Computational Chemistry: Exploring Solar Cell Materials
A day of master classes and workshops were held for key stage 5 students as part of Daresbury Laboratory’s Open Week (Jul 5th to Jul 9th, 2016); one of the workshops involved exploring solar cell materials using a computer. Students worked through a series of tasks that introduced modelling visualisation software, familiarising themselves with the computational chemist’s view of the particular solar cell materials, titanium dioxide and organic dyes. The tasks were interspersed with presentations given by Daresbury scientists and researchers from Bangor University, who are our experimental colleagues working on the aforementioned solar cell materials.
The aim of the workshop was two-fold: to introduce the concept of performing ‘experiments’ on a computer; to produce input files ready for processing on Daresbury Laboratory’s high performance computing (HPC) system. If you would like to read a bit more about the science behind this project, jump to Science Overview.
Following the workshop, the input files were collected by the first of two (paid) work experience students, both of whom had been selected from an impressive list of Daresbury Career Academy applicants to work with the chemistry group. The files were sent to the HPC system, calculations were performed, and the results analysed. The first report can be found here: Solar Cell Report 1.
In true scientific style, more questions than answers were generated from the first batch of calculations! These were further explored by the second Daresbury Career Academy student whose report can be found here: Solar Cell Report 2.
A definitive, scientific conclusion is a work-in-progress because a lot of data (100’s Gb) were generated that will take a few weeks to analyse in-depth. However, a few preliminary conclusions have been inferred from the data and these can be found in the above reports.
The overall, general conclusions drawn from this activity are that the workshop students produced a large range of input files that have been, and continue to be used in research into solar cell materials, and that our two work experience students have both produced some first-class scientific work. From the chemistry group at Daresbury and our colleagues at Bangor we would like to extend a big Thank You! to everyone who has participated so far.
For more information about the Chemistry Group at Daresbury: http://www.scd.stfc.ac.uk/support/40491.aspx
Solar cell materials comprise an important area of research for obvious reasons. The majority of solar cells were originally made from semi-conducting material such as silicon and more recently, rare Earth metals. The latter are both rare(!) and some are toxic, driving the exploration of Earth-abundant, benign materials as alternatives. Titanium dioxide (TiO2) is one such mineral that, in combination with organic dye molecules, (i.e. containing carbon and hydrogen) have together been identified as viable solar cell materials.
The chemically active particles in solar cells are electrons, and when light of the correct energy (i.e. wavelength) hits the organic dye, an electron becomes energised and is transferred to the TiO2 surface.
In the physical solar cell, the organic dye positions itself above a region of a TiO2 surface, (see Figure 1) and the electrons in the TiO2 surface ‘see’ different atomic environments of the organic dye depending on the dye’s position and orientation with respect to the surface. Both the type of dye and how it is positioned above the surface affect the efficiency of the solar cell.
Figure 2 An atomistic model of how an organic dye molecule can position itself with respect to the TiO2 surface. Blue dashed lines show other possible angles of the dye with respect to the surface. Each orientation of the molecule to the surface will have a corresponding energy value, and the lowest energy represents the combined TiO2/organic dye system that is most likely to exist in the physical solar cell.
If experimentalists knew the position and orientation of each different dye (with respect to the TiO2 surface) corresponding to their measured, most efficient solar cell, they could begin to design solar cells where the positions and orientations of the organic dye were controlled, so that the solar cell was always optimally efficient. However, it is not possible to gain this understanding experimentally, which is why we need to use computational chemistry methods. These methods, in this case based on quantum mechanics, enable us to explore electrons in their atomic environments and gain the insight needed to produce efficient solar cells. The calculations are complex and need to be carried out on several computers simultaneously (i.e. ‘in parallel’), so here at Daresbury we are very fortunate to have many computer clusters comprising our HPC systems.
For more information about the Solar Cell Research Group at Bangor University: https://www.bangor.ac.uk/
For more information about the HPC systems at Daresbury: http://community.hartree.stfc.ac.uk/