Research and development of new thin-film solar cells

Thin-film solar cells are composed of semiconductor films deposited on glass, stainless steel, plastic, ceramic substrates or thin films with a thickness of several micrometers or tens of micrometers. Because of its thin semiconductor layer, it can greatly save battery materials and reduce production costs. Therefore, it is the most promising new type of solar cell and has become a key project and hot topic of photovoltaic technology research and development in the world today. The following is a brief introduction to the research and development of several major thin-film solar cells.

① Amorphous silicon thin film solar cells
Amorphous silicon thin film battery was born in 1976 and is a thin film battery made of amorphous silicon semiconductor materials. The most basic feature of amorphous silicon semiconductor materials is that the arrangement of the constituent atoms is long-range disorder and short-range order, and the bonding between atoms is similar to that of crystalline silicon, forming a covalent random network structure. This structure is not an ideal random network model, which contains a certain amount of structural defects, dangling bonds, broken bonds, and voids. The working principle of amorphous silicon cells is similar to that of monocrystalline silicon cells, and both use the photovoltaic effect of semiconductors to achieve photoelectric conversion. Different from monocrystalline silicon cells, the photo-generated carriers of amorphous silicon cells only drift and have no diffusion motion. The reason is that the strong scattering effect caused by the long-range disorder and random network in the amorphous silicon structure makes the carrier The diffusion length of currents is very short. If there is no electric field at the place where the photo-generated carriers are generated, the photo-generated carriers are limited by the diffusion length and will recombine quickly and cannot be absorbed. In order to effectively collect the photogenerated currents, the battery is designed as a PIN type, where the p layer is the incident light layer and the I layer is the intrinsic absorption layer. In the built-in electric field generated by P and N, when incident light enters the I layer through the P+ layer, electron-hole pairs are generated, and the photo-generated carriers are separated by the built-in electric field as soon as they are generated. The hole originally moved to the P+ layer, and the electron drifted to the N room, forming a photo-generated current and a photo-generated voltage.

Amorphous silicon thin film batteries can use glass, stainless steel, special plastics, ceramics, etc. as the bottom of the village. For the amorphous silicon cell at the bottom of the glass village, light is incident on the glass surface, and the cell’s current is drawn from the transparent conductive film (TCO) and aluminum electrodes. The amorphous silicon battery at the bottom of the stainless steel village is similar to the single crystal silicon battery. The comb-shaped silver electrode is prepared on the transparent conductive film, and the current of the battery is drawn from the silver electrode and the stainless steel. There are two double-stacked structures: one is the two-layer structure using the same amorphous silicon material: the other is the upper layer using amorphous silicon alloy, and the lower layer using amorphous silicon germanium alloy to increase the absorption of long-wave light; the upper layer Use a wide band gap amorphous silicon alloy as the intrinsic layer to absorb blue light photons: the intermediate layer uses a medium band gap amorphous silicon germanium alloy containing about 15% germanium to absorb red light. The structure of the triple laminate is similar to the structure of the double laminate.

② Cadmium telluride (CdTe) thin film solar cells
Cadmium elluride (CdTe) thin-film solar cells are another type of thin-film solar cells that have been commercialized. The cell structure is similar to amorphous silicon solar cells (see Figure 1 ), except that cadmium telluride has been used instead of silicon. Is considered to be a replacement for silicon solar cells. Compared with traditional solar cells, cadmium telluride thin-film solar cells, despite their lower efficiency, have significantly reduced production costs. The manufacture of cadmium telluride solar cells is similar to that of silicon-based thin-film solar cells. Because cadmium telluride can be plated with a thin layer at an appropriate temperature, it can be used after a transparent conductive film (TCO) is formed on the glass substrate during manufacturing. CdS layer and CdTe layer are plated with a very low-cost technology, and CdCl2 is coated to increase speech performance. Although cadmium telluride thin-film solar cells have the advantage of low production costs, cadmium is very toxic and will pollute humans and the environment. Therefore, there are not many global manufacturers. Cadmium telluride thin-film solar cells all use hard glass substrates, and no flexible products are available.

Figure 1 Cadmium telluride thin film solar cell structure

③Copper indium selenide/copper indium gallium selenide (CIS/CIGS) thin film solar cells
Copper indium gallium selenide (CIGS) thin film solar cells were mainly composed of three elements of copper, indium and selenium in the early days, forming copper indium selenide solar cells (CuInSe2, CIS), and then adding gallium or sulfur to make the conversion efficiency better The current laboratory efficiency of copper indium gallium selenide (CuInGaSe2, CIGS) solar cells can reach 19.2%, and the efficiency of large-area modules is up to about 13%. The biggest difference between the structure of CIGS thin-film solar cells and other thin-film solar cells is that the glass substrate is on the bottom layer, not on the light-receiving surface (Figure 2). The copper indium gallium selenium solar cell basically adopts the physical vapor deposition technology to carry out, which requires expensive vacuum equipment, which restricts large-scale production. The biggest problem with copper indium gallium selenium solar cells is the limited indium deposits, coupled with the toxicity of selenium sulfide and cadmium, and the difficulty of precise control of these four materials, making CIGS solar cells unable to mass production.

Figure 2 The structure of CIGS thin-film solar cells

④ Dye-sensitized solar cell (DSSC)
Dye sensitized solar cell (DSSC) (see Figure 3) has the advantages of low material cost and simple manufacturing process due to its simple structure, and can also be mass-produced on a large area by printing. Because it uses organic dyes, DSSC has another name for organic dye solar cells. The structure of the dye-sensitized solar cell is composed of two glass substrates, two transparent conductive films and electrodes. The biggest difference from other thin-film solar cells is that the liquid electrolyte is used in the middle, and the photocatalyst and dye are added. The electrode material The electrolyte is mainly platinum, and the electrolyte is mainly iodide ions (I3/I-). In addition, nano-titanium dioxide (TiO2) is used as a photocatalyst, and dyes are used to absorb sunlight to achieve solar power generation. At present, in order to increase practicability, it has begun to develop flexible substrates to replace glass substrates, and colloidal electrolytes or even solid electrolytes to replace liquid electrolytes. Dye-sensitized solar cells can be produced on a roll-to-roll scale. It can greatly reduce the cost, it is also very easy to realize the integration with the building, and it can work under low light conditions, which is a focus of the current research on membrane solar cells.

Figure 3 Dye-sensitized solar cell

There are various technologies for thin-film solar cells, and their development direction is mainly to find breakthroughs in conversion efficiency, large-area uniformity, large-scale possibility, component reliability, and system cost. At present, there is still a certain gap between the conversion efficiency of thin-film solar cells and traditional solar cells. But with the advancement of thin film technology, thin film solar cells will have broader application prospects.