Symposium F.EL02—Emerging Light-Emitting Materials and Devices—Halide Perovskites, Quantum Dots and Other Nanoscale Emitters

Doping metal ions into lead halide perovskite nanocrystals (NCs) has attracted great attention over the past few years due to the emergence of novel properties relevant to optoelectronic applications. Current studies focus mostly on doping a single type of metal ions into the host perovskite NCs, while few research efforts have been made on simultaneous doping more than one type of metal ions into the same perovskite hosts and to study their synergic effects with intra-particle energy transfer processes. Here, a facile synthesis of Mn²⁺/Yb³⁺ codoped CsPbCl₃ NCs through a hot-injection technique is firstly reported. Owing to the completely separated energy relaxation channels among the intrinsic bandgap (BG), Mn²⁺ and Yb³⁺ dopant emissions, the obtained Mn²⁺/Yb³⁺ codoped CsPbCl₃ perovskite NCs exhibit a unique triple-wavelength emission profile simultaneously covering UV/blue (BG-PL), visible (Mn-PL) and NIR (Yb-PL) spectral regions. By optimizing the dopant concentrations, the total photoluminescence quantum yield (PL QY) of the codoped NCs can reach up to ~125.3% due to quantum cutting effects. Mechanism studies reveal different natures of emission mechanisms can be involved in a single batch of Mn²⁺/Yb³⁺ codoped perovskite NCs: (i) electronic inter-band transition (BG-PL); (ii) electronic transition of ion centers within local molecular complexes (Mn-PL); and (iii) defect-induced energy transfer and subsequent quantum cutting scheme (Yb-PL). Owing to the high PL QYs and minimal reabsorption loss, the codoped perovskite NCs are demonstrated to be used as efficient emitters in luminescent solar concentrators, with greatly enhanced external optical efficiency compared to that of using solely Mn²⁺ doped CsPbCl₃ NCs. Therefore, co-doping both Mn²⁺ and Yb³⁺ ions into perovskite NCs can serve as a unique model system for studies of host-to-dopant and inter-dopant energy transfer mechanisms, as well as associated radiative and non-radiative decay pathways at each step. Most importantly, such codoped NCs with multiple emission channels hold a high potential to be applied in a wide range of applications, including multiplexed biological labelling and sensing, multi-channel photodetectors, stimuli-responsive inks for coding, encryption and decryption, and photon management devices in the future.

Combining the advantageous electrical and optical properties of semiconductors with magnetic characteristics gives access to extraordinary phenomena and applications. Fully inorganic dilute magnetic semiconductors (DMS) have been known for decades, which show diverse functionalities like control of magnetism by electrical fields. The material class of DMS is obtained by introducing a substantial number of magnetic ions to an otherwise non-magnetic host semiconductor.
Due to their outstanding optoelectronic properties and high defect tolerance, organo-metal halide perovskites form an ideal system for efficient magnetic doping. We produced a selection of magnetic perovskites using simple solution processing techniques by doping of the host material, Ruddlesden-Popper hybrid perovskite ((PEA)₂PbX₄ with X = Cl, Br, I), with different magnetic ions like Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺ and Eu²⁺. We investigated the influence of both halides and magnetic impurities on the long-range magnetic ordering of the materials with respect to the structural and optical properties. We investigate how localized excitons couple with the magnetic moments of the introduced impurities by detecting a dependence of the emitted lights polarization on the magnetization. We perform structural, magnetic, optical and magneto-optical temperature-dependent measurements to verify a successful magnetic doping of the perovskite and to show how photoluminescence and magnetization properties relate.

Recently, cesium lead halide perovskite quantum dots (PQDs) have generated tremendous interests, due to the great promise they offer for use in optoelectronic applications. However, their poor colloidal stability, which can be traced to the ionic nature of the crystal cores and the high desorption rates of the native ligands, has impeded progress in fundamental research and slowed integration of such materials in devices. Developing novel surface ligands with strong coordination to both passivate surface traps and enhance fluorescence performance, while improving colloidal stability is urgently needed.
Here, we detail the synthesis of a new family of multifunctional polymer ligands and their use for surface engineering of the CsPbBr₃ nanocrystals. The ligand synthesis relies on the ring opening reaction between distinct nucleophiles and poly(isobutylene-alt-maleic anhydride), PIMA. The modular ligand stoichiometrically displays multiple ammonium and/or imidazolium anchors for coordination onto PQD surfaces, along with several alky chains with different length to promote affinity to various organic solvents. The results show that the PQDs coated with those polysalts exhibit great colloidal stability and significantly enhanced photoluminescence. The coating also allows the resulting colloids to be dispersed a wide range of solvent conditions, including ethanol and methanol, while maintaining strong fluorescence and preserving the nanocrystal integrity after ligand substitution.

Nanocrystals (NCs) with different shapes, are of tremendous interest to material chemists, and precise control of their shapes and sizes through colloidal synthetic routes, with further solution-processability of the NCs, provides us with a great opportunity to tune their optoelectronic behaviour for advanced applications.¹ Interestingly, polar solvents are known to play crucial roles in determining the shapes of CsPbBr₃ perovskite NCs owing to the prevailing anisotropic growth conditions. However, to induce polar environment in the perovskite synthesis, a suitable precursor soluble in the desired polar solvent, is primarily required. In this work, we describe an efficient, MeOH-soluble steroidal Cs-precursor: cesium cholate and the different shapes of CsPbBr₃ NCs (nanorods, nanocubes and nanoplatelets) obtained from it, utilizing miniscule amount of polar solvents (methanol and methanol-water) in conjunction with commonly used 1-octadecene solvent and varying the reaction temperatures (90 °C and 180 °C). The resulting NCs exhibit very high photoluminescence (PL) quantum yields (94% for nanoplatelets and 96% for nanocubes), due to mitigation of Br vacancies by large cholate molecules, slowing down the complex nucleation and lattice construction processes in the synthetic pathway.² To study the application of these luminophores for application in perovskite light emitting diodes (PeLEDs), powdered CsPbBr₃ nanocubes were dispersed in toluene at different concentrations to prepare inks. Inks were spin-coated at different speeds (500, 1000 and 1500 rpm) and film quality was optimized to produce uniform and pinhole free films on an ITO substrate. Thin films produced at optimal deposition conditions and ink compositions were characterized using thin film photoluminescence measurements at 325 nm. A sharp and highly intense peak at 514 nm was observed. Initial device fabrication employing a spin-coated hole transport layer (poly (9-vinylcarbazole), and an electron transport layer (2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) produced promising initial results. These initial results with perovskite nanocubes based thin films invite more studies of complete devices, possibly with other nanomaterial geometries (nanorods and nanoplatelets).
References:
(1) Pan, A.; He, B.; Fan, X.; Liu, Z.; Urban, J. J.; Alivisatos, A. P.; He, L.; Liu, Y. Insight into the Ligand-Mediated Synthesis of Colloidal CsPbBr₃ Perovskite Nanocrystals: The Role of Organic Acid, Base, and Cesium Precursors. ACS Nano 2016, 10, 7943–7954.
(2) Chakrabarty, A.; Satija, S.; Gangwar, U.; Sapra, S. Precursor-Mediated Synthesis of Shape-Controlled Colloidal CsPbBr₃ Perovskite Nanocrystals and Their Nanofiber-Directed Self-Assembly. Chem. Mater. 2020, 32, 721–733.

Perovskite solar cells (PSCs) are promising contenders in the solar cell technology market having achieved photo-conversion efficiencies greater than 25%. The role of the hole transport layer in perovskite solar cells is essential for incredible enhancement of their significant photo-conversion efficiency. Iron oxide is one of the interesting hole transport materials that can potentially improve the performance; however, the frequently used chemical synthesis technique of the micaceous iron oxide (MiO) precursors is very toxic, harmful and time-consuming. Molybdenum dioxide (MoO₃) is a commonly studied hole transport layer utilized in perovskite solar cell; This study analyzes a chemically casted process preferably hot-casting technique to prepare a non-toxic molybdenum dioxide precursor that would fabricate a highly effective and densely packed MoO₃ films. Various temperatures below 100 ^oC results in intermittent deposition of the MoO₃ films. When the hot-casting temperature is increased, the MoO₃ thin film are uniformed and resulted in greater charge extraction and insignificant light soaking. Photoluminescence spectroscopy (PL), ultraviolent-visible spectroscopy (UV-VIS), and scanning electron microscopy (SEM) are the characterization techniques utilized to observe the opto-electronic properties of the MoO₃ films. The capacitance, frequency dispersion, and leakage current measurements display stable performance of the MoO₃ perovskite solar cells.

Manganese doped inorganic halide perovskites continue to be of current interest for applications in light emitting devices and down-converters in solar cells. In this work we prepared Mn doped CsPbCl₃ (Mn: CPC) bulk crystals and nano-particles (NPs) and compared their emission properties. Bulk crystals were grown from the melt by vertical Bridgman technique and NPs were synthesized using a microwave assisted method. Under ultra-violet excitation at 350 nm, bulk crystal and NPs of Mn: CPC exhibited a broad orange emission centered at ~605 nm at room temperature. The broadband emission was assigned to the intra-3d transition ⁴T₁ → ⁶A₁ of Mn²⁺ ions incorporated in the CPC host lattice. The Mn²⁺ emission lifetimes were nearly exponential with values of 1.07 ms for NPs and 0.73 ms for the bulk crystal. Mn: CPC NPs also showed exciton emission peaking at ~402 nm, whereas the bulk crystal exhibited no significant emission near the band edge. Instead, the bulk material of Mn: CPC revealed a weak below-gap emission in the 450-550 nm region suggesting the existence of defect states. The excitation spectra for the orange Mn²⁺ emission from Mn: CPC NPs and bulk crystals were significantly different indicating distinct excitation pathways. The excitation spectrum of the orange Mn²⁺ emission for Mn: CPC NPs showed excitonic structure similar to the absorption spectrum suggesting energy transfer from excitons to Mn²⁺ ions. In contrast, UV excitation was less efficient for the bulk Mn: CPC crystal and the excitation was dominated by well defined below-gap excitation bands centered at ~425 and ~530 nm. More details of the material preparation emission spectroscopy, and initial hybrid LED experiments will be discussed at the conference.

Present study analyses the salient features of the newly synthesized phosphor powders of 1, 3, 5, 7, 9 and 11 molar concentrations of Ytterrbium activated Strontium Aluminate by solution Combustion method using Urea. The prepared phosphors were calcined at 950degree celcius for 4 hours. The Powder X-ray diffraction (PXRD) profiles of nanophosphors showed crystalline face centered cubic phase. The average crystallite sizes of the phosphors were in the range 34 to 48 nm. The samples showed pores and voids on their surface that formed by the escaping gases during the combustion method. The morphology showed the inherent nature of combustion process. The Scanning electron microscope (SEM) images showed non-uniform and irregular shapes of the particles. The Particle sizes were further confirmed by Transmission electron microscope (TEM). Energy dispersive analysis of X-ray techniques were used to study the structural characteristics of calcined phosphors. Fourier infrared spectral studies were carried out to evaluate the stretching and vibrational bonds in the product. They showed an absorption peak at 1456 cm^-1 that indicates the formation of crystallized Strontium Aluminate and a sharp peak at 3670 cm^-1 due to the presence of AlO₄ group in the powders. Energy dispersive X-ray analysis (EDAX) with SEM image, revealed the uniform distribution of Yb³⁺ ions. The energy band gaps of the prepared phosphors from Kubelka-Munk plots of the Diffused reflectance Studies (DRS) were found to be in the range 5.31 eV to 5.78 eV. The Photoluminescence studies showed an emission peak at 708 nm when phosphors were excited with 355 nm. All the phosphors were found to emit red light as their Commission Internationale de L’Eclairage (CIE) co-ordinates have fallen in red region of chromaticity diagram. The average correlated color temperature of the phosphors was found to be 3950 degree Kelvin.

Two-dimensional (2D) lead halide perovskites have been shown to exhibit broadband or white-light emission under certain conditions. Broadband emission in lead iodide 2D perovskites has been alternately attributed to self-trapped excitons (STEs) or permanent structural defects and/or impurities. Here, we investigate the photophysical characteristics of broadband emission in six different multi-layered (n > 1) 2D lead iodide perovskites as a function of sample temperature from 5 K to 300 K. We distinguish shallow defect-associated emission from a broad near-infrared (NIR) spectral feature, which we assign to the STE. Sub-gap photoexcitation experiments and NIR transient absorption spectroscopy rule out permanent structural defects as the origin of this feature. When we varied the thickness (n = 2, 3, 4), A-site cation (methylammonium vs. formamidinium), and organic cation spacer (butylammonium vs. hexylammonium vs. phenylethylammonium), we found that the temperature dependence of broad NIR emission was strongly correlated with both the strength of electron-phonon coupling and the extent of structural deformation of the ground state lattice, strongly supporting the assignment of this spectral feature to a STE.

InAs quantum dots (QDs) embedded in MBE grown GaAs/Al_0.30Ga_0.70As/AlGaInAs heterostructures have been investigated in as grown state and after thermal annealing at 640^oC or 710^oC for two hours. Two types of QD structures with the different compositions of AlGaInAs capping layers (GaAs (#1) or Al_0.10Ga_0.75In_0.15As (#2)) and the same composition of buffer layers (In₀₁₅Ga_0.85As) are compared and studied by means of photoluminescence (PL), X-ray diffraction (XRD) and high resolution HR-XRD methods. XRD and HR-XRD techniques are applied with the aim to control the crystal structures, the material compositions and elastic strains in quantum well (QW) layers.

The highest PL intensity, smaller PL band half width and lower energy of ground state (GS) emission are detected in the structure with the Al_0.10Ga_0.75In_0.15As capping layer. Thermal annealing leads to the shift of PL spectra into higher energy range and the value of this shift is more essential in the structure with the Al_0.10Ga_0.75In_0.15As capping layer as well.

The variation of GS emission peak versus temperature has been monitored within the range of 10-450K for as grown and annealed states and it compared with temperature shrinkage of band gap in the InAs and GaAs bulk crystals. It permits to reveal the QD composition that in #2 was closer to InAs than in #1. Finally, the reasons of PL spectrum transformation at annealing, the mechanism of PL thermal decay, and the advantages of the QD structure with strain reduced Al_0.10Ga_0.75In_0.15As capping layer have been analyzed and discussed. The composition variation of QDs and QWs at annealing has been modeled using the simulation of HR-XRD results with Xpert Expitaxy software.

In this study, we developed a novel approach to synthesize high-quality metal-free Nitrogen-doped graphene quantum dots (N-GQDs) with high quantum yield, via irradiation of s-triazene in a solution with benzene by using pulsed laser. The TEM, HRTEM, XPS, XRD, Raman spectroscopy and FTIR were carried out to observe the morphology, size distribution, crystalline structure and to prove successful doping of GQDs with nitrogen atoms. Furthermore, for the first time, to our knowledge, their magnetic properties were investigated. The results indicate that N-GQDs exhibit superparamagnetic behavior. The specific size, shape and zigzag edge structure of N-GQDs were considered to explain the origin of the observed magnetism. The magnetization dependence led to estimating the N-GQD material magnetic permeability for different ambient temperatures. From the zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements, carried out at 50 Oe magnetic field strength, we estimated the blocking temperature T_B to be around 300 K. Based on the experimental data analysis, the magnetic permeability, number of correlated spins per single N-GQD, and number density of superparamagnetic N-GQD per gram of material were estimated. The excellent superparamagnetic properties together with optical properties manifested by N-GQDs have the potential to lead to high performance biomedical applications.

Semiconducting nanocrystals, also known as quantum dots (QDs), are a promising color converting materials for next-generation display application due to their unique photophysical properties such as size-dependent color tunability, narrow spectral bandwidth, and high photoluminescence (PL) quantum yield (QY). However, the vulnerability of QDs to oxygen and moisture is a huge obstacle to practical applications. The commercialized QD for display has been a form of color converting QD films in liquid crystal display (LCD) backlight unit (BLU) using blue light-emitting diode (LED) (i.e., On-Surface type), where QDs were physically blended in polymer matrixes sandwiched between two barrier layers. It is more desirable for LCD BLU using LEDs encapsulated the QDs directly into the packaging (i.e., On-Chip type) in terms of performance, amount of QDs, and cost. Nevertheless, LEDs that are directly encapsulated by QDs have not been used in display applications due to their fatal instability. Therefore, reliable LEDs using highly stable luminescent encapsulation material that contains uniformly dispersed QDs in optical resin should be developed without additional protecting layers (such as SiO₂ or Al₂O₃) and ligand-exchange processes.
Herein, we developed luminescent LED encapsulation material (TSE-QD) using CdSe/CdZnS core/shell QDs (passivated by oleic acid) dispersed in a thermally resistant sol-gel derived siloxane matrix with high refractive index (> 1.57) [1,2]. The TSE-QD was fabricated via hydrosilylation of QD dispersed siloxane resin that was synthesized via in situ sol-gel reactions of silane precursors in the presence of QDs. The QDs were uniformly distributed in a linear-structured siloxane matrix in which functional groups of siloxane matrix is chemically cross-linked with ligands of the surface on QDs, and retained their initial optical properties during the fabrication processes [3]. Notably, we achieved dramatically enhanced thermal (at 120 ^οC in ambient air) and chemical stability (in acid or base solutions and alcohol) without additional barrier films and ligand-exchange process. We demonstrated a natural-white emitting LED using a blue LED chip directly encapsulated (i.e., On-Chip type) by yellow-emitting TSE-QD that consist of red and green QDs in a siloxane matrix. Fabricated white LED exhibited wide color gamut (116% of NTSC 1931), excellent reliability (under 85 ^οC & 85% relative humidity), and high photostability (under continuous blue light irradiation).
[1] H. Y. Kim et al., J. Am. Chem. Soc., 138, 16478-16485 (2016).
[2] J.-Y. Bae et al., RSC Adv., 3, 8871-8877 (2013).
[3] J. Jang et al., Nanoscale, 11, 2828-2834 (2019).

The structural, optical, and magnetic properties of the vacancy-ordered quadruple perovskites Cs₄CdBi₂Cl₁₂ and Cs₄MnBi₂Cl₁₂ and their solid solution have been investigated. Both compounds are found to crystallize with R-3m space group symmetry that arises from ordering of Bi³⁺, Mn²⁺/Cd²⁺, and cation vacancies into layers that run perpendicular to the <111> direction of the cubic perovskite structure. Magnetic measurements reveal that Cs₄MnBi₂Cl₁₂ is paramagnetic down to 2 K with a Weiss constant of −2.88(3) K and an effective moment of 5.840(1) μ_B. This compound exhibits weak orange-red luminescence, which involves Bi³⁺ ions absorbing near UV photons followed by energy transfer to Mn²⁺ ions and finally radiative decay that is attributed to a spin-forbidden ⁴T₁(G) to ⁶A₁(S) d–d transition. The emission peak is centered near 605 nm and has a full-width-half-maximum of ~90 nm and a photoluminescent quantum yield (PLQY) of ~ 4%. The isostructural Cs₄CdBi₂Cl₁₂ is neither magnetic nor does it show detectable photoluminescence at room temperature. Replacing Mn²⁺ with Cd²⁺ to form Cs₄Cd_1−xMn_xBi₂Cl₁₂ leads to a zero-dimensional electronic structure that inhibits energy migration to defect sites where non-radiative decay can occur, increasing the room temperature PLQY to 57% in the x = 0.27 sample. Cs₄Cd_1−xMn_xBi₂Cl₁₂ phosphors are easily synthesized from solution, free of rare-earth ions, and possess emission spectra that compare favorably to narrow-band red emitting phosphors based on Eu²⁺ ions.

Fluorene-based organic phosphorescent materials (OPMs) were explored for organic light-emitting electrochemical cells (LECs) taking advantages of their tunability in color and film processibility. TO tackle the problem of the apparent low emission efficiency of OPMS, compared to inorganic or organometallic materials, fluorene unit was designed as a π-bridge between halogen and n-π transition groups. These OPMs provided a long-lived phosphorences at film state, possibly due to enhanced intersystem crossing (ISC) to the triplet state in the optimized structure. Thus OPMs were applied to light-emitting cells (LECs) along with a host and electrolyte, to show the highest luminescence among the organic phosphorescent LECs up to date. Synthesis of OPMs, mechanism of enhanced emission, and fabrication details on LECs will be presented.

