Research status and development trend of thin film solar cells

Solar energy is the most important basic energy source among various renewable energy sources. Biomass energy, wind energy, hydropower, etc. all come from solar energy. Solar cells are a device that converts solar energy into electrical energy through the photovoltaic effect and are an important form of utilizing solar energy.

　　At present, people divide the application technology of solar cells into two categories: crystalline silicon and thin film according to the selected semiconductor materials. Crystalline silicon solar cells dominate the large-scale application and industrial production at this stage, but their development is limited due to their high cost. Compared with other solar cells such as crystalline silicon, thin-film solar cells have the advantages of low production cost, less raw material consumption, and excellent weak light performance. With the shortage of energy in the world, thin-film solar cells, as a photoelectric functional film, can effectively solve the problem of energy shortage, and are pollution-free. They can also realize photovoltaic building integration and are easy to promote on a large scale.

　　Amorphous silicon thin film solar cells

　　The conversion efficiency of amorphous silicon thin-film solar cells is relatively low, with a laboratory conversion efficiency of only 13%, but the process is mature, the cost is lower than that of crystalline silicon, and it is easy to prepare, making it suitable for large-scale production.

　　Amorphous silicon thin-film solar cells usually have a stacked structure, with three thin films deposited on a glass substrate: a transparent conductive oxide (TCO) layer, an amorphous silicon layer (a-Si layer), and a back electrode layer (Al/ZnO layer). The amorphous silicon layer is deposited by magnetron sputtering.

　　Compared with single-crystal silicon solar cells, amorphous silicon thin film is a material that is very promising to significantly reduce the cost of solar cells. Amorphous silicon thin film solar cells have many advantages that make them an excellent photovoltaic thin film device. (1) Amorphous silicon has a large light absorption coefficient, so when used as a solar cell, the required thickness of the film is much smaller than that of other materials such as gallium arsenide; (2) Compared with single-crystal silicon, the manufacturing process of amorphous silicon thin film solar cells is simple and the energy consumption of the manufacturing process is low; (3) Large-scale and continuous production can be achieved; (4) Glass or stainless steel and other materials can be used as substrates, which makes it easy to reduce costs; (5) It can be made into a stacked structure to improve efficiency.

　　However, there are still some problems that need to be solved in amorphous silicon thin-film solar cells. (1) Due to the existence of the Staebler-Wronski effect, the efficiency of amorphous silicon thin-film solar cells will decay when exposed to sunlight for a long time, resulting in a decrease in the efficiency of the entire cell; (2) The deposition rate is low, which affects the large-scale production of amorphous silicon thin-film solar cells; (3) Subsequent processing is difficult, such as the treatment of Ag electrodes; (4) There are a large number of impurities in the thin-film deposition process, such as O2, N2, C, etc., which affect the quality of the film and the stability of the battery.

　　The next step of research on amorphous silicon thin-film solar cells will focus on the following directions: first, to use high-quality bottom cell i-layer materials; second, to develop towards stacked structure cells; third, to develop and produce stacked amorphous silicon solar cell module technology while ensuring efficiency; and finally, to use cheap packaging materials to reduce costs.

　　Polycrystalline silicon thin film solar cells

　　Poly-Si thin film batteries have the advantages of high efficiency, stability, non-toxicity and abundant material resources of crystalline silicon batteries, as well as the advantages of material saving and low cost of thin film batteries. They have high photosensitivity in the long-wave band, effectively absorb visible light energy, and have the same light stability as crystalline silicon. At the same time, the material preparation process is relatively simple. Poly-Si thin film battery technology is expected to reduce the cost of solar cell modules to a greater extent, thereby making the cost of photovoltaic power generation competitive with conventional energy.

　　There are many factors that limit the conversion efficiency of solar cells. Improving the absorbance and reducing carrier recombination are the two most important methods to improve the conversion efficiency.

　　As we all know, the greater the absorbance, the higher the cell conversion efficiency and the greater the short-circuit current density. The optical absorption length of Si for visible light is about 150um. It can be seen that the thickness of traditional single crystal and amorphous silicon solar cells is about 200um, which is conducive to fully absorbing the energy of sunlight. According to internationally recognized standards, the thickness of a new generation of thin-film solar cells should be less than 50um. This means that the light of a longer wavelength must be reflected back and forth between the upper and lower surfaces of the film to increase its optical path and achieve the purpose of increasing the absorbance. To make the absorbance A (λ) reach a high value in a wide spectrum range, the following two methods can be adopted.

