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

Abstract

We used a variety of optical spectroscopies to investigate the charge excitations and correlated infrared (IR)-active and Raman-active vibrations in poly[(difluoro-benzothiadiazoldiyl)-alt-(di(2-octyldodecyl)-quaterthiophen-diyl)], PffBT4T, a π-conjugated donor-acceptor (DA) copolymer, which, when blended with fullerene PCBM molecules, serves as an active layer in high-performance photovoltaic solar cells. The applied optical spectroscopies in films of pristine PffBT4T and PffBT4T/PCBM blend include absorption, photoluminescence, electroabsorption, photoinduced absorption (PA), and resonant Raman scattering. We found that the PffBT4T copolymer chain contains 11 strongly coupled Raman-active vibrational modes, which are renormalized upon photogeneration of charge polarons onto the chain. As the lower energy polaron absorption band overlaps with the renormalized vibrational modes, they appear in the PA spectrum as antiresonance (AR) lines superposed onto the induced polaron absorption band. We show that the Raman scattering, doping induced, and photoinduced AR spectra in PffBT4T are well explained by the amplitude mode model (AMM), where a single vibrational propagator describes the renormalized Raman modes and their related photoinduced AR intensities in detail. Surprisingly, we found that two of the IR-active modes in the pristine copolymer must be included in the AMM propagator for explaining the complete photoinduced AR spectrum. This feature is unique to DA-copolymers and indicates that some intrachain C 2v symmetry breaking occurs because of the different electron affinities of the donor and acceptor moieties.

Driving Force and Optical Signatures of Bipolaron Formation in Chemically Doped Conjugated Polymers

“…This design provides the advantage that the polymer bandgap can be tuned by changing the offset between the donor and acceptor energy levels, which also furnishes the ability to create lowbandgap materials. Many studies have demonstrated successful chemical doping of push-pull polymers, [8,[13][14][15][16][17][18] and there is strong evidence that charge transfer only occurs when the dopant is located near one of the donor units on the copolymer backbone and not near one of the acceptor units. [19] Molecular dopants are often added to semiconducting polymers to improve electrical conductivity.…”

“…PBTDTP has a relatively low ionization potential of 4.8 eV, [26] making it easy to dope. It is worth noting that the donor group in PBTDTP extends over 5 conjugated rings, which is larger than other push-pull copolymers whose chemical doping has been studied, [6,8,[13][14][15][16][17]26,27] including PTB7. [18,28] We will argue below that the large donor size and thus ability to delocalize the holes is what causes doping of PBTDTP to directly create bipolarons without first creating single polarons, as seen with PTB7 and most other push-pull polymers.…”