Femtosecond Exciton and Carrier Relaxation Dynamics of Two-Dimensional (2D) and Quasi-2D Tin Perovskites.

The dynamics of exciton and free-carrier relaxation of low-dimensional tin iodide perovskites, BA2FAn-1SnnI3n+1, where n = 1 (N1), 2 (N2), 5 (N5), and 10 (N10), were investigated with femtosecond transient absorption spectra (TAS). The absorption and photoluminescence spectra of N1 and N2 show exciton characteristics due to quantum confinement, whereas N5 and N10 display a free-carrier nature, the same as for bulk three-dimensional (3D) films. The TAS profiles were fitted according to a global kinetic model with three time coefficients representing the interactions of biexcitons, trions, and excitons for N1 and N2 and hot carriers, cold carriers, and shallow trap carriers for N5 and N10. The carrier relaxation dynamics of N5 and N10 were similar to those of 3D FASnI3 except for the absence of surface recombination in the deep-trap states due to passivation of the grain surfaces by the long alkyl chain for these quasi-2D samples (N5/N10 vs 3D).

Slow Passivation and Inverted Hysteresis for Hybrid Tin Perovskite Solar Cells Attaining 13.5% via Sequential Deposition.

Herein, we report a sequential deposition procedure to passivate the surface of a hybrid mixed cationic tin perovskite (E1G20) with phenylhydrazinium thiocyanate (PHSCN) dissolved in trifluoroethanol solvent. The photoluminescence lifetime of the PHSCN film was enhanced by a factor of 6, while the charge-extraction rate from perovskite to C60 layer was enhanced by a factor of 2.5, in comparison to those of the E1G20 film. A slow surface passivation was observed; the performance of the PHSCN device improved upon increasing the storage period to attain an efficiency of 13.5% for a current-voltage scan in the forward bias direction. An inverted effect of hysteresis was observed in that the efficiency of the forward scan was greater than that of the reverse scan. An ion-migration model as a result of the effect of the phenylhydrazinium surface passivation is proposed to account for the observed phenomena. The device was stable upon shelf storage in a glovebox for 3000 h.

Mixing of Azetidinium in Formamidinium Tin Triiodide Perovskite Solar Cells for Enhanced Photovoltaic Performance and High Stability in Air.

Overcoming the issue of the stability of tin-based perovskites is a major challenge for the commercial development of lead-free perovskite solar cells. To attack this problem, a new organic cation, azetidinium (AZ), is incorporated into the crystal structure of formamidinium tin triiodide (FASnI3 ) to form the mixed-cation perovskite AZx FA1-x SnI3 . As AZ has a similar size to FA but a larger dipole moment, hybrid AZx FA1-x SnI3 films exhibit variation in optical and electronic properties on increasing the proportion of AZ. Trifluoromethylbenzene (CF3 C6 H5 ) serves as antisolvent to fabricate smooth and uniform perovskite films for the devices with an inverted planar heterojunction structure. The device performance is optimized to produce the greatest efficiency at x=0.15 (AZ15), for which a power conversion efficiency of 9.6 % is obtained when the unencapsulated AZ15 device is stored in air for 100 h. Moreover, the device retains 90 % of its initial efficiency for over 15 days. The significant performance and stability of this device reveal that the concept of mixed cations is a promising approach to stabilize tin-based perovskite solar cells for future commercialization.

Environmentally Compatible Lead-Free Perovskite Solar Cells and Their Potential as Light Harvesters in Energy Storage Systems.

Next-generation renewable energy sources and perovskite solar cells have revolutionised photovoltaics research and the photovoltaic industry. However, the presence of toxic lead in perovskite solar cells hampers their commercialisation. Lead-free tin-based perovskite solar cells are a potential alternative solution to this problem; however, numerous technological issues must be addressed before the efficiency and stability of tin-based perovskite solar cells can match those of lead-based perovskite solar cells. This report summarizes the development of lead-free tin-based perovskite solar cells from their conception to the most recent improvements. Further, the methods by which the issue of the oxidation of tin perovskites has been resolved, thereby enhancing the device performance and stability, are discussed in chronological order. In addition, the potential of lead-free tin-based perovskite solar cells in energy storage systems, that is, when they are integrated with batteries, is examined. Finally, we propose a research direction for tin-based perovskite solar cells in the context of battery applications.

