Junction studies on electrochemically fabricated p-n Cu(2)O homojunction solar cells for efficiency enhancement.

p-n Cu(2)O homojunction solar cells were electrochemically fabricated by consecutively depositing an n-Cu(2)O layer on a p-Cu(2)O layer. In order to better understand the Fermi levels of the electrochemically grown polycrystalline p- and n-Cu(2)O layers and maximize the overall cell performance, the back and front contacts of the Cu(2)O homojunction cells were systematically changed and the I-V characteristics of the resulting cells were examined. The result shows that the intrinsic doping levels of the electrochemically prepared p-Cu(2)O and n-Cu(2)O layers are very low and they made almost Ohmic junctions with Cu metal with which previously studied p-Cu(2)O layers prepared by thermal oxidation of Cu foils are known to form Schottky junctions. The best cell performance (an η of 1.06%, a V(OC) of 0.621 V, an I(SC) of 4.07 mA cm(-2), and a fill factor (ff) of 42%) was obtained when the p-Cu(2)O layer was deposited on a commercially available ITO substrate as the back contact and a sputter deposited ITO layer was used as the front contact on the n-Cu(2)O layer. The unique features of the p-n Cu(2)O homojunction solar cell are discussed in comparison with other Cu(2)O-based heterojunction solar cells.

Photocurrent enhancement of n-type Cu2O electrodes achieved by controlling dendritic branching growth.

Cu(2)O electrodes composed of dendritic crystals were produced electrochemically using a slightly acidic medium (pH 4.9) containing acetate buffer. The buffer played a key role for stabilizing dendritic branching growth as a pH drop during the synthesis prevents formation of morphologically unstable branches and promotes faceted growth. Dendritic branching growth enabled facile coverage of the substrate with Cu(2)O while avoiding growth of a thicker Cu(2)O layer and increasing surface areas. The resulting electrodes showed n-type behavior by generating anodic photocurrent without applying an external bias (zero-bias photocurrent under short-circuit condition) in an Ar-purged 0.02 M K(2)SO(4) solution. The zero-bias photocurrent of crystalline dendritic electrodes was significantly higher than that of the electrodes containing micrometer-size faceted crystals deposited without buffer. In order to enhance photocurrent further a strategy of improving charge-transport properties by increasing dendritic crystal domain size was investigated. Systematic changes in nucleation density and size of the dendritic Cu(2)O crystals were achieved by altering the deposition potential, Cu(2+) concentration, and acetate concentration. Increasing dendritic crystal size consistently resulted in the improvement of photocurrent regardless of the method used to regulate crystal size. The electrode composed of dendritic crystals with the lateral dimension of ca. 12000 microm(2) showed more than 20 times higher zero-bias photocurrent than that composed of dendritic crystals with the lateral dimension of ca. 100 microm(2). The n-type nature of the Cu(2)O electrodes prepared by this study were confirmed by linear sweep voltammetry with chopped light and capacitance measurements (i.e., Mott-Schottky plots). The flatband potential in a 0.2 M K(2)SO(4) solution (pH 6) was estimated to be -0.78 vs Ag/AgCl reference electrode. The IPCE measured without applying an external bias was approximately 1% for the visible region. With appropriate doping studies and surface treatment to improve charge transport and interfacial kinetics more efficient n-type Cu(2)O electrodes will be prepared for use in various photoelectrochemical and photovoltaic devices.

Ni and p-Cu2O nanocubes with a small size distribution by templated electrodeposition and their characterization by photocurrent measurement.

A method for the reproducible formation of Ni and Cu2O nanocubes with dimensions of 200-500 nm and a small size distribution is introduced. For this, the well-known templated electrodeposition technique was extended to cubic PMMA templates made by nanoimprint lithography. When making cubic templates in larger quantities, this method has the potential to become simple and cost-effective. This method was successfully used for the formation of Ni and p-Cu2O nanocubes as well as for the formation of segmented nanobars containing both phases. The lateral dimensions of the nanocubes exactly resembled the dimensions of the template, and the height could be varied by adjusting the deposition time. Nanocubes formed via this method can remain attached to the substrate or can be dispersed in solution. p-Cu2O is considered to be one of the most promising photocathode materials for solar water splitting. It is demonstrated that the activity of the p-Cu2O nanocubes for photocatalytic water splitting can be measured, and it was found that the nanocube morphology enhances the photocatalytic activity compared to thin films.

