Origin of long lifetime of band-edge charge carriers in organic-inorganic lead iodide perovskites

Monitoring Electron-Phonon Interactions in Lead Halide Perovskites Using Time-Resolved THz Spectroscopy

Lead halide perovskite semiconductors have low-frequency phonon modes within the lead halide sublattice and thus are considered to be soft. The soft lattice is considered to be important in defining their interesting optoelectronic properties. Electron-phonon coupling governs hot-carrier relaxation, carrier mobilities, carrier lifetimes, among other important electronic characteristics. Directly observing the interplay between free charge carriers and phonons can provide details on how phonons impact these properties (e.g., exciton populations and other collective modes). Here, we observe a delicate interplay among carriers, phonons, and excitons in mixed-cation and mixed-halide perovskite films by simultaneously resolving the contribution of charge carriers and phonons in time-resolved terahertz photoconductivity spectra. We are able to observe directly the increase in phonon population during carrier cooling and discuss how thermal equilibrium populations of carriers and phonons modulate the carrier transport properties, as well as reduce the population of carriers within band tails. We are also able to observe directly the formation of free charge carriers when excitons interact with phonons and dissociate and to describe how free carriers and exciton populations exchange through phonon interactions. Finally, we also time-resolve how the carriers are screened via the Coulomb interaction at low and room temperatures. Our studies shed light on how charge carriers interact with the low-energy phonons and discuss implications.

Hybrid light emitting diodes based on stable, high brightness all-inorganic CsPbI3 perovskite nanocrystals and InGaN

Despite important advances in the synthesis of inorganic perovskite nanocrystals (NCs), the long-term instability and degradation of their quantum yield (QY) over time need to be addressed to enable the further development and exploitation of these nanomaterials. Here we report stable CsPbI3 perovskite NCs and their use in hybrid light emitting diodes (LEDs), which combine in one system the NCs and a blue GaN-based LED. Nanocrystals with improved morphological and optical properties are obtained by optimizing the post-synthesis replacement of oleic acid ligands with iminodibenzoic acid: the NCs have a long shelf-life (>2 months), stability under different environmental conditions, and a high QY, of up to 90%, in the visible spectral range. Ligand replacement enables the engineering of the morphological and optical properties of the NCs. Furthermore, the NCs can be used to coat the surface of a GaN-LED to realize a stable diode where they are excited by blue light from the LED under low current injection conditions, resulting in emissions at distinct wavelengths in the visible range. The high QY and fluorescence lifetime in the nanosecond range are key parameters for visible light communication, an emerging technology that requires high-performance visible light sources for secure, fast energy-efficient wireless transmission.

Stability of Cubic FAPbI3 from X-ray Diffraction, Anelastic, and Dielectric Measurements

Among the hybrid metal-organic perovskites for photovoltaic applications, FAPbI₃ (FAPI) has the best performance regarding efficiency and the worst regarding stability, even though the reports on its stability are highly contradictory. In particular, since at room temperature the cubic α phase, black and with high photovoltaic efficiency, is metastable against the yellow hexagonal δ phase, it is believed that α-FAPI spontaneously transforms into δ-FAPI within a relatively short time. We performed X-ray diffraction and thermogravimetric measurements on loose powder of FAPI, and present the first complete dielectric and anelastic spectra of compacted FAPI samples under various conditions. We found that α-FAPI is perfectly stable for at least 100 days, the duration of the experiments, unless extrinsic factors induce its degradation. In our tests, degradation was detected after exposure to humidity, strongly accelerated by grain boundaries and the presence of δ phase, but it was not noticeable on the loose powder kept in air under normal laboratory illumination. These findings have strong implications on the strategies for improving the stability of FAPI without diminishing its photovoltaic efficiency through modifications of its composition.

