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The synthesis of ultrathick concentrated poly(methyl methacrylate) (PMMA) brushes by atom transfer radical polymerization (ATRP) was investigated. The reactions were performed with a catalyst syste...

Despite great increases in photovoltaic performance due to the insertion of ligand-exchanged PbS quantum dot layer via 1,2-ethanedithiol (EDT), detailed carrier transport characteristics of solar cells with solution-phase ligand-exchanged PbS quantum dot (QD) still remains unknown. In this paper, relaxation time, recombination resistance, chemical capacitance, carrier lifetime, carrier diffusion length, and defect density corresponding to the depletion width were measured through electrochemical impedance spectroscopy using the Schottky diode model to understand limiting factors for photovoltaic performance. The activation energy, which is estimated from the temperature-dependence of resistance, was reduced from 0.06 eV to 0.02 eV by inserting the PbS-EDT layer, leading to the improved photovoltaic performance. We also evaluated QD solar cells with varying thicknesses of PbS QD layers to investigate the impedance analysis and photovoltaic measurements. The depletion width as well as the carrier diffusion length was larger than the physical thickness of the layer, and this indicates that an increased short-circuit current density can be realized at the thick PbS QD layer due to the resulting large absorbance. Furthermore, the reduced trap density of the QD layer improves the open circuit voltage and fill factor. As a result, a high photoconversion efficiency of 9.41% was achieved.

Background: In this paper, we report on the topics of one-dimensional (1D) and two-dimensional (2D) functional materials. Single-Walled Carbon Nanotubes (SWCNTs) are seamless hollow cylinders made of hexagonal lattice graphite sheets. The SWCNTs have attracted considerable attention due to the applicability of their enclosed nanospaces to engineering, and many types of guest materials are encapsulated inside their 1D space, expecting unusual properties. The poly Transition Metal (TM) phthalocyanine, in which phthalocyanine units are extended in two dimensions by sharing benzene rings, is one of the examples of the TM containing 2D carbon materials. Because of strong correlation between localized d-electrons in the TM atom and delocalized π-electrons on the poly phthalocyanine frame, it is expected that spin-polarized conduction, which is useful for the spintronic applications. Objectives: The objective of the first work is to synthesize SWCNTs encapsulating oxygen molecules having spin one, whose O-O bond directions are aligned to the longitudinal direction of the SWCNTs. The objective of the second work is to synthesize Poly Cu Phthalocyanine (PCuPc) through a bottom-up method by using copper octacyanophthalocyanine as a building block and to elucidate its crystal structure and magnetic properties. Methods: SWCNTs with inner diameter of ca 0.8 nm were prepared by the CoMoCAT method, and encapsulated together with oxygen molecules (~400 Torr) into a high-purity quartz tube. To subtract the background signals of the SWCNTs and the quartz tube, we prepared the same SWCNTs inducing He gas after evacuating oxygen molecules. Magnetization measurements of these SWCNTs samples were conducted by means of a SQUID magnetometer and a pulse magnet using an induction method. PCuPc were synthesized by a solid state reaction of octacyanophthalocyanine, tetracyanobenzene, and CuCl2·2H2O in glass ampoules sealed after evacuation. The as-synthesized samples were characterized using XRD analysis and TEM microscopy. Magnetization measurement of the samples were done by using a SQUID magnetometer. Results: The intrinsic magnetization data from oxygen molecules inside the SWCNTs (temperature and magnetic field dependence) show magnetic properties typical of the spin-one Heisenberg antiferromagnet named a Haldane magnet. PCuPc and its half-filling counterpart were obtained by solid state reaction. Both magnetic susceptibility and magnetization of PCuPc are larger than those of half-filling PCuPc, but the magnitudes of the former sample are about 1.5 times larger than those of the latter one, which is expected to be twice from the geometric superlattice structure. Conclusion: We have studied magnetic properties (magnetic susceptibility and magnetization) of oxygen molecules encapsulated into Single Walled Carbon Nanotubes (SWCNTs) with diameters of about 0.8 nm, regarded as a 1D functional magnetic material, and Poly Copper Phthalocyanine (PCuPc) and poly half-filling copper phthalocyanine (half-filling PCuPc), regarded as 2D functional magnetic materials.

We constructed ZnO/PbS quantum dot (QD) heterojunction solar cells using liquid-phase ligand exchange methods. Colloidal QD solutions deposited on ZnO-dense layers were treated at different temperatures to systematically study how thermal annealing temperature affected carrier transport properties. The surface of the layers became dense and smooth as the temperature approached approximately 80 °C. The morphology of layers became rough for higher temperatures, causing large grain-forming PbS QD aggregation. The number of defect states in the layers indicated a valley-shaped profile with a minimum of 80 °C. This temperature dependence was closely related to the amount of residual n-butylamine complexes in the PbS QD layers and the active layer morphology. The resulting carrier diffusion length obtained on the active layers treated at 80 °C reached approximately 430 nm. The solar cells with a 430-nm-thick active layer produced a power conversion efficiency (PCE) of 11.3%. An even higher PCE is expected in solar cells fabricated under optimal annealing conditions.

