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

Gunn, F., Ghosh, P., Maciejczyk, M., Cameron, J. , Nordlund, D., Krishnamurthy, S., Tuttle, T., Skabara, P. , Robertson, N. and Ivaturi, A. (2021) Understanding the dopant induced effects on SFX-MeOTAD for perovskite solar cells: a spectroscopic and computational investigation. Journal of Materials Chemistry C, 9(45), pp. 16226-16239. (doi: 10.1039/D1TC04172J)

Abstract

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.

• Item Type: Articles
• Additional Information: A.I. acknowledges UK Research and Innovation (UKRI), Engineering and Physical Sciences Research Council (EPSRC) for the Fellowship grant (EP/P011500/1), EPSRC DTP (2120149) and Strathclyde’s Research Excellence Award (REA) for funding Fraser’s studentship. DFT, TD-DFT results were obtained using the ARCHIE-WeSt High-Performance Computer (www.archiewest.ac.uk) based at the University of Strathclyde. S.K. acknowledges Royal Society IES\R2\170272 and Royal Academy of Engineering Newton fund for the funding support. M. R. M. and N. R. acknowledge the European Union’s Horizon 2020 Research and Innovation Programme H2020-MSCA-IF-2014- 659237 and the EPSRC Supersolar Hub for financial support.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Cameron, Dr Joseph and Skabara, Professor Peter
• Authors: Gunn, F., Ghosh, P., Maciejczyk, M., Cameron, J., Nordlund, D., Krishnamurthy, S., Tuttle, T., Skabara, P., Robertson, N., and Ivaturi, A.
• College/School: College of Science and Engineering > School of Chemistry
• Journal Name: Journal of Materials Chemistry C
• Publisher: Royal Society of Chemistry
• ISSN: 2050-7526
• ISSN (Online): 2050-7534
• Published Online: 12 October 2021
• Copyright Holders: Copyright © 2021 The Royal Society of Chemistry
• First Published: First published in Journal of Materials Chemistry C 9(45): 16226-16239
• Publisher Policy: Reproduced under a Creative Commons License