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In a recent publication [1], we have shown using a drift-diffusion model that mobile iodide vacancies within the bulk methylammonium lead tri-iodide (CH3NH3PbI3) active layer of perovskite solar cells are the most probably cause of the hysteresis observed in these devices. Reasonable agreement with experiment was obtained by setting the mobility of the vacancies to within an order of magnitude of that predicted by Eames et al. [2]. Eames et al. assumed that the vacancy motion was thermally activated and that therefore the diffusion coefficient was subject to a Boltzmann-type temperature dependence. By incorporating this assumption into our drift-diffusion model, we have been able to reproduce temperature-dependent transient current decay measurements, suggesting that the vacancy motion is indeed thermally activated. The model can be used to explore the effects of different recombination mechanisms on the current-voltage characteristics; in particular, we have confirmed the hypothesis of Van Reenen et al. [3] that both vacancy motion and a trap-based recombination mechanism are required to observe significant hysteresis. Indeed, the current-voltage curves predicted for bimolecular recombination show a strong similarity to those published in reports of cells with “no hysteresis” [4]. Despite qualitative agreement of the model with experiment there is still further work required before results can produce quantitative agreement with a variety of experimental data. In particular we are looking at extending the model to investigate why certain cell designs are able to limit hysteresis and what effects this has on cell performance. References [1] Giles Richardson et al., En. Environ. Sci., Feb. 2016 DOI: 10.1039/c5ee02740c [2] Chris Eames et al., Nat. Commun., vol. 6, pp. 7497, May 2015 [3] Van Reenen et al., J. Phys. Chem. Lett., vol. 6, pp. 3808-3814, Sept. 2015 [4] Nam Joong Jeon et al., Nat. Mater., vol. 13, pp. 897-903, July 2014 [5] Aron Walsh et al., Angew. Chem., vol. 127, no. 6, pp. 1811-1814, Dec. 201

Can slow-moving ions explain hysteresis in the current–voltage curves of perovskite solar cells?