Carrier Mobility Research Articles

The Structural, Electrical and Dielectric Studies of CMC Based Biopolymer Gel Electrolytes for Ecofriendly Device Applications

ABSTRACTThe solution casting procedure has been effectively used to synthesize biopolymer gel electrolytes (BGEs) using carboxymethyl cellulose (CMC) and ammonium thiocyanate (NH4SCN). The XRD patterns of the bio‐based green emulsions (BGEs) have provided evidence of the non‐crystalline structure of the films. Optical micrographs of BGEs have revealed the formation of homogeneous gel electrolyte films. Complex impedance spectroscopy has been used to investigate the ion transport mechanism and dielectric relaxation dynamics of biopolymer gel electrolyte films over a wide range of temperatures. The conductivity of the gel electrolyte samples has been shown to rise with the amount of salt present. The optimum ionic conductivity, determined at room temperature, is σ = 3.97 × 10−3 Scm−1, and it is achieved in the gel electrolyte sample containing 35 wt% NH4SCN. The variation in ionic conductivity with temperature has shown a blend of Arrhenius and Vogel‐Tamman‐Fulcher (VTF) characteristics. An analysis has been conducted on the impedance data to investigate the ion transport process using the formalisms of ac conductivity, permittivity, and electric modulus. The dielectric impedance of the BGEs films has been used to determine the charge carrier density and mobility. The electrolyte with the best conductivity has shown a broad range of electrochemical stability, spanning from −1.49 to +1.27 V. The I‐t investigations has shown a high transference number (tion ~ 0.99), indicating that ions are the primary contributors to conductivity. The cyclic voltammetry tests have shown excellent reusability, indicating its potential use in devices, such as supercapacitors and rechargeable batteries.

Linkage Regulation of β-Ketoamine Covalent Organic Frameworks for Boosting Photocatalytic Overall Water Splitting.

Two dimensional β-ketoamine covalent organic frameworks (2D TP-COFs) are one category of promising metal-free catalysts for photocatalytic overall water splitting (OWS) because of their unusual stability and versatile electronic/optical properties. However, none of the currently reported TP-COFs can accomplish the hydrogen evolution (HER) and oxygen evolution reactions (OER) simultaneously without adding any sacrificial agents and cocatalysts. To address this challenging issue, we rationally designed 23 2D TP-COFs by regulating the linkage groups and comprehensively evaluated their OWS activity by using the first-principles method. First, the electronic band structure calculations at the HSE06 level reveal that the band gap can be reasonably adjusted with values ranging from 1.67-3.16 eV. Among these 23 systems, 10 TP-COFs are realized to match well with both the chemical potentials of H2/H+ and O2/H2O, which are capable of visible-light-driven OWS from an electronic perspective. Further thermal activity results on OWS demonstrate that only Hep-BDA (heptazine-aniline) and Bpy-4 (bipyrimidinamine) based COFs can satisfy the completely spontaneous of HER and OER under light irradiation and neutral conditions. Importantly, the calculated small exciton binding energies and high carrier mobility for Hep-BDA and Bpy-4 TP-COFs propose they are potentially applied in photocatalytic OWS. We also achieved the theoretical energy conversion efficiency of Hep-BDA can reach as high as 13.01%. Because there are very few successful applications of TP-COFs on OWS, this theoretical work not only offers valuable insights and innovative ideas for the exploration of novel metal-free photocatalysts for OWS but also supplies a direction for the development of new TP-COF derivatives.

Evolution of Two-Dimensional Perovskite Films Under Atmospheric Exposure and Its Impact on Photovoltaic Performance.

Two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs) have garnered significant attention due to their enhanced stability compared with their three-dimensional counterparts. However, the power conversion efficiency (PCE) of 2D perovskite solar cells (2D-PSCs) remains lower than that of 3D-PSCs. Understanding the microstructural evolution of 2D perovskite films during fabrication is essential for improving their performance. This study demonstrates that exposing bare 2D perovskite films to the atmosphere during device fabrication can significantly enhance the PCE of 2D-PSCs. The performance improvement is primarily attributed to the gradual evaporation of organic butylammonium (BA+) spacer cations from the film as butylamine (BA) gas. This evaporation refines the film's composition and structure, leading to the spontaneous formation of larger-n phase RPPs, which exhibit superior carrier mobility. Consequently, the PCE of 2D-PSCs is enhanced. This work offers new insights into the structural evolution of 2D RPP films under ambient conditions and provides a feasible strategy for optimizing the performance of 2D-PSCs and other optoelectronic devices.

