Kazuhiko Murata

High-performance carbon counter electrode for dye-sensitized solar cells

Here, we reported that a new carbon electrode prepared with an activated carbon was superior to a Pt sputtered electrode as the counter electrode ofdye-sensitized solar cells. The photovoltaic performance was largely influenced by the roughness factor of carbon electrode. The open-circuit voltage increased by about 60 mV using the carbon counter electrode compared to the Pt counter electrode because of positive shift of the formal potential for I � =I �

Simple synthesis of water-soluble conducting polyaniline

Abstract A water-soluble externally (HCl)-doped conducting polyaniline (ED-SPAN) is prepared by such a simple synthetic method that emeraldine salts are sulfonated by chlorosulfonic acid in dichloroethane at 80°C and subsequently hydrated in water at 100°C. Sulfonating any emeraldine salts (counter anion X− = Cl−, SO42−, and BF4−) or emeraldine base results in the production of HCl-doped sulfonated polyaniline, where HCl dopant from hydrolysis of chlorosulfonic group exchanges with the original dopant. The degree of sulfonation, namely, sulfur-to-nitrogen (S/N) ratio, can be controlled by adjusting the amount of chlorosulfonic acid. With increasing S/N ratio from 0.65 to 1.3, the solubility in neutral water increases from 22 to 88 g/l and the four-probe conductivity for a compressed pellet decreases from 0.023 to 1.7 × 10−5 S/cm, showing sulfonation-induced undoping.

Relation between carrier mobility and cell performance in bulk heterojunction solar cells consisting of soluble polythiophene and fullerene derivatives

The effect of carrier mobility on the cell performance was examined in the bulk heterojunction solar cells consisting of soluble polythiophene (PHTh) and fullerene derivatives (PCBM). The hole mobility decreased from 6.3×10−3cm2V−1s−1 in the pure PHTh by blending PCBM, while the electron mobility decreased from 2.4×10−2cm2V−1s−1 in the pure PCBM by blending the PHTh. When blending ratio R=PHTh∕(PHTh+PCBM) by weight was 0.5–0.85, ambipolar carrier conduction was possible, showing the best-balanced ambipolar carrier mobility of ca. 10−4cm2V−1s−1 at R=0.7. The power conversion efficiency (η) of the PHTh:PCBM bulk heterojunction solar cells under AM1.5, 85mWcm−2 illumination significantly depended on the blending ratio R and the maximum η of 2.6% was observed at R=0.7, where both electrons and holes are conducting well. Thus, it was concluded that the ambipolar carrier conduction limits the performance of PHTh:PCBM bulk heterojunction solar cells.

Long‐lived excited state of C60 in C60/phthalocyanine heterojunction solar cell

The photovoltaic properties of ITO (indium tin oxide)/C60/H2(pc), (pc=phthalocyanine)/Au sandwich solar cells are investigated. The cell prepared at a higher vacuum (1×10−6 Torr) shows an open‐circuit photovoltage (VOC) of 0.18 V, a short‐circuit photocurrent (JSC) of 89 μA cm−2, a fill factor (ff) of 0.25, and an energy conversion yield (η) of 0.03% when illuminated by white light with 12.5 mW  cm−2 intensity. The photocurrent action spectra of the cell reveal that photocurrent is generated at C60/H2(pc) interface with diffusion of C60 excitons because the excited state of C60 has a relatively long lifetime, while the cell prepared at a lower vacuum (3×10−5 Torr) shows a much smaller photocurrent (JSC=1.4 μA cm−2) because oxygen in C60 acts as carrier traps and increases the resistance of C60.

Efficient organic solar cells by penetration of conjugated polymers into perylene pigments

We report here efficient air-stable p-n heterojunction organic solar cells with a structure consisting of an n-type insoluble perylene pigment penetrated by a p-type-conjugated polymer, where the interfacial area for photocurrent generation increases. The solar cells are easily produced by infiltrating a soluble-conjugated polymer intentionally into an opening among insoluble microcrystalline perylene layer under a saturated chloroform vapor. This approach can be regarded as an alternative convenient way to achieve bulk heterojunction solar cells. The cell performance is further enhanced by inserting an additional layer between the electrode and the photoactive layer to confine exciton in the photoactive layer. The overall attempt to improve the cell performance, so far, results in maximum quantum efficiency up to 45% under illumination of 485-nm monochromatic light and power conversion efficiency up to 1.9% under a simulated solar light (AM1.5) with a 100mWcm−2 intensity. The approach is promising to ach...

