Recently, the efficiency of crystalline silicon-perovskite tandem cells independently developed by LONGi Green Energy has reached 33.9%, which is also the highest efficiency record of crystalline silicon-perovskite tandem cells in the world. Previously, the record was 33.7 percent, achieved by King Saud University of Science and Technology in May this year.

Photovoltaic cell technology is rapidly iterating, from this year's situation, P-type PERC (passivated emitter and back cell technology) cells still occupy a large market share, but with the increase in the penetration rate of N-type cell technology, the industry is generally expected to surpass the proportion of N-type next year.

Of course, both N-type and P-type are based on crystalline silicon technology, while perovskite cells are separated from silicon-based materials and are expected to achieve lower cost and higher efficiency, so they are also the focus of market attention. It should be noted that perovskites are available in both single-cell cells and tandem cells combined with crystalline silicon.

Xu Xixiang, chief scientist of LONGi Green Energy and vice president of Academia Sinica, said in an interview with a reporter from Securities Times E Company that the efficiency limit of crystalline silicon cells is 29.4%, and mass production is expected to reach 28% in the future. In Xu Xixiang's view, in the past period, crystalline silicon cells can improve efficiency at a rate of 0.4%~0.5% per year, and as they gradually approach the theoretical limit, the pace of efficiency improvement has begun to slow down, and may only be 0.3% or even 0.1% a year, "The reason why crystalline silicon-perovskite tandem cells are made is because the ultimate efficiency can be increased by 14 percentage points compared with crystalline silicon, from about 29% to 43%, which can further expand the cell efficiency improvement space." ”

In an interview with reporters, LONGi Green Energy also revealed a number of technical details. Xu Xixiang said that the tandem cell that set a new record this time is partly based on crystalline silicon heterojunction technology, and the top cell is perovskite, which mainly uses the characteristics of perovskite wide bandgap; He revealed that in addition to perovskites, the company is also developing other wide bandgap materials.

As for why perovskites and heterojunctions were chosen for tandem, Xu Xixiang said that on the one hand, heterojunction technology is a low-temperature process, which is conducive to the growth of perovskite films; On the other hand, although perovskites can also be stacked with technologies such as TOPCon solar cell (tunneling oxide passivation) and even PERC solar cell, their conversion efficiency is 2~3 percentage points lower than that of heterojunction stacks in the case of stacking at both ends. Xu Xixiang particularly emphasized that this is based on the situation of two-end stacking, and if it is a four-end stack or other cases, there is no such limitation.

He Bo, head of LONGi's R&D team, said that the company launched the heterojunction cell project in February 2021, and then began the research and development of tandem cells. In the past, the stack conversion efficiency of 23% was achieved in June 2021, reached 29.55% at the end of last year, and exceeded 31.8% and 33.5% in May and June this year, until the efficiency of 33.9% was achieved this time.

He Bo revealed that the above data is based on a total of about 83,000 cells out of 1,700 furnaces of solar cells produced in the past 900 days. At the same time, LONGi's crystalline silicon perovskite tandem cells will achieve a conversion efficiency of 35% in the second half of next year.

Whether it is a single-cell perovskite or a perovskite tandem cell, how to achieve large-area preparation is a core problem and an unavoidable problem for industrialization. In an interview with reporters, LONGi Green Energy did not clearly disclose the efficiency record of 33.9% on what area of the cell was achieved.

However, Xu Xixiang still mentioned that LONGi Green Energy is implementing a perovskite-related "K project" internally, and it is already in the quasi-pilot stage, "The K project team can already make 0.2 square meters of perovskite tandem modules." ”

He Bo pointed out that last year, LONGi Green Energy prepared four-terminal laminated modules on M6 size silicon wafers and achieved a module conversion efficiency of 26.5%, which is significantly superior to the 23%~24% conversion efficiency of existing crystalline silicon modules.

Regarding the situation of the "K project", He Bo said that LONGi Green Energy has calculated the mass production time and cost of perovskite modules. "There is no problem with the efficiency of perovskite modules above 27%. As for the cost, since the cost of perovskite itself is relatively low, the cost will not increase much after superimposing a layer of perovskite on the basis of crystalline silicon cells, so the cost performance is advantageous. In addition, perovskite tandem cells will be the first to be used in high-power scenarios in the future. ”

In terms of perovskite industrialization, there has been a lot of progress in the industry. However, in the view of LONGi Green Energy, there is still a long way to go in the mass production of perovskite cells, "If there is no empirical data to fully grasp the power generation law and reliability of perovskite, we will not blindly mass production, and it will take about 7 years to achieve industrialization." He Bo thinks.

In this regard, Xu Xixiang added, "In the next seven years, whether it is perovskite or other wide bandgap technologies, we will continue to innovate to solve problems such as long-term reliability." For example, the current efficiency attenuation of crystalline silicon modules is less than 1% in the first year and about 0.3% per year in the future life cycle, but the attenuation of perovskite modules is significantly faster, "which is a problem that needs to be solved."