Surface Trap States Research Articles

Activating the surface and bulk of hematite photoanodes to improve solar water splitting.

A simple electrochemical activation treatment is proposed to improve effectively the photoelectrochemical performance of Nb,Sn co-doped hematite nanorods. The activation process involves an initial thrice cathodic scanning (reduction) and a subsequent thrice anodic scanning (oxidation), which modifies both the surface and bulk properties of the Nb,Sn:Fe2O3 photoanode. First, it selectively removes the surface components to different extents endowing the hematite surface with fewer defects and richer Nb-O and Sn-O bonds and thus passivates the surface trap states. The surface passivation effect also enhances the photoelectrochemical stability of the photoanode. Finally, more Fe2+ ions or oxygen vacancies are generated in the bulk of hematite to enhance its conductivity. As a result, the photocurrent density is increased by 62.3% from 1.88 to 3.05 mA cm-2 at 1.23 VRHE, the photocurrent onset potential shifts cathodically by ∼70 mV, and photoelectrochemical stability improves remarkably relative to the pristine photoanode under simulated sunlight (100 mW cm-2).

Synergistic ligand exchange and UV curing of PbS quantum dots for effective surface passivation.

Lead sulfide (PbS) quantum dots (QDs) are promising materials in solution-processed photovoltaic (PV) devices due to their tunable bandgap and low-cost processing. Replacing the long oleic acid ligands of the as-synthesized QDs with shorter ligands is a key step for making functional QD PVs with correctly tuned band energies and reduced non-radiative recombination centers. In this work, we study the effect of ultraviolet (UV) treatment of PbS QD layers on the QD surface states during ligand exchange. We demonstrate that this straightforward approach effectively reduces the surface trap states and passivates the surface of QDs. We find that UV treatment reduces the density of hydroxyl groups attached to the QD surface and improves the bonding of short ligands to the QD surface. Multiple analyses show the reduction of nonradiative recombination centers for the UV-treated sample. The power conversion efficiency (PCE) of our optimized PbS QD device reached 10.7% (vs. 9% for the control device) and was maintained above 10% after 230 h of constant illumination.

Rationalizing and Controlling the Surface Structure and Electronic Passivation of Cesium Lead Halide Nanocrystals.

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as versatile photonic sources. Their processing and luminescent properties are challenged by the lability of their surfaces, i.e., the interface of the NC core and the ligand shell. On the example of CsPbBr3 NCs, we model the nanocrystal surface structure and its effect on the emergence of trap states using density functional theory. We rationalize the typical observation of a degraded luminescence upon aging or the luminescence recovery upon postsynthesis surface treatments. The conclusions are corroborated by the elemental analysis. We then propose a strategy for healing the surface trap states and for improving the colloidal stability by the combined treatment with didodecyldimethylammonium bromide and lead bromide and validate this approach experimentally. This simple procedure results in robust colloids, which are highly pure and exhibit high photoluminescence quantum yields of up to 95–98%, retained even after three to four rounds of washing.

Developing 1D Sb-Embedded Carbon Nanorods to Improve Efficiency and Stability of Inverted Planar Perovskite Solar Cells.

To overcome the zigzag pathway transport of the electron diffusion process and eliminate the surface trap states of phenyl-C61-butyric acid methyl ester (PCBM) nanofilms in inverted perovskite solar cells, novel 1D N-type doped carbon nanorods (CNRs) are developed by a stibonium (Sb) auxiliary ball milling method and introduced into the PCBM film to prepare the PCBM:Sb-CNRs hybrid transport layer. In this way, the N-type doped Sb-CNRs can extend the built-in electric field between CH3 NH3 PbI3 and PCBM to facilitate the separation of electron/hole pairs. The discontinuous band with the built-in potential in the PCBM/Sb-CNRs heterojunction can boost interfacial charge redistribution and promote electrons diffusion from PCBM to electrode through 1D Sb-CNRs network. As a result, the high device efficiency of 19.26% with enhanced air stability and little hysteresis are achieved. This work demonstrates a simple strategy to improve the efficiency and stability of perovskite photovoltaic devices using low-cost carbon nanomaterials.

