Ultrafast charge generation in a semiconducting polymer studied with $\mathrm{THz}$ emission spectroscopy

We study the ultrafast charge generation in a semiconducting polymer (MEH-PPV) by measuring the radiated $\mathrm{THz}$ field after photoexciting the biased polymer with a femtosecond visible pulse. The subpicosecond temporal characteristics of the emitted wave reflects the ultrafast photoconductivity dynamics and sets an upper limit for charge generation of $200\mathrm{fs}$ following photoexcitation, and reveals the dispersive nature of charge transport in MEH-PPV. A comparison of the fields radiated from MEH-PPV and the well-characterized model semiconductor system $\left(\mathrm{GaAs}\right)$ allows for an accurate estimate of the quantum efficiency for charge generation in the polymer, found to be less than $1\%$. Both observations are consistent with ultrafast charge generation in semiconducting polymers through hot exciton dissociation.

Figure 1

Electric fields radiated from (a) $\mathrm{GaAs}$, excited using $10{\mu \mathrm{J}∕\mathrm{cm}}^2$, $180\mathrm{fs}$, $400\mathrm{nm}$ pulses, and (b) MEH-PPV, excited using the same pulses at $225{\mu \mathrm{J}∕\mathrm{cm}}^2$, both biased with an external field of $10\mathrm{kV}∕\mathrm{cm}$. The broad positive peak in the generated field in (a) is due to a slow rise in photocurrent resulting from electron cooling effects. In (b), the sharp change in electric field direction suggests a peak in photoconductivity: the initial rise in photocurrent due to generation of carriers is offset by a rapid decrease in carrier mobility due to the finite conjugation length of the polymer backbone. The dashed lines show the response calculated using time dependent mobilities from Refs. 23, 25, respectively, plotted in Fig. 2, assuming instantaneous generation of charges. The dotted line in (b) shows the corresponding pulse assuming charges generated by hot excitons with a dissociation rate of ${\left(200\mathrm{fs}\right)}^{-1}$ (Ref. 29).

Figure 2

Comparison of time scales involved in charge generation and cooling; the solid lines in both figures represent the calculated time dependent mobility $\mu \left(t\right)$ in (a) $\mathrm{GaAs}$ (excitation with $400\mathrm{nm}$, calculated using the formalism introduced in Ref. 23) and (b) MEH-PPV (calculated from Ref. 25—see text). Dashed lines represent the generation rate $G\left(t\right)$ assuming instantaneous generation of charges upon excitation, i.e., described by the Gaussian intensity profile of the excitation pulse (FWHM $180\mathrm{fs}$). The dotted line in (b) describes the generation rate for charges generated from hot excitons with a dissociation rate of ${\left(200\mathrm{fs}\right)}^{-1}$ (Ref. 29). In (a) the rise in photocurrent is dominated by the slow evolution in $\mu \left(t\right)$, while in (b) the rise in $G\left(t\right)$ and decay in $\mu \left(t\right)$ occur on the same time scale.