| Developments of photovoltaic (PV) devices are driven by increasing needs for economically competitive renewable energy conversion. To improve the efficiency of PV devices for outdoor applications, the concept of intermediate band solar cell (IBSC) has been proposed to boost the conversation efficiency to 63% under concentrated suns illumination, which requires two-step photon absorption (TSPA) dominates among other competing processes: carrier thermal escape, tunneling and recombination. To optimize the design of III-V QD-IBSCs, first, the effect of electric field on band structure and carrier dynamics and device performances were quantitative investigated via simulation and experiments. Second, to experimentally increase TSPA at room temperature, novel QD systems related QD-IBSCs were designed, fabricated and characterized. The InAs/Al0.3GaAs QD-IBSC shows high TSPA working temperature towards 110K, promising for a room temperature IBSC under concentrated sunlight. Alternative QD systems including GaSb/GaAs and type II InP/InGaP were also investigated via band structure simulations. Meanwhile, developments of PV devices under indoor low intensity light (0.1 muW/cm2-1 mW/cm2) illumination not only enable long lifetime radio-isotope based batteries, but also, more important for the daily life, have the potential to promote an emerging market of internet of things by efficiently powering wireless sensors. Single junction InGaP PV devices were optimized for low intensity light sources using via simulations and statistical control. To reduce the dark current and increase the absorption at longer wavelengths (>550 nm), several parameters including doping and thickness were evaluated. The experimental results on the devices show higher conversion efficiencies than other commercial PVs under varied indoor light sources: 29% under 1muW/cm2 phosphor spectrum and over 30% efficiency under LEDs illumination. In addition, the work includes developments of InAs nanowires epi-growth for PV applications. Several marks for selective area growth were successfully made. |
