Posted on Mon, Jul 20, 2015
There was another great SuperSolar consortium meeting here in the UK last week, with all sorts of interesting photovoltaic technologies being discussed. One of the highlights for us was beginning to see what is possibly the start of a consensus on Perovskite hysteresis curves based upon physical impurities and/or lattice type defects (stoichiometric impurity).
Of further interest was the (at this point qualitative) agreement between different research groups coming at the problem from different angles. For example, Simon O'Kane from the University of Bath presented his theoretical models in "Can slow-moving ions explain hysteresis in peroskite solar cells?" which showed convincing similarities to what is often seen in experimental hysteresis data. Meanwhile, Piers Barnes from Imperial College London presented work looking at the time-constants involved  and how it can help to eliminate some of the other theories. Computation modelling also suggested the activation energy required for an iodine atom to hop onto an adjacent defect site could be below 0.7 eV, which when compared to the high flux of 3 eV photons being thrown at these samples under solar illumination, suggests that the lattice defects are highly mobile.
From our own work we have some qualitative and quantitative data suggesting that as we improve the purity of the system the hysteresis decreases and stability increases. As such, we're looking forward to drilling down into this in more detail as part of the new Photovoltaic Lifetime and Degradation Project in conjunction with The University of Sheffield, Imperial College London and Swansea University.
Hopefully things will become clearer in time, but it currently looks like peering through the fog of data, maybe there is a glimmer of light shining through and by improving the lifetime and reducing the hysteresis of perovskite photovoltaic devices may just be as simple as making them purer (both chemically and stoichiometrically). Of course given that perovskite lifetime needs to improve by around two orders of magnitude, it's likely that this may just be the first hurdle of many. It's also true that getting perfect crystal lattices with low defect densities from a low-temperature solution-processed semiconductor will not actually be simple at all. However, time will tell and at least it gives a direction to work in.
 Optoelectronic Studies of Methylammonium Lead Iodide Perovskite Solar Cells with Mesoporous TiO2: Separation of Electronic and Chemical Charge Storage, Understanding Two Recombination Lifetimes, and the Evolution of Band Offsets during J–V Hysteresis
Brian C. O’Regan, Piers R. F. Barnes, Xiaoe Li, Chunhung Law, Emilio Palomares, and Jose M. Marin-Beloqui
J. Am. Chem. Soc., 2015, V137, P5087 (2015)