Ultrafast spectroscopic investigation of a fullerene poly(3-hexylthiophene) dyad

We present the femtosecond spectroscopic investigation of a covalently linked dyad, PCB-P3HT, formed by a segment of the conjugated polymer P3HT (regioregular poly(3-hexylthiophene)) that is end capped with the fullerene derivative PCB ([6,6]-phenyl-C${}_{61}$-butyric acid ester), adapted from PCBM. The fluorescence of the P3HT segment in tetrahydrofuran (THF) solution is reduced by 64% in the dyad compared to a control compound without attached fullerene (P3HT-OH). Fluorescence upconversion measurements reveal that the partial fluorescence quenching of PCB-P3HT in THF is multiphasic and occurs on an average time scale of 100 ps, in parallel to excited-state relaxation processes. Judging from ultrafast transient absorption experiments, the origin of the quenching is excitation energy transfer from the P3HT donor to the PCB acceptor. Due to the much higher solubility of P3HT compared to PCB in THF, the PCB-P3HT dyad molecules self-assemble into micelles. When pure C${}_{60}$ is added to the solution, it is incorporated into the fullerene-rich center of the micelles. This dramatically increases the solubility of C${}_{60}$ but does not lead to significant additional quenching of the P3HT fluorescence by the C${}_{60}$ contained in the micelles. In PCB-P3HT thin films drop-cast from THF, the micelle structure is conserved. In contrast to solution, quantitative and ultrafast (<150 fs) charge separation occurs in the solid-state films and leads to the formation of long-lived mobile charge carriers with characteristic transient absorption signatures similar to those that have been observed in P3HT:PCBM bulk heterojunction blends. While π-stacking interactions between neighboring P3HT chains are weak in the micelles, they are strong in thin films drop-cast from ortho-dichlorobenzene. Here, PCB-P3HT self-assembles into a network of long fibers, clearly seen in atomic force microscopy images. Ultrafast charge separation occurs also for the fibrous morphology, but the transient absorption experiments show fast loss of part of the charge carriers due to intensity-induced recombination and annihilation processes and monomolecular interfacial trap-mediated or geminate recombination. The yield of the long-lived charge carriers in the highly organized fibers is however comparable to that obtained with annealed P3HT:PCBM blends. PCB-P3HT can therefore be considered as an active material in organic photovoltaic devices.

Figure 1

Molecular structure (a) of PCB-P3HT together with a schematic representation of the molecule and (b) of P3HT-OH.

Figure 2

(a) Steady-state absorption spectra (solid lines), emission spectra (dotted lines), and fluorescence excitation spectrum (line with markers) of PCB-P3HT (black), P3HT-OH (orange), and PCBM (light blue) in THF solution. (b) Normalized time-resolved emission spectra of PCB-P3HT in THF solution (2 mg/mL) after excitation at 500 nm, reconstructed from the global analysis of the femtosecond fluorescence time profiles at several emission wavelengths. The inset sows the decay-associated amplitude spectra.

Figure 3

(a) Ultrafast fluorescence time profiles following 500-nm excitation recorded at 575 nm for PCB-P3HT and P3HT-OH solution (see legend for solvents and concentration). (b) Difference between the scaled fluorescence time profiles obtained at 575 nm for PCB-P3HT (2 mg/mL) and P3HT-OH (1 mg/mL) in THF solution. (c) Fluorescence time profiles recorded at various emission wavelengths for PCB-P3HT in THF (2 mg/mL). (d) Time evolution of the fluorescence polarization anisotropy probed at 575 nm after 500-nm excitation of PCB-P3HT (2 mg/mL) and P3HT-OH (1 mg/mL) in THF solution.

Figure 4

Transient absorption spectra recorded at various time delays after 400-nm excitation for (a) P3HT-OH in THF solution (0.5 mg/mL) and (b) PCB-P3HT in THF solution (0.5 mg/mL). The black curves represent the steady-state absorption spectra, while the black dotted curves show the stimulated emission spectra calculated from the steady-state emission spectra (negative, arbitrary unit). In (c) the normalized TA spectra of P3HT-OH and PCB-P3HT are compared for a short and a long time delay.

Figure 5

Transient absorption dynamics recorded at various wavelengths after 400-nm excitation for P3HT-OH in THF solution (0.5 mg/mL, full triangles) and PCB-P3HT in THF solution (0.5 mg/mL, empty circles). The solid black lines represent the best multiexponential fit. The fitting parameters (time constants and absolute amplitudes obtained with the normalized dynamics) are shown in the inset tables.

Figure 6

(a) Steady-state absorption spectra of PCB-P3HT (red), PCB-P3HT with C${}_{60}$ (blue), and a saturated solution of C${}_{60}$ (gray) in a THF:CS${}_2$ solvent mixture (19:1 by volume) and of C${}_{60}$ in pure CS${}_2$ (black). The concentrations are shown in the legend. The inset shows the fluorescence time profiles of the first two solutions recorded at 575 nm after excitation at 500 nm. (b) Fluorescence time profiles of a more concentrated solution of PCB-P3HT (2 mg/mL) without (orange) and with 2 mg/mL C${}_{60}$ (light blue) in a 3:1 THF:CS${}_2$ mixture.

Figure 7

AFM images (phase, on sapphire) of PCB-P3HT drop cast from (a) THF (2.0 mg/mL) and (b) DCB (12 mg/mL). The insets show a schematic representation of the proposed self-assembly.

Figure 8

Transient absorption spectra recorded at various time delays after 400-nm excitation for (a) P3HT-OH drop-cast from THF (2 mg/mL), (b) PCB-P3HT drop-cast from THF (2 mg/mL), and (c) PCB-P3HT drop-cast from DCB (12 mg/mL). The black dotted curves represent the steady-state absorption spectra (negative, arbitrary unit).

Figure 9

Transient absorption dynamics recorded at various wavelengths after 400-nm excitation with 190 μJ/cm${}^2$ (or 250 μJ/cm${}^2$ if indicated) for (a) P3HT-OH drop-cast from THF (2 mg/mL), (b) PCB-P3HT drop-cast from THF (2 mg/mL), and (c) PCB-P3HT drop-cast from DCB (12 mg/mL). The solid lines represent the best multiexponential global fit.