Conjugates of semiconductor quantum dots (QDs) and antibodies have emerged as a promising bioprobes due to their great combination of QD's efficient fluorescence and the high specificity of antigen-antibody reactions. For further developments in this field, it is essential to understand the molecular conformation of the QD-antibody conjugates at the single-molecule scale. Here, we report on the direct imaging of QD-antibody conjugates at the single-molecule scale by using high-speed atomic force microscopy (HS-AFM). Owing to the high spatiotemporal resolution of HS-AFM, we observed the dynamic splitting of individual antibodies during the conjugation process. QD-antibody conjugates were also clearly visualized at the single-molecule scale details. Several important features were even discovered through dynamic observation of the QD-antibody conjugates. We observed an intermediate state of conjugation, where the antibodies attached and detached to QDs repeatedly. We also revealed that the attached antibodies were not steady but drastically fluctuated in their recognition areas due to the Brownian motion. We also demonstrated that HS-AFM observation is useful for the quantitative analysis of fabricated conjugates.

We report on the syntheses, crystal structure, and magnetic properties of the transition metal coordination polymer [Co3(SBA)2(OH)2(H2O)2]n, (SBA = 4-sulfobenzoate) in which CoO6 octahedra are linked through their edges, forming one-dimensional (1D) Co(II) arrays running along the crystal a-axis. These arrays are further perpendicularly bridged by SBA ligand to construct a three-dimensional framework. Its magnetic properties have been investigated, and ferromagnetic interactions within the arrays have been found. From heat capacity measurements, we have found that this compound exhibits a three-dimensional ferromagnetic phase transition at TC = 1.54 K, and the specific heat just above TC shows a Schottky anomaly which originates from an energy gap caused by uniaxial magnetic anisotropy. These results suggest that [Co3(SBA)2(OH)2(H2O)2]n consists of weakly coupled 1D ferromagnetic Ising arrays.

Spray coating, in competition with conventional spin coating, has attracted extensive attention among large-scale solution casting techniques for producing high-performance organic solar cell (OSC) devices. Solution-processed vanadium oxide (VOx) is used extensively as an anode interlayer (AIL) in OSCs because of its unique optical and electronic properties. In this study, we optimized VOx AILs prepared using electrospray deposition (ESD-VOx) and compared them with AILs prepared using spin coating (SC-VOx); various ESD-VOx properties were found to be optimized at a flow rate of 6.25 μL/min. The OSC active layer used was poly(4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl3- fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-bthiophenediyl): [6,6]phenyl-C71-butyric acid methyl ester (PTB7:PC71BM). The ESD-VOx devices had transmittance and surface morphology comparable to SC-VOx interlayers used in high-efficiency OSC devices. X-ray photoelectron spectroscopy indicated that the obtained ESD-VOx surfaces had a favorable chemical composition. Measurement of hole mobilities using the space charge limited current method showed values of 4.36 × 10−4 and 1.39 × 10−4 cm2 V−1 s−1 for ESD- and SC-VOx, respectively. The charge transport resistance, measured by impedance spectroscopy, was very low for ESD-VOx devices, indicating that there faster charge transportation occurs within these devices because of enhanced hole mobility. These data are consistent with the carrier lifetimes of 1.59 × 10−8 and 4.53 × 10−4 s for ESD- and SC-VOx, respectively. The enhanced charge transportation properties, which can reduce charge recombination, can be attributed to the advantageous surface and interfacial properties of ESD-VOx AILs. Results indicate that optimized ESD had unperturbed diode characteristics with an improved power conversion efficiency, compared to spin coated VOx, of 7.61%.

