多田 和也

The dye-sensitized solar cell (DSSC) has been on the market as a permanent power source for indoor IoT edge devices. In recent years, indoor illumination technology has been experiencing a drastic transition from incandescent and fluorescent lamps toward solid-state lighting devices with light-emitting diodes (LEDs). In addition to the high power efficiency, a virtue of LEDs is their prompt response, which enables precise change of the illumination level using pulse-width modulation (PWM) of the current source, and thus PWM illumination is commonly installed in society. The light intensity change from off to on states of an LED under PWM driving is literally infinity, which causes the lighting to flicker. The lighting flicker induces not only an optical illusion but also biological effects, including serious health problems, which can be mitigated by raising the modulation frequency. Because the peak intensity of a PWM illumination can be 100 times that of the average intensity, the indoor solar cell, which has a relatively high series resistance, is expected to underperform. In this paper, the characteristics of a commercial indoor DSSC under PWM illumination are studied. It is found that while PWM illumination at low frequency seriously deteriorates the performance of the DSSC, it recovers at high frequency. The latter feature is not found in indoor amorphous-Si solar cells, and the electrochemical impedance spectroscopy revealed that it stems from the electrochemical nature of some components of the series impedance in the DSSC, offering a key piece of evidence of the superiority for use in the modern indoor application of the DSSC over traditional amorphous-Si solar cells.

Abstract Electrophoretic deposition provides material-efficient film formation on large area electrodes. In this study, it has been found that there is a significant difference in the colloidal particle formation process between a thiophene-based copolymer poly(3-octylthiophene- 2,5-diyl-co-3-decyloxythiophene-2,5-diyl) (POT-co-DOT) and C₆₀ in preparation of suspension for electrophoretic deposition by reprecipitation method. This difference is attributed to the difference between low molecular weight materials with specific molecular weight and polymers with molecular weight distribution. The composition of POT-co-DOT:C₆₀ composite film by electrophoretic deposition has also been estimated.

© 2020 The Japan Society of Applied Physics. Pulse width modulation (PWM) driving is common in emerging solid-state or LED lighting. Unlike conventional lighting devices, the peak illumination intensity of a PWM-driven LED can be a hundredfold of average illumination intensity, causing strong lighting flicker. This work addresses the impact of the flicker on indoor amorphous Si (a-Si) photovoltaic cell performance. The power conversion efficiency of an amorphous silicon photovoltaic cell is found to be a function of not the average, but the peak illumination intensity. Indoor photovoltaic cells can thus seriously underperform under PWM-driven solid-state lighting when the peak illumination intensity is high enough to decrement photovoltaic cell performance.

© 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. The error between the apparent series/shunt resistances of solar cells estimated by the current–voltage slope at the open-/short-circuit condition and the true series/shunt resistances is calculated by the exact analytical expressions of the one-diode equivalent circuit model for the solar cell. In order to avoid the difficulty in computation in known expressions, the exact analytical expressions based on the g-function proposed by Roberts are derived and employed. Significant deviation is found between the apparent and the true series/shunt resistances. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

© 2019 The Japan Society of Applied Physics. 1,2,4-trimethylbenzene is a halogen-free solvent which dissolves unmodified and modified fullerenes and conjugated polymers. The unique feature enables the study of how the substituent of fullerene affects thermal robustness in bulk heterojunction solar cells using a conjugated polymer PTB7-Th. While 175 °C is the best annealing temperature for a solar cell with unmodified C70, it deteriorates a device with a substituted C70 (C70-PCBM). Additionally, annealing at 175 °C does not change the surface of PTB7-Th:C70 film but makes the surface of PTB7-Th:C70-PCBM film bumpy. The results suggest that the substituent promotes the migration of fullerene in a polymer:fullerene solid composite.

Copyright © 2020 The Institute of Electronics, Information and Communication Engineers S-shaped nonlinearity is found in the electrical resistance-length relationship in an electroactive supercoiled polymer artificial muscle. The modulation of the electrical resistance is mainly caused by the change in the contact condition of coils in the artificial muscle upon deformation. A mathematical model based on logistic function fairly reproduces the experimental data of electrical resistance-length relationship.

