Novel Cobalt Complexes as a Dopant for Hole-transporting Material in Perovskite Solar Cells

We report herein the synthesis of new cobalt complexes tris[2-(1H-pyrazol-1-yl)-4-trifluoromethylpyridine]-cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co1), tris[4-(1,1-difluoroethyl)-2-(1H-pyrazol-1-yl)pyridine]-cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co2), bis[6-(1H-pyrazol-1-yl)-2,2'-bipyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co3) and bis[4,4'-dinnethyl-6-(1H-pyrazol-1-yl)-2,2'-bipyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co4), which we use as a dopant to create a hole-transporting material in perovskite solar cells. The addition of Co1-Co4 to 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) in chlorobenzene changes the color of the material, which indicates that charge transfer occurs between spiro-OMeTAD and Co1-Co4. Devices made from spiro-OMeTAD doped by Co1, Co2, and Co4 perform better than those made from Co3 because of the presence of hydrophobic alkyl groups on the ligands. An overall power conversion efficiency of 14.8% is obtained by using Co2 as a dopant, which exceeds that of tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (FK209) under the same conditions with spiro-OMeTAD as a hole-transporting material. (C) The Electrochemical Society of Japan, All rights reserved.