Improved p-n heterojunction solar cells employing thin film organic semiconductors

Drop-Dry Deposition of SnO2 Using a Complexing Agent and Fabrication of Heterojunctions with Co3O4

The drop-dry deposition (DDD) is a simple chemical technique of thin film deposition, which can be applied to metal oxides. The deposition solution is an aqueous solution including a metal salt and an alkali. However, some metal ions react spontaneously with water and precipitate. This work is the first attempt to use complexing agents in DDD to suppress the precipitation. SnO2 thin films are fabricated using DDD with Na2S2O3 as a complexing agent and via annealing in air. The results of the Auger electron spectroscopy measurement show that the O/Sn composition ratio of the annealed films approached two, indicating that the annealed films are SnO2. The photoelectrochemical measurement results show that the annealed films are n-type. Co3O4/SnO2 heterojunction is fabricated using p-type Co3O4 films which are also deposited via DDD. The heterojunction has rectification and photovoltaic properties. Thus, for the first time, a metal oxide thin film was successfully prepared via DDD using a complexing agent, and oxide thin film solar cells are successfully prepared using only DDD.

Photophysical properties and semiempirical calculations of perylene-3,4,9,10-tetracarboxylic tetramethylester (PTME)

The spectral behavior and fluorescence quantum yield of perylene-3,4,9,10-tetracarboxylic tetramethylester (PTME) have been measured in different solvents. Both electronic absorption and fluorescence spectra are not sensitive to medium polarity. The dye exhibits high fluorescence quantum yield and high photostable. Crystalline solid of PTME gives excimer-like emission at 530 nm. The laser activity of PTME has been investigated. The dye solution in N,N-dimethylformamide (DMF) gives laser emission around 480 nm upon excitation by 337.1 nm nitrogen laser pulse. The excitation energy transfer from 7-dimethylamino-4-methylcoumarine (DMC) to PTME has also has been studied and the value of energy transfer rate constant, k(ET), and critical transfer distance, R(0) indicate a Förster-type mechanism. The photodecomposition of PTME in chloromethane solvents has been also studied. We applied semiempirical MO calculations using (PM3 and ZINDO-CI) calculations to explain the geometric and electronic behaviors of the PTME molecule in both ground and excited states and make a correlation with the experimental observations.

Nanopatterned monolayers of an adsorbed chromophore

A simple lift-off process was developed to rapidly fabricate nanopatterned photofunctional surfaces. Dye molecules of a perylene derivative (PDID) were adsorbed irreversibly on clean silicon through the holes of an electron-beam lithographied polymer mask. The subsequent removal of the mask in a proper solvent results in PDID nanosized regions of width as small as 30 nm for stripes and of diameter as small as 120 nm for dots. Numerical analyses of atomic force microscopy and laser-scanning confocal microscopy images show that the dye molecules are confined to the regions defined by the lithographic process, with the integrated fluorescence intensity being essentially proportional to the size of the nanofeatures. This demonstrates that a simple organic lift-off process compatible with clean-room technology, and not involving any chemical step, is able to produce photofunctional nanopatterned surfaces, even though the dye is not chemically bonded to the silicon surface.

Highly stable 3,4,9,10-perylenediimide radical anions immobilized in robust zirconium phosphonate self-assembled films

Self-assembled thin films of 3,4,9,10-perylenediimides (PDIs) containing up to 50 PDI layers were grown on quartz slides using the zirconium phosphonate technique. When the films were immersed in aqueous solutions of the sodium dithionite reducing agent, in situ reduction of the dye was observed, generating a purple film containing PDI radical anions. The PDI radical anions formed within the films were rather stable, persisting for several minutes in the presence of atmospheric oxygen. Atomic force microscopy (AFM) images showed that the film surface was rather smooth and pinhole-free.

