David E. Starr

Investigation of solid/vapor interfaces using ambient pressure X-ray photoelectron spectroscopy

Heterogeneous chemical reactions at vapor/solid interfaces play an important role in many processes in the environment and technology. Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a valuable tool to investigate the elemental composition and chemical specificity of surfaces and adsorbates on the molecular scale at pressures of up to 130 mbar. In this review we summarize the historical development of APXPS since its introduction over forty years ago, discuss different approaches to minimize scattering of electrons by gas molecules, and give a comprehensive overview about the experimental systems (vapor/solid interfaces) that have been studied so far. We also present several examples for the application of APXPS to environmental science, heterogeneous catalysis, and electrochemistry.

Direct observation of an inhomogeneous chlorine distribution in CH3NH3PbI3−xClx layers: surface depletion and interface enrichment

We have used hard X-ray photoelectron spectroscopy (HAXPES) at different photon energies and fluorescence yield X-ray absorption spectroscopy (FY-XAS) to non-destructively investigate CH3NH3PbI3−xClx perovskite thin films on compact TiO2. This combination of spectroscopic techniques allows the variation of information depth from the perovskite layer surface to the top-most part of the underlying compact TiO2 layer. We have taken advantage of this to understand the distribution of chlorine throughout the perovskite/TiO2 layer stack. No Cl is detected using HAXPES, indicating surface depletion of Cl and allowing us to place an upper limit on the amount of Cl in the perovskite layer: x 0.40) consistent with both enhanced concentrations of Cl deep beneath the perovskite film surface and near the CH3NH3PbI3−xClx perovskite/TiO2 interface. The consequences of this distribution of Cl in the CH3NH3PbI3−xClx perovskite layer on device performance are discussed.

Observation and Mediation of the Presence of Metallic Lead in Organic-Inorganic Perovskite Films.

We have employed soft and hard X-ray photoelectron spectroscopies to study the depth-dependent chemical composition of mixed-halide perovskite thin films used in high-performance solar cells. We detect substantial amounts of metallic lead in the perovskite films, which correlate with significant density of states above the valence band maximum. The metallic lead content is higher in the bulk of the perovskite films than at the surface. Using an optimized postanneal process in air, we can reduce the metallic lead content in the perovskite film. This process reduces the amount of metallic lead and a corresponding increase in the photoluminescence quantum efficiency of the perovskite films can be observed. This correlation indicates that metallic lead impurities are likely a key defect whose concentration can be controlled by simple annealing procedures in order to increase the performance for perovskite solar cells.

Redox chemistry of CaMnO3 and Ca0.8Sr0.2MnO3 oxygen storage perovskites

Perovskite oxides CaMnO3 and Ca0.8Sr0.2MnO3 show continuous non-stoichiometry over a range of temperatures and oxygen partial pressures. In this work a thermobalance equipped with an oxygen pump was used to measure the equilibrium non-stoichiometry of both materials for temperatures in the range 400–1200 °C and oxygen partial pressures in the range 1–10−5 bar. Analysis of the data showed that Ca0.8Sr0.2MnO3 has a lower enthalpy of reduction and thus can be more easily reduced. The strontium added sample was also robust against a phase transition that was seen in CaMnO3 at high temperatures. A statistical thermodynamic model of the system suggests that the defects form clusters of the form . The oxidation kinetics were also investigated with Ca0.8Sr0.2MnO3 showing faster kinetics and maintaining activity at lower temperatures. Overall, Ca0.8Sr0.2MnO3 shows very promising properties for redox applications, including gravimetric oxygen storage up to 4% by mass, high stability and rapid reversibility, with re-oxidation in less than 1 min at 400 °C. Finally, the redox chemistry of Ca0.8Sr0.2MnO3 was also investigated using in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption measurements at near ambient pressure in oxygen atmospheres.

In-Situ Probing of H2O Effects on a Ru-Complex Adsorbed on TiO2 Using Ambient Pressure Photoelectron Spectroscopy

Dye-sensitized interfaces in photocatalytic and solar cells systems are significantly affected by the choice of electrolyte solvent. In the present work, the interface between the hydrophobic Ru-complex Z907, a commonly used dye in molecular solar cells, and TiO2 was investigated with ambient pressure photoelectron spectroscopy (AP-PES) to study the effect of water atmosphere on the chemical and electronic structure of the dye/TiO2 interface. Both laboratory-based Al Κα as well as synchrotron-based ambient pressure measurements using hard X-ray (AP-HAXPES) were used. AP-HAXPES data were collected at pressures of up to 25xa0mbar (i.e., the vapor pressure of water at room temperature) showing the presence of an adsorbed water overlayer on the sample surface. Adopting a quantitative AP-HAXPES analysis methodology indicates a stable stoichiometry in the presence of the water atmosphere. However, solvation effects due to the presence of water were observed both in the valence band region and for the S 1s core level and the results were compared with DFT calculations of the dye-water complex.

