Enhanced Charge Carrier Mobility Research Articles

Unveiling heterogeneity of hysteresis in perovskite thin films

The phenomenon of current–voltage hysteresis observed in perovskite-based optoelectronic devices is a critical issue that complicates the accurate assessment of device parameters, thereby impacting performance and applicability. Despite extensive research efforts aimed at deciphering the origins of hysteresis, its underlying causes remain a subject of considerable debate. By employing nanoscale investigations to elucidate the relationship between hysteresis and morphological characteristics, this study offers a detailed exploration of photocurrent–voltage hysteresis at the nanoscale within perovskite optoelectronic devices. Through the meticulous analysis of localized I–V curve arrays, our research identifies two principal hysteresis descriptors, uncovering a predominantly inverted hysteresis pattern in 87% of the locations examined. This pattern is primarily attributed to the energetic barrier encountered at the interface between the probe and the perovskite material. Our findings underscore the pronounced heterogeneity and grain-dependent variability inherent in hysteresis behavior, evidenced by an average Hysteresis Index value of 0.24. The investigation suggests that the localized hysteresis phenomena cannot be exclusively attributed to either photocharge collection processes or organic cation migration at grain boundaries. Instead, it appears significantly influenced by localized surface trap states, which play a pivotal role in modulating electron and hole current dynamics. By identifying the key factors contributing to hysteresis, such as localized surface trap states and their influence on electron and hole current dynamics, our findings pave the way for targeted strategies to mitigate these effects. This includes the development of novel materials and device architectures designed to minimize energy barriers and enhance charge carrier mobility, thereby improving device performance and longevity. This breakthrough in understanding the microscale mechanisms of hysteresis underscores the critical importance of surface/interface defect trap passivation in mitigating hysteretic effects, offering new pathways for enhancing the performance of perovskite solar cells.

Correlation between morphology and transport properties of Au/Co/Au/Si wedge ultra-thin film

We report the thermoelectric power (TEP) and electrical resistivity characterization supported by structural and surface morphological properties on a wedge shaped ultra-thin sample of Au/Co/Au/Si (100) with Co have varying thickness from 0.5 nm to 4 nm across the sample. The sample was prepared by Ion Beam Sputtering method, with an ultrathin layer of Co sandwiched between two layers of Au (3 nm thicknesses each). The observed TEP results showed that the system is semiconductor in nature and the concentration of the charge carrier is inversely proportional to the Seebeck coefficient (S) and also concentration of charge carrier modifies with the film thickness leading to a change in thermo power, which is found to be higher at lower thicknesses of Co sandwiched layer. The type of charge carrier was also determined to be electrons in this system. The observed results are in good agreement with the resistivity measurement carried out as a function of film thickness indicating a decrease in resistivity as a function of film thickness due to enhancement in charge carrier mobility. The thermal activation energy was seen to increase from ∼29.5 meV to ∼280.3 meV as the film thickness increased from the minimum to the highest. Corresponding Atomic Force Microscopy (AFM) measurement revealed island type grain growth with the introduction of a significant amount of roughness, while Magnetic Force Microscopy (MFM) experiments exhibit randomly oriented magnetic domain structures. The overall results suggest that lowest thickness film shows semiconducting nature, while variable range hopping phenomena due to some localized energy levels near the valence and conduction band is observed in the highest thickness film. This result is also well supported by XRD measurement, which revealed crystalline nature of the films. Such low cost, easy to fabricate materials may find applications in Peltier cooling or energy harvesting applications without the harmful carbon dioxide emission. For this, advance experiments will be done in future.

Sustainability-driven photocatalysis: oxygen-doped g-C3N4 for organic contaminant degradation

The unique characteristics of oxygen-doped g-C3N4 (O@g-C3N4), including enhanced charge carrier mobility and changed electronic structure, make it especially appealing for photocatalytic applications.

A Water-Processed Mesoscale Structure Enables 18.5% Efficient Binary Layer-by-Layer Organic Solar Cells.

