Showing papers on "Work function published in 2022"

Stability-limiting heterointerfaces of perovskite photovoltaics

Abstract: Optoelectronic devices consist of heterointerfaces formed between dissimilar semiconducting materials. The relative energy-level alignment between contacting semiconductors determinately affects the heterointerface charge injection and extraction dynamics. For perovskite solar cells (PSCs), the heterointerface between the top perovskite surface and a charge-transporting material is often treated for defect passivation1-4 to improve the PSC stability and performance. However, such surface treatments can also affect the heterointerface energetics1. Here we show that surface treatments may induce a negative work function shift (that is, more n-type), which activates halide migration to aggravate PSC instability. Therefore, despite the beneficial effects of surface passivation, this detrimental side effect limits the maximum stability improvement attainable for PSCs treated in this way. This trade-off between the beneficial and detrimental effects should guide further work on improving PSC stability via surface treatments.

Work function tunable laser induced graphene electrodes for Schottky type solar-blind photodetectors

Abstract: Laser-induced graphene (LIG) is a simple, environmentally friendly, efficient, and less costly method, as well as can form various shapes on a flexible substrate in situ without the use of masks. More importantly, it can tune the work function of LIG easily by changing laser parameters to control the transportation of carriers. In this work, the work functions of LIG were controlled by adjusting the frequency or speed of the laser, and a series of LIG/GaOx Schottky photodetectors were formed. When the work function of the graphene increases, the Fermi energy is shifted below the crossing point of the Π and Π* bands, and then more electrons or holes can be activated to participate in the conduction process, resulting in low resistance. Meanwhile, a large built-in electric field can be formed when using a high work function LIG, which is more beneficial to separate photo-generated carriers. Enabled by the controllable LIG, LIG/GaOx Schottky photodetectors can be modulated to have high photoresponsivity or self-powered characteristics. Our work provides a high-performance photodetector with excellent mechanical flexibility and long-life stability, promising applications in the flexible optoelectronic fields.

Modulating built-in electric field via variable oxygen affinity for robust hydrogen evolution reaction in neutral media.

Abstract: Work function strongly impacts the surficial charge distribution, especially for metal-support electrocatalysts when a built-in electric field (BEF) is constructed. Therefore, studying the correlation between work function and BEF is crucial for understanding the intrinsic reaction mechanism. Herein, we present a Pt@CoO x electrocatalyst with a large work function difference (ΔΦ) and strong BEF, which shows outstanding hydrogen evolution activity in a neutral medium with a 4.5-fold mass activity higher than 20% Pt/C. Both experimental and theoretical results confirm the interfacial charge redistribution induced by the strong BEF, thus subtly optimizing hydrogen and hydroxide adsorption energy. This work not only provides fresh insights into the neutral hydrogen evolution mechanism but also proposes new design principles toward efficient electrocatalysts for hydrogen production in a neutral medium.

Amino‐Functionalized Niobium‐Carbide MXene Serving as Electron Transport Layer and Perovskite Additive for the Preparation of High‐Performance and Stable Methylammonium‐Free Perovskite Solar Cells

Abstract: Perovskite solar cells have shown great potential in commercial applications due to their high performance and easy fabrication. However, the electron transport layer (ETL) materials with good optoelectrical properties and energy levels matching that of the perovskite layer still need to be explored to meet the need of commercialization. In this work, 2D Nb2CTx MXene nanosheets are prepared and their work function (WF) is reduced from 4.65 to 4.32 eV to match the conduction band minimum of perovskite layer by replacing the surface ‐F groups with NH2 groups through hydrazine (N2H4) treatment. Besides, the N2H4 treated (T‐Nb2CTx) MXene nanosheets with abundant NH2 groups are incorporated into the perovskite precursor to retard the crystallization rate by forming hydrogen bond with iodine ions, which promotes the formation of high‐quality and oriented growth perovskite films. Consequently, the PVSCs with T‐Nb2CTx MXene ETLs and T‐Nb2CTx MXene nanosheets additive exhibit the highest power conversion efficiency (PCE) of 21.79% and the corresponding flexible and large‐area devices achieve the highest PCE of 19.15% and 18.31%. Meanwhile, the unencapsulated devices maintain 93% of the original PCEs after 1500 h of storage. This work demonstrates the considerable application prospects of 2D Nb2CTx MXene in photoelectric devices.

