- CdTe thin film process
In order to reduce the cost of CdTe films, polycrystalline structures are often the mainstay, which can be prepared by various methods, including evaporation, close-spaced sublimation (CSS), and vapor transport deposition (vapor transport deposition). , VTD ) , chemical vapor deposition ( chemical vapor deposition , CVD ) , electrochemical deposition ( electro deposition . ED ) , screen printing ( screen printing ) , thermal plate spraying ( spray pyrolysis ) , sputtering ( sputtering ), etc. has been used . Figure 7.4 organizes various CdTe coating processes and related process conditions for reference and comparison . Because CdTe has ionic bonds. It is easy to form Cd and Te bonds during the film formation process, and different coating methods are used. It is not difficult to control the stoichiometric composition. Even elemental films of Cd and Te can be plated separately at room temperature and then heated up. to form compounds.
It is worth mentioning that CdTe solar cells often require an additional heat treatment procedure. Also CdCl2 is usually mixed into the air with a partial pressure of 0.04 and heat treatment at a temperature of 425 ℃ for 20 min. This promotes the growth of CdTe grains and the passivation of the crystal interface, so that the high resistance at the crystal interface disappears, and at the same time, it promotes the mutual diffusion of materials between PN heterojunctions, forming ternary compounds (such as CdS/CdTe near the interface. Produces CdTe1, S,) with a graded composition, thereby reducing the interface defect density caused by the lattice mismatch between CdTe and CdS. This procedure helps to greatly improve the efficiency of solar cells; while 0 in the air has The effect of increasing the P-type conductivity of CdTe and promoting CdS/CdTe interdiffusion. There is also a saying: Since this is a necessary follow-up process. It does not matter which method is used for the CdTe coating, as much as possible using the low-cost method.
- Electrical properties and device technology of CdTe
The combination of N-CdS/P-CdTe in II-VI solar cells can reach 16.5% energy conversion efficiency. The large-area CdTe solar cell module has entered the mass production stage in recent years, and its energy conversion efficiency has exceeded 10%. Different companies use different preparation methods for the P-CdTe film in the device structure, which are coating technologies such as close-range volatilization, electrochemistry, and vapor transport.
The P-type doping of CdTe is dominated by P or As, and it is usually doped under Cd overpressure, that is, Cd-rich conditions. The hole concentration of 2 × 101; cm – 3 can be obtained for the CdTe epitaxial film prepared by CVD or MOCVD, while the polycrystalline film is only 6 × 101 cm . As doped CdTe prepared by photo-assisted molecular beam epitaxy (PAMBE) has the highest hole concentration of 6. 2 × 1018 cm – 3, which is conducive to the activity of atoms on the surface under illumination , and also makes Te more easily desorbed, and the effect on the doping amount is not obvious. In the MBE process, some people have also used low-energy ion beam doping, and its hole concentration is only 2 × 1017 cm3, but the material defects caused by the ion beam impact make its electrical properties worse, and it often needs to be regrown on it. A layer of undoped CdTe to avoid reducing the short-circuit current of the solar cell.
In terms of ohmic contact, because the work function of P-CdTe is as high as 5.7 eV, no metal can form an ideal ohmic contact with it, so the metal alloy containing Cuo.12Au0.88 is rich in Te after treatment. The surface of CdTe is diffused from Cu into the lattice position where CdTe replaces Cd, which promotes the formation of high content of P-type doping on the surface of CdTe, and holes are conducted through strong electric field tunneling. In the module mass production process, the 1-contact resistance contact of the P-CdTe polycrystalline film is often fabricated by electron beam evaporation on the surface of CdTe after CdCl2 annealing treatment → Cu thin layer, and then heated to promote the diffusion of Cu into CdTe, achieve surface doping.
In summary, the fabrication of high-efficiency N-CdS/P-CdTe solar cells should be based on the following designs and steps: ① Coating the CdTe film on the transparent CdS/TCO/Glass substrate; ② The thinner the CdS light-transmitting layer, the more It can improve the spectral response of short wavelength; ③ After the CdTe coating, a heat treatment procedure is required in the atmosphere of CdCl2 and O2 to improve the grain structure and P-type conductivity of CdTe, and also promote the interaction between CdTe and CdS. The interdiffusion of CdTeS is formed to reduce the number of interface defects; ④ The CdTe layer is chemically treated to make its surface rich in Te, then plated with Cu and heated to diffuse into CdTe to increase the amount of P-type doping, and finally plated on Conductive materials such as Ni or graphite.
- Future development of CdTe thin film solar cells
CdTe solar cells are mass-produced by one or two companies in the United States and Germany. The energy conversion efficiency of large-area CdTe modules can currently reach 10.5% and the service life is as long as 25 years. Its unit cost has the opportunity to be $1 per watt. The following are the thin-film solar cells with the lowest cost at present. Furthermore, Cd and Te elements can be obtained from the smelting process of basic metals such as copper and iron ore. However, a negative factor in the application of this product is that the Cd contained in the material is a heavy metal element, and the environmental protection problems caused by its process and disposal have attracted much attention. Although the industry emphasizes that the pollution of Cd compounds is very different from that of elemental Cd, and that the process and waste recycling treatment procedures can meet strict requirements, a company in the United States even takes long-term samples of its employees in the production line. Examination of the Cd content in the body has not detected higher than usual results, but it is still worrying. Under the competition of other materials, although the EU’s restrictions on the Cd content of most electronic products have not yet included solar cells, the manufacturers of CdTe solar cells mass-produced in the short term have fewer competitors to produce the same products, and the products are more expensive. This is lower than solar cells made of other materials, thus making profits. Manufacturers even give a plan to pay the cost of recycling solar panels in the future, but in the long run, under the stricter environmental protection requirements in the future, it is not conducive to CdTe solar cells. development of.