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Frontiers of Optoelectronics

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EDITORIAL
A special issue on Optoelectronics in Energy
Changqing CHEN, Jun ZHOU, Yanyan FANG
Front Optoelec. 2013, 6 (4): 357-358.  https://doi.org/10.1007/s12200-013-0388-9
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REVIEW ARTICLE
Monolithic all-solid-state dye-sensitized solar cells
Yaoguang RONG, Guanghui LIU, Heng WANG, Xiong LI, Hongwei HAN
Front Optoelec. 2013, 6 (4): 359-372.  https://doi.org/10.1007/s12200-013-0346-6
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As a low-cost photovoltaic technology, dye-sensitized solar cell (DSSC) has attracted widespread attention in the past decade. During its development to commercial application, decreasing the production cost and increasing the device stability take the most importance. Compared with conventional sandwich structure liquid-state DSSCs, monolithic all-solid-state mesoscopic solar cells based on mesoscopic carbon counter electrodes and solid-state electrolytes present much lower production cost and provide a prospect of long-term stability. This review presents the recent progress of materials and achievement for all-solid-state DSSCs. In particular, representative examples are highlighted with the results of our monolithic all-solid-state mesoscopic solar cell devices and modules.

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Recent developments in sensitizers for mesoporous sensitized solar cells
Kun CAO, Mingkui WANG
Front Optoelec. 2013, 6 (4): 373-385.  https://doi.org/10.1007/s12200-013-0347-5
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Sensitizers have proven to be extremely important in determining the performance of dye-sensitized solar cells (DSCs). The design and understanding of sensitizers, especially D-π-A structured porphyrins, has become a recent focus of DSC research. In this perspective article, advances in the conception and performance of various sensitizers including ruthenium complexes, organic dyes and porphyrins are reviewed with respect to their structure and charge transfer dynamics at the dye-sensitized mesopours heterojunction interface. In particular, the discussion focuses on the trends that perovskite would be the most effective and most likely to be used in DSCs combining with innovative hole transporting materials.

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Review on one-dimensional ZnO nanostructures for electron field emitters
Meirong SUI, Ping GONG, Xiuquan GU
Front Optoelec. 2013, 6 (4): 386-412.  https://doi.org/10.1007/s12200-013-0357-3
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The emission of electrons from the surface of a solid caused by a high electric field is called field emission (FE). Electron sources based on FE are used today in a wide range of applications, such as microwave traveling wave tubes, e-beam evaporators, mass spectrometers, flat panel of field emission displays (FEDs), and highly efficient lamps. Since the discovery of carbon nanotubes (CNTs) in 1991, much attention has been paid to explore the usage of these ideal one-dimensional (1D) nanomaterials as field emitters achieving high FE current density at a low electric field because of their high aspect ratio and “whisker-like” shape for optimum geometrical field enhancement. 1D metal oxide semiconductors, such as ZnO and WO3 possess high melting point and chemical stability, thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications. In addition, unlike CNTs, in which both semiconductor and metallic CNTs can co-exist in the as-synthesized products, it is possible to prepare 1D semiconductor nanostructures with a unique electronic property. Moreover, 1D semiconductor nanostructures generally have the advantage of a lower surface potential barrier than that of CNTs due to lower electron affinity and the conductivity could be enhanced by doping with certain elements. As a consequence, there has been increasing interest in the investigation of 1D metal oxide nanostructure as an appropriate alternative FE electron source to CNT for FE devices in the past few years. This paper provides a comprehensive review of the state-of-the-art research activities in the field. It mainly focuses on FE properties and applications of the most widely studied 1D ZnO nanostructures, such as nanowires (NWs), nanobelts, nanoneedles and nanotubes (NTs). We begin with the growth mechanism, and then systematically discuss the recent progresses on several kinds of important nano-structures and their FE characteristics and applications in details. Finally, it is concluded with the outlook and future research tendency in the area.

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RESEARCH ARTICLE
High efficiency monobasal solid-state dye-sensitized solar cell with mesoporous TiO2 beads as photoanode
Heng WANG, Peng XIANG, Mi XU, Guanghui LIU, Xiong LI, Zhiliang KU, Yaoguang RONG, Linfeng LIU, Min HU, Ying YANG, Hongwei HAN
Front Optoelec. 2013, 6 (4): 413-417.  https://doi.org/10.1007/s12200-013-0353-7
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A monobasal solid-state dye-sensitized solar cell (ssDSC) with mesoporous TiO2 beads was developed and an efficiency of 4% was achieved under air mass (AM) 1.5 illumination. Scattering properties and electron diffusion coefficients of TiO2 mesoporous beads and P25 nano-particles were investigated. The results show that TiO2 mesoporous beads display higher scatterance than P25 nano-particles, and TiO2 mesoporous beads have higher electron diffusion coefficients (2.86 × 10-5 cm2?s-1) than P25 nano-particles (2.26 × 10-5 cm2?s-1).