DNA oligonucleotide templated nanohybrids using electronic type sorted carbon nanotubes for light harvesting.

Light harvesting nanostructure hybrids have been designed and demonstrated using single-wall carbon nanotubes (SWCNTs) and porphyrin chromophores. DNA oligonucleotides are used to conjugate SWCNTs with light-absorbing chromophores for transparent films which generate photocurrents. High-purity semiconducting SWCNTs demonstrate significant enhancement in the photocurrent compared to metallic or unsorted tubes.

[10]Annulene: bond shifting and conformational mechanisms for automerization.

We report density-functional and coupled-cluster calculations on conformation change and degenerate bond shifting in [10]annulene isomers 1-5. At the CCSD(T)/cc-pVDZ//CCSD/6-31G level, conversion of the twist (1) to the heart (2) has a barrier of 10.1 kcal/mol, compared to Ea = 16.2 kcal/mol for degenerate "two-twist" bond shifting in 1. Pseudorotation in the all-cis boat isomer (3) proceeds with a negligible barrier. The naphthalene-like isomer 4 has a 3.9 kcal/mol barrier to degenerate bond shifting. The azulene-like isomer 5 is the only species for which the nature of the bond-equalized form (5-eq) depends on the method. At the CCSD(T)/cc-pVDZ//CCSD/6-31G level, 5-eq is 1.2 kcal/mol more stable than the bond-alternating form 5-alt. Conversion of 5-eq to 4 has a barrier of 12.6 kcal/mol. Despite being significantly nonplanar, both 5-eq and the transition state for bond shifting in 4 are highly aromatic based on magnetic susceptibility exaltations. On the basis of a detailed consideration of these mechanisms and barriers, we can now, with greater confidence, rule out 4 and 5 as candidates to explain the NMR spectra observed by Masamune. Our results support Masamune's original assignments for both isolated isomers.

Dynamic processes in [16]annulene: Möbius bond-shifting routes to configuration change.

Density functional and ab initio methods have been used to study the mechanisms for key dynamic processes of the experimentally known S4-symmetric [16]annulene (1a). Using BH&HLYP/6-311+G** and B3LYP/6-311+G**, we located two viable stepwise pathways with computed energy barriers (Ea = 8-10 kcal/mol) for conformational automerization of 1a, in agreement with experimental data. The transition states connecting these conformational minima have Möbius topology and serve as starting points for non-degenerate pi-bond shifting (configuration change) via Möbius aromatic transition states. The key transition state, TS1-2, that connects the two isomers of [16]annulene (CTCTCTCT, 1 --> CTCTTCTT, 2) has an energy, relative to the S4 isomer, that ranged from 6.9 kcal/mol (B3LYP/6-311+G**) to 16.7 kcal/mol (BH&HLYP/6-311+G**), bracketing the experimental barrier. At our best level of theory, CCSD(T)/cc-pVDZ(est), this barrier is 13.7 kcal/mol. Several other Möbius bond-shifting transition states, as well as Möbius topology conformational minima, were found with BH&HLYP energies within 22 kcal/mol of 1a, indicating that many possibilities exist for facile thermal configuration change in [16]annulene. This bond-shifting mechanism and the corresponding low barriers contrast sharply with those observed for cis/trans isomerization in acyclic polyenes, which occurs via singlet diradical transition states. All Möbius bond-shifting transition states located in [16]- and [12]annulene were found to have RHF --> UHF instabilities with the BH&HLYP method but not with B3LYP. This result appears to be an artifact of the BH&HLYP method. These findings support the idea that facile thermal configuration change in [4n]annulenes can be accounted for by mechanisms involving twist-coupled bond shifting.