There are two main types of photovoltaic cells: crystal cells and thin film cells.
Examples of different types of photovoltaic cells are shown in Figure 1.
Research in this area is continuing, and manufacturers are always looking for lower cost and higher efficiency photovoltaic cells. For example, there is currently a hybrid heterojunction cell on the market, which contains both a crystalline (single) component and a thin-film (amorphous) component.
1. Crystal silicon cell
The two common types of crystal silicon cells are monocrystalline silicon cells and polycrystalline silicon cells. Many manufacturers also produce some special crystal cells, including polycrystalline silicon cells, ribbon silicon cells and crystalline silicon thin film cells, EFG (Fixed Edge Feeding Film Growth Method) polycrystalline silicon cells, polycrystalline SR band silicon cells, monocrystalline branched silicon cells , Polycrystalline APEC cell.
1). Monocrystalline silicon cell
Metallurgical grade silicon (obtained from sand) is purified through a chemical process until semiconductor grade silicon is produced, melted and added with a certain amount of dopants (such as adding boron to produce P-type silicon). The seed crystal is introduced into the molten silicon and slowly extracted from the molten silicon. The silicon solidifies around the seed crystal to produce single crystal silicon. The size of the crystal depends on the rate at which the seed crystal is extracted from the molten silicon. Crystals above 15 cm in diameter are not uncommon. Monocrystalline silicon components are shown in Figure 2.
Once the crystal (solid cylinder of silicon) is formed, it is cut into 0.2~0.4mm wafers, and then the texture is etched to increase the incidence of light. Phosphorous impurities are introduced into the surface of the wafer through a diffusion process, and metal grids are attached to the front and back of the wafer to increase electron capture.
Recently, the single crystal silicon cell developed in the laboratory can reach an efficiency of more than 24% after experimental tests. In this case, factors that cause inefficiency are reduced, such as reflections and grid coverage. The product control of laboratory batches is much stricter than that of large-scale equipment production. Commercial products currently have an efficiency of 15% to 18%.
2). Polycrystalline silicon battery
Different from single crystal silicon, polycrystalline silicon is a material produced by casting silicon ingots, resulting in many small crystals being pieced together. Some manufacturers take advantage of the ease of producing small wafers than large wafers, and have introduced a process for mass production of low-cost polysilicon cells. One disadvantage of polycrystalline silicon cells is that they are easy to trap electrons at the boundaries of small crystals. These boundaries can hinder the slow movement of electrons or form a short-circuit path along the battery. Manufacturers of polycrystalline silicon cells must ensure that the crystal is large enough so that the electrons generated in the photoelectric effect can be captured by the PN junction and grid before reaching the crystal boundary. Although the efficiency of the battery during the study can reach 21%, the usual efficiency is 13% to 16%.
Polysilicon components are shown in Figure 3.
2. Thin film cell
In order to produce single crystal silicon cells and polycrystalline silicon cells, it is necessary to continuously extract crystals from the molten silicon pool. Once this material is produced, it must be cut into wafers. The only active part of the photovoltaic cell is the area around the PN junction, which is only about a few millionths of a centimeter thick. Since it cannot be cut to this thickness, a lot of materials are wasted in photovoltaic cells. One solution is to abandon this crystalline state and apply photosensitive semiconductors on a thin-film substrate, which is a thin-film cell. Unlike crystalline batteries made of silicon, thin-film batteries are different due to different semiconductor materials.
1). Amorphous silicon (a-Si) thin film cell
Amorphous means no lattice structure. Using gas silicon condensation technology, it is possible to produce a cell with the atomic layer thickness as the measuring unit. The atoms in this silicon film are arranged in a completely random form, which is called an amorphous silicon film cell, as shown in Figure 4.
Because this material is very thin, it is difficult for free electrons to exist in the PN junction. Therefore, a non-doped (intrinsic) I layer is applied between the N layer and the P layer to form a PIN structure as shown in Figure 5.
Although this cell is very cheap, the abandonment of the lattice structure also reduces its efficiency. For multilayer batteries, the average efficiency of each layer is about 5% to 8%, and the maximum efficiency is 13%. At present, the average efficiency of the three-layer structure module is 10%. Over time, the stability and performance degradation of the cell also poses technical challenges for R&D personnel and manufacturers.
2). Copper indium selenium (CIS) thin film cell
Copper indium selenium is an active semiconductor material, which is usually synthesized with gallium or sulfur. This material is usually deposited on a glass substrate.The P-type copper indium selenium absorption layer is formed by simultaneously evaporating the elements copper: indium and selenium, and aluminum-doped zinc oxide (ZnO: Al) is used to make an N-type conductive transparent conductive oxide layer. Intrinsic zinc oxide is located between N-type ZnO:Al and P-type copper steel selenium. Copper indium selenium has an N-type cadmium sulfide (CdS) located between the intrinsic layer and copper indium selenium. Unlike amorphous modules, copper indium selenium will not be decomposed under light, but it will be unstable under high temperature and humid conditions, so it must be well sealed.
3). Cadmium telluride (CdTe) thin film cell
The cadmium telluride thin film cell is made on a glass substrate. Indium tin oxide is usually used as the transparent conductive oxide layer. As shown in Figure 6, the N-type layer composed of cadmium sulfide (CdS) is connected to the P-type layer composed of cadmium telluride through the back contact layer. The main problem of cadmium telluride thin film batteries is the toxicity of cadmium, but cadmium telluride is a non-toxic compound.
3. Development Trends of Photovoltaic Cells
The exploration of new photovoltaic modules has never stopped. In recent years, some new technologies have been announced or are in the pilot manufacturing stage.
Such technologies include the following:
(1) Dye-sensitized photovoltaic cells: light is absorbed by organic dyes containing titanium dioxide (TiO2).
(2) The deposition temperature of microcrystalline and microcrystalline silicon photovoltaic cells is 900~1000℃ and 200~300℃, respectively. The former is based on the deposition of a high-quality silicon film on the substrate, which has properties similar to polysilicon; the latter produces a film by using a fine-textured microcrystalline structure, but is deposited in the same way as an amorphous module.
(3) Heterojunction (HIT) photovoltaic cell with intrinsic thin layer. This is a hybrid cell that combines a crystal call and a thin film cell.
(4) Silver cell. It is made up of many small “silver” batteries connected in series or in parallel.
The efficiency of different types of photovoltaic cells is shown in Table 1
|Photovoltaic cell materials||Battery efficiency (laboratory)||Battery efficiency (product)|
|Crystalline silicon film||19.2||9.5|
|Dye-sensitized photovoltaic cell||12.0||7.0|
Table 1 Maximum efficiency% of different types of photovoltaic cells