Synergistic Crystal and Interface Engineering for Efficient and Stable Perovskite Photovoltaics

AbstractThe presence of bulk and surface defects in perovskite light harvesting materials limits the overall efficiency of perovskite solar cells (PSCs). The formation of such defects is suppressed by adding methylammonium chloride (MACl) as a crystallization aid to the precursor solution to realize high‐quality, large‐grain triple A‐cation perovskite films and that are combined with judicious engineering of the perovskite interface with the electron and hole selective contact materials. A planar SnO2/TiO2 double layer oxide is introduced to ascertain fast electron extraction and the surface of the perovskite facing the hole conductor is treated with iodine dissolved in isopropanol to passivate surface trap states resulting in a retardation of radiationless carrier recombination. A maximum solar to electric power conversion efficiency (PCE) of 21.65% and open circuit photovoltage (Voc) of ≈1.24 V with only ≈370 mV loss in potential with respect to the band gap are achieved, by applying these modifications. Additionally, the defect healing enhances the operational stability of the devices that retain 96%, 90%, and 85% of their initial PCE values after 500 h under continuously light illumination at 20, 50, and 65 °C, respectively, demonstrating one of the most stable planar PSCs reported so far.

Charge/energy transfer dynamics in CuO quantum dots attached to photoresponsive azobenzene ligand

A series of stable CuO quantum dots (QDs) in aqueous alkaline medium are prepared and protected against rapid oxidation by capping with mercaptoacetic acid (MAA). The MAA not only regulates the size of the CuO QDs but it also acts as pendent group for further functionalization of QDs. A specific concentration of MAA improves the crystallinity and increases the fluorescence lifetime by decreasing the density of surface trap states. The MAA capped CuO QDs are attached to (E)-5-((4-chlorophenyl) diazenyl)-2-hydroxybenzaldehyde (CPDHB) and upon UV irradiation at 300 nm the CPDHB experiences a reversible trans- to cis-isomerization. The absorption band of the cis-isomer of CPDHB overlaps with the fluorescing band of the CuO QDs, as a result the fluorescence of the QDs is strongly quenched. The photoluminescenc (PL) quenching is attributed due to the photoinjected electron transfer from the conduction band (CB) of the CuO QDs to the lowest unoccupied molecular orbital (LUMO) of the cis-isomer of CPDHB. Förster resonance energy transfer (FRET) is also observed but its contribution is minor. The fluorescence quenching of CuO QDs in QDs - CPDHB assembly upon ultra-violet excitation and reversible trans-cis interconversion of CPDHB may potentially suggests to design fluorescent probes for bioimaging and to develop data and energy storage devices.

Surface Modification of Hematite Photoanodes for Improvement of Photoelectrochemical Performance

Solar water splitting is a promising method for producing renewable fuels. Thermodynamically, the overall water splitting reaction is an uphill reaction involving a multiple electron transfer process. The oxygen evolution reaction (OER) has been identified as the bottleneck process. Hematite (α-Fe2O3) is one of the best photoanode material candidates due to its band gap properties and stability in aqueous solution. However, the reported efficiencies of hematite are notoriously lower than the theoretically predicted value mainly due to poor charge transfer and separation ability, short hole diffusion length as well as slow water oxidation kinetics. In this Review Article, several emerging surface modification strategies to reduce the oxygen evolution overpotential and thus to enhance the water oxidation reaction kinetics will be presented. These strategies include co-catalysts loading, photoabsorption enhancing (surface plasmonic metal and rare earth metal decoration), surface passivation layer deposition, surface chemical etching and surface doping. These methods are found to reduce charge recombination happening at surface trapping states, promote charge separation and diffusion, and accelerate water oxidation kinetics. The detailed surface modification methods, surface layer materials, the photoelectrochemical (PEC) performances including photocurrent and onset potential shift as well as the related proposed mechanisms will be reviewed.