In order to realize high performance of organic photovoltaic cells (OPVs), it is indispensable to optimize device structures according to composing materials. In particular, the optical density distribution in stacked layers is one of important factors to enhance the optical absorption within the photoactive layer, resulting in the improved photoconversion efficiency. In this study, we optimized the film thickness of the photoactive layer as a fundamental way to control the optical density distribution. The influence of the photoactive layer thickness on photovoltaic performances of OPVs was first investigated using, poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6diyl-alt-(4-(2-ethylhexyl)-3-uorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] and [6,6]-phenyl C 71 -butyric acid methyl ester blend films in normal and inverted OPV structures, respectively. The short circuit current density (J sc ) showed a peak at the thickness at around 100–120 nm and 80–100 nm in normal and inverted OPVs, respectively. In addition, J sc of inverted OPVs were higher than those of normal OPVs for all the thicknesses. The FF decreased continuously with increasing the thickness in both device structures. The fill factor of normal OPV decreased with increasing thickness more slowly than that of the inverted OPV at the thickness of over than 200 nm. Furthermore, we simulated the optical density distribution in the stacked layers using the algorithm utilized matrix formulas based on Abeles's formulas. The simulated optical intensity distribution as a function of the position and the wavelength is in a good agreement with the experimental spectra of external quantum efficiency. The result indicates a close correspondence between the optical density distribution and the photovoltaic performance. At the optimum photoactive layer thickness of 116 nm in the normal and 76 nm in the inverted OPVs, we obtained the highest PCEs of 9.25 and 10.4%, respectively.

As poly[4,8- bis (5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b ; 4,5- b ′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4- b ]thiophene-)-2-carboxylate-2-6-diyl)] has good potential as a low-band gap donor polymer for organic photovoltaic cells (OPVs), we investigated the optimized electrospray deposition condition for realizing suitable polymer ordering and/or crystallite size by controlling the solvent evaporation time. Previous studies on the electrospray process have mainly focused on novel device structure owing to its unique characteristic of small droplet size, which is less than 1 μ m. However, in this research, we investigated the spontaneous formation of interpenetrating continuous networks of the donor- and acceptor-rich domains of solvent evaporation during the electrospray process. By evaluating the ultraviolet–visible absorption spectrum, Raman spectroscopy, and direction of polymer ordering, it was shown that the polymer-stacking condition was not influenced by solvent evaporation time, even though poly(3-hexylthiophene-2,5-diyl) along the face-on direction was well stacked under the slow solvent evaporation condition. In contrast, the crystallite size, which was estimated from the full width at half maximum X-ray diffraction pattern, increased as the solvent evaporation time increased. This means that the crystalline grain spontaneously grew in the droplet and that the large crystalline grain was formed during the slow evaporation condition. Furthermore, the photovoltaic performance trend was the same as the performance trend of the crystallite size and were increased with increasing solvent evaporation time for both polymers. Therefore, the crystalline grain size was a dominant factor in determining the photovoltaic performance. Additionally, the crystalline grain size could be controlled by the solvent evaporation time. Finally, by optimizing the active-layer thickness, the highest photoconversion efficiency of 8.6% was achieved. This is the highest value of an electrospray-based device. These results indicate that the solvent evaporation time is an important factor in determining the crystallite size of an organic thin film, which directly affects the photoconversion efficiency of OPVs.

Since an interface barrier between a ZnO layer and an adjacent organic active layer affects the carrier injection efficiency, we investigated the relationship between the annealing temperature for ...

α-Co hydroxide nanosheets have been synthesized and their magnetic properties were investigated. By using a soft chemical exfoliation technique, exfoliated α-Co hydroxide nanosheets, typically with lateral dimensions of few 100 nm, were obtained in a colloidal suspension. The magnetic responses of a sample consisting of a colloidal suspension of the nanosheets indicates a ferromagnetic phase transition occurs at TC=37.8 K. The magnetization possesses a linear temperature dependence at low temperatures below TC. In addition to this observation, the magnetization is proportional to (1-T/TC)β with β=0.8±0.1 near TC, which imply that the surface magnetism dominates in the exfoliated α-Co hydroxide nanosheets.

We report on a simple method to fabricate a nitrogen N-containing nano-graphite with magnetic moments. By using chlorinated phthalonitrile as the building block and potassium K as the coupling reagent, a paramagnetic N-containing nano-graphite has been successfully prepared. From transmission electron microscopy images, X-ray photoelectron spectroscopy, and X-ray diffraction experiments, it is evident that the reaction product consists of randomly connected flakes of nano-graphite that contains large amount of N-decorated vacancy and edges. Interestingly, this N-containing nano-graphite shows a Curie-like paramagnetism that originates isolated magnetic moments on N-decorated vacancies and edges. These results make the N-containing nano-graphite a promising candidate for versatile applications in the fields of magnetics and spintronics.

Though the crystal growth technology of SiC is improving steadily, it is still crucial to reduce crystalline defects which act as carrier recombination (CR) centers and deteriorate device performance. We detected CR centers in a p-type 4H-SiC substrate by observing the intensity change of photoluminescence due to the addition of a below-gap excitation (BGE) light of 0.93[eV]. We noticed the temperature and the BGE density dependence of band edge (BE) emission in addition to donor acceptor pair (DAP) emission and discriminated the temperature effect from that of BGE. The BGE density dependence of the PL intensity quenching is different among the BE emission, B0- and C0-lines of the DAP, respectively. It gives us an important clue for understanding CR mechanisms inside the bandgap of SiC.