The two-step layer-by-layer (LBL) deposition of donor and acceptor films enables desired vertical phase separation and high performance in organic solar cells (OSCs), which becomes a promising technology for large-scale printing devices. However, limitations including the use of toxic solvents and unpredictable infiltration between donor and acceptor still hinder the commercial production of LBL OSCs. Herein, we developed a water-based nanoparticle (NP) ink containing donor polymer to construct a mesoscale structure that could be infiltrated with an acceptor solution. Using non-halogen o-xylene for acceptor deposition, the LBL strategy with a mesoscale structure delivered outstanding efficiencies of 18.5% for binary PM6:L8-BObased LBL OSCs. Enhanced charge carrier mobility and restricted trap states were observed in the meso-LBL devices with optimized vertical morphology. It is believed that the findings in this work will bring about more research interest and effort on eco-friendly processing in preparation for the industrial production of OSCs.

Tunable Electronic Properties, Carrier Mobility, and Contact Characteristics in Type-II BSe/Sc2CF2 Heterostructures toward Next-Generation Optoelectronic Devices.

Conducting heterostructures have emerged as a promising strategy to enhance physical properties and unlock the potential application of such materials. Herein, we conduct and investigate the electronic and transport properties of the BSe/Sc2CF2 heterostructure using first-principles calculations. The BSe/Sc2CF2 heterostructure is structurally and thermodynamically stable, indicating that it can be feasible for further experiments. The BSe/Sc2CF2 heterostructure exhibits a semiconducting behavior with an indirect band gap and possesses type-II band alignment. This unique alignment promotes efficient charge separation, making it highly promising for device applications, including solar cells and photodetectors. Furthermore, type-II band alignment in the BSe/Sc2CF2 heterostructure leads to a reduced band gap compared to the individual BSe and Sc2CF2 monolayers, leading to enhanced charge carrier mobility and light absorption. Additionally, the generation of the BSe/Sc2CF2 heterostructure enhances the transport properties of the BSe and Sc2CF2 monolayers. The electric fields and strains can modify the electronic properties, thus expanding the potential application possibilities. Both the electric fields and strains can tune the band gap and lead to the type-II to type-I conversion in the BSe/Sc2CF2 heterostructure. These findings shed light on the versatile nature of the BSe/Sc2CF2 heterostructure and its potential for advanced nanoelectronic and optoelectronic devices.

Uniform, Strain-Free, Large-Scale Graphene and h-BN Monolayers Enabled by Hydrogel Substrates.

Translation of the unique properties of 2D monolayers from non-scalable micron-sized samples to macroscopic scale is a longstanding challenge obstructed by the substrate-induced strains, interface nonuniformities, and sample-to-sample variations inherent to the scalable fabrication methods. So far, the most successful strategies to reduce strain in graphene are the reduction of the interface roughness and lattice mismatch by using hexagonal boron nitride (h-BN), with the drawback of limited uniformity and applicability to other 2D monolayers, and liquid water, which is not compatible with electronic devices. This work demonstrates a new class of substrates based on hydrogels that overcome these limitations and excel h-BN and water substrates at strain relaxation enabling superiorly uniform and reproducible centimeter-sized sheets of unstrained monolayers. The ultimate strain relaxation and uniformity are rationalized by the extreme structural adaptability of the hydrogel surface owing to its high liquid content and low Young's modulus, and are universal to all 2D materials irrespective of their crystalline structure. Such platforms can be integrated into field effect transistors and demonstrate enhanced charge carrier mobilities in graphene. These results present a universal strategy for attaining uniform and strain-free sheets of 2D materials and underline the opportunities enabled by interfacing them with soft matter.

Rational Design of Vinylene-Linked Covalent Organic Frameworks for Modulating Photocatalytic H2 Evolution.

Vinylene-linked covalent organic frameworks (COFs) have attracted enormous attention for photocatalytic H2 evolution from water because of their fully conjugated structures, high chemical stabilities, and enhanced charge-carrier mobilities. In this work, two novel vinylene-linked COFs with tuned cyano contents were successfully synthesized and then employed as photocatalysts for H2 generation. Notably, the photocatalytic H2 production rate of the COF with the higher cyano content reached 73 μmol h-1 under visible light irradiation, which is 2.4 times higher than that with the lower content (30 μmol h-1 ). Both the experimental and computational results demonstrated that the rational design incorporating cyano groups into COF skeletons could precisely tune the corresponding energy levels, expand the visible-light absorption, and improve the photoinduced charge separation. This work not only provides a simple method for modulating the photocatalytic activities of COFs at the molecular level, but also affords interesting insights into the relationship between their structures and photocatalytic performance.

