Browsing by Subject "Organic Photovoltaics"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Engineering Excited State Transport and Relaxation in Organic Semiconductors
    (2020-05) Rai, Deepesh
    Organic semiconductors are an important class of optoelectronic materials that are characterized by high degree of conjugation within the molecule. These are thin films of conjugated molecules in organic optoelectronic devices such as organic light-emitting devices (OLEDs) and organic photovoltaic cells (OPVs). In organic semiconductors, the excited state is characterized by a tightly bound electron-hole pair called an exciton. The migration and relaxation of the exciton strongly dictates material optical properties, as well as the subsequent design and operation of semiconductor devices. For example, OPVs rely on the efficient harvesting and dissociation of photogenerated excitons at heterointerfaces in the device layer stack. The transport of long-lived dark excitons is of special interest as they play an important role as energetic intermediates in OLEDs while also being a potential active material in OPVs. Despite this, their spatial migration is challenging to probe accurately. The focus of this thesis is on demonstrating new characterization techniques to track exciton migration as well as on engineering unique device architectures for enhancing energy transport in OPVs. This work has brought insight into the role of spin and molecular structure in impacting exciton diffusion using a novel sensitizer-based methodology to selectively excite and probe dark exciton transport. Furthermore, the normally diffusive aspect of energy transport is overcome by excitonic gates that required development of new experimental and modeling tools.
  • Thumbnail Image
    Item
    Measuring Nanoscale Exciton Transport and Carrier Recombination in Organic Solar Cells
    (2018-08) Curtin, Ian
    Organic photovoltaic devices (OPVs) have the potential to provide low cost solar energy to unique applications which are not accessible by traditional photovoltaics. These devices are made from abundant materials and can be deposited on lightweight flexible substrates with low cost roll-to-roll manufacturing techniques. However, to date they have suffered from relatively low power conversion efficiencies compared to their inorganic counterparts. As such, a deeper understanding of the fundamental processes which govern photoconversion in OPVs is needed in order to better inform materials and device design and realize more efficient devices. Optical absorption in OPVs leads to the formation of Coulombically bound electron-hole pairs, called excitons, which must be dissociated into free carriers to collect photocurrent. Inefficiencies in these devices result from the short diffusion length (LD) of excitons and the subsequent recombination of generated carriers. In this dissertation, material parameters which affect the magnitude of L¬D and new techniques to quantify and decouple transport and recombination mechanisms will be presented. Covered topics include the effects of molecular impurities on L¬D, techniques to measure LD and the exciton lifetime in previously inaccessible dark (non-luminescent) materials through photovoltage measurements, methodologies to quantitatively decouple recombination mechanisms at device relevant operating conditions, and the effects of polycrystalline grain size on LD in singlet fission materials, which are capable of producing two excitons per absorbed photon. These studies provide tools to better understand the underlying physics which govern photoconversion and material parameters which can be manipulated to enhance exciton transport to realize more efficient devices.
  • Thumbnail Image
    Item
    Polythiophene-containing block copolymers for organic photovoltaic applications.
    (2009-08) Boudouris, Bryan W.
    Poly(3-alkylthiophene)s (P3ATs) have become the most common electron-donating material in organic photovoltaics (OPVs), and recent advances in the fabrication of polythiophene-fullerene bulk heterojunction solar cells have allowed for devices with power conversion efficiencies of up to ~6% to be realized. This efficiency has only been possible through enhancements in the active layer microstructure. This key factor allowed for better separation of the bound electron-hole pair (exciton), generated by absorption of light. Understanding how exciton dissociation and the active layer morphology affect device performance will facilitate cell optimization, ultimately leading to higher device efficiencies. Consequently, we developed two new classes of polythiophene-based block copolymers to better understand these phenomena. First, we synthesized well-defined diblock and triblock copolymers with the structures: poly(3-alkylthiophene)-b-polylactide (P3AT-PLA) and polylactide-b-poly(3-alkylthiophene)-b-polylactide (PLA-P3AT-PLA). We have observed that kinetic factors dominate phase separation for a semicrystalline polythiophene block. However, if the polythiophene moiety was amorphous the polymers self-assembled into thermodynamically stable, ordered microstructures with domain spacings on the scale of interest for charge separation in OPV cells (ca 30 nm). Polylactide was chosen as the second moiety in the block copolymers because it could be selectively etched from the polythiophene matrix with a gentle alkaline bath. This procedure led to the formation of nanoporous templates that could generate ordered bulk heterojunctions. In the second approach, P3AT chain ends were terminated with fullerene to create an internal electron acceptor-donor-acceptor, methylfulleropyrrolidine-poly(3-alkylthiophene)-methylfulleropyrrolidine (C60-P3AT-C60). Microphase separation occurred between the polymer chain and fullerene end groups, which suggested the creation of two distinct semicrystalline regimes. A compositionally similar blend of P3HT and C60 showed a similar microstructure. This comparable domain formation, coupled with the possibility of enhanced charge transfer, makes C60-P3AT-C60 a promising candidate as a material in bulk heterojunction organic photovoltaic devices.