Solution-processed organic photocell is one of the most promising candidates for the permanent power source for wireless devices. Since the device is not necessarily to be placed outdoor, and indoor light is generally weaker than outdoor light by 2-3 orders of magnitude, the characterization of the photocell under low-light illumination is important for this purpose. In this study, bulk heterojunction photocells based on a low-energy gap polymer poly[[4,8-bis[5-(2-ethylhexyl)thiophene-2-yl]benzo[1,2-b:4,5-b]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-Th) as well as unmodified C70 prepared with a halogen-free solvent 1,2,4-trimethylbenzene, are characterized under low-light illumination. The combination of a halogen-free solvent with an unmodified fullerene potentially provides a way to develop environmentally friendly organic photocells. It is found that the photocells show higher power conversion efficiency (PCE) under lower light illumination intensity. That is, a device showing PCE of 4.9% under 1 sun illumination shows PCE over 9% at 3 10(-4) sun. A sublinear dependence of the short-circuit photocurrent to the light intensity, as well as the increased fill-factor at reduced illumination, is found to be a key to the high PCE at low-light. The latter originates from the photoconduction in the composite. The critical effect of dark parallel resistance on the low-light performance of the photocells is also demonstrated. (C) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Although processing additives such as 1,8-diiodooctane are known to effectively modify the microscopic structure of polymer bulk heterojunction composite using PCBM-type fullerene derivatives and yield efficient photovoltaic cells, it seems not yet to be clarified which part of the PCBM-type fullerene derivatives, i.e. bucky ball or side group, is essential to the phenomena. Here, the effect of processing additive 1,2-dimethylnaphthalene (DMN), which is reported to be effective for polymer bulk heterojunction composite film with [6,6]-phenyl-C-71-butyric acid methyl ester (C-70-PCBM) coated from 1,2,4-trimethylbenzene, on the composite film with unmodified C-70 is examined. It has been observed that DMN shows negligible effects on the photovoltaic performance including the low-light performance as well as the surface morphology. Therefore, not the bucky ball but the side group in the fullerene derivatives seems to be critical to the positive effect of processing additives such as DMN.

The S-shaped current-voltage characteristic is a sign of poorly performing organic photocell, since it gives low fill-factor. In this paper, the characteristics of a bulk heterojunction device with PTB7-Th:C-70 composite without thermal annealing, which shows S-shaped current-voltage curve with the power conversion efficiency (PCE) of 1.5% under AM 1.5G 1 sun illumination, under low light illumination are studied. It is found that the current-voltage characteristic of the device under 10(-3) sun illumination does not show the kink and thus the PCE exceeds 5%. A photocell which shows both S-shaped and normal current-voltage characteristics seems to be rarely reported so far.

The opposed two-diode equivalent-circuit model consisting of a traditional one-diode photocell model and a parasitic diode with a parallel resistance is known to somehow reproduce the S-shaped current-voltage curve of poor organic photocells. Here, the light-intensity dependencies of the parameters in the model are experimentally studied. On the assumption that the parasitic part is light-insensitive, the parameters of the main part, which are extracted by fitting the current-voltage curves under various illumination intensities with the model, are found to change as those of healthy photocells do. The reasonable separation of light-sensitive main and light-insensitive parasitic parts validates the model. (C) 2016 Elsevier B.V. All rights reserved.

Increasing social demands for clean and cheap renewable energy sources have been promoting the researches on polymer photocells based on bulk heterojunction composites consisting of conjugated polymers and fullerenes. The replacement of traditional chloroaromatic solvents such as chlorobenzene with a halogen-free solvent 1,2,4-trimethylbenzene (TMB), which enables the preparation of bulk heterojunction composites with unmodified fullerenes, potentially reduces the economic and environmental costs of organic bulk heterojunction photocells. In this study, photovoltaic characteristics of PTB7:C-70 bulk heterojunction photocells prepared with TMB at low light intensity are studied. It has been found that the power conversion efficiency (PCE) of a photocell with ITO/PEDOT:PSS/PTB7:C-70/Al structure whose PCE is approximately 3 % under AM1.5G 1 sun (100 mW/cm(-2)) illumination increases with decreased illumination light intensity, being 5.9 % at 10(-3) sun illumination. An analysis of current-voltage characteristics suggests that the increased PCE is mainly due to increased shunt resistance and decreased saturation current of the diode in the model. The results indicate that the solution-processed organic photocell based on unmodified fullerene prepared with halogen-free solvent is a promising candidate for environmental-friendly device for indoor light harvesting.

