Location via proxy:   [ UP ]  
[Report a bug]   [Manage cookies]                
Skip to main content
  • Golden, Colorado, United States

Xerxes Steirer

Metal halide perovskite materials (MHPs) are a family of next-generation semiconductors that are enabling low-cost, high-performance solar cells and optoelectronic devices. The most-used halogen in MHPs, iodine, can supplement its octet... more
Metal halide perovskite materials (MHPs) are a family of next-generation semiconductors that are enabling low-cost, high-performance solar cells and optoelectronic devices. The most-used halogen in MHPs, iodine, can supplement its octet by covalent bonding resulting in atomic charges intermediate to I− and I0. Here, we examine theoretically stabilized defects of iodine using density functional theory (DFT); defect formation enthalpies and iodine Bader charges which illustrate how MHPs adapt to stoichiometry changes. Experimentally, X-ray photoelectron spectroscopy (XPS) is used to identify perovskite defects and their relative binding energies, and validate the predicted chemical environments of iodine defects. Examining MHP samples with excess iodine compared with near stoichiometric samples, we discern additional spectral intensity in the I 3d5/2 XPS data arising from defects, and support the presence of iodine trimers. I 3d5/2 defect peak areas reveal a ratio of 2:1, matching the...
Solid-state electrolytes such as LiS-PS compounds are promising materials that could enable Li metal anodes. However, many solid-state electrolytes are unstable against metallic lithium, and little is known about the chemical evolution of... more
Solid-state electrolytes such as LiS-PS compounds are promising materials that could enable Li metal anodes. However, many solid-state electrolytes are unstable against metallic lithium, and little is known about the chemical evolution of these interfaces during cycling, hindering the rational design of these materials. In this work, operando X-ray photoelectron spectroscopy and real-time in situ Auger electron spectroscopy mapping are developed to probe the formation and evolution of the Li/LiS-PS solid-electrolyte interphase during electrochemical cycling, and to measure individual overpotentials associated with specific interphase constituents. Results for the Li/LiS-PS system reveal that electrochemically driving Li to the surface leads to phase decomposition into LiS and LiP. Additionally, oxygen contamination within the LiS-PS leads initially to LiPO phase segregation, and subsequently to LiO formation. The spatially non-uniform distribution of these phases, coupled with diffe...
Magnesium-based batteries possess potential advantages over their lithium counterparts. However, reversible Mg chemistry requires a thermodynamically stable electrolyte at low potential, which is usually achieved with corrosive components... more
Magnesium-based batteries possess potential advantages over their lithium counterparts. However, reversible Mg chemistry requires a thermodynamically stable electrolyte at low potential, which is usually achieved with corrosive components and at the expense of stability against oxidation. In lithium-ion batteries the conflict between the cathodic and anodic stabilities of the electrolytes is resolved by forming an anode interphase that shields the electrolyte from being reduced. This strategy cannot be applied to Mg batteries because divalent Mg cannot penetrate such interphases. Here, we engineer an artificial Mg-conductive interphase on the Mg anode surface, which successfully decouples the anodic and cathodic requirements for electrolytes and demonstrate highly reversible Mg chemistry in oxidation-resistant electrolytes. The artificial interphase enables the reversible cycling of a Mg/VO full-cell in the water-containing, carbonate-based electrolyte. This approach provides a new ...
Solar water splitting is often performed in highly corrosive conditions, presenting materials stability challenges. Gu et al. show that an efficient and stable hydrogen-producing photocathode can be realized through the application of a... more
Solar water splitting is often performed in highly corrosive conditions, presenting materials stability challenges. Gu et al. show that an efficient and stable hydrogen-producing photocathode can be realized through the application of a graded catalytic–protective layer on top of the photoabsorber.
Herein, we consider the heterojunction formation of two prototypical metal oxides: p-type NiO and n-type ZnO. Elementally abundant, low-cost metal oxide/oxide' heterojunctions are of interest for UV optical sensing, gas sensing,... more
Herein, we consider the heterojunction formation of two prototypical metal oxides: p-type NiO and n-type ZnO. Elementally abundant, low-cost metal oxide/oxide' heterojunctions are of interest for UV optical sensing, gas sensing, photocatalysis, charge confinement layers, piezoelectric nanogenerators, and flash memory devices. These heterojunctions can also be used as current rectifiers and potentially as recombination layers in tandem photovoltaic stacks by making the two oxide layers ultrathin. In the ultrathin geometry, understanding and control of interface electronic structure and chemical reactions at the oxide/oxide' interface are critical to functionality, as oxygen atoms are shared at the interface of the dissimilar materials. In the studies presented here the extent of chemical reactions and interface band bending is monitored using X-ray and ultraviolet photoelectron spectroscopies. Interface reactivity is controlled by varying the near surface composition of nicke...
