“The future of all technologies is ‘smart’ and this is why Organic Solar Cells (OSCs) research is vital. OSCs can act as efficient energy sources in these contexts, given the advantages they offer over current silicon solar cells due to their unique properties – from abundant materials to scalable production processes. The goal is to meet sustainable development goals (SDGs) and the growing global demand through innovative, world class solar energy research, in which Britain is a leading player. The GCRF START grant has provided us with access to these important research opportunities.”Dr Pascal Kaienburg, University of Oxford, UK
As countries move toward rebuilding their economies in the wake of the COVID-19 pandemic, the UN and COP26 call for recovery plans to create “a profound, systemic shift to a more sustainable economy that works for both people and the planet”. OSCs are solar cells made primarily from earth-abundant carbon-based compounds. They have great potential to significantly contribute to the COP26 vision of a “cleaner, greener, energy resilient” future through the development of the next generation of affordable renewable solar energy sources, from small scale-applications to large power stations around the world. This could be to supply the grid with electricity or integrated into off grid solutions and combined with storage to unlock novel applications which current silicon solar cells struggle to or cannot provide.
To this end, researchers from the University of the Witwatersrand (Wits) in South Africa and the University of Oxford in the UK are investigating the OSC microstructure to understand how they perform under varying conditions to find ways of enhancing the OSCs’ performance. Funded by the GCRF START grant, the aim is to gain unique insights at unprecedented resolution using state-of-the-art synchrotron techniques at the UK’s national synchrotron, Diamond Light Source (Diamond), amongst other techniques and facilities.
The UK has a strong record of solar cell research and the Department of Physics at the University of Oxford is an essential part of this research landscape. Led by Professor Moritz Riede, who is also a GCRF START Co-Investigator (Co-I), scientists in Oxford’s AFMD group conduct research into vacuum processing of OSCs. The precedent for upscaled vacuum fabrication of the related multi-billion-pound organic light-emitting diode (OLED) industry highlights the potential of vacuum-based processing for a range of ‘smart’ applications including OSCs. Indeed, leading companies commercialising OSCs apply vacuum deposition in their ‘solar film’ PV solutions.
To meet this growing demand for vacuum-based research opportunities, a National Thin-Film Cluster Facility for Advanced Functional Materials is currently being installed at the University of Oxford funded by the Engineering and Physical Sciences Research Council (EPSRC), the Wolfson Foundation and the University of Oxford, with Prof. Riede as the Technical Lead. The facility will place the UK at the centre of the development of next-generation materials and devices for applications in energy, photonics and electronics and act as an epicentre for novel thin film development within the UK.
Quantifying Organic Solar Cell charge transport
OSCs can be readily processed from solution or created by vacuum deposition. They have the advantage of being flexible, lightweight, affordable in production, and perform well under indoor lighting and elevated temperatures. With the potential to be made semi-transparent and to selectively absorb certain colors in the sun spectrum, they can, for example, enable energy-harvesting semi-transparent office windows or solar energy to be harvested and at the same time grow crops with the transmitted light. These advantages make OSCs suitable for integration into building facades and lightweight roof constructions – such as greenhouses – as well as to power sensors and transmitters for the rapidly evolving Internet of Things (IoT). However, with increasing OSC performance, the transport of charges that carry the energy absorbed from incoming light to the extracting contact in the solar cell has become a bottleneck for the technology over the past years that researchers across the world seek to tackle.
Dr Pascal Kaienburg is a GCRF START-funded Postdoctoral Research Assistant (PDRA) in Prof. Riede’s AFMD group making great strides in his research on quantifying organic solar cell charge transport, amongst other research areas. The aim of Pascal’s research is to link OSC opto-electronic characterisation with the microstructure of the light-absorbing thin films. This involves fabricating OSCs by thermal evaporation in vacuum in corresponding deposition chambers at the University of Oxford together with collaborators testing the microstructure at various synchrotron and neutron facilities worldwide including Diamond, which sits at the heart of the GCRF START grant.
“Solar energy has to be cheap enough to install where needed and useable with ‘smart’ applications, as well as large scale integrated options,” says Pascal. “Ultimately, with my research we want charges to become faster and reduce losses for better systems to make solar cells more efficient and the technology more competitive. Currently, there are silicon solar parks and silicon solar panels on buildings around the world, but to tap into new markets and applications, we need cost effective technologies with unique physical properties and organic solar cells offer these.”
This research involves quantifying and investigating different behaviours of the carbon-based, i.e., organic molecules in an OSC, such as better or worse charge transport, the nanoscale morphology in which the mix of molecules arrange, and most importantly, the interplay between the two. To gain control over the OSC’s behaviour, deposition conditions such as mixing ratio and deposition temperature are varied and optimised. Diamond in the UK, the Advanced Light Source in Berkeley, USA, the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, and the UK’s ISIS neutron and muon source are being used to test OSCs in-situ and ex-situ to generate microstructural data. The data gathered in opto-electronic measurements allows Pascal to quantify the charge transport, before linking them with morphological data at Diamond regularly acquired by his colleague, Dr Thomas Derrien, who is also part of the START network; then working with chemists to design new organic solar cell molecules using this data to guide the design.
