Showing papers by "Xiaoming Wen published in 2021"

Metal-Organic Framework Decorated Cuprous Oxide Nanowires for Long-lived Charges Applied in Selective Photocatalytic CO2 Reduction to CH4.

Abstract: Improving the stability of cuprous oxide (Cu2 O) is imperative to its practical applications in artificial photosynthesis. In this work, Cu2 O nanowires are encapsulated by metal-organic frameworks (MOFs) of Cu3 (BTC)2 (BTC=1,3,5-benzene tricarboxylate) using a surfactant-free method. Such MOFs not only suppress the water vapor-induced corrosion of Cu2 O but also facilitate charge separation and CO2 uptake, thus resulting in a nanocomposite representing 1.9 times improved activity and stability for selective photocatalytic CO2 reduction into CH4 under mild reaction conditions. Furthermore, direct transfer of photogenerated electrons from the conduction band of Cu2 O to the LUMO level of non-excited Cu3 (BTC)2 has been evidenced by time-resolved photoluminescence. This work proposes an effective strategy for CO2 conversion by a synergy of charge separation and CO2 adsorption, leading to the enhanced photocatalytic reaction when MOFs are integrated with metal oxide photocatalyst.

The critical role of composition-dependent intragrain planar defects in the performance of MA1–xFAxPbI3 perovskite solar cells

Abstract: Perovskite solar cells show excellent power conversion efficiencies, long carrier diffusion lengths and low recombination rates. This encourages a view that intragrain defects are electronically benign with little impact on device performance. In this study we varied the methylammonium (MA)/formamidinium (FA) composition in MA1–xFAxPbI3 (x = 0–1), and compared the structure and density of the intragrain planar defects with device performance, otherwise keeping the device nominally the same. We found that charge carrier lifetime, open-circuit voltage deficit and current density–voltage hysteresis correlate empirically with the density and structure of {111}c planar defects (x = 0.5–1) and {112}t twin boundaries (x = 0–0.1). The best performance parameters were found when essentially no intragrain planar defects were evident (x = 0.2). Similarly, reducing the density of {111}c planar defects through MASCN vapour treatment of FAPbI3 (x ≈ 1) also improved performance. These observations suggest that intragrain defect control can provide an important route for improving perovskite solar cell performance, in addition to well-established parameters such as grain boundaries and interfaces. The role of intragrain planar defects in halide perovskite solar cell devices remains elusive. Now, Li et al. tune the composition of the perovskite layer to minimize the planar defect density and observe an improvement in the device performance.

Hexagonal metal oxide monolayers derived from the metal–gas interface

Abstract: Two-dimensional (2D) crystals are promising materials for developing future nano-enabled technologies1-6. The cleavage of weak, interlayer van der Waals bonds in layered bulk crystals enables the production of high-quality 2D, atomically thin monolayers7-10. Nonetheless, as earth-abundant compounds, metal oxides are rarely accessible as pure and fully stoichiometric monolayers owing to their ion-stabilized 'lamellar' bulk structure11-14. Here, we report the discovery of a layered planar hexagonal phase of oxides from elements across the transition metals, post-transition metals, lanthanides and metalloids, derived from strictly controlled oxidation at the metal-gas interface. The highly crystalline monolayers, without the support of ionic dopants or vacancies, can easily be mechanically exfoliated by stamping them onto substrates. Monolayer and few-layered hexagonal TiO2 are characterized as examples, showing p-type semiconducting properties with hole mobilities of up to 950 cm2 V-1 s-1 at room temperature. The strategy can be readily extended to a variety of elements, possibly expanding the exploration of metal oxides in the 2D quantum regime.

Photophysics of 2D Organic-Inorganic Hybrid Lead Halide Perovskites: Progress, Debates, and Challenges.

Abstract: 2D organic-inorganic hybrid Ruddlesden-Popper perovskites (RPPs) have recently attracted increasing attention due to their excellent environmental stability, high degree of electronic tunability, and natural multiquantum-well structures Although there is a rapid development of photoelectronic applications in solar cells, photodetectors, light emitting diodes (LEDs), and lasers based on 2D RPPs, the state-of-the-art performance is far inferior to that of the existing devices because of the limited understanding on fundamental physics, especially special photophysics in carrier dynamics, excitonic fine structures, excitonic quasiparticles, and spin-related effect Thus, there is still plenty of room to improve the performances of photoelectronic devices based on 2D RPPs by enhancing knowledge on fundamental photophysics This review highlights the special photophysics of 2D RPPs that is fundamentally different from the conventional 3D congeners It also provides the most recent progress, debates, challenges, prospects, and in-depth understanding of photophysics in 2D perovskites, which is significant for not only boosting performance of solar cells, LEDs, photodetectors, but also future development of applications in lasers, spintronics, quantum information, and integrated photonic chips

