New CO2 doping method yields efficient perovskite solar cells

Researchers at the New York University (NYU) have designed a perovskite solar cell using a doping technique based on carbon dioxide (CO₂) instead of the commonly used oxygen doping approach.

The scientists described the common oxygen-based p-type doping process as one of the major hurdles to remove to bring perovskite closer to commercial production, as the technique is particularly time consuming and often requires hours to spread oxygen into a hole transporting layer of a perovskite cell.

The process, which aims to increase the electrical conductivity of cell interlayers, consists of exposing the hole transport layer (HTL) based on Spiro-OMeTAD blended with lithium bis(trifluoromethane)sulfonimide (LiTFSI) films to air and light. Spiro-OMeTAD is the most common organic semiconductor used for hole-transport material in perovskite solar cells and LiTFSI is a hydrophilic lithium salt that is widely used as a p-'dopant for Spiro-OMeTAD.

The implementation of this technique, in addition to being dependent on ambient conditions, may often result in leftover unreacted reactants or detrimental by-products in the layer itself, the researchers explained.

As part of their new approach, the research group pre-doped spiro-OMeTAD molecules by bubbling the spiro-OMeTAD:LiTFSI solution with CO₂ for one minute and under ultraviolet light. The CO₂-bubbled spiro-OMeTAD:LiTFSI films were incorporated in the cell as the HTL, and the cell performance was compared to that of a similar solar cell using O₂-treated spiro-OMeTAD:LiTFSI films.

The latter showed an efficiency of 17.3%, an open-circuit voltage of 1.14 V, a short-circuit current density of 20.9 mA cm² and a fill factor of 0.74. The former, by contrast, exhibited an efficiency of 19.1%, an open-circuit voltage of 1.14 V, a short-circuit current density of 21.2 mA cm² and a fill factor of 0.79.

According to the scientists, the CO₂-bubbled spiro-OMeTAD:LiTFSI also showed a conductivity that is around five times higher than that of O₂-bubbled spiro-OMeTAD:LiTFSI. 'The CO₂ treated film also resulted in stable, high-efficiency perovskite solar cells without any post-treatments,' they further explained, noting that the O₂-treated cells need post-process exposure to air to see their efficiency slightly increase.

'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,' explained NYU researcher Jaemin Kong. 'That's partly because most of the detrimental lithium ions in the spiro-OMeTAD:LiTFSI solution were stabilized as lithium carbonates during the CO₂ bubbling process.'

The novel technique is also claimed to be able to sequester the CO₂ used for its implementation. The research team included researchers from Samsung, Yale University, Korea Research Institute of Chemical Technology, Graduate Center of the City University, Wonkwang University, and the Gwangju Institute of Science and Technology.