Hole transfer dynamics in polymer:PC 71 BM blends
2.6. Global fit – discussion
. Analogously, decaying fraction at short delay t, before a depolarization of transition dipole moment and charge transport or recombination processes become important, should be equal to the normalized anisotropy: , where – indicates the anisotropy of pristine polymer at delay t.
In order to check the consistency of the mentioned assumptions, the decaying fraction of the PIA response ( – obtained from fit) at 0.5 ps obtained from global fit was depicted in fig. 2.5 as a function of PC71BM fraction. Lines which represent fractions of decaying components ( ), reasonably well match anisotropy at 0.5 ps (symbols) for all blends.
Fig. 2.5 Global fit results: the contribution of the sum of decaying exponents to the total response.
Symbols represent the normalized transient anisotropy at 0.5 ps; lines represent the share of growth exponents (representing the hole transferred charges) obtained by the global fit. Color/symbol code:
RRa-P3HT – green circles, MDMO-PPV – red triangles, RRe-P3HT – blue squares.
2.6. Global fit – discussion
After having tested the model for self-consistency, the fit of the response originating from the blends with different types of polymers resulted in different global time constants (Table 2.2). The contributions of growing components are shown in the supplementary information fig. S2.10. The global fit resulted in two growing exponents: 1) 30 fs and 2)
200 fs. The fits of the response of RRa-P3HT:PC71BM blends did not exhibit any decaying exponent within 1 ps. The response of the pristine MDMO-PPV polymer had the ultrafast decay of 25 fs, whereas the response of RRe-P3HT had a decay of 30 fs up to 5% of the PC71BM load. The 30 fs decay time constant was attributed to zero-time artifact74 because, first of all, its amplitude depends linearly on the excitation density up to the intensity, which was used for measurements targeting the hole transfer process, and secondly, it is observed only during the pump-probe overlap (fig. S2.7 and S2.8 and also discussion in S2.2.6).
RRe-P3HT blends were fitted also with an additional decaying exponent with a time constant of 10 ps. The exact value has no particular meaning due to too short measurement time-delay (up to 1 ps) but it signifies that there is a difference between the RRa-P3HT and RRe-P3HT blends. As was already discussed before in this Chapter, the RRe-P3HT blends with PC71BM form up CT states that are directly photoexcited and probed in the experiments described in this Chapter. These CT states must be responsible for the relatively fast decay, which is also consistent with the report by Drori et al.64.
Table 2.2. Global time constants of decaying and delayed growing exponents for various blends. Time
The ~30 fs growing component appears immediately with the addition of the smallest amount of PC71BM. The 200 fs exponential growth time constant appears at the concentration of 5% of PC71BM in RRa-P3HT and MDMO-PPV blends (fig. S2.10), whereas it was observed with 20% or higher PC71BM loads in RRe-P3HT blends (fig. S2.10). This observation is consistent with the association of slower hole transfer process originating from the PC71BM clusters (section 1.5.2). The difference of threshold PC71BM load for the appearance of 200 fs component between different blends appears most probably due to the
43 limited experimental capability to resolve 200 fs growth with different PC71BM excitation contrast.
To be more specific, the slower (~200 fs) observable hole transfer time can have several explanations: (i) hole transfer time is modified (slowed) by the presence of neighboring PC71BM molecules because the interface dipole moment between PC71BM molecule/cluster and polymer is reduced; (ii) long-range hole transfer occurs (similarly to the long range electron transfer21); (iii) the PC71BM aggregates may form delocalized band-like states21, for instance due to the formation of nanocrystals73. According to a number of previous reports21-24, 34, 75-79
, the delocalized states play an important role in efficient and ultrafast charge transfer and separation of electron-hole pairs, therefore it is not very likely that exciton delocalization over a few PC71BM molecules is responsible for a slower hole transfer rate.
The third growth component (> 1 ps) has a time constant much longer than 1 ps therefore, it cannot be resolved within 1 ps window. This slow growth has a time-scale comparable to exciton diffusion, which is addressed in detail in Chapter 3. A more detailed description and discussion are present in the supplementary materials (section S2.3 and fig.
One report has claimed an exceptionally slow hole transfer time of approximately 100 ps54. The authors claimed that the energy is transferred from photoexcited RRe-P3HT to PC61BM, and the hole is back transferred to the polymer. The films in that study were initially prepared as ultrathin bilayer structures, although real morphology is most likely more mixed due to PC61BM miscibility in RRe-P3HT. In any case, the initial bilayer structure must have caused preferentially phase separated regions of RRe-P3HT and PC61BM. Trusting the judgment of the authors that energy is transferred from RRe-P3HT to PC61BM, the excitons have a chance to diffuse inside these PC71BM aggregates. The authors did not consider the possibility of a diffusion and simply ascribed the slow rise to the hole transfer time.