Source Material Design for Realizing >50% Indium-Saving Transparent Electrode toward Sustainable Development of Silicon Heterojunction Solar Cells.

Indium (In) reduction is a hot topic in transparent conductive oxide (TCO) research. So far, most strategies have been focused on reducing the layer thickness of In-based TCO films and exploring In-free TCOs. However, no promising industrial solution has been obtained yet. In our work, we adopt the emerging reactive plasma deposition (RPD) approach and provide our In-reduced solution by directly reducing the In content from the source material. We designed the indium zinc oxide (IZO) target with a composition of Zn3In2O6 (i.e., (ZnO)3·In2O3). Density functional theory (DFT) calculation shows that the introduction of a large amount of ZnO significantly perturbs the conduction band of the In2O3 host, resulting in a limitation of exploring high-mobility IZO films. For TCOs used in solar cell application, low resistivity with high carrier mobility is required. Via RPD process optimization, we obtained the minimal resistivity value of 6.08 × 10-4 Ω·cm, which is comparable to our lab-standard tin-doped indium oxide (ITO) film. The corresponding electron mobility and carrier concentration are 31 cm2 V-1 s-1 and 3.37 × 1020 cm-3, respectively. Our IZO film is in an amorphous state. The optical band gap is ∼3.6 eV. X-ray photoelectron spectroscopy (XPS) data show that the film composition is In:Zn:O = 21.60:28.75:49.65 (at. %). Damp heat tests show strong stability of our IZO film, and no aging effects have been observed. Furthermore, we demonstrated wafer-scale silicon heterojunction (SHJ) solar cells with IZO films. As compared with our reference hydrogenated cerium-doped indium oxide (ICO)-based solar cells, the IZO-based devices show even higher fill factor parameters. Our amorphous state stable In-reduced IZO film could find versatile application in the sustainable development of temperature-sensitive devices such as SHJ and perovskite/silicon tandem solar cells, as well as flexible OLEDs.

Calculation of Efficiency Limit of Heterojunction Solar Cells Based on S-Q Theory

The ideal solar cell defined by the Shockley-Queisser (S-Q) theory is an important milestone in the analysis of photovoltaic devices based on some assumptions. One or more of the above assumptions are gradually evaded and even exceed or close to S-Q efficiency limit, so the development and improvement of S-Q theory is necessary. Heterojunction solar cells are one of the hot research fields in photovoltaics. In order to address the hindering effect of energy band discontinuity in the spatial barrier region of heterojunction solar cells on the transport of photogenerated carriers, this paper revised the assumptions of S-Q theory based on the original S-Q theory of photovoltaic cells. It is assumed that the carrier mobility in the barrier region is finite and the infinite mobility in the S-Q model is abandoned. But the mobility in the N-type and P-type neutral region is still infinite. The lumped relationship between carrier mobility and resistance in the barrier region is derived. Thus the physical process of charge transport is described in detail in this paper based on the continuity equation for semiconductors considering the effect of absorption coefficients to prevent the quasi-Fermi level from crossing the conduction or valence band. Thus, the revised S-Q theoretical limit model of heterojunction solar cell was constructed. The diode equivalent circuit diagram is deduced and the photovoltaic conversion efficiency is evaluated eventually. The loss effects of charge transmission and band gap mismatch on the performance of heterojunction solar cells are analyzed in detail in this paper. The calculation results under the condition of 5780K blackbody radiation and 300K cell temperature with N-type wide bandgap(EH) and P-type narrow bandgap(EL) materials show that the highest conversion efficiency is about 31% with hole resistance of 0.01 Ω·cm^2 and electronic resistance of 0.01 Ω·cm^2. The electronic resistance has more negative and complicated effects on solar cell performance than hole resistance based on the results of the calculation. When Re and Rh are small, the best conversion efficiency is achieved between 1.22 and 1.32 of the narrow bandgap. Increasing Re can increase the open circuit voltage of solar cells, but there are losses in efficiency and fill factor of solar cells. When Re is large enough, for example, Re=1000, the open circuit voltage of solar cells is not limited by EL and can exceed the bandgap limit of the narrow bandgap material. Increasing Rh also reduces efficiency and fill factor but has less effects than Re. The change of absorption coefficient makes the photogenerated current of L and H branches change, and the radiation recombination loss of both branches can be regulated.

