Constant Final State Research Articles

Revisiting Sub-Band Gap Emission Mechanism in 2D Halide Perovskites: The Role of Defect States.

Understanding the sub-band gap luminescence in Ruddlesden-Popper 2D metal halide hybrid perovskites (2D HaPs) is essential for efficient charge injection and collection in optoelectronic devices. Still, its origins are still under debate with respect to the role of self-trapped excitons or radiative recombination via defect states. In this study, we characterized charge separation, recombination, and transport in single crystals, exfoliated layers, and polycrystalline thin films of butylammonium lead iodide (BA2PbI4), one of the most prominent 2D HaPs. We combined complementary defect- and exciton-sensitive methods such as photoluminescence (PL) spectroscopy, modulated and time-resolved surface photovoltage (SPV) spectroscopy, constant final state photoelectron yield spectroscopy (CFSYS), and constant light-induced magneto transport (CLIMAT), to demonstrate striking differences between charge separation induced by dissociation of excitons and by excitation of mobile charge carriers from defect states. Our results suggest that the broad sub-band gap emission in BA2PbI4 and other 2D HaPs is caused by radiative recombination via defect states (shallow as well as midgap states) rather than self-trapped excitons. Density functional theory (DFT) results show that common defects can readily occur and produce an energetic profile that agrees well with the experimental results. The DFT results suggest that the formation of iodine interstitials is the initial process leading to degradation, responsible for the emergence of midgap states, and that defect engineering will play a key role in enhancing the optoelectronic properties of 2D HaPs in the future.

Rubidium Iodide Reduces Recombination Losses in Methylammonium‐Free Tin‐Lead Perovskite Solar Cells

AbstractOutstanding optoelectronic properties of mixed tin‐lead perovskites are the cornerstone for the development of high‐efficiency all‐perovskite tandems. However, recombination losses in Sn‐Pb perovskites still limit the performance of these perovskites, necessitating more fundamental research. Here, rubidium iodide is employed as an additive for methylammonium‐free Sn‐Pb perovskites. It is first investigated the effect of the RbI additive on the perovskite composition, crystal structure, and element distribution. Quasi‐Fermi level splitting and transient photoluminescence measurements reveal that the RbI additive reduces recombination losses and increases carrier lifetime of the perovskite films. This finding is attributed to an approximately ten‐fold reduction in the defect density following RbI treatment, as probed using constant final state yield photoelectron spectroscopy. Additionally, the concentration of Sn vacancies is also reduced, and the perovskite film becomes less p‐type both in the bulk and at the interface towards the electron contact. Thus, the conductivity for electrons increases, improving carrier extraction. As a result, the open‐circuit voltage of RbI‐containing solar cells improves by 61 mV on average, with the best efficiency >20%. This comprehensive study demonstrates that RbI is effective at reducing recombination losses and carrier trapping, paving way for a new approach to Sn‐Pb perovskite solar cell research.

Field Effect Passivation in Perovskite Solar Cells by a LiF Interlayer

AbstractThe fullerene C60 is commonly applied as the electron transport layer in high‐efficiency metal halide perovskite solar cells and has been found to limit their open circuit voltage. Through ultra‐sensitive near‐UV photoelectron spectroscopy in constant final state mode (CFSYS), with an unusually high probing depth of 5–10 nm, the perovskite/C60 interface energetics and defect formation is investigated. It is demonstrated how to consistently determine the energy level alignment by CFSYS and avoid misinterpretations by accounting for the measurement‐induced surface photovoltage in photoactive layer stacks. The energetic offset between the perovskite valence band maximum and the C60 HOMO‐edge is directly determined to be 0.55 eV. Furthermore, the voltage enhancement upon the incorporation of a LiF interlayer at the interface can be attributed to originate from a mild dipole effect and probably the presence of fixed charges, both reducing the hole concentration in the vicinity of the perovskite/C60 interface. This yields a field effect passivation, which overcompensates the observed enhanced defect density in the first monolayers of C60.

Revisiting the Determination of the Valence Band Maximum and Defect Formation in Halide Perovskites for Solar Cells: Insights from Highly Sensitive Near-UV Photoemission Spectroscopy.

