Performance Enhancement of Planar Heterojunction Perovskite Solar Cells through Tuning the Doping Properties of Hole-Transporting Materials.

Chemical doping has been widely used to finely tune the electrical properties of organic hole-transporting materials (HTMs) that find widespread applications in perovskite solar cells (PSCs). Here, to shed light on the precise role of chemical p-doping in affecting the charge-transport properties of HTMs and photovoltaic performance of PSCs, two kinds of representative dopants, including lithium bis(trifluoromethane)sulfonimide (LiTFSI) and two Co(III) complexes tris[2-(1-pyrazol-1-yl)-4--butylpyridine]cobalt(III)tris[bis(trifluoromethylsulfonyl)imide] (FK209) and tris[2-(1-pyrazol-1-yl)pyridine]cobalt(III)tris[bis(trifluoromethylsulfonyl)imide] (FK102), are employed as the p-type dopant models to dope the 2,2',7,7'-tetrakis[,-di--methoxyphenylamine]-9,9'-spirobifluorene (spiro-OMeTAD) HTM. Both dopants can facilitate the generation of oxidized spiro-OMeTAD radical cation and improve hole mobility. Co-doping of FK209 and LiTFSI is necessary to achieve an optimal doping property and best device performance with power conversion efficiency of 17.8% compared to that of the FK209-doped device (13.5%) and the LiTFSI-doped device (15%). UV-vis absorption, space-charge-limited current measurements, and steady-state and time-resolved photoluminescence measurements have confirmed that with the co-doping of the two kinds of p-dopants in a proper ratio the doped spiro-OMeTAD exhibits a high charge carrier mobility and charge carrier transfer/collection capability.