Photovoltaic technology has expanded its presence in the electricity market, thanks to the high-power conversion efficiency and low cost of modern photovoltaic (PV) solar cells. Professor Antonio Urbina and his collaborators look at ways to improve the properties of emerging PV cell technologies by inspecting the molecular organisation of the raw materials at the nanometre scale. The team use a technique known as neutron scattering, which can analyse the composition of PV cell polymeric layers with unprecedented resolution – knowledge that can be used for increasing their performance.
Photovoltaic technology will play a pivotal role in the transition to sustainable sources of renewable energy. Photovoltaic (PV) cells harness energy from the Sun and convert this into electricity. When the particles of light, or photons, hit the special materials that make up the Sun-exposed active layer in a PV cell, minuscule, negatively charged particles called electrons are excited by the energy of the incoming photon and then transferred to the electrodes that deliver them to an external load in the form of an electrical current. Each individual cell can be combined into large assemblies, or modules, to produce power outputs that reliably meet the needs of a growing network of homes and businesses connected to the electrical grid.
To make sure that most of the energy generated at the PV cell’s surface is efficiently converted into electricity, it’s important that the materials making up the PV ‘sandwich’ are carefully chosen. While more than 94% of PV cell assemblies consist of crystalline silicon, emerging technologies employ organic and hybrid solar cells.
Organic solar cells
Organic solar cells have an active layer composed of a new class of semiconducting polymers that resulted in the Nobel prize for chemistry in 2000. Organic solar cells are made of carbon-containing polymers and organic/inorganic hybrid materials, consisting of carbon-containing building blocks linked to inorganic materials, such as metals and metal oxides.
Professor Antonio Urbina and his collaborators are exploring ways to improve the properties of organic and hybrid solar cells by inspecting their molecular structure and dynamics at the nanometre scale. The researchers believe that these emerging assemblies are capable of delivering solar electricity at a lower cost than other conventional technologies.
The raw materials of these novel PV cells can be deposited as thin films on flexible substrates, reducing manufacturing costs through roll-to-roll printing methods from tailored ink solutions. Explaining this revolutionary technique, Urbina says, ‘the solutions of conjugated polymers can be “printed” by means of technologies such as ink-jet or screen printing and slot-die or spray coating, or a combination of them.’ Once printed in the form of thin films from tailored solutions, known as ‘inks’, the organic and hybrid materials can self-organise and create the different layers that comprise the solar cells. Urbina’s team employs a technique known as neutron scattering, which analyses the composition of the PV layers and the motion of the molecules with nanometric resolution. This enables scientists to investigate how important parameters – such as solvent interaction, temperature, and molecular weight – influence the behaviour of the polymer assemblies in solution and in the films. These parameters can then be tweaked to improve the manufacturing of the polymer layers, to increase PV cell performance.
Exploring properties at the nanoscale
Neutron scattering is carried out in large international facilities, and Urbina has extensive experience working at these state-of-the-art research centres. He has investigated the use of Small Angle Neutron Scattering (SANS), Neutron Reflectometry (NR), and Quasielastic Neutron Scattering (QENS) for the study of emerging photovoltaic technologies. Now, he is applying these three techniques to study the structure and dynamics of inks used to create PV cells at the nanoscale.
SANS is a technique that enables the structural exploration of solids and liquids at scales of one to 100 nanometres. In a SANS instrument, a beam of neutrons is directed at the sample under analysis. The particles making up the sample will deflect some of the neutrons from their trajectory. The different angles of deflection will be measured by a detector and plotted to form a distinct scattering curve that can be associated with specific molecular shapes with the aid of computational models. NR involves shining a beam of neutrons onto a flat thin film, followed by a measurement of the intensity of the reflected beam. The reflected radiation profile built by the interference of reflections from different layers can provide important information about the surface under examination, such as the thickness, the density, or the roughness of all the layers in the analysed film. Finally, QENS allows researchers to precisely measure very small energy transfers between a beam of neutrons and the atoms in the sample. This can give information about molecular dynamics phenomena, such as molecular reorientations and relaxation processes within the molecular architecture of a material.
Urbina’s laboratory has published work showing how NR and QENS could be applied to study the structure of novel solar cells and to analyse the molecular dynamics of their active layers. ‘The control of the atomic and molecular organisation of these new solar cells requires the understanding of the structure and dynamics of complex materials at the nanoscale,’ says Urbina. ‘This information will improve the manufacturing process of the devices and increase its power conversion efficiency and operational lifetime.’ The researchers found that the smart materials start to self-organise in the initial stages of the ink preparation, a process that continues during the thin film printing and subsequent annealing procedures (as a function of temperature). The neutron-scattering studies confirmed that the optimal structure of organic PV cells can be finely tuned to maximise performance, by improving the ink preparation and controlling molecular additives and temperature cycles.
Perovskite solar cells
Perovskite solar cells (PSCs) are thin-film devices consisting of hybrid organic/inorganic materials containing lead or tin halide-based materials, where the halides consist of halogen ions, such as chlorides, bromides, or iodides. In 2020 Urbina carried out an extensive review of the scientific literature available on the evolution of PSCs, with a focus on their environmental impact and economic cost considerations. Perovskite solar cells are attractive for their flexibility, their low weight, and their demonstrated ability to achieve power conversion efficiencies higher than 25%. Their main drawback holding back a strong penetration in the market is the poor stability of the devices, due to physical and chemical degradation mechanisms that are still under study.
