Publications by Laura Herz

Crystallization of CsPbBr<sub>3</sub> single crystals in water for X-ray detection.

Nature communications 12 (2021) 1531-

J Peng, CQ Xia, Y Xu, R Li, L Cui, JK Clegg, LM Herz, MB Johnston, Q Lin

Metal halide perovskites have fascinated the research community over the past decade, and demonstrated unprecedented success in optoelectronics. In particular, perovskite single crystals have emerged as promising candidates for ionization radiation detection, due to the excellent opto-electronic properties. However, most of the reported crystals are grown in organic solvents and require high temperature. In this work, we develop a low-temperature crystallization strategy to grow CsPbBr<sub>3</sub> perovskite single crystals in water. Then, we carefully investigate the structure and optoelectronic properties of the crystals obtained, and compare them with CsPbBr<sub>3</sub> crystals grown in dimethyl sulfoxide. Interestingly, the water grown crystals exhibit a distinct crystal habit, superior charge transport properties and better stability in air. We also fabricate X-ray detectors based on the CsPbBr<sub>3</sub> crystals, and systematically characterize their device performance. The crystals grown in water demonstrate great potential for X-ray imaging with enhanced performance metrics.

Highly Absorbing Lead-Free Semiconductor Cu<sub>2</sub>AgBiI<sub>6</sub> for Photovoltaic Applications from the Quaternary CuI-AgI-BiI<sub>3</sub> Phase Space.

Journal of the American Chemical Society 143 (2021) 3983-3992

HC Sansom, G Longo, AD Wright, LRV Buizza, S Mahesh, B Wenger, M Zanella, M Abdi-Jalebi, MJ Pitcher, MS Dyer, TD Manning, RH Friend, LM Herz, HJ Snaith, JB Claridge, MJ Rosseinsky

Since the emergence of lead halide perovskites for photovoltaic research, there has been mounting effort in the search for alternative compounds with improved or complementary physical, chemical, or optoelectronic properties. Here, we report the discovery of Cu<sub>2</sub>AgBiI<sub>6</sub>: a stable, inorganic, lead-free wide-band-gap semiconductor, well suited for use in lead-free tandem photovoltaics. We measure a very high absorption coefficient of 1.0 × 10<sup>5</sup> cm<sup>-1</sup> near the absorption onset, several times that of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>. Solution-processed Cu<sub>2</sub>AgBiI<sub>6</sub> thin films show a direct band gap of 2.06(1) eV, an exciton binding energy of 25 meV, a substantial charge-carrier mobility (1.7 cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup>), a long photoluminescence lifetime (33 ns), and a relatively small Stokes shift between absorption and emission. Crucially, we solve the structure of the first quaternary compound in the phase space among CuI, AgI and BiI<sub>3</sub>. The structure includes both tetrahedral and octahedral species which are open to compositional tuning and chemical substitution to further enhance properties. Since the proposed double-perovskite Cs<sub>2</sub>AgBiI<sub>6</sub> thin films have not been synthesized to date, Cu<sub>2</sub>AgBiI<sub>6</sub> is a valuable example of a stable Ag<sup>+</sup>/Bi<sup>3+</sup> octahedral motif in a close-packed iodide sublattice that is accessed via the enhanced chemical diversity of the quaternary phase space.

Halide segregation in mixed-halide perovskites: influence of a-site cations

ACS Energy Letters American Chemical Society 6 (2021) 799-808

A Knight, AJ Borchert, RDJ Oliver, J Patel, PG Radaelli, H Snaith, MB Johnston, LM Herz

Mixed-halide perovskites offer bandgap tunability essential for multijunction solar cells; however, a detrimental halide segregation under light is often observed. Here we combine simultaneous in situ photoluminescence and X-ray diffraction measurements to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br0.5I0.5)3 and FA0.83Cs0.17Pb(Br0.4I0.6)3 films. We report evidence for low-barrier ionic pathways in MAPb(Br0.5I0.5)3, which allow for the rearrangement of halide ions in localized volumes of perovskite without significant compositional changes to the bulk material. In contrast, FA0.83Cs0.17Pb(Br0.4I0.6)3 lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation. Our work elucidates links between composition, ionic pathways, and halide segregation, and it facilitates the future engineering of phase-stable mixed-halide perovskites.

Nanotechnology for catalysis and solar energy conversion.

