University of Groningen. Organic donor-acceptor systems Serbenta, Almis

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1 University of Groningen Organic donor-acceptor systems Serbenta, Almis IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2016 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Serbenta, A. (2016). Organic donor-acceptor systems: Charge generation and morphology. [Groningen]: University of Groningen. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date:

2 Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy Part of this chapter is published in Ultrafast Phenomena XIX Proceedings of the 19th International Conference, Okinawa Convention Center, Okinawa, Japan, July 7-11, 2014: Maxim S. Pshenichnikov, Almis Serbenta, and Paul H. M. van Loosdrecht. The material of this chapter is submitted to journal: Almis Serbenta, Oleg V. Kozlov, Paul H.M. van Loosdrecht and Maxim S. Pshenichnikov 3. Abstract Morphology of organic photovoltaic blends is one of the key factors influencing power conversion efficiency of organic solar cells. We demonstrate that visible pump infrared (IR) probe spectroscopy is a powerful tool to investigate the morphology of the polymer:fullerene blends on the fly. The soluble fullerene derivative PC 71 BM was used as a light absorber while the appearance of charges on the polymer was probed with MID-IR pulses. The essence of the proposed method is the observation of the appearance of charges on the polymer, which is directly related to exciton diffusion, inside and out of PC 71 BM clusters. The details of the PC 71 BM exciton dissociation dynamics provide the necessary information to estimate the characteristic size of PC 71 BM clusters. Our studies demonstrate noticeable differences in the polymer:fullerene mixing behavior for different polymers. These results have direct implication for organic solar cell design where fullerene derivatives are the key players Introduction Organic materials have recently gained interest for their potential use in renewable energy technology, energy efficient lighting applications, and in other optoelectronic devices 1. Photophysical properties of organic materials strongly depend on their molecular packing and nanoscale structure, called morphology 2. Morphology, in particular, plays a crucial role in organic photovoltaics where charge extraction is the final goal 1, 3-5. Light absorption in organic materials typically creates strongly bound electron-hole pairs, called the Frenkel excitons 6. These excitons are separated efficiently at the interfaces between an electron donor (D) and acceptor (A) materials in the bulk heterojunction (BHJ) 7 67

3 architecture 1, 3-5. The diffusion length of excitons in organic materials typically does not exceed 5-15 nm 1, 3-5. Therefore, it is crucial to provide a large interfacial area between donor and acceptor materials, requiring maximal sizes of the phase-separated regions of ~10 nm. Moreover, pathways for charge carriers to move towards electrodes are necessary for their extraction. These strong, but conflicting, requirements make morphology one of the key factors determining the overall power conversion efficiency of organic solar cells. So far, there is no solid theory to predict, or provide a systematic method to control, the selforganization of BHJ except for a few cases where general self-organization patterns were qualitatively computed 8, 9. These challenges have driven the development of morphology characterization techniques. Standard characterization methods such as electron or X-ray microscopy/spectroscopy are capable of achieving a spatial resolution below 10 nm with the help of contrast enhancement techniques 2 such as energy filtering or selective staining of one of the materials with iodine vapors 10. Sub-10 nm spatial resolution, however, is not easily achieved, in particular for the typically low contrast combinations of donor:acceptor materials used in organic photovoltaics. Though the list of available morphology characterization methods is fairly extensive 2, morphology characterization methods have limitations, which depend on the particular method, on the particular types of donor:acceptor blends studied, and on the specific sample preparation methods 2. The major limiting factors in achieving a high spatial resolution in organic photovoltaic blends are usually the poor contrast between materials 2, even when contrast enhancement techniques are used, and the sample degradation under exposure of the imaging X-ray or electron beams. Another drawback of many of the standard methods is that they cannot be applied to actual working devices; usually at least one of the electrodes has to be absent. This requirement poses a serious problem because the morphology typically changes by deposition or removal of the electrode 11. A popular microscopy technique is Atomic Force Microscopy (AFM), which only provides information about the surface topography, a feature that is not necessarily representative for the bulk morphology. This technique is also not suitable for working devices, and the needed sub-10-nm resolution is problematic 12. Aiming to overcome these limitations, alternative methods were developed based on spectroscopic approaches, e.g. monitoring exciton diffusion by photoluminescence (PL) quenching 13, 14 or by pump-probe spectroscopy 15, 16. So far, these methods focused on exciton 68

4 Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy generation in the polymer domain, which provides information on the polymer domain sizes. Polymer excitons, however, are known to be delocalized over several repeating units 17, 18, which limits the highest attainable spatial resolution. Additionally, the PL quenching method functions properly only if the PL quantum yield is reasonably high, which is incompatible with a high efficiency in organic solar cells. Besides the strong absorption by the donor polymer, some fullerene derivatives, e.g. 10, PC 71 BM, show a substantial absorption of sunlight as well. This opens a new opportunity to shed light on the size of the fullerene domains by studying the exciton diffusion in these domains, which can complement information obtained from other experiments on the length scale of the polymer domains. In this Chapter, we demonstrate a new all-optical method for characterizing the nanoscale morphology of the fullerene domains in BHJs. The method is based on spectrally selective excitation of the soluble fullerene C 70 derivative PC 71 BM, and detecting the time which is taken by the exciton to diffuse to the PC 71 BM:polymer interface where it splits into separated charges. The latter are detected by the charge-induced (polaron) absorption 12 induced at the polymer backbone by the presence of positive charges (holes). For three polymers, regiorandom (RRa) and regioregular (RRe) P3HT as well as MDMO-PPV, which were selected as benchmark materials for exemplary cases of BHJ morphologies, we show that their blends contain charge-producing PC 71 BM clusters up to ~7 nm in size dispersed within the polymer matrix. The presence of larger PC 71 BM clusters is evidenced by the observation of a reduced efficiency of photon-to-charge harvesting. Unique spectroscopic signatures of subtle changes in BHJ morphology hold great promise for applications of the proposed technique for on-the-fly device characterisation of fully functional devices Results and discussion Probing the morphology. Implementation Spectrally selective excitation of the PC 71 BM followed by spectrally selective probing of the polymer allows spatial decoupling of the excitation and the probing processes. Selective photoexcitation of PC 71 BM was achieved by tuning the excitation wavelength below the bandgap of the polymer were the PC 71 BM has a significantly higher absorption coefficient. Figure 3.1a-c shows the red edge of the absorption spectra of the three pristine polymers (dotted curves) and their blends with PC 71 BM (dashed curves). Based on the 69

