Polymer solar cells consist of a blend of a semiconducting polymer donor with a fullerene derivative electron acceptor. The photoinduced dynamics at the interface between the donor and acceptor components are fundamental to light-to-current conversion. The precise mechanisms occurring on ultrafast time scales (<100 fs), which involve excitons, interfacial states, and free charge carriers, remain to be explained. Using ultrafast absorption spectroscopy with very high, sub-20 fs time resolution, G. Grancini, G. Cerullo, G. Lanzani, and their colleagues from the Istituto Italiano di Tecnologia, the Politecnico di Milano, and Konarka Technologies (now Belectric) have now explored the impact of the excitation energy on the formation of excited states in polymer solar cells.
The researchers used ultrafast transient absorption spectroscopy to reveal early time formation mechanisms of free charge carriers in poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C60-butyric acid methyl ester (PCBM) heterojunctions, as described in the December 9, 2012 online issue of Nature Materials (DOI: 10.1038/nmat3502). First, the researchers elucidate explicitly the time scale for exciton dissociation into interfacial states and free carriers: this process occurs in 50 fs for bandgap excitation. When the heterojunction is excited by photons with energies above the bandgap, hot excitons dissociate even faster into hot interfacial states, which in turn participate in the formation of free polarons. These fast processes compete effectively with other relaxation pathways.
Density functional theory calculations for dimers of PCBM and a cyclopentadithiophene-benzothiazole (CPDTBT) oligomer also show that high-energy excitons and interfacial states are well coupled, and that the charge-transfer states are more delocalized when they are more energetic. Hence, rapid transition through hot states avoid the lowest lying, bound interfacial state and favor charge dissociation. Hot exciton dissociation through hot charge-transfer states also explains the internal quantum efficiency rise for high-energy excitations.
These findings are expected to encourage research in exploiting the energy collection from hot excitons and hot interfacial state manifolds in organic low-bandgap materials.
