Investigating energy materials for efficient and cost effective conversion of sunlight into electric
- Diamond Comms
Alternative energy at increased efficiency with lower cost and improved environmental footprints has a domino effect on socio-economic landscape.
[This article is supplied by the START (Synchrotron Techniques for African Research and Technology), a collaborative project between scientists in Africa and the UK working together on research using synchrotron science.]
The case for localised energy generation
The rising global demand for energy and the depletion of fossil-based fuels has increased the research focus on new materials which could contribute to efficient localised energy generation, particularly in remote areas with a scarcity of electricity.
Of the nearly 1 billion people globally functioning without electricity, 50% are found in Sub-Saharan Africa alone (UN, 2019), where the focus on finding localised energy generation solutions is a welcome and timely opportunity, especially in schools and clinics located in rural areas far from the existing electricity supply or grid.
Investigating energy materials for efficient conversion of sunlight into electricity
Dr Daniel Wamwangi is a Co-Investigator within the GCRF START programme conducting fundamental research into energy materials for solutions which could one day revolutionise the energy landscape across Africa and beyond. Originally from Kenya and based as Associate Professor in the School of Physics at the University of the Witwatersrand in South Africa, Professor Wamwangi’s research on energy materials focuses on energy conversion with the aim of converting sunlight into electricity in the most efficient and cost effective way.
“The availability of alternative energy at increased efficiencies with lower costs and improved environmental footprints has domino effects on the social economic landscape,” explains Dr Wamwangi. “Benefits such as pumped water supply and purification through local solar power generators, solar based lamps and solar powered electronic devices such as cell phones could radically improve the living standards of populations in these areas.”
Finding the right energy conversion parameters
First the right parameters must be established and tested; only then can prototyping begin. There are two prongs of energy conversion that are of prime interest and focus, namely photovoltaic and thermoelectric conversion. The former involves harnessing sunlight to produce electricity, while the latter entails the conversion of heat into electricity.
The conversion of sunlight into electricity is popularly known as photovoltaics and the devices that enable the conversion are known as solar cells.
“The performance parameters that determine the commercial viability for sustainable renewable energy especially in photovoltaics include efficiency of conversion, lifetime and costs,” explains Dr Wamwangi. “In the current materials-science landscape, Silicon (Si), an element in the periodic table, has exhibited the highest efficiency of light to electricity conversion at 28%. However, the costs of processing are prohibitive and thus alternative materials and technologies are crucial to replace Si.”
This is the crux of Professor Wamwangi’s research in which cost-effective materials such as Organic materials (organic polymer based solar cell) and Inorganic materials (halide perovskites – a hybrid between inorganic and organic materials, which can be solution-based), and control of solar energy in materials (supplementary light management schemes) are investigated and developed. 1
“Hybrids are found in most cases in powder form and retain their properties even when in solution,” Professor Wamwangi explains. “They can be combined from a composite solution that is photo-responsive, which means they can take light and produce electrons and positive charges (holes) leading to voltage and electrical current.”
Supplementary light management schemes
“Supplementary light management schemes are one way to improve efficiency and this is where a lot of solar research these days is focusing. If we look at the energy budget not all the light from the sun is useful to solar devices because every material has a unique energy value,” says Dr Wamwangi. “This means that some of the light that is not used would be wasted. Therefore, we combine the energy of the light particles from the wasted light in order to increase the energy of the wasted light so it can be used by the solar cell. This increases the useful light that can be absorbed by the materials in the device.”
Light management uses nanostructures to modify the emission of light within the visible spectrum with a select number of atoms in a patterned manner to capture light (plasmonics), increasing (up conversion) and decreasing (down conversion) the energy from the sun, as Professor Wamwangi explains,
“When the energy from the sun is very large it produces hot electrons and energy is lost in the form of heat; when the temperature increases, the efficiency of the solar cell decreases, so this energy has to be decreased through the down conversion as part of a light management process.”
Organic-based solar cell devices operate on an entirely different conversion mechanism involving a combination of two interconnected materials (donor and acceptor of electrons) with entirely different electrical properties2. More recently, a hybrid of organic and inorganic materials also popularly known as Halide perovskites is intensively studied due to its associated high photo conversion efficiency.
These low-cost energy materials are predicted to replace Si-based photovoltaics through the addition of a third component to form a ternary system (three interconnected networks of materials) in consumer electronics. However, the production of current in these materials is dependent on the structure at the micro level (as previously investigated by Professor Wamwangi’s PhD student Dr. F. Otieno et al 3).
Access to state-of-the-art synchrotron techniques, collaboration and upskilling
This is where GCRF START makes a significant difference, providing researchers like Professor Wamwangi and his colleagues with access to the UK’s Diamond Light Source synchrotron and sophisticated techniques known as GIWAXS and GISAXS (Glancing incidence Wide/Small Angle X-ray Scattering). These techniques are used to study the arrangement of molecules or atoms in a solid to nanometer length scales in order to probe the type of microstructure of these materials to elucidate the electron (negative charge) and hole transport (positive) charge within the network.
“The factors that determine the microstructure of interconnected networks include temperature and time during processing,” says Professor Wamwangi, “Using the facilities available at Diamond through START, as well as expertise within the START family, we can correlate the microstructure with the production of current (photocurrent) and with the absorption of light on a dynamic basis.”
Besides the research fundamentals, Dr Wamwangi speaks highly of the fact that START provides a collaborative forum for scientists in Africa working in this field of Energy Materials,
“As seen in the recent publication on perovskite solar cells with START collaborators from both Africa and the UK, START increases the impact, quality and sustainability of our research publications and outputs, as well as the overall visibility and influence of African scientists and innovators globally.”
Footnote:
1) F.Otieno, B. Mutuma, M. Airo, K. Ranganathan, R. Erasmus, N. Coville D. Wamwangi 2018, https://www.sciencedirect.com/science/article/pii/S0040609017300573
2) F.Otieno, B. Mutuma, M. Airo, K. Ranganathan, R. Erasmus, N. Coville D. Wamwangi 2018
3) F.Otieno, B. Mutuma, M. Airo, K. Ranganathan, R. Erasmus, N. Coville D. Wamwangi 2018
4) Collaborators in the microstructure project of organic photovoltaics: Prof. D. Billing the principal Investigator in the START project, Wits University; Dr. Moritz Riede, Department of Physics, Oxford University, Dr. Thomas Derrien, Dr. Francis Otieno, School of Physics/Chemistry, Wits University (Professor Wamwangi’s former PhD student).
[This article is supplied by GCRF START (Global Challenges Research Fund Synchrotron Techniques for African Research and Technology), a collaborative project between scientists in Africa and the UK working together on research using synchrotron science]