A photonic method of sawtooth waveform generation by using one single-drive Mach-Zehnder modulator is proposed and experimentally demonstrated. Depending on the polarization-sensitive characteristic of the modulator, the modulation sidebands and optical carrier can independently exist on two orthogonal polarization directions. Therefore, the required Fourier components can be manipulated on two polarization dimensions separately, and the superposition of the orthogonal optical envelopes synthesize a sawtooth waveform in time domain. The feasibility of the scheme is theoretically analyzed. In the experiment, sawtooth waveforms with full duty cycle at 3, 5, and 8 GHz are obtained, which agree with the simulation results well.
A novel method (AC-DC-AC) to produce symmetrical four-phase voltage is presented in this paper, The main circuit is the four-bridge topology and the control method is the three-dimensional voltage space vector control. This method has the merits of novel design, high conversion efficiency, good waveform quality and the method is easily to be realized.The power circuit topology of realizing four- phase power transmission is three-phase four-bridge arms inverter. In this paper, firstly, the four-phase inverter mathematical model is analyzed, via the analysis, the phase voltage and line voltage switching function expressions and their conversion matrixes are got, The method of describing four-phase symmetrical sine voltage with three-dimensional space vector is analyzed, four-phase / three-phase coordinate conversion matrix is derived, through the three- dimensional space vector analyses, these are reached: (a) The movement locus of four-phase symmetrical voltage parameters is a ellipse;(b) The control model of four-phase symmetrical voltage may be three-dimensional space vector pulse-width modulation, lastly, these proposed techniques have been verified by simulation and experimental results.
Abstract Thanks to the development of novel electron acceptor materials, the power conversion efficiencies (PCE) of organic photovoltaic (OPV) devices are now approaching 20%. Further improvement of PCE is complicated by the need for a driving force to split strongly bound excitons into free charges, causing voltage losses. This review discusses recent approaches to finding efficient OPV systems with minimal driving force, combining near unity quantum efficiency (maximum short circuit currents) with optimal energy efficiency (maximum open circuit voltages). The authors discuss apparently contradicting results on the amount of exciton binding in recent literature, and approaches to harmonize the findings. A comprehensive view is then presented on motifs providing a driving force for charge separation, namely hybridization at the donor:acceptor interface and polarization effects in the bulk, of which quadrupole moments (electrostatics) play a leading role. Apart from controlling the energies of the involved states, these motifs also control the dynamics of recombination processes, which are essential to avoid voltage and fill factor losses. Importantly, all motifs are shown to depend on both molecular structure and process conditions. The resulting high dimensional search space advocates for high throughput (HT) workflows. The final part of the review presents recent HT studies finding consolidated structure–property relationships in OPV films and devices from various deposition methods, from research to industrial upscaling.
Abstract Minimizing energy loss in organic solar cells (OSC S ) is critical for attaining high photovoltaic performance. Among the parameters that correlated to photovoltaic performance, the energy offsets between donor–acceptor pairs play a vital role in photoelectric conversion processes. For so far reported a large number of non‐fullerene acceptors (NFAs), only Y6 and its derivatives can achieve external quantum efficiencies (EQEs) over 80% with negligible energy offsets when combined with polymeric donors. Thus, understanding the relationship between energy offsets and energy losses in representative NFAs is the key to further enhancing the efficiency of OSCs. In this study, a series of wide‐bandgap polymer donors based on pyrrolo[3,4‐ f ]benzotriazole‐5,7(6 H )‐dione (TzBI) and benzo[1,2‐ c :4,5‐ c ′] dithiophene‐4,8‐dione building blocks are combined with representative NFAs, including ITIC and Y6, to gain deep insights into their photovoltaic performances and related energy losses. Outstanding EQEs (≈70%) and suppressed non‐radiative recombination are achieved at negligible energy offsets. Moreover, it is noted that a prolonged exciton lifetime of acceptor is not essential to obtain high EQEs in OSCs with negligible energy offsets. Eventually, ITIC derivatives with high electroluminescence efficiencies and near‐infrared absorptions have the potential to be assembled to obtain high‐efficiency OSCs.
We demonstrate that low-fluence neutron irradiation can be a promising way to reduce the reverse leakage current of AlGaN/GaN heterostructures grown by MOCVD on sapphire substrates while maintaining other electronic properties almost unchanged.
The optimization of band alignment at the buffer/absorber interface is realized by tuning compositions of Cd and Zn chalcogenides as the buffer layer toward high-efficiency Cu(In,Ga)Se2 (CIGS) solar cells. Using the special quasi-random structure (SQS) approach, we construct randomly disordered ZnxCd1–xSySe1–y alloys and ZnSxO1–x alloys as alternatives to the traditional CdS buffer layer. The compositional dependence of formation energies, lattice parameters, band-gap energies, and band alignments of ZnxCd1–xSySe1–y and ZnSxO1–x alloys is investigated by first-principles density functional theory calculations. For quaternary ZnxCd1–xSySe1–y alloys, we find that the miscibility temperatures and the bandgap bowing coefficients are proportional to the lattice mismatch between the mixing elements. The linear dependence of lattice parameters, trinomial dependence of band-gap energies and band-edge positions on the alloy-composition of ZnxCd1–xSySe1–y alloys are established. For ZnSxO1–x alloys, we find the lattice parameters also exhibit a linear dependence on its composition. Because of the large lattice mismatch and the chemical disparity between ZnO and ZnS, the bowing coefficient for the bandgap energies of ZnSxO1–x alloys is composition dependent, and is larger for dilute ZnSxO1–x alloys. With the optimization criteria of moderate spike-like conduction band offset, large valance band offset, sufficiently wide bandgap, and lattice match with respect to the CIGS absorber, we illustrate the optimal composition range of both ZnxCd1–xSySe1–y alloys and ZnSxO1–x alloys as the buffer layer of the CIGS solar cells. Our work demonstrates that ZnxCd1–xSySe1–y alloys and ZnSxO1–x alloys are promising buffer layers for high-efficiency CIGS solar cells.
Leaf water content (LWC) is regarded as an important indicator of crop health in agriculture. It is in great need to develop a rapid, non-destructive and accurate diagnosis method which can measure and assess water level of different types of crops. In this research, winter wheat leaves were selected and the reliability and applicability of THz imaging for destructive and non-destructive water status monitoring were studied. Based on the measured THz transmission amplitude, we find that the water loss in the distal region is less than that in the basal region during the nature dehydration process. A high correlation is shown between the transmitted THz signal and water content level measured by gravimetric weighing method during dehydration and after rehydration. The obtained results show that the water content in winter wheat leaves can be measured destructively and non-destructively with a high accuracy, using terahertz waves in transmission geometry, which suggests THz imaging can be a good potential tool to measure the water content to diagnosis crop health in agriculture in a rapid, non-destructive and accurate way.
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