The statement of the authors that energy transfer (ET) from RRe-P3HT to PC61BM is efficient54, is in conflict with the current paradigm of very efficient electron transfer in modern organic materials for photovoltaics20, 21, 31, 34
. The conclusion that ET occurs was based on the disappearance of photobleaching in the polymer. However, there might be an alternative explanation, for example, photobleaching competes against the overlapping photoinduced absorption of charges. Additionally, an overlap between the emission of
RRe-P3HT and absorption of PC61BM is very weak so that ET should not be efficient as predicted by Förster energy transfer (FRET) theory80. FRET could explain the results obtained by Soon et al.81 where the difference between two opposing systems was investigated: 1) ET was efficient in the system with high overlap integral between fluorescence and absorption of donor and acceptor 2) in contrast, inefficient ET was observed in the system with low overlap integral. Hence, the theory of FRET and experimental results of Soon et al.81 contradicts the idea of efficient ET from RRe-P3HT to PC61BM54.
To summarize, the goal of this Chapter was to investigate the hole transfer process in polymer:PC71BM blends, where donor polymers were RRa-P3HT, RRe-P3HT and MDMO-PPV. The hole transfer can occur either by 30 fs – interfacial hole transfer, or slower 200 fs – hole transfer in the presence of PC71BM clusters (fig. 2.6). The slower hole transfer might occur due to one (or more than one) of the following reasons: (i) reduced interface dipole moment; (ii) long range hole transfer (fig. 2.6b). At this point we do not have proofs to support either of the possibilities (change of interface dipole moment vs. long range hole transfer), therefore further research is necessary to attribute the slower hole transfer rate to the dominating particular phenomenon. The HOMO-HOMO energy difference (>0.5 eV) of polymer:PC71BM systems studied here and reported earlier31 is sufficient for very efficient hole transfer. It is very likely that other polymer:fullerene systems would exhibit similar hole transfer times if the HOMO-HOMO level offset is large enough.
Despite the obvious dominance of the selective PC71BM excitation followed by hole transfer in majority of polymer:PC71BM blends studied here, the interpretation of the measurement results can be done only in a broader context considering that polymer:PC71BM photoexcitation occurs as the following: 1) a direct excitation of polymer (CT-like inter-chain transitions in energy disordered regions47), 2) Charge Transfer excitations (RRe-P3HT:PC71BM blends in this study), and 3) excitations of PC71BM molecules. With an addition of PC71BM molecules, CT excitations become available in RRe-P3HT blends and have to compete with excitations of PC71BM molecules. The larger the clusters of PC71BM the smaller is the probability of CT excitations due to the decreased fraction of the interface.
Excitations of PC71BM molecules followed by the hole transfer are responsible for the delayed rise of the charge-induced response.
45 The hole transfer time is very short in all blends. Similar hole transfer rates allow making two tentative conclusions: a) most probably, the nanoscale structure is not very critical for transfer rate, and/or b) geometrical arrangement of interfacial donor-acceptor molecules is very similar in all blends, therefore, overlap of the electronic wavefunctions does not change dramatically. In order to answer to the question which of the two scenarios is a more legitimate further investigations (experiments) are needed. On the other hand, formation of PC71BM nanoclusters does result in a slightly slower hole transfer. Additionally, anisotropy of the transient photoinduced absorption data contains information related to nanoscale structure and reveals such subtle changes as disappearance of nanocrystals of polymer.
Fig. 2.6 Schematic representation of the hole transfer process occurring in the blends of polymer:PC71BM: 30 fs hole transfer on the left and slower 200 fs hole transfer on the right.
Poly(3-hexylthiophene-2,5-diyl) (P3HT), both regiorandom (RRa-P3HT) and regioregular (RRe-P3HT) types, and Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) were purchased from Sigma-Aldrich. Regioregular P3HT had a regioregularity exceeding 90% head-to-tail regiospecific conformation. The soluble C70 derivative [6,6]-Phenyl C71 butyric acid methyl ester (PC71BM, a mixture of isomers36) with a purity >99%with respect to total fullerene content (by HPLC) was purchased from Solenne.
All materials were used as received without any further purification.
