Fast and Sensitive Solution-Processed Visible-Blind Perovskite UV Photodetectors

Endogenous Intermediate Template Guided Crystallization for High Performance FAPbCl3 UV Detector

Lead-chloride perovskite is a promising candidate for the active layer of UV detectors. However, due to the high ionic character of chloride compound, it is difficult to form a ligand-lead adduct that guides the crystallization of perovskite, resulting in poor film quality and device performance. Herein, an endogenous intermediate template strategy is developed to guide the crystallization of lead-chloride perovskite FAPbCl3 (FA = formamidine). Based on the enrichment of hydrogen bonds between ligands, a low dimensional N-Pb-O adduct FAPbCl3-DMSO (DMSO = dimethyl sulfoxide) is observed in precursor liquid. This results in the endogenous formation, not the external introduction of low dimensional FAxPbCly (x>1, y>3) intermediate phase, serving as a crystallization template guiding the oriented growth of FAPbCl3 perovskite film with large and closely packed grain. As a result, the film shows much stronger PL intensity and significantly reduced defect density. Therefore, a self-driven lead-chloride perovskite UV detector is realized with responsivity, detectivity and response time of 0.63 A W-1, 1.3 × 1013 Jones, and 205/53 µs at 365 nm and zero bias. Furthermore, for the first time, a uniform 64 × 64 lead-chloride perovskite UV array detector is successfully fabricated and achieves a promising image sensing function.

Synergistic Control of Organic Lead Chloride Perovskite Crystallization through the Precursor and Growth Substrate for High-Performance and Stable Transparent Optoelectronics

Transparent optoelectronics are crucial in modern applications, advancing display technologies in smartphones and smart windows, and supporting high-speed communication systems and advanced sensors. CH3NH3PbCl3 (MAPbCl3) has garnered significant attention due to its ideal optical bandgap and outstanding optoelectronic performance. However, the fabrication of high-quality MAPbCl3 thin films faces significant challenges, primarily due to the uncontrolled nucleation process, which results in nonuniform crystallization, poor surface coverage with numerous voids, and high roughness. In this work, we utilized methylammonium acetate (MAAc) as a solvent to form the MAPbCl3 precursor. This approach not only enables the air-processed fabrication of MAPbCl3 but also produces uniform colloidal particles, which are beneficial for the formation of compact thin films. We investigated the influence of common hole transport layers (NiOx, PTAA, PEDOT:PSS) on the crystallization of MAPbCl3 films. By synergistically controlling both the precursor and the growth substrate, we significantly improved the quality of the MAPbCl3 film. The resulting photodiode, based on the high-quality MAPbCl3 film, demonstrated potential for transparent photovoltaics and exhibited excellent performance as a photodetector. Specifically, it achieved a responsivity R of 162.5 mA/W and a detectivity (D*) of 8.9 × 1012 Jones at 390 nm, with high response speed (1.37 μs rise time and 1.68 μs fall time) even under self-powered operation (0 V). Furthermore, the device was successfully integrated into an optical communication system. These results highlight the great potential of high-quality MAPbCl3 devices in transparent optoelectronic applications.

Progress in UV Photodetectors Based on ZnO Nanomaterials: A Review of the Detection Mechanisms and Their Improvement

Recent advancements in ultraviolet (UV) photodetection technology have driven intensive research on zinc oxide (ZnO) nanomaterials due to their exceptional optoelectronic properties. This review systematically examines the fundamental detection mechanisms in ZnO-based UV photodetectors (UVPDs), including photoconductivity effects, the threshold dimension phenomenon and light-modulated interface barriers. Based on these mechanisms, a large surface barrier due to surface-adsorbed O2 is generally constructed to achieve a high sensitivity. However, this improvement is obtained with a decrease in response speed due to the slow desorption and re-adsorption processes of surface O2 during UV light detection. Various improvement strategies have been proposed to overcome this drawback and keep the high sensitivity, including ZnO-organic semiconductor interfacing, defect engineering and doping, surface modification, heterojunction and the Schottky barrier. The general idea is to modify the adsorption state of O2 or replace the adsorbed O2 with another material to build a light-regulated surface or an interface barrier, as surveyed in the third section. The critical trade-off between sensitivity and response speed is also addressed. Finally, after a summary of these mechanisms and the improvement methods, this review is concluded with an outlook on the future development of ZnO nanomaterial UVPDs.

Polar Three-dimensional Organic-inorganic Hybrid Perovskite Realize Highly Sensitive Self-driven Ultraviolet Photodetection

Bulk photovoltaic effect (BPVE) triggered by spontaneous polarization in polar organic-inorganic hybrid perovskites (OIHPs) has brought unprecedented development opportunities for self-powered ultraviolet photodetection. However, the currently reported ultraviolet optoelectronic devices are dominated by low-dimensional hybrid perovskites, in which low carrier mobility limits the photoelectric conversion efficiency. Herein, we report for the first time a polar three-dimensional OIHPs, namely MHyPbBr3 (1, MHy=methylhydrazine), that achieves high-performance self-driven ultraviolet photodetection. Benefitting from the large spontaneous polarization and excellent semiconductor attributes, 1 exhibits 0.33 V BPVE and high carrier mobility lifetime product (μτ) of 1.972×10-3 cm-2 V-1 under ultraviolet illumination. Notably, such merits contribute to efficient self-driven photodetection, where responsivity (R) and detectivity (D*) reach up to 198 mA W-1 and 1.42×1013 Jones, surpassing most reported ultraviolet photodetectors. Furthermore, based on the reversible phase transition of 1, we have achieved controllable ultraviolet photoelectric detection. This work will shed light on the fabrication of high-response self-driven ultraviolet optoelectronic devices.

Regulated Charge Transfer in Donor-Acceptor Metal-Organic Frameworks for Highly-Sensitive Photodetectors

In photo-induced charge separation, organic thin films with donor and acceptor chromophores are vital for uses such as artificial photosynthesis and photodetection. The main challenges include optimizing charge separation efficiency and identifying the ideal acceptor/donor ratio. Achieving this is difficult due to the variability in molecular configurations within these typically amorphous organic aggregates. Metal-organic frameworks (MOFs) provide a structured solution by enabling systematic design of donor/acceptor blends with adjustable ratios within a crystalline lattice. We demonstrate this approach by incorporating donor and acceptor naphthalenediimide (NDI) chromophores as linkers in a highly oriented, monolithic MOF thin film. By adjusting the NDI acceptor linker concentration during the layer-by-layer assembly of surface-anchored MOF thin films (SURMOFs), we significantly enhanced charge separation efficiency. Surprisingly, the optimum acceptor concentration was only 3 %, achieving a forty-fold increase in photodetection efficiency compared to baseline NDI donor-based SURMOFs. This unexpected behaviour was clarified through theoretical analysis enabled by the well-defined crystalline structure of the SURMOFs. Using density functional theory and kinetic Monte Carlo simulations, we identified two opposing effects from acceptors: the positive effect of suppressing undesirable charge carrier recombination is offset at high concentrations by a reduction in charge-carrier mobility.

