Subpicosecond Photon-Energy-Dependent Hole Transfer from PbS Quantum Dots to Conjugated Polymers

We use transient absorption (TA) spectroscopy to study the origin of photon energy dependent hole transfer yields in blends of PbS quantum dots with the conjugated polymer poly(3-hexylthiophene-2,5-diyl) (P3HT). We selectively excite only the quantum dots at two different wavelengths and measure the polymer ground state bleach resulting from the transfer of photoexcited holes. The higher photon-energy pump shows a greater prompt yield of hole transfer compared to the lower photon-energy excitation, on time scales sufficient to out-compete hot carrier cooling in lead chalcogenide quantum dots. We interpret the results as evidence that the excess energy of nonthermalized, or "hot," excitons resulting from higher photon-energy excitation allows more efficient charge transfer to the polymer in these systems. The data also demonstrate slow charge transfer rates, up to similar to 1 ns, of the relaxed excitations on the PbS dots. These findings help to clarify the role of excess photon energy and carrier relaxation dynamics on free carrier generation in donor/acceptor solar cells.