Yosei Shibata

Crystallization Dynamics of Organolead Halide Perovskite by Real-Time X-ray Diffraction

We analyzed the crystallization process of the CH3NH3PbI3 perovskite by observing real-time X-ray diffraction immediately after combining a PbI2 thin film with a CH3NH3I solution. A detailed analysis of the transformation kinetics demonstrated the fractal diffusion of the CH3NH3I solution into the PbI2 film. Moreover, the perovskite crystal was found to be initially oriented based on the PbI2 crystal orientation but to gradually transition to a random orientation. The fluctuating characteristics of the crystallization process of perovskites, such as fractal penetration and orientational transformation, should be controlled to allow the fabrication of high-quality perovskite crystals. The characteristic reaction dynamics observed in this study should assist in establishing reproducible fabrication processes for perovskite solar cells.

A hybrid heterojunction based on fullerenes and surfactant-free, self-assembled, closely packed silicon nanocrystals

We demonstrate that nanosecond-pulsed laser chemistry in water leads to closely packed and stable luminescent assemblies of silicon nanocrystals (SiNCs) that can be electronically coupled with fullerenes (C60) without any additional surfactant or catalyst. We show that the fragmentation time in water determines the photoluminescence (PL) intensity (>40%) and redshifts the PL maxima (45nm) of the SiNCs. Heterojunction solar cells made out of these laser-produced self-assemblies of SiNCs and C60 show photovoltaic action with increased quantum efficiency in the region where the absorption of SiNCs appears. (Some figures in this article are in colour only in the electronic version)

Uniaxially oriented polycrystalline thin films and air-stable n-type transistors based on donor-acceptor semiconductor (diC8BTBT)(FnTCNQ) [n = 0, 2, 4]

We report the fabrication of high quality thin films for semiconducting organic donor-acceptor charge-transfer (CT) compounds, (diC8BTBT)(FnTCNQ) (diC8BTBT = 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene and FnTCNQ [n = 0,2,4] = fluorinated derivatives of 7,7,8,8,-tetracyanoquinodimethane), which have a high degree of layered crystallinity. Single-phase and uniaxially oriented polycrystalline thin films of the compounds were obtained by co-evaporation of the component donor and acceptor molecules. Organic thin-film transistors (OTFTs) fabricated with the compound films exhibited n-type field-effect characteristics, showing a mobility of 6.9 × 10−2 cm2/V s, an on/off ratio of 106, a sub-threshold swing of 0.8 V/dec, and an excellent stability in air. We discuss the suitability of strong intermolecular donor–acceptor interaction and the narrow CT gap nature in compounds for stable n-type OTFT operation.

Structural influences on charge carrier dynamics for small-molecule organic photovoltaics

We investigated the structural influences on the charge carrier dynamics in zinc phthalocyanine/fullerene (ZnPc/C60) photovoltaic cells by introducing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and 2,5-bis(4-biphenylyl)-bithiophene (BP2T) between indium tin oxide and ZnPc layers. ZnPc films can be tuned to be round, long fiber-like, and short fiber-like structure, respectively. Time-resolved microwave conductivity measurements reveal that charge carrier lifetime in ZnPc/C60 bilayer films is considerably affected by the intra-grain properties. Transient photocurrent of ZnPc single films indicated that the charge carriers can transport for a longer distance in the long fiber-like grains than that in the round grains, due to the greatly lessened grain boundaries. By carefully controlling the structure of ZnPc films, the short-circuit current and fill factor of a ZnPc/C60 heterojunction solar cell with BP2T are significantly improved and the power conversion efficiency is increased to 2.6%, which...

Single crystalline growth of a soluble organic semiconductor in a parallel aligned liquid crystal solvent using rubbing-treated polyimide films

For directional control of organic single crystals, we propose a crystal growth method using liquid crystal as the solvent. In this study, we examined the formation of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) single crystals using a parallel aligned liquid crystal (LC) cell and rubbing-treated polyimide films in order to clarify the effects of LC alignment on anisotropic C8-BTBT crystal growth. Based on the results, we found that the crystal growth direction of C8-BTBT single crystals was related to the direction of the aligned LC molecules because of rubbing treatment. Moreover, by optical evaluation, we found that the C8-BTBT single crystals have a aligned molecular structure.

