Posted on 15 Dec 11:28
Another good project meeting on Friday 11th December with our academic colleagues from the University of Sheffield, Imperial College London and Swansea University as part of the Engineering and Physical Sciences Research Council (EPSRC) Solar Grand Challenge ‘High Resolution Mapping of Performance and Degradation Mechanisms in Printable Photovoltaic Devices’.
There was considerable discussion surrounding perovskite hysteresis and understanding what's happening. It seems like we are still in the early days of truly understanding what is going on but it seems increasingly accepted that a lot of hysteresis stems from ionic content.
There is also a range of optical imaging techniques being developed to explore inhomogeneity within the perovskite layer, with the hope to develop a new technique to map out ion migration pathways. However, in the mean time from our perspective we are still working on a three-pronged process to reduce hysteresis and improve lifetime:
1) Start with highly pure materials with little "stray" ionic content to begin with (e.g. excess free Iodine).
2) Try to produce a stoichiometrically pure perovskite crystal layer so there isn't too much of an excess of any of the ionic precursor materials (eg. MA+, Pb+, Cl- or I-) lingering at the grain boundaries or interfaces.
3) Don't add salty ionic dopants to the interface materials (especially those dopants containing light and mobile ions such as lithium).
In practice, this means that while we are still working with doped spiro as a short term measure to increase hole extraction efficiency, long term our focus is still likely to be on undoped (or at least intrinsically doped) interface materials to help avoid the issue of ion migration.