Communicating Two States in Perovskite Revealed by Time-Resolved Photoluminescence Spectroscopy

Organic-inorganic perovskite as a promising candidate for solar energy harvesting has attracted immense interest for its low-cost preparation and extremely high quantum efficiency. However, the fundamental understanding of the photophysics in perovskite remains elusive. In this work, we have revealed two distinct states in MAPbI₃ thin films at low temperature through time-resolved photoluminescence spectroscopy (TRPL). In particular, we observed a photo-induced carrier injection from the high energy (HE) state to the low energy (LE) state which has a longer lifetime. The strong interaction between the two states, evidenced by the injection kinetics, can be sensitively controlled through the excitation power. Understanding of the interacting two-states not only sheds light on the long PL lifetime in perovskite but also helps to understand the different behavior of perovskite in response to different excitation power. Further efforts in modifying the low energy state could significantly improve the quantum efficiency and lead to novel application in optoelectronics based on perovskite.

Diagrammatic Monte Carlo Approach to Angular Momentum in Quantum Many-Particle Systems

We introduce a diagrammatic Monte Carlo approach to angular momentum properties of quantum many-particle systems possessing a macroscopic number of degrees of freedom. The treatment is based on a diagrammatic expansion that merges the usual Feynman diagrams with the angular momentum diagrams known from atomic and nuclear structure theory, thereby incorporating the non-Abelian algebra inherent to quantum rotations. Our approach is applicable at arbitrary coupling, is free of systematic errors and of finite-size effects, and naturally provides access to the impurity Green function. We exemplify the technique by obtaining an all-coupling solution of the angulon model; however, the method is quite general and can be applied to a broad variety of systems in which particles exchange quantum angular momentum with their many-body environment.

Modifying Perovskite Films with Polyvinylpyrrolidone for Ambient-Air-Stable Highly Bendable Solar Cells

One major drawback that prevents the large-scale practical implementation of perovskites is their susceptibility to performance degradation in humid environments. Here, we achieved uniform, stable perovskite films within a polyvinylpyrrolidone (PVP) polymer frame via mild solution processing in ambient air with over 60% relative humidity. In addition to facilitating film formation, the hydrophobic PVP served to protect the perovskite grains from atmospheric moisture. Use of PVP, coupled with optimization of the deposition parameters, provided for compact, smooth, pinhole-free perovskite films that when incorporated into a photovoltaic device exhibited highly reproducible efficiencies in the range of up to 17%. In the absence of encapsulation, the devices exhibited stable performance characteristics during exposure to humid ambient air for 600 h. Furthermore, on flexible substrates, the 8 wt % PVP-perovskite samples provided for device efficiencies of ca. 15%. The devices retained ca. 73% of their efficiency after bending 1000 times with a bending radius of 0.5 cm.

Structural fluctuations cause spin-split states in tetragonal (CH3NH3)PbI3 as evidenced by the circular photogalvanic effect

Lead halide perovskites are used in thin-film solar cells, which owe their high efficiency to the long lifetimes of photocarriers. Various calculations find that a dynamical Rashba effect could significantly contribute to these long lifetimes. This effect is predicted to cause a spin splitting of the electronic bands of inversion-symmetric crystalline materials at finite temperatures, resulting in a slightly indirect band gap. Direct experimental evidence of the existence or the strength of the spin splitting is lacking. Here, we resonantly excite photocurrents in single crystalline ([Formula: see text])[Formula: see text] with circularly polarized light to clarify the existence of spin splittings in the band structure. We observe a circular photogalvanic effect, i.e., the photocurrent depends on the light helicity, in both orthorhombic and tetragonal ([Formula: see text])[Formula: see text] At room temperature, the effect peaks for excitation photon energies [Formula: see text] meV below the direct optical band gap. Temperature-dependent measurements reveal a sign change of the effect at the orthorhombic-tetragonal phase transition, indicating different microscopic origins in the two phases. Within the tetragonal phase, both [Formula: see text] and the amplitude of the circular photogalvanic effect increase with temperature. Our findings support a dynamical Rashba effect in this phase, i.e., a spin splitting caused by thermally induced structural fluctuations which break inversion symmetry.

