Potassium-doped organometal halide perovskite solar cells (PSCs) of more than 20% power conversion efficiency (PCE) without I-V hysteresis were constructed. The crystal lattice of the organometal halide perovskite was expanded with increasing of the potassium ratio, where both absorption and photoluminescence spectra shifted to the longer wavelength, suggesting that the optical band gap decreased. In the case of the perovskite with the 5% K+, the conduction band minimum (CBM) became similar to the CBM level of the TiO2-Li. In this situation, the electron transfer barrier at the interface between TiO2-Li and the perovskite was minimised. In fact, the transient current rise at the maximum power voltages of PSCs with 5% K+ was faster than that without K+. It is concluded that stagnation-less carrier transportation could minimise the I-V hysteresis of PSCs.

No abstract

Alkanethiolates (ATs) forming self-assembled monolayers (SAMs) on coinage metal and semiconductor substrates have been used successfully for decades for tailoring the properties of these surfaces. Here, we provide a detailed analysis of a highly promising class of AT-based systems, which are modified by one or more dipolar carboxylic acid ester groups embedded into the alkyl backbone. To obtain comprehensive insight, we study nine different embedded-dipole monolayers and five reference non-substituted SAMs. We systematically varied chain lengths, ester group orientations, and number of ester groups contained in the chain. To understand the structural and electronic properties of the SAMs, we employ a variety of complementary experimental techniques, namely infrared reflection absorption spectroscopy (IRS), highresolution X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM), and Kelvin probe (KP) AFM. These experiments are complemented with state-of-the-art electronic band-structure calculations. We find intriguing electronic properties like large and variable SAM-induced work function modifications and dipole induced shifts of the electrostatic potential within the layers. These observations are analyzed in detail by joining the results of the different experimental techniques with the atomistic insight provided by the quantum-mechanical simulations.orientation of this group in the backbone, the work function of the system can be changed by either +0.57 or −0.42 eV relative to a reference oligophenylene SAM. This variation is achieved without changing the chemistry for docking to the substrate or the chemical composition of the SAMambient interface. 28 Tuning the work function is, however, not the only effect of the embedded pyrimidine group in these systems, since it also induces a potential discontinuity inside the monolayer. This effect significantly changes the transport properties of the SAM, shifting the transition voltage and resulting in current rectification. 30,31 The potential discontinuity also shifts the core-level energies in the regions above and below the embedded dipoles relative to each other. These shifts can be observed directly by X-ray photoelectron spectroscopy (XPS), reflected as different binding energies (BEs) for the photoemission peaks associated with both regions. This observation, along with others, [32][33][34][35] lead us to question the generally accepted chemical shift model that assumes that shifts in the core-level BEs in monomolecular films are solely a consequence of different chemical environments of the respective atoms, 36 with the energy referenced to the Fermi level of the substrate. In contrast, it suggests that electrostatic shifts not related to the immediate chemical environment of an atom are similarly important. Generally, such electrostatic shifts are superimposed on the chemical ones and can under certain circumstances even play a dominant role. 37 The respective electrostatic effects in photoemiss...

The penetration behavior of thermally evaporated Au on S(CH(2))(15)CH(3), S(CH(2))(15)CO(2)CH(3), S(CH(2))(15)CO(2)H, K-modified S(CH(2))(15)CO(2)CH(3), and K-modified S(CH(2))(15)CO(2)H self-assembled monolayers (SAM) on Au substrates is investigated. Gold is a particularly interesting metal since vapor-deposited Au atoms are known to pass through alkanethiolate SAMs on Au{111} substrates at room temperature. Here we show that it is possible to control Au penetration by adjusting the interactions between terminal groups. It is found that Au atoms evenly penetrate into the CH(3) and CO(2)CH(3) films, forming smooth buried layers below the organic thin films. For the CO(2)H film, although Au atoms can still penetrate through it, filaments and mushroomlike clusters form due to H-bonding between film molecules. In the case of the K-modified CO(2)CH(3) or CO(2)H films, however, most Au atoms form islands at the vacuum interface. These results suggest that van der Waals forces and H-bonds are not strong enough to block Au from going through but that ionic interactions are able to block Au penetration. The measurements were performed primarily using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and atomic force microscopy (AFM). The combination of these highly complementary probes provides a very useful strategy for the study of metal atom behavior on SAMs.

