Symposium EP05 : Emerging Light-Emitting Materials and Devices---Halide Perovskite and Low-Dimensional Nanoscale Emitters

Hybrid (inorganic-organic) perovskites have demonstrated an extraordinary potential for clean sustainable energy technologies and low-cost optoelectronic devices such as solar cells; light emitting diodes, detectors, sensors, ionic conductors etc. In spite of the unprecedented progress in the past six years, one of the key challenges that exists in the field today is the large degree of processing dependent variability in the structural and physical properties. This has limited the access to the intrinsic properties of hybrid perovskites and led to to multiple interpretations of experimental data. In addition to this, the stability and reliability of devices has also been strongly affected and remains an open question, which might determine the fate of this remarkable material despite excellent properties.
In this talk, I will describe our recent work on Ruddlesden-Popper halide perovskites as a potential alternative to the bulk hybrid perovskites. I will describe the versatility and tunability of this novel system through our efforts on achieving high-efficiency light emitting diodes with stability. I will describe the design principles based on structure, grain-size and composition of phase-pure layered 2D perovskites and demonstrate proof-of-concept color tunable light emitting diodes and discuss their stability.

Atomically thin 2D halide perovskites are a promising new class of semiconductor nanomaterials, exhibiting bright luminescence, tunable and spectrally narrow absorption and emission features, strongly confined excitonic states, and facile synthesis. In this talk, I will demonstrate recent progress from our group in the synthesis and understanding of optical properties in colloidal perovskite nanoplatelets and 2D layered perovskites. Results to be highlighted include synthetic tuning of the exciton energy, fabrication of perovskite light-emitting devices, structural transformations in 2D, and the origin of dimensionality-dependent vibrational modes.

Hybrid (inorganic-organic) halide perovskites have demonstrated an extraordinary potential for clean, sustainable energy technologies and low-cost optoelectronic devices. In spite of the unprecedented progress over the past five years, one of the key challenges that exists in the field today is the stability and reliability of devices when exposed to the environment, external electric-field and light. This vulnerability remains an open question, which might determine the fate of this remarkable material despite excellent properties.

In this talk, I will introduce a new class of hybrid perovskite material known as Ruddlesden-Popper phase layered perovskites (RPLP) as promising alternatives to the bulk hybrid perovskites, which exhibits intrinsic structural stability and unique optoelectronic properties. These are solution-processed quantum wells with a general formula of A’₂A_n-1B_nX_3n+1 where the inorganic slab (B) is separated by bulky organic molecules (A’). I will first describe the crystal structure and versatility of this novel system through our efforts on synthesis of phase-pure crystals (with well-defined inorganic slab thickness). A major breakthrough was realized through our discovery of growing single-crystalline thin-films using hot-casting method ^[1] with the ability to control the orientation at the molecular scale, which facilitates efficient charge separation ^[2] and has enabled high-efficiency solar cells. More importantly, we demonstrated that solar cells fabricated using vertically oriented single-crystalline films exhibit technologically relevant stability exceeding 2500 hours of operation under continuous light and 65% relative humidity ^[2].

Following this result, our study has shown that the crystalline 2D thin-film with preferred orientation can tremendously enhance carrier injection and transport and lead to strong radiative recombination with high photoluminescence quantum yield. We utilize this property to demonstrate stable light emitting diodes (LEDs) with an external quantum efficiency of electroluminescence approaching 1%, radiance of 35 W/Sr.m² and without any droop in efficiency with injection currents as large as few Amperes/cm2^[3-4]. Furthermore, the LEDs fabricated with phase-pure RPLPs exhibit wide range of color tuning and good stability thus paving the path of realizing the dream of electrically injected lasers using solution-processed semiconductors.

In summary, we believe that these results have opened the door for a wide range of applications using low-temperature processed semiconductor grade single-crystalline thin-films, which will lead to new scientific and technological discoveries.

Reference:
[1] Tsai-Nie et al., Science 347, 522 (2015); [2] H. Tsai et al., Nature 536, 312(2016); [3] Blancon-Tsai et al., Science 355, 1288 (2017); [4] H. Tsai et al., Adv. Mater. DIO:10.1002/adma.201704217 (2017)

Organic-inorganic layered perovskite type compounds with a basic formula of (RNH₃)₂(CH₃NH₃)_n-1Pb_nHal_3n+1 (R is usually an alkyl chain, and X – halogen) are widely studied due to their intriguing electronic, optical properties, and structural characteristics. These compounds are made of slabs of Pb-X corner sharing octahedra with small methylammonium (CH₃NH₃⁺) cations filling inter-slab cavities. Long alkyl chain ammonium cations are situated in-between the layers, they play a role of a potential wall. Tuning of the band gap energy of these compounds is possible through the modification of the carbon chain length in organic amines as well as by changing the thickness of the inorganic layer.
In our study, we eliminated primary amines and replaced typically used methylammonium by cesium expecting higher thermal and chemical stability. As an interlayer component a guanidinium was chosen due to its thermodynamic stability, high basicity (pKa = 13.6), and strong hydrogen-bonds capabilities. The guanidinium cation is not able to maintain a three dimensional network in APbI₃ system due to its crystal ionic radii, therefore, it could situate itself in the interlayer space.
All obtained compounds: Cs[C(NH₂)₃]PbI₄ (I), Cs[C(NH₂)₃]PbBr₄ (II), and Cs₂[C(NH₂)₃]Pb₂Br₇ (III), are two–dimensional, air stable, and possess a reasonable temperature stability (up to 300 ^°C). Compounds I-II are luminescent at moderate cooling, and the compound III is emissive already at room temperature. Photoresponsivities in the range of 1-10 mA×W^-1 were measured with compounds I and III. According to DFT calculations I, II and III are semiconducting compounds with resulting (calculated) band gaps of approximately 2.7 eV for II, 2.2 eV for III, and 2.0 eV for I. It is common to the three that the highest occupied bands and lowest empty ones have dominant halide-p and Pb-p contributions, respectively.

Recently, there is a renaissance of halide perovskites as a promising class of light-emitting materials because of their unusual optoelectronic properties. Particularly, the inorganic halide perovskites attracted more and more attention, owing to their enhanced stability toward moisture, oxygen, and heat, compared to the organic-inorganic hybrid perovskites (e.g. methylammonium lead iodide). Low-dimensional nanostructured perovskites provide controllable morphology, tunable emission, and improved quantum efficiency. This talk will focus on the new strategies towards inorganic halide perovskite nanostructures and heterojunctions, as well as their emerging applications in nanoscopic lasers and high-definition displays.

We develop the advanced synthetic methodology of CsPbX₃ nanostructures with desired sizes, compositions, and optical properties, including colloidal, solution-phase and vapor-phase approaches. Colloidal nanowires exhibit highly efficient photoluminescence, with well-controlled morphology and tunable diameter range from 10 to 2 nm. Mesoscopic single-crystal nanostructures from solution-phase growth were demonstrated as the efficient optical medium for high-performance and robust nanoscopic lasers. Vapor-phase methods generally have better crystalline quality and lower defect density for improved optoelectronic properties. Due to the relatively weak bonding in halide perovskites, anion exchange was demonstrated in these materials with high PLQY throughout the exchange reaction. Moreover, we demonstrate spatially resolved multi-color CsPbX₃ nanowire heterojunctions through localized anion exchange. These perovskite heterojunctions show tunable photoluminescence over the entire visible spectrum with high resolution down to 500 nm, which represent key building blocks for high-resolution displays.

In 2014 we observed white-light emission from layered lead-halide perovskites. When excited by UV light, these bulk solids emit across the entire visible spectrum, approximating sunlight. These hybrid phosphors have high color rendering indices (accurately representing illuminated colors) and tunable chromaticity coordinates, where halide substitution affords both 'warm' and 'cold' white light. They are promising as phosphors for solid-state lighting.

Most inorganic phosphors contain emissive dopants or surface sites. I will share our understanding of how a bulk material, with no obvious emissive sites, can emit every color of visible light. I will discuss the experiments that led us to propose that the broad emission stems from excited electron-hole pairs (or excitons) that couple strongly to the lattice, thereby forming transient lattice defects. Importantly, although we attribute the broad emission to these 'excited-state defects', it responds to systematic variation in the inorganic sublattice, allowing synthetic control over the emission. I will explain synthetic design rules we have uncovered for obtaining white light from layered perovskites. The understanding we have developed of perovskite white-light emitters can be applied to other low-dimensional systems. I will also describe other inorganic topologies that afford broadband emission with a large Stokes shift, which we attribute to self-trapping by analogy to the perovskite white-light emitters.

The remarkable performance of lead halide perovskites in solar cells can be attributed to the excellent photophysical properties that are also ideal for lasers and light-emitting devices (LEDs). Here we first report new insights on the crystal growth of the perovskite materials and developed the solution growth of single crystal nanowires, nanorods, and nanoplates of methylammonium (MA), formamidinium (FA) and all-inorganic cesium (Cs) lead halides perovskites (APbX₃) via a dissolution-recrystallization pathway. We also developed the vapor phase epitaxial growth of CsPbX₃ perovskite nanowires and single-crystal thin films. Moreover, chemical strategies to stabilize the nanostructures of metastable perovskite phases, such as FAPbI₃ and CsPbI₃, have been developed by using surface ligands. We demonstrated high performance room temperature lasing with broad tunability of emission color from 420 nm to 824 nm from single-crystal lead halide perovskite nanowires with estimated lasing quantum yields approaching 100%. LEDs can also be fabricated with nanoscale structures of 3D or 2D perovskites. The excellent properties of these single-crystal perovskite nanostructures of diverse families of perovskite materials with different cations, anions, and dimensionality make them ideal for fundamental physical studies of carrier transport and decay mechanisms, and for enabling high performance semiconductor lasers, LEDs, and other optoelectronic applications.

Metal halide perovskites have emerged as the only solution-processable photovoltaic technology to outperform multicrystalline silicon, by virtue of its intrinsic properties such as large absorption coefficient, balanced charge carrier transport, highly crystalline film formation, weak exciton binding energies, and slow bimolecular recombination. Since 2014, perovskites have made strides in light-emitting applications with first demonstrations of amplified spontaneous emission and LEDs. However, low luminescence, poor efficiencies of the light-emitting diodes (LEDs), and complex preparation methods currently limit further progress towards applications.

The prospects for advancing device efficiencies are contingent upon exploring new perovskite compositions and structures. Lower-dimensionality layered perovskites formulations permit for band gaps and exciton binding energy tuning, carrier mobility facilitated by the inorganic moeities, with the organic moieties providing additional controls for stability, light harvesting, and intralayer charge transport. Nanocrystals is yet another variant in perovskites that promises to yield new opportunities in advancing device performance. By carefully controlling the reaction conditions such as temperature, solvent, and ligands, hybrid perovskites of morphologies ranging from 0D quantum dots to 3D single crystals; and sizes stretching 6 orders of magnitude can be prepared.

This presentation will show that the incorporation of alkylated bromide molecules into the framework of a fully inorganic 3D-perovskite film yields remarkable improvement in the photophysical and morphological properties. Another approach highlighted would be a mesoscopic film architecture of 3D self-assembled nanocrystals, coupled with of lead bromide perovskites platelets that enable high-performance light-emitting diodes. These strategies yield photoluminescence quantum yields of over 80% and intense luminance (>56,000 cd m⁻²) LEDs associated with efficiencies in excess of 57.6 cd A⁻¹ with an external quantum efficiency above 13%. Challenges and opportunities relating to stability, and further improvement in performance will also be discussed, with emphasis on their optoelectronic properties and recombination dynamics. <div id="UMS_TOOLTIP" style="position: absolute; cursor: pointer; z-index: 2147483647; background-color: transparent; top: -100000px; left: -100000px; background-position: initial initial; background-repeat: initial initial;"> </div>

The growing attention to perovskite nanocrystals is connected with their unusual and potentially useful electronic and optical properties. I will discuss the bulk energy band structure of CsPbX₃ (X = I, Cl, and Br) perovskites and show that all of them have the band edge at R-point of the Brillouin zone. To describe electronic and optical properties of perovskite nanocrystals we have derived the four band effective mass Hamiltonian, which describes the electronic properties of electron and holes near the band edge. Using this Hamiltonian we calculate the lowest quantum confined levels of electrons and holes and the spectra of the allowed optical transitions. The calculations takes into account the cubic shape of the perovskite nanocrystals, that results into inhomogeneous electric field of emitted and absorb photons. The symmetry of the ground exciton state has been analyzed and the radiative decay time has been calculated. The results of our theoretical calculations have explained 200 ps radiative decay time and polarization properties measured in single CsPb(BrCl₂) quantum dot experiments .^1
1 M. A. Becker, R. Vaxenburg, G. Nedelcu, P. C. Sercel, A. Shabaev, M. J. Mehl, J. G. Michopoulos, S. G. Lambrakos, N. Bernstein, J. L. Lyons, T. Stöferle, R. F. Mahrt, M. V. Kovalenko, D. J. Norris, G. Rainò, and Al. L. Efros, to be published.

In this presentation, the optical properties of the newly developed near-infrared emitting formamidinium lead triiodide (FAPbI₃) nanocrystals (NCs) and their polycrystalline thin film counterpart are comparatively discussed. The excitonic emission is dominant in NC ensemble because of the localization of electron-hole pairs. A promisingly high quantum yield above 70 %, and a large absorption cross-section (5.2×10^-13 cm^-2) were measured. At high pump fluence, biexcitonic recombination was observed, featuring a slow recombination lifetime of 0.4 ns. In polycrystalline thin films, the quantum efficiency is limited by nonradiative trap-assisted recombination that turns to bimolecular at high pump fluences. From the temperature-dependent photoluminescence (PL) spectra, a phase transition is clearly observed in both NC ensemble and polycrystalline thin film. It is interesting to note that NC ensemble show PL temperature anti-quenching, in contrast to the strong PL quenching displayed by polycrystalline thin films. This difference is explained in terms of thermal activation of trapped carriers at the nanocrystal’s surface, as opposed to the exciton thermal dissociation and trap-mediated recombination, which occur in thin films at higher temperatures.

The halide perovskite CsPbBr3 has shown its promise for green lightemitting diodes. The optimal conditions of photoluminescence and the underlying photophysics, however, remain controversial. To address the inconsistency seen in the previous reports and to offer high-quality luminescent materials that can be readily integrated into functional devices with layered architecture, we created thin films of CsPbBr3/Cs4PbBr6 composites based on a dual-source vapor-deposition method. With the capability of tuning the material composition in a broad range, CsPbBr3 is identified as the only light emitter in the composites. Interestingly, the presence of the photoluminescenceinactive Cs4PbBr6 can significantly enhance the light emitting efficiency of the composites. The unique negative thermal quenching observed near the liquid nitrogen temperature indicates that a type of shallow state generated at the CsPbBr3/Cs4PbBr6 interfaces is responsible for the enhancement of photoluminescence.

Metal halide perovskite, a new type of optoelectronic material, has received extraordinary attention in photovoltaics, light-emitting diodes and photodetector applications. In this study, we combined all inorganic cesium lead iodide (CsPbI₃) perovskite nanoparticles (PNPs) and perovskite nanowires (PNWs) with single layer graphene (SLG) to obtain 0D–2D PNP–SLG and 1D–2D PNW–SLG hybrids with improved light harvesting. Time-resolved single nanostructure photoluminescence studies of PNPs, PNWs and related hybrids revealed (i) quasi two-state photoluminescence blinking in PNPs, (ii) highly polarized photoluminescence emitted by PNWs and (iii) efficient interfacial electron transfer between perovskite nanostructures and SLG in both PNP–SLG and PNW–SLG hybrids. Doping of poorly absorbing, highly conductive single layer graphene with perovskite nanocrystals and nanowires provides a simple, yet efficient path to obtain hybrids with increased light harvesting properties for potential utilization in the next generation photodetectors and photovoltaic devices.

