Organic π-type thermoelectric module supported by photolithographic mold: a working hypothesis of sticky thermoelectric materials.

To examine the potential of organic thermoelectrics (TEs) for energy harvesting, we fabricated an organic TE module to achieve 250 mV in the open-circuit voltage which is sufficient to drive a commercially available booster circuit designed for energy harvesting usage. We chose the π-type module structure to maintain the temperature differences in organic TE legs, and then optimized the p- and n-type TE materials' properties. After injecting the p- and n-type TE materials into photolithographic mold, we eventually achieved 250 mV in the open-circuit voltage by a method to form the upper electrodes. However, we faced a difficulty to reduce the contact resistance in this material system. We conclude that TE materials must be inversely designed from the viewpoints of the expected module structures and mass-production processes, especially for the purpose of energy harvesting.

Thermoelectric materials and applications for energy harvesting power generation.

Thermoelectrics, in particular solid-state conversion of heat to electricity, is expected to be a key energy harvesting technology to power ubiquitous sensors and wearable devices in the future. A comprehensive review is given on the principles and advances in the development of thermoelectric materials suitable for energy harvesting power generation, ranging from organic and hybrid organic-inorganic to inorganic materials. Examples of design and applications are also presented.

Efficient Synthesis and Photosensitizer Performance of Nonplanar Organic Donor-Acceptor Molecules.

Nonplanar organic donor-acceptor molecules bearing a carboxylic acid group were synthesized by the formal [2+2] cycloaddition-retroelectrocyclization reaction between aniline-substituted alkynes and tetracyanoethylene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ). This reaction offers an atom-economic one-step approach to donor-acceptor chromophores in satisfactory high yields. The resulting donor-acceptor molecules were characterized by conventional analytical techniques. In addition, the nonplanarity and intermolecular interactions were investigated by X-ray crystallography. The energy levels and intramolecular charge-transfer (CT), evaluated by UV-Vis-near IR spectroscopy and electrochemistry, suggested that there is a linear correlation between the optical and electrochemical band gaps. Based on these structural and electronic analyses, the photosensitizer performances of the donor-acceptor molecules in dye-sensitized solar cells (DSSCs) were initially investigated using TiO2 or SnO2 electrodes. Although the power conversion efficiencies were limited, the incident-photon-to-current-conversion efficiency (IPCE) spectra indicated a better photocurrent generation for the devices on SnO2 as compared to those on TiO2.

Metastability of anatase: size dependent and irreversible anatase-rutile phase transition in atomic-level precise titania.

Since crystal phase dominantly affects the properties of nanocrystals, phase control is important for the applications. To demonstrate the size dependence in anatase-rutile phase transition of titania, we used quantum-size titania prepared from the restricted number of titanium ions within dendrimer templates for size precision purposes and optical wave guide spectroscopy for the detection. Contrary to some theoretical calculations, the observed irreversibility in the transition indicates the metastablity of anatase; thermodynamics cannot explain the formation of metastable states. Therefore, we take into account the kinetic control polymerization of TiO6 octahedral units to explain how the crystal phase of the crystal-nucleus-size titania is dependent on which coordination sites, cis- or trans-, react in the TiO6 octahedra, suggesting possibilities for the synthetic phase control of nanocrystals. In short, the dendrimer templates give access to crystal nucleation chemistry. The paper will also contribute to the creation of artificial metastable nanostructures with atomic-level precision.

Chemical input and I-V output: stepwise chemical information processing in dye-sensitized solar cells.

As a complex system, a dye-sensitized solar cell (DSC) exhibits emergent photovoltaics not obvious from the properties of the individual components. The chemical input of 4-tert-butylpyridine (TBP) into DSC improves the open circuit voltage (V(oc)) and reduces the short circuit current (I(sc)) in I-V output through multiple interactions with the components, yet it has been difficult to distinguish the multiple interactions and correlate the interactions with the influences on I-V output due to the complexity of the system. To deal with the multiple interactions, we have adapted a conceptual framework and methodology from coordination chemistry. First, we titrated the photovoltaic interface and electrolyte with TBP to identify the stepwise chemical interaction processes. An isopotential point observed in I-V output indicates that most of the inputted chemicals interact with the electrolyte. Cyclic voltammetric titration of the electrolyte demonstrates asymmetric redox peaks and two different isopotential points, indicating that the two-step coordination-decoordination process inhibits the reduction current of the electrolyte. Second, we set an interaction model bridging the hierarchical gaps between the multiple interactions and the I-V output to address the influences on outputs from the amount of the inputs. From the viewpoint of the interaction model and interactions observed, we are able to comprehend the processes of the complex system and suggest a direction to improve V(oc) without sacrificing I(sc) in DSCs. We conclude that the conceptual framework and methodology adapted from coordination chemistry is beneficial to enhance the emergent outputs of complex systems.

Successive large perturbation method for the elimination of initial value dependence in I-V curve fitting.

