Introduction To Types Of Flexible Solar Cells

Amorphous silicon
The thickness of amorphous silicon (a-Si) flexible batteries is 1/300 of that of crystalline silicon batteries, which can further reduce the cost of raw materials. A breakthrough in amorphous silicon flexible batteries was the proposed triple junction stack battery structure in 1997, which improved conversion efficiency and stability, and achieved a stable conversion efficiency of 8.0% to 8.5%.
Taking the amorphous silicon flexible battery of United Solar Ovonic Company in the United States as an example, the amorphous silicon triple junction stack battery structure includes three layers of p-n junction absorption layers with different band gaps. The top battery uses amorphous silicon a-Si with a band gap of 1.8 eV to absorb blue light. The intermediate battery uses a silicon germanium alloy a-SiGe with a band gap of 1.6 eV, which absorbs green light and has a Ge content of 10% to 15%. The silicon germanium alloy a-SiGe with a 1.4 eV band gap for the bottom battery absorbs 40% to 50% of red and infrared light, with a high content of Ge. After the sunlight passes through the three layers of semiconductor absorption layer in turn, there is still a portion of the light that has not been absorbed. After being reflected by the back reflection layer of Al/ZnO, it returns to the three layers of semiconductor absorption layer and undergoes another absorption process. The back reflection layer plays a light trapping role. In this way, amorphous silicon flexible batteries can more effectively absorb incoming and outgoing light, improve conversion efficiency and output power, and achieve better performance under low incident and scattered light conditions.
As of 2016, only Xunli Solar was producing amorphous silicon flexible thin film batteries and components in China, 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 components, there are also folding charging packs that expand the application of flexible amorphous silicon.
Copper indium gallium selenium
In the mid-1970s, people began to study copper indium gallium selenium (CIGS) thin film batteries. CIGS thin films belong to chalcopyrite crystals with adjustable band gaps. Due to the requirement of solar cells for a band gap of 1 to 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 to amorphous silicon, CIGS crystal has fewer internal defects, more stable performance, and a module life of up to 25 years. During the use of the module, the movement of copper ions can repair defects, so the performance of the module will continue to improve, which is in contrast to the photoinduced decay effect or the S-W effect (Staebler Wronskieflect) of amorphous silicon.
Organic
In organic photovoltaic (OPV) solar cells, the organic semiconductor absorbing medium is usually composed of a mixture of donor and acceptor materials. Donor materials are good at giving electrons, absorbing holes, and having a positive charge after mixing. Conjugated polymers are typical donor materials. The acceptor material is good at absorbing electrons and giving holes, and has a negative charge after mixing. Fullerene (C60) is a typical acceptor material.
Excitons are bound electron hole pairs, which are stimulated quasi particles. After being stimulated, the electrons and holes separate, but the electron hole pairs still attract each other through the electrostatic Coulomb force, which cannot be completely separated due to Coulomb binding, forming excitons. There are two types of excitons, watts
Wannier Mottexcition and Frenkel exciton. Varni model excitons exist in crystalline silicon semiconductors, where electrons excited into the conduction band and holes in the valence band form bound states, with a weak Coulomb force of about 0.01 eV. Frenkel excitons exist in donor materials in organic media, and the Coulomb force between them is strong, around 0.3 eV