Directly purifiable Pre-oxidation of Spiro-OMeTAD for stability enhanced perovskite solar cells with efficiency over 23%

Spiro-OMeTAD so far remains the most popular material as the hole-transport layer (HTL) for efficient perovskite solar cells (PSCs). However, the residual dopant, by-products and the hygroscopic lithium salt that remained in the HTL are commonly harmful to the PSCs performance. We develop here a new approach to pre-oxidize the spiro-OMeTAD by using the Cobalt (III) trifluoride (CoF3) as a p-type dopant. Both the CoF3 and its reduced form (CoF2) are insoluble in the spiro-OMeTAD:LiTFSI mixture and the oxidation reaction occurs at the solid/liquid interface. Therefore, the unreacted dopant and by-products (CoF2) can be readily removed by filtering, leading to a purifiable pre-oxidation of spiro-OMeTAD and thus a high-quality HTL with improved conductivity and hole mobility. Additionally, part of detrimental Li+ ions can also be purified out of the spiro-OMeTAD solution in the form of LiF sediments, which improves the stability and reliability of the devices. The resulting PSC achieves an efficiency of 23.05% together with improved stability, maintaining 91% of its initial PCE after one month of storage in the atmosphere with 20-30% relative humidity. These findings provide a new strategy to dope HTL for efficient and stable PSCs.