High-concentration photovoltaic (HCPV) cells, as the third-generation solar power generation technology, are gradually becoming a new focus in the solar energy field.

After more than 30 years of development, high-concentration photovoltaic (HCPV) cells, as the third-generation solar power generation technology, are gradually becoming a new focus in the solar energy field, attracting the pursuit of companies in the industry. In several European and American countries with good sunlight, preferential grid-connected electricity price laws have been passed. With the popularization and cost reduction of III-V semiconductor multi-junction solar cells with a conversion efficiency of 40%, the high-concentration photovoltaic cell market has entered a period of rapid growth. Compared with the previous two generations of cells, HCPV uses multi-junction gallium arsenide cells, which have the advantages of wide spectrum absorption, high conversion efficiency, good temperature characteristics, and low energy consumption manufacturing process, so that it can still maintain a high photoelectric conversion efficiency under high-frequency focusing and high-temperature environment. The technical threshold of high-concentration photovoltaic systems is high and the industry spans a large range, covering semiconductor materials and process manufacturing, semiconductor packaging, optical design and manufacturing, automation control, mechanical design and manufacturing, metal processing and other fields. The products of the HCPV industry include multi-junction cell epitaxial materials, photoelectric conversion chips, optical receiver components, concentrators, photovoltaic modules, dual-axis trackers, etc.

The battery chip uses multi-junction technology to greatly improve the photoelectric conversion efficiency

Compared with silicon-based materials, III-V semiconductor multi-junction solar cells have the highest photoelectric conversion efficiency, which is about 50% higher than silicon solar cells. III-V semiconductors have much higher high temperature resistance than silicon and still have high photoelectric conversion efficiency under high illumination, so high-power concentration technology can be used, which means that only a few solar cell chips are needed to generate the same amount of electricity. A unique aspect of multi-junction technology is the material - different materials can be selected and combined to make their absorption spectrum close to the sunlight spectrum, which is a huge advantage over crystalline silicon. The conversion efficiency of the latter is close to the limit (25%), while the theoretical conversion efficiency of multi-junction devices can reach 68%. Currently, the most commonly used is three pn junctions formed by three different semiconductor materials: germanium, gallium arsenide, and gallium indium phosphide. In this multi-junction solar cell, not only are the lattice constants of these three materials basically matched, but each semiconductor material has a different bandgap width, which absorbs different bands of the sunlight spectrum, so that the full spectrum of sunlight can be absorbed.

The production process of HCPV chips is as follows: first, MOCVD technology is used to epitaxially grow gallium arsenide and indium gallium phosphide on a 4-inch germanium substrate to form the material of a triple-junction cell. Then, photolithography, PECVD, evaporation and other technologies are used on the epitaxial wafer to prepare an anti-reflection film and a metal electrode whose main component is silver. After scribing and cleaning, HCPV chips are produced. The main manufacturers of HCPV chips include Spectrolab and Emcore in the United States, Azurspace in Germany, Cyrium in Canada, Arima and Epistar in Taiwan, China, etc. Substrate-stripped chips and quantum dot technology are the new hot spots in the field of HCPV chips.

The receiver should be used in the system safely, reliably and stably

The concentrated solar cell chip is packaged into the optical receiver. The receiver package protects the solar cell, homogenizes the concentrated light, and also plays a role in heat dissipation. The receiver component also includes a bypass diode and a lead terminal. The main welding processes for the chip are reflow soldering and eutectic soldering. The main difference between the two is that the former uses flux soldering, and the residual flux needs to be cleaned and removed after soldering, while the eutectic soldering uses flux-free solder pads. In order to conduct electricity from the chip, gold ribbon bonding is required to connect the chip and the peripheral circuit. The inspection indicators of the receiver component mainly include void rate and electrical performance test. The void rate is the standard for testing whether the welding is good or not. In terms of electrical performance, the photoelectric conversion rate of the 5.5mm×5.5mm receiver component under 500 times the sunlight is as high as 38.5%. In actual use, the receiver component needs to be packaged together with the secondary optical device and the heat sink to form a complete receiver. The secondary optical device can reduce the high-precision requirements of the tracker and make the light spot focused by the Nel lens more uniformly irradiated on the battery chip.