Besides conventional optoelectronic devices (LEDs and Laser), colloidal nanocrystals (NCs) are pursued as non-classical light sources (i.e. single photon emitters) that might be playing a pivotal role in future quantum technologies (quantum cryptography, quantum sensing, quantum communication). Due to strongly reduced charge trapping on surface states, perovskite NCs become attractive as alternative single photon emitters.
Fully inorganic cesium lead halide (CsPbX₃, where X=Cl, Br, I) perovskite NCs are characterized by narrow emission lines, achieve ultrahigh photoluminescence quantum yields of up to 90% and are tunable over a wide energy range.¹ Furthermore, they are interesting due to their facile solution processability and their potential in diverse optoelectronic devices. Nevertheless, the origin of their exceptional photophysical properties at the single quantum dot level still needs to be completely uncovered.
Here we show that single CsPb(Br/Cl)₃ NCs exhibit stable, blinking-free emission at cryogenic temperatures.² An in-depth investigation of the trion dynamics revealed the absence of non-radiative quenching processes, such as Auger recombination, even without any shell passivation. We examined the origin of the composition dependent ultrafast (sub-ns) radiative recombination dynamic, and assigned it to a giant oscillator transition. For CsPb(Br/Cl)₃ nanocrystals the radiative lifetime is in the order of 200-250 ps, representing a significant enhancement compared to other colloidal II-VI NCs or organic molecules. Using polarization dependent high resolution spectroscopy, we further elucidated the complex nature of the exciton fine structure splitting revealing the unique character of bright triplet excitons.³
Due to their high oscillator strength, high quantum yield and wide range of composition and size tunability, CsPbX₃ NCs are versatile quantum light sources and offer a clear pathway for integration into optical microcavities and for the generation of more complex quantum states of ligth (i.e. multiphoton entangled-states).

References:

1) Protesescu, L. et al. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX ₃ , X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 15, 3692–3696 (2015).
2) Rainò, G. et al. Single Cesium Lead Halide Perovskite Nanocrystals at Low Temperature: Fast Single-Photon Emission, Reduced Blinking and Exciton Fine Structure. ACS Nano 10, 2485 (2016).
3) Becker, M. et al. Bright triplet excitons in lead halide perovskites. arXiv:1707.03071

Metal halide perovskite nanocrystals (NCs)¹ are one of the most attractive materials for optoelectronic applications. The most advantageous properties of this class of nanocrystals are their high photoluminescence (PL) quantum yield (PLQY) and color tunability. Very recently, it has been demonstrated that CsPbBr₃ NCs can reach near-unity PLQY in solution.² Yet, retaining the PLQY in film is not trivial, since the NCs are not as well passivated as in solution and close packing can lead to energy-transfer to trap-states and increased self-absorption.
Here, we report a room temperature synthesis of CsPbBr₃ NCs with relatively short-ligands (octanoic acid/octylamine) displaying near-unity PLQY in solid state films.³ The synthesis is carried out in air at room temperature via injection of the PbBr₂ precursor into a flask containing Cs acetate (instead of the more common Cs⁺ complexes prepared using Cs₂CO₃ and a fatty acid) dispersed in a mixture of hexane and 1-propanol. The as-synthesized nanocrystals show a PLQY of 83% in spin-coated films but a facile treatment of the solution employing Pb²⁺ complexes before spin-coating further enhances the PLQY, which reaches values close to 100%. The high PLQY is further confirmed by the single-exponential decay of the PL (5.8 ns) indicating a nearly complete suppression of non-radiative channels.
Despite the use of relatively short surface ligands in our synthesis, anion exchange with iodide can be carried out at room temperature and in air leading to a high PLQY in film of 65%. Solar cells operating in the wavelength range 350-660 nm can be fabricated in air, and they display a photo-conversion efficiency of 5.3% with an open circuit voltage (Voc) up to 1.31V, among the highest reported for perovskite based solar cells with bandgap below 2 eV.⁴ Similarly, the pristine nanocrystals (CsPbBr₃) have been used in light-emitting diodes whose performance will be here presented and discussed.

References:
(1) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nano Lett. 2015, 15 (6), 3692.
(2) Koscher, B. A.; Swabeck, J. K.; Bronstein, N. D.; Alivisatos, A. P. J. Am. Chem. Soc. 2017, 139, 6566−6569.
(3) Di Stasio, F.; Christodoulou, S.; Huo, N.; Konstantatos, G. Chem. Mater. 2017, 29 (18), 7663.
(4) Christodoulou, S.; Di Stasio, F.; Pradhan, S.; Stavrinadis, A.; Konstantatos, G. Submitted

Halide perovskite nanocrystals with an ABX₃ stoichiometry have become an increasingly interesting material system for optoelectronic applications due to their high fluorescence quantum yield, bandgap tunability through halide composition and now more recently due to the variation of sizes and dimensionalities accessible. However, there are still many open questions concerning the fundamental properties of perovskite nanocrystals especially pertaining to their recombination dynamics.
Here, we present results from optical spectroscopy experiments on hybrid organic-inorganic and all-inorganic nanoplatelets. In these studies, we were able to control the nanoplatelet thickness incrementally down to a single perovskite monolayer yielding absorption spectra similar to established two-dimensional semiconductors. In order to investigate the relaxation dynamics of photoexcited electron-hole pairs, we performed temperature-dependent transient absorption spectroscopy. We find that the carrier cooling behavior as well as the exciton formation dynamics depend on the thickness of the nanoplatelets. Moreover, we study the effect of nanoplatelet thickness and A-site cation on radiative and non-radiative recombination rates.

Heterostructures constructed from two-dimensional (2D) van der Waals materials such as transition metal dichalcogenides (TMD), graphene, and boron nitride, have sparked wide interest in both device physics and materials science.¹ Apart from these inorganic 2D materials, two-dimensional organic-inorganic hybrid lead halide perovskites (2D PVSKs) have recently emerged as promising materials for solar cells, with power conversion efficiencies over 12% and stability over 2000 hours, compared to 10 hours for traditional 3D PVSKs.² 2D PVSKs also show higher photoluminescence quantum yield (~26%) than their 3D counterparts (<1%), suggesting their intrinsic optoelectronic quality may be much higher.³

Fundamental discovery of 2D PVSK materials can be enabled by building a heterostructure with TMDs. In this study, we obtain evidence of the formation of interlayer excitons in 2D PVSK-TMD heterostructures through photoluminescence spectroscopy. Interlayer excitons are bound electron-hole pairs that live across the interface between two disparate materials, rather than being localized in an individual monolithic material. We find that the photoluminescence spectra of a 2D PVSK/WS₂ heterostructure shows a new peak at a longer wavelength (~680nm), signifying the existence of an interlayer exciton. Using ultraviolet photoemission spectroscopy as well as time-resolved photoluminescence spectroscopy, we performed a systematic study on the possible interlayer excitons that can be formed between different 2D PVSKs and TMDs. Interlayer exciton observed in highly luminescent 2D PVSKs may provoke further studies of Van der Waals heterostructures, with novel applications in atomically thin solar cells, light-emitting diodes, and lasers.


Reference
1. Rivera, P. et al. Observation of long-lived interlayer excitons in monolayer MoSe2–WSe2 heterostructures. Nat. Commun. 6, 6242 (2015).
2. Tsai, H. et al. High-efficiency two-dimensional Ruddlesden–Popper perovskite solar cells. Nature 536, 312–316 (2016).
3. Dou, L. et al. Atomically thin two-dimensional organic-inorganic hybrid perovskites. Science 349, 1518–1521 (2015).

I will discuss two applications of luminescent nanoparticles: perovskite nanoparticles for phosphors and light-emitting diodes (LEDs), and lead sulfide semiconductor nanoparticles for luminescent downconverters in photovoltaics.

Cesium lead halide perovskite nanoparticles can give efficient emission in LEDs, but mixing these nanoparticles to give white light emission is problematic due to rapid halide exchange between nanoparticles of different compositions. I will show how mixtures of certain nanoparticle compositions can be stabilised, and will discuss the physics of energy transfer in the mixed films. I will also present measurements of defect-state emission in 2-d perovskite structures.

The efficiency of photovoltaic devices could be improved beyond the Shockley-Queisser limit if it were possible to convert higher-energy photons in the solar spectrum into two bandgap-energy photons. The process of singlet exciton fission in organic semiconductors is a promising route to achieve this, but the challenge is to achieve photon emission from both of the triplet excitons generated. Triplet energy transfer into emissive lead sulfide nanoparticles has been demonstrated, with the potential to achieve a “photon multiplier film” than could be applied to the front surface of a silicon solar cell. I will present recent progress in this area, aiming to achieve highly luminescent nanoparticles whilst still allowing triplet excitons to tunnel easily onto the particles from a surrounding singlet fission material.

Metal halide perovskites have emerged as a new class of solution processable semiconducting materials with applications in a variety of optoelectronic devices, from photovoltaic, to photodetectors, lasers and light emitting diodes (LEDs). While electrically driven perovskite LEDs have shown great promise with the device efficiency approaching to those of organic and quantum dot LEDs, a number of challenges, such as long term stability and color tenability, remain to be addressed.

In this talk, I will report our work on developing efficient red emitting perovskite LED with great spectral stability by using PEO/qausi-2D perovskites composites as light emitting layer. Cesium lead iodide perovskites were chosen for the consideration of their good thermal and moisture stability, as compared the organic methylammonium counterparts. By facile one step solution processing followed by low temperature thermal treatment, composite thin films of PEO/quasi-2D perovskites can be prepared to exhibit tunable photoluminescence from red to deep red. The addition of PEO leads to a significantly enhanced thin film photoluminescence quantum efficiency (PLQE) and superior thin film morphology. Electrically driven red emitting LEDs with emission peaked at 680 nm have been fabricated to exhibit high brightness of 1500cd/m² and external quantum efficiency of 4.7%. More importantly, these red perovskite LEDs show great spectral stability and device performance stability during operation.

Perovskite materials exhibit high photoluminescence quantum yield (PLQY, greater than 90% in solution for nanocrystals) and high color purity with narrow emission line-widths less than 20 nm, which make them as a good candidate material for efficient light-emitting diodes (LEDs). According to electron injection layer/perovskite layer interface engineering, we have improved the perovskite emission layer morphology, suppressed the interfacial non-radiative recombination and also balanced the electron-holes injection. As a result, an inorganic perovskite light-emitting diodes with the brightness of 91000 cd/m² and external quantum efficiency of 10.4% was achieved. Recently, we have pushed the external quantum efficiency of perovskite LED to over than 14% by balancing the charge carrier confinement and charge injection via composition engineering of perovskite layer.


References
L. Zhang, J. You* et al., Nature Communications, 8, 15640 (2017).

Organometal halide perovskites are emerging as potential materials for light-emitting diodes due to superior color purity, tunable band gaps, low cost and solution processability. However, the device stability, which was proven to be a vital issue in perovskite solar cells (PSCs), has not been addressed in PeLEDs as well. Herein, we for the first time exploited the undesirable δ -FAPbI₃ to enable structurally stable, pure FAPbI₃ films with a controllable α/δ phase junction at low annealing temperature (60^oC) through stoichiometrically modified precursors (FAI/PbI₂=1.1–1.5). The α/δ phase junction contributes to a striking stabilization of the perovskite phase of FAPbI₃ at low temperature and significantly enhanced near-infrared emission (NIR) emission at 780 nm, which is markedly different from pure α-FAPbI₃ (815 nm). In particular, the optimal α/δ phase junction with FAI/PbI₂ =1.2 exhibited preferable long-term stability against humidity and high PLQY of 6.9%, nearly 10-fold higher than that of pure α-FAPbI₃ (0.7%). Based on this α/δ phase junction, we fabricated high-performance NIR emitting PeLEDs through the integrated utilization of solvent engineering and moisture exposure. Strikingly, the controlled exposure to a relative humidity (RH) around 65% significantly improved the photoluminescence lifetime of the FAPbI₃ with α/δ phase junction from 14 ns to 35 ns. We revealed that moisture treatment could be exploited to PeLEDs for significantly decreasing trap density of perovskite films, thus enabling fortyfold increase of external quantum efficiency from 0.03% up to 1.20% as well as a low turn-on voltage at 2.4 V. The present study opens a new approach to realize highly stable and efficient emitting perovskite materials by utilizing the phase junctions and could also be extended from FAPbI₃ to other compositional perovskites providing guidelines for optimum integrated post-treatments to realize high-performance PeLEDs.

The organic-inorganic halide perovskites (OIHPs) as a next-generation light emitter have been vigorously studied. To achieve highly efficient OIHP-based optoelectronic devices, the addition of additives to the OIHPs is one of the promising approaches. However, we found that the conventional crystal growth mechanism of spin-coated OIHP films is dramatically affected by impurity effect when additives with an above a critical concentration are directly added to the OIHP precursor solutions to fabricate an additive mixed OIHP hybrid film. In this research, we have identified the crystallization kinetics including a crystal coarsening, which is not suitable for LED applications, of the OIHP film with additives, and have devised a method to effectively overcome a crystal coarsening and to fabricate additive:OIHP hybrid film effective for LED application, thus realizing a highest EQE of pure green PeLED. The fabricated additive:OIHP hybrid film showed high radiative recombination rate by the defect healing effect, and low charge accumulation, which is due to low free carrier density by high radiative recombination rate and effective exciton confinement, and dramatically improved a half-lifetime of PeLEDs were also investigated in PeLEDs. Moreover, the highly changed optical parameters, which induced a low optical loss in the PeLEDs, were firstly observed, and this optical effect is also confirmed by performing the optical simulation.

Hybrid organic-inorganic halide perovskite materials such as methylammonium lead iodide have garnered significant interest in the thin film optoelectronics community due to their promising optoelectronic properties. We have determined that the fabrication of perovskite thin films displays all of the hallmarks of sol-gel processing, an aspect that we exploit to improve the quality of spin coated thin films. In particular, we realize films with roughness on the order of 1 nanometer that consist of nanoscale crystallites, formed by incorporating a bulky organoammonium halide in addition to the stoichiometric 3D perovskite precursors. These bulky ligands passivate the 3D crystal, lead to considerably enhanced luminescence quantum yields, and increase stability. LEDs produced in this way are capable of exceeding 10% external quantum efficiency and exhibit significantly improved stability. Finally, they allow for stabilizing mixed halide (I and Br) stoichiometries such that we can tune emission color from the green to red wavelengths.

Organic-inorganic metal halide hybrids have recently emerged as a highly promising class of functional materials with excellent optical and electronic properties for a variety of applications. The exceptional structural tunability enables these materials to possess three- (3D), two- (2D), one- (1D), and zero-dimensional (0D) structures at the molecular level. Remarkable progress has been realized in this research area in recent years, focusing mainly on 3D and 2D structures, but left low dimensional 1D and 0D structures significantly underexplored. In this talk, I will discuss the most exciting developments that our group has achieved in the low dimensional organic-inorganic metal halide hybrids with 1D and 0D structures. Due to the strong quantum confinement and site isolation, bulk assemblies of 1D and 0D organic metal halide hybrids exhibit remarkable and unique properties that are significantly different from those of well known 3D and 2D metal halide perovskites. For instance, broadband white emissions have been achieved in single crystalline bulk assemblies of 1D organic metal halide nanowires and nanotubes; and near-unity photoluminescence quantum efficiencies have been realized for a number of 0D organic metal halide hybrids. The excitement about the recent developments of organic-inorganic metal halide hybrids with controlled dimensionalities lies not only in the specific achievements, but also in what these materials represent in terms of a new paradigm in materials design. There is a vast parameter space to explore organic-inorganic hybrid materials beyond metal halide perovskites, and we expect to encounter a lot of new science over the coming years.

