Photo-spectroscopic properties of benzothiadiazole-containing pendant polymers for photovoltaic applications

The photophysics of a homopolymer containing pendant phenoxy-benzothiadiazole-bis(thiophene) (OPhBTDT2) moieties, a block copolymer containing both OPhBTDT2 and triphenylamine-based (TPA) pendant units, and a benzothiadiazole model compound, were investigated using steady-state and time-resolved photo-spectroscopic techniques, and quantum mechanical calculations. Electronic excitation of the OPhBTDT2 chromophores leads to rapid intra-molecular charge re-distribution in the lowest unoccupied molecular orbital resulting in substantially increased electron density on the BTD component. In dilute fluid solution, the fluorescence lifetime of the OPhBTDT2 moieties in the block co-polymer was partially quenched due to photo-oxidation of TPA. The triplet excited-state lifetime of the OPhBTDT2 groups in the block co-polymer in solution was unaffected by the TPA moieties signifying that triplet excited-state OPhBTDT2 groups do not oxidize the TPA moieties. In spin-cast films, the OPhBTDT2 singlet and triplet excitons are shorter-lived than the corresponding excited states of the polymers or the OPhBTDT2 model compound in dilute solution, and the lifetimes are essentially independent of the presence of the TPA groups in the block co-polymer. This quenching of OPhBTDT2 exciton lifetimes in the films suggests efficient non-radiative energy migration to low-energy traps, possibly non-emissive OPhBTDT2 molecular aggregates. The complete quenching of fluorescence from OPhBTDT2 in a 1:1 blend of the OPhBTDT2 homoploymer and the electron acceptor [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) is attributed to efficient photo-induced reduction of PC61BM by OPhBTDT2 singlet excitons based on evidence for radical ion formation obtained from nanosecond transient absorbance measurements. The decay kinetics of the absorbance by the resulting charge carriers is consistent with a slow, trap-limited bimolecular recombination mechanism, so the low performance of photovoltaic devices produced using the blend is thought to be limited by extensive phase separation and/or low hole mobility.