Development and optimisation of thin-film photovoltaic devices (solar cells) for sustainable solar energy

Scientists in the UK’s University of Sheffield’s Electronic and Photonic Molecular Materials Group (EPMM) work on the development and optimisation of thin-film photovoltaic devices (solar cells) for sustainable energy solutions. The advantage of these type of devices is that they can convert sunlight to electrical power with high efficiency, with such materials being easy and cheap to process, potentially allowing ‘sustainable’ solar cells to be manufactured at high volume and low cost. This is important given the rising global energy demand and need for electrification in remote, rural areas, particularly in Africa, where national grids are often over-constrained.

The EPMM Group, which collaborates with the GCRF START grant, focuses on two classes of materials: perovskites[1], [2] and polymer fullerene blends. Industry standard silicon solar cells require very high temperature processing and expensive controlled environments (clean rooms) for their manufacture, whereas hybrid-perovskites solar can be fabricated at low temperatures using liquid-based processing which reduces costs and makes their production easily scalable. However, one of the biggest challenges with these hybrid-perovskites is their reduced stability compared to silicon. The Group is therefore investigating these materials to understand how best they can be optimised for the next generation of solar energy devices.

Fig 1. This figure shows both small and large area perovskite PV devices fabricated by spray coating. The figure also shows a SEM cross-sectional image through a typical device. Part (a) shows a photograph of small and large-area fully spray-coated perovskite solar cells. Parts (b,c) show a cross-sectional SEM image of complete devices incorporating a spray-cast perovskite layer. The device in part (b) utilises spin cast SnO2 and spiro-OMeTAD layers whereas the device in part (c) is fully spray-coated. Source: Bishop, J.E., Read, C.D., Smith, J.A. et al. Fully Spray-Coated Triple-Cation Perovskite Solar Cells. Sci Rep 10, 6610 (2020).

Investigating affordable energy solutions with the GCRF START grant

In their research, the EPMM scientists have benefitted from the GCRF START grant, enabling them to get involved in new international collaborations as well as access the UK’s national synchrotron, Diamond Light Source (Diamond), where they use synchrotron techniques such as X-ray scattering to research halide perovskites, a potential material for the next generation of low-cost photovoltaics.

“Using Diamond allows us to explore the structure of these materials at length-scales corresponding to atomic and molecular bonds, we have found that understanding the structure of these materials is absolutely critical in developing our understanding of how they ‘work’ in devices. Ultimately, this understanding helps in developing new materials or new ways to process existing materials to get optimum performance (efficiency) out of our solar cells.”

Prof. David Lidzey, GCRF START Co-I and Director of the EPMM Group
From left to right: Adam Shnier from the Energy Materials Research Group at the University of the Witwatersrand, South Africa, and Rachel Kilbride, Joel Smith and Mary O’Kane from the Electronic and Photonic Molecular Materials Group at the University of Sheffield, UK. The scientists are conducting wide-angle X-ray scattering measurements at the UK’s national synchrotron Diamond Light Source (Diamond). The students are using one of Diamond’s 2D X-ray detectors to monitor in-situ crystallisation and degradation processes with perovskite thin film samples held in an environmental chamber. Photo credit: Onkar Game. ©University of Sheffield / EPMM Group

The GCRF START grant has also funded Dr Onkar Game’s one-year Postdoctoral position in the EPMM Group during which time he has both undertaken his own experiments on perovskites and worked very closely with various PhD students in the Group. Prof. Lidzey reports that Dr Game’s input has been invaluable, this flexible work-pattern maximising the amount of research using the funding available.

“The GCRF START grant provided me a unique opportunity to make use of world class structural characterisation facilities at Diamond” says Dr Game. “Working with PhD student Joel Smith, we utilised the in-situ Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) facility at the I22 beamline in Diamond to identify the factors affecting the degradation of perovskite crystal commonly used in perovskite-PV devices[3]. This helped us to tune the composition of perovskite crystal to make it more stable towards moisture and light induced degradation. I also enjoyed setting up the in-situ measurements with Joel on understanding of solvent-induced liquification and crystallisation of perovskite using GIWAXS on Diamond’s I07 beamline,” Dr Game adds.  