Encapsulated room-temperature synthesized CsPbX3 perovskite quantum dots with high stability and wide color gamut for display

Room temperature recrystallization is an intriguing method of fabricating CsPbX3 perovskite quantum dots since it does not involve high temperature or inert atmosphere, offering a promising route to the mass production of CsPbX3 quantum dots at low cost. However, their performance stability during work was seldom investigated and was far from the requirements for practical applications. Here, we demonstrate a facile and low-cost method to significantly improve the thermal, photo- and water stability of room-temperature synthesized perovskite quantum dots by effectively suppressing the unfavored grain growth and surface trap states. The fabricated quantum dots of green-emitting CsPbBr3 and red-emitting CsPbBr1.2I1.8 were applied for quantum dot-converted white LEDs, which are capable of achieving a wide color gamut of 135% NTSC and 101% Rec. 2020.

Repairing Defects of Halide Perovskite Films To Enhance Photovoltaic Performance.

On account of the low-temperature solution fabrication, the much high defect density at the interfaces and grain boundaries of halide perovskite films is recognized as one of the big obstacles toward high-efficiency solar cells. Here, the time-resolved photoluminescence (TRPL) with incident light exciting from the upper surface and bottom of halide perovskite films, respectively, showed very different results, verifying the much more surface trap states in the film. To eliminate the defects and enhance the photovoltaic properties of perovskite solar cells (PSCs), we designed a facile and effective method to repair the defects of the perovskite film using formamidinium iodine (FAI) solution. The dissociative FA+ and I- ions could compensate for the loss of volatile organic cations and also fill the I- vacancies of halide perovskites. After repairing defects with proper concentration of FAI solution, the TRPL curves obtained by light exciting from the different sides of the perovskite film nearly overlap together, indicating the reduction of surface traps. As a result, both the total carrier lifetime and charge extractions were improved by removing the nonradiative channels (surface traps), which universally enhanced the power conversion efficiency and stability of the planar heterojunction structural PSCs.

Assembly-enhanced fluorescence from metal nanoclusters and quantum dots for highly sensitive biosensing

Fluorescent nanoparticles exhibit unique optical properties for applications in imaging and sensing. Herein, the enhancement of luminescence emission from self-assembled nanoparticles including gold nanoclusters, cadmium sulphide and near-infrared silver selenide quantum dots are reported. Addition of zinc and pyrophosphate ions induces assembly and disassembly of the particle aggregates and the corresponding intensified and recovery of fluorescence intensity, respectively. The enhancement is due to assembly-enhanced emission of metal nanoclusters and passivation of the surface trap states of quantum dots. With the hydrolysis of pyrophosphate catalyzed by alkaline phosphatase, the released zinc ions from pyrophosphate-zinc ion complexes mediate nanoparticle assembly and lead to luminescence regeneration. The assembly-induced fluorescence enhancement can be applied for sensitive assay of alkaline phosphatase with a detection limit of 3.4 pM in a continuous and real-time way. This method is capable of screening enzyme inhibitor and analyzing biological samples.

Fabrication of cost effective and efficient dye sensitized solar cells with WO3-TiO2 nanocomposites as photoanode and MWCNT as Pt-free counter electrode

