Perovskites: it’s your time to shine

Unravel the potential of perovskite solar cells, featuring an interview with Cambridge’s Dr. Sam Stranks

By Marriyum Hasany

One of the perks of being a part of EPS at the University of Birmingham is the exposure to brilliant minds with exciting ideas on how to use Science and Technology to improve the world. The evening of the 16th of October was no exception, as a Poynting building lecture theatre was filled with intrigued physicists and chemists awaiting a lecture by Dr Sam Stranks on ‘The Future of Perovskites for Solar Power and Lighting’.

Introduced by Professor Martin Freer—the head of the School of Physics and Astronomy—Dr Stranks’ incredible career led him to be awarded the 2012 Institute of Physics Roy Thesis Prize for his PhD thesis, followed by a post-doctoral position at the University of Oxford. Currently, he is a Royal Society Research Fellow and the group leader of StranksLab, at the University of Cambridge, which focuses on semiconductors.

Dr Strank’s lecture gave an insight into current research being conducted on perovskite solar cells and their potential to be used as a more effective source of power than the crystalline silicon solar cells, which are presently the commercial norm. The current system in place for the generation of solar power lacks the efficiency and cost-effectiveness required to realistically be considered as a main source of power worldwide. That is where perovskites come in! Perovskites have not only proven to be more efficient, cheaper and versatile but would also be easy to transport due to their general thickness of just 500 nm, making them incredibly lightweight. They can also be placed on top of current silicon crystalline solar cells to work synergistically. 

Perovskites are a class of material composed of a crystal structure (arrangement of the different atoms within a structure) closely resembling that of the naturally occurring mineral, calcium titanite (CaTiO3), as pictured in Figure 1. The sites of the cations and anions within the structure can be occupied by various atoms and molecules, allowing perovskites to take on a wide variety of properties depending on the combinations of cations and anions used. This variability allowed a team in Tokyo lead by A. Kojima in 2009, to synthesise perovskite nanocrystals made of organic-inorganic materials, with the capability to absorb and convert light energy into power, with lower than 4% efficiency.

Fast forward to today and this conversion efficiency has increased to as high as 21%. The perovskite solar cell is contained within a simple system of a three layered, electrode-perovskite-electrode cell; this cuts down on power losses caused by too many interfaces within the system, and having a continuous crystalline structure rather than a porous structure. The most advanced perovskite cells being tested are made from methylammonium, formamidinium, caesium and lead cations, and iodide and bromide anions, which give them the highest stability and performance. Yet, somehow, they are still relatively easy to prepare.

Varying the ratio and the types of different anions present in the perovskites can determine the colour of the cell, and therefore the wavelength of light they can absorb. This may mean that colourful perovskite solar cells could be placed on buildings to not only generate electricity, but to give buildings like the Muirhead Tower the opportunity to serve an aesthetic purpose within the University of Birmingham’s skyline.

Another potential application of perovskites is their capability to act as light emitting diodes (LEDs). LEDs are semiconductors that emit light when a current is passed through them, as the electrons from the current fall into “electron holes” within the LED, losing energy as they do so. This lost energy is released as photons of light. Perovskites are also able to accommodate electrons in this way; however, this technology comes with its fair share of challenges.

The second part of the lecture mainly focused on the challenges that keep these materials from being used on a commercial scale; the two main areas being the loss of power due to defects within the solar cells, and the movement of ions leading to decomposition of the solar cells, thus significantly reducing their efficiency. However, research is currently underway at full force to combat these issues.  

After the lecture, I had the wonderful opportunity to briefly speak to Dr Stranks. I voiced my concern about the presence of lead in the perovskite cells, to which Dr Stranks assured me that “while we are searching for lead-free absorbers, it’s probably not an issue because the amount of lead is very small, so, if the entire panel washed off, it would still be below the safe levels of lead… and to put it in context, a silicon panel solder probably has more lead than the perovskite solar cells do.”

I asked him about the feasibility of transporting perovskite solar cells to remote communities throughout the world, with frequent power outages, to which he gave me a very hopeful answer: “you could easily put them in spools of rolls and carry them in like that… you could fit them easily on a truck or a plane because the weight is significantly less [than silicon solar panels.]” 

Following that, I inquired about the challenges of upscaling the technology, to which he replied, “we know from many other solid technologies that when you take it over a larger area, the efficiency drops. [This is because] when you have a larger area you’re more likely to have defective regions in the area, which is partly a deposition challenge, but there’s no reason we couldn’t make it more uniform over a larger area, so there’s work being done on that. The other issue is that when you connect different modules, there are electrical losses… Considering silicon solar cells, the record is 27% but most modules operate at about 15–20% maximum even at the best technology.” 

A very interesting question put forward by one of my fellow Chemistry students, Harry Fell, concerned how exactly the researchers begin to identify the cations that should be utilised. Dr Stranks told me that “at least with the 3-D perovskites, it is more limited because it’s a very small site, so you can only fit small molecules, and typically they are best when they are nitrogen or sulphur-rich. There is a bit more leeway with 2-D long chain, and there is a lot of work out there looking at cations… There are some that are working well in the community, but in principle you could screen through many molecules, which is probably a very tedious task.”

Relating to our theme of time, I asked Dr Stranks about a time in his research that had the biggest impact in his life. After thinking about it for a few seconds he said, “I think moving into the perovskite space since 2012, because it was completely unexplored, and the community did not know much about this material… It was serendipitous that I joined in at that time because there was [suddenly] a lot of excitement and it was a new space; there was no textbook or any other paper you could refer to, to check if you were right. So, it was very exciting and it’s probably the time that transformed my view on science and my career!”  

With that being my last question, the night was concluded. However, the research continues, and the prospect of commercialisation of perovskite solar cells carries light and hope for the future. With the potential to cheaply power houses, and easily be transported and installed in rural areas, this much-needed sustainable source of energy could allow underdeveloped areas to thrive; with the added bonus of looking beautiful at the same time. Perovskite solar cells could be a means of averting climate catastrophe; perhaps we are not heading for dark days after all!

From Issue 16