Organic photovoltaic cells (OPVs) have been investigated for a long time, and practical realizations have been also in progress due to their high photovoltaic performance over 12%. In this study, we fabricated normal OPVs and investigated the reason of an improved optical-to-electrical conversion efficiency by annealing the MoOx layer. The photoconversion efficiency was improved up to 5.65% from 2.05% after annealing at 160°C for 5 min, and the external quantum efficiency also increased for all the measurement wavelength ranging from 300 to 900 nm. Especially, the short circuit current density increased among photovoltaic parameters. The carrier transport resistance of photoactive layer was found to be reduced by evaluating the impedance measurement. These results indicate that defects at the MoOx/organic interface were successfully reduced by the thermal annealing process of MoOx layer. The efficient carrier transport was realized for the annealed-device, resulting in the high device performance. In addition, the relaxation and electron recombination times were also reduced by the annealing process, which lead to the improved photovoltaic performance.

Molybdenum oxide (MoO x ) has been widely used as a hole transport layer in organic photovoltaic cells (OPVs), whose performance can be improved by inserting a MoO x layer between an organic active layer and a transparent anode because of efficient carrier dissociation. In this study, the influence of thermally annealed MoO x on the photovoltaic performance of OPVs was first investigated using low-bandgap polymer and [6,6]-phenyl C 71 -butyric acid methyl ester (PC 71 BM) blend films as the active layer. We used three low-bandgap polymers: poly[2,6-(4,4-bis-(2-ethylhexyl)-4 H -cyclopenta[2,1- b ;3,4- b ′]dithiophene)- alt -4,7(2,1,3-benzothiadiazole)] (PCPDTBT), poly(4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b :4,5- b ′]dithiophene-2,6-diyl3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl) (PTB7), and poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b,3,3-b]dithiophene]3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl) (PTB7-Th). Power conversion efficiencies were drastically increased for all investigated polymers when the as-deposited MoO x layer was annealed at 160 °C for 5 min. In particular, a high efficiency of 6.57% was achieved when PTB7 was used; for comparison, the efficiency of a reference device with an as-deposited MoO x layer (not subjected to annealing) was 1.40%. Specifically, the short-circuit current density and fill factor were remarkably improved after annealing, which means that efficient carrier dissociation was achieved in the active layer. We evaluated optical absorption and surface morphology to elucidate reasons behind the improved photovoltaic performance, and these parameters only slightly changed after annealing. In contrast, angle-dependent X-ray photoelectron spectroscopy revealed that the MoO x layer was oxidized after annealing. In general, the oxygen vacancies of MoO x act as carrier traps; a reduction in the number of carrier traps causes high hole mobility in the organic layer, which, in turn, results in an improved photovoltaic performance. Therefore, our results indicate that the annealing-induced oxidation of MoO x is useful for achieving high photovoltaic performance.

Poly(3-hexylthiophene-2,5-diyl) (P3HT) is one of the most popular donor polymers for organic photovoltaic cells; however, annealing-induced molecular ordering is necessary for realizing high photoconversion efficiency. In this study, the ratio of ordered-disordered P3HT Raman signals was calculated as the crystallinity parameter, which can be evaluated as the molecular ordering of P3HT. In the case of spin-coated devices, the crystallinity parameter increases with increasing annealing temperature, and this agrees with the photovoltaic performance. Furthermore, the direct molecular ordering of P3HT during electrospray deposition is reported, detailing the relation between the solvent evaporation time and the crystallinity of P3HT, and is evaluated by Raman spectroscopy and grazing incidence X-ray diffraction. To observe the solvent evaporation phenomena of the electropsray process, in situ measurement of solvent evaporation time was also successfully realized for the first time by placing a CCD camera below the substrate in electrospray deposition. The solvent evaporation time was controlled from 0.036 to 2.8 s by changing the applied voltage and solvent. By investigating the relationship between the solvent evaporation time and the molecular ordering of P3HT, the long solvent evaporation time caused the high crystallinity of P3HT. In addition, the population of P3HT crystallinity with an edge-on orientation also increased with increasing solvent evaporation time by evaluating the grazing incidence X-ray diffraction pattern, in good agreement with the estimated crystallinity from Raman spectroscopy. Finally, a photoconversion efficiency of 2.0% was achieved by electrospray deposition without post thermal annealing.

