Triiodide Thin Films Research Articles

Mechanism of Hot-Carrier Photoluminescence in Sn-Based Perovskites.

Metal halide perovskites have shown exceptionally slow hot-carrier cooling, which has been attributed to various physical mechanisms without reaching a consensus. Here, experiment and theory are combined to unveil the carrier cooling process in formamidinium (FA) and caesium (Cs) tin triiodide thin films. Through impulsive vibrational spectroscopy and molecular dynamics, much shorter phonon dephasing times of the hybrid perovskite, which accounts for the larger blueshift in the photoluminescence seen at high excitation density for FASnI3 compared to CsSnI3 is reported. Density functional theory investigations reveal that the largest contribution to the blueshift is accounted by a giant, dynamic band-filling effect in Sn-based perovskites, which in turn can explain the cooling disparity with the Pb-based counterparts. Several years after the first experimental observations, here a deeper understanding of the cooling mechanism of these materials is provided. Design principles for hot-carrier materials, which may be useful for future implementations of hot-carrier solar cells are further provided.

Cation Influence on Hot-Carrier Relaxation in Tin Triiodide Perovskite Thin Films.

Slow hot-carrier cooling may potentially allow overcoming the maximum achievable power conversion efficiency of single-junction solar cells. For formamidinium tin triiodide, an exceptional slow cooling time of a few nanoseconds was reported. However, a systematic study of the cation influence, as is present for lead compounds, is lacking. Here, we report the first comparative study on formamidinium, methylammonium, and cesium tin triiodide thin films. By investigating their photoluminescence, we observe a considerable shift of the emission peak to high energy with the increase of the excited-state population, which is more prominent in the case of the two hybrid organic-inorganic perovskites (∼45 meV vs ∼15 meV at 9 × 1017 cm-3 carrier density). The hot-carrier photoluminescence of the three tin compositions decays with a 0.6-2.8 ns time constant with slower cooling observed for the two hybrids, further indicating their importance.

Quantitative Predictions of Moisture-Driven Photoemission Dynamics in Metal Halide Perovskites via Machine Learning.

Metal halide perovskite (MHP) photovoltaics may become a viable alternative to standard Si-based technologies, but the current lack of long-term stability precludes their commercial adoption. Exposure to standard operational stressors (light, temperature, bias, oxygen, and water) often instigate optical and electronic dynamics, calling for a systematic investigation into MHP photophysical processes and the development of quantitative models for their prediction. We resolve the moisture-driven light emission dynamics for both methylammonium lead tribromide and triiodide thin films as a function of relative humidity (rH). With the humidity and photoluminescence time series, we train recurrent neural networks and establish their ability to quantitatively predict the path of future light emission with 18% error over 4 h. Together, our in situ rH-PL measurements and machine learning forecasting models provide a framework for the rational design of future stable perovskite devices and, thus, a faster transition toward commercial applications.

Photosensitive antimony triiodide thin films by rapid iodization of chemically deposited antimony sulfide

In this work, we report the synthesis of antimony triiodide (SbI3) thin films via a simple and facile process of iodizing chemically deposited antimony sulfide (Sb2S3) thin films. Density functional theory (DFT) calculations were employed to determine the electronic band structure of SbI3. The crystal structure, morphology, composition, and optoelectronic properties of SbI3 films formed at different conditions were investigated. The structural and morphological analyses revealed hexagonal SbI3 thin films with compact and granular surface morphology. EDS analysis showed a nearly stoichiometric composition of the SbI3 phase. The experimentally estimated direct bandgaps were in the range of 2.3−2.1 eV in correlation with the DFT calculations. Further, stable photodetector devices were fabricated using the SbI3 films. The devices displayed good reproducibility and a significant response to the visible light. This novel strategy for the synthesis opens a new opportunity for this metal iodide based optoelectronic and photovoltaic devices.

Echoes from quantum confinement.

The discovery of intrinsic quantum confinement effects in the form of oscillations in the optical absorption of formamidinium lead triiodide thin films is a vivid example of the surprising physical properties of these hybrid organic–inorganic materials.

Intrinsic quantum confinement in formamidinium lead triiodide perovskite.

Understanding the electronic energy landscape in metal halide perovskites is essential for further improvements in their promising performance in thin-film photovoltaics. Here, we uncover the presence of above-bandgap oscillatory features in the absorption spectra of formamidinium lead triiodide thin films. We attribute these discrete features to intrinsically occurring quantum confinement effects, for which the related energies change with temperature according to the inverse square of the intrinsic lattice parameter, and with peak index in a quadratic manner. By determining the threshold film thickness at which the amplitude of the peaks is appreciably decreased, and through ab initio simulations of the absorption features, we estimate the length scale of confinement to be 10-20 nm. Such absorption peaks present a new and intriguing quantum electronic phenomenon in a nominally bulk semiconductor, offering intrinsic nanoscale optoelectronic properties without necessitating cumbersome additional processing steps.

