Improving and Stabilizing Perovskite Solar Cells with Incorporation of Graphene in the Spiro-OMeTAD Layer: Suppressed Li Ions Migration and Improved Charge Extraction

Perovskite solar cells (PSCs) have achieved a huge success in power conversion efficiency (PCE), although they still suffer from the long-term stability problem caused by the intrinsic sensitivity of perovskites to moisture. 2,2',7,7'-Tetrakis (N,N-di-p-methoxyphenylamine) 9,9'-spirobifluorene (Spiro-OMeTAD) is widely used as the hole transport layer (HTL) in typical PSCs; meanwhile, bis(trifuoromethane)sulfonimide lithium salt (Li-TFSI) is necessary as an additive in the Spiro-OMeTAD HTL to improve the hole mobility. However, the Li+ ions bring in high hygroscopicity and a water-uptake effect that both aggravate degradation of the Spiro-OMeTAD HTL and thereby of the perovskite layers. Here, we modify the Li-TFSI-based Spiro-OMeTAD HTL by adding reduced graphene oxide (rGO). We verify that rGO provides adsorption sites for Li+ ions and subsequently suppresses Li+ migration. The water-uptake effect originating from Li+ ions is thus restrained, and unfavorable pinholes in HTL caused by Li+ ion migration are eliminated. Consequently, the rGO- incorporated HTL remarkably improves the device stability that maintains the initial PCEs within 3% loss after 700 h under 40% humidity; however, the pristine devices almost lose the efficiency after 620 h. In addition, the good conductivity of the rGO favors hole transport in the Spiro-OMeTAD, resulting in a promotion in PCEs from 17.7% to 19.3% by incorporating rGO in HTL. Our work takes an insight into the function of rGO in the HTL and demonstrates an effective way of improving the efficiency and stability of PSCs simultaneously.