Because co-deposition method has been utilized in a conventional thermal evaporation process to realize graded donor-acceptor architectures, we investigated an alternative intermittent electrospray co-deposition method for solution-processed organic photovoltaic cells. In this method, two solutions of poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61 butyric acid methyl ester (PCBM) were alternatively deposited using high-voltage pulse. Thus, the P3HT:PCBM blend thin film could be deposited even in a vacuum-free experimental setup. The optimum pulse width was found to be greater than 6 s to avoid an unexpected charge to the adjacent glass capillary, which causes the instable electrospray. The P3HT molecular ordering estimated from Raman spectroscopy and grazing incidence X-ray diffraction patterns was comparable to that estimated from the spin-coated device. In addition, the P3HT:PCBM ratio of the deposited thin film could be controlled by changing the ratio of the pulse width for the P3HT and PCBM solutions and was evaluated from the ultraviolet–visible absorption spectrum. Finally, a two layered bulk heterojunction structure with P3HT:PCBM was successfully demonstrated, leading to a maximum photoconversion efficiency of 3.1%. This value was 1.4-fold higher than that of the uniformly mixed bulk heterojunction device because of the high carrier-collection efficiency.

We investigated a molecular order of poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester thin film by judging from the Raman spectrum, and a crystallinity parameter was estimated as the molecular order of poly(3-hexylthiophene-2,5-diyl). The crystallinity parameter of spin-coated sample increased with increasing annealing temperature, and this result relates to the photoconversion efficiency of photovoltaic cell. This means that the high photoconversion efficiency can be obtained due to the high crystallinity parameter of poly(3-hexylthiophene-2,5-diyl), and this result is reasonable compared to the previous researches. In addition, the crystallinity parameter of electrosprayed thin film increased with increasing applied voltage, corresponding to the supply speed of solution. Since the droplet size increased with increasing supply speed, the evaporation speed of solvent (o-dichlorobenzene) became slow at the high applied voltage. Therefore, the slow evaporation speed of solvent caused the molecular order in the active layer. As a result, the maximum of approximately of 95 % was achieved even though the sample was not thermally annealed, and this value corresponds to the thermal annealing at 100 °C.

A reprecipitation method has been investigated for fabricating colloidal nanoparticles using Eu-complex. Herein, we investigated optical degradation characteristics of (1,10-phenanthroline)tris [4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionato]europium(III) colloidal nanoparticles, which were embedded into a silica glass film fabricated by a conventional sol–gel process. At first, we tried several types of good solvents for the reprecipitation method, and dimethyl sulfoxide (DMSO) is found to be a suitable solvent for realizing the small diameter and the high long-term stability against the ultraviolet irradiation even though the boing point of DMSO is higher than that of water used as a poor solvent. By optimizing the good solvent and the concentration of Eu-complex, the relative photoluminescence intensity of 0.96 was achieved even though the ultraviolet light was continuously irradiated for 90 min. In addition, the average diameter of 106 nm was achieved when DMSO was used as a good solvent, resulting in the high transmittance at a visible wavelength region. Therefore, we can achieve the transparent emissive thin film with a center wavelength of 612 nm, and the optical degradation was drastically reduced by forming nanoparticles.

We investigated crystallinity parameters and ordering domains of poly(3-hexylthiophene-2,5-diyl) blended in [6,6]-phenyl-C61-butyric acid methyl ester films, which were fabricated by pulsed electrospray deposition methods. The crystallinity parameter and the ordering domain were estimated from Raman and optical absorption spectra, respectively. As a result, they were improved with decreasing the off time of pulse voltage, corresponding to the slow evaporation speed of solvent. In addition, both the space-charge limited current mobility and the photoconversion efficiency showed same trend. A highest photoconversion efficiency of 1.11 % was achieved without the thermal annealing process after depositing the active layer.

In this research, we investigated improved photobleaching characteristics of (1,10-phenanthroline)tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionato]europium(III) by forming nano-particles embedded into a sol-gel derived silica glass film by a conventional sol-gel process. The relative photoluminescence intensities after the UV irradiation for 90 min were 88, 76, and 67% for nano-particles in the sol-gel derived glass film, powders in the sol-gel derived glass film, and raw powders, respectively. This result indicates that the phtobleaching of this Eu-complex can improved by forming nano-partcile structures by a reprecipitation method and embedding in the sol-gel derived silica glass.

We present a solution-processed planar fluorene bilayer by an ultrasonic atomized deposition method in combination with a needle electrode as an ionization part for an atomized droplet. An important advantage of our method is that the atomized droplet is efficiently charged using a needle electrode, which speeds up the deposition rate of the polymer thin film. The deposition rate increases 2 to 3 times compared to a that obtained with a conventional technique without using the ionization method, and real-time monitoring of landed droplets indicates that the number of droplets increased as the voltage applied to the needle electrode was increased, owing to the highly charged atomized droplets. Furthermore, the TFB/F8BT bilayer was achieved by optimizing the substrate temperature, and the polymer organic light-emitting diode exhibits a luminance value exceeding 12,000 cd/m 2 by insertion of the TFB as an electron blocking layer. The maximum current efficiency of the fluorene bilayer device was 6.64 cd/A, which was a 3.2-fold increase compared to that obtained with the reference device without the TFB electron blocking layer.

