Electron Trap States Research Articles

Steering Electron–Hole Migration Pathways Using Oxygen Vacancies in Tungsten Oxides to Enhance Their Photocatalytic Oxygen Evolution Performance

AbstractThe overall water splitting efficiency is mainly restricted by the slow kinetics of oxygen evolution. Therefore, it is essential to develop active oxygen evolution catalysts. In this context, we designed and synthesized a tungsten oxide catalyst with oxygen vacancies for photocatalytic oxygen evolution, which exhibited a higher oxygen evolution rate of 683 μmol h−1 g−1 than that of pure WO3 (159 μmol h−1 g−1). Subsequent studies through transient absorption spectroscopy found that the oxygen vacancies can produce electron trapping states to inhibit the direct recombination of photogenerated carriers. Additionally, a Pt cocatalyst can promote electron trap states to participate in the reaction to improve the photocatalytic performance further. This work uses femtosecond transient absorption spectroscopy to explain the photocatalytic oxygen evolution mechanism of inorganic materials and provides new insights into the design of high‐efficiency water‐splitting catalysts.

Micro structurally engineered hysteresis-free high efficiency perovskite solar cell using Zr-doped TiO2 electron transport layer

The high trap-state density and low conductivity in compact TiO2 (c-TiO2) layer limits the power conversion efficiency (PCE) of organic-inorganic hybrid perovskite solar cells (HPSCs). Metal doping in c-TiO2 has been proven to be a successful strategy for enhancing the PCE of HPSCs. Herein, Zr is incorporated in the c-TiO2 layer with different doping concentrations, and its impact on the photovoltaic performance of methylammonium lead iodide (MAPbI3) HPSCs is investigated. Inclusion of Zr enhances the charge carrier collection at TiO2/perovskite junction as well as conductivity of TiO2 layer. It is demonstrated that Zr-doping reduces the dark current significantly by suppressing non-geminated recombinations, leakage path, and electron trap-states. As a result, the HPSCs with optimum Zr-doping (10 vol%) exhibits open-circuit voltage (VOC) of 1.076 V, short-circuit current density (JSC) of 23.57 mA cm−2, and PCE of 18.16% under one-sun illumination conditions. UV absorption confirms the increase in bandgap upon Zr-doping that leads to favorable band alignment with the perovskite layer. Further, the engineered solar cells are probed for current density-voltage (J-V) hysteresis, operational stability, leakage current, intensity-dependence behavior, and electrochemical impedance spectroscopy (EIS).

A Commercial Benchmark: Light‐Soaking Free, Fully Scalable, Large‐Area Organic Solar Cells for Low‐Light Applications

AbstractLow‐light applications provide an exciting market opportunity for organic solar cells (OSCs). However, so far, studies have only considered OSCs of limited commercial viability. Herein, the applicability of a fully‐scalable, flexible, inverted non‐fullerene acceptor (NFA) containing OSC is demonstrated by showing its superior performance to silicon under low‐light, achieving 40 µW cm−2 maximum power output at 1300 lx illumination. The effect of parasitic resistance and dark current on low‐light performance are identified. Furthermore, an atmosphere sensitive light‐soaking (LS) effect, critical for low‐light performance and resulting in undesirable S‐shaped current‐voltage characteristics, is analyzed. By employing different interlayers and photoactive layers (PALs) the origin of this LS effect is identified as poor electron extraction at the electron transport layer (ETL)/PAL interface when the common ETL ZnO is used. Two strategies are implemented to overcome the LS effect: replacement of ZnO with SnO2 nanoparticles to reduce ETL sub‐gap electron trap states or tuning the NFA energy levels to optimize interfacial energetics. Finally, the commercial viability of these LS‐free devices is demonstrated by fabricating fully printed large‐area modules (21.6 cm2) achieving a maximum power output of 17.2 µW cm−2, providing the most relevant example of the currently obtainable performance in commercial low‐light OSCs.