Perovskite nanoparticles (PeNPs) are promising emitters owing to their high color purity, low-cost synthesis, and facile wavelength control with halide composition control. However, chloride-based blue-emitting colloidal PeNPs have suffered from low photoluminescence quantum efficiency (PLQE) due to chloride-induced defect states. Their low PLQE hinders their use of blue-emitting devices.
In this work, we propose a method to improve the efficiency of blue PeNPs. At first, blue-emitting perovskite was synthesized by the hot injection method. Near Unity PLQE of as-synthesized PeNPs was obtained. For the efficient perovskite light-emitting diodes (PeLEDs), additional washing steps are required to remove excess ligands on the surface. But after washing steps, the PLQE rapidly decreased to ~50% because of surface defects.
A simple post-treatment process using short organic ligands solution was done after washing steps. PLQE increased again because the halide anions passivate defect sites. Moreover, those short organic ligands do not impede charge injection. By these methods, highly efficient blue-emitting PeNPs with PLQE > 92 % and high device efficiency of 2% was obtained.

Despite organic light-emitting diode (OLED) technology has already left an indelible mark on the display industry, conventional OLEDs are still susceptible to degradation when exposed to air. A greater hurdle is to achieve the stretchable OLED as protecting devices from the penetration of moisture and oxygen is even more difficult under stretching. Here, we propose an air-stable intrinsically stretchable display without using any encapsulations by integrating a stretchable light-emitting device (SLED) with a stretchable color conversion layer that contains perovskites nanocrystals (PeNCs). PeNCs normally decay when exposed to air; counterintuitively, the moisture-assisted surface passivation of PeNCs is first observed. This work is believed to stimulate considerable research on fundamental aspects of PeNCs and furthermore into the practical applications both in academia and industries. The above-mentioned SLED can also open new ways toward intrinsically stretchable OLEDs.

Recently, lead halide perovskite nanoparticles became promising candidates for light emitters due to narrow full-width at half maximum (FWHM), solution processability, easily tunable bandgap.¹ Excitons are well confined with the aids of long passivating ligands promoting radiative recombination, thus, lead halide perovskite nanoparticles exhibit high photoluminescence quantum yield (PLQY). ² However, the synthesis of perovskite nanoparticles is complicated requiring several centrifugations and production yield is low because only the small amount of precipitate is utilized by reprecipitation method.
So, we took in-situ strategy for synthesizing perovskite nanoparticles by incorporating bulky ligand, phenyl ethyl ammonium bromide (PEABr).³ To achieve stable in situ fabricated perovskite nanoparticles, dispersion of PEABr is essential because PEABr aggregates form local quasi-2D structure.⁴ Here, we successfully synthesized perovskite nanoparticles by exploiting crown additive of 1,4,7,10,13,16-Hexaoxacyclooctadecane (18-crown-6) as a dispersing agent. From UV-vis spectroscopy, it was confirmed that low n phases (~440 nm) considerably decreased with 18-crown-6 and the average lifetime also enhanced due to reduced defect sites. Moreover, temperature-dependent PL represented increased exciton binding energy from 24.6 to 63.9 (meV) owing to the fine dispersion of PEABr and dielectric confinement effect. Finally, with optimized concentration, highly bright perovskite films with maximum PLQY of 93.036 % could be achieved, which is 47 times higher than pristine films.

References
1. Kim Y-H, Cho H, Lee T-W. Metal halide perovskite light emitters. Proc Natl Acad Sci. 2016;113(42):11694-11702.
2. Kim YH, Cho H, Heo JH, et al. Multicolored organic/inorganic hybrid perovskite light-emitting diodes. Adv Mater. 2015;27(7):1248-1254. doi:10.1002/adma.201403751
3. Zhao L, Yeh YW, Tran NL, et al. In Situ Preparation of Metal Halide Perovskite Nanocrystal Thin Films for Improved Light-Emitting Devices. ACS Nano. 2017;11(4):3957-3964. doi:10.1021/acsnano.7b00404
4. Ban M, Zou Y, Rivett JPH, et al. Solution-processed perovskite light emitting diodes with efficiency exceeding 15% through additive-controlled nanostructure tailoring. Nat Commun. 2018;9(1):1-10. doi:10.1038/s41467-018-06425-5

Perovskite nanoparticles (PeNPs) is attractive next-generation sources for light-emitting diodes (LEDs) due to their efficient luminescent, solution processability, tunable bandgap and extremely high color purity which is superior to conventional light emitters (e.g. organic emitters, traditional quantum dots). However, due to halide segregation, emmision wavelength shift while LED operation and precise control of emission wavelength is limited. Here, we develop facile atomic surface defect passivation method exploited post-synthesis treatment using high bandgap perovskite precursor. With this strategy, we synthesized color-stable and pure-red-emitting CsPbI₃ PeNPs with 630 nm of peak emission wavelength with core-shell structure. Highly efficient PeNP LEDs without electroluminescence wavelength shift was also demonstrated.

Quasi two-dimensional (2D) perovskites are currently considered as materials of choice for light emitting diodes (LEDs) due to their unique optoelectronic properties such as strong exciton binding energy, multiple quantum well structure, and reasonable ambient stability for practical applications. Tremendous progress has been achieved in 2D perovskites LEDs in terms of external quantum efficiency (EQE) (~17%), luminescence (6785 cd/m²) and stability through compositional and device engineering. However, the excessive surface defect states generated due to the reduced crystal size, the presence of un-coordinated Pb⁺² ions or shallow vacancy defects generated by MA or halide ions at the surface and grain boundaries, limits the radiative recombination and in turn the electro-luminescence (EL) efficiency of the LEDs. It has been realized that surface passivation with suitable organic/inorganic moieties can play a vital role in overcoming the adverse effect of defect states through their interaction with uncoordinated ions and can improve the stability.
In the present work, we have chosen n=4 composition quasi-2D perovskite (BA)₂(MA)₃Pb₄Br₁₃ films as emissive layer by optimal phase engineering and have shown the order of magnitude enhancement of radiative emission after passivating them with two different molecules namely- triphenyl phosphine oxide (TPPO) and an insulating polymer polymethyl methacrylate (PMMA). Both the steady state and time resolved study confirm up to 400% enhancement in fluorescence intensity and improved average lifetime (=19.4 ns) after surface passivation. Additionally, the effect of passivating agents on film morphology and radiative emission efficiency is studied in details. Also, the interaction mechanism between layered perovskite and passivating agents has been probed with FTIR spectroscopy, XPS and KPFM study. Our systematic investigation with detailed mechanistic study will pave the way for further development of the highly efficient LEDs.
References: -
Yang, Xiaolei et.al Nat. Commun. 2018, 9 (1), 2–9.
Bi, Wentao et.al ACS Appl. Mater. Interfaces 2020, 12 (1), 1721–1727.
Na Quan, L et.al Nat. Commun. 2020, 11 (1), 1–9.

The type I heterostructure forms between Organic semiconductor molecule (4,4’-bis[9-dicarbazolyl]-2,2’-biphenyl [CBP] and bathocuproine [BCP]) and methylammonium lead halide (MAPbX₃), assists easy growth of perovskite nanocrystals (PNCs) in an organic matrix. Herein, we present the modulation of electronic states of MAPbX₃ PNCs (MA = CH3NH3⁺ and X= Br^- and I^-) as a function of crystallite size. This results in the growth of MAPbX₃ PNCs of tunable sizes from ~110 nm to ~10 nm in the organic semiconductor matrix. A blue shift is observed in the photoluminescence peak (PL_max) energy over a range of ~200 meV for MAPbI₃ PNCs embedded in the BCP matrix. However, PL_max energy tunes over ~32 meV only with a similar volumetric concentration in the case of MAPbBr₃ PNCs in the same BCP matrix. Moreover, the PL blue shift is even lower in the case of the CBP matrix in comparison with the BCP matrix for both the perovskites. This discrepancy could be resolved by determining the resultant crystallite size using X-ray diffraction studies and the Debye–Scherrer formula. Results about the blue shift in the PL peak can be explained using the classic particle-in-a-box versus excitonic Bohr radius model under a weak confinement regime.
References

1. S. Singh, A. Dey, S. Kolay and D. Kabra, Modulation of electronic states of Hybrid lead perovskites embedded in organic Matrix. Energy Technol. 2019, 1900894.
2. D. Di, K. P. Musselman, G. Li, A. Sadhanala, Y. Ievskaya, Q. Song, Z.-K. Tan, M. L. Lai, J. L. MacManus-Driscoll, N. C. Greenham, R. H. Friend, Size-dependent photon emission from organometal halide perovskite nanocrystals embedded in organic matrix. J. Phys. Chem. Lett. 2015, 6, 446.

Quantum dots (QDs) are very important light emitting materials that can be used in various applications (LED, bioimaging, luminescent solar concentrators and so forth)[1]. The photophysical properties of QDs are not only size and composition dependent, but also determined by the shape and morphology of the sample[2]. The investigation on the synthesis of Cd-based binary anisotropic nanorod QDs have been well explored to date, but using of heavy toxic metal (Cd) in the reaction impede the practical application of Cd-based QDs regarding serious environmental concerns and the unavoidable health damage for biomedical applications. Therefore, there is an urgent need for the development of toxic metal free fluorescent Cu-based quantum nanostructures with anisotropic morphology.
In this project, anisotropic fluorescent Cu-based multicomponent core/shell/shell heteronanonails (HNNs) have been prepared by using the CIZSe/CIZS core/shell structure with wurtzite (WZ) crystal structure as the seed to coat the ZnS shell with a seeded growth method under fine control. Aliquots have been taken out from the reaction vessel at certain time intervals for further characterization and the growth mechanism of the particular anisotropic nanorod structure has been proposed based on the characterization results. Then the Au nanoparticles have been deposited on the surface of the preformed HNNs to give the Au-CIZSe/CIZS/ZnS nanocomposites. These nanomaterials are expected to find important applications in energy harvesting, sensing and nanobiotechnology.

References:

[1] X. Bai, F. Purcell-Milton, Y. K. Gun'ko, Nanomaterials (Basel) 2019, 9.
[2] D. Sahin, B. Ilan, D. F. Kelley, Journal of Applied Physics 2011, 110.

The ability to dynamically tune light emission through controlled structural transformations has promise in research and application areas. Structural transformations are ubiquitous at all length scales in chemistry, spanning from isomerization reactions of small molecules to solid-solid transformations in bulk crystals, which have generally been studied in isolation. Despite attempts to merge understanding of these disparate regimes by reducing domain size in solids to nanocrystalline dimensions, bulk-like solid-solid transformation behavior still predominates at length scales approaching those of molecules. In-between small molecules and nanocrystals, magic-sized clusters (MSCs) provide an advantageous experimental platform to study isomerization in well-defined atomically precise systems. Additionally, MSCs are atomically identical species, with a narrow optical absorption and potential for precise color emission. Here we show here that a reversible transformation between CdS cluster isomers with distinct stable configurations possesses essential characteristics of both solid-solid transformations and molecular isomerization reactions. These isomers, termed α- and β-(Cd₂S)_x, interconvert (α-to-β/β-to-α) reversibly, as identified by a 140 meV shift in the species’ excitonic energy gap, that is, the absorption edge and peak emission wavelength is precisely tunable and reversible. A characteristic displacive reconfiguration of the inorganic core (solid-solid transformation), as evidenced by our reconstruction of the atomic pair distribution function from x-ray scattering, accompanies the change in electronic structures. The first order kinetics of the transformation—indicative of molecular isomerization—are driven by a distortion of the ligand binding motifs due to the presence of hydroxyl species in the ligand shell. Chemical control over the surface energy boundary conditions via adsorbates appears to be the exclusive determinant of “phase” stability in this system. The reversible transformation of MSCs reported here presents the missing bridge between molecular isomerization and solid-solid transformations. Manipulation of MSCs, which serve as seeds and as a monomer reservoir in nanocrystal growth, will provide an additional strategy for control over the structure of colloidal nanocrystals.

Fluorescent carbon nanoparticles are stable materials that offer wide application potential ranging from the health to sustainable catalysis. Various carbon nanodots (CNDs) already reported, mainly synthesized from organic molecular precursors as a carbon source, and variety of amines as a nitrogen dopant. Photoluminescence (PL) emission of CNDs proposed to originate from the polycyclic aromatic carbon-core and in-situ synthesized molecular fluorophores. Higher temperatures and longer times of pyrolysis known to form higher order of polycyclic aromatic carbon-core. This work reports the CNDs and their fluorescence emission parameters in variety of solvents by means of steady state and time-resolved emission spectroscopies. Response of fluorescence emission lifetime and emission quenching rate constants to changes in solvent parameters such as polarity and tumbling lifetime were essentially independent, unlike molecular fluorophores that display solvent-dependent emission parameters. Fluorescence emission quenched in nitromethane additionally indicating to the polycyclic aromatic carbon-core as a predominant structural feature of the CNDs. The quenching of CNDs emission in the presence of benzophenone that has strong triplet component in excited state was observed. Quenching demonstrate the Stern-Volmer behavior and reveals the additional nonradiative decay pathways of CNDs. The main photophysical features of CNDs are discussed in terms of fluorescence emission originated from the excited state of the polycyclic aromatic carbon-core where contribution from the potential molecular fluorophores considered minimal. Contribution of the molecular fluorophores to emission mechanism of CNDs brings extra complexity and has strong impact on further development of CNDs, therefore, considering the importance of their minimal effect in emission parameters is discussed.

Carbon dots (CDs) are nanoparticles that exhibit strong photoluminescence (PL) emission throughout the visible spectrum. In addition to strong PL emission, they possess several other promising properties, e.g., high water solubility, easy functionalization, excellent biocompatibility, electron donating and accepting capabilities, and low toxicity. Due to these attractive properties, with easy and low-cost synthesis, and abundance of the raw materials, they have been extensively studied as an alternative to conventional semiconductor quantum dots. Different top-down and bottom-up synthesis methods have been utilized to synthesize CDs. Recent studies highlight the presence of fluorescent byproducts in CD dispersions, especially in CDs synthesized by chemical methods.
Here, we synthesize CDs using citric acid and 1,5-diaminonaphthlene by the solvothermal method, and study their optical and structural properties utilizing UV-visible, PL, Raman and Fourier Transform-Infrared (FTIR) spectroscopy techniques. The long-term impact of reaction byproducts on the stability of the physical and optical properties of CDs is also studied. A significant increase in particle size is observed as a function of time after synthesis. Transmission electron microscopy analysis of unpurified CDs shows their average size increases from 23.5 ± 9.7 nm to 63.2 ± 16.5 nm during the 13-month period after synthesis. UV-visible and FTIR spectra show strong absorption bands associated with C=O bonds. These absorption bands become less intense with time after synthesis, implying that C=O states decrease with time. PL emission from CD dispersions is measured using different excitation wavelengths. CDs that have not been purified show strong blue emission with a quantum yield of 30%, which is attributed to C=O states mostly associated with molecular fluorophores. In addition to blue emission, CDs also show weak green emission which becomes distinctive only at excitation wavelengths longer than 400 nm. With storage time, the blue emission is substantially reduced (quantum yield of 3% after 13 weeks) as C=O states decrease. This reduction in C=O states is accompanied by an increase in the average size of the CDs. From these observations it is concluded that the blue emission is mostly from the molecular fluorophores which slowly decompose and attach to the CDs, and cause an increase in particle size.
When CDs are properly purified, the intensities of absorption bands in UV-visible and FTIR spectra corresponding to C=O states and the blue PL emission decrease considerably. The quantum yield of purified CDs is just 2.6 %. Besides the blue emission, a solvent-dependent yellow emission arises, which is almost negligible when CDs are dispersed in water; however, the intensity of yellow emission increases significantly when CDs are dispersed in ethanol. This emission is attributed to solvent-activated surface states on CDs. Considerable reduction of blue PL emission along with C=O states after purification is accompanied by stable particle size. These observations strengthen our conclusions that the blue emission originates from free floating molecular fluorophores instead of CDs, and particle size increase is caused by deposition of impurities and subsequent deposition on the CD surface.

Quasi-two-dimensional (quasi-2D) perovskite is rising as an efficient luminescent material for blue light emitting diodes (LEDs). However, typically reported quasi-2D perovskites with a wide distribution of small-n phases show low LED efficiency owing to the inefficient internal energy transfer. Meanwhile, the inevitable defects and traps generated during the perovskite crystallization increase non-radiative recombination, which further aggravate the emission efficiency. In this work, we demonstrate a new class of mixed ligand quasi-2D perovskite system of PEA_2-xPA₂(CsPbBr₃)_n-1PbBr₄ (where PEA is phenethylammonium and PA is propylamine) by introducing a 2D perovskite (PEA₂PbBr₄) into PA₂(CsPbBr₃)_n-1PbBr₄ quasi-2D perovskite solution. The mixed ligand quasi-2D perovskite shows the unique feature of simultaneous suppression of low-order perovskite phase and defect passivation in quasi-2D perovskite for achieving efficient energy transfer and light emission. We theoretically and experimentally unveil the phase transformation mechanism. By fabricating blue perovskite light-emitting diodes (PeLEDs) from the mixed ligand quasi-2D perovskites, a high external quantum efficiency (EQE) of 7.51% with a brightness of 1765 cd m^-2 and a very low turn-on voltage of 3.07 V is achieved for the blue perovskite light-emitting diodes (PeLEDs). Our most recent result show that by further modifying the mixed ligand quasi-2D perovskites, EQE can reach over 10%. the which is among the highest efficiency of blue PeLEDs. Equally important, the PeLEDs show a very long operational stability of 3961 sec under continuous constant driving current together with very stable EL spectra without any peak shift during the entire stability test [1], which are superior to the reported blue PeLEDs with unstable EL spectra during operation, and thus contribute to the evolution of highly efficient and stable PeLEDs.

[1] Z. Ren, L. Li, J. Yu, R. Ma, X. Xiao, W. Yin, R. Chen, K Wang, X.W. Sun, and W. C. H. Choy*. submitted.

Lead halide perovskite nanocrystals exhibit tunable photoemission properties across the entire visible spectrum, which make them highly promising materials for use in photovoltaic and light-emitting devices. Whereas green-emitting perovskite nanocrystals have achieved near-unity photoluminescence quantum yield (PLQY), their blue-emitting counterparts still lag-behind in terms of both PLQY and colloidal stability. Recently, two-dimensional colloidal perovskite nanoplates (NPLs), with strong 2-D confinement of charge carriers and large exciton binding energy, have attracted attention as an alternative to the mixed-halide core nanocrystals; these tend to experience phase segregation when used as light-emitting materials, for example. However, colloidal NPLs are strongly susceptible to surface defects, resulting in low PLQYs. They also spontaneously undergo morphological transformation manifesting in a shift of their emission to the green region of the spectrum.
Herein, we introduce a surface ligand-engineering strategy that enhances the PLQY, while preserving the shape of perovskite NPLs, using multi-coordinating poly-salts as ligand based on quaternary ammonium or imidazolium anchoring groups. These ligands are prepared using ring opening reaction starting from polymaleic anhydride copolymers. When complexed with PbBr₂ these salt groups strongly interact with the NPL surfaces, which significantly enhances structural stability while increasing the PLQY of these blue-emitting materials up to 70-80%. As such, these nanocrystals can be stored and processed using a wide range of solvents, while maintaining color purity. These finding are highly promising for use in developing blue light emitting devices.

Perovskite light-emitting diodes (PeLEDs) have showed significant progresses in recent years, the external quantum efficiency (EQE) of electroluminescence in green and red regions have been beyond 20%, while the efficiency in blue is far lag behind. Here, we propose to add a large cation CH₃CH₂NH₂⁺ (EA) into the Cs⁺ site in PEA₂(CsPbBr₃)₂PbBr₄ perovskite with green emission to decrease the Pb-Br orbit coupling and increase the bandgap for blue emission. X-ray diffraction and nuclear magnetic resonance results confirmed that the EA has been successfully incorporated into CsPbBr₃ lattice for formation a PEA₂(Cs_1-xEA_xPbBr₃)₂PbBr₄ perovskite. This method provide a versatile tool to modulate the photoluminescence from green region (508 nm) into blue (466 nm), and over than 70% photoluminescence quantum yield (PLQY) in blue is obtained. In addition, the emission spectra is stable under light and thermal stress. While configuration of PeLEDs with 60% EABr, as high as 12.1% EQE of sky-blue electroluminescence located at 488 nm has been demonstrated. As we know, this is the highest efficiency for PeLEDs in blue region up to now, which will pave the way for the full color display for the PeLEDs.

Perovskite Light Emitting Diodes (LEDs) are a rapidly exploding field, driven by their low cost, facile synthesis, and color purity. Indeed, quantum efficiencies over 20% in both red and green emission have been demonstrated. Yet, key weaknesses in both spectral and operational stability hinder the long term application of these materials, and these issues are particularly exacerbated in blue devices, where both efficiencies and stabilities remain quite low. In this talk, we will demonstrate how atomic doping into the perovskite structure allows us to address these challenges, imparting improved efficiency and stability across different emission wavelengths and compositions. Doping is an effective strategy to drive perovskite LEDs towards commercial applications.