We report a real time method to monitor the chemical reaction in microdroplets, which contain an organic dye, 5(6)carboxynaphthofluorescein and a CdSe/ZnS quantum dot using fluorescence spectra. Especially, the relationship between the droplet diameter and the reaction rate of the two reagents was investigated by changing an inlet flow rate. key words: Microfluidic Device, Fluorescence Spectroscopy, Real-Time Monitoring, Semiconductor Quantum Dot, Fluorescence Energy Transfer

The nonradiative recombination processes in undoped InGaN single quantum well (SQW) grown by MOCVD method have been studied by time-resolved and two-wavelength excited photoluminescence. External quantum efficiency and carrier recombination lifetime decreases with increasing the excitation density which can be explained by an enhanced nonradiative recombination rate. A dominant nonradiative center with an optical activation energy of around 1.17 eV has been detected inside and/or hetero-interface of SQW, which might be an important clue for determining their origin and eliminating them during growth process.

Control of the crystallinity parameter and phase segregation of poly(3-hexylthiophene-2,5-diyl) (P3HT) and the soluble fullerene derivative [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) is a critical issue for polymer/fullerene-type organic photovoltaic cell applications. In this paper, P3HT thin films and P3HT:PCBM blend thin films were fabricated using the electrospray deposition method. The Raman spectral features of the samples were estimated using a two-peak fitting with a mixture of Gaussian and Lorentzian functions to determine their crystallinity parameters. Then, the relationship between the film morphology and crystallinity parameter was investigated. Homogenous P3HT thin films with high crystallinity parameters were obtained under dry/wet conditions, and a similar trend was found for P3HT:PCBM blend films.

The below-gap emission bands of undoped GaN grown by MOCVD on sapphire substrate have been investigated by photoluminescence technique. The intensity of yellow luminescence decreases due to the saturation of deep states with increasing excitation power that reflects the contribution of deep states at actual device operating condition. The excitation induced blue-shift of emission energy rationalizes the DAP characteristics. The integrated photoluminescence intensity of each band is analysed experimentally and theoretically by a power law model, and the reasonable agreement between experiment and model calculation justify our model consideration. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A high-voltage rectangular pulse was applied to the electro-spray deposition (ESD) to control the evaporation-induced self-assembly of poly(3-hexylthiophene-2,5-diyl) (P3HT). Two groups of P3HT thin films were deposited by a series of high-voltage rectangular pulses. Compared with the ESD driven by a constant voltage, the pulse-driven ESD enables to probe the effect of solvent evaporation on the self-assembly of P3HT molecules. The droplet size and the evaporation of residual solvent in the droplet determine the film morphology. Ultraviolet–visible absorption spectroscopy was used to identify the ordering of P3HT molecules in the films. The self-assembly of P3HT molecules took place during the solvent evaporation which can be controlled by a combination of the pulse amplitude and the pulse interval. With an appropriate combination of the amplitude and the interval, the ESD produced a P3HT thin film with high chain ordering. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Because color-selective organic photoconductive devices are required for image-sensing applications, high external quantum efficiency (EQE) is necessary. In our previous work, silole derivatives have been used to improve the device performance. In this study, we investigate the carrier kinetics of silole-derivative-doped organic photoconductive devices with a fluorene polymer used as an active layer by measuring the transient absorption spectrum. Transient absorption characteristics show that the silole-induced absorption change corresponds to a negative polaron of the excited fluorene polymer being formed via hole transfer from the silole derivative, and efficient hole transfer is realized by doping with the silole derivative. In addition, peak device performance was also achieved when the doping concentration of the silole derivatives are around 50 wt%. This result can be explained by the high dislocation density of electron–hole pair in individually migration electrons and holes owing to the silole doping in the fluorene polymer.

By choosing zinc acetate dehydrate (Zn(CH3COO)2·2H2O) and ammonium metavanadate (NH4VO3) as starting materials, we succeeded in fabricating Zn3V2O8 phase suitable for broad light emission by sol–gel process. X-ray diffraction (XRD), scanning electron microscope, photoluminescence (PL) and PL quantum yield (PL-QY) measurements have been performed for analyzing the relation between crystallographic and light emission characteristics. The PL-QY is determined by both the crystalline phase purity of the Zn3V2O8 phase defined by XRD patterns and the crystalline size. With optimizing the sintering condition at a slightly Zn-rich side, a pure Zn3V2O8 phase with enough large crystalline size was obtained with the PL-QY value of 52 %, which was higher than our previously obtained value by solid state reaction.