Characterization of a novel water-soluble 3,4,9,10-perylenetetracarboxylic diimide in solution and in self-assembled zirconium phosphonate thin films

The properties of N,N'-bis(2-phosphonoethyl)-3,4,9,10-perylenetetracarboxylic diimide (PPDI), a water-soluble perylene dye, have been studied in solution and in thin films. Absorption spectra showed that PPDI exists in the monomeric form in water/ethanol (1:1) and water/dimethyl sulfoxide (DMSO) (3:7) mixtures, but forms dimers in water and higher aggregates in ethanol. The PPDI monomer is highly fluorescent, in contrast to the dimers and aggregates, which are nonfluorescent. The monomer/dimer equilibrium was conveniently followed in a water/ethanol (7:3) mixture by varying the dye concentration. An equilibrium constant of K = 1.25 x 10(5) M(-1) was estimated for the dimerization process in this solvent mixture. The addition of cetyl trimethylammonium bromide (CTAB), a cationic surfactant, to aqueous solutions of PPDI resulted in the dissociation of the dimers, showing that the dye was incorporated into the micellar phase. Self-assembled thin films of PPDI were grown on both silica gel particles and flat surfaces, using zirconium phosphonate chemistry. The growth of multilayered films on flat surfaces was monitored by ellipsometry (silicon substrates) and UV/Vis spectroscopy (quartz slides), and was linear with the number of deposition cycles. No fluorescence was detected from the PPDI films, and the absorption spectra of the films were quite similar to the spectrum of the compound in ethanol, indicating that the dye molecules were stacked in the films. Mixed monolayers containing PPDI and N,N'-bis(2-phosphonoethyl)-1,4,5,8-naphthalenediimide (PNDI) on quartz were also prepared. Monolayers obtained by codeposition from solutions containing both PPDI and PNDI were richer in PPDI, even when the solution contained a large excess of the naphthalene derivative, showing that pi-stacking of PPDI is an important driving force in the formation of the films.

Fluorescent J-type aggregates and thermotropic columnar mesophases of perylene bisimide dyes

A series of perylene tetracarboxylic acid bisimides 3a-e bearing 3,4,5-tridodecyloxyphenyl substituents on the imide N atoms and zero, two, or four phenoxy-type substituents in the bay positions of the perylene core were synthesized. From investigations of their spectroscopic properties and aggregation behavior in low-polarity solvents by absorption and fluorescence optical spectroscopy, not only were these compounds found to form fluorescent J-type aggregates, but also binding constants for aggregation could be derived which reflect the number and steric demand of the phenoxy substituents for bisimides 3a-d. In the pristine state, 3a-d form thermotropic hexagonal columnar mesophases which exist over a broad temperature range from below -30 degrees C to over 300 degrees C. For the tetraphenoxy-substituted compound 3e, however, a layered crystalline structure was found. This difference in behavior can be explained by the concept of microphase segregation of the aromatic cores of the molecules and the alkyl chains at the periphery. The high stability and bright fluorescence of the mesophase of several of the compounds make them promising for applications as polarizers or components in (opto)electronic devices.

Surface enhanced resonance Raman scattering imaging of Langmuir-Blodgett monolayers of bis (benzimidazo) thioperylene

The synthesis, spectroscopic characterization and surface-enhanced spectroscopy of a new electro active organic material bis (benzimidazo) thioperylene (Monothio BZP) are reported. Langmuir monolayers of Monothio BZP were successfully formed on water subphase and characterized by the pi-A surface-pressure area isotherm. Langmuir-Blodgett (LB) monomolecular layers of Monothio BZP were fabricated onto glass substrates and onto silver island films for surface-enhanced spectroscopic studies. The results of surface-enhanced resonance Raman scattering (SERRS), SERRS imaging and surface-enhanced fluorescence (SEF) studies for Monothio BZP LB monolayers are reported. Raman imaging (global imaging and point-by-point mapping) of the SERRS signal for a single monomolecular layer on silver islands were obtained using the 514.5 nm laser line. The SERRS imaging permits a visualization of the variation of the SERRS intensity across of the rough metal surface. The SEF was recorded for the excimer emission of aggregates in the LB film. The distance dependence and the enhancement factor of SEF were determined using fatty acid spacing layers. A temperature dependence study of the LB monolayer SERRS and SEF spectra was carried out between -190 degrees and + 200 degrees C confirming the thermal stability of the LB monolayer on silver. The specificity and the sensitivity of SERRS signal on metal island films was probed using mixed LB films with 0.01% molecular ratio of Monothio BZP in Arachidic Acid (AA). The micro-Raman SERRS spectra from ca. 10(-3) attomole of the dye were recorded.