Defect and Dielectric Engineering of Transition Metal Dichalcogenides for photovoltaic applications

Transition metal dichalcogenides (TMDs) are an emerging class of materials that are akin to graphene, but in many ways more useful as they provide a non-zero bandgap. They have interesting structural and electronic properties that make them very attractive for a range of applications including; photovoltaics, transistors, low-power microelectronics and sensors. Several recent reviews cover the interesting physics of TMDs as well as their applications [1-4]. Similar to graphite, these are essentially two-dimensional layered materials, they have the chemical composition MX2 (M=transition metal like Mo, W, Nb,Ta, Re etc. and X=S, Se, Te), with a hexagonal microstructure similar to graphene. The electronic properties range from metallic to semiconducting. Native defects such as chalcogenide or metal vacancies can be used to make semiconducting TMDs n-type or p-type. These can be introduced during the synthesis or by ion beam sputtering after synthesis. Doping by other transition metal atoms can modify the band gap as well as carrier characteristics such as mobility. The ability to engineer defects, band-gaps and mobility what makes these materials of great interest for device application.

Many current solar cell architectures have photovoltaic absorbers synthesised on molybdenum. For example, CdTe and Cu2ZnSnS(Se)4 (CZTS or CZTSe) are grown on molybdenum films and during processing form a thin layer of TMD at the interface. The Mo back contact facilitates collection of holes and in some architectures, a barrier layer of ZnO, which could have the effect of dielectric screening of charged defects in the TMD layer, can improve carrier mobility. For microelectronic applications there is also great interest in dielectric engineering on top of TMDs. For example, there have been recent studies of HfO2 gate oxides on TMDs for transistors [5].

Of the many TMDs, only MoS2 has been extensively studied to date. Even then, there are many open questions as to the nature of the defects and interfaces on the electronic properties. The proposed PhD project is to characterize pristine thin films of MoS2, MoSe2 and MoTe2 and the interfaces with a range of oxide thin film materials (such as HfO2, Al2O3, ZnO and Ga2O3) to investigate the bulk and interface material using a variety of techniques including photoemission, X-ray diffraction, Scaning Electron Microscopy, Electrical measurements etc, in order to get both physical and electronic characterization. Advanced analytical techniques will be used to characterise test structures fabricated in-house using a range of film deposition techniques including atomic layer deposition. Measurements will help to understand how interface engineering affects carrier dynamics, and the process-structure-performance relationship of the back-contact part of PV cells, which will provide valuable information relating to the enhancement of cell performance. Measurements will also provide valuable information on the quality and band line-up between the TMD and possible gate dielectric. The student will gain cross disciplinary skills relating to advanced materials and device characterisation as well as film deposition.

Please direct enquiries to:
Dr Vin Dhanak [Email Address Removed]
Dr Ivona Mitrovic [Email Address Removed]
Dr Richard Potter [Email Address Removed]