Electronic Structure of PbS Colloidal Quantum Dots on Indium Tin Oxide and Titanium Oxide

The size of colloidal quantum dot (CQD) materials and their surface modification by chemical ligands can change electronic properties thereby affecting device performances. In this study, direct measurement of the electronic structure within CQD thin film upon solid-state ligand exchange from oleic acid to 1,2-ethanedithiol has been made by photoelectron spectroscopy. Specifically, we analyzed valence band structures as a function of PbS CQD thickness on two kinds of substrates, indium tin oxide and titanium oxide, which give the trace of band bending and its saturation. Consequently, the energy-level alignment of the PbS CQD reveals downward band bending to the substrate but with different magnitude and depletion width depending on substrate. Wide depletion width and barrierless electron injection on TiO2 substrate indicate the importance of junction design and drift length for efficient CQD photovoltaics, which can be addressed discernibly via photoelectron spectroscopy

Variation of Coverage-Dependent Attachment of Multifunctional Groups in Alanine and Leucine to the Ge(100)-2×1 Surface: Bonding Configuration and Adsorption Stability

The coverage-dependent attachment of multifunctional groups included in alanine and leucine molecules adsorbed to the Ge(100)-2×1 surface was investigated and compared using core-level photoemission spectroscopy (CLPES) and density functional theory (DFT) calculations. The bonding configuration, stability, and adsorption energies were evaluated for two different coverage levels. In both molecules, the core-level spectra at a low coverage indicated that both the carboxyl and amine groups participated in the bonding with the Ge(100) surface by “O–H dissociated and N-dative bonded structure”. This is consistent with the DFT calculation results showing that such adsorption geometry is the most stable and follows the minimum reaction pathway. However, at high coverage level, an additional adsorption geometry of “O–H dissociation bonded structure” appeared possibly to minimize the steric hindrance between adsorbed molecules

Porous Networks of CdSe Nanocrystal Chains from Ultrafine Cd(OH)₂ Nanowires and Their Composite Materials

Long ultrathin Cd(OH)2 nanowires have been selectively grown on silica colloids in a basic aqueous condition. The Cd(OH)2 nanowires could be detached from the surface of the silica colloids by simply applying ultrasonication and then transformed into isolated CdSe nanocrystal chains. When the transformation into CdSe was conducted without detaching the Cd(OH)2 nanowires, nanoporous CdSe shells composed of wire-like nanocrystal chains were produced. The good solubility of the Cd(OH)2 nanowires in both hydrophilic and hydrophobic solvents facilitated the formation of composites with quantum dots, magnetic particles, organic molecules, and polymers. Embedding premade quantum dots possessed broad light absorption range and enhanced photoluminescence. Large amount of superparamagnetic particles endowed a fast magnetic response in addition to the fluorescence. Composites of organic/nanocrystal chains were readily fabricated by employing the electrostatic attraction between the positively charged Cd(OH)2 nanowires and negatively charged polymers or small molecules

Order–Disorder Transition in the Molecular Orientation during Initial Growth of Organic Thin Film

We report on the identification of molecular orientation and its order–disorder transition during the initial growth of 1,3-bis­(N-carbazolyl)­benzene (mCP) thin films on a highly ordered pyrolytic graphite (HOPG) surface by using photoelectron spectroscopy (PES). Theoretical PES amplitudes using a quantum mechanical calculation that adapts independent atomic center approximation (IAC) were calculated to compare with experimental observations. At low coverage, an equilibrium orientation of isolated adsorbate was estimated. As the coverage increases, the interaction between adsorbates becomes dominant and raises the disorder, which results in changes in the PES shapes as well as the line broadening of each peak

Diameter-Tunable CdSe Nanotubes from Facile Solution-Based Selenization of Cd(OH)₂ Nanowire Bundles for Photoelectrochemical Cells

Diameter-tunable CdSe nanotubes are synthesized via a sacrificial template approach in solution phase by reacting Cd(OH)2 nanowire bundles with NaHSe. The sacrificial templates of diameter-controlled Cd(OH)2 nanowire bundles undergo an interdiffusion process between Se2− and Cd2+ species, and subsequently core/shell structures of Cd(OH)2/CdSe are formed at intermediate states. The crystalline CdSe hollow nanotubes are finally formed when all Cd reagent sources are chalcogenized with Se in the solution phase, and their diameter can be readily controlled from ca. 20 to 60 nm depending on the dimension of nanowire bundle templates. With heat treatment at 300 °C, crystallinity of the CdSe nanotubes can be enhanced with removal of amorphous selenium present on the nanotube surface, which shows a photo-conversion efficiency of 0.57% in CdSe/polysulfide liquid-junction solar cell along with a bandgap energy of 1.89 eV. A possible scheme for the CdSe nanotube synthesis from the template of Cd(OH)2 nanowire bundles is also proposed in this article

Electronic Mechanism of <i>In Situ</i> Inversion of Rectification Polarity in Supramolecular Engineered Monolayer