A Direct Mapping Approach to Understand Carrier Relaxation Dynamics in Varied Regions of a Polycrystalline Perovskite Film.

We developed a direct mapping approach to overlay the image of a polycrystalline perovskite film obtained from the transient absorption microscope (TAM) with that from the scanning electron microscope (SEM). By mapping these imaging data pixel by pixel, we are able to observe the relaxation dynamics of the photo-generated charge carriers on varied regions of the film. The carrier relaxation dynamics contain a dominated single-exponential decay component owing to the recombination of charge carriers. The lifetime distribution of charge recombination shows a bimodal feature, for which the rapid and slow distributions are assigned as free and trapped carriers, respectively. The charge recombination was slower in the grain boundary (GB) region than in the grain interior (GI) region. The small grains have longer lifetimes than the large grains for the crystal size smaller than 500 nm. Therefore, GB with retarded charge recombination might play a positive role in a perovskite solar cell.

Femtosecond Transient Absorption Spectra and Dynamics of Carrier Relaxation of Tin Perovskites in the Absence and Presence of Additives.

The effects of additives SnF2 (10%) and EDAI2 (1%) on the dynamics of carrier relaxation of formamidinium tin triiodide (FASnI3) perovskite were studied using femtosecond transient absorption spectra (TAS) with excitation at 600 and 870 nm. The TAS were analyzed according to a parallel sequential kinetic model with a global fit through singular-value decomposition. For excitation at 600 nm, two relaxation paths were found: one involved hot and cold carriers in the bulk state undergoing shallow bulk-defect-mediated charge recombination; the other involved trap carriers in the surface state undergoing deep surface-defect-mediated charge recombination. For excitation at 870 nm, only cold carriers were subjected to the bulk-state relaxation channel. Our spectral results indicate significant effects of the additives on retarding charge recombination in both bulk and surface states as well as decreasing the bandgap renormalization energy, the bandwidth of the photobleaching (PB) band, and the Stokes shift between the PB and photoluminescence bands, explaining how the device performance of FASnI3 solar cells became enhanced in the presence of SnF2 and EDAI2.

Inter- and Intragrain Inhomogeneity in 2D Perovskite Thin Films Revealed by Relative Grain Orientation Imaging Using Low-Frequency Polarized Raman Microspectroscopy.

Two-dimensional (2D) organic-inorganic hybrid lead halide perovskites make up an emerging class of semiconductor materials for optoelectronic applications such as solar cells. The grain structure of polycrystalline 2D perovskites is one of the key factors that dictate their functionality in the devices, but currently available methods for in situ, chemically specific characterization of 2D perovskite grains are scarce. Here we show that ultra-low-frequency polarized Raman microspectroscopy is a facile yet powerful tool for visualizing relative grain orientations within 2D perovskite thin films. We demonstrate this method on the simplest 2D perovskite, (CH3(CH2)3NH3)2PbI4. Hierarchical clustering and detailed band decomposition analysis of the low-frequency polarized Raman imaging data reveal not only relative grain orientations but also intragrain inhomogeneity. We envisage that with high chemical specificity, this method will find broad applications ranging from other 2D perovskites to perovskite-based optoelectronic devices.

Mechanism of Photocatalytic CO2 Reduction by Bismuth-Based Perovskite Nanocrystals at the Gas-Solid Interface.

