Symposium ES17—Perovskite-Based Light-Emission and Frontier Phenomena—Single Crystals, Thin Films and Nanocrystals

The excellent optoelectronic properties of halide perovskite materials and the easy growth of single crystals from solution make them ideal candidates for other electronic devices beyond solar cells. Here we will report the birth of perovskite radiation detectors, and advances in imaging detectors in terms of scaling up and integration with silicon read-out circuitry. Single photon detectors will also be presented to outline the challenges and opportunties for perovskite materials. Finally, we will present recent progress in making perovskite to be very efficient blue emitters.

Two-dimensional (2D) metal halide perovskites have made an impressive entry in the field of solar cells and LEDs as highly promising semiconductors. They feature a high degree of structural flexibility and tunable optoelectronic properties. They have a general formula of (A’)₂(A)_n-1M_nX_3n+1, where A = Cs⁺, CH₃NH₃⁺ (MA), HC(NH₂)₂⁺ (FA), M = Ge²⁺, Sn²⁺, Pb²⁺ and X = Cl^-, Br^-, I^-, are the perovskite components and A’⁺ = RNH₃ is an organic spacer. There are four kinds of 2D organic inorganic hybrid perovskites so far: Ruddelsden-Popper, Cation-ordered, Jacobson-Dion and Diammonium Cation. These vary from one another in ways the inorganic slabs stack and the way the spacer cations interact with the inorganic slabs. Generally, 2D perovskites form from solution via the bottom-up self-assembly of individual, semiconducting perovskite sheets having an adjustable slab thickness of up to few nanometers, separated by insulating bulky organic molecules. As a result, they behave as natural multiple quantum wells (QWs) with the semiconducting perovskite layers representing the wells and the insulating organic spacers representing the barriers. The width of the barrier is fixed and depends only on the length of the A’ cation, while the width of the well can be adjusted by varying the thickness of perovskite slabs, which is defined by the n variable in (A’)₂(A)_n-1M_nX_3n+1. It is critical to understand the thermodynamic and chemical limitations of the maximum layer thickness that can be sandwiched between the organic bilayers while retaining the structural integrity of the 2D perovskite.

The spatial localization of charge carriers to promote the formation of bound excitons and concomitantly enhance radiative recombination has long been a goal for luminescent semiconductors. Zero-dimensional materials structurally impose carrier localization and result in the formation of localized Frenkel excitons. We present fully inorganic, perovskite-derived zero-dimensional Sn^II material Cs₄SnBr₆ that exhibits room-temperature broad-band photoluminescence centered at 540 nm with a quantum yield (QY) of 10-20 % [1]. A series of analogous compositions following the general formula Cs_4-xA_xSn(Br_1-yI_y)₆ (A=Rb, K; x≤1, y≤1) can be prepared. The emission of these materials ranges from 500 nm to 620 nm with the possibility to compositionally tune the Stokes shift and the self-trapped exciton emission bands.
We also present the synthesis, the structure as well as electronic and optical properties of a family of hybrid tin (II) bromide compounds comprising guanidinium [G, C(NH₂)₃⁺] and mixed cesium-guanidinium cations: G₂SnBr₄, CsGSnBr_4, and Cs₂GSn₂Br₇. G₂SnBr₄ has a one-dimensional structure that consists of chains of corner-shared [SnBr₅]^2- square pyramids and G cations situated in-between the chains. G₂SnBr₄ is a luminescent phase with a broad emission band resulting from trapped excitonic states. Cs⁺ exhibits pronounced structure-directing effect: with a mixture of Cs⁺ and G cations mono and bilayer two-dimensional perovskites, CsGSnBr₄ and Cs₂GSn₂Br₇ are formed. The dimensionalities of the crystallographic structures have a direct impact on the electronic structures and the experimental optical band gaps are consistent with quantum confinement effects predicted by first principle simulations. Moreover, the flat shape of guanidinium cations induces anisotropic out-of-plane tilts of the [SnBr₆]^4- octahedra in the CsGSnBr₄ and Cs₂GSn₂Br₇ compounds. The related strong anisotropies of the halide perovskite lattice distortions have in turn a direct influence on their electronic and optical properties.

References:
1. B. Benin et al. Angew. Chem. 2018, 57, 11329–11333
2. O. Nazarenko et al. submitted.

In 2014 we observed white-light emission from the inorganic sheets of layered lead-halide perovskites (1). Upon UV excitation, these bulk crystalline solids emit light that spans the entire visible spectrum, similar to sunlight. These hybrid phosphors have high color rendering indices and easily tunable chromaticity coordinates. They are promising as phosphors for solid-state lighting, especially as neat large-area coatings. Recently, a number of other white-light-emitting perovskites and closely related materials have been reported, making this a burgeoning field of study (2). However, the vast majority of layered perovskites display a narrow blue/green emission, and white-light emission remains rare, leading to the question: "what is special about the white-light emitters"?

Over the past several years, we have investigated the generality of obtaining broad photoluminescence from low-dimensional metal-halide lattices. We attributed the white-light emission mechanism to exciton self-trapping, or the trapping of photogenerated electron-hole pairs in transient lattice deformations (1,3), and showed that it is common to all Pb-Br perovskites, although it is highly temperature dependent (4). I will describe our most recent work that provides a more complete picture of the emission mechanism and highlights the generality of exciton self-trapping. Our studies provide design rules for obtaining both the broad white-light emission as well as the narrow blue/green-light emission from these materials. The understanding we have developed of perovskite white-light emitters can be applied to many other low-dimensional systems.

(1) Dohner, Hoke, Karunadasa J. Am. Chem. Soc. 2014, 136, 1718 and Dohner, Jaffe, Bradshaw, Karunadasa J. Am. Chem. Soc. 2014, 136, 13154
(2) Smith, Karunadasa Acc. Chem. Res., 2018, 51, 619
(3) Hu, Smith, Dohner, Sher, Wu, Trinh, Fisher, Corbett, Zhu, Karunadasa, Lindenberg J. Phys. Chem. Lett. 2016, 7, 2258
(4) Smith, Jaffe, Dohner, Lindenberg, Karunadasa Chem. Sci. 2017, 8, 4497

Metal halide perovskites (MHPs) have emerged as a very high promising materials for optoelectronics and photonics, mostly due to their large absorption coefficient and excellent quantum yield of emission at room temperature, among other electro-optical properties of MHPs. The most surprising fact is that these properties are not very different from monocrystalline (or epitaxial films) direct semiconductors, even if MHPs are prepared as polycrystalline thin films by simple deposition methods, as spin-coating, inkjet printing and thermal evaporation, for example. The absorption coefficient is dependent on the material electronic structure and hence mostly intrinsic, but other electro-optical parameters, as the emission quantum yield, will depend on radiative and non-radiative recombination channels for free (and bound) excitons and carriers. In this way, slow carrier recombination in MHPs is considered the origin of the observed large charge carrier diffusion length, whose origin would the so-called "delayed luminescence" due to the existence of shallow non-quenching traps in these materials [1, 2]. In the talk a revision of recombination dynamics in MHPs will be presented. These materials have been successfully integrated in optical waveguides where stimulated emission is observed with very low thresholds both on rigid and flexible substrates [3-5]. Furthermore, perovskite-based photodetectors can be also integrated in the same platform paving the road towards wearable integrated photonics [5].
Finally, we will present the optical properties of MHP thin films using different organic cations alone or their mixture with methylammonium that produce multi-quantum-well structures (or 2D/3D) perovskites. This is an emerging field of work within the scientific community of perovskites, because of the higher stability of photovoltaic devices based on these 2D/3D MHPs. Moreover, quantum confinement in these structures introduces a new way to tune the optical properties, especially useful for emitting devices.


REFERENCES
[1] Chirvony, V. S.; González-Carrero, S.; Suárez, I.; Galian, R. E.; Sessolo, M.; Bolink, H. J.; Martínez-Pastor, J. P.; Pérez-Prieto, J. Delayed luminescence in lead halide perovskite nanocrystals. J. Phys. Chem. C 2017, 121, 13381-13390.
[2] Chirvony, V. S.; Martínez-Pastor, J. P. Trap-limited dynamics of excited carriers and delayed luminescence in metal halide perovskites. J. Phys. Chem. Lett. 2018, accepted.
[3] I. Suárez, E. J. Juárez-Pérez, I. Mora-Seró, J. Bisquert and J. P. Martínez-Pastor, “Polymer/perovskite amplifying waveguides for active hybrid silicon photonics”,Advanced Materials 27,6157(2015).
[4] T. T. Ngo, I. Suarez, G. Antonicelli, D. Cortizo-Lacalle, J. P. Martinez-Pastor, A. Mateo-Alonso and I. Mora-Sero, “Enhancement of the Performance of Perovskite Solar Cells, LEDs and Light Amplifiers by Anti-Solvent Additive Deposition”, Advanced Materials 29, 1604056 (7 pp) (2017).
[5] I. Suarez, E. Hassanabadi, A. Maulu, N. Carlino, C. A. Maestri, M. Latifi, P. Bettotti, I. Mora-Sero and J. P. Martinez-Pastor, “Integrated Optical Amplifier-Photodetector on a Wearable Nanocellulose Substrate”, Advanced Optical Materials, 1800201 (8 pp)(2018).

Ruddlesden-Popper (RP) halide perovskites are promising optoelectronic materials due to their high-performance in PV applications and excellent ambient stability. Their ferroelectric properties provide an exciting opportunity to further improve PV performance based on the nature of their layered structure. Polar domains in ferroelectric materials play an important role in separating electrons and holes. However, the relation between structure and function which lead to the RP ferroelectrics remains unknown. Herein, we realize tunable ferroelectricity in 2-phenylethylammonium (PEA) and methylammonium (MA) RP halide perovskite (PEA)₂(MA)_n-1Pb_nI_3n+1 by varying the number of inorganic layers and tuning the correlation length of the ferroelectric order. Firstly, the non-centrosymmetric nature of RP thin films is confirmed by nonlinear optics. Secondly, switchable polarity leading to ferroelectric properties is validated by piezo force microscopy and polarization-electric field measurements microscopically and macroscopically. Then, the origin of ferroelectricity in the RP halide perovskites is investigated by MD simulations. Finally, dark current-voltage hysteresis phenomenon implies potential issues for light harvesting and light emitting applications. Importantly, our findings reveal an exciting approach to engineer tunable RP ferroelectrics, which could pave the way to new functionalities for perovskite optoelectronics.

In the world of growing energy demand, perovskite materials have emerged as a favorable alternative for next-generation solar cell devices owing to their high power conversion efficiency. The perovskite family is rich in multitudes, and the probability of discovering new and exciting photovoltaic materials are high. To date, most studies on this topic are focused around methyl and ethyl lead iodide structures. Recent advancement in the field of high throughput methodologies have paved a way towards the pursuit of new perovskite complexes beyond the conventional structures, facilitating an understanding of the basic physiochemical properties of these materials. Inverse Temperature Crystallization (ITC) methods were adapted to be compatible with robotic liquid-handler syntheses, resulting in the growth of large single crystals while maintain a high reaction throughput. Chemical spaces were mapped in several related systems and a large reaction dataset was generated for use with machine learning algorithms. A novel approach to prepare and characterize robot-ready perovskite crystals is described for lead halide perovskites.

Lead halide perovskites have been used as emission layers in perovskite light-emitting diodes (PeLEDs), and have many advantages such as high charge-carrier mobility, solution processability, high color purity, color tunability and low material cost. However, low electroluminescence (EL) efficiency of PeLEDs at room temperature is a challenge that must be overcome. Here, we present high-efficiency PeLEDs by controlling the dimension and dimensionality (D) of perovskite grains/crystals to overcome the EL efficiency limitations. First, we kinetically control the grain size in 3D bulk polycrystalline films and achieve uniform methylammonium lead bromide (MAPbBr₃) and CsPbBr₃ films with reduced grain size. By using fine stoichiometry control, additive-based nanocrystal pinning and ideal buffer layer, we prevent the formation of strong luminescence quenchers (metallic Pb atoms), suppress the non-radiative recombination of charge carriers at the interfaces and in the emitting layers, and achieve high external quantum efficiency (11.7%) in PeLEDs based on 3D perovskite bulk films. High-efficiency flexible MAPbBr₃ PeLEDs based on graphene anode and polymeric anode are also first developed. We also develop efficient quasi-2D (Ruddlesden-Popper phase) bulk polycrystalline films with improved film morphology, exciton confinement and reduced trap density, and then demonstrate efficient PeLEDs based on them. Furthermore, by synthesizing ligand-engineered colloidal perovskite 0D nanoparticles (NPs) with high photoluminescence quantum efficiency, we fabricate high-efficiency PeLEDs based on MAPbBr₃ and formaminidium lead bromide NPs. The effects of ligand engineering on photo-physical and surface-chemical properties of perovskite NPs, and EL efficiencies of PeLEDs are also systematically studied.

Metal halide perovskites have emerged as a promising family of functional materials for advanced optoelectronic applications with high performance and low costs. Various chemical methods and processing approaches have been employed to modify the compositions, structures, morphologies, and electronic properties of hybrid perovskites, great progress has been achieved. Whereas, challenges still remain such as the low stability and the lack of an insightful understanding of the structure-property relationships. In this talk, we will present our efforts in using an alternative means, pressure, to tune the structures and physical properties of halide perovskites. Using state-of-the-art high-pressure techniques coupled with in situ synchrotron-based and in-laboratory property measurements, we characterized the changes in their structural, electrical, and optical properties. Pressure-enhanced properties, such as higher electron transport and stronger photoluminescence, were observed. Our findings reveal that high pressure can potentially be able to realize enhanced and/or emergent properties of halide perovskites, and further our understanding of the structure-property relationships.

Perovskite light-emitting diodes (PeLEDs) with white color EL is highly desirable for practical applications in display and lighting. We demonstrate the all-perovskite light-emitting diodes (PeLEDs) with white emission on the basis of simultaneously solving a couple of issues including the ion exchanges between different perovskites, solvent incompatibility in the solution process of stacking different perovskites and carrier transport layers, as well as the energy level matching between each layer in the whole device. The PeLEDs are built with a 2D (CH₃CH₂CH₂NH₃)₂CsPb₂I₇ perovskite that emitting red light, CsPb(Br,Cl)₃ quantum dots that emitting cyan color, and an interlayer composed of bis(1-phenyl-1H-benzo[d]imidazole)phenylphosphine oxide (BIPO) and poly(4-butylphenyl-diphenyl-amine) (Poly-TPD). The interlayer is designed to realize desirable white electroluminescence by tuning the electron and hole transportation and distribution in-between multilayers. With this PeLED configuration, we achieve the typical white light at (0.32, 0.32) in Commission Internationale de L'Eclairage (CIE) 1931 color space, and steady CIE coordinates in a wide range of driving-current density (from 2.94 to 59.29 mA/cm²). Consequently, our work, as the starting point for future research of all-perovskite white PeLEDs, will contribute to the future applications of PeLEDs in lighting and display. In addition, we believe that the proposed material and all-perovskite concept will leverage the design and development of more perovskite-based devices.