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Side chains and backbone structures influence on 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based low-bandgap conjugated copolymers for organic photovoltaics
Debin NI, Dong YANG, Shuying MA, Guoli TU, Jian ZHANG
Front Optoelec. 2013, 6 (4): 418-428.  https://doi.org/10.1007/s12200-013-0343-9
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Five 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based conjugated copolymers with controlled molecular weight were synthesized to explore their optical, energy level and photovoltaic properties. By tuning the positions of hexyl side chains on DTBT unit, the DTBT-fluorene copolymers exhibited very different aggregation properties, leading to 60 nm bathochromic shift in their absorptions and the corresponding power conversion efficiencies (PCEs) value of photovoltaic cells varied from 0.38%, 0.69% to 2.47%. Different copolymerization units, fluorene, carbazole and phenothiazine were also investigated. The polymer based on phenothiazine exhibited lower PCE value due to much lower molecular weight owing to its poor solubility, although phenothiazine units were expected to be a better electron donor. Compared with the fluorene-based polymer, the carbazole-DTBT copolymer showed higher short circuit current density (Jsc) and PCE value due to its better intermolecular stacking.

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Improvement of blue InGaN light-emitting diodes with gradually increased barrier heights from n- to p-layers
Wu TIAN, Xiong HUI, Yang LI, Jiangnan DAI, Yanyan FANG, Zhihao WU, Changqing CHEN
Front Optoelec. 2013, 6 (4): 429-434.  https://doi.org/10.1007/s12200-013-0342-x
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The advantages of blue InGaN light-emitting diodes (LED) with the active region of gradually increased barrier heights from n- to p-layers are studied. The energy band diagram, hole concentration, electrostatic field near the electron blocking layer (EBL), and the internal quantum efficiency (IQE) are investigated by Crosslight simulation program. The simulation results show that the structure with gradually increased barrier heights has better performance over the equal one, which can be attributed to the mitigated polarization effect near the interface of the last barrier/EBL due to less interface polarization charges. Moreover, reduced barrier height toward the n-layers is beneficial for holes injection and transportation in the active region. As a result, holes are injected into the active region more efficiently and distributed uniformly in the quantum wells, with which both the IQE and the total lighting power are increased. Although it can lead to the broadening of the spontaneous emission spectrum, the increase is slight such that it has little effect on the application in solid-state lighting.

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A simple unilateral homogenous PhOLEDs with enhanced efficiency and reduced efficiency roll-off
Shaoqing ZHUANG, Wenzhi ZHANG, Xiao YANG, Lei WANG
Front Optoelec. 2013, 6 (4): 435-439.  https://doi.org/10.1007/s12200-013-0349-3
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In this paper, highly efficient phosphorescent organic lighting emitting diodes (PhOELDs) with low efficiency roll-off are demonstrated by using a unilateral homogenous device structure with wide band-gap material 4, 4', 4″-tri(N-carbazolyl)-triphenylamine (TCTA) as hole transporting layer and emitting layer (EML). The optimized blue device exhibits a high power efficiency of 40 lm/W, external quantum efficiency of 19.2% and current efficiency of 37.7 cd/A. More importantly, the device exhibits a low efficiency roll-off at 1000 cd/m2. In addition, the white homogenous PhOLEDs only exhibits the efficiency roll-off 5.6% and 17.5%, corresponding to the brightness of 1000 and 5000 cd/m2 respectively. These interesting results demonstrate that the simple unilateral homogenous device structure is a promising way to enhance the device efficiency and reduce the efficiency roll-off.

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Selective growth and characterization of ZnO nanorods assembled a hexagonal pattern on H2-decomposed GaN epilayer
Yu TIAN, Huiquan CHEN, Xiaolong ZHU, Guang ZHENG, Jiangnan DAI
Front Optoelec. 2013, 6 (4): 440-447.  https://doi.org/10.1007/s12200-013-0350-x
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This paper reported a simple and effective method for fabricating and patterning highly ordered ZnO nanorod arrays on H2-decomposed GaN epilayer via hydrothermal route. The edge of pattern, which has been decomposed by H2 flow, provides appropriate nucleation sites for the selective-growth of aligned ZnO nanorods. The density of ZnO nanorod arrays assembled the hexagonal pattern can be tuned by varying the solution concentrations, growth time and reaction temperatures. The results have demonstrated that the ZnO nanorods are highly uniform in diameter and height with perfect alignment and are epitaxially grown along [0001] direction. This work provides a novel and accessible route to prepare oriented and aligned ZnO nanorod arrays pattern. And the aligned ZnO nanorods form an ideal hexagonal pattern that might be used in many potential applications of ZnO nanomaterials.