Nanotechnology 32 (2021) 042003-

U Banin, N Waiskopf, L Hammarström, G Boschloo, M Freitag, EMJ Johansson, J Sá, H Tian, MB Johnston, LM Herz, RL Milot, MG Kanatzidis, W Ke, I Spanopoulos, KL Kohlstedt, GC Schatz, N Lewis, T Meyer, AJ Nozik, MC Beard, F Armstrong, CF Megarity, CA Schmuttenmaer, VS Batista, GW Brudvig

This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: 'high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing' to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as described by Spanopoulos et al 'Next generation' solar cells using multiple exciton generation (MEG) from hot carriers, described in the article by Nozik and Beard, could lead to remarkable improvement in photovoltaic efficiency by using quantization effects in semiconductor nanostructures (quantum dots, wires or wells). These challenges will not be met without simultaneous improvement in nanoscale characterization methods. Terahertz spectroscopy, discussed in the article by Milot et al is one example of a method that is overcoming the difficulties associated with nanoscale materials characterization by avoiding electrical contacts to nanoparticles, allowing characterization during device operation, and enabling characterization of a single nanoparticle. Besides experimental advances, computational science is also meeting the challenges of nanomaterials synthesis. The article by Kohlstedt and Schatz discusses the computational frameworks being used to predict structure-property relationships in materials and devices, including machine learning methods, with an emphasis on organic photovoltaics. The contribution by Megarity and Armstrong presents the 'electrochemical leaf' for improvements in electrochemistry and beyond. In addition, biohybrid approaches can take advantage of efficient and specific enzyme catalysts. These articles present the nanoscience and technology at the forefront of renewable energy development that will have significant benefits to society.

Terahertz Conductivity Analysis for Highly Doped Thin-Film Semiconductors


AM Ulatowski, LM Herz, MB Johnston

Impact of tin fluoride additive on the properties of mixed tin-lead iodide perovskite semiconductors

Advanced Functional Materials Wiley 30 (2020) 2005594

KJ Savill, AM Ulatowski, MD Farrar, MB Johnston, HJ Snaith, LM Herz

Mixed tin‐lead halide perovskites are promising low‐bandgap absorbers for all‐perovskite tandem solar cells that offer higher efficiencies than single‐junction devices. A significant barrier to higher performance and stability is the ready oxidation of tin, commonly mitigated by various additives whose impact is still poorly understood for mixed tin‐lead perovskites. Here, the effects of the commonly used SnF2 additive are revealed for FA0.83Cs0.17SnxPb1−xI3 perovskites across the full compositional lead‐tin range and SnF2 percentages of 0.1–20% of precursor tin content. SnF2 addition causes a significant reduction in the background hole density associated with tin vacancies, yielding longer photoluminescence lifetimes, decreased energetic disorder, reduced Burstein–Moss shifts, and higher charge‐carrier mobilities. Such effects are optimized for SnF2 addition of 1%, while for 5% SnF2 and above, additional nonradiative recombination pathways begin to appear. It is further found that the addition of SnF2 reduces a tetragonal distortion in the perovskite structure deriving from the presence of tin vacancies that cause strain, particularly for high tin content. The optical phonon response associated with inorganic lattice vibrations is further explored, exhibiting a shift to higher frequency and significant broadening with increasing tin fraction, in accordance with lower effective atomic metal masses and shorter phonon lifetimes.

Efficient energy transfer mitigates parasitic light absorption in molecular charge-extraction layers for perovskite solar cells

Nature Communications Springer Science 11 (2020) 5525

HJ Eggimann, JB Patel, MB Johnston, LM Herz

Organic semiconductors are commonly used as charge-extraction layers in metal-halide perovskite solar cells. However, parasitic light absorption in the sun-facing front molecular layer, through which sun light must propagate before reaching the perovskite layer, may lower the power conversion efficiency of such devices. Here, we show that such losses may be eliminated through efficient excitation energy transfer from a photoexcited polymer layer to the underlying&#xA0;perovskite. Experimentally observed energy transfer between a range of different polymer films and a methylammonium lead iodide perovskite layer was used as basis for modelling the efficacy of the mechanism as a function of layer thickness, photoluminescence quantum efficiency and absorption coefficient of the organic polymer film. Our findings reveal that efficient energy transfer can be achieved for thin (&#x2264;10&#xA0;nm) organic charge-extraction layers exhibiting high photoluminescence quantum efficiency. We further explore how the morphology of such thin polymer layers may be affected by interface formation with the perovskite.