Thiazole‐Modified C3N4 Interfacial Layer for Defect Passivation and Charge Transport Promotion in Perovskite Solar Cells

Despite the conspicuous achievements in perovskite solar cells (PSCs), further improvement of the power conversion efficiency (PCE) is hindered by substantially detrimental carrier recombination resulting from the high interfacial charge defect density and inferior charge transport kinetics. Herein, an interface engineering strategy is developed to introduce a Lewis base thiophene or thiazole–modified C3N4 layer at the electron transfer layer (ETL)/perovskite interface to constitute a stepwise energy band alignment and passivate defects at interfaces of the perovskite film. Attributed to its well‐matched energy level with TiO2 and perovskite, the charge extraction efficiency and charge transfer dynamics can be promoted remarkably, greatly inhibiting charge recombination at the interface. Furthermore, thiophene and thiazole can donate the lone pair electrons in S or N atoms to undercoordinated Pb2+, which effectively passivates the electronic trap states caused by halogen vacancies, thereby greatly minimizing trap‐assisted nonradiative recombination in the PSCs. Eventually, the thiazole–C3N4/perovskite‐based devices acquire an outstanding efficiency of 19.23%, supported by an enhanced open‐circuit voltage (VOC) of 1.11 V with improved moisture stability. This work provides an avenue for interfacial energy level modulation and defect passivation strategies for a rational interface microstructure design for meliorating the performance of PSCs.

A single layer deposition of Li-doped mesoporous TiO2 beads for low-cost and efficient dye-sensitized solar cells

Cost-effective DSCs with superior electronic properties are gained by a reduction in electronic trap states and outstanding light scattering and harvesting.

Efficient promotion and transfer of excited charge carriers in phosphorus doped and Ni complex modified g-C3N4

Solar hydrogen energy generation via photocatalysis can play a vital role in reducing global warming, environmental pollution, and the ever-increasing energy demand. Herein, phosphorus and Ni complex modified g-C3N4 were prepared by a hydrothermal method. The bonding of electron-rich phosphorous heteroatom to the g-C3N4 changed the morphology of the g-C3N4 from nanosheets into nano-rods and significantly altered the electronic structure, as evident by the shift in the conduction band edge and bandgap of g-C3N4. FTIR and XPS measurements revealedthat the triazine ring is oxygen functionalized and the P atom bonded to g-C3N4 via PN bond. Furthermore, EPR, UPS, and UV-DRS measurements revealed that the P and Ni complex reduced the localized π-conjugated structure defects of g-C3N4 and extended the light absorption into the near IR region. The Ni complex in the photocatalysts provided the additional charge transfer band of IMCT and OMCT. EDS color mapping revealed that the Ni complex is distributed uniformly on the P-g-C3N4 surface. These surface embedded Ni complex acts as an electron trap state, thereby suppressing the charge pair recombination rate which was confirmed by the PL and TRPL measurements. The synergetic effect of Ni complex and P resulted in 86 % tetracycline degradation in 20 min. and 3272 μmol/g of H2 production in 4 h. Compared to bulk g-C3N4, 818 times higher H2 production activity was achieved for the P and Ni complex modified g-C3N4.

Conjugated polyelectrolyte doped perovskite films with enhanced photovoltaic performance and stability

Perovskite solar cells (PSCs) have achieved great success in the past few years because of outstanding photovoltaic performance. However, the existence of electronic trap states in the perovskite film is an obstacle for the fabrication of high-performance and stable PSCs. Herein, a linear conjugated polyelectrolyte was developed and employed as a dopant to improve the performance and stability of the perovskite film. The PPNNA doping can passivate surface defects to reduce the electronic trap states in the perovskite film, while simultaneously enhancing the electron extraction and transport to achieve high-efficiency solar cells. Furthermore, the PPNNA-doped perovskite film shows better stability from efficient surface passivation. The optimal 0.4% PPNNA-doped device exhibits better photovoltaic efficiency (20.38%) with enhanced device stability as compared to the control device (18.51%). This novel method by doping with the conjugated polyelectrolyte provides a pathway to fabricate high-efficiency and stable inverted PSCs.

Boosting the Efficiency of NiOx-Based Perovskite Light-Emitting Diodes by Interface Engineering.