2.8.2. Film preparation
Blends with the polymers and PC71BM were prepared with a PC71BM content ranging from 0 to 100% (by weight). The preparation procedure was the following: polymer and PC71BM were dissolved separately in 1,2-dichlorobenzene, also called ortho-dichlorobenzene (ODCB). MDMO-PPV and PC71BM were dissolved separately 3 g/L in ODCB while both types of P3HT with PC71BM were dissolved separately10 g/L in ODCB. Solutions were stirred overnight on the hot plate with elevated temperature of 60˚. The solution of PC71BM was filtered using polytetrafluoroethylene (PTFE) filter with pore size of 0.2 μm. Two solutions of polymer and fullerene were mixed together with appropriate volumes in order to obtain the desired fractions of PC71BM in mixed solutions. The final solutions were drop cast using equal volumes (0.2 ml) on glass microscopic cover slides with a thickness of 150 μm and were allowed to dry. Evaporation of ODCB took, at least, several hours thereby ensuring solvent assisted annealing26 of the films.
The reference sample, which was used to verify the time overlap position of the excitation and probe laser pulses, 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 1:0.382. The materials were dissolved in chlorobenzene 2 g/L separately and mixed together. The final solution was drop cast on the same substrate as the samples and allowed to dry.
The linear absorption of the films was measured using a standard Perkin Elmer Lambda 900 spectrometer with a bare substrate as a reference (fig. S2.1). The film thicknesses have been estimated from the measured profiles across a scratch in the films using a Dektak profilometer (fig. S2.2).
2.8.3. Visible pump – IR probe setup
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)83 producing visible light output (30 fs, 40 μJ) with wavelength tunability in the range of 500-700 nm, and a 3-stages IR OPO84 producing ~80 fs, 350 cm-1 FWHM spectral width transform-limited pulses at ~3 μm wavelength. The excitation wavelength was selected for the best absorption ratio between fullerene and polymer: for P3HT:PC71BM and MDMO-PPV:PC71BM it was 680 nm and 630 nm wavelength, respectively (insets of fig. 2.1). The IR probe pulse wavelength was tuned to 3.3
47 μm, close to the maximum of charge-induced absorption known as the low energy polaron (charge) absorption in MDMO-PPV50 and RRa-P3HT48, 64. The convoluted Gaussian width of pump-probe overlap was 120 fs FWHM, however the design of the experiment allows to resolve hole transfer time from the delay of the half-maximum of the response, which leads to the measured hole transfer time error < 5 fs. The visible pump pulse was focused into a spot of 200µm, which was a factor of 2 wider than IR probe pulse to minimize the spatial inhomogeneity.
2.8.4. Calculation of isotropic and anisotropic signals
PIA response was calculated as the relative transmission change ΔT/T, where T – stands for transmission and ΔT – change in transmission with and without excitation pulse. For the purpose of avoiding bimolecular (biexciton) recombination processes within the measurement time scale, the pump flux was carefully adjusted using a gradient neutral density filter for the response to be in the linear regime. The used pump flux was equal to 75 μJ/cm2 for P3HT blends, and 125 μJ/cm2 for MDMO-PPV blends. This corresponds to absorbed photon densities below 10-3 photons/nm3. The polarization of the probe beam with respect to the pump was rotated by 45˚ using a half-wave plate. The IR probe beam after having passed the sample was split into two using a 50-50 beam splitter. In the two arms, wire-grid polarizers (1:100 extinction) selected either the parallel or the perpendicular components of the IR beam with respect to the pump polarization. Two indium antimonide (InSb) detectors, cooled with liquid nitrogen, were used for detection of both components.
Signals with parallel and perpendicular polarizations were used to determine the isotropic and/or anisotropic components of the PIA signal using the following equations66:
Simultaneous detection of both polarizations dramatically enhanced the transient anisotropy signal-to-noise ratio. A third InSb detector measured the intensity of incoming IR pulses which was used to normalize the PIA signal.
During the measurement, the samples were kept under a nitrogen atmosphere to prevent degradation. The pump-probe time-overlap position was carefully checked before and after
each measurement by measuring the reference sample, MEH-PPV:TNF. This blend forms a ground-state charge transfer complex which provides a step-like response (<100 fs) limited only by system time resolution50, 61, 62, 64
(fig. S2.3a). The long-term (~1 day) drift of the pump-probe time-overlap position measured on the reference sample was ~5.5 fs root mean square (RMS) (fig. S2.3b) and was caused by thermal and humidity fluctuations of the air in the lab.