Session X20: Hybrid Perovskites -- Low Dimensions

Interplay between structure and physical properties in organic-inorganic 2D perovskites

Organic-inorganic (hybrid) 2D perovskites feature unique structural characteristics related to the hybrid nature of their crystal lattice, which includes soft and dynamic lattice structure and organic-inorganic atom-thick interfaces.There is still little knowledge of the interplay between, on the one hand, their photo-excited states and electronic properties and, on the other hand, their structural characteristics. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modelling, we report the dependence of the formation, dynamics, and recombination of exciton states on the structural and compositional details of hybrid 2D perovskites [Nature Communications 9, 2254, 2018 & Science 355, 1288, 2017]. First, our work reveals that the exciton properties are tailored by both the thickness of the 2D perovskites and the size of the organic molecules in the lattice. Second, we demonstrate the existence of unique electronic states located at the edges of the 2D perovskite layers, which result from local distortions of the lattice at the edges, and which promote exciton dissociation and localization. Finally, we will provides insight into the hetero-coupling between hybrid 2D perovskites and graphene-like layers such as graphene and transition metal dichalcogenides.

  

Electronic Trends in Oligothiophene Based Layered Lead Halide Perovskites

We investigate and analyze electronic trends in aminoethyl-oligothiophene (AEnT, n=1-5) based lead halide (PbX$_4$, X=Cl, Br, I) layered perovskites using hybrid density functional theory including spin orbit coupling. Analysis in terms of band edges (PRL 121, 146401 (2018)) demonstrates that one can understand these materials as quantum wells of four different subtypes (called type Ia, type Ib, type IIa, type IIb), depending on whether the organic or the inorganic component contributes the frontier orbitals. We provide a detailed analysis of the electronic properties of the full group in terms of frontier orbitals, densities of states, band curvatures and other key electronic characteristics of interest for the selection of semiconductor materials.   

Studies of Spin-Orbit Coupling strength in 2D and 3D organic/inorganic hybrid lead based perovskites by magnetic field effect

Organic/inorganic hybrid lead-based perovskites have been studied as advanced materials for novel optoelectronic devices.¹ In those materials, strong spin-orbit coupling (SOC) from lead atoms plays a decisive role in lowering the band gap and consolidating the optical absorption against the local distortions of the lattice, necessary for high performance solar cells.² Nevertheless, fundamental studies on SOC strength of these materials have still been very limited. Here, we present the magnetic field effect (MFE) in light emitting diodes (LEDs) made of the 3D hybrid perovskite CH₃NH₃PbI₃ (MAPbI₃) and 2D Ruddlesden-Popper hybrid perovskite (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)Pb₂I₇, where the width and the magnitude of the magnetic response in conductivity and electroluminescence are measures of the SOC strength in the materials.³ We observed the magneto-conductance (MC) and magneto-electroluminescence (MEL) in the 3D perovskite devices in which MC and MEL with 0.07% magnitude and 250 G half width indicate the strong SOC strength. The absence of the MFE in the 2D perovskite devices indicates that the SOC strength is much larger due to the larger charge confinement effect in these materials.⁴ The results were confirmed by our MFE studies on the photocurrent of the devices.   

Freestanding Crystalline Monolayers of Oxide Perovskites

Two-dimensional (2D) materials like graphene have demonstrated new electronic phases can emerge when a bulk crystal is thinned down to a mono-layer. As transition metal oxide perovskites host a variety of correlated phases, realizing the analogs with oxide perovskite materials would open the door to a rich spectrum of exotic 2D correlated phases that have not yet been explored. Here we report on the fabrication of freestanding perovskite films with high crystalline quality down to a single unit cell. Using the recently developed method based on water-soluble Sr₃Al₂O₆ as the sacrificial buffer layer we synthesize freestanding SrTiO₃ and BiFeO₃ ultrathin films by reactive molecular beam epitaxy and transfer them to different substrates such as crystalline silicon wafers and holey carbon films. We find that freestanding BiFeO₃ films exhibit an unexpected giant tetragonality and polarization when approaching the ultimate 2D limit. The ability to synthesize crystalline freestanding perovskite films without thickness limitation opens a new field for the 2D correlated electronic phases and interfacial phenomena.   

