Optical probes of $\pi$-conjugated polymer blends with strong acceptor molecules

We used a variety of optical probes to study the primary and long-lived photoexcitations in blends of polyphenylene-vinylene derivative poly(2-methoxy-5($2^{\prime }$-ethyl)hexoxy-phenylenevinylene) (MEH-PPV) with various concentrations of a strong electron-acceptor molecule 2,4,7-trinitro-9-fluorenone (TNF) for photovoltaic applications. The optical probes include fs transient photomodulation in a broad spectral range from 0.25 to 2.5 eV and a variety of cw spectroscopies such as steady-state photomodulation and its excitation spectrum, photoluminescence, and electroabsorption. With excitation above the polymer optical gap we found that the fs transient midinfrared-photoinduced absorption band and cw photoluminescence, which are both due to photogenerated excitons, are dramatically quenched in MEH/TNF blends with increasing TNF concentration. In addition significant charge-transfer (CT) species are also instantaneously photogenerated but they undergo fast geminate recombination within $\sim 10\text{ps}$; this explains a major inefficiency of this blend for photovoltaic applications. Indeed, $I\text{−}V$ measurement of a photovoltaic cell made from (1:1) MEH-PPV/TNF blend under illumination yields photocurrent density in the $\mu \text{A}/{\text{cm}}^2$ range, three orders smaller than a device made from $\text{MEH-PPV}/{\text{C}}_{60}$ blend. Nevertheless the few photogenerated CT species that escape geminate recombination are subsequently captured in traps forming long-lived polarons. When using “below-gap” excitation in the near-infrared spectral range we found that short-lived CT species and long-lived polarons are also photogenerated with high quantum efficiency. This shows that a CT complex state is formed below the polymer optical gap in these blends, as verified by electroabsorption spectroscopy but has low dissociation efficiency; this is in contrast to polymer blends with fullerene molecules, where apparently the CT complex state below the gap has much higher dissociation efficiency.

Figure 1

(Color online) (a) Absorption spectra in a neat MEH-PPV film (green circles), TNF (violet squares), and 1:1 molar ratio MEH-PPV:TNF (red triangles). The solar irradiance spectrum (AM1.5; dashed line) is also shown for comparison. Inset shows the chemical structure of the polymer and TNF molecules. (b) Electroabsorption spectrum in a blend film (black circles) compared with the absorption spectrum (red squares). Inset depicts respective HOMO and LUMO energy levels of MEH-PPV (Ref. 36) and TNF (Ref. 37).

Figure 2

(Color online) Current-voltage dependence of (1:1) MEH-PPV/TNF (blue squares) and (1:1) $\text{MEH-PPV}/{\text{C}}_{60}$ (green circles) in bulk-heterojunction photovoltaic device configuration.

Figure 3

(Color online) (a) Clockwise photomodulation spectrum of (1:1) MEH-PPV/TNF blend excited at 2.4 eV (blue dotted line), at 1.55 eV (red solid line), compared to (b) the PM spectrum of (1:1) blend of $\text{MEH-PPV}/{\text{C}}_{60}$ excited at 2.4 eV. Samples were excited at the same pump intensity of $\sim 70\text{mW}$. The polaron bands ($P_1$ and $P_2$) and IR-active vibrations (IRAVs) are denoted; the spectra are normalized to the first polaron band, $P_1$.

Figure 4

(Color online) (b) Excitation dependence (action spectrum) of (1:1) mixture of MEH-PPV/TNF per incident photon (black solid line) of $P_1$ PA band probed at 0.6 eV. The linear absorption spectrum of the blend is also shown for comparison (red dashed line). Data have not been normalized to lifetime (see text). Inset shows lifetime determined by PA frequency-dependence measurement of $P_1$, comparing photon excitation at 2.4 eV $\left(\tau =8\text{ms}\right)$ and 1.55 eV $\left(\tau =10\text{ms}\right)$.

Figure 5

(Color online) (a) Femtosecond PM spectrum at $t=0$ of (1:1) MEH-PPV/TNF pumped at 3.2 eV (blue squares) compared to that in pristine MEH-PPV (green circles). The PA bands $P_1$, $P_2$, ${\text{CT}}_1$, and ${\text{CT}}_2$ are denoted. Inset shows the energy levels and optical transitions of polarons, singlet excitons, and triplet excitons in MEH-PPV. (b) PM spectrum of (1:1) MEH-PPV/TNF blend pumped at 1.6 eV. Inset shows the PA vs pump intensity of (1:1) MEH-PPV/TNF probed at 0.69 eV $\left({\text{CT}}_1\right)$ for both BG and AG excitation

Figure 6

(Color online) PM spectral dynamics of (1:1) MEH-PPV/TNF blend. The PA bands ${\text{CT}}_1$ and ${\text{CT}}_2$ of the CT excitons, and ${\text{PA}}_1$ of the polymer singlet excitons are denoted.

Figure 7

(Color online) Femtosecond dynamics vs TNF concentration in various MEH-PPV/TNF blends for the ${\text{CT}}_1$ PA band probed at 0.67 eV and pumped at (a) 3.2 eV (AG) and (b) 1.55 eV (BG). All data have been normalized to their respective peaks at $t=0$.

Figure 8

(Color online) Transient pump/probe polarization in parallel (black circles) and perpendicular (red squares) directions to each other and the obtained transient polarization memory decay, $P\left(t\right)$ (green diamonds); see text for P definition.