R. Jaramillo

3.88% Efficient Tin Sulfide Solar Cells using Congruent Thermal Evaporation

Tin sulfide (SnS), as a promising absorber material in thin-film photovoltaic devices, is described. Here, it is confirmed that SnS evaporates congruently, which provides facile composition control akin to cadmium telluride. A SnS heterojunction solar cell is demons trated, which has a power conversion efficiency of 3.88% (certified), and an empirical loss analysis is presented to guide further performance improvements.

Direct measurement of antiferromagnetic domain fluctuations

Measurements of magnetic noise emanating from ferromagnets owing to domain motion were first carried out nearly 100 years ago, and have underpinned much science and technology. Antiferromagnets, which carry no net external magnetic dipole moment, yet have a periodic arrangement of the electron spins extending over macroscopic distances, should also display magnetic noise. However, this must be sampled at spatial wavelengths of the order of several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets. Here we present a direct measurement of the fluctuations in the nanometre-scale superstructure of spin- and charge-density waves associated with antiferromagnetism in elemental chromium. The technique used is X-ray photon correlation spectroscopy, where coherent X-ray diffraction produces a speckle pattern that serves as a ‘fingerprint’ of a particular magnetic domain configuration. The temporal evolution of the patterns corresponds to domain walls advancing and retreating over micrometre distances. This work demonstrates a useful measurement tool for antiferromagnetic domain wall engineering, but also reveals a fundamental finding about spin dynamics in the simplest antiferromagnet: although the domain wall motion is thermally activated at temperatures above 100 K, it is not so at lower temperatures, and indeed has a rate that saturates at a finite value—consistent with quantum fluctuations—on cooling below 40 K.

Origins of bad-metal conductivity and the insulator-metal transition in the rare-earth nickelates

Bad metals, such as the copper oxide superconductors, do conduct electricity but the origin of their poor conductivity is unclear. A study of disordered rare-earth nickelates now provides microscopic insights into bad-metal behaviour

Variations of ionization potential and electron affinity as a function of surface orientation: The case of orthorhombic SnS

We investigated the dependence of absolute SnS band-edge energies on surface orientation using density functional theory and GW method for all surfaces with Miller indices −3≤h,k,l≤3 and found variations as large as 0.9 eV as a function of (hkl). Variations of this magnitude may affect significantly the performance of photovoltaic devices based on polycrystalline SnS thin-films and, in particular, may contribute to the relatively low measured open circuit voltage of SnS solar cells. X-ray diffraction measurements confirm that our thermally evaporated SnS films exhibit a wide distribution of different grain orientations, and the results of Kelvin force microscopy support the theoretically predicted variations of the absolute band-edge energies.

Signatures of quantum criticality in pure Cr at high pressure

The elemental antiferromagnet Cr at high pressure presents a new type of naked quantum critical point that is free of disorder and symmetry-breaking fields. Here we measure magnetotransport in fine detail around the critical pressure, Pc ∼ 10 GPa, in a diamond anvil cell and reveal the role of quantum critical fluctuations at the phase transition. As the magnetism disappears and T → 0, the magntotransport scaling converges to a non-mean-field form that illustrates the reconstruction of the magnetic Fermi surface, and is distinct from the critical scaling measured in chemically disordered Cr∶V under pressure. The breakdown of itinerant antiferromagnetism only comes clearly into view in the clean limit, establishing disorder as a relevant variable at a quantum phase transition.

High-Current-Density Monolayer CdSe/ZnS Quantum Dot Light-Emitting Devices with Oxide Electrodes

Films of semiconductor quantum dots (QDs) are promising for lighting technologies, but controlling how current flows through QD films remains a challenge. A new design for a QD light-emitting device that uses atomic layer deposition to fill the interstices between QDs with insulating oxide is introduced. It funnels current through the QDs themselves, thus increasing the light emission yield.

