyl)‐Pentacene

Triplet population dynamics of solution cast films of isolated polymorphs of 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐Pn) provide quantitative experimental evidence that triplet excitation energy transfer is the dominant mechanism for correlated triplet pair (CTP) separation during singlet fission. Variations in CTP separation rates are compared for polymorphs of TIPS‐Pn with their triplet diffusion characteristics that are controlled by their crystal structures. Since triplet energy transfer is a spin‐forbidden process requiring direct wavefunction overlap, simple calculations of electron and hole transfer integrals are used to predict how molecular packing arrangements would influence triplet transfer rates. The transfer integrals reveal how differences in the packing arrangements affect electronic interactions between pairs of TIPS‐Pn molecules, which are correlated with the relative rates of CTP separation in the polymorphs. These findings suggest that relatively simple computations in conjunction with measurements of molecular packing structures may be used as screening tools to predict a priori whether new types of singlet fission sensitizers have the potential to undergo fast separation of CTP states to form multiplied triplets.     

Photon Upconversion Hydrogels for 3D Optogenetics

The ability to optically induce biological responses in 3D has been dwarfed by the physical limitations of visible light penetration to trigger photochemical processes. However, many biological systems are relatively transparent to low-energy light, which does not provide sufficient energy to induce photochemistry in 3D. To overcome this challenge, hydrogels that are capable of converting red or near-IR (NIR) light into blue light within the cell-laden 3D scaffolds are developed. The upconverted light can then excite optically active proteins in cells to trigger a photochemical response. The hydrogels operate by triplet–triplet annihilation upconversion. As proof-of-principle, it is found that the hydrogels trigger an optogenetic response by red/NIR irradiation of HeLa cells that have been engineered to express the blue-light sensitive protein Cry2olig. While it is remarkable to photoinduce the clustering of Cry2olig with blanket NIR irradiation in 3D, it is also demonstrated how the hydrogels trigger clustering within a single cell with great specificity and spatiotemporal control. In principle, these hydrogels may allow for photochemical control of cell function within 3D scaffolds, which can lead to a wealth of fundamental studies and biochemical applications.     

Synthesis of Highly Luminescent SnO2 Nanocrystals: Analysis of their Defect‐Related Photoluminescence Using Polyoxometalates as Quenchers

Colloidal semiconductor nanocrystals (NCs), called quantum dots (QDs), have been intensively studied because of their excellent photoluminescence (PL) quantum yields. However, commercial QDs such as CdSe and InP contain toxic or expensive rare elements, limiting their sustainable use. This study focuses on nontoxic, stable, and cheap tin oxides, and synthesized luminescent SnO₂ NCs of ≈2 nm in size by a heating‐up method. Tin precursors and diols in a high‐boiling point solvent with oleylamine as the surfactant are heated at 240 °C. SnO₂ NCs show defect‐related photoluminescence at 400–460 nm by excitation at 370 nm, achieving a high quantum yield of more than 60%. The PL intensity is stable even when the NCs are stored in atmospheric air at room temperature for over 1 year. The defect‐related emissions of the SnO₂ NCs are studied using polyoxometalates (POMs) as the PL quencher. POMs efficiently quench the PL emissions by extracting excited electrons from the conduction band and shallow surface defects. The results reveal that PL emissions from SnO₂ NCs are associated with radiative charge recombination via shallow defect levels on the surface and in the bulk, demonstrating the effectiveness of the PL quenching technique using POMs in studying the PL emission mechanism in QDs.     

Ruddlesden–Popper Perovskite Nanocrystals Stabilized in Mesoporous Silica with Efficient Carrier Dynamics for Flexible X-Ray Scintillator

The development of metal-halide perovskite nanocrystals (NCs) that yield bright and stable emission is of great importance. Previous reported perovskite NCs are mostly based on APbX₃-type family fabricated via ligand- or surfactant-assisted chemical approaches. However, realizing bright and stable emission remains a challenge because of desorption of ligands/surfactants during long-term operation. Herein, Ruddlesden–Popper (RP)-type (A)₂(MA)_n-1Pb_nBr_3n+1 NCs with size less than Bohr radius stabilized in mesoporous silica scaffold, which are prepared in situ via physical approach at low temperature are introduced. The RP NCs in mesoporous silica exhibit the formation of spatially and electronically separated quantum wells, efficient energy funneling between different n phases for bright emission (photoluminescence quantum yields of ≈99%), high irradiation stability (T₇₀ = 110 days), and long-term stability (T₉₀ = 110 days). These RP NCs have broad potential for bright light-emitting diodes, high-resolution PL imaging, and waterproof inks. Importantly, for the first time, stretchable perovskite X-ray scintillator is demonstrated with excellent X-ray imaging with resolution greater than 14 line pairs mm⁻¹. These findings offer a paradigm to motivate future research toward stable and efficient perovskite optoelectronics.