Secondary optical elements are usually optical glass prisms or hollow inverted pyramid metal reflectors. In order to maximize the use of solar energy resources and save chip materials to reduce costs, the concentration multiple of the battery can be increased, which puts higher requirements on the heat dissipation system. At present, Sanan Optoelectronics in China has achieved 1000 times of concentration, and the corresponding DBC uses AlN materials with higher thermal conductivity than Al2O3. It is worth noting that the system that combines photovoltaics and photothermal heat can convert more energy generated after concentration into electricity or hot water again, greatly improving the utilization rate of energy. In order to improve the reliability of concentrating solar cells, the International Electrotechnical Commission (IEC) has formulated an international standard as the evaluation standard for concentrating solar receivers and components-IEC62108. Through a series of inspection standards such as thermal cycles and insulation, it stipulates the minimum design standards and quality requirements for concentrating solar receivers to ensure their safe, reliable and stable application in photovoltaic systems in open air environments.

Solar Tracker Accuracy and Effectiveness Improve HCPV Performance

Solar trackers are power devices used to keep solar panels facing the sun at all times, so that the sunlight always shines vertically on the solar panels. Trackers are mainly divided into single-axis trackers and dual-axis trackers. The single-axis type is suitable for Si solar cells or trough-type concentrator systems with relatively low tracking accuracy requirements, such as systems used on rooftops, and its development direction tends to be small and light. The dual-axis type is suitable for concentrated solar cell power generation systems with high tracking accuracy requirements. Its tracking accuracy can reach 0.1°, and it is mainly used in large power stations. The product is developing towards large and medium-sized and high stability.

Solar trackers are classified into sensor tracking, solar motion trajectory tracking and hybrid tracking according to the tracking method. The sensor tracking method uses a photoelectric sensor to detect the deviation between sunlight and the normal of the solar panel to achieve feedback tracking. Its tracking accuracy is 0.1°, but it is greatly affected by the weather. The solar motion trajectory tracking method adjusts the tracking device according to the actual trajectory of the sun according to a predetermined program. This tracking method can track in real time around the clock, and its tracking accuracy is about 0.5°. The hybrid type combines the advantages of both. When the weather conditions are good, sensor tracking is used to ensure tracking accuracy; when the weather conditions are bad, the tracking method is changed from sensor tracking to visual solar motion trajectory tracking. According to statistics, 90% of HCPV system failures are caused by tracker failures.

The threshold for enterprises to enter the concentrated solar cell industry should be lowered

From a technical perspective, there are five main ways to reduce the cost of photovoltaic cells: first, further improve the photoelectric conversion efficiency of battery chips through full-spectrum absorption, second, reduce chip manufacturing costs through substrate reuse, third, increase the system's concentration ratio and improve the optical efficiency of the entire component, fourth, make comprehensive use of light energy and thermal energy, and fifth, adopt a focusing system that greatly reduces costs.

From the market perspective, as concentrated photovoltaic technology matures and production scale expands, its comprehensive cost is expected to be lower than that of crystalline silicon and thin-film batteries in the next few years. If the cost per kilowatt-hour is to be reduced to less than $0.1, the cost of the installed system must be reduced from the current $5-8/watt to $2/watt, and the cost of the chip must be reduced from $8-10/square centimeter to $3-5/square centimeter. To achieve this goal, it is impossible without a certain industrial scale, which requires more companies to participate. At present, major development companies in the world have vertically integrated the entire industrial chain of HCPV power generation systems starting from chips, such as Solfocus, Amonix, Emcore in the United States, and Concentrix in Germany (now acquired by Soitec). These companies are at the forefront of HCPV technology research and development and have their own independent design systems, making it difficult for other companies to participate. This has raised the technical threshold to a certain extent and hindered the development of the industry. Invotech has been committed to flattening the technical threshold of high-power concentrated solar power generation technology from the perspective of making the industry bigger and stronger, so as to help more companies enter this industry smoothly. The company's main product now is the receiver assembly, which is the core component in the photovoltaic module.