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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

Light absorption and recycling in hybrid metal halide perovskites photovoltaic devices

Advanced Energy Materials Wiley 10:10 (2020) 1903653

Authors:

Jay Patel, Adam Wright, Kilian Lohmann, Kun Peng, Chelsea Xia, James Ball, Nakita Noel, Timothy Crothers, Henry Snaith, Laura Herz, Michael Johnston

Abstract:

The production of highly efficient single‐ and multijunction metal halide perovskite (MHP) solar cells requires careful optimization of the optical and electrical properties of these devices. Here, precise control of CH3NH3PbI3 perovskite layers is demonstrated in solar cell devices through the use of dual source coevaporation. Light absorption and device performance are tracked for incorporated MHP films ranging from ≈67 nm to ≈1.4 µm thickness and transfer‐matrix optical modeling is utilized to quantify optical losses that arise from interference effects. Based on these results, a device with 19.2% steady‐state power conversion efficiency is achieved through incorporation of a perovskite film with near‐optimum predicted thickness (≈709 nm). Significantly, a clear signature of photon reabsorption is observed in perovskite films that have the same thickness (≈709 nm) as in the optimized device. Despite the positive effect of photon recycling associated with photon reabsorption, devices with thicker (>750 nm) MHP layers exhibit poor performance owing to competing nonradiative charge recombination in a “dead‐volume” of MHP. Overall, these findings demonstrate the need for fine control over MHP thickness to achieve the highest efficiency cells, and accurate consideration of photon reabsorption, optical interference, and charge transport properties.
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Trap states, electric fields, and phase segregation in mixed-halide perovskite photovoltaic devices

Advanced Energy Materials Wiley 10:9 (2020) 1903488

Authors:

Alexander Knight, Jay Patel, Henry Snaith, Michael Johnston, Laura Herz

Abstract:

Mixed-halide perovskites are essential for use in all-perovskite or perovskite–silicon tandem solar cells due to their tunable bandgap. However, trap states and halide segregation currently present the two main challenges for efficient mixed-halide perovskite technologies. Here photoluminescence techniques are used to study trap states and halide segregation in full mixed-halide perovskite photovoltaic devices. This work identifies three distinct defect species in the perovskite material: a charged, mobile defect that traps charge-carriers in the perovskite, a charge-neutral defect that induces halide segregation, and a charged, mobile defect that screens the perovskite from external electric fields. These three defects are proposed to be MA+ interstitials, crystal distortions, and halide vacancies and/or interstitials, respectively. Finally, external quantum efficiency measurements show that photoexcited charge-carriers can be extracted from the iodide-rich low-bandgap regions of the phase-segregated perovskite formed under illumination, suggesting the existence of charge-carrier percolation pathways through grain boundaries where phase-segregation may occur.
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Charge-Carrier Cooling and Polarization Memory Loss in Formamidinium Tin Triiodide

Fundacio Scito (2019)

Authors:

Kimberley Savill, Matthew Klug, Rebecca Milot, Henry Snaith, Laura Herz
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Dual-source co-evaporation of low-bandgap FA1-xCsxSn1-yPbyI3 perovskites for photovoltaics

ACS Energy Letters American Chemical Society 4 (2019) 2748-2756

Authors:

JM Ball, L Buizza, HC Sansom, Farrar, MT Klug, J Borchert, J Patel, LM Herz, Michael Johnston, Henry Snaith
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Charge-carrier cooling and polarization memory loss in formamidinium tin triiodide

Journal of Physical Chemistry Letters American Chemical Society 10:20 (2019) 6038-6047

Authors:

Kimberley Savill, Matthew Klug, RL Milot, Henry Snaith, Laura Herz

Abstract:

Combination of a cryogenic ion-trap machine, operated at 4.7 K, with the free-electron-laser FELIX allows the first experimental characterization of the unusually bright antisymmetric stretch (ν3) and π-bending (ν2) fundamentals of the He–X+–He (X = H, D) chromophore of the in situ prepared HHen+ and DHen+ (n = 3–6) complexes. The band origins obtained are fully supported by first-principles quantum-chemical computations, performed at the MP2, the CCSD(T), and occasionally the CCSDTQ levels employing extended basis sets. Both the experiments and the computations are consistent with structures for the species with n = 3 and 6 being of T-shaped C2v and of D4h symmetry, respectively, while the species with n = 4 are suggested to exhibit interesting dynamical phenomena related to large-amplitude motions.

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