Prof Laura Herz FRS

Abstract:

Mixed halide perovskites can provide optimal bandgaps for tandem solar cells which are key to improved cost-efficiencies, but can still suffer from detrimental illumination-induced phase segregation. Here we employ optical-pump terahertz-probe spectroscopy to investigate the impact of halide segregation on the charge-carrier dynamics and transport properties of mixed halide perovskite films. We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities, and that charge carriers within the I-rich phase are not strongly localised. We further demonstrate enhanced lattice anharmonicity in the segregated I-rich domains, which is likely to support ionic migration. These phonon anharmonicity effects also serve as evidence of a remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains. Our analysis demonstrates how minimal structural transformations during phase segregation have a dramatic effect on the charge-carrier dynamics as a result of charge funnelling. We suggest that because such enhanced recombination is radiative, performance losses may be mitigated by deployment of careful light management strategies in solar cells.

Abstract:

A newly reported compound, CuAgBiI5, is synthesized as powder, crystals, and thin films. The structure consists of a 3D octahedral Ag+/Bi3+ network as in spinel, but occupancy of the tetrahedral interstitials by Cu+ differs from those in spinel. The 3D octahedral network of CuAgBiI5 allows us to identify a relationship between octahedral site occupancy (composition) and octahedral motif (structure) across the whole CuI–AgI–BiI3 phase field, giving the ability to chemically control structural dimensionality. To investigate composition–structure–property relationships, we compare the basic optoelectronic properties of CuAgBiI5 with those of Cu2AgBiI6 (which has a 2D octahedral network) and reveal a surprisingly low sensitivity to the dimensionality of the octahedral network. The absorption onset of CuAgBiI5 (2.02 eV) barely changes compared with that of Cu2AgBiI6 (2.06 eV) indicating no obvious signs of an increase in charge confinement. Such behavior contrasts with that for lead halide perovskites which show clear confinement effects upon lowering dimensionality of the octahedral network from 3D to 2D. Changes in photoluminescence spectra and lifetimes between the two compounds mostly derive from the difference in extrinsic defect densities rather than intrinsic effects. While both materials show good stability, bulk CuAgBiI5 powder samples are found to be more sensitive to degradation under solar irradiation compared to Cu2AgBiI6.

Abstract:

Silver-bismuth based semiconductors represent a promising new class of materials for optoelectronic applications because of their high stability, all-inorganic composition, and advantageous optoelectronic properties. In this study, charge-carrier dynamics and transport properties are investigated across five compositions along the AgBiI4–CuI solid solution line (stoichiometry Cu4x(AgBi)1−xI4). The presence of a close-packed iodide sublattice is found to provide a good backbone for general semiconducting properties across all of these materials, whose optoelectronic properties are found to improve markedly with increasing copper content, which enhances photoluminescence intensity and charge-carrier transport. Photoluminescence and photoexcitation-energy-dependent terahertz photoconductivity measurements reveal that this enhanced charge-carrier transport derives from reduced cation disorder and improved electronic connectivity owing to the presence of Cu+. Further, increased Cu+ content enhances the band curvature around the valence band maximum, resulting in lower charge-carrier effective masses, reduced exciton binding energies, and higher mobilities. Finally, ultrafast charge-carrier localization is observed upon pulsed photoexcitation across all compositions investigated, lowering the charge-carrier mobility and leading to Langevin-like bimolecular recombination. This process is concluded to be intrinsically linked to the presence of silver and bismuth, and strategies to tailor or mitigate the effect are proposed and discussed.

Abstract:

Using ultrafast optical-pump terahertz-probe spectroscopy (OPTP) and ultrafast terahertz emission spectroscopy, we showcase the electron and hole dynamics in Cd3As2 nanowires (NWs), a well-known 3D Dirac semimetal a subgroup of the newly discovered . A temperature-dependent photoconductivity measurement was carried out yielding an incredibly high electron mobility ~ 16,000 cm2/Vs at 5K. Strong THz emission due to helicity-dependent surface photocurrents was also observed for both nanowires and single crystal (SC) which is highly desirable for devices such as THz sources.