Dr. Chao-Han Cheng Profile

Dr. Chao-Han Cheng

at Academia Sinica

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Proceedings Article | 13 September 2012 Paper

In situ Raman study of ordered molecular aggregates during freeze-dry fabrication of bulk heterojunction solar cells

Yu-Bing Lan, Pin-Hao Sher, Chao-Han Cheng, Ping-Tsung Huang, Juen-Kai Wang

Proceedings Volume 8477, 84770S (2012) https://doi.org/10.1117/12.930356

KEYWORDS: Raman spectroscopy, Solar cells, Polymers, Heterojunctions, Molecular aggregates, Annealing, Excitons, Molecular spectroscopy, Temperature metrology, Molecules

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Extensive interface between donors and acceptors and respective connected networks of both created in bulk heterojunction (BHJ) solar cells have been proposed to effectively boost the photovoltaic efficiency via facilitating exciton dissociation and charge transport. The multi-scale nature of intermolecular interaction involved however renders the fabrication of such nano-morphology to try-and-error. Our recently proposed freeze-dry method to fabricate the BHJ polymer solar cells has demonstrated comparable efficiencies, regardless the intermolecular interaction strengths of polymers. A fibrous polymer scaffold, being first concocted with the simultaneously grouted PCBM in solution, sustains while the solid-phase solvent sublimates at a low temperature. The formation of such polymer structure can only be unraveled with in-situ monitoring means. Here, we report in-situ characterization of such structure during the initial cooling process with Raman spectroscopy. Raman spectroscopy – revealing molecular vibrational signatures – scrutinizes short-range structural regularity. In comparison with the Raman spectrum of the thermally annealed films, the sequential Raman spectra, acquired during cooling drop-cast o-dichlorobenzene solution of pristine P3HT and its blend with PCBM, show promptly emergent Raman signatures below -5°C – significantly narrowed peaks and new prominent peaks, signifying homogeneously packed P3HT agglomerates. These distinct Raman characteristics accompanied by real-time photoluminescence and absorption measurements suggest extended conjugation and high homogeneity of the P3HT network formed under the dynamic cooling process. This in-situ study thus opens a new utility of Raman spectroscopy to investigate intricate molecular packing that is relevant to the efficient transport of excitons and charges in polymer solar cells.

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