Although photocells are commonly characterized under AM1.5G 100 mW cm(-2) (1 sun) illumination, their performance under low light illumination is also important, because photocells are frequently used for indoor applications. In this study, polymer photocells based on a bulk heterojunction composite consisting of a low energy gap polymer PTB7 and unmodified C-70 prepared with a halogen-free solvent 1,2,4-trimethylbenzene have been characterized under the illumination of 1 sun or below. A typical photocell with the power conversion efficiency (PCE) of 4% at 1 sun shows the PCE of approximately 7% at 10(-3) sun, which seems to fit for some indoor applications such as a permanent power source for a wireless sensor node. The sublinear dependence of short-circuit photocurrent on light intensity as well as the increase of fill-factor under low light illumination yields the increased efficiency under low light illumination. An analysis employing a one-diode equivalent circuit model suggests that the increased parallel resistance as well as the decreased saturation current of the diode under low light illumination accounts for the latter feature. It is also pointed out that the parallel resistance and/or the saturation current under dark strongly influence the PCE of a photocell under low light illumination. In addition, the dependence of the device performance on the light intensity is found to be useful for analyzing the effects of the thermal treatment and the PFN interlayer at cathode. (C) 2016 Elsevier B.V. All rights reserved.

In terms of the environmentally-friendliness, it is desirable that the energy and toxicity embodied in a photocell are as low as possible. In this context, a solution-processed organic photocell with unmodified fullerene as electron acceptor coated with halogen-free solvent is an attractive object to study. Here, the dependences of the photovoltaic performance of PTB7-Th:C-70 bulk heterojunction photocells prepared from a chlorine-free solvent 1,2,4-trimethylbenzene on composition and annealing temperature have been studied. Similar to the PTB7:C-70 bulk heterojunction photocells, the device performance of PTB7-Th:C-70 photocells is improved by the thermal annealing up to 175 degrees C, suggesting that the PTB7-Th:C-70 photocells are superior in thermal robustness. The optimal device using PTB7-Th:C-70=1:1 annealed at 175 degrees C showed the power conversion efficiency of 5.1% under 1 sun illumination, which is the highest value reported for solution-processed organic photocells using unmodified fullerene so far. (c) 2015 Elsevier B.V. All rights reserved.

Although the spectral response is a key data for photocells, traditional measurement setup for it consists of expensive optical precision components including white-light source, collimator and monochromator, and has relatively large footprint. Here, it is demonstrated that a compact, portable, and low-cost device based on an array of light-emitting diodes can provide quantitatively reliable spectral response characterization of photocells. The device seems to be useful for the purpose of preliminary survey of the spectral response of novel material and/or materials system prior to precise measurement with the traditional equipment. (C) 2015 AIP Publishing LLC.

Non-traditional photocells such as organic photocells frequently show S-shaped current-voltage characteristics, which can be reproduced with not the simple one-diode equivalent-circuit model but an opposed two-diode model. In this study, a facile method for the extraction of the device parameters from S-shaped characteristics of a photocell using the opposed two-diode model is demonstrated. It was found that the fitting accuracy is insensitive to the ideality factors of the diodes. Moreover, contrary to the preceding studies, the exclusion of the series resistance was found to have a noticeable impact on the fitting results.

The combination of a halogen-free solvent 1,2,4-trimethylbenzene and unmodified fullerene potentially provides a way to develop environmentally-friendly and cost-effective solution-processed organic photocells. In this paper, the thermal annealing effect on the optical absorption spectra in poly(3-hexylthiophene): unmodified-C-60 composites with various compositions is reported. It is found that the onset temperature of the absorption spectrum change is higher in the composites with higher fullerene content. It is speculated that strong interaction between the polymer main chain and C-60 tends to suppress the reorientation of polymer main chains in a composite with high C-60 content.

