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Ultrafast exciton and charge dynamics in novel conjugated oligomers

Liu, X (2016) Ultrafast exciton and charge dynamics in novel conjugated oligomers. Master's Thesis / Essay, Applied Physics.

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Bulk heterojunction (BHJ) organic solar cells (OSCs) based on solution processable organic material have recently attracted much attention for being a very promising alternative to silicon solar cells. The demand for new, more efficient materials has always been one of the main concerns for the organic photovoltaic field. Recently, it was proposed to replace typical OSC materials, conjugated polymers, with organic small molecules (SMs). OSCs based on SM combine advantages of the polymer-based OSCs with such benefits of small molecules as better reproducibility and higher thermal stability. Moreover, because of small molecular weight, vacuum processing of SM-based OSCs is possible, which opens new possibilities for device engineering. In a constant hunt for best-performing material, a lot of SMs with different chemical structures have been synthesized nowadays. However, alternating of chemical structure of the molecule may lead to unexpected changes of chemical and/or photophysical properties, like decreased charge production efficiency or enhanced recombination. This calls for extensive photophysical studies of each new material. In this Thesis, early-time photophysics of a novel push-pull small molecule TPA-2T-DCV-Me (TV38) are investigated with steady-state absorption spectroscopy and ultrafast photoinduced absorption (PIA) spectroscopy. To provide a direct connection with OSC operation, charge separation dynamics are investigated in solution-processes TV38:[70]PCBM BHJ and vacuum-deposited TV38/C60 layers. We found that in mixtures with [70]PCBM acceptor, TV38 acts as well-performing OSC donor material and provide sufficient amount of separated charges. Both intermolecular electron transfer from TV38 to [70]PCBM and hole transfer from [70]PCBM to TV38 contribute to the separated charges with comparable fraction. However, the intramolecular exciton recombination in TV38 phase and/or recombination of intermolecular charge transfer states compete with charge separation process and lead to losses of charges. Overall, BHJ TV38:[70]PCBM blends at the optimized ratio demonstrated that more than 50% of initially generated charges are survived. If these charge lost channels can be suppressed by tuning the blend properties and/or the structure of the molecule, the capability and efficiency for producing long-lived charge will be highly improved. In layer-structured OSCs, high exciton diffusion distances are required to maximize the exciton harvesting. By employing PIA spectroscopy and analytical modeling, the exciton diffusion distance in vacuum-deposited TV38 is estimated as ~10 nm. This value is in line with the typical exciton diffusion distance in organic material but is much shorter than the light penetration depth in TV38, which is around 80 nm. This rather short exciton diffusion distance significantly limits the exciton harvesting efficiency in the layered structure TV38-based OSCs. Based on the fact that the diffusion distance for TV38 is short, BHJ architecture seems to be more promising choice for device designing. In total, the TV38 molecule appears as a promising donor material for OSCs. The results presented in this Thesis provide valuable feedback for device engineers and suggest the ways of further OSCs optimization. The methods presented are universal and applicable for wide range of donor and acceptor materials, and therefore may be utilized for thorough photophysical studies of new photovoltaic compounds (polymers, small molecules and fullerene derivatives).

Item Type: Thesis (Master's Thesis / Essay)
Degree programme: Applied Physics
Thesis type: Master's Thesis / Essay
Language: English
Date Deposited: 15 Feb 2018 08:30
Last Modified: 15 Feb 2018 08:30

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