Introducing Ionic Liquid Dopant to Spiro-OMeTAD for Improving Thermal Stability in Perovskite Solar Cells

Perovskite solar cells (PSCs) are attracting attention as next-generation solar cells because of their high-power conversion efficiencies (PCEs) and lightweight. Lightweight PSCs can be installed on roofs with weight limitations, further increasing the potential for renewable energy. In practical out-door use, PSCs require measures for environmental stability such as heat and light. It is well known that there are issues regarding the thermal stability of PSCs. There are issues with thermal stability in the perovskite itself and in the components such as carrier transport layer in PSCs. Thermal stability of PSCs with conventional structure has present a challenge in doping materials of hole transport layer (HTL). In conventional PSCs, LiTFSI is widely used as a doping material for Spiro-OMeTAD of HTL. However, LiTFSI is known to change to Li₂O₃ or become Li⁺ and diffuse inside PSCs. To improve the thermal stability of PSCs, it is important to avoid diffusion into the device by fixation Li⁺ at HTL. We focused on the doping method of HTL in conventional PSCs for improving thermal stability. For immobilization of the dopant in the HTL, bulky organic cations were investigated introducing PSCs. Since TFSI is an effective doping for Spiro-OMeTAD, a combination of TFSI and organic cations were selected as dopants for spiro-OMeTAD. Our PSCs structure is FTO/SnO₂/Cs_0,05FA_0.95PbI₃/Spiro-OMeTAD/Au. N-Ethyl-N-(2-methoxyethyl)-N,N-dimethylammonium TFSI (EMe-TFSI) and LiTFSI were used as dopants for HTL and compared, respectively. Optimized PSCs have PCE of 18.1% with LiTFSI and 18.2% with EMe-TFSI. The solar cell performance with EMe-TFSI was found to be equivalent to LiTFSI, and it was found that EMe-TFSI can function as a dopant. We evaluated thermal stability of both PSCs at 85°C and 30% RH. The PSC with EMe-TFSI exhibited 50% of its initial properties after 300 hours at thermal stability test. On the other hand, the PSC with LiTFSI reduced to 10% of its initial characteristics in just 24 hours. These results suggest that the thermal stability of PSCs can be improved by using bulky organic ions rather than small and easy to diffuse ions such as Li.