Since some PSCs contain lead, toxicity represents one of the barriers to commercialisation, in close competition with finding more cost-effective production routes. Not only is lead intoxication a concern in light of the large-scale deployment of commercial PSCs, but it poses a significant health hazard among researchers at laboratories or workers at future PSCs production plants. Urbina suggests that recycling should be considered as part of a research strategy for perovskite solar cells. One solution he proposes is the design and fabrication of embedded layers or encapsulation materials that may act as lead-capturing traps to avoid the release of the toxic metal, while at the same time facilitating its recovery for use in a new generation of cells.
Life-cycle assessment (LCA) is a well-established methodology aimed at quantifying the environmental impacts of a product or service over its lifetime, from sourcing the raw materials, through the production, distribution and use phases, to the end-of-life phase, which includes waste management and disposal. This systematic analysis covers a range of environmental considerations, such as the use of ores and crude oil, water and land, as well as carbon dioxide emissions into the air, water, and soil. Urbina explains that all the LCA studies carried out so far indicate that solar electricity produces much lower impacts than any fossil fuel alternative, in several categories including climate change, ozone depletion, human toxicity, and resource depletion.
In his book, Urbina debunks the early ‘80s myth that the energy input required to manufacture a PV module is larger than the energy it supplies along its lifetime. According to the researcher, the energy balance of solar electricity is positive after a period of between 1.2 and three years for crystalline silicon technology, compared with a lifetime of between 25 and 30 years or longer.
Emerging technologies, such as organic or hybrid solar cells, have demonstrated energy payback times as low as a few days or weeks, although their lifetime is still low in comparison to inorganic technologies. The book also explores the issue of raw materials that could face supply tensions. Thin-film technologies in the market may face higher risks in materials supply; in particular, tellurium and indium are key elements. Similarly, some other emerging technologies rely on indium supply for transparent conducting oxides, but good replacements have been made available.
Next-generation PV composites
A new class of emerging photovoltaic technology based on organic and hybrid materials is about to reach the market, which is currently dominated by commercial technologies based on crystalline silicon. These new technologies offer the possibility of reduced manufacturing costs, but more research is needed to explore the molecular dynamics of the raw materials at the nanometre scale. The Urbina laboratory employs neutron-scattering techniques to investigate the structural properties and the molecular interactions of novel PV assemblies, such as organic, hybrid, and perovskite PV cells. Urbina and his colleagues have shown that the new generation PV composites start their self-organisation in the initial stages of the ink preparation, which continues during the thin film printing. The optimal structure of the devices can be tuned by improving the ink preparation and controlling molecular additives and temperature cycles. This year, a significant milestone for renewable energy will be reached when the global installed PV capacity will hit 1 tera watts (TW) – or one trillion watts. Urbina and his team’s research is contributing to this incredible sustainable record for photovoltaic technology, which is set to be significantly surpassed in the near future.
- Urbina, A, (2022) Sustainable Solar Electricity. Cham: Springer. www.doi.org/10.1007/978-3-030-91771-5
- Urbina, A, (2020) The balance between efficiency, stability and environmental impacts in perovskite solar cells: a review. Journal of Physics: Energy, 2(2). www.doi.org/10.1088/2515-7655/ab5eee
- Urbina, A, Abad, J, Fernández Romero, AJ, et al, (2019) Neutron reflectometry and hard X-ray photoelectron spectroscopy study of the vertical segregation of PCBM in organic solar cells. Solar Energy Materials and Solar Cells, 191, 62–70. www.doi.org/10.1016/j.solmat.2018.10.004
- Guilbert, AA, Urbina, A, Abad, J, et al, (2015) Temperature-dependent dynamics of polyalkylthiophene conjugated polymers: a combined neutron scattering and simulation study. Chemistry of Materials, 27(22), 7652–61. www.doi.org/10.1021/acs.chemmater.5b03001
Antonio Urbina employs neutron-scattering techniques to investigate the structural properties and the molecular interactions of novel PV assemblies, such as organic, hybrid, and perovskite solar cells.
Agencia Estatal de Investigación (AEI – MICINN, Spain), including European FEDER Funds, project PID2019-104272RB-C55.
- Professor Jenny Nelson (Imperial College London, UK)
- Professor Vicky García-Sakai (ISIS-Rutherford Appleton Laboratory, UK)
- Professor Jaime Colchero (UM), Professor Wolfgang Maser and Professor Ana Benito (ICB-CSIC), Professor Mónica Lira (ICN2), Professor Ana Cross and Professor Nuria Garro (UV, Spain).
- Dr Lucía Serrano (URJC, Spain), Dr Nieves Espinosa (JRC, EU), Dr Jose Abad, Dr Antonio Fernández and Dr Javier Padilla (UPCT, Spain).
Professor Antonio Urbina researches the physics of solar cells and the optimisation of their manufacturing processes, especially for organic and hybrid technologies, by applying neutron-scattering techniques to the study of their structure and dynamics. He has also quantified its environmental and health impacts by using the life-cycle assessment (LCA) methodology.
Institute for Advanced Material sand Mathematics (INAMAT2) and
Department of Sciences
Public University of Navarra (UPNA)
Campus de Arrosadía