Charge-Carrier Dynamics in 2D Hybrid Metal-Halide Perovskites.

Incorporating extended pi‐conjugated organic cations in layered lead halide perovskites is a recent trend promising to merge the fields of organic semiconductors and lead halide perovskites. Herein, we integrate benzodithiophene (BDT) into Ruddlesden–Popper (RP) layered and quasi‐layered lead iodide thin films (with methylammonium, MA) of the form (BDT)2MAn−1PbnI3n+1. The importance of tuning the ligand chemical structure is shown as an alkyl chain length of at least six carbon atoms is required to form a photoactive RP (n=1) phase. With N=20 or 100, as prepared in the precursor solution following the formula (BDT)2MAN−1PbNI3N+1, the performance and stability of devices surpassed those with phenylethylammonium (PEA). For N=100, the BDT cation gave a power conversion efficiency of up to 14.7 % vs. 13.7 % with PEA. Transient photocurrent, UV photoelectron spectroscopy, and Fourier transform infrared spectroscopy point to improved charge transport in the device active layer and additional electronic states close to the valence band, suggesting the formation of a Lewis adduct between the BDT and surface iodide vacancies.

Understanding carrier recombination in semiconductors is a critical component when developing practical applications. Here we measure and compare the monomolecular, bimolecular, and trimolecular (Auger) recombination rate constants of CH3NH3PbBr3 and CH3NH3PbI3. The monomolecular and bimolecular recombination rate constants for both samples are limited by trap-assisted recombination. The bimolecular recombination rate constant for CH3NH3PbBr3 is ∼3.3 times larger than that for CH3NH3PbI3 and both are in line with that found for radiative recombination in other direct-gap semiconductors. The Auger recombination rate constant is 4 times larger in lead-bromide-based perovskite compared with lead-iodide-based perovskite and does not follow the reduced Auger rate when the bandgap increases. The increased Auger recombination rate, which is enhanced by Coulomb interactions, can be ascribed to the larger exciton binding energy, ∼40 meV, in CH3NH3PbBr3 compared with ∼13 meV in CH3NH3PbI3.

Hybrid\nmetal-halide perovskites have potential as cost-effective\ngain media for laser technology because of their superior optoelectronic\nproperties. Although lead-halide perovskites have been most widely\nstudied to date, tin-based perovskites have been proposed as a less\ntoxic alternative. In this Letter, we show that amplified spontaneous\nemission (ASE) in formamidinium tin triiodide (FASnI<sub>3</sub>)\nthin films is supported by an observed radiative monomolecular charge\nrecombination pathway deriving from its unintentional doping. Such\na radiative component will be active even at the lowest charge-carrier\ndensities, opening a pathway for ultralow light-emission thresholds.\nUsing time-resolved THz photoconductivity analysis, we further show\nthat the material has an unprecedentedly high charge-carrier mobility\nof 22 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> favoring\nefficient transport. In addition, FASnI<sub>3</sub> exhibits strong\nradiative bimolecular recombination and Auger rates that are over\nan order of magnitude lower than for lead-halide perovskites. In combination,\nthese properties reveal that tin-halide perovskites are highly suited\nto light-emitting devices.

Hybrid metal-halide perovskites have potential as cost-effective gain media for laser technology because of their superior optoelectronic properties. Although lead-halide perovskites have been most widely studied to date, tin-based perovskites have been proposed as a less toxic alternative. In this Letter, we show that amplified spontaneous emission (ASE) in formamidinium tin triiodide (FASnI3) thin films is supported by an observed radiative monomolecular charge recombination pathway deriving from its unintentional doping. Such a radiative component will be active even at the lowest charge-carrier densities, opening a pathway for ultralow light-emission thresholds. Using time-resolved THz photoconductivity analysis, we further show that the material has an unprecedentedly high charge-carrier mobility of 22 cm2 V-1 s-1 favoring efficient transport. In addition, FASnI3 exhibits strong radiative bimolecular recombination and Auger rates that are over an order of magnitude lower than for lead-halide perovskites. In combination, these properties reveal that tin-halide perovskites are highly suited to light-emitting devices.

Hybrid organic/inorganic lead iodide perovskites of the formula APbI3, where A is a molecular cation such as methylammonium, exhibit remarkably slow photoinduced charge carrier recombination rates, for reasons that remain uncertain. Prevalent hypotheses credit this behavior to the unique dipolar nature of the molecular cation. Herein, transient terahertz spectroscopy is applied to solution-processed, all-inorganic, perovskite-phase cesium lead iodide (CsPbI3) thin films, which lack such a dipole. The recombination kinetics are studied as a function of the initial photoinduced carrier concentration and the wavelength of excitation. A kinetic model combining diffusion and recombination is fit to the data, from which the rate constants are determined, revealing a bimolecular recombination rate of 10–10 cm3 s–1, comparable to high-quality, single-crystal, direct-gap semiconductors. This rate, as well as a charge carrier mobility > 30 cm2 V–1 s–1 measured herein for CsPbI3, are similar to values reported for the hybrid perovskites, strongly suggesting that the organic cation does not confer a fundamental advantage.

