Efficient hole transport material formed by atmospheric pressure plasma functionalization of Spiro-OMeTAD

Ghosh, Paheli; Ivaturi, Aruna; Bhattacharya, Debabrata; Bowen, James; Nixon, Tony; Kowal, Jan; Braithwaite, Nicholas St John and Krishnamurthy, Satheesh (2020). Efficient hole transport material formed by atmospheric pressure plasma functionalization of Spiro-OMeTAD. Materials Today Chemistry, 17, article no. 100321.

DOI: https://doi.org/10.1016/j.mtchem.2020.100321

Abstract

A technique to increase the conductivity of Spiro-OMeTAD using an easily scalable, non-thermal atmospheric pressure plasma jet (APPJ) is reported. An investigation of plasma functionalization demonstrated an enhancement in hole conductivity by over an order of magnitude from 9.4 × 10⁻⁷ S cm−1 for the pristine film to 1.15 × 10⁻⁵ S cm−1 for films after 5 minutes of plasma treatment. The conductivity value after plasma functionalization was comparable to that reported for 10–25% Li-TFSI-doped Spiro-OMeTAD. The increase in conductivity was correlated with a reduction in phase value observed using electrostatic force microscopy. Kelvin probe force microscopy showed an increase in work function after plasma exposure corresponding to the p-type nature of the doping. X-ray photoelectron spectroscopy revealed surface oxidation of plasma-functionalized films, as well as variation in nitrogen chemistry, with the formation of a higher binding energy quaternary nitrogen tail. Oxidation of Spiro-OMeTAD was also confirmed by the appearance of the 500 nm absorption peak using UV–vis spectroscopy. The synergistic contribution of increase in charge density in Spiro-OMeTAD due to the energetic species in the plasma jet coupled with improvement in π-π stacking of the molecules is thought to underlie the conductivity enhancement. The enhancement in positive charges can also be attributed to the formation of quinoid structures with quaternary nitrogen +N=C formed due to loss of methyl groups during plasma surface interaction. This work opens up the possibility of using an atmospheric pressure plasma jet as a simple and effective technique for doping and functionalizing Spiro-OMeTAD thin films to circumvent the detrimental issues associated with chemical dopants.

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