Fluorinated carbon nanotubes as nonvolatile additive to the active layer of polymer/fullerene solar cells

Highly dispersed fluorinated SWCNTs of submicron length were prepared, characterized and tested as nonvolatile additive to the active layer of polymer/fullerene organic solar cells. Two types of initial SWCNT materials - TUBALL (R) (OCSiAl) and Super purified plasma tubes (Nanointegris Inc.) - were used. The absence of big CNT aggregates was revealed by atomic force microscopy. The absence of metallic SWCNTs is confirmed by optical spectra of fluorinated SWCNT dispersions. Reproducible increase of the main performance parameters of solar cells (short circuit current, open circuit voltage, fill factor and power conversion efficiency) was obtained upon admixing a small amount of fluorinated SWCNTs (less than 1% weight) into the polymer/fullerene active layer, with either P3HT or PCDTBT donor and either PC60BM or PC70BM acceptor for both direct and inverted device architectures. Since effective charge mobility did not change upon fluorinated SWCNT admixture, the improvement of solar cell performance is not related to SWCNT-induced modification of the electronic structure of the active layer components or charge transfer via fluorinated SWCNTs. Presumably, the origin of this improvements is geometric optimization of the active layer morphology (improving connectivity of donor and acceptor domains) by fluorinated SWCNTs due to their high aspect ratio.