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Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy

3.4. Experimental methods

Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), regiorandom and regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) were purchased from Sigma-Aldrich. Regioregular P3HT has regioregularity greater than 90% head-to-tail regiospecific conformation. The soluble C70 derivative [6,6]-Phenyl C71 butyric acid methyl ester (a mixture of isomers19) (PC71BM) purity: >99% by HPLC with respect to the total fullerene content was purchased from Solenne.

Blends of MDMO-PPV and both P3HTs were prepared with different PC71BM content ranging from 0.02 to 9 as a PC71BM to polymer weight ratio w. The preparation procedure was the following: the polymer and PC71BM were dissolved separately with concentrations of 3 g/L for MDMO-PPV and 10 g/L for RRa/RRe-P3HT in 1,2-dichlorobenzene (ODCB) and stirred overnight on the hot plate with elevated temperature of 60˚. A solution of PC71BM was filtered using polytetrafluoroethylene (PTFE) filter with pore size of 0.2 μm. The two solutions of polymer and fullerene were mixed together with the appropriate volumes to


83 obtain the variety of PC71BM content in the solution. The final solutions were drop cast by equal volumes of 0.2 ml on the glass microscope cover slides with the thickness of 150 μm, and were allowed to dry. Evaporation of ODCB took, at least, several hours making the solvent-assisted annealing46, 47. During all the measurements, the samples were kept under the nitrogen atmosphere to prevent degradation; none was observed. Linear absorption was measured using standard Perkin Elmer spectrometer. Film thicknesses were measured using Dektak profilometer.

Time-resolved photo modulation spectroscopy was performed with a home-built setup.

The output of a 1 kHz Ti:Sapphire multipass amplifier was split to pump a noncollinear optical parametric amplifier (NOPA)48 and a 3-stages IR OPO49. NOPA was producing visible light output 30 fs, 40 μJ with wavelength tunability in the range of 500-700 nm. A 3-stage IR OPO49 was producing ~80 fs 350 cm-1 FWHM spectral width transform-limited pulses at 3.3 μm wavelength. The excitation wavelength was selected for the best absorption ratio between fullerene and polymer: for PC71BM:RRa/RRe-P3HT and PC71BM:MDMO-PPV it was 680 nm and 630 nm, respectively. The probe pulse was tuned to IR wavelength suitable for probing charge (polaron) appearance on MDMO-PPV24 and RRa-P3HT23 (fig. 4.7 in Chapter 4) as well as charge/Charge Transfer (CT) exciton in RRe-P3HT23, 50 at 3.3 μm. The system time resolution was ~100 fs. The visible pump was focused into a factor of 2 wider spot than the IR probe to minimize the spatial inhomogeneity of the pump.

The photoinduced absorption (PIA) response was calculated as the relative transmission change ΔT/T, where T – stands for transmission and ΔT – change in transmission. Pump flux was carefully tuned using gradient neutral density filter for the response to be in the linear regime. This resulted in pump fluxes of 75 μJ/cm2 for the P3HT blends and 120 μJ/cm2 for the MDMO-PPV blends. In all cases, the absorbed photon density was below 10-3 photons/nm3 (i.e. ~1 photon per 10 nm of length) to ensure a low probability of bi-exciton (non-geminate) annihilation.

The polarization of the probe beam, with respect to the pump, was rotated by 45˚ using the half-wave plate. The beam splitter was placed after the sample, splitting the IR probe beam into two. Two wire-grid polarizers (1:100 extinction), placed in the path of the two beams, were set to parallel and perpendicular directions with respect to the pump polarization. Two indium antimonide (InSb), liquid nitrogen cooled photodiodes were


simultaneously detecting two different polarizations of the signal. Parallel and perpendicular polarizations were used to recalculate isotropic component using the following relation51:

, (3.2)

where indices and denote parallel and perpendicular components, and t is a time delay.

The third InSb detector was used as a reference for IR pulses. The reference measurements together with a renormalization of digital signals greatly enhanced the signal-to-noise ratio of the PIA signal.

The precise pump-probe time-overlap position (zero delay) was carefully checked and if necessary, corrected before and after each scan (every 30 minutes) by measuring the reference sample. The reference sample was a blend of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) mixed with 2,4,7-trinitrofluorenone (TNF) by weight ratio of 1:0.3. This blend forms a ground-state charge transfer complex for which response is limited only by apparatus resolution24, 50, 52, 53

. The materials were dissolved in chlorobenzene 2 g/L separately and mixed together. The final solution was drop-cast from chlorobenzene solution of 2 g/L on the same substrate as samples and allowed to dry. The root-mean-square drift of the reference zero delay was 5.5 fs during the 15-hour measuring session, which results in better than 5 fs accuracies in the zero position between the sample scans.

spectroscopy fraction, the absorption coefficient was calculated using the following equation for each polymer:PC71BM blend and shown in fig. S3.1:

, (S3.1)

where OD – the optical density of a sample and L is the length of a sample in centimeters.

0 20 40 60 80

Fig. S3.1 Absorption coefficients (symbols) of (a) PC71BM:RRa-P3HT, (b) PC71BM:MDMO-PPV and (c) PC71BM:RRe-P3HT, calculated as optical densities divided by the film thickness at 680 nm (a and c) and 630 nm (b). The lines are the fits with the linear function (equations are given in the graphs) to the experimental data. The difference in the slope between (a), (c) and (b) is due to different wavelengths: at 630 nm, PC71BM absorbs more, by approximately a factor of 2 than it does at 680 nm.

The measured IR PIA response of PC71BM:polymer blends in this work originally had small (negligible in most cases) background response from pristine PC71BM and polymer materials (fig. S3.2). The shares of these responses depend on the relative absorption of the respective compounds. Therefore, the pristine polymer response (i.e. response from separated charges inside the polymer such as CT excitons) is expected to contribute more at low PC71BM loads while the response from pristine PC71BM becomes more visible at high PC71BM loads. For background subtraction, the responses of pristine PC71BM and pristine polymer, weighted by their relative absorption fractions, were calculated. fig. S3.2 (symbols) demonstrates the original data of photoinduced absorption -ΔT/T (isotropic component recalculated using eq. 3.1 presented in experimental methods of the main text) of the blends, where T and ΔT are the total transmission and change in transmission respectively. The share of the response of pristine PC71BM in the blends (fig. S3.2, solid blue lines) was estimated by rescaling the transient of pristine PC71BM film response (bottom panels) using the following equation:

a) b) c)