Hoi Nok Tsao

Porphyrin-Sensitized Solar Cells with Cobalt (II/III)–Based Redox Electrolyte Exceed 12 Percent Efficiency

Simultaneous modification of the dye and redox shuttle boosts the efficiency of a dye-sensitized solar cell. The iodide/triiodide redox shuttle has limited the efficiencies accessible in dye-sensitized solar cells. Here, we report mesoscopic solar cells that incorporate a Co(II/III)tris(bipyridyl)–based redox electrolyte in conjunction with a custom synthesized donor-π-bridge-acceptor zinc porphyrin dye as sensitizer (designated YD2-o-C8). The specific molecular design of YD2-o-C8 greatly retards the rate of interfacial back electron transfer from the conduction band of the nanocrystalline titanium dioxide film to the oxidized cobalt mediator, which enables attainment of strikingly high photovoltages approaching 1 volt. Because the YD2-o-C8 porphyrin harvests sunlight across the visible spectrum, large photocurrents are generated. Cosensitization of YD2-o-C8 with another organic dye further enhances the performance of the device, leading to a measured power conversion efficiency of 12.3% under simulated air mass 1.5 global sunlight.

Cyclopentadithiophene Bridged Donor-Acceptor Dyes Achieve High Power Conversion Efficiencies in Dye-Sensitized Solar Cells Based on the tris-Cobalt Bipyridine Redox Couple

Keywords: cobalt ; dye-sensitized solar cells ; electrochemistry ; photovoltaics ; sensitizers ; Photovoltaic Cells ; Transport ; Recombination ; Electrolyte ; Performance ; Mediators Reference EPFL-ARTICLE-166673doi:10.1002/cssc.201100120View record in Web of Science Record created on 2011-06-08, modified on 2017-05-12

Tailoring Structure−Property Relationships in Dithienosilole−Benzothiadiazole Donor−Acceptor Copolymers

Four new DTS-BTD copolymers (P1-P4) differing by the concentration of electron-donating and -withdrawing substituents along the backbone have been synthesized and characterized by 2D-WAXS and in bottom-contact FETs. While all copolymers can self-assemble into lamellar superstructures, only P2 and P4 show a propensity to pi-stack. P4 exhibits a hole mobility as high as 0.02 cm(2) V(-1) s(-1) in excellent agreement with the close pi-stacking and lamellar distances found by structural analysis (0.36 and 1.84 nm, respectively) and absorbs homogenously across the entire visible spectrum as solar cell applications require.

Influence of the interfacial charge-transfer resistance at the counter electrode in dye-sensitized solar cells employing cobalt redox shuttles

We highlight the effect of the interfacial charge-transfer resistance at the counter electrode in dye-sensitized solar cells based on two cobalt redox shuttles, namely cobalt(III/II) tris(2,2′-bipyridine) and cobalt(III/II) tris(1,10-phenanthroline). Highly porous counter electrodes based on poly(3,4-ethylenedioxythiophene) (PEDOT) prepared by electro-oxidative polymerization are compared to the typically employed platinized FTO glass, with the former showing much lower charge transfer resistances for both cobalt complexes, leading to improved fill factors and to linear response of the short circuit photo-current density to light intensity up to one sun. Based on these findings, an excellent power conversion efficiency of 10.3% was achieved with a recently reported organic sensitizer and PEDOT as counter electrode.

From Ambi‐ to Unipolar Behavior in Discotic Dye Field‐Effect Transistors

Ambipolar, solution-processed thin-film transistors based on a discotic dye turn into unipolar behavior after thermal annealing. No evidence for temperature-induced change in injection barrier or interface trapping can be found to explain this phenomenon. Instead, a variation in morphology is considered as the cause for the observed transition from ambipolar to unipolar charge transport.

