Open-Circuit Photovoltage Exceeding 950 mV with an 840 mV Average at Sb2S3-Thianthrene+/0 Junctions Enabled by Thioperylene Anhydride Back Contacts.

Covalently attached perylene monolayers serve as back contacts for Sb2S3 photoelectrochemical cells with a thianthrene+/0 front, rectifying contact. Covalent attachment of perylenetetracarboxylic dianhydride, PTCDA, to Si(111) utilizes an anhydride-to-imide conversion at surface-attached amines. For Sb2S3 solar absorbers, we hypothesized that a terminal thioperylene anhydride, i.e., S=C-O-C=S, formed from thionation of the terminal perylene anhydride would serve as a soft, electron-selective and hole-blocking back contact. We explored several routes to convert carbonyls to thiocarbonyls on surface-attached perylene anhydrides including Lawesson's reagent, P4S10, and a P4S10-pyridine complex. Here, P4S10 in toluene yielded the highest conversion as quantified by thioperylene-anhydride-S-to-imide-N ratios in X-ray photoelectron spectroscopy (XPS). Spectra demonstrated minimal residual reagent as determined by the absence of quantifiable phosphorus following sonication and rinsing. Photoelectrochemistry yielded an average |V oc| = 840 ± 90 mV with the highest value of 952 mV under ELH-simulated AM1.5G illumination for chemical-bath-deposited Sb2S3 in the strongly oxidizing thianthrene+/0 redox couple when thioperylene-anhydride-tethered surfaces formed the back contact. Sb2S3 absorbers in which perylene anhydride, esters, thionoesters, and thiols form the back contact yielded significantly decreased |V oc| magnitudes vs Sb2S3 on perylene-thioanhydride-terminated surfaces. We attribute the large V oc to the combination of favorable sulfur-functionalized surfaces for deposition, charge transfer properties of the perylene layer, and use of the thianthrene+/0 redox couple.

Quantification of Surface Reactivity and Step-Selective Etching Chemistry on Single-Crystal BiOI(001).

To bridge the gap between the cleanliness of a freshly cleaved surface of 2D BiOI and that available from a purely chemical-etching means, we subjected single-crystal BiOI to a series of surface treatments and quantified the resulting chemical states and electronic properties. Vapor transport syntheses included both physical vapor transport from single-source BiOI, as well as chemical vapor transport from Bi2O3 + BiI3 and from Bi + I2 + Bi2O3. Surface treatments included tape cleaving, rinsing in water, sonication in acetone, an aqueous HF etch, and a sequential HF etch with subsequent sonication in acetone. X-ray diffraction, XRD, and X-ray photoelectron spectroscopy, XPS, probed the resulting bulk crystalline species and interfacial chemical states, respectively. In comparison with overlayer models of idealized oxide-terminated or iodide-terminated BiOI, angle-resolved XPS elucidated surface terminations as a function of each treatment. Ultraviolet photoelectron spectroscopy, UPS, established work-function, and Fermi-level energies for each treatment. Data reveal that HF etching yields interfacial BiI3 at BiOI steps that is subsequently removed with acetone sonication. UPS establishes n-type behavior for the vapor-transport-synthesized BiOI, and surface work function and Fermi level shifts for each chemical treatment under study. We discuss the implications for processing BiOI nanofilms for energy-conversion applications.

ZSM-5 decrystallization and dealumination in hot liquid water.