Ge Bidirectional Diffusion to Simultaneously Engineer Back Interface and Bulk Defects in the Absorber for Efficient CZTSSe Solar Cells

Abstract: Aiming at a large open‐circuit voltage (VOC) deficit in Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, a new and effective strategy to simultaneously regulate the back interface and restrain bulk defects of CZTSSe absorbers is developed by directly introducing a thin GeO2 layer on Mo substrates. Power conversion efficiency (power‐to‐efficiency) as high as 13.14% with a VOC of 547 mV is achieved for the champion device, which presents a certified efficiency of 12.8% (aperture area: 0.25667 cm2). Further investigation reveals that Ge bidirectional diffusion simultaneously occurs toward the CZTSSe absorber and MoSe2 layer at the back interface while being selenized. That is, some Ge element from the GeO2 diffuses into the CZTSSe absorber layer to afford Ge‐doped absorbers, which can significantly reduce the defect density and band tailing, and facilitate quasi‐Fermi level split by relatively higher hole concentration. Meanwhile, a small amount of Ge element also participates in the formation of MoSe2 at the back interface, thus enhancing the work function of MoSe2 and effectively separating photoinduced carriers. This work highlights the synergistic effect of Ge element toward the bulk absorber and the back interface and also provides an easy‐handling way to achieve high‐performance CZTSSe solar cells.

Work‐Function‐Tunable MXenes Electrodes to Optimize p‐CsCu2I3/n‐Ca2Nb3‐xTaxO10 Junction Photodetectors for Image Sensing and Logic Electronics

Abstract: Tunable work function has a high profile for the MXene‐based optoelectronic devices, and surface modification provides the huge potential to shift its Fermi level and modulate the work function. In this work, the window of MXene's work function is engineered from 4.55 to 5.25 eV by surface modification with LiF, Se, and polyethylenimine ethoxylated (PEIE). The vertical p‐CsCu2I3/n‐Ca2Nb3‐xTaxO10 junction photodetectors are constructed on the basis of the above surface‐modified MXenes, which changes the Schottky barrier between n‐Ca2Nb3‐xTaxO10 and the electrodes. In particular, the rectification effect is significantly enhanced by utilizing PEIE‐decorated MXene electrodes, resulting in a high rectification ratio of 16 136 and improved UV responsivity of 81.3 A W–1. Such high‐performance devices based on MXenes electrodes are compatible with the standard clean room fabrication process, realizing large‐scale flexible UV detectors that maintain 80% of the original current after 5000 times bending. Meanwhile, a photodetector array stimulated with UV of different wavelengths is constructed to reveal its potential for image sensing. Finally, functional “AND” and “OR” optoelectronic logic gates are developed for UV communication using Au/CsCu2I3/Ca2Nb3‐xTaxO10/MXene–PEIE photodetectors, enriching the application of MXene‐based optoelectronic devices. This work on tuning MXene work function via surface modification demonstrates that MXene is a promising candidate for future optoelectronics.

Work-function-induced Interfacial Built-in Electric Fields in Os-OsSe2 Heterostructures for Active Acidic and Alkaline Hydrogen Evolution.

Abstract: Theoretical calculations unveil that the formation of Os-OsSe2 heterostructures with neutralized work function (WF) perfectly balances the electronic state between strong (Os) and weak (OsSe2) adsorbents and bidirectionally optimizes the hydrogen evolution reaction (HER) activity of Os sites, significantly reducing thermodynamic energy barrier and accelerating kinetics process. Then, heterostructural Os-OsSe2 is constructed for the first time by a molten salt method and confirmed by in-depth structural characterization. Impressively, due to highly active sites endowed by the charge balance effect, Os-OsSe2 exhibits ultra-low overpotentials for HER in both acidic (26 mV @ 10 mA cm-2) and alkaline (23 mV @ 10 mA cm-2) media, surpassing commercial Pt catalysts. Moreover, the solar-to-hydrogen device assembled with Os-OsSe2 further highlights its potential application prospects. Profoundly, this special heterostructure provides a new model for rational selection of heterocomponents.

Work‐Function‐Tunable MXenes Electrodes to Optimize p‐CsCu ₂ I ₃ /n‐Ca ₂ Nb _3‐ <i> _x </i> Ta <i>

Abstract: Tunable work function has a high profile for the MXene-based optoelectronic devices, and surface modification provides the huge potential to shift its Fermi level and modulate the work function. In this work, the window of MXene's work function is engineered from 4.55 to 5.25 eV by surface modification with LiF, Se, and polyethylenimine ethoxylated (PEIE). The vertical p-CsCu2I3/n-Ca2Nb3-xTaxO10 junction photodetectors are constructed on the basis of the above surface-modified MXenes, which changes the Schottky barrier between n-Ca2Nb3-xTaxO10 and the electrodes. In particular, the rectification effect is significantly enhanced by utilizing PEIE-decorated MXene electrodes, resulting in a high rectification ratio of 16 136 and improved UV responsivity of 81.3 A W–1. Such high-performance devices based on MXenes electrodes are compatible with the standard clean room fabrication process, realizing large-scale flexible UV detectors that maintain 80% of the original current after 5000 times bending. Meanwhile, a photodetector array stimulated with UV of different wavelengths is constructed to reveal its potential for image sensing. Finally, functional “AND” and “OR” optoelectronic logic gates are developed for UV communication using Au/CsCu2I3/Ca2Nb3-xTaxO10/MXene–PEIE photodetectors, enriching the application of MXene-based optoelectronic devices. This work on tuning MXene work function via surface modification demonstrates that MXene is a promising candidate for future optoelectronics.

