Charge Carrier Mobility Research Articles

Green synthesis of nitrogen-doped TiO2 nanoparticles with exposed {001} facets using Chromolaena odorata leaf extract for photodegradation of pollutants under visible light

Facet-tailored TiO2 nanoparticles (NPs) exhibit exceptional properties due to high surface energy. Conventional strategies for the fabrication of such TiO2 NPs involve harmful chemicals, which necessitates the development of environmentally benign pathways. Plant extract-assisted synthesis has emerged as a promising green alternative to conventional nanomaterial synthesis. This work introduces an innovative method for the synthesis of nitrogen-doped TiO2 (N-TiO2) NPs with exposed {001} facets using the leaf extract of a weed plant Chromolaena odorata, which is commonly known as Siam weed. The synthesis was carried out by sol-gel process with triethylamine (TEA), hydrazine hydrate and urea being the nitrogen precursors. The synthesised N-TiO2 NPs exhibited exposed {001} facets and showed a reduction in band gap. Photo-induced degradation of methylene blue dye was used to analyse the photocatalytic capability of N-TiO2 NPs in the visible range. The effect of N precursor, N dosage and light exposure time on the catalytic efficacy was studied. N-TiO2 NPs derived from TEA with 1 mol.% dopant achieved 98 % degradation in 180 minutes, while those synthesized with hydrazine and urea attained 96 % and 93 %, respectively when compared to 90 % degradation for undoped samples. The N-doping leads to significant advancement of photocatalytic effectiveness of the TiO2 NPs by introducing mid-gap levels in the forbidden energy gap that diminishes the charge carrier-recombination and boost the charge-carrier mobility of TiO2. This along with the existence of high energy facets causes a substantial advancement in the photocatalytic function in the visible region. The proposed method is a sustainable way for synthesising N-TiO2 NPs with exposed {001} facets for environment remediation applications.

Enhanced Efficiency and Intrinsic Stability of Wide-Bandgap Perovskite Solar Cells through Dimethylamine-Based Cation Engineering.

High efficiency and stable wide-bandgap perovskite solar cells (PSCs) are important for perovskite-based tandem solar cells. However, the efficiency and stability of wide-bandgap PSCs suffers from severe phase segregation and surface defects. In this study, we propose a cation engineering strategy for wide-bandgap perovskite deposited by a two-stepsequential methodthrough the incorporation ofdimethylamine hydroiodide (DMAI)into the lead halide complex in the first step. The DMAI additive modifies the crystal structure and grain growth of perovskite film, resulting in enhancing crystal quality, suppressing photo-induced halide segregation, reducing defect density, and improving charge carrier mobility. As a result, we achieved a champion photoelectric conversion efficiency (PCE) of 21.9% for 1.68 eV wide-bandgap PSCs. Additionally, the stability of PSCs based on DMA doped perovskite was strongly enhanced, after being exposed to ambient air for 1500 hours, the unencapsulated device maintained an impressive 80.6% of their initial efficiency, demonstrating great potential for stable and efficient wide-bandgap PSCs.

Graphene Oxide-Doped Indium Oxide Buffer Film Sandwiched between Titanium Oxide Layers for the Development of Photosensitive Resistive Memory Devices.

Over the past decade, metal oxide semiconductors have attracted considerable attention because of their transparency, high intrinsic charge carrier mobility, and charge carrier density. Metal oxide semiconductors also provide a promising route to develop resistive memory devices because of the tunability of their conductivity via the removal of oxygen ions, forming oxygen vacancies that can act as electron donors. Here, this paper reports the fabrication of a resistive random-access memory (ReRAM) device with TiO2 and TiO2-x layers and introduces a solution-processed In2O3-graphene oxide (GO) buffer layer from a water-based precursor solution to tune the switching characteristics. The devices were compared with a ReRAM device with no buffer layer, and the device composition was optimized by tuning the GO concentration in the layers. The devices showed hysteresis under cyclically scanned bias between positive and negative voltages with clear switching between the low resistance state (LRS) and the high resistance state (HRS). The optimized device also showed a more than 3 orders of magnitude difference in resistance between the LRS and HRS and a stable current retention. The devices also showed photoresponsive behaviors. In the LRS, the current increased significantly under illumination, while in the HRS, the current deteriorated slowly when constantly illuminated. These results highlight the dual role of the GO flakes in the buffer layer by increasing the resistance in the HRS and providing a photoresponsive switching capability. The ReRAM device was connected to a TFT device, and its feasibility as a memory component in a digital circuit was successfully demonstrated. These results provide a new avenue for developing metal-oxide-based ReRAM devices with GO-containing active layers.