The remarkable solar performance of lead halide perovskites can be attributed to their excellent physical properties that present many mysteries, challenges, as well as opportunities. Better control over the crystal growth of these fascinating materials and better understanding of their complex solid state chemistry would further enhance their applications. Here I will first report new insights on the crystal growth of perovskite materials and the solution growth of single crystal nanowires and nanoplates of various 3D lead halides perovskites (APbX₃, A = MA, FA, Cs) via a dissolution-recrystallization pathway. These perovskite nanowires have demonstrated high performance room temperature lasing with broad tunability of emission. Moreover, chemical strategies to stabilize the metastable perovskite phases, such as FAPbI₃ and CsPbI₃, have been developed by using surface ligands to manipulate the delicate thermodynamic and kinetic balance between 3D and 2D layered perovskites. Such stabilization also allows the incorporation of unconventional large A site cations, such as guanidinium (GA) to be incorporated into n 2D Ruddlesden−Popper (RP) perovskites, and their effects on the physical properties of perovskites are studied. The excellent properties of these 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 lasers, LEDs, and other optoelectronic applications.

ZnO nanocrystal films grown at ultrasonic spray pyrolysis and doped by In atoms have been investigated. The In impact on optical and structural parameters, and the evolution of O- and In- related defects versus In contents have been studied by means of the scanning electron microscopy, Raman scattering, photoluminescence and X ray photoelectron spectroscopy (XPS) methods.
Raman scattering spectra revealed the first and second order Raman peaks at In doping 0.5-2.5at% In. Raman peaks shift to higher energy, such as: 231, 245, 285, 324, 438, 481 and 481cm^-1 owing to decreasing the inter-planar distances in crystal lattice in comparison with un-doped ZnO stimulated by elastic strains. Simultaneously, the intensity of near band edge (NBE) bands enlarges. At In contents (1.0-3.0at%) the PL band (3.034eV) appears in PL spectra and its peak shifts to lower energy with increasing In concentrations. At higher In contents (3.0-5.5at%) the intensities of Raman peaks and NBE bands falling down owing to the formation of O- and In-related defects and decreasing the ZnO film crystallinity.
To analysis In – complex defects the high resolution XPS spectra were measured for lines: i) In 3d_3/2 and In3d_5/2, and ii) In4d_3/2 and In4d_5/2. In the XPS spectrum of ZnO:1.0%In NCs the doublets: In 3d_3/2 and In3d_5/2 with the energies of 451.7eV and 444.2eV, and overlapped In4d_3/2 and In4d_5/2 doublet peaks with the energies of 18.48eV and 17.28eV, respectively, have been detected. These peaks are typical for In³⁺ ions in the In₂O₃ crystal lattice. The In3d and In4d doublets shift to lower binding energies versus In contents, approaching the energies of 450.9 and 443.5 eV, as well as 17.58ev and 16.38eV, respectively, in the ZnO:5.5% In NC films. The last peaks are closer to the In^o doublet lines in the metallic In crystal. Thus, this is a trend connected with the In atom coagulation and formation of In complex defects at higher In contents in ZnO:In NCs. Simultaneously, the oxygen vacancies have appeared in the studied films together with film lattice disordering. The optimal In concentration range to fabricate the ZnO films with the high optical parameters has been estimated.

Two-dimensional (2D) perovskite nanoplatelets have attracted interest for light-emitting applications due to their strong quantum- and dielectric-confinement, directional emission, and tunable color throughout the visible spectrum. In this work, we demonstrate room-temperature synthesis of atomically-thin colloidal organic-inorganic hybrid perovskite nanoplatelets (Chemical formula: L₂[ABX₃]_n-1BX₄, L: alkylammonium, A: methylammonium or formamidinium, B: lead, X: halide, n(=1 or 2): number of PbBr₆^4- octahedral layers in out-of-plane direction) incorporating manganese (Mn²⁺) dopants using the ligand-assisted reprecipitation method. Substitutional doping of manganese for lead enhances the photoluminescence quantum yield (PLQY) and introduces a broad, red-shifted emission feature in bromide nanoplatelets. We find that the choice of organic cation – in addition to the Pb:Mn ratio - strongly affects the final dopant concentration and the overall emission spectrum. We investigate the photophysics of Mn-doped lead bromide nanoplatelets (varying thickness and A-site cation) using time-resolved photoluminescence, excitation intensity-dependent PLQY, and spectral analysis, and we propose a kinetic model describing exciton dynamics between free-exciton and dopant-bound excitonic states. This work demonstrates a materials platform for tunable dual-wavelength emission from a 2D material and provides new understanding of photophysical interactions between localized Mn²⁺ dopants and delocalized band-edge excitons in 2D perovskites.

Besides photovoltaics, which represents the main application focus of metal halide perovskites in the last decade, lighting has emerged as another appealing field. Perovskite films demonstrate readily tunable emission color, sharp luminescence spectra and high external quantum efficiencies exceeding 21 % as emissive layers in perovskite LED (PeLED). Limited long-term stability of perovskite layers under electrical stress and the lack of deposition tools with process control on an industrial level remain as the critical obstacles for the commercialization of PeLED. Chemical vapor deposition (CVD) is an attractive highly scalable alternative to established solution-based techniques, enabling thin-film fabrication with high purity and reproducibility on large-area substrates.
Here, we introduce a custom-developed perovskite hot-wall showerhead CVD tool along with an optimized low-vacuum (5 hPa) CVD process to obtain dense perovskite films. We employ the organo-halide CH₃NH₃Br (MABr) and the metal-halide PbBr₂ which are simultaneously introduced into the reactor to synthesize MAPbBr₃ layers. To achieve this, the individual halide precursors are thermally sublimed (MABr: 175 °C, PbBr₂: 410 °C) and transported via separate N₂ carrier gas flows (100 sccm each) to a static mixing unit. Due to a laminar flow regime, forced intermixing is essential to obtain uniformly distributed precursor molecules in the merged gas stream. A high-temperature run-vent system is employed to prevent undesired deposition during heat-up and cool-down phases. The heated showerhead spreads the carrier gas flow, enabling homogeneous deposition on temperature-controlled substrates up to 108 cm² in area. The substrates consist of a layer stack of poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA, 10 nm) / ITO (100 nm) / glass (substrate size 2.5 cm x 2.5 cm). We investigated the influence of the substrate temperature in the range of 60 °C – 110 °C on the morphology, optical and photophysical properties of the deposited films. The 110 °C-sample exhibits a highly porous morphology and a pronounced vertical growth mode, being unsuitable for device integration. Photoluminescence (PL) spectroscopy reveals green light emission at 530 nm, which is characteristic for MAPbBr₃. However, the emission is of low intensity, which likely can be attributed to increased non-radiative recombination losses induced by thermal perovskite degradation, e. g. during reactor cool-down. A reduction of substrate temperature provides a lower energy barrier of nucleation, leading to the formation of smaller crystallites and less porous layers at 100 °C. Besides light emission at 530 nm, a second peak at 500 nm is observed in PL measurements. Decreasing the substrate temperature further to 60 °C leads to dense films and light emission exclusively at 500 nm. The blueshift of 30 nm may be attributed to the formation of MAPbBr₃-nanoparticles with a size of 7 – 8 nm, emitting higher-energy photons due to the quantum confinement effect. Furthermore, the emission peak of the 60 °C-sample features a 200x enhanced intensity. The lower substrate temperature induces stronger MABr condensation, passivating perovskite grain boundaries. Thus, the high-intensity PL at 500 nm can be explained by suppressed non-radiative recombination due to MAPbBr₃-nanoparticles incorporated in an MABr matrix.

Ongoing work will examine the electrical properties of fabricated CVD layers in PeLED structures and will also focus on CsPbBr₃ as a thermally more robust perovskite compound.

Perovskite light-emitting diodes have recently exceeded the 20% external quantum efficiency (EQE) milestone. In particular, we have shown in our recent work that with careful device design, it is possible to achieve near-unity internal quantum efficiencies (IQEs) [1], demonstrating the potential of perovskite materials for extremely efficient electroluminescence. In this presentation, we discuss some design considerations behind the high IQEs, and for achieving EQEs that can approach the theoretical limits [1, 2]. We discuss the lessons learned from our recent studies in high-efficiency OLEDs [3. 4], and the important role of charge-transport interfaces in the realization of this goal [5]. Transient spectroscopy experiments are carried out to illustrate how radiative and non-radiative processes play their roles in the high-efficiency devices. Our work on perovskite LEDs may provide useful implications in the design of high-performance perovskite solar cells.
1. B. Zhao, R. H. Friend*, D. Di* et al, “High-efficiency perovskite-polymer bulk heterostructure light-emitting diodes”, Nature Photonics 12, 783–789 (2018).
2. C. Cho, R. H. Friend, D. Di*, F. Deschler*, N. C. Greenham* et al, “The role of photon recycling in perovskite light-emitting diodes”, Nature Communications 11, 611 (2020).
3. D. Di, A. S. Romanov, L. Yang, R. H. Friend, M. Linnolahti, M. Bochmann, D. Credgington et al, “High-performance light-emitting diodes based on carbene-metal-amides”, Science 356, 159-163 (2017).
4. L. Yang, V. Kim, Y. Lian, B. Zhao, D. Di* , “High-efficiency dual-dopant polymer light-emitting diodes with ultrafast inter-fluorophore energy transfer”, Joule 3, 2381-2389 (2019).
5. B. Zhao, D. Di*, R. H. Friend* et al, under review (2020).

Perovskite light emitting devices have drawn considerable attention for their favorable optoelectronic properties. High carrier mobilities make perovskites excellent candidates as host materials in electroluminescent devices, and iridium complexes serve as highly efficient triplet emitters. To achieve high performance in a simple single layer device, we employed a CsPbBr₃ perovskite host and a novel ionic iridium complex guest along with a polyelectrolyte to demonstrate efficient light emitting electrochemical cells (PeLECs). For an optimal guest/host blend, 10600 cd/m² luminance at 11.6 cd/A and 9.04 Lm/W were achieved. These devices showed voltage-dependent electroluminescence color proceeding from orange-red to green, facilitated by the reconfigurable ionic materials blend. Optimized devices exhibited enhanced stability under constant current driving. Our rationally-designed ionic guest at an optimal concentration of the host produced efficient (>90%) Förster energy transfer and improved thin film morphologies for high performance PeLEC operation enabled by ionic migration to interfaces.

Traditionally, CMOS is not considered a light emitting device. Electronic instruments based on advanced CMOS technology function without lights in closed dark packages. Similar to laser or LED, photonic CMOS is a light emitting device. A laser is fabricated in the MOSFET drain region. A photon sensor or avalanche photo diode (APD) is fabricated in the MOSFET channel / well region. CMOS, laser and photon sensor are manufactured as one integral device. When the MOSFET is off, laser and photon sensor are also off. When the MOSFET is on, light from the laser is absorbed by the photon sensor or avalanche photo diode, triggering a large photonic current flowing back to the MOSFET drain to produce more lights. This process forms a positive feedback loop for conversion of light and electrons, and reduces heats.

Very low series resistance laser diodes, with forward voltage near 0V, are available, and suitable for the photonic CMOS.

In this paper we will discuss techniques to process small RF light signals with photonic CMOS under certain DC biases. In order to process the light signals, optical waveguide or air optical paths need to be created in an RF ASIC or analog ULSI circuit.

Additional features, such as polarization, optical filter, and grating thin films, can be added to the circuit for processing lights.

For digital switching devices in ULSI or ASIC, the design of a CMOS circuit may need to be optimized for optical processing. For traditional ICs, an electronic voltage is applied to the CMOS gate to turn the transistors on. In a optoelectronic IC, in additional to the electronic voltage, an optical voltage can be generated by directing a laser beam to the CMOS gate. Together electronic and photonic voltage may switch the CMOS invertor to "on". This new function not only increases the CMOS speed, but also reduces heat and power consumption.

A traditional SRAM is a 4 or 6 transistor bi-stable circuit, which consists of two back-to-back CMOS invertors. The speed of SRAM depends on how efficiently the pull-up and pull-down transistors operate for transferring electrons. A "pre-charging" step is always necessary prior to memory operations (WRITE / READ / ERASE).

In theory, the speed of an optoelectronic SRAM, which consists of two back-to-back photonic CMOS invertors, may surpass a traditional SRAM, because it takes less time to remove photon-generated electronics. For instance, when a positive voltage is applied to a pull down NMOSFET, holes need to be removed from the pull-up PMOSFET. If part of the holes are generated by laser from a photonic MOSFET, they can be more quickly removed when the laser if off.

To accomplish this much higher speed photonic SRAM, local interconnect must include optical waveguide or air optical paths, in addition to the traditional metal wires.

Finally, we would like to discuss how to expand the functions of photonic signal processors using nonlinear optics.

Nonlinear optical films can be fabricated in a photonic CMOS device in the isolation, buried oxide (in the case of FDSOI or UTSOI), or as optical waveguides connecting the CMOS. The nonlinear optical operation may increase the photon absorption rate of the embedded photon senor or APD in a photonic CMOS, and quantum efficiencies. Promising near IR nonlinear optical materials will be illustrated, and the requirements for efficient second harmonic generation (SHG).

Although perovskite light emitting diodes (PeLEDs) have shown fast advances in external quantum efficiency (EQE), their operational stability is still low compared to other light emitting diode (LED) technologies. Here, we show that by engineering the device stack for optimal band alignment and high mobility, the operating voltage of the PeLED is remarkably reduced, i.e. reaching high current density at low voltage. In depth analysis of the device characteristics shows that the low operating voltage of the stack leads to a significant mitigation of Joule heating. This mitigation leads to a better device performance at high current densities where the PeLED shows < 10% EQE roll-off at 800 mA/cm². Furthermore, pulsed excitation and low temperature measurements allow us to show that ion-migration is the root cause for device degradation, while Joule heating accelerates this process. Based on this understanding, we resort to a perovskite layer with high ion-migration activation energy. Together with the optimized device stack, this allows us to demonstrate an unprecedented half-life time (T₅₀) of 110 hours at room temperature, when biased at 100 mA/cm². At 163 K, it even displays T₉₈ = 90 h at 300 mA/cm². Our optimal device shows an EQE of 10.5% at 400 mA/cm² and a radiance of 351 W Sr^-1 m^-2 at 3.15 V, which rivals some of the highest performing stacks. Achieving an operationally stable high-performance PeLED at high current densities is crucial for a potential demonstration of an electrically pumped perovskite laser. To this end, we fabricate scaled PeLEDs down to 20 μm in diameter, and demonstrate reliable electrical pulsing at current densities > 1 kA/cm². Pulsed excitation provides important insights about EQE roll-off at current densities relevant for injection lasing. Furthermore, unlike commonly used PeLED stacks, our PeLED shows optimal charge injection at low temperatures down to 163 K, which provides an opportunity to study PeLEDs at intense excitation at low temperatures.

Noble metal nanoparticles are well-known for presenting plasmonic coupling with molecular excitons to enhance the performances of opto-electronic devices, especially light emitting diodes and solar cells. The higher momentum associated with metal surface plasmons can alter the lifetime of molecular excitons, thereby increasing the efficiency in the respective devices and enhancing the outcoupling. Solution processed metal nanoparticles are usually capped with selective surfactants which play significant role in their applications. In particular, for perovskite light-emitting diodes (PeLEDs), this kind of application positively contributes to the efficiency and stability and helps overcome the interface and charge transfer problems between hole injection layer (HTL) and emission layer (EML).

Herein, we have investigated the effect of incorporating size dependent gold nanoparticles (AuNPs) for altering the charge balance in PeLEDs. In addition, we studied the far-field interaction of AuNPs plasmons and charge trap by AuNPs through theoretical and experimental tools. To clearly understand the efficiency improvement tactics, we have fabricated PeLED samples with methylammonium lead bromide (MAPbBr₃). AuNPs were incorporated in PeLED through the poly(3,4-ethylenedioxythio-phene):poly(styrene sulfonate) (PEDOT:PSS) solution as hole injection layer (HIL). To understand size dependent behaviors, AuNPs with sizes 10, 50, 90, 100 nm were inserted in PEDOT:PSS. Interestingly, even though we used the conventional perovskite (MAPbBr₃) material, the obtained results speculated the possibility of enhancement in other perovskite materials such as CsPbBr3 or FAPbBr3. By controlling the size of Au-NPs, we got the highest efficient inflection point at Au-NPs 90 nm.
By monitoring the lifetime of the excitons through time resolved photoluminescence (TRPL), the effect of plasmon over the perovskite photons can be delved. Each sample except the reference (w/o AuNPs) showed improved exciton lifetime, owing to far-field scattering effect by the AuNPs. We analyzed the modifications by AuNPs in the PEDOT:PSS thin film using ultraviolet photoelectron spectroscopy(UPS) and observed that the work function increment with increased AuNP size. It is beneficial for reducing the hole injection barrier between HTL and the deep HOMO of perovskite. As expected, an increase in efficiency was observed with the increase in size of AuNPs, however the AuNPs with 90nm size was the inflection point. Using LT₅₀ measurement, it was cross-evaluated that the efficiency of the device and the AuNPs size are directly proportional. To further investigate this issue, we fabricated hole-only-device (HOD) for the Richardson-schottky equation. It contains variables for carrier trapping or defect, so additional analysis was conducted to confirm the effects of trap and defect through the temperature-dependent I-V curve.
Device lifetime increasement with the addition of AuNP might attributed to the capping layer PVP. We believed that the hydrophobic nature of PVP ligand on top of AuNPs helps to obstruct the indium diffusion from ITO as well as halide ion from MAPbBr₃. Even though PVP serves as a capping layer, it exhibits hydrophobic passivation which prevented water penetration into perovskite. By applying PVP-capped AuNPs, we demonstrated the improved device efficiency and stability in perovskite LED in parallel.

The reproducibility of results is one of the corner stones of scientific research. However, often in emerging technologies, the reported results tend to be sensitive to the chosen measurement protocol. This can stem from measurement artifacts or from poorly understood complex underlying phenomena. Since their rise as an exciting material system for optoelectronic devices, metal halide perovskites have shown anomalous hysteresis effects in the current density-voltage (J-V) characteristics. In the case of perovskite solar cells, it has been shown that this anomalous hysteresis effect, which is highly dependent on the voltage scan rate, range, direction as well as the device structure, could significantly influence the extracted device metrics and the reported power conversion efficiency value. This has triggered wide discussions about the best practices to report perovskite solar cell performance to achieve consistency between different research groups. On the other hand, less attention is so far given to the anomalous effects that can affect the reporting of the characteristics of perovskite light emitting diodes (PeLEDs). Here, we show that even for PeLEDs with little J-V hysteresis, the electroluminescence (EL) intensity changes by an order of magnitude on a timescale ranging from 10 msec to few seconds. This change is directly observed in the measured value of external quantum efficiency (EQE). Such reversible EL transient which is likely caused by ion-migration, can be followed by a roll-off regime dominated by Joule heating, and potentially device degradation when measured at high current densities. Naturally, this transient response leads to significantly different device characteristics depending on the measurement procedures for the same device. We directly explain those implications using the commonly used J-V-R scans with different sweep parameters, and show that these scans are not portraying unique device characteristics, but rather a snapshot in which the true device characteristics are entangled with inter-dependent underlying processes of ion-migration, Joule heating and device degradation in a non-linear way. Therefore, we propose a method based on pulsed excitation that allows better reproducibility and interpretation of the measured device characteristics. Furthermore, we also provide suggestions on reporting PeLED characteristics, as input for further discussions in the scientific community.