Quantification of mobile charge carrier yield and transport lengths in ultrathin film light-trapping ZnFe2O4 photoanodes.

Zinc ferrite (ZnFe2O4, ZFO) has gained attention as a candidate material for photoelectrochemical water oxidation. However, champion devices have achieved photocurrents far below that predicted by its bandgap energy. Herein, strong optical interference is employed in compact ultrathin film (8-14 nm) Ti-doped ZFO films deposited on specular back reflectors to boost photoanode performance through enhanced light trapping, resulting in a roughly fourfold improvement in absorption as compared to films deposited on transparent substrates. The spatial charge carrier collection profile and wavelength-dependent photogeneration yield of mobile charge carriers was then extracted via spatial collection efficiency analysis based on optical and external quantum efficiency measurements. We demonstrate that despite the enhanced performance enabled by the light trapping structure, substantial recombination occurs for thin film ZFO photoanodes even within the space charge region of an ultrathin film photoanode. Furthermore, the excitation-wavelength-dependent yield of mobile charge carriers in ZFO is shown to be less than unity across the visible spectrum, ultimately limiting the attainable photocurrent density. These results explain the underperformance of ZFO as a photoanode material and suggest that reduction of the mobile charge carrier yield due to the existence of ligand field states is a dominant loss mechanism for metal-oxides containing Fe metal centers with open d-shell configuration.

First-principles investigation of the photocatalytic properties of two-dimensional CdO/ZrSSe heterojunctions.

Two-dimensional van der Waals heterojunction materials have demonstrated significant potential for photocatalytic water splitting in hydrogen production, owing to their distinct electronic and optical properties. Among these materials, direct Z-scheme heterojunctions have attracted considerable attention in recent research. In this study, a novel CdO/ZrSSe heterojunction is designed using first-principles calculations. The structural stability, electronic properties, carrier mobility, optical absorption, and photocatalytic performance of this heterojunction are systematically investigated, with a particular focus on the influence of strain on the band structure. The results reveal that both type-I and type-II heterojunctions exhibit staggered, indirect bandgap structures, with bandgap values of 0.80 eV and 0.60 eV, respectively. A built-in electric field is established from the CdO layer to the ZrSSe layer, facilitating oxidation in the ZrSSe layer and reduction in the CdO layer. The highest carrier mobilities are calculated to be 462 cm2 V-1 s-1 and 2738 cm2 V-1 s-1, respectively. Under compressive strain, the bandgap widths of both I-CdO/ZrSSe and II-CdO/ZrSSe heterojunctions decrease, whereas tensile strain results in an increase in the bandgap width. Correspondingly, the optical absorption peaks of both heterojunctions are enhanced under compressive strain and diminished under tensile strain. These findings suggest that the performance of both type-I and type-II CdO/ZrSSe heterojunctions surpasses that of their individual monolayers, exhibiting a typical Z-scheme photocatalytic mechanism, and thus positioning them as promising high-efficiency catalysts for water splitting.

Ultrahigh carrier mobility and multidirectional piezoelectricity in 2D Janus copper-containing chalcogenide monolayers.

Two-dimensional (2D) materials have attracted enormous research attention due to their remarkable properties and potential applications in electronic and optoelectronic devices. In this work, Janus 2D copper-containing chalcogenides, CuP2Se0.5S0.5 and CuP2Te0.5Se0.5 monolayers, are proposed and studied systematically based on first-principles calculations. These two Janus-structured materials possess the same thermal and dynamic stability as the perfect CuP2Se structure. Remarkably, we observe multiple VBM and CBM points with negligible energy differences in the band structures of perfect CuP2Se and Janus CuP2Se0.5S0.5 and CuP2Te0.5Se0.5 monolayers. This will significantly impact the electronic and transport properties of the material. The calculated anisotropic carrier mobilities can reach 104-105 cm2 V-1 s-1 orders of magnitude, which are higher than those of most reported materials. Meanwhile, the two Janus derivatives, CuP2Se0.5S0.5 and CuP2Te0.5Se0.5 monolayers, exhibit outstanding multidirectional piezoelectricity, which are comparable with those of traditional piezoelectric materials. The combination of ultrahigh carrier mobility and multidirectional piezoelectricity indicates that these novel 2D Janus materials could be promising for applications in electronic and piezoelectric devices under special conditions.