A New alcohol‐soluble polymer PFN‐ID as cathode interlayer to optimize performance of conventional polymer solar cells by increasing electron mobility

Abstract: A new alcohol-soluble polymer PFN-ID is successfully synthesized by combining N,N-di(2-ethylhexyl)-6,6′-dibromoisoindigo and an amino-containing fluorene subunits, and applied to polymer solar cells (PSCs) with PTB7-Th:PC71BM as an active layer. The n-type backbone of the PFN-ID improves electron transfer performance and thus optimizes device performance. The PSCs with PFN-ID as cathode interfacial layers (CILs) have significantly improved compared to the device without the interface layer, especially the optimum power conversion efficiency (PCE) of PSCs reaches up to 9.24%, which is 1.62 times higher than that of devices without CILs. The I–V curves show that the introduction of the n-type backbone leads to a significant increase in the conductivity of PFN-ID compared to PFN. The UV photoelectron spectroscopy and Mott–Schottky curves further confirm that PFN-ID can decrease the work function of Al electrode, and increase its built-in potential, giving higher open-circuit voltage. The resulting conventional PSCs using PFN-ID as cathode interlayer achieve high photovoltaic performance, and the research results can provide a new strategy for the advancement of PSCs.

Fermi‐Level Pinning‐Free WSe2 Transistors via 2D Van der Waals Metal Contacts and Their Circuits

Abstract: Precise control over the polarity of transistors is a key necessity for the construction of complementary metal–oxide–semiconductor circuits. However, the polarity control of 2D transistors remains a challenge because of the lack of a high‐work‐function electrode that completely eliminates Fermi‐level pinning at metal–semiconductor interfaces. Here, a creation of clean van der Waals contacts is demonstrated, wherein a metallic 2D material, chlorine‐doped SnSe2 (Cl–SnSe2), is used as the high‐work‐function contact, providing an interface that is free of defects and Fermi‐level pinning. Such clean contacts made from Cl–SnSe2 can pose nearly ideal Schottky barrier heights, following the Schottky–Mott limit and thus permitting polarity‐controllable transistors. With the integration of Cl–SnSe2 as contacts, WSe2 transistors exhibit pronounced p‐type characteristics, which are distinctly different from those of the devices with evaporated metal contacts, where n‐type transport is observed. Finally, this ability to control the polarity enables the fabrication of functional logic gates and circuits, including inverter, NAND, and NOR.

Performance enhancement of (FAPbI3)1-x(MAPbBr3)x perovskite solar cell with an optimized design

Abstract: In this paper, an optimized design of (FAPbI3)1-x(MAPbBr3)x perovskite solar cell is numerically investigated using SCAPS-1D software package. A variety of potential charge transport materials are investigated. Cu2O as HTL and ZnO as ETL outperform other choices; they are therefore considered as the best candidates. The impact of the electronic properties of both ZnO/perovskite and Perovskite/Cu2O interfaces on the solar cell performance is thoroughly investigated. We discovered that appropriate values of the conduction band offset (CBO+ = 0.29) and valence band offset (VBO+ = 0.09) assure a “spike-type” band alignment at both interfaces. This choice lowers the unwanted interfacial recombination mechanism, resulting in a challenging PCE. In addition, the impact of the work function of back contact is also investigated. According to simulation findings, Ni back electrodes with a work function of 5.04 eV is appropriate for Zn0.8Mg0.2O/(FAPbI3)0.85(MAPbB3)0.15/Cu2O perovskite solar cell. The optimized FTO/MgZnO/(FAPbI3)0.85(MAPbBr3)0.15/Cu2O/Ni PSC reaches a conversion efficiency as high as 25.86%. These findings will pave the way for the design of low-cost, high-efficiency solar cells.

