Chen Wei believes that under the guidance of the "dual carbon" policy, the photovoltaic market has room for long-term and substantial growth in the next few decades. Although there will be some involution in the short term due to the impact of supply relations, in the long term it will continue to improve.
Chen Wei said that although the development time of perovskite cells is very short compared with crystalline silicon cells, the efficiency and certain key performance parameters of the former are very competitive, and the process flow of perovskite cells is very short, and the difficulty of industrialization will be Significantly lower than crystalline silicon cells. "Perovskite cells are a disruptive technology, and there is no need to be stuck in the rules of crystalline silicon cells. I believe that in the next 2-3 years, the efficiency record of perovskites in the laboratory will definitely exceed that of crystalline silicon."
Chen Wei mentioned that the life span of perovskite batteries is still being discussed and explored. In the short term, there are still two bottlenecks in perovskite industrialization. One is the inconsistency between small area and large area high efficiency; the other is the insufficient stability of perovskite materials and devices.
Chen Wei believes that compared with the efficiency competition that many perovskite photovoltaic companies currently focus on, "a fair, open and just stability competition is more critical. The resolution of stability is related to the future success or failure of perovskite and the scale of its final application." "
The following is the full text of Chen Wei's speech, edited and published by China Energy Network:
First of all, let’s talk about the general background. Under the guidance of the “dual carbon” policy, the future development of solar energy is unlimited. Even if there is a supply imbalance in the short term, in the long term, it will continue to improve. Because to achieve carbon neutrality, there is no way without widespread use of solar energy to replace fossil energy on a large scale. It is also against this background that perovskites have become popular among the investment community in recent years.
So what are perovskite solar cells? First of all, starting with perovskite itself, the earliest perovskite material is indeed calcium titanate. It was first discovered by a Russian mineralogist in the Ural Mountains in 1839. It has a body-centered cubic crystal as shown in the picture. structure. From now on, people will refer to materials with this crystal structure as perovskite structure materials. Well, oxide perovskites have been widely used in many fields, such as lead zirconate titanate in piezoelectric ceramics, and the previous star superconducting material yttrium barium copper oxide are all perovskite oxide materials.
In photovoltaic perovskites, O ions are replaced by halogen ions, Ti ions are replaced by Pb Sn, and the original Ca ion position is occupied by organic formamidine, methylamine and larger Cs ions at the center of the body. It is a large class of halide materials with a perovskite crystal structure. According to the following formula, its ionic radius must meet a certain coefficient range before this crystal structure can exist stably, otherwise it will become a substance without photoactivity. Because halide perovskites have some ionic crystal characteristics, they can be dissolved in polar solvents and therefore can be solution processed. It can also be combined with a variety of elements to continuously modulate the optical band gap, giving it very rich optoelectronic properties.
It can be seen from the structural evolution that the earliest perovskite battery was a variant extension of dye- Henry Snaith of the University of Oxford was the first to abandon mesoporous TiO₂. This discovery was based on the bipolar conductive properties of the perovskite film itself, which can both pass electrons and pass holes. Put the electron transport layer underneath, and you have the prototype of a modern formal structure perovskite cell. The trans structure reverses the order of the hole transport layer and the electron transport layer, and the electron flow direction is exactly opposite to that of the formal structure battery.
In addition to the above-mentioned single-junction perovskite cells, perovskites can also form stacked cells with crystalline silicon and narrow-bandgap perovskites. They use two materials with different band gaps to absorb light of different wavelengths to maximize the use of solar energy, thereby breaking the theoretical upper limit and increasing it from 33% to 42%. The current calcium-silicon stack efficiency record and perovskite-perovskite stack efficiency record were both achieved by Chinese R&D teams.
