Study of germanium carbide and zirconium nitride as an absorber material for hot carrier solar cells

Hot Carrier Solar Cell {HC-SC) is one of the advanced photovoltaic technologies. The two main building blocks of a HC-SC are: the absorber, where electrons- holes pairs are photo generated by absorbing the incoming photons, and the energy selective contacts, which allow extraction of these carriers to the external circuit in a narrow range of energies. HC-SCs offer the possibility of operating at very high efficiencies (Limiting efficiency above 65% for un-concentrated light) with structure of reduced complexity. An ideal HC-SC operates at very high efficiency by absorbing wide range of photon energies and extracting the photo-generated carriers at elevated temperatures before they thermalize to the band-edge of the absorber material. The finding of suitable absorber material and its fabrication along with characterisation are the initial steps to implement the HC-SC with a better efficiency. Germanium Carbide {Gee) and Zirconium nitride (ZrN) have a large mass difference between their consecutive elements and exhibit a large phonon band gap sufficient to block the dominant carrier cooling mechanism by Klemens decay route. In this thesis, Gee and ZrN have been studied to investigate their suitability as absorbers for HC-SCs. These material have been fabricated using sputtering and characterised using an array of techniques such as X-ray photoelectron spectroscopy, transmission microscopy, X-ray diffraction, Raman spectroscopy, Atomic Force Microscopy and optical absorption spectroscopy to study structural, compositional and optical properties. Transient absorption spectroscopy was used to study the carrier dynamics. Nano-crystalline Gee has been fabricated which may be suitable for photovoltaic application. Germanium carbide with the record of 15.5 % Gee has been demonstrated. Growth of ZrN have been demonstrated in the range of process parameters. Epitaxial growth of ZrN on MgO substrate has been achieved with DC sputtering at 500°C. Carrier dynamics in ZRN film have been investigated with Ultrafast transient absorption measurements which have indicated a long relaxation time of order of hundreds of picoseconds. The finding in the thesis provide initial insight into the properties of Gee and ZrN and show the possibilities of these materials as an absorber for the HC-SC