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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
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Publons/WoS
  • About
  • Publications

Thermally stable perovskite solar cells by all-vacuum deposition

ACS Applied Materials and Interfaces American Chemical Society 15:1 (2022) 772-781

Abstract:

Vacuum deposition is a solvent-free method suitable for growing thin films of metal halide perovskite (MHP) semiconductors. However, most reports of high-efficiency solar cells based on such vacuum-deposited MHP films incorporate solution-processed hole transport layers (HTLs), thereby complicating prospects of industrial upscaling and potentially affecting the overall device stability. In this work, we investigate organometallic copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) as alternative, low-cost, and durable HTLs in all-vacuum-deposited solvent-free formamidinium-cesium lead triodide [CH(NH2)2]0.83Cs0.17PbI3 (FACsPbI3) perovskite solar cells. We elucidate that the CuPc HTL, when employed in an “inverted” p–i–n solar cell configuration, attains a solar-to-electrical power conversion efficiency of up to 13.9%. Importantly, unencapsulated devices as large as 1 cm2 exhibited excellent long-term stability, demonstrating no observable degradation in efficiency after more than 5000 h in storage and 3700 h under 85 °C thermal stressing in N2 atmosphere.

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Bending a photonic wire into a ring

Nature Chemistry Springer Nature 14 (2022) 1436-1442

Authors:

Henrik Gotfredsen, Jie-Ren Deng, Jeff M Van Raden, Marcello Righetto, Janko Hergenhahn, Michael Clarke, Abigail Bellamy-Carter, Jack Hart, James O'Shea, Timothy DW Claridge, Fernanda Duarte, Alex Saywell, Laura M Herz, Harry L Anderson

Abstract:

Natural light-harvesting systems absorb sunlight and transfer its energy to the reaction centre, where it is used for photosynthesis. Synthetic chromophore arrays provide useful models for understanding energy migration in these systems. Research has focused on mimicking rings of chlorophyll molecules found in purple bacteria, known as ‘light-harvesting system 2’. Linear meso–meso linked porphyrin chains mediate rapid energy migration, but until now it has not been possible to bend them into rings. Here we show that oligo-pyridyl templates can be used to bend these rod-like photonic wires to create covalent nanorings that consist of 24 porphyrin units and a single butadiyne link. Their elliptical conformations have been probed by scanning tunnelling microscopy. This system exhibits two excited state energy transfer processes: one from a bound template to the peripheral porphyrins and one, in the template-free ring, from the exciton-coupled porphyrin array to the π-conjugated butadiyne-linked porphyrin dimer segment.
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Topological Dirac semi-metals as novel, optically-switchable, helicity-dependent terahertz sources

2022 47th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz) IEEE (2022)

Authors:

Jessica L Boland, Chelsea Q Xia, Djamshid A Damry, Piet Schonherr, Dharmalingam Prabhakaran, Laura M Herz, Thorsten Hesjedal, Michael B Johnston

Abstract:

The generation and control of terahertz pulses is vital for realizing the potential of terahertz radiation in several sectors, including 6G communication, security and imaging. In this work, we present the topological Dirac semimetal cadmium arsenide as a novel helicity-dependent terahertz source. We show both broadband (single-cycle) and narrowband (multi-cycle) terahertz pulses upon near-infrared photoexcitation at oblique incidence. By varying the incident angle of the photoexcitation pulse, control of the emission frequency can also be achieved, providing a candidate for a tuneable narrowband terahertz source.
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Advances and challenges in understanding the microscopic structure-property-performance relationship in perovskite solar cells

Nature Energy Springer Nature 7:9 (2022) 794-807

Authors:

Yuanyuan Zhou, Laura M Herz, Alex K-Y Jen, Michael Saliba

Abstract:

The emergence of perovskite photovoltaic technology is transforming the landscape of solar energy. Its rapid development has been driven by the advances in our understanding of the thin-film microstructures of metal halide perovskites and their intriguing correlations with optoelectronic properties, device efficiency and long-term stability. Here we discuss the morphological characteristics of three key microstructure types encountered in perovskites, which include grain boundaries, intragrain defects and surfaces. To reveal detailed structural information of these microstructure types via tailored characterizations is crucial to probe their detrimental, neutral or beneficial effects on optoelectronic properties. We further elaborate the impacts of these microstructures on the degradation modes of perovskites. Representative examples are also presented, which have translated fundamental understandings to achieve state-of-the-art perovskite solar cells. Finally, we call for more attention in probing hidden microstructures and developing high-spatiotemporal-resolution characterizations, as well as harnessing the potential merits of microstructural imperfections, towards an elevated understanding of microstructure–property–performance relationships for the next solar cell advances.
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Strong absorption and ultrafast localisation in NaBiS2 nanocrystals with slow charge-carrier recombination

Nature Communications Springer Nature 13 (2022) 4960

Authors:

Yi-Teng Huang, Seán R Kavanagh, Marcello Righetto, Marin Rusu, Igal Levine, Thomas Unold, Szymon J Zelewski, Alexander J Sneyd, Kaiwen Zhang, Linjie Dai, Andrew J Britton, Junzhi Ye, Jaakko Julin, Mari Napari, Zhilong Zhang, James Xiao, Mikko Laitinen, Laura Torrente-Murciano, Samuel D Stranks, Akshay Rao, Laura M Herz, David O Scanlon, Aron Walsh, Robert Hoye

Abstract:

I-V-VI2 ternary chalcogenides are gaining attention as earth-abundant, nontoxic, and air-stable absorbers for photovoltaic applications. However, the semiconductors explored thus far have slowly-rising absorption onsets, and their charge-carrier transport is not well understood yet. Herein, we investigate cation-disordered NaBiS2 nanocrystals, which have a steep absorption onset, with absorption coefficients reaching >105 cm−1 just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-time scale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS2 nanocrystals. These unusual features arise because of the localised, non-bonding S p character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes. This work reveals the critical role of cation disorder in these systems on both absorption characteristics and charge-carrier kinetics.

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