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Frontiers of Optoelectronics

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EDITORIAL
A special issue on solar cells
Yi-Bing CHENG
Front Optoelec Chin. 2011, 4 (1): 1-1.  https://doi.org/10.1007/s12200-011-0212-3
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REVIEW ARTICLE
Intermediate-band solar cells based on dilute alloys and quantum dots
Weiming WANG, Jun YANG, Xin ZHU, Jamie PHILLIPS
Front Optoelec Chin. 2011, 4 (1): 2-11.  https://doi.org/10.1007/s12200-011-0151-z
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This paper describes our recent developments of intermediate-band solar cells, with a focus on the use of dilute alloys and nanostructured materials such as quantum dots (QDs). The concept of “full-spectrum” solar cells and their working mechanism with various material structures are first illustrated. A comprehensive review of ZnTe:O-based intermediate-band solar cells, including material growth, structural and chemical analysis, device modeling and testing, are presented. Finally, the progress and challenges of quantum-dot-based solar cells are discussed.

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Conjugated dendritic oligothiophenes for solution-processed bulk heterojunction solar cells
Chang-Qi MA
Front Optoelec Chin. 2011, 4 (1): 12-23.  https://doi.org/10.1007/s12200-011-0206-1
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This mini-review summarizes the recent achievements of developing conjugated dendritic oligothiophenes (DOT) for use in solution-processed bulk heterojunction (BHJ) solar cells. These DOTs are structurally defined molecules with relatively high molecular weight. Therefore, this novel class of thiophene based material possesses not only some advantages of oligomers, such as defined and monodispersed molecular structure, high chemical purity, but also some characteristics of polymers, for example, good solution-processability. In addition, the step-by-step approach of its synthesis allows precise functionalization of dendritic backbones with desired moieties, which is helpful to finely tune the optical and electronic properties of materials. Power conversion efficiencies (PCE) of BHJ solar cells were achieved up to 2.5% when functionalized thiophene dendrimers were used as electron donor and electron acceptor was a fullerene derivative. These results indicated that dendritic oligothiophenes are a novel class of the materials of electron donor for solution-processed organic solar cells.

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Dye-sensitized solar cells based on ZnO nanotetrapods
Wei CHEN, Shihe YANG
Front Optoelec Chin. 2011, 4 (1): 24-44.  https://doi.org/10.1007/s12200-011-0207-0
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In this paper, we reviewed recent systematic studies of using ZnO nanotetrapods for photoanodes of dye-sensitized solar cells (DSSCs) in our group. First, the efficiency of power conversion was obtained by more than 3.27% by changes of conditions of dye loading and film thickness of ZnO nanotetrapod. Short-circuit photocurrent densities (Jsc) increased with the film thickness, Jsc would not be saturation even the film thickness was greater than 35 μm. The photoanode architecture had been charactered by good crystallinity, network forming ability, and limited electron-hopping interjunctions. Next, DSSCs with high efficiency was devised by infiltrating SnO2 nanoparticles into the ZnO nanotetrapods photoanodes. Due to material advantages of both constituents described as above, the composite photoanodes exhibited extremely large roughness factors (RFs), good charge collection, and tunable light scattering properties. By varying the composition of the composite photoanodes, we had achieved an efficiency of 6.31% by striking a balance between high efficiency of charge collection for SnO2 nanoparticles rich films and high light scattering ability for ZnO nanotetrapods rich films. An ultrathin layer of ZnO was found to form spontaneously on the SnO2 nanoparticles, which primarily was responsible for enhancing open-circuit photovoltage (Voc). We also identified that recombination in SnO2/ZnO composite films was mainly determined by ZnO shell condition on SnO2, whereas electron transport was greatly influenced by the morphologies and sizes of ZnO crystalline additives. Finally, we applied the composite photoanodes of SnO2 nanoparticles/ZnO nanotetrapods to flexible DSSCs by low temperature technique of “acetic acid gelation-mechanical press-ammonia activation.” The efficiency has been achieved by 4.91% on ITO-coated polyethylenenaphtalate substrate. The formation of a thin ZnO shell on SnO2 nanoparticles, after ammonia activation, was also found to be critical to boosting Voc and to improving inter–particles contacts. Mechanical press, apart from enhancing film durability, also significantly improved charge collection. ZnO nanotetrapods had been demonstrated to be a better additive than ZnO particles for the improvement of charge collection in SnO2/ZnO composite photoanodes regardless of whether they were calcined.

