EXAMINING ELECTRON TRANSFER AND CHARGE TRANSPORT IN ORGANIC PHOTOVOLTAIC MATERIALS

Open Access
Author:
Barbour, Larry W
Graduate Program:
Chemistry
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 07, 2008
Committee Members:
  • John B Asbury, Thesis Advisor
Keywords:
  • OPV
  • PCBM
  • PPV
  • Charge Transfer Dynamics
  • Charge Seperation
Abstract:
Energy consumption is extremely high and costly in the world today. The use of fossil fuels is climbing at a rapid rate; and with the demand so high, costs could skyrocket. Also, the use of more fossil fuels is leading to worldwide issues such as global warming. A change to more renewable energy resources is needed to eliminate emissions and potentially lower the cost of energy. Several options are available for substituting or even replacing fossil fuels. The most promising renewable resource is solar energy. Solar radiation hits the Earth around the clock and will not become depleted in the seeable future. Current inorganic solar technology has been used for decades, but the cost of production is too high to replace current energy sources. A new and cheaper technology is emerging that has the potential to become cheaper than fossil fuels in terms of the amount of energy produced. This technology is known as organic photovoltaics (OPVs). These materials use electron donating polymers in conjunction with electron accepting materials (such as fullerenes). Current efficiencies of OPVs are drastically low in order to become implemented into energy collection methods. Improvement on solar energy harnessing and conversion to electrical energy is currently being examined to make these materials efficient enough to succeed in making solar energy a feasible substitute for an energy source. Current issues of OPVs include charge carrier separation, degradation, charge carrier mobility and extraction. This study will focus on examining charge separation and mobility in OPV materials. Studies utilizing two-dimensional infrared (2D IR) spectroscopy were used to examine the local structure of PCBM molecules within the OPV bulk heterojunction (BHJ). In conjunction with SEM and FTIR studies, it was found that the fullerene molecules tend to cluster together into roughly spherical domains surrounded by a layer of polymer. The addition of the polymer perturbs the fullerene molecules which allows for the measure of electron transfer from the polymer to the PCBM domains using Vis-IR spectroscopy. Vis-IR studies give a measure of the charge transfer process and allow for a kinetic study of charge transfer from optically mobile charge carriers. In conjunction with these kinetic studies, CELIV experiments began to measure electrically mobile charge carriers. All together, these experiments allow for the understanding of rates of the charge transfer process as a function of morphology of the OPV samples.