Imaging ultra-weak UV light below 100 pW cm-2 using a 4H-SiC photodetector with an Al2O3 interfacial layer.

Weak light detection is crucial in various practical applications such as night vision systems, flame monitoring, and underwater operations. Decreasing the dark current of a photodetector can effectively mitigate noises, consequently enhancing the signal-to-noise ratio and overall weak light detection performance. Herein, we demonstrate a 4H-SiC UV photodetector capable of detecting extremely weak UV light. This device comprises a photosensitive layer of 4H-SiC, two TiN electrodes and an atomically thin Al2O3 interfacial layer between TiN and the C surface of 4H-SiC. Under 360 nm UV light illumination, the proposed Al2O3 device demonstrates an ultra-low dark current of 18 fA, possibly benefiting from the effective passivation of interfacial carriers, and a boosted photo-to-dark current ratio of 6.7 × 107. Consequently, it achieves a weak-light detection limit as low as 31.8 pW cm-2, significantly outperforming the control device lacking Al2O3. When compared to previously reported SiC photodetectors, our Al2O3 device boasts an exceptional large linear dynamic range of 172 dB. Leveraging this, we construct a photodetector array capable of clearly imaging an object under ultra-weak light illumination below the 100 pW cm-2 level. The proposed photodetector represents a significant advancement in the development of highly sensitive image sensors for weak light detection.

Detecting Visible to Near-Infrared II Light via CsPbBr3 Nanocrystals/Y6 Heterojunctions.

In the endeavor to develop advanced photodetectors (PDs) with superior performance, all-inorganic perovskites, recognized for their outstanding photoelectric properties, have emerged as highly promising materials. Due to their unique electronic structure and band characteristics, the majority of all-inorganic perovskite materials are not sensitive to near-infrared (NIR) light. Here, we demonstrate the fabrication of a high-performance broadband PD comprising CsPbBr3 perovskite NCs/Y6 planar heterojunctions. The incorporation of Y6 not only facilitates charge transfer from CsPbBr3 NCs to Y6 for enhancing photodetection performance under visible illumination but also broadens the absorption spectrum range of the whole device toward the NIR regime. As a result, the heterojunction PD exhibits a photo-to-dark-current ratio above 105, a dynamic range of 149.5 dB, and an impressive lowest detection limit of incident power density of 1.6 nW/cm2 under 505 nm illumination. In the NIR regime, where photon energy is below the bandgap of CsPbBr3, electron-hole pairs can still be produced in the Y6 layer even when illuminated at 1120 nm. Consequently, photodetection is uniquely possible in PDs that incorporate heterojunctions when the illumination wavelength is longer than 565 nm. At 850 nm, the heterojunction device is capable of detecting light with power densities as low as 1.3 µW/cm2 corresponding to a LDR of 99.8 dB. The exceptional performance is attributed to the creation of a heterojunction between CsPbBr3 NCs and Y6. These findings propose a novel approach for developing broadband PDs based on perovskite NC materials.

High-Quality Solution-Processed Quasi-2D Perovskite for Low-Threshold Lasers.

Spin-coated quasi-two-dimensional halide perovskite films, which exhibit superior optoelectronic properties and environmental stability, have recently been extensively studied for lasers. Crystallinity is of great importance for the laser performance. Although some parameters related to the spin-coating process have been studied, the in-depth understanding and effective control of the acceleration rate on two-dimensional perovskite crystallization during spin-coating are still unknown. Here we investigate the effect of solvent evaporation on the microstructure of the final perovskite films during the spin-coating process. The crystallization quality of the film can be significantly improved by controlling solvent evaporation. As a result, the prepared quasi-2D perovskite film exhibits a stimulated emission threshold (pump: 343 nm, 6 kHz, 290 fs) of 550 nm as low as 16.2 µJ/cm2. Transient absorption characterization shows that the radiative biexciton recombination time is reduced from 738.5 to 438.3 ps, benefiting from the improved crystallinity. The faster biexciton recombination significantly enhanced the photoluminescence efficiency, which is critical for population inversion. This work could contribute to the development of low-threshold lasers.