Recent discoveries of perovskite electroluminescence and lasing, as well as the realisation of highly-luminescent perovskite nanocrystals have triggered intense research into the use of perovskite semiconductors for display applications. Perovskites' facile bandgap tuning, that allow light-emission across the entire visible spectrum, and their narrow emission bandwidth are remarkably useful properties for implementation into wide color-gamut displays with vibrant and accurate color reproduction. In this talk, we will share some of our recent developments on perovskite emissive devices as well as discuss our new findings on the degradation mechanism and stability enhancement solutions to perovskite materials. In particular, we will share our recent work on photo-activated halide exchange, which enables an alternative approach towards bandgap tuning, and discuss some of its potential applications in emissive display devices.

The electroluminescence (EL) intensity usually is low for perovskite-based light-emitting diodes (LEDs) biased at the low current density regime and increases nonlinearly with the current density as found elsewhere in the previous studies. The traps in the active layer and the unbalanced injection of the diodes at the low current regime possibly cause the nonlinear EL intensity versus current density curve (L-I curve). In this work, by adding a small amount of additives to passivate the ionic defects of methylammonium lead bromide (CH₃NH₃PbBr₃) polycrystalline film markedly enhances the device performance of perovskite-based LEDs. We observe the linear correlations of L-I curve for devices biased at different current regimes, which should be the feature for a decent LED. We attribute the passivation of ionic defects suppresses the trap-assisted non-radiative recombination in perovskite polycrystalline active layer and therefore elevates the output performance of devices. As characterized by SEM, XRD, and photoluminescence (PL) measurement, adding the additives reduces the averaging crystalline size, enhances PL intensity, and elongates the carrier lifetime, but did not change the basic crystal structure of CH₃NH₃PbBr₃ perovskite. The reduced light turn-on voltage of devices also suggests the balanced injection of the carriers. Our work poses the direction for preparing the good quality of perovskite polycrystalline film for real LED applications.

The organic-inorganic hybrid perovskite materials have been interested because of their outstanding characteristics that enable a remarkable enhancement of device efficiency, raising them promising semiconductor candidates for light-emitting diodes (LEDs) and solar cells. High-quality perovskite films are required to realize highly efficient and stable perovskite devices. However, solution-processed perovskite films inevitably contain defect sites such as voids, pinholes, grain boundaries and under-coordinated ions, creating a large number of undesired electronic trap sites. Here, we investigate the significant beneficial effects using a new treatment based on amine-based passivating materials (APMs) to passivate the defect sites of methyl ammonium lead tribromide (MAPbBr₃), resulting in improved efficiency and long-term stability of perovskite light-emitting diodes (PeLEDs).

The electroluminescence efficiency of metal halide perovskite light-emitting diodes (PeLEDs) has been rapidly increased during recent years. Conducting polymer anodes (CPAs) have been very effective to realize high efficiency in simplified PeLEDs. An ideal CPA requires high conductivity, high work function WF, and prevention of exciton quenching between an anode and an overlying metal halide perovskite layer. However, increasing the conductivity and WF at the same time has been very challenging because chemical or molecular approaches to increase the WF have reduced the films’ conductivity. Chemical post-treatment on CPA films to increase the conductivity is not suitable for mass production of the reliable CPA. Here, we introduce a chemical additive approach without chemical post-treatment, to simultaneously increase conductivity and WF of CPAs. The CPA composition includes a WF-tuning fluoropolymer, but the chemical additives affect only the PEDOT:PSS conducting polymer. The additives weaken the coulombic interaction between positively-charged PEDOT and negatively-charged PSS, and thereby improve the interaction of conducting PEDOT chains. This change resulted in a record current efficiency up to 52.86 cd A^-1 (10.93 % ph el^-1) in green PeLEDs. Our results provide a significant clue to develop effective CPAs for highly-efficient PeLEDs.

Semiconductor quantum dots have proven to be promising materials for optoelectronic devices, such as light emitting devices (LEDs) and solar cells, due to their thin linewidth emission spectra, high photoluminescence quantum yield and high absorption coefficient.^1-3 By controlling the reaction temperature, and reactants concentrations, researchers are also able to synthesise pure inorganic perovskite quantum dots, CsPbX₃ (X = Cl, Br, I) with precise colour tuneability and high quantum-yield which are soluble in non-polar solvents.⁴ All of these properties make them promising materials for fabricating nanocrystal LEDs (NC-LEDs). Here, we synthesize high quantum yield (50- 80%) monodispersed quantum dots, with tuneable emission spectra over the entire visible region, by a colloidal synthesis method and successfully process them produce thin films as the emitting layer in an organic LED-type device architecture. The electroluminescence emission wavelength is tuned by varying the temperature of the growth solution and the halide ratio in the reactants. We demonstrate the feasibility of producing LEDs with electroluminescence from blue to red using solution processing techniques for the charge transport layers. Most importantly, we demonstrate the field induced evidence of halide separation in mixed halide CsPb(Br/I)₃ NCs which causes color instability in the NC-LEDs. Moderate iodide content mixed halide devices seemed to have greater compositional stability.

References:
1. Yang, Y.; Zheng, Y.; Cao, W.; Titov, A.; Hyvonen, J.; Manders, J. R.; Xue, J.; Holloway, P. H.; Qian, L., High-efficiency light-emitting devices based on quantum dots with tailored nanostructures. Nat. Photonics 2015.
2. Anikeeva, P. O.; Halpert, J. E.; Bawendi, M. G.; Bulovic, V., Quantum dot light-emitting devices with electroluminescence tunable over the entire visible spectrum. Nano Lett. 2009, 9 (7), 2532-2536.
3. Chuang, C.-H. M.; Brown, P. R.; Bulović, V.; Bawendi, M. G., Improved performance and stability in quantum dot solar cells through band alignment engineering. Nat. Mater. 2014, 13 (8), 796.
4. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V., Nanocrystals of cesium lead halide perovskites (CsPbX3, X= Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut. Nano Lett. 2015, 15 (6), 3692-3696.

This presentation we report a new strategy to demonstrate a very stable high EL brightness by using our one-step solution processing method to prepare high quality perovskite (MAPbBr₃) films with mixed large/small grains in micrometers/nanometers. Essentially, our design of using mixed large/small grains presents a self-passivation mechanism of grain boundary defects through ion migration under EL operation, consequently leading to a high brightness of 21,233 cd/m² at the current density of 562 mA/cm² in our device without current efficiency roll-off even at current densities. On the other hand, the self-passivation of grain boundary defects allows the turn-on voltage (1.9 V) lower than the bandgap (2.25 eV) to operate EL actions. The self-passivation of grain boundary defects is verified by the slow relaxation of both spectral intensity and shift when the photoluminescence (PL) is modulated by an electrical injection in our perovskite LEDs. Specifically, we observe that the PL intensity is slowly relaxed in the order of ~ 1 minutes after removing electrical injection while constant photoexcitation is applied. Simultaneously, the spectrum is blue shifted by ~ 15 nm due to the de-passivation upon removing electrical injection in the mixed large/small grains. Therefore, using mixed large/small grains with one-step solution processing method presents an important strategy to develop high-performance perovskite light-emitting devices based on self-passivation of mechanism of grain boundary defects.

Owed to increasing demand for high color-purity displays, there have been various attempts to realize natural colors. In this point of view, light sources with narrow full width half maximums (FWHM) are essentially needed. Metal halide perovskites are a promising alternative emitter material owed to their superior optical properties such as high color-purity (i.e., narrow FWHM <20 nm), solution processability, and low-material cost. Nonetheless, i) small exciton binding energies, ii) long exciton diffusion lengths, and iii) high degree of surface defects induce non-radiative exciton or free carrier dissociation that degrades luminous properties of light-emitting diodes (LEDs).
Here, we report recent developments on methods for controlling optoelectronic properties, and surface defects of metal halide perovskites for high-brightness light-emitting applications. Two-step solution processing techniques have enabled uniform and small sized crystals, which promotes good charge carrier and exciton confinement effects to reduce exciton diffusion lengths and non-radiative recombination in perovskite emitting layers. All-inorganic perovskite nanocrystals were also synthesized to lead better carrier and exciton confinement for high blue electroluminescence. By incorporating yellow-lighting polymer into the perovskite nanocrystals, white LED (CIEx: 0.33, CIEy: 0.34) were fabricated. In addition, intermediate phase engineering of organic-inorganic hybrid metal halide perovskites was used to control crystallization and to passivate surface defects for high-efficiency LEDs.

Recently, metal-halide perovskite light-emitting diodes have attracted a great attention due to not only the high efficiency but tunable color and color purity, low material cost and easy fabrication. However, use of conventional transparent conducting oxide (TCO) electrodes has limited the development of perovskite light-emitting diodes (PeLEDs) as a flexible and efficient device. We developed light-emitting diodes with a methylammonium lead bromide (MAPbBr₃) emitter using metal-oxide free graphene anode (Gr-PeLEDs). The device achieved high maximum current efficiency (CE_max) = 18.8 cd/A and maximum external quantum efficiency (EQE_max) = 4.23 %, which are both higher than those of PeLEDs based on TCO electrodes. We confirmed that the use of graphene anode can avoid formation of exciton quenching sites due to the diffused metallic species from TCO anodes to the overlying layers. Reduction of exciton quenching by using the chemically inert graphene anode for the MAPbBr₃ emitter which has long exciton diffusion length resulted in increase of device efficiency. In this regard, graphene is a promising anode material to solve the intrinsic problems of metal-halide perovskite emitters in terms of exciton quenching due to the low exciton binding energy. We have also fabricated highly flexible Gr-PeLEDs on PET substrate; the graphene anode withstood repeated bending (>1,000 bending cycles) and high bending strain (5.3%). Therefore, graphene enables high-efficiency flexible PeLEDs that have high color purity and low fabrication cost, which will provide practical application of next-generation flexible displays and solid-state lighting.

Perovskite materials have recently attracted considerable interest in photovoltaics and light-emitting diodes. In general, a number of hybrid and all-inorganic perovskite materials are excellent optical gain materials, due to strong optical absorption, large exciton binding energy and oscillator strength. In this talk, I will first discuss our effort in vapor phase synthesis of high quality inorganic-organic or all-inorganic perovskite crystals, with controllable emission band from ultraviolet to near-infrared. Steady-state and transient spectroscopy approaches can elaborate the exciton binding energy and excitonic emission nature at room temperature. Then I will discuss the optically pumped photonic lasing based on the intrinsic whispering gallery mode cavity will then be presented, while the lasing quality factor can be as high as 5000 in all-inorganic perovskite crystals. Lastly, I will introduce our latest work on room temperature exciton-polariton lasing in all-inorganic perovskite CsPbCl₃ crystals embedded in optical microcavities. Those crystals have exceptionally large exciton binding energy, strong oscillator strength and can be grown by facile epitaxy-free techniques. Polariton lasing is unambiguously evidenced by a superlinear power dependence, macroscopic ground state occupation, blueshift of ground state emission, and the build-up of long-range spatial coherence. Our work suggests considerable promise of lead halide perovskites towards large-area, low-cost, high performance room temperature polariton devices and coherent light sources extending from the ultraviolet to near infrared range.

Halide perovskites have shown great potential as building blocks for low-cost optoelectronics for their exceptional optical and electrical properties, leading to applications with versatile optoelectronic functionalities such as light emitting diodes (LED), lasing, solar cells and photodetectors. Despite the remarkable progress in device demonstration, fundamental understanding of the physical processes in halide perovskites remains limited, especially the unusual electronic behaviors such as the current-voltage hysteresis and the switchable photovoltaic effect and light-emitting diodes(LED) without LED junctions. These phenomena are of particular interests for being closely related to device functionalities and performance. In this work, a microscopic picture of electric fields in halide perovskite thin films was obtained using scanning laser microscopy. Unlike conventional semiconductors, distribution of the built-in electric fields in the halide perovskite evolves dynamically under the stimulation of external biases. The observations can be well explained using a model based on field-assisted ion migration, indicating that the mechanism responsible for the evolving charge transport observed in this material is not purely electronic. The anomalous dynamic responses to the applied bias are found to be effectively suppressed by operating the devices at reduced temperature or processing the materials at elevated temperature, which provides potential strategies for designing and creating halide perovskites with more stable charge transport properties in the development of viable perovskite-based optoelectronics.

Solution-processed halide perovskites possess exceptional photovoltaic properties. Amazingly, these materials are also outstanding optical gain media. Halide perovskites are the latest member of solution-processed optical gain media, joining organics and traditional semiconductor colloidal quantum dots. Amplified spontaneous emission and lasing have been demonstrated in various halide perovskite configurations and nanostructures with wavelengths tunable over the visible and infrared wavelengths (400 – 1000 nm). Despite the success of optically-pumped perovskite lasing, electrically-pumped lasing is still elusive. Challenges include the high current densities needed for electrical driven lasing and their heating effects and perovskite degradation (given the low thermal conductivity of halide perovskites). In this talk, I review the milestones, state-of-the-art and prospective outlook of this new family of lasers.

Polariton lasing is the coherent emission arising from a macroscopic polariton condensate first proposed in 1996. Over the past two decades, polariton lasing has been demonstrated in a few inorganic and organic semiconductors in both low and room temperatures. Polariton lasing in inorganic materials significantly relies on sophisticated epitaxial growth of crystalline gain medium layers sandwiched by two distributed Bragg reflectors in which combating the built-in strain and mismatched thermal properties is nontrivial. On the other hand, organic active media usually suffer from large threshold density and weak nonlinearity due to the Frenkel exciton nature. Further development of polariton lasing toward technologically significant applications demand more accessible materials, ease of device fabrication, and broadly tunable emission at room temperature. Herein, we report the experimental realization of room-temperature polariton lasing based on an epitaxy-free all-inorganic cesium lead chloride perovskite nanoplatelet microcavity. Polariton lasing is unambiguously evidenced by a superlinear power dependence, macroscopic ground-state occupation, blueshift of the ground-state emission, narrowing of the line width and the buildup of long-range spatial coherence. Our work suggests considerable promise of lead halide perovskites toward large-area, low-cost, high-performance room-temperature polariton devices and coherent light sources extending from the ultraviolet to near-infrared range.

Hybrid lead halide perovskites are unique solution-processed semiconductors with very large optical absorption coefficients in the visible spectrum, yet very long excited state lifetimes and large carrier diffusion lengths. Recent studies have proposed that the slow recombination of photoexcitations stems from Rashba spin orbit coupling, which gives rise to an indirect gap, few tens of meV lower in energy than the direct one. Radiative recombination through the indirect gap is inefficient, but a fraction of the optical excitations are thought to be able to recombine through the direct gap thanks to thermal energy; as a consequence, radiative recombination in such scenario becomes slower at low temperature.
Here, the radiative recombination rates in hybrid perovskites were extracted from the instantaneous intensity of photoluminescence under pulsed excitation. The radiative recombination is measured as a function of temperature in CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ samples, both in polycrystalline and single crystal form. Results are compared against the prediction for three-dimensional Rashba and for direct bandgap semiconductors.

Lead halide perovskite nanowires (NWs) have been demonstrated in lasing with high quantum yields, low thresholds, and broad tunability. Here we show that both pulsed and CW lasing in CsPbBr₃ perovskite NWs originate from strong light-matter interaction and the formation of polaritons in the bottleneck region. Analysis of the cavity modes and their temperature dependence reveals a vacuum Rabi splitting of 0.20±0.01 eV. Time resolved measurement provides a direct view of the the kinetic condensation process and reveals that polariton-polariton scattering is inhibited on the lower polariton branch in a pseudo 1D system. Instead, polariton population in the bottleneck region is determined by cooling from the exciton reservoir via acoustic phonon emission. These findings suggest that lead halide perovskite NWs may serve as low-power CW coherent light sources and as model systems for polaritonics in the strong-coupling regime.