Compared with crystalline silicon, perovskite was born much later, but it has many advantages and features in many technical performance indicators. For example, the ultimate efficiency of perovskite laboratories is very close to that of crystalline silicon, surpassing CdTe (cadmium telluride) and CIGS (copper indium gallium selenide) thin film batteries that have been commercialized for many years. At the same time, because the amount of perovskite materials is much lower, the purity and energy consumption are much lower, so the ultimate cost of perovskite will be significantly lower than that of crystalline silicon. The only weakness of perovskites is that they don't last long enough.
From this picture, we can see that perovskites are the fastest to achieve efficiency breakthroughs among the new generation of photovoltaic technologies, and through stacking technology, they can also surpass crystalline silicon cells by a large margin.
The perovskite production line process is also very short, and the engineering technology involved is much simpler than that of crystalline silicon. Unlike crystalline silicon, which has certain technical thresholds for silicon materials, silicon wafers, cells, and components, perovskites start from chemical raw materials and can obtain efficient components through simple manufacturing of a few film layers.
It was said before that the cost of perovskite materials is low, and you may not be able to understand it clearly, but by comparing this energy recovery cycle, you can see it clearly. The energy required to manufacture this solar cell will take about 1-2 years to recover by generating electricity from crystalline silicon, while it only takes 3 months for perovskite. Since the supply relationship is not connected upstream and downstream, perovskite is still relatively expensive for the time being, but the Energy Payback time (energy payback period) will not lie. Perovskite will definitely be an extremely cheap technology in the future.
To summarize, this solar cell performance triangle. At present, the mass production efficiency of perovskite is still lower than that of crystalline silicon, and the cost is also more expensive. However, through continued investment in R&D, the mass production efficiency and cost will inevitably drop to where they should be, and the mass production efficiency will catch up with crystalline silicon. , the cost is significantly lower than crystalline silicon, these are bound to be realized. The only technical risk is how many years will perovskites last? The ideal expectation of some scientists is that with some years of hard work, perovskites are expected to have a service life of 10-15 years.
So could an ultra-cheap, short-lived perovskite be commercially competitive? My personal judgment is OK. This is a profit calculation table for ground power stations. If we bring in perovskite components with high enough efficiency and low enough cost, and also calculate the cost of replacing components, we can use two perovskite components to produce a 25-year service life . Over their lifetimes, perovskites may also gain economic advantages.
The main obstacle to large-scale commercialization of perovskite is that the mass production efficiency is not high enough . Perovskite cells can achieve more than 26% in the laboratory with a size of 0.1c㎡, and the components coming off the production line are currently the highest. 18%-19%. Although it is still slightly inferior to crystalline silicon, compared with the CIGS developed by Hanergy and the CdTe currently produced by China National Building Materials Corporation, this progress is already very fast. It is also just a matter of engineering amplification. The difficulty lies in the solution coating to make large-area perovskite films. The wet coating of photovoltaic semiconductor films has never happened in the history of semiconductors, so it will take some time to improve and optimize. .
Another problem is the short lifespan mentioned before, because perovskite is an organic-inorganic hybrid material crystallized at low temperature. Under the action of light and heat, perovskite will slowly decompose . This is completely different from silicon smelted at high temperatures.
This is the current production capacity planning situation of enterprises. Several fast-moving companies have built 100MW production lines and are currently building some MW-level demonstration power plants, and several companies are planning to build GW-level production lines.
This is the product display of these so-called leading companies. In addition to these, traditional photovoltaic giants such as Longi have also begun to engage in perovskite research and development. New energy companies CATL and BYD are also developing perovskites. Recently, panel industry giant BOE has also entered perovskite, and many of their industrial equipment processes are very similar to perovskite. China National Nuclear Power and Huaneng, two of the five largest and four largest energy companies in the energy industry, also took action personally.
Perovskite solar cells actually have several markets. Among them, for indoor solar energy, perovskite has no technical bottlenecks and has no rivals. Consumer power supplies for outdoor use do not have high requirements on lifespan, and perovskites are easily suitable. The larger market lies in building-integrated photovoltaic and ground power stations, which require further maturity of technical elements before they can be applied on a large scale.
