CHROMTISOL

New light on solar cells Solar cells are an increasingly important element of overall energy provision, yet there is still room to improve their efficiency and performance. Researchers in the Chromtisol project are utilising Titanium dioxide nanotubes to develop a new physical concept of a solar cell which could help improve solar-to-electricity conversion efficiency, as Dr. Jan M. Macak explains.

The development of renewable sources of energy is widely recognised as a research priority, with scientists looking to efficiently harness solar power to meet our energy needs. Layers of ordered nanotubular titanium dioxide (TiO2), shown illustratively on scanning electron microscope micrographs, offer a lot of potential in this respect, says Dr Jan M. Macak. “It has become clear that they are unique, they posses large surface area in a small volume, are very stable upon irradiation and can be produced by a simple technology. In combination with suitable chromophores, they can very efficiently absorb both sunlight and artificial light and convert this light into electrons.”

This is a topic Dr Macak is exploring further in the Chromtisol project, an EU-backed initiative based at the University of Pardubice in the Czech Republic, which aims to develop a new, more efficient physical concept of a solar cell. “If you want to make a good solar cell, you have to make sure that it absorbs as much light as possible, and reflects as little as possible,” he explains.

TiO2 nanotubes

This topic is central to the project’s overall agenda, with researchers aiming to utilise TiO2 nanotube layers in the development of a new type of solar cell. The nanotube layers act as a functional scaffold, and provide a relatively large surface area to the cell. “This is very important, because the larger the surface area, the better for the solar cell,” explains Dr Macak. When light is shone onto a regular flat surface, a certain proportion is reflected; by increasing the surface area of the cell and adapting the morphology, Dr Macak aims to improve absorption efficiency. “The first major challenge in this project is to make sure that we absorb as much light as possible.

Currently, as we recently showed in two publications published in journal Nanoscale, the absorption rate is typically somewhere between 80-90 percent, which means that approximately 10 percent of light is reflected and not used,” he outlines.

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Simplified sketch of the developed solar cell illustrating the absorption of light and consequent generation of electrons within TiO2 nanotube layers coated with suitable chromophore. The inset graph shows increase of the photon-to-electron conversion efficiency (IPCE) of the nanotube layer with added chromophore.

The type of light that is absorbed is also an important parameter in this respect. While the TiO2 nanotubes perform effectively in absorbing UV light, it’s also important to absorb visible and infrared light, an issue Dr Macak is working to address. “I have put some additional materials called chromophores in the solar cell. In nature, chromophores absorb sunlight and make energy out of it for plants,” he outlines.

These chromophores inside the nanotubes are designed to capture the ultraviolet, the visible and the infrared light. “The aim is to utilise, as efficiently as possible, the space inside the tubes,” continues Dr Macak. “Putting these chromophores inside nanotubes is not easy however, as the scale is so small. So the project is not just about

developing the solar cell, it’s also about finding the best strategy to put the correct type of chromphore inside the tubes.”

A number of different strategies are available for this task. In one of the more complex approaches, researchers utilise a thin-film deposition technique called atomic layer deposition to coat the interior of the nanotubes. “It’s kind of like a large vacuum tool,” says Dr Macak.

This technique has already been exploited by various researchers and industries to make thin functional coatings of different materials for different purposes; Dr Macak says it holds rich potential in terms of developing a high-quality solar cell. “The costs would probably be a little bit higher than other cells, but the light management

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and the efficiencies have the potential to be really very high,” he outlines. “The chromophores in the tubes, and the solar cells in general, are also able to capture photons from indirect light. This means things that are in shadow - for example, they could be placed on the sides of cars or houses.”

This is very different to the conventional approach of putting large silicon solar cells in fields or deserts to absorb direct sunlight.

