Energy Level Shifts in Spiro-OMeTAD Molecular Thin Films When Adding Li-TFSI
2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 50, 26300-26305 p.Article in journal (Refereed) Published
Hard X-ray photoelectron spectroscopy (HAXPES) has been used to study the effects of adding Li-TFSI to hole conducting molecular thin films of 2,2',7,7'-tetrakis(N,N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD). The work shows that a procedure of mixing a Li-TFSI solution into a spiro-OMeTAD solution, and subsequent spin-coating this mixture into a solid thin film causes the Fermi level of the molecular film to move closer to the HOMO level. Hence, adding the Li-TFSI gives similar effects to spiro-OMeTAD as a p-dopant. Specific effects from doping on the valence levels were also characterized. Absorbance measurements also showed that the spiro-OMeTAD film was partially oxidized when Li-TFSI was added before spin-coating. By varying the photon energy in the photoelectron spectroscopy measurements, the probe depth varies between being surface sensitive (<1 nm) and bulk sensitive (inelastic mean free path >= 10 nm). This property was used to follow differences in the composition at different depth of the spiro-OMeTAD/Li-TFSI film. It could be concluded that there was a concentration gradient in the molecular film and that the concentration of Li-TFSI was dominating at the interface between the spiro-OMeTAD/Li-TFSI film and vacuum.
Place, publisher, year, edition, pages
2012. Vol. 116, no 50, 26300-26305 p.
IdentifiersURN: urn:nbn:se:uu:diva-220596DOI: 10.1021/jp306433gOAI: oai:DiVA.org:uu-220596DiVA: diva2:705732