Evaluating Charge Transport in Light Harvesting, Metal-Organic Frameworks (MOFs) Thin Films via Wall-jet Voltammetry

Mina Guli^a, Benjamin M Klahr^a, Monica C So^a, M Hassan Beyzavi^a, Omar K Farha^a^,^b, Joseph T Hupp^a^,^c

^aNorthwestern University, Department of Chemistry and International Institute for Nanotechnology, Sheridan Road, 2145, Evanston, United States

^bDepartment of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah

^cMaterials Science Division, Argonne National Laboratory, South Cass Avenue, 9700, Lemont, United States

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Mina Guli, 365
Publication date: 5th February 2015

Spatially oriented, porphyrin-based, light harvesting MOF thin films were grown on transparent conductive substrates by via an automated layer-by-layer (LbL) technique that offers near-molecular-scale control over MOF thickness.^{[1, 2]} Our goal was in to examine their electrochemical response, especially their ability to accomplish directional charge transport relevant to solar energy conversion by capitalizing on the material’s characteristic long-range order. All the porphyrins examined are oriented perpendicular to the substrate in 2D sheets connected with 1,4-diazabicyclo[2.2.2]octane molecules. The oriented porphyrins proved to be fully oxidatively electrochemically accessible, as corroborated by film thickness and absorption measurements. Hopping-type charge-transport through films, and charge transfer between individual porphyrins, was probed using potential-step chronoamperometry and wall-jet electrochemistry. Wall-jet ^[3] alters the flow of a redox couple toward the electrode surface, thereby modulating the contributions of hydrodynamics to observed steady-state currents. Extrapolation to infinite solution flow rate permits the hydrodynamic contributions to be removed and the contributions due to intra-film charge hopping to be isolated and quantified. We have thus far demonstrated that MOF films can be grown in an oriented fashion where all redox centers are able to be electrochemically accessed - an important requirement for energy conversion and catalysis schemes. This work represents a promising starting point for preparing and studying oriented and, potentially electrocatalytic,MOF thin films as candidate components for molecule-based solar cells.

[1] Shekhah, O.; Wang, H.; Paradinas, M.; Ocal, C.; Schupbach, B.; Terfort, A.; Zacher, D.; Fischer, R. A.; Woll, C. Controlling interpenetration in metal-organic frameworks by liquid-phase epitaxy. Nat. Mater. 2009, 8, 481-484. [2] So, M. C.; Jin, S.; Son, H.-j.; Wiederrecht, G. P.; Farha, O. K.; Hupp, J. T. Layer-by-Layer Fabrication of oriented porous thin films based on porphyrin-containing metal-organic frameworks. J. Am. Chem. Soc. 2013, 135, 15698-15701. [3] Lindgren, A.; Munteanu, F.-D.; Gazaryan, I. G.; Ruzgas, T.; Gorton, L. Comparison of rotating disk and wall-jet electrode systems for studying the kinetics of direct and mediated electron transfer for horseradish peroxidase on a graphite electrode. J Electroanal Chem. 1998, 458, 113-120.

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