Diffraction mechanism of a light-diffusing film with an alternate-polymer-layer structure

A diffraction mechanism, and an optical model to reflect that mechanism, for a light-diffusing film with an alternate-polymer-layer structure was proposed and validated. According to this model, the film forms an angular distribution of light intensity that is almost constant in a certain scattering-angle (cutoff angle) range and drastically decreases outside that range; that is, the profile is similar to a trapezoid. Although the trapezoid intensity distribution (TID) is a fundamental distribution of the film, the mechanism to form the TID has not previously been clarified. A key mechanism is that the refractive-index distribution of the layer structure is expressed as stacked phase gratings, some of which should diffract zeroth-order light to higher orders strongly, and the repetition of this multiple diffraction spreads light over the whole cutoff angle range, thereby generating the TID. To verify the proposed mechanism, intensity distributions were calculated by using the proposed model. The calculation results indicate that the proposed model reproduces the TID.

Innovative polymer technologies for flexible liquid crystal displays

It is generally thought that the development of flexible display technologies will soon substantially broaden the possibilities for displays. The specific characteristics of the flexible technologies (in addition to their excellent flexibility, they can be easily handled and stored) mean that flexible displays could find several applications in everyday life (e.g., for posters and calendars, rollscreen TVs and signboards, large maps and design charts, wearable costumes and seals, as well as for packaging and painting materials). To realize these flexible displays, however, it is necessary for the display panel to be made sufficiently thin, light, and flexible for easy handling. Organic LEDs are generally thought to be the most promising candidates for achieving super-flexible displays. Such devices, however, come with a number of drawbacks. For example, organic LEDs have limited lifetimes because of the poor gas barriers of their plastic substrates. In addition, the complex matrix driving systems of organic LEDs mean that they are associated with high production costs. In our work, we have therefore previously shown that liquid crystal (LC) and plastic substrates1–3 are good options for achieving the desired (i.e., improved) properties for flexible displays. For example, flexible LCDs have excellent storability, portability, and designability and can thus be used to create new viewing methods and human interfaces. Low-cost fabrication of large-area panels with flexible LCDs is also possible. The strongest benefit of flexible LCDs, however, is their excellent stability and reliability for the passage of moisture and oxygen through their substrates. Although LCDs exhibit these many attractive features (compared with ultraslim displays that include organic LEDs), the bending tolerance of flexible LCDs Figure 1. A slightly curved liquid crystal (LC) device fabricated from polycarbonate substrates and conventional etched spacers. As the degree of bending increases, the substrate gap decreases at the curving center. This causes degradation of the displayed image (see Figure 2).

Silicon nanocrystal surface engineering and their electronic interaction with carbon based materials

Hybrid nanomaterials that consist of silicon nanocrystals (Si-ncs) and carbon-based nanostructures (i.e. nanotubes, fullerenes C60, etc.) may represent a new class of materials for photovoltaics (PV) with potential for improvements in efficiency and ease of device integration. In this contribution we present results on photovoltaic applications of surfactant-free, quantum confined and surface-engineered Si-ncs combined with semiconducting single walled carbon nanotubes (SWCNTs) or C60. We show that both types of carbon-based nanomaterials allow for electronic interactions with the Si-ncs. Firstly, the electronic interactions between Si-ncs and purified semiconducting SWCNTs has shown opto-electronic conversion over a large spectral range (300–1600 nm). but, in order to enhance interface interactions, the accurate control of Si-ncs surface properties is essential. Secondly, our approach to achieve effective Si-ncs surface engineering is based on nanosecond pulsed laser processing in liquid media. We provide evidence that laser processing in water induces Si-ncs dipole-dipole surface interactions that result in self-organized Si-ncs patterns. The subsequent deposition of a C60 nano-layer on the Si-ncs forms a bulk-type heterojunction. Solar cell devices made out of these surface-engineered Si-ncs and the C60 nano-layer showed photovoltaic action with increased conversion efficiency due to Si-ncs surface engineering.

Improvement of photo-current density by alternative deposition method for organic photovoltaic cell

Recently, power conversion efficiency of organic photovoltaic cell (OPV) has been upgraded due to high photo current density. An aggregation of organic molecules causes a reduction of photovoltaic performance owning to decline in film uniformity of organic layer. Here, we investigate the effect of alternative deposition method on controlling the aggregation and crystallization of organic layers. Advantage of the alternative deposition method is to improve photo current density due to the improved quality of p-n heterojunction. We succeeded in improving the photo current density by our alternative deposition method of tetraphenyldibenzoperiflanthene/C60 OPV cells.