Nickel oxide (NiOx) is a promising hole-transporting material for perovskite light-emitting diodes (PeLEDs) because of its low cost, excellent stability, and simple fabrication process. However, the electroluminescence efficiencies of NiOx-based PeLEDs are greatly limited by inefficient hole injection and exciton quenching at the NiOx-perovskite interfaces. Here, a novel interfacial engineering method with sodium dodecyl sulfate-oxygen plasma (SDS-OP) is demonstrated to simultaneously overcome the aforementioned issues. Experimental results reveal that a short OP treatment on the top of the SDS-coated NiOx significantly deepens the NiOx work function (from 4.23 to 4.85 eV) because of the formation of a large surface dipole, allowing for efficient hole injection. Moreover, the SDS-OP layer passivates the electronic surface trap states of perovskite films and suppresses the exciton quenching by NiOx. These improvements inhibit the nonradiative decays at the NiOx-perovskite interface. As a result, the external quantum efficiency of CsPbBr3 LEDs is increased from 0.052 to 2.5%; that of FAPbBr3 nanocrystals LEDs is increased from 5.6 to 7.6%.

Switchable Asymmetric Moiré Patterns with Strongly Localized States

Most moiré pattern structures are constructed by twisting the angle of two similar 2D materials. The corresponding electronic structures are fixed in device applications. Here we study moiré patterns constructed with monolayers of InSe and ferroelectric In2Se3. The ferroelectricity of In2Se3 induces deep electron trap states and allows the switch of moiré pattern by an applied electric field. Using a unique linear scaling computational method, we systematically studied the electronic structures, localized state sizes, and strong correlation effects of switchable moiré patterns of systems containing close to 10 000 atoms.

Influence of Surface Ligands on Charge-Carrier Trapping and Relaxation in Water-Soluble CdSe@CdS Nanorods

In this study, the impact of the type of ligand at the surface of colloidal CdSe@CdS dot-in-rod nanostructures on the basic exciton relaxation and charge localization processes is closely examined. These systems have been introduced into the field of artificial photosynthesis as potent photosensitizers in assemblies for light driven hydrogen generation. Following photoinduced exciton generation, electrons can be transferred to catalytic reaction centers while holes localize into the CdSe seed, which can prevent charge recombination and lead to the formation of long-lived charge separation in assemblies containing catalytic reaction centers. These processes are in competition with trapping processes of charges at surface defect sites. The density and type of surface defects strongly depend on the type of ligand used. Here we report on a systematic steady-state and time-resolved spectroscopic investigation of the impact of the type of anchoring group (phosphine oxide, thiols, dithiols, amines) and the bulkiness of the ligand (alkyl chains vs. poly(ethylene glycol) (PEG)) to unravel trapping pathways and localization efficiencies. We show that the introduction of the widely used thiol ligands leads to an increase of hole traps at the surface compared to trioctylphosphine oxide (TOPO) capped rods, which prevent hole localization in the CdSe core. On the other hand, steric restrictions, e.g., in dithiolates or with bulky side chains (PEG), decrease the surface coverage, and increase the density of electron trap states, impacting the recombination dynamics at the ns timescale. The amines in poly(ethylene imine) (PEI) on the other hand can saturate and remove surface traps to a wide extent. Implications for catalysis are discussed.

Approach of fermi level and electron-trap level in cadmium sulfide nanorods via molybdenum doping with enhanced carrier separation for boosted photocatalytic hydrogen production

Doping semiconductor with non-noble metal is a promising strategy to modulate the electronic structures and therefore develop efficient photocatalysts. In this study, we report a facile one-pot solvothermal strategy to synthesize Mo-doped CdS nanorods (NRs) using ammonium tetrathiomolybdate as the sources for both of S and Mo, cadmium acetate as the Cd source, and ethanediamine as the solvent heated at 180 °C for 24 h. The experimental characterizations and theoretical calculations reveal that Mo in the form of Mo4+ is incorporated into the CdS lattice to substitute Cd2+ ions and the Mo-S-Cd bonds are formed accordingly. The Mo doping not only introduces localized electron-trapping states at the bottom of conduction band minimum, but also elevates the Fermi level towards the defect level, which endows the doped system with enhanced n-type characteristic and the defect state with strong electron-trapping ability. Moreover, a nonuniform distribution of charge density is formed for the Mo-doped CdS NRs, facilitating the separation of photoexcited charge carriers. Therefore, the Mo-doped CdS NRs exhibit remarkably enhanced photocatalytic activity, with an average H2 production rate of 14.62 mmol·g−1·h−1 without using Pt as the co-catalyst, about 5.8 times higher than that of bare CdS. This work provides new insight into the facile synthesis of visible-light-driven photocatalysts as well as the effect of metal ion doping on the modulation of electronic structures.