Synthesis, Structures and Air-stable N-type Organic Field-effect Transistor (OFET) Properties ofFunctionalized-phenanthrene Conjugated Asymmetric N-heteroacenes.

The development of stable high-performance n-type organic semiconductors for applications in organic field-effect transistors (OFETs) under ambient conditions is desirable but challenging. To address this issue, we here synthesized a series of functionalized-phenanthrene conjugated asymmetric N-heteroacenes, where the phenanthrene moiety was modified by N substitution or Br functionalization at different positions to induce various degrees of asymmetry in their structures. The photophysical and electrochemical properties of these molecules were studied, and their packing patterns were analysed. The OFETs based on these materials were fabricated through simple spin-coating method, and the as-resulted thin films were treated with different conditions. The devices exhibit typical n-type performances under ambient conditions with charge carrier mobilities up to 4.27 × 10-3 cm2V-1s-1. The crystallinities and morphologies of these thin films were studied to investigate the correlations between the device performances and thin-film characteristics. Our study suggests that phenanthrene conjugated N-heteroacenes can be developed as promising air-stable solution-processable n-type semiconducting materials, and Br modification at certain positions of phenanthrene is beneficial in adjusting the thin-film properties for the improvement of OFET performances.

Regulated Charge Transfer in Donor-Acceptor Metal-Organic Frameworks for Highly-Sensitive Photodetectors.

In photo-induced charge separation, organic thin films with donor and acceptor chromophores are vital for uses such as artificial photosynthesis and photodetection. The main challenges include optimizing charge separation efficiency and identifying the ideal acceptor/donor ratio. Achieving this is difficult due to the variability in molecular configurations within these typically amorphous organic aggregates. Metal-organic frameworks (MOFs) provide a structured solution by enabling systematic design of donor/acceptor blends with adjustable ratios within a crystalline lattice. We demonstrate this approach by incorporating donor and acceptor naphthalenediimide (NDI) chromophores as linkers in a highly oriented, monolithic MOF thin film. By adjusting the NDI acceptor linker concentration during the layer-by-layer assembly of surface-anchored MOF thin films (SURMOFs), we significantly enhanced charge separation efficiency. Surprisingly, the optimum acceptor concentration was only 3%, achieving a forty-fold increase in photodetection efficiency compared to baseline NDI donor-based SURMOFs. This unexpected behaviour was clarified through theoretical analysis enabled by the well-defined crystalline structure of the SURMOFs. Using density functional theory and kinetic Monte Carlo simulations, we identified two opposing effects from acceptors: the positive effect of suppressing undesirable charge carrier recombination is offset at high concentrations by a reduction in charge-carrier mobility.

High Charge Carrier Mobility in Porphyrin Nanoribbons

Polydisperse edge‐fused nickel(II) porphyrin nanoribbons have been synthesized by Yamamoto coupling followed by gold(III)‐mediated fusion, with average degrees of polymerization of up to 37 repeat units (length 31 nm). Time‐resolved optical‐pump terahertz spectroscopy measurements indicate that photo‐generated charge carriers have dc mobilities of up to 205 cm2 V–1 s–1 in these nanoribbons, exceeding the values previously reported for most other types of nanoribbon or π‐conjugated polymer.

High Charge Carrier Mobility in Porphyrin Nanoribbons.

Polydisperse edge-fused nickel(II) porphyrin nanoribbons have been synthesized by Yamamoto coupling followed by gold(III)-mediated fusion, with average degrees of polymerization of up to 37 repeat units (length 31 nm). Time-resolved optical-pump terahertz spectroscopy measurements indicate that photo-generated charge carriers have dc mobilities of up to 205 cm2 V-1 s-1 in these nanoribbons, exceeding the values previously reported for most other types of nanoribbon or π-conjugated polymer.

