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

Ghosh, Paheli and Ivaturi, Aruna and Bhattacharya, Debabrata and Bowen, James and Nixon, Tony and Kowal, Jan and Braithwaite, Nicholas St. J. and Krishnamurthy, Satheesh (2020) Efficient hole transport material formed by atmospheric pressure plasma functionalization of Spiro-OMeTAD. Materials Today Chemistry, 17. 100321. ISSN 2468-5194

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    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 × 10-7S cm-1 for the pristine film to 1.15 × 10-5S 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 2 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. We also attribute the formation of quinoid structures with quaternary nitrogen +N=C to the enhancement in positive charge centres due to loss of methoxy 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.