Linking Phase Segregation and Photovoltaic Performance of Mixed-Halide Perovskite Films through Grain Size Engineering

Abstract: Mixed-halide perovskites are attractive candidates as wide-bandgap absorber layers in tandem solar cells However, photoinduced phase segregation leads to the formation of Br-rich and I-rich domain

Lead-free metal-halide double perovskites: from optoelectronic properties to applications

Abstract: Abstract Lead (Pb) halide perovskites have witnessed highly promising achievements for high-efficiency solar cells, light-emitting diodes (LEDs), and photo/radiation detectors due to their exceptional optoelectronic properties. However, compound stability and Pb toxicity are still two main obstacles towards the commercialization of halide perovskite-based devices. Therefore, it is of substantial interest to search for non-toxic candidates with comparable photophysical characteristics. Metal-halide double perovskites (MHDPs), A2BBʹX6, are recently booming as promising alternatives for Pb-based halide-perovskites for their non-toxicity and significantly enhanced chemical and thermodynamic stability. Moreover, this family exhibits rich combinatorial chemistry with tuneable optoelectronic properties and thus a great potential for a broad range of optoelectronic/electronic applications. Herein, we present a comprehensive review of the MHDPs synthesized so far, and classified by their optical and electronic properties. We systematically generalize their electronic structure by both theoretical and experimental efforts to prospect the relevant optoelectronic properties required by different applications. The progress of the materials in various applications is explicated in view of the material structure-function relationship. Finally, a perspective outlook to improve the physical and optoelectronic properties of the materials is proposed aiming at fostering their future development and applications.

Self-assembled carbon dot-wrapped perovskites enable light trapping and defect passivation for efficient and stable perovskite solar cells

Abstract: Simultaneously improving photovoltaic performance and longevity has become the main focus towards the commercialization of metal halide perovskite solar technology. Herein, we demonstrate resilient, high-efficiency triple-cation perovskite solar cells (PSCs) by incorporating carbon dots (CDs) derived from human hair into the perovskite film synthesis. It is found that a toluene-based antisolvent containing CDs results in the formation of a bilayer structure where a wave-like textured top perovskite layer is assembled on the bottom dense perovskite counterpart, enabling reduced optical losses through light trapping. Further characterization has revealed that the CDs are formed around and over the surface of perovskite crystals, serving as a full armour to preserve the perovskite stoichiometry during the crystallization and operation. Accordingly, the CD-wrapped perovskite film demonstrates a reduced density of interfacial defects including metallic lead clusters and uncoordinated halide vacancies, improved carrier recombination lifetime, better energy alignment with the adjacent hole transport layer, and enhanced hydrophobicity. By leveraging these advantages to enhance the efficiency of PSCs, we have achieved a maximum power conversion efficiency of 20.22%, higher than 18.72% for PSCs without CDs, and the device stability is also significantly enhanced.

Layer number dependent exciton dissociation and carrier recombination in 2D Ruddlesden-Popper halide perovskites

Abstract: It has been a consensus that the exciton binding energies in two-dimensional (2D) Ruddlesden–Popper perovskites is closely correlated with the number of layers. However, the detailed recombination dynamics of excitons and free carriers in these 2D perovskites are still highly controversial. Using transient spectroscopic techniques, carrier dynamics of 2D Ruddlesden–Popper perovskites are investigated at both room and low temperatures. We confirmed that 2D perovskites of (BA)2PbI4, with the number of [PbX6]4− layers n = 1, exhibit clearly exciton characteristic properties. Meanwhile, (BA)2(MA)Pb2I7 and (BA)2(MA)2Pb3I10, n = 2 and 3, exhibit free carrier behaviour, which is inconsistent with their binding energies of more than 100 meV. Such anomalous exciton and free carrier behaviours are attributed to the effective exciton in-plane transport and dissociation by the edge state. This investigation provides novel insights into exciton and charge carrier dynamics of 2D perovskites as well as the influence of edge states.

Spectroscopic Insight into Efficient and Stable Hole Transfer at the Perovskite/Spiro-OMeTAD Interface with Alternative Additives.