Organic-inorganic hybrid perovskites have attracted immense attention primarily due to its outstanding photovoltaic and light emission properties. Specifically, certified power conversion efficiencies exceeding 20 % have been demonstrated in perovskite solar cells – attributed to their large absorption coefficients and long, balanced, ambipolar diffusion lengths, large grains and its unique defect tolerance. Careful morphological control is needed to form dense, uniform films essential for high performance devices. However, the resultant fundamental optoelectronic properties of such process controls are not well understood. In addition, the higher excited states that can aid in breaking the detailed balance limit is also unexplored. This thesis reports on a series of studies using ultrafast optical spectroscopy on the archetypal bulk 3D perovskite, MAPI, to provide insights on the photophysics and recombination dynamics of this class of materials. Here, process controls in fabricating perovskite thin films refers to treatments to the film, in the form of solvent engineering and additives, and to the substrate for hydrophilicity. Our findings reveal that solvent engineering of MAPI, i.e., dripping of toluene during spincoating which is key to improving film morphologies and subsequent solar cell efficiencies, had resulted in increased trap densities. We attribute this anomalous behaviour to an interplay of factors where the improved film morphology had also resulted in the lengthening of the carrier recombination lifetimes. Interestingly, depending on the combination of treatments, the charge extraction interface can go from injecting to non-injecting. Exceptionally low carrier recombination rates in lead halide perovskites is crucial for its high performance. By careful optimisation of the additive concentration, improvements to device performance were observed when H2O was added to MAPI as an additive. Trace amounts of H2O passivates the trap states leading to reduced recombination rates, markedly improved carrier lifetimes and Jsc. At the optimal additive concentration of 1 vol% H2O, higher order carrier recombination is suppressed, and the greatly reduced monomolecular and bimolecular recombination rates was correlated with an increase in power conversion efficiencies. Process controls have all but ensured the unprecedented growth in record efficiencies of perovskite solar cells. However, it slows as it approaches the detailed balance limit of solar cells. One way to surpass this limit is to exploit the concept of hot carriers -- photoexcited carriers in higher excited states. A technique to directly probe these states is presented, and through this, the broad photoinduced absorption band was revealed to be attributed to the promotion of photoexcited carriers to higher energy states. However, the observed sub-picosecond thermalisation times of these higher excited states may prove difficult for hot carrier extraction. Importantly, our results underscores the importance of judiciously choosing process controls to optimise optoelectronic properties of perovskite devices.

Optical and structural properties of Zn-doped lead iodide thin films

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH3NH3PbI3 are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50 and 533 nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 × 10-10 cm3s-1 is common to all samples irrespective of film thickness. Hence, we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.

We monitored the occupancy of a functionally important non-coordinated water molecule in the distal heme pocket of sperm whale myoglobin over the pH range 4.3-9.4. Water occupancy was assessed by using time-resolved spectroscopy to detect the perturbation of the heme visible band absorption spectrum caused by water entry after CO photodissociation ( Goldbeck, R. A., Bhaskaran, S., Ortega, C., Mendoza, J. L., Olson, J. S., Soman, J., Kliger, D. S., and Esquerra, R. M. (2006) Proc. Natl. Acad. Sci. U. S. A. 103, 1254-1259 ). We found that the water occupancy observed during the time interval between ligand photolysis and diffusive recombination decreased by nearly 20% as the pH was lowered below 6. This decrease accounted for most of the concomitant increase in the observed CO bimolecular recombination rate constant, as the lower water occupancy presented a smaller kinetic barrier to CO entry into the pocket at lower pH. These results were consistent with a model in which the distal histidine, which stabilizes the water molecule within the distal pocket by accepting a hydrogen bond, tends to swing out of the pocket upon protonation and destabilize the water occupancy at low pH. Extrapolation of this model to lower pH suggests that the additional increase in ligand association rate constant observed previously in stopped-flow studies at pH 3 may also be due in part to reduced distal water occupancy concomitant with further His64 protonation and coupled protein conformational change.