[1] J. Mao, H. Lin, F. Ye, M. Qin, J.M. Burkhartsmeyer, H. Zhang, X. Lu, K.S. Wong, W.C.H. Choy*, “All-Perovskite Emission Architecture for White Light-Emitting Diodes", ACS Nano, in press.

We report the application of calcium niobate (CNO) perovskite nanosheets for electron injection layers (EILs) in organic light-emitting diodes (OLEDs). Four kinds of tetraalkylammonium hydroxides having different alkyl lengths were utilized as the exfoliation agents of a layered compound precursor HCa₂Nb₃O₁₀ to synthesize CNO nanosheets, including tetramethylammonium hydroxide (TMAOH), tetraethylammonium hydroxide (TEAOH), tetrapropylammonium hydroxide (TPAOH), and tetrabutylammonium hydroxide (TBAOH). CNO nanosheet EILs were applied in fluorescent poly(9,9-dioctylfluorene-alt-benzothiadiazole), poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) polymer-based OLEDs. The effects of dispersion concentrations and alkyl chain length on the device performances were investigated. The results demonstrated that OLED performances were related to the coverage ratio of the CNO nanosheets, their thicknesses, and their workfunction values. Among the four exfoliation agents, the device with CNO nanosheets exfoliated by TPAOH showed the lowest driving voltage. The OLEDs with the CNO nanosheet EILs showed lower driving voltages compared with the devices with conventional EIL material lithium 8-quinolate.

Hybrid organic-inorganic halide perovskite materials such as methylammonium lead iodide have captured the interest of the thin film optoelectronics community due to their promising optoelectronic properties. For light emitting diodes (LEDs), we have established a general protocol to prepare ultrathin, smooth, passivated, and pinhole free films of metal halide perovskites with various compositions, by incorporating bulky organoammonium halide additives to the stoichiometric 3D perovskite precursors. LEDs produced in this way are capable of exceeding 17% external quantum efficiency, exhibit significantly improved stability, and are capable of being as flexible as organic electronic thin films. Finally, they allow for stabilizing mixed halide (I and Br) and mixed Pb-Sn stoichiometries such that we can tune emission from the green to near infrared.

Lead halide perovskite is a new type of semiconductor optoelectronic material, which owns large absorption coefficient, long diffusion length, and also it shows high emission efficiency. This advantages feature it a great potential in in solar cells and also in light-emitting diodes. Recently, there are great breakthrough in these two types of optoelectronic devices, the power conversion efficiency (PCE) and the electroluminescence external quantum efficiency (EQE) have been pushed to 23.3% and over than 20% for perovskite solar cells and light-emitting diodes, respectively. In this talk, I will talk about how we achieve high performance and stable of perovskite based optoelectronic devices according to perovskite film growth control, interface engineering and surface passivation [1-5].

References
[1] Q. Jiang, …X. W. Zhang*, J. B. You*. Nat Energy, 2, 16177 (2016).
[2] Q. Jiang,…X. W. Zhang*, J. B. You*, Adv. Mater. 29, 1703852 (2017).
[3] P. Y. Wang,…J. B. You*. Nat. Commun. 9, 2225. (2018).
[4] X. L. Yang,…J. B. You*. Nat. Commun. 9, 570 (2018).
[5] L. Q. Zhang, …X. W. Zhang*…J. B. You*. Nat. Commun. 8, 15640 (2017)

Metal halide perovskite generates extremely sharp emission peak and high luminescence quantum yield, enabling it to be a promising candidate as the new generation of emitting materials for displays. However, the development of perovskite LEDs is impeded by their fast carriers diffusion and poor stability in air. Herein, quasi 2D CsPbBr₃ quantum wells homogeneously surrounded by inorganic crystalline Cs₄PbBr₆ of large bandgap are grown. The centralization of carriers in nanoregion facilitates radiative recombination and brings much enhanced luminescence quantum yield. The external quantum efficiency and luminescence intensity of the LEDs based on this nanocomposite are one order of magnitude higher than the conventional low dimensional perovskite. Meanwhile, the use of inorganic nanocomposite materials brings much improved device operation lifetime under constant electrical field.
By using bidentate 2,2’-Iminodibenzoic acid as conductive ligand, we preapared CsPbI₃ NCs based high efficiency red color light emitting diode. The passivated NCs enabled us to realize red light emitting diodes (LEDs) with 5.02% external quantum efficiency and 748 cd/m² luminance, which is the higest efficiency realized at that time.

References
1. Yuequn Shang, Gang Li, Weimin Liu, and Zhijun Ning*, Quasi 2D Inorganic CsPbBr₃ Perovskite for Efficient and Stable Light-Emitting Diodes, Adv. Funct. Mater., 2018, 1801193
2. Jun Pan , Yuequn Shang, Jun Yin, Michele De Bastiani, Wei Peng, Ibrahim Dursun, Lutfan Sinatra, Ahmed M. El-Zohry, Mohamed N. Hedhili, Abdul-Hamid Emwas, Omar F. Mohammed, Zhijun Ning*, and Osman M. Bakr*, Bidentate Ligand-Passivated CsPbI3 Perovskite Nanocrystals for Stable Near-Unity Photoluminescence Quantum Yield and Efficient Red Light-Emitting Diodes. J. Am. Chem. Soc., 2018, 140, 562–565.

Ruddlesden Popper (RP) perovskites are of great interest in light emitting diodes (LED), due to the efficient energy transfer (funneling) from high bandgap (donor) domains to low bandgap (acceptor) domains which lead to enhanced photoluminescence (PL) intensity, long PL lifetime, and high efficiency LED. However, the influence of reduced effective emitter centers in the active emissive film as well as the implications of electrical injection into the larger bandgap donor material have not been addressed in the context of an active device. We critically assess and modulate the electrical and optical signatures of the energy cascading mechanisms in a model Ruddlesden Popper perovskite series ((C₈H₁₇NH₃)₂(CH(NH₂)₂)_m-1Pb_mBr_3m+1). Optimised devices demonstrated a current efficiency of 22.9 cd A^-1 and 5% external quantum efficiency, more than 5 times higher than systems where funneling was absent. The signature of non-ideal funneling in RP perovskites is revealed by the appearance of donor electroluminescence from the device, followed by a reduction in the LED performance

Power conversion efficiency (PCE) of lead halide perovskite solar cell (over 23%) has surpassed those of CIGS and CdTe, approaching the top value of crystalline Si cell. Our group has been able to achieve PCE over 21% by low cost ambient fabrication. However, high PCE of single-cell enabled by lead halide-based perovskite absorbers are now being saturated, taking the Shockley Queisser (SQ) limit of open-circuit voltage (V_OC) (ca.1.32V) into account. Tandem cell making, which can further increases PCE up to 28% or more, leads to higher material and process cost and will raise a question if performance/cost ratio can be accepted in industry. Therefore, a smart way is to create a single cell which has high PCE comparable with that of GaAs (>28%) by reducing bandgap energy to <1.4 eV without accompaniment of increase in V_OC loss. This possibility will be in a family of metal halide perovskite out of those depending on use of lead. In addition to such efficiency issue, high performance of organo lead halide materials is not compatible with robust high stability required for practical use. Ensuring the intrinsic thermal stability (desirably >200^oC) of the perovskites is a key issue before industrialization. In addition, toxicity of lead-based perovskites are going to become the most formidable challenges for real use (commercialization), in particular, for applications to IoT society, which is one of the most promising field of perovskite photovoltaic device in terms of high voltage output even under weak illumination. These thoughts urge us to concentrate our next research of perovskite photovoltaics (PV) more on development of non-lead high efficiency absorbers. Sn perovskite is still a strong candidate because Sn(II) has been found to be stabilized against ambient air by metal doping method (such as Ge). Regarding Bi-based perovskites, we found AgBi₂I₇ as a promising all-inorganic absorber having high thermal and moisture stability. Stability also highly depends on the property of charge transport materials (CTMs), especially, the kind of hole transporter. Spiro-OMeTAD does not work at high temperature while P3HT, for example, is thermally stable. In our collaboration with JAXA, P3HT-based perovskite devices showed robust stability by exposure to high (100^oC) and low (-80^oC) temperatures and also to high energy particle radiations (iScience, 2018, 2, 148). Selection of CTMs is another important key in combination with non-lead perovskite materials. In conclusion, next direction of perovskite PV should be to enhance PV performance of non-lead all-inorganic semiconductor materials by extended compositional engineering, in parallel with developing thermally stable CTMs. Our on-going studies on non-lead perovskite materials in our group will be introduced in the talk.

Since the first report on the high efficiency, stable solid-state perovskite solar cell (PSC) in 2012 by our group, following two seed works on perovskite-sensitized liquid junction solar cells in 2009 and 2011, PSC demonstrated its power conversion efficiency (PCE) of 23.3% in 2018. According to Web of Science, publications on PSC increase exponentially since 2012 and total number of publications reaches already over 10,000 as of October 2018, which is indicative of a paradigm shift in photovoltaics. Although small area cell exhibited superb efficiency surpassing the performance of thin film technologies, scale-up technology is required toward commercialization. In addition, further higher efficiency toward Shockley–Queisser limit is required in parallel. In this talk, Large-area coating technology is introduced using perovskite cluster embedded coating solution, followed by brief introduction on history of perovskite solar cell. Bi-facial stamping method was developed for not only scale-up technique but also interface modification and low-temperature phase stabilization. For higher efficiency, managing recombination is critical. Methodology reducing recombination is developed via interface and bulk engineering. Current-voltage hysteresis is also discussed because hysteresis is related to the stability of perovskite solar cell. Ion migration is now visualized and confirmed to correlate with hysteresis.

Organic-inorganic perovskites enable a combination of useful organic and inorganic properties within a single molecular-scale composite and have attracted substantial interest for use within organic-inorganic electronic devices [1], in part due to the high carrier mobilities, long minority carrier lifetimes, tunable band gaps and relatively benign defects and grain boundaries for systems based on Group 14 metals (e.g., Ge, Sn and Pb) [2]. Indeed, these materials have enabled unprecedented rapid improvement in perovskite photovoltaic performance to levels above 20% power conversion efficiency and with open circuit voltages above 1V for a single junction photovoltaic (PV) device [3]. This talk will provide an historical perspective on foundational work related to the organic-inorganic perovskite semiconductors, including discussion of crystal structure flexibility [4,5], semiconducting properties, film deposition approaches and electronic device applications of the three-dimensional and lower-dimensional perovskite structures. Recent trends in the field, as they relate to application in photovoltaics and related devices, will also be coupled into this discussion.

[1] D. B. Mitzi, K. Chondroudis, C. Kagan, IBM J. Res. Develop. 45, 29 (2001).
[2] W.-J. Yin, T. Shi, Y. Yan, Adv. Mater. 26, 4653 (2014).
[3] W. S. Yang et. al., Science 356, 1376 (2017).
[4] B. Saparov and D. B. Mitzi, Chemical Reviews 116, 4558 (2016).
[5] D. B. Mitzi, Prog. Inorg. Chem. 48, 1 (1999).

The contribution is concerned with the significance of ionic conduction for hybrid perovskites. It starts with outlining the analysis of the nature of the ionic charge carriers and the quantification of their contributions to the overall transport in methyl-ammonium lead iodide. Apart from NMR, tracer exchange and doping experiments, detailed electrochemical experiments form the basis of these investigations. The prevailing mobile carriers are iodine vacancies, holes and conduction electrons. The analysis also reveals the adjusting screws that can be used to tune the charge carrier chemistry – mostly component partial pressure (iodine activity) and doping content [1-3].
The consequence of the mixed conductivity is not only carrier and mass transport, as it is decisive for chemical kinetics (e.g. decomposition kinetics), but also the occurrence of a substantial chemical capacitance [1,4]. These phenomena are well understood for oxide perovskites the behavior of which is analogous to the situation in the hybrid perovskites under dark conditions. Of particular relevance is the occurrence of a distinct long-time polarization (also the hysteresis in CV experiments) which is not a dielectric or space charge polarization, but a true bulk phenomenon. This stoichiometric effect is known as Wagner-Hebb polarization and is one of the basic phenomena in Solid State Ionics involved e.g. in resistance degradation or electro-coloration.
A special role is played by oxygen because depending on the time window, its influence leads either to a varied iodine potential, to a doping effect or to a global decomposition. The understanding of this complex behavior reflects our far-reaching defect-chemical understanding of methyl-ammonium lead iodide [5].
In addition to understanding and influencing ionic and electronic carrier concentrations by classic Solid State Ionics principles, we also applied strategies of Nanoionics (e.g. heterogeneous doping) [6]. In addition to offering further degrees of freedom for charge carrier tuning, such research reveals new insight into the interface chemistry, such as ionically dominated space charge zones. Most recent results are presented [7].
Last but not least, the striking finding of an enormously light-enhanced ion conductivity will be addressed. This is – also from the viewpoint of Solid State Ionics – completely unexpected. The reason for this phenomenon is presented and implications for photo-decomposition but also for the design of novel “opto-ionic” devices are discussed [8].
References
[1] T.-Y. Yang, G. Gregori, N. Pellet, M. Grätzel, and J. Maier, Angew. Chem. Int. Ed. 54 (2015) 7905.
[2] A. Senocrate, I. Moudrakovski, G. Y. Kim, T.-Y. Yang, G. Gregori, M. Grätzel, and J. Maier, Angew. Chem. Int. Ed. 56 (2017) 7755.
[3] A. Senocrate, T.-Y. Yang, G. Gregori, G. Y. Kim, M. Grätzel, and J. Maier, Solid State Ionics 321 (2018) 69.
[4] J. Jamnik and J. Maier, Phys. Chem. Chem. Phys. 3 (2001) 1668.
[5] A. Senocrate, I. Moudrakovski, T. Acartürk, R. Merkle, G. Y. Kim, U. Starke, M. Grätzel, and J. Maier, J. Phys. Chem. C 122 (2018) 21803.
[6] J. Maier, Nature Materials 4 (2005) 805.
[7] G. Y. Kim, A. Senocrate and J. Maier, in preparation.
[8] G. Y. Kim, A. Senocrate, T.-Y. Yang, G. Gregori, M. Grätzel, and J. Maier, Nat. Mater. 17 (2018) 445.

Mixed halide lead perovskites are good candidates for designing tandem solar cells because one can tune the bandgap by varying halide ion composition. For example, by varying the iodide/bromide composition (CH3NH3PbBrxI3-x (x=0 to 3)) it is possible to tune the bandgap between 1.55 eV and 2.43 eV. Of particular interest are the composition dependent absorption and emission properties. The halide ions are mobile both in the dark as well as under light irradiation. The photoluminescence and absorption spectra offer a convenient means to track the movement of halide ions. By employing gradient and homogeneous films of mixed halide perovskites we have probed the movement of iodide ions and factors influencing their mobility. The effect of halide ion segregation on the photovoltaic properties will also be discussed.