In the past six years, perovskite light-emitting diodes (LEDs) have experienced rapid development. Thanks to the superior optoelectronic properties, perovskite material has made itself as a competitive candidate for next-generation displays and lighting. With the great efforts of researchers in this field, the external quantum efficiencies (EQEs) have already surpassed the 20% milestone for both red and green colored perovskite LEDs. Lately, the blue perovskite LEDs have been reported to achieve an EQE higher than 12%.
With the fantastic material quality, perovskite material can achieve a very high photoluminescent quantum efficiency (PLQY); therefore, the internal QE for perovskite LEDs can also reach the level as high as more than 90%. As a result, the key solution to the further enhancement of the perovskite LEDs performance lies in the light extractions/ outcoupling for perovskite LEDs.
During the past few years, our group has devoted ourselves to the development of nanophotonic strategies for the performance enhancement of the perovskite LEDs. The strategies include two main categories. The first kind is to use a nanophotonic substrate for perovskite LEDs devices. The structure we designed was a combination of a nanodome coupler and the photonic crystal optical antennas. When the geometry of the structure was properly optimized, the light originally trapped inside the active layer can be first coupled into the photonic crystals. After that, the photonic crystals, working as optical antennas, can convert the guided modes to the leaky modes and finally couple out the leaky modes to the air. With this delicate strategy, we successfully improved the EQE of the perovskite LEDs from about 8% to 17.5%, which was the record for the MAPbBr₃ based perovskite LEDs at that time.
The second category of the nanophotonic strategies is making nanostructured active materials instead of using the nanostructured out-coupling substrates. With the exploration of varieties of nanostructures, we finally decided to use the nanowire arrays for the perovskite LEDs study. Very intriguingly, we found a solution method with the nanoporous template for perovskite nanowires growth. By studying the growth with different template geometries, we found out that the nanowire growth had a strong relationship with the capillary effect. Then we carefully designed the template thickness and the nanowire lengths, and we made the three-dimensional nanowire arrays based perovskite LEDs. Compared to the planar counterparts, the nanowire-based perovskite LEDs showed an enhanced EQE (from 11% to 16%), which is due to the improved light coupling inside the nanowires (from about 20% to about 45%). More importantly, the operational stability of the nanowire-based perovskite LEDs was also enhanced (T50 increased from 9 min to 35 min). And the long-term stability of the nanowire-based perovskite LEDs was also increased with an enhancement factor of 3 times. It’s also worth noting that, the mechanical robustness of the nanowire-based perovskite LEDs was also enhanced a lot due to the protection of the porous template, and much fewer cracks were caused inside nanowires during bendings when compared to the planar counterparts.
With the above mentioned two nanophotonic strategies for perovskite LEDs performance (both light-coupling efficiency and stability) enhancement, we demonstrated the advantages of applying nanostructures in perovskite LEDs. By further developing the perovskite material qualities and meanwhile improving the deposition techniques that are more compatible with nanostructures, we believe the EQEs of perovskite LEDs can be further improved to a much higher milestone soon. Last but not least, the nanophotonic strategies we developed here are not just limited to LEDs; they might be also applied for other device applications such as solar cells, photodetectors, and lasers.

Solution-processable metal halide perovskites have sparked major interest for their use in (opto)electronic applications, but until now metal halide perovskite light emitting diodes (PeLEDs) have not been fabricated by inkjet printing. This is because of the difficulty in printing defect-free perovskite films as opposed to other printable optoelectronic materials.
Our work represents the first demonstration of inkjet-printed PeLEDs by specifically templating the crystallization behaviour within the perovskite. This effect is achieved by adding KCl to the commonly used hole injection material poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) underneath the inkjet-printed perovskite layer, which significantly influences the device performance of the PeLEDs.
Firstly, the resulting “salty” PEDOT:PSS works as a seeding template to induce crystal nucleation of the metal halide perovskites inkjet-printed on top. It does so while having only a minor effect on the intrinsic electrical properties of the PEDOT:PSS. Secondly, the perovskite-polymer composite ink specifically developed for inkjet printing renders the commonly used antisolvent treatment unnecessary to induce favourable perovskite crystallization.
PeLEDs incorporating this PEDOT:PSS/KCl layer with the MAPbBr₃:PEG perovskite composite show greatly improved performance, predominantly due to improved crystallization compared to PeLEDs containing only PEDOT:PSS. In particular, KCl-modified PEDOT:PSS contact layers enabled the realization of inkjet-printed PeLEDs with a 30-fold increased luminance at the same operating voltage. Using this approach, the first inkjet-printed metal halide PeLEDs with comparable performance parameters to spin-coated reference devices were produced [1].

[1] F. Hermerschmidt, F. Mathies, V. R. F. Schröder, C. Rehermann, N. Zorn Morales, E. L. Unger, E. J. W. List-Kratochvil, Mater. Horiz. 7 (2020) 1773.

Metal halide perovskites and perovskite-related hybrid materials have emerged as new classes of light emitting materials with exceptional structural and property tunability. In these materials, various types of molecular metal halide building blocks can be assembled to form distinct crystalline structures with different dimensionalities at the molecular level, ranging from zero-dimensional (0D) clusters, to one-dimensional (1D) wires, two-dimensional (2D) layers, and three-dimensional (3D) networks. The rich combinations of compositions and structures afford these materials with a wide range of optical characteristics, for instance, direct band narrow emissions from 3D metal halide perovskites, and below-gap broadband emissions from 0D organic metal halide hybrids. In this talk, I will present our recent efforts on the development and study of highly efficient metal halide based light emitting materials, from synthetic control to device integration. The major subjects will include (i) color tuning and surface passivation of metal halide perovskite nanocrystals, (ii) single crystalline multicomponent organic metal halide hybrids with white emissions, and (iii) the applications of metal halide perovskites and perovskite-related hybrid materials in light-emitting diodes and radiation detectors.

A major efficiency limit for solution-processed perovskite optoelectronic devices (e.g. light-emitting diodes, LEDs) is trap-mediated non-radiative losses. Defect passivation using organic molecules has been identified as an attractive approach to tackle this issue. We significantly enhance the interaction with defects sites and minimize non-radiative recombination losses. Consequently, we achieve high-performance near infrared perovskite LEDs (PeLEDs) with a high external quantum efficiency (EQE) of 21.6%. We find that our devices can also work efficiently in an emitting/detector switchable mode, with tens-megahertz speed for both functions. Benefiting from the small Stokes shift of perovskites, the diode shows high specific detectivity (>2×10¹² Jones) at its peak emission (~ 800 nm), allowing optical signal exchange between two identical diodes. We further demonstrate the potential of the dual-functional diode for biomedicine diagnosis applications (as a monolithic heart pulse sensor) and for inter- and intra-chip bidirectional optical communications.

The optical and electronic properties of quantum dots (QDs) are mainly governed not only by their size distribution but also structural quality. Although the synthetic strategies for size control of QDs are well established for the last few decades, no systematic approach yet exists to achieve high structural quality nanocrystals. Here we show that reaction temperature and precursor reactivity must be independently tuned to achieve high crystallinity, uniform morphology, high quantum yield, and a well-defined core-shell structure. Reaction temperature governs the surface instability of QDs and lattice rearrangement. Precursor reactivity determines reaction kinetics at the surface, affecting the crystallinity and morphology of the resulting crystals. Prior to this work, the reactivity of conventional precursors has not been modulated in temperature-independent manner. As a result, the simultaneous optimization of reaction temperature and precursor reactivity has not been possible. Accordingly, growth conditions for QDs with various sizes and materials are optimized by trial-and-error. To allow temperature-independent modulation of precursor reactivity, we designed a new sulfur precursor employing a boron-sulfur bond, 9-mercapto-9-borabicyclo[3.3.1]nonane. The reactivity of the precursor is predictably modulated by the addition of a Lewis base, which modifies the boron-sulfur bond strength upon coordination to the boron atom. The new precursor chemistry enables the systematic optimization of growth conditions at a wide range of reaction temperatures. Adapting the growth method established here, high-quality QDs were grown from cores of various sizes and materials. The QD growth principles using the new design of precursors are universally applicable to different types of nanoparticles, including non-sulfur-based QDs. This work provides critical insights into the nanoparticle growth process, the design of new precursors, and the optimization of reaction conditions for high-quality QD synthesis.

Metal-halide perovskites (MHPs) has been considered as a promising candidate for solid-state light emission because of their superior optoelectronic properties such as extremely high color purity (full-width at half maximum (FWHM) ≈ 20 nm at room temperature), high photoluminescence quantum efficiency > 90%, and easily tunable bandgap.¹ Further, the efficiency of perovskite light-emitting diodes (PeLEDs) based on three-dimensional (3D) polycrystalline perovskites has been greatly increased reaching external quantum efficiency (EQE) over 20% within only 5 years since the first high-efficiency PeLEDs reported in 2015.^2–4 However, a short operational lifetime of PeLEDs has been a critical drawback for their practical applications and commercialization.⁵ One of the most critical causes of the problem is ion migration. PeLEDs’ behavior can be more significantly influenced by ion migration as it requires higher bias to operate compared to perovskite solar cells.
Here, we demonstrate a proton-transfer-induced 3D/2D hybrid perovskite by adding a small amount of neutral molecule, benzylamine (BnA). The proton transfer from organic ammonium in a precursor to the added BnA enabled the crystallization of 2D perovskite without destroying the 3D phase. As a result, the lifetime of PeLEDs was highly extended by more than 20 times, and an overshooting of electroluminescence at the initial stage of device operation was extremely suppressed; the 2D perovskite passivates trap sites of the 3D perovskite and blocks possible pathways of ion migration in the emitter. We also identified the mechanism of the suppressed luminance overshooting and the lifetime extension by relating with the retarded reorientation of BnA⁺ in the perovskite lattice. Our work provides a helpful guideline to improve the stability of PeLEDs with important insights regarding defect engineering.

Reference
1. Kim, Y.-H., Cho, H. & Lee, T.-W. Metal halide perovskite light emitters. Proc. Natl. Acad. Sci. U. S. A. 113, 11694–11702 (2016).
2. Cho, H. et al. Overcoming the electroluminescence efficiency limitations of perovskite light-emitting diodes. Science (80-. ). 350, 1222–1225 (2015).
3. Park, M.-H. et al. Boosting Efficiency in Polycrystalline Metal Halide Perovskite Light-Emitting Diodes. ACS Energy Lett. 4, 1134–1149 (2019).
4. Xu, W. et al. Rational molecular passivation for high-performance perovskite light-emitting diodes. Nat. Photonics 13, 418–424 (2019).
5. Quan, L. N., García de Arquer, F. P., Sabatini, R. P. & Sargent, E. H. Perovskites for Light Emission. Adv. Mater. 30, 1801996 (2018).

Colloidal nanocrystals made of all-inorganic lead halide cores, referred to as perovskite quantum dots (PQDs), exhibit quantum confinement of charge carriers. This endows these materials with size- and composition-dependent photophysical and electronic properties. They have been successfully grown using high temperature reaction routes. Along with organic metal halide compounds, PQDs are highly sought for applications that include light-emitting and photovoltaic devices. However, tangible progress in probing their fundamental properties and/or their integration in device design, has been hampered by issues of colloidal and photophysical instability. These problems can be traced to the ionic nature of the cores combined with the fast desorption rate of the surface capping ligands.

To address those problems, we introduce a promising surface coating strategy relying on a poly-zwitterionic polymer, where high affinity binding onto the QDs is driven by multi-coordinating electrostatic interactions with the ion-rich surfaces of CsPbBr₃ PQDs. The polymer ligands were synthesized by grafting a stoichiometric mixture of amine-modified sulfobetaine anchors and solubilizing motifs on a poly(isobutylene-alt-maleic anhydride), PIMA, via nucleophilic addition reaction. We find that this coating imparts enhanced colloidal stability to the nanocrystals over a broad range of solvent conditions and in powder form. It allows easy phase transfer of the PQDs to an array of solutions with varying polarities and properties. Additionally, this stabilization strategy preserves the photophysical and structural characteristics of the nanocrystals in a variety of polar solvents over a period extending to 1.5 years.

InP quantum dots emerge as a non-toxic and industrially-applicable alternative to CdSe quantum dots in light-emitting materials. While the recent progress in synthesis and optimization of InP quantum dots led to quantum yields above 90%, the industrial goal of 95% quantum yield remains yet to be achieved, especially for the blue light-emitting quantum dots. As a viable strategy towards further optimizing quantum yields, reducing the trap states originating from the surfaces has been proposed. Dissimilar to CdSe quantum dots, controlling the surface facets and equilibrium crystal shapes of InP have been difficult, with largely spherical quantum dots synthesized and reported in the literature. In this work, we discuss and examine the thermodynamic stability of (100), (110) and (111) surfaces according to the ligand adsorption of CH₃COO^-, P(CH₃)₃ and CH₃NH₂, using density functional theory calculations. The change in equilibrium crystal shapes are examined with respect to the ligand species, concentration and the chemical potential of In, in order to account for the various synthesis conditions

Metal halide perovskites have been widely used as the light absorber of solar cells, with the highest certified power conversion efficiency has reaching 25.2%, which is comparable to silicon-based solar cell technology. This high efficiency is inherent in perovskite’s characteristics, such as high absorption coefficient, bandgap tunablity, long carrier diffusion length, high carrier mobility, and compatibility with simple solution processing or vacuum evaporation. By taking these characteristics, perovskites can be used as the emitter of light-emitting diodes (LEDs) and laser devices. In this context, we show our recent results on such optoelectronic applications based on perovskites.
We found that singlet and triplet excitons are formed in quasi-two dimensional perovskite films with many domains of different phase orders (Nat. Photon., 14, 70, 2020). When we used organic cations with the low triplet energy for the fabrication of quasi-two dimensional perovskite films, the triplets generated by the recombination of electrons and holes injected from electrodes are strongly quenched. On the other hand, the use of organic cations with the sufficiently high triplet energy enabled the generated triplets to be harvested for emission as a result of efficient reverse intersystem crossing. With this method, we demonstrated a four-fold increase in external quantum efficiency of quasi-two-dimensional perovskite LEDs.
To demonstrate the potential of using perovskites for other applications, we used the wide-energy-gap perovskite as the host layer of organic LEDs (Adv. Mater., 30, 1802662, 2018). We doped an organic fluorescent emitter into the perovskite host layer. The fluorescent molecules could exist at grain boundaries of perovskite films. Because of the better overlap between the emission band of the perovskite and the absorption band of the fluorescent molecules, the energy transfer from the perovskite to the organic emitter happened. We obtained low-voltage driving in the LEDs because of the high hole and electron mobilities of the perovskite. The radiative singlet exciton generation efficiency of the LEDs achieved ~40%, which is higher than the theoretical limit (25%) of emission from single excitons. We already demonstrated ~100% radiative singlet generation efficiency in a similar fluorescence/perovskite composite system.
To overcome poor carrier transport in organic films, we used the perovskite as the transport layers of organic LEDs (Nature, 572, 502, 2019). We were able to increase the total organic LED thickness by more than 10 times without reducing efficiency or operational durability or increasing driving voltage . This overturns 30 years of thinking that organic LEDs are limited to thin organic films. Additionally, some of thick organic LEDs with the perovskite transport layers had the same electroluminescence color from every viewing angle because of the absence of the optical interference. These results open new paths for low-cost, reliable, and uniform fabrication of organic LED displays and lighting.
We found that thresholds of amplified spontaneous emission under optical pumping strongly depend upon phase orders of perovskite films; thresholds decrease when a higher phase order is used and a typical three-dimensional perovskite with the highest phase order has the lowest threshold (Phys. Chem. Chem. Phys. 20, 15030, 2018). By embedding three-dimensional perovskite films into two-dimensional distributed feedback grating substrates, we saw lasing from the perovskite under optical pumping, with a peak width being lower than 1 nm (ACS Photon. 6, 460, 2019). We demonstrated a way of replicating grating patterns in lasing perovskite films on low-cost plastic substrates, which can reduce complexity and cost of the perovskite laser fabrication in the future (Appl. Phys. Lett. 115, 141106, 2019).

Transparent displays have attracted considerable interest as the next-generation display technology in various fields, such as automotive industries, defense or military technology, and internet of things (IoT) applications. These applications require simultaneous projection or display of information and visibility of the surroundings through the device. However, the widely commercialized organic light-emitting diodes (OLEDs) are only used in semi-transparent displays due to their opaque and reflective metal thin film cathodes using Al and Ag despite the numerous excellent properties of OLEDs such as high color contrast, wide viewing angles and low-cost solution-based fabrication. For the transparent OLEDs (T-OLEDs), all the electrodes and organic functional materials in an OLED should be transparent. Especially, the opaque metal cathode should be replaced by a transparent material and blue EML materials owing to their wide electronic band gap sholud be used for the colorless characteristic. This study presents transparent organic light-emitting diodes (T-OLEDs) with excellent light-emission performance achieved by virtue of optically transparent functional layers. In this T-OLED, blue polymer poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) is used as a host material for the emission layer (EML) of the T-OLEDs, to achieve a high optical light transmittance in the visible range of the light spectrum. By controlled blending of the commercial emitting material Super Yellow with the PFO, the light-emission efficiency of the EML was significantly improved and the electroluminescence color could be changed from blue to green through a Förster resonance energy transfer (FRET). Furthermore, indium tin oxide (ITO) thin films was used not only as a transparent bottom anode but also as a transparent top cathode. Indium tin oxide (ITO) is one of the best candidates for transparent cathodes, owing to its excellent combination of optical transparency and electrical conductivity, along with its highly developed and mature deposition technology. However, the direct-current (DC) magnetron sputtering deposition approaches cause detrimental plasma damages to the organic layers ITO is deposited. Hence, solution-processable electron-transport composed of the zinc oxide (ZnO) nanoparticle layers and cesium carbonate-doped polyethyleneimine ethoxylated (d-PEIE), which aside from facilitating the efficient injection of electrons into the T-OLED, protected the organic emission layer (EML) of the T-OLED against the plasma damage during top ITO cathode sputtering deposition. Furthermore, semi-hydrophilic poly(methyl methacrylate) (PMMA) interlayer coated between the EML and ETL insured the good wettability and contact of the hydrophilic ETL with the hydrophobic EML. The even distribution of ZnO-NPs in the uniformly coated newly designed solution-processable composite ETL significantly enhanced the blue light emission performance of the conventionally structured T-OLED with top ITO cathode. The solution-processed T-OLED emitted the total maximum luminance (L_max) of about 2,417 cd m^-2 (bottom ITO anode emission at 1,455 cd m^-2 and top ITO cathode emission at 962 cd m^-2), total maximum current efficiency (ε_max) at 3.12 cd A^-1 (bottom and top emissions at about 1.78 and 1.34 cd A^-1, respectively), and total maximum power efficiency (η_max) at 1.64 lm W^-1 (bottom and top emissions at about 0.95 and 0.69 lm W^-1, respectively) while having a very high optical transmittance of around 85% at 550 nm light wavelength.

Light-emission efficiency of organic light emitting diodes (OLEDs) strongly depends on the photophysical processes: charge recombination, population of triplet excitons, nonradiative decay, steered by the electronic structure of molecules, their orientation and morphology. Various techniques to enhance luminescence and device quantum yield were reported. Among them, incorporation of thermally activated delayed fluorescent (TADF) chromophores allowing to convert unwanted triplets to singlets through reverse intersystem crossing (RISC) is a promissing approach. Yet, controlled and stable assembly of TADF chromophores, modulating their functionality, is still not known, therefore, it is highly desirable to find a synthetic platform promoting the achievement of theoretically predicted device efficiencies.

Due to their vast chemical composition space, metal organic frameworks (MOFs) have emerged as a versatile class of functional materials with tunable properties for various applications. Here, we demonstrate new surface-anchored MOF (SURMOF) thin films with TADF activation, which results in efficient green electroluminescence of a LED device with luminance of 270 Cd/m² at 14.25 V. The TADF functionality of films originates from controlled aggregation of chromophores during their packing in a MOF scaffold. There, restriction of the rotational freedom of MOF linkers triggers new photophysical phenomena based on the “hot-exciton” RISC, enabling high population of singlet excited states and efficient SURMOF emitting layer [1].

In my talk, in depth explanation of the observed RISC processes and emission efficiency on the basis of the time-dependent density functional theory calculations within quantum mechanics/molecular mechanics (QM/MM) approach will be given. The temperature dependence of RISC will be demonstrated as a function of the microscopic parameters between singlet and triplet excited states, i.e. energy splitting, reorganization energy and spin-orbit coupling, as predicted from the Marcus theory of charge transport. Perspectives of future computational screening of TADF chromophores [2] leading to a pronounced increase in luminescence of SURMOF-LEDs will be shown.

References
[1] R. Haldar, M. Jakoby, M. Kozlowska, M. R. Khan, H. Chen, Y. Pramudya, B. S. Richards, L. Heinke, W. Wenzel, F. Odobel, S. Diring, I. A. Howard, U. Lemmer, C. Wöll, JACS, submitted.
[2] R. Haldar, A. Mazel, M. Krstić, Q. Zhang, M. Jakoby, I. Howard, B. S. Richards, N. Jung, D. Jacquemin, W. Wenzel, S. Diring, F. Odobel, C. Wöll, A de novo strategy for predictive crystal engineering to tune excitonic coupling, Nat. Commun. (2019) 10:2048.