Enhanced thermoelectric performance of p-type BiSbTe through incorporation of magnetic CrSb

There is evidence that magnetism can potentially increase the thermopower of materials, most likely due to magnon scattering, suggesting the incorporation of intrinsic magnetic semiconductors in non-magnetic thermoelectric materials. Here, samples of p-type Bi0.5Sb1.5Te3 with 10 at. % excess Te are ball-milled with varying ratios of the antiferromagnetic semiconductor CrSb (0, 0.125, 0.5, and 1 wt. %) to prepare bulk samples by spark plasma sintering technique. The thermopower of samples containing CrSb is increased due to an increase in the effective mass of the charge carriers, indicating that there is a drag effect originating from the magnetic particles. However, this was at the expense of reduced electrical conductivity caused by reduced charge carrier mobility. While overall only marginal improvements in power factors were observed, these samples exhibited significantly lower thermal conductivity compared to the single-phase material. As a result, a peak zT value of ∼1.4 was achieved at 325 K for the sample with 0.125 wt. % CrSb. These results highlight the potential of incorporating magnetic secondary phases to enhance the thermoelectric performance of materials.

Effect of Ni doping on the acetone vapor sensing performance of ZnO nanofibers

Nickel-doped zinc oxide (NiZ) nanofibres (NFs) were fabricated using sol-gel electrospinning (ES) technique followed by pyrolysis from an aqueous solution of polyvinyl alcohol/zinc acetate/nickel acetate tetrahydrate. The morphology of the NiZ NFs was analyzed using Scanning Electron Microscopy (SEM), which revealed a uniform and well-defined fibrous structure. X-ray diffraction (XRD) results indicated complete removal of the organic phase from NiZ NFs during pyrolysis. The structural analysis confirmed the incorporation of Nickel (Ni) into the Zinc oxide (ZnO) lattice without altering its wurtzite crystal structure. The optical properties and bandgap variations were evaluated using UV-visible spectroscopy, which indicated a bandgap narrowing with increasing Nickel doping. The Photoluminescence (PL) spectroscopy confirmed the presence of defect states and recombination processes in the NFs. The gas sensing performance was investigated by measuring the response to various analytes at a concentration of 50 ppm with varying operating temperatures. The results indicated that the highest response was observed for 5 w% NiZ NFs towards acetone vapors. The response and recovery time were recorded at 80 seconds and 60 seconds. The enhanced sensitivity is attributed to the optimal doping concentration, which significantly improves the surface reaction and charge carrier mobility.

Efficient photocatalytic activity over α-MnS/ZIF-67 p-n junction: Revealing the synergistic effects of exposed crystal facets and built-in electric field mechanism

In this investigation, we explored two series of composites comprising p-type α-MnS with selectively exposed {001} or {111} facets, coated with n-type ZIF-67 nanoparticles, to enhance the photocatalytic activity on Rhodamine B (RhB) degradation. The influence of the crystal facet orientation and the p-n junction on the photocatalytic efficacy was also well studied. The α-MnS/ZIF-67 composites displayed enhanced separation of charge carriers and superior light absorption capabilities, benefiting for efficient photocatalysis. The composites with cubic α-MnS morphology exhibited a pronouncedly higher photocatalytic activity relative to those with octahedral α-MnS morphology. The composition of α-MnS with ZIF-67 constructed the formation of a p-n junction with build-in electric field, extending the response into the visible region and promoting the mobility of charge carriers, thereby improving the photocatalytic performance. Our findings revealed that superoxide radicals (O2-) were the major reactive species in the photocatalytic degradation process. This study contributes novel insights into the development of high-performance and stable metal-organic framework (MOF)-based photocatalysts through the crystal facet engineering and the construction of p-n junctions.

