Published: October 27, 2011 r2011 American Chemical Society 25125 dx.doi.org/10.1021/jp2069946 | J. Phys. Chem. C 2011, 115, 25125–25131 ARTICLE pubs.acs.org/JPCC Photoinduced Optical Transparency in Dye-Sensitized Solar Cells Containing Graphene Nanoribbons Josef A. Velten, † Javier Carretero-Gonz  alez, †,‡ Elizabeth Castillo-Martínez, †,‡ Julia Bykova, † Alex Cook, † Ray Baughman, † and Anvar Zakhidov* ,† † The Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, Texas 75083, United States ‡ CIC energigune, Alava Technology Park, Albert Einstein 48—ED, CIC 01510 Mi~ nano,  Alava, Spain 1. INTRODUCTION Dye-sensitized solar cells (DSSCs) are a type of nano- structured solar cell based on a photoelectrochemical reaction by charge injection from an excited dye to a photoelectrode. Their basic components incorporate nanostructures of TiO 2 , a photo- sensitive dye, a redox couple for charge mediation, and a platinized counter electrode. 1 The highest reported power conversion eff- iciencies are for DSSCs that use the iodine/triiodide (I  /I 3  ) redox couple, which absorbs some of the useful light in the visible range (390750 nm), thereby shading the photoactive layer. 2 This shading may become a major issue for optically inverted 3 and tandem DSSCs 4 in which light must pass through the electrolyte to reach the photoelectrode. Alternative chemistries for electrolytes that are more transparent have been devised, 57 but these more transparent electrolytes have lower power con- version efficiencies than the standard I  /I 3  redox couple. We have observed that we can induce a reversible increase of transparency of the I  /I 3  -based electrolyte by using few- layered graphene nanoribbons (GNRs) in the DSSC. This is surprising, since there have been no reports of similar photo- bleaching induced by other tested carbon nanostructures, such as single-walled carbon nanotubes, 8 multiwalled carbon nanotubes, 9 graphene flakes, 10 and carbon black. 11 In this paper we report reversible photoinduced modification of the absorbance spectrum for an electrolyte utilizing an I  /I 3  redox couple when using thin films of a chemically and thermally reduced high-aspect-ratio GNR as a counter electrode and as an additive to the electrolyte. When applied as a counter electrode, this nonoptimized device exhibits an efficiency of over 5.8%, in addition to the photoinduced transparency of the electrolyte. In fact, the photobleaching even slightly enhances the fill factor at the expense of slightly decreased photocurrent, leaving perfor- mance nearly unchanged. This effect was tested in an optically inverted setup, where the DSSC geometry is set up in which photons enter though a transparent cathode and must travel through the electrolyte to reach a titania/dye photoelectrode, which is usually coated on an opaque substrate as a cost-savings measure. Few-layer graphene nanoribbons were then used as an additive to the electrolyte in an optically inverted setup, and the photocurrent improved by ∼2.2 mA/cm 2 , an increase of over 21%. This ability to change the electrolyte’s transparency by this photobleaching effect is very useful for inverted DSSCs, as any strong deviations of the iodine concentrations from the standard electrolyte will result in much poorer photovoltaic performance. 12,13 2. EXPERIMENTAL SECTION Synthesis of Few-Layer Graphene Nanoribbons. Vertically aligned multiwalled carbon nanotube (carbon MWNT) forests were produced by catalytic chemical vapor deposition (CVD) on Received: July 21, 2011 Revised: October 17, 2011 ABSTRACT: We describe the use of few-layer graphene nanoribbons, either attached to counter electrodes or dispersed into electrolyte, to induce optical transparency of an iodide/triiodide redox couple in a dye-sensitized solar cell (DSSC). We then evaluate the effect of reversible bleaching of the electrolyte on the DSSC performance. This bleaching effect is related to an energy transfer from photoexcited quantum-dot-like regions to the triiodide (I 3  ) radical ions in the electrolyte, saturating their absorption in the visible optical range. DSSC power conversion efficiency using few-layer graphene nanoribbons at the counter electrode (5.8%) did not deteriorate when the electrolyte became optically transparent. The increased transparency of the electrolyte resulted in a decreased photocurrent density (from 17.6 to 14.2 mA/cm 2 ), an unchanged open circuit voltage of 750 mV, and a slightly increased fill factor (from 0.45 to 0.55). When the few-layer graphene nanoribbons were introduced into the electrolyte directly by ultrasonication, a semitransparent DSSC was found to have increased its power conversion efficiency in an optically inverted setup from 5.75% to 7.01%, arising from an increase in photocurrent from 9.9 to 12.1 mA/cm 2 . This significant photocurrent increase demonstrates that the effect of electrolyte bleaching can be used for further improving power conversion efficiency for inverted and tandem DSSCs, in which light has to pass through the electrolyte to generate photocurrent on one or more photocells.