Organic light emitting diodes (OLEDs) are becoming ubiquitous with applications in television and computer screens, smart windows and wearable diagnostic tools due to their high resolution, energy efficiency and novel flexibility. However, the growth of this $48Bn industry is facing stagnation due to dearth of suitable blue emitters. OLEDs need excellent red, green and blue emitters. Although the industry has high performing red and green emitters, the search is still ongoing for suitable blue emitters in solid state.
Presently, blue emission is obtained from rare-earth metal complexes which have complex synthesis routes and difficult processability. These materials have low thermal stability and poor operation lifetimes. As a result, the final devices are inefficient and expensive.
Among other potential candidates, thiazolothiazole (TTz) based materials have been widely reported as a molecular semiconductor that can emit blue. This blue emission has been used for a plethora of applications. TTzs are solution processible materials with facile synthesis routes using inexpensive and environment friendly precursors. They are thermally stable, and their planar structure enhances conduction of charge carriers in devices. Despite these properties, TTzs have been relatively unexplored for OLED applications. We hypothesize that TTz based materials can represent a new generation of affordable display materials with high device efficiency. In this study, we report the effect of different molecular spacers on electronic, structural and photophysical properties of seven novel TTz materials to help identify most suitable candidate for OLEDs.
We have explored the effect of molecular spacers on TTz packing, by resolving the crystal structures of the synthesized materials using single crystal X-ray diffractometry. Combined with fluorescence studies, we have thus correlated the tuning of inter-TTz core distances in a unit cell to their emission properties. We have determined their thermal stability using thermogravimetry and differential scanning calorimetry.
We have also determined their quantum yield and photoluminescence emission lifetime decay profiles. To establish exciton diffusion through these molecules, we have determined their singlet exciton diffusivity coefficient and the exciton diffusion length by employing a time resolved bulk quenching model with Monte Carlo eDiffusion computation simulation. We have also explored the molecular orientation of these materials in spin cast thin films using grazing incidence wide angle X-ray scattering. We establish the crystal orientation in thin films and how it affects charge mobilities in devices. This study will enable future researchers to engage in further possibilities with these materials, within and beyond the scope of OLEDs.

We present a pure blue fluorescent organic light emitting diode (OLED) where the device structure is optimized to yield a Commission Internationale de l’Eclairage (CIE) coordinate of (0.15, 0.13) which is very close to the National Television Standards Committee (NTSC) standard of blue color (0.14, 0.08). The emissive layer (EML) is composed of a host-guest matrix of 4,4’-di[N-carbazolyl] diphenyl (CBP, host) and 4,4’-Bis[(N-carbazole) styryl] biphenyl (BSB4, guest). We show, from photoluminescence (PL) studies, a blue-shift in the emission spectrum of the host-guest matrix from that of the pristine guest molecules resulting in a pure blue emission suitable for display application. We demonstrate methods to balance the electron and hole concentration in the EML of the OLED in order to improve the efficiency and its roll-off and optimize the device to obtain a maximum external quantum efficiency (EQE) of 4.08%. This is very close to the theoretical limit for EQE (5%) of fluorescent OLEDs without the use of any light extraction layer. This is achieved by the choice of organic materials and their thicknesses in the OLED stack. The charge carrier mobility and molecular energy levels of the organic materials are taken into consideration for this optimization.

Metal halide perovskite nanocrystals (MHP NCs) exhibit many novel and unique phenomena both in terms of photophysics and materials chemistry. For NIR emission and solar energy conversion, the use of all-inorganic CsPbI₃ NCs is especially appealing, combining a narrow band gap with improved stability due to the absence of volatile organic constituents. However, the cubic black phase of CsPbI₃ is not thermodynamically stable at room temperature and transformation to the yellow orthorhombic phase occurs. This problem can be overcome by the use of molecular iodine as the halogen source during synthesis. It results in an improved surface passivation of the NCs and long-term air- and phase-stability of CsPbX₃ NCs.
Substitution of lead by the lighter group-14 homologue tin enables to shift the absorption onset and photoluminescence peak further to the near infrared, beyond 900 nm. However, Sn²⁺ is much more prone to oxidation to the +IV state than Pb²⁺ and can give rise to the formation of undesired intraband trap states. In this regard, mixed Cs(Pb,Sn)I₃ NCs exhibit enhanced stability and further possibilities of luminescence and morphology control, which will be discussed. Complete substitution of lead is also possible with europium, which gives access to nanoparticles strongly emitting in the deep blue spectral range (400-420 nm).
Finally, the application of the obtained MHP NCs in two approaches for solar energy conversion will be presented. First, they are incorporated into mesoporous TiO₂, which constitutes to date the best performing electron transport material in perovskite solar cells. Efficient migration of the NCs into the mesoporous structure is achieved by means of interface engineering, enhancing at the same time significantly the electron injection rate. As a result, highly reproducible perovskite NC solar cells with an efficiency exceeding 14% could be obtained.
As an example for a further emerging application, the obtained air-stable MHP NCs can also be used as photocatalysts in organic syntheses. As an example, we demonstrate their use as efficient visible-light photocatalyst for the oxidative aromatization of a wide range of heterocyclic substrates.

The M13 bacteriophage, a nature-inspired environmentally friendly biomaterial, is used as the perovskite crystal growth template and the grain boundary passivator in perovskite solar cells. The amino groups and carboxyl groups of the four types of amino acids on the M13 bacteriophage surface function as Lewis bases, interacting with the perovskite materials. The M13 bacteriophage-added perovskite films show a larger grain size and reduced trap-sites compared with the reference perovskite films. In addition, the existence of the M13 bacteriophage induces light scattering effect, which enhances the light absorption particularly in the long-wavelength region around 825 nm. Both the passivation effect of the M13 bacteriophage coordinating to the perovskite defect sites and the light scattering effect of the M13 bacteriophage intensify when the M13 virus-added perovskite precursor solution is heated at a temperature of 90 °C prior to the film formation. Heating the solution denatures the M13 bacteriophage by breaking their inter- and intra-molecular bondings. The denatured M13 bacteriophage-added perovskite solar cells exhibit a power conversion efficiency of 20.1% while the reference devices without the addition of the virus give a power conversion efficiency of 17.8%. The great improvement in efficiency comes from all of the three photovoltaic parameters, namely short-circuit current, open-circuit voltage, and fill factor, which corresponds to the perovskite grain size, trap-site passivation, and charge transport, respectively.

For the past ten years, perovskite solar cells have known a tremendous development among photovoltaic technologies, rapidly increasing from 3,8% [1] to 25.2% [2] power conversion efficiency (PCE). However, before transferring this technology to commercial applications, one of the main challenges to overcome is the fabrication of high quality perovskite absorber films on large surfaces. Various alternatives to spin-coating have been explored in the literature such as blade coating [3], inkjet coating [4], roll-to-roll [5] or spray coating [6]. But, slot-die coating is one of the most promising techniques. It is easily compatible with high rate fabrication technique like roll-to-roll and allows a sharp control over the uniformity on large surfaces.

In this work, we emphasis on the influence of the drying method upon the absorber properties. After the slot-die coating deposition, we chose to use a vacuum quench method, which allow a uniform extraction of the solvent, few minutes before annealing. The change of the quench system, from anti-solvent to vacuum aspiration, modify the speed with which the solvent is extracted. Then, it is possible to influence the crystallization mechanisms to fit the new drying environment by changing the perovskite ink composition. We herein propose a method to fabricate CsFA perovskite with a double-additives protocol, combining a surfactant and chloride addition through MACl.

A particular difficult challenge is to isolate the action of the surfactant and the chloride during crystallization. We combined PL and morphological characterisations, under different annealing and quench conditions, in order to successfully bring their respective effects out. For example, including MACl into precursor solutions seem to increase the film coverage and grain sizes during secondary grain growth, generating a smooth and pinhole-free perovskite layer with large grain size while surfactant enhance film uniformity and passivate surface defects. We reached over 16% PCE on small-scale cells (0.09 cm² aperture) with optimized ratio of chloride, thermal annealing and vacuum conditions.

This approach offers a better understanding of perovskite growth under large-scale fabrication conditions and is an important step towards industrial perovskite devices.

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[2] NREL, ‘Best research-cells efficiency’, 2020.
[3] Y. Deng, et al., Energy & Environmental Science, vol. 8, no. 5, pp. 1544–1550, 2015, doi: 10.1039/C4EE03907F.
[4] F. Mathies, et al., ACS Applied Energy Materials, vol. 1, no. 5, pp. 1834–1839, May 2018, doi: 10.1021/acsaem.8b00222.
[5] C. Zuo, et al., Nano Energy, vol. 46, pp. 185–192, Apr. 2018, doi: 10.1016/j.nanoen.2018.01.037.
[6] Y. Jiang et al., Nano Energy, vol. 53, pp. 440–448, Nov. 2018, doi: 10.1016/j.nanoen.2018.08.062.

Disorder is intrinsic to ionic systems and gives rise to specific electronic processes. In recently developed perovskite ionic crystals, this dynamic lattice disorder is inferred to give rise to properties of interest, such as defect tolerance. Here, the elementary excitation of interest is the polaron, a localized lattice distortion. We employ state resolved pump-probe spectroscopy to monitor electron and lattice dynamics in bulk CsPbBr₃ perovskite nanocrystals. The data report spectral lineshape dynamics that monitor the system from optical birth through polaron formation and exciton formation to fully thermalized, confined exciton. The formation of quantum confined excitons arises from the liquid-like polaronic potential. This polaronic confinement creates a quantum drop, as opposed to physical confinement creating a quantum dot. Disorder is intrinsic to ionic systems and gives rise to specific electronic processes. In recently developed perovskite ionic crystals, this dynamic lattice disorder is inferred to give rise to properties of interest, such as defect tolerance. Here, the elementary excitation of interest is the polaron, a localized lattice distortion. We employ state resolved pump-probe spectroscopy to monitor electron and lattice dynamics in bulk CsPbBr₃ perovskite nanocrystals. The data report spectral lineshape dynamics that monitor the system from optical birth through polaron formation and exciton formation to fully thermalized, confined exciton. The formation of quantum confined excitons arises from the liquid-like polaronic potential. This polaronic confinement creates a quantum drop, as opposed to physical confinement creating a quantum dot.

Metal halide perovskites (MHPs) are promising materials for active layers in solar cells (SCs) because they have outstanding advantages such as low exciton binding energy (~14 meV for CH(NH₂)₂PbI₃)^[1] and high carrier mobility (100 cm²/(Vs))^[2]. Despite the superior properties of MHPs, MHP polycrystalline (PC) bulk film based SCs had bad reproducibility due to bad surface morphology of MHP PC films. Furthermore, controlling the surface morphology of MHP PC films is very difficult because the environmental conditions such as temperature and moisture have a strong influence on the morphology. Here, we present MHP nanoparticles (NPs) synthesized by hot injection method and purified with ligand-engineering method. Moreover, perovskite NP layers with uniform morphology were fabricated by using a ligand washing method. The ligand engineering methods which reduce the amount of ligand improved the charge extraction/transport characteristics in MHP NP films. Also, more ligands can be removed by washing MHP NP films with organic solvent. The washing method is applied by dropping the organic solvent onto the film followed by subsequent spin-coating. Our research suggests a promising way to increase PCE of perovskite NP SCs.

In this work, we report on a systematic investigation of the effect of tellurium (Te)-doping on the morphological, optical, and electrical properties of the GaAsSb nanowires (NWs) grown by Ga-assisted molecular beam epitaxy (MBE) and realization of the ensemble device. Te-incorporation in the NWs was assessed qualitatively from the structural, optical, and electrical properties, whereas the quantitative assessment was made based on the dark I-V simulations. Schottky barrier ensemble photodetector (PD) fabricated by capitalizing on optimized Te-doped NW exhibited a broad spectral range with the longer wavelength cutoff at ~1.2 µm, responsivity in the range of 580- 620 A/W and detectivity in the 1-3x 10¹² Jones. Padovani-Stratton parameter (E₀₀) revealed field emission mechanism being dominant current conduction mechanism, backed by the higher ideality factor. Low-frequency noise (LFN) spectroscopy revealed the generation-recombination noise being the dominant noise source attributed to the Te induced localized states. The doped GaAsSb NWs have tremendous potential for further improvement, paving the path for high-performance NIR photodetection applications.
Author for correspondence: [email protected]
Acknowledgment:
This work is supported by the Air Force Office of Scientific Research (AFOSR) under (grant# W911NF1910002) and National Science Foundation (grant# ECCS-1832117).

Metal halide perovskites with optimal opto-electronic properties contain lead (Pb). The replacement of lead with less toxic or nontoxic element, such as tin, has been studied in bulk thin film, but few in nanocrystal form. In this research, we synthesized lead-free FASnI-based (FA: formamidinium) perovskite nanoparticles through the ligand-assisted reprecipitation method (LARP) at low temperature, and studied the impact of precursor solution molarity, the precursor solvent and the “poor” solvent on the phase of perovskite nanoparticle as well as optical properties. Different kinds of ligand were used and their function during the synthesis process were investigated. By controlling the molarity of precursor solution, we were able to tune the bandgap of FASnI-based perovskite from 700nm (1.77eV) to more than 830nm (1.49eV). For the same synthesis condition, the compositional variation of precursor solvent affected the phase of perovskite with either blue or red shift in PL spectra. For example, when 20% of DMSO was introduced into precursor solution, there is a ~70nm blue shift in photoluminescence spectra. A series of “poor solvents” were used in this research, such as chlorobenzene, chloroform, toluene, and hexane, among which the synthesis with toluene exhibited the better performance. With the interplay between precursor molarity, type of solvents, and ligands, we were able to achieve a low temperature synthesis of single phase FAPbI₃ nanoparticles. The PL, XRD and TEM data will be presented along with synthesis details.

Colloidal semiconductor quantum dots (CQDs) are attractive materials for realizing highly flexible, solution-processable optical gain media with readily tunable operational wavelengths [1, 2]. However, CQDs are difficult to use in lasing due to extremely short optical gain lifetimes limited by nonradiative multicarrier Auger recombination [3]. This, in particular, is a serious obstacle for realizing cw optically and electrically pumped lasing devices. Recently, we have explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell CQDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination [4, 5]. Using these specially engineered CQDs, we have been able to realize optical gain with direct-current electrical pumping [4], which has been a long-standing goal in the field of colloidal nanostructures. Further, we have applied these dots to practically demonstrate 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 [5, 6]. Such charged QDs are optical-gain-ready without excitation, which has allowed us to reduce the lasing threshold to record-low values that are well below the fundamental single-exciton-per-dot limit [6]. Most recently, we have developed CQD devices that operate as both an electroluminescent (EL) structure and a distributed feedback optically pumped laser [7]. By carefully engineering a refractive-index profile across the device stack, we have been able to demonstrate low-threshold lasing even with a very thin EL-active region, which comprises only three monolayers of the QDs. Yet another recent advance has been the realization of CQD-LEDs that achieve ultrahigh current densities exceeding 1,000 A cm^-2. This has allowed us to inject ~10 excitons per dot and thereby realize population inversion of both the ground-state (1S) and the excited-state (1P) transitions. All of these recent developments suggest that CQD laser diodes (QLDs) are just around the corner. The availability of such devices will benefit numerous fields from integrated photonic circuits and optical interconnects to lab-on-a-chip platforms and wearable devices.

[1] V.I. Klimov, A.A. Mikhailovsky, S. Xu, A. Malko, J.A. Hollingsworth, C.A. Leatherdale, H.J. Eisler, M.G. Bawendi, Science 290, 314 (2000).
[2] V.I. Klimov, S.A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J.A. McGuire, A. Piryatinski, Nature 447, 441 (2007).
[3] V.I. Klimov, A.A. Mikhailovsky, D.W. McBranch, C.A. Leatherdale, M.G. Bawendi, Science 287, 1011 (2000).
[4] J. Lim, Y.-S. Park, V.I. Klimov, Nature Materials 17, 42 (2018).
[5] K. Wu, Y.-S. Park, J. Lim, V.I. Klimov, Nature Nanotechnology 12, 1140 (2017).
[6] O.V. Kozlov, Y.-S. Park, J. Roh, I. Fedin, V.I. Klimov, Science 365, 672 (2019).
[7] J. Roh, Y.-S. Park, J. Lim, V.I. Klimov, Nature Communications 11, 271 (2020).

Silicon photonic integrated circuits operating at visible and near-infrared wavelengths have enormous application potential; however, a scalable on-chip light source remains elusive. We address this challenge by introducing a new paradigm for halide perovskite light-emitting diodes (LEDs) and lasers that enables them to be integrated on silicon in a final solution processing step in the back-end-of-line (BEOL). In contrast to traditional sandwich-type device architectures, our approach makes use of co-planar interdigitated electrodes with ~100 nm spacing that facilitates both low voltage current injection and first order distributed feedback for lasing. This strategy overcomes the limited compatibility of perovskite materials with silicon processing by essentially pre-fabricating all components for LEDs and lasers expect light-emitting materials so that they become functional once the perovskite is deposited from solution in the last step.
We demonstrate this approach by fabricating methylammonium lead iodide (MAPI) perovskite LEDs on SiO₂/Si using alumina-coated Au interdigitated electrodes patterned by electron beam lithography. Despite the nominally-symmetric electrical structure of the devices, we achieve bipolar injection and bright MAPI electroluminescence with turn-on voltages below 2V. We propose that ionic conduction in MAPI, which is often viewed as detrimental for photovoltaics, is advantageous in this case because it can be exploited to break the electrical symmetry of the device by accumulating ions at the anode and cathode contacts so that the resultant interfacial band-bending facilitates hole and electron injection, respectively. We go on to investigate the operation of these devices under GHz bandwidth electrical pulsing enabled by an on-chip graded-impedance matching network and discuss the remaining hurdles that must be overcome to achieve electrically-pumped lasing.

Organic and halide-perovskite semiconductors are capable of low threshold optically-pumped lasing over a wide range of wavelengths. Combined with their capacity for non-epitaxial deposition, these materials could facilitate a new range of coherent light sources for integrated photonics if electrically-pumped lasing can be established. In contrast to mature III-V laser diode technology where gain is achieved by diffusion current in a pn junction, the active layer in organic lasers cannot be doped because the dopants severely quench emission, and simply doping perovskites remains an ongoing challenge to begin with. In both cases, this means that gain will need to be achieved in an undoped active layer by a space charge-limited (SCL) drift current – a fundamentally different regime for diode lasing than III-V lasers operate in. While unipolar SCL current frequently occurs in organic materials and bipolar SCL current has been claimed in some organic light-emitting diodes, the implications of bipolar SCL current for lasing in organic and perovskite semiconductors has not been explored. Here, we establish analytical bounds on the threshold current, voltage, and power density for laser diodes operating in the bipolar SCL current regime. This formalism also provides useful insight into scaling relationships for SCL lasing in regard to electric field strength, carrier concentrations, recombination uniformity, and the impact of mobility imbalance on bimolecular quenching processes such as exciton-polaron annihilation (organics) and Auger recombination (perovskites). Importantly, we show how sub-Langevin recombination in perovskite semiconductors implies substantially higher threshold current density than in organics and motivates the need for quantum dot/well-like architectures to exploit carrier concentration/funneling effects. Our results establish basic bounds for the operation of organic and perovskite laser diodes and provide analytical guidelines for designing devices and identifying materials that are best suited for lasing in the SCL current regime.

Solid-state room-temperature lasing with tunability in a wide range of wavelengths is desirable for many applications. To achieve so, besides an efficient gain material with a tunable emission wavelength, a high quality-factor optical cavity is essential. Here, we combine a film of colloidal CdSe/CdZnS core-shell nanoplatelets with square arrays of nanocylinders made of TiO₂ to achieve optically-pumped lasing at visible wavelengths and room temperature. The all-dielectric arrays support bound states in the continuum (BICs), which result from lattice-mediated Mie resonances and boast infinite quality factors in theory. In particular, we demonstrate lasing from a BIC that originates from out-of-plane magnetic dipoles oscillating in phase. By adjusting the diameter of the cylinders, we tune the lasing wavelength across the gain bandwidth of the nanoplatelets. The spectral tunability of both the cavity resonance and nanoplatelet gain, together with efficient light confinement in BICs, promises low-threshold lasing with wide selectivity in wavelengths.

Amplified spontaneous emission (ASE) have become a common phenomenon in hybrid perovskites by establishing coherent light-emitting states. This paper reports a new phenomenon where spin alignment is occurred between coherent light-emitting excitons in the ASE regime in the perovskites. It is observed that the ASE exhibits higher and lower amplitudes when spin alignment is operated differently by using circularly and linearly polarized pumping. This shows as an increase on ASE amplitude upon switching pumping beam from linear to circular polarization, leading to a positive △ASE. Essentially, this positive △ASE identifies that the orbital magnetic dipoles between coherent light-emitting excitons are conserved during ASE development, realizing spin-aligned coherent light-emitting excitons. Furthermore, increasing pumping fluence gives rise to an enhancement on △ASE phenomenon, proportionally following the ASE trend. This result indicates that the spin alignment can be gradually established between coherent light-emitting excitons by increasing pumping density. Clearly, our studies present a fundamental understanding on spin alignment between coherent light-emitting excitons towards developing spin-dependent lasing actions in metal halide perovskites.