Mechanisms of exceptional grain growth and stability in formamidinium lead triiodide thin films for perovskite solar cells

Pure formamidinium lead triiodide (α-FAPbI3) organic-inorganic halide perovskite (OIHP) semiconductor is very attractive for use as light absorber in the new thin-film perovskite solar cells (PSCs) technology. This is primarily because of its superior thermal stability, more suitable bandgap, and compositional simplicity. However, the existence of the photo-inactive non-perovskite δ-FAPbI3 polymorph (‘yellow’ phase) is a major hurdle in the path towards the development of α-FAPbI3-based PSCs. Also, there is general consensus that the fine-grained nature of OIHP thin films is detrimental to the environmental stability and performance of the resulting PSCs. In this context, here we take advantage of the polymorphism in FAPbI3, and use solvent-vapor-assisted δ-to-α phase transformation to induce exceptional grain coarsening (up to 50-fold) in 0.3-μm thickness FAPbI3 thin films, resulting in an unprecedented average grain size of up to ~9 μm. The underlying mechanisms are elucidated based on the results from a combination of some key experiments, which involve studying systematically the effects of time, temperature, initial grain size, and solvent polarity index (PI). The ultra-coarse-grained α-FAPbI3 thin films show dramatically improved environmental stability over their medium-grained counterparts, which is explained based on grain-boundary density arguments. PSCs made using the ultra-coarse-grained α-FAPbI3 thin films have improved photovoltaic (PV) performance, but it is somewhat modest. This is attributed to the underestimation of the effective grain size relevant to photocarrier dynamics.

Amine additive reactions induced by the soft Lewis acidity of Pb2+ in halide perovskites. Part II: impacts of amido Pb impurities in methylammonium lead triiodide thin films

The beneficial and detrimental effects of amido Pb impurities incorporated in methylammonium lead triiodide thin films are characterized by photoemission spectroscopy.

Electronic structures and chemical states of methylammonium lead triiodide thin films and the impact of annealing and moisture exposure

Methylammonium lead triiodide (CH3NH3PbI3) is the fundamental material used in perovskite solar cells, and its electronic properties have, therefore, attracted a great deal of attention as a potential key to highly efficient solar cell performance. However, the deterioration of perovskite solar cells when exposed to high temperature and humidity remains a serious obstacle to the material's use, and the clarification of the degradation mechanisms has been keenly anticipated. In this study, the valence electronic structures and depth-dependence of the chemical states of CH3NH3PbI3 thin films are investigated using ultraviolet photoelectron spectroscopy and excitation energy dependent X-ray photoelectron spectroscopy. Additionally, the effects of high temperature and a moisture rich atmosphere on the CH3NH3PbI3 thin films are examined. It is confirmed that the high temperature and moist atmosphere facilitate the oxidation of CH3NH3PbI3, whereas the Pb:I stoichiometry of the CH3NH3PbI3 thin films is found to be preserved at its original ratio (1:3) after thermal annealing and exposure to a moist atmosphere.

Impact of grain boundaries on efficiency and stability of organic-inorganic trihalide perovskites

Organic–inorganic perovskite solar cells have attracted tremendous attention because of their remarkably high power conversion efficiencies. To further improve device performance, it is imperative to obtain fundamental understandings on the photo-response and long-term stability down to the microscopic level. Here, we report the quantitative nanoscale photoconductivity imaging on two methylammonium lead triiodide thin films with different efficiencies by light-stimulated microwave impedance microscopy. The microwave signals are largely uniform across grains and grain boundaries, suggesting that microstructures do not lead to strong spatial variations of the intrinsic photo-response. In contrast, the measured photoconductivity and lifetime are strongly affected by bulk properties such as the sample crystallinity. As visualized by the spatial evolution of local photoconductivity, the degradation process begins with the disintegration of grains rather than nucleation and propagation from visible boundaries between grains. Our findings provide insights to improve the electro-optical properties of perovskite thin films towards large-scale commercialization.

Optical Analysis of Ag-NPs Containing Methyl Ammonium Lead Tri-Iodide Thin Films

Methyl ammonium lead tri-iodide hybrid thin films were grown using solution technique. They were doped with silver nano-particles at different concentrations at concentrations of 0.05, 0.06, 0.07, 0.08, and 0.09 mM. Their reflectance and transmittance were recorded in the wavelength range 300–900 using UV-Vis double - beam spectrophotometer. Using these measurements, other optical parameters were simulated using scout software. The effect of silver nanoparticles was investigated. Results revealed that the thin films had highest transmittance of about 79 % as their band gap varied from 1.921–1.832 eV. Electrical conductivity varied from 1.4–1.6 × 10 5 S cm –1 while optical conductivity varied in the range of 0.3–0.6 × 10 10 sec -1 . They had a significantly low refractive index, suitable for optical applications within the range of 1.6–1.8. The extinction coefficient varied in the range as 1.0–1.7 × 10 -5 while the absorption coefficient varied varies in the range of 2.1-4.2 cm - 1 . It was concluded that the thin films were suitable for photonic device applications