All‐van‐der‐Waals Barrier‐Free Contacts for High‐Mobility Transistors

Abstract: Ultrathin 2D semiconductor devices are considered to have beyond‐silicon potential but are severely troubled by the high Schottky barriers of the metal–semiconductor contacts, especially for p‐type semiconductors. Due to the severe Fermi‐level pinning effect and the lack of conventional semimetals with high work functions, their Schottky hole barriers are hardly removed. Here, an all‐van‐der‐Waals barrier‐free hole contact between p‐type tellurene semiconductor and layered 1T′‐WS2 semimetal is reported, which achieves a zero Schottky barrier height of 3 ± 9 meV and a high field‐effect mobility of ≈1304 cm2 V–1 s–1. The formation of such contacts can be attributed to the higher work function of ≈4.95 eV of the 1T′‐WS2 semimetal, which is in sharp contrast with low work function (4.1–4.7 eV) of conventional semimetals. The study defines an available strategy for eliminating the Schottky barrier of metal–semiconductor contacts, facilitating 2D‐semiconductor‐based electronics and optoelectronics to extend Moore's law.

Highly sensitive and selective room-temperature gas sensors based on B6N6H6 monolayer for sensing SO2 and NH3: A first-principles study

Abstract: We applied first-principles calculations and studied the structures and electronic properties of SO2, NH3, H2, N2, CH4, H2O, N2O, CO, CO2, HCN, and H2S molecules adsorbed on the B6N6H6 monolayer to understand its performance as a sensor. The crystalline structure of the B6N6H6 monolayer remains thermodynamic stable under T = 1000 K, and the rippled structure is more stable than the planar one. The NH3 and SO2 molecules are chemically and physically, respectively, adsorbed on the B6N6H6 monolayer with moderate adsorption energy and charge transfer. By adsorbing NH3 and SO2, B6N6H6 monolayer electronic properties (especially conductivity) could be adjusted. Adsorption of other molecules did not have any similar effects. The recovery times of NH3 and SO2 desorption from the monolayer were only 0.42 ms and 1.1 s, respectively, at room temperature. The existence of humidity would have, to a certain extent, influence on the sensing of SO2 or NH3 gas using B6N6H6 monolayer. It is thus predicted that the B6N6H6 monolayer could be a room-temperature NH3 and SO2 sensor in a dry environment with high selectivity and sensitivity and fast response and recovery time. We also believe that the B6N6H6 monolayer will be a good candidate for NH3 work-function-type gas sensors.

Bimetallic AgAualloy@ZnO Core-Shell Nanoparticles for Ultra-High Detection of Ethanol: Potential Impact of Alloy Composition on Sensing Performance

Abstract: The detection/sensing properties of chemiresistive gas sensors are greatly improved by catalytically triggered bimetallic alloyed nanoparticles, because of their synergistic effect. In this work, well-defined AgAualloy@ZnO core-shell nanoparticles are synthesized with the various compositions of Ag and Au. Alloying Au with Ag gave superior thermal stability to Ag, which favored gas sensing performance. As the amount of Ag within the AgAualloy core increased, the optimum working temperature was lowered to 300 ℃ with the dramatically enhanced ultra-high response of 1755 was obtained for Ag70Au30 @ZnO NPs sensor to 100 ppm ethanol. The outstanding sensing performance of AgAualloy@ZnO were endorsed to the improved the electronic and catalytic properties of AgAualloy core, aplenty chemisorbed oxygen on the surface of ZnO, unique core-shell structure that bring large active surface area and enhanced electron transfer process at the interface between core and shell. Ultraviolet photoelectron spectroscopy (UPS) analysis shows that with increasing Ag concentration, the work function (ϕ) decreases and the chemisorbed oxygen increases that brings wider depletion layer and Schottky barrier between the AgAualloy core and the ZnO shell, enhancing the gas sensing response. Our findings suggest that the incorporation of noble metal alloy-metal oxide semiconductor based core-shell nanostructures enhance the gas sensing performance with selectivity in the practical field of applications in environmental issues and human health monitoring.

Non-oxidized bare copper nanoparticles with surface excess electrons in air

Abstract: Abstract Copper (Cu) nanoparticles (NPs) have received extensive interest owing to their advantageous properties compared with their bulk counterparts. Although the natural oxidation of Cu NPs can be alleviated by passivating the surfaces with additional moieties, obtaining non-oxidized bare Cu NPs in air remains challenging. Here we report that bare Cu NPs with surface excess electrons retain their non-oxidized state over several months in ambient air. Cu NPs grown on an electride support with excellent electron transfer ability are encapsulated by the surface-accumulated excess electrons, exhibiting an ultralow work function of ~3.2 eV. Atomic-scale structural and chemical analyses confirm the absence of Cu oxide moiety at the outermost surface of air-exposed bare Cu NPs. Theoretical energetics clarify that the surface-accumulated excess electrons suppress the oxygen adsorption and consequently prohibit the infiltration of oxygen into the Cu lattice, provoking the endothermic reaction for oxidation process. Our results will further stimulate the practical use of metal NPs in versatile applications.