Zeolites have recently attracted attention for upgrading renewable resources in the presence of liquid water phases; however, the stability of zeolites in the presence of liquid-phase water is not completely understood. Accordingly, the stability of the ZSM-5 framework and its acid sites was studied in the presence of water at temperatures ranging from 250 to 450 °C and at pressures sufficient to maintain a liquid or liquid-like state (25 MPa). Treated samples were analyzed for framework degradation and Al content and coordination using a variety of complementary techniques, including X-ray diffraction, electron microscopy, N2 sorption, 27Al and 29Si NMR spectroscopy, and several different types of infrared spectroscopy. These analyses indicate that the ZSM-5 framework retains >80% crystallinity at all conditions, and that 300-400 °C are the most aggressive. Decrystallization appears to initiate primarily at crystal surfaces and share many characteristics in common with alkali promoted desilication. Liquid water treatment promotes ZSM-5 dealumination, following a mechanism analogous to that observed under steaming conditions: initiation by Al-O hydrolysis, Al migration to the surface, and finally deposition as extra framework Al or possibly complete dissolution under some conditions. As with the framework, dealumination is most aggressive at 300-400 °C. Several models were evaluated to capture the non-Arrhenius effect of temperature on decrystallization and dealumination, the most successful of which included temperature dependent values of the water auto-ionization constant. These results can help interpretation of previous studies on ZSM-5 catalysis in hot liquid water and suggest future approaches to extend catalyst lifetime.

Covalent Attachment and Characterization of Perylene Monolayers on Si(111) and TiO2 for Electron-Selective Carrier Transport.

We functionalized chemically oxidized Si(111) and TiO2 surfaces with covalently attached rylene molecules and demonstrated further chemical conversion of the attached species. Base-catalyzed activation of perylene tetracarboxylic dianhydride (PTCDA) preceded reaction with phenylaminosilane-terminated surfaces, yielding surface-bound perylene via an imide linkage. Transflection infrared (IR) spectroscopy of the carbonyl vibrational region elucidated the presence of anhydride, imide, and ester species following each reaction stage. The presence of both anhydride and imide IR features following reaction with PTCDA validates successful perylene attachment. Subsequent functionalization of the surface-attached perylenes yielded IR spectra with little or no detectable anhydride features that indicate successful conversion to ester or imide species based on respective reactions with alkyl bromides or aryl amines. X-ray photoelectron spectroscopy quantified fractional coverages of surface-attached perylene species following a post-deposition derivatization with fluorine-containing alkyl bromides and with anilines. Overlayer model interpretation of the photoelectron results determined a perylene surface coverage of ∼15% relative to the surface density of Si(111) atop sites and a ∼10% surface coverage of imide-terminated perylene species. The interpreted coverage data yield an approximate conversion efficiency for the anhydride-to-imide derivatization at surface-attached perylenes of ∼66%. We discuss the present results in terms of possible coverage and packing on oxide-free silicon surfaces and the utilization of covalently attached rylene species as electron-transporting and hole-blocking connecting layers in molecular electronics and tandem-junction photovoltaic designs.

Detection of adsorbates on emissive MOF surfaces with X-ray photoelectron spectroscopy.

Luminescent metal-organic frameworks (MOFs) have been explored extensively as potential probes for nitroaromatic molecules, which are common constituents of explosive devices. Guest encapsulation within MOF pores is often cited as the prerequisite for emission changes, but the evidence for this signal transduction mechanism is often inadequate. Using the unique bipyridyl ligand AzoAEpP (2,2'-bis[N,N'-(4-pyridyl)ethyl]diaminoazobenzene), we constructed two luminescent pillared paddle-wheel Zn2+ MOFs using aryl dicarboxylate ligands 1,4-naphthalenedicarboxylic acid (ABMOF-1) and benzene 1,4-dicarboxylic acid (ABMOF-2). While both MOFs exhibit luminescence, 2,4-dinitrophenol only extinguishes ABMOF-1 emission. Since the size of the pores in ABMOF-1 precludes guest inclusion, we used X-ray photoelectron spectroscopy (XPS) to confirm the surface interaction and obtain insight into the nature of the quenching process. XPS experiments utilized a fluorinated nitroaromatic molecule, 4-trifluoromethyl-2,6-dinitrophenol, that extinguishes ABMOF-1 emission, and verified surface adsorption through a series of angle-resolved (ARXPS) and argon-ion sputter depth profile experiments. By further developing these techniques, we hope to develop a general approach for distinguishing between the various intermolecular interactions between MOFs and analytes that lead to changes in luminescence.