　　The first method is to make the reflection coefficient Rf of the upper surface of the thin-film battery close to 0. For this purpose, a single-layer or multi-layer anti-reflection film composed of ZnS, MgF, TiO2 and Si is usually used. The second method is to make the reflection coefficient Rb of the back of the thin-film battery close to the ideal 100%. Usually, a metal film is evaporated on the substrate as a reflective layer to increase the reflection coefficient of the back of the battery.

　　Whether it is bulk silicon or thin-film silicon solar cells, the carrier recombination inside them is inevitable. In Si thin-film solar cells, a large number of carrier recombination occurs at the impurity center, surface, interface and grain boundary. In polycrystalline silicon thin films and microcrystalline silicon thin films, there will be grain boundary recombination at the grain boundary. In order to reduce these recombination, the unnecessary impurities in the film should be reduced as much as possible, and the grain size in polycrystalline silicon and microcrystalline silicon thin films should be increased.

　　CIGS thin film solar cells

　　Copper indium gallium selenide thin film solar cells are the first choice for the third generation of solar cells and have the highest output power per unit weight. The so-called third generation solar cells are compound thin film solar cells such as copper indium gallium selenide (CIGS) that are highly efficient, low-cost, and can be industrially produced on a large scale.

　　CIGS has excellent anti-interference and radiation resistance, so there is no performance degradation effect caused by light radiation, and the service life is long. CIGS is a direct bandgap semiconductor material, so the thickness of the CIGS film required in the battery is very small (generally around 2um). Its absorption coefficient is as high as 10-5cm-1, and it also has very good response characteristics over a very wide range of solar spectra. The bandgap of CIGS can be changed by adjusting Ga/(In+Ga), and the adjustment range is 1.04eV~1.72eV. CIGS-based cells can be easily made into multi-junction systems. In the case of four junctions, they are arranged in descending order according to the bandgap width from the direction of light incidence, and the theoretical conversion efficiency limit of solar cells can exceed 50%.

　　CIGS thin film is deposited on a glass substrate coated with Mo at a temperature above 500°C, and with a CdS layer formed by chemical deposition, a CdS/CIGS heterojunction solar cell is formed. The efficiency of solar cells made of gallium-doped CIS (CIGS) and CdS as a buffer layer has reached 21.5%.

　　At present, most CIGS battery components contain CdS buffer layer, but there are also some disadvantages in using CdS buffer layer. From the perspective of recovering short-wave photocurrent, a buffer layer with wider bandgap should be used. From the perspective of the environment, the toxicity of cadmium will have a negative impact on the environment. Therefore, in recent years, the buffer layer materials used in research include ZnS, In2S3, ZnSe, ZnO, SnO2, ZnIn2Se, etc., to replace CdS as a buffer layer to achieve the preparation of green cadmium-free and high-efficiency CIGS thin-film solar cells. At the same time, in order to save raw materials and energy, it should also be considered to reduce the film thickness as much as possible.

　　Organic thin film solar cells

　　Organic thin film solar cells mainly include: single-layer Schottky cells, double-layer pn heterojunction cells, and bulk heterojunction cells with interpenetrating structures of P-type and n-type semiconductor networks. It is currently believed that the working process of organic thin film solar cells is divided into three steps: light excitation to generate excitons, splitting of excitons at the donor/acceptor (D/A) interface, drift of electrons and holes and their collection at their respective electrodes. Organic thin film solar cells have significant advantages such as potential low material prices, easy processing, large-area film formation, designability of molecular and thin film properties, light weight, and flexibility. However, the current photoelectric conversion efficiency of organic thin film solar cells is very low and the stability is poor. Large-scale application is only possible if the photoelectric conversion efficiency is increased to more than 5%.

　　In summary, thin-film solar cells will play an increasingly important role in the future development of photovoltaic cell technology due to their low cost, low material consumption and continuously improving conversion efficiency. Many researchers are committed to the research and development of thin-film solar energy. Different types of thin-film solar cells have their own advantages and disadvantages. The cost of a-Si thin-film solar cells is lower than that of single-crystal Si solar cells, but due to the existence of light-induced degradation effect, it is currently difficult to develop into a solar cell with stable and high efficiency. Poly-Si thin-film solar cells have the advantages of both single-crystal Si and a-Si, and the preparation process is relatively simple, which is suitable for industrial large-scale production. CIGS thin-film solar cells have high efficiency and superior performance, and it is recommended that scientific researchers pay more attention to them. Organic thin-film solar cells are of great significance for achieving low energy consumption, low cost and pollution-free, but they have low conversion efficiency and poor long-term stability. It takes a long research process to achieve commercial use. It can be imagined that in the near future, with the continuous deepening of scientific research, the problems currently faced by thin-film solar cells will be solved one by one, and the performance will be continuously improved and enhanced, thereby meeting the urgent needs of future consumers for energy.