Amplitude-Mode Spectroscopy of Charge Excitations in PTB7 π -Conjugated Donor-Acceptor Copolymer for Photovoltaic Applications

Abstract

We measured the spectra of resonant Raman scattering and doping induced absorption of pristine films of the π-conjugated donor-acceptor (D-A) copolymer, namely Thieno[3,4 b]thiophene-altbenzodithiophene (PTB7), as well as photoinduced absorption spectrum in blend of PTB7 with fullerene PCBM molecules used for organic photovoltaic (OPV) applications. We found that the D-A copolymer contains six strongly coupled vibrational modes having relatively strong Raman scattering intensity, which are renormalized upon adding charge polarons onto the copolymer chains either by doping or photogeneration. Since the lower energy charge polaron absorption band overlaps with the renormalized vibrational modes, they appear as anti-resonance lines superposed onto the induced polaron absorption band in the photo-induced absorption spectrum, but less so in the doping induced absorption spectrum. We show that the Raman scattering, doping-and photoinduced absorption spectra of PTB7 are well explained by the amplitude mode model, where a single vibrational propagator describes the renormalized modes and their related intensities in detail. From the relative strengths of the induced infrared activity of the polaronrelated vibrations and electronic transitions we obtained the polaron effective kinetic mass in PTB7 using the amplitude mode model to be ~3.8m * , where m * is the electron effective mass. The enhanced polaronic mass in PTB7 may limit the charge mobility, which, in turn reduces the OPV solar cell efficiency based on PTB7/fullerene blend.

Electronic and vibrational spectroscopy studies of PffBT4T π-conjugated donor–acceptor copolymer

“…As these two modes are both IR-active and Raman-active, we conclude that the additional IR-active modes in D 0 ðωÞ of PffBT4T originate from the C 2v intrachain symmetry breaking that occurs in the copolymer chain due to the different donor and acceptor moieties. This has been observed before in the PA spectrum of PTB7 copolymer 29 but has not been properly analyzed. We therefore conclude that both Raman and IR activities for few modes in the PffBT4T chain are unique to the class of π-conjugated copolymers.…”

“…23 This was successfully applied to π-conjugated homopolymers 23 as well as copolymers. 29 We bring here the essence of the AMM to show that, through its correct analysis in the case of PffBT4T, we need to include two additional modes that are IR-active in the pristine copolymer film and form their own ARs. An important ingredient of the AMM is that all IRAVs are interconnected by being coupled to the same phonon propagator.…”

“…In principle, we could use the AMM parameters and the DIA spectrum to estimate the polaronic mass in this copolymer. 29 However, we choose not to go this route at this stage since we lack the theoretical justification for this. We can only say from the large IRAV oscillator Table 1 The best fitting parameters for the bare phonon propagator, D 0 ðωÞ, that describes the 13 most strongly coupled vibrational modes in PffBT4T.…”

“…24,27 Recently, the AMM has also been shown to describe the vibrational modes of PTB7, a prototype DA-copolymer. 29 If the absorption bands of the charge polaron in π-conjugated polymers do not overlap with the vibrational modes, then the IRAVs appear in the absorption spectrum as intense absorption lines. 24 However, when the charge polaron lower absorption band overlaps with the vibrational modes, then quantum interference between the two features may occur in the form of multiple Fano effect, where the IRAVs manifest themselves in the form of antiresonances (ARs) superimposed on the polaron lower absorption band.…”

“…Indeed, ARs have been observed in the photoinduced absorption (PA) spectrum of PTB7/PCBM blend. 29 However, important information that could have been obtained from the photoinduced ARs spectrum about the loss of the C 2v symmetry in the copolymer chain 22 has been ignored so far. Due to the combined effects of strong electron correlations and loss of C 2v symmetry ("symmetry breaking") caused by a different electron affinity, Δ af , between the donor and acceptor moieties, theory predicts 22 that there is considerable mixing between a charge-transfer exciton that defines the optical gap and an energetically proximate triplet-triplet (TT) state with overall spin singlet.…”