Er@C82 as a Bifunctional Additive to the Spiro-OMeTAD Hole Transport Layer for Improving Performance and Stability of Perovskite Solar Cells

Perovskite solar cells (PSCs) based on 2,2 ',7,7 '-tetrakis-(N,N-di-p-methoxyphenylamino)-9,9 '-spirobifluorene Spiro-OMeTAD hole transport layer (HTL) have achieved a huge success in power conversion efficiency (PCE), but the required lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) dopant in Spiro-OMeTAD HTL is hygroscopic, not only impairing the charge transport but also inducing the instability of PSCs. Herein, Er@C-82, which consists of a hydrophobic fullerene cage encapsulating an Er3+ ion, is first introduced as a novel additive to modify the Li-TFSI-based Spiro-OMeTAD HTL. By adding a tiny amount of Er@C-82 (0.09 mg mL(-1)) in the Spiro-OMeTAD HTL, the PSC exhibits an efficiency promotion from 17.53% to 19.22%. The PCE enhancement is mainly attributed to the improved film quality of HTL after adding Er@C-82, which promotes the oxidation of Spiro-OMeTAD, resulting in faster hole transport and less charge recombination. Simultaneously, the hydrophobic Er@C-82 and the improved film quality of HTL lead to a dramatically enhanced stability of PSCs. Accordingly, the Er@C-82-modified devices can maintain over 70% and 80% of the initial efficiencies after exposure in air for 400 h and in an Ar atmosphere for 2000 h, respectively. Therefore, this bifunctional Er@C-82 additive provides a promising pathway for fabricating highly efficient and stable PSCs.