Rafael Jaramillo

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

Are you Rafael Jaramillo?

Claim your profile

Publications (54)244.74 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We quantify the effects of growth temperature on material and device properties of thermally evaporated SnS thin-films and test structures. Grain size, Hall mobility, and majority-carrier concentration monotonically increase with growth temperature. However, the charge collection as measured by the long-wavelength contribution to short-circuit current exhibits a non-monotonic behavior: the collection decreases with increased growth temperature from 150°C to 240°C and then recovers at 285°C. Fits to the experimental internal quantum efficiency using an opto-electronic model indicate that the non-monotonic behavior of charge-carrier collection can be explained by a transition from drift-to diffusion-assisted components of carrier collection. The results show a promising increase in the extracted minority-carrier diffusion length at the highest growth temperature of 285°C. These findings illustrate how coupled mechanisms can affect early-stage device development, highlighting the critical role of direct materials property measurements and simulation.
    Applied Physics Letters 05/2015; 106(20):203901. DOI:10.1063/1.4921326 · 3.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm(2). Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.
    Journal of Visualized Experiments 01/2015; DOI:10.3791/52705
  • [Show abstract] [Hide abstract]
    ABSTRACT: Secondary phase segregation is hypothesized to have detrimental impacts on Cu2ZnSnS4 (CZTS) thin-film solar cells. In this study, we demonstrate the potential of using kinetic stabilization to inhibit phase decomposition in CZTS. By growing CZTS films at low temperature, we achieve a kinetically stabilized alloy with an expanded solid solution window in the pseudoternary CuS-ZnS-SnS phase diagram. Using X-ray absorption spectroscopy, we study the structural evolution and stability of this metastable alloy upon annealing. For near-stoichiometric samples, we observe a continuous emergence of short-range order toward crystalline CZTS that is nearly complete after a 1-min anneal at 450 °C. For Zn-rich samples, we detect precipitation of ZnS upon annealing, which suggests that the excess Zn exists as cation antisite defects in metastable CZTS.
    IEEE Journal of Photovoltaics 01/2015; 5(1):372-377. DOI:10.1109/JPHOTOV.2014.2360334 · 3.00 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Tin sulfide (SnS) as a promising absorber material in thin-film photovoltaic devices is described. Here, we confirm that SnS evaporates congruently, which provides facile composition control akin to cadmium telluride. We demonstrate a SnS heterojunction solar cell with a power conversion efficiency of 3.88% (certified), and we present an empirical loss analysis to guide further performance improvements.
    Advanced Materials 11/2014; 26(44). DOI:10.1002/adma.201402219 · 15.41 Impact Factor
  • Source
    IEEE 40th Photovoltaic Specialists Conference, Denver, USA; 06/2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Applied Physics Letters 05/2014; 104(21):211603. DOI:10.1063/1.4879558 · 3.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Thin-film solar cells consisting of earth-abundant and non-toxic materials were made from pulsed chemical vapor deposition (pulsed-CVD) of SnS as the p-type absorber layer and atomic layer deposition (ALD) of Zn(O,S) as the n-type buffer layer. The effects of deposition temperature and annealing conditions of the SnS absorber layer were studied for solar cells with a structure of Mo/SnS/Zn(O,S)/ZnO/ITO. Solar cells were further optimized by varying the stoichiometry of Zn(O,S) and the annealing conditions of SnS. Post-deposition annealing in pure hydrogen sulfide improved crystallinity and increased the carrier mobility by one order of magnitude, and a power conversion efficiency up to 2.9% was achieved. Copyright © 2014 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 05/2014; DOI:10.1002/pip.2504 · 9.70 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have investigated the evolution of work function in epitaxial correlated perovskite SmNiO3 (SNO) thin films spanning the metal–insulator transition (MIT) by Kelvin probe force microscopy (KPFM). Combining contact-mode atomic force microscopy, KPFM and electrostatic force microscopy (EFM), we present charge writing processes associated with point defect engineering in SNO thin films. Surface potential tuning in two-terminal devices is demonstrated and compared to thermal control by proximity to the phase transition boundary. The charge distribution, retention, and diffusion on SNO were systematically examined. Local compositional changes by AFM-tip induced electric fields are shown to be a viable approach to spatially engineer electronic properties of correlated oxides towards eventual applications in electronics.
    04/2014; 2(19). DOI:10.1039/C4TC00030G