Bulk heterojunction (BHJ) photocells using low energy-gap polymers such as poly[[4,8-bis[(2-ethylhexyl) oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyllthieno[3,4-b]thiophen ediyl]] (PTB7) and modified fullerenes such as [6,6]-phenyl-C-71-butyric acid methyl ester (C-70-PCBM) show relatively high power conversion efficiency. It is known that the thermal annealing, which effectively improves the photovoltaic performance of BHJ photocells with semi-crystalline polymers such as poly (3-hexylthiophene), is not applicable to BHJ photocells with amorphous-type polymers such as PTB7. In this study, it is found that PTB7:C70-PCBM BHJ photocells degrade shortly at 80 degrees C, which is within the range of the temperature shock cycling test condition specified in IEC 61646, the international standard for testing thin-film photovoltaic module. On the other hand, the photovoltaic performance of PTB7:C-70 BHJ photocells composite has been found to significantly improve after the thermal annealing at 160 degrees C. The results strikingly indicate not only that the lack of thermal robustness in PTB7:C-70-PCBM BHJ photocells does not mainly arise from the nature of the polymer but the substituents of the fullerene used, but also that neat fullerenes should not be considered as just cheap and nasty substitutes of fullerene derivatives. (C) 2014 Elsevier B.V. All rights reserved.

Because of ease in ensuring chemical integrity and purification, small molecular materials are promising as alternatives for polymeric electron-donor materials in solution-processed bulk heterojunction (BHJ) photocells. On the other hand, replacement of modified fullerene with neat one potentially reduces both economical and environmental costs of the photocells. In this paper, solution-processed BHJ photocells with a small molecular electron-donor 7,7'[4,4-bis(2-ethylhexyl)-4H-silolo [3,2-b:4,5-b']dithiophene-2,6-diyl]bis[6-fluoro-4-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole] (p-DTS(FBTTh2)(2)) and neat C-70 are studied. The devices showed a power conversion efficiency of approximately 1.3% after optimal thermal annealing. Analysis of current-voltage characteristics using an equivalent circuit model suggests that the thermal annealing enhances not only the generation but also the internal recombination of photocarriers. (C) 2014 Elsevier B.V. All rights reserved.

The effect of the insertion of a very thin layer of a conjugated polyelectrolyte poly[9,9-bis(3'-(N,N-dimethylamino)-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) at the composite/cathode interface in bulk heterojunction photocells based on a low-energy-gap polymer PTB7 and neat C-70 is studied. The device performance is sensitive to the deposition condition of the PFN layer. An analysis based on a simple one-diode equivalent circuit model reveals that the PFN interlayer deposited under an appropriate condition effectively reduces the saturation current of the internal diode as well as the series resistance, resulting in a power conversion efficiency (PCE) of over 4%. (C) 2014 The Japan Society of Applied Physics

A lot of efforts have been devoted to synthesize novel fullerene derivatives to improve the performance of polymer-based bulk heterojunction photocells. Although the archetypical soluble fullerene derivative [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) is frequently treated as a reference material, PCBM, which carries both aromatic and aliphatic moieties, is not very simple molecule. In this context, unmodified C-60, which is the simplest molecule among the C-60-based fullerenes, seems to be the most suitable for the reference material. Here, a set of systematic experiments on the device performances of the bulk heterojunction composites consisting of P3HT and unmodified C-60 over various compositions and annealing temperatures has been carried out. It has been found that the apparent optimum composition strongly depends on the annealing temperature chosen. Additionally, a monotonic relationship between the shift in the open-circuit voltage and the multiplication factor of the short-circuit photocurrent has been also found, directly indicating that the fullerene aggregation results in increased electron affinity to enhance the charge separation in the composites. The simple structure of the C-60 molecule seems to contribute for maintaining the coherency in the present study. (C) 2013 Elsevier B.V. All rights reserved.