Optoelectronic properties of layered and 3D hybrid perovskites: recent results

In the past five years, solution-processed organometallic perovskite based solar cells have emerged as a promising thin-film photovoltaic technology with record conversion efficiencies as high as 23.3%. The presentation will review recent optical spectroscopy and diffraction results on 3D halide perovskites, as well as colloidal nanocrystals that also exhibit attractive light emission properties. Presently, the intended optoelectronic applications of this class of 3D materials are in the realm of semiconductors. But they can be described as unconventional semiconductors, because the spin-orbit coupling is giant and shows up in the conduction band. More, the band gap is direct with a critical wavevector located at one of the edges of the reference Brillouin zone, and among others, excitonic and Rashba-Dresselhaus effects, as well as electron-phonon coupling and lattice anharmonicity may play a crucial role. Related 2D multilayered phases, composed of perovskites multilayers sandwiched between two layers of large organic cations, have recently demonstrated improved solar cells photostability under standard illumination as well as humidity resistance, leading to conversion efficiencies in the 10-15% range.In this case, intrinsic quantum and dielectric carrier confinements are afforded by the organic inner barriers and a tunable perovskite layer thickness, which lead to stable Wannier excitons at room temperature.These excitations share some similarities with the ones observed in Van der Waals heterostructures. Solar cell and LED device operations are related to internal exciton dissociation through low energy states in multilayered perovskites, as shown from the investigations of both thin films and small exfoliated single crystals. Multilayered halide perovskites shall lead to extensive chemical engineering in the future.   

First-Principles Studies of the Electronic and Optical Properties of Halide Perovskites: From Three to Two Dimensions

Metal-halide perovskites have emerged in the past six years as a disruptive new class of optoelectronic materials. Solar cells based on 3D lead-halide perovskites have recently reached a record power conversion efficiency of 23.3%. In addition, the 2D variants are gaining momentum as a promising new class of versatile materials for applications in light emission and solar energy conversion [1,2,3]. Despite these developments, fundamental optoelectronic properties of this novel family of layered materials are not yet fully understood. Here, we will explore the role of structural effects on the optoelectronic properties of 2D lead-halide perovskites with DFT and ab-initio many-body perturbation theory. We will examine the effects of various mean field starting points and self-consistency in the GW approximation, while explicitly incorporating spin-orbit interactions. Lastly, we will analyze the importance of electron-hole interactions upon optical excitation, within the Bethe-Salpeter equation, and discuss the optical spectra and photophysics of these complex class of compounds.

[1] Tsai et al,Nature 536,312(2016)
[2] Gong et al,Nat.Mater.17,550(2018)
[3] Smith et al,Chem.Mater.29(5)1868(2017)   

Making and breaking the exciton in layered halide hybrid perovskites

Layered halide hybrid organic−inorganic perovskites (LHP) [1] have been the subject of intense investigation before the rise of three-dimensional (3D) halide perovskites and their impressive performance in solar cells. Recently, LHP have also been proposed as attractive alternatives for photostable solar cells [2] and revisited for light-emitting devices. LHP present inherent quantum and dielectric confinements imposed by the organic layers sandwiching the inorganic core, and computational approaches have successfully help rationalized their properties [4-6].
Here combine theoretical and spectroscopic studies inspect the various ingredients of LHP strong exciton binding energy and the ways to break the electron-hole pair [3,4]. In particular, we propose an elastic model providing design principles for future layered perovskites with optimized properties for photovoltaics or light emission [7].

1. L. Pedesseau et al., ACS Nano10, 9776 (2016).
2. H. Tsai et al., Nature536, 312 (2016), 536.
3. J.-C. Blancon et al., Science355, 1288 (2017).
4. M. Kepenekian et al., ACS Nano12, 11557 (2015).
5. M. D. Smith et al., Chem. Sci.8, 1960(2017).
6. D. Sapori, M. Kepenekian, L. Pedesseau, C. Katan, J. Even, Nanoscale8, 6369 (2016).[7] M. Kepenekian et al., Nano Lett. 18, 5603 (2018).   

Rashba Splitting in Hybrid Lead Halide Perovskites probed by Photogalvanic Spectroscopy

Hybrid Lead Halide Perovskites (HLHPs) such as MAPbX₃ (where MA is methyl-ammonium and X is halogen) have shown unprecedented performance in optoelectronic device applications such as solar cells, photodetectors and light emitting diodes. The recent discoveries of giant Rashba splitting in both bulk (3D) and two-dimensional (2D) HLHPs have raised promise that these compounds may be excellent candidates for spintronic device applications. Using both photogalvanic (PGA) and photoluminescence spectroscopies we evaluate the Rashba splitting value in both polycrystalline and single crystal HLHPs. We have uncovered the morphological influence on the Rashba splitting in these compounds by measuring angular, temperature and excitation dependencies of the PGA. We also report on the influence of defects and grain boundaries on the Rashba splitting in various HLHP crystallographic phases.
  