Organic-inorganic hybrid perovskites (OIHPs) have attracted extensive interest for potential use in next-generation photovoltaic devices due to the demonstrated high power conversion efficiency, low materials cost, and low fabrication cost. For conventional inorganic perovskites, commercial applications mainly rely on the strong electromechanical coupling, including both piezoelectricity and electrostriction. OIHPs materials possess a similar lattice structure as conventional inorganic perovskites, however, the electromechanical coupling of OIHPs has barely been explored. Here, we report the electrostrictive response of the MAPbI₃ hybrid perovskite crystals as large as 1% compressive strain under a field of 3.7 V/µm along the electric field direction with a corresponding to an electrostrictive coefficient of –730 nm²/V² and mechanical energy density of 0.74 J/cm³. Superior to other electrostrictive materials, MAPbI₃ demonstrates both large electrostrictive strain and large elastic energy densities under a small applied field. The electrostrictive strain is independent of the electric field polarity and has a quadratic dependence on the electric field. The electrostrictive response time is in the order of millisecond. The influences of piezoelectricity, thermal expansion, intrinsic electrostrictive effect, Maxwell stress, ferroelectricity, local polar fluctuation, and methylammonium cation ordering on this electromechanical response are excluded. We speculate, using density functional theory, that electrostriction of MAPbI₃ likely originates from lattice deformation due to the formation of additional defects under applied bias. Since almost every electronic device either operates under applied electric field or experiences an internal built-in field which is close to the field of 3.7 V/µm inducing 1% strain, the discovery of electrostriction and its origin may also impact the design and performance of OIHP-based electronic devices, including solar cells, LEDs, photodetectors, and radiation detectors. The discovery of large electrostriction in lead-iodide perovskites may lead to new potential applications in actuators, sonar, and micro-electromechanical systems, and aid the understanding of other field-dependent material properties.

Mixing different halogen ions provided a convenient means to tune the bandgaps of halide perovskites (e.g. CsPb(Br_xI_1-x)₃). The so-called mixed-halide perovskites, however, suffered from severe phase separation under optical illumination. In this talk, we will present a facile approach to suppress such phase separation by making the mixed halide perovskites into composites. The tuned bandgap remained remarkably stable even under optical illumination 440 W/cm² at 405-nm (The AM1.5 spectrum has an integrated power of 0.1 W/cm²). The mechanism responsible for the suppression of phase separation pointed to a model based on thermodynamic nucleation, which was further verified by the temperature dependent photoluminescence. The tuned and stabilized bandgap is expected to be essential for the development of perovskite-based optoelectronics, such as tandem or concentrated solar cells, full-color LED displays, or wavelength-specific photodetectors.

The perovskite crystal structure hosts a wealth of intriguing properties, and the renaissance of interest in halide (and hybrid organic-inorganic) perovskites (HOIPs) has further broadened the palette of exciting physical phenomena. HOIPs have recently received great attention as candidates for commercially viable and efficient conversion of solar energy. Breakthroughs in HOIP synthesis, characterization, and solar cell design have led to remarkable increases in reported photovoltaic efficiency. However, the observed long carrier lifetime and PV performance have eluded comprehensive physical justification, and the performance and stability of these materials is greatly affected by water and moisture in the environment.

In this talk, recent theoretical progress in understanding HOIPs will be reviewed and integrated with experimental findings, and we investigate the nature of water interactions in hybrid perovskite MAPbI3. By varying water concentration over two orders of magnitude, we study the process of water infiltration and the surface and bulk chemistry that follows and leads to first reversible, and then irreversible changes to the material’s structure. We discuss, based on electronicstructure analyses, how water changes the optical properties of the material in different concentrations. In addition, the large amplitude motions of HOIPs will be highlighted, including ionic diffusion, anharmonic phonons, and dynamic incipient order on various length and time scales. The intricate relationships between correlated structural fluctuations, polar order, and excited charge carrier dynamics will also be discussed.

This understanding of the nature of water-HOIP interactions and ionic dynamics should lead to the design of better and more stable solar cells.

The electroluminescence (EL) intensity as well as the efficiency are usually low for perovskite-based light-emitting diodes (PeLEDs) biased at the low current density regime, but markedly raise to a maximal magnitude at the high voltage or current biasconditions. We observe a nonlinear EL intensity versus current density curve (L-I curve) of devices and attribute this observation to the bias-induced migration of ions in perovskite active layer (CH₃NH₃PbBr₃) to modulate the device performance. In this work, we utilize the ac impedance and photoluminescence (PL) spectroscopy to characterize the effect of ion migration in polycrystalline CH₃NH₃PbBr₃ layer. The passivation of ionic defects by the mobile ions as induced by the electric bias markedly enhances PL magnitude of the polycrystalline CH₃NH₃PbBr₃layer in PeLEDs. Here, by adding a small amount of additives to the polycrystalline CH₃NH₃PbBr₃ layer possibly passivates the surface ionic defects and suppress the inter-grain ion migration. We find a decrease of ionic conductivity in perovskite active layer by the additives in impedance measurement. In addition, as characterized by SEM, XRD, and PL measurement, adding the additives reduces the average crystalline size, enhances PL intensity, and elongates the carrier lifetime, but did not change the basic crystal structure of CH₃NH₃PbBr₃perovskite. We observe the linear correlations of L-I curve for devices biased at different current regimes and the markedly enhanced brightness and efficiency of PeLEDs at the low current regime to be the feature of a decent LED.

Halide perovskite nanocrystals exhibit exceptional brightness in vivid colors and with narrow emission bandwidths, making these materials potential “game-changers” for display technologies and light-emitting devices more generally. Bandgap tunability over the entire visible spectrum can be easily accomplished via halide mixing, with mixed chloride-bromide perovskites emitting in the blue-green range and mixed bromide-iodide perovskites emitting in the green-red range. However, at sufficiently high excitation intensities or in larger-dimension mixed-halide perovskites, light exposure causes phase separation of bromide-iodide perovskites into Br-rich and I-rich regions, limiting the applicability of these materials. Because the I-rich regions formed have narrower bandgap than the Br-rich regions, charges become trapped in the I-rich regions, which limits carrier mobility and causes large anomalous red photoluminescence. Strategies to suppress this light-induced phase separation (LIPS) have involved reducing perovskite crystal size or filling halide vacancies. However, much still remains unknown regarding LIPS – in particular, the potential roles played by solvent complexes/ perovskite intermediates in enhancing or mitigating LIPS has not been explored. This talk will discuss the contributions of solvent complexes to the kinetics and reversibility of LIPS in methylammonium lead bromide-iodide thin films, and will illustrate LIPS mechanistic pathways that are not tied to solvates. The insights gained by this study are expected to aid future design of LIPS-free mixed-halide perovskite devices.

LIPS is characterized by an intense red photoluminescence signal near 1.7 eV that grows with time as the I-rich phase forms, combined with a reduction in emission from the original bandgap of the mixed-halide perovskite. The rate of red photoluminescence (red-PL) growth increased strongly when large amounts of solvent remained in the film, suggesting a dynamic process in which solvent complexes aid formation of I-rich phases upon light exposure. Correspondingly, in harshly-annealed films, longer exposure times were needed to induce LIPS. LIPS occurred more slowly in Br-rich films than in I-rich films (all harshly-annealed), suggesting that defect accumulation near iodide is necessary to induce LIPS. In harshly-annealed films, LIPS red-emission spectra exhibited multiple previously-unreported emissive states, which was attributed to formation of intermediary I-rich and B-rich phases before formation of the equilibrium I-rich and Br-rich phases. Finally, solvent complexes enhanced red-PL growth reversibility, as evidenced by greater reversibility in unannealed films. These results provide guidelines for synthesizing solvate-free mixed-halide perovskite films and potentially identify novel LIPS kinetic pathways. We anticipate that the insights gained by this study will aid modelling of LIPS and design of LIPS-free mixed-halide perovskite optoelectronic devices.

Colloidal nanocrystals of inorganic cesium lead halide (CsPbX₃, X = Cl, Br, I, or combinations thereof) perovskites have attracted much attention for photonic and optoelectronic applications due to the high carrier mobility, easily tunable optical absorption range, manufacturing simplicity and low cost. The optical and electrical properties of the halide perovskites significantly depend on their crystal structure, shape and size. And there were many studies on the shape and size of perovskite nanocrystals related to reaction time, reaction temperature, and ligand changes. However, the roles of the acid ligand and the amine ligand were not identified.
In this study, we are able to control the shape and size of CsPbBr₃ perovskite nanocrystals by using different kinds of the ligands and changing the concentration of the ligands. By observing these phenomena, we could understand the respective effects of acid ligand and amine ligand on the structure, shape and size of perovskite nanocrystals. The acid ligands influenced the structure and shape of the perovskite nanocrystals. With acetic acid ligands, the 2D perovskite structure CsPb₂Br₅ nanorods were synthesized through the facile ligand-mediated synthesis. The 3D perovskite structure CsPbBr₃ nanoplates were synthesized using other acid ligands. And small nanorods and nanospheres were mixed in the some cases. The amine ligands had a large effect on the size of the perovskite nanocrystals. The size of the nanocrystals decreased as the amine ligand became longer, and the crystal size could be finely controlled by adjusting the amount of the amine ligand. We believe that our report helps to understand the synthesis of halide perovskite for the application in optoelectronic devices.

Metal halide perovskite nanocrystals have been deeply studied in the last years due to their multiple properties and applications. These materials usually have a high photoluminescence quantum yield (PLQY), narrow emission bandwidth and their band-gap is tuned easily by changing the chemical composition of the nanocrystals or by varying their morphology from cubic structures to nanoplatelets. All these outstanding properties make them suitable for optoelectronic applications, sensors and lasers.
For the entire visible range, emissive perovskite nanoparticles have been achieved. However, high PLQY blue emissive nanoparticles are still a challenge, with high chloride content they have relatively low PLQY. Recently, 70% was reported for CsPbBr₃ nanoplatelets and in films 88%, obtained with a mixture of different perovskites. [1,2]
In this work, a combination of CsPbBr₃ nanoparticles and a gelator agent leads to a blue-emissive gel, with PLQY of ~70% which is a high value compared with the PLQY of the nanoparticles themselves (~75%). Interestingly, depending on the structure of the gelator agent, different behaviors are observed, tuning the emission. In addition, they present a great stability in ambient conditions, after 4 months the PLQY decreased only 7%. The nanoparticles and the gel with nanoparticles were also characterized by TEM.

[1] B. J. Bohn, Y. Tong, M. Gramlich, M. L. Lai, M. Doblinger, K. Wang, R. L. Z. Hoye, P. Muller-Buschbaum, S. D. Stranks, A. S. Urban, L. Polavarapu and J. Feldmann, Nano Letters, 2018, 18, 5231-5238.
[2] J. Xing, Y. Zhao, M. Askerka, L. N. Quan, X. Gong, W. Zhao, J. Zhao, H. Tan, G. Long, L. Gao, Z. Yang, O. Voznyy, J. Tang, Z.-H. Lu, Q. Xiong and E. H. Sargent, Nature Communications, 2018, 9, 3541

Phosphor-based white light-emitting diodes (WLEDs) have become a kind of attractive solid light source due to their merits of being environmentally friendly and exhibiting energy savings, high brightness, high luminous efficiency and a long lifetime [1]. The white LEDs based on the near-UV (NUV) light chip and red/green/blue (RGB) primary colors phosphors have been widely studied for their tunable color rendering index and color temperature [2]. However, the red phosphors which can be excited by NUV light in the range of 380-410 nm are facing challenges such as their low efficiency, instability and lack of red-color purity, resulting in low color rendering index and high color temperature of white light [3]. Therefore, it is of paramount importance to find a desirable red-emitting phosphor, with good thermal stability and strong luminescent intensity, which can be successfully excited by the NUV light.
In this work, Ba(Mg_1/3Nb_2/3)O₃ was employed as a host of the phosphor with the considering of its low phonon energy [4] and good optical properties [5], and a new red phosphor, Eu³⁺ ions doped Ba(Mg_1/3Nb_2/3)O₃ was prepared by wet chemical method. Structure characterization results show that all the BMN:Eu phosphors, with various doping contents, have long-range ordering hexagonal perovskite structures and Eu³⁺ ions replace Ba²⁺ ions occupying at 2d sites with C_3v point symmetry in BMN. The phosphors exhibit the apparent absorption located at 395 nm which matches well with the emission of NUV LED chip, and show the bright red emission of ⁵D₀-⁷F₂ transitions under the excitation of 395 nm NUV light. The Commission Internatianale de L'Eclairage (CIE) chromaticity coordinate (0.657,0.343) of BMN:5%Eu phosphor is close to the standard value (0.670, 0.330) of red phosphors, which indicates that BMN:Eu is a promising red-emitting phosphor for NUV-based WLEDs.

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Recently, inorganic III-nitride compound semiconductors have been applied in various fields. In particular, light-emitting diodes (LEDs) are being used in a lot of lighting applications with long-term stability, high efficiency and the field of their use is expanding. The GaN-based LEDs has advantages of high efficiency and light emission of various wavelengths by the composition of In and Al. However, a high level of epitaxial growth technology is required, and it is difficult to form Ohmic electrodes according to the compositional change. To emit white light, a blue LEDs chip and encapsulated phosphors to emit yellow light and mixed with blue light. However, the emitted light reduces the conversion efficiency of 50~80 % caused by Stokes loss.
Conventional inorganic white LEDs suffer not only loss of conversion efficiency but also heat generation and possible package deterioration. There have been many studies in many groups to overcome this problem. However, the efficiency of white LEDs using phosphor is not remarkable. Recently, optical device research using halide perovskites has been reported. Perovskite is a semiconductor material structure with cubic AMX3 (A, M cation, X anion) composed of metal and halogen elements. It can control the emission spectrum by changing the element.
Metal halide perovskite has been extensively studied as an active layer of solar cells and optical devices. Perovskite has advantages such as high carrier mobility, wavelength change due to halogen element composition, simplicity of process and low cost, so that it is competitive as a photoelectric material. However, it has a drawback that it is very vulnerable to oxygen and moisture, and research is underway to improve it. Many studies have used the high absorbance of perovskite as an active layer. However, we did not use it as an active layer but as a phosphor to convert light emitted from UVLEDs.
In this study, we synthesized perovskite and studied the fabrication method of LEDs which emits yellow light by using synthesized peovskite with UVLEDs phosphor. The synthesized MAPbBr3 was confirmed to have a wavelength peak of 539 nm using the photoluminescence (PL). The Br-containing perovskite was coated under the substrate of the fabricated UVLEDs and the electroluminescence (EL) spectrum was analyzed using a probe-station system equipped. The wavelengths of typical UVLEDs were observed 365 nm main peak wavelength and deep-level wavelength in the range of 500-600 nm. In contrast, UVLEDs coated with perovskite exhibit a stronger light emission at 539 nm with a wavelength of 365 nm and a distinct difference is observed through the emission image. The wavelength of the emitted UV region emitted from the UVLEDs emitted light at 539 nm due to absorption in the coated perovskite. Since the emission spectrum of perovskite is easily adjustable, our novel method will be a new avenue for white LEDs using perovskite instead of conventional phosphors.