Moisture degradation of halide perovskites is the Achilles heel of perovskite solar cells. A surprising revelation in 2014 about the beneficial effects of controlled humidity in enhancing device efficiencies overthrew established paradigms on perovskite solar cell fabrication. Despite the extensive studies on water additives in perovskite solar cell processing that followed, detailed understanding of the role of water from the photophysical perspective remains lacking; specifically, the interplay between the induced morphological effects and the intrinsic recombination pathways. Through ultrafast optical spectroscopy, we show that both the monomolecular and bimolecular recombination rate constants decrease by approximately 1 order with the addition of an optimal 1% H2O by volume in CH3NH3PbI3 as compared to the reference (without the H2O additive). Correspondingly, the trap density reduces from 4.8 × 1017 cm–3 (reference) to 3.2 × 1017 cm–3 with 1% H2O. We obtained an efficiency of 12.3% for the champion inverted CH3NH3PbI3 perovskite solar cell (1% H2O additive) as compared to the 10% efficiency for the reference cell. Increasing the H2O content further is deleterious for the device. Trace amounts of H2O afford the benefits of surface trap passivation and suppression of trap-mediated recombination, whereas higher concentrations result in a preferential dissolution of methylammonium iodide during fabrication that increases the trap density (MA vacancies). Importantly, our study reveals the effects of trace H2O additives on the photophysical properties of CH3NH3PbI3 films. DOI: 10.1021/acsami.9b00793

Thermally processed lead iodide (PbI2) thin films were prepared by the vacuum evaporation method in a constant ambient. Measured thickness of the film was verified analytically from the optical transmittance data in a wavelength range between 300 and 1600 nm. From the Tauc relation for the non‐direct inter band transition, the optical band gap of the film was found to be 2.58 eV for film thickness 300 nm. X‐ray diffraction analysis confirmed that PbI2 films are polycrystalline, having hexagonal structure. The low fluctuation in Urbach energy indicates that the grain size is quite small. The present findings are in agreement with the other results. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spray pyrolysis was used for the deposition of lead iodide (PbI2) thin films using N.N-dimethylformamide (DMF) as an alternative solvent under varying deposition parameters. Final thickness of 60 μm was obtained for a total deposition time of 2.5 hours. The films were characterized mainly by using Raman and photoluminescence, but additional techniques such as X-ray diffraction, scanning electron microscopy and dark conductivity as a function of temperature were also employed. Thick PbI2 films deposited by spray pyrolysis using DMF as a solvent are promising to be used in medical systems as X-ray imaging.

This study describes the metastability properties of lead iodide (PbI2) thin films which were deposited by thermal chemical vapor deposition (CVD) and spin coating on ultrasonically and chemically cleaned microscope glass substrates. Morphological differences due to deposition techniques were defined by scanning electron microscopy (SEM). Deposited PbI2 thin films have been exposed to laboratory atmosphere ambient, vacuum, and enhanced deionize water vapor (EDIWV) in order to define metastability changes. Samples metastability changes have been investigated by current–voltage (I–V), time-dependent conductivity, and annealing cycle method depending on atmospheric conditions. Activation energy of PbI2 thin films was calculated from temperature-dependent dark conductivity. Based on results gathered from this work, degradation of PbI2 dependent on atmospheric condition and surface morphology with the aspect of meta/instability view has been tried to analyzed.

The stabilization of black-phase formamidinium lead iodide (α-FAPbI3) perovskite under various environmental conditions is considered necessary for solar cells. However, challenges remain regarding the temperature sensitivity of α-FAPbI3 and the requirements for strict humidity control in its processing. Here we report the synthesis of stable α-FAPbI3, regardless of humidity and temperature, based on a vertically aligned lead iodide thin film grown from an ionic liquid, methylamine formate. The vertically grown structure has numerous nanometer-scale ion channels that facilitate the permeation of formamidinium iodide into the lead iodide thin films for fast and robust transformation to α-FAPbI3 A solar cell with a power-conversion efficiency of 24.1% was achieved. The unencapsulated cells retain 80 and 90% of their initial efficiencies for 500 hours at 85°C and continuous light stress, respectively.

Seemingly contradictory reports on polar domains and their origin have surrounded the controversial discussion about the ferroelectricity of the methyl ammonium lead iodide (MAPbI3) thin films that are commonly employed in perovskite solar cells. In this work, microscopic modulations of the polar domain patterns upon application of an electric poling field are correlated with macroscopic changes to the currents through the MAPbI3 layer. Piezoresponse force microscopy is used to monitor the widening, narrowing, generation or extinction of polar domains, as well as shifts of the domain walls at room temperature under an in‐plane electric poling field that is applied between two laterally organized electrodes. This poling leads to a net polarization of individual grains and the thin film itself. Macroscopically, this net polarization results in a persistent shift of the diode characteristics that is measured across the channel between the electrodes. Both the modulation of the polar domains upon electric poling and the concurrent persistent shift of the electric currents through the device are the unambiguous hallmarks of ferroelectricity, which demonstrate that MAPbI3 is a ferroelectric semiconductor.