Theory of interfacial charge-transfer complex photophysics inπ-conjugated polymer-fullerene blends

Abstract

We present a theory of the electronic structure and photophysics of 1:1 blends of derivatives of polyparaphenylenevinylene and fullerenes. Within the same Coulomb-correlated Hamiltonian applied previously to interacting chains of single-component π-conjugated polymers, we find an exciplex state that occurs below the polymer's optical exciton. Weak absorption from the ground state occurs to the exciplex. We explain transient photoinduced absorptions in the blend, observed for both above-gap and below-gap photoexcitations, within our theory. Photoinduced absorptions for above-gap photoexcitation are from the optical exciton as well as the exciplex, while for below-gap photoexcitation induced absorptions are from the exciplex alone. In neither case are free polarons generated in the time scale of the experiment. Importantly, the photophysics of films of singlecomponent π-conjugated polymers and blends can both be understood by extending Mulliken's theory of ground-state charge transfer to the case of excited-state charge transfer.

Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions

“…Sheng et al 48 demonstrated that polarons in P3HT:PCBM are created through two different channels: a direct, ultrafast one-step process and a slower, two-step process involving the creation and the dissociation of CT states, while Aryanpour et al 49 applied a Coulomb-correlated Hamiltonian to find that fully separated charge-pairs are not a direct photoexcitation in polymer:fullerene heterojunctions. These two works would suggest that in some systems the rapid charge separation dynamics argued here are not observed.…”

Charge carrier dynamics in organic semiconductors and their donor-acceptor composites: Numerical modeling of time-resolved photocurrent

“…7,8 In order to further improve performance of BHJs, including SMBHJs, it is important to understand mechanisms of charge carrier photogeneration, transport, and recombination, as well as their contribution to the photocurrent, in both pristine organic semiconductor materials and their D/A composites. Many recent studies have addressed dynamics and propensity for dissociation of CT states, [9][10][11][12][13][14][15][16][17][18][19] effects of D/A LUMO and HOMO energy offsets, [20][21][22][23][24] and of D/A molecular packing at the D/A interfaces, 25 in polymer-based BHJs, whereas relatively few studies have explored similar issues in SMBHJs. [26][27][28][29][30] For SMBHJs, of particular interest are solution-processable photoconductive organic semiconductors with high charge carrier mobilities and solid-state packing favoring efficient charge separation at the D/A interfaces.…”

Correlation between charge transfer exciton recombination and photocurrent in polymer/fullerene solar cells

“…[5][6][7] The energetic position of the CTE, often determined by absorption spectroscopy, 5 exhibits a strong correlation with the V oc , as the CTE is an excitation described by a hole localized on the HOMO ͑polymer͒ and an electron on the LUMO ͑fullerene͒ . 8,9 The influence of CTE on J sc is more difficult to estimate but as well important for the photovoltaic efficiency. In polymer/ polymer blends it has been observed how the intensity of CTE ͑exciplex͒ emission is influencing the photocurrent, 10,11 although nonradiative decay of exciplexes into triplet intrachain excitons has been shown to compete with free carrier generation, 11,12 thus influencing a direct correlation between exciplex intensity and J sc .…”