Efforts to stabilize photoactive formamidinium (FA)–based halide perovskites for perovskite photovoltaics have focused on the growth of cubic formamidinium lead iodide (α-FAPbI
) phases by ...empirically alloying with cesium, methylammonium (MA) cations, or both. We show that such stabilized FA-rich perovskites are noncubic and exhibit ~2° octahedral tilting at room temperature. This tilting, resolvable only with the use of local nanostructure characterization techniques, imparts phase stability by frustrating transitions from photoactive to hexagonal phases. Although the bulk phase appears stable when examined macroscopically, heterogeneous cation distributions allow microscopically unstable regions to form; we found that these transitioned to hexagonal polytypes, leading to local trap-assisted performance losses and photoinstabilities. Using surface-bound ethylenediaminetetraacetic acid, we engineered an octahedral tilt into pure α-FAPbI
thin films without any cation alloying. The templated photoactive FAPbI
film was extremely stable against thermal, environmental, and light stressors.
Halide perovskite materials have promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power ...conversion efficiencies above 25 per cent in single-junction devices and 28 per cent in tandem devices
. This strong performance (albeit below the practical limits of about 30 per cent and 35 per cent, respectively
) is surprising in thin films processed from solution at low-temperature, a method that generally produces abundant crystalline defects
. Although point defects often induce only shallow electronic states in the perovskite bandgap that do not affect performance
, perovskite devices still have many states deep within the bandgap that trap charge carriers and cause them to recombine non-radiatively. These deep trap states thus induce local variations in photoluminescence and limit the device performance
. The origin and distribution of these trap states are unknown, but they have been associated with light-induced halide segregation in mixed-halide perovskite compositions
and with local strain
, both of which make devices less stable
. Here we use photoemission electron microscopy to image the trap distribution in state-of-the-art halide perovskite films. Instead of a relatively uniform distribution within regions of poor photoluminescence efficiency, we observe discrete, nanoscale trap clusters. By correlating microscopy measurements with scanning electron analytical techniques, we find that these trap clusters appear at the interfaces between crystallographically and compositionally distinct entities. Finally, by generating time-resolved photoemission sequences of the photo-excited carrier trapping process
, we reveal a hole-trapping character with the kinetics limited by diffusion of holes to the local trap clusters. Our approach shows that managing structure and composition on the nanoscale will be essential for optimal performance of halide perovskite devices.
Hybrid organic–inorganic perovskites show remarkable charge transport properties despite their deposition via low-temperature solution phase methods. It has recently been shown that this includes the ...ballistic transport of charges following photoexcitation, with ballistic transport lengths as large as 150 nm measured in MAPI3 films, which is almost twice the value reported for GaAs. Here we explore the ballistic transport regime in high-performance triple-cation and K-passivated triple-cation perovskite films, using femtosecond transient absorption microscopy, which allows us to image carrier motion with 10 fs temporal resolution and 10 nm spatial precision. We observe ballistic transport lengths of 160 and 220 nm in triple-cation and K-passivated triple-cation perovskite films, respectively. We propose that the ballistic transport is limited by nanoscale trap clusters at grain boundaries and interfaces, which can be passivated via chemical treatments to enhance the ballistic transport length, which implies that further enhancements are possible as passivation methods are improved.
Metal halide perovskites have emerged as exceptional semiconductors for optoelectronic applications. Substitution of the monovalent cations has advanced luminescence yields and device efficiencies. ...Here, we control the cation alloying to enhance optoelectronic performance through alteration of the charge carrier dynamics in mixed-halide perovskites. In contrast to single-halide perovskites, we find high luminescence yields for photoexcited carrier densities far below solar illumination conditions. Using time-resolved spectroscopy we show that the charge carrier recombination regime changes from second to first order within the first tens of nanoseconds after excitation. Supported by microscale mapping of the optical bandgap, electrically gated transport measurements and first-principles calculations, we demonstrate that spatially varying energetic disorder in the electronic states causes local charge accumulation, creating p- and n-type photodoped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells and light-emitting diodes.Localized photodoping in mixed-cation perovskites is shown to modify charge-carrier recombination and thus offer a route for more efficient light emission.
All-inorganic double perovskites (elpasolites) are a promising potential alternatives to lead halide perovskites in optoelectronic applications. Although halide mixing is a well-established strategy ...for band gap tuning, little is known about halide mixing and phase segregation phenomena in double perovskites. Here, we synthesize a wide range of single- and mixed-halide Cs2AgBiX6 (X = Cl, Br, and I) double perovskites using mechanosynthesis and probe their atomic-level microstructure using 133Cs solid-state MAS NMR. We show that mixed Cl/Br materials form pure phases for any Cl/Br ratio while Cl/I and Br/I mixing is only possible within a narrow range of halide ratios (<3 mol % I) and leads to a complex mixture of products for higher ratios. We characterize the optical properties of the resulting materials and show that halide mixing does not lead to an appreciable tunability of the PL emission. We find that iodide incorporation is particularly pernicious in that it quenches the PL emission intensity and radiative charge carrier lifetimes for iodide ratios as low as 0.3 mol %. Our study shows that solid-state NMR, in conjunction with optical spectroscopies, provides a comprehensive understanding of the structure–activity relationships, halide mixing, and phase segregation phenomena in Cs2AgBiX6 (X = Cl, Br, and I) double perovskites.
Mixed lead–tin halide perovskites have sufficiently low bandgaps (∼1.2 eV) to be promising absorbers for perovskite–perovskite tandem solar cells. Previous reports on lead–tin perovskites have ...typically shown poor optoelectronic properties compared to neat lead counterparts: short photoluminescence lifetimes (<100 ns) and low photoluminescence quantum efficiencies (<1%). Here, we obtain films with carrier lifetimes exceeding 1 μs and, through addition of small quantities of zinc iodide to the precursor solutions, photoluminescence quantum efficiencies under solar illumination intensities of 2.5%. The zinc additives also substantially enhance the film stability in air, and we use cross-sectional chemical mapping to show that this enhanced stability is because of a reduction in tin-rich clusters. By fabricating field-effect transistors, we observe that the introduction of zinc results in controlled p-doping. Finally, we show that zinc additives also enhance power conversion efficiencies and the stability of solar cells. Our results demonstrate substantially improved low-bandgap perovskites for solar cells and versatile electronic applications.
A wide variety of desirable antenna beam patterns can be synthesized by optimal excitation and phasing of the HE 11 and HE 12 modes in scalar corrugated feedhorns. However, the bandwidth of such ...two-mode horns is often limited by modal dispersion. In this paper we introduce a class of low dispersion, two-mode feedhorns that can operate, in some cases, over operating bandwidths of 40-50%. We provide example designs that include horns with high coupling efficiency to: 1) a pure HE 11 mode for single-mode excitation of corrugated pipe transmission lines; 2) a LG 00 and LG 02 combination for radiometry, with narrow beams; 3) a pure Laguerre Gaussian LG 00 mode for quasi-optical instrumentation with constant phase centers; 4) a constant gain antenna for uniform illumination with frequency; 5) Airy patterns or "top hat" patterns for radar or communications applications, designed to maximize aperture efficiencies when used with larger reflect or lens antennas. More generally, we show methods to generate and phase multiple <inline-formula> <tex-math notation="LaTeX">{\text{HE}}_{1n} </tex-math></inline-formula> modes, to synthesize symmetric output beams at any desired frequency or gain.
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