Quanyou Feng

Flexible, Light‐Weight, Ultrastrong, and Semiconductive Carbon Nanotube Fibers for a Highly Efficient Solar Cell

Carbon nanotubes have been widely introduced to fabricate high-efficiency organic solar cells because of their extremely high surface area (e.g., ca. 1600 mg 1 for single-walled nanotubes) and superior electrical properties. In one direction, nanotubes are used in electrode materials. For example, the incorporation of nanotubes onto titania nanoparticle films has been shown to increase the roughness factor and decrease the charge recombination of electron/hole pairs, and the replacement of platinum with nanotubes as counter electrode catalyzed the reduction of triiodide to improve the cell performance. In another direction, the distribution of nanotubes within the photoactive layer improved the short circuit current density and fill factor owing to rapid charge separation at the nanotube/electron donor interface and efficient electron transport through nanotubes. However, the degrees of improvement are far from what is expected for nanotubes, mainly because of random aggregation of nanotubes in the cells. For a random nanotube network, the electrons have to cross many more boundaries. Therefore, alignment of nanotubes will further greatly improve cell performance as charge transport is more efficient. Solar cells have typically been fabricated from rigid plates, which are unfavorable for many applications, especially in the fields of portable and highly integrated equipment. As a result, flexible devices have recently become the subject of active research as a good solution. In particular, weavable fiber solar cells are very promising and have attracted increasing attention in recent years. Fiber solar cells based on metal wires, glass fibers, or polymer fibers have been investigated. Herein, we first made a family of novel organic solar cells with excellent performance from the highly aligned nanotube fiber. Compared with traditional solar cells fabricated from rigid plates or recently explored flexible films/fibers, nanotube fiber solar cells demonstrate some unique and promising advantages. Firstly, as the building nanotubes are highly aligned, the fiber shows excellent electrical properties, which provide the novel solar cell with higher short-circuit photocurrent, better maximum incident monochromatic photon-to-electron conversion efficiency, and higher power conversion efficiency. Secondly, nanotube fibers show excellent mechanical properties, much stronger than Kevlar and comparable to the strongest commercial fibers of zylon and dyneema in tensile strength. Thirdly, these fibers are flexible, light-weight, and weavable and have tunable diameters ranging from micrometers to millimeters. The above properties provide nanotube fiber solar cells with a broad spectrum of applications, including power regeneration for space aircraft and clothing-integrated photovoltaics. To produce desired nanotube fibers, high-quality nanotube arrays were first synthesized by a typical chemical vapor deposition method. The synthetic details are reported elsewhere. To summarize, Fe/Al2O3 was used as the catalyst, ethylene served as the carbon source, and Ar with 6%H2 was used to carry the precursor to a tube furnace, where the growth took place. The reaction temperature was controlled at 750 8C and the reaction time was typically between 10 and 20 min. Fibers were directly spun from the nanotube array (see Figure S1 in the Supporting Information), and the fiber diameter was controlled from 6 to 20 mm by varying the initial ribbon, that is, a bunch of nanotubes pulled out of the array at the beginning of the spinning. The nanotube fiber can be spun with lengths of tens of meters or even longer, and the fiber is uniform in diameter. The density of the nanotube fiber was calculated to be on the order of 1 gcm , and its linear density was on the order of 10 mgm , relative to 10 mgm 1 and 20– 100 mgm 1 for cotton and wool yarns, respectively. As shown in Figure 1a, the nanotube fiber is flexible and will not break after being bent, folded, or even tied many times. Highresolution transmission electron microscopy (see Figure 1b) [*] T. Chen, S. Wang, Z. Yang, Q. Feng, X. Sun, Dr. L. Li, Prof. Z.-S. Wang, Prof. H. Peng Laboratory of Advanced Materials, Fudan University Shanghai 200438 (China) E-mail: zs.wang@fudan.edu.cn penghs@fudan.edu.cn T. Chen, Z. Yang, X. Sun, Dr. L. Li, Prof. H. Peng Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Fudan University (China) T. Chen, Z. Yang, X. Sun, Dr. L. Li, Prof. H. Peng Department of Macromolecular Science, Fudan University (China) S. Wang, Prof. Z.-S. Wang Department of Chemistry, Fudan University (China) [] These authors contributed equally to this work.

