Doping Organic Interlayers in Perovskite Solar Cells with Carbon Dioxide

The π-conjugated small molecule spiro-OMeTAD is the most frequently used semiconductor for the hole-conducting layer in perovskite solar cells. To enhance the electrical conductivity of spiro-OMeTAD, LiTFSI is typically used in a doping process that is conventionally initiated by exposing spiro-OMeTAD:LiTFSI films to air and light for several hours. This process, in which oxygen acts as the p-type dopant, is highly time-intensive and largely depends on ambient conditions. Here we highlight our recent publication [Kong <i>et al</i>, Nature, <b>594</b> (2021)] wherein we report a fast and reproducible doping method that involves bubbling a spiro-OMeTAD:LiTFSI solution with CO₂ (or O₂) under ultraviolet light for a few minutes. During this process, CO₂ obtains electrons from photoexcited spiro-OMeTAD, rapidly promoting its p-type doping and resulting in the precipitation of carbonates that can be extracted by filtering, greatly lowering the Li-content in both the doped spiro-OMeTAD film and in the rest of the device. The CO₂-treated interlayer exhibits approximately 100 times higher conductivity than a pristine film while realizing stable, high-efficiency perovskite solar cells without any post-treatments. Finally, we also show that this method is not reserved for spiro-OMeTAD but can be used to dope other organic systems, such as π-conjugated polymers, opening the potential for other applications beyond interlayers in perovskite solar cells.<br/>Kong, J. <i>et al.</i> CO2 doping of organic interlayers for perovskite solar cells. <i>Nature</i> <b>594</b>, 51–56 (2021).