Air-exposure induced dopant redistribution and energy level shifts in spin-coated spiro-Meotad films

Zafer Hawash, Luis K. Ono, Sonia R. Raga, Michael V. Lee, Yabing Qi

Research output: Contribution to journalArticleResearchpeer-review

228 Citations (Scopus)

Abstract

Doping properties of 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) hole transport layer are investigated by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and atomic force microscopy under air exposure. XPS results reveal that 3 h exposure of Li-bis(trifluoromethanesulfonyl)-imide (LiTFSI) doped spiro-MeOTAD to air results in the migration of LiTFSI from the bottom to the top across the spiro-MeOTAD film. AFM images reveal the presence of pinholes with an average diameter of ∼135 nm and a density of ∼3.72 holes/μm2. In addition, cross-sectional scanning electron microscope images reveal that these pinholes form channels across the doped spiro-MeOTAD film. Optical microscopy and Fourier transform infrared microscopy images confirm the presence of large pinholes with diameters in the range of 1-20 μm and a density of ∼289 holes/mm2 as well. The presence of pinholes may play a major role in the migration processes of the LiTFSI within the spiro-MeOTAD film as well as on the degradation processes of solar cells. This is further confirmed by the rapid decreasing efficiency of perovskite solar cells with solution prepared doped spiro-MeOTAD layers when exposed to air.

Original languageEnglish
Pages (from-to)562-569
Number of pages8
JournalChemistry of Materials
Volume27
Issue number2
DOIs
Publication statusPublished - 29 Dec 2015
Externally publishedYes

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