Investigating the Molecular Orientation and Thermal Stability of Spiro‐OMeTAD and its Dopants by Near Edge X‐Ray Absorption Fine Structure

• Published in: Advanced Physics Research 2023-11, Vol.2 (11), p.n/a
• Main Authors: Brady‐Boyd, Anita, Hazeldine, Kerry, Cross, Rachel E., Ren, Gongxizi, Connell, Arthur, Kershaw, Christopher P., Holliman, Peter J., Evans, D. Andrew
• Format: Article
• Language: English
• Subjects: Absorption; Dopants; F4‐TCNQ; Fine structure; Glass transition temperature; hole transporting materials; Investigations; LiTFSI‐tBP; Lithium; Molecular structure; near edge x‐ray absorption fine structures; Oxidation; Photovoltaic cells; Radiation; Solar cells; Spectra; Spiro‐OMeTAD; Surface stability; Tetracyanoquinodimethane; Thermal stability; Thermal utilization; Transition temperatures; Valence band
• ISSN: 2751-1200; 2751-1200
• DOI: 10.1002/apxr.202200045

This study describes the utilization of near edge X‐ray absorption fine structure (NEXAFS) to investigate the hole transporting material (HTM) 2,2ʹ,7,7ʹ‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐ 9,9ʹ‐spirobifluorene (Spiro‐OMeTAD) and its most common dopants, lithium bis‐(trifluoromethylsulfonyl) imide (LiTFSI), 4‐tert‐butylpiridine (tBP), and 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ). By changing the angle of the sample with respect to the beam, the orientation of the molecules on the surface can be observed. The data suggest that it is difficult to determine any orientational preference for Spiro‐OMeTAD deposited on a surface due to the 3D propeller‐like geometry of this molecule. Both doped and undoped samples show thermal stability beyond the glass transition temperature of the molecules. Significant changes to the Spiro‐OMeTAD spectra are observed with the addition of the dopants, in particular the C K‐edge. Differences are also observed in the valence band spectra when dopants are added. It is also demonstrated how the doping combination of LiFTSI with tBP and, F4‐TCNQ act as p‐type dopants by altering the position of the HOMO levels. The F4‐TCNQ induces a larger change in the HOMO levels when compared to the LiTFSI and tBP. These results are important to increase the understanding of Spiro‐OMeTAD and the effect dopants have on this material for next generation solar cells. Spiro‐OMeTAD (doped and undoped) is thermally stable beyond its glass transition temperature, although no molecular orientation is observed. It is observed that the F4‐TCNQ acts as a p‐type dopant pushing the HOMO level toward the Fermi level. Inducing a larger shift than the favored LiTFSI and tBP dopant combination.