Polythienylene–Vinylene Structure–Function Correlations Revealed from Resonance Raman Spectroscopy and Photocurrent Imaging

Abstract

New structure–function correlations are revealed in poly(3-alkyl-thienylenevinylene) (P3ATV) derivatives using resonance Raman spectroscopy techniques. P3ATVs are prototype conjugated polymers that have received increased attention recently due to reports of efficient singlet fission following excitation of the lowest energy allowed (1Bu) excited state. Raman spectra are measured with excitation wavelengths resonant with the P3ATV 1Bu absorption line shape (∼450–750 nm) to probe geometrical rearrangements incurred in this initial excited state. Rich spectral features are resolved, namely, multiple overtone and combination progressions involving the main P3ATV symmetric skeletal stretching vibrational modes. We use a time-dependent wavepacket approach to calculate the Raman cross-correlation wavepacket overlap function for each displaced mode and their extent of displacement in the excited state. Fit parameters are checked by simulating optical absorption spectra that show good agreement with experimental absorption line shapes. Excitation wavelength-dependent Raman frequency dispersion and Raman excitation profiles of symmetric CC skeletal vibrations showed significant variation across the P3ATV absorption envelope and anomalously large enhancements for lower energy (longer wavelength) excitation, respectively. We demonstrate these behaviors are due to excitation of aggregated and amorphous P3ATV chains with distinct absorption and Raman signatures. This assignment was confirmed by measuring Raman spectra to selectively excite each limiting form and spectral decomposition of absorption line shapes using simulated absorption spectra of aggregates. Resonance Raman and photocurrent imaging of model P3ATV/fullerene blend solar cells were next performed to spatially correlate local morphology to photocurrent generation efficiency. Raman images of aggregated and amorphous P3ATV regions were constructed that show larger photocurrent production in the former, suggesting these structures are well mixed with PCBM. Conversely, amorphous zones have diminished PCBM content, resulting in lower photocurrents. Absorption and Raman spectra demonstrate that the P3ATV aggregate packing characteristics and amounts are largely unaffected by the addition of PCBM, indicating weak interactions with P3ATV chain backbone that lead to poor charge generation efficiencies in solar cells.