Variable-Temperature Resonance Raman Studies to Probe Interchain Ordering for Semiconducting Conjugated Polymers with Different Chain Curvature

Abstract

The morphology and crystallinity of the polymers used to fabricate bulk heterojuction (BHJ) solar cells significantly influences the efficiency of the cells. We have used variable-temperature (VT) spectroscopy techniques, namely VT emission and VT resonance Raman spectroscopy (VT-RRS), to examine how the backbone linearity of a conducting polymer affects its electronic response to temperature and variations in solution behavior. We have studied two types of donor-acceptor polymers used in BHJ cells with differing backbone structures; they are poly-{5,6-bis(tetradecyloxy)-4-(thiophen-2-yl)benzo[c]-1,2,5-thiadiazole} (PTBT) which has a curved backbone and poly-{5,6-bis(tetradecyloxy)-4-(thieno[3,2-b]-thiophen-2-yl)benzo[c]-1,2,5-thiadiazole} (PTTBT) which has a linear chain structure. Time-dependent density functional theory (TD-DFT) calculations and resonance Raman spectra (RRS) of PTTBT revealed the presence of three electronic transitions, with character that varies between π to π*, mixed π to π*/charge transfer and pure charge transfer in nature. Emission spectra of PTTBT showed spectral changes at 650 and 710 nm with varied temperature (-10 to 60 °C). Variable-temperature RRS was measured in resonance with the lowest and highest energy electronic transitions. The changes were interpreted using two-dimensional correlation spectroscopy (2DCOS) analysis. PTTBT showed gradual shifts to lower wavenumbers of modes at around 1425, 1450 and 1500 cm^-1 . For PTBT larger and more rapid spectral changes are observed at 1440 and 1460 cm^-1 consistent with greater variation in the electronic nature upon heating. Further study into the influence of polymer linearity on crystallinity and long range order was carried out using low-frequency Raman (LFR) to examine drop cast films under a variety of different conditions. LFR spectra showed that PTTBT has a band at 73 cm^-1 . This is observed under a variety of film-forming conditions. PTBT does not show distinct low frequency modes, consistent with its low crystallinity.

Keywords: conducting polymers; long range order; low-frequency Raman; resonance Raman spectroscopy; variable temperature.