In conventional organic light emitting diode (OLED) as well as perovskite light emitting diode (pLED) device, polyvinyl carbazole (PVK) and Poly-TPD have been widely used as a hole transporting layer (HTL) in the past. However, PVK and Poly-TPD have disadvantage in terms of soluble property limitation during stacked-soluble process device preparation. Therefore, we synthesized new cross-linkable HTL polymer, SMPF-DNPB for pLED device. It has non-soluble property to general solvent after thermal treatment and curing process. It includes phenyl naphtyl group, spirofluorene, and vinyl group. The device configuration is ITO/PEDOT:PSS-PSS:Na/ SMPF-DNPB or others/2D perovskite/TOPO/TPBi/LiF/Al. When new HTL materials were used, it showed high luminance efficiency of more than 10 Cd/A in green pLED device.

Photoluminescence mapping provides spatial information about a semiconductor’s (photovoltaic’s) defects, efficiency, bandgap, uniformity, and other important properties. Near liquid helium temperatures, photoluminescence peaks in the spectra become sharper than those observed at higher temperatures, and transitions not resolved at room temperature become observable. An automation system was designed to collect low temperature photoluminescence maps of solar cells to exploit the extra information obtained from cooling the sample being mapped. The photoluminescence spectra were collected from the sample at different temperatures by inserting the sample in a helium-based closed-cycle cryostat.
Automation was developed in which a LabView application controls the low temperature PL mapping process. This involved controlling the following mapping components: motion stages to scan the incoming light source and optics, the temperature of the cryostat, and the PL data collection via a spectrometer/camera combination. The result of combining these components was a system that generates PL maps at temperatures as low as ~8 K. A spatial filter was added to the system to remove signal generated from anywhere other than the point of interest. This addition helped the system approach the theoretical resolution achievable. The completed prototype will be used to study degradation of perovskite solar cells among other types of solar cells.

The state-of-the-art high efficiency perovskite solar cells (PSCs) contain lead and organic cations in the perovskite light-absorber. However, the toxicity associated with the lead and the volatility of the organic cations are hurdles in the path towards the future commercialization of PSCs. While there has been effort towards replacing lead cation with less toxic cations, typical lead-free PSCs still suffer from the low power-conversion efficiency (PCE) and poor stability. Herein, we were firstly inspired by calculation results from Ti-based robust double perovskite materials and then realized the PCE (> 3%) on such materials. Meanwhile, we utilized an alloying strategy in all-inorganic Sn-based perovskite materials and realized those PSCs with a promising PCE (> 7%). Furthermore, the formation native-oxide layer passivation effect resulted in superior air-stability for these perovskite materials. Thus, this work provides a new avenue for the design and development of high performance and stable Lead-free PSCs.

Formamidinium Lead Iodide (FAPbI₃) based perovskites have attracted a great deal of interest as solar cell absorbers due to their superior thermal stability and more suitable band gap compared to methylammonium based perovskites. Here, we report a new phenomenon, where fine-grained (∼175 nm) δ-FAPbI₃ thin films transform rapidly to phase-pure α-FAPbI₃ OIHP thin films with ultra large grain size exceeding an unprecedented ∼5μm. The large grain nature of the films is confirmed using appropriate materials characterization. The improved kinetics of transformation is explained by studying the phase and morphological evolutions during film-solvent interaction. The nature of phase nucleation and growth is studied through in-situ microscopy techniques. In-situ X ray diffraction and solvent polarity effect on the transformation rate are also studied to corroborate the proposed mechanism. Photoluminescence microscopy was carried out to probe into carrier diffusion within grains and grain boundaries. Eventhough, the entire grain gives identical EBSD signal, it was observed that there exist sub-grain boundaries which obstruct carrier diffusion within the grains and likely limit the electronic properties. The passivation or tampering of sub-grain boundaries can become critical in improving the optoelectronic properties and hence need to be investigated further.

Bismuth ferrite BiFeO₃ is promising as magnetoelectric material because both ferroelectric and antiferromagnetic orders coexist in this material at room temperature. Doping of BiFeO₃ or the substitution of bismuth by other chemical elements can modify its magnetic properties. The goal of this paper is to determine the influence of bismuth substitution by rare earth elements La, Nd, and Gd on magnetic ordering in bismuth ferrite.

Substitutional alloys of BiFeO₃ of the types Bi_1-xNd_xFeO₃, Bi_1-xGd_xFeO₃, and Bi_1-x-yLa_xGd_yFeO₃ were synthesized by solid-state reaction method using powders of Bi₂O₃, Nd₂O₃, Gd₂O₃, La₂O₃, and Fe₂O₃ oxides of pure grade quality. The X-ray diffraction method was applied using the diffractometer Dron-3 on Cu K_α radiation to control crystal structure. Experimental data were collected during scanning, which was repeated ten times in the 2Θ range from 20° to 90° at the scanning speed of 10°/6 min. The magnetization was measured using an automated vibrating sample magnetometer (VSM, Oxford instruments) in magnetic fields up to 10 T. Temperature magnetic measurements were carried out using Physical Property Measurement System equipped with a 9 T superconducting magnet (PPMS: Model 6000, Quantum design).

It was found that undoped samples of bismuth ferrite BiFeO₃ show typical temperature dependence of magnetization, namely the jump of magnetization at 640 K and—following antiferromagnetic ordering—below this temperature. Total substitution of 0.10 - 0.15 atomic part of Bi by La, Nd, and Gd leads to the paramagnetic behavior of the doped bismuth ferrite at low temperatures in a wide range of magnetic field. Strong nonlinear dependence of magnetization on the magnetic field at low temperatures was detected. A ferromagnetic-like dependence of magnetization was observed for small magnetic fields. It can be explained by the exchange interaction between doping magnetic ions, as well as by the exchange interaction of these ions with ions of iron. It can be concluded that it impossible to change the initial magnetic ordering of bismuth ferrite by its doping by rare earth elements for the concentrations smaller than 0.10 atomic part of the doping element.

Recently, bismuth-based perovskites have aroused as a promising choice in replacing the lead-based perovskites due to its non-toxic and stable 6p-block structure properties. In addition, computational predictions suggest that Bi-based perovskite exhibits lower intrinsic trap densities and defect states, long carrier lifetime and its defect-tolerate properties merit the progression of lead-free photovoltaic.Alternatively, engineering double perovskite material by replacing the divalent Pb²⁺ions with a monovalent and a trivalent ions, in forming A₂B⁺B³⁺X₆(also denoted as elpasolite) double perovskite structure is a way towards synthesizing a stable and environmental benign perovskite material.
We here present our work on the succession in synthesizing high crystallinity, stable, and novel Cs₂NaBiI₆(CNBI) lead-free double perovskite via one-step facile hydrothermal process.In this work, we provided an in-depth study on the CNBI crystal growth process under the influence of various acid concentrations and its plausible crystal growing mechanism. Our synthesized CNBI double perovskite possesses bandgap of 1.66 eV, excellent stability, and good light absorption performances.We also revealed that hydroiodic acid plays an important role in controlling the crystal growth preference during the crystal assembling process. XRD are performed to determine the crystal structure of CNBI synthesized using different acid concentrations such as 1, 3, 6 and 9 mol.L^-1. Overall, the XRD data of all the synthesized CNBI matches perfectly and are reproducible regardless of the acid concentrations employed. It shows that the intensities of the CBI peaks gradually decrease in conjunction with the increment of acid concentration, showing that the decomposition became less significant while the formation of CNBI is more dominating. We can say that increasing acid concentration would supress the decomposition and promote the formation of our target CNBI compound.
In conclusion, the amount of hydroiodic acid (HI) used in the synthesis process is the main factor in favouring the formation of CNBI phase. The unique shuttle-like 3D CNBI material has a bandgap of 1.66 eV. In addition, the CNBI double perovskite possesses wide absorption range and its remarkable stability (negligible degradation after 5 months of storage at relative humidity of 70%) matches the criteria of photovoltaic application.

The hybrid organic-inorganic perovskites are an emerging class of semiconductors that have excellent optoelectronic properties, even being solution processed. These compounds also hold great promise for the field of spintronics due to their large and tunable spin-orbit coupling, spin-dependent optical selection rules, and predicted electrically tunable Rashba spin splittings. We demonstrate the optical orientation of spin-polarized excitons and spin coherence in polycrystalline films of MAPbI_3 using time-resolved Faraday rotation measurement. The exciton spin coherence is manifested as oscillations of spin polarization in a transverse magnetic field, with a life time exceeding 1 ns at 4 K. The nanosecond spin coherence is quite surprising given that Pb and I exhibit large spin-orbit couplings. There are two frequencies in the oscillations show linear relationship with the transverse magnetic field, and the slopes give two g-factors which we assign to electrons and holes as g_e = 2.63, and g_h = -0.33. The energy dependence of the Faraday rotation follows the exciton absorption band at low temperatures, confirming its excitonic origin. However, exchange couplings of the excitons are very small (< 1 micro eV) in comparison to traditional semiconductors. We will discuss the possible spin relaxation mechanisms and unusual exciton spin physics.

Organic-inorganic semiconducting perovskites have demonstrated very attractive room-temperature magneto-optical response, remarkable photovoltaic actions, high light-emitting properties, and low-threshold lasing actions, to become emerging multifunctional materials. On the other hand, organic-inorganic semiconducting perovskites possess a strong spin-orbital coupling within electrically polarizable semiconducting framework consisting of organic and inorganic components in ABX₃ structure. In general, spin-orbital coupling can generate three major outcomes: (i) Rashba effect, (ii) spin mixing between different states, and (iii) electric-magnetic coupling in such hybrid perovskites. It should be pointed out that organic-inorganic semiconducting perovskites show significant orbital momentum to form a strong spin-orbital coupling with spin momentum. Therefore, using orbital momentum presents a unique mechanism to control the optoelectronic properties in such hybrid perovskites. We found that changing from 3D to 2D perovskites chamges the internal interaction from short-range spin-spin interaction to long-distance orbital-orbital interaction, leading to distinct SOC effects on the populations on dark and bright states towards developing photovoltaic and light-emitting actions. On the other hand, we observed that the spin-orbital coupling can be changed by grain boundary polarization, leading to a convenient method to tune the spin-orbital coupling through doping and mechanical stress. Moreover, using the spin-orbital coupling presents a practical approach to remove the light-emitting loss from dark states in perovskite LEDs. In summary, this presentation will discuss the spin-orbital coupling effects involved in photovoltaic and light-emitting devices from 3D to 2D perovskites.

Inorganic CsPbX₃ (X=I,Br,Cl) lead halide perovskites are of interest for photovoltaic and light-emitting applications due to long charge carrier diffusion lengths and tuneability of the bandgap to generate photoluminescence (PL) with efficient quantum yields (QY). In the bulk morphology, the long charge carrier diffusion lengths are attributed to large polaron formation due to a ‘soft’ crystal lattice, which is inferred from infrared (IR) absorption¹. Polaron formation significantly reduces PL in films of CsPbX₃ NCs due to localization of photo-excited electrons and holes. A way to enhance IR polaron emission would be to confine the polaron which would significantly increase the spatial overlap of electron and hole states. CsPbX₃ nanocrystals (NCs) provide an excellent framework to explore the possibility of efficient IR polaron emission. Using a fully-passivated CsPbBr₃ NC atomistic model ^2,3 we compute spinor Kohn-Sham orbitals (SKSOs) with spin-orbit coupling (SOC) interaction as a basis to calculate efficiency of polaron emission. Efficiency of emission is determined from non-radiative recombination (k_nr) and radiative recombination (k_r) as k_r/(k_r + k_nr). Implications of this work provide framework for utilizing CsPbX₃ NCs as IR sensors.

1. Munson, K. T.; Kennehan, E. R.; Doucette, G. S.; Asbury, J. B., Dynamic Disorder Dominates Delocalization, Transport, and Recombination in Halide Perovskites. Chem 2018.
2. Forde, A.; Inerbaev, T. M.; Kilin, D. S., Spinor Dynamics in Pristine and Mn2+ Doped CsPbBr3 NC: Role of Spin-Orbit Coupling in Ground and Excited State Dynamics. The Journal of Physical Chemistry C 2018.
3. Forde, A.; Inerbaev, T.; Kilin, D., Role of Cation-Anion Organic Ligands for Optical Properties of Fully Inorganic Perovskite Quantum Dots. MRS Advances, 1-7.

Optical control of electron’s spin in a material system will lead to novel technological branch called as opto-spintronics. Conventional inorganic semiconductors (e.g. III-V and II-VI systems) face major challenges such as of lattice-matching and arduous fabrication processes. Moreover, optical spin control in most of these systems also requires cryogenic cooling. Herein, we propose lead halide perovskites as robust alternatives, where their band structures allows resonant excitation of 100% spin-polarized carriers/excitons. Few of their robustness to be highlighted: (i) ultra-large photoinduced Faraday rotation up to 10°/μm in 3D perovskites, which is the largest reported up to our knowledge; and (ii) room-temperature tunable spin-selective optical Stark effect in 2D perovskites, which is a few times stronger than in the inorganics. Time-resolved studies on the spin-populations show that the spin-coherence live for a few ps in the 3D and few hundreds of fs in the 2D, due to Elliot-Yafet scattering and exciton exchange-interaction, respectively. Importantly, these works demonstrate robust optical control over spin-states, our result suggests the potential of perovskites for opto-spintronics applications.

Vapor-based epitaxial synthesis of halide perovskites remains rather limited. In this work, we present our recent progress in synthesizing epitaxial halide perovskite via a couple of epitaxy mechanisms. By van der Waals epitaxy, centimeter-scale in-plane interconnecting networks of CsPbBr₃ microwires and CsPb (Br_xI_1−x)₃ heterojunctions have been synthesized. By ionic epitaxy, single-crystalline CsPbBr₃ and CsSnBr₃ films were grown on water-soluble NaCl substrates, which provides solutions for free-standing wafer-scale 3D-perovskite single-crystalline films. It is also shown that the concept of van der Waals epitaxy could be applied to the growth of Ruddlesden-Popper phase halide perovskite films. The epitaxial halide perovskites show unprecedented optoelectronic properties and phase transition behaviors.

Halide Perovskites have shown potential for developing high-performance cost-effective optoelectronics. Previous work in this field was primarily focused on pristine perovskites. We discovered that perovskites in form of composites can exhibit more desirable physical properties with improved material stability. In this talk, I will discuss the correlation between material morphology and physical properties in composite halide perovskites, and their performance as light emitters.

The synthesis and excitonic properties of methylammonium lead halide perovskites and their mixed halides are presented with a focus on time-resolved photoluminescence and transient absorption spectroscopy. The substitution of a fraction of Iodid with chloride anions leads to a distorted unit cell due to the smaller radius of the chloride anion relative to the Iodide ion and thus to decreased symmetry and an increased band gap. Femtosecond laser induced transient absorption and photoluminescence measurements show that interface defects contribute to the relaxation processes in photoexcited perovskites. Under two-photon excitation, longer excited state lifetimes could be assigned to the lowest exciton with surprisingly different characteristics compared to the one-photon created states. Origins and implications of these materials properties will be discussed.

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 methylammonium (MA), formamidinium (FA), and all-inorganic cesium (Cs) lead halides perovskites (APbX₃) via a dissolution-recrystallization pathway. We also developed the epitaxial growth of perovskite materials and 2D heterostructures with controlled phases. 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. We demonstrated high performance room temperature lasing with broad tunability of emission with these single-crystal perovskite nanowires. The excellent properties of these single-crystal perovskite nanostructures of diverse families of perovskite materials with different cations, anions, and dimensionality make them ideal for fundamental physical studies of carrier transport and decay mechanisms, and for enabling high performance lasers, LEDs, and other optoelectronic applications.

