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Representation of THz spectroscopy of a metamaterial with a Nanowire THz sensor

Representation of THz spectroscopy of a metamaterial with a Nanowire THz sensor

Credit: Rendering by Dimitars Jevtics

Prof Michael Johnston

Professor of Physics

Research theme

  • Photovoltaics and nanoscience

Sub department

  • Condensed Matter Physics

Research groups

  • Terahertz photonics
  • Advanced Device Concepts for Next-Generation Photovoltaics
michael.johnston@physics.ox.ac.uk
Johnston Group Website
  • About
  • Publications

Bandgap-universal passivation enables stable perovskite solar cells with low photovoltage loss

Science American Association for the Advancement of Science 384:6697 (2024) 767-775

Authors:

Yen-Hung Lin, Vikram, Fengning Yang, Xue-Li Cao, Akash Dasgupta, Robert DJ Oliver, Aleksander M Ulatowski, Melissa M McCarthy, Xinyi Shen, Qimu Yuan, M Greyson Christoforo, Fion Sze Yan Yeung, Michael B Johnston, Nakita K Noel, Laura M Herz, M Saiful Islam, Henry J Snaith

Abstract:

The efficiency and longevity of metal-halide perovskite solar cells are typically dictated by nonradiative defect-mediated charge recombination. In this work, we demonstrate a vapor-based amino-silane passivation that reduces photovoltage deficits to around 100 millivolts (>90% of the thermodynamic limit) in perovskite solar cells of bandgaps between 1.6 and 1.8 electron volts, which is crucial for tandem applications. A primary-, secondary-, or tertiary-amino–silane alone negatively or barely affected perovskite crystallinity and charge transport, but amino-silanes that incorporate primary and secondary amines yield up to a 60-fold increase in photoluminescence quantum yield and preserve long-range conduction. Amino-silane–treated devices retained 95% power conversion efficiency for more than 1500 hours under full-spectrum sunlight at 85°C and open-circuit conditions in ambient air with a relative humidity of 50 to 60%.

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The Role of Chemical Composition in Determining the Charge‐Carrier Dynamics in (AgI)x(BiI3)y Rudorffites

Advanced Functional Materials Wiley (2024) 2315942

Authors:

Snigdha Lal, Marcello Righetto, Benjamin WJ Putland, Harry C Sansom, Silvia G Motti, Heon Jin, Michael B Johnston, Henry J Snaith, Laura M Herz

Abstract:

Silver‐bismuth‐based perovskite‐inspired materials (PIMs) are increasingly being explored as non‐toxic materials in photovoltaic applications. However, many of these materials exhibit an ultrafast localization of photogenerated charge carriers that is detrimental for charge‐carrier extraction. In this work, such localization processes are explored for thermally evaporated thin films of compositions lying along the (AgI)x(BiI3)y series, namely BiI3, AgBi2I7, AgBiI4, Ag2BiI5, Ag3BiI6, and AgI, to investigate the impact of changing Ag+/Bi3+ content. A persistent presence of ultrafast charge‐carrier localization in all mixed compositions and BiI3, together with unusually broad photoluminescence spectra, reveal that eliminating silver will not suppress the emergence of a localized state. A weak change in electronic bandgap and charge‐carrier mobility reveals the resilience of the electronic band structure upon modifications in the Ag+/Bi3+ composition of the mixed‐metal rudorffites. Instead, chemical composition impacts the charge‐carrier dynamics indirectly via structural alterations: Ag‐deficient compositions demonstrate stronger charge‐carrier localization most likely because a higher density of vacant sites in the cationic sublattice imparts enhanced lattice softness. Unraveling such delicate interplay between chemical composition, crystal structure, and charge‐carrier dynamics in (AgI)x(BiI3)y rudorffites provides crucial insights for developing a material‐by‐design approach in the quest for highly efficient Bi‐based PIMs.
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Unraveling loss mechanisms arising from energy-level misalignment between metal halide perovskites and hole transport layers

Advanced Functional Materials Wiley 34:30 (2024) 2401052

Authors:

Jae Eun Lee, Silvia G Motti, Robert DJ Oliver, Siyu Yan, Henry J Snaith, Michael B Johnston, Laura M Herz

Abstract:

Metal halide perovskites are promising light absorbers for multijunction photovoltaic applications because of their remarkable bandgap tunability, achieved through compositional mixing on the halide site. However, poor energy-level alignment at the interface between wide-bandgap mixed-halide perovskites and charge-extraction layers still causes significant losses in solar-cell performance. Here, the origin of such losses is investigated, focusing on the energy-level misalignment between the valence band maximum and the highest occupied molecular orbital (HOMO) for a commonly employed combination, FA0.83Cs0.17Pb(I1-xBrx)3 with bromide content x ranging from 0 to 1, and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA). A combination of time-resolved photoluminescence spectroscopy and numerical modeling of charge-carrier dynamics reveals that open-circuit voltage (VOC) losses associated with a rising energy-level misalignment derive from increasing accumulation of holes in the HOMO of PTAA, which then subsequently recombine non-radiatively across the interface via interfacial defects. Simulations assuming an ideal choice of hole-transport material to pair with FA0.83Cs0.17Pb(I1-xBrx)3 show that such VOC losses originating from energy-level misalignment can be reduced by up to 70 mV. These findings highlight the urgent need for tailored charge-extraction materials exhibiting improved energy-level alignment with wide-bandgap mixed-halide perovskites to enable solar cells with improved power conversion efficiencies.

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The Role of the Organic Cation in Developing Efficient Green Perovskite LEDs Based on Quasi‐2D Perovskite Heterostructures

Advanced Functional Materials Wiley 34:14 (2024)

Authors:

Alexandra J Ramadan, Woo Hyeon Jeong, Robert DJ Oliver, Junke Jiang, Akash Dasgupta, Zhongcheng Yuan, Joel Smith, Jae Eun Lee, Silvia G Motti, Olivia Gough, Zhenlong Li, Laura M Herz, Michael B Johnston, Hyosung Choi, Jacky Even, Claudine Katan, Bo Ram Lee, Henry J Snaith
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Intermediate-Phase Engineering via Dimethylammonium Cation Additive for Stable Perovskite Solar Cells

Fundacio Scito (2024)

Authors:

Philippe Holzhey, Henry Snaith, Sebastian Fürer, Steven Harvey, Laura Schelhas, James Ball, Suhas Mahesh, David McMeekin, Nicholas Hawkins, Jianfeng Lu, Michael Johnston, Joseph Berry, Udo Bach, Seongrok Seo
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