Adjusting Energy Levels Research Articles

Trap-Filling of ZnO Buffer Layer for Improved Efficiencies of Organic Solar Cells.

Trap-assisted recombination loss in the cathode buffer layers (CBLs) is detrimental to the electron extraction process and severely restricts the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Herein, a novel organic–inorganic hybrid film composed of zinc oxide (ZnO) and 2,3,5,6-tetrafluoro-7,7,8, 8-tetracyanoquinodimethane (F4TCNQ) is designed to fill the intrinsic charge traps of ZnO-based CBLs by doping F4TCNQ for high-performance inverted OSCs. Thus, constructed ZnO:F4TCNQ hybrid film exhibits enhanced surface hydrophobicity and adjustable energy levels, providing favorable interfacial condition for electron extraction process. Consequently, trap-assisted recombination loss in the CBLs was efficiently suppressed, leading to the significantly improved fill factor and PCEs of both fullerene- and non-fullerene-based OSCs using the ZnO:F4TCNQ hybrid CBLs. This work illustrates a convenient organic acceptor doping approach to suppress the internal charge traps of traditional inorganic CBLs, which will shed new light on the fabrication of high-performance CBLs with facile electron extraction processes in inverted OSC devices.

Continuously-tunable light\u2013matter coupling in optical microcavities with 2D semiconductors

A theoretical variation between the two distinct light–matter coupling regimes, namely weak and strong coupling, becomes uniquely feasible in open optical Fabry—Pérot microcavities with low mode volume, as discussed here. In combination with monolayers of transition-metal dichalcogenides (TMDCs) such as WS2, which exhibits a large exciton oscillator strength and binding energy, the room-temperature observation of hybrid bosonic quasiparticles, referred to as exciton–polaritons and characterized by a Rabi splitting, comes into reach. In this context, our simulations using the transfer-matrix method show how to tailor and alter the coupling strength actively by varying the relative field strength at the excitons’ position – exploiting a tunable cavity length, a transparent PMMA spacer layer and angle-dependencies of optical resonances. Continuously tunable coupling for future experiments is hereby proposed, capable of real-time adjustable Rabi splitting as well as switching between the two coupling regimes. Being nearly independent of the chosen material, the suggested structure could also be used in the context of light–matter-coupling experiments with quantum dots, molecules or quantum wells. While the adjustable polariton energy levels could be utilized for polariton-chemistry or optical sensing, cavities that allow working at the exceptional point promise the exploration of topological properties of that point.

Electron Transport Bilayer with Cascade Energy Alignment for Efficient Perovskite Solar Cells

Energy alignment between electron transport layers (ETLs) and perovskite has a strong influence on the device performance of perovskite solar cells (PSCs). Two approaches are deployed to tune the energy level of ETLs: 1) doping ETLs with aliovalent metal cations and 2) constructing heterojunction bilayers with different materials. However, the abrupt interfaces in the heterojunction bilayers introduce undesirable carrier recombination. Herein, a homojunction bilayer ETL is developed by stacking Sb‐doped SnO2 (Sb‐SnO2) and SnO2 ETLs via low‐temperature spin‐coating processes. The energy levels of ETLs are tuned by the incorporation of Sb and altering stacking orders. Bilayer ETL of Sb‐SnO2/SnO2 with cascade energy alignment promotes the best power conversion efficiency of 20.73%, surpassing single‐layer ETLs of SnO2 (18.23%) and Sb‐SnO2 (19.15%), whereas the SnO2/Sb‐SnO2 bilayer with barricade energy alignment receives the poorest device performance. The cascade bilayer ETL facilitates charge separation and suppresses carrier recombination in PSCs, which is verified by photoluminescence, conductivity, and impedance characterizations. The homojunction bilayer ETLs with adjustable energy levels open a new direction for interface engineering toward efficient PSCs.

Influence of virtual monochromatic spectral image at different energy levels on motion artifact correction in dual-energy spectral coronary CT angiography.