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: For most metals, increasing temperature (T) or disorder will quicken electron scattering. This hypothesis informs the Drude model of electronic conductivity. However, for so-called bad metals this predicts scattering times so short as to conflict with Heisenberg's uncertainty principle. Here we introduce the rare-earth nickelates (RNiO_3, R = rare earth) as a class of bad metals. We study SmNiO_3 thin films using infrared spectroscopy while varying T and disorder. We show that the interaction between lattice distortions and Ni-O bond covalence explains both the bad metal conduction and the insulator-metal transition in the nickelates by shifting spectral weight over the large energy scale established by the Ni-O orbital interaction, thus enabling very low \sigma while preserving the Drude model and without violating the uncertainty principle.
    Nature Physics 09/2013; 10(4). DOI:10.1038/nphys2907 · 20.60 Impact Factor
  • Source
  • [Show abstract] [Hide abstract]
    ABSTRACT: The rare-earth nickelates (LnNiO3, Ln = lanthanide) are interesting from both fundamental and applied perspectives, but synthesis remains a bottleneck to research due to their thermodynamic instability. Here we report the synthesis of SmNiO3 thin films on oxidized silicon wafers by physical vapor deposition followed by high pressure oxygen annealing at intermediate temperatures. The high pressure annealed films show an insulator–metal transition characteristic of bulk samples. Our experimental observations then allow us to estimate bounds on the phase stability regime, which are particularly useful given the dearth of direct thermodynamic data available for LnNiO3. We examine the limitations of these thermodynamic analyses applied to ultra-thin films. The stabilization of SmNiO3 on a canonical semiconductor template creates opportunities to study the utility of the above room temperature insulator–metal transition (at TIM = 400 K) in electronic devices.
    03/2013; 1(13):2455-2462. DOI:10.1039/C3TC00844D
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The rare-earth nickelates (RNiO3) exhibit interesting phenomena such as unusual antiferromagnetic order at wavevector q = (1/2, 0, 1/2) and a tunable insulator-metal transition that are subjects of active research. Here we present temperature-dependent transport measurements of the resistivity, magnetoresistance, Seebeck coefficient, and Hall coefficient (RH) of epitaxial SmNiO3 thin films with varying oxygen stoichiometry. We find that from room temperature through the high temperature insulator-metal transition, the Hall coefficient is hole-like and the Seebeck coefficient is electron-like. At low temperature the N\'eel transition induces a crossover in the sign of RH to electron-like, similar to the effects of spin density wave formation in metallic systems but here arising in an insulating phase ~200 K below the insulator-metal transition. We propose that antiferromagnetism can be stabilized by bandstructure even in insulating phases of correlated oxides, such as RNiO3, that fall between the limits of strong and weak electron correlation.
    Physical review. B, Condensed matter 01/2013; 87(12). DOI:10.1103/PhysRevB.87.125150 · 3.66 Impact Factor
  • Source
    R Jaramillo, Yejun Feng, T F Rosenbaum
    [Show abstract] [Hide abstract]
    ABSTRACT: We describe a technique for making electrical transport measurements in a diamond anvil cell using an alcohol pressure medium, permitting acute sensitivity while preserving sample fidelity. The sample is suspended in the liquid medium by four gold leads that are electrically isolated by a composite gasket made of stainless steel and an alumina-loaded epoxy. We demonstrate the technique with four-probe resistivity measurements of chromium single crystals at temperatures down to 4 K and pressures above 10 GPa. Our assembly is optimized for making high precision measurements of the magnetic phase diagram and quantum critical regime of chromium, which require repeated temperature sweeps and fine pressure steps while maintaining high sample quality. The high sample quality enabled by the quasi-hydrostatic pressure medium is evidenced by the residual resistivity below 0.1 μΩ cm and the relative resistivity ratio ρ(120 K)∕ρ(5 K) = 15.9 at 11.4 GPa. By studying the quality of Cr's antiferromagnetic transition over a range of pressures, we show that the pressure inhomogeneity experienced by the sample is always below 5%. Finally, we solve for the Debye temperature of Cr up to 11.4 GPa using the Bloch-Gruneisen formula and find it to be independent of pressure.
    The Review of scientific instruments 10/2012; 83(10):103902. DOI:10.1063/1.4757178 · 1.58 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We use high-pressure magnetic x-ray diffraction and numerical simulation to determine the low-temperature magnetic phase diagram of stoichiometric CeFe2. Near 1.5 GPa we find a transition from ferromagnetism to antiferromagnetism, accompanied by a rhombohedral distortion of the cubic Laves crystal lattice. By comparing pressure and chemical substitution we find that the phase transition is controlled by a shift of magnetic frustration from the Ce-Ce to the Fe-Fe sublattice. Notably the dominant Ce-Fe magnetic interaction, which sets the temperature scale for the onset of long-range order, remains satisfied throughout the phase diagram but does not determine the magnetic ground state. Our results illustrate the complexity of a system with multiple competing magnetic energy scales and lead to a general model for magnetism in cubic Laves phase intermetallic compounds.