Semiconductor quantum wells and superlattices, which are usually fabricated through metal-organic chemical vapor deposition or molecular beam epitaxy, are key building blocks in modern optoelectronics. The ability to simultaneously realize defect-free epitaxial growth and to individually fine-tune the chemical composition and band structure of each layer is essential for achieving the desired performance. Such structures are challenging to realize using organic or hybrid materials because of the difficulty of controlling the materials growth. In this talk, I will present a molecular approach to the synthesis of high-quality organic-inorganic hybrid perovskite quantum wells through incorporating widely tunable organic semiconducting building blocks. By introducing sterically tailored groups into the molecular motif, the strong self-aggregation of the conjugated organic molecules can be suppressed, and single crystalline organic-perovskite hybrid quantum wells (down to one mono-layer thick) and superlattices can be easily obtained via one-step solution-processing. Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically-flat interface and ultra-small interlayer distance. The 2D hybrid perovskite superlattices are surprisingly stable, due to the protection of the bulky hydrophobic organic groups. The molecularly engineered 2D semiconductors are promising candidates for use in next-generation nanoelectronics, optoelectronics, and photonics.

Perovskite quantum dots (QDs) as a new type of colloidal nanocrystals have gained significant attention for both fundamental research and commercial applications owing to their appealing optoelectronic properties and excellent chemical processability^[1]. For their wide range of potential applications, synthesizing colloidal QDs with high crystal quality is of crucial importance. However, like most common QD systems, those reported perovskite QDs still suffer from a certain density of trapping defects, giving rise to detrimental non-radiative recombination centers and thus quenching luminescence. Very recently, we have proposed an improved synthetic protocol that involves introducing organolead compound trioctylphosphine-PbI₂ (TOP-PbI₂) as the reactive precursor, which also leads to a significantly improved stability for the resulting perovskite CsPbI₃ QDs and Sn-Pb alloyed QDs ^[2-4] (named as TOP-QDs here). In this talk, I would like to focus on the synthesis and various fundamental physical properties of the TOP-QDs as well as their applications to optoelectronic devices.
HRTEM image of the product shows monodisperse cubic-shaped CsPbI₃ QDs with high crystallinity. XRD patterns of the resulting QDs can be well indexed to the bulk cubic CsPbI₃.^[1] Urbach energies were derived from absorption spectra of CsPbI₃ QDs, varying from ~18 to 19 meV, while for traditional QDs, this value is around 30 meV, indicating TOP-QDs have a lower level of electronic disorder and/or defect density.
Very encouragingly, twice-washed (by MeOAc) QDs in all sizes prepared from TOP route exhibit the best-so-far QYs of up to near 100%, higher than that of the traditional route-produced ones with 78-84%. This behavior, indicative of the absence of non-radiative pathways within the TOP-QDs, is very unusual and rarely observed, even in those traditional high-quality QDs such as CuInS₂ and CdS/Se QDs employing those advanced core/shell passivation techniques. In addition, TOP-CsPbI₃ QDs yield stable QY of 100% for the first 9 days and retain ~85% of its initial value after storage for 1 month. In contrast, traditional QDs show a decreased QY from the initial 86% to 60% after 30 days storage, which in turn suggests a better chemical stability of the resulting TOP-QDs. we couldn’t fit the TA curve of TOP-QDs using the same exponential function because no decay can be resolved in the initial 1 ns time scale, which in turn confirms the negligible electron or hole trapping pathways in these TOP-QDs. In order to gain more insight into the fundamental physics behind these appealing optical properties, we therefore monitored the ultrafast exciton relaxation dynamics through transient absorption (TA) spectrum measurements. TA responses of the traditional QDs measured under a low pump light intensity show a slow single-exponential decay in 1 ns time scale with a background signal y₀. Decay of the photo-excited excitons suggests the presence of recombination pathway due to defects or surface trap states in traditional QDs. However, we couldn’t fit the TA curve of TOP-QDs using the same exponential function because no decay can be resolved in the initial 1 ns time scale, which in turn confirms the negligible electron or hole trapping pathways in these TOP-QDs. Solar cells based on these high-quality CsPbI3 QDs exhibit power conversion efficiency of over 12%, showing great promise for practical application. We expect the successful synthesis of the “ideal” perovskite QDs will exert profound influence on their applications to both QD-based light-harvesting and -emitting devices in the near future.

Crystal instability has been a major drawback hindering the commercialization of Perovskite based devices. In this work, we demonstrate enhanced crystal stability and phase arresting at low temperatures in Perovskite quantum dots, surface functionalized with 3-aminopropyl triethoxysilane. We studied temperature dependent crystal phase transitions in CH₃NH₃PbBr₃ Perovskite quantum dots (PQDs) ligated with octylaminebromide (P-OABr) and 3-aminopropyl triethoxysilane (P-APTES), using a framework of static and dynamic spectroscopy. P-OABr undergoes the expected structural phase transition from tetragonal to orthorhombic phase at ~ 140 K, established by the emergence of a higher energy band at 2.64 eV in the photoluminescence (PL) spectrum, while no phase transition was observed in the case of P-APTES. Such phase stabilization is a result of variation in their respective surface energies, an important contributing factor to the Gibbs free energy for nanomaterials. On further investigating the consequences of this altered crystal phase diagram using time-resolved PL, excitation power dependent PL and Raman microscopy over a range of 300 – 20 K, we observe significant differences in recombination rates and charge carrier types between P-APTES and P-OABr. Our findings highlight how aspects of PQD phase stabilization are linked to nanoscale morphology and the surface energy manipulation of the crystal phase diagram, providing critical insights into the synthesis of stable perovskite crystals for device implementation.
This work was supported by NASA MIRO grant No. NNX15AQ01A
(1) Sarang, S.; Bonabi Naghadeh, S.; Luo, B.; Kumar, P.; Betady, E.; Tung, V.; Scheibner, M.; Zhang, J. Z.; Ghosh, S. Stabilization of the Cubic Crystalline Phase in Organometal Halide Perovskite Quantum Dots via Surface Energy Manipulation. J. Phys. Chem. Lett. 2017, 8 (21), 5378–5384.

Lead halide perovskite (LHP) nanocrystals (NCs) and their light emitting properties have recently moved into the focus of optoelectronic applications. Due to their narrow emission linewidths and high quantum yields, LHP NCs are promising materials for display and lighting technology. Especially within the blue emission regime, they could even outperform their organic counterparts. However many properties of emissive LHP nanoparticles remain to be investigated for a complete understanding of their optoelectronic properties.
One of the core concepts for efficient organic light-emitting diodes (OLEDs) is the preferential alignment of the emissive transition dipole moments within the emitting layer of the device. Using this effect the external quantum efficiency of purely organic devices can easily exceed 30%. However, within the context of LHP NCs this effect is rarely investigated. Recently the orientation of the emissive transition dipole of LHP nanocubes could be determined to be vertical with respect to the film surface, hampering the maximum achievable quantum efficiency of LED applications. ^[1]
To overcome this limitation we investigated the orientation of the emissive transition dipole moment of CsPbBr3 nanoplatelets. Compared to their cubic counterpart, which do not show any form of quantum confinement, these NCs exhibit weak quantum confinement within the dimensions of the plates. This feature leads to huge differences in the optical properties of the excited state. As a first consequence the energy of the emission peak shifts to shorter wavelengths, resulting in an emission peak at 460nm for the investigated nanoplatelets. Further the orientation of the transition dipole moments is confined in space and therefore aligns within the plane of the substrate. Combined with the high refractive index of the perovskite material, the NCs have the potential to outperform existing solutions for blue emitting devices within display and lighting applications.

[1] M. J. Jurow, T. Lampe, E. Penzo, J. Kang, M. A. Koc, T. Zechel, Z. Nett, M. Brady, L.-W. Wang, A. P. Alivisatos, S. Cabrini, W. Brütting, Y. Liu, Nano letters 2017, 17, 4534.

Since the discovery of colloidal metal halide perovskite quantum dots (QDs) three years ago, [1] they have rapidly grown to become one of the most promising classes of nanomaterials for applications in low-cost and highly efficient optoelectronic devices. Anion exchange reactions of the highly luminescent perovskite QDs provide a facile post-synthetic route for tuning of the absorption/emission bandgap of these exciting nanocrystals. The post-synthetic anion exchange reactions allow precise bandgap tuning of perovskite QDs tailored for the desired application. Synthesis, screening, and optimization of colloidal QDs are conventionally conducted using the time- and material-intensive flask-based approaches. Process optimization is therefore limited by the sampling rate, off-line analysis time, and batch reactor/reaction process control. Batch reactors suffer from mixing and heat transfer inefficiencies that degrade the resulting physicochemical properties of the QDs and worsen with the reaction scale (i.e., polydispersed nanocrystals after scale-up).
Our group has recently developed a modular intelligent flow reactor integrated with a translational in situ spectral monitoring probe for continuous synthesis and systematic studies of the colloidal synthesis and anion-exchange reactions of perovskite QDs. [2] Utilizing the developed flow synthesis platform, we have demonstrated, for the first time, a mixing-controlled growth kinetics of cesium lead tribromide perovskite nanocrystals.[2]
The intelligent flow synthesis platform consists of modular heating units equipped with a unique in-situ translational flow cell (UV-Vis absorption and fluorescence spectroscopy). The translational movement of the spectral monitoring probe along the tubular reactor decouples the effect of early timescale mixing of QD precursors from the residence time (i.e., growth time) along the microreactor. Automated sampling along the continuous flow reactor enables rapid photoluminescence and absorption spectra sampling across 68 ports (i.e., reaction times) spanning residence times ranging four orders of magnitude – from 100 ms to 17 min. Varying the average droplet velocity moving in the flow reactor tunes the degree of QD precursor mixing within droplets, resulting in perovskite nanocrystals with different optical properties.
The developed flow synthesis approach enables rapid discovery, screening, and optimization of perovskite QDs with desired optoelectronic properties via high-throughput screening (>10,000 experimental conditions per day) of the accessible synthesis parameter space. In addition, the modularity of the developed QD synthesis platform enables direct scale up using a numbered-up strategy (i.e. parallel flow reactors) for large-scale continuous nanomanufacturing of high-quality perovskite QDs.

[1] Protesescu et al., Nano Lett. 15, 3692–3696 (2015).
[2] Epps et al., Lab Chip 17,4040–4047 (2017).

Recently, broadband white-light emission has been observed in one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) organic-inorganic lead halide perovskites. In addition to the broadband feature, the photoluminescence (PL) spectra of these materials show a massive Stokes shift compared to the corresponding absorption onset. Broadband emission is attributed to the formation of self-trapped excitons (STEs) due to the strong electron-phonon coupling. Interestingly, it has been observed in only certain three-dimensional and low-dimensional metal halide perovskites. In this talk, we will show by density-functional theory calculation that multiple STE structures exist in each perovskite exhibiting broadband emission. However, only the STE with Jahn-Teller like octahedral distortion is mainly responsible for the observed broadband emission, though it may not be the lowest energy structure. Our results provide important insights for designing perovskite materials for broadband emissions with preferred chromaticity coordinator or color temperature.

Halide Perovskites are under intense scrutiny for both light harvesting and light emission applications. Presently, record power conversion efficiencies (PCE) >23% in solar cells and external quantum efficiencies (EQE) >20% in LEDs have been demonstrated. The perovskite field has since exploded with increased research activities spanning the domains of single crystals, thin films, and nanocrystals. An exciting range of frontier phenomena in optical gain, hot-carriers, spin, and ferroelectric properties etc. have been uncovered. In this talk, I will discuss some of these exciting phenomena and highlight our group’s efforts in some of these areas. A succinct overview of the state-of-the-art as well as the prospective outlook will also be presented.

Prospects of widespread perovskite optoelectronics are contingent on the ability to exploit the unique photophysics of such class of materials for device functionality [1]. Photoconverting devices, like solar cells, as well as light-emitting ones, like LEDs and lasers, work best when non-radiative recombination is reduced, either by passivating traps and defects or by boosting radiative recombination rates. Concerning solar cells, the optimal Shockley-Quessier photoconversion efficiency is achieved precisely in the radiative limit, when the only carrier loss is optical emission. We demonstrate a technique, based on the absolute photoluminescence quantum yield as a function of the excitation intensity, to measure the photoconversion open circuit voltage (the free energy) and the ideality factor without any current flowing through the film [2]. A picture emerges of selective traps creating unbalanced free electron and hole populations, universally shared by perovskite materials with various compositions and fabrication routes. We identify interfaces or materials that are the limiting the solar cell photoconversion efficiencies and highlight a rational method to optimize solar cells.
The very nature of the optical emission process in perovskites, whether it comes from a direct or indirect bandgap, is actively debated, as recent studies have proposed that the Rashba spin orbit coupling gives rise to an indirect gap, few tens of meV lower in energy than the direct one. The instantaneous intensity of photoluminescence under pulsed excitation provides measurements for the radiative recombination rates in halide perovskites and shows that radiative recombination becomes faster with decreasing temperature, as in all direct bandgap materials and contrary to what expected for 3D Rashba semiconductors [3].
As for strategies to boost light emission at low excitation levels, carrier confinement enhances local concentration of photoexcitations either in mixed 2D-3D materials or quantum-dot-in-a-matrix architectures, where a low concentration of localized excitated states can be filled efficiently even by photoexcitation even in the low excitation regime [4], leading to efficient LEDs or low-threshold lasers.
Given the major progress achieved in perovskite devices by understanding their photophysics, deployment of optical spectroscopy tools will provide crucial guidance for the undergoing efforts to engineer novel perovskite materials with improved stability, more efficient in emitting light and overall more sustainable in composition through substitution of lead.
References
[1] M. Saba, F. Quochi, A. Mura, G. Bongiovanni, Acc. Chem. Res 49, 166–173 (2015).
[2] V. Sarritzu, et al. Sci. Rep. 7, 44629 (2017).
[3] V. Sarritzu, et al. Advanced Optical Materials 356, 1701254 (2018).
[4] D. Marongiu, et al. ACS Energy Lett. 2, 769–775 (2017).

Metal halide perovskite have received great attention in recent years for applications in various types of optoelectronic devices, including solar cells, light emitting diodes, and X-ray detectors.^1,2 The emission properties show tunability with various perovskite structure. However, exploring an effective strategy to further improve their emission performance, and understanding its correlation with crystal and electronic structures, remains unclear. Here, we report that the photoluminescence of a white light emitting perovskite, C₄N₂H₁₄PbBr₄, can be greatly enhanced under pressure. With 2.0 GPa of applied pressure, the photoluminescent intensity is nearly five times higher than the pristine sample. Simultaneously, a structural transition from orthorhombic to monoclinic was observed from 2.2-2.8 GPa, along with a transition from direct bandgap to indirect bandgap. Structure characterization shows that the Pb-Br₆ octahedron has larger distortion to keeping the original crystal structure with increasing pressure. At 2.0 GPa, the Pb-Br6 Octahedral reach the largest distortion and the photoluminescent intensity also reach its maximum. A reasonable explanation of the enhanced emission is that the pressure-induced distortion of the Pb-Br6 octahedral stabilized the self-trapped excitonic state and promotes it to be more optically active. Our finding demonstrate that pressure can be an efficient tool to improve photoluminescence of halide perovskite, and provide insight of structure distortion and optical properties under extreme conditions.