Using advanced near-UV photoemission spectroscopy (PES) in constant final state mode (CFSYS) with a very high dynamic range, we investigate the triple-cation lead halide perovskite Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 and gain detailed insights into the density of occupied states (DOS) in the valence band and band gap. A valence band model is established which includes the parabolic valence band edge and an exponentially decaying band tail in a single equation. This allows us to precisely determine two valence band maxima (VBM) at different k-vectors in the angle-integrated spectra, where the highest one, resulting from the VBM at the R-point in the Brillouin zone, is found between -1.50 to -1.37 eV relative to the Fermi energy EF. We investigate quantitatively the formation of defect states in the band gap up to EF upon decomposition of the perovskites during sample transfer, storage, and measurements: during near-UV-based PES, the density of defect states saturates at a value that is around 4 orders of magnitude below the density of states at the valence band edge. However, even short air exposure, or 3 h of X-ray illumination, increased their density by almost a factor of six and ∼40, respectively. Upon prolonged storage in vacuum, the formation of a distinct defect peak is observed. Thus, near-UV CFSYS with modeling as shown here is demonstrated as a powerful tool to characterize the valence band and quantify defect states in lead halide perovskites.

A scripting LabVIEW based program for experiment automation in synchrotron radiation applications

We have established a soft X-ray beamline and an end-station for the material research community, multiple experiment techniques have been implemented to satisfy the applications in synchrotron radiation, and the challenge is to get control of many commercial instruments from different vendors shipping with various control software in a single experiment platform. A LabVIEW based program has been developed for such purpose to allow automation of multiple experiments including angle-resolved photoemission spectroscopy, near edge X-ray absorption spectroscopy with detection of total electron yield and Auger electron yield, constant final state spectroscopy, constant initial state spectroscopy, and X-ray magnetic circular dichroism. The option of scan-on-the-fly mode for photon energy and magnetic field scans can reduce measurement time effectively. This program has become a necessary tool in routine measurements with numerous applications, of which we present several examples.

Near ambient pressure photoemission spectroscopy of metal and semiconductor surfaces

AbstractWe describe a novel photoemission technique utilizing a traditional Kelvin probe as a detector of electrons/atmospheric ions ejected from metal and semiconductor surfaces (Al, Ag, Au, Si) illuminated by a Deep Ultra‐Violet (DUV) source at ambient pressure. In Constant Final State Yield Spectroscopy (CFSYS) the incident photon energy is rastered rather than applying a variable retarding electric field as in conventional UPS. For both ambient‐ and near ambient pressure‐photoemission spectroscopy (NAP‐PES) the CFSYS configuration overcomes the limitation of inelastic electron scattering in air. This arrangement can be applied in several operational modes: using the DUV source to determine the absolute work function (ϕ) of the metal with 50‐100 meV resolution and also the Kelvin probe, under dark conditions, to measure Contact Potential Difference (CPD) with an accuracy of 1‐3 meV. We show that the metal photoresponse agrees with Fowler theory. We have used CPD and linear extrapolated photoemission measurements to produce an energy level diagram for the native‐oxide covered Si. We propose a model of photoemission in air involving atmospheric ions. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

RIXS at the cerium L3-edge of Ce(III) and Ce(IV) systems: some observations

We report recent observations of resonant inelastic X-ray scattering (RIXS) at the cerium L3-edge of CePO4, CeO2, and Ce3+ ion in aqueous solution. The intensity of the emission spectrum, including the dispersive Raman below the edge and the emerging nondispersive fluorescence above the edge, was recorded with a solid-state detector with low-energy resolution and a WDX detector with modestly high-energy resolution. The yield of the emission was used to monitor the cerium L3-edge X-ray absorption near edge structures (XANES) in a constant initial state (CIS) mode as the photon energy sweeps across the cerium L3-edge. The CIS XANES is compared with the XANES recorded using the nondispersive fluorescence yield, Lα, in a constant final state (CFI) mode. It is found that the Raman yield dominates the total emission in the pre-edge region and diminishes rapidly at threshold. The RIXS of Ce3+ in aqueous solution exhibits similar behavior as CePO4. The post-whiteline RIXS intensity of CePO4 exhibits a correlation with excitation energy. CeO2 also exhibits a less intense whiteline in CIS XANES compared with the XANES detected with normal fluorescence. The implications of these observations with improved instrumental resolution are noted.

