Science
Solar Energy Cu2ZnSnSâ‚„4
From 2013 to 2018 Chris worked as a materials-physics researcher at the University of Durham, investigating the novel solar-cell material Cuâ‚‚ZnSnSâ‚„ (known as CZTS).
​
Most solar cells are made out of silicon, which requires relatively expensive, high-energy processing and must be relatively thick to absorb enough light. Alternative 'thin-film' materials, like CZTS, have the potential to be less expensive to process and can be much thinner, which could allow a wider range of applications, including flexible cells. However, most thin-film materials include rare (and therefore expensive) elements or are toxic. CZTS does not have these downsides. Chris' research aimed to clarify the precise quantum-mechanical processes that occur within CZTS in order to improve the photovoltaic efficiency of devices made with it and so make it an economically viable solar technology.
​
CZTS forms in the kesterite crystal structure shown below. Note that, because of its symmetry, every copper atom in the theoretical structure shown is on either a 2a or a 2c lattice site and every zinc atom on a 2d site. However, in reality some copper atoms are actually found on sites supposed to be zinc and some zinc atoms on sites supposed to be copper. These defects are collectively referred to as cation disorder and significantly affect the properties of the material. They are dictated by precise chemical composition and by the process used to make the material (primarily the temperatures it is subjected to).
​
​
The kesterite crystal structure of CZTS​
Chris used compositional, structural, and optoelectronic analysis techniques to study the effect of precise chemical composition on material properties. Previously, a theoretical quasi-ternary phase diagram was commonly used to show which materials would form as impurities in CZTS from a mixture of its four constituent elements in any ratio. Chris demonstrated that this diagram was incorrect. He also proved that that no common analysis technique can quantify cation disorder in CZTS, despite Raman spectroscopy commonly being used to do so prior to Chris' research (a Raman spectrum for a sample of CZTS is shown below).
​
​
The Raman spectrum of a sample of CZTS​
​
Chris used neutron diffraction to study the order-disorder phase transition that occurs in CZTS at around 550 K. As shown in the plot of lattice site occupancy below, at temperatures below the transition the 2a and 2c sites mostly contain copper and the 2d site mostly zinc (as expected from the kesterite structure), but above the transition, all sites are approximately equal. Chris discovered that the transition temperature is dependent on elemental composition and that Cu-Zn disorder is present on all cation lattice sites, not merely the 2c and 2d sites of the kesterite crystal structure as had previously been assumed.
​
​
The order-disorder phase transition occurring at around 550 K in a sample of CZTS
​
Chris then used anomalous X-ray diffraction to study cation disorder further (an X-ray diffraction pattern for a sample of CZTS is shown below). He discovered that two distinct phases of CZTS can be present in the same sample, with different elemental compositions resulting from the prevalence of different point-defect complexes (i.e. groups of defects in each of which a lattice site does not contain the atom it is supposed to according to the overall crystal structure of the material). He discovered two new such phases of CZTS (G-type and H-type, shown in the ternary cation composition diagram below) and proposed a mechanism that explains the formation of the various known phases based on the order-disorder phase transition.
​
​
The high-resolution X-ray diffraction pattern of a sample, showing two distinct phases of CZTS
​
The quasi-ternary composition diagram showing the distinct types of CZTS, including the two that Chris discovered
​
For more detail on Chris' work, see his publications.