The new generation of multi-chromophoric poly(3-hexylthiophene) (P3HT) analogs display broad absorption bands both in visible and near IR due to the incorporation of electron deficient units in the backbone of P3HT. Despite this extended absorption however, different semi-random polymers exhibit markedly different photovoltaic performance. The femtosecond dynamics governing charge carrier generation and recombination in composites of these donor-acceptor polymers with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were measured to understand the differences in the device performance. Our studies indicate that unlike P3HT:PCBM, these semi-random donor acceptor polymer:PCBM blends exhibit rapid polaron generation (˛ 200 fs), but the geminate recombination of charges must be reduced to achieve further higher efficiencies. The fitting of our transient absorption (TA) data with a diffusive escape model shows that geminate recombination losses can be minimized by achieving a large charge separation length at the polymer:fullerene interface, possible if the initial exciton states can inject electrons via a delocalized hot CT state. A correlation has also been established between the trends in the IQE measurements and the wavelength-dependent charge recombination obtained from time-resolved TA spectroscopy. Optical modeling of different thickness films suggest that varying long wavelength IQEs for the same blend are due to difference in morphology under different spin-coating conditions rather than optical wave-guiding. Our current hypothesis is that both the morphology of the composite at the interface and the donor-acceptor interactions present in the polymer can influence the charge separation length and hence the charge recombination.
We are also exploring via time-resolved spectroscopy how the incorporation of a strong donor or different acceptor units, as well as film thickness and other processing conditions can influence the charge recombination dynamics in the semi-random donor acceptor polymer:PCBM blends. Moreover, absolute polaron yields are currently being extracted from experimental data using the polaron cross-section values obtained from chemical doping measurements, which may give additional insight in quantifying the initial charge generation in these polymer:PCBM blends.