Solar cells require the presence of two chemically different donor and acceptor materials to create an electric current from solar energy. Researchers from Graz University of Technology now demonstrated that the donor and acceptor electronic properties can be locally manipulated in a single material by arranging its polar groups in a regular pattern.
The discovery has great potential for a novel design of organic solar cells but also offers the possibility to realise new quantum structures.
The working principle of solar cells is based on the generation of an electron-hole pair that is then separated in the solar cell to produce electrical energy. The separation is realised by so-called donor and acceptor building blocks in the semiconducting solar cell material, with a donor part that exhibits spare electrons and an acceptor part that lacks electrons.
How likely the donor material is to give away electrons, and how likely the acceptor material is to take on electrons is determined by the ionisation energy and electron affinity of the respective materials. Traditionally, tuning these properties has been the only way to adjust the electron-hole separation in a solar cell.
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Tuning electronic levels in a material by arranging polar groups in a regular pattern
Now, researchers from the Institute of Solid State Physics at Graz University of Technology in Austria have discovered a method to control charge carriers in a single semiconducting material by shifting the electronic levels at different locations within the material using collective electrostatic effects. In that way, they could mimic the donor-acceptor situation used in solar cells.
Their approach has the advantage that the electronic levels can be continuously tuned over a wide range, and that only one material needs to used. The research was recently published in the high-impact journal Advanced Materials.
Computational simulations are commonly used to understand electronic effects in existing materials and to suggest ways for improving them. Principal investigator Egbert Zojer, PhD student Veronika Obersteiner and their colleagues have taken computational simulations a considerable step further by proposing a radically new way to control the electronic properties of materials.
This was achieved by assembling the polar groups within a material in a regular fashion, which resulted in an overlap of their electric fields, which gave rise to collective electrostatic effects.
The collective electrostatic effects led to that some locations within the material exhibited higher and other parts lower electrostatic energies. How high or low the energy levels of the regions were, was dependent on the dipole moment of the polar groups and how far they were located from each other.
“The basic approach of the electrostatic design concept is to modify the electronic states of semiconductor via the periodic arrangement of dipolar groups. In this way, we are able to locally manipulate energy levels in a controlled way. In doing so, we do not try to find ways to bypass such effects, which are inevitable at interfaces. Rather, we make deliberate use of them for our own purposes”, comments Zojer.
Tuning charge transport is possible in 1D, 2D and 3D materials of all kinds
The team around Zojer has been researching this topic for a while. First, they demonstrated the electrostatic design in molecular monolayers, on for example gold electrodes. The scientists predicted energy shifts in the material, and the charge transport through the monolayers could indeed be tuned experimentally. In the second step, they applied the concept to the two-dimensional material graphene.
The current study proves that the electrostatic manipulation also works in a three-dimensional material. “This work is the climax to our intensive research on the electrostatic design of materials”, says Zojer.
Exemplary demonstrated for a three-dimensional covalent organic network, the researchers could tune the energy levels by means of collective electrostatic effects. This created spatially confined pathways for electrons and holes. “In this way, charge carriers can, for instance, be separated and the electronic properties of the material can be designed as desired”, says Zojer.
The researchers stress that the electrostatic approach is not limited to a certain class of materials but that it can be achieved in any type of structure (both organic or inorganic), where a regular arrangement of dipoles can be realised.
For organic solar cells, this means that they could potentially be fabricated without chemically different donor and acceptor components. Instead, the polar groups in the organic material could be arranged in a way that provides the desired electron-hole separation. Also, the energy levels could be continuously tuned.
Electrostatic design in three-dimensional materials could also be used to realise complex quantum structures, such as quantum cascades and quantum-checkerboards. “Only the imagination of the materials designer can set limits to our concept”, says Zojer.
- V. Obersteiner et al. (2017) Electrostatic Design of 3D Covalent Organic Networks, Advanced Materials, 1700888, DOI: 10.1002/adma.201700888.
- Image Credit: Shutterstock.com/sakkmesterke