Efficiency of Exciton and Charge Carrier Photogeneration in a Semiconducting Polymer

We determine the efficiencies for the formation of excitons and charge carriers following ultrafast photoexcitation of a semiconducting polymer (MEH-PPV). The simultaneous, quantitative determination of exciton and charge photoyields is achieved through subpicosecond studies of both the real and the imaginary components of the complex conductivity over a wide frequency range. Predominantly excitons, with near-unity quantum efficiency, are generated on excitation, while only a very small fraction ($<{10}^{-2}$) of free charges are initially excited, consistent with rapid ($\sim 100\mathrm{fs}$) hot exciton dissociation. These initial charges are very short lived, decaying on subpicosecond time scales.

Figure 1

(a) Inset: Photoexcitation occurs with 150 fs pulses (266 or 400 nm), while the conductivity at delay $\tau$ is probed using pulsed THz radiation (1 ps, 0–1.5 THz). By varying the delay between pump and probe pulses and measuring the magnitude of the THz field modulation, $\Delta E(t,\tau )$, the photoconductivity dynamics can be studied. The dynamics of MEH-PPV show two distinct decays, designated fast and slow. (b) The fast time dynamics decays exponentially with time constant $0.9\pm 0.2\mathrm{ps}$, while the rise time is $\sim 0.3\mathrm{ps}$. (c) The slow time dynamics occur with a time constant of $200\pm 40\mathrm{ps}$.

Figure 2

(a) The complex conductivity of an MEH-PPV film measured 0.5 ps after excitation (circles). The lines represent the conductivity of a hole charge on a PPV chain simulated using the formalism introduced in Ref. [21], for a density $n_0^{\mathrm{free}}\sim 3.2\times {10}^{21}{\mathrm{m}}^{-3}$ (lines). (b) The complex conductivity measured 10 ps after initial excitation (circles) and the conductivity expected for bound excitons (lines) generated with unit quantum efficiency and polarizability, $\alpha \sim 800{\AA }^3$.

Figure 3

The complex conductivity measured 10 ps after photoexcitation on an MEH-PPV/PCBM blended sample, demonstrating the response for thermalized hole charges. The lines represent the simulated conductivity also shown in Fig. 2a, with a density $n_0^{\mathrm{free}}\sim 8.0\times {10}^{21}{\mathrm{m}}^{-3}$.