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The effect of metal sulfides on hole and electron transport buffer layers in organic photovoltaics: experimental and numerical device simulation investigations.

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Organic solar cells (OSCs) are promising alternative renewable energy sources that often suffer from insufficient absorption of solar radiation, short exciton lifetimes and small diffusion length of their charge carriers. Several strategies are being investigated to overcome these challenges in a move towards the commercialization of this solar cell technology. Increasing the path-length of incident electromagnetic radiation within the photo-absorbing layer of the solar cell, may elongate the time that light spends within the solar cell, thereby increasing the light-matter interaction time and consequently the photo-absorption within the photo-active material of the solar cell. The process described may be accomplished if suitable plasmonic metal nano-structures are added into the solar cell matrix. This intervention may also enhance the collection of the photo-generated charge carriers. The effects of metal sulphide nanoparticles incorporation in organic solar cells were studied and presented in this thesis. The metal sulphide nanoparticles were characterized and introduced into the hole- and electron- transport layers of fullerene and non-fullerene electron acceptors based solar cells, to elicit improved photoabsorption via the localized surface plasmon effects and facilitate better charge collection at the electrodes. Devices were fabricated both in an ambient environment and in a controlled environment (Nitrogen filled glovebox). The metal sulphide nanoparticles were incorporated in the fabricated solar devices using both the conventional and inverted device architectures. The power conversion efficiencies of the devices improved significantly after the incorporation of these nanoparticles. Device numerical simulation studies were also performed to reproduce some of the experimental results with a view to further investigating the devices and discussing their charge transport characteristics. The simulation results show improved charge carrier characteristics from the metal-sulphide doped devices by way of improved conductivity and shifted Fermi level offsets which were aided by the presence of the metal-sulphides. Asides from successfully achieving improved device performances in the investigations and simulations carried out in this thesis, this thesis successfully demonstrated the incorporation of nano-composites in non-fullerene acceptors-based organic solar cells for the first time to the best of our knowledge.


Doctoral degree. University of KwaZulu-Natal, Pietermaritzburg.