Wolfram Jaegermann

Role of the Selective Contacts in the Performance of Lead Halide Perovskite Solar Cells

The effect of electron- and hole-selective contacts in the final cell performance of hybrid lead halide perovskite, CH3NH3PbI3, solar cells has been systematically analyzed by impedance spectroscopy. Complete cells with compact TiO2 and spiro-OMeTAD as electron- and hole-selective contacts have been compared with incomplete cells without one or both selective contacts to highlight the specific role of each contact. It has been described how selective contacts contribute to enhance the cell FF and how the hole-selective contact is mainly responsible for the high Voc in this kind of device. We have determined that the recombination rate is mainly governed by the selective contacts. This fact has important implication for the future optimization of perovskite solar cells. Finally, we have developed a method to analyze the results obtained, and it has been applied for three different electron-selecting materials: TiO2, ZnO, and CdS.

Nanostructured SnO2–ZnO Heterojunction Photocatalysts Showing Enhanced Photocatalytic Activity for the Degradation of Organic Dyes

Nanoporous SnO(2)-ZnO heterojunction nanocatalyst was prepared by a straightforward two-step procedure involving, first, the synthesis of nanosized SnO(2) particles by homogeneous precipitation combined with a hydrothermal treatment and, second, the reaction of the as-prepared SnO(2) particles with zinc acetate followed by calcination at 500 °C. The resulting nanocatalysts were characterized by X-ray diffraction (XRD), FTIR, Raman, X-ray photoelectron spectroscopy (XPS), nitrogen adsorption-desorption analyses, transmission electron microscopy (TEM), and UV-vis diffuse reflectance spectroscopy. The SnO(2)-ZnO photocatalyst was made of a mesoporous network of aggregated wurtzite ZnO and cassiterite SnO(2) nanocrystallites, the size of which was estimated to be 27 and 4.5 nm, respectively, after calcination. According to UV-visible diffuse reflectance spectroscopy, the evident energy band gap value of the SnO(2)-ZnO photocatalyst was estimated to be 3.23 eV to be compared with those of pure SnO(2), that is, 3.7 eV, and ZnO, that is, 3.2 eV, analogues. The energy band diagram of the SnO(2)-ZnO heterostructure was directly determined by combining XPS and the energy band gap values. The valence band and conduction band offsets were calculated to be 0.70 ± 0.05 eV and 0.20 ± 0.05 eV, respectively, which revealed a type-II band alignment. Moreover, the heterostructure SnO(2)-ZnO photocatalyst showed much higher photocatalytic activities for the degradation of methylene blue than those of individual SnO(2) and ZnO nanomaterials. This behavior was rationalized in terms of better charge separation and the suppression of charge recombination in the SnO(2)-ZnO photocatalyst because of the energy difference between the conduction band edges of SnO(2) and ZnO as evidenced by the band alignment determination. Finally, this mesoporous SnO(2)-ZnO heterojunction nanocatalyst was stable and could be easily recycled several times opening new avenues for potential industrial applications.

Enhanced specific grain boundary conductivity in nanocrystalline Y2O3-stabilized zirconia

Abstract Stabilized zirconia samples containing 1.7 and 2.9 mol% Y 2 O 3 with average grain sizes of 25–50 nm were prepared by pressureless sintering and hot-pressing. Phase content, microstructure and average grain size of the samples were examined using X-Ray Diffraction (XRD) and High Resolution Scanning Electron Microscopy (HRSEM). The density of the samples measured based on the Archimedes Principle was in the range of 93–96% of the theoretical density. The samples were also investigated using X-ray Photoelectron Spectroscopy (XPS) for traces of Si. Impedance Spectroscopy was used to determine the dc ionic conductivities of the grain interior (bulk) and of the grain boundaries. The activation energies of both processes were slightly lower as for comparable microcrystalline samples as reported in the literature. This result is attributed to the complete tetragonal structure and the low Si-content of the samples. The conductivities of the bulk and the grain boundary process were in the same range as for microcrystalline samples. Therefore, due to the much smaller grain size the specific grain boundary conductivities of the nanocrystalline samples is 1–2 orders of magnitude higher than that of the microcrystalline samples. This is also attributed to the low Si-content and its grain size-dependent segregation in the nanocrystalline samples.

Interfacial properties of semiconducting transition metal chalcogenides

Abstract This review is aimed at the correlation of structural and electronic properies of semiconducting transition metal chalcogenides with molecular surface processes and mechanisms in photoelectrochemistry, (photo)catalysis, geochemistry and hydrometallurgy. Layer-type, pyrite structured and transition metal cluster containing chalcogenides are selected as model systems to explain the principles involved. Special emphasis is given to the discussion of materials which involve transition metal d- states in the interfacial reaction pathways of holes and electrons. Since they initiate and control heterogeneous coordination chemistry at the surfaces they may provide the possibility of tailoring selective and catalytically demanding reactions. Examples of such mechanisms are presented and discussed in relation to surface properties involved.