Abstract: A stable and efficient carrier transfer is a prerequisite for high-performance perovskite solar cells With optimized additives, a significantly improved charge carrier transfer can be achieved at the interface of perovskite/2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,90-spirobifluorene (Spiro-OMeTAD) with significantly boosted photostability Using time-dependent spectroscopic techniques, we investigated charge carrier and mobile-ion dynamics at the perovskite/Spiro-OMeTAD interface, where the Spiro-OMeTAD contains different bis(trifluoromethanesulfonyl)imide (TFSI) salts additives (Li-TFSI, Mg-TFSI2, Ca-TFSI2) The pristine response and the dynamic changes under continuous illuminations are presented, which is correlated to the different behaviors of mobile-ion accumulations at the perovskite/Spiro interface and ascribed to the improved hole mobilities in Spiro-OMeTAD, ultimately contributing to the favorable behaviors in solar cells It is demonstrated that the hole mobility and conductivity of hole transport layers play an important role in suppressing mobile-ion accumulation at the interfaces of solar cells With the engineering of mixed-cation mixed-halide perovskite, optimal engineering of additives in hole transport materials is an efficient strategy Therefore, it should be emphasized for accelerating perovskite photovoltaic commercialization

Manipulating the Fate of Charge Carriers with Tungsten Concentration: Enhancing Photoelectrochemical Water Oxidation of Bi2WO6

Abstract: Bismuth tungstate (Bi2 WO6 ) thin film photoanode has exhibited an excellent photoelectrochemical (PEC) performance when the tungsten (W) concentration is increased during the fabrication. Plate-like Bi2 WO6 thin film with distinct particle sizes and surface area of different exposed facets are successfully prepared via hydrothermal reaction. The smaller particle size in conjunction with higher exposure extent of electron-dominated {010} crystal facet leads to a shorter electron transport pathway to the bulk surface, assuring a lower charge transfer resistance and thus minimal energy loss. In addition, it is proposed based on the results from conductive atomic force microscopy that higher W concentration plays a crucial role in facilitating the charge transport of the thin film. The "self-doped" of W in Bi2 WO6 will lead to the higher carrier density and improved conductivity. Thus, the variation in the W concentration during a synthesis can be served as a promising strategy for future W based photoanode design to achieve high photoactivity in water splitting application.

Intermediate phase-enhanced Ostwald ripening for the elimination of phase segregation in efficient inorganic CsPbIBr2 perovskite solar cells

Abstract: Mixed halide perovskites with the ability to tune bandgaps exhibit attractive applications in tandem solar cells, building integrated photovoltaic and wavelength-tunable light-emitting devices. However, halide demixing under illumination or in the dark with a charge-carrier injection in both hybrid and inorganic perovskites results in bandgap instability and current-density-voltage (J-V) hysteresis, which can significantly hamper their application. Here, we demonstrate that halide segregation and J-V hysteresis in mixed halide inorganic CsPbIBr2 solar cells can be effectively mitigated by introducing an intermediate phase-enhanced Ostwald ripening through the control of the chemical composition in the CsPbIBr2 precursor solution. Excess amounts of either PbBr2 or CsI are incorporated into originally even molar amounts of PbBr2 and CsI precursor solutions. With the PbBr2-excess, we observed an enlarged perovskite grain size, no detectable halide phase segregation at the grain boundaries nor the perovskite/TiO2 interface, an increased minority carrier lifetime, a reduced J-V hysteresis, and an improved solar-cell performance. However, different CsI:PbBr2 stoichiometric ratios were found to have different effects on the performance of the perovskite solar cell. The excessive lead phase is reactive with the dimethyl sulfoxide (DMSO) in the precursor solution to form the Pb(I, Br)2-DMSO complex and the quasi-two-dimensional (2D) CsPb2(I, Br)5, which are conducive to Ostwald maturation and defect extinction. Finally, the CsPbIBr2 solar cell with a PbBr2-excess precursor composition reaches a power conversion efficiency (PCE) of 9.37% (stabilized PCE of 8.48%) and a maximum external quantum efficiency of over 90%.

