Highly efficient and stable planar perovskite solar cells with K3[Fe(CN)6]-doped spiro-OMeTAD

Spiro-OMeTAD has been widely employed as one of the most frequently used hole transport materials (HTMs) for high-performance perovskite solar cells (PSCs). Nevertheless, raw spiro-OMeTAD suffers from inferior electronic properties. To date, doping has been considered to be a promising strategy to address this dilemma. In this work, we successfully introduce K3[Fe(CN)6], a cheap and effective inorganic oxidant, into spiro-OMeTAD together with lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpyridine (tBP) for highly efficient PSCs. The addition of the dopant K3[Fe(CN)6] could not only promote the oxidation of spiro-OMeTAD to generate a higher hole density, but also adjust its highest occupied molecular orbital (HOMO) to pursue an appropriate energy level alignment with the perovskite active layer, resulting in a high conductivity and hole extraction ability of spiro-OMeTAD. As a consequence, the PSC incorporated with K3[Fe(CN)6] achieves a champion efficiency of 20.84%, which is higher than that of the undoped device (18.09%). Furthermore, the K3[Fe(CN)6]-doped devices demonstrate an enhanced reproducibility and alleviated hysteresis. Our results indicate that K3[Fe(CN)6] is an efficient and facile dopant of spiro-OMeTAD for highly efficient and stable PSCs.