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Lasing characteristics of curved semiconductor nanowires
Weisong YANG, Yipei WANG, Yaoguang MA, Chao MENG, Xiaoqin WU, Qing YANG
Front Optoelec. 2013, 6 (4): 448-451.  https://doi.org/10.1007/s12200-013-0344-8
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The characteristics of curved semiconductor nanowire (NW) lasers were investigated. The red-shift in the laser spectra with increasing bending angles can be observed much more clearly than that in the photoluminescence (PL) spectra. Due to oscillation of light in resonant cavity, the bending loss of laser exhibits multiple times amplification of that of PL. Furthermore, an abnormal phenomenon of dominant peak switching is found in curved NWs when increasing the pump power, which has been first discovered and reported.

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Preparation, structure and properties of fluorescent nano-CdTe/poly (1, 4–butanediol-citrate) bioelastomer nanocomposite in-situ dispersion technique
Li JIANG, Aimiao QIN, Kunpeng JIANG, Lei LIAO, Xiulan WU, Chaojian WU
Front Optoelec. 2013, 6 (4): 452-457.  https://doi.org/10.1007/s12200-013-0345-7
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Hydrophilic photoluminescent CdTe/poly (1, 4–butanediol-citrate) (PBC) bioelastomer nanocomposite was successfully synthesized by a two-step method and characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, Uv-vis spectroscopy, photoluminescence (PL) spectroscopy and scanning electron microscope (SEM). The differential scanning calorimetry analysis shows that the bioelastomer nanocom-posites with different mass fractions of CdTe have low glass-transition temperature, which indicates that they possess elastic property in the range from room temperature to the expected applied temperature (37°C). The measurements of the hydrophilicity, in vitro degradation and PL show that the nanocomposite has good hydrophilicity, degradation and high fluorescence properties.

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Condensed state fluorescence switching of hexaarylbiimidazole-tetraphenylethene conjugate for super-resolution fluorescence nanolocalization
Wen-Liang GONG, Zhe HU?§?, Chong LI, Guo-Feng ZHANG, Tao CHEN, Matthew. P. ALDRED, Zhen-Li HUANG, Ming-Qiang ZHU
Front Optoelec. 2013, 6 (4): 458-467.  https://doi.org/10.1007/s12200-013-0330-1
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This paper reported the synthesis of hexaarylbiimidazole-tetraphenylethene (HABI-TPE) conjugated photochromic fluorophore, which simultaneously exhibited photochromic property, condensed state enhanced emission and reversible fluorescence switching. Upon UV irradiation, a green species with a broad absorption band between 550 and 800 nm ( the absorption maximum at 697 nm ) was observed, which readily faded to colorless in the darkness. HABI-TPE launched strong fluorescence with the maximum emission wavelength at 520–580 nm under the excitation with 450–500 nm visible light in condensed state, which is in contrast to nonfluorescence in solution. The maximum emission wavelength in condensed state was dependent of excitation wavelength. More interestingly, HABI-TPE exhibited reversible fluorescence switching upon alternating irradiation with blue or near-UV light (wavelength less than 490 nm) and green light (more than 490 nm) in condensed state. Our evaluation demonstrated that HABI-TPE exhibited great photoswitchable fluorescence, which is a promising photoswitchable fluorophore for localization-based super-resolution microscopy, evidencing by resolving nanostructures with sub-100 nm resolution in polymethylmethacrylate films.

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Tunable and programmable fiber ring laser based on digital-controlled chirped fiber Bragg grating
Huiqi LIAO, Ming TANG, Hailiang ZHANG, Yiwei XIE
Front Optoelec. 2013, 6 (4): 468-471.  https://doi.org/10.1007/s12200-013-0356-4
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This paper proposed and experimentally demonstrated an all-fiber tunable and programmable bandpass filter using a linearly chirped fiber Bragg grating (CFBG). The center wavelength and spacing of the transmission peaks could be independently tuned via computer. The tunable range is about 18 nm. With this filter we demonstrated a tunable fiber ring laser which has an output power of about -7 dBm, full-width at half-maximum linewidth of ~0.017 nm which is limited by the resolution of the optical spectrum analyzer (OSA). Furthermore, a spacing tunable dual-wavelength fiber laser was achieved with the same setup. This all-fiber laser features advantages like simple structure, low cost, flexible and digital tuning capability.

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