Investigation on properties of CuI/InSe heterojunction with Z–alignment for photocatalytic water splitting

The energetic, thermodynamic and mechanically stability as well as the electronic and optical properties of the monolayers CuI, InSe and the constructed CuI/InSe heterojunction have been investigated by first–principles calculations. The solar–to–hydrogen production efficiency of CuI/InSe heterojunction with or without applied electric field or biaxial strain has been predicted and elucidated in view of electronic structure systematically. The exciton binding energy and carrier mobility in CuI, InSe and CuI/InSe as well as the hydrogen and the oxygen evolution reaction have been analyzed. The predicted results show that the constructed 2D–CuI/InSe van der Waals heterojunction with Z–alignment is energetic, thermodynamic and mechanically stable. It enables the hydrogen production to take place on the CuI layer meanwhile oxygen is generated on the InSe layer during photocatalytic water splitting. The solar–to–hydrogen production efficiency of the constructed CuI/InSe1×1×1, CuI/InSe2×2×1 and CuI/InSe3×3×1 is predicted as 31.16%, 25.61% and 24.84% respectively. The exciton binding energy of CuI/InSe is lower than that of monolayers. A moderate negative applied electric field can promote the solar–to–hydrogen production efficiency of CuI/InSe while the positive electric field makes it decrease. Similarly, a moderate positive applied strain can also promote solar–to–hydrogen production efficiency of CuI/InSe significantly while the negative applied strain decreases the solar–to–hydrogen production efficiency.

Optimizing carrier transport properties in the intrinsic layer of a-Si single and double junction solar cells through numerical design

This research aims to improve the performance of a-Si: H solar cells, particularly in terms of carrier transport properties, through a numerical design approach utilizing AFORS-HET simulation software. By performing a series of rigorous computer simulations, we explore the potential regulation of the intrinsic layer thickness, carrier mobility, loading factor, and density of states (DoS) distribution in the solar cell's intrinsic layer. Recombination losses are reduced, and light absorption efficiency is significantly increased when the intrinsic layer thickness is adjusted, as shown by simulation findings. Moreover, reduction of transit times and enhancement of the total efficiency of the solar cells depend on increased carrier mobility. Parameters can be adjusted to attain optimal performance under various operating situations by adjusting the DoS and load factors. Furthermore, the simulations provide insightful information about the interactions between the junctions in solar cells with double junctions. Our results of this research provide an important contribution to efforts to develop more efficient and sustainable a-Si: H solar cells and emphasize the importance of numerical design approaches in photovoltaic technology.

Structural features of sodium- and halogen-containing fluorite-like rare-earth molybdates.

Compounds of nominal composition NaLa4Mo3O15F1-xClx (x = 0-0.7) have been obtained for the first time as ceramics and single crystals. All the compounds belong to the cubic crystal system and are isostructural to undoped crystals of the Ln5Mo3O16 family (Ln = La-Gd). A study of the physical properties of the obtained phases has shown their significant change compared to undoped compounds and a reversible jump in conductivity near 600 °C. According to X-ray diffraction analysis, sodium atoms are localized near the positions of rare-earth cations La2, and halogen impurities enter partially occupied general positions of O2 atoms and the interstices formed by six La1 atoms. As the temperature increases, chlorine ions leave O2 positions and interstices. This increases the number of vacancies and creates new ion transport pathways. A jump in conductivity occurs when three conditions are simultaneously met: an increase in the number of vacancies, the appearance of new charge transport pathways, and, possibly, an increase in the mobility of the main charge carriers, oxygen ions, due to electrostatic repulsion between anions (O2-, F-, Cl-).

Band structure engineering, optical, transport, and photocatalytic properties of pristine and doped Nb3O7(OH): a systematic DFT study.

