Talking about the future development of CIGS thin film solar cells

by admin II-VI and I-III-VI compound semiconductor solar cells

In the future, thin-film solar cell products will be similar to today’s computer CPUs, and the demand for products can also be stimulated by the ever-improving energy conversion efficiency. Judging from a certain area. The roof of the house, the exterior of the building, the surface of the clothes bag, etc. Some possible application and installation places of thin-film solar cells. Needless to say, its power output increases proportionally with efficiency. How to further improve the energy conversion efficiency of the product is a major challenge, and it is also the key to the decisive victory of the product. Although reducing the manufacturing cost of the product is another factor, compared with the two, the former also has the effect of reducing costs. should be given more attention.

The more traditional method to improve the efficiency of CIGS solar cells is to use a tandem structure, that is, PN junctions made of different materials are stacked in order from top to bottom according to the size of the energy gap. According to theoretical calculations, the series connection of three cells can achieve the most economical Affordable results, see Figure 1. The light-transmitting conductive layers can be connected in series between the cells, each responsible for the absorption of sunlight in different bands, which can improve the efficiency to more than 30%. Taking the Hybrid V compound semiconductor as an example, the current maximum has reached 40%. The IHB VI family also has different material combinations with adjustable energy gaps for device design. The related material data is shown in the series structure of the compound, as shown in Figure 2.

Figure 1 - Calculated energy conversion efficiency of tandem solar cells, Figure 2 - IHB group VI compounds can change the energy gap through the adjustment of different material compositions to meet the design of tandem solar cell devices
Figure 1 – Calculated energy conversion efficiency of tandem solar cells, Figure 2 – IHB group VI compounds can change the energy gap through the adjustment of different material compositions to meet the design of tandem solar cell devices

Another research direction to improve the efficiency of CIGS solar cells is to deposit an ultra-thin absorber layer (ETA cell for short) in nanostructures, as shown in Figure 3. Its operation principle is similar to that of dye-sensitized TiO2 solar cells. similar. If quantum dots can be deposited and formed, using their light absorption characteristics different from those of bulk materials, when the size of the material is reduced to the point where the particle size is lower than the mean free path of carriers. It can reduce the carrier recombination generated by light, and can control the energy gap value of nanocrystals by adjusting the size, improve the utilization rate of sunlight energy, and also allow photons with more energy than the energy gap to generate more than one pair of electrons. and holes to improve battery efficiency. This new type of design is called a third-generation solar cell. Since it is an emerging technology, there are still problems to be overcome. If the efficiency can be effectively improved to more than 10% in the future, it may become one of the market mainstreams of thin-film solar cells. To read more about batteries click here to open.

Figure 3 ETA cell compares with crystalline silicon solar cells up to 200µm thick, and thin film solar cells can build their product features on flexible substrates. Figure 4 is a comparison of the measured efficiency values ​​of flexible CIGS solar cells fabricated on different substrates. High-efficiency CIGS solar cells are usually prepared at temperatures above 500 °C,

Figure 4-(a) Efficiency comparison of CIGS solar cells fabricated on various flexible substrates; (b) Polyimide is the efficiency of CIGS solar cells fabricated on substrates at different temperatures
Figure 4-(a) Efficiency comparison of CIGS solar cells fabricated on various flexible substrates; (b) Polyimide is the efficiency of CIGS solar cells fabricated on substrates at different temperatures

It is feasible to use metals such as stainless steel as the substrate. However, if the mass is to be greatly reduced in order to facilitate the use of personal power sources or to facilitate the application of power sources in outer space, lightweight polymer substrates are often used. From Figure 4(b), it can be seen that the efficiency value of CIGS solar cells fabricated at different substrate temperatures starts to drop when the temperature is above 450 °C [19], because this type of substrate has cracking phenomenon at this temperature, so a low-temperature process is required. Preparation of CIGS films. The German company Solarion has developed a roll-to-roll process to manufacture CIGS solar cells on flexible polymer substrates. As shown in Fig. 5, the device structure is fabricated by Polyimide/Mo/CIGS/CdS/ZnO stack, in which the CIGS film is fabricated by ion beam assisted evaporation, but the cell efficiency is only about 8%. Defects will affect the film properties. resulting in low power generation efficiency. In our laboratory, high-quality CIS epitaxial films can be grown at 300°C by UV-assisted evaporation. This method should be a better choice for the research and development of low-temperature processes.

Figure 5 - Solarion's roll-to-roll process using flexible polymers as substrates
Figure 5 – Solarion’s roll-to-roll process using flexible polymers as substrates

There is a worrying question in the mass production of CIGS solar cells, that is, whether In will run out of material when there is a lot of demand. Although some arguments suggest that In mineral stocks are sufficient, they are not necessarily credible. When planning ahead, there is indeed a research on the idea of ​​replacing In with Zn and Sn, but the method is successful because of the concomitant generation of three phases. It can be tried to use CuGaTe2 with similar energy gap. In addition, reducing the amount of use is also one of the solutions. If you can seek a breakthrough in the design of the device structure, try to make use of the extremely high light absorption coefficient of CIGS to reduce the thickness of CIGS by half or even to a quarter, which is another solution. Way. These may be the focus of future research and development.

Due to the significant increase in demand for crystalline silicon solar cells in recent years, raw material manufacturers of silicon materials strategically do not cooperate with the simultaneous expansion of production, resulting in the dilemma of material shortages. Those who are eager to catch a ride and participate in the investment and establishment of factories are discouraged. Turning to the production of thin-film solar cells, because the mass production technology of amorphous silicon solar cells is mature and easy to obtain, several amorphous silicon solar cell manufacturing plants were established around the beginning of 2007. However, the efficiency of amorphous silicon solar power modules Low, it can meet the demand of the towel field in the short term, but it is not optimistic in the long term. Another option on the tabletop of static film solar cells is CIGS, whose power and module efficiency is close to that of products made of polysilicon materials. More turn-key mass production equipment manufacturers with a full set of technologies came to Taiwan to sell.

The above information and development have made this industry attract attention. We can see that in the field of solar cells, there can be different materials, different processes, and different products. Higher efficiency and lower cost are the keys to winning, and there is no clear answer at present. Research in this area is still worthy of further exploration, and we have noticed that new ideas are advancing with the times, and new combinations of technologies and materials are constantly being introduced. Today, the research and development of solar cells has escaped the old framework and is heading for a new future, which has the opportunity to become a cheap and clean energy source in human daily life.

Read more: What are semiconductor solar cells and blackbody radiation