Highly stable and efficient all-inorganic lead-free perovskite solar cells with native-oxide passivation.

There has been an urgent need to eliminate toxic lead from the prevailing halide perovskite solar cells (PSCs), but the current lead-free PSCs are still plagued with the critical issues of low efficiency and poor stability. This is primarily due to their inadequate photovoltaic properties and chemical stability. Herein we demonstrate the use of the lead-free, all-inorganic cesium tin-germanium triiodide (CsSn0.5Ge0.5I3) solid-solution perovskite as the light absorber in PSCs, delivering promising efficiency of up to 7.11%. More importantly, these PSCs show very high stability, with less than 10% decay in efficiency after 500 h of continuous operation in N2 atmosphere under one-sun illumination. The key to this striking performance of these PSCs is the formation of a full-coverage, stable native-oxide layer, which fully encapsulates and passivates the perovskite surfaces. The native-oxide passivation approach reported here represents an alternate avenue for boosting the efficiency and stability of lead-free PSCs.

Selective CO2 Reduction Catalyzed by Single Cobalt Sites on Carbon Nitride under Visible-Light Irradiation.

Framework nitrogen atoms of carbon nitride (C3N4) can coordinate with and activate metal sites for catalysis. In this study, C3N4 was employed to harvest visible light and activate Co2+ sites, without the use of additional ligands, in photochemical CO2 reduction. Photocatalysts containing single Co2+ sites on C3N4 were prepared by a simple deposition method and demonstrated excellent activity and product selectivity toward CO formation. A turnover number of more than 200 was obtained for CO production using the synthesized photocatalyst under visible-light irradiation. Inactive cobalt oxides formed at relatively high cobalt loadings but did not alter product selectivity. Further studies with X-ray absorption spectroscopy confirmed the presence of single Co2+ sites on C3N4 and their important role in achieving selective CO2 reduction.

Synthesis and Characterization of Alkylamine-Functionalized Si(111) for Perovskite Adhesion With Minimal Interfacial Oxidation or Electronic Defects.

We investigated synthetic strategies for the functionalization of Si(111) surfaces with organic species containing amine moieties. We employed the functionalized surfaces to chemically "glue" perovskites to silicon with efficient electron transfer and minimal oxidation leading to deleterious recombination at the silicon substrate. A two-step halogenation-alkylation reaction produced a mixed allyl-methyl monolayer on Si(111). Subsequent reactions utilized multiple methods of brominating the allyl double bond including reaction with HBr in acetic acid, HBr in THF, and molecular bromine in dichloromethane. Reaction with ammonia in methanol effected conversion of the bromide to the amine. X-ray photoelectron spectroscopy (XPS) quantified chemical states and coverages, transient-microwave photoconductivity ascertained photogenerated carrier lifetimes, atomic force microscopy (AFM) quantified perovskite-silicon adhesion, and nonaqueous photoelectrochemistry explored solar-energy-conversion performance. The HBr bromination followed by the amination yielded a surface with ∼10% amine sites on the Si(111) with minimal oxide and surface recombination velocity values below 120 cm s-1, following extended exposures to air. Importantly, conversion of amine sites to ammonium and deposition of methylammonium lead halide via spin coating and annealing did not degrade carrier lifetimes. AFM experiments quantified adhesion between perovskite films and alkylammonium-functionalized or native-oxide silicon surfaces. Adhesion forces/interactions between the perovskite and the alkylammonium-functionalized films were comparable to the interaction between the perovskite and native-oxide silicon surface. Photoelectrochemistry of perovskite thin films on alkylammonium-functionalized n+-Si showed significantly higher Voc than n+-Si with a native oxide when in contact with a nonaqueous ferrocene+/0 redox couple. We discuss the present results in the context of utilizing molecular organic recognition to attach perovskites to silicon utilizing organic linkers so as to inexpensively modify silicon for future tandem-junction photovoltaics.