Generation and Recombination Kinetics of Optical Excitations in Poly(3-dodecylthienylenevinylene) with Controlled Regioregularity

Abstract

Regioregularity in π-conjugated polymers is known to have desirable effects on chain organization in the solid state leading to improved characteristics for electronic device applications. In this report, we studied the photophysics of a new series of π-conjugated polymers, namely, poly(3-dodecylthienylenevinylene), with controlled regioregularity (PTV-CR), using the steady state photoinduced absorption (PIA), doping-induced absorption (DIA), thermally modulated absorption (TMA), and photoluminescence (PL) techniques in a broad spectral range, as well as Grazing incidence X-ray diffraction (GIXRD). PTV-CR has a low band gap of 1.65-1.70 eV from the most random PTV-5:5 to regular PTV-0:10. We have found that regioregularity affects optical properties of PTV-CR in different ways than in the case of well-studied poly(3-alkylthiophene). We discovered that even the most random PTV-5:5 shows submicrometer crystalline order, and there is no correlation between regioregularity and crystallinality. We found that PTV is intrinsically dark due to the reverse order of 1B(u) and 2A(g) excitons, with PL quantum efficiency ranging from 10(-5) to 2 × 10(-4). Spectroscopic data confirm the existence of long-lived neutral (triplets) and charged (polaron) excitations, and thus PTV is a unique nondegenerate ground state π-conjugated polymer which is intrinsically nonluminescent. The recombination mechanism and lifetime of such long-lived excitations are investigated through the pump intensity and pump frequency dependencies as well as temperature dependence of the sample. Increasing regioregularity is seen to reduce the thermal responsivity and intersystem crossing in these films. The novel, intrachain character of these effects and their implications for use of PTV derivatives for photovoltaic application are discussed.

Heavy-atom effects on intramolecular singlet fission in a conjugated polymer

“…In agreement with previous work by ourselves and others, 31,[67][68][69] we find that band-edge excitation of P3TV results in extremely rapid excited-state decay, with no signal detectable beyond ∼20 ps. In addition to the prominent ground-state bleach signal (ΔT/T > 0) in the region of the steady-state absorption, we identify four regions of the transient absorption signal which can be distinguished based on their different kinetics and/or pumpwavelength dependence.…”

Effect of the Side‐Chain‐Distribution Density on the Single‐Conjugated‐Polymer‐Chain Conformation

“…These data suggest that a lower side‐chain‐distribution density substantially weakens the effect of regioregularity on the polymer‐chain conformation and can produce well‐organized single‐chain conformations even in the presence of regionrandom order. Furthermore, the single‐molecule results for P3HTV with different regioregularities correspond well to the data from bulk films,2e,f for which the regioregularity variation of P3HTV does not induce an evident difference in film structural order. From the discussions above, it is clear that the distribution density of side chains plays a critical role in affecting the isolated single‐chain conformation.…”

“…the proportion of head‐to‐tail (HT) coupling between monomer units in the polymer backbone) leads to a decrease in film crystallinity, hole mobility, and hence device performance 2b. 3 In contrast, for poly(3‐hexyl‐2,5‐thienylene vinylene) (P3HTV) (Scheme ), the variation in regioregularity does not result in apparent differences in film crystallinity or device function 2e,f. The major difference in the chemical structures between P3HT and P3HTV lies in the vinyl unit between the adjacent thiophene rings, which serves to increase the distance between the alkyl side chains therefore decreasing the distribution density of the alkyl side chains along the polymer backbone.…”

Ultrafast optical studies of ordered poly(3-thienylene-vinylene) films

“…4 It has a lower optical gap (1.7-1.8 eV) compared to other NDGS polymers and thus could, in principle, provide a better match with the solar spectrum for organic solar cell applications. 5,6 However, so far PTV-based solar cells have shown low power-conversion efficiency, indicating poor charge photogeneration. 7 Low PLQE may be extrinsic in origin, where the photogenerated 1 1 B u exciton falls into traps.…”

“…We note that the PLQE in RRa-PTV is even smaller than that in RR-PTV. 6 The EA spectra of RR-and RRa-PTV films are given in Fig. 1(d).…”