Unleashing the Potential: High Responsivity at Room Temperature of Halide Perovskite-Based Short-Wave Infrared Detectors with Ultrabroad Bandwidth

Short-wave infrared (SWIR) imaging systems offer remarkable advantages, such as enhanced resolution and contrast, compared to their optical counterparts. However, broader applications demand improvements in performance, notably the elimination of cryogenic temperature requirements and cost reduction in manufacturing processes. In this manuscript, we present a new development in SWIR photodetection, exploiting the potential of metal halide perovskite materials. Our work introduces a cost-effective and easily fabricated SWIR photodetector with an ultrabroad detection range from 900 to 2500 nm, a room-temperature responsivity of 1.57 × 102 A/W, and a specific detectivity of 4.18 × 1010 Jones at 1310 nm. We then performed comprehensive static and time-resolved optical and electrical measurements under ambient conditions, complemented by extensive density functional theory simulations, validating the formation of heterojunctions within the intrinsic n-type and extrinsic p-type perovskite structures. The potential of our perovskite-based SWIR materials extends from photodetectors to photovoltaic cells and introduces a possibility for high SWIR responsivity at room temperature and atmospheric pressure, which promotes its economic efficiency.

Visible-ultraviolet dual-band photodetectors based on an all-inorganic CsPbCl3/p-GaN heterostructure

All-inorganic metal halide perovskites (MHPs) have attracted increasing attention because of their high thermal stability and band gap tunability. Among them, CsPbCl3 is considered a promising semiconductor material for visible-ultraviolet dual-band photodetectors because of its excellent photoelectric properties and suitable band gap value. In this work, we fabricated a visible-ultraviolet dual-band photodetector based on a CsPbCl3/p-GaN heterojunction using the spin coating method. The formation of the heterojunction enables the device to exhibit obvious dual-band response behavior at positive and negative bias voltages. At the same time, the dark current of the device can be as low as 2.42 × 10-9 A, and the corresponding detection rate can reach 5.82 × 1010 Jones. In addition, through simulation calculations, it was found that the heterojunction has a type II energy band arrangement, and the heterojunction response band light absorption is significantly enhanced. The type II energy band arrangement will separate electron-hole pairs more effectively, which will help improve device performance. The successful implementation of visible-ultraviolet dual-band photodetectors based on a CsPbCl3/p-GaN heterojunction provides guidance for the application of all-inorganic MHPs in the field of multi-band photodetectors.

Defect Engineering and Emission Tuning of Wide-Bandgap MAPbCl3 Perovskite

Lead-chloride perovskites are promising candidates for optoelectronic applications, such as visible-blind UV photodetection. It remains unclear how the deep defects in this wide-bandgap material impact the carrier recombination dynamics. In this work, we study the defect properties of MAPbCl3 (MA = CH3NH3) based on photoluminescence (PL) measurements. Our investigations show that apart from the intrinsic emission, four sub-bandgap emissions emerge, which are very likely to originate from the radiative recombination of excitons bound to several intrinsic vacancy and interstitial defects. The intensity of various emission features can be tuned by adjusting the type and ratio of precursors used during synthesis. Our study not only provides important insights into the defect property and carrier recombination mechanism in this class of material but also demonstrates efficient strategies for defect passivation and engineering, paving the way for further development of lead-chloride perovskite-based optoelectronic devices.

Enabling Broadband Solar-Blind UV Photodetection by a Rare-Earth Doped Oxyfluoride Transparent Glass-Ceramic

Oxyfluoride transparent glass-ceramics (GC) are widely used as the matrix for rare-earth (RE) ions due to their unique properties such as low phonon energy, high transmittance, and high solubility for RE ions. Tb3+ doped oxyfluoride glasses exhibit a large absorption cross section for ultraviolet (UV) excitation, high stability, high photoluminescence quantum efficiency, and sensitive spectral conversion characteristics, making them promising candidate materials for use as the spectral converter in UV photodetectors. Herein, a Tb3+ doped oxyfluoride GC is developed by using the melt-quenching method, and the microstructure and optical properties of the GC sample are carefully investigated. By combining with a Si-based photo-resistor,a solar-blind UV detector is fabricated, which exhibits a significant photoelectric response with a broad detection range from 188 to 400 nm. The results indicate that the designed UV photodetector is of great significance for the development of solar-blind UV detectors.

Insight into the interface engineering between methylammonium lead halide perovskites and gallium oxide: a first-principles approach

Interface engineering of the organo-lead halide perovskite devices has shown the potential to improve their efficiency and stability. In this study, the atomic, electronic, optical and transport characteristics of MAPbI3/Ga2O3 and MAPbCl3/Ga2O3 interfaces were investigated by using first-principles calculations. Eight different interfacial models were established and the interfacial properties were discussed. The results show that the PbI/O configuration exhibits the largest bonding strength out of all eight interfacial configurations. Owing to the larger interfacial interaction, the charge transfer at the PbI/O interface is significantly more than that at the other interfaces. The analysis of absorption spectra indicates that the Ga-terminated perovskite/Ga2O3 heterostructures are expected to have great potential for efficient optoelectronic applications. The analysis of transmission spectra shows that the MA/O configurations with more transmission peaks near the Fermi level exhibit lower resistance compared to others. The results of our study could help understand the interfacial engineering mechanism between perovskite and Ga2O3.