The combination of halogen-free solvent and neat fullerene is expected to reduce the economical and environmental costs of polymer photovoltaic devices. Recently, it has been found that a naturally produced solvent, 1,2,4-trimethylbenzene, enables the preparation of bulk heterojunction composites with neat C-60. However, the power conversion efficiency (PCE) of such devices reported so far was less than 1%. In this research, the annealing effects on and film thickness dependence of the performance of photovoltaic devices based on a composite consisting of equivalent weights of poly(3-hexylthiophene) and C-60 have been studied to improve the device performance. The optimized device showed a PCE of 1.21%, suggesting that neat C-60 is a promising candidate electron acceptor material for polymer bulk heterojunction composites for environmentally friendly and cost-effective photovoltaic devices. (C) 2014 The Japan Society of Applied Physics

The combination of halogen-free solvent and neat fullerene is expected to provide a way to develop environment-friendly and cost-effective polymer photocells. Here, polymer photocells using neat C-70 and a low energy-gap polymer poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b]dithiophene-2,6-diyI]d[3fluoro-2-1(2-ethylhexyl)carbonyllthieno[3,4-b]thiophenediyl]] (PTB7) prepared by using 1,2,4-trimethyl-benzene as a halogen-free solvent have been studied. After a preliminary optimization, a device with the power conversion efficiency of 3.02% has been obtained. The C-70-based composites contain less amount of fullerene than the composites using C-70-PCBM at their optimal compositions, reflecting the different electron mobility and/or the aggregation tendency of the fullerenes. (C) 2013 Elsevier B.V. All rights reserved.

It has been reported that the temporal change of current during the deposition shows a plateau and a break, similar to those found in a photocurrent profile taken by the time-of-flight technique for the investigation of photocarrier dynamics in condensed matters, enabling the estimation of electrophoretic mobility of colloidal particles in the suspension. The estimation of the electrophoretic mobility from transient current during the deposition by the simple drift model is based on the assumption that a constant electric field is uniformly applied between the positive and negative electrodes. Therefore, it is important to check if this assumption is satisfied. It is also important to measure the temporal evolution of film thickness, because this may give information about uniformity of colloidal size in the suspension. This study addresses these topics and validity of the assumption is confirmed.

Electrophoretic deposition is a useful and efficient technique to deposit conjugated polymers, if suitable suspension of the target polymer is obtained. Unfortunately, neither general theory nor universal procedure for the preparation of a suspension suitable for electrophoretic deposition has seemed to be established yet. Thus, accumulation of individual knowledge is still important in this area. Here, the preparation of suspensions of a thiophene-based polymer poly(3-octylthiophene-2,5-diyl-co-3-decyloxythiophene-2,5-diyl) (POT-co-DOT) and their application to the electrophoretic deposition have been reported. The suspensions of POT-co-DOT with various good/poor solvents ratios were easily obtained by mixing a toluene solution of the polymer and acetonitrile. The composition of the dispersion medium significantly affects the surface morphology of the film prepared by the electrophoretic deposition in the suspension. Composite films consisting of POT-co-DOT and C-60 have also been successfully prepared by electrophoretic deposition. Because the optical absorption peak at 333 nm in C-60 showed neither significant broadening nor a red shift, C-60 molecules were expected to be well-dispersed in the composite films. A photovoltaic device with a composite film prepared from a POT-co-DOT/C-60 = 2:1 suspension showed 10 times larger short-circuit current density and 5 times larger power conversion efficiency than a device with pure POT-co-DOT, although further work is required to improve the device performance.

Chemically modified fullerenes and chlorinated solvents have been predominantly chosen in the recent studies of the photovoltaic devices based on polymer bulk heterojunction composites. However, these items seem to be undesirable because of their potential impacts on the environment as well as the consumption of resources. In this context, a systematic study on the annealing effect and film composition dependence of bulk heterojunction photovoltaic devices based on composites consisting of poly(3-hexylthiophene) (P3HT) and neat C-70 prepared with a chlorine-free solvent 1,2,4-trimethyl-benzene has been carried out. For the device using P3HT:C-70 composite (2:1 by weight) film as an active layer, power conversion efficiency of 1.47%, with open-circuit voltage of 0.52 V, short-circuit current density of 6.2 mA/cm(2) and fill-factor of 45%, has been obtained after post-production annealing at 160 degrees C. The combination of neat fullerenes with naturally obtained solvents will open up a new way to produce cost-effective and environmentally-friendly photovoltaic devices based on polymer:fullerene bulk heterojunction composites. (C) 2012 Elsevier B.V. All rights reserved.

