Letter pubs.acs.org/JPCL
Boosting the Photovoltage of Dye-Sensitized Solar Cells with Thiolated Gold Nanoclusters Hyunbong Choi, Yong-Siou Chen, Kevin G. Stamplecoskie, and Prashant V. Kamat* Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *
ABSTRACT: Glutathione-capped gold nanoclusters (Aux-GSH NCs) are anchored along with a sensitizing squaraine dye on a TiO2 surface to evaluate the cosensitizing role of Aux-GSH NCs in dye-sensitized solar cells (DSSCs). Photoelectrochemical measurements show an increase in the photoconversion efficiency of DSSCs when both sensitizers are present. The observed photoelectrochemical improvements in cosensitized DSSCs are more than additive effects as evident from the increase in photovoltage (ΔV as high as 0.24 V) when Aux-GSH NCs are present. Electron equilibration and accumulation within gold nanoclusters increase the quasi-Fermi level of TiO2 closer to the conduction band and thus decrease the photovoltage penalty. A similar beneficial role of gold nanoclusters toward boosting the Voc and enhancing the efficiency of Ru(II) polypyridyl complex-sensitized solar cells is also discussed.
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revealed ligand-to-metal type transitions with an excited state lifetime of 780 ns.40 One approach to broaden the photoresponse of a solar cells is to employ two or more sensitizers with different spectral responses.41−45 For example, organic dyes such as zinc phthalocyanine and triarylamine-bithiophene-cyanoacrylate, Dπ-A based organic dyes, which are complementary in their spectral responses, are employed to fabricate a panchromatic solar cell.42 However, these efforts have been hindered by low open-circuit voltages as these cells adopt the potential of the component with the lowest open-circuit voltage.41−43 The loss in photovoltage as compared with the theoretically expected value (referred commonly as photovoltage penalty) arises from the poor accumulation of electrons at the photoanode stemming from the losses due to back electron transfer reactions. Rational design of coupling organic dyes with semiconducting QDs in a cosensitized solar cell has led to some synergistic effects.44,45 For example, the squaraine dye coupled to semiconductor QDs exhibits a dual role in extending the spectral response of QDSCs into the near-IR region and promoting hole capture processes.44,45 In the present study, we have employed glutathione-capped gold nanoclusters (Aux-GSH NCs) as a cosensitizer in DSSCs employing a squaraine (SQ) dye. Aux-GSH NCs and the SQ dye selectively absorb in the spectral region below 500 nm and between 550 and 800 nm, respectively, and thus enable the broadening of the overall photoresponse of the DSSCs. Photoelectrochemical measurements that elucidate the role of
ensitization of semiconductor nanostructures with dyes is attractive for capturing a broader range of visible−near-IR photons and improving the performance of solar energy conversion devices.1−3 Synthetic approaches such as structural modification, surface treatment of mesoscopic oxide films, and use of cosensitizers have been attempted to improve the performance of dye-sensitized solar cells (DSSCs). For example, structural manipulation of ligands of Ru(II) bipyridyl complexes is found to be quite effective for tailoring the absorptivity and excited-state properties of the sensitizing molecule.4−6 Semiconductor quantum dots (QDs) are another class of sensitizers that are widely used in liquid junction and solid-state solar cells.2,7−9 Of particular interest are metal chalcogenides such as CdS, CdSe, PbS, and Sb2S3. These materials have high extinction coefficients, and their photophysical properties are tunable through size,10,11 shape,12,13 and surface modifications.14,15 By engineering the band alignments of PbS QDs layers in quantum dot solar cells (QDSCs), power conversion efficiency as high as 8.55% has been reported.16 Thiolated gold nanoclusters are a new class of photosensitizers, which are quite effective in the operation of DSSCs and photocatalytic generation of H2 in a photoelectrolysis cell.17−19 Glutathione-capped gold nanoclusters, which consist of a few metal atoms in the core and a thiolated shell, exhibit molecule-like properties with discrete electronic transitions. The photophysical properties of these nanoclusters are strongly correlated to their size,20−24 ligand,25,26 and composition.27−30 Applications of such metal clusters in catalysis,31−34 sensing,35−37 and biolabeling and imaging38,39 have been reported. Glutathione-capped gold nanoclusters were able to inject electrons into mesoscopic TiO2 films with a relatively high photon to charge carrier generation efficiency (IPCE) and deliver power conversion efficiency of 2%.17 The excited state, as characterized from transient absorption spectroscopy, © XXXX American Chemical Society
Received: November 25, 2014 Accepted: December 22, 2014
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DOI: 10.1021/jz502485w J. Phys. Chem. Lett. 2015, 6, 217−223
Letter
The Journal of Physical Chemistry Letters
Figure 1. (A) Absorption and (B) IPCE spectra recorded using (a) Aux-GSH, (b) SQ dye, and (c) Aux-GSH + SQ dye-sensitized TiO2 photoanode. Note that nonzero absorbance above 520 nm in trace a is due to light scattering by the TiO2 layer.
electrode in a sandwich configuration with a 50 μm hot-melt ionomer film (Surlyn SX 1170−25, Solaronix) as a spacer. A redox electrolyte of [Co(III)(bpy)3](PF6)3/[Co(II)(bpy)3](PF6)2 was introduced between the two electrodes. Because Aux-GSH undergoes oxidation in I3−/I− redox electrolyte, we employed the common Co(III)/Co(II)-based redox system. Trace a in Figure 1B shows the incident photon to chargecarrier generation efficiency (IPCE) response of solar cells employing sensitized TiO2 photoanodes. The IPCE of the photoelectrochemical cell employing Aux-GSH NCs sensitized TiO2 electrode shows photocurrent response below 520 nm, in agreement with the absorption feature seen in Figure 1A (trace a). The maximum IPCE observed for this solar cell is ∼65% in the region of 350−425 nm. The TiO2 photoanode consisting of both SQ dye and Aux-GSH NCs as sensitizers shows the characteristic photoresponse in the red (550−800 nm) and blue (