Reference:
1 Wei, H. et al. Sensitive X-ray detectors made of methylammonium lead tribromide perovskite single crystals. Nature Photonics 10, 333 (2016).
2 Kovalenko, M. V., Protesescu, L. & Bodnarchuk, M. I. Properties and potential optoelectronic applications of lead halide perovskite nanocrystals. Science 358, 745-750 (2017).

Cesium lead iodide perovskite (CsPbI₃) with excellent optical and electrical properties have attracted numerous academic attentions. Specifically, the black cubic phase CsPbI₃ with a direct band gap of 1.74 eV has been most appropriate materials for various optoelectronic applications, especially for photovoltaic (PV), Light-Emitting Diodes (LED) and photodetector applications. However, the preferred cubic phase of bulk CsPbI₃ (α-CsPbI₃) is usually only stable at high temperatures and it will undergo an immediate phase transformation to orthorhombic phase (δ-CsPbI₃) after fabrication at room temperature. Attempts to stabilize the cubic phase in ambient temperature have been focused on complicated procedures such as quantum dot-induced films, poly-vinylpyrrolidone (PVP)-induced surface passivation engineering and chemical composition tuning. In the meantime, most of current efforts have been focused on solution process. Up to now, it has been still a challenge to synthesize the CsPbI₃ through chemical vapor deposition (CVD) method. In this work, we have discovered a convenient CVD method to investigate the growth behavior of the cubic α-CsPbI₃ film on the porous alumina substrate. The lead iodide and cesium iodide were used as the precursors for the deposition of CsPbI₃. The porous alumina with high surface area and large pore volume was used as growth substrate. It was shown that the porous alumina promoted the growth of CsPbI₃ film by absorbing the precursor and increasing the nucleation density. The prepared CsPbI₃ film emitted strong and stable red light (ca. 1.9 eV) under ultraviolet light excitation at room temperature and ambient atmosphere. The lead iodide was absorbed on the surface of the porous alumina firstly then reacted with cesium iodide to form the CsPbI₃. The successful preparation of the CsPbI₃ by the CVD method paves the way for its large scale growth and application in optoelectronic devices.

A description of the photophysical properties of perovskite thin films, microcrystals and nanocrystals, and their dependence with the chemical and optical environment, will be provided. The exposure of thin films and microcrystals to different atmospheres has a strong effect on their photoemission, which reveals relevant information about the mechanism behind ion migration, one of the most intriguing observations reported for these semiconductors.[1,2,3] A detailed analysis of the processes triggered by photoexcitation that lead to activation and latter deactivation of luminescence will be presented. Also, it will be shown that precise control of the spectral features of the luminescence, along with enhanced stability, can be achieved from perovskite nanocrystals synthesized in different types of nanoporous matrices.[4,5] Interestingly, under these conditions, increase of the photoluminescence quantum yield results from the improved optical environment they are exposed to.[6] All this opens the door to the development of novel color converting coatings of application in LED technology, as it will be demonstrated in this talk.

[1] Environmental effects on the photophysics of organic-inorganic halide perovskites. F.J. Galisteo-López, M. Anaya, M.E. Calvo, H. Míguez, J. Phys. Chem. Lett. 2016, 6, 2200.
[2] Three-Dimensional Optical Tomography and Correlated Elemental Analysis of Hybrid Perovskite Micro-Structures. F.J. Galisteo-López, Y. Li, H. Míguez, J. Phys. Chem. Lett. 2016, 7, 5227.
[3] Improving the Bulk Emission Properties of CH₃NH₃PbBr₃ by Modifying the Halide-Related Defect Structure. D. Tiede, M.E. Calvo, F.J. Galisteo-López, H. Míguez, J. Phys. Chem. C 2018, under review.
[4] Strong Quantum Confinement and Fast Photoemission Activation in CH₃NH₃PbI₃ Perovskite Nanocrystals Grown within Periodically Mesostructured Films. M. Anaya, A. Rubino, T.C. Rojas, J.F. Galisteo-López, M.E. Calvo, H. Míguez, Adv. Opt. Mater. 2017, 5, 1601087.
[5] Highly Efficient and Environmentally Stable Flexible Color Converters Based on Confined CH₃NH₃PbBr₃ Nanocrystals. A. Rubino, M. Anaya, J.F. Galisteo-López, T.C. Rojas, M.E. Calvo, H. Míguez. ACS Appl. Mater. Interfaces 2018, DOI: 10.1021/acsami.8b11706.
[6] Absorption and Emission of Light in Optoelectronic Nanomaterials: The Role of the Local Optical Environment. A. Jiménez-Solano, J.F. Galisteo-López, H. Míguez, J. Phys. Chem. Lett. 2018, 9, 2077.

Monomolecular and bimolecular recombination constants in metal halide perovskite thin films of various monovalent cations with consistent morphology and charge trap density were studied. Our results reveal that the monovalent cation plays different roles in monomolecular and bimolecular recombination, indicating that the remarkably customizable metal halide perovskites may be more tunable than previously thought.

Perovskites have emerged as low-cost, high efficiency photovoltaics with certified efficiencies of 22.1% approaching already established technologies. The perovskites used for solar cells have an ABX₃ structure where the cation A is methylammonium (MA), formamidinium (FA), or cesium (Cs); the metal B is Pb or Sn; and the halide X is Cl, Br or I. Unfortunately, single-cation perovskites often suffer from phase, temperature or humidity instabilities. This is particularly noteworthy for CsPbX₃ and FAPbX₃ which are stable at room temperature as a photoinactive “yellow phase” instead of the more desired photoactive “black phase” that is only stable at higher temperatures. Moreover, apart from phase stability, operating perovskite solar cells (PSCs) at elevated temperatures (of 85 °C) is required for passing industrial norms.
Recently, double-cation perovskites (using MA, FA or Cs, FA) were shown to have a stable “black phase” at room temperature.(1,2) These perovskites also exhibit unexpected, novel properties. For example, Cs/FA mixtures supress halide segregation enabling band gaps for perovskite/silicon or perovskite/perovskite tandems.(3) In general, adding more components increases entropy that can stabilize unstable materials (such as the “yellow phase” of FAPbI₃ that can be avoided using the also unstable CsPbI₃). Here, we take the mixing approach further to investigate triple cation (with Cs, MA, FA) perovskites resulting in significantly improved reproducibality and stability.(4) We then use multiple cation engineering as a strategy to integrate the seemingly too small rubidium (Rb) (that never shows a black phase as a single-cation perovskite) to study novel multication perovskites.(5)
One composition containing Rb, Cs, MA and FA resulted in a stabilized efficiency of 21.6% and an electroluminescence of 3.8%. The V_oc of 1.24 V at a band gap of 1.63 eV leads to a very small loss-in-potential of 0.39 V, one of the lowest measured on any PV material indicating the almost recombination-free nature of the novel compound. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking. This is a crucial step towards industrialisation of perovskite solar cells.

Lastly, to explore the theme of multicomponent perovskites further, molecular cations were revaluated using a globularity factor. With this, we calculated that ethylammonium (EA) has been misclassified as too large. Using the multication strategy, we studied an EA-containing compound that yielded an open-circuit voltage of 1.59 V, one of the highest to date. Moreover, using EA, we demonstrate a continuous fine-tuning for perovskites in the "green gap" which is highly relevant for lasers and display technology.
The last part elaborates on a roadmap on how to extend the multicatio to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years.(6)
(1) Jeon et al. Nature (2015)
(2) Lee et al. Advanced Energy Materials (2015)
(3) McMeekin et al. Science (2016)
(4) Saliba et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science (2016)
(5) Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science (2016).
(6) Turren-Cruz et al. Methylammonium-free, high-performance and stable perovskite solar cells on a planar architecture Science (2018)

In recent years, the organic-inorganic hybrid perovskite has gained an increasing research interest in academia for applications in highly efficient thin film solar cells and LEDs, due to the low-cost fabrication process, low non-radiative recombination, tunable bandgap [¹], and material availability [²]. For highly efficient LEDs, the highly uniform ultra-flat films with nanometer-sized grains are required. Among the hybrid perovskites, MAPbI₃ (CH₃NH₃PbI₃) based LEDs has shown high efficiency but with several obstacles such as instability of un-encapsulated LEDs, and hysteresis loss. Therefore, commercialization of these LEDs is still a challenge. Understanding and resolving these issues necessitates the investigation of the perovskite thin films at the atomic scale to determine the underlying fundamental processes.
Here, we present the growth and experimental characterization of thin MAPbI₃ films on Au (111) under ultra-high vacuum conditions. The thin films were prepared by vacuum evaporation of the precursor molecules MAI and PbI₂ with a thickness of a few monolayers (approx. 4 nm). We characterize the sample with scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS), obtaining information about the atomic structure, and chemical composition. For the electronic properties analysis, we used Ultraviolet photoemission spectroscopy (UPS), and Inverse Photoemission spectroscopy (IPES). Our study will provide the basis for further exploration of mixed perovskite materials for the future highly efficient LED applications.

Reference
1. Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G., Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorganic Chemistry 2013, 52 (15), 9019-9038.
2. Xiao, Z.; Kerner, R. A.; Zhao, L.; Tran, N. L.; Lee, K. M.; Koh, T.-W.; Scholes, G. D.; Rand, B. P., Efficient perovskite light-emitting diodes featuring nanometre-sized crystallites. Nature Photonics 2017, 11, 108.

Lead-halide perovskites are an exciting new class of low-cost semiconductors with optoelectronic properties suitable for LED applicaitons. MAPbBr₃ has been utilized to create LEDs with an external quantum efficiency of 9.3%.(Xiao et al., 2017) However, defects in perovskite films can affect device performance. It has been hypothesized that vacancy defects enable ion migration,² which has been implicated to cause current-voltage hysteresis and long-term material degradation. Additionally, defects at the surface of the perovskite film may affect the interfaces in a device, and interface engineering is seen as an important avenue for improving device performance. Scanning tunneling microscopy (STM) offers the ability to probe the surface of OHPs with atomic resolution, including resolving individual vacancy defects.^3-5 Scanning the same area multiple times allows for observation of dynamic events. Here, multiple types of defects were resolved and dynamic ion migration to and from the surface was imaged at the atomic scale. DFT calculations indicate vacancy defects are MABr vacancies and that vacancy defects at the surface of the film change the local work function, which has important implications for energy level alignment between layers in an LED device.

References
¹ Z. Xiao, R.A. Kerner, L. Zhao, N.L.Tran, K.M. Lee, T.W. Koh, G.D. Scholes, and B.P. Rand, Nat Photonics 11, 108 (2017).
² J. Azpiroz, E. Mosconi, J. Bisquert and F. De Angelis, Eng. Environ. Sci. 8, 2118 (2015).
³ L. She, M. Liu, D. Zhong, ACS Nano. 10, 1126 (2016).
⁴ R. Ohmann, L.K. Ono, H.S. Kim, H. Lin, M.V. Lee, Y. Li, N.G. Park, Y.B. Qi, J. Am. Chem. Soc. 137, 16049 (2015).
⁵ Y. Liu, K. Palotas, X. Yuan, T. Hou, H. Lin, Y. Li and S.T. Lee, ACS Nano. 11 2060 (2017).

Lead-halide perovskites have emerged as a promising class of highly efficient and affordable semiconductors with applications in solar cells and optoelectronic devices. One of the key features of these lead-halide perovskites is their ability to retain their performance even in the presence of a large concentration of defects that are generated from their fast solution-based synthesis. Point defects in lead-halide perovskites either lead to shallow defects levels or resonant levels within the valence or conduction bands, which allow efficient carrier transport.¹ In addition to point defects, the solution-deposited polycrystalline perovskites also have a large concentration of planar-defects, such as grain boundaries. These defects are critical to the charge transport and electron-hole recombination characteristics of a semiconductor, and therefore, to the overall device performance. Nevertheless, there are diverging reports on the electrical activity of grain boundaries in lead-halide perovskites. While some reports have suggested them to be benign or even beneficial for charge separation and transport,^2,3 others have suggested them to be detrimental.⁴ A major reason of this controversy lies in the fact that, till date, there have been no experimental reports on the atomic structure and composition of planar defects in these materials.

We have combined aberration-corrected scanning transmission electron microscopy (STEM) and first-principles density-functional theory (DFT) calculations to image grain boundaries and other planar faults in CsPbBr₃ nanocrystals and elucidate their impact on the electronic properties. We have employed a novel post-synthesis process to trigger the fusion of as-synthesized nanocrystals that leads to the formation of planar faults.⁵ Using atomic resolution STEM imaging, we observe that the fusion process is accompanied by the formation of predominantly two types of planar faults: previously unreported Br-rich ∑5 grain boundaries and Ruddlesden-Popper (RP) planar faults.⁶ Generally, in conventional semiconductors planar defects — such as grain boundaries — can be detrimental to their performance as they can introduce mid-gap states, which act as nonradiative recombination centers. However, using DFT calculations, we reveal that neither of the planar faults observed in CsPbBr₃ induce deep defect levels, but their Br-deficient counterparts do. We find that the ∑5 grain boundaries repel electrons and attract holes, similar to an n-p-n junction, while the RP planar defects repel both electrons and holes, similar to a semiconductor-insulator-semiconductor junction. We will discuss the implications of these findings along with strategies to improve the performance of lead-halide perovskites by tailoring the planar faults, for instance, to achieve stable photoluminescence. Finally, we extend the insights obtained from CsPbBr₃ to organic-inorganic lead-halide perovskites and explain the diverging experimental reports on their electrical activity.

Acknowledgments: This work was supported by NSF grant DMR-1806147. This work used computational resources of the Extreme Science and Engineering Discovery Environment, which is supported by the NSF grant number ACI-1053575. A portion of the STEM experiments was conducted at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory (ORNL), which is a Department of Energy (DOE) Office of Science User Facility, through a user project (J.A.H. and J.C.I.).

1. Yin, W.-J. et al., Appl. Phys. Lett., 104, 063903, doi:10.1063/1.4864778 (2014).
2. Edri, E. et al., Nat. Commun., 5, 3461, doi:10.1038/ncomms4461 (2014).
3. Yun, J. S. et al., J. Phys. Chem. Lett. 6, 875, doi:10.1021/acs.jpclett.5b00182 (2015).
4. deQuilettes, D. W. et al., Science, 348, 683, doi:10.1126/science.aaa5333 (2015).
5. Morrell, M. V. et al., ACS Appl. Nano Mate., doi:10.1021/acsanm.8b01298 (2018).
6. Thind, A. S. et al., Microsc. Microanal., 24, 100, doi:10.1017/s1431927618000995 (2018).

