Overview of Solar Cells and Challenges and Perovskite Solar Cells

Introduction.

According to “China’s Solar Cell Market, Expectations for Perovskite Solar Cells Ahead of Severe Oversupply.

On May 29, Longi Green Energy (Shanghai 601012), the industry’s largest solar cell manufacturer, announced a 30.8% price cut for its mainstay P-type M10 150 micron monocrystalline wafers, and a 30% reduction for M6 wafers. The company had just lowered the price of its mainstay P-type M10 150 micron monocrystalline wafers by a modest 3% on April 27. In response, competitor TCL Zhonghuan (002129) also lowered its prices by 16% to 24% on June 1 for several products of different sizes.” The report states.

In the solar cell industry as a whole, competition is intensifying as more and more companies from other industries enter the market. For example, according to the Securities Daily (June 5), the annual production capacity of silicon, the raw material for solar panels, is expected to reach 2.1 million tons in 2023, but actual demand is only about 1.1 million tons.

Li Zhenguo, founder and chairman of LONGI Green Energy, attended the SNEC2023 international photovoltaic power generation exhibition held in Shanghai from May 23 to 26, and in his keynote speech, he discussed the future outlook for the industry. The shift to clean energy is already the global consensus, and the industry as a whole will continue to grow. However, the development will not be linear, but will proceed in stages. We are currently in the stagnant stage, and more than half of the companies will be eliminated in the next two to three years.

This stagnation phase (oversupply) is caused by the fact that development proceeds in stages. Technological progress does not proceed in a linear fashion. The Securities Daily reported the opinion of Dr. Wenchu Shen, director of the Institute of Solar Energy Research at Shanghai Jiao Tong University, that “PERC (Passivated Emitter and Rear Cell) technology is currently the mainstream for solar cells, but a shift to TOPCon (Tunnel Oxide Passivated Contact) will occur in the next three to five years. However, a shift to TOPCon (Tunnel Oxide Passivated Contact) is expected to occur in the next three to five years. 　Beyond that, perovskite solar cells, which are thin, light, bendable, and can be installed on places that have been difficult in the past, such as car roofs, are likely to replace them. These incremental technological advances will replace conventional equipment. Companies that cannot keep up with technological advances will be weeded out, while ventures that specialize in the development of new technologies will have a chance to grow, and the overall market will grow in stages.

The market as a whole will grow in stages,” the report states. The Sunshine Project, which began in Japan in 1973, has brought several decades of technological accumulation to solar cell technology, which has now reached a plateau and is about to take the next step forward. In this article, I would like to give an overview of solar cells and their future.

About Solar Cells

Solar cells, the leading clean energy technology, are devices that directly convert solar energy into electrical energy by using semiconductor materials to receive light energy and convert it into the movement of electrons.

As described in “Computational Elements of Computers and Semiconductor Chips” semiconductors are materials that can control the electrons (and holes) in the film, and when exposed to light of a specific wavelength, the light is absorbed and the electrons (or holes) are excited by the energy (photovoltaic effect ( Solar cells use this principle to generate DC electricity by connecting two types of materials that produce negative charge (electrons) and positive charge (holes) when exposed to light, as shown in the figure below.

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There are various combinations of semiconductors with photovoltaic effect, but Si solar cells have the advantage that the chip fabrication technology described in “Overview of Semiconductor Manufacturing Technology and Application of AI Technology” can be applied without modification and that mass production at low cost is easy because microfabrication technology is not required. In addition, the technology does not require microfabrication technology, making it easy to mass-produce at low cost. There are various types of Si solar cell structures as shown below.

• Crystalline silicon solar cells: Crystalline silicon solar cells are made from wafers (thin disk-shaped silicon substrates) of mono or polycrystalline silicon. Silicon is a semiconductor material; when a photon strikes silicon, electrons are activated to produce electrical energy. Crystalline silicon solar cells are the current mainstream in solar panels (97% share in 2022 statistics).
• Polycrystalline solar cells: Polycrystalline silicon solar cells are made primarily from crystalline silicon. Polycrystalline silicon is expected to be a low-cost, high-efficiency material (current market share is less than 1%).
• Thin-film solar cells: Thin-film solar cells are made of thin films of silicon or other semiconductor materials. Thin-film solar cells use less material and are more flexible than crystalline silicon solar cells, but tend to be somewhat less efficient (market share around 3%).

The following issues exist in such Si-based solar cells.