Intrinsic quantum confinement in formamidiniumlead triiodide perovskite

Nature Materials Nature Research 19 (2020) 1201-1206

AD Wright, G Volonakis, J Borchert, M Johnston, L Herz

Understanding the electronic energy landscape in metal halide perovskites is essential for further improvements in their promising performance in thin-film photovoltaics. Here, we uncover the presence of above-bandgap oscillatory features in the absorption spectra of formamidinium lead triiodide thin films. We attribute these discrete features to intrinsically occurring quantum confinement effects, for which the related energies change with temperature according to the inverse square of the intrinsic lattice parameter, and with peak index in a quadratic manner. By determining the threshold film thickness at which the amplitude of the peaks is appreciably decreased, and through ab initio simulations of the absorption features, we estimate the length scale of confinement to be 10–20 nm. Such absorption peaks present a new and intriguing quantum electronic phenomenon in a nominally bulk semiconductor, offering intrinsic nanoscale optoelectronic properties without necessitating cumbersome additional processing steps.

Atomic-scale microstructure of metalhalide perovskite

Science American Association for the Advancement of Science 370 (2020) eabb5940

J Kim, H Snaith, M Johnston, L Herz, AJ Borchert, M Rothmann

Hybrid organic-inorganic perovskites are exciting materials for solar-energy applications whose microscopic properties are still not well understood. Atomic-resolution (scanning) transmission electron microscopy, (S)TEM, has provided invaluable insights for many crystalline solar-cell materials, and is used here to successfully image CH(NH2)2PbI3 thin films with low electron-radiation dose. Such images reveal a highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI2 interfaces, with a striking absence of long-range disorder in the crystal. We demonstrate that beaminduced degradation of the perovskite leads to an initial loss of CH(NH2)2 + ions, leaving behind a partially unoccupied perovskite lattice, which explains the unusual regenerative properties of these materials. We further observe aligned point defects and climbdissociated dislocations. Our findings thus provide an atomic-level understanding of technologically important lead-halide perovskites.

Charge‐carrier trapping and radiative recombination in metal halide perovskite semiconductors

Advanced Functional Materials Wiley 30 (2020) 2004312

MJ Trimpl, AD Wright, K Schutt, LRV Buizza, Z Wang, MB Johnston, H Snaith, P Müller‐Buschbaum, LM Herz

Trap‐related charge‐carrier recombination fundamentally limits the performance of perovskite solar cells and other optoelectronic devices. While improved fabrication and passivation techniques have reduced trap densities, the properties of trap states and their impact on the charge‐carrier dynamics in metal‐halide perovskites are still under debate. Here, a unified model is presented of the radiative and nonradiative recombination channels in a mixed formamidinium‐cesium lead iodide perovskite, including charge‐carrier trapping, de‐trapping and accumulation, as well as higher‐order recombination mechanisms. A fast initial photoluminescence (PL) decay component observed after pulsed photogeneration is demonstrated to result from rapid localization of free charge carriers in unoccupied trap states, which may be followed by de‐trapping, or nonradiative recombination with free carriers of opposite charge. Such initial decay components are shown to be highly sensitive to remnant charge carriers that accumulate in traps under pulsed‐laser excitation, with partial trap occupation masking the trap density actually present in the material. Finally, such modelling reveals a change in trap density at the phase transition, and disentangles the radiative and nonradiative charge recombination channels present in FA0.95Cs0.05PbI3, accurately predicting the experimentally recorded PL efficiencies between 50 and 295 K, and demonstrating that bimolecular recombination is a fully radiative process.

Terahertz conductivity analysis for highly doped thin-film semiconductors

International Journal of Infrared and Millimeter Waves Springer 41 (2020) 1431-1449

A Ulatowski, L Herz, M Johnston

The analysis of terahertz transmission through semiconducting thin films has proven to be an excellent tool for investigating optoelectronic properties of novel materials. Terahertz time-domain spectroscopy (THz-TDS) can provide information about phonon modes of the crystal, as well as the electrical conductivity of the sample. When paired with photoexcitation, optical-pump-THz-probe (OPTP) technique can be used to gain an insight into the transient photoconductivity of the semiconductor, revealing the dynamics and the mobility of photoexcited charge carriers. As the relation between the conductivity of the material and the THz transmission function is generally complicated, simple analytical expressions have been developed to enable straightforward calculations of frequency-dependent conductivity from THz-TDS data in the regime of optically thin samples. Here, we assess the accuracy of these approximated analytical formulas in thin films of highly doped semiconductors, finding significant deviations of the calculated photoconductivity from its actual value in materials with background conductivity comparable to 102Ω− 1cm− 1. We propose an alternative analytical expression, which greatly improves the accuracy of the estimated value of the real photoconductivity, while remaining simple to implement experimentally. Our approximation remains valid in thin films with high dark conductivity of up to 104Ω− 1cm− 1 and provides a very high precision for calculating photoconductivity up to 104Ω− 1cm− 1, and therefore is highly relevant for studies of photoexcited charge-carrier dynamics in electrically doped semiconductors. Using the example of heavily doped thin films of tin-iodide perovskites, we show a simple experimental method of implementing our correction and find that the commonly used expression for photoconductivity could result in an underestimate of charge-carrier mobility by over 50%.