Carrier-gas assisted vapor deposition (CGAVD) is used to systematically control the growth of CH₃NH₃SnI_xBr_3-x thin films across a wide range of stoichiometries and morphologies. Our home-built CGAVD system enables more precise morphological control as compared to high-vacuum physical vapor deposition methods, as well as the ability to grow multi-layered perovskite stacks which are typically inaccessible using solution processing. Here, tin-halide perovskite films are grown via co-deposition of CH₃NH₃X and SnX₂ (X = I, Br), where each component vapor is carried from a hot source to a cool substrate by means of a N₂ carrier gas stream. CGAVD offers a broad selection of processing parameters that can be varied to tailor stoichiometry, grain size, and film texturing. Chief among these are the source material temperature (T_source~140 °C – 290 °C), carrier gas flow rate (V_dot~2 sccm – 100 sccm), substrate temperature (T_sub~ 8 °C – 40 °C), and chamber pressure (P~350 mTorr – 10 Torr). In varying these parameters, we have realized corresponding changes in grain orientation and grain size from ~100 nm to over 1 μm. Moreover, the morphologies accessed using this technique are consistent across many substrates, including quartz, silicon, ITO, c-TiO₂, and PEDOT:PSS, allowing facile integration of CGAVD films into different device-relevant architectures while maintaining the same morphology. Having established optimized deposition conditions for CH₃NH₃SnI_xBr_3-x, CGAVD can be applied to other halide perovskite systems to enable to growth of previously inaccessible morphologies and multi-layer perovskite structures.

Colloidal Perovskite Quantum Dots (QDs) have some outstanding optoelectronic properties. Here we present results on such properties of perovskite QDs in solution and in films measured through a variety of optical and electrical techniques. Such Perovskite QD solar cells show high V_OC of 90% of their maximum in the radiative limit, but also show promise in non-PV applications such as light emitters for LED, lasing, displays, or sensing such as photoFETs, which will be the focus of this talk. Perovskite QDs possess unique properties that are not accessible in their bulk or thin-film counterparts. For example, inorganic perovskite materials of CsPbI₃ are unstable in ambient condition in bulk or thin-film, but they are phase stable in their quantum confined form. Another interesting advantage of these QD materials is that their compositions can be tuned without changing the crystal framework either by direct synthesis or by post-synthetic ion exchanges. Particularly, X-site ion exchange in the perovskite QDs with general formula ABX₃ (where A= Cesium-Cs, methylammonium-MA, formamidinium-FA etc.; B= Pb or Sn; X= Cl, Br, I) has shown to be very facile. On the other hand, A-site composition tunability is very limited in these materials, or even in the corresponding thin films. For example, high FA concentration Cs_1-xFA_xPbI₃ has not previously been demonstrated in either QDs and thin films, and it has been shown that only compositions with (1-x)>0.4 can be realized in the pure usable perovskite phase. This is due to thermal instability of FAPbI₃ (crystallizes at around 130 ^oC) at temperatures required to crystallize CsPbI₃ (above 300 ^oC). Here, we present a simple post synthetic cross-cation exchange reaction between colloidal solutions of CsPbI₃ and FAPbI₃ nanocrystals just by mixing them at temperatures slightly above the room temperature that enables us to achieve compositions in the whole range of 0<x<1. This helps us to realize compositions that were not known previously. The photoluminescence (PL) kinetics studies reveal that the activation energy required to inter-exchange the Cs⁺ and FA⁺ ions is around 0.65 eV, higher than that for X-site exchange in lead halide perovskites. We have studied a wide range of perovskite QD compositions with time resolved photoluminescence and microwave conductivity as well as studied the application toward photosensing FETs.

Mixed perovskites towards high efficiency photovoltaics have been pioneered within the photovoltaic community and extensive efforts have been made in order to enable the technology. The impact however, of chemical mixing of such dynamic systems on the morphological structures is largely understudied. Within our study we show that not only does chemical engineering of perovskites affect electronic structure within the material, but the colloidal precursor of the material behaves in strongly dynamic ways and leads to the emergence of intense hierarchical structures within thin films. The phenomenon is similar to those within vivid natural and material systems. Unlike other studies which emphasize on refining film morphologies, we explain the science behind the emergence of natural microstructures, with the aim of elucidating the inherent behaviour of perovskite precursors, which would assist in better understanding and treatment of the material and its’ properties. We utilize simple and highly intuitive measurements (microscopy and scattering) which strongly support the narrative and can be easily understood by a broad range of scientists and engineers- spanning from those interested in photovoltaic material processing, colloidal and soft materials, fluid dynamics, pattern formation, crystallography, to name a few. We explain the emergence of different lengthscales of film microstructure from different perspectives and how they all are intertwined and dependant on the lowest common multiple of all systems- thermodynamics and energetics, during non-equilibrium conditions.

Metal halide perovskite materials are an emerging class of semiconductor with a great potential for use in optoelectronic devices. Recent work, reported the formation of a perovskite based light-emitting diode with external quantum efficiency (EQE) exceeding 20% [1]. This record EQE for perovskite based technology was achieved by managing the compositional distribution in a CsPbBr₃/MABr system, to provide high luminescence and balanced charge injection. The MABr species is suggested to passivate defects and grain boundaries in the perovskite film. However, the exact location of the methylammonium species and its interplay with the perovskite lattice is still unclear. Therefore, the determination and the control of the perovskite morphology [2] as well as the locations of the cations in the material are of prime importance for developing perovskite based LED with high EQE.
In this work, combining scanning tunneling microscopy (STM) and density functional theory (DFT), we reveal the exact location of MA (CH₃NH₃) and Cs cations in MA_1-XCs_xPbBr₃ mixed perovskite lattice at the atomic scale. Additionally, using UV/X-ray photoelectron spectroscopy (UPS/XPS), we demonstrate the impact of cations mixing on the material electronic properties. The understanding of the structure/property relationship in mixed MA_1-XCs_xPbBr₃ cations perovskite provide new tools for the precise design of light emitting devices.

References:
[1] Lin, K.; Xing, J.; Quan, L.N.; Pelayo Garcia de Arquer, F.; Gong, X.; Lu, J.; Xie, L.; Zhao, W.; Zhang, D.; Yan, C.; Li, W.; Liu, X.; Lu, Y.; Kirman, J.; Sargent, E.H.; Xiong, Q.; Wei, Z., Perovskite light-emitting diodes with external quantum efficiency exceeding 20 per cent. Nature 2018, 562, 245-249.
[2] Xiao, Z.; Kerner, R.A.; Zhao, L.; Tran, N.L.; Lee, K.M.; Koh, T.-W.; Scholes, G.D.; Rand, B.P., Efficient perovskite light-emitting diodes featuring nanometre-sized crystallites. Nature Photonics 2017, 11, 108-116.

Halide perovskites have been astonishing the semiconductor community in recent years due to intriguing optoelectronic performance, such as high quantum yields and large band gap tunability. Nevertheless, a lack of stability still hinders many applications, the two main problems being environmentally-induced degradation and migration of halide ions. Here we present a block copolymer-templated synthesis for perovskite nanocrystals (NCs) providing a drastically enhanced stability. The polymer spontaneously forms micelles which act both as nanoreactors and as a protective shell. While unprotected bulk MAPI NCs degraded within a few days under ambient conditions, nanoreactor-NCs showed reliable photoluminescence after more than 200 days. Even more remarkable is the possibility to immerse nanoreactor-NC thin films in water and still obtain characteristic emission after 75 days. Furthermore, the polymer shell prevents halide ion migration in thin films, enabling the possibility to fabricate heterostructures of different types of NCs.

Metal halide perovskites have been used in various optoelectronic applications, such as solar cells, lasers and photodetectors. The material class is of particular interest for the photodetector field since the band gap varies with its composition. Using tin-based hybrid inorganic-organic perovskites leads to near-infrared sensitive photodetectors [1]. By changing the composition of lead-based perovskites one can achieve band gaps varying over the visible spectrum from 550 to 800 nm [2, 3]. With copper as the metal cation it is possible to synthesize materials with even higher band gaps [4]. The fact that all of these perovskites can easily be made in single crystal form makes them even more attractive for photodetector applications since they are typically more stable and have lower trap densities than their thin film counterparts. Especially the two-dimensional perovskites are known for their superior stability against humidity compared to the 3D compounds [5].

Here we report the performance of photodetectors made of various 2D perovskite single crystals and compare these to their 3D counterparts. Among the 2D materials investigated are phenylethylamine lead iodide ((PEA)₂PbI₄) and copper-based perovskites as MA₂CuCl₄. They are used in the lateral type photoconductor layout, in which two metal electrodes are deposited on the same side of the perovskite crystal. This typically has the advantage of high photocurrent and high responsivity, at the expense of relatively slow response times [6].

We demonstrated in our earlier work that perovskite single crystals made of methylammonium lead bromide can be used as a gas detector [7]. This effect is governed by the surface trap state density and its modulation by physisorption of gases, and it is evident from a change in photocurrent upon light exposure. Previously we focused on the effects of nitrogen, carbon dioxide, argon, oxygen, air and water on the MAPbBr₃ crystal’s optoelectronic properties. Here we expand this to the two-dimensional perovskite photoconductors mentioned above. In addition, we characterize the behavior of these devices under additional gases such as alcohols and ketones, which might play a role in lung cancer detection.

[1] Adv. Funct. Mater., 2017, 27, 1701053
[2] Nano Lett., 2013, 13 (4), pp 1764–1769
[3] Energy Environ. Sci., 2014, 7, 982–988
[4] Inorg. Chem., 2016, 55, 1044−1052
[5] Energy Environ. Sci., 2018, 11, 234 - 242
[6] Chem. Soc. Rev., 2017, 46, 5204
[7] Sci. Adv., 2016, 2, e1600534

Formamidinium lead iodide (FAPbI₃) perovskite has emerged as one of the most promising materials in the field of optoelectronics. However, FAPbI₃ perovskite exhibits very poor ambient stability, because the FAPbI₃ non-perovskite polymorph (δ-phase) is thermodynamically more stable than the desired perovskite polymorph (α-phase) at ambient temperature. In order to address this issue, we report a novel method to stabilize α-FAPbI₃ in the ambient environment by integrating perovksite nanocrystals with various compositions into α-FAPbI₃ thin films. The nanocrystals/thin-film integration retards the transformation of α-phase into δ phase , contributing to the enhanced ambient stability. The stabilization mechanism is studied comprehensively. This work points out a new direction towards more stable FAPbI₃ perovskites for optoelectronic applications.

Lead-based inorganic metal halide perovskite materials have the general chemical formula of CsPbX₃, where X is Cl, Br, or I. The electronic and optical properties of metal halide perovskites (MHP), such as narrow emission wavelengths, readily adjustable emission wavelengths, and high photoluminescence quantum yields (PLQYs) are advantageous for a number of different optoelectronic devices, including photovoltaics, lasing, and light emitting diodes. Though synthesis and anion-exchange of MHPs are relatively facile, synthesized MHP nanocrystals (NCs) via typical ion-exchange methods show instability in air and moisture, and PLQYs of MHPNCs decrease over time due to a continuous loss of labile parent surface ligands. To enable the further development and application of MHPNCs, the stability must be increased while maintaining or improving high PLQYs. Herein, we report a fast and simple anion-exchange reaction for MHPNCs using alkyltrichlorosilane (RSiCl₃) at room temperature (20^o-22^oC), with motivation stemming from the ability of the alkyltrichlorosilane to both provide the chloride for the anion exchange while also forming a protective stabilizing siloxane shells around the exchanged NCs. In this study, we show that colloidal host CsPbBr₃ NCs transform into colloidal CsPbCl₃ NCs upon exposure to RSiCl₃ due to the anion-exchange reaction between Br of CsPbBr₃ and Cl of RSiCl₃. Both colloidal and solid thin film of CsPbCl₃ NCs produced with alkyltrichlorosilane show good moisture stability and relatively high PLQYs of ~13%, whereas colloidal CsPbCl₃ NCs produced via typical methods show low PLQYs (1-10%) and degrade rapidly upon exposure to moisture or ambient air.

Two-dimensional (2D) hybrid perovskites (HPVKs) are structures alternating organic and inorganic layers, arising from inclusion of a large organic cation providing Goldschmidt’s tolerance factor higher than 1. This fact generates separation of a determined number of inorganic layers (n), which can range from 1 to ∞, which corresponds to a 3D arrangement. A variation of this pure organic−inorganic structure can be obtained by addition of a small organic cation, MA (CH3NH3+) in most cases, providing a Goldschmidt’s tolerance factor adequate for perovskite formation, making it so the inorganic part becomes a hybrid structure. These organic−inorganic hybrid structures are called 2D/3D HPVKs. Actually, 2D/3D perovskite-based solar cells have emerged as an alternative to pure 3D perovskites with the aim to improve their long-term stability, which is a key factor in future device commercialization.
In the last years, several studies have been reported on 2D/3D perovskites prepared using mainly two
ammonium salts: (i) phenylethylammonium iodide (PEA), and (ii) butylammonium iodide (BAI). The results obtained so far can be considered very limited because, in general, the studies have focused only on aliphatic nonconjugated ammonium salts. Although it is true that the moisture stability increases, it is also true that the photovoltaic performance of 2D/3D PVK materials is severely limited owing to quantum and dielectric confinement effects. Accordingly, it is necessary the synthesis and deep optical characterization of materials with an adequate management of dielectric contrast between the layers. In this study, we demonstrate the successful tuning of dielectric confinement by the inclusion of a conjugated molecule, anilinium cation, as a bulky cation, in the fabrication of the 2D/3D PVK material (C6H5NH3)2(CH3NH3)n-1PbnI3n+1, where n=3-5.

The structure of organic-inorganic lead halide perovskites (APbI3) has been limited by geometric considerations to what cation can occupy the A-site cavity. Here we have continued to expand beyond this A-site limit by synthesizing colloidal nanoplatelets of the structurally labile Ruddlesden-Popper perovskites (A’A_nPb_n-1I_3n+1) containing a series of new large A-site cations, such as ethylammonium (EA), as well as the commonly observed methylammonium (MA), formamidinium (FA) and cesium. The flexible lattice of the Ruddlesden-Popper’s structure allows the incorporation of large A-site cations into the perovskites A-site cavity. The occupancy of these large A-cations in the perovskite A-site cavity is supported by single-crystal XRD and structural determination of select compounds. The facile ligand assisted reprecipitation method to synthesize colloidal nanoplatelets of (HA)₂(A)Pb₂I₇ with varying A-cations was utilized as a convenient pathway to explore more exotic cations. The nanoplatelets are 100-200 nm in lateral dimension and exhibit clear quantum confinement due to the layered crystal structure. Optical characterization demonstrates strong photoluminescence as well as a strong excitonic feature in the absorbance spectra. The A-site cation is shown to have an impact on the absorbance onset position, PL peak position, QY, and lifetimes. This work highlights a pathway to synthesizing model systems as a way to systematically compare the impact of the A-site cations role in the optoelectronic properties of lead halide perovskites.