可溶性をもつ導電性高分子は比較的低い融点をもつが,溶融状態の導電性高分子を電子素子に利用しようという試みはほとんどなされていない.本論文では溶融状態の導電性高分子を電子素子に利用するための予備的な検討として,2枚のITO電極付ガラス基板間に溶融状態の導電性高分子を挟んだ構造の素子(ITO/ITOセル)と,高分子上に真空蒸着法によりAu電極を形成した素子を試作し,特性評価を行った.後者では高分子層の液化に伴い,素子構造を保てなくなることが予想されたが,高分子層とその上に形成する金属電極層が一定の厚さ以上であれば素子構造を保ち,ITO/ITOセルと同様の結果が得られることが分かった.

Although C-60 is unquestionably not only the cheapest but also the most energy-saving fullerene, lack of suitable solvent to make bulk heterojunction composites with it has prevented its use in photocells. Here, it is shown that a not-so-common organic solvent 1,2,4-trimethylbenzene (TMB) is useful to prepare cost-effective bulk heterojunction composites based on C-60. Additionally, chlorine-free TMB is expected to be environmental friendly compared with chloroaromatic solvents commonly used for the preparation of polymer/fullerene composites. A preliminary evaluation indicated that a poly(3-hexylthiophene) (P3HT)/C-60 bulk heterojunction photocell prepared from TMB solution shows power conversion efficiency comparable to that of a P3HT/PCBM photocell, suggesting that C-60 should be revisited as a promising candidate for an acceptor material in polymer bulk heterojunction composites for low-cost green photocells. (C) 2012 Elsevier B.V. All rights reserved.

In terms of environmental friendliness, halogenated compounds including 1,2-dichlorobenzene (ODCB), which is a solvent frequently used in the preparation of polymer/fullerene bulk heterojunction composites for photocells, are not desirable. As a result of exploration in this context, 1,2,4-trimethylbenzene (TMB) has been found as a useful solvent for the preparation of poly(3-hexylthiophene) (P3HT)/C-70 composite. A preliminary experiment has shown that the power conversion efficiency of a device with a P3HT/C-70 composite film prepared from TMB solution reaches 0.45%, which is higher than the previously reported value for P3HT/C-70-based devices prepared from ODCB solution by one order of magnitude. (C) 2012 The Japan Society of Applied Physics

The fabrication of nanocrystals (NCs) composed of the cationic Au(I) complex was demonstrated, by the reprecipitation method in which colloidal solution of the NCs showed brilliant green phosphorescence with a quantum yield of 83% in n-hexane. Characterization of the prepared NCs was performed by transmission electron microscopy observation and elemental analysis with energy-dispersive-X-ray. spectroscopy. The obtained Au(I) NCs were particles of random shapes with a diameter of 200-400 nm. The selected-area diffraction and X-ray diffraction measurements showed the characteristic diffraction patterns attributable to the crystal structure of the bulk crystal of the Au(I) complex. A similar method was performed with a different counteranion, leading to a colloidal solution of the microcrystals (MCs) with brilliant yellow phosphorescence and a quantum yield of 26% in n-hexane. Luminescence deposition onto an indium tin oxide. (ITO)-coated glass substrate, resulting in characteristic luminescence patterns on the ITO substrates with relatively high photoluminescence quantum yields.

The morphology of composite films consisting of conjugated polymers and C-60 prepared by electrophoretic deposition (EPD) has been studied. While C-60 exists as nanocrystals uniformly embedded in the composite films based on poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene], a spontaneous stratification of the composite films into polymer-C-60-mixture and C-60-microcrystal layers is observed in the films based on a polyfluorene derivative, poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene}]. The results suggest that EPD can be a useful tool for controlling the morphology of bulk heterojunction systems for organic photocells. (C) 2011 Elsevier B.V. All rights reserved.

We synthesized a new oligothiophene derivative with pyridine end groups and investigated their optical properties. Upon adding an aqueous polystyrene sulfonic acid solution, it forms homogeneous polymer complex films with protonated structure. Furthermore, reversible color change in photoluminescence in response to a base due to reversible protonation of pyridyl end groups in oligothiophene was demonstrated, which can be applied to acid sensors.