Unraveling exciton-phonon coupling in individual FAPbI3 nanocrystals emitting near-infrared single photons

Formamidinium lead iodide (FAPbI3) exhibits the narrowest bandgap energy among lead halide perovskites, thus playing a pivotal role for the development of photovoltaics and near-infrared classical or quantum light sources. Here, we unveil the fundamental properties of FAPbI3 by spectroscopic investigations of nanocrystals of this material at the single-particle level. We show that these nanocrystals deliver near-infrared single photons suitable for quantum communication. Moreover, the low temperature photoluminescence spectra of FAPbI3 nanocrystals reveal the optical phonon modes responsible for the emission line broadening with temperature and a vanishing exciton-acoustic phonon interaction in these soft materials. The photoluminescence decays are governed by thermal mixing between fine structure states, with a two-optical phonon Raman scattering process. These results point to a strong Frölich interaction and to a phonon glass character that weakens the interactions of charge carriers with acoustic phonons and thus impacts their relaxation and mobility in these perovskites.

Impact of the Organic Cation on the Optoelectronic Properties of Formamidinium Lead Triiodide

Metal halide perovskites have proven to be excellent light-harvesting materials in photovoltaic devices whose efficiencies are rapidly improving. Here, we examine the temperature-dependent photon absorption, exciton binding energy, and band gap of FAPbI₃ (thin film) and find remarkably different behavior across the β-γ phase transition compared with MAPbI₃. While MAPbI₃ has shown abrupt changes in the band gap and exciton binding energy, values for FAPbI₃ vary smoothly over a range of 100-160 K in accordance with a more gradual transition. In addition, we find that the charge-carrier mobility in FAPbI₃ exhibits a clear T^-0.5 trend with temperature, in excellent agreement with theoretical predictions that assume electron-phonon interactions to be governed by the Fröhlich mechanism but in contrast to the T^-1.5 dependence previously observed for MAPbI₃. Finally, we directly observe intraexcitonic transitions in FAPbI₃ at low temperature, from which we determine a low exciton binding energy of only 5.3 meV at 10 K.

Ferroelectric and Piezoelectric Effects on the Optical Process in Advanced Materials and Devices

Piezoelectric and ferroelectric materials have shown great potential for control of the optical process in emerging materials. There are three ways for them to impact on the optical process in various materials. They can act as external perturbations, such as ferroelectric gating and piezoelectric strain, to tune the optical properties of the materials and devices. Second, ferroelectricity and piezoelectricity as innate attributes may exist in some optoelectronic materials, which can couple with other functional features (e.g., semiconductor transport, photoexcitation, and photovoltaics) in the materials giving rise to unprecedented device characteristics. The last way is artificially introducing optical functionalities into ferroelectric and piezoelectric materials and devices, which provides an opportunity for investigating the intriguing interplay between the parameters (e.g., electric field, temperature, and strain) and the introduced optical properties. Here, the tuning strategies, fundamental mechanisms, and recent progress in ferroelectric and piezoelectric effects modulating the optical properties of a wide spectrum of materials, including lanthanide-doped phosphors, quantum dots, 2D materials, wurtzite-type semiconductors, and hybrid perovskites, are presented. Finally, the future outlook and challenges of this exciting field are suggested.

Real-Time Observation of Order-Disorder Transformation of Organic Cations Induced Phase Transition and Anomalous Photoluminescence in Hybrid Perovskites

A fundamental understanding of the interplay between the microscopic structure and macroscopic optoelectronic properties of organic-inorganic hybrid perovskite materials is essential to design new materials and improve device performance. However, how exactly the organic cations affect the structural phase transition and optoelectronic properties of the materials is not well understood. Here, real-time, in situ temperature-dependent neutron/X-ray diffraction and photoluminescence (PL) measurements reveal a transformation of the organic cation CH₃ NH₃⁺ from order to disorder with increasing temperature in CH₃ NH₃ PbBr₃ perovskites. The molecular-level order-to-disorder transformation of CH₃ NH₃⁺ not only leads to an anomalous increase in PL intensity, but also results in a multidomain to single-domain structural transition. This discovery establishes the important role that organic cation ordering has in dictating structural order and anomalous optoelectronic phenomenon in hybrid perovskites.

