22 February 2021

Q+A: PhD student Mary O'Kane on perovskite solar cells

A postgraduate researcher in the electronic and photonic molecular materials research group explains how she got involved in solar technology research and her work on perovskites.

Mary O'Kane

The University of Sheffield has been working on solar technologies for more than 20 years. Researchers in Professor David Lidzey’s lab are focussed on perovskites, a class of materials that have several advantages over the silicon traditionally used in photovoltaic systems. In this Q+A Mary O’Kane, a PhD student in the electronic and photonic molecular materials (EPMM) research group explains how she got involved in solar research and her work on perovskites.

How did you get involved in photovoltaics research?

For me, the interest started during my undergraduate studies reading Physics here in Sheffield. I worked on the industrial physics module with Dr Al Buckley, and was placed on a project studying smart building materials with TATA Steel. Currently within construction and architecture there are a lot of clever energy-saving techniques being used to make the running, heating and maintenance of buildings more carbon-friendly. One technique that is being thoroughly explored is building-integrated photovoltaics, where solar cells are included in the construction of a building rather than retroactively fitted.

Interest in this, combined with thoroughly enjoying the research elements of my undergraduate degree, lead me to do a summer project working with Professor David Lidzey, reviewing innovative uses and applications of perovskite solar cells (PSCs). This was probably the best thing I could do at this time, reviewing all the interesting things that are being done with PSCs. It just seemed like the field for me. The thing I love most about photovoltaic research is how close you feel to having real world impacts.

Your research group works on perovskites – what makes these materials interesting to photovoltaics researchers?

There are a number of reasons that perovskites have sparked interest in the PV community. In PV research, any solar cell is usually going to be compared with crystalline-Silicon PV. As of 2020, single-junction c-Si PV devices have achieved power conversion efficiencies (PCE) of up to 29.9% – but it has taken over 40 years of development to get to this point. Perovskite solar cells were first used as an active absorber material in 2009 and as of 2020 have highest certified PCE’s of 25.5%. This is much higher performances than any other type thin-film third generation solar cell.

As well as their startling rise in performance, PSCs have a number of interesting material properties and practical advantages. Perovskites are made up of different anions and cations in an ABX3 crystal structure shown below. Therefore, their material properties, such as band gap, can be effectively selected by changing the individual materials used and by varying their combinations. By selecting the appropriate band gap, they can be used in tandem with Silicon solar cells to improve further their combined PCE. The current record PCE for Si/Perov tandem solar cell is 29.5%.

Perovskite crystal structure
A perovskite crystal structure, with an organic or inorganic cation occupying the A-sites (blue), a metal ion, such as lead, occupying the B-site (red) and eight halogen anions (green) creating a metal-halide octahedra.

Additionally, perovskites can be easily processed from a single precursor solution. They could potentially be manufactured at large scales through processes such as spray- or slot-die coating, which are both compatible with roll-to-roll processing. Also, PSCs are very thin compared to c-Si PV, and do not require high temperatures to be produced. PSCs can therefore be deposited on a wide range of surfaces, can be flexible, semi-transparent or lightweight. This give plenty of room for innovative and creative applications where perhaps silicon PV may not be appropriate.

What have you been working on recently?

I have been studying the lifetime of these perovskite precursor solutions, investigating if their shelf-life can be improved. Due to the simplistic deposition demands of PSCs, there is so much interesting chemistry that must happen in these solutions which has not yet been fully explored. It is thought that the key to definitive control of the perovskite film lies in refining this solution chemistry. This is an important matter for the research field at this moment.

Additionally if PSCs are to be solution processed industrially, they need to be stable in solution for significant periods of time, and produce devices with consistently high performances. We have recently completed a study on a high performance perovskite material showing that with optimal storage conditions, its shelf life can be drastically improved without the need for changing stoichiometries or using stabilisers or additive that may limit PSC performance. I am now excited to extend this learning to investigate other perovskite solutions, and to see if solution chemistry is in fact the most effective way to be storing perovskite material. 

There is now a real urgency driving developments in clean energy. What role do you see for researchers such as yourself in combating the climate crisis?

I think the role of research in this pursuit is critical. At sea level, the average power from the sun hitting the earth's surface is ~1000 W/m2 and its our job to utilise as much of this as we can. Only by having the freedom to experiment with new materials and novel structures can we find new ways to push the field forward. This is obviously something that’s hard to achieve in industrial research but really is bread and butter to academic researchers.

Mary O'Kane using a glove box

This is such an interesting time to be involved in energy research. Everyone, be it industry, government or individuals, understands that solving the clean energy crisis won’t come in one fell swoop, but will be a results of many small changes to our everyday lives. It’s quite satisfying to be working in a field where the importance of small improvements is truly appreciated. If we can increase the efficiency of all solar cells by 1%, that’s a huge amount of energy that we can utilise and would be a very big win.

How has your experience of studying at the University of Sheffield been?

It’s been satisfying, stimulating and of course at times, challenging. Sheffield is a fantastic city and I love working in EPMM – although I am truly missing the University Arms. There’s a lot of inter-group and inter-departmental collaboration at Sheffield and we have so much truly fantastic work going on here. Through my PhD, I have collaborated closely with my industrial sponsor, Ossila. They have been so great and it’s been really useful to see how research can impact and inform industrial decisions. I have also found a fantastic support network of Women in Physics, within my group and the faculty generally, and it’s great to have so many amazing women as peers and role models this close to home.

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