From left to right: Prof. David Lidzey and Dr Onkar Game from the University of Sheffield’s EPMM Group, UK. The scientists are investigating perovskite degradation using an environmental chamber and 2D X-ray detector on the I07 beamline at the UK’s national synchrotron Diamond Light Source. Photo credit: Joel Smith. ©University of Sheffield / EPMM Group

Collaborative research on Perovskites: the effects of composition and temperature

Dr Claire Greenland completed her PhD in the EPMM Group supervised by Prof. Lidzey. She has been working on perovskites and has been particularly interested in how the structure of such materials are affected by their composition and local temperature. This work was done in close collaboration through the GCRF START network with the Energy Materials Research Group at the University of the Witwatersrand (Wits), South Africa, led by GCRF START Co-I, Prof. Dave Billing. In this research, Claire studied one type of popular perovskite called a ‘mixed cation’ perovskite, and used X-ray scattering to characterise the structure of the perovskite as it was cooled down to very low temperature (-190 °C). She also measured the ability of the perovskite to absorb and emit light over the same temperature range. In these studies, Claire was looking for changes in the crystal-phase as the temperature was changed.

Understanding such processes is important, as the temperature at which some phase changes occur are within the expected operational temperature of the solar cell and understanding the effect on the properties of the materials in the solar cell forms a critical part of the understanding of how such materials (and devices) work.

“Interestingly, the structure of the perovskite crystal is not fixed, but can vary as a function of temperature, being ‘cubic’ at room temperature and starting to change to a ‘tetragonal’ phase around -13 °C, with a second low-temperature phase identified at -180 °C,” Prof. Lidzey explains. “Through careful analysis, these changes in crystal structure could be correlated with changes in the optical properties of the perovskite.”

This research was supported with assistance from Wits University scientist, Adam Shnier. Adam met members of the Sheffield team at a GCRF START meeting in 2018, hosted by the Energy Materials Research Group at Wits.

“Adam Shnier provided us invaluable support in the analysis of the X-ray scattering data, allowing us to understand how the crystal structure changed with temperature,” Prof. Lidzey says.With support from the GCRF START grant, Adam was able to travel to the UK in 2019 and assist us in some scattering experiments performed at Diamond and became an important part of the team.”  

“At the 2018 GCRF START meeting, I met a PhD student, Joel Smith, from the University of Sheffield who works on the same type of materials,” says Adam. “Joel and his colleagues are experienced at making high quality, efficient devices; while at Wits, we are experienced in making other materials that can be used for these devices and studying their crystal structures. As the 2019 GCRF START meeting was being held in the UK, we planned a research visit where I would spend a week working with Joel and his colleague Dr Onkar Game in Prof. David Lidzey’s laboratory at Sheffield. The purpose was to share technical knowledge. They were more than happy to share their experiences with these materials which instilled me with a plenty of ideas and information to share with my colleagues at home in South Africa.”

Adam Shnier from the Energy Materials Research Group at the University of the Witwatersrand, South Africa, visiting the UK’s national synchrotron, Diamond Light Source. Photo credit: Joel Smith. ©University of Sheffield / EPMM Group

The research was published in January 2020[4] and formed a very important part of Claire’s PhD thesis. Claire has since passed her PhD viva and has come back to Sheffield in the position of ‘University teacher’. While she is not currently working on perovskites, her knowledge of materials physics and experience in understanding complex phenomena are proving invaluable in teaching electromagnetism to 1st year undergraduate students and will also be useful in the 2nd year lab experiments that she is developing and running, says Prof. Lidzey.

‘The GCRF START collaboration enabled me to collaborate with academics from Wits University in South Africa, which greatly enriched my work due to their expertise in X-ray diffraction and crystal structure,” Claire explains.  “I collaborated mainly with Adam Shnier, who was able to computationally model the X-ray diffraction data taken at a range of temperatures on mixed cation perovskites. This modelling revealed the temperature dependence of a variety of crystal parameters, which not only allowed us to identify phase transitions in these materials but also to correlate crystal structure with photoluminescence properties.”

“I really enjoyed working with Adam, and his insight on all things related to crystal structure and phase was invaluable to my work,” Claire adds. “These studies shed light on the fundamental properties of perovskites and how these vary as a function of temperature – such studies are a key part of solar cell optimisation, because real world solar cells must operate under a wide range of temperatures. So, it’s really cool to know that this project was part of the push towards cheap and efficient solar cell materials, which are an essential part of tackling climate change.”  