With the ability to capture photons even from indirect light, the Chromtisol solar cell could potentially be applied in a wider range of locations, not limited to those which often experience high amounts of direct sunlight. “Most silicon solar cell

a major consideration in this respect, which will affect the technology’s range of potential applications. “This would be rather a special solar cell, and so would be used for certain applications, like on certain space applications,” says Dr Macak. While commercialisation is not on the immediate agenda, Dr Macak says a lot has been achieved in the project already, for example in refining and finetuning different tools and methodologies.

“Through our work in this area, we are pushing the limits of atomic layer deposition, which is a prominent topic in research,” he stresses.

The project’s research has also led to the development of other tools and methods,

The first major challenge in this project is to make sure that we absorb in the nanotubular layer with the suitable chromophore as much light as possible.

Currently, the absorption rate is something like 80-90 percent, which means that approximately 10 percent of

light is reflected and not used. installations are sun-facing, to directly absorb sunlight. But there are also places which do not experience so much sun,” points out Dr Macak. The intention is to produce a final prototype of the solar cell at some point over the next year or so, beyond which Dr Macak is also considering the possibility of scaling up the technology.

“We’ll look at scaling it up to the larger sizes needed for further experiments and testing,” he outlines.

Research is still at a relatively early stage, with Dr Macak and his colleagues still working to improve the core methods and techniques, yet he is fully aware of the wider commercial potential of the technology. Economic factors are

including a methodology on how to put the chromophores inside nanotubes, which Dr Macak believes represents an important step forward. “It’s really a very small area within a structure, and placing the chromophores is a big challenge,” he says.

This is quite an exploratory, challenging area of research, and while Dr Macak and his colleagues are keen to translate their work into tangible benefits, it’s also important to note the role of the project in paving the way for further development in future. “We are trying to use these materials in quite an interesting way, and this will be valuable for researchers in future,” he says.

Scanning electron micrographs showing the top view (left) and the cross-sectional view (right) of the TiO2 nanotube layer.

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CHROMTISOL Towards New Generation of Solid-State Photovoltaic Cell: Harvesting Nanotubular Titania and Hybrid Chromophores Project Objectives

A lot of attention in research has been centred on technologies that could boost the solarto-electricity conversion efficiency and power recently unpowerable devices and objects.

The focus of research in the Chromtisol project is a new physical concept of a solar cell that explores extremely promising materials, yet unseen and unexplored in a joint device, whose combination may solve drawbacks commonly associated with solar cells, in particular carrier recombination and narrow light absorption.

The project aims to reach important scientific findings in highly interdisciplinary fields. It is extremely challenging and risky, yet based on feasible ideas and steps that will result in exciting achievements.

Project Funding

ERC Starting Grant / Total cost: EUR 1 644 380 / EU contribution: EUR 1 644 380

Contact Details

Dr. Jan M. Macak, Senior Scientist Center of Materials and Nanotechnologies.

Faculty of Chemical Technology University of Pardubice Nam. Cs. Legii 565 530 02 Pardubice Czech Republic T: +420 466 037 401 E: jan.macak@upce.cz W: https://cordis.europa.eu/project/ rcn/193604_en.html M. Krbal, J. Prikryl, R. Zazpe, H. Sopha, J.M. Macak, CdS-coated TiO2 nanotube layers: downscaling tube diameter towards efficient heterostructured photoelectrochemical conversion, Nanoscale. 9 (2017) 7755–7759. doi:10.1039/C7NR02841E R. Zazpe, H. Sopha, J. Prikryl, M. Krbal, J. Mistrik, F. Dvorak, L. Hromadko, J.M. Macak, 1D conical nanotubular TiO2 / CdS heterostructure with superior photon-to-electron conversion, Nanoscale, in press, DOI: 10.1039/C8NR02418A

Dr. Jan M. Macak

Jan M. Macák is a Senior Researcher at the Center for Materials and Nanotechnologies, University of Pardubice. His main research focus is on materials science, with an emphasis on nanostructured materials and their applications. He is also interested in thin film characterization, self-organisation phenomena and semi-conductor chemistry.

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