A procedure to prepare flat and dense composite films of a conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEHPPV) and various amounts of neat C-60 by means of electrophoretic deposition from dilute suspensions is reported. It has been observed that deposition from suspensions containing both components results in pinhole-free films, which are applicable to photovoltaic devices, while inhomogeneous films are formed from the suspensions containing only one of the components. The applicability of electrophoretic deposition to obtain polymer-C-60 composite films showing photovoltaic effects has been successfully demonstrated. (C) 2010 Elsevier B.V. All rights reserved.

The nanoporosity installed in conjugated polymer films prepared by electrophoretic deposition makes it difficult to measure the amount of polymer deposited on a substrate. Here, an alternative approach, the estimation of material efficiency of the electrophoretic deposition from the optical absorption spectra of the residual suspensions has been studied. The ultimate recovery rate, which becomes smaller in suspensions with lower acetonitrile content, does not depend on the deposition voltage. The light scattering by the colloidal particles seems to be absent in residual suspensions after a deposition long enough to reach the ultimate recovery rate, indicating the exhaustion of the colloidal particles. Although the deposition rate of the polymer markedly lowers upon coating of the deposition electrode with PEDOT, the ultimate recovery rate remains unchanged. These results suggest that the material efficiency in this deposition method is limited by the generation rate of the colloidal particles in the suspension.

The electrophoretic deposition is a method useful to prepare conjugated polymer films for electronic devices. This method provides high material recovery rate on the substrate from the suspension, in contrast to the conventional spin-coating in which most of the material placed on the substrate is blown away. Although manual reprecipitation technique successfully yields suspensions of various conjugated polymers including polyfluorene derivatives, it is favorable to control the preparation process of suspensions. In this context, this paper reports preliminary results on the preparation of suspension of conjugated polymer by using an ultrasonic atomizer. While the resultant films do not show particular difference due to the preparation methods of the suspension, the electric current profiles during the electrophoretic deposition suggests that the ultrasonic atomization of polymer solution prior to be mixed with poor solvent results in smaller and less uniform colloidal particles than the conventional manual pouring method. (C) 2010 Elsevier B.V. All rights reserved.

Electric currents during the electrophoretic deposition of a conjugated polymer in a thin slab vessel directly reflect the electrophoretic motion of polymer nanoparticles in suspensions. The double-logarithmic plot of the current profile was employed to estimate the transit time from the current profile, which is apparently featureless in the linear plot. The electrophoretic mobility was calculated to be 10(-4)-10(-5) cm(2)/(V-s), which tended to decrease with increasing polymer concentration in the suspension. The current profiles obtained at different voltages show a clear scaling behavior. That is, the dimensionless plots of the electric current profiles collapse onto a single curve. This result suggests that the diffusion of the colloidal particles due to the concentration gradient does not seem to play a major role in the present system. (C) 2010 The Japan Society of Applied Physics DOI: 10.1143/JJAP.49.061602

Composite films of a polyfluorene derivative, poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene}], and neat C-60 with various C-60 contents have been obtained by electrophoretic deposition from dilute suspensions. It has been observed that deposition from suspensions containing both components results in pinhole-free films that are applicable to photovoltaic devices, whereas inhomogeneous films are formed from the suspensions containing only one of the components. Much improvement in the power conversion efficiency was achieved in the photovoltaic devices using the polymer-C-60 composites compared with the devices with a pristine polymer film. The applicability of electrophoretic deposition to the fabrication of photovoltaic devices with polymer-C-60 composites has been successfully demonstrated, although the power conversion efficiency was only on the order of 10(-3)% at this stage. (C) 2010 The Japan Society of Applied Physics

Patterned polarized blue-light emission was achieved using a new liquid-crystalline fluorene derivative (F1) on a patterned photoalignment layer. Adjusting the doses of linearly polarized (LP) UV light exposure onto the photoalignment layer controlled the alignment direction of F1, which coincided with the molecular reorientation direction of the photoalignment layer. An electroluminescent device with orthogonally patterned molecular orientation was fabricated, demonstrating patterned polarized blue-light emission with a polarization ratio between 2.7 and 3.5. (c) 2009 The Japan Society of Applied Physics