Abstract
Triplet state formation after photoexcitation of low-bandgap polymer/fullerene blends has recently been demonstrated; however, the precise mechanism and its impact on solar cell performance is still under debate. Here, we study exciton dissociation, charge carrier generation, and triplet state formation in low-bandgap polymer PBDTTT-C/PC60BM bulk heterojunction photovoltaic blends by a combination of fs-mu s broadband vis-NIR transient absorption (TA) pump-probe spectroscopy and multivariate curve resolution (MCR) data analysis. We found sub-ps exciton dissociation and charge generation followed by sub-ns triplet state creation. The carrier dynamics and triplet state dynamics exhibited a very pronounced intensity dependence, indicating nongeminate recombination of free carriers is the origin of triplet formation in these blends. Triplets were found to be the dominant state present on the nanosecond time scale. Surprisingly, the carrier population increased again on the ns-mu s time scale. We attribute this to triplet-triplet annihilation and the formation of higher energy excited states that subsequently underwent charge transfer. This unique dip and recovery of the charge population is a clear indication that triplets are formed by nongeminate recombination, as such a kinetic is incompatible with a monomolecular triplet state formation process.