High-Q two-dimensional perovskite topological laser.

Quasi-two-dimensional perovskites have attracted widespread interest in developing low-cost high-quality small lasers. The nano cavity based on topologically protected valley edge states can be robust against special defects. Here, we report a high-quality two-dimensional perovskite topological photonic crystal laser based on the quantum valley Hall effect. By adjusting the position of the air holes relative to the pillar, radiation leakage in topological edge states is reduced to a large extent, electric field distribution becomes more uniform and the quality factor can be as high as 3.6 × 104. Our findings could provide opportunities for the development of high-power, stable perovskite lasers with topological protection.

Atomic-level chemical reaction promoting external quantum efficiency of organic photomultiplication photodetector exceeding 108% for weak-light detection.

Low-cost, solution-processed photomultiplication organic photodetectors (PM-OPDs) with external quantum efficiency (EQE) above unity have attracted enormous attention. However, their weak-light detection is unpleasant because the anode Ohmic contact causes exacerbation in dark current. Here, we introduce atomic-level chemical reaction in PM-OPDs which can simultaneously suppress dark current and increase EQE via depositing a 0.8 nm thick Al2O3 by the atomic layer deposition. Suppression in dark current mainly originates from the built-in anode Schottky junction as a result of work function decrease of hole-transporting layer of which the chemical groups can react chemically with the bottom surface of Al2O3 layer at the atomic-level. Such strategy of suppressing dark current is not adverse to charge injection under illumination; instead, responsivity enhancement is realized because charge injection can shift from cathode to anode, of which the neighborhood possesses increased photogenerated carriers. Consequently, weak-light detection limit of the forwardly-biased PM-OPD with Al2O3 treatment reaches a remarkable level of 2.5 nW cm-2, while that of the reversely-biased control is 25 times inferior. Meanwhile, the PM-OPD yields a record high EQE and responsivity of 4.31 × 108% and 1.85 × 106 A W-1, respectively, outperforming all other polymer-based PM-OPDs.

Localized Bound Multiexcitons in Engineered Quasi-2D Perovskites Grains at Room Temperature for Efficient Lasers.

Reducing the excitation threshold to minimize the Joule heating is critical for the realization of perovskite laser diodes. Although bound excitons are promising for low threshold laser, how to generate them at room temperature for laser applications is still unclear in quasi-2D perovskite-based devices. In this work, via engineering quasi-2D perovskite PEA2 (CH3 NH3 )n -1 Pbn Br3 n +1 microscopic grains by the anti-solvent method, room-temperature multiexciton radiative recombination is successfully demonstrated at a remarkably low pump density of 0.97 µJ cm-2 , which is only one-fourth of that required in 2D CdSe nanosheets. In addition, the well-defined translational momentum in quasi-2D perovskite grains can restrict the Auger recombination which is detrimental to radiative emission. Furthermore, the quasi-2D perovskite grains are favorable for increasing binding energies of excitons and biexcitons and so as the related radiative recombination. Consequently, the prepared phase quasi-2D perovskite film renders a threshold of room-temperature stimulated emission as low as 13.7 µJ cm-2 , reduced by 58.6% relative to the amorphous counterpart with larger grains. The findings in this work are expected to facilitate the development of solution-processable perovskite multiexcitonic laser diodes.

Organic Photodetectors with Extended Spectral Response Range Assisted by Plasmonic Hot-Electron Injection.