Aziridinium Lead Iodide: A Stable, Low-Band-Gap Hybrid Halide Perovskite for Photovoltaics

The low ionization energy of an A site molecule is a very important factor, which determines the thermodynamical stability of APbI₃ hybrid halide perovskites, while the size of the molecule governs the stable phase at room temperature and, eventually, the band gap. It is challenging to achieve both a low ionization energy and the reasonable size for the PbI₃ cage to circumvent the stability issue inherent to hybrid halide perovskites. Here we propose a new three-membered charged ring radical, which demonstrates a low ionization energy that renders a good stability for its corresponding perovskite and a reasonable cation size that translates into a suitable band gap for the photovoltaic application. We use ab initio calculations to evaluate a polymorphism of the crystal structure of the proposed hybrid halide perovskite, its stability, and electronic properties in comparison with the mainstream perovskites, such as the methylammonium and formamidinium lead iodide.

Unique Optical Properties of Methylammonium Lead Iodide Nanocrystals Below the Bulk Tetragonal-Orthorhombic Phase Transition

Methylammonium (MA) and formamidinium (FA) lead halides are widely studied for their potential as low-cost, high-performance optoelectronic materials. Here, we present measurements of visible and IR absorption, steady state, and time-resolved photoluminescence from 300 K to cryogenic temperatures. Whereas FAPbI₃ nanocrystals (NCs) are found to behave in a very similar manner to reported bulk behavior, colloidal nanocrystals of MAPbI₃ show a departure from the low-temperature optical behavior of the bulk material. Using photoluminescence, visible, and infrared absorption measurements, we demonstrate that unlike single crystals and polycrystalline films NCs of MAPbI₃ do not undergo optical changes associated with the bulk tetragonal-to-orthorhombic phase transition, which occurs near 160 K. We find no evidence of frozen organic cation rotation to as low as 80 K or altered exciton binding energy to as low as 3 K in MAPbI₃ NCs. Similar results are obtained in MAPbI₃ NCs ranging from 20 to over 100 nm and in morphologies including cubes and plates. Colloidal MAPbI₃ NCs therefore offer a window into the properties of the solar-relevant, room-temperature phase of MAPbI₃ at temperatures inaccessible with single crystals or polycrystalline samples. Exploiting this phenomenon, these measurements reveal the existence of an optically passive photoexcited state close to the band edge and persistent slow Auger recombination at low temperature.

Revealing the Chemistry between Band Gap and Binding Energy for Lead-/Tin-Based Trihalide Perovskite Solar Cell Semiconductors

A relationship between reported experimental band gaps (solid) and DFT-calculated binding energies (gas) is established, for the first time, for each of the four ten-membered lead (or tin) trihalide perovskite solar cell semiconductor series examined in this study, including CH₃ NH₃ PbY₃ , CsPbY₃ , CH₃ NH₃ SnY₃ and CsSnY₃ (Y=I_(3-x) Br_x=1-3 , I_(3-x) Cl_x=1-3 , Br_(3-x) Cl _x=1-3 , and IBrCl). The relationship unequivocally provides a new dimension for the fundamental understanding of the optoelectronic features of solid-state solar cell thin films by using the 0 K gas-phase energetics of the corresponding molecular building blocks.

Polaronic Charge Carrier-Lattice Interactions in Lead Halide Perovskites

Almost ten years after the renaissance of the popular perovskite-type semiconductors based on lead salts with the general formula AMX₃ (A=organic or inorganic cation; M=divalent metal; X=halide), many facets of photophysics continue to puzzle researchers. In this Minireview, light is shed on the low mobilities of charge carriers in lead halide perovskites with special focus on the lattice properties at non-zero temperature. The polar and soft lattice leads to pronounced electron-phonon coupling, limiting carrier mobility and retarding recombination. We propose that the proper picture of excited charge carriers at temperature ranges that are relevant for device operations is that of a polaron, with Fröhlich coupling constants between 1<α<3. Under the aspect of light-emitting diode application, APbX₃ perovskite show moderate second order (bimolecular) recombination rates and high third-order (Auger) rate constants. It has become apparent that this is a direct consequence of the anisotropic polar A-site cation in organic-inorganic hybrid perovskites and might be alleviated by replacing the organic moiety with an isotropic cation.