Cesium lead halide perovskite nanocrystals exhibit high photoluminescence quantum efficiencies and tunability across the visible spectrum. This makes these crystals ideal candidates for solar panels, light-emitting diodes, lasers, and especially nanolasers. Due to the versatility of cation substitution in perovskite nanocrystals, they can be grown on amine-functionalized silicon dioxide nanoparticles, where the amine linker replaces the standard cation structure. Selectively growing luminescent nanocrystals on spherical silicon dioxide microspheres results in the opportunity to populate whispering-gallery modes in these spherical silica microspheres. In this case, the nanocrystal halide composition can be used to selectively tune the emission wavelength mode, and microsphere radius to tune the mode spacing. This silicon dioxide attachment also adds to the overall stability of the system. Through photoluminescence microscopy measurements, we show whispering gallery modes in individual perovskite-coated microspheres for CsPbBr₃ and CsPbI₃ nanocrystals on 9.2 μm diameter silica spheres and compare these to theoretically predicted optical modes. In CsPbBr₃, we provide evidence that these modes will lase under optical excitation, with a threshold of 750 μJ/cm². This study presents a novel system that, through optimization, could be a promising pathway to achieve facile and stable perovskite nanolasers.

Photon upconversion describes the process of shortening the wavelength of the light emitted upon irradiation, resulting in a gain in photon energy. To comply with energy conservation laws, this occurs by combining two or more low energy photons through triplet states. Since triplet states are ‘spin-forbidden’, so-called sensitizers are required to indirectly populate the triplet state. Triplet sensitizers span a broad range of material classes including from metal-organic complexes, nanomaterials and bulk films. Understanding the energy transfer mechanism is crucial for the advancement of optoelectronic devices based on this process. In particular, tailoring the role of trap states in the sensitizer is key to further increase the device efficiency.
The exact triplet sensitization mechanism varies depending on several factors including: (i) the absolute alignments of the sensitizer and acceptor energy levels. (ii) The exciton binding energy in the sensitizer, resulting in excited states in form of excitons or free carriers. (iii) Energetic polydispersity of a sample, which varies the energetic driving force for triplet transfer.
I will present the current understanding of the triplet sensitization mechanism based on sensitizer materials with different dimensionalities including 0D quantum dots, 1D nanorods and 2D nanoplatelets and highlight the differences of each upconversion system. We obtain upconversion efficiencies exceeding 15% for quantum dot based green-to-blue upconversion and 5% for nanoplatelet-based green-to-blue upconversion.

Photon upconversion (UC) resulting in blue and ultraviolet wavelengths has been studied for application in photocatalysis, as these high energy wavelengths could be transiently generated from lower energy excitation, thus avoiding photobleaching effects. Specifically, this can occur through the process of triplet-triplet annihilation based upconversion (TTA-UC), where the combination of two spin-triplet states results in a higher energy spin-singlet excited state. TTA-UC, however, requires triplet sensitization from another source, as triplets cannot be directly excited, as given by spin-selection rules.
We investigate 2D CdSe nanoplatelets (NPLs) as potential triplet sensitizers in a TTA-UC scheme. NPLs only have one dimension of quantum confinement, allowing for finer control of the band gap emission. Additionally, as NPLs have not been substituted for QDs previously, we looked to examine if their unique properties, like their giant oscillator strengths and ability to form stacks and other microstructures, would have an effect on TTA-UC.
We investigated the NPLs as a triplet sensitizer while taking advantage of 9-anthracenecarboxylic acid (ACA) as a transmitter ligand to increase the efficiency of energy transfer between the sensitizer and the triplet annihilator, 9,10-diphenylanthracene, while comparing a system sensitized by a CdSe QD for the sake of comparison. While NPL stacking and parasitic singlet back transfer still limit the achievable UC efficiency, we obtain a promising UC quantum yield of 5.4% and a low efficiency threshold, I_th = 237 mW/cm².[1]


References:
[1] VanOrman, Z.; Bieber, A.; Wieghold, S.; Nienhaus, L.; Green-to-blue Triplet Fusion Upconversion Sensitized by Anisotropic CdSe Nanoplatelets. Chem. Mater. 2020. https://doi.org/10.1021/acs.chemmater.0c01354

Quantum dots (QDs) are popular materials for applications that require stable photoluminescence with narrow-band emission. Biological imaging applications impose restrictions on QD size, while significant advantages are gained by tuning emission to the biological near-infrared (NIR) window (700 - 2000 nm) where tissue transparency is high and autofluorescence noise is low. Mercury cadmium chalcogenide alloy QDs are promising materials for these applications because their emission wavelength can be tuned continuously throughout the visible and NIR spectra through cation exchange-mediated alloying without changing the QD physical dimensions. However, precise selection of emission wavelengths through alloying remains challenging. In this context, atomic scale modeling of QD photophysical properties with varying size and composition can inform experimental QD designs. Here we combine first principles simulations of alloyed zinc blende Hg_xCd_1-x(S,Se) QDs with experimental results to describe the optical properties of these materials. First we present experimental results for a series of size- and composition-controlled Hg_xCd_1-xS and Hg_xCd_1-xSe QDs, including optical spectra and materials analysis. We further apply density functional theory to compute the ground state properties and simulated optical spectra of QDs with varying size and composition. In this talk, we will interpret the combination of results from both approaches to inform alloyed QD design.

Semiconductor quantum dots (QDs) have attracted significant interest for wide color gamut display applications. Narrow emission linewidths are required to be achieved for the optical performance. As synthetic techniques have been improved, inhomogeneous linewidths can be reduced close to the single-dot level. However, the ultimate limit and the mechanism of homogeneous broadening of QDs are not clarified yet.
In this study, we develop ab initio method to calculate the single-dot emission spectra of QDs originated from exciton-phonon coupling (EPC) [1]. We calculate emission linewidths of core CdSe QDs, and demonstrate that the results are consistent with the CdSe/Cd_xSe_1-xZe_x QD [2]. In addition, the effect of charged surface traps on linewidth is studied. The partial Huang-Rhys factors of QDs show that the acoustic modes are dominant in EPC for perfectly passivated QDs, while the contribution of optical modes becomes significant with surface defects due to the internal field.
Next, to simulate actual-size InP/ZnSe core/shell QD (3,506 atoms), we develop the theoretical framework combining machine learning and density functional theory (DFT) calculations: the structural relaxation and phonon calculations are performed by neural network potential (NNP), and the exciton is described by constrained DFT in single-particle level. We train NNP over reference structures made of various bulk, surface, and interface of InP and ZnSe using SIMPLE-NN code [3]. Trained NNP is tested for ZnSe QD and InP/ZnSe QD, not included in the training set, and verified to provide a precise description of phonon spectra in similar accuracy to DFT. Combining trained NNP with DFT, we calculate the emission spectrum of InP/ZnSe QD, and find that the result is in good agreement with the experimental measurement on the same size of InP/ZnSe/ZnS QD [4,5]. In addition, we find that the contribution of optical modes significantly increases when introducing ZnSe shell due to the mismatch between electron and hole wave functions.

[1] S. Kang, Y. Kim, E. Jang, Y. Kang and S. Han, ACS Appl. Mater. Interfaces, 12(19), 22012 (2020).
[2] Y.-S. Park, J. Lim and V. I. Klimov, Nat. Mater., 18(3), 249 (2019).
[3] K. Lee, D. Yoo, W. Jeong and S. Han, Comput. Phys. Commun., 242, 95 (2019).
[4] S. H. Lee, Y. Kim, H. Jang, J. H. Min, J. Oh, E. Jang and D. Kim, Nanoscale, 11(48), 23251 (2019).
[5] Y. Kim, S. Ham, H. Jang, J. H. Min, H. Chung, J. Lee, D. Kim and E. Jang, ACS Appl. Nano Mater., 2(3), 1496 (2019).

In the past few decades, many products incorporating quantum dots (QDs) have been introduced to the market. This has led to increasing demands for a simple, economical, and scalable synthesis method to produce high-quality core/shell QDs. The heat-up approach, involving bulk delivery of precursors at low temperature, is simple, scalable, and less sensitive to mixing limitations compared to the commonly used injection method. Its applications, however, have been limited because it inevitably yields a strong overlap between the nucleation and growth stages. We addressed this long-standing problem by introducing a new type of precursor having separated reaction paths for nucleation and growth. Unlike existing precursors, which employs a shared intermediate for both reactions, the new precursor have well-separated reaction mechanisms for growth and nucleation; growth proceeds via surface-assisted bond cleavage while nucleation requires the formation of metal-sulfur clusters in solution. Moreover, this precursor chemistry embeds an efficient mechanism to suppress nucleation during growth, enabling nucleation-free growth even at high precursor concentrations. Another unique benefit is temperature-independent tunability of precursor reactivity; the reactivity of the new precursor can be controllably tuned by chemical additives. By independently varying reaction temperature and precursor reactivity, researchers can systematically identify the optimal growth condition for various cores. We show that the new precursor grows high quality shells during heat up synthesis, comparable to those prepared by the continuous injection protocol. Lastly, we established a simple, economical, and scalable scheme for core/shell QDs by streamlining quantitative core synthesis and heat-up-based shell growth using the new precursor. The chemical insight obtained from this work inspires the design of other new precursors to unleash the full potential of heat-up synthesis as well as the development of a synthesis scheme that yields scale-independent reaction outcomes.

Electron field emission is a process where the electrons are ejected from the material surface through quantum mechanical tunneling under the effect of high electric field. There is a great deal of research on the field emission properties of various nanomaterials, for example carbon nanomaterials, metal oxides, metal nitrides and metal tips. Among these cold cathode materials, carbon nanomaterials including carbon nanotubes (CNTs), graphene and diamond-like carbon are extensively investigated in order to seek high current density, stable and long-life cold cathodes. As expected, the carbon nanomaterials display unique advantages in field electron emission and carbon nanomaterials can potentially be used in cold-electron-source-based vacuum micro/nanoelectronics. Nitrogen doped graphene quantum dots (N-GQDs), chemically stable and with potentially low toxic zero-dimensional nanostructure, are extensively studied in various fields, especially in energy storage, bio-imaging, photodetectors. So far the electron field emission behavior of individual N-GQDs has not been studied so in this research, a facile synthesis of nitrogen doped graphene quantum dots (N-GQDs) and its electron field emission performance will be carried out. Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Raman and FTIR Spectroscopy, UV-vis and PL Spectra characterizations were carried out to study the structural and optical properties of as synthesized N-GQDs. The Current vs. Voltage ( J–E) Curve of the N-GQDs Coated on Si Substrate was measured and Fowler–Nordheim (FN) plot was analyzed to check the Emission Stability of the N-GQDs Cathode with the objectives to achieve Outstanding Emission Stability of the N-GQDs Cathode and Low Turn-on Field (E_to) and Threshold Field (E_th).

Quasi-two-dimensional (2D) semiconductor nano-platelets manifest strong quantum confinement with exceptional optical characteristics of narrow photoluminescence peaks with energies tunable by thickness with monolayer precision.[1] We employed scanning tunneling spectroscopy (STS) in conjunction with optical measurements to probe the thickness-dependent band gap and density of excited states in a series of CdSe nanoplatelets.[2] The tunneling spectra, measured in the double-barrier tunnel junction cofiguration, reveal the effect of quantum cofinement on the band gap taking place mainly through a blue-shift of the conduction band edge, along with a signature of 2D electronic structure intermixed with finite lateral-size and/or defects effects. The STS fundamental band gaps are larger than the optical gaps as expected from the contributions of exciton binding in the absorption, as confirmed by theoretical calculations. The calculations also point to strong valence band mixing between the light- and split-off hole levels. Strikingly, the energy difference between the heavy-hole and light-hole levels in the tunneling spectra are significantly larger than the corresponding values extracted from the absorption spectra. We have shown that this difference is mainly connected with enhancing of dielectric confinement of light holes relative to the heavy hole observed in STS measurement and an increase of light hole exciton binding energy relative to heavy hole exciton binding energy observed in absorption spectra.[2]
1. S. Ithurria, M. D. Tessier, B. Mahler, R. P. S. M. Lobo, B. Dubertret, and Al. L. Efros, A. L. Nat. Mater. 2011, 10, 936−941.
2. B. Ji, E. Rabani, Al. L. Efros, R. Vaxenburg, O. Ashkenazi, D. Azulay, U. Banin, and O. Milo ACS Nano, 2020 https://dx.doi.org/10.1021/acsnano.0c01950.

We have demonstrated theoretically that non-chiral perovskite layers can exhibit circular dichroism (CD) in the absence of a magnetic field and without chiral activation by chiral molecules [1]. The exciton dispersion in orthorhombic perovskites in the presence of Rashba spin-orbit terms creates helical exciton states, which are split from each other. The splitting can be described as a Zeeman effect in an effective magnetic field, whose direction and magnitude depend on the exciton momentum. The selective excitation of these states by helical light gives rise to CD. Using experimentally determined material parameters, we calculate significant circular dichroism of order 30% in orthorhombic perovskites under off-normal, top illumination.These calculations suggest the effect is observable and CD can be measured in non-chiral perovskite nanostructures such as layered-2D perovskites or nanoplatelets.

[1] P.C.Sercel, Z.V. Vardeny, Al.L. Efros, Nanoscale 2020, DOI/10.1039/D0NR05232A

In this talk, I will present recent results from our lab exploring the excitonic and vibrational properties of emerging 2D light-emitting materials, including hybrid organic-inorganic perovskites, few-layer black phosphorous, and hybrid chalcogenides. Topics may include: 1) interlayer vibrational coupling and superlattice modes of layered 2D structures; 2) tunable exciton binding energy in layered perovskites; 3) origin of broadband emission in multilayer (n>2) lead halide perovskites; and 4) demonstration of novel materials with strong excitonic anisotropy.

Magnetic doping of semiconductor quantum dots has generated new materials that can couple magnetism with light, which are attractive for various applications such as spintronics. Here, we report the synthesis of strongly quantum-confined CsPbI₃ quantum dots doped with paramagnetic Mn²⁺ ions and the characterization of their optical and magneto-optic properties evidencing the creation of light-induced magnetism. The magnetic order of the dopant ions forms via exchange interaction of the dopants and long-lived dark exciton at cryogenic temperatures. At the temperature of 5 K, CsPbI₃ quantum dots exhibited photoluminescence (PL) from dark exciton PL with lifetime of 1.5 microsecond in the absence of Mn dopant indicating that the lowest exciton state in this system is dipole forbidden dark state, which is beneficial in obtaining longer-lived light-induced magnetism. In Mn-doped CsPbI₃ quantum dots, the dark exciton PL lifetime decreased by 30 %, which is similar to the effect of applying external magnetic field, suggesting the creation of magnetic order from the doped paramagnetic Mn²⁺ ions. Comparison of the temperature-dependent PL spectra of Mn-doped and undoped CsPbI₃ quantum dots showed that Mn-doped quantum dots have smaller PL energy due to magnified Zeeman splitting under light-induced magnetism. Our results suggest Mn-doped CsPbI₃ quantum dots, combined with long-lived dark ground exciton state, could become a promising material platform for obtaining optically generated magnetism.

In 2D perovskites, the choice of the A-site perovskitizer cations is not entirely limited by the so-called Goldschmidt tolerance factor. The stabilizing effect of the spacer cations makes it possible to tolerate larger A-site cations as perovskitizers. By solving the single-crystal structures of (BA)₂(A)Pb₂I₇ where BA = butylammonium, and A = methylammonium (MA), formamidinium (FA), dimethylammonium (DMA) or guanidinium (GA), we have proven that the relatively large DMA and GA can serve as the A-site cation of 2D perovskites despite the fact that they are too large to formally satisfy the Goldschmidt rule. As the size of the A-site cations across these compounds increases, the Pb−I bonds are elongated, which reduces the orbital overlap of Pb s- and I p-orbitals, thus decreasing the electronic bandwidths and increasing the bandgaps, as confirmed by DFT calculations. This increases the local distortions of the PbI₆ coordination environment and leads to a decrease of PL intensity and shortening of PL lifetime. The expanded cages also result in soft and anharmonic lattices, as supported by Raman spectra. Exciton-exciton annihilation processes occur in these systems with the rates being slower in the structures with larger A cations.

Lead halide perovskites have gained a lot of popularity in the field of optoelectronics in recent times. Where the toxicity of lead remains a major issue, an emerging non-toxic alternative proposed is lead-free double perovskites with the generic stoichiometric formula A₂M^IM^IIIX₆. In double perovskite the divalent lead is replaced with one monovalent (M^I) and one trivalent (M^II) metal cation. Cs₂AgBiBr₆ is one the stable double perovskite with an indirect bandgap where the optical properties and the charge carrier relaxation processes are not fully understood. We applied time-resolved photoluminescence and differential transmission spectroscopy to explore the photo-excited charge carrier dynamics within the indirect band structure of Cs₂AgBiBr₆ nanocrystals. We observed a high energetic emission at the direct bandgap, alongside the emission from the indirect bandgap transition. We assigned this emission to the radiative recombination of the direct bound excitons originating due to trapping of holes. Due to the electron intervalley scattering process, the emission maximum from this direct bound excitons redshifts over 1 eV within 10 ps, leading to its transfer from direct to indirect bound exciton. We conclude that this direct bound exciton has giant oscillator strength which causes the higher energetic emission to occur at the direct bandgap despite the prevailing intervalley scattering process. These results expand the current understanding of the optical properties and the charge carrier relaxation process in double perovskites family, thus, facilitating the further development of optoelectronic devices harnessing lead-free perovskites.

Metal halide perovskites have attracted substantial interest due to their promising properties for optoelectronic applications. Despite much progress made in the field, the exact growth mechanism of perovskite nanocrystals (e.g. CsPbBr₃) remains elusive and further improvement of their controllable synthesis is challenging. Herein, we point out different phenomena during the processes of growth, cooling and purification of high-quality CsPbBr₃ nanocrystals using in-situ PL spectroscopy. The as-synthesized materials have been further characterized by time-resolved transient differential transmission and photoluminescence spectroscopies. Using X-ray scattering, we confirm that nanocrystals form superstructures during the process of cooling already in dispersion, which is frequently ignored. The purification process is explained by a proposed model based on the self-size-selection. On the one hand, such superstructures pave a potential pathway to the fabrication of high-quality devices such as LEDs. On the other hand, the approach to reveal their formation process benefits the comparison and understanding of the difference between nanocrystals and supercrystals. The fact that superstructures form already during synthesis may also apply to the different perovskite systems and thus help to improve the quality of as-prepared nanocrystals.

Radiographical imaging with X-rays, gamma-rays, and thermal neutrons (~25 meV) has transformed fields including medical imaging, industrial inspection, and nuclear security over the past century. Fast neutron (> 1 MeV) imaging is a rising technique which benefits from the high penetration capabilities of fast neutrons which offer low interaction cross sections relative to other radiation sources. This penetrative power makes fast neutrons ideal for imaging large-scale objects such as large fossils, nuclear fuel, and as-built plane turbines, but widespread application of fast neutron imaging is hindered by inefficient fast neutron detection. The current detection scheme is based on detection of recoil protons generated by neutrons scattering off hydrogen, with the leading material in the field consisting of ZnS:Cu embedded in hydrogen-dense plastic. However, these scintillation plate detectors rely on microscale phosphors with drawbacks including significant light scattering at the plastic-phosphor interface, strong absorption of (and sensitivity to) gamma-rays, and long-lived afterglows on the order of minutes. Thus, alternative solutions are required to enhance the performance of detectors for fast neutron imaging.

Separately, the development of semiconductor nanocrystals (NCs) has ushered in a golden age for nanoscale emissive materials with quantum yields near unity and enhanced recombination rates for short decay times, promising maximal detector efficiency while eliminating problematic afterglow. Here, we demonstrate the efficacy of nanocrystals as scintillators for fast neutron imaging, utilizing colloidal nanocrystals in hydrogen-dense solvents to detect recoil protons generated by scattering of fast neutrons. The light yield, spatial resolution, and neutron-vs.-gamma sensitivity of several representative compositions are compared, including both chalcogenide (CdSe and CuInS₂)-based and halide perovskite-based NCs, revealing the key parameters for optimized fast neutron detection.