A signal-to-noise ratio (S/N) is an important factor for an organic image sensor. We investigated the doping effect of silole derivative in the green-sensitive organic photoconductive device for improving S/N. The maximum S/N of 12 was achieved when 1,1-dimethyl-2,5-bis(N,N-dimethylaminophenyl)-3,4-diphenylsilole with the lowest ionization potential was used as an active layer. In addition, the graphene oxide (GO) layer was found to be an important role for decreasing the dark current density, resulting in the high S/N. The maximum S/N of approximately 20 was realized by inserting the GO layer between an indium tin oxide (ITO) anode and an active layer.

Thin films of three types of fullerene derivatives were prepared through the electrospray deposition (ESD) method. The optimized conditions for the fabrication of the thin films were investigated for different types of fullerene derivatives: [6,6]-phenyl-C61-butyric acid methyl ester, [6,6]-phenyl-C71-butyric acid methyl ester, and indene-C60-monoadduct. The spray diameter during the ESD process was observed as a function of the supply rate achieved by changing the applied voltage. In all cases, the spray diameter increased with increasing applied voltage, reaching the maximum diameter (Dmax) in the voltage range 4 to 6 kV. It was clear that Dmax was influenced by the dipole moments of the fullerene derivatives (as calculated by density functional theory methods). Scanning electron microscopy observation of the fabricated thin films showed that imbricated structures were formed through the stacking of the fullerene-derivative sheets. Atomic force microscopy images revealed that the density of the imbricated structure was dependent on the spray diameter during the ESD process, and the root-mean-square roughness of the film surface decreased with increasing applied voltage. These findings suggest that the ESD method will be effective for the preparation of fullerene-derivative thin films for the production of organic devices.

Metal oxide nanofibers have attracted considerable research interest for processing both one-dimensional nanometer scale morphology and unique chemical and electrical properties. A variety of their practical applications in light-emitting diodes, liquid crystal displays, solar cells, and gas sensors have been demonstrated. Electrospinning provides a rapid and facile way to fabricate nanofibers with diameter several orders of magnitude smaller than that produced by conventional spinning methods. In this chapter, we discuss the fabrication of ultrathin metal oxide nanofibers by the electrospinning technique. Priority is given to zinc oxide nanofibers. Major parameters affecting the morphology and diameter of the nanofibers are investigated systematically. The effect of calcination condition on chemical composition and crystallization of the electrospun nanofibers is also addressed. In addition, we show the advantages and problems when applying electrospun nanofibers to solar cells.

Wave functions of N-isoelectronic levels in GaP overlap with increasing N concentration and form an intermediate band (IB) with a suitable energy allocation. Thus GaPN is a promising candidate for IB-type solar cells. Detailed characterization of defect originated recombination levels is the first step to realize the expected efficiency improvement. We have studied carrier recombination processes via the IB in GaPN with N concentration of 0.56% by two wavelength excited photoluminescence (TWEPL) measurement. Spectral behavior of high energy shoulder in the IB luminescence with increasing the density of below gap excitation (BGE) light (hv BGE =1.17eV) discriminated the effect of optical excitation from that of thermal activation. The BGE effect revealed the presence of different carrier recombination levels which are connected with conduction band and with the IB, respectively.

It is still indispensable to reduce defect densities in SiC to realize its potential superiority for power device applications. We have studied carrier recombination (CR) processes in p-type 4H-SiC substrate by our optical scheme of two wavelength excited photoluminescence (TWEPL) at 12K. The Donor acceptor pair (DAP) photo-luminescence (PL) decreased distinctly with the addition of the below gap excitation (BGE) light, which revealed the presence of CR levels whose energy location corresponds to the BGE photon energy of 0.98 eV. Its dependence on the BGE power density together with the difference obtained by the BGE energy of 1.17 eV revealed a distribution of CR levels inside the forbidden gap of SiC. The method is free from electrical contact and is applicable to whole wafer mapping.

We investigated an alternative intermittent electrospray co-deposition method for solution processed-organic photovoltaic cells with the graded active layer. This method uses separated two solutions, which contain poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C 61 butyric acid methyl ester (PCBM), are alternatively deposited by using the pulse high voltage. Thus, the P3HT:PCBM blend thin film can be deposited even though a vacuum free experimental setup. The optimized pulse width was found to be > 6 s to avoid unexpected charge to the adjacent glass capillary, which cases the instable electrospray. The P3HT:PCBM ratio of deposited thin film can be controlled by changing the ratio of pulse width for P3HT and PCBM solutions. Finally, a vertically graded bulk heterojunction structure with P3HT:PCBM was successfully demonstrated, leading to a maximum photoconversion efficiency (PCE) of 3.1%. This value was 1.4-fold compared to the uniformly blended device due to the high carrier collection efficiency.

