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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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