Site-selective mapping of metastable states using electron-beam induced luminescence microscopy

Metastable states created by electron or hole capture in crystal defects are widely used in dosimetry and photonic applications. Feldspar, the most abundant mineral in the Earth’s crust (> 50%), generates metastable states with lifetimes of millions of years upon exposure to ionizing radiation. Although feldspar is widely used in dosimetry and geochronometry, the creation of metastable states and charge transfer across them is poorly understood. Understanding such phenomena requires next-generation methods based on high-resolution, site-selective probing of the metastable states. Recent studies using site-selective techniques such as photoluminescence (PL), and radioluminescence (RL) at 7 K have revealed that feldspar exhibits two near-infrared (NIR) emission bands peaking at 880 nm and 955 nm, which are believed to arise from the principal electron-trapping states. Here, we map for the first time the electron-trapping states in potassium-rich feldspar using spectrally-resolved cathodoluminescence microscopy at a spatial resolution of ~ 6 to 22 µm. Each pixel probed by a scanning electron microscope provides us a cathodoluminescence spectrum (SEM-CL) in the range 600–1000 nm, and elemental data from energy-dispersive x-ray (EDX) spectroscopy. We conclude that the two NIR emissions are spatially variable and, therefore, originate from different sites. This conclusion contradicts the existing model that the two emissions arise from two different excited states of a principal trap. Moreover, we are able to link the individual NIR emission peaks with the geochemical variations (K, Na and Fe concentration), and propose a model that explains the quenching of the NIR emission by Fe4+. Our study contributes to an improved understanding of charge storage in feldspathic minerals, with implications for developing sub-single grain (micrometer scale) measurement techniques in radiation dosimetry.

Passivation Strategy of Reducing Both Electron and Hole Trap States for Achieving High-Efficiency PbS Quantum-Dot Solar Cells with Power Conversion Efficiency over 12%

For the current passivation strategy, pure iodine passivation during solid-state ligand exchange (SSE) cannot completely passivate the entire surface of PbS colloidal quantum dots (CQDs). Here, a s...

Enhancing the electron transfer process of TiO2-based DSSC using DC magnetron sputtered ZnO as an efficient alternative for blocking layer

Abstract Developing dye sensitized solar cell (DSSC) technology by exploiting different alternative semiconductors has attracted research attentions. Among all types of semiconductors, ZnO nanostructures due to their unique electrical properties and the facile preparation of various morphologies, have considered as the promising materials for application in DSSCs. In the present study, DC magnetron sputtering method was utilized to prepare a ZnO thin film as an efficient alternative for TiCl4 pre-treatment to suppress the charge recombination process occurring at a conventional TiO2-based DSSC. Different thicknesses of ZnO seed layers on fluorine tin oxide conductive glass substrates (FTO) were prepared via various sputtering deposition times. Field emission-scanning electron microscopy (FE-SEM) and x-ray diffraction (XRD) analyses were utilized to study the surface uniformity and crystallinity of the ZnO nanostructures. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques were employed to investigate the photoanode interface properties via determination of the electron lifetime and density of electron in conduction band. The results demonstrated that the thickness of ZnO compact layer which either acts as electron trapping states or blocking layer has the important role in cell performance. Finally, by the optimum thickness of ZnO thin film, the highest efficiency was achieved at 5.1%.

Suppression of Persistent Photoconductivity of Rubrene Crystals using Gate-Tunable Rubrene/Bi2 Se3 Diodes with Photoinduced Negative Differential Resistance.

