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    Organic solar cells based on liquid crystalline and polycrystalline thin films

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    Author
    Yoo, Seunghyup
    Issue Date
    2005
    Keywords
    Organic solar cells
    pentacene
    fullerene
    discotic liquid crystals
    organic polycrystalline thin films
    photovoltaic device modeling
    Advisor
    Armstrong, Neal R.
    Kippelen, Bernard
    Committee Chair
    Armstrong, Neal R.
    Kippelen, Bernard
    
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    Publisher
    The University of Arizona.
    Rights
    Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
    Abstract
    This dissertation describes the study of organic thin-film solar cells in pursuit of affordable, renewable, and environmentally-friendly energy sources. Particular emphasis is given to the molecular ordering found in liquid crystalline or polycrystalline films as a way to leverage the efficiencies of these types of cells. Maximum efficiencies estimated based on excitonic character of organic solar cells show power conversion efficiencies larger than 10% are possible in principle. However, their performance is often limited due to small exciton diffusion lengths and poor transport properties which may be attributed to the amorphous nature of most organic semiconductors.Discotic liquid crystal (DLC) copper phthalocyanine was investigated as an easily processible building block for solar cells in which ordered molecular arrangements are enabled by a self-organization in its mesophases. An increase in photocurrent and a reduction in series resistance have been observed in a cell which underwent an annealing process. X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements suggest that structural and morphological changes induced after the annealing process are related to these improvements.In an alternative approach, p-type pentacene thin films prepared by physical vapor deposition were incorporated into heterojunction solar cells with C60 as n-type layers. Power conversion efficiencies of 2.7 % under broadband illumination (350-900 nm) with a peak external quantum efficiency of 58 % have been achieved with the broad spectral coverage across the visible spectrum. Analysis using an exciton diffusion model shows this efficient carrier generation is mainly due to the large exciton diffusion length of pentacene films. Joint XRD and AFM studies reveal that the highly crystalline nature of pentacene films can account for the observed large exciton diffusion length. In addition, the electrical characteristics are studied as a function of light intensity using the equivalent circuit model used for inorganic pn-junction solar cells. Dependences of equivalent-circuit parameters on light intensity are further investigated using a modified equivalent circuit model, and their effects on the overall photovoltaic performance are discussed.
    Type
    text
    Electronic Dissertation
    Degree Name
    Ph.D.
    Degree Level
    doctoral
    Degree Program
    Optical Sciences
    Graduate College
    Degree Grantor
    University of Arizona
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