Relating Structure to Properties in Non-Conjugated Pendant Electroactive Polymers.

Non-conjugated pendant electroactive polymers (NCPEPs) are an emerging class of polymers that offer the potential of combining the desirable optoelectronic properties of conjugated polymers with the superior synthetic methodologies and stability of traditional non-conjugated polymers. Despite an increasing number of studies focused on NCPEPs, particularly on understanding fundamental structure-property relationships, no attempts have been made to provide an overview on established relationships to date. This review showcases selected reports on NCPEP homopolymers and copolymers that demonstrate how optical, electronic, and physical properties of the polymers are affected by tuning of key structural variables such as the chemical structure of the polymer backbone, molecular weight, tacticity, spacer length, the nature of the pendant group, and in the case of copolymers the ratios between different comonomers and between individual polymer blocks. Correlation of structural features with improved π-stacking and enhanced charge carrier mobility serve as the primary figures of merit in evaluating impact on NCPEP properties. While this review is not intended to serve as a comprehensive summary of all reports on tuning of structural parameters in NCPEPs, it highlights relevant established structure-property relationships that can serve as a guideline for more targeted design of novel NCPEPs in the future.

A Direct Z‐Scheme Quasi‐2D/2D Heterojunction Constructed by Loading Photosensitive Metal–Organic Nanorings with Pd Single Atoms on Graphitic–C3N4for Superior Visible Light‐Driven H2Production

Rational design of Z‐scheme composite photocatalysts with 2D/2D heterojunction is a promising method to convert solar energy into chemical fuels. Herein, a Pd2L2‐type metal–organic nanoring (MAC‐1), which can serve as a photochemical molecular device (PMD), is assembled by catalytic Pd2+centers and photosensitive ligands (M‐7). Then MAC‐1 is combined with graphite‐phase carbon nitride (g‐C3N4) to construct a direct Z‐scheme quasi‐2D/2D composite material MAC‐1/g‐C3N4with Pd single atoms. The optimized 3.5% MAC‐1/g‐C3N4single‐atom catalyst (SAC) shows better photocatalytic performance than g‐C3N4, MAC‐1, Pd/g‐C3N4, M‐7/Pd/g‐C3N4, and 3.5% MAC‐1/g‐C3N4‐mixed and also displays good durability for H2production. The H2yield rate of 22.3 mmol g−1 h−1and the corresponding turnover number based on Pd or MAC‐1 amount (TONPd/TONMAC‐1) of 119 172/238 344 within 50 h are achieved for 3.5% MAC‐1/g‐C3N4, which is one of the highest records of all visible light‐driven g‐C3N4‐based photocatalysts reported. Such remarkably enhanced activity can result from the enhanced charge carrier mobility, improved light absorption ability, enlarged heterojunction contact interface, and highly monodispersed Pd active sites. The Z‐scheme SAC composed of metal–organic nanorings and g‐C3N4nanosheets therefore has great potential as an efficient and sustainable photocatalyst for solar‐driven H2production.

Modulating the bridging units of carbon nitride for highly efficient charge separation and visible-light-responsive photocatalytic H2 evolution

Carbon nitrides have attracted broad attention in the field of photocatalytic H2 evolution. However, the design and preparation of carbon nitrides with highly efficient charge separation and transfer are still challenging. Here, we synthesized a novel heptazine derivative-based photocatalyst, named as M+U-x to overcome the inherent limitations of carbon nitrides (C3N4 and C3N5). Experimental and theoretical calculation results reveal that the resultant M+U-x is composed by electron-withdrawing C3N4 and electron-donating C3N5 moieties, which are integrated into the conjugated framework via N-atoms and azo bridging units to form molecular heterojunctions. Among these, M+U-3 can dramatically facilitate the spatial separation of photoinduced charge carriers, delivering significantly high visible-light-responsive H2 evolution activity of 372 μmol h−1 with an AQY of 21.6% at 400 nm. This work provides a novel strategy for rationally modulating the bridging units of carbon nitrides to acquire enhanced charge-carrier mobility and efficient photocatalytic H2 evolution from the molecular level.

