Ultrafast spectroscopic characterization and modelling of hybrid plasmonic organic photovoltaic devices

• Main Author: Wu, Bo
• Other Authors: Sum Tze Chien
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Science::Physics::Optics and light
• Online Access: https://hdl.handle.net/10356/61674
• Institution: Nanyang Technological University

Solar energy is one of the most attractive candidates of alternative energy sources to replace the conventional fossil fuels. Among all kinds of solar cells, polymer-based organic solar cells are one that under spotlight in recent years due to some advantages they possess, e.g., low-cost, solution-processed, properties-tunable, flexible, etc. With the efficiency of organic solar cells exceeding 10%, their shift from lab to factory will materialize soon. Further performance improvement of organic solar cell can be achieved by using plasmonic nanostructures as subwavelength antennas or scattering centres. The surface plasmon, which is the collective oscillation of free electrons in the conduction band of some metals such as gold and silver, can yield high-strength local field and scattering when interacting with incident light. However, concomitant issues including energy/charge transfer, morphology disturbance, resistance change, etc can also contribute the performance change when incorporating metallic nanostructures into organic solar cell. The work explores the integration of different metallic structures into different positions of an organic solar cell and investigates the performance enhancement or loss mechanisms within the plasmonic organic solar cells through ultrafast spectroscopies. It was found that embedding metallic nanostructures (silver nano-triangles, gold/silver nanowires) into the hole transporting layer results in improvement of the performance of an organic solar. Far-field scattering and near-field light coupling contribute to an increased light absorption and exciton/charge generation resulting in an improved short circuit current density. Meanwhile, although the exciton-surface plasmon interaction is present in polymer only films, it becomes much weaker and negligible in polymer:fullerene blend, indicating exciton dynamics are not strongly affected by the surface plasmon in device. Polaron dynamics were also proven to be negligibly affected by metallic nanostructures well-covered by charge transport layers (e.g. PEDOT: PSS). Thus a good fill factor and open-circuit voltage can be attained while short-circuit current can be increased by incorporating appropriate plasmonic nanostructures in PEDOT: PSS layer. With silver nanoparticles embedded in the active layer, it was found the performance degrade with increasing silver nanoparticle loading. The loss mechanism is well explained from ultrafast spectroscopy that it is mainly attributed to the charge carrier traps introduced by the silver nanoparticles. The traps can fast trap charges generated near the silver nanoparticle and accelerate the trap-assisted recombination in organic solar cells. The finding correlates well with the electric behaviour of the plasmonic organic solar cells. Lastly, after realizing that the performance can usually be ameliorated by incorporating plasmonic nanostructures into the hole transporting layer, an academic exercise was also embarked to optimize the ultrathin organic solar cell performance with silver nanodisk array in the hole transporting layer for an ideal structure/scenario. Our finding revealed that the absorption in ultrathin active layer can be significantly boosted using the plasmonic nanodisk and the metal cathode to create cavity-like resonance. A study based on 60 nm thick PCPDTBT: PCBM active layer found that solar light absorption can be increased by 40% through incorporating an appropriate aluminium nanodisk array in the PEDOT:PSS layer. Further improvement of the device performance may also be obtained with silver nanostructure formed cavity.