Understanding exciton formation is of fundamental importance for optoelectronic materials, as excitons are the lowest-energy photoexcitations in semiconductors, are electrically neutral and do not directly contribute to charge transport, but can emit light more efficiently than free carriers. Hybrid organic-inorganic perovskites are an emerging class of solution-processed semiconductors with promising properties for photovoltaics and light-emitting devices. However, despite the increasing attention towards these materials, experimental results on the processes of formation of an exciton population are still elusive. Here, an ultrafast differential photoluminescence technique allows revealing the kinetics of exciton formation and dissociation in CH₃NH₃PbBr₃. Geminate excitons, i.e. primary excitons directly created upon photon absorption, are identified and their dissociation into free electron-hole pairs is detected. The formation of a secondary exciton phase through pairing of the initial population of free carriers is demonstrated. The analysis of the generation of secondary excitons provides an estimate of the Langevin factor, the parameter governing the charge-pairing rate. Understanding and controlling the formation of a bright exciton population instead of a highly conductive free carrier population may help the design new hybrid perovskite materials with tailored optoelectronic functionalities.

Electrically-pumped lasing remains an elusive grand challenge for the organic and thin film electronics community. Recently, hybrid organic-inorganic perovskites have emerged as promising gain media for tunable, solution-processed semiconductor lasers, sparking interest in the use of these materials for an eventual diode laser. However, continuous-wave (CW) operation, which is considered a key stepping stone to electrically-pumped lasing, has not been achieved to date. Previous work has shown that lasing from methylammonium lead iodide (MAPbI₃) perovskite in its tetragonal phase at temperatures T>160 K undergoes an as-yet-unexplained lasing death phenomenon within ~100 ns following turn-on of the pump.
Here, we demonstrate that optically-pumped CW lasing can be realized in MAPbI₃ distributed feedback lasers that are maintained below the MAPbI₃ tetragonal-to-orthorhombic phase transition temperature, T~160 K. At a substrate temperature of 102 K, these lasers achieve threshold at a pump intensity of I_th~17 kW/cm² and sustain CW lasing with a clear output beam for over one hour. Importantly, although transient absorption measurements indicate that the bulk of the perovskite films exist in the orthorhombic phase, we find that CW gain and lasing originates from the tetragonal phase at an emission wavelength λ~785 nm. We propose that small tetragonal phase inclusions are photogenerated by the pump and may act as charge carrier sinks within the larger-bandgap orthorhombic phase host matrix, enhancing population inversion in a fashion analogous to host–guest organic– semiconductor gain media and inorganic quantum wells. These results suggest a general strategy to design perovskite gain media for CW lasing and represent a key step toward the ultimate goal of a perovskite laser diode.

Chemically synthesized quantum dots (QDs) can potentially enable a new class of highly flexible, spectrally tunable lasers processible from solutions [1,2]. Despite a considerable progress over the past years, colloidal-QD lasing, however, is still at the laboratory stage and an important challenge - realization of lasing with electrical injection - is still unresolved. A major complication, which hinders the progress in this field, is fast nonradiative Auger recombination of gain-active multi-carrier species [3,4]. Recently, we explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination by “softening” the electron and hole confinement potentials [5]. Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping [6], which has been a long-standing goal in the field of colloidal nanostructures. Further, we apply these dots to practically demonstrated the viability of a “zero-threshold-optical-gain” concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [7]. Such charged QDs are optical-gain-ready without excitation and, in principle, can exhibit lasing at vanishingly small pump levels. All of these exciting recent developments demonstrate a considerable promise of colloidal nanomaterials for implementing solution-processible optically and electrically pumped laser devices operating across a wide range of wavelengths and fabricated on virtually any substrate using a variety of optical-cavity designs.

[1] Klimov, V. I. et al. Optical gain and stimulated emission in nanocrystal quantum dots. Science 290, 314 (2000).
[2] Klimov, V. I. et al. Single-exciton optical gain in semiconductor nanocrystals. Nature 447, 441 (2007).
[3] Klimov, V. I., Mikhailovsky, A. A., McBranch, D. W., Leatherdale, C. A. & Bawendi, M. G. Quantization of multiparticle Auger rates in semiconductor quantum dots. Science 287, 1011 (2000).
[4] Robel, I., Gresback, R., Kortshagen, U., Schaller, R. D. & Klimov, V. I. Universal Size-Dependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals. Phys. Rev. Lett. 102, 177404 (2009).
[5] Y.-S. Park, J. Lim, N. S. Makarov, V. I. Klimov, Effect of Interfacial Alloying versus “Volume Scaling” on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots. Nano Lett. 17, 5607 (2017).
[6] Lim, J., Park, Y.-S. & Klimov, V. I. Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection. Nat. Mater. in press (2017).
[7] Wu, K., Park, Y.-S., Lim, J. & Klimov, V. I. Towards zero-threshold optical gain using charged semiconductor quantum dots. Nat. Nanotechnol. DOI: 10.1038/NNANO.2017.189 (2017).

Three-dimensional ABX₃ perovskite material has attracted immense interest and applications in optoelectronic devices, because of their enabling properties. Recently, Mn²⁺ doping directly into APbCl₃-type three-dimensional (3D) nanocrystals, manifesting host-to-dopant energy transfer, have been reported for LED display applications. Strongly bound excitons in the doped system can enhance the dopant-carrier exchange interactions, leading to efficient energy transfer. Here, we report the simple and scalable synthesis of Mn²⁺-doped (C₄H₉NH₃)₂PbBr₄ two-dimensional (2D) layered perovskites. The Mn²⁺-doped 2D perovskite shows enhanced energy transfer efficiency from the strongly bound excitons of the host material to the d electrons of Mn²⁺ ions, resulting in intense orange-yellow emission, which is due to spin-forbidden internal transition (⁴T₁ → ⁶A₁) with the highest quantum yield (Mn²⁺) of 37%. Because of this high quantum yield, stability in ambient atmosphere, and simplicity and scalability of the synthetic procedure, Mn²⁺-doped 2D perovskites could be beneficial as color-converting phosphor material and as energy down-shift coating for perovskite solar cells. The newly developed Mn²⁺-doped 2D perovskites can be a suitable material to tune dopant-exciton exchange interactions to further explore their magneto-optoelectronic properties.

Bandgap engineering is a process that aims at acquiring desired bandgap from semiconductors for various applications. This is typically done by controlling the structure and composition of the semiconductors. Forming alloys is a powerful technique in achieving continuous bandgap tunability in a wide spectral range. However, this is often limited by the difficulty to synthesize full composition alloys as well as the band bowing effect. Here we show that full composition GaSe_1-xTe_x nanostructures can be synthesized on GaAs (111) substrates by physical vapor transport. As Te content increases, the nanostructures transform from isotropic hexagonal phase to anisotropic monoclinic phase, with both phases coexist at a certain Te content range. Such phase transition causes an anomalous band bowing behavior and gives a large emission wavelength tunability. Our results demonstrate the synthesis of semiconductor alloys far from equilibrium conditions and open up opportunities for bandgap engineering through the phase engineering approach.

The use of hybrid organic−inorganic perovskites in optoelectronic applications are attracting an interest because of their outstanding characteristics, which enable a remarkable enhancement of device efficiency. However, solution-processed perovskite crystals inevitably contain defects that cause hysteresis in perovskite solar cells (PeSCs) and blinking in perovskite light-emitting diodes (PeLEDs). Here, we report remarkable beneficial effects using a new treatment based on amine-based passivating materials (APMs) to passivate the defects of methyl ammonium lead tribromide (MAPbBr₃) through coordinate bonding between the nitrogen atoms and under-coordinated lead ions. This treatment greatly improved PeLEDs performance, with a luminance of 22,800 cd m^-2, a current efficiency of 28.9 cd A^-1, an external quantum efficiency (EQE) of 6.2 %, enhanced photoluminescence (PL), a lower threshold for amplified spontaneous emission (ASE), a longer PL lifetime and enhanced device stability. Using confocal microscopy, we observed the cessation of PL blinking in perovskite films treated with ethylenediamine (EDA) due to the passivation of the defect sites in the MAPbBr₃.

Research on light-emitting devices has developed along with IT industries such as TV and smart phones, focusing on light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs). Recently, as application for general lightings and automotives have expanded, performance has improved in many areas such as luminous efficiency, color rendering, and reliability. However, there is still limitation in the complex process and difficulty in securing price competitiveness. In this study, we demonstrated a light-emitting devices capable of low cost, large area, solution process by using Electrochemiluminescence (ECL) and organic-inorganic hybrid perovskite compound, which are candidates of next generation light sources. All devices can be applied not only to ITO/Glass electrodes but also to flexible electrode such as ITO/PET and silver nanowire network (AgNWs) embedded substrates expected to be applied to flexible and wearable devices in the future. In this work, the luminescence characteristics of red-orange region were confirmed by using ECL of ruthenium complex, and flexible ECL devices were fabricated. In addition, the luminescence characteristics of the green region were confirmed using a perovskite compound of MAPbBr₃ which is attracting attention as a next generation light source, and its value as a next generation light-emitting devices was confirmed by applying it to a large area flexible device.

Computational modeling of photoexcited dynamics in perovskite quantum dots materials doped by low concentration transition metal ions of Mn(2+), demonstrates how excitation thermalizes to the doping site and participate in the radiative PL processes independent on existence of surface trap sites. The computation is based on dissipative propagation of the excited state by Redfield equation of motion. The nonadiabatic transitions betweeen electronic states are facilitated by nuclear motion and are computed in the basis of non-collinear spin DFT,[1] which accounts for the spin-orbit interactions. The quantum yileld is numerically estimated by comparing rates of radiative and nonradiative recombination pathways [2]The presence of surface traps and surface charge transfer[3] is found to have minimal influence of the doping-related PL.

[1] Barth, U. v.; Hedin, L., A local exchange-correlation potential for the spin polarized case. i. J. Phys. C: Solid State Phys. 1972, 5 (13), 1629-1642.
[2] Vogel, D. J.; Kryjevski, A.; Inerbaev, T.; Kilin, D. S., Photoinduced Single- and Multiple-Electron Dynamics Processes Enhanced by Quantum Confinement in Lead Halide Perovskite Quantum Dots. J. Phys. Chem. Lett. 2017, 8 (13), 3032-3039.
[3] Forde, A.; Kilin, D. Hole Transfer in Dye-Sensitized Cesium Lead Halide Perovskite Photovoltaics: Effect of Interfacial Bonding. The Journal of Physical Chemistry C 2017, 121, 20113-20125.

In recent years, colloidal quantum-dots based light-emitting diodes (QD-LEDs) have been considered as the attractive display device because of remarkable electrical/optical characteristics of colloidal quantum dots. QD-LEDs are of particular interest due to their wide-range color tunability, high brightness and good color purity by narrow emission bandwidth. Challenges remain, however, in achieving the necessary multilayer device structures using printing process. In this study, quantum dots were printed by electrohydrodynamic (EHD)-Jet spray technology and were applied to all solution-processed QD-LEDs. The QD-LED has a structure of ITO/PEDOT:PSS/PVK/ EHD-sprayed QDs/ZnO/Al. Core/shell type red QD of CdS/ZnS/CdSe and ZnO NPs as the carrier transporting layer were synthesized using solution mediated process according to the literatures. Sprayed QD pattern was controlled with the various parameters to obtain uniform surface morphology. The optimized QD-LED device showed a luminance of 1,980 cd/m², current efficiency of 1.07 cd/A, and EQE of 1.01 %.

Organic-inorganic hybrid perovskites have been widely investigated for application in light-emitting devices (LEDs) due to their adjustable optical bandgap by changing the composition of the halide anions. Moreover, perovskites can be easily fabricated by solution process, which enables low-cost and large-area application. However, the perovskite materials are easily crystallized at low temperature, resulting in poor surface coverage with large-sized crystals. These non-uniform perovskite films induce high leakage current and poor contact with the adjacent layers. In addition, large-sized crystals effectively diffuse the generated excitons, which reduce the recombination probability. Thus, smaller-sized crystals are favored to limit the diffusion length of excitons or charge carriers and limit the dissociation of excitons into charge carriers. Therefore, producing uniform perovskite films with minimized grain size is an essential factor to achieve high performance perovskite LEDs (PeLEDs).
In this report, to improve the quality of perovskite films, a methylammonium lead bromide (MAPbBr₃)-polymer composite film was fabricated by using a gas-assisted crystallization (GAC) method. N₂ gas blowing during spin-coating process induced ultra-fast removal of the solvent, resulting in a high degree of supersaturation and a formation of a large number of nuclei, which promoted the growth of nano-sized grains. The optimized MAPbBr₃-polymer composite film deposited by GAC method showed a uniform surface coverage with a grain size, thickness, and root mean square roughness of 79.3 nm, 509.4 nm, and 31.7 nm, respectively. A maximum luminance of 6800 cd/m² and a maximum current efficiency of 1.12 cd/A were measured from the PeLED with MAPbBr₃-polymer composite film deposited by GAC method. Finally, the same device with 1 cm²-area pixel exhibited a strong homogenous green emission centered at 528 nm. Therefore, it is believed that a uniform MAPbBr₃-polymer composite film deposited by GAC method can satisfy high performance and large area application in perovskite-based optoelectronic devices.

Quantum dots(QDs) have been investigated as lighting material because their inherent properties, such as controllable color depending on the size, narrow emission bandwidth and compatibility with solution processing. Recently, quantum dot light emitting diode (QLED), which uses such QDs as emissive layer (EML), has exhibited high brightness and wide color gamut, which is comparable to those of the commercialized organic light emitting diode (OLED). Despite the excellent electro-optical characteristics of QLED, it is still necessary to improve its efficiency and lifetime for commercialization. In a conventional QLED structure, which has a structure of transparent anode electrode/hole injection layer (HIL)/hole transport layer (HTL)/QD EML/electron transport layer (ETL)/cathode electrode with high reflectance and low work function, one of the main causes for the decrease in efficiency is exciton quenching due to metal diffusion from the electrode to the emissive layer. Especially, when we use typical combination of indium-tin oxide (ITO) anode and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) HIL, the metal ion diffusion into the polymer layer is initiated during the conventional device fabrication process because sulfur contained in PEDOT:PSS etches the ITO anode. The metal ions, such as indium or tin, easily migrate into EML by high electric field on the operation of QLED.
In this work, we report the enhanced efficiency of conventional structured QLED device by inserting a Al₂O₃ layer between PEDOT:PSS HIL and ITO anode to prevent reaction of PEDOT:PSS with ITO. Because Al₂O₃ has a very large band gap and a deep valence band level, the thickness of Al₂O₃ should be minimized not to hinder the hole injection while the Al₂O₃ film should uniformly cover ITO substrate in order to block the reaction of PEDOT:PSS with ITO. So we adopted atomic layer deposition (ALD) process, which makes it possible to control the thickness of Al₂O₃ at a monolayer level by using trimethyl aluminium (TMA) and H₂O precursors at low temperatures. In order to optimize the thickness of Al₂O₃ barrier layer, we fabricated the conventional structured QLED devices with various thickness of Al₂O₃ barrier layer. As expected, the device with 1 monolayers of Al₂O₃, which is formed by 2 cycles of ALD process, showed the highly enhanced current efficiency and the efficiency gradually decreased as we increase the thickness of Al₂O₃ layer. The maximum value of the current efficiency was measured to be 33 cd/A, which is enhanced by 80 % compared with the device without Al₂O₃ barrier layer. We believe this result would contribute to commercialization of QLED by enhancing efficiency and improving lifetime of QLED.

Organic-inorganic hybrid perovskites have received great attention due to their high photoluminescence quantum efficiencies, good charge mobilities, high color purity and tunable bandgap. The external quantum efficiency (EQE) of near-infrared (NIR) perovskite light-emitting diodes (PeLEDs) have reached 11.7%.¹ However, in conventional PeLEDs, the light extraction efficiency is only ~20% due to the high refractive index of perovskites, resulting in majority of the light generated in the device is confined in the active layers and indium tin oxide layer. Here, we demonstrate a solution-processed microstructure in PeLEDs by adding additive to the perovskite precursor. The microstructure can significantly increase the light out-coupling efficiency by self-assembling low-index grid in the perovskite layer, resulting in a record EQE of NIR PeLEDs. Moreover, the PeLEDs based on this structure also have good stability, with a device lifetime (time to half of the initial brightness) of about 20 hours under a current density of 100 mA cm^-2. The results demonstrate that the self-assembled microstructure in perovskites provide promise for further development in solution-processed, high performance LEDs.