Tungsten oxide/titanium oxide nanocomposites (nWO3-TiO2) with three different WO3-TiO2 mass ratios (n = 1%, 3% and 5%) were synthesized and used as a photo-anode to fabricate dye sensitized solar cells (DSSC), in conjunction with multiwall carbon nanotube (MWCNTs) as a platinum free (Pt-free) counter electrode. It was observed that [1%WO3-TiO2/N719/MWCNT] DSSC exhibited an efficiency enhancement of about 40% as compared to the conventional TiO2/dye/Pt solar cell, and 18% efficiency enhancement compared to [1%WO3-TiO2/N719/Pt] solar cell. The reduced electron-hole (e, h+) recombination in the nWO3-TiO2 nanocomposite due to reduced surface trap states, the increased accumulation of electrons at the photo-anode mediated by the photo-excitation of the dye, the efficient electron transportation and increased light absorption can be attributed to the superior performance of [nWO3-TiO2/N719/MWCNT] DSSCs in general. Also nWO3-TiO2 nanocomposite, when used as a photo-catalyst in the presence of UV radiation for the photo-catalytic degradation of Methylene blue (MB) dye, demonstrated the dye degradation efficiency as high as 99.99%. The enhanced degradation of MB dye further confirms the reduced charge (e, h+) recombination and enhanced light absorption, the features essential for a good DSSC. In addition to the photovoltaic and photo-catalytic studies, the morphological and optical characterization of nWO3-TiO2 nanocomposite photo-anodes films, the catalytic, electrochemical and electrical characterization of counter electrode and the fabricated DSSCs were carried out.

How Trap States Affect Charge Carrier Dynamics of CdSe and InP Quantum Dots: Visualization through Complexation with Viologen

The depth of surface trap states in semiconductor quantum dots (QDs) is influenced by the degree of covalency, which in turn affects the charge recombination process in hybrid donor–acceptor systems. By taking relatively ionic cadmium selenide (CdSe) QDs with shallow trap states and covalent indium phosphide (InP) QDs having deep trap states as examples, the charge-transfer dynamics are explored using viologen derivative as an electron acceptor. Light-induced electron transfer in a 1:1 stoichiometric complex of both the donor–acceptor systems occurs in a picosecond time scale. The presence of deep hole trap states in InP QDs retards the charge recombination to a submillisecond time scale, which is 7 orders of magnitude lower than that in CdSe QDs in homogeneous solutions. The immobile quenchers in the quenching sphere of InP further stabilize the electron-transfer products to seconds through charge hopping, which extends the potential of these systems for charge-transfer and transport applications in phot...

Surface Trap States Passivation for High‐Performance Inorganic Perovskite Solar Cells

Cesium halide perovskite CsPbX3 has emerged to be a promising candidate for photovoltaic materials due to their componential and thermal stability. During the fabrication of CsPbX3 films, rich halide ions could cause deep trap states on the surface of the perovskite film, leading to much charge recombination. Herein, Pb2+ solution post‐processing strategy is introduced to passivate the deep trap states of CsPbI2Br films. The dissociative Pb2+ in the solution effectively combines with the excess halide ions on the perovskite surface to reduce the deep trap states of Pb vacancy (VPb) and I interstitial (Ii). As a result, the average photoluminescence lifetimes τave of the perovskite film prolonged nearly double after passivation. The trap density of perovskite is effectively decreased from 8 × 1016 to 6.64 × 1016 cm−3. The CsPb2Br solar cell shows an open‐circuit‐voltage as high as 1.29 V and power conversion efficiency of 12.34% with small hysteresis. The postprocessing method would provide an avenue to improve further the efficiency of inorganic perovskite solar cells via reducing surface traps.

Vacuum-Ultraviolet Photodetection in Few-Layered h-BN.

Over the past 20 years, astro and solar physicists have been working hard to develop a new-generation semiconductor-based vacuum-ultraviolet (VUV, 100-200 nm) photodetector with small size and low power consumption, to replace the traditional microchannel detection system, which is ponderous and has high energy consumption, and finally to reduce the power load and launch costs of explorer satellites. However, this expectation has hardly been achieved due to the relatively low photoresponsivity and external quantum efficiency (EQE) of the reported VUV photoconductive detectors based on traditional wide-band-gap materials and structures. Here, on the basis of few-layer h-BN, we fabricated a high-performance two-dimensional photodetector with selective response to VUV light. Typically, it has high sensitivity (EQE = 2133%, at 20 V) to the extremely weak 160 nm light (3.25 pW). This excellent photoresponsivity can be attributed to the high carrier collection efficiency and existing surface trap states of few-layer h-BN. In addition, this device can maintain a stable performance in a wide temperature range (80-580 K), which is quite favorable for application in deep space with huge temperature fluctuation.

