Design Strategies for Controlling Optoelectronic Properties and Solid-State Order of Conjugated Materials in Organic Photovoltaics
- Author(s): Lee, Olivia Pei-Hua
- Advisor(s): Fréchet, Jean M. J
- et al.
Organic photovoltaics (OPVs) have emerged as an alternative technology to current silicon-based systems for harvesting solar energy. The solution-processability of conjugated materials offers the potential for scalable, low-cost production of lightweight, flexible solar cells. In particular, OPVs containing a bulk-heterojunction (BHJ) architecture, where electron-donors and electron-acceptors form an interpenetrating network within the device active layer, have demonstrated high solar cell performance. The active layer components serve a crucial role in light absorption, charge generation and carrier transport. Through synthetic design and device characterization, this work focuses on elucidating design strategies and principles for the development of high-performing electron-donors for OPVs.
Molecular packing parameters of conjugated materials can strongly impact charge transport in the solid state. With a model polymer backbone containing thiophene comonomers, it was shown that strategic substitution of furan for thiophene improved material solubility and allowed for the use of linear alkyl side chains to reduce insulating contents in devices. These structural modifications yielded polymers with sufficient solubility, favorable solid-state morphology, enhanced long-range packing order, and efficient OPV performance. In a separate study, conjugated small molecules were functionalized with symmetric, planar end-groups so that they could self-assemble and form an interpenetrating network with electron-acceptors in the active layer. This investigation demonstrates that directing self-assembly via end-groups is an effective strategy to enhance molecular interconnectivity and improve the performance of molecular semiconductors in organic solar cells.
Alongside solid-state morphology, semiconducting materials need to exhibit optical and electronic properties that enable effective light absorption and charge generation in solar cells. We showed that incorporating antiaromatic units into conjugated molecules effectively redshifted and broadened absorption to cover a significant portion of the visible spectrum. The impact of atomic substitution on optoelectronics was also explored with the design of an electron-poor monomer, oxadiazolopyridine, which yielded efficient OPV materials upon copolymerization with electron-donors. Furthermore, gold and silver nanoparticles were embedded in one of the device interlayers to enhance light absorption via surface plasmon resonance, improving OPV performance.
Finally, influence of chemical structures on photoexcited charge transfer processes was investigated in a system containing organic electron-donors and inorganic electron-acceptor. Characterization by femtosecond-stimulated Raman spectroscopy provided information on electron transfer dynamics and molecular structural changes in the excited state. The results revealed that organic molecules containing trans-double bonds could isomerize into cis-double bonds upon photoexcitation, subsequently decreasing charge separation and device efficiency.