REVIEW https://doi.org/10.32386/scivpro.000004 The Path to Perovskite on Silicon PV Henry J. Snaith1*, Samuele Lilliu2 Henry J. Snaith is Professor of Physics in the Clarendon Laboratory at the University of Oxford and Fellow of the Royal Society. He has pioneered the field of perovskite solar cells and published more than 300 papers. He is the founder and Chief Scientist Officer of Oxford Photovoltaics, which holds the largest perovskite patent portfolio worldwide and focuses on developing and commercialising perovskite PV technology. In this interview, he discusses the present status and future prospects of perovskite PV. The interview is available at https://youtu.be/sbe9Z5oEs5o. Notable Achievements Samuele Lilliu (SL): How did you become one of the most influen- tial scientists in perovskite solar cell research and what have been the most notable achievements to date? Henry Snaith (HS): Perovskite solar cells are simply solar cells containing metal halide perovskites (Figure 1).1,2 I first became aware of metal halide perovskites in 2009, when visiting some colleagues and collaborators in Japan.1 The first paper using metal halide perovskites in solar cells had just been published by the group of Prof. Tsutomu (Tom) Miyasaka (Toin University of Yokohama, Japan).3 Tom gave a presentation on this work and it was very impressive. The most remarkable aspect was that he just took two salts, led bromide (PbBr2) and methylammonium bromide Figure 1 | Prof. Henry Snaith holding a 6 inch perovskite on Si wafer. (CH3NH3Br), put them together in a solvent, and spin coated a film, which crystallized immediately during spin coating.3 At the same time, back in Oxford, I would been playing around with quantum crystallizing the perovskite material, what we were getting was not dots led sulphide (PbS) nanoparticles.4 They were extremely tedious just small nano-crystals but more of a polycrystalline material, to synthetize and it took a long time, weeks of work, to make devic- with very long range crystalline order. We had a suspicion that we es. Whereas this perovskite material appeared to just form instantly. were also getting charge conduction in the perovskite material, so That was something that I found quite interesting. we made some test cells swapping out the porous TiO2, which is an 10 I subsequently returned to the UK and managed to get a collabo- electron conductor, with porous alumina (Al2O3). We tested these rative grant with one of my colleagues Takurou Murakami and I sent cells to see how they worked under the sunlight. We were not ex- a PhD student back to Japan to learn how to make these perovskite pecting highly functional solar cells because they had an insulator materials. Back in Oxford we managed to integrate them into what in place of one of the semiconductors, but surprisingly they actually 5-9 we were working on at the time: dye sensitized solar cells (DSSCs). worked better than the cells without TiO2 and we got a very big im- The device that Tom Miyasaka had developed3 was based on a po- provement in voltage.10 This was the first real breakthrough, where rous TiO2 anode, where the anode is sensitized, typically with a dye. we realized the perovskite material did not just act as a light absorb- He used small perovskite nano-crystals and then infiltrated the de- er but it also acted as a charge conductor in the cell.11 vice with an electrolyte.3 The big problem with this device was that Following on from that work, in close succession, we started in- the electrolyte dissolves the perovskite crystals, so the device lasted vestigating whether we really needed this porous alumina scaffold. seconds. I think this is probably one of the reasons why that work By making it thinner and thinner we improved current and device was not instantly followed. It looked like it was quite interesting, but efficiency at extracting charge.12 When we looked at a cross section just led to unstable cells. of these devices, they had a solid absorber layer of perovskite on top Whereas, in contrast, when we replaced the electrolyte with a of a thin porous scaffold of alumina.12 This told us already that the solid-state organic hole-conductor into the perovskite device, we solid perovskite material could conduct charge out of the device. obtained films that would be stable for a thousand hours under sun- Now that might not seem very surprising, but at the time the light, which was a massive step change. paradigm was that as a solid absorber material you could either have The big shock, if you like, or the first shock, was just our first a very expensive crystalline semiconductor with low defect density, batch of devices were already 6% power conversion efficiency. At or a cheap material that was easy to process from solution at low the time, DSSCs had hit a plateau of about 5% and for 3-5 years temperature, but with a lot of electronic disorder.13 The cheap ma- we could not get them any better. These perovskite devices were al- terials would not work as well as the expensive materials, and to ready more efficient on the first attempt and of course they rapidly get them to work you had to blend multiple materials, typically one stepped up. The first real surprise was that they worked instantly. material that would conduct electrons and one that would conduct Usually when trying a new material if you can get 1% efficiency you holes.14,15 In the case of the DSSC, the absorber was sandwiched are pretty happy. between these two materials in a rather complex meso-struc- Following on from that, with the way we were processing and tured or nano-structured device.16-20 The way of thinking was that 1*Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, UK. Contact Henry. J. Snaith. 2The UAE Centre for Crystallography, UAE SCIENTIFIC VIDEO PROTOCOLS | www.scivpro.com REVIEW SCIENTIFIC VIDEO PROTOCOLS materials either fell into a category of high disorder, but needed to be and the grain boundaries. nano-structured, or low disorder and would work in a solid crystal- The other component in the efficiency improvement, has been line film. The perovskites had the best of both worlds. They worked controlling the crystallization,34-37 working out good ways to process as a solid crystalline film and they were very easy to process at low these perovskite materials. temperature; so they were clearly in a different category, a league of The perovskite absorber itself in principle has a pretty low defect their own. density.38 There are not many defects and the defects that exist are Moving on from that, we demonstrated quite clearly that they not absolutely detrimental.39,40 They do not kill the performance, but could work as a solid thin film and in fact now the embodiment of limit the open circuit voltage.41 One way to improve the voltage is a perovskite solar cell is that on one side you have a charge selective to passivate the crystal. Here the term ‘passivation’ is used in a very material, maybe an n-type organic material, you have the perovskite broad term, because we do not really understand what is happen- absorber as a solid polycrystalline film (about 1 micron in thick- ing chemically. There are lots of routes of post-processing the films ness), then you have a p-type material (an organic charged conduc- that improve, for instance, the radiative efficiency and the fraction of tor or a metal oxide), and an electrode on the top. So it is a very sim- light re-emitted from the perovskite material.42-46 What we indicate ple sandwich structure device that we call a planar heterojunction or with the term ‘passivation’ could be recrystallization of the surface, double heterojunction.21,22 for example.47 Understanding the surface chemistry, the exact chem- Since those first few publications in the end of 2012, starting istry at the grain boundaries and in the buried interface is really im- from 2013, the research field of perovskite solar cells has absolutely portant, because this has in part driven the performance improve- exploded. To put it into context, in 2012 there were four publications ments.48 However, there is still a massive scope for further advances. on perovskite solar cells, in 2013 there were about sixty, last year in We are currently at 23% efficiency for record cells in labs, but actu- 2017 there were about 2,500, and this year there is probably going to ally there is nothing stopping us towards 30% efficiency in a single be 5,000 by the end of the year.23 Publications in the field are growing junction cell. That will happen by achieving better control of the sur- exponentially now. We were very fortunate to be there right at the face properties, in my view, of the perovskite absorber. beginning and we have managed to stay in the front pack, ahead Coupled with that, there has been a lot of tuning and optimiza- in certain aspects of the research. Obviously we have been lucky to tion of the contact materials.49,50 What is quite surprising is that we be riding this wave and hopefully we will be still riding this wave have not had to develop new contact materials. We just had to select through to fruition. So, the explosion of the field is the reason why materials that were already available either from the organic elec- we get so many citations for the work that we do.