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.