Lithium carbonate formation is a big challenge

1. Lithium carbonate formation is a big challenge Significant progress in low-cost solar cell manufacturing has also locked in greenhouse gases Engineers have invented a method that can greatly improve the speed and efficiency of a key doping process of perovskite solar cells, which can also isolate carbon dioxide. Perovskite solar cells have developed rapidly in recent years, and their energy conversion efficiency has increased rapidly (from 3% in 2006 to 25.5% today), which is more competitive than silicon-based photovoltaic cells. However, there are still many challenges before they become competitive business technologies. Now, a team from the Tantong School of engineering of New York University has developed a method to solve one of these problems, which is a bottleneck in the key steps involving p-type doped organic hole transport materials in photovoltaic cells. The research, titled "carbon dioxide doping in organic interlayer of perovskite solar cells", was published in the journal Nature. At present, the p-doping process is realized by oxygen entering and diffusing into the hole transport layer, which is time intensive (from a few hours to a day), making the commercial mass production of perovskite solar cells impractical. Tandon's team, led by associate professor Andr é D. Taylor, postdoctoral jaemin Kong and assistant professor Miguel modesino, both from the Department of chemical and Biomolecular Engineering, found a way to greatly increase the speed of this critical step by using carbon dioxide instead of oxygen. In perovskite solar cells, it is usually necessary to dope organic semiconductors as the charge extraction intermediate layer between the photoactive perovskite layer and the electrode. The traditional method of doping these intermediate layers includes adding lithium salt lithium bis (trifluoromethane) sulfonimide (LiTFSI) to helix ometad, which is a π conjugated organic semiconductor widely used as a hole transport material for perovskite solar cells. Then, the spiro ometad: LiTFSI blend film was exposed to air and light to start the doping process.

2. This method is not only time-consuming, but also largely depends on environmental conditions. In contrast, Taylor and his team reported a rapid and repeatable doping method, including the solution of spiro ometad:litfsi with carbon dioxide bubbles under ultraviolet light. They found that compared with the original mixed membrane, their process rapidly increased the conductivity of the intermediate layer by 100 times, and also increased about 10 times compared with the oxygen bubbling process. Stable and efficient perovskite solar cells can also be produced by CO2 treated films without any post-treatment. "In addition to shortening the device manufacturing and processing time, the application of pre doped spiral ometad in perovskite solar cells makes the cells more stable," explained lead author Kong. "This is partly because in the spiro ometad: LiTFSI solution, most harmful lithium ions are stabilized as lithium carbonate during CO2 bubbling." He added that when the researchers spin the pre doped solution onto the perovskite layer, the lithium carbonate will eventually be filtered out. "Therefore, we can obtain fairly pure doped organic materials for effective hole transport layers." Researchers from Samsung, Yale University, Korea Institute of chemistry, Graduate Center of City University, won Kwang University and Gwangju Institute of science and technology also found that the CO2 doping method can also be used for p-type doping of other π - conjugated polymers, such as PTAA, MEH-PPV, P3HT and pbdb-t. According to Taylor, researchers are seeking to break through the boundaries of typical organic semiconductors used in solar cells. Taylor explained: "we believe that the wide applicability of CO2 doping to various π conjugated organic molecules has promoted the research of organic solar cells, organic light emitting diodes (OLEDs), organic field effect transistors (OFETs) and even thermoelectric devices, all of which semiconductors." He added that since this process consumes a considerable amount of carbon dioxide gas, it can also consider CO2 capture and isolation research in the future. require controlled doping of organic He explained: "at a time when the government and companies are seeking to reduce carbon dioxide emissions, if decarbonization is not carried out, this research provides a way to make a large amount of carbon dioxide react in lithium carbonate, so as to improve the next generation of solar cells and remove this greenhouse gas from the atmosphere." He added that the idea of this new method is a counterintuitive insight in the team's battery research.

3. "From our long history of studying lithium oxygen / air batteries, we know that it is a great challenge to form lithium carbonate when the oxygen electrode is exposed to the air, because it will deplete the lithium ions of the battery, thus destroying the battery capacity. However, in this spiro doping reaction, we are actually taking advantage of the formation of lithium carbonate, which combines lithium and prevents it from becoming a migration harmful to the long-term stability of perovskite solar cells Kinetic ions. We hope that this carbon dioxide doping technology can be a stepping stone to overcome the challenges of existing organic electronics and other fields. " The National Science Foundation of the United States, the Korea National Research Foundation, the China Scholarship Management Committee, and the functional nanomaterials center of Brookhaven National Laboratory of the United States Department of energy provided support for this research.