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REVIEW AIRTICLE
Electrolyte-dependent photovoltaic responses in dye-sensitized solar cells
Hong LIN, Feng HAO, Jianbao LI
Front Optoelec Chin. 2011, 4 (1): 45-52.  https://doi.org/10.1007/s12200-011-0208-z
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Promoted by the growing concerns about the worldwide energy demand and global warming, dye-sensitized solar cells (DSSCs) are currently attracting worldwide scientific and technological interest because of their high energy conversion efficiency and simple fabrication process. Considering long-terms stability and practice applications, growing attentions have been paid to non-volatile, 3-methoxyproprionitrile (MPN)-based electrolyte, ionic liquids (ILs) electrolyte, as well as quasi-solid state electrolyte. In this present review, recent progress in electrolyte for DSSCs made by our group are summarized, including component-optimization of the non-volatile electrolyte, the fluidity-dependent charge transport mechanism in the binary IL electrolytes as well as the structure dominance of the employed ILs. Furthermore, progress on the quasi-solid state electrolyte based on inorganic nanomaterials as gelators in our group has also been outlined.

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RESEARCH ARTICLE
Electrochemical analysis of dye adsorption on aligned carbon nanofiber arrays coated with TiO2 nanoneedles for dye-sensitized solar cell
Jianwei LIU, Jun LI
Front Optoelec Chin. 2011, 4 (1): 53-58.  https://doi.org/10.1007/s12200-011-0154-9
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An electrochemical method has been developed to analyze dye absorption on the aligned carbon nanofiber arrays coated with TiO2 nanoneedles for dye-sensitized solar cell. The unique nanostructure with the roughness factor of 90.6 provides a large effective surface area for dye adsorption. The experimental results showed that the dye molecules cover 39.7% of the TiO2 surface area which influences the performance of dye-sensitized solar cell. The electrochemical method provides the information of the coverage of dye molecules which is a key issue to optimize solar cell performance.

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Dependence of porosity, charge recombination kinetics and photovoltaic performance on annealing condition of TiO2 films
Chang-Ryul LEE, Hui-Seon KIM, Nam-Gyu PARK
Front Optoelec Chin. 2011, 4 (1): 59-64.  https://doi.org/10.1007/s12200-011-0205-2
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Effect of annealing temperature, time of nanocrystalline TiO2 film on porosity, electron transport/recombination and photovoltaic performance on dye-sensitized solar cell (DSSC) had been investigated in this article. Photocurrent density was slightly higher as annealing at 550°C compared to those of annealing at 450°C and 500°C under the given annealing time of 60?min, which was correlated with the amount of adsorbed dye. Thermogravimetric analysis showed there was a more weight loss between 500°C and 550°C, which revealed there were more sites for dye adsorption. Given the annealing temperature of 550°C, as annealing time varied from 60 to 90 and 120 min, results showed that the average size of pore and surface area decreased with longer annealing time, which deteriorated photocurrent density due to less dye loading. Electron diffusion rate remained almost unchanged regardless of annealing condition. However, electron recombination was influenced by annealing condition, it became slower with the increase of the annealing temperature under the given annealing time. In the contray, the electron recombination developed faster for the longer annealing time at a given annealing temperature. These results suggested that heat treatment of TiO2 film at 550°C for 60 min in air would be the optimal annealing condition to achieve high efficiency DSSC.

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Enhanced electron injection/transportation by surface states increment in mesoporous TiO2 dye-sensitized solar cells
Minghui DENG, Shuqing HUANG, Zhexun YU, Dongmei LI, Yanhong LUO, Yubai BAI, Qingbo MENG
Front Optoelec Chin. 2011, 4 (1): 65-71.  https://doi.org/10.1007/s12200-011-0204-3
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A strategy of surface modification to the mesoporous TiO2 photoanode with hydrochloric acid treatment was used in this study, and it was found that short circuit current and photovoltaic efficiency of dye-sensitized solar cells (DSSCs) were increased by 5.5% and 8.9% respectively. The improvement was attributed to the reduced impedances in the TiO2 film and at the TiO2/dye/electrolyte interface. It was showed that the increased surface electronic states could remarkably prolong electron lifetime, which was responsible for the reduction of impedances. Under these quasi-continuous states in mesoporous structure, the electron injection/transportation can be notably facilitated, which will be beneficial for the DSSC performance.