Analysis of electronic structure of amorphous InGaZnO/SiO2 interface by angle-resolved X-ray photoelectron spectroscopy

The electronic structures of amorphous indium gallium zinc oxide (a-IGZO) on a SiO2 layers before and after annealing were observed by constant final state X-ray photoelectron spectroscopy (CFS-XPS) and X-ray adsorption near-edge structure spectroscopy (XANES). From the results of angle-resolved CFS-XPS, the change in the electronic state was clearly observed in the a-IGZO bulk rather than in the a-IGZO/SiO2 interface. This suggests that the electronic structures of the a-IGZO bulk strongly affected the thin-film transistor characteristics. The results of XANES indicated an increase in the number of tail states upon atmospheric annealing (AT). We consider that the increase in the number of tail states decreased the channel mobility of AT samples.

Resonant photoemission at the O1s threshold to characterize In2O3 single crystals

We report on spectroscopic investigations on In2O3 single crystals. We focus on the detailed analysis of the O1s resonance profile by resonant photoelectron spectroscopy. We analyze the electronic structure and assign the O2p to build the valence band and both, O2p and In5s5p states to contribute to the conduction band (CB). We determine the partial density of states for the valence and CBs and find a strong hybridization of O2p and In5s states. This is deduced from constant final state spectra on the O-KLL-Auger over the O K-edge and In M4,5-edge and a comparison to the corresponding X-ray absorption spectroscopy data. We also identify several types of defects. A broad band of oxygen derived defects is identified that extends throughout the gap. These are attributed to small polarons which cause an anti-resonance in the valence states around the O1s threshold. In addition, a separate Auger decay at resonance indicates the existence of localized charge transfer states which involves localized In5s5p states.

Observation of UV-induced Auger features in catechol adsorbed on anatase TiO2 (101) single crystal surface

We have investigated the electronic structure of catechol adsorbed on the anatase TiO2 (101) surface under illumination with ultraviolet (UV) light (4.75 eV) using resonant photoemission spectroscopy. UV illumination results in the appearance of a strong Ti MVV (M refers to photoionization of 3p level and VV the Auger decay process via the valence levels) feature at a kinetic energy of 26.2 eV. This is attributed to the creation of localised states following catechol to Ti-3d excitation by the UV source. A sharp resonance attributed to excitation from Ti 3p states into these localised states is observed in constant final state spectra.

Resonant Photoemission at the O1s threshold to characterize β-Ga2O3 single crystals

We report on spectroscopic investigations on β-Ga2O3 single crystals. We focus on the detailed analysis of the O1s resonance profile by resonant photoelectron spectroscopy. We analyze the electronic structure and assign the O2p to build the valence band and both, O2p and Ga4s4p states to contribute to the conduction band. We determine the partial density of states for the valence and conduction bands and find a strong hybridization of O2p and Ga4s states. This is deduced from constant final state spectra on the O-KLL-Auger over the O K-edge and Ga L2,3-edge and a comparison to the corresponding X-ray absorption spectroscopy data. We also identify several types of defects. A broad band of oxygen derived defects is identified that extends throughout the gap. Small polarons are attributed to cause an anti-resonance in the valence states around the O1s threshold. In addition, a separate Auger decay at resonance indicates the existence of localized charge transfer states which involves localized Ga4sp states.

Doping type and thickness dependence of band offsets at the amorphous/crystalline silicon heterojunction

We conduct a systematic investigation of the valence band offset ΔEv for amorphous/crystalline silicon heterojunctions (a-Si:H/c-Si) using low-energy photoelectron spectroscopy in the constant final state mode. The dependence of ΔEv on a-Si:H thickness as well as on the possible combinations of c-Si substrate and a-Si:H film doping types are explored. ΔEv is found to be independent of both substrate and film doping and amounts to ΔEv¯=0.458(6) eV, averaged over all doping combinations and thicknesses, with a systematic error of 50–60 meV. A slight but statistically significant dependency of ΔEv on the a-Si:H film thickness may be explained by a changing interface dipole due to variations in dangling bond saturation during a-Si:H growth.