FAPbBr₃ NCs exhibit the brightest total light output at 19.3% of the commercial ZnS:Cu(PP) standard (consisting of ZnS:Cu embedded in PP = polypropylene), the light output of the other compositions ranged from 1.5% for CuInS₂ NCs to 6.0% for CdSe/CdS quantum dots. Colloidal NCs also showed significantly lower gamma-ray sensitivity than ZnS:Cu, with the ratio of detected neutrons to gamma-rays ranging from 2.2 in CsPbBrCl₂:Mn NCs to 4.1 for CsPbBr₃ NCs, well above the 1.0-1.1 reported for ZnS:Cu(PP). This permits the use of less gamma-ray shielding and therefore enables a higher flux of fast neutrons, for example, 79% of the FAPbBr₃ light yield results from neutron-induced radioluminescence and hence the neutron-specific light yield of FAPbBr₃ is 30.4% of that of ZnS:Cu(PP), despite the tenfold higher phosphor load of ZnS:Cu(PP). Metal blocks with sharp edges were used to estimate the spatial resolution, with the high Stokes shift CsPbBrCl₂:Mn NCs offering the best spatial resolution at ~2.6 mm while that of FAPbBr₃ NCs is doubled at ~5.2 mm due to reabsorption and re-emission. Importantly, all NCs showed no evidence for afterglow on the order of seconds, in contrast to the minutes-long afterglow of the ZnS:Cu(PP) standard. Concentration and thickness-dependent measurements highlight the importance of increasing concentrations and reducing self-absorption, yielding design principles to optimize and foster an era of NC-based scintillators for fast neutron imaging.

Low dimensional (2D, 1D, 0D) lead halide materials have proven to be quite promising in terms of strong broad light emission with PLQY values above 90%, rendering them suitable for solid state lighting applications. These materials in order to be utilized in real applications must be environmentally stable and of non-toxic composition. Tin(II) based compounds pose as an excellent choice for addressing toxicity issues, however they are moisture and oxygen sensitive.
Addressing the above concerns, we report a new air-stable one-dimensional (1D) hybrid tin(II) halide material (DAO)Sn₂I₆ (DAO: 1,8-octyldiammonium), that is resistant to water for more than 15h.[1] The material exhibits a sharp absorption edge at 2.70 eV and a strong broad orange light emission centered at 634 nm, with a full width at half maximum (FWHM) of 142 nm (0.44 eV). The corresponding emission has a high photoluminescence (PL) quantum yield (PLQY) of at least 20.3% at RT, being among the highest reported for lead-free low-dimensional 2D and 1D perovskites and metal halides, which is coupled by a very long PL lifetime of 582 ns. Additionally, the PL emission intensity and linewidth is constant over a very broad temperature range (145-415 K), underscoring the beneficial high temperature PL robustness. The corresponding CIE chromaticity coordinates are (0.55, 0.42), with a correlated color temperature (CCT) of 1822 K (warm orange light), and a color-rendering index (CRI) value of 75, meaning that this material is suitable for outdoor lighting.
We further evaluated the ability of (DAO)Sn₂I₆ to fabricate thin films, the first step prior to device assembly. Apparently, the corresponding films exhibited the same optical properties of the bulk crystals, yet enhanced with an improved PLQY of 36%, and a much longer PL lifetime of 1100 ns, for a 60 nm thick film. These results demonstrate the versatile nature of hybrid halide materials, laying the path for the design of next generation of water stable lead-free semiconductors.

References
[1] Spanopoulos, I.; Hadar, I.; Ke, W.; Guo, P.; Sidhik, S.; Kepenekian, M.; Even, J.; Mohite, A. D.; Schaller, R. D.; Kanatzidis, M. G., J. Am. Chem. Soc. 2020, 142, 9028-9038

In this study, high specific surface areas (SSAs) of anatase titanium dioxide (TiO2) quantum dots (QDs) were successfully synthesized through a novel one-step microwave–hydrothermal method in rapid synthesis time (20 min) without further heat treatment. XRD analysis and HR-TEM images showed that the as-prepared TiO2 QDs of approximately 2 nm size have high crystallinity with anatase phase. Optical properties showed that the energy band gap (Eg) of as-prepared TiO2 QDs was 3.60 eV, which is higher than the standard TiO2 band gap, which might be due to the quantum size effect. Raman studies showed shifting and broadening of the peaks of TiO2 QDs due to the reduction of the crystallite size. The obtained Brunauer–Emmett–Teller specific surface area (381 m2 g−1) of TiO2 QDs is greater than the surface area (181 m2 g−1) of commercial TiO2
nanoparticles. The photocatalytic activities of TiO2 QDs were conducted by the inactivation of Escherichia coli under ultraviolet light irradiation and compared with commercially available anatase TiO2 nanoparticles. The photocatalytic inactivation ability of E. coli was estimated to be 91% at 60 μg ml−1 for TiO2 QDs, which is superior to the commercial TiO2 nanoparticles. Hence, the present study provides new insight into the rapid synthesis of TiO2 QDs without any annealing treatment to increase the absorbance of ultraviolet light for superior photocatalytic inactivation ability of E. coli.

Recently wide bandgap halide double perovskites have emerged as promising phosphor materials for white light LED applications. These materials demonstrate enhanced photoluminescence properties compared to bulk APbX₃ (A = NH₃CH₃⁺, Cs; X = Cl, Br, I) perovskites due in large part to their higher exciton binding energy, thereby reducing extensive nonradiative decay at defects and grain boundaries. Cs₂NaBiCl₆ is an example of a halide double perovskite which in the pure form exhibits no photoluminescence. Upon doping Mn²⁺ into the lattice (0.06-7.2 mol%) bright orange luminescence emerges under UV excitation originating from the ⁴T₁ to ⁶A₁ transition of Mn²⁺ ions in an octahedral environment. Analysis of absorption and excitation measurements indicate absorption of the host material is dominated by localized 6s² to 6s¹6p¹ transitions of Bi³⁺ ions, indicating a zero-dimensional electronic structure. The localized nature of Bi³⁺ absorption in the perovskite structure is identified as a crucial component for efficient energy transfer to Mn²⁺ sites. Partial replacement of Cl^- with Br^- in the structure forming Cs₂NaBiCl_6-xBr_x:Mn²⁺ is demonstrated to relax the crystal field around Bi³⁺ ions thereby shifting the excitation range towards the visible where it can be excited by conventional GaInN blue LEDs. Lessons learned in this study extend towards several other double perovskite and perovskite-derivative materials, including systems exhibiting intrinsic luminescence via self-trapped excitons as well as activator-doped materials. Routes to optimize photoluminescence in these systems are addressed.

Rare-earth doped wide bandgap (WBG) semiconductors have attracted great attention as efficient luminescent materials for optoelectronic applications, such as photon downshift, down- and up-conversion systems, a new generation of low voltage light-emitting devices, non-contact luminescent temperature sensors and photonic structures.

However, these luminescent mechanisms have yet to be successfully implemented in transparent conductive oxide (TCO) films with a WBG, such as indium tin oxide (ITO) or aluminum zinc oxide (AZO) which often form part of these devices. Such optically active TCOs would therefore be advantageous for these applications since layers of the same TCO are already being used in said devices.

Our work focuses on the production and characterization of terbium (Tb) doped AZO thin films, keeping a high optical transmittance in the visible region and exhibiting Tb related luminescent properties. The effect of Tb doping on the optical and luminescent properties was investigated for different post-deposition annealing conditions. Films were produced on silicon and fused silica substrates by radiofrequency magnetron co-sputtering employing high purity AZO and Tb targets and a rotary sample holder. After deposition, samples were annealed between 200°C - 1000°C in steps of 100°C. The annealing processes were carried out in air or argon atmosphere. Optical transmittance, photoluminescence (PL) and energy dispersive X-ray spectroscopy experiments were made after each annealing step. Characteristic Tb³⁺ related light emission peaks were found in the as grown, as well as in the annealed samples. To the best of our knowledge, this is the first demonstration of Tb³⁺ emission in AZO as host matrix. Absorption coefficient, band gap and Urbach energies were estimated using computational methods and models developed in our group. Furthermore, temperature dependent PL experiments, ranging from 80K to 530K, were carried out on representative samples to discuss possible energy transfer mechanisms between Tb³⁺ ions and AZO matrix.

The origin of the near-infrared photoluminescence (PL) from thiolate-protected gold nanoclusters (Au NCs, <2 nm) has long been controversial, and the exact mechanism for the enhancement of quantum yield (QY) in many works remains elusive. It is still a major challenge for researchers to map out a definitive relationship between the atomic structure and the PL property and understand how the Au kernel and Au–S surface contribute to the PL of Au NCs.
Herein, we first provide a paradigm study to address the above critical issues. By using a correlated series of “mono-cuboctahedral kernel” Au NCs and combined analyses of steady-state, temperature-dependence, femtosecond transient absorption, and Stark spectroscopy measurements, we have explicitly mapped out a kernel-origin mechanism and clearly elucidate the surface–structure effect, which establishes a definitive atomic-level structure–emission relationship. A ∼100-fold enhancement of QY is realized via suppression of two effects: (i) the ultrafast kernel relaxation and (ii) the surface vibrations. The new insights into the PL origin, QY enhancement, wavelength tunability, and structure–property relationship constitute a major step toward the fundamental understanding and structural-tailoring-based modulation and enhancement of PL from Au NCs.
Furthermore, we report the emergence of a photo-induced structural distortion accompanied by an electron redistribution in a series of bi-tetrahedral gold nanoclusters. Such unexpected slow process of excited-state transformation results in near-infrared dual emission with extended photoluminescent lifetime. We demonstrate that this dual emission exhibits highly sensitive and ratiometric response to solvent polarity, viscosity, temperature and pressure. Thus, a versatile luminescent nano-sensor for multiple environmental parameters has been developed based on this strategy. Furthermore, we fully unravel the atomic-scale structural origin of this unexpected excited-state transformation, and demonstrate control over the transition dynamics by tailoring the bi-tetrahedral core structures of gold nanoclusters.

Dark exciton as the lowest-energy (ground) exciton state in metal halide perovskite nanocrystals is a subject of much interest. This is because the superior performance of perovskites as the photon source combined with long lifetime of dark exciton is very attractive for the applications such as quantum information processing. However, the direct observation of the long-lived dark exciton emission confirming the accessibility to dark ground exciton state has remained elusive. In this presentation, we discuss the observation of the intense photoluminescence (PL) from dark exciton with lifetime of 1-10 microsecond in strongly confined CsPbBr₃ nanocrystals of varying dimensionality of confinement. Strongly confined quantum dots, nanowires and nanoplateletes of cesium lead halide perovskite showed dark exciton emission that dominates the PL at cryogenic temperatures with bright-dark splitting energy (ΔE) of ~20 meV, which contrasts to the behavior of weakly confined nanocrystals that exhibit little dark exciton PL. The lifetime of dark exciton PL shortened under magnetic field as expected. ΔE increased with increasing quantum confinement giving an easier access to the dark exciton PL at the higher temperatures, which is also consistent with the prediction from the earlier theoretical prediction. This work establishes the potential of strongly quantum confined perovskite nanostructures as the excellent platform to harvest the benefits of extremely long-lived and intense dark exciton PL.

The replacement of lead in halide perovskites with alternative halide perovskite materials has been driven by the concerns of lead toxicity. Lead-free all-inorganic cesium bismuth halide (Cs₃Bi₂X₉) perovskites nanocrystals have attracted much attention in recent years due to their air-stability and the relatively non-toxic nature of bismuth. Common solvents used to prepare Cs₃Bi₂X₉ nanomaterials under ambient conditions have included dimethyl sulfoxide (DMSO) and dimethylformamide (DMF), which are also desired to be avoided for their own hazards. Herein, we demonstrate a facile, sonication-assisted approach for the preparation of all-inorganic cesium bismuth iodide (Cs₃Bi₂I₉) perovskite nanocrystals (NCs) using propylene carbonate as a green solvent. The photoluminescence (PL) spectra of the Cs₃Bi₂X₉ NCs are further tuned from 410 to 550 nm by controlling the composition of the NCs through anion exchange reactions using tetraalkylammonium halides as a source of halide ions. The rate of reaction for the anion exchange process significantly influences the dimensions of the resulting NCs in contrast to their parent Cs₃Bi₂I₉ NCs. Absolute photoluminescence quantum efficiency (PLQE) of the Cs₃Bi₂I₉ NCs was also determined, and their PL emission was determined to be predominately due to exciton recombination processes. The NCs also exhibited an excellent air-stability for at least 150 days.

Organic-inorganic metal halide perovskites have emerged as attractive materials for light-emitting and light-harvesting devices. We discuss the fundamental processes that have underpin light-matter interactions enabling these applications.

Photon recycling is shown to slow charge losses from thin hybrid perovskite films, depending on light outcoupling.
Interestingly, for thin films comprising a quasi-two-dimensional (2D) perovskite region interfaced with
a 3D MAPbI perovskite layer the blue-shifted emission originating from quasi-2D regions overlaps significantly
with the absorption spectrum of the 3D perovskite, allowing for highly effective “heterogeneous photon
recycling”. We show that this combination fully compensates for the adverse effects of electronic confinement,
yielding quasi-2D perovskites with highly efficient charge transporting properties.
We further discuss how vibrational, charge-carrier recombination and mobility is altered in thin films of
perovskite nanocrystals films with respect to the bulk film. We show in thin films, carrier mobilities are still
mostly dominated by intra-particle motion rather than inter-particle hops.
In tunable bandgaps across the entire visible light emission range, we discuss the mechanisms underlying detrimental halide segregation in mixed halide lead perovskites.

S. G. Motti, T. Crothers, R. Yang, Y. Cao, R. Li, M. B. Johnston, J. Wang, and L. M. Herz,
Nano Lett. 19, 3953 (2019).

S. G. Motti, F. Krieg, A. J. Ramadan, J. B. Patel, H. J. Snaith, M. V. Kovalenko, M. B. Johnston, and L. M. Herz,
Adv. Func. Mater. 30, 1909904 (2020).

E. S. Parrott, J. B. Patel, A.-A. Haghighirad, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Nanoscale 11, 14276 (2019).

A. Knight, A. D. Wright, J. B. Patel, D. McMeekin, H. J. Snaith, M. B. Johnston, and L. M. Herz,
ACS Energy Lett. 4, 75 (2019).

A. J. Knight, J. B. Patel, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Adv. Energy Mater. 10, 1903488 (2020).

On-demand single-photon sources (SPSs) operating at room temperature under electrical pumping play a crucial role in the emerging quantum applications ranging from linear quantum computing [1] to unconditionally secure communication lines [2]. Color centers in silicon carbide are now considered as a promising platform for the realization of such electrically-pumped SPSs operating at room temperature. In the steady-state, they are brighter than other quantum emitters, such as color centers in diamond or epitaxial quantum dots [3]. At the same time, steady-state emission of single photons almost does not have practical quantum applications, and an on-demand SPS is highly needed. However, very little is known about how to control, dynamically tune and quickly switch on and off the single-photon electroluminescence (SPEL) of color centers in SiC that will allow to meet the on-demand requirement when exactly one photon is emitted per excitation pulse.
Here, we propose and numerically demonstrate the SiC gate-tunable single-photon emitting diode (GT-SPED) with a silicon antisite near stacking fault (Si_C-SF) defect placed underneath the gate [4]. Its emission can be controlled via source/drain and gate contacts. When no voltage is applied to the gate, the GT-SPED acts as a conventional p-i-n single-photon emitting diode, where the SPEL of Si_C-SF cannot be switched between ON and OFF states fastly due to the high diode capacitance. However, if one applies voltage to the gate contact and change it in the range of several volts at a fixed source voltage, he can not only control and dynamically tune the SPEL of SiC-SF center from 0.16 Mcps (OFF state) to 40 Mcps (ON state) but what is more important, switch between ON and OFF states of SiC-SF SPEL in less than 0.2 ns, which is less than the radiative lifetime of the SiC-SF center and other defects in solids. These possibilities allow to trigger single-photon emission from the Si_C-SF center by applying small voltage pulses to the gate contact. If the width of such a voltage pulse is chosen correctly, a single photon will be emitted with the probability of multiphoton events of less than 2%. Hence, the proposed device lays the solid foundation for the development of electrically-driven on-demand single-photon sources operating at room temperature.

References

[1] T. D. Ladd, F. Jelezko, R. Laflamme, Y. Nakamura, C. Monroe, and J. L. O’Brien, Nature 464, 45 (2010).
[2] A. V. Sergienko, Quantum Communications and Cryptography (CRC Press, 2018).
[3] A. Lohrmann, N. Iwamoto, Z. Bodrog, S. Castelletto, T. Ohshima, T. J. Karle, A. Gali, S. Prawer, J. C. McCallum, and B. C. Johnson, Nat. Commun. 6, 7783 (2015).
[4] I. A. Khramtsov and D. Y. Fedyanin, ACS Applied Electronic Materials 1, 1859 (2019).

Narrow-band photoluminescence (PL) together with high quantum efficiency from organic molecules is essential for high-color-purity emitters. Supramolecular assemblies like J-aggregates are promising materials due to their narrow PL signal with full-width at half maximum <20 nm. However, their microcrystalline nature and coherent exciton migration results in strong nonradiative exciton recombination at the grain boundaries that diminish the photoluminescence quantum yield (PLQY), and possibilities for improving the crystallinity by tuning the growth mechanism are limited. Here, two distinct routes to grow different J-aggregate morphologies like platelets and lamellar crystals with improved crystallinity by surface-guided molecular assembly are demonstrated, thereby suppressing nonradiative decay and improving PLQY (upto 5% at room temperature). Both platelets and lamellar crystals show similar absorbance at room temperature. However, temperature-dependent PL studies show sevenfold (twofold) higher PLQY for lamellar films compared to platelets at 6 K (300 K). Using time-resolved PL spectroscopy, different nonradiative decay pathways are identified.¹
Revisiting the J-aggregates formation in solution, we demonstrate that cyanine J-aggregates can reach an order of magnitude higher photoluminescence quantum yield (increase from 5% to 60%) in blend solutions of water and alkylamines at room temperature.² By means of time-resolved photoluminescence (TRPL) studies we show an increase in the exciton lifetime as a result of the suppression of non-radiative processes. Small-angle neutron scattering studies suggest a necessary condition for the formation of such highly emissive J-aggregates: the presence of a sharp water/amine interface for J-aggregate assembly and the coexistence of nanoscale-sized water and amine domains to restrict the J-aggregate size and solubilize monomers, respectively.

References
1. Anantharaman, S. B., Stöferle, T., Nüesch, F. A., Mahrt, R. F. & Heier, J. Exciton Dynamics and Effects of Structural Order in Morphology-Controlled J-Aggregate Assemblies. Adv. Funct. Mater. 29, 1806997 (2019).
2. Anantharaman, S. B. et al. Enhanced Room Temperature Photoluminescence Quantum Yield in Morphology-controlled J-aggregates. Under Revision. (2020).

Plasmonic terahertz (THz) emitters are expected as on-chip and coherent THz sources. Up to the present, THz emissions stimulated by plasmonic instability with dc-current-driven Doppler-shift (DS) type and electron-transit-time (ETT) type have been experimentally observed in high-electron-mobility transistors (HEMTs). These instabilities could stimulate resonant-mode THz emission. Here we experimentally investigate the dc-current-driven plasmonic boom instability, the third plasmonic instability that occurs when the electron drift velocity exceeds the plasma velocity. The plasmonic boom instability generates shockwaves so that it stimulates rather broadband THz emission. To obtain a coherent narrow-band emission, a pertinent cavity structure needs to be installed. We use an original dual-grating-gate (DGG) HEMT structure based on an InGaAs high electron mobility channel under photoexcitation of photomixed dual-cw infrared (IR) lasers irradiation.
The InGaAs DGG-HEMT photomixer incorporates two types of grating-gate electrodes having a finger size of 100 nm and spacing between the adjacent fingers of 200 nm. The DGG structure modulates the electron drift velocity and plasma velocity in a complementary manner. One gate G1 (the other gate G2) are high (low) biased working as plasmonic cavities (as high-field, depleted regions) wherein the plasma velocity is higher (lower) than the electron drift velocity. The difference frequency (δf) of the dual-cw-IR lasers was set around the plasmon resonant frequency determined by the G1 grating finger size and the electron density. As above mentioned, the plasmonic boom instability generates a shockwave of rather broadband plasmonic displacement current. Since the photomixed dual-cw-IR lasers irradiation generates coherent narrow spectral photocurrent component around the frequency δf, one can expect the broad plasmonic ac current could be injection-locked to the photogenerated δf component, resulting in an enhanced coherent THz emission around δf. We set δf at 2, 3, and 4 THz. The THz emission from the fabricated device was measured at 100K using a Fourier Transform IR (FTIR) spectrometer and a 4.2-K cooled Si bolometer.
The bias for G1 (VG1) was set at 0 V whereas that for G2 (VG2) was set at -0.5 V. The drain bias (VD) was set at 1V. We compared the THz emission beyond the background blackbody radiation at a fixed biases (VG1, VG2, VD) condition among the different photomixed δf conditions. With laser irradiation, rather broad THz emissions were observed. The observed spectral peaks are mostly independent of δf because of rather high-level of thermally-excited incoherent plasmons. However, the emission spectra for δf = 2 and 4 THz exhibited a higher intensity at a narrow frequency range around 4-to-6 THz than that for δf = 3 THz. To see these intensity differences in detail, the enhancement of mono-peak emission around 5 THz showed a weak blue shift from the emission for δf = 2 THz to that for δf = 4 THz. From these results, in the plasmonic cavities under G1 region, plasmonic resonant modes are situated close to 2 and 4 THz, and a plasmonic anti-resonant mode is situated close to 3 THz. It is worth noting that neither DS type nor ETT type current-driven instability emission could originate such an observed broadband emission. Since the drift velocity could exceed the plasma velocity at the cavity boundaries between G1 and G2, the results suggest that the photomixing-originated THz emission is amplified by stimulated emission of plasmons originated by the plasmonic boom instability.
In summary, the radiation spectra observed at 100K showed two distinctive emission components beyond the black-body radiation, one promoted by δf-independent thermally excited incoherent plasmons and the other promoted by δf-dependent coherent plasmons. The results suggest the occurrence of the plasmonic instability stimulated by the dc current flow in the channel.

