# Photovoltaics

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Recently, significant research interest has been focused on developing conjugated polymer/fullerene mixtures for use in low cost organic photovoltaic (PV) applications. In spite of the large number of characterization techniques applied to these samples, the dynamic mechanisms that modulate the photoelectrical efficiency (and hence economic utility) of these devices are still poorly understood. One of the main impediments to gaining a complete understanding of the dynamic photoelectrical mechanisms is the large importance that morphology plays in controlling device function (e.g., modifying the morphology of the same material mixture can improve the device efficiency by an order of magnitude). The size, shape, crystallinity, and orientation of material domains in the active device layer have been shown to strongly affect important device parameters such as charge mobility, charge separation probability, and optical absorption.

We propose to use a combination of new methods to control the morphology based on the material parameters and to apply rapid time resolved spectroscopies to determine the critical rate constants for the optical excitation and charge transport processes. We will study the samples as nano-particle suspensions and as-cast thin-films, where the major features within a PV film are reproduced, but the size of the nanoparticle limits the length scale for dynamic processes such as exciton formation and charge transfer. We have already shown that comparing fully dissolved polymer and polymer nanoparticles with differing morphologies allows one to define the structural character of the polymer within the film. This proposal outlines a synergistic collaboration between rapid time-resolved optical spectroscopies in the Larsen Group (UCD) and the newly emerging organic PV production techniques in the Moule' Group (UCD) with the goal to generate the necessary rapid time-resolved data to test and refine the microscopic models describing the underlying exciton and charge transfer kinetics in fully functioning polymer/fullerene PV devices as a function of external morphological changes. This knowledge is a required component for the rational design of more efficient instruments for converting solar energy into useful and economic power for the consumer.

References

• Excited-state Self-Trapping and Ground-state Relaxation Dynamics in Poly(3-hexylthiophene) Resolved with Broadband Pump-Dump-Probe Spectroscopy, Erik Busby, Elizabeth C. Carroll, Erin M. Chinn, Lilian Chang, Adam J. Moulé, and Delmar S. Larsen, submitted (2011).
• Acceptor Dependence of the Polaron Recombination in Poly 3-hexyl thiophene:Fullerene Composite Films, Erik Busby, Christopher W. Rochester, Adam J. Moulé, Delmar S. Larsen, Chemical Physics Letters, 513 (2011) 77–83 (2011). pdf