Novel cobalt complexes as a dopant for hole-transporting material in perovskite solar cells
Takashi Funaki a, Nobuko Onozawa-Komatsuzaki a, Kazuhiro Sayama a, Masayuki Chikamatsu a, Said Kazaoui a, Takurou N. Murakami a
a National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Yokohama-shi, Japan, 2017 February 2nd - 4th
Organizers: Tsutomu Miyasaka and Iván Mora-Seró
Poster 053
Publication date: 7th November 2016

Perovskite solar cells (PSCs) have attracted considerable attention of late, because of their low cost, high performance, and facile fabrication. A key component of PSCs is the hole-transport material (HTM), which plays a crucial role in achieving high performance by hindering charge recombination. The most frequently used HTM is wide-band-gap 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD), which to date has proven to be the most effective HTM. However, a major drawback of this compound is its low conductivity, which directly affects the photovoltaic parameters. An important strategy to overcome this limitation and improve device performance is p-type doping to spiro-OMeTAD for inducing more charge carriers.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'-dimethyl-6-(1H-pyrazol-1-yl)-2,2'-bipyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co4), which we use as a dopant to create a HTM in perovskite solar cells. The addition of Co1–Co4 to spiro-OMeTAD in chlorobenzene changes the color of the material, which indicates that charge transfer occurs between spiro-OMeTAD and Co1–Co4. These complexes have suitable redox potentials for oxidation of spiro-OMeTAD and the conductivity is enhanced in films with 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 fabrication conditions.These results indicate that the Co(III) complex Co2 is a good candidate for a p-dopant of the HTM. 

This research was funded by the New Energy and Industrial Technology Development Organization (NEDO).

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