Ultrafast dissociation of triplets in pentacene induced by an electric field

Singlet fission into a pair of triplet excitons in pentacene is envisaged to be a potential carrier multiplication route in organic photovoltaics. In order to shed light on the dissociation dynamics of the triplet states we performed electric field induced pump-probe experiments on polycrystalline films of pentacene. We find that the nascent triplet pair formed immediately after the fission process is susceptible to dissociation in the presence of the electric field. By measuring the temporal dynamics of the dissociation, we estimate its binding energy to be approximately 320 meV. After about 2 ps the triplets diffuse away from each other and the field assisted dissociation is substantially reduced. Our results suggest that if the nascent triplet pairs could be harvested at the interface in a device, the efficiency of the process would be higher.

Figure 1

(a) Femtosecond transient absorption (fs-TA) spectra of pentacene thin films at various pump-probe delays, pump wavelength at 490 nm. Also plotted are the cw-PA spectra taken at various modulation frequencies, pump wavelength at 532 nm. Each of the spectra are plotted with certain offset. The excitation density for the fs-TA is around $1\times {10}^{18}{\mathrm{cm}}^{-3}$. (b) Field induced pump-probe spectra at −5 ps and 20 ps pump-probe delay of pentacene film with an applied field of about 200 kV/cm. Also shown is the rescaled electroabsorption spectrum on the same sample.

Figure 2

(a) Background subtracted field induced pump-probe spectra at various pump-probe delays. Also shown is the negative EA spectrum in the topmost panel. (b) Normalized FPP kinetics at various probe wavelengths. (c) Illustration to show the interpretation of FPP spectra. A negative FPP signal for a PA corresponds to an increase of population due to field and a positive signal corresponds to a reduction. This is assuming zero EA response.

Figure 3

(a) Experimental FPP dynamics of triplet and polaron bands along with monoexponential fits with a time constant of 1.35 ps. (b) Proposed scheme of field induced dissociation in pentacene after singlet fission.