Efficient water splitting using light as the only energy input requires stable semiconductor electrodes with favorable energetics for the water oxidation and proton reduction reactions. Strategies to tune electrode potentials using... more
Efficient water splitting using light as the only energy input requires stable semiconductor electrodes with favorable energetics for the water oxidation and proton reduction reactions. Strategies to tune electrode potentials using molecular dipoles adsorbed to the semiconductor surface have been pursued for decades but are often based on weak interactions and quickly react to desorb the molecule under conditions relevant to sustained photoelectrolysis. Here, we show that covalent attachment of fluorinated, aromatic molecules to p-GaAs(100) surfaces can be employed to tune the photocurrent onset potentials of p-GaAs(100) photocathodes and reduce the external energy required for water splitting. Results indicate that initial photocurrent onset potentials can be shifted by nearly 150 mV in pH -0.5 electrolyte under 1 sun illumination resulting from the covalently-bound surface dipole. Though XPS analysis reveals that the covalent molecular dipole attachment is not robust under extende...
cis,cis-Muconic acid for downstream separation and catalytic upgrading to adipic acid for nylon-6,6 polymerization.
The p-type semiconductor GaInP2 has a nearly ideal bandgap (∼1.83 eV) for hydrogen fuel generation by photoelectrochemical water splitting but is unable to drive this reaction because of misalignment of the semiconductor band edges with... more
The p-type semiconductor GaInP2 has a nearly ideal bandgap (∼1.83 eV) for hydrogen fuel generation by photoelectrochemical water splitting but is unable to drive this reaction because of misalignment of the semiconductor band edges with the water redox half reactions. Here, we show that attachment of an appropriate conjugated phosphonic acid to the GaInP2 electrode surface improves the band edge alignment, closer to the desired overlap with the water redox potentials. We demonstrate that this surface modification approach is able to adjust the energetic position of the band edges by as much as 0.8 eV, showing that it may be possible to engineer the energetics at the semiconductor/electrolyte interface to allow for unbiased water splitting with a single photoelectrode having a bandgap of less than 2 eV.
Performance deficiencies from the too large conduction band offset between Cu2ZnSnSe4/ZnOS heterojunctions are abated by the inclusion of a co-solvent during aqueous growth of the buffer layer.
Decomposition/oxidation correlated to nanoscale c-AFM helps separate selectivity and conductivity.
Chemical bath deposition (CBD) Zn(O,S) buffer layers grown on Cu(In1-xGax)Se2 (CIGS) thin films have recently surpassed CdS in high efficiency cells (20.9%). A critical component of a CIGS device is the buffer layer - the layer that is... more
Chemical bath deposition (CBD) Zn(O,S) buffer layers grown on Cu(In1-xGax)Se2 (CIGS) thin films have recently surpassed CdS in high efficiency cells (20.9%). A critical component of a CIGS device is the buffer layer - the layer that is found between the absorber CIGS layer and the ZnO window layer. Although CBD CdS is an effective buffer layer and traditionally used for devices, it is not entirely effective for high bandgap absorber films. The Zn(O,S)/CIGS interface was studied by X-ray photoelectron spectroscopy to reveal the valence band offset (VBO) and conduction band offset (CBO) as -1.15 eV and 1.17 eV respectively. Band bending that accompanies junction formation is also characterized in both layers.
Recent research has enabled Cu2ZnSnSe4 (CZTSe) to reach efficiencies close to 10% in photovoltaic devices with CdS as the junction partner and over 12% when the CZTSe is alloyed with sulfur. Little work, however, has been reported on the... more
Recent research has enabled Cu2ZnSnSe4 (CZTSe) to reach efficiencies close to 10% in photovoltaic devices with CdS as the junction partner and over 12% when the CZTSe is alloyed with sulfur. Little work, however, has been reported on the potential for wide band gap, Cd-free buffer layers in these devices. Reported here are photoelectron spectroscopy measurements (XPS/UPS) of the band energy positions between CZTSe and zinc oxysulfide (ZnOS) with sputter depth profiling. Measurements indicate the formation of a large conduction band offset (CBO) of 1.2 eV with chemical-bath deposition (CBD) of ZnOS on CZTSe (Eg = 0.96 eV). However, Ar ion sputter depth profiling is shown to produce compositional changes of the ZnOS thin film resulting in an apparent increase of the valence band maximum (VBM) for the buffer layer. With this in mind, the valence band edge energy offsets (VBO) are calculated and used to study solar cells made with the configuration glass/Mo/CZTSe/ZnOS/i-ZnO/Al:ZnO/Ni/Al. Variation of the deposition time of the ZnOS buffer layer during the CBD process has led to device efficiencies above 5%. For the thinnest ZnOS buffer layers, the short-circuit current matches that of devices with CdS buffer layers, but suffers from loss of open-circuit voltage. Interpretation of the solar cell measurements are aided by SCAPS thin-film device modeling.