Oxygen enriched PAni-based counter electrode network toward efficient dye-sensitized solar cells (DSSCs)

Dye-sensitized solar cells (DSSCs) have great potential as a renewable energy technology assisting combat climate change due to its low cost, adaptability, and sustainability. Oxygen plasma ion doping is a promising strategy to improve the capacity of a low-cost, platinum-free counter-electrodes (CEs) to absorb photons and drive high-performance DSSCs via generating an abundance of active absorption sites. In this instance, novel PAni–ZnO (PZ) composite layers were designed as a CE material and received various in-situ oxygen plasma dosages, including 0, 2, 4, 6, 8, and 10 min, to improve their physiochemical and microstructural feature for the first time, to the best of our knowledge. Physical evaluations of the microstructure, porosity, morphology, contact angle, roughness, electrical, and optical, electrochemical impedance spectroscopy (EIS) features of CEs were conducted in along with an evaluation of J–V variables. Compared to pristine CE substance, the surface nature of the modified hybrids was gradually enhanced as the plasma level rose, reaching an optimum after 8 min (i.e. 0.2 µm for average pore size and average roughness Ra = 7.21 µm). Expanded plasma treatment doses also improved PV cell performance even further: after 4 min at a plasma level, η = 5.41% was obtained, and after 6 min in a oxygen plasma environment, η = 5.81% was obtained. Mixing high energetic plasma ions increased the mobility of charge carriers in PAni composites along with lowered charge carrier recombination through generating an environment that was conducive to charge dissociation. Therefore, longer lifespans and more effective charge transfer inside the photovoltaic cell as a consequence of the increased mobility less resistive losses. In this respect, following 8 min of plasma surface modification of the PZ CE, the optimized efficiency of 6.31% and Jsc of 15.6 mA/cm2 were obtained. The improvement in efficiency equated to a proportion growth of 77% versus a pristine one. This gain was explained by the reality that suffusing a quantity of oxygen plasma free radicals into the PAni system developed continuous channels that enabled the mixture to move electrons more rapidly, hence raising the photovoltaic efficiency. Overall, this study highlights the advantages of regulating heteroatom species and their co-doping, offering a new perspective for the application of heteroatom-doped CE in DSSCs.

CsSnBr3 and Cs3Bi2Br9: Structural, Optical Characteristics, and Application in a Schottky Barrier Diode

The search for alternatives to Pb‐based perovskites, due to concerns about stability and toxicity, has led to the exploration of Pb‐free options. Tin (Sn) and bismuth (Bi) are promising candidates, given their similar ionic radii to Pb and the isoelectronic nature of Pb2+ and Bi3+, which suggest comparable chemical properties. Among these, CsSnBr3 and Cs3Bi2Br9 are relatively underexplored but offer lower toxicity and enhanced stability while demonstrating optoelectronic properties suitable for various applications. In this study, CsSnBr3 and Cs3Bi2Br9 nanocrystals are synthesized using a colloidal method and integrated into Schottky diodes. X‐ray photoelectron spectroscopy analysis of the surface chemistry confirms improved thermal and phase stability compared to Pb‐based perovskites. Schottky diode parameters, including ideality factor, barrier height, and series resistance are assessed using conventional thermionic emission, modified Cheung's, and Norde's models. The Cs3Bi2Br9‐based Schottky diode exhibits superior electrical performance with the lowest series resistance and optimal barrier height. Electrical impedance spectroscopy results indicated that CsSnBr3 has higher resistances and lower capacitances than Cs3Bi2Br9, reflecting lower charge carrier mobility and more defects, although the R1C1 regions in both materials demonstrated faster charge dynamics, making them ideal for high‐speed applications.

Ultrafast Charge and Exciton Diffusion in Monolayer Films of 9-Armchair Graphene Nanoribbons.

Determining the electronic transport properties of graphene nanoribbons is crucial for assessing their suitability for applications. So far, this has been highly challenging both through experimental and theoretical approaches. This is particularly the case for graphene nanoribbons that are prepared by chemical vapor deposition, which is a scalable and industry-compatible bottom-up growth method that results in closely packed arrays of ribbons with relatively short lengths of a few tens of nanometers. In this study, the experimental technique of spatiotemporal microscopy is applied to study monolayer films of 9-armchair graphene nanoribbons prepared using this growth method, and combined with linear-scaling quantum transport calculations of arrays of thousands of nanoribbons. Both approaches directly resolve electronic spreading in space and time through diffusion and give an initial diffusivity approaching 200 cm2 s-1 during the first picosecond after photoexcitation. This corresponds to a mobility up to 550 cm2 V-1 s-1. The quasi-free carriers then form excitons, which spread with a diffusivity of tens of cm2 s-1. The results indicate that this relatively large charge carrier mobility is the result of electronic transport not being hindered by defects nor inter-ribbon hopping. This confirms their suitability for applications that require efficient electronictransport.

