Top and Bottom Surfaces Limit Carrier Lifetime in Lead Iodide Perovskite Films

Carrier recombination at defects is detrimental to the performance of solar energy conversion systems, including solar cells and photoelectrochemical devices. Point defects are localized within the bulk crystal while extended defects occur at surfaces and grain boundaries. If not properly managed, surfaces can be a large source of carrier recombination. Separating surface carrier dynamics from bulk and/or grain-boundary recombination in thin films is challenging. Here, we employ transient reflection spectroscopy to measure the surface carrier dynamics in methylammonium lead iodide perovskite polycrystalline films. We find that surface recombination limits the total carrier lifetime in perovskite polycrystalline thin films, meaning that recombination inside grains and/or at grain boundaries is less important than top and bottom surface recombination. The surface recombination velocity in polycrystalline films is nearly an order of magnitude smaller than that in single crystals, possibly due to unintended surface passivation of the films during synthesis.