Postpassivation of Multication Perovskite with Rubidium Butyrate

ACS Photonics American Chemical Society (ACS) (2020) acsphotonics.0c00878

JC Germino, R Szostak, SG Motti, RF Moral, PE Marchezi, HS Seleghini, LG Bonato, FL de Araújo, TDZ Atvars, LM Herz, D Fenning, A Hagfeldt, AF Nogueira

Understanding the Performance-Limiting Factors of Cs2AgBiBr6 Double-Perovskite Solar Cells

ACS ENERGY LETTERS 5 (2020) 2200-2207

G Longo, S Mahesh, LRV Buizza, AD Wright, AJ Ramadan, M Abdi-Jalebi, PK Nayak, LM Herz, HJ Snaith

Preventing phase segregation in mixed-halide perovskites: a perspective

Energy & Environmental Science Royal Society of Chemistry (RSC) (2020)

AJ Knight, LM Herz

&lt;p&gt;Halide segregation represents a severe stability problem for certain mixed-halide perovskites. Here we explore a myriad of methods for mitigating halide segregation, including several largely unexplored approaches that show significant promise.&lt;/p&gt;

Metal composition influences optoelectronic quality in mixed-metal lead-tin triiodide perovskite solar absorbers

Energy & Environmental Science Royal Society of Chemistry 13 (2020) 1776-1787

M Klug, R Milot, J Patel, T Green, H Sansom, M Farrar, A Ramadan, S Martani, Z Wang, B Wenger, J Ball, L Langshaw, A Petrozza, M Johnston, L Herz, HJ Snaith

Current designs for all-perovskite multi-junction solar cells require mixed-metal Pb-Sn compositions to achieve narrower band gaps than are possible with their neat Pb counterparts. The lower band gap range achievable with mixed-metal Pb-Sn perovskites also encompasses the 1.3 to 1.4 eV range that is theoretically ideal for maximising the efficiency of single-junction devices. Here we examine the optoelectronic quality and photovoltaic performance of the ((HC(NH2)2)0.83Cs0.17)(Pb1-ySny)I3 family of perovskite materials across the full range of achievable band gaps by substituting between 0.001% and 70% of the Pb content with Sn. We reveal that a compositional range of "defectiveness"exists when Sn comprises between 0.5% and 20% of the metal content, but that the optoelectronic quality is restored for Sn content between 30-50%. When only 1% of Pb content is replaced by Sn, we find that photoconductivity, photoluminescence lifetime, and photoluminescence quantum efficiency are reduced by at least an order of magnitude, which reveals that a small concentration of Sn incorporation produces trap sites that promote non-radiative recombination in the material and limit photovoltaic performance. While these observations suggest that band gaps between 1.35 and 1.5 eV are unlikely to be useful for optoelectronic applications without countermeasures to improve material quality, highly efficient narrower band gap absorber materials are possible at or below 1.33 eV. Through optimising single-junction photovoltaic devices with Sn compositions of 30% and 50%, we respectively demonstrate a 17.6% efficient solar cell with an ideal single-junction band gap of 1.33 eV and an 18.1% efficient low band gap device suitable for the bottom absorber in all-perovskite multi-junction cells.

Charge-Carrier Trapping Dynamics in Bismuth-Doped Thin Films of MAPbBr3 Perovskite.

The journal of physical chemistry letters American Chemical Society (ACS) 11 (2020) 3681-3688

AM Ulatowski, AD Wright, B Wenger, LRV Buizza, SG Motti, HJ Eggimann, KJ Savill, J Borchert, HJ Snaith, MB Johnston, LM Herz

Successful chemical doping of metal halide perovskites with small amounts of heterovalent metals has attracted recent research attention because of its potential to improve long-term material stability and tune absorption spectra. However, some additives have been observed to impact negatively on optoelectronic properties, highlighting the importance of understanding charge-carrier behavior in doped metal halide perovskites. Here, we present an investigation of charge-carrier trapping and conduction in films of MAPbBr3 perovskite chemically doped with bismuth. We find that the addition of bismuth has no effect on either the band gap or exciton binding energy of the MAPbBr3 host. However, we observe a substantial enhancement of electron-trapping defects upon bismuth doping, which results in an ultrafast charge-carrier decay component, enhanced infrared emission, and a notable decrease of charge-carrier mobility. We propose that such defects arise from the current approach to Bi-doping through addition of BiBr3, which may enhance the presence of bromide interstitials.