    Physical Review B 07/2012; 86(1):014422. DOI:10.1103/PhysRevB.86.014422 · 3.74 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Samarium nickelate (SmNiO3) is a correlated oxide that exhibits a metal–insulator transition (MIT) above room temperature and is of interest for advanced electronics and optoelectronics. However, studies on SmNiO3 thin films have been limited to date, in part due to well-known difficulties in stabilizing the Ni3+ valence state during growth, which are manifested in non-reproducible electrical characteristics. In this work, we show that stable epitaxial SmNiO3 thin films can be grown by rf magnetron sputtering without extreme post-deposition annealing conditions using relatively high growth pressure (>200 mTorr). At low growth pressure, SmNiO3 is insulating and undergoes an irreversible MIT at ∼430 K. As pressure is increased, films become metallic across a large temperature range from 100 to 420 K. At high pressure, films are insulating again but with a reversible and stable MIT at ∼400 K. Phase transition properties can be continuously tuned by control of the sputtering pressure.
    Journal of Solid State Chemistry 06/2012; 190:233–237. DOI:10.1016/j.jssc.2012.02.047 · 2.20 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 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-NbSe(2) 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.
    Proceedings of the National Academy of Sciences 04/2012; 109(19):7224-9. DOI:10.1073/pnas.1202434109 · 9.81 Impact Factor
  • Rafael Jaramillo, Shriram Ramanathan
    [Show abstract] [Hide abstract]
    ABSTRACT: Better understanding and control of band alignment in oxide-semiconductor heterostructures is essential for improving the performance of devices such as sensitized solar cells and quantum dot based light emitting devices. We will present studies of Schottky junctions formed between Al-doped ZnO (AZO) conducting oxide thin films and lightly doped silicon. AZO films with varying oxygen content have been synthesized by control of oxygen pressure during growth. Transport measurements (I-V and C-V) on devices are used to illustrate the degree to which the oxide stoichiometry can be used to engineer the junction characteristics.
  • [Show abstract] [Hide abstract]
    ABSTRACT: CeFe2 is a ferromagnet that exhibits antiferromagnetic fluctuations in its ground state. We use x-ray diffraction and diamond-anvil-cell techniques to directly measure the transition to antiferromagnetism in pure CeFe2 at high pressure which couples to the change in the lattice symmetry. Numerical simulations are adopted to identify the magnetic structure of the ground states and to quantitatively illustrate effects of competing magnetic energy scales and geometrical frustration on the magnetic phase diagram. Comparison of phase transitions under both chemical substitution and applied pressure suggests a general solution to the physics of Laves phase magnets.
  • Rafael Jaramillo, Shriram Ramanathan
    [Show abstract] [Hide abstract]
    ABSTRACT: Controlling the efficiency of electron transport across oxide interfaces is essential for numerous emerging technologies including advanced photovoltaics and light emitting devices. This work illuminates the connections between granular structure, defect chemistry, and the work function of a technologically important transparent conductor, ZnO:Al. Visual evidence is provided for a model of grain boundary oxidation in the form of nanometer-scale heterogeneity in the contact potential between grains and grain boundaries, a phenomenon referred to as electronic granularity. By correlating scanning probe data with photoemission spectroscopy we relate electronic granularity to defect chemistry and, importantly, account for the overall trends in work function. The resulting physical picture connects heterogeneity at the nanoscale to macroscopic properties, informs the design of transparent electrodes, and may be broadly relevant to granular oxide conductors.
    Advanced Functional Materials 11/2011; 21(21). DOI:10.1002/adfm.201101069 · 11.81 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Advanced Materials 10/2011; 23(39):4521-5. DOI:10.1002/adma.201101782 · 15.41 Impact Factor

Publication Stats

288 Citations
244.74 Total Impact Points

Institutions

  • 2014
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 2010–2014
    • Harvard University
      • School of Engineering and Applied Sciences
      Cambridge, Massachusetts, United States
  • 2005–2009
    • University of Chicago
      • • Department of Physics
      • • James Franck Institute
      Chicago, Illinois, United States