Photoelectric properties of glass-ceramics containing KTb2F7 nanocrystals for UV detection

In this work, a glass ceramics (GC) containing KTb2F7 nanocrystals was fabricated by controlled crystallization of an fluorosilicate glass via heat-treatment. The microstructure, luminescence, and photoelectric properties of the GCs are systematically studied by X-ray diffraction, transmission electron microscopy, spectral analysis, and current-voltage (I-V) curves. The results show that the GC containing KTb2F7 nanocrystals exhibit intense visible emission due to the 4f transition of Tb3+: 5Di (i = 3, 4) → 7Fj (j = 0-6) upon excitation of ultraviolet (UV) light. In addition, a UV detector device based on the GC was fabricated, which has a large dynamic linear response range, fast response speed and high sensitivity. This study not only provides a new material for UV detector that can simplify the process of UV detection, but also highlight a new strategy for UV detection.

Ultraviolet-Visible-Short-Wavelength Infrared Broadband and Fast-Response Photodetectors Enabled by Individual Monocrystalline Perovskite Nanoplate

Perovskite-based photodetectors exhibit potential applications in communication, neuromorphic chips, and biomedical imaging due to their outstanding photoelectric properties and facile manufacturability. However, few of perovskite-based photodetectors focus on ultraviolet-visible-short-wavelength infrared (UV-Vis-SWIR) broadband photodetection because of the relatively large bandgap. Moreover, such broadband photodetectors with individual nanocrystal channel featuring monolithic integration with functional electronic/optical components have hardly been explored. Herein, an individual monocrystalline MAPbBr3 nanoplate-based photodetector is demonstrated that simultaneously achieves efficient UV-Vis-SWIR detection and fast-response. Nanoplate photodetectors (NPDs) are prepared by assembling single nanoplate on adjacent gold electrodes. NPDs exhibit high external quantum efficiency (EQE) and detectivity of 1200% and 5.37 × 1012 Jones, as well as fast response with rise time of 80 µs. Notably, NPDs simultaneously achieve high EQE and fast response, exceeding most perovskite devices with multi-nanocrystal channel. Benefiting from the high specific surface area of nanoplate with surface-trap-assisted absorption, NPDs achieve high performance in the near-infrared and SWIR spectral region of 850-1450 nm. Unencapsulated devices show outstanding UV-laser-irradiation endurance and decent periodicity and repeatability after 29-day-storage in atmospheric environment. Finally, imaging applications are demonstrated. This work verifies the potential of perovskite-based broadband photodetection, and stimulates the monolithic integration of various perovskite-based devices.

Multifunctional Perovskite Photodetectors: From Molecular-Scale Crystal Structure Design to Micro/Nano-scale Morphology Manipulation

Multifunctional photodetectors boost the development of traditional optical communication technology and emerging artificial intelligence fields, such as robotics and autonomous driving. However, the current implementation of multifunctional detectors is based on the physical combination of optical lenses, gratings, and multiple photodetectors, the large size and its complex structure hinder the miniaturization, lightweight, and integration of devices. In contrast, perovskite materials have achieved remarkable progress in the field of multifunctional photodetectors due to their diverse crystal structures, simple morphology manipulation, and excellent optoelectronic properties. In this review, we first overview the crystal structures and morphology manipulation techniques of perovskite materials and then summarize the working mechanism and performance parameters of multifunctional photodetectors. Furthermore, the fabrication strategies of multifunctional perovskite photodetectors and their advancements are highlighted, including polarized light detection, spectral detection, angle-sensing detection, and self-powered detection. Finally, the existing problems of multifunctional detectors and the perspectives of their future development are presented.

Advances in the Application of Perovskite Materials

Nowadays, the soar of photovoltaic performance of perovskite solar cells has set off a fever in the study of metal halide perovskite materials. The excellent optoelectronic properties and defect tolerance feature allow metal halide perovskite to be employed in a wide variety of applications. This article provides a holistic review over the current progress and future prospects of metal halide perovskite materials in representative promising applications, including traditional optoelectronic devices (solar cells, light-emitting diodes, photodetectors, lasers), and cutting-edge technologies in terms of neuromorphic devices (artificial synapses and memristors) and pressure-induced emission. This review highlights the fundamentals, the current progress and the remaining challenges for each application, aiming to provide a comprehensive overview of the development status and a navigation of future research for metal halide perovskite materials and devices.

Achieving a Highly Stable Perovskite Photodetector with a Long Lifetime Fabricated via an All-Vacuum Deposition Process

Hybrid organic-inorganic metal halide perovskites (HOIP) have become a promising visible light sensing material due to their excellent optoelectronic characteristics. Despite the superiority, overcoming the stability issue for commercialization remains a challenge. Herein, an extremely stable photodetector was demonstrated and fabricated with Cs_0.06FA_0.94Pb(I_0.68Br_0.32)₃ perovskite by an all-vacuum process. The photodetector achieves a current density up to 1.793 × 10^-2 A cm^-2 under standard one sun solar illumination while maintaining a current density as low as 8.627 × 10^-10 A cm^-2 at zero bias voltage. The linear dynamic range (LDR) and transient voltage response were found to be comparable to the silicon-based photodetector (Newport 818-SL). Most importantly, the device maintains 95% of the initial performance after 960 h of incessant exposure under one sun solar illumination. The achievements of these outstanding results contributed to the all-vacuum deposition process delivering a film with high stability and good uniformity, which in turn delays the degradation process. The degradation mechanism is further investigated by impedance spectroscopy to reveal the charge dynamics in the photodetector under different exposure times.

Vitality surveillance at distance using thin-film tandem-like narrowband near-infrared photodiodes with light-enhanced responsivity

Remote measurement of vital sign parameters like heartbeat and respiration rate represents a compelling challenge in monitoring an individual's health in a noninvasive way. This could be achieved by large field-of-view, easy-to-integrate unobtrusive sensors, such as large-area thin-film photodiodes. At long distances, however, discriminating weak light signals from background disturbance demands superior near-infrared (NIR) sensitivity and optical noise tolerance. Here, we report an inherently narrowband solution-processed, thin-film photodiode with ultrahigh and controllable NIR responsivity based on a tandem-like perovskite-organic architecture. The device has low dark currents (<10^-6 mA cm^-2), linear dynamic range >150 dB, and operational stability over time (>8 hours). With a narrowband quantum efficiency that can exceed 200% at 850 nm and intrinsic filtering of other wavelengths to limit optical noise, the device exhibits higher tolerance to background light than optically filtered silicon-based sensors. We demonstrate its potential in remote monitoring by measuring the heart rate and respiration rate from distances up to 130 cm in reflection.

