Efficient generation of charges via below-gap photoexcitation of polymer-fullerene blend films investigated by terahertz spectroscopy

Using optical-pump terahertz-probe spectroscopy, we have investigated the time-resolved conductivity dynamics of photoexcited polymer-fullerene bulk heterojunction blends for two model polymers: poly[3-hexylthiophene] (P3HT) and poly[2-methoxy-5-($3^{},7^{}$-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) blended with [6,6]-phenyl-${\text{C}}_{61}$ butyric acid methyl ester (PCBM). The observed terahertz-frequency conductivity is characteristic of dispersive charge transport for photoexcitation both at the $\pi -{\pi }^{\ast }$ absorption peak (560 nm for P3HT) and significantly below it (800 nm). The photoconductivity at 800 nm is unexpectedly high, which we attribute to the presence of a charge-transfer complex. We report the excitation-fluence dependence of the photoconductivity over more than four orders of magnitude, obtained by utilizing a terahertz spectrometer based upon on either a laser oscillator or an amplifier source. The time-averaged photoconductivity of the P3HT:PCBM blend is over 20 times larger than that of P3HT, indicating that long-lived hole polarons are responsible for the high photovoltaic efficiency of polymer:fullerene blends. At early times $\left(\sim \text{ps}\right)$ the linear dependence of photoconductivity upon fluence indicates that interfacial charge transfer dominates as an exciton decay pathway, generating charges with mobility of at least $\sim 0.1{\text{cm}}^2{\text{V}}^{-1}{\text{s}}^{-1}$. At later times, a sublinear relationship shows that carrier-carrier recombination effects influence the conductivity on a longer time scale $\left(>1\mu \text{s}\right)$ with a bimolecular charge annihilation constant for the blends that is approximately two to three orders of magnitude smaller than that typical for neat polymer films.

Figure 1

(Color online) Absorption spectra for thin films of (a) pristine P3HT (black solid line) and the P3HT:PCBM blend (red dashed line), and (b) MDMO-PPV (blue solid line) and the MDMO-PPV:PCBM blend (green dashed line). Vertical lines mark the center wavelengths of the excitation used in the experiments—560 and 800 nm. The chemical structures of the materials used are displayed at the top of the figure.

Figure 2

(Color online) Photoinduced change in conductivity for thin films of pristine P3HT (blue circles) and P3HT:PCBM (red diamonds) as a function of time after excitation with an 800 nm optical pulse at a fluence of $425\mu \text{J}{\text{cm}}^{-2}$. The inset shows the photoinduced change in conductivity of the blend after excitation with a 560 nm optical pulse at a fluence of $210\mu \text{J}{\text{cm}}^{-2}$. Lines shown are a guide to the eye.

Figure 3

(Color online) Photoinduced conductivity spectra across in the terahertz region for the P3HT:PCBM film (a) taken on the high-fluence amplifier system at 1.2 ns after excitation with 800 nm photons of fluence $425\mu \text{J}{\text{cm}}^{-2}$ (blue and green lines and circles) and after excitation with 560 nm photons of fluence $210\mu \text{J}{\text{cm}}^{-2}$ (gray dashed lines and squares, divided by ten to allow comparison); (b) taken on the low-fluence oscillator system after excitation with 800 nm photons of fluence $0.64\mu \text{J}{\text{cm}}^{-2}$ for a nominal delay time of 10 ps.

Figure 4

(Color online) Photoinduced change in conductivity for pristine P3HT and P3HT:PCBM blend films as a function of time after excitation at 800 nm using the low-fluence $\left(0.64\mu \text{J}{\text{cm}}^{-2}\right)$, high-repetition rate system. No rise in photoconductivity can be observed at zero delay (corresponding to the arrival of the excitation pulse), indicating that the photoconductivity lifetime is significantly longer than the laser interpulse period $\left(\Delta t=12.5\text{ns}\right)$.

Figure 5

(Color online) Fluence dependence of the photoinduced change in conductivity at 800 nm for MDMO-PPV:PCBM (blue diamonds) and P3HT:PCBM (red squares) measured with the low-fluence system, and for P3HT:PCBM measured with the high-fluence system (black triangles). For the low-fluence regime, the data points are the average of photoconductivity transients measured in the nominal delay range of 2–20 ps (as shown, e.g., in Fig. 4), while for the high-fluence regime the average was taken over the long-term delay scans between 0.08–1.2 ns (as displayed in the right panel of Fig. 2). Both methods yield the spectrally averaged value of the terahertz photoconductivity for the times at which the spectrum carries a charge-carrier signature. The low-fluence data represent a time-averaged photoconductivity value while the high-fluence data corresponds to the charge yield before recombination. The lines shown are power-law fits yielding approximately square root and linear dependencies for low-fluence and high-fluence experiments, respectively, as described in the text.