Fullerenes
have attracted considerable interest as an electron-transporting
layer in perovskite solar cells. Fullerene-based perovskite solar
cells produce no hysteresis and do not require high-temperature annealing.
However, high power conversion efficiency has been only achieved when
the fullerene layer is thermally evaporated, which is an expensive
process. In this work, the limitations of a solution-processed fullerene
layer have been identified as high crystallinity and the presence
of remnant solvents, in contrast to a thermally deposited C60 film, which has low crystallinity and no remaining solvents. As
a solution to these problems, a mixed C60 and C70 solution-processed film, which exhibits low crystallinity, is proposed
as an electron-transporting layer. The mixed-fullerene-based devices
produce power conversion efficiencies as high as that of the thermally
evaporated C60-based device (16.7%) owing to improved fill
factor and open-circuit voltage. In addition, by vacuum-drying the
mixed fullerene film, the power conversion efficiency of the solution-processed
perovskite solar cells is further improved to 18.0%. This improvement
originates from the enhanced transmittance and charge transport by
removing the solvent effect. This simple and low-cost method can be
easily used in any type of solar cells with fullerene as the electron-transporting
layer.