A Quasi-Solid State DSSC with 10.1% Efficiency through Molecular Design of the Charge-Separation and -Transport

Organic-based solar cells potentially offer a photovoltaic module with low production costs and low hazard risk of the components. We report organic dye-sensitized solar cells, fabricated with molecular designed indoline dyes in conjunction with highly reactive but robust nitroxide radical molecules as redox mediator in a quasi-solid gel form of the electrolyte. The cells achieve conversion efficiencies of 10.1% at 1 sun, and maintain the output performance even under interior lighting. The indoline dyes, customized by introducing long alkyl chains, specifically interact with the radical mediator to suppress a charge-recombination process at the dye interface. The radical mediator also facilitates the charge-transport with remarkably high electron self-exchange rate even in the quasi-solid state electrolyte to lead to a high fill factor.


Additoinal Notes
Preparations of the gel electrolyte: The polymer gel electrolytes were prepared with 1.5 M TEMPO, 0.025 M NOBF4, 0-1.0 M tributylphosphate, and 1.2 M lithium bis(trifluoromethanesulfonyl)imide dissolved in the acetonitrile solution of 10 wt% poly(vinylidene fluoride-co-hexafluoropropylene) (Mw ca. 4×10 5 , Sigma-Aldrich Co.). The mixtures were heated at 80 °C for 6 hr to form the homogeneous solutions and then cooled down to r.t. for 3 hr to get the gels. The electrolytes for the experiment under interior lighting were prepared by replacing acetonitrile with 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Tokyo Chemical Industry Co.) as the solvent.

Durability of MD-153:
The new indoline MD-dyes were highly durable upon the photoredox processes. The reduction peak in cyclic voltammograms (ascribed to the radical cation state of the indoline dyes) was clearly observed in the repeated scanning.
Durability of the quasi-solid state gel electrolyte: The gel electrolytes were examined in stability or durability tests. Sandwiching the gel film with a pair of filter papers to squeeze out the electrolyte resulted in no significant mass decrease in the gel film and mass increase in the papers. The filter papers were subjected to ESR; the silent ESR supported that TEMPO was held in the gel. The gel electrolyte films were exposed to open air for 1 week: Around 20 wt% was lost for the gel film, but 96 wt% was lost for the ungelated film. The gel was composed of the imidazolium electrolyte, and none of the sealed cells exhibited any weight loss in the tests.

Materials and Methods
Materials: All starting reagents and solvents were purchased from commercial suppliers and used as received. A silicon photodiode was used for the light intensity correction. The Keithley source meter (used in the J-V characterization) was utilized to measure the incident photon-to-collected electron conversion efficiency.
ESR spectra were recorded using a JEOL JES-TE200 spectrometer. The field modulation frequency and width were 100 kHz and 0.1 mT, respectively. 1 H NMR spectra were obtained using a Ultra Shield 600 MHz (Bruker Co.) spectrometer with chemical shifts downfield from tetramethylsilane as an internal standard. UV-Vis data for the dye solution were acquired with a F7000 UV-Vis spectrometer (Hitachi Co.).
QCM data were recorded using a crystal oscillator chemical measurement system (Seiko Co. QCA922) and an ALS 660DX electrochemical analyzer (BAS Inc.) with the fundamental frequency of the QCM (9.0 MHz), a platinum-sputtered titanium oxide QCM electrode (the area 0.196 cm 2 ), and the shear modulus of quartz (2.95×10 11 dyn cm -2 ). The increment of the mass Δm accompanied by the reduction was determined by employing Sauerbrey's equation.
Fluorescence quenching measurement: Fluorescence spectra were measured using a spectrofluorophotometer (Hitachi Co., F-7000) with a 150 W xenon lamp as the excitation light source. Fluorescence quenching parameters between the dyes and TEMPO were estimated from the following equation: where Ksv is Stern-Volmer-equilibrium constant, I0 is initial fluorescence intensity, I is fluorescence intensity, Ksv is Stern-Volmer constant, kq is quenching rate constant, and t0 is fluorescence lifetime. t0 of the dyes was estimated to be 965 and 691 psec for MD-153 and D205, respectively, using a compact fluorescence lifetime spectrometer (Hamamatsu Photonics Co., C11367) upon excitation laser irradiation at 506 nm.  The λ max and ε values were obtained from UV-vis spectra of 0.1 mM dye DMF solution. The HOMO level was estimated by CV. The LUMO level was calculated from the HOMO level and the absorption edge wavelength of UV-vis spectra.

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The dyes are reductively quenched with TEMPO or iodide. The parameters were estimated with the data in Fig. 3 with Stern-Volmer equation (linearly extrapolated for the MD-153 in the range of low TEMPO concentration 0-0.1 mM). (1400)