Bulk Heterojunction Perovskite Solar Cells Incorporated with Conjugated Polyfluorenes

Ambipolar transport characteristics of metal halide perovskites (MHPs) enable both hole and electron to be transported simultaneously in perovskite solar cells (PSCs); however, unbalanced charge transport is a fundamental limitation in boosting power conversion efficiency (PCE) of PSCs. Compared to the PSCs with either n–i–p or p–i–n device structures, bulk heterojunction (BHJ) PSCs, inspired by organic photovoltaics, are recognized as superior for balancing charge transport and maximizing interface interactions. This study reports high‐performance BHJ PSCs, where the BHJ composites are composted with the n‐type MHPs mixed with the p‐type conjugated polyfluorenes. In the systematic studies, it is indicated that a small amount of conjugated polyfluorenes component can boost the crystallinity and grain size of the MHP thin film. Moreover, BHJ composite thin films possess boosted charge carrier mobility and a tendency of balanced charge transport and suppress both radiative and non‐radiative charge carrier recombinations. Most importantly, an efficient photoinduced charge transport within the BHJ composite thin film boosts photocurrent. As a result, the PSCs based on the n‐type MHPs:p‐type conjugated polyfluorenes BHJ composite thin film exhibit 23.46% of PCE and boost stability. The results demonstrate a facial method to approach high‐performance PSCs.

Soft hydrogel semiconductors with augmented biointeractive functions.

Hydrogels, known for their mechanical and chemical similarity to biological tissues, are widely used in biotechnologies, whereas semiconductors provide advanced electronic and optoelectronic functionalities such as signal amplification, sensing, and photomodulation. Combining semiconducting properties with hydrogel designs can enhance biointeractive functions and intimacy at biointerfaces, but this is challenging owing to the low hydrophilicity of polymer semiconductors. We developed a solvent affinity-induced assembly method that incorporates water-insoluble polymer semiconductors into double-network hydrogels. These semiconductors exhibited tissue-level moduli as soft as 81 kilopascals, stretchability of 150% strain, and charge-carrier mobility up to 1.4 square centimeters per volt per second. When they are interfaced with biological tissues, their tissue-level modulus enables alleviated immune reactions. The hydrogel's high porosity enhances molecular interactions at semiconductor-biofluid interfaces, resulting in photomodulation with higher response and volumetric biosensing with higher sensitivity.

As‐Doped Polycrystalline CdSeTe: Localized Defects, Carrier Mobility and Lifetimes, and Impact on High‐Efficiency Solar Cells

AbstractThe efficiency potential for single‐junction photovoltaics (PV) is described by the detailed balance model, which requires the elimination of nonradiative recombination and perfect minority carrier collection. Improvements in GaAs, Si, and perovskite PV follow this model. It might be more complex for CdTe, a leading thin‐film PV technology. While lifetime, passivation, and doping goals for 25% efficient CdTe solar cells are largely reached, voltage is ≈20% below the detailed balance limit. Why is that? In Se‐alloyed CdSexTe1‐x (Se is required for >20% efficiency) additional losses can occur due to electrostatic and bandgap fluctuations and due to electronic trap states. To understand mechanisms limiting CdSeTe solar cell performance and to suggest improvements, carrier dynamics, and transport in CdSexTe1‐x with variation in Se composition and as doping is analyzed. It is shown that trapping, likely due to anion‐site defects and their complexes, is correlated with low charge carrier mobility of 0.1–0.6 cm2 (Vs)−1. Even with 1000 ns charge carrier lifetimes, carrier diffusion length is less than the absorber thickness, reducing efficiency to ≈23%. Device simulations are used to analyze the performance of CdSexTe1‐x solar cells; thermodynamic models are not sufficient for absorbers with electronic disorder and trapping.

Unusual Cross-Conjugation in Cyclic Dipeptide-Based Building Blocks for Donor-Acceptor Semiconducting Conjugated Polymers.