[1] Wang, N. et al. Perovskite light-emitting didoes based on solution-processed self-organized multiple quantum wells. Nat. Photonics 10, 699-704(2016).

In recent years, metal-halide perovskites have attracted much attention as promising materials for optoelectronic devices such as light-emitting diodes, photovoltaic cells and lasers. Especially, Pb-based perovskites have demonstrated outstanding achievements as emitters in perovskite light-emitting diodes (PeLEDs). However, the toxicity of Pb and the inherent instability of these Pb-based perovskites are the main obstacles against large-scale and commercial application. Sn-based perovskites are currently emerging as an alternative to the Pb-based perovskites but researchers have difficulty developing efficient Sn-based PeLEDs because they have metallic behavior arising from the self-oxidation from Sn²⁺ to Sn⁴⁺ and produce poor film morphology with incomplete surface coverage. Here, we employed Pb-free perovskite, CsSnBr₃, as an emitter of PeLEDs to avoid the toxicity issue and proposed two strategies to overcome the limitations of the Sn-based perovskites. First, we introduced Cs instead of the widely-used organic cations such as methylammonium (MA) or formamidinium (FA) because it acts as a strong oxygen binder which can suppress the oxidation of Sn²⁺ to Sn⁴⁺. Furthermore, the additional introduction of SnF₂ to the perovskite precursor reduced the Sn vacancies, and correspondingly the metallic conductivity. We also improved the perovskite film morphology (i.e., surface coverage) by optimizing the SnF₂ content in the perovskite precursor. We analyzed the crystal structures of CsSnBr₃ using x-ray diffraction (XRD) and observed a sharp and narrow photoluminescence (PL) spectrum. Finally, we demonstrated bright Pb-free PeLEDs based on CsSnBr₃.

The efficiency of the organic light emitting diode (OLED) can be remarkably improved by introducing a hole injection layer (HIL) between the anode and the active layer. Typical examples of TMD are MoS₂, WS₂, and TiS₂. Applying TMD can increase efficiency, but it will also improve efficiency by applying graphite oxide (GO) together. TMD nanosheets are manufactured by the exfoliation method using ultrasonic treatment, and GO is synthesized by the modified Hummers method. TMD nanosheets are synthesized to thicknesses and sizes of about 3.1-4.3 nm and over 100 nm, and GO is synthesized at 400 nm to several micrometers. The large-sized GO can completely cover the surface of the indium tin oxide (ITO) film to reduce the roughness. OLEDs with GO HIL show very high highest maximum power efficiency (PE^max) due to very low roughness. These results indicate that the TMD-GO composites are applied to the HIL layer in OLEDs and show high efficiency.

Acknowledgement
This research was supported in part by National Research Foundation of Korea (NRF) grants provided by the Korean government (MSIP) (Nos 2015K1A3A1A59073839, 2017H1D8A1030599, 2017K1A3A1A67014432) and in part by Korea Agency for Infrastructure Technology Advancement grant funded by Ministry of Land, Infrastructure and Transport (17IFIP-B133622-01).

A new class of multiple quantum well (MQW) perovskites light-emitting diodes (PeLEDs) has been demonstrated, which have high external quantum efficiency (EQE) and good stability, indicating MQW perovskite is promising for realizing high performance PeLEDs. However, as other LEDs, both the EQE and energy conversion efficiency of MQW PeLEDs decrease monotonically with increasing current density after peak efficiency, which is detrimental for displays requiring efficient operation at high brightness. In particular, in MQW structures, the charge carriers are concentrated in the quantum wells with lower bandgaps, which only occupy a small portion of the MQW film, thereby the local carrier density is much higher than the total carrier density. We introduce mixed-cations to control the constitution of MQW perovskites, which increases the width of quantum wells and leads to reduced efficiency roll-off. Moreover, due to the stable structure of mixed-cations, the device lifetime (half of the initial brightness, T₅₀) reaches ~30 hr under a constant current density of 10 mA cm^-2.

Significant progress has been achieved on lead halide perovskites in opto-electronic
applications, with 22.1% power conversion efficiency reached in solar cells and 11.7% external
quantum efficiency (EQE) in light-emitting diodes (LEDs). Since the first report of cadmium
selenide (CdSe) quantum dot (QD) LED, various candidates have been considered as potential
emitting materials such as cadmium sulphide (CdS) and cadmium telluride (CdTe). However,
it is only recently that the use of lead halide perovskites in the form of QDs emerged. Cesium
lead halide (CsPbX₃, X = Cl, Br, and I) QDs were successfully synthesized by hot injection
method and exhibited promising properties such as high quantum yields (50−90%), short
radiative lifetime (1−29 ns), tunable emission wavelength through the entire visible spectra,
and narrow emission (FWHM ~20 nm). With such properties, the interest in CsPbX₃ QDs grew
considerably and their incorporation in LEDs was shortly reported. While the performances of
CsPbX₃ QD LEDs have been rapidly improved, with EQE reaching 6.3%, there are several
issues which cause instability of the devices. Most importantly, the widely used organic charge
transport layers have poor stability and cannot efficiently protect the perovskite layer from
moisture in the atmosphere.
In this research, we chose to approach the issue by focusing on metal oxide semiconductors
as charge transport layers. It is known that metal oxides have much higher carrier mobility
through intrinsic or extrinsic doping and excellent stability compared to organic materials.
Therefore, metal oxide charge transport layers play an important role in the effective protection
of the perovskite layer but also possess the adequate energy-levels and charge transport
properties to improve the efficiency. Here, we investigate the light-emission characteristics and
stability of CsPbX₃ QD LEDs by using nickel oxide (NiO), molybdenum oxide (MoO₃) as the
hole transport layer and zinc oxide (ZnO), titanium oxide (TiO₂) as the electron transport layer.
In addition, CsPbX₃ QDs were synthesized by the supersaturated recrystallization method. This
method allows the synthesis to be performed in air and at room temperature, as opposed to the
hot injection process which used controlled atmosphere and higher temperatures. The CsPbX₃
(X = Cl, and Br) QDs exhibited tunable emission wavelengths between 400 and 520 nm, with
full width half maximum (FWHM) between 15 and 20 nm. Furthermore, we offer important
energy band engineering guidelines for device design with a view to accomplishing both highly
efficient and stable opto-electronic applications.

Metal halide perovskites (MHPs) are attracting materials in the field of light-emitting diodes (LEDs), owing to their advantages on light emission such as extremely high color purity (full width at half maximum ≈ 20 nm), easily tunable bandgap by compositional engineering and low material cost[1]. Especially, MHPs have ionic characteristics originating from ionic soft bonding of metal-halide inorganic networks[2]. Because of the soft bonding, various defects such as halide vacancy and cation vacancy can be easily formed and cause nonradiative recombination and structural degradation which decreases luminescence efficiency and stability. Furthermore, during operation of perovskite LEDs, applied electric field can induce ion migration along grain boundaries, and thereby generate more defects and facilitate degradation of perovskite LEDs. Thus, passivation of defect sites at grain boundary and interfaces is necessary to achieve highly-efficient stable perovskite LEDs[3].
Here, we present perovskite LEDs with increased efficiency and stability achieved by defect passivation using amine additives. Various amine additives with different functional groups such as amino group (-NH₂), phosphono group (-P(=O)(OH)₂), or fluoro group (-F) were incorporated into perovskite films or spin-coated on the perovskite films; the additives passivated defect sites of perovskite films and thus dramatically increased steady-state photoluminescence (PL) intensity and PL lifetime. The nonradiative recombination paths caused by ionic defects (activation energy ≈ 0.6~0.8 eV) were passivated, as evidenced by temperature-dependent PL measurement. Also, efficiency and operating stability of perovskite LEDs with amine additives were much better than those of perovskite LEDs without amine additives. This study will suggest a new strategy to improve efficiency and stability of perovskite LEDs.

References
1. Cho, H. et al. (2015). Overcoming the electroluminescence efficiency limitations of perovskite light-emitting diodes. Science. 350, pp. 1222–1225.
2. Miyata, K., Atallah, T. L. and Zhu, X. (2017). Lead halide perovskites : Crystal-liquid duality , phonon glass electron crystals , and large polaron formation. 3, pp. 1–10.
3. Yun, J. S. et al. (2016). Critical role of grain boundaries for ion migration in formamidinium and methylammonium lead halide perovskite solar cells. Adv. Energy Mater., 6, pp. 1–8.

Metal-halide perovskites (hereafter, perovskite) are promising materials for the application as a light emitter of light-emitting diodes (LED) based on their narrow emission spectrum and the tunable bandgap. However, the perovskite with 3D lattice structure has been considered inefficient to confine excitons leading to dissociation, which is unfavorable for the development of efficient perovskite LEDs. While the 2D or quasi-2D perovskite where organic ammonium (OA) with long chain (e.g. phenethylammonium) is incorporated can effectively confine excitons, the insulating OA in a certain preferred orientation limits efficient charge transfer and transport in the perovskite film. In this work, we induced structural modulation of quasi-2D perovskite, (PEA)₂(MA)_m-1Pb_mBr_3m+1 by applying nanocrystal pinning (NCP) process. The structurally modulated quasi-2D perovskite had random orientation that was confirmed by Grazing Incidence X-ray Diffraction (GIXRD) analysis. Because the modulated quasi-2D perovskite can have connected inorganic layers, charge transfer and transport can easily occur leading to efficient radiative recombination maintaining exciton confinement effect. We developed perovskite LEDs with the modulated quasi-2D perovskite emitter by controlling the ratio between phenethylammonium bromide (PEABr) and methylammonium bromide (MABr). The device with the modulated quasi-2D perovskite, (PEA)₂(MA)₂Pb₃Br₁₀ had the maximum current efficiency of 20.18 cd/A and maximum external quantum efficiency of 4.98 %

Recently, metal halide perovskites (MHPs) have been studied as good light emitting materials due to many advantages such as high color purity (full width at half maximum ~ 20 nm), solution processability and low material cost. Initial perovskite light-emitting diodes (PeLEDs) were demonstrated using perovskite polycrystalline (PC) bulk films. Although perovskite PC films have shown dramatically improved electroluminescence efficiency in PeLEDs, they have still suffered from low photoluminescence quantum efficiency (PLQE) ~ 36% [1]. Perovskite nanoparticles (NPs) can be another good candidate for emission layer in efficient PeLEDs due to their high PLQE ~ 90% and facile synthesis method [2-4]. However, multi-coatings of perovskite NP solution more than 5 cycles, which are needed to fabricate the uniform NP films, can induce aggregation of NPs and thin thickness of perovskite NP films (< 30 nm) can induce leakage current and limit the luminescence efficiencies in PeLEDs.
Here, we report perovskite PC/NP bilayers as emission layers in PeLEDs. Perovskite PC/NP bilayers can reduce the leakage current in devices because of the thick PC films under the NP films. Perovskite PC/NP bilayers also do not need multi-coating of perovskite NP solutions to achieve uniform film morphology and thus, prevent aggregation of NPs. With these strategies, we demonstrated highly bright perovskite PC/NP bilayers, and fabricated efficient PeLEDs based on perovskite PC/NP bilayers. This new emitting layer system reported here provide the new ways to improve the luminescence efficiency of PeLEDs and introduce a new research direction of PeLED

Reference

[1] H. Cho, S.-H. Jeong, M.-H. Park, Y.-H. Kim, C. Wolf, C.-L. Lee, J.H. Heo, A. Sadhanala, N. Myoung, S. Yoo, S.H. Im, R.H. Friend, T.-W. Lee, Science 2015, 350, 1222.
[2] Y.-H. Kim, H. Cho, T.-W. Lee, Proc. Natl. Acad. Sci. U S A 2016, 113, 11694.
[3] Y.-H. Kim, C. Wolf, Y.-T. Kim, H. Cho, W. Kwon, S. Do, A. Sadhanala, C.G. Park, S.-W, Rhee, S.H. Im, R.H. Friend, T.-W. Lee, ACS Nano 2016, 11, 6586.
[4] Y.-H. Kim, G.-H. Lee, Y.-T. Kim, C. Wolf, H.J. Yune, W. Kwon, C.G. Park, T.-W. Lee, Nano Energy 2017, 38, 51

Metal halide perovskites (MHPs) have been studied as active layers in solar cells (SCs) because of its superior optical and electrical properties such as large absorption coefficient ( 5×10³ cm^-1 ~ 5×10⁴ cm^-1 for CH₃NH₃PbI₃) and low exciton binding energy (37–75 meV for CH₃NH₃PbBr₃ and 29–50 meV for CH₃NH₃PbI₃). However, MHP polycrystalline (PC) bulk film based SCs suffered poor reproducibility because surface morphology of MHP PC films were severely affected by the environmental conditions during film formation process such as temperature, atmosphere and surface energy of under-layer.
Here, we report ligand-engineered MHP nanoparticles (NPs) and MHP NP SCs based on uniform MHP NP films. Ligand-engineered MHP NPs which had short organic ligands increased the charge extraction/transfer characteristics in MHP NP films and short-circuit current in MHP NP SCs. We also fabricated the uniform CH₃NH₃PbBr₃ NP/CH₃NH₃PbI₃ NP bilayers by using non-wet stamping process and MHP NP SCs based on them. These bilayer NP films can increase the power conversion efficiency (PCE) of MHP NP SCs because different band gaps of CH₃NH₃PbBr₃ ~ 2.2 eV and CH₃NH₃PbI₃ ~ 1.5 eV increases absorption wavelength, short-circuit current and open-circuit voltage. We also confirmed the effects of ligand engineering and bilayers by conducting photoluminescence (PL) and UV-vis absorption measurements. Our works demonstrated here showed the effective ways to increase the PCE in perovskite NP SCs.

Metal halide perovskites (MHPs) have attracted attention as light emitters due to high color purity (full width at half maximum ~20 nm), low material cost, solution processibility and tunable bandgap. However, perovskite light-emitting diodes (PeLEDs) suffer from poor stability due to degradation by moisture¹ and ion migration of perovskite component². One of the method to overcome these problems is polymer incorporation method, because polymer can effectively prevent ion migration and degradation by moisture. Even though current efficiency (CE) of 21.38 cd/A and external quantum efficiency (EQE) of 4.76% were achieved by polymer incorporation method³, studies concerning ion migration and air stability are still lacking.
Here, we present highly efficient and stable PeLEDs achieved by incorporating polymer additives and investigation regarding ion migration and air stability. Various kinds of polymers that have lone pair electrons were used to passivate Pb²⁺ atoms which act as non-radiative recombination center. To identify whether Pb²⁺ atoms were passivated or not, photoluminescence (PL) and transient photoluminescence (TRPL) were implemented. PL intensity and PL lifetime were dramatically enhanced with polymer additives, which can be attributed to passivation of trap states. To further investigate interaction between polymer additives and perovskites, fourier-transform infrared spectroscopy (FT-IR) and ultraviolet photoelectron spectroscopy (UPS) were implemented. Also, as the polymer impedes the diffusion of perovskite precursor during crystallization, the grain size was decreased, confirmed by scanning Electron Microscopy (SEM) and atomic force microscopy (AFM). This result can also lead to improved luminescent property, by improving special confinement of exciton or free carrier from decreased grain size. Considering the low stability of perovskite emitters, time-dependent electroluminescence and J-V-L measurement were also conducted to figure out effect of polymer additives on air stability and ion migration, respectively. Finally, the device efficiency can be improved by polymer additives, confirmed by J-V-L characteristics. In conclusion, our work has shown that polymer incorporation method can improve device performance and stability. Thus it will contribute to development and commercialization of PeLEDs.