CuInS 2 – CdS heterojunction valence band offset measured with near-UV constant final state yield spectroscopy

The valence band offset of the heterojunction between CuInS2 (CIS) and chemical bath deposited CdS has been determined both by means of combined x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) and by near-UV constant final state (CFS) yield spectroscopy. The use of the latter shows that this spectroscopic method is indeed suitable for the investigation of chalcopyrite thin films. The higher information depth due to the low excitation energies (7.5–4.0 eV) used in CFS makes it possible to obtain a signal from the valence band of the CIS substrate even after a relatively thick (∼5 nm) CdS layer has been deposited. The number of samples needed to determine the band offset is reduced from three to one and the effects of chemical changes on the CIS surface can be accounted for because the valence band edges of both materials are measured directly. The valence band offsets with the XPS/UPS and CFS methods were determined to be 1.25±0.20 and 1.45±0.20 eV, respectively.

Synchrotron Methods of the Conduction Band Electronic Structure Analysis

The experimental and theoretical studies of the density of states for unoccupied conduction band are a valid source of knowledge on the electronic structure of condensed matter. Among known analytical methods of density of states for unoccupied states a significant place is occupied by the methods with the use of intensive synchrotron radiation beam. Adequately chosen monochromatic beam excites electrons from the occupied core states or the valence band to unoccupied states in the conduction band. The most universal experimental method for the studies of the unoccupied conduction band is the X-ray absorption near edge structure analysis for X-ray edges of the constituent atoms of the material. A satisfying description of the unoccupied conduction band density of states can be also reached by the analysis of reflectivity coefficient in the vacuum ultraviolet (above 10 eV). The photoelectron energy distribution curves elaborated by the constant initial state and constant final state procedures deliver valuable information on the high excited states of conduction band. In the paper the menaces of the correct interpretation of the experimental results will be discussed. The efficiency of the unoccupied conduction band density of states studies with application of X-ray absorption near edge structure and analysis of reflectivity coefficient of light will be presented on the basis of several binary and ternary semiconductor compounds of II–VI group with transition metals as well as of some other compounds. To avoid a misinterpretation of the experimental results, the data were compared and discussed with an adequate theoretical band structure calculation.

An extension of the mean free path approach to X-ray absorption spectroscopy

In the partial electron yield (PEY) acquisition mode commonly used in X-ray absorption spectroscopy (XAS) both elastically and inelastically scattered electrons are acquired, the latter contribution dominating the detector signal. Hence, the majority of the inelastic scattering events will not result in signal attenuation as happens in the case of X-ray photoelectron spectroscopy (XPS). To determine the respective changes in the effective mean free paths (MFP) we have performed XPS and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements for a series of self-assembled monolayers of alkanethiols on gold substrates. The length of the alkyl chain and, therefore, the film thickness was varied. In agreement with expectations, the obtained MFPs for the Au 4f photoelectrons and C KLL Auger electrons in the PEY acquisition mode (with the respective inelastic contributions) exceed the corresponding values for the Au 4f and C KLL electrons of the same kinetic energies in the constant final state acquisition mode. Furthermore, the effective PEY-MFP for the C KLL Auger electrons increased with decreasing retarding voltage of the PEY detector, which correlates with the enhanced contribution of the inelastically scattered electrons in the acquired signal. The results obtained are of importance for the analysis of XAS spectra of thin organic films and polymers.

Band structure ofMoS2,MoSe2,andα−MoTe2:Angle-resolved photoelectron spectroscopy andab initiocalculations

In this work the complete valence-band structure of the molybdenum dichalcogenides MoS_2, MoSe_2, and alpha-MoTe_2 is presented and discussed in comparison. The valence bands have been studied using both angle-resolved photoelectron spectroscopy (ARPES) with synchrotron radiation, as well as, ab-initio band-structure calculations. The ARPES measurements have been carried out in the constant-final-state (CFS) mode. The results of the calculations show in general very good agreement with the experimentally determined valence-band structures allowing for a clear identification of the observed features. The dispersion of the valence bands as a function of the perpendicular component k_perp of the wave vector reveals a decreasing three-dimensional character from MoS_2 to alpha-MoTe_2 which is attributed to an increasing interlayer distance in the three compounds. The effect of this k_perp dispersion on the determination of the exact dispersion of the individual states as a function of k_parallel is discussed. By performing ARPES in the CFS mode the k_parallel-component for off-normal emission spectra can be determined. The corresponding k_perp-value is obtained from the symmetry of the spectra along the GammaA, KH, and ML line, respectively.