We report here a series of nontoxic and stable bismuth-based perovskite nanocrystals (PeNCs) with applications for photocatalytic reduction of carbon dioxide to methane and carbon monoxide. Three bismuth-based PeNCs of general chemical formulas A3Bi2I9, in which cation A+ = Rb+ or Cs+ or CH3NH3+ (MA+), were synthesized with a novel ultrasonication top-down method. PeNC of Cs3Bi2I9 had the best photocatalytic activity for the reduction of CO2 at the gas-solid interface with formation yields 14.9 µmol g-1 of methane and 77.6 µmol g-1 of CO, representing a much more effective catalyst than TiO2 (P25) under the same experimental conditions. The products of the photocatalytic reactions were analyzed using a gas chromatograph coupled with a mass spectrometer. According to electron paramagnetic resonance and diffuse-reflectance infrared spectra, we propose a reaction mechanism for photoreduction of CO2 via Bi-based PeNC photocatalysts to form CO, CH4, and other possible side products.

Enhanced Dissociation of Hot Excitons with an Applied Electric Field under Low-Power Photoexcitation in Two-Dimensional Perovskite Quantum Wells.

The dependence of photoluminescence (PL) on excitation power and the effect of an external electric field have been studied for a two-dimensional (2D) perovskite (C4H9NH3)2PbI4 thin-film sample. The efficiency of dissociation of hot excitons to produce free carriers was enhanced with a small excitation power because the relaxation of hot excitons to cold emissive excitons was slow, indicating that the thermal energies of hot carriers can be utilized in solar cells under weak photoirradiation. The dissociation was notably enhanced with an applied electric field, resulting in efficient field-induced quenching of the PL. The present results shed light on an application of 2D perovskite materials to photovoltaic (PV) devices with dim radiation, e.g., for indoor PV applications; the concept of electric field-assisted solar cells might be applicable to next-generation solar cells.

Robust Tin-Based Perovskite Solar Cells with Hybrid Organic Cations to Attain Efficiency Approaching 10.

The stability of a tin-based perovskite solar cell is a major challenge. Here, hybrid tin-based perovskite solar cells in a new series that incorporate a nonpolar organic cation, guanidinium (GA+ ), in varied proportions into the formamidinium (FA+ ) tin triiodide perovskite (FASnI3 ) crystal structure in the presence of 1% ethylenediammonium diiodide (EDAI2 ) as an additive, are reported. The device performance is optimized at a precursor ratio (GAI:FAI) of 20:80 to attain a power conversion efficiency (PCE) of 8.5% when prepared freshly; the efficiencies continuously increase to attain a record PCE of 9.6% after storage in a glove-box environment for 2000 h. The hybrid perovskite works stably under continuous 1 sun illumination for 1 h and storage in air for 6 days without encapsulation. Such a tin-based perovskite passes all harsh standard tests, and the efficiency of a fresh device, 8.3%, is certified. The great performance and stability of the device reported herein attains a new milestone for lead-free perovskite solar cells on a path toward commercial development.

Sulfur-Doped Graphene Oxide Quantum Dots as Photocatalysts for Hydrogen Generation in the Aqueous Phase.

Sulfur-doped graphene oxide quantum dots (S-GOQDs) were synthesized and investigated for efficient photocatalytic hydrogen generation application. The UV/Vis, FTIR, and photoluminescence spectra of the synthesized S-GOQDs exhibit three absorption bands at 333, 395, and 524 nm, characteristic of C=S and C-S stretching vibration signals at 1075 and 690 cm-1 , and two excitation-wavelength-independent emission signals with maxima at 451 and 520 nm, respectively, confirming the successful doping of S atom into the GOQDs. Electronic structural analysis suggested that the S-GOQDs exhibit conduction band minimum (CBM) and valence band maximum (VBM) levels suitable for water splitting. Under direct sunlight irradiation, an initial rate of 18 166 µmol h-1 g-1 in pure water and 30 519 µmol h-1 g-1 in 80 % ethanol aqueous solution were obtained. Therefore, metal-free and inexpensive S-GOQDs hold great potential in the development of sustainable and environmentally friendly photocatalysts for efficient hydrogen generation from water splitting.