Reference

1. Yang, S., Wang, Y., Liu, P., Cheng, Y., Zhao, H. J., & Yang, H. G. (2016). Functionalization of perovskite thin films with moisture-tolerant molecules. Nat. Energy, 1(2), 15016.
2. Azpiroz, J. M., Mosconi, E., Bisquert, J., & Angelis, F. D. (2015). Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation. Energy Environ. Sci., 8(7), 2118-2127.
3. C. Wu, Y. Zou, T. Wu, M. Ban, V. Pecunia, Y. Han, Q. Liu, T. Song, S. Duhm, B. Sun. (2017). Improved Performance and Stability of All-inorganic Perovskite Light-Emitting Diodes by Antisolvent Vapor Treatment. Adv. Funct. Mat, 27, 1700338.

Quantum dot (QD) light-emitting diodes (QLEDs) emerge as a new device for flat-panel displays with superior qualities, such advantages as a narrow emission spectrum (high color purity), colloidal stability, and low cost solution processibility. Since the first demonstration of the efficient QLED, intensive researches have been carried out to improve the device performance such as luminous efficiency, a driving voltage and a lifetime. However, fewer researches on the degradation mechanism of QLEDs have been reported than their efficiency or driving voltages. In this work, we investigated the effect of degradation in the red-emitting QLEDs with an inverted structure of ITO/ZnO/QD/CBP/MoO₃/Al by using an impedance spectroscopy with the applied external electric field. Both the electroluminescence characteristics and the analysis of the charge variation inside the devices before and after constant current aging will be presented.

Given their distinct structural and electronic properties, two dimensional materials, such as transition metal dichalcogenides (TMDs), have received increasing attention due to their potential applications in a new generation of 2D opto-electronics. Laterally connected TMD materials with distinct chemical composition can form heterostructures that are the building blocks for developing 2D p-n diodes, light-emitting diodes, photovoltaic devices, and transistors. A large scale and controlled growth of this kind of heterostructures is necessary to exploit the full potential of TMD materials. Here we report the successful synthesis of 2D heterostructures composed of bilayer and multilayer 2D semiconductor that are laterally connected (MoSe₂-WSe₂ and MoS₂-WS₂), via a modified Chemical Vapor Deposition (CVD) process. The proposed approach allows to in situ produce single- and multi-junction heterostructures by only changing the carrier gas supplied during the growth. For optimal growth conditions, the grown heterostructures can be found over large areas on the substrate surface. Raman and Photoluminescence spatial mapping confirm the chemical and optical homogeneity of the distinct TMD domains in the heterostructures. PL line scan revealed the effective modulation of the optical bandgap across the heterostructures, as well as the interfaces quality. The grown structures have great potential for developing 2D heterogeneous materials with higher degree of complexity such as superlattices and one-dimensional periodic quantum wells.

Laser refrigeration of semiconductors is an emerging field of science with exciting possible applications, one of which is optical refrigerator operating in the sub-10 K regime. Recently, it has been shown that hybrid perovskites can exhibit laser cooling through photoluminescence (PL) up-conversion. Here, we investigate PL up-conversion in CsPbBr₃ nanocrystals (NCs) using temperature-dependent, frequency-dependent, and excitation intensity-dependent up-conversion measurements. We conclude that PL up-conversion is phonon-assisted and demonstrate first observation of PL up-conversion from CsPbBr₃ NCs at both the ensemble and single-NC levels. Ensemble up-conversion efficiencies are estimated to be on the order of 75% for a ΔE = 23 meV excitation detuning into the NC bandgap.

Recently, Organic-Inorgainc hybrid perovskite materials have much attracted due to their excellent optical property. Perovskite LEDs have advantages such as cheap and easy process, color tunability and vivid color. For achieving highly efficient LED, Reduced injection barrier is the key. Highly efficient perovskite light-emitting diodes (PeLEDs) were demonstrated by a solution processable PEDOT:PSS and MoO₃ composite (PEDOT:MoO₃ composite) layer as hole transport layer. As adding different concentration of MoO₃ powder in PEDOT:PSS dispersion, the work function of PEDOT:MoO₃ composite film increases linearly. The PEDOT:MoO₃ composite film reduced the energy barrier between active layer and hole injection layer, leading to dramatically enhance the luminance and luminance efficiency of PeLEDs. The optimized PeLEDs with a PEDOT:MoO₃ composite layer improved the external quantum efficiency (EQE) of 220% and maximum luminous efficiency of 210 % compared to reference PeLEDs with a pristine PEDOT:PSS layer.

Having access to the colloidal quantum dot (QD) concentration is of great importance to avoid lengthy trial-and-error procedures in fundamental studies and practical applications of QDs. An efficient way to determine the QD concentration is based on the Beer-Lambert law (A= εcl). If the molar extinction coefficients (ε) of those QDs were known, the concentration (c) of suspended QDs can be readily determined by means of absorption spectrometry analysis. To date, the size dependent extinction coefficient properties have been focusing on binaries (Cd-, Pb- and Ag-chalcogenides, InP and InAs), while less attention has been paid to ternary QDs.

In this work, we quantitatively investigate the size dependence of the band gap, of the extinction coefficients, and of the absorption cross-section of ternary CuInS₂ QDs over a wide size range (2.7–6.8 nm). We adopt partial cation exchange synthesis protocol to produce nearly stoichiometric and spherical CuInS₂ QDs (polydispersity less than 10%) that are comparable in size, size distribution, shape and composition. The size dependence of the band gap allows us to construct a sizing curve valid from 2.7 to 6.8 nm. The extinction coefficients and absorption cross-section per CuInS₂ formula unit both at high energies (3.1 eV) and at energies around the band gap are analyzed. The results demonstrate that the extinction coefficients of CIS QDs scale with their volume at high energies and the extinction coefficients at first excitonic transition energies follow a power law with a factor of 2.45. We notice that the absorption cross-section per formula unit at high energies (i.e., far above the band edge) is constant, suggesting that the use of extinction coefficients at high energies is better suited for analytical purposes.

Flexible, wearable and stretchable light-emitting fibers are of significance to meet the escalating requirements of increasing complexity and multifunctionality of smart electronics. Here, we report a stretchable alternating current light-emitting fiber by a low-cost and all solution-processed scalable process. The fiber device provides high stretchability, decent light-emitting performance with excellent stability and nearly zero hysteresis. It can be stretched up to 80% strain. Our fiber device kept a stable luminance for over 10,000 stretch-release cycles with 360 degree whole-area light emission. The mechanical stretchability and optical stability of our fiber device provides new possibilities towards next generation stretchable displays, electronic textiles, advanced biomedical imaging and lighting, conformable visual readout on arbitrary shapes, and novel health-monitoring devices.

Red and near-infrared (NIR)-emitting materials have collected rapidly growing attention because of their emerging applications in night-vision, information-secured displays, bio-imaging and diagnosis, etcs. However, in the case of conventional red emitting materials, quantum efficiency decreases as the wavelength become longer due to the lower energy gap, and their LEC devices exhibit relatively lower brightness as compared with materials emitting at shorter wavelengths. In this presentation, we report synthesis of red-emitting Ir complex (IR-red) with 40% improvement in quantum efficiency using quinoline (TQ) as a ligand. As synthesized IR-red was examined as a red light emitting dopant and used as guest molecules into a highly emissive but yellow light emitting Ir complex (IR-yellow). A host-guest system with IR-red and IR-yellow increased the emission output in red region and afforded a red-emitting LEC device with brightness of 300cd / m^² or more. Further optimization of doping solution and a mechanism for drastic increase in LEC emission in red region will be discussed.

Fluorescent materials have attracted increasing interest for their use in new generation of low energy lighting.^[1,2] However, the vast majority of the fluorescent materials currently employed in commercial LED lighting are based on rare earth elements (REE)– particularly in the yellow, orange and red regions of the emissive visible spectrum. The US Department of Energy currently has the development of new low 600 nm wavelengths fluorescent materials as a priority. Furthermore the location and the extraction of rare earth elements remain major geopolitical, environmental, and economic issues of the near future. One solution to these issues would be to develop cheap, sustainable, REE-free, fluorescent materials that exhibit excellent durability and reasonably high emission efficiencies in the key 590-620 nm region.
The species we have identified for this purpose is the simple [S₂⁻] anion. This anion occurs naturally, trapped in certain minerals, including sodalites and scapolites. The low level of [S₂⁻] present produces a very strong orange/yellow/red fluorescence when irradiated with wavelengths between 350 and 450 nm. ^[3–6] Our work concerns the synthesis of new materials containing [S₂⁻] and optimization of their fluorescence efficiencies to levels equivalent to, and better than, those of the natural minerals. In particular we have investigated the synthesis and properties of sodalite, Na₈[AlSiO₄]₆(Cl,SO₄,S₂), with various ratio of sulfate, disulfide and chloride anions. The results of this systematic optimization of the fluorescent properties of Na₈[AlSiO₄]₆(Cl,SO₄,S) compositions will be presented.
Optimized materials show a very strong fluorescence, following 390 nm excitation, centred on 615 nm with quantum yields up to 40% - comparable to natural mineral samples. Modifications to the exact sodalite composition also allow the emission frequency maximum to be tuned between 585 and 620 nm. We continue to develop our understanding of the [S₂⁻] host structures and thus the relationships that exist between the different crystallographic structures, composition and their light emission properties. Full development of these materials should allow the design of very cheap, sustainable, fluorescent materials with high efficiencies and the additional ability to tune the emission over a large wavelength range in the yellow–orange-red region.
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[5] A. Sidike, I. Kusachi, S. Kobayashi, K. Atobe, N. Yamashita, Phys. Chem. Miner. 2008, 35, 137–145.
[6] M. Kaiheriman, A. Maimaitinaisier, A. Rehiman, A. Sidike, Phys. Chem. Miner. 2014, 41, 227–235.

Microdisplay with high resolution, brightness, and efficiency with long-term stability and reliability are highly required for advanced display technologies. Inorganic semiconductors LEDs best suits this purpose because they can emit very high density of light from a small area and they have very high efficiency and long-term stability. To use inorganic LEDs for display applications, various lift-off and transfer techniques of inorganic thin films grown on single crystal substrates, such as sapphire or Si, were developed. However, achieving display devices using inorganic semiconductor thin films is still very challenging because of the limited size and high manufacturing cost of the single crystal substrates, as well as the complicated processes required for lift-off and assembly. To resolve this problem, growths of inorganic semiconductor nanostructures and thin films on graphene substrates have recently been proposed, since graphene has great scalability and extremely thin layered hexagonal lattice structure as an excellent substrate for GaN growth. Moreover, the inorganic semiconductors prepared on large-area graphene can be transferred easily to or grown on elastic substrates to meet the flexibility demand. Here, we suggest a method of fabricating ultrathin, high-resolution inorganic microdisplay based on individually addressable GaN microdisk LED arrays grown on graphene dots. Most of the GaN microdisks prepared by epitaxial lateral overgrowth on patterned graphene microdots were single-crystalline. Furthermore, the discrete and small microdisk LED arrays in the microdisplay also ensured that stress and strain were minimal under various bending conditions, thereby providing excellent flexibility. [1]
Here, we report on the fabrication and EL characteristics of ultrathin and individually addressable GaN microdisk LED arrays grown on graphene dots for microdisplay applications. GaN microdisks were prepared by epitaxial lateral overgrowth on patterned graphene microdots on SiO₂/Si substrates using MOVPE. After preparing the GaN microdisk arrays, p-GaN and InGaN/GaN multiple quantum well, and n-GaN layers were heteroepitaxially grown on the surface of the GaN microdisks. Ultrathin layers composed of GaN microdisk LED arrays on graphene dot were prepared by coating a polyimide layer and lifting-off the entire layers from the substrate. Then, single-walled carbon nanotubes (SWCNTs)/Ni/Au and SWCNTs/Ti/Au multiple electrode lines were formed on the top and bottom surface of GaN microdisk arrays in an aligned manner and crossing each other. The electrical and optical characteristics of the individually addressable GaN microdisk array on graphene dots were investigated by measuring their I–V curves and EL characteristics at various bending conditions. We also confirmed that the ultrathin micro-LED display worked reliably under flexible conditions and continuous operation mode.
[1] K. Chung et al., Adv. Mater. 28, 7688-7694 (2016)

Spectral conversion has been widely applied in the field of luminescence and is potentially beneficial in photovoltaics. Among different approaches, down conversion shows a theoretical potential to enhance the limit of energy conversion efficiency of a single-junction solar cell from about 32% to 40%. For CIGS solar cells, the main current loss in the short wavelength region (<550 nm) comes from absorption of the CdS buffer layer with a relative small bandgap. As the spectral response of CIGS has an optimal efficiency in the longer wavelength (550-1000 nm) region, spectral down converter that converts photons with wavelength shorter than 550 nm to longer wavelength at a high luminescent efficiency has the potential to reduce the above current loss for CIGS solar cells. In this work, transparent ceramics of Y₃Al₅O₁₂: Ce³⁺have been fabricated and applied as the down convertor for CIGS solar cells. Their absorption and photoluminescence properties have been investigated. The device conversion efficiencies of CIGS solar cells with and without the down converter have been measured and compared. The results show that optimized Y₃Al₅O₁₂: Ce³⁺ceramics may bring about 9% enhancement to the current density, which enhanced the efficiency of CIGS solar cells from 17.3% to 18.9%. Compared with a pure antireflection coating with an enhancement of ~6%, the great enhancement in down-converter device demonstrates that the Y₃Al₅O₁₂: Ce³⁺ceramics has a potential to further improve the performance of CIGS solar cells.

We report controllable anisotropic light emission of photons originating from vertically aligned transition dipole moments in spun-cast fi lms of CsPbBr3 nanocubes. By depositing films of nanocrystals on precoated substrates we can control the packing density and resultant radiation pattern of the emitted photons. We develop a technical framework to calculate the average orientation of light emitters, i.e., the angle between the transition dipole moment vector (TDM) and the substrate. This model is applicable to any emissive material with a known refractive index. Theoretical modeling indicates that oriented emission originates from an anisotropic alignment of the valence band and conduction band edge states on the ionic crystal lattice and demonstrates a general path to model the experimentally less accessible internal electric field of a nanosystem from the photoluminescent anisotropy. The uniquely accessible surface of the perovskite nanoparticles allows for perturbation of the normally isotropic emissive transition. The reported sensitive and tunable TDM orientation and control of emitted light will allow for applications of perovskite nanocrystals in a wide range of photonic technologies inaccessible to traditional light emitters.

Developing narrowband light sources in the mid- and far-infrared spectral regions has long been a major scientific and engineering challenge. Developments in quantum cascade laser (QCL) technology has been successful in providing single frequency sources in the mid and far infrared. However, these devices have limitations, most notably the lack of emission between 15-50µm, but additionally high-power consumption and limited tunability for far-infrared QCLs. Solving these challenges ultimately requires new material approaches to emitter design as semiconductor heterostructures do not currently provide a complete solution. Two dimensional (2D) materials have been the subject of intense scrutiny for their optoelectronic properties – such as polarized excitons, tuneable bandgap absorption and polariton behaviour. Indeed, there has been extensive research into making inter-band emitters and lasers from 2D materials, but realizing these at mid- and far-IR frequencies has proven a more significant challenge. This is mainly because achieving efficient emission requires careful design of both the optical mode and electronic states in the 2D material device. Here we will discuss an alternative approach; using polaritons within two dimensional materials to control an existing radiative process. This allows us to focus on efficient design of the optical modes in the structure, without the constraint of maintaining band-gap emission in the 2D material.
Specifically, we investigate graphene and hexagonal boron nitride (hBN) as polariton materials for controlling emission from quantum cascade structures and radiative emitters. Graphene has a carrier concentration that can be electrically controlled to create surface plasmon polaritons from the mid- to far-infrared. Here, we show how graphene can be coupled into a metal grating in the far-infrared to create an electrically tuneable optical response. By using such a structure in the waveguide of a QCL, we subsequently demonstrate spectral tuning of the terahertz laser emission. However, one of the limitations of this approach is that the device is still tied to the frequency of the QCL active region – ultimately limiting the spectral range of this approach. To solve this issue we turn to hexagonal boron nitride, which supports hyperbolic phonon polariton modes in each of its two Reststrahlen bands within the mid-wave and long-wave IR. By patterning resonators into hBN it has already been shown that multiple polariton modes be excited in such structures. By Kirchhoff’s law, an efficient absorber should also act as an efficient emitter for thermal radiation. Here, we demonstrate that hBN nanostructures can produce narrowband thermal emission by heating the device. Such emitters could be made compact by integrating local heat sources such as conductive elements such as graphite or graphene. We envision that a multiple material approach in 2D heterostructures will allow for better control of IR emission processes.

