We study the effects of a series of post-deposition ligand treatments on the photoluminescence (PL) of polycrystalline methylammonium lead triiodide perovskite thin films. We show that a variety of Lewis bases can improve the bulk PL quantum efficiency (PLQE) and extend the average PL lifetime, ⟨τ⟩, with large enhancements concentrated at grain boundaries. Notably, we demonstrate thin-film PLQE as high as 35 ± 1% and ⟨τ⟩ as long as 8.82 ± 0.03 μs at solar equivalent carrier densities using tri-n-octylphosphine oxide-treated films. Using glow discharge optical emission spectroscopy and nuclear magnetic resonance spectroscopy, we show that the ligands are incorporated primarily at the film surface and are acting as electron donors. These results indicate it is possible to obtain thin-film PL lifetime and PLQE values that are comparable to those from single crystals by control over surface chemistry.

We study the impact of excitation energy on the photostability of methylammonium lead triiodide (CHNHPbI or MAPI) perovskite thin films. Light soaking leads to a transient increase of the photoluminescence efficiency at excitation wavelengths longer than 520 nm, whereas light-induced degradation occurs when exciting the films with wavelengths shorter than 520 nm. X-ray diffraction and extinction measurements reveal the light-induced decomposition of CHNHPbI to lead iodide (PbI) for the high-energy excitation regime. We propose a model explaining the energy dependence of the photostability that involves the photoexcitation of residual PbI species in the perovskite triggering the decomposition of CHNHPbI.

With perovskite solar efficiencies exceeding 23%, more investigations are needed to understand and enhance the open-circuit voltage (V oc ) for further efficiency gains. Here, by changing the perovskite film annealing conditions, we achieve significant V oc variations in a p−i−n cell architecture. While the morphology remains unaffected, we observe that the annealing conditions affect the surface stoichiometry. Subsequent variations in the V oc are identified to be caused by differences in trap-assisted recombination processes as well as also largely by the suppression of charge collection due to surface band bending. These findings highlight the impact of surface band bending on V oc which can be permanently introduced by normal annealing treatments and impede the charge transport process. Our study suggests that the detailed states of the band bending in perovskite films should be carefully examined and designed to maximize the V oc and consecutively improve the performance of perovskite solar cells.

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