Highly Sensitive Broadband Phototransistors Based on Gradient Tin/Lead Mixed Perovskites

Highly sensitive broadband photodetectors are critical to numerous cutting-edge technologies such as biomedical imaging, environment monitoring, and night vision. Here, phototransistors based on mixed Sn/Pb perovskites are reported, which demonstrate ultrahigh responsivity, gain and specific detectivity in a broadband from ultraviolet to near-infrared region. The interface properties of the perovskite phototransistors are optimized by a special three-step cleaning-healing-cleaning treatment, leading to a high hole mobility in the channel. The highly sensitive performance of the mixed Sn/Pb perovskite phototransistors can be attributed to the vertical compositional heterojunction automatically formed during the film deposition, which is helpful for the separation of photocarriers thereby enhancing a photogating effect in the perovskite channel. This work demonstrates a convenient approach to achieving high-performance phototransistors through tuning compositional gradient in mixed-metal perovskite channels.

Cs3Cu2I5/ZnO Heterostructure for Flexible Visible-Blind Ultraviolet Photodetection

Wearable, portable, and biocompatible optoelectronic devices made of all-green and abundant materials and fabricated by low-temperature solution method are the key point in the development of next generation of intelligent optoelectronics. However, this is usually limited by the weaknesses of mono-component materials, such as non-adjustable photoresponse region, high carrier recombination rate, high signal-to-noise ratio, as well as the weak mechanical flexibility of bulk films. In this work, the Cs₃Cu₂I₅/ZnO heterostructure flexible photodetectors were constructed by a low-temperature solution method combined with spin-coating technique. The heterostructure combines the low dark current and strong deep ultraviolet absorption of Cs₃Cu₂I₅ quantum dots with the high carrier mobility of ZnO quantum dots as well as the efficient charge separation of the vertical p-n junction, to improve the photodetection performance. The heterostructure shows enhanced light/dark current ratio and ultraviolet-to-visible rejection ratios. Under an illumination of 280 nm light, an optical detectivity as high as 1.26 × 10¹¹ Jones was obtained; the optical responsivity and response time are much better than those of control devices. After 300 times of 180° bending cycles, the photocurrent had no obvious change. The results demonstrate that the Cs₃Cu₂I₅/ZnO heterostructure has great potential in wearable and portable visible-blind ultraviolet optoelectronic devices.

Excitation-dependent perovskite/polymer films for ultraviolet visualization

Ultraviolet (UV) visualization has extensive applications in military and civil fields such as security monitoring, space communication, and wearable equipment for health monitoring in the internet of things (IoT). Due to their remarkable optoelectronic features, perovskite materials are regarded as promising candidates for UV light detecting and imaging. Herein, we report for the first time the excitation-dependent perovskite/polymer films with dynamically tunable fluorescence ranging from green to magenta by changing the UV excitation from 260 to 380 nm. And they still render dynamic multi-color UV light imaging with different polymer matrixes, halogen ratios, and cations of perovskite materials. The mechanism of its fluorescence change is related to the chloride vacancies in perovskite materials. A patterned multi-color ultraviolet visualization pad is also demonstrated for visible conversion of the UV region. This technique may provide a universal strategy for information securities, UV visualizations, and dynamic multi-color displays in the IoT.

Two-photon absorption in halide perovskites and their applications

Active research on halide perovskites has given us a deep understanding of this family of materials and their potential for applications in advanced optoelectronic devices. One of the prominent outcomes is the use of perovskite materials for nonlinear optical applications. Two-photon absorption in perovskites, in particular their nanostructures, has been extensively studied and shows huge promise for many applications. However, we are still far from a thorough understanding of two-photon absorption in halide perovskites from a micro to macro perspective. Here we summarize different techniques for studying the two-photon absorption in nonlinear optical materials. We discuss the in-depth photophysics in two-photon absorption in halide perovskites. A comprehensive summary about the factors which influence two-photon absorption provides the direction to improve the two-photon absorption properties of halide perovskites. A summary of the recent applications of two-photon absorption in halide perovskites provides inspirations for engineers to utilize halide perovskites in two-photon absorption device development. This review will help readers to have a comprehensive and in-depth understanding of the research field of two-photon absorption of halide perovskites from microscopic mechanisms to applications. The article can serve as a manual and give inspiration for future researchers.

Photophysical and photoelectrochemical properties of CsPbBr3 films grown by an electrochemically assisted deposition

Perovskite have had a great impact on the solid-state physics world in the last decade not only achieving great success in photovoltaics but, more recently, also in the implementation of other optoelectronic devices. One of the main obstacles for the adoption of Pb-based perovskite technologies are the high amounts of Pb needed in the conventional preparation methods. Here we present for the first time a detailed analysis of the photophysical and photoelectrochemical properties of CsPbBr3 films directly grown on FTO coated glass through a novel technique based in the electrodeposition of PbO2 as CsPbBr3 precursor. This technique allows to save up to 90 % of the Pb used compared to traditional methods and can be scalable compared with the commonly used spin-coating process. The low temperature analysis of their photoluminescence spectra, performed in both steady state and time dependence, revealed a strong interaction between electrons and longitudinal optical phonons dominant at high temperatures. On the other hand, the electrochemical and photoelectrochemical analysis proves that CsPbBr3 prepared using this new method has state-of-the-art features, showing a p-type behavior under depletion regime. This is also confirmed by photoelectrochemical measurements using p-benzoquinone as target molecule. These results prove that the proposed method can be used to produce excellent CsPbBr3 films, saving much of the lead waste.

Multiple Responsive CPL Switches in an Enantiomeric Pair of Perovskite Confined in Lanthanide MOFs

Circularly polarized luminescence (CPL) switches have attracted widespread attention due to their potential applications in advanced information technologies. However, the design and fabrication of solid-state multiple-responsive CPL switches remain challenging. Here, through self-assembly of chiral metal-organic frameworks (MOFs) and perovskite nanocrystals (NCs), a pair of crystalline enantiomeric (P)-(+)/(M)-(-)-EuMOF⊃MAPbX₃ (MA = CH₃ NH₃ ⁺ , X = Cl^- , Br^- , I^- ) adducts is prepared, where the achiral MAPbBr₃ perovskite NCs embedded into chiral MOFs inherit the chirality of host MOFs by host-guest EuBr and PbO coordination bonds, which is demonstrated by synchrotron-radiation-based X-ray absorption spectroscopy. The chiral adducts show enhanced photoluminescence quantum yield (PLQY), good thermal stability of CPL in air, and photoswitchable CPL properties upon altering different UV irradiation. Based on two chiral emission centers and their different characteristics, reversible CPL switches are realized upon a diversity of external stimuli, for example, chemicals (water /CH₃ NH₃ Br solution) or temperatures (room temperature/high temperature). Benefiting from the extraordinary stimuli-responsive and highly reversible switchable CPL, multiple information encryptions and decryptions integrated with CPL, together with a chiroptical logic gate are successfully designed. This work opens a new avenue to generally fabricate solid-state CPL composite materials and develops new applications based on switchable CPL.