Semiconductor colloidal quantum dots have attracted great attentions in recent decades due to their unique optical properties including high quantum yields, tunable optical properties, and narrow-band emission. The solution processibility of these nanomaterials also enables low-cost, high throughput processes for device manufacturing. Built upon the success in controlling the size, shape, and composition of quantum dots during colloidal synthesis, researchers have employed quantum dots as the light emitters to achieve efficient electroluminescence, for potential applications in flat-panel displays and solid-state lighting. In this talk, I will present some of my group’s work on improving the manufacturability and performance of quantum dot based light-emitting devices, as well as in terms of understanding the basic operation principles. These multilayer devices combine quantum dot based light emitting layer with organic and nanoparticle layers as charge transport layers. Maximum external quantum efficiencies in the range of 12-15% and good operational stability have been now achieved for blue/violet, green and red emitting devices. The composition gradient inside the graded core-shell quantum dots, the thickness of the shell layer, and the organic ligands on the quantum dot surfaces all have significant consequences on device performance. Finally, I will report the use of nanoplatelets with uniform thicknesses to realize supersaturated light emission with a spectral width of only 14 nm.

The recent progress in formation of two-dimensional (2D) GaN by a migration-enhanced encapsulated technique [1] opens up new possibilities for group III-V 2D semiconductors with a band gap within the visible energy spectrum. The drawback of the planar monolayer GaN is its indirect band gap. Using first-principles calculations we explored alloying of 2D-GaN to achieve an optically active material with a tuneable band gap [2]. The effect of isoelectronic III-V substitutional elements on the band gaps, band offsets, and spatial electron localization is studied using a set of first-principle assays [3,4]. In addition to optoelectronic properties, the formability of alloys is evaluated using impurity formation energies. A dilute highly-mismatched solid solution 2D-GaN:P features an efficient band gap reduction in combination with a moderate energy penalty associated with incorporation of phosphorous in 2D-GaN. The energy penalty is substantially lower than in the case of the bulk GaN. The group-V alloying elements also introduce significant disorder and localization at the valence band edge that facilitates direct band gap optical transitions thus implying the feasibility of using III-V alloys of 2D-GaN in light-emitting devices.

[1] Z. Y. Al Balushi, K. Wang, R. K. Ghosh, R. A. Vilá, S. M. Eichfeld, J. D. Caldwell, X. Qin, Y.-C. Lin, P. A. DeSario, G. Stone, S. Subramanian, D. F. Paul, R. M. Wallace, S. Datta, J. M. Redwing, and J. A. Robinson, Nat. Mater. 15, 1166 (2016).
[2] C. Pashartis and O. Rubel, Phys. Rev. B 96, 155209 (2017).
[3] C. Pashartis and O. Rubel, Phys. Rev. Appl. 7, 64011 (2017).
[4] O. Rubel, A. Bokhanchuk, S. J. Ahmed, and E. Assmann, Phys. Rev. B 90, 115202 (2014).

Perovskite NCs are gaining increasing attention in many fields ranging from chemistry to physics and engineering owing to their extremely interesting properties such as high photoluminescence quantum yields, tunable optical bandgap, enhanced stability, large diffusion lengths and shape controllability [1-4]. In spite of the rapid progress in the shape-controlled synthesis of perovskite NCs, very few attempts have been made toward the understanding of their growth mechanisms and morphology-dependent optical properties. These studies require interdisciplinary research collaborations.

In this talk, we will present a facile synthesis of single-crystalline CsPbBr₃ perovskite nanowires (NWs) directly by ultrasonication of their precursor powders [5]. The optical and morphological evolution revealed that initially CsPbBr₃ nanocubes are formed, which transformed into NWs through an oriented-attachment mechanism. The optical bandgap of the NWs can be controlled over the entire visible range by varying the halide (Cl, Br, and I) composition through a subsequent halide ion exchange step. Furthermore, we have demonstrated that these NWs can self-assemble in a quasi-oriented fashion by the Langmuir–Blodgett technique. This work not only provides a facile method to synthesize highly monodisperse perovskite NWs, but also expands our current understanding of the growth mechanism and optical properties, and open new avenues for the fabrication of highly ordered architectures using perovskite NC building blocks for future optical and optoelectronic devices.


(1) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nano Lett. 2015, 15, 3692.
(2) Tong, Y.; Bladt, E.; Aygüler, M. F.; Manzi, A.; Milowska, K. Z.; Hintermayr, V. A.; Docampo, P.; Bals, S.; Urban, A. S.; Polavarapu, L.; Feldmann, J. Angew. Chem. Int. Ed. 2016, 55, 13887.
(3) Hintermayr, V. A.; Richter, A. F.; Ehrat, F.; Döblinger, M.; Vanderlinden, W.; Sichert, J. A.; Tong, Y.; Polavarapu, L.; Feldmann, J.; Urban, A. S. Adv. Mater. 2016, 28, 9478.
(4) Polavarapu, L.; Nickel, B.; Feldmann, J.; Urban, A. S. Adv. Energy Mater. 2017, 7, 1700267
(5) Tong, Y.; Bohn, B. J.; Bladt, E.; Wang, K.; Müller-Buschbaum, P.; Bals, S.; Urban, A. S.; Polavarapu, L.; Feldmann, J. Angewandte Chemie International Edition 2017, 56, 13887.

Organic-inorganic metal halide hybrids, consisting of a wide range of inorganic anions and organic cations, are an important class of hybrid crystalline materials with exceptional structure and property tunability. By choosing appropriate organic and inorganic components, the crystallographic structures can be finely controlled with the inorganic metal halide units forming zero- (0D), one- (1D), two- (2D), and three-dimensional (3D) structures in the hybrids. The applications of these materials in optoelectronic devices have been extensively explored in recent years, including photovoltaic cells (PVs), light emitting diodes (LEDs), and optically pumped lasers. The lowering of the structure dimensionality leads to the emerging of unique properties. For instance, unlike narrow emissions with small Stokes shift observed in typical 3D metal halide hybrids, strongly Stokes shifted broadband photoluminescence has been realized in corrugated-2D, 1D, and 0D metal halide hybrids, as a result of exciton self-trapping or excited state structural deformation. The versatility of this class of hybrid materials suggests there is a vast parameter space to explore novel structures to exhibit new and useful properties.
In this talk, I will present our recent efforts in developing and studying new classes of 0D organometal halide hybrids containing tin halide species with different structures. Due to the structural deformation on the excited states, highly luminescent broadband emissions with large Stokes shift have been realized for these 0D organometal halide hybrids. Our work significantly advances the research in organic metal halide hybrids, and provides a platform for fundamental studies of structure-property relationship in bulk crystalline materials.

Solution-grown films of CsPbBr₃ nanocrystals imbedded in Cs₄PbBr₆ are incorporated as the recombination layer in light-emitting diode structures, and optical response is studied to shed light on why the very poor light emitter CsPbBr₃ becomes a high-efficiency fast emitter when imbedded as nanocrystals in the wider gap host Cs₄PbBr₆. Kinetics at high carrier density of pure (extended) CsPbBr₃ and the nano-inclusion composite are measured and analyzed, indicating second order kinetics in extended and mainly first order kinetics in the confined CsPbBr₃, respectively. In these terms, the nano-confinement may be viewed as enforcing mainly geminate recombination of electrons and holes in a material (CsPbBr₃) that does not support stable excitons at room temperature. The resulting first-order recombination competes with trapping much more effectively than does bimolecular recombination at the moderate carrier densities typical of LEDs and usual photo-excitation. Analysis of absorption strength of this all-perovskite, all-inorganic imbedded nanocrystal composite relative to pure CsPbBr₃ indicates enhanced oscillator strength consistent with earlier published attribution of the subnanosecond exciton radiative lifetime in nano-precipitates of CsPbBr₃ in melt-grown CsBr host crystals and CsPbBr₃ evaporated films.

Hybrid organic-inorganic perovskite semiconductors have gained considerable attention for use in light emitting diodes (LEDs) and lasers due to their tunable bandgap (400 nm ≤ λ ≤ 780 nm) and attractive gain characteristics. In particular, these materials have renewed hope for the prospect of a solution-processed laser diode; however, the nature of the perovskite LED operation and efficiency roll-off in the high current density regime required for lasing is largely unexplored. Here, we investigate the external quantum efficiency (EQE) of a small area (radius of 100 μm) methylammonium lead iodide (MAPbI₃) perovskite LEDs at current densities exceeding 150 A/cm² under low duty cycle pulsed drive conditions. In contrast to continuous drive, where Joule heating limits the EQE of our devices to a peak of ~8% at J~0.03 A/cm², we find that the true peak EQE under pulsed drive reaches >15% at J~10 A/cm², which corresponds to nearly 100% internal quantum efficiency given the large refractive index of MAPbI₃ (n~2.4). Analyzing the quantum efficiency roll-off within the framework of the usual ABC rate model for inorganic LEDs, we find that the EQE transitions from being limited by Shockley-Read-Hall recombination at low current to charge imbalance due to carrier leakage at high current. Taken together, our results support the feasibility of a future perovskite diode laser and point to a general opportunity to significantly increase the efficiency of perovskite LEDs simply by reducing Joule heating to allow for operation at higher current.

Although currently their main application is as highly efficient light absorber materials for solar cell applications, halide perovskites have recently also attracted attention due to their prospective potential as emitters in electroluminescent devices. Among their particularly beneficial properties are their balanced charge carrier mobilities as well as the tuneability and high color purity of their emission spectra. From an industrial point of view, they have a number of additional features which are particularly interesting: the synthetic procedures are quite straightforward, the starting materials are available at low cost and they can be deposited from solution by large-area and roll-to-roll compatible coating methods rather than by vacuum based technologies. This contribution presents the development of LED stack architectures which have the potential to be processed completely from solution with 3D-perovskite as the emitter layer . Peak luminances of more than 13000 cd/m² have been demonstrated with efficiencies of up to 14 cd/A. Photophysical studies indicate that ionic redistribution induced by the electrical field during operation might have a significant influence on device performance and stability. In order to pave the way towards industrial-style mass production, also the results from initial roll-to-roll slot die coating experiments using halide perovskite based inks will be presented.

Multiple types of organic metal halide hybrid materials have been explored in our group over the past few years. The structures of these organic metal halides range from three-dimensional (3D) to zero-dimensional (0D) classified by the connection method of the metal halide units. And this time we have discovered a single crystalline material that contains large arrays of one-dimensional (1D) metal halide nanotubes surrounded by organic ligands.
The single crystalline bulk assembly of organic metal halide nanotubes possess a chemical formula of (C₆H₁₃N₄)₃Pb₂Br₇. In the nanotube structure, six face-sharing metal halide dimers (Pb₂Br₉^5-) connect in corners to form rings that extend in one dimension, of which the inside and outside surfaces are coated with protonated hexamethylenetetramine (HMTA) cations (C₆H₁₃N₄⁺). The individual nanotubes further assemble to bulk crystals in the form of hexagonal close compact. Since the nanotubes have minimum interaction with each other, the bulk quantum material could exhibit the intrinsic properties of an individual organic metal halide nanotube. Computational and experimental results show that this unique 1D tubular structure possesses highly localized electronic states with strong quantum confinement, resulting in the formation of self-trapped excitons that give strongly Stokes shifted broadband yellowish-white emission with photoluminescence quantum efficiency (PLQE) of ~ 7 %. Other than the application in the field of solid state lighting, this functional nanotube could have many potential applications in catalysis, gas storage, electronic devices, molecular machines, etc.
Having realized single crystalline bulk assemblies of two-dimensional (2D) wells, 1D wires, and now 1D tubes using organic metal halide hybrids, our work significantly advances the research on bulk assemblies of quantum-confined materials. The excitement of this work lies not only in the specific achievements, but also in what it represents in terms of new opportunities of organic metal halide hybrids with structures beyond conventional perovskites.

Organometal halide perovskites have recently emerged as a new generation of semiconducting materials due to their extraordinary optoelectronic properties along with their solution processability. It has been discovered that a homogeneous p−i−n junction can be developed in situ due to the ionic transport in the perovskites when an external bias is applied. Here, we report efficient perovskite light-emitters with thickness tolerance (300 nm to 30 μm) based on the investigation of the junction formation dynamics in methylammonium lead tribromide (MAPbBr₃)/polymer composite thin films. Despite a large thickness variation (300 nm to 3.6 μm), the average turn-on voltages differed only 0.2 V. It was concluded that the p- and n- doped regions propagated into the intrinsic region with an increasing bias, leading to a reduced intrinsic perovskite layer thickness, and the formation of an effective light-emitting junction regardless of perovskite layer thicknesses. The efficient perovskite-polymer thin film emitters with high tolerance to thickness in this work can potentially lead to large area printed perovskite LEDs in the future.

Interest in light emitting diodes utilizing perovskite nanocrystals has exploded in the past several years. Indeed, green and red devices have shown high efficiencies and brightness. Blue devices, however, have lagged significantly behind. In this work, we show that a key reason for the lag is the architecture of the device itself. By monitoring their brightness and lifetime, we demonstrate that the perovskite nanocrystals are significantly impacted by the presence of nickel oxide, a hole transport layer in one of the highest efficiency devices to date. We develop a transport layer structure which maintains robust dot emission, allowing the LEDs to achieve external quantum efficiencies of 0.50% at 469 nm, much higher than previous devices at that wavelength. We demonstrate that these improvements are most robust for blue devices, and the gains reduce as we tune the emission energy back towards the green. The improvements demonstrated open the door towards efficient blue perovskite LEDs.

Recently, solution-processable organic lead halide perovskites have gained significant interest for their application in high efficiency light-emitting diodes[1-2]. However, the energy conversion efficiency (ECE) and operation stability of perovskite light-emitting diodes (PeLEDs) are still major obstacles . Here, by controlling the crystallites inside perovskite grains, we obtained ideal perovskite film for near-infrared LEDs which integrates merits of complete film coverage, a high PLQE and good charge transport. The PeLEDs show a record peak external quantum efficiency (EQE) up to 14.2%, remarkable peak ECE of 10.7 % and low efficiency droop ( i. e. 14.0% EQE and 8.3% ECE at 300 mA cm^-2 ). Fourier-transform photocurrent spectroscopy (FTPS) and time-resolved photoluminescence were used to understand how the trap density affect device performance in high efficiency PeLEDs. In addition, by further modification of the perovskite layer, the device lifetime (half of the initial brightness, T₅₀) is ~23.7 hr under a constant current density of 100 mA cm^-2 which is comparable to the lifetime of near-infrared organic LEDs[3.] These results suggest that PeLED is a promising technology for achieving low-cost, large-size, high-efficiency and high-brightness lighting and displays.
References
[1] Wang, J. et al. Interfacial control toward efficient and low-voltage perovskite light-emitting diodes. Adv. Mater. 27, 2311–2316 (2015).
[2] Wang, N. et al. Perovskite light-emitting diodes based on solution-processed self-organized multiple quantum wells. Nat. Photonics 10, 699–704 (2016).
[3] Sassi, M. et al. Near-infrared roll-off-free electroluminescence from highly stable diketopyrrolopyrrole light emitting diodes. Sci. Rep. 6, 34096 (2016).