Self-Assembled Perovskite Nanoislands on CH3NH3PbI3 Cuboid Single Crystals by Energetic Surface Engineering

Abstract: Organometal perovskite single crystals have been recognized as a promising platform for high-performance optoelectronic devices, featuring high crystallinity and stability. However, a high trap density and structural nonuniformity at the surface have been major barriers to the progress of single crystal-based optoelectronic devices. Here, the formation of a unique nanoisland structure is reported at the surface of the facet-controlled cuboid MAPbI(3) (MA = CH3NH3+) single crystals through a cation interdiffusion process enabled by energetically vaporized CsI. The interdiffusion of mobile ions between the bulk and the surface is triggered by thermally activated CsI vapor, which reconstructs the surface that is rich in MA and CsI with reduced dangling bonds. Simultaneously, an array of Cs-Pb-rich nanoislands is constructed on the surface of the MAPbI(3) single crystals. This newly reconstructed nanoisland surface enhances the light absorbance over 50% and increases the charge carrier mobility from 56 to 93 cm(2) V-1 s(-1). As confirmed by Kelvin probe force microscopy, the nanoislands form a gradient band bending that prevents recombination of excess carriers, and thus, enhances lateral carrier transport properties. This unique engineering of the single crystal surface provides a pathway towards developing high-quality perovskite single-crystal surface for optoelectronic applications.

A high-performance visible-light-driven all-optical switch enabled by ultra-thin gallium sulfide

Abstract: On-chip optical switches have emerged as a new class of photonic components for high-performance optical communication networks and on-chip interconnects, in which the all-optical configuration without the incorporation of other control-means is highly desired. While two-dimensional (2D) ultrathin materials demonstrate their great potential in developing ultrafast all-optical switches owing to their unique light–matter interaction, such investigations have so far been limited to the fiber optic platform or free space. Here, we realize an all-optical on-chip switch from a silicon waveguide-based asymmetric Mach–Zehnder interferometer (MZI) structure enabled by 2D ultrathin Ga2S3. Upon the visible light excitation at 532 nm, excessive photocarriers in Ga2S3 cause a change of the refractive index and subsequently a phase variation between MZI arms at the 1550 nm operation wavelength, triggering on the optical switch. On the other hand, the switch is off without the visible light stimulation, as the phase variation is recovered due to the ultrafast photo-exciton relaxation behavior of Ga2S3. The Ga2S3-enabled all-optical switch is driven at an extremely small optical power density of 0.12 W cm−2 and exhibits a response and recovery time of 26.3 and 43.5 μs, respectively, which as a combination is superior to those of fiber optic-based all-optical switches enabled by 2D materials. This work may provide a viable approach to develop on-chip all-optical photonic components for practical integrated photonic chips.

Ni2+ doping induced structural phase transition and photoluminescence enhancement of CsPbBr3

Abstract: All-inorganic CsPbBr3 perovskites exhibit great potential in optoelectronic application due to their outstanding physical properties for optoelectronics. However, the relatively low luminescence efficiency of the bulk CsPbBr3 crystal hampers its further application. Here, we report a facile method to enhance the optical efficiency of halide perovskite powder via doping the transition metal Ni ions. It is demonstrated that Ni doping results in the formation of the Cs(NiPb)Br3 alloy and also triggers the phase transition of CsPbBr3 from orthorhombic to cubic. Both the Cs(NiPb)Br3 alloy and cubic CsPbBr3 are found to contribute to the asymmetric photoluminescence (PL) spectra in the green-yellow light band. The splitting-peak fitting experiments for the overlapped PL spectrum based on the bi-Gaussian line model reveal that the Cs(NiPb)Br3 alloy dominates the contribution of the observed PL enhancement. The Ni2+ doping is also beneficial for optimizing the crystallinity of the CsPbBr3 perovskite, suppressing the nonradiative recombination and significantly enhancing the photoluminescence consequently. Our investigation provides a novel insight into the effect of environmentally friendly metal doping in perovskites.

Ultralong free-standing single crystal perovskite microwires with extremely low dark current

Abstract: Super long perovskite microwires (PMWs) are in a great demand in many fields such as low-loss microcables and integrated optical waveguide. Despite decades of research into PMWs, single crystal PMWs with several centimeters long have not been obtained. Here, ultralong (up to 7.6 centimeters) monoclinic crystal structure CH3NH3PbI3·DMF PMWs have been synthesized. The high-quality microwire exhibits long carrier lifetime of 1775.7 ns. The as-prepared free-standing PMWs can be integrated to any arbitrary substrate and 808 nm near-infrared photodetectors have been successfully demonstrated. The fabricated device shows a high light on/off ratio of 1.79×106 and an extremely low dark current of 2.5 fA at 1 V bias. This work provides a strategy for the solution growth of ultralong microwires.