Control over ultrafast Rashba splitting dynamics in quasi-2D arrays of 3D hybrid perovskite nanocrystals by the interfacial electric field

Snapshot spectral imaging has been performed with 100 fs steps in delay-times for fully encapsulated quasi-2D arrays of 3D hybrid perovskite MAPbBr₃ nanocrystals (~20 nm in size) spin-coated to either the clean sapphire plate or to that initially ALD-coated by a ~30 or ~100 nm thick ZnO layer. The absorption bleaching band for the bare MAPbBr₃ nanocrystal arrays reveals a ~200 meV splitting appearing within ~1 ns, which has been assigned to the Rashba splitting effect. However, the splitting unexpectedly disappears for the MAPbBr₃/ZnO samples at delay-times longer than ~500-600 fs when the interfacial electric field at the MAPbBr₃/ZnO heterointerface develops due to charge separation. The effect has been treated theoretically in the frame of the model taking into account the interfacial-field-induced structural phase transition in MAPbBr₃ nanocrystals.   

Revealing nanocrystal growth within CsPbI2xBr3-x perovskite thin films

Inorganic lead halide perovskites have greatly impacted the optoelectronics field due to their band-tunability and solution-processability in high performance light emitting diodes (LEDs). However, the bulk material is unstable under room temperature which tends to phase segregate under ambient or device operational conditions. Recent breakthrough has successfully demonstrated a stabilized CsPbX₃ phase at room temperature by fabricating into nanostructured crystals (NC). Further, photo-physical property enhancement is attributed to the nano-scale structure control which are thus the most promising candidate for the lighting applications. It is therefore critical to systematically understand the structure of the CsPbX₃ NC synthesized via controlled approach and connect that with device operation efficiency and stability. In this study, we systematically investigated the synthetic approach to obtain room temperature stabilized mixed halide nanocrystals, focusing on the effect of the molar ratio between Cs source and Pb source on the optical performance in the mixed CsPbI₂Br perovskite thin films. The findings revealed the nanocrystal growth mechanism of the in situ processed perovskite layer which enabled us to fabricate bright and efficient red perovskite LEDs.   

Above-bandgap emission properties of CsPbBr3 as probed by fine-scale photoluminescence excitation spectroscopy

Halide perovskites are next-generation materials for solar cells and light emitting diodes. Despite rapid progress in applied perovskite technology, understanding of their basic properties is not complete yet. Especially, it is necessary to clarify the bandgap and the nature of the photoluminescence (PL) process (whether excitonic or electron-hole recombination) for the high performance of the devices. As a case study, we investigated unusual bandgap and recombination behaviors of CsPbBr₃ using PL excitation spectroscopy. Significant variation of both shape and intensity of the PL was observed when the excitation wavelength was tuned across the bandgap. Intriguingly, PL emission occurs above the bandgap, which arises presumably from radiative recombination at halide vacancies. Excitation power dependence of the PL shows that the power exponent continuously varies from 2 to 1 and to sublinear with increasing the laser intensity. The rate equation for pulsed excitation was modeled to account for this anomalous effect. Our model implies that the PL arises from electron-hole recombination at room temperature.   

Optical and transport properties of mixed halide all-inorganic quantum dot inkjet-printed films

Inorganic halide perovskite quantum dot inks may have great potential in applications related to the field of inkjet-printed photovoltaics. The photoactive inks are synthesized by a wet chemical reaction and are printed into thin films using a commercial inkjet printer. The inkjet-printed films were then characterized using optical absorption spectroscopy, photoluminescent spectroscopy, X-ray diffraction, X-ray photoemission spectroscopy, and electronic transport measurements. Characterization indicates that interlayer printing, alternating printed layers of CsPbBr₃ and CsPbI₃ , results in a halide exchange producing a high quality (220) CsPbBr_2.1I_0.9 quantum dot thin film. The repeatable properties of the inkjet-printed thin films suggest high quality, customizable, photovoltaic films are realizable with an inkjet printing method.