Two independent research teams published research in the journal Science on July 6 that the photoelectric conversion efficiency of solar cells has exceeded the 30% mark - to be precise, perovskite/crystalline laminated solar cells.

For decades, monocrystalline silicon solar cells have been the main force in the photovoltaic industry. The current conversion efficiency can reach more than 24%, but it is subject to the theoretical efficiency limit of about 29.4%. In order to break through this limitation and further promote the use of solar energy, scientists have been exploring new material architectures to innovate devices, including connecting two or more solar cells in series.

Perovskite solar cells have received widespread attention since they first appeared in 2009. On the one hand, it is trying to improve the photoelectric conversion efficiency in order to squeeze into the existing silicon-based photovoltaic market, and on the other hand, it is trying to combine with silicon to form a new battery - perovskite/crystalline silicon laminated solar cells.

In this laminated battery, the perovskite battery will be deposited above the crystalline silicon battery. "The two can effectively capture different parts of the solar spectrum to make better use of solar energy." Qin Xinyu of the Federal Institute of Technology in Lausanne, Switzerland (the first author and corresponding author of one of the new studies), said. For example, compared with silicon, perovskite materials can use higher energy parts such as blue light or ultraviolet light. In this way, "by combining the output of two sub-battery, the laminated battery can achieve a higher energy output than a single-junction battery."

However, at present, perovskite/crystalline silicon laminated solar cells are still facing some key challenges, one of which is the recombination losses at the interface between the top surface of the perovskite sub-cell and the electronic transport layer. Compound loss refers to the recombination of photoeous carriers (electrons and holes) before they are collected and used, resulting in efficiency loss. Qin Xinyu explained.

In order to solve this problem, Qin Xinyu and others introduced an additive based on phosphoric acid to regulate the crystallization process of perovskite. The results show that this method can effectively passuate the interface between the perovskite layer and the electronic transmission layer, thus reducing the composite loss that will affect the overall performance of the battery. The resulting perovskite/crystalline silicon laminated solar cells have a certified photoelectric conversion efficiency of 31.25% - more than 30%. This once again confirms the great potential of laminated solar cell technology," Qin Xinyu said.

Kong Wenchi (who did not participate in these two new studies) of Tan Hairen's research group of Nanjing University also commented on this: "The efficiency of laminated batteries has exceeded the theoretical efficiency limit of single-junction solar cells, which is of great significance for promoting the development of the photovoltaic industry." In January this year, Tan Hairen's team and collaborators published a study in the journal Advanced Materials, developing a strategy of organic anionic additives to assist perovskite crystallization, and finally achieved a photoelectric conversion efficiency of about 28% perovskite/crystalline. Silicon laminated solar cells.

In addition, another new study was led by Steve Albrecht (corresponding author of another study) of the Berlin Center for Materials and Energy in Helmholz, Germany. Unlike the research of Qin Xinyu's team, Albrecht's team uses a dual-functioning molecule piperazinium iodide (PI), which is located between the perovskite layer and the electron transport layer. It mainly plays a bridge role at this interface and reduces composite losses. Kong Wenchi said. At the same time, it promotes the electron transport layer to extract electrons from the perovskite layer, thus improving the electronic transmission efficiency. The certified conversion efficiency of this laminated solar cell has reached 32.5%.

It is worth noting that the prototypes described in these two new studies are limited to the size of the laboratory, and the lifespan is not satisfactory - after dozens or hundreds of hours, the conversion efficiency will be reduced to the initial 80%. In contrast, the monocrystalline silicon solar cells on the market can still maintain more than 85% of the initial efficiency after 25 years of work.

Qin Xinyu said that he was "not surprised" by this breakthrough, because they knew that these materials had the potential to achieve more than 30% conversion efficiency, and the results now do prove that this solar technology is promising. However, in order to enter the photovoltaic market, it is necessary to further improve and improve the relevant technology, which is "expected to take 5 to 10 years," Qin Xinyu said.