Large Transconductance of Electrochemical Transistors Based on Fluorinated Donor-Acceptor Conjugated Polymers.

Organic electrochemical transistors (OECTs) have enormous potential for use in biosignal amplifiers, analyte sensors, and neuromorphic electronics owing to their exceptionally large transconductance. However, it is challenging to simultaneously achieve high charge carrier mobility and volumetric capacitance, the two most important figures of merit in OECTs. Herein, a method of achieving high-performance OECT with donor-acceptor conjugated copolymers by introducing fluorine units is proposed. A series of cyclopentadithiophene-benzothiadiazole (CDT-BT) copolymers for use in high-performance OECTs with enhanced charge carrier mobility (from 0.65 to 1.73 cm2·V-1·s-1) and extended volumetric capacitance (from 44.8 to 57.6 F·cm-3) by fluorine substitution is achieved. The increase in the volumetric capacitance of the fluorinated polymers is attributed to either an increase in the volume at which ions can enter the film or a decrease in the effective distance between the ions and polymer backbones. The fluorine substitution increases the backbone planarity of the CDT-BT copolymers, enabling more efficient charge carrier transport. The fluorination strategy of this work suggests the more versatile use of conjugated polymers for high-performance OECTs.

Fullerene-Liquid-Crystal-Induced Micrometer-Scale Charge-Carrier Diffusion in Organic Bulk Heterojunction.

The short charge-carrier diffusion length (LD ) (100-300nm) in organic bulk heterojunction (BHJ) impedes the further improvement in power conversion efficiency (PCE) of organic solar cells (OSCs), especially for thick-film (>400nm) devices matching with industrial solution processing. Here a facile method is developed to efficiently increase LD and then improve PCEs of OSCs via introducing a fullerene liquid crystal, F1, into the active layer. F1 combines the inherent high electron mobility of fullerene and strong self-assembly capacity of liquid crystal, providing a fast channel for charge-carrier transportand reducing energetic disorder and trap density in BHJ film via enhancing crystallization. Typically, in PM6:Y6:F1 BHJ, the enhanced charge-carrier mobility (>10-2 cm-2 V-1 s-1 ) and prolonged charge-carrier lifetime (55.3 µs) are acquired to realize the record LD of 1.6 or 2.4µm for electron or hole, respectively, which are much higher than those of the PM6:Y6 binary sample and comparable to or even better than those values reported for some inorganic/hybrid materials, such as CuInx Ga(1- x ) Se2 (CIGS) and perovskite thin films. Benefitting from the micrometer-scale LD , the PM6:Y6:F1 ternary OSCs sustain a remarkable PCE of 15.23% with the active layer thickness approaching 500nm.

Atomically dispersed Ti-O clusters anchored on NH2-UiO-66(Zr) as efficient and deactivation-resistant photocatalyst for abatement of gaseous toluene under visible light

Photocatalytic oxidation (PCO) of volatile organic compounds (VOCs) over MOF-based photocatalysts is considerably impeded by the weak activation of reactant molecules on the catalyst surface and low charge carrier mobility. In this study, we demonstrate that atomically dispersed Ti species anchored on NH2-UiO-66(Zr) (AUiO-66(Zr/Ti)) exhibit high visible-light-responsive photocatalytic activity toward toluene vapor with an 83 % removal efficiency and 89 % CO2 selectivity. These results are markedly superior to those reported in the literature. More importantly, AUiO-66(Zr/Ti) exhibited excellent catalytic stability during a prolonged reaction, while its pristine AUiO-66(Zr) counterpart underwent rapid catalytic deactivation after a few hours. The optimized sample, AUiO-66(Zr/Ti)-4h, provided extended visible light absorption and enhanced charge carrier mobility due to ligand-to-linker metal charge transfer. Meanwhile, the defect-rich surface of AUiO-66(Zr/Ti)-4h facilitated the activation of H2O/toluene molecules into the critical intermediates of hydroxyl, benzoic acid, and maleic anhydride, which were effectively converted under visible light illumination. On the basis of the combined results of the PCO of toluene and material characterization, the structure − activity relationship and the related catalytic mechanism are discussed comprehensively.

