Solar energy is the most important basic energy source for various renewable energy sources. Biomass energy, wind energy, and water energy all come from solar energy. A solar cell is a device that converts solar energy into electrical energy through the photovoltaic effect, and is an important form of utilizing solar energy.

At present, according to the selected semiconductor materials, solar cell applications are divided into crystalline silicon and thin films. 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 to other solar cells such as crystalline silicon, thin-film solar cells have the advantages of low production cost, low raw material consumption, and excellent low-light-light performance. With the world's energy shortage, thin-film solar cells as an optoelectronic functional film can effectively solve the energy shortage problem, and without pollution, can also realize the integration of photovoltaic buildings, and is easy to promote in large areas.

Amorphous silicon thin film solar cells

The conversion efficiency of amorphous silicon thin-film solar cells is low, and the laboratory conversion efficiency is only 13%. However, the process is mature, the cost is lower than that of crystalline silicon, and the preparation is convenient. It is suitable for large-scale production.

Amorphous silicon thin-film solar cells are usually laminated structures. A transparent conductive oxide (TCO) layer, an amorphous silicon layer (a-Si layer) and a back electrode layer (Al/ZnO layer) are deposited on the glass substrate. The amorphous silicon layer is deposited by magnetron sputtering.

Compared to monocrystalline silicon solar cells, amorphous silicon thin films are a promising material for significantly reducing the cost of solar cells. Amorphous silicon thin film solar cells have many advantages, making it an excellent photovoltaic thin film photovoltaic device. (1) Absorption coefficient of amorphous silicon is large, so when used as a solar cell, the required thickness of the film is much smaller than other materials such as gallium arsenide; (2) Compared to monocrystalline silicon, amorphous silicon thin film solar energy The battery manufacturing process is simple and the energy consumption in the manufacturing process is low; (3) large area and continuous production can be realized; (4) materials such as glass or stainless steel can be used as the substrate, so that the cost can be easily reduced; (5) the stack can be made Layer structure, improve efficiency.

However, at the same time, 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 amorphous silicon thin-film solar cell will be attenuated for a long period of time under sunlight, resulting in a decrease in the overall cell efficiency; (2) Low deposition rate, affecting amorphous silicon Large-scale production of thin-film solar cells; (3) Subsequent processing difficulties such as the handling of Ag electrodes; (4) A large amount of impurities such as O2, N2, and C in the film deposition process affect the quality of the thin film and the battery stability.

The next step in the research of amorphous silicon thin-film solar cells is mainly in the following directions: First, the use of high-quality bottom-battery i-layer materials; second-phase stacked-layer structural battery development; and third, development and production under the conditions of ensuring efficiency. Stacked amorphous silicon solar cell module technology; Finally, inexpensive packaging materials are used to reduce costs.

Polysilicon thin film solar cells

The poly-Si thin-film battery not only has the advantages of high efficiency, stability, non-toxicity, rich material resources of the crystalline silicon battery, but also has the advantages of material saving and low cost of the thin-film battery. It has high photosensitivity in the long wavelength band, and effectively absorbs visible light energy. It has the same light stability as crystalline silicon, and the material preparation process is relatively simple. The poly-Si thin-film solar cell technology is expected to reduce the cost of solar cell modules to a greater extent, thus enabling the cost of photovoltaic power generation to compete with conventional energy sources. .

There are many factors that limit solar cell conversion efficiency. Increasing absorbance and reducing carrier recombination are the two most important methods for improving conversion efficiency.

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

The first method is to make the surface reflectance Rf of the thin film battery close to zero. For this purpose, single-layer or multi-layer antireflection films composed of ZnS, MgF, TiO2 and Si are generally used. The second method is to make the reflection coefficient Rb of the back side of the thin film battery close to the ideal 100%, and it is usually used to increase the reflection coefficient of the backside of the battery by vapor-depositing the metal film as a reflection layer on the substrate.

Whether bulk silicon or thin-film silicon solar cells, their internal carrier recombination is unavoidable. In si thin-film solar cells, a large number of carrier recombination occurs at the center, surface, interface and grain boundary of the impurity. L2J is in the polycrystalline silicon thin film and the microcrystalline silicon thin film, and there is grain boundary recombination at the grain boundary. In order to reduce these complexes. The unnecessary impurities in the thin film should be reduced as much as possible, and the grain size in the polysilicon and microcrystalline silicon thin films should be increased.

CIGS Thin Film Solar Cell

The copper indium gallium selenide thin film solar cell is the first choice of the third generation solar cell, and is the solar cell with the highest unit weight output power. 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/large-scale industrial production.

CIGS has excellent anti-interference and radiation resistance, so it has no performance degradation due to light radiation and long service life. CIGS is a direct bandgap semiconductor material, so the required CIGS film thickness in the battery is very small (generally around 2um). Its absorption coefficient is as high as 10-5cm-1, and it also has a very good range of solar spectral response characteristics. The band gap of CIGS can be changed by adjusting Ga/(In+Ga), and the adjustment range is 1.04 eV to 1.72 eV. The CIGS battery can be easily made into a multi-junction system. In the case of four junctions, the order of the light's incident direction is in descending order of the forbidden band width. The theoretical conversion efficiency of a solar cell can exceed 50%.

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

Most current CIGS battery modules contain a CdS buffer layer, but the use of a CdS buffer layer also has some disadvantages. From the viewpoint of recovering shortwave photogenerated currents, a buffer layer with a wider band gap should be used. From the environmental point of view, the toxicity of cadmium will have a negative impact on the environment. Therefore, in recent years, the buffer layer materials used in the study have been ZnS, In2S3, ZnSe, ZnO, SnO2, ZnIn2Se, etc. to replace CdS as a buffer layer to realize the preparation of green cadmium-free CIGS thin film solar cells, and in order to save raw materials and energy, it should also Consider reducing the film thickness as much as possible.

Organic Thin Film Solar Cell

Organic thin-film solar cells mainly include single-layer Schottky cells, double-layer p-n heterojunction cells, and bulk heterojunction cells with P-type and n-type semiconductor network interpenetrating structures. Currently, the process of organic thin-film solar cells is considered to be divided into three steps: exciton generation by photoexcitation, splitting of excitons at the donor/acceptor (D/A) interface, drift of electrons and holes, and their respective electrodes. collect. Organic thin film solar cells have the potential advantages of low cost, easy processing, large area film formation, designability of molecular and thin film properties, light weight, flexibility, etc. However, organic thin film solar cells currently have low photoelectric conversion efficiency and poor stability. Only by increasing the photoelectric conversion efficiency to more than 5% can large-scale applications be possible.

In summary, thin-film solar cells occupy an increasingly important position in the development of photovoltaic cell technology in the future because of low cost, low material consumption, and ever-increasing conversion efficiency. Many researchers are committed to 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 photo-induced recession effect, it is difficult to develop solar cells with stable and high efficiency at present. The poly-Si thin-film solar cells have the advantages of both single-crystal Si and a-Si, and the preparation process is relatively simple, and is suitable for large-scale industrial production. CIGS thin-film solar cells have high efficiency and superior performance, and researchers are advised to pay more attention. Organic thin-film solar cells are of great significance for achieving low energy consumption, low cost, and no pollution, but they have low conversion efficiency and poor long-term stability, and they require a long research process for commercial applications. It can be assumed that in the near future, as the scientific research continues to deepen, the problems facing thin-film solar cells will be solved one by one, and the performance will continue to be improved and improved, so as to meet the urgent needs of consumers in the future.