Conventionally, perovskite solar cells are processed in an inert environment, as the processing of the perovskite films is known to be very sensitive to ambient moisture. By spin coating a combination of lead acetate, lead chloride and methylammonium iodide at elevated temperature on ITO/PEDOT:PSS substrates, we are able to form a dense layer of perovskite crystals in a few seconds during spin coating in ambient air. These perovskites require further annealing in air to obtain high-efficiency devices, but how long to anneal?

By using in-situ methods to measure the absorption spectrum and photoluminescence (PL) intensity during annealing we develop tools to investigate the rate of the processes that occur during annealing. In the first period of annealing, the in-situ absorption measurements show that the perovskite absorption increases. This demonstrates that there is continued conversion of the precursor materials into perovskite. On the same timescale, the PL intensity decreases rapidly in two stages. We attribute this to quenching at the PEDOT:PSS interface, which becomes more efficient due to an increase in the mobility of charges in the perovskite layer. The rate of increase in absorption and decrease in PL intensity changes dramatically with annealing temperature.

The timescale for the annealing required to make efficient solar cells also depends on the annealing temperature, and coincides with the timescales that we observe in the in-situ measurements. Without further annealing after the hot casting, these perovskite films can be processed into working solar cells with a limited efficiency of ~ 5%. The efficiency increase substantially to ~ 13-15% by annealing in ambient air for several minutes. However, further annealing reduces the current density and open-circuit voltage which results in a lower efficiency of ~ 11%.

Because the efficiency is very sensitive to the duration of annealing, the optimization of the recipe is complicated. For example, changes in the layer thickness, the stoichiometry or the annealing temperature will require a change in annealing time. As these simple in-situ methods can predict the required annealing duration these methods will help in the optimization of perovskite recipes. We propose that these in-situ methods are useful tools as in-line quality monitor in perovskite fabrication.

This research forms part of the research programme of the Dutch Polymer Institute (DPI), project #734.