Fig. 1. (a) Schematic diagram of perovskite type solar cells; (b) Perovskite energy level diagrams with different silver injection rates; (c) Voltage-current efficiency curves of perovskite devices with different silver injection rates.

Figure 2. (a) Cross-section scanning Kelvin probe microscopy to characterize the energy level arrangement of trans-structured organic solar cells; (b) Restoring the true potential distribution of the device by deconvolution.

In recent years, new types of thin-film solar cells, such as organic/inorganic hybrid perovskite devices and organic photovoltaic devices, have attracted widespread attention due to their advantages of low cost, high efficiency, simple structure, and flexible carrying. For thin-film solar cells, the arrangement of the device's energy levels determines the microscopic physical processes such as separation, recombination, transmission, and collection of photo-generated carriers, and is one of the important determinants of device performance. How to effectively regulate and characterize the arrangement of device energy levels is an important starting point for understanding the working mechanism of devices, guiding the selection of materials and device process optimization.

Recently, the researcher Chen Lizhen of the Institute of Nanotechnology and Nano-Bionics of the Chinese Academy of Sciences has made new progress in the research on the regulation and characterization of the energy level arrangement of thin-film solar cells:

1. In organic/inorganic hybrid perovskite photovoltaic devices, an in-depth understanding of the interrelationships between material doping, energy level regulation, and device performance. First-principles calculations indicate that the outer electron orbitals of metal ions are an important determinant of the organic/inorganic hybrid perovskite band structure, which in turn affects the optical and electrical properties of the material. A number of research teams at home and abroad reported that a small amount of metal ions replacing Pb2+ can effectively improve device performance, but the mechanism of interaction between energy level structure evolution and device performance lacks in-depth research. Chen Lijiu's group used Ag+ to replace part of Pb2+ and introduce acceptor states into the perovskite, making the Fermi level of an originally n-type perovskite material move towards the center of the forbidden band, showing the properties of intrinsic semiconductors. The change effectively reduces the electron concentration in the perovskite, which is beneficial to the balanced transport of carriers. By optimizing the silver doping ratio, the crystallinity of the perovskite material is improved, and the film morphology and carrier kinetics are improved. Under the synergetic promotion of these factors, the efficiency of trans-structured perovskite planar heterojunction devices (ITO/Cu:NiOx/perovskite/PCBM/Ag) increased from 16.0% to 18.4% (Fig. 1). Furthermore, the interaction between perovskite carrier concentration and device performance was confirmed by an equivalent circuit model. Related research results were published on Nano Lett.

2. In organic thin-film photovoltaic devices, the energy level arrangement of trans devices was quantitatively characterized under device conditions. For the vertical closed type structure of thin-film solar cell layer stacks, there is currently no effective means to intuitively measure the energy level structure of the operating conditions of the device. Chen Lizheng's group reported a cross-sectional scanning cross-sectional SKPM at Nat. Commun. It can realize energy-level structure measurement under the operating conditions of short circuit, open circuit, dark state, and light, but the energy level The quantitative measurement of the arrangement is still pending. Further research, Chen Lijiu's group found that the pinpoint/cantilever beam convolution effect is an important factor affecting the quantitative measurement of the energy level structure of the device, in the presence of inversion-level abrupt change in the inversion device (ITO/ZnO/BHJ/MoOx/Al The convolution effect can even conceal the real potential distribution and lead to a misdirected built-in electric field. To solve this problem, they collaborated with Lu Shulong's research group to calibrate the tip transfer function of GaAs/GaInP heterojunction grown by molecular beam epitaxy. The deconvolution algorithm was used to remove the mean effect of the pin tip and restore the true energy level of the trans-device. Cloth (Figure 2). The relevant research results were published on Nano Energy, and they wrote monographs in the Journal of Physical Chemistry.

The research work was supported by professors such as Alex Jen from the University of Washington, Lu Shulong, and researchers from the Suzhou Nanometer Institute. The research work was supported by the National Natural Science Foundation of China, the Ministry of Science and Technology’s key R&D program, and the funding support and R&D conditions of the Chinese Academy of Sciences’ scientific research equipment project.

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