Multi-functional thin films for photovoltaic and battery storage applications
More than 90% of photovoltaic solar cells are made from crystalline silicon. Record-breaking silicon solar cells have efficiencies in excess of 26%, and such cells require silicon substrates in which photogenerated charge carrier diffusion lengths exceed 1 mm. As substrates are typically < 200 µm thick, the properties of the cell are strongly influenced by charge carrier recombination at surfaces. In Warwick, we have recently developed a novel ionic surface passivation scheme which can passivate surfaces extremely well (see open access papers at https://dx.doi.org/10.1109/JPHOTOV.2017.2751511 and https://doi.org/10.1016/j.solmat.2018.03.028). This has enabled us to exceed the so-called “intrinsic limit” of carrier lifetime in silicon, which implies that the current limit of a silicon cell efficiency is slightly conservative. We anticipate the films we have developed will have other uses, including in tandem photovoltaic structures and in the context of surface treatment of silicon anodes for lithium ion batteries, which have the potential to provide an order of magnitude capacity improvement compared to graphitic carbon.
The aim of this PhD project is to develop ultra-thin passivation films (< 1 nm) derived from ionic solutions, and atomic layer deposition (ALD) methods, which exhibit excellent thermal and electrical stability when applied to semiconductor surfaces. Central to the project is Warwick’s new £400k ALD facility which was commissioned in the Science City Cleanroom in early 2018. The objective will be to develop a fundamental understanding of the passivation mechanism at the atomic scale and how processes can be manipulated in order to achieve optimal long-term thermal and electrical properties. The films developed will then be applied in a range of contexts, including silicon photovoltaics, next-generation photovoltaic architectures such as silicon-based tandems, and lithium ion battery anodes. The student will gain experience with cleanroom processing, and cutting-edge materials characterisation by electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and nuclear magnetic resonance. In addition, the student will use silicon-specific analytical techniques available in Dr Murphy’s laboratory in Engineering. This includes minority carrier lifetime measurements, and photoluminescence imaging. The project would suit someone with a materials science, chemistry, physics or electrical engineering background. Dr Murphy leads the multi-institution EPSRC SuperSilicon PV project (EP/M024911/1) and there will be opportunities for collaboration with other universities in the UK and internationally.
Silicon materials for high efficiency photovoltaic solar cells
Photovoltaics have the potential to supply the majority of the world’s electricity. Today’s market is dominated from cells made from bulk silicon (~90%) and silicon’s abundance and properties (chemical stability, density, band gap, and non-toxicity) mean that there is a good chance it will continue to dominate in the longer term. Although uptake of photovoltaic technologies is increasing rapidly, without subsidy all stable photovoltaic technologies are too expensive to compete with incumbent electricity generation sources. Academic research is needed to improve the performance of materials to produce more cost-effective silicon solar cells.
The performance of the very highest efficiency silicon solar cells (e.g. back-junction back-contact cells) is becoming limited by the minority carrier lifetime in the substrate. Although carrier lifetimes in the best silicon wafers now routinely exceed 1ms, the best cells will soon require lifetimes of > 10ms. Although in normal operating conditions such lifetimes are physically possible, defects in the material mean that they are seldom achieved in practice. Recombination centres present in very low concentrations (parts in a million million) can impinge on the carrier lifetime. This project aims to understand the origin of this weak recombination activity, and ideally developing ways to overcome it. Experimental techniques such as photoconductance lifetime measurements and photoluminescence imaging will be used, and there will be a need for occasional cleanroom working. The project will be in collaboration with a leading silicon manufacturer.
This PhD project requires prior knowledge and an interest in semiconductor materials. It would ideally suit a candidate with an undergraduate degree in physics, materials science, or electrical engineering. Informal enquiries can be made to Dr Murphy by e-mail (email@example.com). Please attach a CV.
Note: Should your application for admission be accepted you should be aware that this does not constitute an offer of financial support. Please refer to the scholarships & funding pages.