  Although perovskite solar cells have demonstrated impressive efficiencies in
research labs (above 23%), there is a need of experimental procedures that
allow their fabrication at ambient conditions, which would decrease
substantially manufacturing costs. However, under ambient conditions, a
delicate control of the moisture level in the atmosphere has to be enforced to
achieve efficient and highly stable devices. In this work, we show that it is
the absolute content of water measured in the form of partial water vapour
pressure (WVP) the only determining control parameter that needs to be
considered during preparation. Following this perspective, MAPbI3 perovskite
films were deposited under different WVP by changing the relative humidity (RH)
and the lab temperature. We found that efficient and reproducible devices can
be obtained at given values of WVP. Furthermore, it is demonstrated that small
temperature changes, at the same value of the RH, result in huge changes in
performance, due to the non-linear dependence of the WVP on temperature. We
have extended the procedure to accomplish high-efficient FA0.83MA0.17PbI3
devices at ambient conditions by adjusting DMSO proportion in precursor
solution as a function of WVP only. As an example of the relevance of this
paramater, a WVP value of around of 1.6 kPa appears to be an upper limit for
safe fabrication of high efficiency devices at ambient conditions, regardless
the RH and lab temperature.
