Contactless Visualization of Fast Charge Carrier Diffusion in Hybrid Halide Perovskite Thin Films

Organic–inorganic metal halide perovskite solar cells have recently attracted considerable attention with reported device efficiencies approaching those achieved in polycrystalline silicon. Key for an efficient extraction of photogenerated carriers is the combination of low nonradiative relaxation rates leading to long carrier lifetimes and rapid diffusive transport. The latter, however, is difficult to assess directly with reported values varying widely. Here, we present an experimental approach for a contactless visualization of the charge carrier diffusion length and velocity in thin films based on time-resolved confocal detection of photoluminescence at varying distances from the excitation position. Our measurements on chloride-treated methylammonium lead iodide thin films, the material for which the highest solar cell efficiencies have been reported, reveal a charge carrier diffusion length of 5.5–7.7 μm and a transport time of 100 ps for the first micrometer corresponding to a diffusion constant of about 5–10 cm² s^–1, similar to GaAs thin films

Grain Boundaries Act as Solid Walls for Charge Carrier Diffusion in Large Crystal MAPI Thin Films

Micro- and nanocrystalline methylammonium lead iodide (MAPI)-based thin-film solar cells today reach power conversion efficiencies of over 20%. We investigate the impact of grain boundaries on charge carrier transport in large crystal MAPI thin films using time-resolved photoluminescence (PL) microscopy and numerical model calculations. Crystal sizes in the range of several tens of micrometers allow for the spatially and time resolved study of boundary effects. Whereas long-ranged diffusive charge carrier transport is observed within single crystals, no detectable diffusive transport occurs across grain boundaries. The observed PL transients are found to crucially depend on the microscopic geometry of the crystal and the point of observation. In particular, spatially restricted diffusion of charge carriers leads to slower PL decay near crystal edges as compared to the crystal center. In contrast to many reports in the literature, our experimental results show no quenching or additional loss channels due to grain boundaries for the studied material, which thus do not negatively affect the performance of the derived thin-film devices

Synthesis of Perfectly Oriented and Micrometer-Sized MAPbBr₃ Perovskite Crystals for Thin-Film Photovoltaic Applications

Wide band gap perovskites such as methylammonium lead bromide are interesting materials for photovoltaic applications because of their potentially high open-circuit voltage. However, the fabrication of high-quality planar films has not been investigated in detail for this material. We report a new synthesis approach for the fabrication of bromide-based perovskite planar films based on the control of the deposition environment. We achieve dense layers with large and perfectly oriented crystallites 5–10 μm in size. Our results show that large crystal sizes can be achieved only for smooth indium-doped tin oxide substrates, whereas lateral perovskite crystal growth is limited for the rougher fluorine-doped tin oxide substrates. We additionally correlate photocurrent and perovskite crystal properties in photovoltaic devices and find that this parameter is maximized for ordered systems, with internal quantum efficiencies approaching unity. Hence, our work not only gives a new pathway to tune morphology and crystal orientation but also demonstrates its importance for planar perovskite solar cells

Nanostructures in Te/Sb/Ge/Ag (TAGS) Thermoelectric Materials Induced by Phase Transitions Associated with Vacancy Ordering

Te/Sb/Ge/Ag (TAGS) materials with rather high concentrations of cation vacancies exhibit improved thermoelectric properties as compared to corresponding conventional TAGS (with constant Ag/Sb ratio of 1) due to a significant reduction of the lattice thermal conductivity. There are different vacancy ordering possibilities depending on the vacancy concentration and the history of heat treatment of the samples. In contrast to the average α-GeTe-type structure of TAGS materials with cation vacancy concentrations <∼3%, quenched compounds like Ge_0.53Ag_0.13­Sb_0.27□_0.07Te₁ and Ge_0.61Ag_0.11­Sb_0.22□_0.06Te₁ exhibit “parquet-like” multidomain nanostructures with finite intersecting vacancy layers. These are perpendicular to the pseudocubic ⟨111⟩ directions but not equidistantly spaced, comparable to the nanostructures of compounds (GeTe)_<i>n</i>­Sb₂Te₃. Upon heating, the nanostructures transform into long-periodically ordered trigonal phases with parallel van der Waals gaps. These phases are slightly affected by stacking disorder but distinctly different from the α-GeTe-type structure reported for conventional TAGS materials. Deviations from this structure type are evident only from HRTEM images along certain directions or very weak intensities in diffraction patterns. At temperatures above ∼400 °C, a rock-salt-type high-temperature phase with statistically disordered cation vacancies is formed. Upon cooling, the long-periodically trigonal phases are reformed at the same temperature. Quenched nanostructured Ge_0.53Ag_0.13­Sb_0.27□_0.07Te₁ and Ge_0.61Ag_0.11­Sb_0.22□_0.06Te₁ exhibit ZT values as high as 1.3 and 0.8, respectively, at 160 °C, which is far below the phase transition temperatures. After heat treatment, i.e., without pronounced nanostructure and when only reversible phase transitions occur, the ZT values of Ge_0.53Ag_0.13­Sb_0.27□_0.07Te₁ and Ge_0.61Ag_0.11­Sb_0.22□_0.06Te₁ with extended van der Waals gaps amount to 1.6 at 360 °C and 1.4 at 410 °C, respectively, which is at the top end of the range of high-performance TAGS materials