Pen-writing high-quality perovskite films and degradable optoelectronic devices

Paper is ubiquitous in the daily life and has been widely used for writing and drawing because of their low-cost, widely accessible, and degradable properties. However, simple ways to fabricate paper-based optoelectronic devices remain a great challenge. In this work, we report a facile method to fabricate high-quality perovskite films and optoelectronic devices on paper by direct pen-writing. Through introducing seed layers on papers, planar-integrated single-crystal perovskite films are easily prepared using commercial pens. Based on such a simple and convenient method, perovskite photodetector arrays and image sensors with graphite electrodes are fabricated on paper, and show satisfactory performances. This method provides a simple and effective approach for preparation of paper-based perovskite devices. It will be of significance for the development of degradable optoelectronic devices.

Ferro-Pyro-Phototronic Effect in Monocrystalline 2D Ferroelectric Perovskite for High-Sensitive, Self-Powered, and Stable Ultraviolet Photodetector

2D hybrid perovskite ferroelectrics have drawn great attention in the field of photodetection, because the spontaneous polarization-induced built-in electric field can separate electron-hole pairs, and makes self-powered photodetection possible. However, most of the 2D hybrid perovskite-based photodetectors focused on the detection of visible light, and only a few reports realized the self-powered and sensitive ultraviolet (UV) detection using wide bandgap hybrid perovskites. Here, 2D ferroelectric PMA₂PbCl₄ monocrystalline microbelt (MMB)-based PDs are demonstrated. By using the ferro-pyro-phototronic effect, the self-powered Ag/Bi/2D PMA₂PbCl₄ MMB/Bi/Ag PDs show a high photoresponsivity up to 9 A/W under 320 nm laser illumination, which is much higher than those of previously reported self-powered UV PDs. Compared with responsivity induced by the photovoltaic effect, the responsivity induced by the ferro-pyro-phototronic effect is 128 times larger. The self-powered PD also shows fast response and recovery speed, with the rise time and fall time of 162 and 226 μs, respectively. More importantly, the 2D PMA₂PbCl₄ MMB-based PDs with Bi/Ag electrode exhibit significant stability when subjected to high humidity, continuous laser illumination, and thermal conditions. Our findings would shed light on the ferro-pyro-phototronic-effect-based devices, and provide a good method for high-performance UV detection.

A lead-free halide hybrid perovskite (TMHD)BiCl5 for ultraviolet photodetection

Lead halide hybrid perovskites with a wide bandgap (e.g., CH3NH3PbCl3) have gained tremendous attention in the field of ultraviolet (UV) photodetection due to their brilliant optoelectronic activity.

Photo-enhanced growth of lead halide perovskite crystals and their electro-optical properties

Organic–inorganic hybrid perovskites have attracted much attention for opto-electronic applications due to their low trap density, strong light absorption, and superior charge transport performance. Generally, the high efficiency of perovskite electro-optical thin films depends on their high-quality perovskite single crystals. In this work, the external illumination is applied on the perovskite single crystals growing in solution. It was shown that larger size CH₃NH₃PbI₃ single crystals can be obtained under illumination conditions. In addition, CH₃NH₃PbI₃ films are prepared by a one-step spin coating process under illumination, the crystal size and film quality of which have been obviously improved due to the effect of illumination in the film forming process. The efficiency of the average cells based on CH₃NH₃PbI₃ films increased from 15.2% for the normal conditions to 17.7% for the illumination one, which is consistent with the light enhanced growth of CH₃NH₃PbI₃ single crystals in their solution. The “photo-inhibition nucleation” mechanism is proposed to explain the light-induced enhancement of perovskite crystal growth.

Organic–inorganic hybrid perovskites have attracted much attention for opto-electronic applications due to their low trap density, strong light absorption, and superior charge transport performance.

Efficient Photoluminescence of Manganese-Doped Two-Dimensional Chiral Alloyed Perovskites

In this work, we introduced chiral cations into the achiral two-position layered perovskite system for the first time to form an alloyed system that still retains a clear layered structure. In addition, in order to explore the potential photoelectric properties of the alloyed system, manganese ions were doped into the alloyed system. The XRD pattern shows that the steady-state absorption and emission spectra of the alloyed system have a large structural distance, while the doped manganese system exhibits a two-color photoluminescence phenomenon. In addition, combined with time-resolved fluorescence and testing, the photoluminescence characteristics and ultralong lifetime of Mn-doped samples were further characterized. The exciton band structure of the lead halide perovskite framework can be adjusted through this design strategy. Mn²⁺ ions can form characteristic energy levels in the host system and then energy transfer of excitons occurs, which is of great significance for the development of new functional and high-efficiency photoluminescent materials.

Ultralow dark current in near-infrared perovskite photodiodes by reducing charge injection and interfacial charge generation

Metal halide perovskite photodiodes (PPDs) offer high responsivity and broad spectral sensitivity, making them attractive for low-cost visible and near-infrared sensing. A significant challenge in achieving high detectivity in PPDs is lowering the dark current density (J_D) and noise current (i_n). This is commonly accomplished using charge-blocking layers to reduce charge injection. By analyzing the temperature dependence of J_D for lead-tin based PPDs with different bandgaps and electron-blocking layers (EBL), we demonstrate that while EBLs eliminate electron injection, they facilitate undesired thermal charge generation at the EBL-perovskite interface. The interfacial energy offset between the EBL and the perovskite determines the magnitude and activation energy of J_D. By increasing this offset we realized a PPD with ultralow J_D and i_n of 5 × 10^-8 mA cm^-2 and 2 × 10^-14 A Hz^-1/2, respectively, and wavelength sensitivity up to 1050 nm, establishing a new design principle to maximize detectivity in perovskite photodiodes.