b S Supporting Information N anosized metal oxide semiconductors have been actively studied for the past few decades, especially for their potential applications for photocatalytic reactions, photobreaching of toxic compounds, artificial photosynthesis, and photovoltaics. 1À5 Among all metal oxide semiconductors, TiO 2 is one of the most important materials because of the abundance of titanium as a natural resource, corrosion resistivity, transparency in the visible region, and nontoxicity. The kinetically favorable anatase crystalline phase is the most widely studied due to its ease of preparation, stability of the crystal up to ∼600°C, and advantages for catalytic and electronic properties. The anatase crystal of TiO 2 has a few fundamental low-index facet systems, such as {101}, {001}, {100}, {110}, and {103}. 2 Typical anatase crystal of TiO 2 , including naturally grown crystals, consists of mostly the {101} facet due to the much greater stability of this surface than other surfaces due to its lower surface energy. Therefore the characteristics of TiO 2 nanoparticles, in most cases, have been supposed to be attributed to the {101} facet, regradless of whether it is mentioned in the literature, since it had been extremely difficult to selectively synthesize other unstable surfaces, such as the (001) surface. 2,6 Recently, however, Lu et al. proposed a novel synthesis to selectively increase the fraction of {001} facet and thus introduced a new area of study, the chemically reactive (001) surface, into the widely studied TiO 2 nanoparticles. 7 The proposed novel process is composed of the addition of a relatively large amount of hydrofluoric acid (HF) for hydrothermal growth of TiO 2 . Although a few groups also reported the follow-up work of {001} facet-dominating TiO 2 preparation, most groups used the same procedure using HF. 8À11 The role of HF in TiO 2 synthesis was also suggested to be that the fluorine-termination of TiO 2 surface stabilizes the (001) surface more than the other facets and results in a larger fraction of intrinsically unstable (001) surface; the result of the first-principle calculations also supported this explanation. 7 The typical fraction of {001} facet in reports varies from 40 to 80%, usually determined by the characteristic bipyramidal structure of particles revealing the trapezoidal (101) and rectangular (001) surfaces observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). 7À12 Since the surface energy of the {001} facet is higher than that of the {101} facet, recent reports indicated a few characteristic features of the {001} facet, a site-selective reaction, enhancement of reactivity, adsorption of molecules, and water dissociation with {001} facetdominating TiO 2 nanoparticles 13À15 for application to photocatalytic reactions 10,11 and photovoltaics. 12,16 Additionally, a few theoretical investigations based on first-principle calculations also reported the characteristic reactivity of the {001} facet. 17,18 Nevertheless, detailed analysi...

We report a crossed-nanowire molecular junction array platform that enables direct measurement of current-voltage-temperature characteristics simultaneously with inelastic electron tunneling and Raman vibrational spectra on the same junction. Measurements on dithiol-terminated oligo(phenylene-ethynylene) junctions show both spectroscopies interrogate the gap-confined molecules to reveal distinct molecular features. This versatile platform allows investigation of advanced phenomena such as molecular switching and cooperative effects with the flexible ability to scale both the junction geometries and array sizes.

Solid state dye-sensitized solar cell using polypyrrole as a hole transport layer was improved. Carbon-based counter electrode gave a good electric contact with the hole transport layer of polypyrrole to give higher cell performance compared to the cell with gold or platinum counter electrode.

Highly controlled morphology Au nanoparticle films can be formed on the surfaces of self-assembled monolayers (SAMs) by vapor deposition at cryogenic temperatures (approximately 10 K) with intervening condensed Xe layers on the SAMs serving as a buffer to reduce the kinetic energy of the Au atoms impinging on the surface (buffer layer assisted growth or BLAG). Under these conditions pristine Au nanoparticles (AuNp) of a uniform shape and size were deposited onto two SAMs differing only by their terminal groups, 4-benzenedithiol (BDT) and 4-methylbenzenethiol (MBT), to form -S/Au and -CH(3)/Au interfaces with essentially identical AuNp overlayer morphologies. A surface enhanced Raman (SERS) enhancement factor ratio EF(BDT)/EF(MBT) of approximately 130 was observed uniformly across the surfaces (approximately <10% variation). Since equal electromagnetic contributions to the SERS enhancements are expected from the two identically structured Au overlayer films, the observed SERS intensity ratio accordingly reflects a pure chemical enhancement (CE) contribution arising from the -S/Au relative to the -CH(3)/Au interface and thereby provides the first quantitative experimental data for the magnitude of the SERS CE for well-defined Au-molecule contacts.