Transient terahertz photoconductivity measurements of minority-carrier lifetime in tin sulfide thin films: Advanced metrology for an early stage photovoltaic material

Materials research with a focus on enhancing the minority-carrier lifetime of the light-absorbing semiconductor is key to advancing solar energy technology for both early-stage and mature material platforms alike. Tin sulfide (SnS) is an absorber material with several clear advantages for manufacturing and deployment, but the record power conversion efficiency remains below 5%. We report measurements of bulk and interface minority-carrier recombination rates in SnS thin films using optical-pump, terahertz (THz)-probe transient photoconductivity (TPC) measurements. Post-growth thermal annealing in H_2S gas increases the minority-carrier lifetime, and oxidation of the surface reduces the surface recombination velocity. However, the minority-carrier lifetime remains below 100 ps for all tested combinations of growth technique and post-growth processing. Significant improvement in SnS solar cell performance will hinge on finding and mitigating as-yet-unknown recombination-active defects. We describe in detail our methodology for TPC experiments, and we share our data analysis routines as freely-available software.

Narrow band defect luminescence from Al-doped ZnO probed by scanning tunneling cathodoluminescence

We present an investigation of optically active near-surface defects in sputtered Al-doped ZnO films using scanning tunneling microscope cathodoluminescence (STM-CL). STM-CL maps suggest that the optically active sites are distributed randomly across the surface and do not correlate with the granular topography. In stark contrast to photoluminescence results, STM-CL spectra show a series of sharp, discrete emissions that characterize the dominant optically active defect, which we propose is an oxygen vacancy. Our results highlight the ability of STM-CL to spectrally fingerprint individual defects and contribute to understanding the optical properties of near-surface defects in an important transparent conductor.

Order parameter fluctuations at a buried quantum critical point

Quantum criticality is a central concept in condensed matter physics, but the direct observation of quantum critical fluctuations has remained elusive. Here we present an X-ray diffraction study of the charge density wave (CDW) in 2H-NbSe2 at high pressure and low temperature, where we observe a broad regime of order parameter fluctuations that are controlled by proximity to a quantum critical point. X-rays can track the CDW despite the fact that the quantum critical regime is shrouded inside a superconducting phase; and in contrast to transport probes, allow direct measurement of the critical fluctuations of the charge order. Concurrent measurements of the crystal lattice point to a critical transition that is continuous in nature. Our results confirm the long-standing expectations of enhanced quantum fluctuations in low-dimensional systems, and may help to constrain theories of the quantum critical Fermi surface.

Invited Article: High-pressure techniques for condensed matter physics at low temperature

Condensed matter experiments at high pressure accentuate the need for accurate pressure scales over a broad range of temperatures, as well as placing a premium on a homogeneous pressure environment. However, challenges remain in diamond anvil cell technology, including both the quality of various pressure transmitting media and the accuracy of secondary pressure scales at low temperature. We directly calibrate the ruby fluorescence R1 line shift with pressure at T=4.5 K using high-resolution x-ray powder diffraction measurements of the silver lattice constant and its known equation of state up to P=16 GPa. Our results reveal a ruby pressure scale at low temperatures that differs by 6% from the best available ruby scale at room T. We also use ruby fluorescence to characterize the pressure inhomogeneity and anisotropy in two representative and commonly used pressure media, helium and methanol:ethanol 4:1, under the same preparation conditions for pressures up to 20 GPa at T=5 K. Contrary to the accepted wisdom, both media show equal levels of pressure inhomogeneity measured over the same area, with a consistent DeltaP/P per unit area of +/-1.8 %/(10(4) microm(2)) from 0 to 20 GPa. The helium medium shows an essentially constant deviatoric stress of 0.021+/-0.011 GPa up to 16 GPa, while the methanol:ethanol mixture shows a similar level of anisotropy up to 10 GPa, above which the anisotropy increases. The quality of both pressure media is further examined under the more stringent requirements of single crystal x-ray diffraction at cryogenic temperature. For such experiments we conclude that the ratio of sample-to-pressure chamber volume is a critical parameter in maintaining sample quality at high pressure, and may affect the choice of pressure medium.