Gold nanoparticles inlaid TiO2 photoanodes: a superior candidate for high-efficiency dye-sensitized solar cells

By designing a fine-controlled nanocomposite with Au nanoparticles (∼2 nm in size) directly inlaid in TiO2 as working electrode, efficiency (η) of 10.1% for a dye-sensitized solar cell with an open-circuit photovoltage of 863 mV and a short-circuit photocurrent of 15.71 mA cm−2 have been achieved, giving an enhancement of 97 mV in photovoltage, 63% in photocurrent and 84% in efficiency compared to the cell with pure TiO2 photoanode (η = 5.5%). As compared to pure TiO2, besides the local-field optical enhancement near the TiO2 surface caused by plasma resonance of Au nanoparticles which increases the dye absorption and hence the amount of photogenerated charge contributing to the photocurrent, it is evidenced that not only the quasi-Fermi level of Au–TiO2 photoanode can be modulated to more negative potentials by controlling the mass ratio of Au–TiO2, but their mosaic nanostructure reduces the charge recombination rate effectively, both leading to a marked enhancement of photovoltage. Our results prove that the unique nanostructural, physical and chemical properties of the direct mosaic nanoarchitecture of ∼2 nm Au and TiO2 make them valuable materials as working electrodes for DSSCs to achieve high photovoltage and hence further improve the performance.

In situ growth of oriented polyaniline nanowires array for efficient cathode of Co(III)/Co(II) mediated dye-sensitized solar cell

To improve the electrocatalytic performance of polyaniline thin films, an oriented polyaniline nanowires array has been successfully grown in situ on conductive glass substrates without templates and applied as the cathode of dye-sensitized solar cells (DSSCs) mediated with a Co(bpy)33+/2+ (bpy = 2,2′-bipyridine) redox couple. Compared to the polyaniline film with a random network, the oriented polyaniline nanowires array exhibits much better electrocatalytic performance, and even outperforms the typical Pt electrode in both electrocatalytic performance and electrochemical stability when exposed to the acetonitrile solution of the Co(bpy)33+/2+ redox couple. Owing to the higher electrocatalytic performance, the DSSC with the oriented nanowires array produces a higher short-circuit photocurrent and fill factor than the DSSCs with the random polyaniline network or Pt cathodes. Consequently, the power conversion efficiency of DSSCs based on a typical D–π–A organic dye sensitizer increases from 5.97% for the polyaniline random network cathode to 8.24% for the oriented polyaniline nanowires array cathode, which is also higher than the efficiency (6.78%) of the DSSC with the Pt cathode.

Near infrared thieno[3,4-b]pyrazine sensitizers for efficient quasi-solid-state dye-sensitized solar cells

Three near infrared (NIR) metal-free organic sensitizers (FNE32, FNE34, FNE36) based on the thieno[3,4-b]pyrazine derivative have been designed and synthesized for application in quasi-solid-state dye-sensitized solar cells (DSSCs). These organic dyes demonstrate maximum absorption bands at 596-625 nm due to the presence of the thieno[3,4-b]pyrazine derivative, which facilitates the intramolecular electron transfer from the donor to the acceptor. Quasi-solid-state DSSCs based on FNE34 display efficient photoelectric conversion over the whole visible range extending into the NIR region up to 900 nm with maximum incident monochromatic photon-to-electron conversion efficiency (IPCE) of 77%, yielding a short-circuit photocurrent density of 16.24 mA cm(-2) and a power conversion efficiency of 5.30%. To the best of our knowledge, this is the highest efficiency for quasi-solid-state DSSCs based on an organic NIR dye. When exposed to one-sun illumination for 1000 h, the quasi-solid-state DSSC based on FNE34 exhibits good long-term stability with almost constant power conversion efficiency.

Enhanced performance of quasi-solid-state dye-sensitized solar cells by branching the linear substituent in sensitizers based on thieno[3,4-c]pyrrole-4,6-dione.

Thieno[3,4-c]pyrrole-4,6-dione-based organic sensitizers with triphenylamine (FNE38 and FNE40) or julolidine (FNE39 and FNE41) as electron-donating unit have been designed and synthesized. A linear hexyl group or a branched alkyl chain, the 2-ethylhexyl group, is incorporated into molecular skeleton of the dyes to minimize intermolecular interactions. The absorption, electrochemical, and photovoltaic properties for these sensitizers were then systematically investigated. It is found that the sensitizers have similar photophysical and electrochemical properties, such as absorption spectra and energy levels, owing to their close chemical structures. However, the quasi-solid-state dye-sensitized solar cells (DSSCs) based on the two types of sensitizers exhibit very different performance parameters. Upon the incorporation of the short ethyl group on the hexyl moiety, enhancements in both open-circuit voltage (V(oc)) and short-circuit current (J(sc)) are achieved for the quasi-solid-state DSSCs. The V(oc) gains originating from the suppression of charge recombination were quantitatively investigated and are in good agreement with the experimentally observed V(oc) enhancements. Therefore, an enhanced solar energy conversion efficiency (η) of 6.16%, constituting an increase by 23%, is achieved under standard AM 1.5 sunlight without the use of coadsorbant agents for the quasi-solid-state DSSC based on sensitizer FNE40, which bears the branched alkyl group, in comparison with that based on FNE38 carrying the linear alkyl group. This work presents a design concept for considering the crucial importance of the branched alkyl substituent in novel metal-free organic sensitizers.

