Daisuke Tadaki

Annealing-induced chemical and structural changes in tri-iodide and mixed-halide organometal perovskite layers

The annealing process is crucial for obtaining high-quality perovskite layers used in highly efficient planar perovskite solar cells. In this study, we have investigated the annealing-induced chemical and structural changes of tri-iodide (TI) and mixed-halide (MH) organometal perovskite layers using infrared absorption spectroscopy, scanning electron microscopy and X-ray diffraction measurements. For TI layers, the solvent molecules, dimethylformamide (DMF), remained in the form of the PbI2/DMF compound after drying at room temperature. During annealing, the DMF evaporated to form PbI2 crystals. When the MH perovskite film was annealed, both CH3NH3PbCl3 and CH3NH3PbI3 crystals were initially formed from an amorphous phase. With further annealing, the CH3NH3PbI3 crystals gradually grew through the incorporation of source materials supplied from the CH3NH3PbCl3 crystals and the amorphous phase and the slow evaporation of methylammonium (MA) and chloride ions. The resultant MH perovskite layer after annealing was mainly composed of large CH3NH3PbI3 grains with a trace of chloride ions. We suggest that the difference in composition and structure leads to different charge transport properties of the TI and MH perovskite layers.

Effects of interfacial chemical states on the performance of perovskite solar cells

We showed that the widely used solvent molecule, N,N-dimethyl-formamide (DMF), readily adsorbs on the surface of TiO2 electrodes of perovskite solar cells (PSCs), and that the adsorbed DMF molecules remain intact on the TiO2 surface even after long-term annealing of the perovskite layer, resulting in an increase in the contact resistance of the PSCs. We found that the absorption of DMF is significantly suppressed by modifying the TiO2 electrode surface with a fullerene derivative, [6,6]-phenyl-C61-butyric acid (PCBA). We also suggested that the high electron affinity of PCBA enhances the charge transportation at the perovskite/TiO2 interface and reduces the contact resistance.

Fabrication and characterization of p+-i-p+ type organic thin film transistors with electrodes of highly doped polymer

Organic thin film transistors (OTFTs) have been explored because of their advantageous features such as light-weight, flexible, and large-area. For more practical application of organic electronic devices, it is very important to realize OTFTs that are composed only of organic materials. In this paper, we have fabricated p+-i-p+ type of OTFTs in which an intrinsic (i) regioregular poly (3-hexylthiophene) (P3HT) layer is used as the active layer and highly doped p-type (p+) P3HT is used as the source and drain electrodes. The 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) was used as the p-type dopant. A fabricating method of p+-i-p+ OTFTs has been developed by using SiO2 and aluminum films as capping layers for micro-scaled patterning of the p+-P3HT electrodes. The characteristics of the OTFTs were examined using the photoelectron spectroscopy and electrical measurements. We demonstrated that the fabricated p+-i-p+ OTFTs work with carrier injection through a built-in potential at p+/i interfaces. We found that the p+-i-p+ OTFTs exhibit better FET characteristics than the conventional P3HT-OTFT with metal (Au) electrodes, indicating that the influence of a carrier injection barrier at the interface between the electrode and the active layer was suppressed by replacing the metal electrodes with p+-P3HT layers.

Fabrication and Characterization of High-Quality Perovskite Films with Large Crystal Grains

Solution-processable organometal perovskite materials have been widely used in various kinds of devices. In these devices, the perovskite materials normally act as active layers. Grain boundaries and structural disorder in the perovskite layer would interfere the charge transport and increase recombination probability. Here we proposed a novel fabrication method to dramatically increase the crystal size by more than 20 times as compared with previously reported values. Exceptional structural order in the large crystals is illustrated by nanoscale surface morphology and a simple recrystallization method. Because of reduced grain boundaries and increased crystal order in perovskite layers, the lateral charge transport is significantly improved, as demonstrated by conductive atomic-force microscopy and performance of photodetectors. This deposition technology paves the way for future high-performance devices based on perovskite thin films.

Investigation of TiO2 Surface Modification with [6,6]-Phenyl-C61-butyric Acid for Titania/Polymer Hybrid Solar Cells

We have investigated modification of TiO2 surfaces with [6,6]-phenyl-C61-butyric acid (PCBA) used for fabrication of TiO2/poly(3-hexylthiophene-2,5-diyl) (P3HT) hybrid solar cells. The surface modification process was monitored using in-situ infrared absorption spectroscopy in the multiple-internal reflection geometry (MIR-IRAS). IR data showed that longer exposure of TiO2 surfaces to an organic solution of PCBA leads to undesirable formation of a physisorbed PCBA overlayer that cannot be removed by rinsing the surface in pure solvent. We found that ultrasonic cleaning of the TiO2 surface removed most of the physisorbed PCBA molecules. Modification of TiO2 surfaces with PCBA molecules drastically increased the short circuit current of TiO2/P3HT-based hybrid solar cells, which is ascribed to improved charge separation efficiency at the TiO2/P3HT interface. The physisorbed PCBA molecules decreased the open circuit voltage and the fill factor. We demonstrated that the power conversion efficiency is improved by ultrasonic cleaning following PCBA deposition.