Lead halide perovskite nanocrystals have been widely studied in recent years due to their huge application potential as a cost-effective UV-VIS semiconductor. In this work we prepare and study Yb-doped all-inorganic perovskite nanocrystals (IP-NCs). Introduction of rare earth (RE) ions is a popular way to modify photonic properties of semiconductors, due to the RE’s sharp electronic transitions in VIS-NIR. However, because of the small absorption cross-sections, for practical applications of RE-doped materials, there is an urgent need for efficient sensitization. Perovskites are a natural candidate to host RE dopants, providing strong absorption and high resistance to crystal defects. Here we explore several synthesis techniques of RE-doped IP-NCs, characterizing them by their optical properties. In particular, we show NIR photoluminescence quantum yields in excess of 100%, and discuss the excitation mechanism of Yb ions, their photoluminescence quantum yield and its limits. Keeping in mind applications such as down-conversion layers for solar cells, the application potential of these doped perovskites looks very promising.

Metal halide perovskites are promising candidates for use in light emitting diodes (LEDs), due to their potential for colour tuneable and high luminescence efficiency. While recent advances in perovskite based light emitting diodes (PeLEDs) have resulted in external quantum efficiencies (EQEs) exceeding 10% for the green emitters, the EQEs of the red and blue emitters still lag behind. A critical issue to date is creating highly emissive and stable perovskite emitter with the desirable emission band gap to achieve full-colour displays and white LEDs. Herein, we report the preparation and characterization of a stable suspension of cubic CH₃NH₃PbI₃ perovskite nanocrystals (NCs), where we synthesise the NCs via a ligand-assisted re-precipitation technique, using an acetonitrile/methylamine compound solvent system to solvate the ions, and toluene as the anti-solvent to induce crystallisation. Through tuning the ratio of the ligands, the ligand to toluene ratio, and the temperature of the toluene, we obtain a stable solution of CH₃NH₃PbI₃ NCs with a photoluminescence quantum yield exceeding 90%, and tuneable red emission between 660 and 705nm. We demonstrate the utility of these nanocrystals in PeLEDs, resulting in a maximum EQE of 2%.

As organic-inorganic lead halide perovskite emerges as a potential emissive material for light-emitting devices such as light emitting diodes (LEDs) and lasers, the necessity of understanding its fundamental emission characteristics is being emphasized. In general, the temperature-dependent photoluminescence of the material comprises fruitful photo-physical information such as phonon scattering, activation energy for the radiative recombination of the excitons etc. In this study, we report on the temperature-dependent emission properties of three different types of CH₃NH₃PbBr₃ perovskite: single crystal, quantum dots and polycrystalline thin films. The evolution of emission peaks for every photoluminescence spectra shows continuous blue shift with increasing temperature from 20 to 300 K, except when the phase transition appears as sudden red shift in certain temperature range (~120 K) for single crystal. The photo-physical properties such as exciton binding energy and exciton-phonon scattering were also investigated for different types of CH₃NH₃PbBr₃ perovskite samples. Among three types of CH₃NH₃PbBr₃ perovskite, quantum dots show lower phonon scattering with higher exciton binding energy that ensure better emission property for their use in light emitting applications, especially, LEDs.

Perovskites quantum dots (PQDs) have rapidly emerged as a new generation of light emitting nanocrystals despite their very recent report.¹ However their transport^2,3 and electronic properties⁴ remain poorly investigated which prevents the device optimization. In this paper we propose a procedure for the integration of PQD into conductive and photoconductive film.⁵ We demonstrate that the proposed film preparation can be applied to other halide PQDs compositions and shapes. We use X-ray photoemission to unveil the electronic spectrum of the PQDs, bring evidence of their n-type character and report a value of 4.1 eV for the work function of CsPb(Br_0.65,I_0.35)₃ PQDs. Using a combination of high bandwidth transient photocurrent and DC photocurrent measurements, we show that the large exciton binding energy prevents efficient dissociation of the generated photocharges. We thus propose to use nanotrench electrodes as a path to boost the material photoresponse and report an enhancement by a factor 1000.

References

1. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX₃, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692-3696.
2. Kwak, D. H.; Lim, D. H.; Ra, H. S.; Ramasamy, P.; Lee, J. S., High Performance Hybrid Graphene-CsPbBr_3-xI_x Perovskite Nanocrystal Photodetector. RSC Adv. 2016, 6, 65252-65256.
3. Palazon, F.; Di Stasio, F.; Lauciello, S.; Krahne, R.; Prato, M.; Manna, L., Evolution of CsPbBr₃ Nanocrystals upon Post-Synthesis Annealing under an Inert Atmosphere. J. Mater. Chem. C 2016, 4, 9179-9182.
4. Ravi, V. K.; Markad, G. B.; Nag, A. Band Edge Energies and Excitonic Transition Probabilities of Colloidal CsPbX₃ (X = Cl, Br, I) Perovskite Nanocrystals. ACS Energy Lett. 2016, 1, 665-671.
5. Swarnkar, A.; Marshall, A. R.; Sanehira, E. M.; Chernomordik, B. D.; Moore, D. T.; Christians, J. A.; Chakrabarti, T.; Luther, J. M. Quantum Dot–Induced Phase Stabilization of α-CsPbI₃ Perovskite for High-Efficiency Photovoltaics. Science 2016, 354, 92-95.

Organic-inorganic hybrid materials have sparked the interest of the materials research community due to their excellent optoelectronic properties and their broadly tunable crystal structures and chemical compositions. This rapidly growing field is mostly known for its high efficiency photovoltaic material CH₃NH₃PbI₃.^1-4 However, through compositional and chemical modifications the use of hybrid compounds can be expanded to include scintillation materials, multiferroics, light-emitting diodes, and photodetectors.^5-7

Here we report the solution syntheses, crystal and electronic structures, and optical properties of the novel hybrid organic-inorganic compounds R-M-X (R = C₆H₅C₂H₄NH₃, C₆(CH₃)₅CH₂N(CH₃)₃; M = Zn, Cd, Hg; X = Cl, Br, I). All compounds were grown under ambient conditions in various solvent systems, resulting in colorless or light-yellow millimeter-sized crystals. The stabilities of the resultant materials under ambient conditions vary depending on their chemical compositions and crystal structures. Generally, these materials have large, tunable band gaps ranging from 2.5 – 6 eV and exhibit broad photoluminescence peaks that span the entire visible spectrum. Thus, the full width at half maximum values can measure up to 220 nm. In several examples, broadband white- and bluish-white light emission can be observed at room temperature upon excitation using a 325 nm source. Based on the considerations of crystal chemistry, compositions, band structures, and past literature reports the PL emission in these materials are attributed to self-trapped exctions. On the basis of our results these R-M-X compounds may be of interest for white-light-emitting phosphors or scintillator applications.

References
1. Saparov, B.; Mitzi, D. B. Chem. Rev. 2016, 116 (7), 4558-4596.
2. Mitzi, D. B. J. Chem. Soc., Dalton Trans. 2001, (1), 1-12.
3. Albero, J.; Asiri, A. M.; Garcia, H. Journal of Materials Chemistry A, 2016, 4 (12), 4353-4364.
4. Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Nat. Mater. 2014, 13 (9), 897-903.
5. Si, J.; Liu, Y.; Wang, N.; Xu, M.; Li, J.; He, H.; Wang, J.; Jin, Y. Nano Research, 2017, 10 (4), 1329-1335.
6. Deschler, F.; Price, M.; Pathak, S.; Klintberg, L. E.; Jarausch, D.-D.; Higler, R.; Hüttner, S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J.; Atatüre, M.; Phillips, R. T.; Friend, R. H., J. Phys. Chem. Let. 2014, 5 (8), 1421-1426.
7. Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Nature Nanomaterials, 2014, 9, 687.

Introducing a few impurity atoms in semiconductor quantum dots (QDs) drastically changes the electronic property and provide new functionalities towards the photonic applications. Controlled formation of localized impurity states in the energy gap extends the freedom to engineer the luminescence properties exceeding the range that can be achieved by size and shape control. In fact, superior luminescence properties have been demonstrated in copper or silver-doped cadmium-chalcogenide QDs and manganese-doped zinc-chalcogenide QDs.[1] In addition to compound semiconductors, silicon (Si) is earth abundant and highly biocompatible material and thus colloidal Si QDs emerge as excellent luminescent materials for optoelectronic and biophotonic applications. However, doping in Si QDs is more difficult than II-VI QDs due to the small solid solubility of typical impurity atoms and resultant strong self-purification effects.
In this work, we introduce donor and acceptor levels in the band gap of Si QDs by simultaneously doping phosphorus and boron.[2] It is known that codoping of n- and p-type impurities reduces the self-purification effects due to the charge compensation and makes very high concentration doping possible. Furthermore, charge carrier-induced Auger processes, which reduces the luminescence quantum yield significantly, can be avoided by codoping, In this work, we first discuss the distribution of impurity atoms in phosphorus and boron codoped Si QDs from the atom-probe tomography analyses [3]. We then discuss the size dependence of the HOMO and LUMO level energies measured from the vacuum level by combining photoemission yield spectroscopy and photoluminescence spectroscopy. Furthermore, we show density of state spectra of single codoped Si QDs obtained by scanning tunneling spectroscopy and discuss the size dependence of the in-gap donor and accepter states, onset of the conduction and valence band edges, and the ionization energies of dopants quantitatively.[4] From these data, we will identify the origin of the luminescence in impurity codoped Si QDs. In parallel, we perform comprehensive studies to maximize the luminescence quantum yield of codoped Si QDs in a wide wavelength range. We demonstrate that the codoped Si QDs exhibit efficient luminescence in the energy range below bulk Si band gap. This opens up a new application of codoped Si QDs as a phosphor in the second biological window (1000-1350 nm).
[1] Pradhan, et al., Angew. Chem. Int. Ed. 56, 7038 (2017) [2] Sugimoto, et al., J. Phys. Chem. C, 120, 17845 (2016) [3] Nomoto, Sugimoto, et al., J. Phys. Chem. C, 120, 17845 (2016) [4] Hori, et al., Nano Letters, 16, 2615 (2016) [5] O. Ashkenazi, et al, submitted

Colloidal semiconductor nanocrystals are promising for optically pumped lasers that can practically emit at any wavelength from UV to IR. However, lasing performance of the conventional nanocrystals has been severely limited due to Auger recombination which depletes gain-active excitonic species before they could contribute to gain. To this end, shape-controlled and composition-tuned nanocrystals have been proposed towards low-threshold optical gain since they can suppress Auger recombination. Among these, nanocrystals having Type-II band alignment have been highly promoted for lasing, yet to date their performance has suffered from small modal gain coefficients and short gain lifetimes due to their diminishing oscillator strength and modest absorption cross-section. Overcoming these challenges, here we accomplish unprecedented optical gain performance in Type-II nanocrystals by developing an alloyed colloidal quantum well architecture. By optimizing the composition of the core/alloyed-crown CdSe/CdSe_xTe_1-x quantum wells, we realized amplified spontaneous emission with a threshold as low as 26 µJ/cm², accompanied with large net modal gain coefficients up to ~930 cm^-1 and long gain lifetimes (τ_gain ~400 ps). The performance of the Type-II quantum wells studied here represents more than an order of magnitude improvement over the previous best reports in Type-II nanocrystals. Also, the measured modal gain coefficient is record high among all colloidal semiconductors. Moreover, we corroborated the underpinning mechanism of this efficient gain via ultrafast transient absorption spectroscopy and revealed that the gain surprisingly arises from the carriers that are localized to the alloyed-crown region. The gain scheme in these alloyed quantum wells resembles a “threshold-less” four-level gain system, which makes them extremely promising towards realization of continuous-wave (CW) pumped and electrically-injected nanocrystal lasers.

Charged defects in 2D materials have emerging applications in quantum technologies such as
quantum emitters and quantum computation. Advancement of these technologies requires rational
design of ideal defect centers, demanding reliable computation methods for quantitatively accurate
prediction of defect properties. We present an accurate, parameter-free and efficient procedure to
evaluate quasiparticle defect states and thermodynamic charge transition levels of defects in 2D
materials. Importantly, we solve critical issues that stem from the strongly anisotropic screening
in 2D materials, that have so far precluded accurate prediction of charge transition levels in these
materials. Using this procedure, we investigate various defects in monolayer hexagonal boron nitride
(h-BN) for their charge transition levels, stable spin states and optical excitations, through which we
select most promising defect candidates for scalable quantum bit and emitter applications[1].
[1] Wu, Feng; Andrew, Galatas; Sundararaman, Ravishankar; Rocca, Dario; Ping, Yuan*,
"First-principles Engineering of Charged Defects for Two-dimensional Quantum Technologies", arXiv:1710.00257

Various concepts have been investigated to produce deep-blue and efficient organic light emitting diodes (OLEDs) for displays, and to ultimately generate white light for general lighting applications. Among them, fluorescent emitters, phosphorescent emitters and thermally activated delayed fluorescence emitters.
In this work, we demonstrate the emission of deep-blue photons from an organic upconversion layer that is applied atop a high-brightness green phosphorescent OLED. Whereas, formerly, efficient photon-upconversion has been realized mostly in solution, here, its integration into OLEDs calls for solid-state upconversion concepts that can be applied to thin-films and that is compatible with thin-film device concepts which will facilitate fabrication, e.g., by additive manufacturing by roll-to-roll printing. We realized the solid-state upconversion layer by gelation of a suitable sensitizer/emitter pair. Additional band-pass filters between the OLED and the upconversion layer as well as on the outer side of the upconverter reduce the re-absorption of upconverted light in the OLED and enhance the photon confinement in the upconversion layer.

III-V compound semiconductor nanowires (NWs) have attracted significant attention as nanoscale light sources in the integrated photonics due to their nanoscale size, good optical properties and strain relaxation feature enabling the monolithic growth on lattice mismatched substrates [1, 2]. In particular, NWs grown by selective area epitaxy (SAE) technique have many benefits such as controllability of their size and position, compatibility with silicon technology platform and high uniformity in diameter and length, facilitating the integration with other electronic devices. In addition to these advantages, a single standing NW itself can act as a vertical optical cavity [3] and an array composed of few tens to thousands of NWs also can act as a photonic crystal [4, 5], which is convenient for design of high power light emitting diodes (LEDs) and lasers. With suitable wavelength ranging from 1.3 to 1.6 μm and lattice matching of constituent materials, the InGaAs/InP multi quantum well (MQW) system has been being widely used for the optical communication devices [6]. Nonetheless, there is limited understanding on the growth of InGaAs/InP MQW nanowires [7] and their application for LEDs has not reported.
In this work, we demonstrated the growth of highly uniform InP/InGaAs MQW NW array by metalorganic chemical vapour deposition (MOCVD) on the pre-patterned substrate defined by electron-beam lithography (EBL). The structural properties of the MQW NW were studied by aberration corrected high resolution scanning transmission electron microscopy (HR-STEM). The optical properties were characterized by micro photoluminescence (micro-PL) and cathodoluminescence (CL). From the HR-STEM study, it is found that MQWs are formed in both radial and vertical directions of the NW. From the in-depth microstructure analyses, we reveal a unique lateral growth evolution process occurring during the MQW NW growth which is induced by the phase difference between the InGaAs QW and barrier InP.
The InGaAs/InP MQW structure was further incorporated between the growth of a p- and n-InP segment for the formation of a diode. Single NWs were then transferred onto a SiO₂-on-Si substrate and fabricated with Ti/Au electrodes. Based on the preliminary I-V measurements and electron-beam induced current (EBIC) characterisation, the fabricated single NW devices showed typical diode behaviour and EBIC also revealed a clear p-n junction location of the device. Further work is underway to optimise the growth and design of the p-i(MQW)-n diode structures for efficient MQW NW based LEDs.