Controllable Perovskite Crystallization at a Gas–Solid Interface for Hole Conductor-Free Solar Cells with Steady Power Conversion Efficiency over 10%

Depositing
a pinhole-free perovskite film is of paramount importance
to achieve high performance perovskite solar cells, especially in
a heterojunction device format that is free of hole transport material
(HTM). Here, we report that high-quality pinhole-free CH3NH3PbI3 perovskite film can be controllably
deposited via a facile low-temperature (<150 °C) gas–solid
crystallization process. The crystallite formation process was compared
with respect to the conventional solution approach, in which the needle-shaped
solvation intermediates (CH3NH3PbI3·DMF and CH3NH3PbI3·H2O) have been recognized as the main cause for the incomplete
coverage of the resultant film. By avoiding these intermediates, the
films crystallized at the gas–solid interface offer several
beneficial features for device performance including high surface
coverage, small surface roughness, as well as controllable grain size.
Highly efficient HTM-free perovskite solar cells were constructed
with these pinhole-free CH3NH3PbI3 films, exhibiting significant enhancement of the light harvesting
in the long wavelength regime with respect to the conventional solution
processed one. Overall, the gas–solid method yields devices
with an impressive power conversion efficiency of 10.6% with high
reproducibility displaying a negligible deviation of 0.1% for a total
of 30 cells.