Imperial Perovskite PV Symposium

Students and staff form the CPE and beyond gathered in December to share the latest research on hybrid perovskite materials and solar cells.

Recent experimental and theoretical results were presented from eight different groups in Chemistry, Physics, and Materials. 

Hybrid perovskites have attracted major international research interest, primarily because of their use as the active layer of solar cells. This class of materials can be prepared from cheap solution based ingredients which can be deposited to form thin crystalline films which absorb light strongly and show remarkably good semiconducting properties. The perovskite crystal structure has an inorganic framework held together by cations, which are typically small organic molecules – hence the term ‘hybrid’. Perovskite solar cells have reached more than 19 % efficiency for devices of 1 cm2 within a few years. Although this progress is impressive, many interesting scientific challenges remain to be answered, ranging from understanding their physical properties to improving their stability as devices. These materials are the subject of a number of current CPE students research projects.

Artem Bakulin, new Lecturer in the Chemistry Department, kicked the afternoon off with a description of his amazing work using ultra-fast 2D vibrational spectroscopy to characterise the time scales methylammonium ion motions in the material. The measurements showed excellent agreement with the theoretical calculations of Jarvist Frost who spoke later in the afternoon. Jarvist is Imperial’s ‘prodigal son’, he has just returned to Imperial from the Unversity of Bath, where he was working as a post-doc with Aron Walsh, he also persuaded Aron to come with him! Jarv’s condensed his recent work on the energetic landscape with-in methyl ammonium lead iodide. He showed that a variety of factors ranging from disorder in the methyl ammonium cation alignments in the lattice to Rashba splitting in the band structure are likely to result in energetic peaks and troughs in the material. This gives some insight into the unusually long lifetimes of excited charge carriers in these materials.

Iain Hamilton, a PhD student from Ji Seon Kim’s group, gave us an introduction to some amazing new measurement techniques he has been trying out on perovskite materials. These included combined atmospheric pressure ultraviolet photoelectron spectroscopy measurements, kelvin probe and surface photovoltage measurements. These tools allow the relative surface energies of the different components used in perovskite devices to be measured, and they should prove to be invaluable for evaluating the various possible perovskite and contact material combinations. Jinhyun Kim, a PhD student from James Durrant’s group then presented his recent results on photoluminescence quenching in perovskites with different contact layers. He gave us a number of interesting insights into the relative rate and efficiency of charge transfer of photogenerated electrons and holes into the contacts.

After a break, Nic Harrison presented the work of his PhD student Andrew McMahon on phase diagram of the hydrated phases of hydrated perovskites. One of the degradation pathways for perovskites is via their transformation to a hydrated phase in the presence of water vapour. In this really fascinating study (I’m a little biased here) they have used ab initio thermodynamic calculations to show that the phase transition only occurs below a certain temperature, and that temperature is determined in part by the amount of entropy in the perovskite phase – disorder in the crystal actually appears to stabilise it relative to the hydrate. Following this Tian Du a PhD student from Martyn McLachlan’s group presented a really nice study looking at the effects of using different precursor stoichiometry on defects in perovskites and devices. The presentation included some beautiful transmission electron micrographs of the interfaces in devices subject to different conditions, these physical observations were coupled with transient measurements of device behaviour.

Next up was Davide Moia who showed us some of the preliminary results he has gathered on different perovskite alloys using electroabsorbance measurements. Electroabsorbance measures the change in transmission of light that occurs through a device when an extranal voltage changes its internal electric field. Davide’s technique looks very promising as it allows the movement of charged defects with the material to be directly monitored. The programme was concluded by Saif Haque who presented his ground breaking results on the effects of oxygen on the degradation of hybrid perovskites. His results show that oxygen in the presence of free electrons in the crystal, such as those generated by light, results in the formation of superoxide species which can then attack methyl ammonium ions. Saif then discussed strategies for controlling this effect.

The afternoon was rounded off by drinks, crisps and plenty of good discussions, with all who attended much the wiser about what was going on in the other groups. This was the second symposium in the series so far and I hope it will help to catalyse yet more exciting results and collaborations by the time of the next one!