Study on III-V materials for hot carrier solar cell absorbers

Hot carrier solar cell is an idea of third generation solar cells, which could boost the energy conversion efficiency greater than 60% at one sun condition through minimizing the carrier thermalization loss. These cells consist of one hot carrier absorber sandwiched by two energy selective contacts. The energy selective contacts are able to extract hot carriers at a certain energy level preventing energy dissipation. On the other hand, the hot carrier absorber plays a more important role, which aims to extend thermalization time to several nanoseconds at least. Some III-V semiconductor materials have advantages over the other materials such as small electronic bandgap and large phononic bandgap, which could dramatically reduce the carrier cooling rate through Klemens and/or Ridley decay mechanisms. Furthermore, some unknown mechanisms which might also affect the carrier dynamics are worth for investigation. Therefore it is essential to investigate these specific III-V semiconductor materials comprehensively. In this project, specific bulk and multiple quantum well III-V semiconductors have been studied in terms of their optical and crystal properties. Energetic neutral atom beam lithography and epitaxy-molecular beam epitaxy (ENABLE-MBE) and conventional molecular beam epitaxy (MBE) were employed to fabricate good quality materials with required parameters. Then a variety of characterization techniques are applied on these samples for their structural and optical properties measurement. Among these techniques, the time resolved photoluminescence (TRPL) in picosecond is the most important technique to indicate carrier cooling rate. On the other hand, several fitting methods on TRPL had been developed and applied on the optical results to figure out the carrier thermalization time. An extend thermalization time was observed in InGaN alloy and carrier lifetime in nanoseconds was achieved in GaAs/AlAs multiple quantum wells in different thicknesses due to bottleneck effect and carrier screening. In general, this project give a comprehensive understanding on the studied materials and concludes that the multiple quantum wells structure might be the most appropriate candidate as a hot carrier absorber for hot carrier solar cells