Two-Dimensional Ultrathin Graphic Carbon Nitrides with Extended π-Conjugation as Extraordinary Efficient Hydrogen Evolution Photocatalyst.

Construction of 2D graphic carbon nitrides (g-CNx ) with wide visible light adsorption range and high charge separation efficiency concurrently is of great urgent demand and still very challenging for developing highly efficient photocatalysts for hydrogen evolution. To achieve this goal, a two-step pyrolytic strategy has been applied here to create ultrathin 2D g-CNx with extended the π-conjugation. It is experimentally proven that the extension of π-conjugation in g-CNx is not only beneficial to narrowing the bandgap, but also improving the charge separation efficiency of the g-CNx . As an integral result, extraordinary apparent quantum efficiencies (AQEs) of 57.3% and 7.0% at short (380nm) and long (520nm) wavelength, respectively, are achieved. The formation process of the extended π-conjugated structures in the ultrathin 2D g-CNx has been investigated using XRD, FT-IR, Raman, XPS, and EPR. Additionally, it has been illustrated that the two-step pyrolytic strategy is critical for creating ultrathin g-CNx nanosheets with extended π-conjugation by control experiments. This work shows a feasible and effective strategy to simultaneously expand the light adsorption range, enhance charge carrier mobility and depress electron-hole recombination of g-CNx for high-efficient photocatalytic hydrogen evolution.

Efficient Ternary Polymer Solar Cells with Two Structurally Similar Fullerene-Free Acceptors to Redshift Absorption Peaks and Improve Exciton Dissociation

In this work, Y6 and BTP-eC9 blends are used as a acceptor and D18-Cl as donor to construct ternary PSCs and to boost the power conversion efficiency (PCE) of ternary polymer solar cells (PSCs). Compared with Y6, BTP-eC9 shows a red-shifted absorption peak. As a result, the optimized ternary PSCs show the highest short-circuit current density (JSC). Compared with Y6, BTP-eC9 shows a shallower LUMO level and paly the role of broaden energy level difference. Thus, the blend acceptor endows a high open-circuit voltage (VOC) to the ternary PSCs. In addition, the of Y6 and BTP-eC9 blend as acceptors to enhance crystalline and improve surface and bulk morphology of ternary films, which favor the enhancement of charge carrier mobility and suppression of charge carrier recombination. Thus, the blend of two acceptors with similar molecular structures boosts a higher photovoltaic performance than binary PSCs.

Efficient and Stable Perovskite Solar Cells by B-Site Compositional Engineered All-Inorganic Perovskites and Interface Passivation.

Perovskite solar cells (PSCs) have emerged as a cost-effective solar technology in the past years. PSCs by three-dimensional hybrid inorganic-organic perovskites exhibited decent power conversion efficiencies (PCEs); however, their stabilities were poor. On the other hand, PSCs by all-inorganic perovskites indeed exhibited good stability, but their PCEs were low. Here, the development of novel all-inorganic perovskites CsPbI2Br:xNd3+, where Pb2+ at the B-site is partially heterovalently substituted by Nd3+, is reported. The CsPbI2Br:xNd3+ thin films possess enlarged crystal sizes, enhanced charge carrier mobilities, and superior crystallinity. Thus, the PSCs by the CsPbI2Br:xNd3+ thin films exhibit more than 20% enhanced PCEs and dramatically boosted stability compared to those based on pristine CsPbI2Br thin films. To further boost the device performance of PSCs, solution-processed 4-lithium styrenesulfonic acid/styrene copolymer (LiSPS) is utilized to passivate the surface defect and suppress surface charge carrier recombination. The PSCs based on the CsPbI2Br:xNd3+/LiSPS bilayer thin film possess reduced charge extraction lifetime and suppressed charge carrier recombination, resulting in 14% enhanced PCEs and significantly boosted stability compared to those without incorporation of the LiSPS interface passivation layer. All these results indicate that we developed a facile way to approach high-performance PSCs by all-inorganic perovskites.