Two-wavelength excited photoluminescence, a non-destructive and non-contacting method for defect study has been used to investigate the AlGaN multiple quantum well (MQW) emitting at ∼260 nm. We have succeeded to detect the non-radiative recombination (NRR) centers in both wells and barrier layers using a below-gap ex- citation (BGE) light of 1.17 eV. The PL intensity of both well and barrier layers decreases after irradiation of BGE which can be explained by the well-established two-level model. The normalized PL intensity decreases with increasing BGE power density for both cases but more pronounced for the barrier layer. The normalized PL intensity increases for both well and barrier layer's emission with increasing AGE density and becomes saturated at higher excitation due to the fill-up of NRR centers.

An electrospray deposition (ESD) is a solution process, which has several advantages, such as the utilization of low concentration solution, the large device area using a continuous roll-to-roll process, and the simple setup without vacuum condition. By applying a high voltage to a solution, mists are generated from a tip of glass capillary, leading to the formation of organic thin film. Halogenated solvents are often used, but the amount of solvent should be reduced from the environmental aspect. Thus, by using the ESD and the non-halogenated solvent, the manufacturing process is friendly to environment. In this research, by measuring current density-voltage (J-V) characteristics and atomic force microscope (AFM) images, we evaluated OPV characteristics and root mean square (RMS) roughness of devices fabricated using the non-halogenated solvent. By adding 1,8-diiodooctane (DIO), the photo conversion efficiency (PCE) was drastically improved, and the PCE of 5.45% was achieved.

To obtain a highly sensitive and color selective photodetector for an image sensor, silole-derivative-doped coumarin 30 photoconductive films were fabricated. The silole-doped films showed excellent color selectivites only in the blue region of visible light. The dark current in the films decreased when the silole-doping concentration was increased, which reached down to 1.0 × 10−9 A/cm2 at −10 V in the 80%-silole-doped film. The 50%-silole-doped device showed high external quantum efficiency up to 60% along with the excellent color selectivity.

Instabilities of Eu-complex against ultraviolet (UV) light irradiation and annealing are important problems to solve before they can be practically applied in several applications, such as white light-emitting diodes, bioimaging sensors, and wavelength conversion films for silicon photovoltaic cells. By now, our research group demonstrated an encapsulation technique of tris (2-thenoyltrifluoroacetonato)(1,10-phenanthroline) europium (III) (Eu (TTA)3phen) using the solvothermal process as a final annealing process of sol–gel synthesis. In this paper, we investigated the optimized starting solution of the sol–gel process to improve the long-term stability of Eu (TTA)3phen by changing the silane alkoxide and solvent. The long-term stability was improved by annealing at the high pressure condition (solvothermal process), and the longest half brightness time of 9.8 h was achieved while irradiating with the UV light of 360 nm and 5 mW/cm2. One possible reason for this result is that the hydrolysis of the sol–gel based glass network occurs more efficiently with the high-pressure annealing. As a result, the high-encapsulating efficiency was achieved due to the low organic component in the sol–gel-derived glass network around Eu (TTA)3phen. Furthermore, the particle structure with the diameter of less than 50 nm was obtained by coating the sol–gel-derived silica glass around Eu (TTA)3phen.

Since rhodamine 6G (R6G) has the selective absorption band at the green wavelength regions, the green-sensitive organic photoconductive device was demonstrated using R6G as a photoconductive layer. By optimizing the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) used as the carrier transport layer and the concentration of R6G, the maximum external quantum efficiency of 32.6% was obtained at the electric field of −34 MV/m. We found that the low-conductivity of the PEDOT:PSS layer was most important factor for the improved device performance. In addition, the signal-to-noise ratio (photocurrent/dark current) was improved by reducing the conductivity of the PEDOT:PSS layer, and the highest value of 60 was achieved., This is an Author's Accepted Manuscript of an article published in Solution-Processed Green-Sensitive Organic Photoconductive Device Using Rhodamine 6G in the Molecular Crystals and Liquid Crystals (12 Sep 2012) [copyright Taylor & Francis], available online at: http://www.tandfonline.com/10.1080/15421406.2012.701830.

The initial growth stage of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films by the electrospray deposition (ESD) method was investigated using 70% N , N -dimethylformamide (DMF) as a solvent solution through the real-time spectroscopic ellipsometric (SE) characterization. The uniaxial anisotropic optical property was prominent for the film growth on c-Si wafer, whereas the homogeneous growth occurred with random molecular orientation on indium-tin-oxide (ITO) coated glass. These findings probably originate from a preferential orientation of the polymer chains as obtained in the preparation process, sort of substrate, its morphology, and film thickness.