CO2 doping of organic interlayers for perovskite solar cells

The team reporting on this research was led by André D. Taylor, a professor of chemical and biomolecular engineering at NYU Tandon, and post-doctoral associate Jaemin Kong.

Perovskite solar cells have progressed in recent years with rapid increases in power conversion efficiency (from 3% in 2006 to 25.5% today), making them more competitive with silicon-based photovoltaic cells. However, a number of challenges remain before they can become a competitive commercial technology.
One of these challenges involves inherent limitations in the process of p-type doping of organic hole-transporting materials within the photovoltaic cells.

This process, wherein doping is achieved by the ingress and diffusion of oxygen into hole transport layers, is time intensive (several hours to a day), making commercial mass production of perovskite solar cells impractical. The Tandon team, however, discovered a method of vastly increasing the speed of this process through the use of carbon dioxide instead of oxygen.

In perovskite solar cells, doped organic semiconductors are normally required as charge-extraction interlayers situated between the photoactive perovskite layer and the electrodes. The conventional means of doping these interlayers involves the addition of lithium bis(trifluoromethane)sulfonimide (LiTFSI), a lithium salt, to spiro-OMeTAD, a π-conjugated organic semiconductor widely used for a hole-transporting material in perovskite solar cells, and the doping process is then initiated by exposing spiro-OMeTAD:LiTFSI blend films to air and light. Besides being time consuming, this method largely depends on ambient conditions. By contrast, Taylor and his team reported a fast and reproducible doping method that involves bubbling a spiro-OMeTAD:LiTFSI solution with carbon dioxide (CO2) under ultraviolet light.

They found that the CO2 bubbling process rapidly enhanced electrical conductivity of the interlayer by 100 times compared to that of a pristine blend film, which is also approximately 10 times higher than that obtained from an oxygen bubbling process. The CO2 treated film also resulted in stable, high-efficiency perovskite solar cells without any post-treatments.

The lead author Jaemin Kong explained that “Employing the pre-doped spiro-OMeTAD to perovskite solar cells shortens the device fabrication and processing time. Further, it makes cells much more stable as most detrimental lithium ions in spiro-OMeTAD:LiTFSI solution were stabilized to lithium carbonate, created while the doping of spiro-OMeTAD happened during CO2 bubbling process. The lithium carbonates end up being filtered out when we spincast the pre-doped solution onto the perovskite layer. Thus, we could obtain fairly pure doped organic materials for efficient hole transporting layers.”

Moreover, the team found that the CO2 doping method can be used for p-type doping of other π-conjugated polymers, such as PTAA, MEH-PPV, P3HT, and PBDB-T. Taylor said the team is looking to push the boundary beyond typical organic semiconductors used for solar cells.

"We believe that wide applicability of CO2 doping to various π-conjugated organic molecules stimulates research ranging from organic solar cells to OLEDs and OFETs even to thermoelectric devices that all require controlled doping of organic semiconductors,” and added that “Since this process consumes a quite large amount of CO2 gas during the process, it can be also considered for CO2 capture and sequestration study in the future. We are hoping that the CO2 doping technique could be a stepping stone for overcoming existing challenges in organic electronics and beyond.”