Enhance the Optical Absorptivity of Nanocrystalline TiO2 Film with ...

High Performance Dye-Sensitized Solar Cellsbeen partially passivated by guanidinium thiocyanate in deviceA, even with lithium ions inside. However, in device E withthe solvent-free ionic liquid electrolyte guanidinium may notpassivate titanium species very efficiently due to the extraordinarilyhigh ion strength. This also indicates that TiO 2nanocrystals are not fully covered by dye molecules and partiallyin contact with electrolytes. Furthermore, we have noted thatthe tetracyanoborate anion in ionic electrolytes does not have astrong interaction with TiO 2 , different from the behavior ofdicyanoamide used in our previous work. 21f The upliftedconduction band-edge of sensitized titania film, positive-shiftedequilibrium potential of electrolyte, and higher photocurrentdensity together explains the observed higher V oc for device Ain contrast to device E.It is known that the charge recombination at the titania/electrolyte interface depends on the thermodynamic driving forceas well as charge densities. The pseudo-first-order recombinationrate (k r ) was determined from small perturbation photovoltagetransient decays at different V oc , by adjusting output lightintensities of white light-emitting diodes. Figure 7B presentsthe semilogarithmic plot of k r versus V oc or the energy level ofphotoanodes (E versus vacuum). The linear fitting of all thedata gave a similar slope value of 11.5 ( 0.1 for both devicesA and E, indicative of a similar recombination mechanism inboth devices. This slope corresponds to ∼90 mV per decade,which is smaller than the values of 120 mV per decade obtainedfor solid-state DSCs. 22a Obviously, along with increasing V ocor up-lifting the energy levels of titania films, the recombinationrates become higher due to the higher electron densities in thetitania film as well as larger driving forces for charge recombination.If we just consider the same driving force forrecombination by looking at the plot of k r versus V oc , it is clearthat the logarithmic recombination rate ratio of device E to Ais ∼2. Note that even if there is not much difference in theDOS profiles of these two devices, at the same V oc , device Ehas a higher electron density due to uplifting the equilibriumpotential of its electrolyte relative to that of device A. Thus,we also compared the recombination rates at the same energylevels of titania films, while in this case the driving force ofrecombination is smaller for device E than A. Even though,the logarithmic k r ratio is still ∼1.9, as shown in Figure 7B.Thus, we believe that the faster recombination in the ionic liquidbased device is mainly caused by the higher triiodide concentrationof ∼0.24 M compared to that of 0.03 M in device A. Inaddition, it is possible to have an even higher local concentrationof triiodide close to photoanode in device E due to the lowfluidityof its electrolyte, enhancing the charge recombinationat the titania/electrolyte interface.We further resorted to the electrical impedance measurements23 to clarify the charge transport in devices A and E withdifferent electron densities in the titania film. As shown in Figure8A, at the same DOS, device E always shows a feature of shorterelectron lifetime, consistent with the above transient photovoltagedecay measurements. Because there is not too muchdifference in the DOS profiles of device A and E, it is surprisingto see the higher electron diffusion coefficient in device Acompared to E in terms of the trapping-detrapping electrontransport model in the titania film. Actually, this is in keeping(23) (a) Bisquert, J. J. Phys. Chem. B 2002, 106, 325. (b) Bisquert, J. Phys.Chem. Chem. Phys. 2003, 5, 5360. (c) Bisquert, J. Phys. Chem. Chem.Phys. 2008, 10, 49.Figure 8. Plots of electron lifetime, diffusion coefficient, and diffusionlength versus DOS: (a) device A; (b) device E.with the notion 24 that the diffusion coefficient is ambipolar,reflecting apart from the electron motion also the diffusion ofcations that screen the photoinjected electrons in the mesoporoustitania film. 25 Although the total concentration of cations indevice A is ∼1.15 M compared with that of ∼5.86 M in deviceE, the fluidity ratio of 184 for electrolyte A to C endows a muchhigher diffusion flux of cations in device A, efficiently speedingup the transport of electrons in the mesoporous titania film. Thelarge electron diffusion length (L n ) directly related to a highcharge collection yield is well consistent with the high J scobserved for device A.ConclusionsARTICLESIn summary, two new heteroleptic polypyridyl rutheniumcomplexes with high molar extinction coefficients have beensynthesized and demonstrated as efficient sensitizers. The opticalabsorptivities of mesoporous titania film have been enhancedwith these sensitizers by extending the π conjugated system ofancillary ligands. Shortening of the light-absorption length ofsemiconducting film endowed by these sensitizers are veryimportant to achieve a good charge collection yield for highefficiencydye-sensitized solar cells. On the basis of the newC101 sensitizer, several new DSC benchmarks under theillumination of AM 1.5G full sunlight have been reached, suchas a 11.0% efficiency along with an acetonitrile based electrolyte,a long-term stable >9% device using a low volatilityelectrolyte, and a long-term stable ∼7.4% device employingan ionic liquid electrolyte. We are now actively taking theadvantage of the C101 sensitizer and its analogues for all-solidsatedye-sensitized solar cells, where the conflict between lightabsorption length and charge diffusion length is even moreserious.Furthermore, we remark that, for the future efficiencyenhancement of DSCs with solvent-free ionic liquid electrolytesby improving the charge collection yield, it is pertinent toconsiderably reduce the charge recombination and enhance theelectron transport. This could be realized by reducing theviscosity of practical electrolyte systems, through which not onlythe diffusion coefficients of cations screening electrons in thetitania film can be improved, but also the use of low triiodideconcentration will be allowed without causing the mass transportlimit of photocurrent. Also, the smart design of a more efficientsensitizer along with rational interface engineering such as(24) Kopidakis, N.; Schiff, E. A.; Park, N.-G.; van de Lagemaat, J.; Frank,A. J. J. Phys. Chem. B 2000, 104, 3930.(25) Lanning, O. J.; Madden, P. J. Phys. Chem. B 2004, 108, 11069.J. AM. CHEM. SOC. 9 VOL. 130, NO. 32, 2008 10727