Organic photodetectors (OPDs) have aroused intensive attention for signal detection in industrial and scientific applications due to their advantages including low cost, mechanical flexibility, and large-area fabrication. As one of the most common organic light-emitting materials, 8-hydroxyquinolinato aluminum (Alq3) has an absorption wavelength edge of 460 nm. Here, through the introduction of Ag nanoparticles (Ag NPs), the spectral response range of the Alq3-based OPD was successfully extended to the near-infrared range. It was found that introducing Ag NPs can induce rich plasmonic resonances, generating plenty of hot electrons, which could be injected into Alq3 and then be collected. Moreover, as a by-product of introducing Ag NPs, the dark current was suppressed by around two orders of magnitude by forming a Schottky junction on the cathode side. These two effects in combination produced photoelectric signals with significant contrasts at wavelengths beyond the Alq3 absorption band. It was found that the OPD with Ag NPs can stably generate electric signals under illumination by pulsed 850 nm LED, while the output of the reference device included no signal. Our work contributes to the development of low-cost, broadband OPDs for applications in flexible electronics, bio-imaging sensors, etc.

Enhancing Hot-Electron Photodetection of a TiO2/Au Schottky Junction by Employing a Hybrid Plasmonic Nanostructure.

Hot-electron photodetectors (HEPDs) are triggering a strong surge of interest in applications of image sensors and optics communication, since they can realize photoelectric responses when the incident photon energy is lower than the bandwidth of the semiconductor. In traditional HEPD systems, the metal layers are dressed with regular gratings, which can only excite plasmonic resonance over a narrow bandwidth, limiting the hot-electron photoelectric effect. To break this limitation, hybrid plasmonic nanostructures should be applied in HEPDs. Here, we propose a TiO2 based HEPD device incorporated with a hybrid plasmonic nanostructure, which consists of Au nanoparticles (Au NPs) and a conformal transparent Au film. With the assistance of the plasmonic resonances excited in this hybrid nanostructure, the spectrum of the photocurrent response is significantly broadened from the UV band to the visible and near-infrared (NIR) ranges. It is demonstrated that at the wavelengths of 660 nm and 850 nm, the photocurrent in the hybrid HEPD device is enhanced by 610% and 960%, respectively, compared with the counterparts without the addition of Au NPs. This work promotes the development of high performances HEPDs, offering an alternative strategy for realizing photodetection and image sensing in the NIR range.

Transfer Printing of Perovskite Whispering Gallery Mode Laser Cavities by Thermal Release Tape.

The outstanding optoelectrical properties and high-quality factor of whispering gallery mode perovskite nanocavities make it attractive for applications in small lasers. However, efforts to make lasers with better performance have been hampered by the lack of efficient methods for the synthesis and transfer of perovskite nanocavities on desired substrate at quality required for applications. Here, we report transfer printing of perovskite nanocavities grown by chemical vapor deposition from mica substrate onto SiO2 substrate. Transferred perovskite nanocavity has an RMS roughness of ~ 1.2 nm and no thermal degradation in thermal release process. We further use femtosecond laser to excite a transferred perovskite nanocavity and measures its quality factor as high as 2580 and a lasing threshold of 27.89 µJ/cm2 which is almost unchanged as compared with pristine perovskite nanocavities. This method represents a significant step toward the realization of perovskite nanolasers with smaller sizes and better heat management as well as application in optoelectronic devices.

2D Perovsktie Substrate-Assisted CsPbI3 Film Growth for High-Efficiency Solar Cells.

