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Jinghui Electronics Introduction: Flexible Solar Cells

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Flexible solar cells are a type of thin-film solar cells, and have better technology, excellent performance, low cost, and wide use. Can be applied to solar backpacks, solar hoods, solar flashlights, solar cars, solar sailing boats and even solar aircraft. An important application area of flexible solar energy is BIPV (Building Integrated Photovoltaic), which can be integrated on windows or roofs, exterior walls or interior walls.

Battery principle

Photoelectric effects can occur when ultraviolet light is irradiated on a metal surface. As Einstein concluded, free electrons are generated because the photon energy of the incident light is greater than the binding energy of the electrons. The function of solar cells is to convert sunlight into voltage and current, which is a type of photoelectric conversion. The photovoltaic effect is much more efficient than the photovoltaic effect. Because in a photovoltaic cell where a photovoltaic effect occurs, two types of semiconductors of opposite polarity form a pn junction, which forms a built-in electric field, drives electrons into the circuit, and creates voltage and current in the circuit. Type introduction "Amorphous silicon" Amorphous silicon (a-Si) flexible batteries are 1 / 300th the thickness of crystalline silicon batteries, which can further reduce raw material costs.

A breakthrough of the amorphous silicon flexible battery was the three-junction stack structure proposed in 1997, which improved the conversion efficiency and stability, and the stable conversion efficiency reached 8.0% to 8.5%. Taking the amorphous silicon flexible battery of United Solar Ovonic Company in the United States as an example, the structure of an amorphous silicon three-junction stacked battery includes three pn junction absorbing layers with different band gaps. The top battery uses a 1.8 eV amorphous silicon a- Si, absorbs blue light. The intermediate cell uses a 1.6 eV silicon-germanium alloy a-SiGe with a band gap, which absorbs green light, and the Ge content is 10% -15%. The bottom cell uses a 1.4 eV silicon-germanium alloy a-SiGe with a band gap of 40% -50% to absorb red and infrared light, and the Ge content is high. After the sunlight passes through the three semiconductor absorption layers in sequence, a part of the unabsorbed light is reflected by the Al / ZnO back reflection layer, and then returns to the three semiconductor absorption layers, and the absorption process is performed again. Light trapping effect. In this way, the amorphous silicon flexible battery can more effectively absorb incident light, improve conversion efficiency and output power, and perform better under conditions of low incident light and scattered light.

As of 2016, only Xunli Solar is producing amorphous silicon flexible thin film batteries and modules with a conversion efficiency of 8-10% and an overall thickness of only 1.5mm. In product applications, in addition to roll-to-roll flexible thin-film modules, there are also folding charging bags, which expands the application of flexible amorphous silicon.

Copper indium gallium selenium

Copper indium gallium selenium copper copper indium gallium selenium (Cu (In, Ga) Se2, CIGS) thin film batteries have been studied in the mid-1970s. CIGS films are chalcopyrite crystals, whose band gap can be adjusted. Since the band gap requirement of solar cells is 1 ~ 1.7eV, the band gap of CIGS can be adjusted as needed by changing the content of Group III cations In, Ga, Al, and Group VI anions Se, S. Compared with amorphous silicon, CIGS crystals have fewer internal defects, more stable performance, and a component life of 25 years. During the use of the component, the movement of copper ions can repair defects, so the performance of the component will be continuously improved, which is the exact opposite of the photo-induced decay effect or S-W effect (Staebler-Wronskieflect) of amorphous silicon. Organic In organic photovoltaic (OPV), organic semiconductor absorption media are usually composed of a donor material and an acceptor material. Donor materials are good at giving electrons and absorbing holes, and have positive charge after mixing. Conjugated polymers are typical donor materials. Acceptor materials are good at absorbing electrons and giving holes, and have negative charge after mixing. Fullerene (C 60) is a typical acceptor material. An exciton is a bound electron-hole pair and is a quasiparticle after being excited. After being excited, the electrons and the holes are separated, but the electron-hole pair still attracts each other by the electrostatic Coulomb force. Due to the Coulomb restraint, they cannot be completely separated to form excitons. There are two kinds of excitons, Wannier-Mottexcition and Frenkel exciton. Varney-model excitons exist in crystalline silicon semiconductors. The electrons excited into the conduction band and the holes in the valence band form a bound state. The Coulomb force is weak, about 0.01 eV. Frenkel excitons exist in the donor material of the organic medium, and the Coulomb force between them is strong, about 0.3eV.

Dye sensitization

As early as the 1970s, people hoped to develop new solar cells by simulating photosynthesis. At that time, people coated a layer of chlorophyll dye on the surface of the semiconductor crystal material titanium dioxide (TiO2). Although the concept of a dye-sensitized solar cell (DSC) was proposed, the conversion efficiency was only 0.01% due to the difficulty in transporting electrons in chlorophyll. It was not until 1991 that Swiss chemist Michael Gratzel applied nanotechnology to promote the substantial development of dye-sensitized batteries. Gratzel replaced the large-grained TiO2 crystals with small-particle 20nm-diameter sponge-like TiO2, and the outer layer was wrapped with a thin layer of dye to form a 10um thick optically transparent film. The finished dye-sensitized battery has a conversion efficiency of 7.1% and a current density of 12 mA / cm.

Now, the world record for conversion efficiency of dye-sensitized cells is 11%. In the structure of fuel sensitized cells, photosensitizers are covered by crboxyl (-COOH), phosphoric acid (-PO3H2) or boric acid (Bonic acid -B (OH) 2) functional groups. On the surface of TiO2 particles, a charge transfer complex is formed, and then immersed in a redox mediator solution. The TCO glass and metal substrate serve as the cathode and anode, respectively.

Wuhu Jinghui Electronics understands that in 2016, Chinese scientists researched into a new type of flexible solar cell, which may realize wearable solar devices in the future.


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