Highly Stable Dye-Sensitized Solar Cells Based on Novel 1,2,3-Triazolium Ionic Liquids

We describe the design and synthesis of novel low viscosity bicyclic 1,2,3-triazolium ionic liquids. These new salts are applied as nonvolatile electrolytes in dye-sensitized solar cells, affording efficiencies up to 7.07% at low light intensities, and 6.00% when illuminated at 100 mW cm(-2). The devices are highly stable, retaining ca. 90% of their initial performance even after 1000 h of sun testing at 60 °C. The results obtained with these new ionic liquids compare very favorably to benchmark ionic liquid-based devices and illustrate the potential of the triazolium family of salts to compete with their imidazolium counterparts.

Extrinsic corrugation-assisted mechanical exfoliation of monolayer graphene.

Figure 1 . a) The optical microscopy (OM) image of ≈ 100 nm thick graphite fl akes on silicon substrate after thermal annealing at 350 ° C for 2 h. b) The OM image of the same position after the reverse exfoliation [∗] S. Pang , Dr. H. N. Tsao , Dr. Y. Hernandez , Dr. X. Feng , Prof. K. Mullen Max Planck Institute for Polymer Research Ackermannweg 10, D-55128 Mainz (Germany) E-mail: feng@mpip-mainz.mpg.de; muellen@mpip-mainz.mpg.de J. M. Englert , Prof. A. Hirsch Zentralinstitut fur Neue Materialien und Prozesstechnik Dr.-Mack Str. 81 D-90762 Furth (Germany) Since the reports of the fi rst isolation and observation of the exceptional electronic, mechanical, and chemical properties, single layer graphene has attracted intense interest from both academic and industrial communities. [ 1–3 ] While the mechanical exfoliation method led to many exciting discoveries, several promising approaches have been reported for the preparation of monolayer graphene including solution exfoliation of graphite, [ 4 , 5 ] epitaxial growth from SiC, [ 6 , 7 ] reduction of graphene oxide, [ 8–10 ] and chemical vapor deposition (CVD) on metal surfaces. [ 11–13 ] Up to now, the CVD growth method seems to be the most promising technique for the production of large-scale few layer graphene fi lms. [ 13 , 14 ] However, this surface-mediated process requires very high temperatures, and tedious additional steps involving etching and transfer, thus rendering the production of graphene-based electronic devices diffi cult. Ultra-large monolayer graphene [ 15–17 ] and patterned graphene structures [ 18–22 ] constitute two important aspects for graphene fabrication technology. A simple way to directly “print” a high-quality graphene monolayer on insulating substrates from a graphite stamp would be particularly appealing for electronic applications. Covalent immobilization [ 23 , 24 ] and electrostatic forces [ 18 , 25 ] have been devised for the modifi cation of mechanical exfoliation, with the aim to improve the yield of monolayer graphene or to achieve patterned graphene structures. Despite the success in deposition of graphene patterns over a large area, these procedures suffer from a low monolayer yield ( < 10%). [ 18 , 19 , 26 ] Here, we describe an extrinsic corrugationassisted mechanical exfoliation (ECAME) for synthesizing monolayer graphene on substrates. This strategy involves a simple thermal treatment of deposited graphite on a silicon wafer in association with a wafer processing tape peeling process. This work reveals that underlayer graphene sheets can corrugate following the rough SiO 2 surface when the thick graphite fl ake is thermally annealed. Such a surface-mediated extrinsic corrugation process thus serves as the key driving force for exfoliation leading to more than 60% high-quality monolayer graphene. This protocol can be further employed to fabricate graphene patterns on the surface, a technique that may be explored for graphene-based electronic device fabrication.

Enhancing the Stability of Porphyrin Dye‐Sensitized Solar Cells by Manipulation of Electrolyte Additives

The use of porphyrin-based photosensitizers with superior light-harvesting properties has enabled the power conversion efficiency of dye-sensitized solar cells (DSCs) to reach 13 % under full sun illumination. However, a major limitation of such devices corresponds to the volatility of the solvent used so far for the electrolyte, which prevents practical applications. In this work, we describe a porphyrin-ionic liquid DSC, which not only affords the highest efficiency reported to date, but is also stable for more than 300 h under continuous full sun illumination at 60 °C. Furthermore, we identify a previously unreported pathway for device degradation, and show that the addition of N-methylbenzimidazole and a thiocyanate salt to the electrolyte is critical to obtaining long-lived devices.