Facile and Selective Synthesis of Oligothiophene-Based Sensitizer Isomers: An Approach toward Efficient Dye-Sensitized Solar Cells

Two sets of isomeric organic dyes with n-hexyl (DH and AH) or 2-ethylhexyl (DEH and AEH) groups substituted at the spacer part have been designed and straightforwardly synthesized via a facile and selective synthetic route. The structure difference between the isomers stands at the position of the incorporated alkyl chains which are introduced into the terthiophene spacer close to the donor (D) or anchor (A) side. The relationship between the isomeric structures and the optoelectronic properties are systematically investigated. It is found that, in the D series dyes, the alkyl group is much closer to the aromatic donor moiety, which brings about strong steric hindrance and therefore causes a remarkable twist in the molecular skeleton. In contrast, a more planar chemical structure and more effective π-conjugation are realized in the A series dye isomers. Consequently, the A series isomeric dyes demonstrate bathochromically shifted absorption bands, resulting in the improved light-harvesting capability and enhanced photo-generated current. However, the D series isomeric dyes with more twisted molecular skeleton have suppressed the intermolecular interactions and retarded the charge recombination more efficiently, which induces higher open-circuit photovoltage. Combining the two effects on the performance of the fabricated dye-sensitized solar cells (DSSC), the influence from the short-circuit photocurrent plays a more significant role on the power conversion efficiency (η). As a result, isomer AEH-based DSSC with quasi-solid-state electrolyte displays the highest η of 7.10% which remained at 98% of the initial value after continuous light soaking for 1000 h. Promisingly, a η of 8.66% has been achieved for AEH-based DSSC with liquid electrolyte containing Co(II)/(III) redox couple. This work presents the crucial issue of molecular engineering and paves a way to design organic sensitizers for highly efficient and stable DSSCs.

Novel Thiazolo[5,4‐d]thiazole‐Based Organic Dyes for Quasi‐Solid‐State Dye‐Sensitized Solar Cells

A series of novel metal-free organic dyes containing the thiazolo[5,4-d]thiazole moiety were designed and synthesized for quasi-solid-state dye-sensitized solar cells (DSSCs). Different alkoxy chains were introduced into the electron donor part of the dye molecules for comparison. The optical, electrochemical, and photovoltaic properties for all sensitizers were systematically investigated. It was found that the sensitizers with the different alkoxy groups have similar photophysical and electrochemical properties, such as absorbance and energy levels, owing to their close chemical structures. However, the quasi-solid-state DSSCs based on the resulting sensitizers exhibit different performance parameters. The quasi-solid-state DSSC based on sensitizer FNE74 with two octyloxy chains possessed the highest solar energy conversion efficiency of 5.10 % under standard AM 1.5G sunlight illumination without the use of coadsorbant agents.

Reduced graphene oxide–Ta3N5 composite: a potential cathode for efficient Co(bpy)33+/2+ mediated dye-sensitized solar cells

Polypyridine complexes of Co(II)/Co(III) coupled with donor–π bridge–acceptor structured organic dyes are by far the most promising route to boost the efficiency of dye-sensitized solar cells (DSSCs). We report herein the first investigation of Ta3N5 nanorods incorporated on reduced graphene oxide (RGO) sheets as a counter electrode for application in Co(bpy)33+/2+ (bpy = 2,2′-bipyridine) mediated DSSCs. RGO–Ta3N5 composite was fabricated by mixing graphene oxide (GO) with pre-synthesized Ta3N5 nanorods (aspect ratio of ∼3) followed by facile hydrazine hydrate reduction. The composite film on conductive glass was prepared by a drop-casting process at room temperature without heat treatment. Compared with a Pt cathode with a power conversion efficiency of 7.59%, RGO–Ta3N5 exhibited comparable electrocatalytic performance for the reduction of Co(bpy)33+ species and better electrocatalytic stability in Co(bpy)33+/2+ acetonitrile solution, which was attributed to the synergetic catalytic effect of RGO and Ta3N5 nanorods and the high electrical conductivity derived from the RGO network, resulting in a power conversion efficiency of 7.85%. The present result is expected to lead to a family of composites for highly efficient cathode materials.