To investigate the influence of virtual monochromatic spectral (VMS) CT images at different energy levels on the effectiveness of a motion correction technique (SSF) in dual-energy Spectral coronary CT angiography (CCTA). 29 cases suspected of or diagnosed with coronary artery disease underwent Spectral CCTA using a prospective ECG triggering with 250ms padding time. SSF was applied to the determined least-motion phase to generate 6 additional sets of VMS images with energy levels from 40 to 100keV. CT value and standard deviation (SD) in the aortic root and epicardial adipose tissue were measured. Image quality of the RCA, LAD and LCX was evaluated on a per-vessel basis in each patient. Two reviewers evaluated the artery using the score of the segment. The low energy VMS images increased CT value and image noise compared with higher-energy VMS images, except 90keV and 100keV. The CNR of 40-70keV were higher than those of 80-100keV (P < 0.05). The image quality scores for images at 50-80keV were higher than those of 40, 90, and 100keV (P < 0.05), and the VMS image quality at 50keV and 60keV with SSF was the highest. SSF can effectively reduce the motion artifacts when coronary vessels have suitable contrast enhancement which can be achieved by adjusting energy levels of VMS images.

Electron transport through two one-dimensional multi-quantum dot arrays coupled to four leads

Employing the non-equilibrium Green’s function technique, we have obtained the formula for dc current of two one-dimensional multi-quantum dot arrays, which couple to each other via tunneling coupling between two quantum dots connected to four leads, respectively. The retarded Green’s function is a staircase type, terminating at the four leads. Furthermore, the four quantum dots case is demonstrated. The influence of inter-dot coupling strength and quantum dot energy level on the transmission probability for TL, TM and TN branches is investigated. A non-resonant band is observed. By adjusting energy levels of quantum dots, a resonance emerges in the region of the non-resonance band. The system can be used as a quantum switch.

Dithienonaphthalene-Based Non-fullerene Acceptors With Different Bandgaps for Organic Solar Cells.

Compared to the traditional fullerene derivatives, non-fullerene acceptors show more tunable absorption bands as well as adjustable energy levels which are favorable for further PCE enhancement of organic solar cells. In order to enhance light-harvesting property of dithienonaphthalene (DTN)-based acceptors, we designed and synthesized two novel non-fullerene acceptors (DTNIF and DTNSF) based on a ladder-type DTN donor core flanked with two different acceptor units. In combination with a benchmark wide bandgap copolymer (PBDB-T), the best performance device based on DTNIF displayed a high PCE of 8.73% with a short-circuit current (Jsc) of 13.26 mA cm−2 and a large fill factor (FF) of 72.77%. With a reduced bandgap of DTNSF, the corresponding best performance device showed an increased Jsc of 14.49 mA cm−2 although only a moderate PCE of 7.15% was achieved. These findings offer a molecular design strategy to control the bandgap of DTN-based non-fullerene acceptors with improved light-harvesting.

Rectification‐Regulated Memristive Characteristics in Electron‐Type CuPc‐Based Element for Electrical Synapse

As one novel electronic strategy, the integration of memristive behaviors with rectification has shown its potential application. Herein, based on the interfacial effects of a LiF buffer layer at the anode of an electron‐type CuPc memristor on reconfiguring the device energy levels and redistributing the charge accumulations, a new class of organic memristor with rectifying characteristics is proposed. Memristive behaviors characterized by smooth consecutive changes are obtained, which are illustrated in detail by the adjustable device energy levels and relevant carrier processes. For this device, aside from its appeal in solving the well‐known cross‐talk problem, based on the consecutive rectification‐regulated memristive behaviors, this study paves the way for devising a new type artificial synapse—an electrical rectifying synapse for effective comprehensive bioinspired computations. Thus these results can invite further investigations of the memristor through energy‐level reconfiguration and open original perspectives for rectification‐regulated memristors in design for novel brain‐like information processing.

Single-Junction Binary-Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency.

A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push-pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (JSC ). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing JSC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC71 BM (PCE = 5.22%).

