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UBC Theses and Dissertations

Organic photovoltaic films : the commissioning of and preliminary measurements on an organic molecular beam epitaxy system Tully, Alexandra


Organic Photovoltaics (OPVs) may provide a means of achieving flexible and transparent solar cells, comprised of inexpensive materials and created through scalable processes. Compared to today’s dominant silicon-based solar cells, OPVs suffer from lower power conversion efficiency, and a principal barrier to efficient power conversion in OPVs lies in the separation of generated charges. In OPVs, photoabsorption results in a coulombically bound exciton; in order to generate free charges, we must engineer exciton dissociation. Thus, an understanding of the dynamics involved in exciton dissociation and the underlying electronic states that drive this separation is requisite to increasing the power conversion efficiency and developing commercially-viable OPV devices. In order to do this, we intend to map the energy landscapes of the system on a femto- to picosecond timescale, as well as an Angström length scale. To facilitate this mapping, we will use a combination of time- and angle-resolved photoemission spectroscopy (TR-ARPES) and scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) to analyze our films. Using a femtosecond pump-probe scheme, TR-ARPES measures the dynamic spectral properties of a system by monitoring a material’s electronic states after excitation. STM/S provides local information on the electronic structure, including both occupied and unoccupied states. Combined, these measurements will facilitate the understanding of energy level alignment, the band structure of the system, and the evolution of the excited states. Because of the quality and purity requirements for the samples, as well as the fragility of organic thin films, we must grow our films in-situ in a UHV environment. Over the past two years, we have designed and commissioned an organic molecular beam epitaxy (OMBE) growth chamber as well a home-built low energy electron diffraction (LEED) characterization chamber that is attached to an ARPES system. This thesis discusses the motivation and background information for this project in further detail, presents the experimental techniques required to understand and operate the OMBE and LEED chamber, describes the commissioning process of the OMBE, and touches on our preliminary growth recipes and data acquisition.

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