Organic single-crystalline semiconductors show great potential in high-performance photodetectors. However, they suffer from persistent photoconductivity (PPC) due to the charge trapping, which has severely hindered high-speed imaging applications. Here, a universal strategy of solving the PPC by integrating with topological insulator Bi2 Se3 is provided. The rubrene/Bi2 Se3 heterojunctions are selected as an example for general demonstration due to the reproducibly high mobility and broad optoelectronic applications of rubrene crystals. By virtue of high carrier concentration on Bi2 Se3 surface and the strong built-in electrical field, the photoresponse of the heterotransistor is significantly reduced for more than two orders (from over 10 s to 54ms), meanwhile the photoresponsivity can reach 124 A W-1 . To the best of knowledge, this operating speed is among the fastest responses in organic-inorganic heterojunctions. The heterotransistor also shows unique negative differential resistance under positive gate bias, which can be explained by photoinduced de-trapping of electron trap states in the bulk rubrene crystals. Besides, the rubrene/Bi2 Se3 heterojunction behaves as a gate-tunable backward-like diode due to the inhomogenous carrier distribution in the thick rubrene crystal and inversion of relative Fermi level positions. The findings demonstrate versatile functionalities of the rubrene/Bi2 Se3 heterojunctions for various emerging optoelectronic applications.

The Interplay Between Lead Vacancy and Water Rationalizes the Puzzle of Charge Carrier Lifetimes in CH3NH3PbI3: Time‐Domain Ab Initio Analysis

AbstractThe perovskite CH3NH3PbI3 excited‐state lifetimes exhibit conflicting experimental results under humid environments. Using ab initio nonadiabatic (NA) molecular dynamics, we demonstrate that the interplay between lead vacancy and water can rationalize the puzzle. The lead vacancy reduces NA coupling by localizing holes, slowing electron–hole recombination. By creating a deep electron trap state, the coexistence of a neutral lead vacancy and water molecules enhances NA coupling, accelerating charge recombination by a factor of over 3. By eliminating the mid‐gap state by accepting two photoexcited electrons, the negatively charged lead vacancy interacting with water molecules increases the carrier lifetime over 2 times longer than in the pristine system. The simulations rationalize the positive and negative effects of water on the solar cell performance exposure to humidity.

The Interplay Between Lead Vacancy and Water Rationalizes the Puzzle of Charge Carrier Lifetimes in CH3 NH3 PbI3 : Time-Domain Ab Initio Analysis.

The perovskite CH3 NH3 PbI3 excited-state lifetimes exhibit conflicting experimental results under humid environments. Using ab initio nonadiabatic (NA) molecular dynamics, we demonstrate that the interplay between lead vacancy and water can rationalize the puzzle. The lead vacancy reduces NA coupling by localizing holes, slowing electron-hole recombination. By creating a deep electron trap state, the coexistence of a neutral lead vacancy and water molecules enhances NA coupling, accelerating charge recombination by a factor of over 3. By eliminating the mid-gap state by accepting two photoexcited electrons, the negatively charged lead vacancy interacting with water molecules increases the carrier lifetime over 2 times longer than in the pristine system. The simulations rationalize the positive and negative effects of water on the solar cell performance exposure to humidity.

Direct Correlation of Nanoscale Morphology and Device Performance to Study Photocurrent Generation in Donor-Enriched Phases of Polymer Solar Cells.

The nanoscale morphology of polymer blends is a key parameter to reach high efficiency in bulk heterojunction solar cells. Thereby, research typically focusing on optimal blend morphologies while studying nonoptimized blends may give insight into blend designs that can prove more robust against morphology defects. Here, we focus on the direct correlation of morphology and device performance of thieno[3,4-b]-thiophene-alt-benzodithiophene (PTB7):[6,6]phenyl C71 butyric acid methyl ester (PC71BM) bulk heterojunction (BHJ) blends processed without additives in different donor/acceptor weight ratios. We show that while blends of a 1:1.5 ratio are composed of large donor-enriched and fullerene domains beyond the exciton diffusion length, reducing the ratio below 1:0.5 leads to blends composed purely of polymer-enriched domains. Importantly, the photocurrent density in such blends can reach values between 45 and 60% of those reached for fully optimized blends using additives. We provide here direct visual evidence that fullerenes in the donor-enriched domains are not distributed homogeneously but fluctuate locally. To this end, we performed compositional nanoscale morphology analysis of the blend using spectroscopic imaging of low-energy-loss electrons using a transmission electron microscope. Charge transport measurement in combination with molecular dynamics simulations shows that the fullerene substructures inside the polymer phase generate efficient electron transport in the polymer-enriched phase. Furthermore, we show that the formation of densely packed regions of fullerene inside the polymer phase is driven by the PTB7:PC71BM enthalpy of mixing. The occurrence of such a nanoscale network of fullerene clusters leads to a reduction of electron trap states and thus efficient extraction of photocurrent inside the polymer domain. Suitable tuning of the polymer-acceptor interaction can thus introduce acceptor subnetworks in polymer-enriched phases, improving the tolerance for high-efficiency BHJ toward morphological defects such as donor-enriched domains exceeding the exciton diffusion length.

