Prof Michael Johnston

Abstract:

Mixed lead-tin triiodide perovskites are promising absorber materials for low band-gap bottom cells in all-perovskite tandem photovoltaic devices. Key structural and electronic properties of the FAPb1-xSnxI3 perovskite are presented here as a function of lead:tin content across the alloy series. Temperature-dependent photoluminescence and optical absorption measurements are used to identify changes in the band-gap and phase transition temperature. The large band-gap bowing parameter, a crucial element for the attainment of low band-gaps in this system, is shown to depend on the structural phase, reaching a value of 0.84 eV in the low-temperature phase and 0.73 eV at room temperature. The parabolic nature of the bowing at all temperatures is compatible with a mechanism arising from bond bending to accommodate the random placement of unevenly sized lead and tin ions. Charge-carrier recombination dynamics are shown to fall into two regimes. Tin-rich compositions exhibit fast, mono-exponential recombination that is almost temperature independent, in accordance with high levels of electrical doping. Lead-rich compositions show slower, stretched-exponential charge-carrier recombination that is strongly temperature-dependent, in accordance with a multi-phonon assisted process. These results highlight the importance of structure and composition for control of band-gap bowing and charge-carrier recombination mechanisms in low band-gap absorbers for all-perovskite tandem solar cells.

Abstract:

Terahertz (THz) photonic devices are now exploiting emerging materials systems, while novel device designs utilise plasmonic effects, nanophotinics, and metamaterials. The scope of this special topic highlights and reviews the recent cutting-edge THz photonic devices which have been revolutionised from the advances in the above research areas.

Abstract:

InAsSb nanowires are promising elements for thermoelectric devices, infrared photodetectors, high-speed transistors, as well as thermophotovoltaic cells. By changing the Sb alloy fraction the mid-infrared bandgap energy and thermal conductivity may be tuned for specific device applications. Using both terahertz and Raman noncontact probes, we show that Sb alloying increases the electron mobility in the nanowires by over a factor of 3 from InAs to InAs0.65Sb0.35. We also extract the temperature-dependent electron mobility via both terahertz and Raman spectroscopy, and we report the highest electron mobilities for InAs0.65Sb0.35 nanowires to date, exceeding 16,000 cm2 V–1 s–1 at 10 K.