Photophysics and morphology of poly (3-dodecylthienylenevinylene)-[6,6]–phenyl-C61-butyric acid methyl ester composite

Abstract

A series of low band gap poly(3-dodecylthienylenevinylene) (PTV) with controlled morphological order have been synthesized and blended with the electron acceptor [6,6]–phenyl-C61-butyric acid methyl ester (PCBM) for organic photovoltaic devices. Two polymers with the most and least side chain regioregularity were chosen in this work, namely the PTV010 and PTV55, respectively. Using photoluminescence, photo-induced absorption spectroscopy, and atomic force microscopy, we find no direct evidence of photoinduced charge transfer between the two constituents, independent of the bulk-heterojunction morphology of the film, although the possibility of formation of P+/C60− charge transfer complex was not completely ruled out. The large exciton binding energy (Eb = 0.6 eV) in PTV inhibits the photoinduced electron transfer from PTV to PCBM. In addition, excitons formed on polymer chains suffer ultrafast (<ps) intrachain decay to the dark 2Ag state in both PTV010 and PTV55 cases, whereas excitons generated on PCBM molecules undergo energy transfer only to PTV55 in the blend film. Thus, the addition of PCBM increases the photoluminescence yield with respect to neat polymer yield. The efficiency of the energy transfer process is shown to depend on the degree of polymer and PCBM intermixing within the film, which in turn is governed by the polymer chain orders. The effect of such intermixing on the resulting kinetics of photo-induced excitations is also discussed. Our results show limited effect of polymer crystallinity of PTV to its excitonic properties, much the contrary of the case with poly (3-hexylthiophene) which has similar chemical structure with PTV.

Conjugation-length dependence of regioregular oligo 3-alkyl(thienylene-vinylene)s demonstrates polyene-like behaviour with weak electron–electron correlations

“…The photoinduced dynamic behaviour of the nTV trimer in solution is similar to other polyene systems 4,15,56 and consistent with rapid (B280 AE 50 fs) internal conversion from 1B u to 2A g . This finding, together with literature reports of transient absorption spectroscopy of related polymers, 11,12,57,58 and the steady-state spectroscopy described above, demonstrate that similar to diphenyl-polyenes and related carotenoids, P3TV and its oligomers can be assigned as polyenes. 20,21 Poly 3-alkyl(thienylene-vinylene)s have weaker electronic correlations than pure polyenes While the photophysical behaviour of P3TV and its oligomers are qualitatively very similar to that of diphenyl-polyenes and related carotenoids, there is one important difference: the magnitude of the 1B u -2A g energy gap is significantly smaller for alkyl-thienylene-vinylenes compared with pure polyenes of the same conjugation length.…”

Resonance Raman Spectroscopy and Imaging of Franck–Condon Vibrational Activity and Morphology in Conjugated Polymers for Solar Cells

“…The persistence of overtone-combination progressions in both Raman spectra in Figure c,d also reflects relatively long-lived wavepacket dynamics since overlaps of these transitions (eq ) require longer excited state evolution to develop appreciable intensities (e.g., at least 2–3 vibrational periods). Interestingly, Franck–Condon vibrational dynamics in P3DTV are comparable to measured time scales of excited state relaxation from transient absorption studies. ,, Although RRS cannot directly interrogate OPV processes decided on longer time scales, analysis with eqs – offers useful perspectives of excited state structural stabilities that are sensitive to the local environment (namely, morphology). Relationships between Franck–Condon relaxation and material performance are relatively unexplored although considerable potential exists for additional diagnostic capabilities in selecting and optimizing BHJ systems for maximum performance.…”

“…Interestingly, Franck−Condon vibrational dynamics in P3DTV are comparable to measured time scales of excited state relaxation from transient absorption studies. 24,27,28 Although RRS cannot directly interrogate OPV processes decided on longer time scales, analysis with eqs 1−4 offers useful perspectives of excited state structural stabilities that are sensitive to the local environment (namely, morphology). Relationships between Franck−Condon relaxation and material performance are relatively unexplored although considerable potential exists for additional diagnostic capabilities in selecting and optimizing BHJ systems for maximum performance.…”

“…Most notably, many recurrences appear consistent with long-lived wavepacket dynamics on similar time scales of excited state relaxation from transient absorption spectroscopy studies. 24,27,28,39 Despite evidence of a well-mixed polymer/fullerene phase, the highly localized and distorted nature of P3DTV excited states favors efficient nonradiative relaxation detrimental to OPV processes. These studies shed light on the relationship between polymer chain flexibility and excited state structural stability which must be taken into account along with solubility and acceptor miscibility.…”

Activated Singlet Exciton Fission in a Semiconducting Polymer

“…In this work, we evaluate the role of the low-lying A g state and study the mechanism of singlet fission in a different conjugated polymer, poly(3-dodecylthienylenevinylene) (P3TV, Figure ). This polymer fits into the polyene class, with the important added advantages of solution-processability and stability. , Several groups have recently studied P3TV without noting the presence of singlet fission. − Their results instead reflect the conventional model of Figure b, in which photoexcited states decay rapidly and nonradiatively via the 2A g state. Our study takes these investigations further to demonstrate conclusively the presence in P3TV of activated (i.e., occurring only above a threshold pump photon energy) intrachain singlet fission, depicted in Figure c, as a parallel decay pathway upon midband excitation.…”