Geometric Characteristics of Three-Phase Porous Microstructures for All Solid-State Lithium Ion Batteries

Fast lithium ion conducting inorganic solid materials are considered superior candidates for lithium ion batteries (LIBs) to overcome flammable characteristics, leakage, and tight electrochemical window issues of organic liquid electrolytes. Besides high ion conductivities of the inorganic materials, their wide electrochemical window and good chemical stability with electrode materials are great advantages as an electrolyte [1]. However, there are remained challenges to utilize the inorganic solid-state electrolytes for LIBs because of a low specific capacity, rate capability, and cycle life. Our previous study [2] showed a large capacity decay after the first charge and poor cycle life of an all solid-state LIB (Li1+xMn2O2 + Li1.3Ti1.7Al0.3(PO4)3 | Li1.3Ti1.7Al0.3(PO4)3 | Li1-xMn2O2 + Li1.3Ti1.7Al0.3(PO4)3). We concluded that the large capacity decay was caused by dismantled ion and electron pathway due to the structural change after the first charge. The electrochemical impedance spectroscopy results showed that the internal resistance of the solid-state cell has reduced by applying higher pressing pressure. Therefore, it is necessary to investigate the realistic microstructure to understand the capacity decay and poor cycle life of the all solid-state LIB. By using synchrotron nano-computed tomography (nano-CT) at Argonne National Lab, we implemented quantitative study of the microstructures of all solid-state LIBs. The three-phase electrode was fabricated from a 47:47:6 (wt%) mixture of Li(Ni1/3 Mn1/3 Co1/3)O2 as active material (NMC), Li1.3Ti1.7Al0.3(PO4)3as Li-ion conductor (LTAP), and Super P carbon as electron conductor. The electrodes were packed under two different pressures (700 psi and 1300 psi) to control the volume fractions of void phase. To obtain the morphology data of the electrodes, TXM measurements were carried out adjusting energy levels of a synchrotron x-ray source. The x-ray generated images of solid-state electrodes with a 60 nm of spatial resolution at 8 keV are shown in Fig. 1. The gray-scaled intensity values of the images indicate the x-ray absorption rate of the materials. From the Fig. 1 (a) and (b), NMC particles, LTAP particles, and pores can be distinguished due to their different intensity values. Based on the reconstructed microstructure, the geometric characteristics will be presented to understand the lithium ion and electron pathways in all solid-state LIBs. Acknowledgments: This work was supported by US National Science Foundation under Grant No. 1335850.

The pilot experience upon surgical ablation of large liver tumor by microwave system with tissue permittivity feedback control mechanism.

BackgroundMicrowave ablation (MWA) is used to treat patients with unresectable liver cancer. Our institution applied a novel microwave generator capable of automatically adjusting energy levels based on feedback related to tissue permittivity. This approach is meant to facilitate ablations over larger areas and provide results of greater predictablility. This paper reports on the safety, efficacy, and feasibility of this new system in the treatment of patients with large liver tumors.MethodsBetween July 2012 and December 2012, a total of 23 patients with malignant liver tumors exceeding 4 cm in diameter underwent surgical MWA using a 902–928 MHz generator. The proposed system used a 14-gauge antenna without internal-cooling. Follow up on tumor recurrence was performed using contrast-enhanced computed tomography or magnetic resonance imaging at 1 month and then at 3 month intervals for a period of at least 12 months following ablation.ResultsAmong the cancers treated, 10 were primary hepatocellular carcinomas (HCCs) and 13 were metastatic lesions from primary colorectal cancer (CRLM). The mean tumor size was 5.40 cm (range of 4.0-7.0 cm). A total of 18 patients underwent MWA via open surgery, and 5 received laparoscopic MWA. The mean ablation time was 1982 seconds, with a range of 900-3600 seconds, and the median number of ablation sessions was 2.0 (range of 1–4 sessions). The rate of complete ablation, as defined by a total loss of contrast-enhancement one month post-treatment, was 82.6% (19 of 23 patients), and the rate of local recurrence was 26.3% (5 of 19 patients). For tumors with a diameter of 4.0-7.0 cm, the technical success rate of MWA was higher for HCC patients (70%) than for metastatic liver cancer (53.8%) patients; however, the difference was not statistically significant. All patients survived throughout the observation period, and the morbidity rate was 8.6%.ConclusionsMWA treatment using the proposed system with tissue permittivity feedback control resulted in a high rate of complete ablation and reduced morbidity. This approach proved to be a fast, easy, and effective option for the ablation of large liver cancers, particularly HCCs.

