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The performance of photodetectors fabricated from emerging semiconductors such as perovskites, quantum dots, two-dimensional materials or organics, for example, can be prone to misinterpretation. This Comment exposes the problems and proposes some guidelines for accurate characterization.

Photon recycling, that is, iterative self-absorption and re-emission by the photoactive layer itself, has been speculated to contribute to the high open-circuit voltage in several types of high efficiency solar cells. For organic–inorganic halide perovskites that have yielded highly efficient photovoltaic devices, however, it remains unclear whether the photon recycling effect is significant enough to improve solar cell efficiency. Here we quantitatively evaluate the re-absorption and re-emission processes to determine photon recycling efficiency in hybrid perovskite with its single crystals by measuring the ratio of the re-emitted photons to the initially excited photons, which is realized by modulating their polarization to differentiate them. The photon recycling efficiencies are revealed to be less than 0.5% in CH 3 NH 3 PbI 3 and CH 3 NH 3 PbBr 3 single crystals under excitation intensity close to one sun, highlighting the intrinsically long carrier recombination lifetime instead of the photon-recycling-induced photon propagation as the origin of their long carrier diffusion length. Fang et al . develop a method to determine the photon recycling efficiency for organic-inorganic hybrid single crystal perovskites by differentiating between emitted and re-absorbed photons based on their polarization difference. For these systems efficiencies of less than 0.5% are reported.

A low defect density in metal halide perovskite single crystals is critical to achieve high performance optoelectronic devices. Here we show the reduction of defect density in perovskite single crystals grown by a ligand-assisted solution process with 3‐(decyldimethylammonio)‐propane‐sulfonate inner salt (DPSI) as an additive. DPSI ligands anchoring with lead ions on perovskite crystal surfaces not only suppress nucleation in solution, but also regulate the addition of proper ions to the growing surface, which greatly enhances the crystal quality. The grown CH 3 NH 3 PbI 3 crystals show better crystallinity and a 23-fold smaller trap density of 7 × 10 10  cm −3 than the optimized control crystals. The enhanced material properties result in significantly suppressed ion migration and superior X-ray detection sensitivity of CH 3 NH 3 PbI 3 detectors of (2.6 ± 0.4) × 10 6  µC Gy −1 air cm −2 for 60 kVp X-ray and the lowest detectable dose rate reaches (5.0 ± 0.7) nGy s −1 , which enables reduced radiation dose to patients in medical X-ray diagnostics. The performance of a metal halide perovskite single crystal is governed by the defect density. Here, the authors report a high quality single crystal perovskite grown by a ligand-assisted solution process with DPSI achieving 23-fold smaller trap density than that without DPSI.

Organolead trihalide perovskite is an emerging low-cost, solution-processable material with a tunable bandgap from the violet to near-infrared, which has attracted a great deal of interest for applications in high-performance optoelectronic devices. Here, we present hybrid perovskite single-crystal photodetectors that have a very narrow spectral response with a full-width at half-maximum of <20 nm. The response spectra are continuously tuned from blue to red by changing the halide composition and thus the bandgap of the single crystals synthesized by solution processes. The narrowband photodetection can be explained by the strong surface-charge recombination of the excess carriers close to the crystal surfaces generated by short-wavelength light. The excess carriers generated by below-bandgap excitation locate away from the surfaces and can be much more efficiently collected by the electrodes, assisted by the applied electric field. This provides a new design paradigm for a narrowband photodetector with broad applications where background noise emission needs to be suppressed. Perovskite-based devices typically exhibit broadband spectral responses. Here narrowband (< 20 nm FWHM) response is achieved for a photodetector application.

Layered perovskites have been shown to improve the stability of perovskite solar cells while its operation mechanism remains unclear. Here we investigate the process for the conversion of light to electrical current in high performance layered perovskite solar cells by examining its real morphology. The layered perovskite films in this study are found to be a mixture of layered and three dimensional (3D)-like phases with phase separations at micrometer and nanometer scale in both vertical and lateral directions. This phase separation is explained by the surface initiated crystallization process and the competition of the crystallization between 3D-like and layered perovskites. We further propose that the working mechanisms of the layered perovskite solar cells involve energy transfer from layered to 3D-like perovskite network. The impact of morphology on efficiency and stability of the hot-cast layered perovskite solar cells are also discussed to provide guidelines for the future improvement. It is well-accepted that the two dimensional layered halide perovskite can improve the device stability of perovskite solar cells but the operation mechanism remains unclear. Here Lin et al. reveal the real morphology of the hot-cast layered perovskite solar cells and understand the working mechanism.

