Reports: DNI1049008-DNI10: Uncovering and Understanding Morphology- Dependent Charge Transport and Trapping in Polymer Photovoltaic Materials

John K. Grey, University of New Mexico

This research focused on the structure-function relationships of conjugated polymer molecules in device environments, such as solar cells.  In particular, we sought an understanding of structural factors that promote efficient charge transport as well as those responsible for detrimental charge trapping and recombination processes.  Our initial efforts involved the development of spectroscopic probes capable of resolving the packing (aggregation) state of the polymer which could then be spatially mapped with sub-micron resolution.1  We then extended this approach to generated correlated photocurrent images which revealed for the first time the efficacies of polymer aggregates in generating and transporting charges.2,3  This PRF supported work formed the basis of more advanced studies on polymer aggregates in the form of nanofibers and resulted in a long-term grant from the National Science Foundation. 

More recently, my group has undertaken studies of polymer donor/acceptor charge transfer complexes that play key roles in determining solar cell performance characteristics, namely, open circuit voltages.  A student from the PI’s group was partially supported from PRF funds to study various charge transfer complex systems using resonance Raman spectroscopy.  A key finding from this preliminary work was that long and detailed progressions of overtone and combination band features are apparent and intensity distributions of these transitions are sensitive to the specific donor/acceptor interface interactions.  Importantly, overtone-combination bands reveal the extent to which the nuclear framework distorts upon transfer of a charge between donor and acceptor systems and quantitative estimates can be obtained from time-dependent quantum mechanical simulations.  These findings allow us to quantitatively estimate vibrational reorganization energies of the charge transfer complex which cannot be obtained from any other physical approach.  The results from this PRF-supported research are presently being prepared for publication4 and the PI is seeking additional funding support from national funding agencies to continue this work.

Throughout the duration of this award, undergraduate students played a significant role in conducting experimental work and, while not directly supported by the PRF, made substantial contributions to the above projects. 

(1) Gao, Y.; Grey, J. K. J. Am. Chem. Soc. 2009, 131, 9654-9662.

(2) Gao, Y.; Martin, T. P.; Thomas, A. K.; Grey, J. K. J. Phys. Chem. Lett. 2010, 1, 178-182.

(3) Gao, Y.; Martin, T. P.; Niles, E. T.; Wise, A. J.; Thomas, A. K.; Grey, J. K. J. Phys. Chem. C 2010, 114, 15121-15128.

(4) Gao, J.; Martin, E. J.; Thomas, A. K. Grey, J. K.  In preparation.