“The data looks good. I am not sure yet when we go to publication, but it won’t be long,” Pascal adds. “I’m very grateful to the GCRF START grant which has funded me to do this research and boosted my skills at the same time. Engaging with experts on microstructure characterisation has taught me and also allowed me to publish as co-author on other aspects of organic solar cells.”
Understanding how organic solar cells organise themselves and the effect on performance
Dr Thomas Derrien was a PDRA at Diamond from 2018-2021. His project focused on understanding how OSCs organise themselves and how this affects the performance of the solar cells. This work in collaboration with Prof. Riede’s group, made extensive use of an evaporation chamber, MINERVA, developed at Diamond. MINERVA enables researchers to thermally evaporate organic solar cell molecules while under synchrotron X-ray illumination enabling X-ray scattering images to be collected as the molecules are deposited on surfaces. From the X-ray scattering images, the structure, orientation, and crystallinity of the molecules can be deduced, all of which can have effects on solar cells performance. Another aspect of the project was to upgrade the MINERVA chamber to be include a third evaporation source in order to allow the study of more complex molecular mixtures and multilayer structures.
New UK-Africa solar energy research collaborations through the GCRF START grant
With the GCRF START grant, researchers like Pascal, Thomas, and Prof. Riede have teamed up with their counterparts in Africa, providing new perspectives on renewable solar energy research across different continents and research institutes. For example, the scientists from Africa contribute novel interesting materials that the UK scientists have not yet explored, and the UK scientists share knowledge about characterisation techniques with their African collaborators.
Dr Francis Otieno and Professor Daniel Wamwangi – both originally from Kenya – are based at the University of the Witwatersrand (Wits), South Africa. A member of the Energy Materials Research Group at Wits, which is led by GCRF START Co-I, Professor Dave Billing, Francis is a GCRF START funded Postdoctoral Research Fellow (PDRF) exploring the fabrication and testing of thin films-based solar cell devices including organics, perovskites and dye sensitised solar cells. In addition, he is looking into ways of enhancing their performance through nanoparticles technology such as plasmonics and spectral conversion thin films – research that is aimed at realising an efficient, cheaper source of solar energy and device-making for local and global markets. Prof. Wamwangi focuses on supplementary light management schemes and cost-effective materials, including organic molecules based on polymers and inorganic materials (metal halide perovskites – a hybrid between inorganic and organic materials, which can be solution- or vacuum-processed).
The GCRF START grant has been instrumental in exposing both scientists to expertise in the characterisation of OSC devices, such as evaporated fabrication techniques used by the AFMD Oxford University researchers, and access to synchrotron techniques at Diamond. The learning is mutual and in 2019, the grant enabled the Oxford University researchers to visit the University of Cape Town, South Africa, for a joint Energy Materials workshop. There they learnt more about African perspectives on Solar energy and other aspects of energy materials research, sharing their learning and ideas with their African colleagues.
“The GCRF START grant is very interesting because most research grants are focused exclusively on the science but START has the additional key element of capacity building both in Africa and the UK,” says Thomas. “It makes you think as a researcher about different angles on research… and allows us to build a network of collaborators. Pascal and I are in different institutions, but we have been able to work together and learn from each other through START, much like the collaborators we work with in Africa. Ultimately, the need for solar cells is huge in Africa so there is a large potential market for the UK to engage with. The potential to deploy the solar cells in Africa is much, much bigger than in Europe due to more sunlight, less existing electricity coverage and a large and growing population.”
In another example, Mohamed Abdelaal – an MSc. student from Egypt’s Ain Shams University in Cairo – was seconded in 2020 to the AFMD group in Oxford for several weeks. Despite the disruption of having to cut short his stay due to Covid-19 pandemic restrictions, he was able to conduct experiments at the University of Oxford and Diamond prior to returning home, with access provided by the GCRF START grant. Mohamed learnt new experimental techniques and applied computer modelling to explore the potential of simulating growth to better understand solar cell microstructure and how it affects performance. As a result, Mohamed has a joint publication from this work.
“The GCRF START grant allows us all to do better research, and with Daniel and Francis we collaborate on through several characterisation techniques,” Pascal explains. “By learning from each other’s knowledge and perspectives – including joint research visits and co-authoring new projects and publications – START has initiated new UK-African research opportunities which we aim to continue.”
Beyond ‘GCRF START 1’: creating a legacy of solar energy solutions for global challenges
To ensure the continuation of vital research collaborations and networks supported by ‘GCRF START’, follow-on opportunities are being explored. One option is through access to facilities like the new National Thin-Film Cluster Facility for Advanced Functional Materials, which will process a range of advanced functional materials including those for OSC and perovskite solar cells. Building on the success of the GCRF START grant, collaborators from this project and Dr Celine Omondi and Dr Victor Odari (MMUST, Kenya) – new partners from the African continent – are piloting how African countries can best benefit from these new capabilities established in Oxford.
