Optical Spectroscopy of Organic Solar Cell Materials

Today global energy demands are satisfied by nonrenewable sources.  Population growth and environmental concerns require a larger fraction of our energy needs to be met by renewable sources. Sunlight can provide clean, large-scale electricity production if the cost of solar cells can be reduced. The application of conventional silicon solar cells is limited by high production costs; however, organic (“plastic”) photovoltaic (OPV) cells, processed by inexpensive roll-to-roll techniques, have the potential to be cost-competitive with existing energy sources if their efficiencies can be improved. Alignment and ordering of the organic materials that make up the OPV device are critical for maximizing the absorption of light and the mobility of electrical charges through the solar cell, both of which strongly affect device efficiency. In order to study how light absorption and molecular ordering are affected by processing, we use optical spectroscopic techniques to investigate OPV materials fabricated in our lab. Light absorption is measured via UV-vis spectroscopy, and subtle changes in the peak shape and position in UV-vis and low temperature photoluminescence spectra provide information on molecular ordering. Preliminary results suggest that we can achieve better molecular ordering and absorb a wider region of the solar spectrum using thicker organic layers, and provide interesting insight on the interplay of light absorption and molecular ordering when two commonly-used OPV materials are blended together. This information will provide connections between processing and structure so we can better understand and control OPV materials and increase device efficiency.