Solar Cell Challenges

• Efficiency challenges: Current solar cell technology is limited in its efficiency in converting sunlight into electricity, and some solar cells can only capture a portion of the sunlight and are unable to maximize the light energy.
• Scarcity of materials: Current solar cells are challenged by the scarcity and high cost of some materials, especially silicon used in crystalline silicon solar cells, which is in limited supply and is at risk of price increases and supply shortages.
• Durability and stability challenges: Solar cells need to perform consistently over long periods of time, but can be degraded by external environmental factors.
• Manufacturing cost and scalability challenges: Reducing the manufacturing cost of solar cells and accommodating large-scale manufacturing processes are also challenges.

To address these challenges, what are called next-generation solar cells are being developed. They are described below.

Next Generation Solar Cells

Next-generation solar cells are being developed with the aim of providing more efficient and sustainable energy conversion technology than conventional solar cells, and research is being conducted on materials such as non-Si semiconductors, organic semiconductors, and their hybrid structures. Some of them are described below.

• Compound semiconductor solar cells: Compound semiconductor solar cells are based on III-V compound semiconductor materials such as gallium arsenide (GaAs), indium arsenide (InAs), and gallium indium arsenide (GaInAs), which have been studied for many years. These materials have high optical absorption coefficients and excellent carrier transfer characteristics, and are expected to achieve high photovoltaic conversion efficiency, but their production cost is higher than that of Si.
• Organic Solar Cells: Organic solar cells use carbon-based organic materials to capture sunlight and are composed of organic semiconductors, charge transport materials, and substrate materials. Organic materials are flexible and lightweight, and are characterized by their ability to create large-area solar panels at low cost; however, the efficiency of current organic solar cells is lower than that of Si.
• Multi-Junction Solar Cells: Multi-junction solar cells will have a structure that combines several different material layers and has a structure that efficiently converts a wide range of the solar spectrum. These solar cells are used in applications requiring high energy density, especially in concentrator solar cell systems and in the aerospace industry. However, due to the challenges of manufacturing costs and complex manufacturing processes, further research and technological advances are needed for more practical deployment.
• Quantum dot solar cells: These cells use quantum dots, which are nanoscale semiconductor crystals, and are characterized by high conversion efficiency, multiple effects, thermal stability, flexibility, etc. However, various issues remain, such as stability of quantum dots and manufacturing issues, and the technology is far from practical.
• Perovskite Solar Cells: Perovskite solar cells use perovskite materials with a special crystal structure to convert light. These solar cells can be manufactured at low cost and are considered highly efficient. However, perovskite materials are sensitive to humidity and heat, and improving their durability is a challenge.

Among these, organic and perovskite solar cells, which are attracting the most attention as next-generation solar cells, will be discussed further.

Organic solar cells

Organic solar cells use carbon-based organic materials to convert sunlight into electricity. The materials used include organic semiconductors (perovskite, various polymers, etc.) that play the roles of electron conduction and light absorption, charge transport materials (fullerene, perovskite, etc.) necessary to effectively collect electrons and holes generated from light and generate electric current, and substrate materials (polyester, etc.) necessary to take advantage of the flexibility and lightweight characteristics. perovskite, etc.), and substrate materials (polyester (PET), polyimide (PI), etc.) necessary to take advantage of their flexibility and lightweight characteristics. Below we discuss the advantages and challenges of these organic solar cells.

Advantages:

• Low cost: Organic solar cells can be manufactured at a lower cost than conventional silicon solar cells. Organic materials are relatively inexpensive and the manufacturing process is simple, such as printing or coating, which can reduce production costs.
• Flexibility: Because organic material solar cells are made of flexible materials, they can be bent and flexed. This allows them to be incorporated into flexible electronic devices and flexible surfaces. Their lightweight properties also make them suitable for a wide variety of applications.
• Performance under low light conditions: Organic material solar cells exhibit relatively high conversion efficiencies for weak or indirect light. This is due to the ability of organic materials to absorb light of a wide range of wavelengths, allowing them to effectively generate power even in low light conditions, such as indoors or on cloudy days.
• Environmental friendliness: Organic materials are generally environmentally friendly and are often made from renewable resources. The manufacturing process may also have lower energy consumption and environmental impact than conventional solar cells. This makes them an attractive option for sustainable energy generation.
• Product Flexibility: The flexibility and light weight of organic material solar cells allow them to be used in areas that are difficult for conventional solar cells. This would be, for example, in cell phones, wearable devices, roofing materials, and flexible displays.