Completely quenching of the trap states emission of CdSe QDs by CdS/ZnS shell growth using a one pot photochemical approach and application for dye photo-degradation

In this research CdSe QDs was grown by microwave activated approach. Then using UV-sensitivity of TGA, CdS shell was grown on the CdSe cores and subsequently ZnS was shelled on the CdSe/CdS QDs. Synthesized QDs were characterized by means of XRD, FTIR, EDAX, UV–Vis and PL analysis. CdSe QDs indicated a broad band surface trap states emission between 500 and 800 nm with a peak at about 600 nm. This emission was decreased after CdS shell growth and was completely quenched after ZnS shell growth and a narrow band edge emission confirming successfully growth of the shell. Photocatalyst activity of the synthesized QDs was investigated by using methyl orange (MO) and methylene blue (MB) as pollutant. The CdSe indicated good photocatalyst activity for photo-degradation and it׳s activity was considerably increased after CdS shell and CdS/ZnS shell growth.

Ti(OBu)4 and cobalt iron oxide co-modified hematite nanoparticles synthesized by a facile anodic electrodeposition for highly efficient water oxidation

Based on the bare hematite prepared by a facile anodic electrodeposition, Ti(OBu)4 modified hematite through FeOOH film surface is a better method for obtaining more excellent photoelectrochemical performance. This is due to the significantly increased bulk charge separation efficiency caused by the refined grain, preferred orientation of (110) plane and higher donor density. Coupled with Cobalt Iron Oxide (CIO) nanoparticles, Ti(OBu)4 modified hematite photoanode has a 3 times enhanced photocurrent density of 1.02 mA cm−2 and a negative shift of onset potential about 40 mV compared with bare hematite photoanode. In which, Ti(OBu)4 can increase the electron-hole separation and passivate the surface trap states simultaneously. The CIO nanoparticles coated on the surface of Ti/FeOOH sample not only act as a catalyst for promoting the surface hole injection for water oxidation, but also boost the charge separation with passivating the surface states. This work provides an indepth understanding of the effect of Ti(OBu)4 and CIO nanoparticles on morphology, crystalline structure and photoelectrochemical performance of anodic electrodeposited hematite photoanode via systematical characterization with scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis spectra, X-ray diffractometry (XRD), X-ray photoelectron spectra (XPS), and photoelectrical measurements.

Interface Modification by Ionic Liquid: A Promising Candidate for Indoor Light Harvesting and Stability Improvement of Planar Perovskite Solar Cells

AbstractOrganic–inorganic hybrid perovskite solar cells (PSCs) are currently attracting significant interest owing to their promising outdoor performance. However, the ability of indoor light harvesting of the perovskites and corresponding device performance are rarely reported. Here, the potential of planar PSCs in harvesting indoor light for low‐power consumption devices is investigated. Ionic liquid of 1‐butyl‐3‐methylimidazolium tetrafluoroborate ([BMIM]BF4) is employed as a modification layer of [6,6]‐phenyl‐C61‐butyric acid methyl ester) (PCBM) in the inverted PSCs. The incorporation of [BMIM]BF4 not only paves the interface contact between PCBM and electrode, but also facilitates the electron transport and extraction owing to the efficient passivation of the surface trap states. Moreover, [BMIM]BF4 with excellent thermal stability can act as a protective layer by preventing the erosion of moisture and oxygen into the perovskite layer. The resulting devices present a record indoor power conversion efficiency (PCE) of 35.20% under fluorescent lamps of 1000 lux, and an impressive PCE of 19.30% under 1 sun illumination. The finding in this work verifies the excellent indoor performance of PSCs to meet the requirements of eco‐friendly economy.