ABSTRACT We demonstrate the use of chemical vapor deposition (CVD) to create unique thin (12–36 nm) and conformal TiO2 interlayers on indium-tin oxide (ITO) electrodes, for use as electron collection contacts in inverted bulk... more
ABSTRACT We demonstrate the use of chemical vapor deposition (CVD) to create unique thin (12–36 nm) and conformal TiO2 interlayers on indium-tin oxide (ITO) electrodes, for use as electron collection contacts in inverted bulk heterojunction P3HT/PC61BM organic photovoltaics (OPVs). Optimized CVD formation of these oxide films is inherently scalable to large areas, and may be a viable non-contact alternative to electron-selective interlayer formation. Oxide-based electron-selective interlayers, such as TiO2, need to be thin, conformal and sufficiently electronically conducting films without sacrificing electron harvesting selectivity. Using volatile titanium-tetraisopropoxide (TTIP) precursors in a flowing N2 gas stream, the CVD process provides nanometer control of film thickness to produce 12–36 nm thickness device-quality films. The best performing CVD films, processed at substrate temperatures of ca. 210 °C, characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were found to be amorphous but stoichiometric TiO2. Solution electrochemistries (voltammetry) of probe molecules were shown to be easily accessible indicators of film porosity and are predictive for electron harvesting selectivity (and hole-blocking) in an inverted configuration OPV platform. Small molecules whose redox potentials placed them energetically above the conduction band edge energy (ECB) were reduced/oxidized at nearly the same rates as for bare ITO. Probe molecules whose redox potentials place them energetically within the band gap region, below ECB, show almost complete blocking of their oxidation/reduction processes, for optimized conformal (and nonporous) TiO2 films. In addition, background oxidation current densities for solution probe molecules correlate inversely with the shunt resistance (RP) measured in OPVs. OPVs with the configuration: ITO/CVD-TiO2/P3HT:PC61BM/MoO3/Ag, using TiO2 films of 12, 24 and 36 nm, were evaluated for short-circuit photocurrent (JSC), open-circuit photopotential (VOC), and fill-factor (FF), versus bare ITO. OPVs using amorphous, conformal 24 nm TiO2 interlayers showed the highest fill factors, lowest RS, highest RP and power conversion efficiencies of ca. 3.7%.
Page 1. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim FULL P APER 1 wileyonlinelibrary.com Adv. Energy Mater. 2011, XX, 1–8 www.MaterialsViews. com www.advenergymat.de molecule based OPVs these gains ...
Using time-resolved in situ X-ray photoelectron spectroscopy, we identify and suppress rapid degradation mechanisms for cesium-stabilized formamidinium lead iodide perovskite materials used in state-of-the-art photovoltaics. Accelerated... more
Using time-resolved in situ X-ray photoelectron spectroscopy, we identify and suppress rapid degradation mechanisms for cesium-stabilized formamidinium lead iodide perovskite materials used in state-of-the-art photovoltaics. Accelerated degradation under high light intensity and heating reveals a photocatalytic reaction pathway responsible for rapid decomposition in iodide-rich compositions and a slower pathway for more stoichiometric samples. Using Avrami−Erofe'ev kinetic analysis, we find that the fast process is consistent with a 2D crystallization and growth mechanism fueled by excess halide salt at grain boundaries and surfaces. Moreover, the rate of decomposition varies dramatically with the wavelength of light used to illuminate the thin films. Our results reveal the photodegradation mechanisms of PbI 2 and excess iodide and provide a path to increasing perovskite stability under photoexcitation.