Band lineup of layered semiconductor heterointerfaces prepared by van der Waals epitaxy: Charge transfer correction term for the electron affinity rule

The occurrence of quantum dipoles at layered materials semiconductor heterointerfaces was investigated by photoemission spectroscopy (PES). Due to the unique properties of layered compounds the prepared interfaces are essentially free of the structural problems known from the usually investigated heterosystems composed of III–V, IV or II–VI materials allowing the detailed investigation of electronic phenomena at the interfaces. We investigated heterostructures composed of epitaxial layers of SnS2 and SnSe2 on different single crystalline layered chalcogenide substrates (WSe2, MoS2, MoTe2, and GaSe). The epilayers were grown by van der Waals epitaxy (vdWe) on the (0001) plane of the substrate crystals. For every system the valence band offset was determined by careful evaluation of the PES data as a function of the film thickness. Using published values for the band gaps and the experimentally determined work functions and surface potentials the band lineup for each system was determined. The band offsets ...

Efficient Planar Heterojunction Perovskite Solar Cells Based on Formamidinium Lead Bromide

The development of medium-bandgap solar cell absorber materials is of interest for the design of devices such as tandem solar cells and building-integrated photovoltaics. The recently developed perovskite solar cells can be suitable candidates for these applications. At present, wide bandgap alkylammonium lead bromide perovskite absorbers require a high-temperature sintered mesoporous TiO2 photoanode in order to function efficiently, which makes them unsuitable for some of the above applications. Here, we present for the first time highly efficient wide bandgap planar heterojunction solar cells based on the structurally related formamidinium lead bromide. We show that this material exhibits much longer diffusion lengths of the photoexcited species than its methylammonium counterpart. This results in planar heterojunction solar cells exhibiting power conversion efficiencies approaching 7%. Hence, formamidinium lead bromide is a strong candidate as a wide bandgap absorber in perovskite solar cells.

Interface properties and band alignment of Cu2S/CdS thin film solar cells

The stoichiometry and electronic properties of bulk Cu2S thin films obtained by vacuum evaporation were investigated by optical spectroscopy, X-ray diffraction and photoemission spectroscopy. The Cu2S/CdS heterojunction interface has been prepared in situ and characterized by photoelectron spectroscopy (X-ray photoemission spectroscopy and ultraviolet photoelectron spectroscopy) after each growth step under ultra high vacuum conditions. The XPS core level spectra as well as valence band spectra of the substrate Cu2S and overlayer CdS were acquired after each step. From these measurements, a large overall band bending of 0.9 eV is observed. The valence band offset is determined to be ΔEVB=1.2 eV and the conduction band offset is ΔECB=0±0.1 eV.

Surface states on cubic d-band semiconductor pyrite (FeS2)

A model for the electronic structure of the (100) surface of the d-band semiconductor pyrite (FeS2) is presented based on ligand field theory. The existence of a new type of surface states at the valence band edge resulting from a symmetry reduction of the Fe coordination sphere is deduced. The model is able to explain the inversion layer observed on n-type crystals as well as surface core level shifts in S 2p photoemission spectra.

Thin photoactive FeS2 (pyrite) films

Abstract Thin pyrite films (0.05 – 2 μm) have been prepared by metal organic chemical vapor deposition (MOCVD) employing ironpentacarbonyl and sulphur or hydrogen sulfide. Best photoconductivity was found at a deposition temperature of 130°C, where polycrystalline layers were obtained. The photoactivity of the prepared layers was measured by time-resolved microwave conductivity (TRMC) and by steady state photoconductivity measurements.

Reactivity of layer type transition metal chalcogenides towards oxidation

Abstract X-ray excited photoemission spectroscopy has been used to correlate the electronic structure of the layered compounds MoS 2 , MoSe 2 , WSe 2 , MoTe 2 and PtS 2 to their reactivity. We find that the ionicity decreases in the sequence given above, indicated by the changes observed in the density of states in the valence band. The reactivity towards oxidation of the investigated materials increases in the sequence given above. After oxidation in air (several hours to days) or after electrochemical oxidation (0.01 to 1 C) we detect only small amounts of Mo oxide for MoS 2 , the selenides are more easily oxidized, whereas MoTe 2 fully corrodes. The results indicate that the differences in reaction behavior are correlated to the relative contributions of metal d-states in the valence band and conduction band of the compounds.