Thick Layered Semiconductor Devices with Water Top-Gates: High On–Off Ratio Field-Effect Transistors and Aqueous Sensors

Layered semiconductors show promise as channel materials for field-effect transistors (FETs). Usually, such devices incorporate solid back or top gate dielectrics. Here, we explore deionized (DI) water as a solution top-gate for field-effect switching of layered semiconductors including SnS2, MoS2, and black phosphorus. The DI water gate is easily fabricated, can sustain rapid bias changes, and its efficient coupling to layered materials provides high on-off current ratios, near-ideal subthreshold swing, and enhanced short-channel behavior even for FETs with thick, bulk-like channels, where such control is difficult to realize with conventional back gating. Screening by the high-k solution gate eliminates hysteresis due to surface and interface trap states and substantially enhances the field-effect mobility. The onset of water electrolysis sets the ultimate limit to DI water gating at large negative gate bias. Measurements in this regime show promise for aqueous sensing, demonstrated here by the amperometric detection of glucose in aqueous solution. DI water gating of layered semiconductors can be harnessed in research on novel materials and devices, and it may with further development find broad applications in microelectronics and sensing.

AlGaN barrier thickness dependent surface and interface trapping characteristics of AlGaN/GaN heterostructure

The aluminium gallium nitride (AlGaN) barrier thickness dependent trapping characteristic of AlGaN/GaN heterostructure is investigated in detail by frequency dependent conductance measurements. The conductance measurementsin the depletion region biases (−4.8 V to −3.2 V) shows that the Al0.3Ga0.7N(18 nm)/GaN structure suffers from both the surface (the metal/AlGaN interface of the gate region) and interface (the AlGaN/GaN interface of the channel region) trapping states, whereas the AlGaN/GaN structure with a thicker AlGaN barrier (25 nm) layer suffers from only interface (the channel region of AlGaN/GaN) trap energy states in the bias region (−6 V to −4.2). The two extracted time constants of the trap levels are (2.6–4.59) μs (surface) and (113.4–33.8) μs (interface) for the Al0.3Ga0.7N(18 nm)/GaN structure in the depletion region of biases (−4.8 V to −3.2 V), whereas the Al0.3Ga0.7N (25 nm)/GaN structure yields only interface trap states with time constants of (86.8–33.3) μs in the voltage bias range of −6.0 V to −4.2 V. The extracted surface trapping time constants are found to be very muchless in the Al0.3Ga0.7N(18 nm)/GaN heterostructure compared to that of the interface trap states. The higher electric field formation across the AlGaN barrier causes de-trapping of the surface trapped electron through a tunnelling process for the Al0.3Ga0.7N(18 nm)/GaN structure, and hence the time constants of the surface trap are less.

NiO Nanoparticles Anchored on Phosphorus-Doped α-Fe2 O3 Nanoarrays: An Efficient Hole Extraction p-n Heterojunction Photoanode for Water Oxidation.

The photoelectrochemical (PEC) water-splitting efficiency of a hematite-based photoanode is still far from the theoretical value due to its poor surface reaction kinetics and high density of surface trapping states. To solve these drawbacks, a photoanode consisting of NiO nanoparticles anchored on a gradient phosphorus-doped α-Fe2 O3 nanorod (NR) array (NiO/P-α-Fe2 O3 ) was fabricated to achieve optimal light absorption and charge separation, as well as rapid surface reaction kinetics. Specifically, a photoanode with the NR array structure allowed a high mass-transport rate to be achieved, while phosphorus doping effectively decreased the number of surface trapping sites and improved the electrical conductivity of α-Fe2 O3 . Furthermore, the p-n junction that forms between NiO and P-α-Fe2 O3 can further improve the PEC performance due to efficient hole extraction and the water oxidization catalytic activity of NiO. Consequently, the NiO/P-α-Fe2 O3 NR photoanode produced a high photocurrent density of 2.08 mA cm-2 at 1.23 V versus a reversible hydrogen electrode and a 110 mV cathodic shift of the onset potential. This rational design of structure offers a new perspective in exploring high-performance PEC photoanodes.