Examination of charge transport in smart matrix light-harvesting arrays for use in molecular-based solar cells

Abstract

Films assembled employing this layer-by-layer (LbL) technique have been shown to possess a novel property in which they can effectively be used as a passivation layer when immobilized on an electrode. Due to the insulating nature of these films, when incorporated into a typical DSC, undesirable current flow in which a redox shuttle recombines with the underlying electrode can be physically blocked, while still allowing desirable current flow mediated by the conductive porphyrin film upon reaching its electrochemical potential. Cyclic voltammetry was applied to qualitatively examine surface rectification of our films towards five outer-sphere redox probes, while interfacial rates were measured using forced-convection hydrodynamic flow electrochemical methods with a wall-jet instrumental set-up we built specifically for this purpose. Using wall-jet electrochemical methods, we were able to determine a quantitative interfacial electron transfer rate for these five different outer-sphere redox probes, with the goal of ultimately modeling photocurrent enhancement in a DSC.

The research described herein was focused with the intent of having a substantially broad impact, as this design has the potential to be applied to a number of solar-energy harvesting devices. As society as a whole moves towards alternative sources of energy in order to supplant our ever-growing energy needs, solar has the potential to be a viable competitor in this race towards a cleaner-energy planet.

In order to apply the aforementioned results to an actual device, through-film electron transfer rates must also be examined and optimized to ensure optimal current flow upon oxidation of the porphyrin film into its conductive state. Chronoamperometry was used to calculate through-film electron transfer rates in varying numbers of layers of ZnTPEP, as well as in mixed assemblies of porphyrins constructed with the goal of incorporating a redox gradient with an intrinsic driving force for electron movement. A Butler-Volmer parameterization was used to fit chronoamperometry results, and a quantitative electron transfer rate was determined for each assembly with the goal of optimizing current flow for application in devices with enhanced photocurrent. The Laviron Method was also applied in a similar manner with the same ultimate goals.

Herein we examine charge transfer through molecular multilayer films assembled via copper(I) catalyzed azide-alkyne cycloaddition "click" chemistry towards application in dye-sensitized solar cells (DSCs). These films were designed to incorporate a number of different porphyrin chromophores into Smart Matrix arrays with highly tunable photophysical and electrochemical properties. Preliminary device studies were conducted with the goal of optimizing the number of layers required to maximize photocurrents and photovoltages when in the presence of various outer-sphere redox mediators. The results of these studies implied multilayer films do in fact improve device function by minimizing recombination, and through the use of outer-sphere mediators, we were able to improve photovoltages overall.