Organic solid-state solar cells with a mixture of monomeric porphyrins for light-harvesting and regioregular polythiophene for charge transport

Abstract An enhanced photocurrent was observed for a Schottky-barrier cell with a mixed solid consisting of metal-free porphyrin (H 2 tpp) and regioregular polythiophene (PTh) compared to similar cells with pure metal-free porphyrin or pure PTh. This is because produced holes efficiently transport in the high-mobility PTh solid soon after a photoinduced electron-transfer from the PTh molecule to the photon harvesting H 2 tpp molecule.

Photocurrent from photocorrosion of aluminum electrode in porphyrin/Al Schottky-barrier cells

Current–voltage characteristics under illumination of monochromatic light for metal-free tetrabenzporphyrin, metal-free triazatetrabenzporphyrin, and metal-free phthalocyanine in (indium–tin–oxide)/porphyrin/Al sandwich cells seem to show high-energy conversion efficiencies of 14%, 7.7%, and 5.5%, respectively. However, according to the simultaneous measurement of both short-circuit photocurrent and transmittance of the aluminum electrode with time, the transmittance of Al linearly increases as the photocurrent flows into the external circuit. It is concluded that all photocurrents in the above cells arise from photoaccelerated corrosion of aluminum electrode and do not originate from conversion of light energy to electrical energy.

Enhanced quantum yield in porphyrin/electron-donor double-layer solar cells

When the layer of 3-carboxymethyl-5-[(3-ethyl-2(3H)-benzothiazolylidine)ethylidene (MC(COOH)) is inserted into the Au/Zntpyp interface in Al/Zntpyp/Au sandwich-type solar cell (Zntpyp: 5,10,15,20-tetra(3-pyridyl)porphyrinatozinc), the photovoltaic properties are remarkably improved. For the Al/Zntpyp(thickness 10 nm)/MC(COOH)(20 nm)/Au cell, a short-circuit photocurrent (Jsc) of 0.93 μ Acm−2, open-circuit photovoltage (Voc) of 0.71 V, fill factor (ff) of 0.41, and energy conversion yield (η) of 3.6% are obtained when illuminated at the Al/Zntpyp interface with 455 nm monochromatic light of 7.5 μ Wcm−2 intensity. A rapid electron-transfer from the donor MC(COOH) to photogenerated holes in Zntpyp suppresses the charge recombination of the photogenerated carriers. The energetically well-arranged valence band levels eventually enhance the η value about 9 times compared with the Al/Zntpyp/Au cell. Further the Al/HD(9 nm)/MC(COOH)(20 nm)/Au cell using a longer-lived sensitizer (HD) instead of Zntpyp gives a Jsc value of 2.36 μ Acm−2, Voc value of 0.69 V, ff value of 0.34, and η value of 4.8% when illuminated with 445 nm monochromatic light of 11.7 μ Wcm−2 intensity at the Al/HD interface, where HD represents a heterodimer consisting of 5,10,15-tri(4-chlorophenyl)-20-(3-pyridyl)porphyrin(H2pyp3p(Cl)) and 5,10,15,20-tetra(2,5-dimethoxyphenyl)porphyrinatozinc(Zntpp(OMe)2).

Enhanced photocurrent by Schottky-barrier solar cell composed of regioregular polythiophene with merocyanine dye

Abstract When electron-transfer from poly(3-hexylthiophene-2, 5-diyl) (PTh6) to merocyanine dye (MC) occurred in the mixed solid under illumination, the photocurrent of Al/MC+PTh6 mixed solid Schottky-barrier cell was enhanced compared to that of pure MC or pure PTh6 cell. This is because discrete pairs of electron and hole created by the photoinduced electron-transfer are effectively separated in the electric field formed at the Al/organic solid interface.

Photoinduced electron transfer from porphyrin to C60 in a C60|porphyrin double-layer photoelectrochemical cell

Photoelectrochemical properties of double-layer electrodes consisting of a very thin film of C60 and porphyrin solids were investigated in a quinhydrone aqueous solution. A larger anodic photocurrent is obtained in the photoelectrochemical cell using an ITO (indium tin oxide)|C60|Zntpp (5,10,15,20-tetraphenylporphyrinatozinc) electrode compared with that for ITO|C60 or ITO|Zntpp ele when the ITO|Zntpp|C60 electrode, deposited in reverse order, is employed, a cathodic photocurrent is obtained. Neither of the double-layer electrodes exhibits rectification. The results suggest that the primary process of the photocurrent generation is a photoinduced electron transfer from Zntpp to C60 enhancing the photocurrent, and that the Zntpp and C60 solids do not act as semiconductors but as photosensitizers.