Effect of Porous Counter Electrode with Highly Conductive Layer on Dye-Sensitized Solar Cells

A Pt/porous titanium (Ti)/dense Ti/aluminum (Al)/glass composite counter electrode for a dye-sensitized solar cell (DSC) was fabricated and characterized. The introduction of a highly conductive Al film drastically reduced the sheet resistance of the counter electrode to improve the short circuit current and fill factor of the DSC. Porous and dense Ti layers were deposited by the DC magnetron sputtering process. The dense Ti layer protected the Al film from dissolution in the electrolyte solution to stabilize the composite counter electrode. The porous Ti layer with a larger electrochemical active area at the electrolyte/counter electrode interface mainly improved the short circuit current of the DSC. The results indicate the importance of the introduction of porous materials with highly conductive films into the counter electrode of a DSC and the suitability of composite materials of Al and Ti for the counter electrode.

Mechanically stable solvent-free lipid bilayers in nano- and micro-tapered apertures for reconstitution of cell-free synthesized hERG channels

The self-assembled bilayer lipid membrane (BLM) is the basic component of the cell membrane. The reconstitution of ion channel proteins in artificially formed BLMs represents a well-defined system for the functional analysis of ion channels and screening the effects of drugs that act on them. However, because BLMs are unstable, this limits the experimental throughput of BLM reconstitution systems. Here we report on the formation of mechanically stable solvent-free BLMs in microfabricated apertures with defined nano- and micro-tapered edge structures. The role of such nano- and micro-tapered structures on the stability of the BLMs was also investigated. Finally, this BLM system was combined with a cell-free synthesized human ether-a-go-go-related gene channel, a cardiac potassium channel whose relation to arrhythmic side effects following drug treatment is well recognized. Such stable BLMs as these, when combined with a cell-free system, represent a potential platform for screening the effects of drugs that act on various ion-channel genotypes.

Fabrication of polymer/TiO2-nanotube-based hybrid structures using a solvent-vapor-assisted coating method

The incomplete infiltration and ununiform coating of polymer material in inorganic nanostructures have hindered the applications of hybrid nanostructure. In this work, a novel solvent-vapor-assisted coating (SVAC) method is proposed for uniform coating of polymers in inorganic nanostructure. It is demonstrated that the thickness of the polymer layer in TiO2 nanotubes can be simply controlled using processing temperatures. At moderate temperatures, the polymer nanotubes which showed unique chain ordering formed in ordered TiO2 nanotubes. Hybrid solar cells, made by filling the tube-in-tube structure with hole transporting material, produced drastically improved short circuit current and serial resisitance. The results imply that the proposed method has potential to improve performance of devices with hybrid nanostructures by controlling the morphology of polymer layer.

Amphiphobic Septa Enhance the Mechanical Stability of Free-Standing Bilayer Lipid Membranes

Artificial bilayer lipid membranes (BLMs) provide well-defined systems for investigating the fundamental properties of membrane proteins, including ion channels, and for screening the effect of drugs that act on them. However, the application of this technique is limited due to the low stability and low reconstitution efficiency of the process. We previously reported on improving the stability of BLM based on the fabrication of microapertures having a tapered edge in SiO2/Si3N4 septa and efficient ion channel incorporation based on vesicle fusion accelerated by a centrifugal force. Although the BLM stability and incorporation probability were dramatically improved when these approaches were used, some BLMs were ruptured when subjected to a centrifugal force. To further improve the BLM stability, we investigated the effect of modifying the surface of the SiO2/Si3N4 septa on the stability of BLM suspended in the septa. The modified surfaces were characterized in terms of hydrophobicity, lipophobicity, and surface roughness. Diffusion coefficients of the lipid monolayers formed on the modified surfaces were also determined. Highly fluidic lipid monolayers were formed on the amphiphobic substrates that had been modified with long-chain perfluorocarbons. Free-standing BLMs formed in amphiphobic septa showed a much higher mechanical stability, including tolerance to water movement and applied centrifugal forces with and without proteoliposomes, than those formed in the septa that had been modified with a short alkyl chain. These results demonstrate that highly stable BLMs are formed when the surface of the septa has amphiphobic properties. Because highly fluidic lipid monolayers that are formed on the septa seamlessly connect with BLMs in a free-standing region, the high fluidity of the lipids contributes to decreasing potential damage to BLMs when mechanical stresses are applied. This approach to improve the BLM stability increases the experimental efficiency of the BLM systems and will contribute to the development of high-throughput platforms for functional assays of ion channel proteins.

Micro- and nanofabrication methods for ion channel reconstitution in bilayer lipid membranes

The self-assembled bilayer lipid membrane (BLM) forms the basic structure of the cell membrane and serves as a major barrier against ion movement. Ion channel proteins function as gated pores that permit ion permeation across the BLM. The reconstitution of ion channel proteins in artificially formed BLMs represents a well-defined system for investigating channel functions and screening drug effects on ion channels. In this review, we will discuss our recent microfabrication approaches to the formation of stable BLMs containing ion channel proteins as a potential platform for next-generation drug screening systems. BLMs formed in a microaperture having a tapered edge exhibited highly stable properties, such as a lifetime of ~65 h and tolerance to solution changes even after the incorporation of the human ether-a-go-go-related gene (hERG) channel. We also explore a new method of efficiently incorporating human ion channels into BLMs by centrifugation. Our approaches to the formation of stable BLMs and efficient channel incorporation markedly improve the experimental efficiency of BLM reconstitution systems, leading to the realization of a BLM-based high-throughput platform for functional assays of various ion channels.