Water vapour pressure as determining control parameter to fabricate high efficiency perovskite solar cells at ambient conditions

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.