Herein, we consider the heterojunction formation of two prototypical metal oxides: p-type NiO and n-type ZnO. Elementally abundant, low-cost metal oxide/ oxide' heterojunctions are of interest for UV optical sensing, gas sensing,... more
Herein, we consider the heterojunction formation of two prototypical metal oxides: p-type NiO and n-type ZnO. Elementally abundant, low-cost metal oxide/ oxide' heterojunctions are of interest for UV optical sensing, gas sensing, photocatalysis, charge confinement layers, piezoelectric nanogenerators, and flash memory devices. These heterojunctions can also be used as current rectifiers and potentially as recombination layers in tandem photovoltaic stacks by making the two oxide layers ultrathin. In the ultrathin geometry, understanding and control of interface electronic structure and chemical reactions at the oxide/oxide' interface are critical to functionality, as oxygen atoms are shared at the interface of the dissimilar materials. In the studies presented here the extent of chemical reactions and interface band bending is monitored using X-ray and ultraviolet photoelectron spectroscopies. Interface reactivity is controlled by varying the near surface composition of nickel oxide, nickel hydroxide, and nickel oxyhydroxide using standard surface-treatment procedures. A direct correlation between relative percentage of interface hydroxyl chemistry (and hence surface Lewis basicity) and the local band edge alignment for ultrathin p−n junctions (6 nm NiO/30 nm ZnO) is observed. We propose an acid−base formulism to explain these results: the stronger the acid−base reaction, the greater the fraction of interfacial electronic states which lower the band offset between the ZnO conduction band and the NiO valence band. Increased interfacial gap states result in larger reverse bias current of the p−n junction and lower rectification ratios. The acid−base formulism could serve as a future design principle for oxide/oxide' and other heterojunctions based on dissimilar materials.
Research Interests:
Photovoltaic applications of perovskite semiconductor material systems have generated considerable interest in part because of predictions that primary defect energy levels reside outside the bandgap. We present experimental evidence that... more
Photovoltaic applications of perovskite semiconductor material systems have generated considerable interest in part because of predictions that primary defect energy levels reside outside the bandgap. We present experimental evidence that this enabling material property is present in the halide-lead perovskite, CH 3 NH 3 PbI 3 (MAPbI 3), consistent with theoretical predictions. By performing X-ray photoemission spectroscopy, we induce and track dynamic chemical and electronic transformations in the perovskite. These data show compositional changes that begin immediately with exposure to X-ray irradiation, whereas the predominant electronic structure of the thin film on compact TiO 2 appears tolerant to the formation of compensating defect pairs of V I and V MA and for a large range of I/Pb ratios. Changing film composition is correlated with a shift of the valence-band maximum only as the halide−lead ratio drops below 2.5. This delay is attributed to the invariance of MAPbI 3 electronic structure to distributed defects that can significantly transform the electronic density of states only when in high concentrations.
Research Interests:
An organometallic ink based on the nickel formate–ethylenediamine (Ni(O2CH)2(en)2) complex forms high performance NiOx thin film hole transport layers (HTL) in organic photovoltaic (OPV) devices. Improved understanding of these HTLs... more
An organometallic ink based on the nickel formate–ethylenediamine (Ni(O2CH)2(en)2) complex forms high performance NiOx thin film hole transport layers (HTL) in organic photovoltaic (OPV) devices. Improved understanding of these HTLs functionality can be gained from temperature-dependent decomposition/ oxidation chemistries during film formation and corresponding chemical structure-function relationships for energetics, charge selectivity, and transport in photovoltaic platforms. Investigations of as-cast films annealed in air (at 150 C–350 C), with and without subsequent O2-plasma treatment, were performed using thermogravimetric analysis, Fourier transform infrared spectroscopy, ultraviolet and X-ray photoelectron spectroscopy, and spectroscopic ellipsometry to elucidate the decomposition and oxidation of the complex to NiOx. Regardless of the anneal temperature, after exposure to O2-plasma, these HTLs exhibit work functions greater than the ionization potential of a prototype donor polymer poly(N-90-heptadecanyl-2,7- carbazole-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole) (PCDTBT), thereby meeting a primary requirement of energy level alignment. Thus, bulk-heterojunction (BHJ), OPV solar cells made on this series of NiOx HTLs all exhibit similar open circuit voltages (Voc). In contrast, the short circuit currents increase significantly from 1.7 to 11.2 mA cm2 upon increasing the anneal temperature from 150 C to 250 C. Concomitantly, increased conductivity and electrical homogeneity of NiOx thin films are observed at the nanoscale using conductive tip-AFM. Similar Voc observed for all the O2-plasma treated NiOx interlayers and variations to nanoscale conductivity suggest that the HTLs all form charge selective contacts and that their carrier extraction efficiency is determined by the amount of precursor conversion to NiOx. The separation of these two properties: selectivity and conductivity, sheds further light on charge selective interlayer functionality.
