  Perovskite solar cells employing self assembled monolayers such as Me-4PACz
as hole transport layer has been reported to demonstrate high device
efficiency. However, the poor perovskite wetting on the Me-4PACz caused by poor
perovskite ink interaction with the underlying Me-4PACz presents significant
challenges for fabricating efficient perovskite devices. A triple co-solvent
system comprising of dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and
N-methyl-2-pyrrolidone (NMP) is employed to improve the perovskite
ink-substrate interaction and obtain a uniform perovskite layer. In comparison
to DMF, DMSO-based inks, the inclusion of NMP shows considerably higher binding
energies of the perovskite ink with Me-4PACz as revealed by density-functional
theory calculations. With the optimized triple co-solvent ratio, the perovskite
devices deliver high power conversion efficiencies of >20%, 19.5% and ~18.5%
for active areas of 0.16 cm2, 0.72 cm2 and 1.08 cm2 respectively. Importantly,
this perovskite ink-substrate interaction approach is universal and helps in
obtaining a uniform layer and high photovoltaic device performance for other
perovskite compositions such as MAPbI3, FAMAPbI3-xBrx, and MA-free
FACsPbI3-xBrx.
