  Non-radiative recombination processes are the biggest hindrance to
approaching the radiative limit of the open-circuit voltage for wide-band gap
perovskite solar cells. In addition, to high bulk quality, good interfaces and
good energy level alignment for majority carriers at charge transport
layer-absorber interfaces are crucial to minimize non-radiative recombination
pathways. By tuning the lowest-unoccupied molecular-orbital of electron
transport layers via the use of different fullerenes and fullerene blends, we
demonstrate open-circuit voltages exceeding 1.35 V in CH3NH3Pb(I0.8Br0.2)3
device. Further optimization of mobility in binary fullerenes electron
transport layer can boost the power conversion efficiency as high as 18.6%. We
note in particular that the Voc-fill factor product is > 1.085 V, which is the
highest value reported for halide perovskites with this band gap.