Solution-processable perovskites have recently emerged as extraordinary semiconductor materials for light-emitting diodes (LEDs). So far most of the high efficiency LEDs have been demonstrated on small area and by employing difficult-to-control anti-solvent engineering techniques, whereby the required processes do not permit uniform coating over a large area, hindering the potential commercialization of the technology in large area lightings and display devices. Here we report a scalable method to deposit large-area and uniform nanostructured perovskite thin film for active layer in LEDs. The large area deposition is enabled by the pre-synthesized colloidal nanocrystal inks, which allows the formation of unique self-assembled perovskite nanostructure featuring 3D formamidinium lead bromide (FAPbBr₃) nanocrystals of graded size coupled with microplatelets of octylammonium lead bromide perovskites. The nanostructure thin film, formed by a simple spincoating deposition of colloidal inks, exhibits photoluminescence quantum yields of over 80%, enabled by the energy cascading effect that significantly improve bimolecular recombination properties at lower charge injection density. Our small area (3 mm²) LEDs reach high external quantum efficiency exceeding 13% and brightness above 50k cd m^-2. Moreover we also demonstrate large area LED (>5 cm²) with a uniform emission and high brightness (>20k cd m^-2). These results demonstrate the benefits of the pre-synthesized colloidal inks for fabrication of large area LEDs.

Lead -Halide Perovskites exhibit favorable properties when compared to conventional optoelectronic materials. The single-crystalline hybrid perovskites have important photoelectronic properties for semiconductor devices, such as solar cells and light emitting diodes. Here we implement a first principle study to identify and analyze dependence of optoelectronic properties of Cesium Lead Bromide perovskite, CsPbBr₃ on composition, quantum confinement, and surface morphology. The calculations include combination of ground state modeling by DFT and dynamics modeling of excited state. The dynamic of excited states is modeled via Redfield equation of motion for electronic degrees of freedom, parameterized by nonadiabatic couplings, i.e. response of electronic states to thermal dynamics of nuclei. Specifically, we model how dynamics of photoinduced electronic state at the surface of 2D quantum confined thing films of perovskite depends on presence or absence of different adsorbates and symmetry of exposed crystallographic surface. By analyzing dynamics of charge redistribution between inner area of thin film of the perovskite and surface defects we can identify the winner in the competition between radiative and nonradiative recombination pathways, and, thus, quantum yield of photoluminescence. The obtained results allow for theory-guided design of optimal morphology of 2D perovskite films for light emitting applications.

Organic-inorganic hybrid perovskite (OIHP) based flexible electronic devices have attracted great deal of interest due to their low-cost fabrication and potential light weight and portable applications. However, limited flexibility and unfavourable bending stability have been major challenges to their advancement. In this context, multiple studies have demonstrated that the films exhibit enhanced thermal/moisture tolerance under compressive strain and lower tolerance under tensile strain by physically bending the substrates. Here, we follow the evolution of stresses and highlight the thermal effects and structural changes in the film. For doing so, we also propose a processing method to achieve flat films with different stress states on transparent-conducting flexible substrates. The effect of residual stress on the optoelectronic properties and thermal and environmental stability of the films is studied. The observed changes in the film properties are correlated with the nature of the defects generated/annihilated as the film passes through a bending cycle. Understanding the fundamental role of stresses on film properties would help in discovering ways to mitigate its detrimental effects and achieve a more robust technology.

The intensive research impetus on halide perovskite materials, originating from their exceptional progress in solution-processed photovoltaics, has rapidly extended to various optoelectronic applications and fundamental studies due to their abundant elemental compositions and crystal structures as well as many unusual, tunable physical properties. This is exemplified by all-inorganic halide perovskites, including CsBX₃ (B = Pb, Sn and X = Cl, Br, I). These compositions have attracted increasing attention because of their enhanced ambient stability compared to organic-inorganic hybrid counterparts in which organic cations occupy Cs site. Remarkable features such as efficient charge transport and unique thermoelectric properties have been discovered to establish structure-property correlations. However, heterostructure formation in this class of materials is largely unexplored thus far.

Specifically, nano-scale heterostructure interface, in which geometric and electronic structures of nanomaterials are effectively coupled, brings about novel and unique functionalities that cannot be achieved by single constituents. Abundant research on the interfaces formed by dissimilar solid-state nanomaterials attracts worldwide attention due to the interest in both fundamental understanding of interfacial charge transfer mechanisms and their potential applications in a variety of fields.

Here we introduce our recent two works about the precise engineering and systematic characterization of the functional heterostructure in all-inorganic halide perovskite nanowires. On one work, we show that the p-n junctions can be readily induced through a localized thermal-driven phase transition in a single-crystalline halide perovskite CsSnI₃ nanowire. This material undergoes a phase transition from a double chain yellow phase to an orthorhombic black phase. The formation energies of the cation and anion vacancies in these two phases are significantly different, which leads to n- and p- type electrical characteristics for yellow and black phases respectively. Interface formation between these two phases and directional interface propagation within a single NW are directly observed under cathodoluminescence microscopy. Further, we demonstrate the current rectifying behavior of the resulting p-n heterostructure originating from the distinctly different charge transport properties of these two phases. On the other work, taking advantage of the soft and reconfigurable ionic lattice characteristics in halide perovskite materials, we design a CsPbBr₃-CsPbCl₃ heterostructure nanowire that bridges the pre-patterned parallel Au electrodes, via a precisely controlled anion exchange approach. Quantitative correlation between halide ion ratios and photoluminescence emission wavelengths in halide perovskites allows us to visualize the halide ion migration across the heterostructure interface under electric fields. More interestingly, we show that the halide ion migration is heavily dependent on the bias polarity across the nanowire, leading to the field-induced halide ion migration rectification in this solid-state material. The present approaches to heterostructure formation could inspire the precise control over the design of functional heterostructures using halide perovskites as building blocks.

Recently, the appearance of halide perovskites has catalyzed an unprecedented revolution in the field of (opto)electronics with tunability in the crystal-structure dimensionality and chemical composition. Our group has previously proposed a useful methodology for the discovery of new rare earth phosphors for LED applications by mineral-inspired prototype evolution and new phase construction via cosubstitution strategy, and many useful phosphors have been reported.^1,2 Since it is a challenge to discover the counterparts for rare earth phosphor, our group firstly conducted the investigations on perovskite quantum dots represented by CsPbX₃ to target the improved quantum efficiency and enhanced stability, and they also demonstrated potential for liquid crystal display backlight application.^3-4 Secondly, in the search of stable lead-free perovskites, hybrid perovskites or perovskites double have been studied by utilizing heterovalent substitution or chemical unit cosubstituion proposed by our group in the phosphor studies.^5-6 Our pioneer work on Cs₂AgInCl₆ crystals, as an example, focused on the structural design principles, crystal structure, and morphology related with solution-based crystal growth habit, band structure, optical properties, and the stability,⁵ and this compound has now become the “star” materials in the luminescent perovskites. Finally, our recent work on the correlation of the crystal chemistry and photoluminescence behavior of the new zero-dimensional (0D) or two-dimensional (2D) structures will be mainly addressed in this talk, moreover the Mn²⁺ or other ions doped perovskites, as well as the photoluminescence and optoelectronic applications will be also discussed.

References:
Zhiguo Xia^*, and Quanlin Liu, Progress in discovery and structural design of color conversion phosphors for LEDs, Progress in Materials Science, 2016, 84, 59-117.
Ming Zhao, Hongxu Liao, Lixin Ning, Qinyuan Zhang, Quanlin Liu, Zhiguo Xia^*, Next-Generation Narrow-band Green-emitting RbLi(Li₃SiO₄)₂:Eu²⁺ Phosphor for Backlight Display Application, Advanced Materials, 2018, 30(38) 1802489.
Ying Liu, Fei Li, Quanlin Liu, Zhiguo Xia^*, Synergetic Effect of Postsynthetic Water Treatment on the Enhanced Photoluminescence and Stability of CsPbX₃ (X = Cl, Br, I) Perovskite Nanocrystals, Chemistry of Materials, 2018, 30 (19), pp 6922–6929.
Fei Li, Ying Liu, Hongliang Wang, Qian Zhan, Quanlin Liu, Zhiguo Xia^*, Postsynthetic Surface Trap Removal of CsPbX₃ (X = Cl, Br, I) Quantum Dots via ZnX₂-Hexane Solution toward Enhanced Luminescence Quantum Yield, Chemistry of Materials, 2018, doi: 10.1021/acs.chemmater.8b03442.
Jun Zhou, Zhiguo Xia^*, Maxim S. Molokeev, Xiuwen Zhang, Dongsheng Peng, QuanLin Liu, Composition design, optical gap and stability investigations of lead-free halide double perovskite Cs₂AgInCl₆, Journal of Materials Chemistry A, 2017, 5, 15031–15037.
Jun Zhou, Ximing Rong, Maxim S. Molokeev, Xiuwen Zhang^*, Zhiguo Xia^*, Exploring the Transposition Effects on Electronic and Optics Properties of Cs₂AgSbCl₆ via a Combined Computational- Experimental Approach, Journal of Materials Chemistry A, 2018, 6, 2346-2352.

Lab scale (<0.1 cm²) metal halide perovskite solar cells were demonstrated to achieve power conversion efficiencies (PCEs) comparable to several other photovoltaic technologies. However, development of up-scaling processes (>10 cm²) with high PCE and stability is important for moving forward this technology towards commercialization [1,2]. At OIST, a team of researchers in the Energy Materials and Surface Sciences Unit has been making concerted efforts to develop processes aiming at high PCE, high-throughput, and minimum batch-to-batch variation, and compatible with large-area perovskite solar cells and modules. In this talk, we will present our current progress to use chemical vapor deposition CVD [3-6] to fabricate perovskite solar cells and light-emitting diodes [7]. Also, we will introduce a novel methylamine (CH₃NH₂) gas induced crystallization process [8-10], which provides valuable insight into the formation of perovskite films.

[1] L.K. Ono and Y.B. Qi*, Research Progress on Organic-Inorganic Halide Perovskite Materials and Solar Cells. J. Phys. D Appl. Phys. 51 (2018) 093001.
[2] L.K. Ono, T. Kim, Y. Jiang, Y.B. Qi, and S.F. Liu, “Heat Wave” of Metal Halide Perovskite Solar Cells Continues in Phoenix. ACS Energy Lett. 3 (2018) 1898-1903.
[3] Y. Jiang, M.R. Leyden, L. Qiu, S. Wang, L.K. Ono, Z. Wu, E.J. Juarez-Perez, and Y.B. Qi*, Combination of Hybrid CVD and Cation Exchange for Up-Scaling Cs-Substituted Mixed Cation Perovskite Solar Cells with High Efficiency and Stability. Adv. Funct. Mater. 27 (2018) 1703835.
[4] M.R. Leyden, Y. Jiang, and Y.B. Qi*, Chemical Vapor Deposition Grown Formamidinium Perovskite Solar Modules with High Steady State Power and Thermal Stability. J. Mater. Chem. A 4 (2016) 13125-13132.
[5] M.R. Leyden, M.V. Lee, S.R. Raga, and Y.B. Qi*, Large Formamidinium Lead Trihalide Perovskite Solar Cells Using Chemical Vapor Deposition with High Reproducibility and Tunable Chlorine Concentrations. J. Mater. Chem. A 3 (2015) 16097-16103.
[6] M.R. Leyden, L.K. Ono, S.R. Raga, Y. Kato, S.H. Wang, and Y.B. Qi*, High Performance Perovskite Solar Cells by Hybrid Chemical Vapor Deposition. J. Mater. Chem. A 2 (2014) 18742-18745.
[7] M.R. Leyden, L. Meng, Y. Jiang, L.K. Ono, L. Qiu, E.J. Juarez-Perez, C. Qin, C. Adachi, and Y. Qi, Methylammonium Lead Bromide Perovskite Light-Emitting Diodes by Chemical Vapor Deposition. J. Phys. Chem. Lett. 8 (2017) 3193-3198.
[8] S.R. Raga, L.K. Ono, and Y.B. Qi*, Rapid Perovskite Formation by CH₃NH₂ Gas-Induced Intercalation and Reaction of PbI₂. J. Mater. Chem. A 4 (2016) 2494-2500.
[9] Y. Jiang, E.J. Juarez-Perez, Q. Ge, S. Wang, M.R. Leyden, L.K. Ono, S.R. Raga, J. Hu, and Y.B. Qi*, Post-Annealing of MAPbI₃ Perovskite Films with Methylamine for Efficient Perovskite Solar Cells. Mater. Horiz. 3 (2016) 548-555.
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In the past few years, the hybrid lead perovskites materials are the megastar in photovoltaics research field and has grab enormous attention due to its excellent absorption coefficient, charge-transport properties and long carrier life time. This thus has enabled a wide range of optoelectronic applications, notably in high efficiency photovoltaics with power conversion efficiency exceeding 23%; and bright light-emitting diodes (LEDs), sensitive photo-detector. Very recently, perovskites have been recognized as promising semiconductor for solid-state radiation detectors due to their high radiation absorption cross-section enhanced by the presence of heavy ions, such as lead, cesium and halide etc. State-of-the-art solid-state X-ray detector utilize germanium and silicon that exhibits an excellent detectivity. However, such performance is only achievable under bulk volume of materials due to its low z number, whereas operation suffers from charge diffusion and drifting that is detrimental to the detector sensitivity. Therefore, perovskite-based semiconductor with high mobility-lifetime product, low defect density would be a novel candidate for X-ray detector.

Here, for the first time we demonstrate a thin film X-ray detector using a P-I-N diode fabricated with highly crystalline Ruddles-Popper layered Perovksites thin films. It features with ultra-low dark current and high X-ray induced photocurrent signal, greater than state-of-the-art silicon diode. Most impressively, using merely 200 nm active layer, we could detect high energy hard X-ray above 10 keV with on/off ratio over 10^-11. We attribute such performance to be the superior crystallinity in layered perovskite that suppressed recombination induced dark current that thus enhance the X-ray detection sensitivity. Our demonstration holds promise for efficient high energy radiation detection such as X-ray and low energy Gamma-Ray, using thin film PIN junction configuration. Our findings also inform a new generation of highly efficient and low-cost X-ray detectors based on Ruddles-Popper Layered Perovksites thin film.

Organic-inorganic hybrid methylammonium lead halide perovskite (MAPbX₃, X= Cl, Br, I) is an emerging candidate of diverse fields, including the light emitting diode, solar cell, photodetector, and photocatalysis. Due to their low formation energy, MAPbX₃ materials are sensitive to many factors, including polar solvents, moisture, light, and high temperature. To address this issue, a straightforward method, encapsulating MAPbX₃ nanoparticles (NPs) with stable polymer-like shells is commonly reported. Meanwhile, the previously reported encapsulated MAPbX₃ were limited to the macroscale, which makes it difficult to develop the applications requiring monodisperse particles with high uniformity. In this work, monodisperse MAPbBr₃@ poly(norepinephrine) (PNE) NPs were successfully prepared by reverse micelle method and in-situ polymerization. In the polymerization process, the MAPbBr₃ NPs were encapsulated with the stable and thin layer of PNE homogenously, maintaining the intrinsic crystalline structure and monodisperse morphology. Meanwhile, PNE shell bearing alkyl hydroxyl groups resulted in activated MAPbBr₃ NPs with remarkable features in photocatalysis tests even after treatment with water. Our work not only proposes a novel approach to produce core-shell perovskite NPs on the single particle level but also enhances the stability and photocatalytic efficiency of pristine perovskite.