Kohshin Takahashi

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 �

Characterization of Inverted-Type Organic Solar Cells with a ZnO Layer as the Electron Collection Electrode by ac Impedance Spectroscopy

An inverted-type organic bulk-heterojunction solar cell inserting zinc oxide (ZnO) as an electron collection electrode, fluorine-doped tin oxide (FTO)/ZnO/[6,6]-phenyl-C(61)-butyric acid methyl ester:regioregular poly(3-hexylthiophene) (PCBM:P3HT)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS)/Au, was fabricated in air and characterized by an alternating current impedance spectroscopy (IS). In the IS measurement, we observed reproducibly the electric resistance and capacitance components originating from ZnO and organic active layers, and we found that the depletion layer functioning to take out the photocurrent to the external circuit was formed in both the ZnO and PCBM:P3HT layers at the ZnO/PCBM:P3HT interface. In this letter, we propose that this IS measurement is effective for evaluating the electric properties of several layers with capacitance components in organic thin-film solar cells.

Mechanistic Insights into UV-Induced Electron Transfer from PCBM to Titanium Oxide in Inverted-Type Organic Thin Film Solar Cells Using AC Impedance Spectroscopy

An inverted organic bulk-heterojunction solar cell containing amorphous titanium oxide (TiOx) as an electron collection electrode with the structure ITO/TiO(x)/[6,6]-phenyl C(61) butyric acid methyl ester (PCBM): regioregular poly(3-hexylthiophene) (P3HT)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid)/Au (TiO(x) cell) was fabricated. Its complicated photovoltaic properties were investigated by photocurrent-voltage and alternating current impedance spectroscopy measurements. The TiO(x) cell required a significant amount of time (approximately 60 min) to reach its maximum power conversion efficiency (PCE) of 2.6%. To investigate the reason for this slow photoresponse, we investigated the influences of UV light and water molecules adsorbed on the TiO(x) layer. Surface treatment of the TiO(x) cell with water induced a rapid photoresponse and enhanced the performance, giving a PCE of 2.97%. However, the durability of the treated cell was considerably inferior that of the untreated cell because of UV-induced photodegradation. The cause of the rapid photoresponse of the treated cell was attributed to the formation of hydrogen bonds between adsorbed water molecules and carbonyl oxygen atoms in PCBM close to the TiO(x) surface. When the TiO(x) surface was positively charged by UV-induced holes, the carbonyl oxygen in PCBM close to the TiO(x) surface can quickly join to the TiO(x) surface, rapidly transporting photogenerated electrons from PCBM to TiO(x) in competition with the photocatalyzed degradation. The experimental results suggested that the slow photoresponse of the untreated TiO(x) cell was because the morphology of the photoactive organic layer changed gradually upon irradiation to improve the transport of photocarriers at the TiO(x)/PCBM:P3HT interface.

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.

Charge-transporting properties of electropolymerized phenosafranin in aqueous media

The redox properties of polyphenosafranin were investigated at its different oxidation and protonation levels by potential scan voltammetry and electrochemical impedance spectroscopy. The polymer comprised phenosafranin units linked via a secondary amino nitrogen. A two-electron, two-proton process is proposed as its main redox reaction; its reduction yields the corresponding 5-hydro-phenazine form. In an isolated electroactive site, the polymer resembled redox polymers rather than conducting polymers. Low-frequency capacitance versus potential curves showed their maxima near the formal potential. Since the low-frequency capacitance increased linearly with the film thickness, it was identified with the redox capacitance. The width of the Warburg region showed a minimum at the half-reduced state of the polymer. This relationship indicated that electron transport was a key factor controlling the rate of charge transport in the polymer. The potential of the minimum Warburg width shifted to more negative values as the solution pH increased, with the minimum increasing exponentially with the pH. These observations lead to a conclusion that electron transport occurs via a process of sequential electron self-exchange between neighboring redox sites. This electron hopping is accompanied by proton transfer via intermolecular acid/base reactions.

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...

Characterization of ZnS-layer-inserted bulk-heterojunction organic solar cells by ac impedance spectroscopy

A fluorine-doped tin oxide (FTO)/zinc sulfide (ZnS)/[6,6]-phenyl C61 butyric acid methyl ester (PCBM): regioregular poly(3-hexylthiophene) (P3HT)/poly(3,4-ethylenedioxylenethiophene): poly(4-styrene sulfonic acid) (PEDOT:PSS)/Au type organic solar cell (FTO/ZnS/PCBM:P3HT/PEDOT:PSS/Au, ZnS cell) with a 1 cm2 active area was first developed using transparent ZnS prepared on a FTO electrode by a chemical bath deposition method. The ZnS inserted solar cells were investigated by photocurrent-voltage (I-V) and ac impedance spectroscopy (IS) measurements. In photo I-V measurements, the ZnS cell exhibited scattered power conversion efficiencies (η) of 0.7%–1.2% when the FTO/ZnS electrode was not immersed in various aqueous solutions before the fabrication of the ZnS cells. In contrast, the solar cells with a surface-modified FTO/ZnS electrode by immersing in 0.1M Na2S solutions at pH 7–9 containing hydrosulfide ions (HS−) exhibited reproducible η of 1.5%–1.7%. The electric resistance components in the cell consis...

Efficient Small‐Molecule Photovoltaic Cells Using a Crystalline Diindenoperylene Film as a Nanostructured Template

A cascade-type small-molecule organic photovoltaic cell using a crystalline diindenoperylene film as a nanostructured template is demonstrated. This cell architecture simultaneously realizes organic nanostructure and cascade energy concepts, which significantly improves the photocurrent generation and fill factor, leading to a power conversion efficiency of 5.2±0.3%.