Nb3O2(OH) has emerged as a highly attractive photocatalyst based on its chemical stability, energetic band positions, and large active lattice sites. Compared to other various photocatalytic semiconductors, it can be synthesized easily. This study presents a systematic analysis of pristine and doped Nb3O7(OH) based on recent developments in related research. The current study summarizes the modeling approach and computationally used techniques for doped Nb3O7(OH) based photocatalysts, focusing on their structural properties, defects engineering, and band structure engineering. This study demonstrates that the Trans-Blaha modified Becke-Johnson approximation (TB-mBJ) is an effective approach for optoelectronic properties of pristine and Ta/Sb-doped Nb3O7(OH). The generalized gradient approximation is used for structure optimization of all systems studied. Spin-orbit (SO) coupling is also applied to deal with the Ta f orbital and Sb d orbital in the Ta/Sb-doped systems. Doping shifts the energetic band positions and relocates the Fermi level i.e. both the valence band maximum and the conduction band minimum are relocated, decreasing the band gap from 1.7 eV (pristine), to 1.266 eV (Ta-doped)/1.203 eV (Sb-doped). The band structures of pristine and doped systems reflect direct band behavior. Investigation of the partial density of states reveals that the O p orbital and Nb d/Ta d/Sb-d orbitals contributed to the valence and conduction bands, respectively. Optical properties like real and imaginary components of the dielectric function, reflectivity, and electron energy loss function are calculated using the OPTIC program implemented in the WIEN2k code. Moreover, doped systems shift the optical threshold to the visible region. Transport properties like effective mass and electrical conductivity are calculated, reflecting that the mobility of charge carriers increases with the doping of Ta/Sb atoms. The reduction in the band gap and red-shift in the optical properties of the Ta/Sb-doped Nb3O7(OH) to the visible region suggest their promising potential for photocatalytic activity and photoelectrochemical solar cells.

Enhanced thermoelectric performance of Bi2O2Se by MXene addition via liquid assisted shear exfoliation-restacking process

In this study, Bi2O2Se-x%MXene (Ti3C2Tx) composite samples were prepared using the liquid assisted shear exfoliation-restacking (LASE-R) method, and their thermoelectric properties were thoroughly examined. The findings indicate that the introduction of MXene effectively boosts the carrier concentration and mobility, thereby enhancing the electrical conductivity of the composite. Notably, the absolute Seebeck coefficient exhibited a distinct trend of increasing initially and then declining with an increase in the MXene composite content, leading to an improved Power Factor (PF). Furthermore, the inclusion of MXene suppressed the thermal conductivity of the composites, attributed to enhanced phonon scattering. Consequently, the Bi2O2Se-2 %MXene composite sample achieved a ZTmax value of 0.58 at 773 K, representing a significant enhancement of 1.66 times compared to the pristine sample.

A linearly polarized AC-driven perovskite light emitting device with nanoscale metal contact.

Electroluminescent (EL) devices consisting of a single metal-semiconductor contact and a gate effect structure have garnered significant attention in the field of perovskite light-emitting devices. This interest is largely due to the thermal stability of the active layer and the simplicity of the device structure. However, the application of these devices in large-area light-emitting applications is hindered by the inherently low carrier mobility in perovskite materials. In our study, we addressed this limitation by optimizing the nanostructure within the electrodes, which resulted in enhanced electroluminescence and linear polarization. To confirm the luminescence mechanism and the observed enhancement, we conducted comprehensive electrical and optical characterization studies. These characterization studies demonstrated the effectiveness of our approach in improving the performance of perovskite-based EL devices, paving the way for their broader application in large-area light-emitting technologies.

Point defects at grain boundaries can create structural instabilities and persistent deep traps in metal halide perovskites.

Metal halide perovskites (MHPs) have attracted strong interest for a variety of applications due to their low cost and excellent performance, attributed largely to favorable defect properties. MHPs exhibit complex dynamics of charges and ions that are coupled in unusual ways. Focusing on a combination of two common MHP defects, i.e., a grain boundary (GB) and a Pb interstitial, we developed a machine learning model of the interaction potential, and studied the structural and electronic dynamics on a nanosecond timescale. We demonstrate that point defects at MHP GBs can create new chemical species, such as Pb-Pb-Pb trimers, that are less likely to occur with point defects in bulk. The formed species create structural instabilities in the GB and prevent it from healing towards the pristine structure. Pb-Pb-Pb trimers produce deep trap states that can persist for hundreds of picoseconds, having a strong negative influence on the charge carrier mobility and lifetime. Such stable chemical defects at MHP GBs can only be broken by chemical means, e.g., the introduction of excess halide, highlighting the importance of proper defect passivation strategies. Long-lived GB structures with both deep and shallow trap states are found, rationalizing the contradictory statements in the literature regarding the influence of MHP GBs on performance.