Challenges:

• Improving efficiency: The conversion efficiency of organic material solar cells is still low and not comparable to conventional silicon solar cells. Currently, the efficiency of organic material solar cells is around 10% to 15%, and higher efficiencies are required for commercialization.
• Stability and durability: Organic materials are relatively vulnerable to environmental conditions and tend to be easily degraded by factors such as light, oxygen, and humidity. Breakthroughs are needed to improve the long-term stability and durability of these solar cells.
• Scalability: Organic material solar cells are currently manufactured on a relatively small scale and are not yet suitable for large-scale commercial production. The scalability of the manufacturing process needs to be improved to enable mass production.
• Material hazards: Some organic materials are hazardous to the environment and can cause problems in waste disposal. In response, it is important to develop and use environmentally friendly materials.
• Leveling difficulties: Organic material solar cells are flexible because they can be applied directly to the surface of the solar panel. However, technical challenges such as difficulties in uniform thin film formation and patterning have made it necessary to devise ways to produce solar cells with uniform efficiency.

Thus, although organic solar cells have features such as low cost and product flexibility, their low efficiency has been a hurdle to their widespread use. In contrast, perovskite solar cells have attracted particular attention in recent years as a technology that solves the problem of low efficiency.

Overview of Perovskite Solar Cells

Perovskite solar cells are a type of thin-film solar cell that uses light-absorbing layers with a perovskite structure. Perovskite structure refers to a crystal structure whose chemical composition is expressed as ABX3.

The above is a schematic diagram of an ABX3-type perovskite structure. The structure consists of A ions (CH3NH3, CH”(NH2)2, Cs, etc.) in the corners of the cube, B ions (Pb, Sn) in the center, and X ions (I, Br, Cl) in the position of the quiescence.

In general semiconductors, electrons and holes that can move freely are formed when irradiated by light as described above, but in perovskite semiconductors the time until they disappear is considerably longer than in conventional semiconductors, which is the main reason for their high performance. However, the speed of electrons and success moving through the semiconductor is slower than in conventional semiconductors, and electrical conductivity is not as good, which reduces efficiency.

This challenge is believed to be caused by the soft lattice of perovskite semiconductors, which allows electrons to distort the surrounding lattice system. This is a state of electrons called polarons, which are thought to be caused by the distortion of the lattice, so that if a heavy ball is placed on top of a sponge, the ball distorts the sponge and sinks, effectively increasing the weight of the electrons.

To solve these problems, perovskite solar cells use perovskite materials with high efficiency of photovoltaic effect in the light-absorbing layer, and electrons and holes extracted therefrom are sandwiched between the electron transport layer and hole transport layer, thereby improving electrical conductivity and increasing overall efficiency.

Perovskite solar cells gained attention in the early 2010s and are developing rapidly. Their unique feature is that they can be manufactured at low cost and achieve very high photovoltaic conversion efficiencies; in the laboratory, perovskite solar cells have shown the potential to be more efficient than existing solar cell technologies.

Currently, there are many efforts underway to improve the stability and commercialization of perovskite solar cells. Regarding these, an earlier article stated, “In November 2022, Nanjing University of Technology in China published research results on “perovskite solar cells based on screen-printing technology” in the British scientific journal Nature. 　In addition, Taisho (Jiangsu) Micronergy Technology, a venture company for the production of perovskite solar cells, was created in 2018. Among listed companies, Longi Green Energy is actively continuing its development, announcing at the aforementioned SNEC2023 exhibition that its prototype perovskite solar cell achieved a conversion efficiency of 31.8% (as tested by Fraunhofer ISE, Germany), the third highest in the world.” The report states.

Despite these promising prospects for perovskite solar cells, there are some challenges to overcome, as shown below.

Solar Cells and Artificial Intelligence

Finally, approaches to improving solar technology through artificial intelligence techniques are described. They are mainly used for energy efficiency and system optimization.

• Predicting and Optimizing Performance: Artificial intelligence techniques are used to predict and optimize solar cell performance; AI models can analyze multiple input parameters such as weather data, solar radiation, temperature, and solar cell characteristics to predict solar cell output. This can improve the efficiency of photovoltaic systems and determine the optimal timing and amount of power generation.
• Maintenance and Troubleshooting: Artificial intelligence technology can also be used for maintenance and troubleshooting of solar systems: AI algorithms can analyze solar cell output data and sensor data in real time to detect signs of abnormal performance or failure, This enables early detection of problems and more efficient maintenance and repair work.
• System optimization and control: Artificial intelligence technology will also be used to optimize and control the entire solar system: AI models will continuously analyze solar cell output data, power demand, power prices, energy storage systems, and other information to suggest optimal power supply strategies. This enables efficient use of electricity and coordination with power networks.
• Cell Design and Material Exploration: Artificial intelligence technology is also applied to solar cell cell design and material exploration; AI models can learn information from material databases and physical property data to predict the properties of new materials, thereby facilitating the development of more efficient solar cells and the exploration of new materials.