Characterisation of GaAsBi/GaAs Multiple Quantum Wells for Photovoltaic Applications

The incorporation of bismuth (Bi) into GaAs creates many potentials in different areas of technology such as telecommunications, spintronics and photovoltaics applications. The ability for GaAsBi to reach 1 eV bandgap is highly anticipated in solar cells industry as an alternative to replace InGaAs in achieving higher efficiency multi-junction solar cells. Although it is known that the growth of Bi-based material is challenging, the characterisations of this material device will help in providing its properties and open up opportunity for improvement and development. A series of GaAsBi/GaAs multiple quantum well p-i-n diodes was grown using molecular beam epitaxy and the material characterisations are presented in this thesis. From the electrical characterisations, the current-voltage measurements of the devices demonstrate good diode behaviours with clear differences in dark current value between strained and strain-relaxed devices. Meanwhile, the reverse bias current-voltage measurements show the dominance of reverse leakage current for all devices. The devices also experience hole trapping in the valence band, causing poor carrier extractions when light is absorbed during photocurrent measurements. Carrier enhancement can be achieved by applying slight reverse bias when the measurement was taken. Besides that, the absorption coefficient of the devices was confirmed to be similar with other work. Finally, the device’s performance under solar illuminator is lower compared to InGaAs/GaAsP strained-balanced multiple-quantum well device due to its poor value of open-circuit voltage and it has higher bandgap offset compared to GaAs. Overall, these results suggest than GaAsBi/GaAs multiple quantum well(s) do have a lot of room for improvement especially on growth, structure and strain level of the material. If these components can be catered, GaAsBi can be a competitive alternative for 1 eV junction in multiple junction solar cells.