High-quality perovskite films are beneficial for fabricating perovskite solar cells (PSCs) with excellent photoelectric performance. The substrate on which the perovskite film grows plays a profound role in improving the crystallization quality of the perovskite film. Here, we proposed a novel method for optimizing CsPbI3 perovskite films, that is, two-dimensional (2D) perovskite substrate-assisted growth (2D-PSAG) method. The prepared PEA2PbI4 2D perovskite with proper wettability and roughness is used as a substrate to fabricate the high-quality CsPbI3 film. Moreover, it is found that PEA cations show a vertical gradient distribution within the whole CsPbI3 film because of their bottom-up self-diffusion. Also, PEA cations induce the moderate distortion of [PbI6]4- octahedron and slight lattice contraction of CsPbI3 by chemically bonding between Pb and N atoms. Surprisingly, the trace amounts of PEA cations lead to a bottom-up gradient phase transition from γ-CsPbI3 to ß-CsPbI3. Therefore, the energy-level alignment becomes more matched at the interface of the perovskite layer/hole transport layer (poly3-hexylthiophene, P3HT), which denotes a large improvement of hole transport and extraction in PSCs made with the 2D-PSAG method. As a result, the CsPbI3-based PSCs with P3HT as a hole transport layer exhibit a champion efficiency of 17.13%, while the control device exhibits a PCE of only 14.16%. The PSCs made by the 2D-PSAG method retain above 70% of the initial PCE value after storage of 9 days in air (RH 10-20%), while the control device decomposes completely after 9 days. The improved stability could originate from the steric effects of PEA cations and the high crystallization quality of the mixed-phase CsPbI3 film. Therefore, 2D-PSAG is a novel and promising strategy to develop all-inorganic PSCs with high performance and stability.

Multifunction Sandwich Structure Based on Diffusible 2-Chloroethylamine for High-Efficiency and Stable Tin-Lead Mixed Perovskite Solar Cells.

Low-bandgap tin-lead mixed perovskites (PVKs) are necessary for all-perovskite tandem solar cells. This work proposes a multifunctional sandwich structure with 2-chloroethylamine (CEA) as the top and bottom interface layer and perovskite as the core layer. The sandwich structured CEA allows large ClCH2CH2NH3+ and small Cl- to diffuse into the crystal lattice and grain boundaries of perovskites, endowing an excellent antioxidation property by forming Sn(0), coordinating with SnI2, and controlling the perovskite crystallization process. Moreover, the energy level alignment at the interface of the perovskite and transport layer becomes more matched. As a result, the CEA-modified champion device acquires a power conversion efficiency of 18.13% with an open-circuit voltage of 0.82 V and a short-circuit current density of 30.06 mA cm-2. Meanwhile, the environmental stability of CEA-modified devices is substantially enhanced. This work introduces a new strategy for improving the performance and stability of tin-lead mixed perovskite solar cells.

Boosting Continuous-Wave Laser-Driven Nonlinear Photothermal White Light Generation by Nanoscale Porosity.

The irradiation of an optically absorptive medium by a continuous-wave (CW) near-infrared (NIR) laser can result in a spectral continuum emission covering both the visible and NIR regions, which is attractive for applications as continuum light sources in diverse fields. It is shown here that this NIR-laser-driven light emission can be effectively modulated with nanoscale architecture in the medium. By using porous silica as the model matrix and Yb3+ ions as the photothermally active centers, up to 100 folds increment in NIR-laser-induced emission intensity and dramatic decrease in threshold excitation density are demonstrated. It is observed that the emission intensity exhibits a strong nonlinear dependence on the power of the NIR excitation laser, featuring clear excitation power thresholds. Based on combined numerical simulation and spectral and temperature measurements, the improved broadband emission and photothermal nonlinearity are interpreted by enhanced optical energy localization around the laser spot that results in boosting the photon-to-photon conversion efficiency. The use of the nonlinear photothermal emission process as a broadband NIR light source, which could be exploited for applications including NIR spectroscopy, imaging, and sensing, is further demonstrated as a proof-of-concept.

Sunlight-Activated Orange Persistent Luminescence from Bi-Doped SrBaZn2Ga2O7 for Warm-Color Optical Applications.