Heterogeneous Integration of Chiral Lead-Chloride Perovskite Crystals with Si Wafer for Boosted Circularly Polarized Light Detection in Solar-Blind Ultraviolet Region

Chiral hybrid organic-inorganic perovskites (HOIPs) have been well developed for circularly polarized light (CPL) detection, while new members that target at solar-blind ultraviolet (UV) region remain completely unexplored. Here, an effective design strategy to demonstrate circular polarization-sensitive solar-blind UV photodetection by growing wide-bandgap chiral HOIP [(R)-MPA]₂ PbCl₄ ((R)-MPA = methylphenethylammonium) single crystals onto silicon wafers, with well-defined heterostructures, is reported. The solid mechanical and electrical connection between the chiral HOIP and silicon wafer results in strong built-in electric field at heterojunction, providing a desirable driving force for separating/transporting carriers generated under CPL excitation at 266 nm. Unexpectedly, during such a transport process, not only the chirality of HOIP crystal is transferred to the heterostructure, but also the circular polarization sensitivity is significantly amplified. Consequently, anisotropy factor of the resultant detectors can reach up to 0.4 at zero bias, which is much higher than that of the pristine single-phase chiral HOIP (≈0.1), reaching the highest among the reported CPL-UV photodetectors. As far as we know, the integration of chiral HOIP crystals with silicon technology is unprecedent, which paves a way for designing boosted-performance CPL detectors in solar-blind UV region as well as for other advanced optoelectronic devices.

Metal Halide Perovskite/2D Material Heterostructures: Syntheses and Applications

The past decade has witnessed the great success achieved by metal halide perovskites (MHPs) in photovoltaic and related fields. However, challenges still remain in further improving their performance, as well as, settling the stability issue for future commercialization. Recently, MHP/2D material heterostructures that combining MHPs with the low-cost and solution-processable 2D materials have demonstrated unprecedented improvement in both performance and stability due to the distinctive features at hetero-interface. The diverse fabrication techniques of MHPs and 2D materials allow them to be assembled as heterostructures with different configurations in a variety of ways. Moreover, the large families of MHPs and 2D materials provide the opportunity for the rational design and modification on compositions and functionalities of MHP/2D materials heterostructures. Herein, a comprehensive review of MHP/2D material heterostructures from syntheses to applications is presented. First, various fabrication techniques for MHP/2D material heterostructures are introduced by classifying them into solid-state methods and solution-processed methods. Then the applications of MHP/2D heterostructures in various fields including photodetectors, solar cells, and photocatalysis are summarized in detail. Finally, current challenges for the development of MHP/2D material heterostructures are highlighted, and future opportunities for the advancements in this research field are also provided.

Deactivation/Activation of Quenching Defects in CH3NH3PbI3 Perovskite by Direct Electron Injection/Extraction

Organometal halide perovskites (OMHPs) have emerged as advisible materials for application in optoelectronic devices over the past decade. However, a variety of complex slow responses in OMHPs under an external electric field have been observed, and the mechanisms for these responses remain a topic of intense debate. In this work, with an external voltage applied to the CH₃NH₃PbI₃ crystal, reversible photoluminescence (PL) enhancement and quenching behaviors respectively near the anode and the cathode were observed under wide-field fluorescence microscopy. Further experiments attribute the reversible PL enhancing responses to the electron injection effect increasing the radiative recombination, while PL quenching was attributed to be due to the electron extraction effect increasing the nonradiative recombination. The control of PL by external applied voltage indicates brilliant carrier mobility in the CH₃NH₃PbI₃ crystal and also reminds us to focus on the effect of hole/electron injection on the materials which may limit the performance of perovskite-based optoelectronic devices.

Benzyltrimethylammonium cadmium dicyanamide with polar order in multiple phases and prospects for linear and nonlinear optical temperature sensing

Coordination polymers with multiple non-centrosymmetric phases have sparked substantial research efforts in the materials community. We report the synthesis and properties of a hitherto unknown cadmium dicyanamide coordination polymer comprising benzyltrimethylammonium cations (BeTriMe+). The room-temperature (RT) crystal structure of [BeTriMe][Cd(N(CN)2)3] (BeTriMeCd) is composed of Cd centers linked together by triple dca-bridges to form one-dimensional chains with BeTriMe+ cations located in void spaces between the chains. The structure is polar, the space group is Cmc21, and the spontaneous polarization in the c-direction is induced by an arrangement of BeTriMe+ dipoles. BeTriMeCd undergoes a second-order phase transition (PT) at T1 = 268 K to a monoclinic polar phase P21. Much more drastic structural changes occur at the first-order PT observed in DSC at T2 = 391 K. Raman data prove that the PT at T2 leads to extensive rearrangement of the Cd-dca coordination sphere and pronounced disorder of both dca and BeTriMe+. On cooling, the HT polymorph transforms at T3 = 266 K to another phase of unknown symmetry. Temperature-resolved second harmonic generation (TR-SHG) studies at 800 nm confirm the structural non-centrosymmetry of all the phases detected. Optical studies indicate that BeTriMeCd exhibits at low temperatures an intense emission under 266 nm excitation. Strong temperature dependence of both one-photon excited luminescence and SHG response allowed for the demonstration of two disparate modes of optical thermometry for a single material. One is the classic ratiometric luminescence thermometry employing linear excitation in the ultraviolet region while the other is single-band SHG thermometry, a thus far unprecedented subtype of nonlinear optical thermometry. Thus, BeTriMeCd is a rare example of a dicyanamide framework exhibiting polar order, SHG activity, photoluminescence properties and linear and nonlinear optical temperature sensing capability.

Multifunctional Charge Transporting Materials for Perovskite Light-Emitting Diodes

Despite their low exciton-binding energies, metal halide perovskites are extensively studied as light-emitting materials owing to narrow emission with high color purity, easy/wide color tunability, and high photoluminescence quantum yields. To improve the efficiency of perovskite light-emitting diodes (PeLEDs), much effort has been devoted to controlling the emitting layer morphologies to induce charge confinement and decrease the nonradiative recombination. The interfaces between the emitting layer and charge transporting layer (CTL) are vulnerable to various defects that deteriorate the efficiency and stability of the PeLEDs. Therefore, the establishment of multifunctional CTLs that can improve not only charge transport but also critical factors that influence device performance, such as defect passivation, morphology/phase control, ion migration suppression, and light outcoupling efficiency, are highly required. Herein, the fundamental limitations of perovskites as emitters (i.e., defects, morphological and phase instability, high refractive index with poor outcoupling) and the recent developments with regard to multifunctional CTLs to compensate such limitations are summarized, and their device applications are also reviewed. Finally, based on the importance of multifunctional CTLs, the outlook and research prospects of multifunctional CTLs for the further improvement of PeLEDs are discussed.