Research Interests:
A co-solvent, dimethylsulfoxide (DMSO), is added to the aqueous chemical ‘‘bath’’ deposition (CBD) process used to grow ZnOS buffer layers for thin film Cu2ZnSnSe4 (CZTSe) solar cells. Device performance improves markedly as fill factors... more
A co-solvent, dimethylsulfoxide (DMSO), is added to the aqueous chemical ‘‘bath’’ deposition (CBD) process used to grow ZnOS buffer layers for thin film Cu2ZnSnSe4 (CZTSe) solar cells. Device performance improves markedly as fill factors increase from 0.17 to 0.51 upon the co-solvent addition. X-ray photoelectron spectroscopy (XPS) analyses are presented for quasi-in situ CZTSe/CBD-ZnOS interfaces prepared under an inert atmosphere and yield valence band offsets equal to 􏰈1.0 eV for both ZnOS preparations. When combined with optical band gap data, conduction band offsets exceed 1 eV for the water and the water/ DMSO solutions. XPS measurements show increased downward band bending in the CZTSe absorber layer when the ZnOS buffer layer is deposited from water only. Admittance spectroscopy data shows that the ZnOS deposited from water increases the built-in potential (Vbi) yet these solar cells perform poorly compared to those made with DMSO added. The band energy offsets imply an alternate form of transport through this junction. Possible mechanisms are discussed, which circumvent the otherwise large conduction band spike between CZTSe and ZnOS, and improve functionality with the low-band gap absorber, CZTSe (Eg = 0.96 eV).
Research Interests:
Zinc Oxysulfide (ZnOS) has demonstrated potential in the last decade to replace CdS as a buffer layer material since it is a wide- band-gap semiconductor with performance advantages over CdS (Eg = 2.4 eV) in the near UV-range for solar... more
Zinc Oxysulfide (ZnOS) has demonstrated potential in the last decade to replace CdS as a buffer layer material since it is a wide- band-gap semiconductor with performance advantages over CdS (Eg = 2.4 eV) in the near UV-range for solar energy conversion. However, questions remain on the growth mechanisms of chemical bath deposited ZnOS. In this study, a detailed model is employed to calculate solubility diagrams that describe simple conditions for complex speciation control using only ammonium hydroxide without additional base. For these conditions, ZnOS is deposited via aqueous solution deposition on a quartz crystal microbalance in a continuous flow cell. Data is used to analyze the growth rate dependence on temperature and also to elucidate the effects of dimethylsulfoxide (DMSO) when used as a co-solvent. Activation energies (EA) of ZnOS are calculated for different flow rates and solution compositions. The measured EA relationships are affected by changes in the primary growth mechanism when DMSO is included.
Research Interests:
The use of oxide materials as a hole transport layers (HTL) offers the opportunity to optimize hole collection in a bulk heterojunction organic photovoltaic (OPV) device. We discuss the use of NiOx deposited by three different methods,... more
The use of oxide materials as a hole transport layers (HTL) offers the opportunity to optimize hole collection in a bulk heterojunction organic photovoltaic (OPV) device. We discuss the use of NiOx deposited by three different methods, pulsed laser deposition, sputtering and a solution precursor as an alternative to the standard OPV HTL. We also examine the ability of the HTL to improve device performance in a bulk heterojunction device utilizing a donor that has a deeper highest occupied molecular orbital (HOMO) level..
Organic photovoltaics (OPVs) are realizing power conversion efficiencies that are of interest for commercial production. Consequently, understanding device lifetime and mitigating degradation pathways have become vital to the success of a... more
Organic photovoltaics (OPVs) are realizing power conversion efficiencies that are of interest for commercial production. Consequently, understanding device lifetime and mitigating degradation pathways have become vital to the success of a new industry. Historically, the active organic components are considered vulnerable to photo-oxidation and represent the primary degradation channel. We present several (shelf life and light soaking) studies pointing to the relative stability of the active layers and instabilities in commonly used electrode materials. We show that engineering of the metal electrode and hole/electron injection layer can lead to environmentally stable devices without encapsulation.
Organic photovoltaics devices may pose one of the least expensive routes toward conversion of solar power. Two significant obstacles are low intrinsic material stabilities as well as poor interfacial charge transfer kinetics between the... more
Organic photovoltaics devices may pose one of the least expensive routes toward conversion of solar power. Two significant obstacles are low intrinsic material stabilities as well as poor interfacial charge transfer kinetics between the transparent conducting oxide and organic semiconductor. Presented is a series of investigations for several surface preparations on a popular metal-oxide (indium tin oxide) using cyclic voltammetry, four-point probe, work function, and contact angle measurement techniques. Surface treatments are correlated with device results in a prototypical organic photovoltaic architecture with an eye toward enhanced charge transfer and material stability at the metal-oxide/organic interface. Included is an overview of main organic photovoltaic operation and degradation mechanisms in the context of surface modification studies.