The stress hormone, Abscisic acid (ABA), is crucial for plants to respond to changes in their environment. It triggers changes in cytoplasmic Ca 2+ levels, which activate plant responses to external stresses. However, how Ca 2+ sensing and signaling feeds back into ABA signaling is not well understood. Here we reveal a calcium sensing module that negatively regulates drought stress via modulating ABA receptor PYLs. Mutants cbl1 / 9 and cipk1 exhibit hypersensitivity to ABA and drought resilience. Furthermore, CIPK1 is shown to interact with and phosphorylate 7 of 14 ABA receptors at the evolutionarily conserved site corresponding to PYL4 Ser129, thereby suppressing their activities and promoting PP2C activities under normal conditions. Under drought stress, ABA impedes PYLs phosphorylation by CIPK1 to respond to ABA signaling and survive in unfavorable environment. These findings provide insights into a previously unknown negative regulatory mechanism of the ABA signaling pathway, which is mediated by CBL1/9-CIPK1-PYLs, resulting in plants that are more sensitive to drought stress. This discovery expands our knowledge about the interplay between Ca 2+ signaling and ABA signaling. ABA signaling and Ca 2+ signaling regulatory networks are crucial for how plants respond to drought stress. Here, the authors reported that the functional module of the CBL1/9-CIPK1-PYLs regulatory network plays a negative role in ABA signaling and its response to drought stress.

Grain boundaries have been established to impact charge transport, recombination and thus the power conversion efficiency of metal halide perovskite thin film solar cells. As a special category of grain boundaries, ferroelastic twin boundaries have been recently discovered to exist in both CH 3 NH 3 PbI 3 thin films and single crystals. However, their impact on the carrier transport and recombination in perovskites remains unexplored. Here, using the scanning photocurrent microscopy, we find that twin boundaries have negligible influence on the carrier transport across them. Photoluminescence (PL) imaging and the spatial-resolved PL intensity and lifetime scanning confirm the electronically benign nature of the twin boundaries, in striking contrast to regular grain boundaries which block the carrier transport and behave as the non-radiative recombination centers. Finally, the twin-boundary areas are found still easier to degrade than grain interior. Grain boundaries are known to be generally detrimental to the operation of metal halide perovskite solar cells. Here Xiao et al. show with scanning photocurrent microscopy that ferroelastic twin boundaries are benign to the electrical properties in crystalline samples, in contrast to grain boundaries.

The efficiency of perovskite solar cells has surged in the past few years, while the bandgaps of current perovskite materials for record efficiencies are much larger than the optimal value, which makes the efficiency far lower than the Shockley–Queisser efficiency limit. Here we show that utilizing the below-bandgap absorption of perovskite single crystals can narrow down their effective optical bandgap without changing the composition. Thin methylammonium lead triiodide single crystals with tuned thickness of tens of micrometers are directly grown on hole-transport-layer covered substrates by a hydrophobic interface confined lateral crystal growth method. The spectral response of the methylammonium lead triiodide single crystal solar cells is extended to 820 nm, 20 nm broader than the corresponding polycrystalline thin-film solar cells. The open-circuit voltage and fill factor are not sacrificed, resulting in an efficiency of 17.8% for single crystal perovskite solar cells. Thin films of halide perovskites are promising for solar cell technology but they do not perform well at the band edge due to the low optical absorption. Herein, Chen et al. fabricate a high efficiency single crystal perovskite solar cell with thicker single crystals to harvest the below-bandgap photons.

The efficiencies of perovskite solar cells (PSCs) are now reaching such consistently high levels that scalable manufacturing at low cost is becoming critical. However, this remains challenging due to the expensive hole-transporting materials usually employed, and difficulties associated with the scalable deposition of other functional layers. By simplifying the device architecture, hole-transport-layer-free PSCs with improved photovoltaic performance are fabricated via a scalable doctor-blading process. Molecular doping of halide perovskite films improved the conductivity of the films and their electronic contact with the conductive substrate, resulting in a reduced series resistance. It facilitates the extraction of photoexcited holes from perovskite directly to the conductive substrate. The bladed hole-transport-layer-free PSCs showed a stabilized power conversion efficiency above 20.0%. This work represents a significant step towards the scalable, cost-effective manufacturing of PSCs with both high performance and simple fabrication processes. The existing hole-transporting materials cause problems in the cost and scalability of the perovskite solar cells. Here Wu et al. fabricate high efficiency cells by molecularly doping the perovskite layer without using hole-transporting layers, thus simplify the device architecture and processing steps.

The ionic defects at the surfaces and grain boundaries of organic–inorganic halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells. Here, we show that quaternary ammonium halides can effectively passivate ionic defects in several different types of hybrid perovskite with their negative- and positive-charged components. The efficient defect passivation reduces the charge trap density and elongates the carrier recombination lifetime, which is supported by density-function-theory calculation. The defect passivation reduces the open-circuit-voltage deficit of the p–i–n-structured device to 0.39 V, and boosts the efficiency to a certified value of 20.59 ± 0.45%. Moreover, the defect healing also significantly enhances the stability of films in ambient conditions. Our findings provide an avenue for defect passivation to further improve both the efficiency and stability of solar cells. Losses in solar cells can be caused by material defects in the bulk or at interfaces. Here, Zheng et al.  use quaternary ammonium halides to passivate various perovskite absorbers and prepare solar cells with certified efficiency above 20%, suggesting that both anionic and cation defects are affected.