Research Progress of Non-Fullerene Small-Molecule Acceptor Materials for Organic Solar Cells

Bulk heterojunction organic photovoltaics have been the subject of intensive academic interest over the past two decades. Numerous recent efforts have been directed towards this area with the vision of developing next-generation low-cost solar cells. In the field of bulk heterojunction organic photovoltaics, fullerene and its derivatives are an important class of n-type electron acceptor materials. However, their disadvantages such as narrow wavelength absorption, high affinity, poor solubility, have severely limited their wide application as electron acceptors for organic solar cells and have largely hampered further improvement of the device performance. Recently, a number of research efforts have been focused on the develop- ment of novel non-fullerene n-type small-molecule acceptors. Various design rules and interesting new materials have been explored. The non-fullerene n-type small-molecule acceptors usually appear to possess lots of attractive advantages, such as adjustable energy levels, facile synthesis, good solubility, low processing cost. More important, when compared to the fullerene and its derivatives, this kind of small-molecule acceptors has wider spectral absorption that allows to absorb more sunlight to generate electricity. Recent breakthroughs rely mostly on the development of novel high-performance acceptor materials and optimization of the device structures. The up-to-date power conversion efficiencies exceeding 4% with using non-fullerene small-molecule acceptor materials in bulk heterojunction organic solar cells have been achieved. In this review, recent advances of non-fullerene small-molecule n-type acceptor materials for organic solar cells are reviewed, including rylene diimide-based acceptors, pentacene-based acceptors, benzothiadiazole-based acceptors, 1,4-diketopyrrolo-(3,4-c)- pyrrole (DPP)-based acceptors, fluorene-based acceptors, fluoranthene-fused imide-based acceptors and so on. Meanwhile, the future trends on material design and development have also been discussed. This review on illustrating the influence of the molecular structures and corresponding photovoltaic properties would thus be helpful to further unravel the role of elec- tron acceptors and shed light on exploring efficient n-type electron acceptor materials for high performance organic photo- voltaic devices. Keywords n-type small-molecular acceptors; wide spectral absorption; solution processing; organic photovoltaics

Electronic promotion effect of double proton transfer on conduction of DNA through improvement of transverse electronic communication of base pairs

The effect of double proton transfer (DPT) on charge migration of DNA was investigated by the nonequilibrium Green's function method combined with density functional theory. The results revealed that DPT not only lowers ionization potentials, but also improves the delocalization of the localized π-orbitals at each base moiety through adjusting energy levels and spatial distributions of their molecular orbitals. Furthermore, DPT leads to both the strengthening of the second-order interactions of the Watson-Crick H-bond zones, and the promotion of the charge transfer transitions between two pairing bases in the UV absorption spectra. Electronic transport calculations indicated that DPT can improve the charge migration along the DNA duplex for specific sequences through enhancing transverse base-to-base electronic communication. This work will provide a new insight into the understanding of DNA charge conduction which can be electronically promoted or regulated by DPT.

High photovoltage achieved in low band gap polymer solar cells by adjusting energy levels of a polymer with the LUMOs of fullerene derivatives

Solar cells based on organic molecules or conjugated polymers attract great attention due to their unique advantages, such as low cost, and their use in flexible devices, but are still limited by their low power conversion efficiency (PCE). To improve the PCEs of polymer solar cells, more efforts have been made to increase short-circuit current (Jsc) or open-circuit voltage (Voc). However, the trade-off between Jsc and Voc in bulk heterojunctions solar cells makes it tricky to find a polymer with a low band gap to efficiently absorb photons in the visible and near infrared region of the solar spectrum, while maintaining a high Voc in solar cells. Therefore, it is crucial to design and synthesize polymers with energy levels aligning with the LUMO (lowest unoccupied molecular orbital) of an electron acceptor to minimize the LUMO level difference between donor and acceptor to keep enough driving force for charge generation, thereby maximizing photovoltage in solar cells. Here a novel copolymer APFO-Green 9 was synthesized. Polymer solar cells based on APFO-Green 9 blended with a derivative of fullerene demonstrate high photovoltage by fine tuning the HOMO and LUMO level of APFO-Green 9. Solar cells based on APFO-Green 9 and [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) present a photoresponse extended to 900 nm with Jsc of 6.5 mA cm−2, Voc of 0.81 V and PCE of 2.3% under illumination of AM1.5 with light intensity of 100 mW cm−2. As a low band gap polymer with a Voc bigger than 0.8 V, APFO-Green 9 is a promising candidate for efficient tandem solar cells.