Fabrication and characterization of electro-photonic performance of nanopatterned organic photovoltaics Public Deposited

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  • March 20, 2019
Creator
  • Ko, Doo-Hyun
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • Incommensurate length scales conspire to degrade photovoltaic efficiencies in organic photovoltaic (OPV) solar cells: The exciton diffusion length is of order 10 nm while the absorption length is typically more than one order of magnitude larger. And when this discrepancy is ameliorated by co-precipitating a bicontinuous donor and accepter phase to form a bulk heterojunction (BHJ), the tortuous carrier transport path and the electric field distribution across the nano-phase separated components become problematic. Photonic crystal solar cells have the potential for addressing the disparate length scales in polymer photovoltaic materials, thereby confronting the major challenge in solar cell technology: efficiency. One must achieve simultaneously an efficient absorption of photons with effective carrier extraction. Unfortunately the two processes have opposing requirements. Efficient absorption of light calls for thicker PV active layers whereas carrier transport always benefits from thinner ones, and this dichotomy is at the heart of an efficiency/cost conundrum that has kept solar energy expensive relative to fossil fuels. This dichotomy persists over the entire solar spectrum but increasingly so near a semiconductor's band edge where absorption is weak. I report a 2-D photonic crystal geometry that enhances the efficiency of organic photovoltaic cells relative to conventional planar cells. The PC geometry is developed by patterning an organic photoactive bulk heterojunction via PRINT, a nano-embossing method that lends itself to large area fabrication of nanostructures. The photonic crystal cell morphology generally increases photocurrents, and particularly through the excitation of resonant modes near the band edge of the OPV material. The device performance of the photonic crystal cell showed a nearly doubled increase in efficiency relative to conventional planar cell designs. Replication flexibility for various shapes of nanopatterns and materials by PRINT provides further feasibility for PC cell fabrication. The optical interference of PC cells depending on device architecture was investigated theoretically and experimentally. Moreover, the PRINT provides flexibility to fabricate PC geometry for inverted OPV (iOPV) as well as standard OPV. For the PC behavior, the large contrast of refractive index of nanopatternes in the adjacent material is essential. Here, the incorporated layer for the PC behavior affects the device performance electrically as well as optically. In particular, incorporating UV-sensitive electron transport layers (ETL) into organic bulk heterojunction photovoltaic devices dramatically impacts short-circuit current (Jsc) and fill factor characteristics. Resistivity changes induced by UV illumination in the ETL of inverted BHJ devices suppress bimolecular recombination producing up to two orders of magnitude changes in Jsc. Electro-optical modeling and light intensity experiments effectively demonstrate that bimolecular recombination, in the form of diode current losses, controls the extracted photocurrent and is directly dependent on the ETL resistivity.
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  • "... in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry."
Advisor
  • Samulski, Edward T.
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  • Chapel Hill, NC
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  • Open access
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