Understanding the dopant induced effects on SFX-MeOTAD for perovskite solar cells: a spectroscopic and computational investigation

SFX-MeOTAD [2,2′,7,7′-Tetrakis(N,N-di(4-methoxyphenyl)amino)-spiro-(fluorene-9,9′-xanthene)] (also known as X60) has emerged as a cost-effective alternative to the ubiquitous, but excessively-expensive, spiro-MeOTAD hole transport material (HTM) in perovskite solar cells. Using its pre-oxidised dicationic salt, SFX-(TFSI)2, a controlled concentration dependent conductivity tuning of this HTM without the requirement of air (oxygen) exposure is carried out. This study details the modifications in optical and electrical properties of this low cost HTM as a function of concentration of the dicationic salt (0-100 mol %) using UV-Vis absorption and electrical conductivity measurements. X-ray absorption and photoelectron spectroscopic investigations have been carried out to elucidate the role of the dicationic salt on the enhanced electronic properties of SFX-MeOTAD. By incorporating the dicationic SFX-(TFSI)2 it was shown that the conductivity of SFX-MeOTAD increased by 4 orders of magnitude from 2.55 × 10-8 S cm-1 to 9.4 × 10-4 S cm-1 using an optimal dopant concentration of 20.5 mol %. The degree of oxidation of SFX-MeOTAD was determined through UV-Vis absorption and consolidated by the computational calculations. XPS study reveal that doping SFX-MeOTAD with SFX(TFSI)2 results not only in oxidation of the HTM but also leads to variation in the local chemistry around carbon and nitrogen which directly influence the conductivity of the doped films. NEXAFS studies indicates that doping enhances the aromatic nature of the molecule initially but increasing the dopant concentration further affects the aromaticity and possibly the π stacking, similar to the trend seen in dopant concentration dependent conductivity of the SFX-MeOTAD films. These findings have implications on the choice of dopant concentration and counterions more generally for triarylamine based HTMs.