CsPbBr₃ nanocrystals (NCs) have attracted much attention over the past five years due to their exceptional optoelectronic properties and potential applications in devices such as light-emitting diodes (LEDs), lasers, and single-photon emitters. However, their fundamental photophysical properties, especially at low temperatures, are still under active debate. To date, almost all of the reports have used photoluminescence (PL) alone to infer the lattice dynamics in these materials. Here, we measure both the temperature-dependent (35 K - 300 K) absorption and PL spectra of zwitterionic ligand-capped CsPbBr₃ NCs with four different edge lengths (L = 4.9 - 13.2 nm). The excitonic transitions observed in the absorption spectra can be explained with an effective mass model considering the quasicubic NC shape and non-parabolicity of the electronic bands. We observe a temperature-dependent Stokes shift; while the trend is similar to the Stokes shift observed in both MAPbBr₃ and CsPbBr₃ single crystals, it does not approach zero at cryogenic temperatures, pointing to an additional contribution intrinsically present in the NCs. Surprisingly, the effective dielectric constant determined from the best fit model parameters is independent of temperature, contrary to the previous report that the change in dielectric constant leads to the Stokes shift temperature dependence. Overall, our study sheds light on the fundamental lattice dynamics in these materials, and can potentially be used to guide future material optimization for device applications.

Chemical doping of perovskites holds promise for enhancing luminescence efficiencies, which is crucial for applications such as efficient light-emitting diodes (LEDs). Using transient optical spectroscopy we study the recombination dynamics in manganese-doped Cs(Mn:Pb)(Cl,Br)₃ perovskite nanocrystals. We find an increased excitonic radiative recombination rate upon doping, together with reduced non-radiative losses. Supported by ab initio calculations, we propose the enhanced luminescent recombination to arise from an increased electron hole overlap induced by the dopant ions. Thus, the stronger exciton localization leads to a higher probability of luminescent decay, important for optimising LED efficiencies.

In recent years, the interest in halide perovskites rose at a rapid pace due to their tremendous success in the field of photovoltaic while other fields, like light emitting diodes, show great potential as well. One intriguing property of this material class is the wide tunability of the band-gap that can be induced by changing the perovskite composition. While changes in band gap are regularly reported, it is unclear how the respective conduction and valence band positions change and what the underlying origins of these changes are. Knowing the band positions is however crucial for device design, i.e. ensuring efficient charge transport across the various interfaces.
In this talk, I will present our results regarding variations in valence and conduction band position for a wide variety of lead and tin based perovskites [1]. Next to the primary ABX3 type composition, also mixed metal structures will be shown. Using a combination of photoelectron spectroscopy and density functional theory we are able to explain the origin of these energy level and band gap changes based on changes in hybridization strength, atomic level positions, and lattice distortion.
References
[1] S. Tao, Ines Schmidt, G. Brocks, J. Jiang, I. Tranca, K. Meerholz, and S. Olthof , Absolute energy level positions in tin- and lead-based halide perovskites, Nature Communications 10, 2560 (2019)

Halide perovskites exhibit a smorgasbord of emission properties, including tunable emission energies, linewidths and luminescence yields through judicious composition and dimensionality choice. This opens up a number of exciting applications in lighting and displays, as well as highly efficient solar cells. Achieving absolute control over the emission properties will further enable these novel semiconductors.

In this talk, I will cover recent developments in our group focusing on the light emission from halide perovskite semiconductors. I will demonstrate several approaches to dynamically control energy or charge transfer processes and ultimately tune emission colour, yields and carrier recombination by tuning either the 2D/3D components, halide components on the surface and/or bulk of thin films, or synthesizing well-passivated nanoplatelet structures. I will also show that local chemical variations in alloyed perovskites used in solar cells allow accumulation of photo-excited holes in small regions, leading to high emission yields even at low carrier densities in those regions. Finally, I will present new optical spectroscopy approaches to quantify luminescence yields and related device parameters in both solar cells and light-emitting diodes, providing unique insight into power loss processes in devices.

Near-infrared optoelectronic devices have found significant applications in facial recognition, eye tracking, motion sensing and health monitoring technologies. In this talk, I will discuss some of our new developments on efficient near-infrared light-emitting devices based on colloidal semiconductor quantum dots and metal halide perovskites. I will also highlight some of the design principles, synthesis and processing methods that we have employed to achieve high-performance devices with improved efficiency, radiance and reliability. Finally, I will showcase some novel implementations of large-area and flexible NIR devices in new wearable devices and technologies.

Existing gels are mostly polar, whose nature limits their role in soft devices. The intermolecular interactions of the nonpolar polymer-liquid system are typically weak, which makes the gel brittle or opaque. Here we report highly soft and transparent nonpolar organogels. Even though their elements are only carbon and hydrogen, their elastic modulus (~30 kPa), transparency (~98 %), and stretchability (~1000 %) are comparable to common soft hydrogels. A key strategy is introducing aromatic interaction into the polymer-solvent system, resulting in a high swelling ratio that enables efficient plasticization of the polymer networks. As a proof of applicability, soft perovskite nanocomposites are synthesized, where the organogels are in-situ polymerized with the two types of perovskite nanocrystals: physically dispersible or chemically addressable. The highly nonpolar environment of organogels enables stable formation and preservation of highly concentrated perovskite nanocrystals without mechanical sacrifice, and especially the chemically anchored perovskite-nonpolar organogel nanocomposites show high photoluminescence quantum efficiency (PLQE) that reaches 92.1 %, while their luminescence properties are environmentally stable against air, water, acid, base, heat, ultraviolet light, and mechanical deformation. Furthermore, when exposed to water, the chemically anchored nanocomposites show luminescence increasing behavior that their PLQE reaches near unity (~99.8 %) and is maintained for more than 110 days, which performance in the form of elastomeric perovskite emitters has not been reported. These superb properties enable the demonstration of fully soft electroluminescent devices that stably emit bright and pure green light under diverse deformations with no luminescence hysteresis. The aromatic interaction-induced nonpolar organogels may enable a step forward in soft optoelectronics.

Semiconductor nanowires are routinely grown on high-priced crystalline substrates as it is extremely challenging to grow directly on plastics and flexible substrates due to high temperature requirements and substrate preparation. At the same time, plastic substrates can offer many advantages such as extremely low price, light weight, mechanical flexibility, shock and thermal resistance, and biocompatibility. We explore the direct growth of high-quality III−V nanowires on flexible plastic substrates by metal-organic vapor phase epitaxy (MOVPE). We synthesize InAs, InP, InSb and InAlAs nanowires on polyimide and show that the fabricated NWs are optically active with strong light emission in the mid-infrared range. We create a monolithic flexible nanowire-based p−n junction device on plastic in just two fabrication steps. Overall, we demonstrate that III−V nanowires can be synthesized directly on flexible plastic substrates inside a MOVPE reactor, and we believe that our results will further advance the development of the nanowire-based flexible electronic devices.

Highly flexible ultrathin light sources and displays can play a key role in realizing future optoelectronic devices owing to their outstanding form factors. They may be utilized in wearable, body-attachable, and imperceptible electronic devices. Foldable displays are a good example of such devices that make most of such form-factor advantages. With the emerging interest in folding displays, it is expected that the market of foldable devices is growing bigger and bigger over the next few decades. Organic light-emitting diodes (OLEDs) or quantum-dot light-emitting diodes (QLEDs) are regarded as promising candidates for foldable displays because these light-sources are surface-emitting and their low-temperature processing is compatible with flexible substrates. Among these light-sources, perovskite light-emitting diodes (PeLEDs) exhibit excellent light-emitting properties such as high color purity thanks to their exceptional optoelectronic properties. For example, PeLEDs using organic-inorganic hybrid perovskite as light-emitting material have shown not only high external quantum efficiency over 20% but also narrow full width at half maximum (FWHM). For their superb material properties, several studies have been conducted to realize highly flexible light sources using perovskite as a light-emitting material. However, the bending radii in most of the initial attempts were larger than 1mm, which is the bench-mark number desired by foldable or imperceptible devices. Even if the bending radius was smaller than 1mm, the devices had shown a quick degradation upon repeated bending cycles.

In this study, we propose ultrathin perovskite light-emitting diodes using a substrate formed from UV-curable resins. In order to reduce the bending radius below 1mm, the thickness of the substrate should be decreased to a few tens of microns. Mechanical design has been done carefully to retain the device performance even after the repeated bending test by considering the crack onset strain (COS) for each layer. To this end, we measured COS for each layer including the perovskite emitting layer. The COS value of the perovskite layer is found to be ca. 3%, which is greater than that of typical transparent conductive oxides. Furthermore, to overcome any fabrication issues such as carrier leakage or solution penetration between a substrate and a carrier substrate, we ensured each layer to be formed as uniform as possible.

With these considerations, we were able to fabricate foldable perovskite light-emitting diodes that were shown to retain its EQE after 10,000 repeated bending cycles at the bending radius as small as 0.1 mm. The overall thickness was 20 μm including the top encapsulation layer which was used for forming a neutral plane in the middle of the device, as well as protecting the device from ambient air. We believe the proposed foldable perovskite light-emitting diodes can be an important component for wearable or body-attachable devices, which can play a key role in realizing human-centric Internet-of-Things (IoT) applications.

Halide double perovskites provide a flexible platform for manipulating electronic structure through synthetic design. I will present some recent studies from our group, where we have attempted to increase the compositional and structural complexity of halide perovskites through chemical substitution. In our quest to understand the electronic structure of double perovskites, we developed a simple model that allows us to predict the bandgap symmetry based on chemical composition. Our work constructs an intuitive picture of how the molecular orbitals of metal-hexahalide coordination complexes give rise to electronic bands in the extended perovskite lattice and describes how these molecular orbitals vary across the bands to give rise to the overall band structure. Although the effects of quantum confinement of Pb and Sn perovskites have been well studied, 2D analogs of 3D double perovskites are notably lacking. I will present a series of 2D double perovskites and discuss the optical and electronic consequences of dimensional reduction of the 3D double perovskite lattice to 2D derivatives.

Halide Perovskites exhibit outstanding light-harvesting and emissive properties. Presently, record power conversion efficiencies (PCE) >25% in solar cells and external quantum efficiencies (EQE) >20% in light-emitting devices have been demonstrated. The perovskite field has since expanded well beyond photovoltaics and LEDs into other optoelectronic domains of lasers, x-ray detectors, memristors etc. Emerging frontier areas include multiphoton, multi-exciton generation and hot carrier applications. In this talk, I will focus on the hot carrier phenomena in halide perovskites and our recent work in this area.

Low-dimensional perovskites are rapidly emerging due to their distinctive emission properties. Here, first I will present the fundamental properties of pristine 2D perovskites, such as the nature of the primary photo-excitations, their relaxation through electronic states and their diffusion across thin films. Such properties will be correlated and modulated by the structural characteristics of the 2D perovskites. Then, I will discuss the possibility of perovskite doping by controlled addition of impurity ions which may enable new electronic, magnetic, and optical performances. I will explore new doping strategies based on transition and rare earth metals for tuning of the emission wavelength (from UV to NIR, further extending the typical range covered with perovskites) and luminescent enhancement. The suitability of doping for the realization of 4 level perovskite lasers will be assessed. Doping with heterovalent ions will be tested to selectively increase p and n type conductivity of the material. The addition of alkaline-earth metals as B-site dopants will be tested to improve the material stability

In this report, the photoluminescent properties of a lead-free double perovskite Cs₂NaInCl₆ doped with Sb³⁺ are explored. The host crystal structure is a cubic double perovskite with Fm-3m symmetry, a = 10.53344(4) Å, and rock salt ordering of Na⁺ and In³⁺. It is a wide bandgap compound (Eg ≈ 5.1 eV), and substitution with Sb³⁺ leads to strong absorption in the UV due to localized 5s² → 5s¹5p¹ transitions on Sb³⁺ centers. Radiative relaxation back to the 5s² ground state, via a ³P₁ → ¹S₀ transition, leads to intense blue luminescence, centered at 445 nm, with a photoluminescent quantum yield of 79%. The Stokes shift of 0.94 eV is roughly 33% smaller than it is in the related vacancy ordered double perovskite Cs₂SnCl₆. The reduction in Stokes shift is likely due to a change in coordination number of Sb³⁺ from 6-coordinate in Cs₂NaInCl₆ to 5-coordinate in Cs₂SnCl₆. In addition to the high quantum yield, Cs₂NaInCl₆:Sb³⁺ exhibits excellent air/moisture stability and can be prepared from solution; characteristics make it a promising blue phosphor for applications involving near-UV excitation.

Two-dimensional (2D) halide perovskites are a class of emitter materials that are enjoying growing attention recently. The materials are related to 3D halide perovskites but either part or all of the small A-site cations are replaced by large organic molecules with an amonium binding moiety, for example butylamonium or phenyl-ethylamonium. This (generally) leads to the formation of a layered structure where a layer of metal-halide octahedra is formed and the large organic cations separate the layers from each other. Mixing of large and small A-site cations can lead to quasi-2D multi-layer materials where multiple layers of metal halide octahedra are separated by the organic molecules, for instance forming Ruddlesden-Popper or Dion-Jacobson structures. The emissive properties of these materials are intricately linked to the structural dynamics and formation of possible defects in the inorganic layers.

In this work we explore how modifications of the large organic amonium ion can affect the structural dynamics of the inorganic layer, combining molecular dynamics simulations with time-resolved spectroscopy. Using time-resolved fluorescence and ultrafast transient absorption spectroscopy we have studied the photophsyical behavior of 2D perovskites with a range of different organic cations inorder to gain insight in the relation between the structure and the exciyted state dynamics, These measurements indicate marked differences that can not solely be attributed to changes in the average structure of the materials.

To explain these differences we have performed classical molecular dynamics simulations for a variety of structures, for instance aliphatic alkyl chains, amonium ions containing aromatic moieties such as phenyls or perylene, but also di-functional ions containing two binding sites that can connect to an inorganic pervskite layer on both sides. The results indicate that there are marked differences between the different structures in terms of their structural rigidity. Multi-layer Ruddlesden-Popper structures are generally more rigid than single layers, however, the rigidity of the single layer structures can be strongly enhanced by engineering of the interactions between organic cations, for instance by introduction of large conjugated ions. In addition, di-functional ligands resulting in Dion-Jacobson like structures also lead to more rigid structures. Comparison with the experimental results allows us to derive some design rules that indicate how engineering of the organic part of the structure can help in improving the the emissive properties of the materials that is mostly due to the inorganic layer structure.

Halide double perovskites provide a flexible platform for manipulating electronic structure through synthetic design. I will present some recent studies from our group, where we have attempted to increase the compositional and structural complexity of halide perovskites through chemical substitution. In our quest to understand the electronic structure of double perovskites, we developed a simple model that allows us to predict the bandgap symmetry based on chemical composition. Our work constructs an intuitive picture of how the molecular orbitals of metal-hexahalide coordination complexes give rise to electronic bands in the extended perovskite lattice and describes how these molecular orbitals vary across the bands to give rise to the overall band structure. Although the effects of quantum confinement of Pb and Sn perovskites have been well studied, 2D analogs of 3D double perovskites are notably lacking. I will present a series of 2D double perovskites and discuss the optical and electronic consequences of dimensional reduction of the 3D double perovskite lattice to 2D derivatives.

Halide Perovskites exhibit outstanding light-harvesting and emissive properties. Presently, record power conversion efficiencies (PCE) >25% in solar cells and external quantum efficiencies (EQE) >20% in light-emitting devices have been demonstrated. The perovskite field has since expanded well beyond photovoltaics and LEDs into other optoelectronic domains of lasers, x-ray detectors, memristors etc. Emerging frontier areas include multiphoton, multi-exciton generation and hot carrier applications. In this talk, I will focus on the hot carrier phenomena in halide perovskites and our recent work in this area.

Low-dimensional perovskites are rapidly emerging due to their distinctive emission properties. Here, first I will present the fundamental properties of pristine 2D perovskites, such as the nature of the primary photo-excitations, their relaxation through electronic states and their diffusion across thin films. Such properties will be correlated and modulated by the structural characteristics of the 2D perovskites. Then, I will discuss the possibility of perovskite doping by controlled addition of impurity ions which may enable new electronic, magnetic, and optical performances. I will explore new doping strategies based on transition and rare earth metals for tuning of the emission wavelength (from UV to NIR, further extending the typical range covered with perovskites) and luminescent enhancement. The suitability of doping for the realization of 4 level perovskite lasers will be assessed. Doping with heterovalent ions will be tested to selectively increase p and n type conductivity of the material. The addition of alkaline-earth metals as B-site dopants will be tested to improve the material stability

In this report, the photoluminescent properties of a lead-free double perovskite Cs₂NaInCl₆ doped with Sb³⁺ are explored. The host crystal structure is a cubic double perovskite with Fm-3m symmetry, a = 10.53344(4) Å, and rock salt ordering of Na⁺ and In³⁺. It is a wide bandgap compound (Eg ≈ 5.1 eV), and substitution with Sb³⁺ leads to strong absorption in the UV due to localized 5s² → 5s¹5p¹ transitions on Sb³⁺ centers. Radiative relaxation back to the 5s² ground state, via a ³P₁ → ¹S₀ transition, leads to intense blue luminescence, centered at 445 nm, with a photoluminescent quantum yield of 79%. The Stokes shift of 0.94 eV is roughly 33% smaller than it is in the related vacancy ordered double perovskite Cs₂SnCl₆. The reduction in Stokes shift is likely due to a change in coordination number of Sb³⁺ from 6-coordinate in Cs₂NaInCl₆ to 5-coordinate in Cs₂SnCl₆. In addition to the high quantum yield, Cs₂NaInCl₆:Sb³⁺ exhibits excellent air/moisture stability and can be prepared from solution; characteristics make it a promising blue phosphor for applications involving near-UV excitation.

Two-dimensional (2D) halide perovskites are a class of emitter materials that are enjoying growing attention recently. The materials are related to 3D halide perovskites but either part or all of the small A-site cations are replaced by large organic molecules with an amonium binding moiety, for example butylamonium or phenyl-ethylamonium. This (generally) leads to the formation of a layered structure where a layer of metal-halide octahedra is formed and the large organic cations separate the layers from each other. Mixing of large and small A-site cations can lead to quasi-2D multi-layer materials where multiple layers of metal halide octahedra are separated by the organic molecules, for instance forming Ruddlesden-Popper or Dion-Jacobson structures. The emissive properties of these materials are intricately linked to the structural dynamics and formation of possible defects in the inorganic layers.

In this work we explore how modifications of the large organic amonium ion can affect the structural dynamics of the inorganic layer, combining molecular dynamics simulations with time-resolved spectroscopy. Using time-resolved fluorescence and ultrafast transient absorption spectroscopy we have studied the photophsyical behavior of 2D perovskites with a range of different organic cations inorder to gain insight in the relation between the structure and the exciyted state dynamics, These measurements indicate marked differences that can not solely be attributed to changes in the average structure of the materials.

To explain these differences we have performed classical molecular dynamics simulations for a variety of structures, for instance aliphatic alkyl chains, amonium ions containing aromatic moieties such as phenyls or perylene, but also di-functional ions containing two binding sites that can connect to an inorganic pervskite layer on both sides. The results indicate that there are marked differences between the different structures in terms of their structural rigidity. Multi-layer Ruddlesden-Popper structures are generally more rigid than single layers, however, the rigidity of the single layer structures can be strongly enhanced by engineering of the interactions between organic cations, for instance by introduction of large conjugated ions. In addition, di-functional ligands resulting in Dion-Jacobson like structures also lead to more rigid structures. Comparison with the experimental results allows us to derive some design rules that indicate how engineering of the organic part of the structure can help in improving the the emissive properties of the materials that is mostly due to the inorganic layer structure.