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

We measure the spectra of resonant Raman scattering and doping-induced absorption of pristine films of the $\pi$-conjugated donor-acceptor ($D\text{−}A$) copolymer, namely, thieno[3,4 b]thiophene-alt-benzodithiophene (PTB7), as well as photoinduced absorption spectrum in a blend of PTB7 with fullerene phenyl–C61–butyric acid methyl ester molecules used for organic photovoltaic (OPV) applications. We find that the $D\text{−}A$ 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 antiresonance lines superposed onto the induced polaron absorption band in the photoinduced 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 polaron-related vibrations and electronic transitions, we obtain the polaron effective kinetic mass in PTB7 using the amplitude mode model to be approximately $3.8{\mathit{m}}^{*}$, where ${\mathit{m}}^{*}$ 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 the PTB7-fullerene blend.

Figure 1

(a) The backbone structure of the D-A copolymer that contains two different moieties. (b) The photoluminescence and absorption spectra of PTB7 film at ambient conditions. O.D. is the film’s optical density.

Figure 2

The Raman-scattering (a) and IR-absorption spectra (b) of PTB7 film measured at ambient conditions. The most strongly coupled modes 1 to 6 are assigned. The (*) symbol denotes the impurity-related mode [37].

Figure 3

The evolution of the doping-induced absorption of PTB7 film upon exposure to iodine vapor at various time durations as given. The $P1$ polaron band and related IRAVs are assigned.

Figure 4

The DIA spectra of PTB7 film (a),(b) and PIA spectra of PTB7/PCBM blend film (c),(d). The polaron bands $P1$ and $P2$ are assigned [see inset in panel (c)]; the triplet PIA band $T$ is also denoted in (c). The IRAVs are shown in more detail in panels (b) and (d), where the ARs are denoted with arrows pointing to dips superimposed on the polaron $P1$ band.

Figure 5

The measured spectrum (lines) and calculated vibrational mode frequencies (symbols) in PTB7 film for (a) PIA, (c) DIA, and (d) resonant Raman scattering. The “bare” phonon propagator $D_0(\omega )$ is shown in (b), where the horizontal lines are obtained using $2{\lambda }^{*}=0.73$ (RRS, down triangles), ${\alpha }^{\prime }(\mathrm{PIA})=0.44$ (ARs, circles), and ${\alpha }^{\prime }(\mathrm{DIA})=0.44$ (ARs, up triangles). The modes 1 to 6 are assigned. $D_0(\omega )$ parameters are given in Table 1. The (*) symbol in panels (b) and (c) denotes an IRAV at $1115{\mathrm{cm}}^{-1}$ that is not an AR dip (see text for discussion).

Figure 6

The RRS spectrum of PTB7 as measured (a) and calculated (b) using the AMM parameters given in Table 1.