Dr Claire Greenland from the University of Sheffield’s Electronic and Photonic Molecular Materials Group, UK. ©University of Sheffield / EPMM Group

Perovskite ‘healing’ for high-speed manufacture processes

PhD student Joel Smith has also been working on perovskites. Here, however, he has been interested perovskite recrystallisation processes using solvent molecules. In this process, a perovskite film is exposed to a solvent gas, with this exposure causing the solid perovskite film to melt into a different solid or liquid form. Once the gas is removed or when heated, this material crystallises back into a ‘healed’ perovskite. The advantage of this process is that the quality of the perovskite film can be substantially improved. Practically, this could be used to improve the quality of perovskite films deposited in a high-speed manufacture process such as spray coating.

The first part of this work was led by Dr Onkar Game and investigated how treatment with one type of solvent had some unexpected effects on the perovskite film microstructure. This included measurements on thin films at Diamond to understand how these changes in microstructure affected the perovskite’s ability to withstand different challenging environments3. As part of his PhD, Joel undertook experiments at Diamond where he used X-ray scattering to monitor the very rapid changes in the structure of the perovskite film as it turned into a liquid and then back into the perovskite.

“Here we were able to resolve crystal structures of new intermediate compounds that formed in the liquid, and we could evidence the improvement in crystal structure caused by the healing process,” explains Prof. Lidzey, “Importantly, we also showed that this process could be enhanced by changing the temperature at which healing took place.”

Joel is currently writing a paper on this work which has formed part of his PhD thesis. He is set to start his Postdoctoral research in Prof. Henry Snaith’s group at the University of Oxford, working on advanced perovskite solar cell devices.

“Bringing together expertise from our different institutions in the UK and Africa through collaborating with the GCRF START grant has allowed us to investigate the crystallisation behaviour and stability of perovskites in new ways at Diamond Light Source,” says Joel. “More generally, we have been able to assist each other in fabricating, characterising and understanding these materials by sharing experience and facilities. The wider START community has been valuable as a mutually supportive network for us to develop as independent researchers, and most importantly, to grow the synchrotron research community in Africa.”

Joel Smith from the University of Sheffield’s Electronic and Photonic Molecular Materials Group, UK. Photo credit: Onkar Game. ©University of Sheffield / EPMM Group

Commenting on the need for affordable, renewable energy solutions in Africa and the importance of collaboration to tackle global energy challenges, Prof. Billing said,

The synergy in GCRF START collaborations of having people from different backgrounds tackle a problem makes the solution more robust. Creativity is important to most Africans, and we need to be involved in these creative solutions. Also, if you co-develop through collaboration, you have a sense of ownership which is what we set out to do through the GCRF START grant.”

“Everything costs energy, fundamentally,” Prof. Billing adds. “Silicon uses a lot of energy in its making to make solar cells so I think the best we can do as humans is look at remediation and balance. The GCRF START grant asks us to consider questions in our research around the global challenges like: am I using components that are sustainable? Are we using elements which are abundant, affordable, and environmentally compatible? If you think about a rural village which is cooking using wood charcoal, they are energy poor and lighting will be paraffin or candles. If you can find a cheap source of energy there and you can bring in lighting, that is life changing!”

Traditional rondavel house in a rural South African village. Photo Credit: Rebekka Stredwick. ©Diamond Light Source
Adam Shnier at the University of the Witwatersrand taken in front of a Bruker D8 diffractometer which he uses in the analysis of thin film materials. Photo credit: Adam Shnier. ©Diamond Light Source

Read more about the Sustainable Development Goals for Energy (SDG 7).

[1] Here, “perovskite” is a name for a class of crystalline material, and there are many different combinations of starting materials that can be used to make a perovskite. Some of these materials absorb sunlight more efficiently, and others have greater environmental stability (both important characteristics for practical applications of solar cells).

[2] Bishop, J.E., Read, C.D., Smith, J.A. et al. Fully Spray-Coated Triple-Cation Perovskite Solar Cells. Sci Rep 10, 6610 (2020). This work demonstrates the possibility for spray-coating to fabricate high efficiency and low-cost perovskite solar cells at speed.

[3] Onkar S. Game, Joel A. Smith et al.  Solvent vapour annealing of methylammonium lead halide perovskite: what’s the catch? J. Mater. Chem. A, 8, 2020, 10943-10956 DOI: 10.1039/D0TA03023F 

[4] Greenland, Claire, Adam Shnier, Sai K. Rajendran, Joel A. Smith, Onkar S. Game, Daniel Wamwangi, Graham A. Turnbull, Ifor DW Samuel, David G. Billing, and David G. Lidzey. “Correlating Phase Behavior with Photophysical Properties in Mixed‐Cation Mixed‐Halide Perovskite Thin Films.” Advanced Energy Materials 10, no. 4 (2020): 1901350 (