Showing papers by "Xiaoming Wen published in 2018"

Universal passivation strategy to slot-die printed SnO 2 for hysteresis-free efficient flexible perovskite solar module

Abstract: Perovskite solar cells (PSCs) have reached an impressive efficiency over 23%. One of its promising characteristics is the low-cost solution printability, especially for flexible solar cells. However, printing large area uniform electron transport layers on rough and soft plastic substrates without hysteresis is still a great challenge. Herein, we demonstrate slot-die printed high quality tin oxide films for high efficiency flexible PSCs. The inherent hysteresis induced by the tin oxide layer is suppressed using a universal potassium interfacial passivation strategy regardless of fabricating methods. Results show that the potassium cations, not the anions, facilitate the growth of perovskite grains, passivate the interface, and contribute to the enhanced efficiency and stability. The small size flexible PSCs achieve a high efficiency of 17.18% and large size (5 × 6 cm2) flexible modules obtain an efficiency over 15%. This passivation strategy has shown great promise for pursuing high performance large area flexible PSCs.

Chemical Dopant Engineering in Hole Transport Layers for Efficient Perovskite Solar Cells: Insight into the Interfacial Recombination

Abstract: Chemical doping of organic semiconductors has been recognized as an effective way to enhance the electrical conductivity In perovskite solar cells (PSCs), various types of dopants have been developed for organic hole transport materials (HTMs); however, the knowledge of the basic requirements for being efficient dopants as well as the comprehensive roles of the dopants in PSCs has not been clearly revealed Here, three copper-based complexes with controlled redox activities are applied as dopants in PSCs, and it is found that the oxidative reactivity of dopants presents substantial impacts on conductivity, charge dynamics, and solar cell performance A significant improvement of open-circuit voltage (Voc) by more than 100 mV and an increase of power conversion efficiency from 132 to 193% have been achieved by tuning the doping level of the HTM The observed large variation of Voc for three dopants reveals their different recombination kinetics at the perovskite/HTM interfaces and suggests a model of an

Template-Free Synthesis of High-Yield Fe-Doped Cesium Lead Halide Perovskite Ultralong Microwires with Enhanced Two-Photon Absorption.

Abstract: Doping in perovskite is challenging and competitive due to the inherently fast growth mechanism of perovskite structure. Here, we demonstrate successful synthesis of high-yield Fe-doped cesium lead halide perovskite ultralong microwires (MWs) that have diameters up to ∼5 μm and lengths up to millimeters via an antisolvent vapor-assisted template-free method. Microstructure characterization confirms the uniformly doped Fe in the high-quality crystal perovskite MWs. Significantly, doping the Fe(III) concentration can affect both the MW morphology and photoluminescence (PL). The band edge emission of the MW at variable excitation has been accounted for by the superposition and combination of optical transitions of nearby singlet, triplet, and magnetic polaronic excitons. High-quality two-photon PL emission and the enhanced nonlinear absorption coefficient of Fe-doped MWs have been experimentally demonstrated. This superhigh nonlinear absorption coefficient and high-quality optical properties endow it with pr...

Slow Response of Carrier Dynamics in Perovskite Interface upon Illumination

Abstract: The current–voltage hysteresis, as well as the performance instability of perovskite solar cells (PSCs) under a working condition, is serving as the major obstacle toward their commercialization while the exact fundamental mechanisms to these issues are still in debate In this study, we investigated the slow variation of photogenerated carrier dynamics in a (FAPbI3)085(MAPbBr3)015 perovskite interface under continuous illumination Different response behaviors of carrier dynamics in the perovskite interfaces with and without the hole transport layer, Spiro-OMeTAD (Spiro), were systematically studied by time-dependent, steady-state, and time-resolved photoluminescence It was demonstrated that a light-induced defect curing process is dominantly responsible for the carrier dynamics evolution for the perovskite interface without Spiro, whereas both defect curing process and mobile ion migration should be accounted for the dynamic response of the perovskite interface contact with Spiro When contacted with

Improving the Photo-Oxidative Performance of Bi2MoO6 by Harnessing the Synergy between Spatial Charge Separation and Rational Co-Catalyst Deposition.

Abstract: It has been reported that photogenerated electrons and holes can be directed toward specific crystal facets of a semiconductor particle, which is believed to arise from the differences in their surface electronic structures, suggesting that different facets can act as either photoreduction or photo-oxidation sites. This study examines the propensity for this effect to occur in faceted, plate-like bismuth molybdate (Bi2MoO6), which is a useful photocatalyst for water oxidation. Photoexcited electrons and holes are shown to be spatially separated toward the {100} and {001}/{010} facets of Bi2MoO6, respectively, by facet-dependent photodeposition of noble metals (Pt, Au, and Ag) and metal oxides (PbO2, MnO x, and CoO x). Theoretical calculations revealed that differences in energy levels between the conduction bands and valence bands of the {100} and {001}/{010} facets can contribute to electrons and holes being drawn to different surfaces of the plate-like Bi2MoO6. Utilizing this knowledge, the photo-oxidative capability of Bi2MoO6 was improved by adding an efficient water oxidation co-catalyst, CoO x, to the system, whereby the extent of enhancement was shown to be governed by the co-catalyst location. A greater oxygen evolution occurred when CoO x was selectively deposited on the hole-rich {001}/{010} facets of Bi2MoO6 compared to when CoO x was randomly located across all of the facets. The elevated performance exhibited for the selectively loaded CoO x/Bi2MoO6 was ascribed to the greater opportunity for hole trapping by the co-catalyst being accentuated over other potentially detrimental effects, such as the co-catalyst acting as a recombination medium and/or covering reactive sites. The results indicate that harnessing the synergy between the spatial charge separation and the co-catalyst location on the appropriate facets of plate-like Bi2MoO6 can promote its photocatalytic activity.