This Communication describes polarity inversion in molecular rectification and the related mechanism. Using a supramolecular engineered, ultrastable, binary-mixed self-assembled monolayer (SAM) composed of an organic molecular diode (SC11BIPY) and an inert reinforcement molecule (SC8), we probed a rectification ratio (r)–voltage relationship over an unprecedentedly wide voltage range (up to |3.5 V|) with statistical significance. We observed positive polarity in rectification at |1.0 V| (r = 107), followed by disappearance of rectification at ∼|2.25 V|, and then eventual emergence of new rectification with the opposite polarity at ∼|3.5 V| (r = 0.006; 1/r = 162). The polarity inversion occurred with a span over 4 orders of magnitude in r. Low-temperature experiments, electronic structure analysis, and theoretical calculations revealed that the unusually wide voltage range permits access to molecular orbital energy levels that are inaccessible in the traditional narrow voltage regime, inducing the unprecedented in situ inversion of polarity

Tailored Band Edge Positions by Fractional Ligand Replacement of Nonconductive Colloidal Quantum Dot Films

The tunable band edge position of colloidal quantum dot (CQD) films is a key part of efficient optoelectronic device design of various forms such as photovoltaics, light-emitting diodes, and photodetectors. An accurate estimation of shifts in the band edge position of CQD layers is still considered challenging, especially when the CQD films are nonconductive. Here, we investigate the effect of nonconductive CQD films on photoelectron spectroscopy (PES) and photoelectron yield spectroscopy (PYS). We demonstrate control of systematic band edge positions by fractional ligand replacement of nonconductive CQD film characterized with photoelectron yield spectroscopy and density functional theory calculations. As-synthesized CQDs with insulating oleate ligands were fractionally replaced with trans-3,5-difluorocinnamic acid molecules in a nonpolar solution. The fractionally replaced surface-bound ligands are quantitatively analyzed using 1H and 19F nuclear magnetic resonance (NMR) spectroscopy. We found that the energy levels of nonconducting CQD films shift linearly as a function of the number of bound trans-3,5-difluorocinnamates with specific dipole moments while retaining hydrophobic wettability

Hole Injection Enhancement by a WO₃ Interlayer in Inverted Organic Light-Emitting Diodes and Their Interfacial Electronic Structures

The interfacial energy level alignment of hole injection layers with WO3 insertion in an inverted organic light-emitting diode structure and its influence on electroluminescence have been studied using in situ X-ray, ultraviolet photoelectron spectroscopy, and device measurements. The hole injection barrier for N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) upon Al deposition was estimated to be 1.37 eV, which makes an Al anode unfavorable for hole injection. With a thin WO3 layer deposited onto the NPB, the NPB highest occupied molecular orbital (HOMO) level bends dramatically to 0.15 eV below the Fermi level. This NPB HOMO level and WO3 conduction band close to the Fermi level form a charge generation layer without an interfacial chemical reaction. This is why the WO3 interlayer dramatically helps charge injection even with a low work-function Al metal anode. Indeed, the electroluminescence and current efficiency from this structural device were greatly enhanced by 3 orders of magnitude compared with that without a WO3 interlayer

Defect Passivation of Low-Temperature-Sputtered Tin Oxide Electron Transport Layers through Magnesium Doping for Perovskite Solar Cells

The optimal choice of electron transporting materials is of vital importance in improving the efficiency and reducing the cost of perovskite solar cells (PSCs) as electron transport layers (ETLs) play a key role in charge extraction and transfer. Despite SnO2 being a commonly used ETL, magnetron-sputtered SnO2 continues to be constrained by oxygen vacancy (VO)-related point defects, which result in severe interface charge recombination, thereby limiting the open-circuit voltage and fill factor of PSCs using magnetron-sputtered SnO2 ETLs. Herein, a doping strategy was adopted to suppress the defect density in magnetron-sputtered SnO2, in which Mg:SnO2 (MTO) was prepared by magnetron co-sputtering of MgO and SnO2 at room temperature. After Mg doping, the VO defects were passivated, the density of the trap states in the SnO2 ETL was reduced, and the energy level alignment between the ETL and perovskite layer was optimized. As a result, the undesired charge recombination was effectively suppressed, thus leading to an approximately 8.7% increase in the average device efficiency and approximately 11% increase in the stabilized power output. The best-performing device achieved an efficiency of 19.55%, therefore indicating the high potential of the magnetron-sputtered Mg:SnO2 ETL toward the commercialization of PSCs

Photovoltaic Performance and Interface Behaviors of Cu(In,Ga)Se₂ Solar Cells with a Sputtered-Zn(O,S) Buffer Layer by High-Temperature Annealing

We selected a sputtered-Zn­(O,S) film as a buffer material and fabricated a Cu­(In,Ga)­Se₂ (CIGS) solar cell for use in monolithic tandem solar cells. A thermally stable buffer layer was required because it should withstand heat treatment during processing of top cell. Postannealing treatment was performed on a CIGS solar cell in vacuum at temperatures from 300–500 °C to examine its thermal stability. Serious device degradation particularly in <i>V</i>_<i>OC</i> was observed, which was due to the diffusion of thermally activated constituent elements. The elements In and Ga tend to out-diffuse to the top surface of the CIGS, while Zn diffuses into the interface of Zn­(O,S)/CIGS. Such rearrangement of atomic fractions modifies the local energy band gap and band alignment at the interface. The notch-shape induced at the interface after postannealing could function as an electrical trap during electron transport, which would result in the reduction of solar cell efficiency