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Electron transportation and optical properties of micro-structure TiO2 films: applied in dye-sensitized solar cells
Shuangying XU, Linhua HU, Jiang SHENG, Dongxing KOU, Huajun TIAN, Songyuan DAI
Front Optoelec Chin. 2011, 4 (1): 72-79.  https://doi.org/10.1007/s12200-011-0202-5
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Micro-structure of TiO2 films in dye-sensitized solar cells (DSSCs) can affect light absorption and electron transportation that impact on the characteristics of current-voltage (J-V). In this paper, films with different surface area, pore size and porosity were obtained by adding different ratio of ethyl cellulose (Ec-S) to pastes, and a photo-electric conversion efficiency (η) of 7.55% with a short-circuit current density (Jsc) of 16.81 mA·cm-2 was obtained when the ratio of Ec-S was 10∶5. BET results showed that film with this optimum ratio had the most suitable pore size and surface area for good properties of photovoltaic, which had a low reflectivity and high transmission rate, and the efficiency of light utilization was improved. Moreover, measurements by intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) implied that the electron transport time (τd) increased as the content of Ec-S increased, which was related to the larger surface area. Results of steady-state cyclic voltammetry indicated that diffusion-limited current density (Jlim) of I3- in TiO2 film increased with its porosity, which revealed that the transportation of redox mediators in the electrolyte was speeded up.

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Effect of deoxycholic acid on performance of dye-sensitized solar cell based on black dye
Quanyou FENG, Hong WANG, Gang ZHOU, Zhong-Sheng WANG
Front Optoelec Chin. 2011, 4 (1): 80-86.  https://doi.org/10.1007/s12200-011-0209-y
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The effect of coadsorption with deoxycholic acid (DCA) on the performance of dye-sensitized solar cell (DSSC) based on [(C4H9)4N]3[Ru(Htcterpy)(NCS)3](tcterpy= 4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine), a so-called black dye, had been investigated. Results showed that the coadsorption of DCA with the black dye results in significant improvement in the photocurrent and mild increase in the photovoltage, which leads to an enhancement of overall power conversion efficiency by 9%. The enhancement of photocurrent was attributed to the increased efficiency of charge collection and/or electron injection. The coadsorption with DCA suppressed charge recombination and thus improved open-circuit photovoltage.

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Synthesis and photovoltaic property of pyrrole-based conjugated oligomer as organic dye for dye-sensitized solar cells
Qianqian LI, Wenjun WU, Aoshu ZHONG, Jianli HUA, Ming PENG, Jing HUANG, Jie SHI, He TIAN, Jingui QIN, Zhen LI
Front Optoelec Chin. 2011, 4 (1): 87-92.  https://doi.org/10.1007/s12200-011-0210-5
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A new pyrrole-based conjugated oligomer (P1) was obtained with phenyl-linked triphenylamine moieties as an isolation group. Little aggregations were observed whether oligomer P1 was absorbed on titanium dioxide (TiO2) surface or in solid state. Since the pyrrole-based moieties in donor-π-acceptor type was the core component of oligomer P1 for light absorption, the corresponding dye-sensitized solar cell (DSSC) demonstrated the efficiency of light-to-electrical conversion by 0.48%. Higher conversion efficiency could be achieved by tuning the size of the isolation groups and the structure of the donor-π-acceptor type dyes.

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Quantum dot photoelectrochemical solar cells based on TiO2-SrTiO3 heterostructure nanotube array scaffolds
Jun ZHANG, Chengchun TANG
Front Optoelec Chin. 2011, 4 (1): 93-102.  https://doi.org/10.1007/s12200-011-0211-4
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Titania-Strontium titanate (TiO2-SrTiO3) nanotube array with heterostructure has been demonstrated as an efficient scaffold applied to quantum dot photoelectrochemical solar cells. Quantum dot CdS serviced as solar light absorbent is chosen as an example to illustrate superior performance and deposited on scaffolds by successive ionic layer adsorption and reaction (SILAR) technique. The photoelectrochemical performance of such solar cell is strongly dependent on the structure of heterostructured scaffolds. Only well-dispersed SrTiO3 nanocrystallites on TiO2 could improve the overall conversion efficiency. Transient absorption spectra and photoelectrochemical measurements show that the formation of SrTiO3 energy gradient between TiO2 and electrolyte slows down the rate of electronic injection from 19.3 × 108 to 6.30 × 108 s-1, while it greatly increases electronic collection efficiency via reduced charge recombination. Cadmium sulfide (CdS) quantum dots used to decorate TiO2-SrTiO3 (1 h hydrothermal treatment) electrode exhibits superior photoelectrochemical performance with nearly 70% increase in external quantum efficiency at 460 nm and also in overall cell conversion efficiency. The photostability and high efficiency properties of TiO2-SrTiO3 composites would enable its practical application in solar energy conversion devices.