Photoelectron imaging using an ellipsoidal display analyzer

We have built an ellipsoidal display analyzer (EDA) for angle-resolved photoelectron spectroscopy and related techniques. The instrument is an improved version of a design by Eastman et al. [Nucl. Instrum. Methods 172, 327 (1980)] and measures the angle-resolved intensity distribution of photoelectrons at fixed energy I(θ,φ)|E=const.. Such two-dimensional cuts through the Brillouin zone are recorded using a position-sensitive detector. The large acceptance angle (Δθ=43° in the polar direction and Δφ=360° in the azimuthal direction) leads to a collection efficiency which exceeds that of conventional hemispherical analyzers by a factor of about 3000. Using ray-tracing calculations we analyze the electron optical properties of the various analyzer components and optimize their arrangement. This minimizes distortions and aberrations in the recorded images and greatly improves the performance compared to previous realizations of this analyzer. We present examples demonstrating the performance of the analyzer and its versatility. Using a commercial He-discharge lamp we are able to measure complete angular distribution patterns in less than 5 s. The energy and angular resolution are ΔEEDA=85 meV and Δθ=1.2°, respectively. Complete stacks of such cuts through the Brillouin zone at different kinetic energies E can be acquired automatically using custom software. The raw data are processed leading to a three-dimensional set (I(EB,k∥) of photoelectron intensity versus binding energy E and wave vector k∥. From this all relevant information, like the dispersion relations EB(k∥) along arbitrary directions of the Brillouin zone or Fermi-surface maps, can then be computed. An additional electron gun enables low-energy electron diffraction, Auger electron spectroscopy, and electron energy-loss spectroscopy. Switching between electrons and photons as the excitation source is possible without any movement of the sample or analyzer. Because of the high acquisition speed it is possible to study the electronic structure of solids as a function of an external parameter (i.e., temperature) or to make animated movies showing, for example, the evolution of electronic states in reciprocal space. After installation of this EDA at a synchrotron providing tunable photon energy, the full power of the instrument will come into play by adding techniques like constant final state or constant initial state spectroscopy, and x-ray photoelectron diffraction.

Photoelectron spectroscopy studies of microcrystalline/amorphous silicon interfaces

Hydrogenated microcrystalline silicon (μc-Si:H) films and microcrystalline/amorphous silicon heterojunctions have been studied using in situ X-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS), total yield and constant final state (CFS) spectroscopy. We present XPS valence band spectroscopy as a new method of determining the near-surface (∼5 nm) crystallinity of microcrystalline films. The analysis of the CFS spectra allows the direct determination of the valence band discontinuity at the μc-Si:H/a-Si:H interface. The experimental ΔE V=300±30 meV is in agreement with results obtained using indirect methods.

Resonant photoemission characterization of SnO

A thick layer of SnO, equivalent in its electronic properties to the bulk material, has been investigated by means of resonant photoemission and the mathematical method of factor analysis. This study has shown that $\mathrm{O}2p,$ $\mathrm{Sn}5s,$ and $\mathrm{Sn}5p$ partial density of states are the main contributions to the valence band of this material. The distribution through the valence band of these partial contributions has been determined by spectral subtraction and factor analysis of the resonance photoemission spectra, as well as by band structure calculation. The resonance behavior (i.e., change in intensity with the photon energy) of these three contributions has been analyzed. The $\mathrm{Sn}5p$ levels present a typical Fano-like behavior with a minimum intensity at about 28 eV and a maximum at 40 eV. The $\mathrm{Sn}5s$ partial density distribution also depicts a change in intensity as a function of the photon energy with a minimum situated at 35 eV and a maximum at 55 eV. Tentatively, this behavior has been linked to the existence of a broad absorption feature detected by electron energy loss spectroscopy and constant final state spectra, both depicting a broad maximum at about 50 eV.

Dichroism in constant final-state photoemission for Gd(0 0 0 1) 4d–4f giant resonance

The dichroism in constant final state photoemission for the 4d–4f giant resonance and its pre-edge structure is studied from thin epitaxial gadolinium films. This study provides information of the maximum magnetic linear dichroism in angular dependence (MLDAD) effect which can be seen for certain photon energies. The MLDAD results can be compared with the magnetic circular dichroism on the same Gd films grown on W(1 1 0). These are clear differences between these two techniques at the main resonance.