A novel dopant for spiro-OMeTAD towards efficient and stable perovskite solar cells

2,2 ',7,7 '-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9 '-spirobifluorene (spiro-OMeTAD), as the most common used hole transport material (HTM), plays a significant role in the normal structured (n-i-p) high-efficiency perovskite solar cells (PSCs). In general, it is prepared by a halogen solvent (chlorobenzene, CBZ) and needs an ion dopant (lithium bis (trifluoromethanesulfonyl)imide, Li-TFSI) to improve its conductivity and hole mobility. However, such a halogen solvent is not environmentally friendly and the widely used Li-TFSI dopant would affect the stability of PSCs. Herein, we develop a non-halogen solvent-tetrahydrofuran (THF)-prepared spiro-OMeTAD solution with a new p-type dopant, potassium bis(fluorosulfonyl)imide (K-FSI), to apply into PSCs. By this strategy, high-hole-mobility spiro-OMeTAD film is achieved. Meanwhile, the potassium ions introduced by diffusion into perovskite surface passivate the interfacial defects. Therefore, a hysteresis-free champion PSC with an efficiency of 21.02% is obtained, along with significantly improved stability against illumination and ambient conditions. This work provides a new strategy for HTMs toward hysteresis-free high-efficiency and stable PSCs by substituting dopants.