Insights into the hole transport properties of LiTFSI-doped spiro-OMeTAD films through impedance spectroscopy

Abstract

Hole transport materials are crucial for efficient charge extraction in perovskite solar cells to achieve high power conversion efficiency and stability. Herein, the hole transport properties of the 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD) thin films with a dopant lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) are investigated through impedance spectroscopy. Upon doping, we observe a dispersive hole transport with nearly a 100-fold increase in the hole mobility compared with the pristine spiro-OMeTAD films. The hole mobilities slightly decrease with increasing electric fields for both films, exhibiting a negative electric field dependence of mobility due to the positional disorder. Subsequently, the charge carrier density of the LiTFSI-doped spiro-OMeTAD film is three orders of magnitude higher than that of the pristine film. The LiTFSI dopant induces two different electrical regions in the doped thin film, which can be reflected through impedance spectroscopy. The presented investigation through impedance spectroscopy is of high practical interest for the development of hole transport materials and the optimization of the transport layer doping in perovskite solar cells.