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Understanding the Dielectric Constant in Organic Photovoltaic Materials

  • Author(s): Hughes, Michael
  • Advisor(s): Nguyen, Thuc- Quyen
  • et al.
Abstract

The photovoltaic and electrical properties of organic semiconductors are characterized by their low dielectric constant, which leads to the formation of polarons and Frenkel excitons. These resulting excitons require careful engineering of complex devices to generate electricity using these materials. Furthermore, the low dielectric constant of organic semiconductors has been suggested to play a large role in geminate and bimolecular recombination losses to the performance of organic photovoltaics (OPV). However, despite the critical attention that the dielectric constant has received in discussions in the literature, there has not yet been a thorough study of the dielectric constant in common organic semiconductors and how it may change in the functional OPV device. In fact, there have been some inconsistent and contradictory reports on the trends of dielectric constants, making it difficult to identify trends. This work begins by providing a detailed explanation of the fabrication of organic photovoltaic devices, including fundamental problems encountered and how they relate to the dielectric constant. A specific methodology is then presented to determine the dielectric constant in OPV materials using impedance spectroscopy, including guidelines and illustrations of possible experimental pitfalls. This methodology is utilized to provide the analysis for the dielectric constant of 20 common neat organic semiconductors. Additionally, the dielectric constant in blend systems is studied, and the relationship between the dielectric constant and blend morphology was determined. It is observed that the dielectric constant of a blend system can be very accurately predicted solely based on the dielectric constants of the neat materials, scaled by their respective weight ratios in the blend film. Increasing the dielectric constant of organic photovoltaic materials to reduce recombination rates has long been pursued, however, material modification often results in the modification of multiple device characteristics making system comparison difficult. Once establishing the viability of increased dielectric constant through chemical modification, a model system was developed to study recombination rate changes. A fullerene derivative with an increased dielectric constant by the addition of a triethylene glycol appendage to the fullerene (TEG-PCBM) was synthesized for examination. Devices were fabricated with TEG- PCBM blended with donors P3HT and PTB7-Th and found comparable performance to PC60BM. This model system shows the rarely reported characteristic of an increase in the dielectric constant while leaving its other properties unaltered. Although observing a shift in recombination behavior, a reduction wasn't observed to increase device performance. Sensitivity of morphological conditions consistently prove too important when making such large chemical modifications and bimolecular recombination became trap-assisted monomolecular recombination. While triethlyene glycol appendages may prove to be ineffective in improving recombination through increased dielectric constant, provided is a robust methodology for studying recombination in future high dielectric systems.

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