First-principles study of a novel type-II As2C3/MoSi2N4 heterostructure with high carrier mobility in photocatalytic water splitting

First-principles study of a novel type-II As2C3/MoSi2N4 heterostructure with high carrier mobility in photocatalytic water splitting

Topological electronic structure and transport properties of the distorted rutile-type WO2

We elucidate the transport properties and electronic structures of distorted rutile-type WO2. Electrical resistivity and Hall effect measurements of high-quality single crystals revealed the transport property characteristics of topological materials; these characteristics included an extremely large magnetoresistance of 13 200% (2 K and 9 T) and a very high carrier mobility of 25 700 cm2 V−1 s−1 (5 K). First-principles calculations revealed Dirac nodal lines (DNLs) near the Fermi energy in the electronic structure when spin–orbit interactions (SOIs) were absent. Although these DNLs mostly disappeared in the presence of SOIs, band crossings at high-symmetry points in the reciprocal space existed as Dirac points. Furthermore, DNLs protected by nonsymmorphic symmetry persisted on the ky = π/b plane. The unique transport properties originating from the topological electronic structure of chemically and thermally stable WO2 could represent an opportunity to investigate the potential electronic applications of the material.

The effect of acceptor and donor doping on the electronic properties of the half-Heusler TiNiSn.

Optimizing the electronic transport properties of thermoelectric compounds is commonly achieved by either donor or acceptor atom doping to increase the conduction of the appropriate carrier type, electrons or holes, respectively. Enhancing carrier mobility and carrier concentration will both lead to optimized electronic properties. In this work the effect of various dopants on the electronic properties of TiNiSn was explored, by modeling the doping of an ideal compound on the Ti-sublattice with acceptor or donor elements. Using ab initio DFT calculations and a set of analytical expressions of transport properties, the temperature dependencies of the electronic properties were calculated, to examine possible n-type or p-type dopants to be used in further experimental studies.

A low-cost, high-performance, dopant-free phenyl-based hole transport material for efficient and stable perovskite solar cells

Benefiting from the expansion of the central structure, the carrier mobility of HTMs is elevated to 2.56 × 10−4 cm2 V−1 s−1. Thus, the PCE of PSCs with dopant-free HTL is increased from 17.57% to 20.37%.

Reversal of chirality in solutions and aggregates of chiral tetrachlorinated diperylene diimides towards efficient circularly polarized light detection.

Helicenes exhibit promise as active layer materials for circularly polarized light (CPL) detectors due to their strong chiroptical activity. However, their practical application is limited by the complicated synthesis and loosely solid-state packing. This study introduces a chiral induction strategy towards the synthesis of helicene derivatives, chiral tetrachlorinated diperylene diimides ((SSSS)-4CldiPDI or (RRRR)-4CldiPDI). When incorporating the chiral (S/R)-1-cyclohexylethyl (Cy) substituents, the chirality is directly transferred to the π-aromatic core and forms the PP- or MM-helicene subunit. Notably, (SSSS)-Cy induces preferred PP helicity while (RRRR)-Cy leads to the MM helicity in the monomers. However, these molecules exhibit reversed chirality in crystals, where (SSSS)-Cy controls MM helicity and (RRRR)-Cy induces PP helicity. Theoretical calculations reveal that the (SSSS)-PP structure demonstrates lower energy distribution in monomers, whereas the (SSSS)-MM structure exhibits lower energy in crystals. Then, the CPL detection based on n-type PDI-helicene derivatives is achieved by using (SSSS)-4CldiPDI or (RRRR)-4CldiPDI crystals. The maximum photocurrent dissymmetry factor gph of +0.16 for (RRRR)-4CldiPDI and -0.15 for (SSSS)-4CldiPDI is obtained. Our work demonstrates a novel chiral induction strategy for designing helicene-based materials with both high dissymmetry factor and large charge carrier mobility, which offers great potential for the advancement of CPL detection.