Therefore, the current technical route for perovskites is that trans single junction is easier to achieve high efficiency and stability, and is easier to mass produce.
Therefore, half of the perovskite start-ups with the largest planned production capacity are doing trans-single junctions, and there are two companies among them, with my students as founding shareholders and CTO (chief technology officer).
Our team has long been targeting application-oriented bottlenecks and has chosen the trans-perovskite technology route to continue to tackle key problems. Some substantial progress has been made in obtaining higher basic efficiency, higher stability and large-scale process scale-up of trans-perovskites. So far, 12 efficiency certification reports and 1 stability certification report have been obtained. At present, the efficiency of trans batteries exceeds 26.8%, and the efficiency of small components reaches 22.7%. These two items are being submitted to the international efficiency record list.
I published the first Science ("Science", one of the world's authoritative academic journals) and the first efficiency record of trans-perovskites in the world. The oxide interface materials here are still indispensable for the industrialization of trans-perovskite batteries.
Based on the above technical prototype, we have recently found more efficient interface materials and published a Science paper with our collaborators again this year.
In addition, perovskite material systems have also been developed. This FACs perovskite is the most efficient and stable material perovskite system currently widely used in the industry. We continue to make efficiency breakthroughs in this regard. The recent authoritative certification efficiency reached 26.8%, far exceeding the current international efficiency record of 26.1%.
Stability is the biggest bottleneck. We have developed an electrode material that is resistant to perovskite corrosion. It is very special in the periodic table of elements and is unmatched by other metal electrode materials.
Based on inert metal electrodes and strong barrier packaging, we can limit the photothermal decomposition of perovskite to the greatest extent, so that the photothermal decomposition reaction will not occur, greatly improving the photothermal stability of the battery. Looking at this picture, we can see that perovskite originally vaporized and decomposed significantly at 160°C, but after our modification, the decomposition temperature increased to more than 250°C.
In terms of process amplification, we have developed a formula that can be coated and prepared in an air environment, achieving internationally leading large-area module certification efficiency.
As a technical reserve, we have also developed perovskite-perovskite stacked cells. The current certified efficiency is second only to the Nanjing University team and is among the top three in the world.
The next step is to solve the stability problem of perovskite. First, the perovskite must withstand the test of light and heat for a long enough time. The establishment of test standards must be accelerated. Long-term outdoor testing must be carried out to ensure that the perovskite can withstand the test of light and heat for a long time. Stable use outdoors for 10-15 years. These standards are not available in crystalline silicon photovoltaics. For crystalline silicon crystallized above 1000 degrees, there is no difference between 50°C illumination and 75°C illumination, but for perovskite crystallized at 100°C, the impact is very large.
After we completed sufficient technology accumulation, we established a company in Wuhan to carry out industrial transformation of related technologies. Our technology is a complete closed loop, including efficient and stable interface materials, perovskite film formation technology in the air, and unique Bi Electrode material system and highly reliable packaging technology.
Our efficiency indicators and stability indicators can benchmark against the best international standards, and they have all been certified by third-party authorities. Moreover, we have also tried outdoor testing and endured the hottest summer in Wuhan without any decline in efficiency.
Our company will adopt an active but steady expansion strategy. We will first launch products that meet the needs of use into the market in stages, wait for the market response, and then expand production capacity according to demand, and finally enter the main battlefield of ground power stations. I personally judge that it will take 5-10 years for the ground power station scene to mature . It requires upstream and downstream cooperation to open up all links and strive for a small success in 5 years and a big success in 10 years. It is more realistic to consider this time dimension.
In addition, we also provide some stability testing equipment products to the outside world and provide stability certification services to the outside world. Compared with the efficiency competition, a fair, open and just stability competition is more critical. The solution of stability is related to the future success or failure of perovskite and the scale of its final application. We have the ability and willingness to make our own contribution here.