High-Performance Deep Ultraviolet Photodetector Based on a One-Dimensional Lead-Free Halide Perovskite CsCu2I3 Film with High Stability

Lead halide perovskites have received much attention in the field of optoelectronic devices. However, the environment-unfriendly nature and intrinsic instability of these perovskites hamper their commercial applications. In this work, one novel one-dimensional lead-free halide perovskite with high stability, CsCu₂I₃, was prepared via an antisolvent-assisted crystallization method. The prepared CsCu₂I₃ bears a high exciton binding energy of ∼105 meV and a high photoluminescence quantum yield of 12.3%. We fabricated a deep ultraviolet photodetector based on a CsCu₂I₃ film that is nearly blind to 405 nm visible light but is sensitive to 265 and 365 nm illumination. The device exhibits excellent reproducibility and a high I_light/I_dark ratio of 22 under 265 nm illumination. Furthermore, the responsivity, specific detectivity, and external quantum efficiency are as high as 22.1 mA/W, 1.2 × 10¹¹ Jones, and 10.3% under a light density of 0.305 mW/cm², respectively. These findings demonstrate that CsCu₂I₃ perovskites should have great potential for future optoelectronics.

Gradient 2D/3D Perovskite Films Prepared by Hot-Casting for Sensitive Photodetectors

2D Ruddlesden-Popper perovskites have attracted wide attention recently because of tunable optoelectronic properties and have been used as alternatives to their 3D counterparts in various optoelectronic devices. Here, a series of (PEA)2(MA) n -1Pb n I3 n +1 perovskite thin films is designed and fabricated by a convenient hot-casting method to obtain gradient n in the films, which leads to the formation of vertical heterojunctions that can enhance charge separation in the films under light illumination. Based on a single gradient perovskite film, a highly sensitive and stable photodetector with a responsivity up to 149 AW-1 and a specific detectivity of 2 × 1012 Jones is obtained. This work paves a way to realizing high-performance optoelectronic devices with enhanced charge separation by introducing compositional gradient in a perovskite film.

Transparent, flexible MAPbI3 perovskite microwire arrays passivated with ultra-hydrophobic supramolecular self-assembly for stable and high-performance photodetectors

The emergence of organic-inorganic hybrid perovskites (OHPs) has revolutionised the potential performance of optoelectronic devices; most perovskites are opaque and hence incompatible with transparent optoelectronics and sensitive to environmental degradation. Here, we have reported a single-step fabrication of ultra-long MAPbI₃ perovskite microwire arrays over a large area using stencil lithography based on sequential vacuum sublimation. The environmental stability of MAPbI₃ is empowered with a newly designed and synthesized transparent supramolecular self-assembly based on a mixture of two tripodal l-Phe-C₁₁H₂₃/C₇F₁₅ molecules, which showed a contact angle of 105° and served as ultra-hydrophobic passivation layers for more than 45 days in an ambient atmosphere. The MAPbI₃ microwire arrays passivated with the supramolecular self-assembly demonstrated for the first time both excellent transparency of ∼89% at 550 nm and a remarkable photoresponse with a photo-switching ratio of ∼10⁴, responsivity of 789 A W^-1, detectivity of 10¹⁴ Jones, linear dynamic range of ∼122 dB, and rise time of 432 μs. Furthermore, the photodetector fabricated on a flexible PET substrate demonstrated robust mechanical flexibility even beyond 1200 bending cycles. Therefore, the scalable stencil lithography and supramolecular passivation approaches have the potential to deliver next-generation transparent, flexible, and stable optoelectronic devices.

Void-free 3D Bioprinting for In-situ Endothelialization and Microfluidic Perfusion

Two major challenges of 3D bioprinting are the retention of structural fidelity and efficient endothelialization for tissue vascularization. We address both of these issues by introducing a versatile 3D bioprinting strategy, in which a templating bioink is deposited layer-by-layer alongside a matrix bioink to establish void-free multimaterial structures. After crosslinking the matrix phase, the templating phase is sacrificed to create a well-defined 3D network of interconnected tubular channels. This void-free 3D printing (VF-3DP) approach circumvents the traditional concerns of structural collapse, deformation and oxygen inhibition, moreover, it can be readily used to print materials that are widely considered "unprintable". By pre-loading endothelial cells into the templating bioink, the inner surface of the channels can be efficiently cellularized with a confluent endothelial layer. This in-situ endothelialization method can be used to produce endothelium with a far greater uniformity than can be achieved using the conventional post-seeding approach. This VF-3DP approach can also be extended beyond tissue fabrication and towards customized hydrogel-based microfluidics and self-supported perfusable hydrogel constructs.

(1-C5H14N2Br)2MnBr 4 : A Lead-Free Zero-Dimensional Organic-Metal Halide With Intense Green Photoluminescence

Low-dimensional organic-inorganic hybrid materials have attracted tremendous attentions due to their fascinating properties as emerging star materials for light-emitting applications. Taking advantage of their rich chemical composition and structural diversity, here, a novel lead-free organic-manganese halide compound, (1-mPQBr)₂MnBr₄ (1-mPQ = 1-methylpiperazine, 1-C5H14N2) with zero-dimensional structure has been rationally designed and successfully synthesized through solvent-evaporation method. Systematical characterizations were carried out to investigate the structure, thermal and photophysical properties. The (1-mPQBr)₂MnBr₄ was found to crystallized into an orthorhombic crystal (P2₁2₁2₁) with lattice parameters of a = 8.272(6) Å, b = 15.982(10) Å and c = 17.489(11) Å. The structure consists of isolated [MnBr₄]²⁻ clusters and free Br⁻ ions as well as [C5H14N2]²⁺ molecules. Thermal analysis indicates that it is stable up to 300°C. Upon ultraviolet photoexcitation, the (1-mPQBr)₂MnBr₄ exhibits intense green emission centered at 520 nm with a narrow full width at half-maximum of 43 nm at room temperature, which should be assigned to the spin-forbidden internal transition (⁴T₁(G) to ⁶A₁) of tetrahedrally coordinated Mn²⁺ ions. The superior photoluminescence properties coupled with facile and efficient synthesis method of this material suggest its considerable promise to be utilized as light-emitting materials.