Solar cells research investigated through START can be dominated by those with access to the best equipment, which is usually prohibitively expensive for most. With this new facility in Oxford, the goals of a newly commenced GCRF-funded UK-Africa project – ‘A Foundry for Research into emerging Photovoltaic Materials’ – are twofold: a) continue to investigate key physical questions on perovskite and organic solar cells; b) pilot a foundry model – aka “mail-order” for bespoke samples designed in Africa and made at this facility before being sent back to Africa – to see whether this concept is feasible and could be implemented in a similar pan-African research facility located on the African continent.
“I’m really excited about this project, which should demonstrate that something that is state-of-the-art in computer-chip manufacturing – the foundry model – works for research in advanced functional materials as well. We hope that such opportunities will benefit research in Africa and bring both economic and social benefits down the line, as well as being a stepping-stone to future larger projects led by a thriving and growing research community in Africa,” says Prof. Riede, the Principal Investigator of Oxford University’s Foundry project.
In another example, being part of the START collaboration has helped Francis to win a British Council Newton travel grant, allowing him to visit the AFMD group in Oxford for a period of six weeks later this year (2021). This would not have been possible without the skills and opportunities received through the GCRF START grant, enabling him to ‘give back’ to the next generation of students following in his footsteps, as Francis explains below,
“Not only has GCRF START funded my two and a half years’ Postdoctoral Research Fellowship at the University of the Witwatersrand, but it has also really exposed me to new skills, with access to advanced equipment at Diamond Light Source, the Advanced Functional Materials and Devices Group at the University of Oxford, and the Materials Physics Group at Sheffield University in the UK. As a GCRF START PDRF, I was able to buy research materials and made and strengthened South Africa-UK collaborations. But the benefit and legacy does not stop with me. Finding alternative cheaper sources using locally available materials such as organic polymers forms the basis of my research. It is this learning that I pass on to the next generation of scientists. My findings form the basis of my solar cell technology teaching for undergraduate and postgraduate students and research fellows back home in Africa.”
Post-pandemic recovery plans and ‘smart’ solar energy solutions
To address the planetary emergency, “post-pandemic recovery plans need to trigger long-term systemic shifts that will change the trajectory of CO2 levels in the atmosphere” (UN SDG 13 – Climate Action). This is where ‘smart’ solar energy solutions can play a role, says Pascal. Almost all applications use sensors in manufacturing and increasingly in domestic contexts, requiring a decentralised power source for these smart applications. The aim is to move away completely from externally charged batteries and power the sensors with solar cells, although integrated battery storage backup is possible, with batteries charged by OSCs.
Without the need for complex wiring and the frequent changing of batteries, OSCs could make the overseeing of manufacturing processes using sensors more efficient, as well as the heating of homes and workplaces, especially where hundreds of sensors are located throughout a building. From mounting OSCs on smoke detectors to integrating them into the fabric of electric vehicles to extend their travel range, to making a significant impact in the ‘power market’ – the applications of OSCs are numerous and so are the opportunities to reduce carbon footprints while improving lives and livelihoods.
“It makes sense to produce customised solutions locally which in turn creates jobs,” says Pascal. “For example, integrating OSCs into building facades requires customisation, which is an advantage as customisation increases the value. Lightweight OSCs are particularly advantageous in countries with less solidly constructed buildings and housing, particularly in sunny, warmer climates in Africa and South East Asia where thin walls may not bear the heavier loads of silicon solar cells. Another exciting option of growing interest is agricultural Photovoltaics (PV). Solar panels are located on agricultural fields, providing dual use of the area. Covering entire fields with semi-transparent or selectively absorbing OSCs, crops can still be grown while solar energy is harvested from the sunlight.”
With research progress and cost reductions due to growth of production (something that silicon solar cells, the incumbent, have well demonstrated), OSCs have the potential to eventually produce electricity at lower cost than silicon solar cells in large scale applications. This offers an exciting future of inexpensive electricity that can be scaled to the dimensions needed to make a difference in the world’s energy system.
Read more about the UN’s Sustainable Development Goals here
 The UN Secretary-General has proposed six climate-positive actions for governments to take as they build back their economies and societies. These include investing funds “in the future, not the past, and flow to sustainable sectors and projects that help the environment and the climate.” Also see: https://sdgs.un.org/2030agenda
 Thomas Derrien is a GCRF START member and in his role at Diamond was funded as a Postdoctoral Research Assistant by the GCRF START grant.
 Prof Dave Billing is Professor in the School of Chemistry and Co-PI of the Energy Materials Research Group at the University of the Witwatersrand (Wits), South Africa, and also Assistant Dean in the Faculty of Science at Wits.