Fabrication and characterization of ternary sepiolite/g-C3N4/Pd composites for improvement of photocatalytic degradation of ciprofloxacin under visible light irradiation

The development of high-quality photocatalytic materials for the degradation of organic pollutants under visible light irradiation is a vital field of research. In the present study, a composite of natural sepiolite clay and synthetic graphitic carbon nitride (CN) mixed with dispersed palladium nanoparticles was developed for the efficient photocatalytic degradation of ciprofloxacin (CIP) under visible light irradiation. The sepiolite, CN, and composite materials were characterized by several techniques. The sepiolite/CN composite (SC30%) displayed superior activity than pristine sepiolite and CN, resulted from the generation of new electron trap states in the interfacial contract between sepiolite and CN to suppress the charge recombination of CN. Furthermore, the well-dispersed of 1 wt% Pd-nanoparticles in the SC30% composite collectively enhanced CIP degradation by avoiding the recombination of photogenerated electrons and holes. Additionally, the electron trap states on the surface of all samples were studied using novel reversed double-beam photoacoustic spectroscopy to understand electron transfer in the composites related to the photocatalytic degradation mechanism of CIP. The developed sepiolite/CN/Pd(0) composite can act as a potential catalyst for the degradation of organic pollutants in wastewater under visible light irradiation.

Highly Efficient and Air Stable Ternary Organic Solar Cell Enabled By Employing a Perylenediimide-Based Acceptor

Organic solar cells (OSCs) are regarded as one of the promising photovoltaic technologies to address the issues of energy crisis and environmental pollution all over the world, and are thus considered as the next generation of solar cells. 1 However, the devices power conversation efficiency and stability of the devices are still important factors to limit the commercial application of OSCs. 2 To this end, enhancement of photon absorption is an attractive strategy for the increase of PCE. Recently, developing the ternary OSCs with three materials of complementary absorption spectra is an efficient method to enhance the photon harvesting of single junction devices while having a fairly simple fabrication process. Meanwhile, the morphology of active layers can be finely optimized by selecting the third component and adjusting its contents to improve performance of ternary OSCs. 3 Herein, we synthesized highly air-stable perylenediimide (PDI)-based non-fullerene material and integrated it into common organic solar cells as the third component to form the ternary OSCs. This cascade band structure showed a better energy level alignment and the complimentary absorption, thus demonstrating a high ternary device efficiency of 8.71% that represented 14.6% enhancement with respect to the corresponding binary OSCs. Besides, morphology was characterized by two-dimensional grazing-incidence X-ray diffraction (2D-GIXRD) and photo-induced force microscopy (PiFM) and results suggest improved compatibility between organic materials in OSCs, which could reduce the electronic trap states to inhibit the charge recombination and increase the stability of devices. The ternary OSCs thus exhibited good air stability, photo-stability and humidity. Our work provides a very promising approach to enhance the photovoltaic performance while demonstrating highly stable devices, which could contribute to the future development of the large area manufacturing in an ambient condition and roll-to roll processing on flexible substrates. Reference 1. Yu T., Xu X. P., Zhang G. J., Wan J. H., Li Y., Peng Q., advanced functional materials, 2017, 1701491.2. Li P. D., Mainville M., Zhang Y. L., leclerc M., Sun B. Q., Izquierdo R., Ma D. L., small, 2019, 1804671.3. An Q. S., Zhang F. J., Gao w., Sun Q. Q., Zhang M., Yang C. L., Zhang J., nano energy, 2018, 45, 177-183.