Microcrystalline silicon oxides for silicon-based solar cells: impact of the O/Si ratio on the electronic structure

Hydrogenated microcrystalline silicon oxide (μc-SiOx:H) layers are one alternative approach to ensure sufficient interlayer charge transport while maintaining high transparency and good passivation in Si-based solar cells. We have used a combination of complementary x-ray and electron spectroscopies to study the chemical and electronic structure of the (μc-SiOx:H) material system. With these techniques, we monitor the transition from a purely Si-based crystalline bonding network to a silicon oxide dominated environment, coinciding with a significant decrease of the material’s conductivity. Most Si-based solar cell structures contain emitter/contact/passivation layers. Ideally, these layers fulfill their desired task (i.e., induce a sufficiently high internal electric field, ensure a good electric contact, and passivate the interfaces of the absorber) without absorbing light. Usually this leads to a trade-off in which a higher transparency can only be realized at the expense of the layer’s ability to properly fulfill its task. One alternative approach is to use hydrogenated microcrystalline silicon oxide (μc-SiOx:H), a mixture of microcrystalline silicon and amorphous silicon (sub)oxide. The crystalline Si regions allow charge transport, while the oxide matrix maintains a high transparency. To date, it is still unclear how in detail the oxygen content influences the electronic structure of the μc-SiOx:H mixed phase material. To address this question, we have studied the chemical and electronic structure of the μc-SiOx:H (0 ≤ x = O/Si ≤1) system with a combination of complementary x-ray and electron spectroscopies. The different surface sensitivities of the employed techniques help to reduce the impact of surface oxides on the spectral interpretation. For all samples, we find the valence band maximum to be located at a similar energy with respect to the Fermi energy. However, for x > 0.5, we observe a pronounced decrease of Si 3s – Si 3p hybridization in favor of Si 3p – O 2p hybridization in the upper valence band. This coincides with a significant increase of the material’s resistivity, possibly indicating the breakdown of the conducting crystalline Si network. Silicon oxide layers with a thickness of several hundred nanometres were deposited in a PECVD (plasma-enhanced chemical vapor deposition) multi chamber system using an excitation frequency of 13.56 MHz with a plasma power density of 0.3 W/cm2. Glass (Corning type Eagle) and mono-crystalline silicon wafer substrates were coated in the same run at a substrate temperature of 185°C. The deposition pressure was 4 mbar and the substrate-electrode distance 20 mm. Mixtures of silane (SiH4), 1% TMB (B(CH3)3) diluted in helium, hydrogen (H2), and carbon dioxide (CO2) gases were used at flow rates of 1.25 - 0.18/0.32/500/0 – 1.07) sccm (standard cubic centimeters per minute) for the deposition of μc-SiOx:H(B) layers. By changing the CO2/SiH4 gas flow rate ratio from 0 to 6, μc-SiOx:H(B) layers with a composition of 0 ≤uf0a3 x = O/Si uf0a3≤ 1 were prepared using a constant sum of SiH4 and CO2. The TMB flow and the H2 flow were kept constant within the series. For more details see Ref. [1]. The oxygen content in the films was determined using Rutherford Backscattering Spectroscopy (RBS). With RBS, the area-related atomic density of oxygen and silicon can be determined (± 2% [2]), and thus x can be calculated. This quantity considers only the number of silicon / oxygen atoms and not the number of atoms of other elements, such as hydrogen, which is also incorporated to a considerable extent: up to 20% in μc-SiOx:H (measured using the hydrogen effusion method). To avoid charging effects, the measurements were performed on films deposited on a substrate of mono-crystalline silicon wafers. The electrical conductivity was measured in the planar direction of the film in a vacuum cryostat, using voltages from - 100 V to + 100 V. For that two co-planar Ag contacts were evaporated on the film with a gap of 0.5 mm uf0b4 5 mm. In the present study, the optical band E04 is arbitrarily used as a measure for the optical band gap. E04 is defined by the photon energy E for which an optical absorption coefficient of α of 104cm-1 is obtained. The absorption coefficient α(λ) versus the wavelength λ of the films was determined by measuring the transmittance T(λ) and reflectance R(λ), using the Beer-Lambert law, as suggested by Ref. [3]. The film thickness d was measured using the step profiler close to the measurement spot of the spectrophotometer. It is important to measure the transmittance T(λ) and the reflectance R(λ) at the same spot on the sample, to avoid inaccuracies in the calculated absorption spectra that arise from non-uniformity of the film thickness and different positions of the reflectance and transmittance minima and maxima in the spectrum [4]. Hard X-ray photoelectron spectroscopy (HAXPES) experiments were conducted at the HiKE end-station [5] on the KMC-1 beamline [6] of the BESSY-II electron storage ring. This end-station is equipped with a Scienta R4000 electron energy analyzer capable of measuring photoelectron kinetic energies up to 10 keV. A pass energy of 200 eV was used for all measurements. Spectra were recorded with a photon energy of 2003 eV using the first and fourth order supplied by a Si(111) double crystal monochromator. The combined analyzer plus beamline resolution is approx. 0.25 eV for spectra taken at both photon energies. The top surface of the sample was electrically grounded for all measurements. The binding energy was calibrated by measuring the 4f spectrum of a grounded Au foil and setting the Au 4f7/2 binding energy equal to 84.00 eV. In SiO2, the inelastic mean free path of electrons was estimated to be approx. 5 and 13-16 nm for the core levels and valence band measurements performed with 2003 and 8012 eV [7].

EMIL: The energy materials in situ laboratory Berlin

In a concerted effort, the Helmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB) and the Max Planck Society (MPG) will develop, install, and operate EMIL, the Energy Materials In situ Laboratory, which will be a unique facility at the BESSY II synchrotron light source in Berlin, Germany. EMIL will be dedicated to the in situ and in operando X-ray analysis of materials and devices for photovoltaic applications and of (photo)catalytic/photo-electrochemical processes. EMIL provides access to soft and hard X-rays in an energy range of 80 eV - 10 keV, and comprises all characterization and deposition facilities in one integrated ultra-high vacuum system. EMIL allows studying solar energy converting devices including photovoltaic (PV) structures based on silicon- and compound semiconductors, hybrid organic/inorganic heterojunctions, emerging organo-metal halide perovskites as well earth-abundant materials for solar fuel production (water splitting).