Halide perovskite materials offer an ideal playground for easily tuning their color and, accordingly, the spectral range of their emitted light. In contrast to common procedures, this work ...demonstrates that halide substitution in Ruddlesden–Popper perovskites not only progressively modulates the bandgap, but it can also be a powerful tool to control the nanoscale phase segregation—by adjusting the halide ratio and therefore the spatial distribution of recombination centers. As a result, thin films of chloride‐rich perovskite are engineered—which appear transparent to the human eye—with controlled tunable emission in the green. This is due to a rational halide substitution with iodide or bromide leading to a spatial distribution of phases where the minor component is responsible for the tunable emission, as identified by combined hyperspectral photoluminescence imaging and elemental mapping. This work paves the way for the next generation of highly tunable transparent emissive materials, which can be used as light‐emitting pixels in advanced and low‐cost optoelectronics.
Controlled halide substitution in low‐dimensional hybrid perovskites is an effective strategy to vary the material bandgap, obtaining a palette of light emitters, whereby transparent thin films emitting in the green are realized. By fine‐tuning the halide content, nanoscale phase segregation happens, resulting in a controlled nanoscale spatial distribution of recombination centers.
Mixed‐halide lead perovskites have attracted significant attention in the field of photovoltaics and other optoelectronic applications due to their promising bandgap tunability and device ...performance. Here, the changes in photoluminescence and photoconductance of solution‐processed triple‐cation mixed‐halide (Cs0.06MA0.15FA0.79)Pb(Br0.4I0.6)3 perovskite films (MA: methylammonium, FA: formamidinium) are studied under solar‐equivalent illumination. It is found that the illumination leads to localized surface sites of iodide‐rich perovskite intermixed with passivating PbI2 material. Time‐ and spectrally resolved photoluminescence measurements reveal that photoexcited charges efficiently transfer to the passivated iodide‐rich perovskite surface layer, leading to high local carrier densities on these sites. The carriers on this surface layer therefore recombine with a high radiative efficiency, with the photoluminescence quantum efficiency of the film under solar excitation densities increasing from 3% to over 45%. At higher excitation densities, nonradiative Auger recombination starts to dominate due to the extremely high concentration of charges on the surface layer. This work reveals new insight into phase segregation of mixed‐halide mixed‐cation perovskites, as well as routes to highly luminescent films by controlling charge density and transfer in novel device structures.
The changes in photophysical properties of mixed‐halide perovskite films under solar‐equivalent illumination are studied. The illumination generates localized low‐bandgap surface domains, onto which photoexcited charge carriers transfer and recombine with high radiative efficiency. The fraction of radiative and nonradiative (Auger) recombination bandgap can be balanced to achieve extremely high photoluminescence quantum yields at low excitation densities.
Solution‐processable near‐infrared (NIR) photodetectors are urgently needed for a wide range of next‐generation electronics, including sensors, optical communications and bioimaging. However, it is ...rare to find photodetectors with >300 kHz cut‐off frequencies, especially in the NIR region, and many of the emerging inorganic materials explored are comprised of toxic elements, such as lead. Herein, solution‐processed AgBiS2 photodetectors with high cut‐off frequencies under both white light (>1 MHz) and NIR (approaching 500 kHz) illumination are developed. These high cut‐off frequencies are due to the short transit distances of charge‐carriers in the ultrathin photoactive layer of AgBiS2 photodetectors, which arise from the strong light absorption of this material, such that film thicknesses well below 120 nm are sufficient to absorb >65% of NIR to visible light. It is also revealed that ion migration plays a critical role in the photo‐response speed of these devices, and its detrimental effects can be mitigated by finely tuning the thickness of the photoactive layer, which is important for achieving low dark current densities as well. These outstanding characteristics enable the realization of air‐stable, real‐time heartbeat sensors based on NIR AgBiS2 photodetectors, which strongly motivates their future integration in high‐throughput systems.
Fast near‐infrared (NIR) photodetectors are vital for many applications, including optical communication and data transfer. Herein, solution‐processed AgBiS2 photodetectors with a cut‐off frequency approaching 500 kHz under NIR (940 nm) illumination, and >1 MHz under visible light are developed. The high response speed comes about because of the strong absorption of light in AgBiS2, enabling the fabrication of ultrathin photodetectors with fast charge‐extraction, which is demonstrated as heart beat sensors operating in air.
Hybrid integration of n‐type oxide with p‐type polymer transistors is an attractive approach for realizing high performance complementary circuits on flexible substrates. However, the stability of ...solution‐processed oxide transistors is limiting the lifetime and reliability of such circuits. Oxygen vacancies are the main defect degrading metal oxide transistor performance when ambient oxygen adsorbs onto metal oxide films. Here, an effective surface passivation treatment based on negative oxygen ion exposure combined with UV light is demonstrated, that is able to significantly reduce surface oxygen vacancy concentration and improve the field effect mobility to values up to 41 cm2 V−1 s−1 with high on–off current ratio of 108. The treatment also reduces the threshold voltage shift after 2 days in air from 5 to 0.07 V. The improved stability of the oxide transistors also improves the lifetime of hybrid complementary circuits and stable operation of complementary, analog amplifiers is confirmed for 60 days in air. The suggested approach is facile and can be widely applicable for flexible electronics using low‐temperature solution‐processed metal oxide semiconductors.
Combined treatment by ultraviolet radiation and negative oxygen ions can effectively passivate oxygen vacancies on the surface of metal oxide films, resulting in significantly improved field‐effect mobility and stability. It enables complementary analog amplifiers based on n‐type metal oxide and p‐type polymer transistors with improved performance and lifetime. The simple passivation approach is widely applicable for solution‐processed oxide electronics.
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.
Investigation of the inherent field-driven charge transport behaviour of three-dimensional lead halide perovskites has largely remained challenging, owing to undesirable ionic migration effects near ...room temperature and dipolar disorder instabilities prevalent specifically in methylammonium-and-lead-based high-performing three-dimensional perovskite compositions. Here, we address both these challenges and demonstrate that field-effect transistors based on methylammonium-free, mixed metal (Pb/Sn) perovskite compositions do not suffer from ion migration effects as notably as their pure-Pb counterparts and reliably exhibit hysteresis-free p-type transport with a mobility reaching 5.4 cm
V
s
. The reduced ion migration is visualized through photoluminescence microscopy under bias and is manifested as an activated temperature dependence of the field-effect mobility with a low activation energy (~48 meV) consistent with the presence of the shallow defects present in these materials. An understanding of the long-range electronic charge transport in these inherently doped mixed metal halide perovskites will contribute immensely towards high-performance optoelectronic devices.
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