A warm persistent luminescence (PersL) material SrBaZn2Ga2O7:Bi3+ was prepared using the conventional high-temperature solid-phase reaction method. We first investigated the PersL properties of SrBaZn2Ga2O7:Bi3+ in detail via PersL spectra, PersL excitation spectrum, PersL decay curves, and thermoluminescence (TL) spectra. The highlight of this study is that in addition to the 254 nm light source, the low-energy light source of 365 nm and sunlight can effectively excite electrons and charge traps, resulting in preferable orange PersL performance. The PersL decay time of the representative sample can last for 960 s after excitation by a 365 nm light source and 900 s after excitation by simulated sunlight. Meanwhile, the PersL color can be regulated by changing the excitation wavelength. In order to explain the infrequent PersL phenomena after different light source excitations, we recorded a series of TL spectra as a function of different light sources, different charging times, and different decay times to reveal the distribution of traps in the material and the influence of trap distribution on trapping and detrapping processes. This novel sunlight-activated orange PersL material is expected to promote the development of sunlight-activated PersL materials and expand potential applications in solar energy utilization and anticounterfeit marking.

Interface modification of an electron transport layer using europium acetate for enhancing the performance of P3HT-based inorganic perovskite solar cells.

In recent years, although the power conversion efficiency (PCE) of thermally stable all-inorganic CsPbI3 perovskite solar cells (PSCs) had shown a great progress, the most reported CsPbI3 PSCs suffered from the large open-circuit voltage (Voc) loss, which is related to severe nonradiative recombination and a mismatch in energy level at the transport layer/perovskite interface. In this work, europium acetate (EuAc3) as a multifunction interface material is chosen to modify the TiO2/perovskite interface, the crystal quality of CsPbI3 perovskite films is improved, and both bulk and interfacial defects are reduced effectively. Meanwhile, the energy levels arrangement between TiO2 and CsPbI3 perovskites is also optimized, corresponding the raised built-in electric field afford a strength force to accelerate the transport and extraction of charge carriers from CsPbI3 perovskites to TiO2. As a result, the performance of CsPbI3 PSCs is largely enhanced with the PCE of 16.76%. When an Ag electrode was replaced by Au, the PCE further improves to 17.92%, which is the highest for CsPbI3 PSCs with P3HT as the HTL ever reported. Besides, the CsPbI3 PSC with the EuAc3 modification layer maintains 84% of the initial PCE under continuous UV irradiation for 250 h in a nitrogen filled glovebox, being obviously higher than the control devices with only 40% of the initial PCE after UV irradiation for 100 h in the same environment.

Application of quantum dots in perovskite solar cells.

Perovskite solar cells (PSCs) are important candidates for next-generation thin-film photovoltaic technology due to their superior performance in energy harvesting. At present, their photoelectric conversion efficiencies (PCEs) are comparable to those of silicon-based solar cells. PSCs usually have a multi-layer structure. Therefore, they face the problem that the energy levels between adjacent layers often mismatch each other. Meanwhile, large numbers of defects are often introduced due to the solution preparation procedures. Furthermore, the perovskite is prone to degradation under ultraviolet (UV) irradiation. These problems could degrade the efficiency and stability of PSCs. In order to solve these problems, quantum dots (QDs), a kind of low-dimensional semiconductor material, have been recently introduced into PSCs as charge transport materials, interfacial modification materials, dopants and luminescent down-shifting materials. By these strategies, the energy alignment and interfacial conditions are improved, the defects are efficiently passivated, and the instability of perovskite under UV irradiation is suppressed. So the device efficiency and stability are both improved. In this paper, we overview the recent progress of QDs' utilizations in PSCs.

Interface Modification of a Perovskite/Hole Transport Layer with Tetraphenyldibenzoperiflanthene for Highly Efficient and Stable Solar Cells.