Role of Surface Recombination in Halide Perovskite Nanoplatelets.

Abstract: Halide perovskites are an extremely promising material platform for solar cells and photonic devices. The role of surface carrier recombination-well known to detrimentally affect the performance of devices-is still not well understood for thin samples where the thickness is comparable to or less than the carrier diffusion length. Here, using time-resolved microspectroscopy along with modeling, we investigate charge-carrier recombination dynamics in halide perovskite CH3NH3PbI3 nanoplatelets with thicknesses from ∼20 to 200 nm, ranging from much lesser than to comparable to the carrier diffusion length. We show that surface recombination plays a stronger role in thin perovskite nanoplatelets, significantly decreasing photoluminescence (PL) efficiency, PL decay lifetime, and photostability. Interestingly, we find that both thick and thin nanoplatelets exhibit a similar increase in PL efficiency with increasing excitation fluence, well described by our excitation saturation model. We also find that the excited carrier distribution along the depth impacts the surface recombination. Using the diffusion-surface recombination model, we determine the surface recombination velocity. This work provides a comprehensive understanding of the role of surface recombination and charge-carrier dynamics in thin perovskite platelets and reveals valuable insights useful for applications in photovoltaics and photonics.

Oxygen-deficient bismuth tungstate and bismuth oxide composite photoanode with improved photostability

Abstract: A homogeneous layer of Bi2O3-Bi14WO24 composite (BWO/Bi2O3) thin film was fabricated using a combination of electrodeposition and thermal treatment. The evenly distributed Bi14WO24 component within the Bi2O3 layer was found to be important in stabilising the photoelectrochemical performances of Bi2O3 photoanode by promoting the photoelectron transport. The unmodified Bi2O3 suffered from severe photocorrosion as proven by X-ray diffraction (XRD) and inductively coupled plasma (ICP) analyses while the composite thin film was active without noticeable activity decay for at least 3 h of illumination. This strategy might be applicable to other photocatalysts with stability issues.

Free charges versus excitons: photoluminescence investigation of InGaN/GaN multiple quantum well nanorods and their planar counterparts.

Abstract: InGaN/GaN multiple quantum well (MQW) nanorods have demonstrated significantly improved optical and electronic properties compared to their planar counterparts. However, the exact nature of the processes whereby nanorod structures impact the optical properties of quantum wells is not well understood, even though a variety of mechanisms have been proposed. We performed nanoscale spatially resolved, steady-state, and time-resolved photoluminescence (PL) experiments confirming that photoexcited electrons and holes are strongly bound by Coulomb interactions (i.e., excitons) in planar MQWs due to the large exciton binding energy in InGaN quantum wells. In contrast, free electron–hole recombination becomes the dominant mechanism in nanorods, which is ascribed to efficient exciton dissociation. The nanorod sidewall provides an effective pathway for exciton dissociation that significantly improves the optical performance of InGaN/GaN MQWs. We also confirm that surface treatment of nanorod sidewalls has an impact on exciton dissociation. Our results provide new insights into excitonic and charge carrier dynamics of quantum confined materials as well as the influence of surface states.

Ultrafast carrier dynamics in GaN/InGaN multiple quantum wells nanorods

Abstract: GaN/InGaN multiple quantum wells (MQW) is a promising material for high-efficiency solid-state lighting. Ultrafast optical pump-probe spectroscopy is an important characterization technique for examining fundamental phenomena in semiconductor nanostructure with sub-picosecond resolution. In this study, ultrafast exciton and charge carrier dynamics in GaN/InGaN MQW planar layer and nanorod are investigated using femtosecond transient absorption (TA) techniques at room temperature. Here nanorods are fabricated by etching the GaN/InGaN MQW planar layers using nanosphere lithography and reactive ion etching. Photoluminescence efficiency of the nanorods have been proved to be much higher than that of the planar layers, but the mechanism of the nanorod structure improvement of PL efficiency is not adequately studied. By comparing the TA profile of the GaN/InGaN MQW planar layers and nanorods, the impact of surface states and nanorods lateral confinement in the ultrafast carrier dynamics of GaN/InGaN MQW is revealed. The nanorod sidewall surface states have a strong influence on the InGaN quantum well carrier dynamics. The ultrafast relaxation processes studied in this GaN/InGaN MQW nanostructure is essential for further optimization of device application.