Cross conjugation, though prevalent in many organic compounds, is typically considered less effective for electron delocalization compared to linear conjugation. Consequently, it is rarely used as the backbone structure for semiconducting conjugated polymers. In this study, we designed and synthesized a novel building block, TIDP, which features a central cyclic dipeptide with cross conjugation characteristics. Strong intramolecular hydrogen bonding interactions confer TIDP with a highly rigid and coplanar conformation. Importantly, theoretical calculations reveal that π electrons are well delocalized across the entire structure, despite its low aromaticity. Conjugated polymers incorporating TIDP exhibit high charge carrier mobilities, demonstrating the effective π electrons delocalization of this innovative building block. Our findings show that with rational design, cross conjugation can achieve effective π electrons delocalization, providing a valuable approach for developing high-performance conjugated polymers for organic electronic materials.

Modelling of Cut-off Frequency of Armchair Graphene Nanoribbon Tunnel Field Effect Transistor Using Transfer Matrix Method

Graphene is considered as a promising two-dimensional material for electronic device applications due to its high charge carrier mobility and it is extremely thin in size. As a material, graphene has great potential for the development of high-speed nano electric devices. Graphene can be used as a channel material in Tunnel Field Effect Transistor (TFET). In this study, the cut-off frequency of the armchair graphene nanoribbon tunnel field effect transistor (AGNR-TFET) device was modelled quantum mechanically. The solution of the Dirac-like Hamiltonian and Poisson equations self-consistently is used to determine the potential profile of device and the transmittance is calculated using the transfer matrix method (TMM). The Landauer formulation with the help of Gauss Legendre Quadrature Method (GLQM) is used to calculate the tunnelling current and the cut-off frequency of AGNR-TFET. The calculation results showed that the cut-off frequency increased as the drain voltage increased, while the increase in channel length and the N-index graphene made the cut-off frequency decreased. Moreover, the thicker the oxide layer could increase the cut-off frequency and the higher the temperature on the device would reduce the cut-off frequency.

Unusual Cross‐Conjugation in Cyclic Dipeptide‐Based Building Blocks for Donor–Acceptor Semiconducting Conjugated Polymers

AbstractCross conjugation, though prevalent in many organic compounds, is typically considered less effective for electron delocalization compared to linear conjugation. Consequently, it is rarely used as the backbone structure for semiconducting conjugated polymers. In this study, we designed and synthesized a novel building block, TIDP, which features a central cyclic dipeptide with cross conjugation characteristics. Strong intramolecular hydrogen bonding interactions confer TIDP with a highly rigid and coplanar conformation. Importantly, theoretical calculations reveal that π electrons are well delocalized across the entire structure, despite its low aromaticity. Conjugated polymers incorporating TIDP exhibit high charge carrier mobilities, demonstrating the effective π electrons delocalization of this innovative building block. Our findings show that with rational design, cross conjugation can achieve effective π electrons delocalization, providing a valuable approach for developing high‐performance conjugated polymers for organic electronic materials.

CsPbBr3 and Cs2AgBiBr6 Composite Thick Films with Potential Photodetector Applications

This paper investigates the optoelectronic properties of CsPbBr3, a lead-based perovskite, and Cs2AgBiBr6, a lead-free double perovskite, in composite thick films synthesized using mechanochemical and hot press methods, with poly(butyl methacrylate) as the matrix. Comprehensive characterization was conducted, including X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), UV–visible spectroscopy (UV–Vis), and photoluminescence (PL). Results indicate that the polymer matrix does not significantly impact the crystalline structure of the perovskites but has a direct impact on the grain size and surface area, enhancing the interfacial charge transfer of the composites. Optical characterization indicates minimal changes in bandgap energies across all different phases, with CsPbBr3 exhibiting higher photocurrent than Cs2AgBiBr6. This is attributed to the CsPbBr3 superior charge carrier mobility. Both composites showed photoconductive behavior, with Cs2AgBiBr6 also demonstrating higher-energy (X-ray) photon detection. These findings highlight the potential of both materials for advanced photodetector applications, with Cs2AgBiBr6 offering an environmentally Pb-free alternative.