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Studies of high-efficient and low-cost dye-sensitized solar cells
Qingqing MIAO, Mingxing WU, Wei GUO, Tingli MA
Front Optoelec Chin. 2011, 4 (1): 103-107.  https://doi.org/10.1007/s12200-011-0201-6
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Dye-sensitized solar cell (DSSC) is a new type of photoelectric device. To commercialize DSSC successfully, it is necessary to further improve the efficiency of energy conversion and reduce its cost. Nitrogen-doped (N-doped) TiO2 photoanode, the carbon counter electrode (CE), and a new type of hybrid photoanode were investigated in this study. The conversion efficiency of the DSSC reached by 10.10% as the DSSC was fabricated with the N-doped photoanode, and this efficiency is much higher than that of the undoped-DSSC with 8.90%; as the low-cost carbon was used as CE, the efficiency of the DSSC was 7.50%, it was as samilar as that of Pt CE (7.47%); the hybrid DSSC with multilayer photoanode by the film-transfer technique achieved a panchromatic response and a superposed short circuit current density (JSC) by using two complementary dyes.

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Flexible solar cells based on PCBM/P3HT heterojunction
Gentian YUE, Jihuai WU, Yaoming XIAO, Jianming LIN, Miaoliang HUANG
Front Optoelec Chin. 2011, 4 (1): 108-113.  https://doi.org/10.1007/s12200-011-0203-4
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[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) / poly (3-hexylthiophene) (P3HT) heterojunction has not only the absorption in ultraviolet light for PCBM, but also the absorption in visible light for P3HT, which widens the incident light harvest range, improving the photoelectrical response of hybrid solar cell effectively. Using conducting polymers blend heterojunction consisting of C60 derivatives PCBM and P3HT as charge carrier transferring medium to replaceI3-/I- redox electrolyte and dye, a novel flexible solar cell was fabricated in this study. The influence of PCBM/P3HT mass ratio on the photovoltaic performance of the solar cell was also studied. Under a simulated solar irradiation of 100 mW·cm-2, the flexible solar cell achieved a light-to-electric energy conversion efficiency of 1.04%, an open circuit voltage of 0.86 V, short circuit current density of 2.6 mA·cm-2 and fill factor (FF) of 0.46.

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Surface-enhanced fluorescence from copper nanoparticles on silicon nanowires
Shujuan ZHUO, Mingwang SHAO, Liang CHENG, Ronghui QUE, Dorthy Duo Duo MA, Shuit Tong LEE
Front Optoelec Chin. 2011, 4 (1): 114-120.  https://doi.org/10.1007/s12200-011-0152-y
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A method to enhance surface plasmon coupled fluorescence from copper nanoparticles on silicon nanowires is presented. Owing to resonant plasmons oscillation on the surface of Cu/Si nanostructure, the fluorescence peaks of several lanthanide ions (praseodymium ions, Pr3+, neodymium ions Nd3+, holmium ions Ho3+, and erbium ions Er3+) were markedly enhanced with the enhancement of maximal 2 orders of magnitude, which was larger than that caused by unsupported Cu nanoparticles. These results might be explained by the local field overlap originated from the closed and fixed copper nanoparticles on silicon nanowires.

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Photochemical synthesis and photocatalysis application of ZnS/amorphous carbon nanotubes composites
Zhen FANG, Yueting FAN, Yufeng LIU
Front Optoelec Chin. 2011, 4 (1): 121-127.  https://doi.org/10.1007/s12200-011-0165-6
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In this paper, a photochemical synthesis of ZnS-amorphous carbon nanotubes (ACNTs/ZnS) composites using ACNTs was reported, whose surface were modified with carboxylic groups as a support. The size and distribution of ZnS nanoparticles can be controlled by adjusting the initial amount of reactants and the reaction time. The ACNTs/ZnS nanocomposites were characterized by X-ray power diffraction, scanning electron microscopy and transmission electron microscopy. Studies showed that ACNTs/ZnS nanocomposites had high photocatalytic activity toward the photodegradation of dye molecule.

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