Nanostructures in Te/Sb/Ge/Ag (TAGS) Thermoelectric Materials Induced by Phase Transitions Associated with Vacancy Ordering

Te/Sb/Ge/Ag (TAGS) materials with rather high concentrations of cation vacancies exhibit improved thermoelectric properties as compared to corresponding conventional TAGS (with constant Ag/Sb ratio of 1) due to a significant reduction of the lattice thermal conductivity. There are different vacancy ordering possibilities depending on the vacancy concentration and the history of heat treatment of the samples. In contrast to the average α-GeTe-type structure of TAGS materials with cation vacancy concentrations <∼3%, quenched compounds like Ge_0.53Ag_0.13­Sb_0.27□_0.07Te₁ and Ge_0.61Ag_0.11­Sb_0.22□_0.06Te₁ exhibit “parquet-like” multidomain nanostructures with finite intersecting vacancy layers. These are perpendicular to the pseudocubic ⟨111⟩ directions but not equidistantly spaced, comparable to the nanostructures of compounds (GeTe)_<i>n</i>­Sb₂Te₃. Upon heating, the nanostructures transform into long-periodically ordered trigonal phases with parallel van der Waals gaps. These phases are slightly affected by stacking disorder but distinctly different from the α-GeTe-type structure reported for conventional TAGS materials. Deviations from this structure type are evident only from HRTEM images along certain directions or very weak intensities in diffraction patterns. At temperatures above ∼400 °C, a rock-salt-type high-temperature phase with statistically disordered cation vacancies is formed. Upon cooling, the long-periodically trigonal phases are reformed at the same temperature. Quenched nanostructured Ge_0.53Ag_0.13­Sb_0.27□_0.07Te₁ and Ge_0.61Ag_0.11­Sb_0.22□_0.06Te₁ exhibit ZT values as high as 1.3 and 0.8, respectively, at 160 °C, which is far below the phase transition temperatures. After heat treatment, i.e., without pronounced nanostructure and when only reversible phase transitions occur, the ZT values of Ge_0.53Ag_0.13­Sb_0.27□_0.07Te₁ and Ge_0.61Ag_0.11­Sb_0.22□_0.06Te₁ with extended van der Waals gaps amount to 1.6 at 360 °C and 1.4 at 410 °C, respectively, which is at the top end of the range of high-performance TAGS materials

Efficient Planar Heterojunction Perovskite Solar Cells Based on Formamidinium Lead Bromide

The development of medium-bandgap solar cell absorber materials is of interest for the design of devices such as tandem solar cells and building-integrated photovoltaics. The recently developed perovskite solar cells can be suitable candidates for these applications. At present, wide bandgap alkylammonium lead bromide perovskite absorbers require a high-temperature sintered mesoporous TiO₂ photoanode in order to function efficiently, which makes them unsuitable for some of the above applications. Here, we present for the first time highly efficient wide bandgap planar heterojunction solar cells based on the structurally related formamidinium lead bromide. We show that this material exhibits much longer diffusion lengths of the photoexcited species than its methylammonium counterpart. This results in planar heterojunction solar cells exhibiting power conversion efficiencies approaching 7%. Hence, formamidinium lead bromide is a strong candidate as a wide bandgap absorber in perovskite solar cells

Blue-Green Color Tunable Solution Processable Organolead Chloride–Bromide Mixed Halide Perovskites for Optoelectronic Applications

Solution-processed organo-lead halide perovskites are produced with sharp, color-pure electroluminescence that can be tuned from blue to green region of visible spectrum (425–570 nm). This was accomplished by controlling the halide composition of CH₃NH₃Pb­(Br_<i>x</i>Cl_1–<i>x</i>)₃ [0 ≤ <i>x</i> ≤ 1] perovskites. The bandgap and lattice parameters change monotonically with composition. The films possess remarkably sharp band edges and a clean bandgap, with a single optically active phase. These chloride–bromide perovskites can potentially be used in optoelectronic devices like solar cells and light emitting diodes (LEDs). Here we demonstrate high color-purity, tunable LEDs with narrow emission full width at half maxima (FWHM) and low turn on voltages using thin-films of these perovskite materials, including a blue CH₃NH₃PbCl₃ perovskite LED with a narrow emission FWHM of 5 nm