We present a comprehensive review on the optical properties in pristine low bandgap copolymers, namely PTB7 and PDTP–DFBT, which are used as electron donors in copolymer/phenyl C71 butyric acid methyl ester blends for high efficiency solar cell devices. The copolymer backbone chain comprises of donor (D) and acceptor (A) moieties, which lower the band gap to the near-IR spectral region. Unlike traditional π-conjugated polymers in which the primary photoexcitations are singlet excitons (SE), in D–A copolymers we find at short times coexistence of two primary photoexcitation species, namely SE and triplet–triplet (TT) pairs, which are directly photogenerated upon photon absorption from the ground state within 300 fs, our transient time resolution. Using the transient magnetophotoinduced absorption (t-MPA) spectroscopy, we reveal the spin coupling between the SE and TT spin states from their correlated t-MPA responses. In addition, we show that the TT species dissociates into two individual triplet excitons (TE) in the picosecond time scale; however, the two-geminate TEs are entangled and maintain their spin coherence into the microsecond time domain.
We used both cw and transient spectroscopies for studying the optical properties and photoexcitations in the low bandgap
copolymer PTB7 that has been used in organic photovoltaic applications (OPV). Surprisingly we observed two
primary photoexcitations that are generated within ~150 fs (our time resolution); we identify them as singlet exciton (S1)
and triplet-pair (1TT). The singlet exciton has been considered to be the only primary photoexcitation in regular π-conjugated polymers and is related with a transient absorption band that peaks at an energy value close to the exciton
binding energy (~0.4 eV in PTB7). The TT pair is a novel photoexcitation species in low band-gap π-conjugated
copolymers. It has an absorption band close to that of isolated triplet exciton, and may readily dissociate at the donoracceptor
interfaces in the PTB7/fullerene blend. This finding may explain the underlying mechanism for the high
obtained power conversion efficiency in OPV devices based on the PTB7 copolymer.