Interface engineering has been recognized as a very effective method to simultaneously improve both efficiency and stability in perovskite solar cells (PSCs). In this work, we report using an excellent small molecular material tetraphenyldibenzoperiflanthene (DBP) to modify the perovskite/Spiro-OMeTAD interface to achieve significantly improved solar cell performance. It is found that the ultrathin DBP interlayer accelerates hole transfer across the FAxMA1-xPbInBr3-n/Spiro-OMeTAD interface and effectively reduces the nonradiative recombination. The Kelvin probe force microscopy and energy band analyses reveal that the DBP modification helps build better matched energy level alignment and smaller energy loss for more fluent hole transport. Consequently, the DBP-treated PSCs achieve an enhanced open-circuit voltage as high as 1.184 V and fill factor as high as 78.2% as well as the negligible hysteresis. The champion PSC made with DBP gives a PCE of 21.49%, significantly increased compared to 19.68% from the reference cell without the modification. Moreover, DBP also serves as a water-resistant protection for improved moisture stability. The PCE of the DBP-treated cells without encapsulation remains more than 84% of its initial efficiency, which is significantly higher than that of the reference PSCs (65%) after 20 days of storage under an air environment with 50-65% humidity. This study provides an effective interface modification material to address notorious stability problems in Spiro-OMeTAD-based PSCs.

High performance flexible organic photomultiplication photodetector based on an ultra-thin silver film transparent electrode.

Flexible and lightweight photomultiplication-type organic photodetectors (PM-OPDs) have attracted wide attention for their broad application prospects, especially in the field of wearable electronic products. However, the commonly used indium tin oxide (ITO) conductive anode is not conducive to realize high-performance flexible PM-OPDs due to its rigidity and fragility. Here, on the flexible polyethylene terephthalate (PET) substrate, we successfully fabricate highly sensitive poly 3-hexylthiophene:phenyl-C70-butyric acid methyl ester (P3HT:PC70BM, 100:1) based PM-OPDs using ultra-thin silver films as transparent anodes. Specifically, a 1 nm thick MoO3 layer is utilized as the wetting layer for facilitating the silver film percolation, and a 2 nm thick MoO3 layer, as the hole transport layer, is coated on top of the ultra-thin silver film before coating the P3HT:PC70BM film. The as-prepared flexible PM-OPDs based on the ultra-thin silver film exhibit the optimal external quantum efficiency (EQE) and responsivity (R) of 1.3 × 105% and 388.4 A W-1, respectively, under -15 V bias, which are 1.98 times and 2.15 times greater than those of the ITO anode based device. More importantly, the device has good flexibility with the EQE maintaining 70.6% of its initial value after bending 10 times, and 51.4% of its initial value after bending 1000 times. This work paves the way for developing flexible PM-OPDs as well as other flexible optoelectronic devices.

Broadband and wide-angle light absorption of organic solar cells based on multiple-depths metal grating.

A silver grating containing three grooves with different depths in one period was proposed as the back electrode for improving light absorption in organic solar cells. We found that the broadband absorption enhancement of the active layer covering the visible and near-infrared bands can be obtained due to the excitation of surface plasmon resonance and the multiple resonances of cavity mode. The integrated absorption efficiency of the proposed structure under TM polarization between 350 nm to 900 nm is 57.4%, with consideration of the weight of AM 1.5G solar spectrum, and is increased by 13.4% with respect to the equivalent planar device. Besides, the wide-angle absorption in proposed structure can be observed in the range from 0 to 50 degrees. These findings are of great importance for rationally designing composite nanostructures of metal gratings-based absorbers for sensing and photon-detecting applications.

Research Progress in Organic Photomultiplication Photodetectors.

Organic photomultiplication photodetectors have attracted considerable research interest due to their extremely high external quantum efficiency and corresponding high detectivity. Significant progress has been made in the aspects of their structural design and performance improvement in the past few years. There are two types of organic photomultiplication photodetectors, which are made of organic small molecular compounds and polymers. In this paper, the research progress in each type of organic photomultiplication photodetectors based on the trap assisted carrier tunneling effect is reviewed in detail. In addition, other mechanisms for the photomultiplication processes in organic devices are introduced. Finally, the paper is summarized and the prospects of future research into organic photomultiplication photodetectors are discussed.