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Energy and Electron Transfer Cascade in SelfAssembled Bilayer Dye-Sensitized Solar Cells Omotola Olukemi Ogunsolu, Ian A Murphy, Jamie C Wang, Anjan Das, and Kenneth Hanson ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09955 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016
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ACS Applied Materials & Interfaces
Energy and Electron Transfer Cascade in SelfAssembled Bilayer Dye-Sensitized Solar Cells Omotola O. Ogunsolu,a† Ian A. Murphy,b† Jamie C. Wang,b Anjan Das,b and Kenneth Hansona,b* a
Materials Science and Engineering, Florida State University, Tallahassee, Florida 32306, United States b Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States Keywords: self-assembly, energy transfer, electron transfer, bilayer, DSSC
ABSTRACT Current high efficiency dye-sensitized solar cells (DSSCs) rely on the incorporation of multiple chromophores, via either co-deposition or pre-formed assemblies, as a means of increasing broad band light absorption. These strategies have some inherent limitations including decreased total light absorption by each of the dyes, low surface loadings, and complex synthetic procedures. In this report, we introduce an alternative strategy, self-assembled bilayers, as a simple, step-wise method of incorporating two complementary chromophores into a DSSC. The bilayer devices exhibit a 10% increase in Jsc, Voc and η over the monolayer devices due to increased incident photon-to-electron conversion efficiency across the entire visible spectrum and slowed recombination losses at the interface. Directional energy and electron transfer towards the metal oxide surface are key steps in the bilayer photon-to-current generation process. These results are important as they open the door to a new architecture for harnessing broad band light in dyesensitized devices. ACS Paragon Plus Environment
ACS Applied Materials & Interfaces
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INTRODUCTION Dye-sensitized solar cells (DSSCs) are a promising alternative to traditional silicon solar cells due to their ease of fabrication and lower manufacturing costs.1, 2 In DSSCs, the crucial first step in efficient solar energy conversion is to maximize light absorption by the surface bound dye molecule.2,
3
Unfortunately, unlike inorganic crystalline materials which have broad band
absorption features,4 molecules typically exhibit relatively narrow absorption transitions resulting in transmission losses and decreased efficiencies.5 One strategy to fill these absorption gaps is to incorporate, either by co-deposition6,
7
or in pre-formed assemblies,8 two or more
molecules with complementary absorption spectra onto the metal oxide surfaces. The highest efficiency DSSCs yet reported (>13%) utilize co-deposition of two complementary dyes.9,
10
This strategy has the advantage of absorbing a larger portion of the
solar spectrum but, due to limited surface area at a given film thickness, it decreases the total absorbance of each chromophore. The decreased surface area can be mitigated by increasing the film thickness, but thicker films are detrimental because 1) they increase the likelihood of deleterious recombination events,11,
12
and 2) they slow diffusion of redox mediators into the
porous films.13, 14 Alternatively, molecular assemblies composed of two dyes and a surface binding group can be prepared prior to film loading.8,
15-18
This strategy has received less attention than co-
deposition and limited success presumably because these assemblies typically require a complex, multistep synthesis with cumulatively low yields and low surface loadings (
