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Herz Group

Prof Laura Herz FRS

Professor of Physics

Sub department

  • Condensed Matter Physics

Research groups

  • Semiconductors group
  • Advanced Device Concepts for Next-Generation Photovoltaics
Laura.Herz@physics.ox.ac.uk
Google Scholar
Publons/WoS
  • About
  • Publications

Polarons and charge localization in metal-halide semiconductors for photovoltaic and light-emitting devices

Advanced Materials Wiley 33:24 (2021) 2007057

Authors:

Leonardo RV Buizza, Laura M Herz

Abstract:

Metal-halide semiconductors have shown excellent performance in optoelectronic applications such as solar cells, light-emitting diodes, and detectors. In this review the role of charge–lattice interactions and polaron formation in a wide range of these promising materials, including perovskites, double perovskites, Ruddlesden–Popper layered perovskites, nanocrystals, vacancy-ordered, and other novel structures, is summarized. The formation of Fröhlich-type “large” polarons in archetypal bulk metal-halide ABX3 perovskites and its dependence on A-cation, B-metal, and X-halide composition, which is now relatively well understood, are discussed. It is found that, for nanostructured and novel metal-halide materials, a larger variation in the strengths of polaronic effects is reported across the literature, potentially deriving from variations in potential barriers and the presence of interfaces at which lattice relaxation may be enhanced. Such findings are further discussed in the context of different experimental approaches used to explore polaronic effects, cautioning that firm conclusions are often hampered by the presence of alternate processes and interactions giving rise to similar experimental signatures. Overall, a complete understanding of polaronic effects will prove essential given their direct influence on optoelectronic properties such as charge-carrier mobilities and emission spectra, which are critical to the performance of energy and optoelectronic applications.
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Nanowires: A New Horizon for Polarization-resolved Terahertz Time-domain Spectroscopy

2021 Conference on Lasers and Electro-Optics, CLEO 2021 - Proceedings (2021)

Authors:

K Peng, D Jevtics, F Zhang, S Sterzl, DA Damry, MU Rothmann, B Guilhabert, MJ Strain, H Tan, LM Herz, L Fu, MD Dawson, A Hurtado, C Jagadish, MB Johnston

Abstract:

In this study, a novel type of broadband polarization-sensitive photoconductive terahertz detectors based on crossed nanowire networks is demonstrated, enabling fast and precise polarization terahertz time-domain spectroscopy measurements.

Charge-carrier mobility and localization in semiconducting CU2AGBiI6 for photovoltaic applications

ACS Energy Letters American Chemical Society 6:5 (2021) 1729-1739

Authors:

Leonardo RV Buizza, Adam D Wright, Giulia Longo, Harry C Sansom, Chelsea Q Xia, Matthew J Rosseinsky, Michael B Johnston, Henry J Snaith, Laura M Herz

Abstract:

Lead-free silver–bismuth semiconductors have become increasingly popular materials for optoelectronic applications, building upon the success of lead halide perovskites. In these materials, charge-lattice couplings fundamentally determine charge transport, critically affecting device performance. In this study, we investigate the optoelectronic properties of the recently discovered lead-free semiconductor Cu2AgBiI6 using temperature-dependent photoluminescence, absorption, and optical-pump terahertz-probe spectroscopy. We report ultrafast charge-carrier localization effects, evident from sharp THz photoconductivity decays occurring within a few picoseconds after excitation and a rise in intensity with decreasing temperature of long-lived, highly Stokes-shifted photoluminescence. We conclude that charge carriers in Cu2AgBiI6 are subject to strong charge-lattice coupling. However, such small polarons still exhibit mobilities in excess of 1 cm2 V–1 s–1 at room temperature because of low energetic barriers to formation and transport. Together with a low exciton binding energy of ∼29 meV and a direct band gap near 2.1 eV, these findings highlight Cu2AgBiI6 as an attractive lead-free material for photovoltaic applications.
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Limits to electrical mobility in lead-halide perovskite semiconductors

Journal of Physical Chemistry Letters American Chemical Society 12:14 (2021) 3607-3617

Authors:

Chelsea Xia, Jiali Peng, Samuel Poncé, Jay Patel, Adam Wright, Timothy W Crothers, Mathias Rothmann, Anna Juliane Borchert, Rebecca L Milot, Hans Kraus, Qianqian Lin, Feliciano Giustino, Laura Herz, Michael Johnston

Abstract:

Semiconducting polycrystalline thin films are cheap to produce and can be deposited on flexible substrates, yet high-performance electronic devices usually utilize single-crystal semiconductors, owing to their superior charge-carrier mobilities and longer diffusion lengths. Here we show that the electrical performance of polycrystalline films of metal-halide perovskites (MHPs) approaches that of single crystals at room temperature. Combining temperature-dependent terahertz conductivity measurements and ab initio calculations we uncover a complete picture of the origins of charge-carrier scattering in single crystals and polycrystalline films of CH3NH3PbI3. We show that Fröhlich scattering of charge carriers with multiple phonon modes is the dominant mechanism limiting mobility, with grain-boundary scattering further reducing mobility in polycrystalline films. We reconcile the large discrepancy in charge-carrier diffusion lengths between single crystals and films by considering photon reabsorption. Thus, polycrystalline films of MHPs offer great promise for devices beyond solar cells, including light-emitting diodes and modulators.
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Ultrafast excited-state localization in Cs2AgBiBr6 double perovskite

Journal of Physical Chemistry Letters American Chemical Society 12:13 (2021) 3352-3360

Authors:

Adam Wright, Leonardo RV Buizza, Kimberley Savill, Giulia Longo, Henry Snaith, Michael Johnston, Laura Herz

Abstract:

Cs2AgBiBr6 is a promising metal halide double perovskite offering the possibility of efficient photovoltaic devices based on lead-free materials. Here, we report on the evolution of photoexcited charge carriers in Cs2AgBiBr6 using a combination of temperature-dependent photoluminescence, absorption and optical pump–terahertz probe spectroscopy. We observe rapid decays in terahertz photoconductivity transients that reveal an ultrafast, barrier-free localization of free carriers on the time scale of 1.0 ps to an intrinsic small polaronic state. While the initially photogenerated delocalized charge carriers show bandlike transport, the self-trapped, small polaronic state exhibits temperature-activated mobilities, allowing the mobilities of both to still exceed 1 cm2 V–1 s–1 at room temperature. Self-trapped charge carriers subsequently diffuse to color centers, causing broad emission that is strongly red-shifted from a direct band edge whose band gap and associated exciton binding energy shrink with increasing temperature in a correlated manner. Overall, our observations suggest that strong electron–phonon coupling in this material induces rapid charge-carrier localization.
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