Three-dimensional cross-nanowire networks recover full terahertz state

Science American Association for the Advancement of Science 368 (2020) 510-513

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

Terahertz radiation encompasses a wide band of the electromagnetic spectrum, spanning from microwaves to infrared light, and is a particularly powerful tool for both fundamental scientific research and applications such as security screening, communications, quality control, and medical imaging. Considerable information can be conveyed by the full polarization state of terahertz light, yet to date, most time-domain terahertz detectors are sensitive to just one polarization component. Here we demonstrate a nanotechnology-based semiconductor detector using cross-nanowire networks that records the full polarization state of terahertz pulses. The monolithic device allows simultaneous measurements of the orthogonal components of the terahertz electric field vector without cross-talk. Furthermore, we demonstrate the capabilities of the detector for the study of metamaterials.

CsPbBr3 nanocrystal films: Deviations from bulk vibrational and optoelectronic properties

Advanced Functional Materials Wiley 30 (2020) 1909904

SG Motti, F Krieg, AJ Ramadan, JB Patel, HJ Snaith, MV Kovalenko, MB Johnston, LM Herz

Metal‐halide perovskites (MHP) are highly promising semiconductors for light‐emitting and photovoltaic applications. The colloidal synthesis of nanocrystals (NCs) is an effective approach for obtaining nearly defect‐free MHP that can be processed into inks for low‐cost, high‐performance device fabrication. However, disentangling the effects of surface ligands, morphology, and boundaries on charge‐carrier transport in thin films fabricated with these high‐quality NCs is inherently difficult. To overcome this fundamental challenge, terahertz (THz) spectroscopy is employed to optically probe the photoconductivity of CsPbBr3 NC films. The vibrational and optoelectronic properties of the NCs are compared with those of the corresponding bulk polycrystalline perovskite and significant deviations are found. Charge‐carrier mobilities and recombination rates are demonstrated to vary significantly with the NC size. Such dependences derive from the localized nature of charge carriers within NCs, with local mobilities dominating over interparticle transport. It is further shown that the colloidally synthesized NCs have distinct vibrational properties with respect to the bulk perovskite, exhibiting blue‐shifted optical phonon modes with enhanced THz absorption strength that also manifest as strong modulations in the THz photoconductivity spectra. Such fundamental insights into NC versus bulk properties will guide the optimization of nanocrystalline perovskite thin films for optoelectronic applications.

Three-dimensional cross-nanowire networks recover full terahertz state.

Science (New York, N.Y.) 368 (2020) 510-513

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

Terahertz radiation encompasses a wide band of the electromagnetic spectrum, spanning from microwaves to infrared light, and is a particularly powerful tool for both fundamental scientific research and applications such as security screening, communications, quality control, and medical imaging. Considerable information can be conveyed by the full polarization state of terahertz light, yet to date, most time-domain terahertz detectors are sensitive to just one polarization component. Here we demonstrate a nanotechnology-based semiconductor detector using cross-nanowire networks that records the full polarization state of terahertz pulses. The monolithic device allows simultaneous measurements of the orthogonal components of the terahertz electric field vector without cross-talk. Furthermore, we demonstrate the capabilities of the detector for the study of metamaterials.

Control over crystal size in vapor deposited metal-halide perovskite films

ACS Energy Letters American Chemical Society (ACS) 5 (2020) 0c00183

KB Lohmann, JB Patel, MU Rothmann, CQ Xia, RDJ Oliver, LM Herz, HJ Snaith, MB Johnston

Understanding and controlling grain growth in metal halide perovskite polycrystalline thin films is an important step in improving the performance of perovskite solar cells. We demonstrate accurate control of crystallite size in CH3NH3PbI3 thin films by regulating substrate temperature during vacuum co-deposition of inorganic (PbI2) and organic (CH3NH3I) precursors. Films co-deposited onto a cold (−2 °C) substrate exhibited large, micrometer-sized crystal grains, while films that formed at room temperature (23 °C) only produced grains of 100 nm extent. We isolated the effects of substrate temperature on crystal growth by developing a new method to control sublimation of the organic precursor, and CH3NH3PbI3 solar cells deposited in this way yielded a power conversion efficiency of up to 18.2%. Furthermore, we found substrate temperature directly affects the adsorption rate of CH3NH3I, thus impacting crystal formation and hence solar cell device performance via changes to the conversion rate of PbI2 to CH3NH3PbI3 and stoichiometry. These findings offer new routes to developing efficient solar cells through reproducible control of crystal morphology and composition.