New procedure enhances production of perovskite solar cells

A team of scientist have developed a method that boosts the manufacturing efficiency of perovskite solar cells, making them more commercially viable.

The NYU Tandon School of Engineering team has created a novel technique that significantly accelerates the p-doping process of producing perovskite solar cells – a major bottleneck in the building procedure – making the cells economical and efficient.

The research is published in the journal Nature.

Throughout the last 15 years, the technology of perovskite solar cells has advanced dramatically, with their power conversion efficiency increasing from 3% to 25.5%, making their performance comparable with photovoltaic cells.

However, one obstacle that persists is the arduous p-doping process – attained through the ingress and diffusion of oxygen into the hole transporting layer – which can take many hours or even days to complete, meaning producing perovskite solar cells on a mass scale is uneconomical. To combat this issue, the team has created a new method that accelerates this process by utilising carbon dioxide instead of oxygen.

Traditional perovskite solar cells require doped organic semiconductors for the charge-extraction interlayers that are interspersed between the electrodes and the photoactive perovskite layer. Traditionally, these interlayers are doped through the implementation of lithium bis(trifluoromethane)sulfonamide (LiTFSI), a lithium salt, to spiro-OMeTAD, a π-conjugated organic semiconductor widely used for a hole-transporting material in perovskite solar cells. Finally, the doping process is commenced by exposing spiro-OMeTAD: LiTFSI blend films to light and air.

This process is exceptionally impractical not only from a time perspective but also because ambient conditions are crucial to successfully conducting it; contrastingly, the new method involves bubbling a spiro-OMeTAD: LiTFSI solution with CO2 under ultraviolet light and has proved to considerably more efficient. The electrical conductivity of the interlayers exhibited a 100 times increase compared to a pristine blend film, approximately ten times higher than that attained from an oxygen bubbling process, achieving high efficiency and stability in perovskite solar cells with no need for post-treatment.

Jaemin Kong, a post-doctoral associate and lead author of the study, said: “Besides shortening the device fabrication and processing time, application of the pre-doped spiro-OMeTAD in perovskite solar cells makes the cells much more stable. That’s partly because most of the detrimental lithium ions in the spiro-OMeTAD: LiTFSI solution were stabilised as lithium carbonates during the CO2 bubbling process. Thus, we can obtain fairly pure doped organic materials for efficient hole transporting layers.”

Additionally, the team determined that their CO2 doping method can be utilised for p-type doping of different π-conjugated polymers, such as PTAA, MEH-PPV, P3HT, and PBDB-T.

André D. Taylor, an associate professor from the Department of Chemical and Biomolecular Engineering, said: “We believe that wide applicability of CO2 doping to various π-conjugated organic molecules stimulates research ranging from organic solar cells to organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs) even to thermoelectric devices that all require controlled doping of organic semiconductors.

“At a time when governments and companies alike are now looking to reduce CO2 emissions if not de-carbonise, this research offers an avenue for reacting large amounts of CO2 in lithium carbonate to improve next-generation solar cells, while removing this greenhouse gas from the atmosphere,” he explained, adding that the idea for this novel approach was a counterintuitive insight from the team’s battery research.

“From our long history of working with lithium-oxygen/air batteries, we know that lithium carbonate formation from exposure of oxygen electrodes to air is a big challenge because it depletes the battery of lithium ions, which destroys battery capacity. In this Spiro doping reaction, however, we are actually exploiting lithium carbonate formation, which binds lithium and prevents it from becoming mobile ions detrimental to the long-term stability of the Perovskite solar cell. We are hoping that this CO2 doping technique could be a stepping stone for overcoming existing challenges in organic electronics and beyond.”

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