Selenium: A Unique Member in the Chalcogen Family for Conjugated Materials Used in Perovskite and Organic Solar Cells

Organic conjugated materials play an extremely important role in the development of perovskite and organic solar cells (PVSCs and OSCs). Among different molecular design strategies, the introduction of selenium is an efficient way to optimize material properties and improve the performance of solar cells. The benefits can be attributed to the looser electron cloud delocalization and more polarizable nature of selenium, which help enable stronger intermolecular Se···Se interaction and extended conjugation length, leading to enhanced charge carrier mobility and redshifted absorption in selenium‐incorporated organic conjugated materials. Herein, the design of the selenium‐incorporated materials applied in the PVSCs and OSCs to provide a systematic study on the relationship between chemical structures, material properties, and device performance is focused on. The future direction for further development of selenium‐incorporated organic conjugated materials is also provided.

Enhancement in Charge Carrier Mobility by Using Furan as Spacer in Thieno[3,2-b]Pyrrole and Alkylated-Diketopyrrolopyrrole Based Conjugated Copolymers

The structural alteration of semiconducting polymer backbones can improve the optoelectronic properties of organic semiconductors and enhance field-effect mobilities. In our efforts towards improving the performance of organic field-effect transistors (OFETs), we are reporting a donor–acceptor polymer containing thieno[3,2-b]pyrrole (TP) donor and a furan-flanked diketopyrrolopyrrole (DPP) electron acceptor, which yielded an asymmetric poly(methylthienopyrrolo)furanyl)diketopyrrolopyrrol) P(FDPP-TP) organic semiconducting polymer. The introduction of a furan spacer improved thermally induced crystallinity and molecular packing, as confirmed by grazing incidence X-ray diffraction (XRD) and tapping-mode atomic force microscopy (TMAFM). The tested OFET devices gave maximum hole mobility of 0.42 cm2 V−1 s−1 with threshold voltages around 0 V for bottom-gate bottom-contact device configuration.

Benzotriazole-based a conjugated polymer with siloxane-functionalized side chains for non-fullerene organic solar cells

A new benzotriazole based π-conjugated polymer (Nap-UD-SiBTA) with siloxane-functionalized side chains is designed and synthesized for fabrication of non-fullerene organic solar cells (NFOSCs). The application of Nap-UD-SiBTA enhanced charge carrier mobility, good solubility and better surface morphology. The Nap-UD-SiBTA was synthesized by Stille polymerization and showed good compatibility with ITIC as non-fullerene acceptor. Therefore, Nap-UD-SiBTA blended with ITIC exhibited an efficiency of 4.76%, open-circuit voltage (V oc) of 0.77 V, a short-circuit current density (J sc) of 11.60 mA/cm2, and fill factor (FF) of 53.18%, respectively. The study results provide a pacesetting strategy to design and fabricate the highly efficient NFOSCs.

Large‐Area Heteroepitaxial Nanostructuring of Molecular Semiconductor Films for Enhanced Optoelectronic Response in Flexible Electronics

AbstractOrganized nano‐ and microstructures of molecular semiconductors display interesting optical and photonic properties, and enhanced charge carrier mobilities, as compared to disordered thin films. However, known directed‐growth and self‐organization strategies cannot create structured molecular heterojunctions and cannot be practically incorporated into existing device fabrication routines to create large‐area optoelectronic devices. Here, an ultrathin (<2 nm) seed layer of the compound coronene creates 1D nanostructures of an electron‐transporting molecule (IFD) is shown, which possesses an intrinsic proclivity to form disordered thin films in the absence of the seed layer. It is revealed that nanostructured IFD films exhibit enhanced light absorption and emission, and greater electron mobilities, as compared to amorphous counterparts. This seed layer strategy creates uniform IFD nanowires over large areas of up to 18 mm2 at low processing temperatures. Notably, the coronene seed layer creates IFD nanowires when applied over either oxide surfaces or predeposited organic layers, meaning that this structuring approach can be integrated into diode manufacturing routines to realize large‐area flexible optoelectronic devices. Flexible organic light‐emitting diodes and fullerene‐free organic solar cells containing IFD nanowires in the photoactive layer to demonstrate that molecular nanostructures can lead to robust, large‐area device arrays on flexible substrates being fabricated.