Perovskite Transparent Conducting Oxide for the Design of a Transparent, Flexible, and Self-Powered Perovskite Photodetector

Transparent and flexible electronic devices are highly desired to meet the great demand for next-generation devices that are lightweight, flexible, and portable. Transparent conducting oxides (TCOs), such as indium-tin oxide, serve as fundamental components for the design of transparent and flexible electronic devices. However, indium is rare and expensive. Herein, we report the fabrication of low-cost perovskite SrVO₃ TCO films on transparent and flexible mica substrates and further demonstrate their utilization as a TCO electrode for building a transparent, flexible, and self-powered perovskite photodetector. Superior stable optical transparency and electrical conductivity are retained in SrVO₃ after bending up to 10⁵ cycles. Without an external power source, the constructed all-perovskite photodetector exhibits a high responsivity (42.5 mA W^-1), fast response time (3.09/1.23 ms), and an excellent flexibility and bending stability after dozens of cycles of bending at an extreme 90° bending angle. Our results demonstrate that low-cost and structure-compatible transition metal-based perovskite oxides, such as SrVO₃, as TCO electrodes have huge potential for building high-performance transparent, flexible, and portable smart electronics.

Ultrafast and broadband photodetectors based on a perovskite/organic bulk heterojunction for large-dynamic-range imaging

Organic-inorganic hybrid perovskite (OIHP) photodetectors that simultaneously achieve an ultrafast response and high sensitivity in the near-infrared (NIR) region are prerequisites for expanding current monitoring, imaging, and optical communication capbilities. Herein, we demonstrate photodetectors constructed by OIHP and an organic bulk heterojunction (BHJ) consisting of a low-bandgap nonfullerene and polymer, which achieve broadband response spectra up to 1 μm with a highest external quantum efficiency of approximately 54% at 850 nm, an ultrafast response speed of 5.6 ns and a linear dynamic range (LDR) of 191 dB. High sensitivity, ultrafast speed and a large LDR are preeminent prerequisites for the practical application of photodetectors. Encouragingly, due to the high-dynamic-range imaging capacity, high-quality visible-NIR actual imaging is achieved by employing the OIHP photodetectors. We believe that state-of-the-art OIHP photodetectors can accelerate the translation of solution-processed photodetector applications from the laboratory to the imaging market.

2D Perovskite Sr2 Nb3 O10 for High-Performance UV Photodetectors

2D perovskites, due to their unique properties and reduced dimension, are promising candidates for future optoelectronic devices. However, the development of stable and nontoxic 2D wide-bandgap perovskites remains a challenge. 2D all-inorganic perovskite Sr₂ Nb₃ O₁₀ (SNO) nanosheets with thicknesses down to 1.8 nm are synthesized by liquid exfoliation, and for the first time, UV photodetectors (PDs) based on individual few-layer SNO sheets are investigated. The SNO sheet-based PDs exhibit excellent UV detecting performance (narrowband responsivity = 1214 A W^-1 , external quantum efficiency = 5.6 × 10⁵ %, detectivity = 1.4 × 10¹⁴ Jones @270 nm, 1 V bias), and fast response speed (t_rise ≈ 0.4 ms, t_decay ≈ 40 ms), outperforming most reported individual 2D sheet-based UV PDs. Furthermore, the carrier transport properties of SNO and the performance of SNO-based phototransistors are successfully controlled by gate voltage. More intriguingly, the photodetecting performance and carrier transport properties of SNO sheets are dependent on their thickness. In addition, flexible and transparent PDs with high mechanical stability are easily fabricated based on SNO nanosheet film. This work sheds light on the development of high-performance optoelectronics based on low-dimensional wide-bandgap perovskites in the future.

Solvent-Free Mechanochemical Synthesis of a Systematic Series of Pure-Phase Mixed-Halide Perovskites MAPb(Ix Br1-x )3 and MAPb(Brx Cl1-x )3 for Continuous Composition and Band-Gap Tuning

Hybrid perovskites have recently received much attention in optoelectronic applications. However, hybrid perovskites are unstable in a humid environment. Mixed halide perovskites (MHPs) show enhanced stability and band-gap tunability upon engineering of their halide composition. Here, MHPs are prepared through a solvent-free mechanochemical synthesis (MCS) route that allows superior control over halide compositions than the solvent synthesis routes (SS). The MCS route eliminates the problem in the preparation of MAPb(I_x Br_1-x )₃ with continuously varying x, while maintaining the material properties and suppressing phase segregation present in SS routes. UV-vis absorption and X-ray diffraction patterns confirm the production of the desired pure-phase MHPs. For MAPb(I_x Br_1-x )₃ (0≤x≤1), with increased ratio of halide (x), the cubic phase gradually transforms into the tetragonal phase and band-gap tunability is accomplished. The MCS route for the preparation of MHPs is a very promising and efficient technique for superior control in optoelectronic properties, leading to improved control in fabrication approaches.

A two-dimensional bilayered Dion–Jacobson-type perovskite hybrid with a narrow bandgap for broadband photodetection

A unique 2D bilayered Dion–Jacobson type perovskite hybrid semiconductor shows broadband photodetection.

Trilayered Lead Chloride Perovskite Ferroelectric Affording Self-Powered Visible-Blind Ultraviolet Photodetection with Large Zero-Bias Photocurrent

Perovskite ferroelectrics, in which the spontaneous polarization (P_s) is conducive to the separation of photoexcited charge carriers, have shown great potential for self-powered photodetection. Nevertheless, such self-powered ferroelectric photodetectors are mostly dominated by traditional inorganic oxides and exhibit relatively small zero-bias photocurrent, which limit their further application. Herein, we present a wide-bandgap 2D trilayered lead chloride hybrid perovskite ferroelectric, EA₄Pb₃Cl₁₀ (1, EA = ethylammonium), which shows a notable P_s of ∼4.5 μC/cm² and a high Curie temperature (415 K) beyond that of BaTiO₃ (393 K). Significantly, benefiting from the wide bandgap of 3.39 eV, a self-powered visible-blind ultraviolet (UV) photodetector has been successfully realized, with a zero-bias photocurrent as high as 18.6 μA/cm², which is nearly 2 orders of magnitude larger than those of mostly conventional inorganic ferroelectrics. As a pioneering study, this work offers an efficient approach for exploring wide-bandgap perovskite ferroelectrics and will excavate their application in the field of self-powered visible-blind UV photodetection.