A Spectroscopic and DFT Study of the Electronic Properties of Carbazole-Based D–A Type Copolymers

Abstract

The structural and electronic properties of three carbazole containing copolymers used in organic photovoltaic applications, poly[N-1-octylnonyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), poly[N-1-octylnonyl-2,7-carbazole-alt-4,7-(2′,1′,3′-benzothiadiazole)] (PCBT), and poly[N-1-octylnonyl-2,7-carbazole-alt-4,7-(2′,1′,3′-benzoselenadiazole)] (PCBSe) have been studied using resonance Raman (RR) and transient absorption (TA) spectroscopies and density functional theory (DFT) calculations. Enhancement of Raman modes centered on the acceptor unit when a Raman excitation wavelength is coincident with lowest energy electronic excitation suggests that the excitation involves charge transfer from the carbazole donor to the varying benzodiazole acceptors. The pattern of the enhancement when the excitation wavelength is coincident with the higher energy transition indicates that this transition is π to π* in nature; this is consistent with TD-DFT calculations. Nanosecond transient absorption studies show long-lived excited state signals for PCDTBT (126 ± 4 ns and 1.56 ± 0.1 μs) and PCBSe (1.82 ± 0.1 μs), suggesting that population of the triplet state is appreciable. No transient signal could be detected in PCBT. B3LYP TD-DFT calculations of the monomer through to the hexamer indicate a broadly delocalized excited state orbital for PCDTBT as indicated by the linear decrease in excitation energy with an increased number of repeat units, while for PCBSe and PCBT, the reduction in excitation is sublinear. The highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO) of PCBSe and PCBT polymers compared to PCDTBT are similarly diffuse, but the population of higher order orbitals is decreased when compared with PCDTBT. CAM-B3LYP calculations reduce the delocalization of the frontier orbitals and show less reduction in excitation energy with additional repeat units for each polymer.

Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions

“…The evidence presented here in favour of the singlet exciton contradicts this hypothesis. We underline that according to our calculations and other work 21,27 , the singlet exciton exhibits some degree of CT-like character. However, we insist that this picture is distinct from the loosely bound polaron pairs hypothetized for similar 'push-pull' co-polymers as noted above.…”

“…We then identified the modes of the measured Raman peaks by comparing them to the frequency of the bands in the theoretical Raman spectrum ( Supplementary Fig. 1) and to a published resonance-Raman study of PCDTBT 21 . These assignments are summarized in Tables 1 and 2.…”

Sub-picosecond delocalization in the excited state of conjugated homopolymers and donor–acceptor copolymers

“…142 Moreover, calculations on increasingly long oligomers of PCDTBT show that the density of states near the frontier orbitals increases upon polymerization, so that higher energy, more delocalized orbitals can be reached, leading to a more diffused nature of the excited state (up to 10 nm). 138 We note that the 0.2 eV red shi when going from the CDTBT repeat unit to the PCDTBT polymer is relatively small (Fig. 5A and 7A).…”

“…Qualitatively very similar TD-DFT results were reported with other functionals (yielding oen less good agreement with the experimental spectra), and for larger oligomers. 137,138,140,142 There is consensus that absorption into the rst band of PCDTBT leads to partial charge transfer.…”

Multilayer films composed of conductive poly(3‐hydroxybutyrate)/carbon nanotubes bionanocomposites and a photoresponsive conducting polymer

“…The absorption at 395 nm is attributed to p-p* transition, whereas absorption at 570 nm is known to be due to a charge transfer from donor Cz unit to acceptor DTBT unit. 26 When the PC70BM was added a modification of the fine structure was observed with a broader absorption. This result presumably indicates a miscibility of the conjugated polymer with the PC70BM.…”