A successive large perturbation method (SLPM) is proposed to resolve the problem of initial value dependence in the numerical least-square fitting for the extraction of I-V parameters in solar cells. In this method a large perturbation is applied onto certain a parameter before next turn of Newton-like iteration is proceeded and an improved result is usually obtained if the I-V parameters suffer from a latent initial value dependence problem. The numerical insensitivity of mean square of deviation to the variation of large shunt resistance is a critical factor to cause the initial value dependence. An application example for a dye-sensitized solar cell shows that about a 60% change of reverse saturation current I(0) occurs after SLPM is applied to the result obtained by Newton-like method. Our result demonstrates that SLPM is a powerful tool to eliminate the initial value dependence in I-V curve fitting.

Self-assembled monolayers of metal-assembling dendron thiolate formed from dendrimers with a disulfide core.

Novel phenylazomethine dendrimers with a disulfide core (SS-DPA G1-4) were synthesized in nearly quantitative yields. Although the disulfide core is shielded by the rigid dendron shell, direct formation of the self-assembled monolayers of metal-assembling dendron thiolate was observed by XPS and electrochemical reduction of the self-assembled monolayer substrates. The dendrimers showed a similar metal-assembling manner with other derivatives. The metal assembly to the self-assembled monolayers of metal-assembling dendron thiolate was also confirmed.

Quantum size effect in TiO2 nanoparticles prepared by finely controlled metal assembly on dendrimer templates.

The use of dendrimer templates to make metal-based nanoparticles of controlled size has attracted much interest. These highly branched macromolecules have well-defined structures that enable them to bind metal ions to generate precursors that can be converted into nanoparticles. We describe the sub-nanometre size control of both anatase and rutile forms of TiO2 particles with phenylazomethine dendrimers, leading to samples with very narrow size distributions. Such fine tuning is possible because both the number and location of metal ions can be precisely controlled in these templates. Quantum size effects are observed in the particles, and the energy gap between the conduction and valence bands exhibits a blueshift with decreasing particle size and is dependent on the crystal form of the material. The dependency of the bandgap energy on these factors is explained using a semi-empirical effective mass approximation.

Long-lived hole stabilized at a triphenylamine core and shielded by rigid phenylazomethine dendrons: a pulse radiolysis study.

The optical properties and reactivity of one-electron oxidized states (radical cations) of dendrimers are investigated in benzonitrile solutions by nanosecond pulse radiolysis. The hole stabilized at the triphenylamine (TPA) core is effectively shielded by a rigid dendritic phenylazomethine (DPA) shell of four generations, leading to an extension of its lifetime by nearly 2 orders of magnitude in comparison with a core radical cation without dendrons. A continuous red shift of the peak in the photoabsorption spectrum in the visible region and a decrease in the extinction coefficients (oscillator strengths) are found with increasing dendrimer generation number. These experimental observations are compared to the results of time-dependent density functional theory. It is suggested that bulky, rigid, insulating DPA dendrons shield against outer reactants and stabilize hole at the TPA core. Correlating the dendrimer generation number with the optical properties and reactivities of the radical cations could shed light on fundamental aspects of structurally defined nanoenvironments having hyperbranched entities.

Metal-assembling dendrimers with a triarylamine core and their application to a dye-sensitized solar cell.

A series of charge-separable and hole-transporting phenylazomethine dendrimers with a triarylamine core are prepared and evaluated for use as a charge separator in dye-sensitized solar cells (DSSCs). Triphenylamine with dendric phenylazomethine (TPA-DPA) is prepared by synthesizing up to five generations of dendrons using a convergent method. The resultant dendrimer has a rigid sphere structure similar to globular protein, with a hydrodynamic radius of 2.43 nm. Electrochemical oxidation of the TPA core reveals that the dendron units in the dendrimer have 0.35 of the attenuation factor (beta) in the electron transfer. Complexation of TPA-DPA with SnCl2 proceeds in stepwise fashion from the core to the terminal imine following the basicity gradient among imine groups in each dendron shell. DSSCs prepared by casting these dendrimers onto dye-sensitized TiO2 film exhibited a higher open-circuit voltage than the bare film through the suppression of back electron transfer. The generational growth of dendrons increases the radius of the dendrimer, resulting in a stronger association with I3- and higher open-circuit voltage with an increasing number of generations. Complexation with SnCl2 reduces the resistance of TPA-DPA and improves the fill factor. The energy conversion efficiency of the DSSC prepared using fifth-generation TPA-DPA is 21% higher than that for the bare film and, when complexed with SnCl2, provides a 34% improvement.

Novel triarylamine dendrimers as a hole-transport material with a controlled metal-assembling function.

A series of phenylazomethine dendrimers with a triarylamine core (TPA-DPA) were synthesized by dehydration using TiCl4. The complexation of the fourth genereration (G4) TPA-DPA with SnCl2 proceeds in not a random but a stepwise fashion from the core to the terminal imines of the G4 dendrimer. The molecular size of TPA-DPA G4 is larger than that of DPA G4 in THF solution and has a rigid sphere structure like a globular protein. Organic light-emitting diodes (OLEDs) were fabricated, and the EL performances of the devices using the TPA-DPA-metal complexes as the hole-transport materials are drastically increased (ca. 20 times) by metal complexation.