Driscoll Research Group
Device Materials Group
Trinity College Cambridge
1) Exploring new Chemically Processed, Scalable and Stable Bi-based Inorganic Photovoltaic Films
This project is fully funded by the EPSRC UK Centre for Doctoral Training in Photovoltaics
Prof. Judith Driscoll, Dr. Robert Hoye and Dr. Dan Credgington, Dept. of Materials Science and Physics, University of Cambridge, 01223-334468, firstname.lastname@example.org
The photovoltaics field has been revolutionised by the emergence of lead-based hybrid perovskites. These materials are high quality semiconductors that can be deposited cheaply, from solution, at room temperature. However, the toxicity of lead and stability issues for these perovskites are driving a new search for more benign and stable alternatives. We have been exploring BiOI as a possible lead-free contender material. To date, nanostructured BiOI has been explored in photovoltaic devices but has shown poor efficiencies (maximum 1.03%) as a result of low photocurrents,1–3 likely because of high interfacial recombination and/or poor transport properties. However, film growth has yet to be optimised for achieving high efficiency.
We have recently deposited BiOI thin films by chemical vapour transport (CVT) and have achieved excellent, phase pure material (fig. 1a and b). Measurements of the photoluminescence decay indicate a bulk lifetime of approximately 3 ns (fig 1c), which well exceeds the 1 ns lifetime threshold need to potentially achieved 10% efficient 4. The bandgap is measured to be 1.9 eV through absorption measurements and spectrally-resolved photoluminescence measurements. This is ideal for a top-cell absorber in a four-terminal tandem with silicon. We have also found BiOI to be phase-stable in air over a 200-day test period, which is two orders of magnitude better than the very popular methylammonium lead iodide perovskite.
In this project, systematic doping of BiOI with chalcogenides S and Se on the optical and electronic properties of the films will be studied. Literature suggests that substitutional doping of O with other chalcogenides results in a reduced bandgap, making it more suitable for a single-junction device.5 The aim is to form idealised microstructures in large area films, to tune the bandgap in the range of 1.9 - 1.5 eV and to maximise the charge-carrier mobility and minority-carrier lifetimes. A very wide range of state-of-the-art electrical and optical characterisation will be undertaken in collaboration with groups in Physics.
Figure 1 a) X-ray diffractogram b) AFM micrograph and c) time resolved PL decay of a BiOI film grown at 350°C by CVT on a fused silica substrate.
References: 1. K. Zhao, X. Zhang, and L. Zhang, Electrochem. commun., 2009, 11, 612–615; 2. K. Wang, F. Jia, Z. Zheng, and L. Zhang, Electrochem. commun., 2010, 12, 1764–1767; 3. M. Fang, H. Jia, W. He, Y. Lei, L. Zhang, and Z. Zheng, Phys. Chem. Chem. Phys., 2015, 17, 13531–13538; 4. R. Jaramillo, M. J. Sher, B. K. Ofori-Okai, V. Steinmann, C. X. Yang, K. Hartman, K. A. Nelson, A. M. Lindenberg, R. G. Gordon, T. Buonassisi, Journal of Applied Physics, 119; 5. N. T. Hahn, A. J. E. Rettie, S. K. Beal, R. R. Fullon, and C. B. Mullins, J. Phys. Chem. C, 2012, 116, 24878–24886.
2) Development of REBCO Coated Conductors for Spherical Tokamak Fusion Reactors
This project is an EPSRC CASE studentship jointly with Tokamak Energy, Abington, UK.
Prof. Judith Driscoll, Dept. of Materials Science
Tokomak Energy is a private company building compact spherical Tokamaks for nuclear fusion power generation. Critical to the success of these devices is the development of high field and temperature superconducting magnets made from commercial rare-earth barium copper oxide (REBCO) coated conductors. Fusion is an emerging market for these conductors, and improvements in their properties and manufacturing processes could drastically improve the performance and economics of future reactors. Factors such as critical current density in high magnetic fields, tape construction, resilience against neutron irradiation, and the speed and economy of manufacturing processes, may all be customised for this novel application. The aim of this project will be to explore new ways in which REBCO coated conductors can be optimised for use in spherical Tokamaks, in order to accelerate the generation of commercial nuclear fusion reactors.