5 highest contrast between PC 71 BM/polymer excitations (i.e. the ratios of their absorption spectra, solid lines), the excitation wavelength was selected as 680 nm for both RRa-P3HT and RRe-P3HT mixed with PC 71 BM, and 630 nm for PC 71 BM:MDMO-PPV. a) b) c) Fig. 3.1 Absorption of PC 71 BM:polymer blends with three different polymers: (a) RRa-P3HT, (b) MDMO-PPV, (c) RRe-P3HT. The dotted and dashed lines depict absorption spectra of pristine polymers and their blends with PC 71 BM:polymer weight ratio 7:3, respectively. The solid lines present the ratio between optical densities of the blends and pristine polymers. Exciton dissociation into charges was monitored by probing the charge-induced (polaron) absorption of the polymers in the mid-ir For this, the wavelength of the probe IR pulse was set close to the maximum of the low-energy polaron band at 3 μm for all three polymers (for polaron spectrum of RRa-P3HT, see Chapter 4). As the exciton is harvested (i.e. reaches the interface and dissociates into the charges), the charge-induced (polaron) absorption increases proportionally to the amount of holes in the polymer. By changing the delay between the pump and the probe pulses exciton diffusion followed by exciton dissociation, is monitored in the real time. Dynamics of the exciton dissociation to charges are depicted in fig. 3.2 for different PC 71 BM/polymer weight ratios w. The transients were corrected for the pristine polymer response, present due to the finite contrast in excitation (this is especially important for the RRe-P3HT, fig. 3.1f). The correction was also applied for IR response of the PC 71 BM, noticeable for RRa-P3HT and MDMO-PPV at high values of w (see section S3.1 for more details). All transients were also normalized to the PC 71 BM absorption at the excitation wavelength (i.e. the transient amplitudes represent the charge yield per absorbed photon) to allow for direct comparison of the transient amplitudes at different PC 71 BM loads. The charge yields in fig. 3.2 was scaled by a constant number for each set of polymer:pc 71 BM blends separately in order to obtain the maximal amplitude of one for the low PC 71 BM load blends, which are expected to have a 100% exciton dissociation to charges efficiency. A more detailed explanation is presented in the section "Charge yield". 70

6 Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy Fig. 3.2 Charge yield as a function of time for PC 71 BM:polymer blends with different PC 71 BM weight fractions. Three types of polymers were used as donor materials: (a) RRa-P3HT (b) MDMO-PPV (c) RRe-P3HT. Symbols represent experimental data points after background subtraction; lines represent simulations. Irregularities in the transient amplitudes at low PC 71 BM content are due to low absorption over which the transients are normalized. The charge yield dynamics for the blends with the three polymers have similar features that can be summarized in the following way: for the blends with low PC 71 BM content (low w) their respective transients exhibit a large amplitude and a rapid rise time (< 1 ps), whereas for the blends with high PC 71 BM content (high w) the amplitudes are decreased (except of the RRa-P3HT blends) while the rise of the response becomes substantially slower: up to 100 ps. These dynamics are assigned to the PC 71 BM exciton diffusion followed by the dissociation to charges at the PC 71 BM:polymer interface. We attribute this behavior to variations in the PC 71 BM cluster size: the larger the PC 71 BM clusters, the longer it takes for excitons to reach an interface (therefore, the slower rise of the response) and more excitons are lost (therefore, the lower amplitude of the response). 71

7 Data analysis Charge yield Fig. 3.3 shows the PC 71 BM exciton dissociation to charges yield obtained from the maximal amplitudes of charge-induced response (fig. 3.2). Blends with the low PC 71 BM/polymer weight ratio w, for instance, w 0.4, exhibit a high yield because PC 71 BM clusters are either small or not present at all (i.e. there are only isolated PC 71 BM molecules dispersed in the polymer). Taking into consideration ultrafast hole-transfer (HT) time (~30 fs) 25 (see also Chapter 2), we can assume that most excitons, very close to 100%, dissociate into charges for the blends with low PC 71 BM content, e.g. w = The charge yield (fig. 3.3) remains constant within experimental accuracy up to w = 0.4 for all PC 71 BM:polymer blends. Therefore, we assign the signal at low PC 71 BM content to the exciton-to-charges yield of unity, taking the average amplitude (shown in fig. S3.2) for blends with w = as the normalization factor. a) b) c) Fig. 3.3 PC 71 BM charge yield calculated from the amplitudes of the pump-probe transients (fig. 3.2) for the blends of (a) PC 71 BM:RRa-P3HT, (b) PC 71 BM:MDMO-PPV and (c) PC 71 BM:RRe-P3HT. Symbols represent the experimental data, the lines present the results of the Monte-Carlo simulations. The red open symbols represent amplitudes at 1 ps, which are assigned to dissociation of the interface excitons, the green closed symbols are the maximal amplitudes, associated with all excitons that have reached interface, and the blue semi-closed symbols are the difference of the two, i.e. the excitons which originate from the bulk (i.e. outside the interface) of the PC 71 BM clusters. 72

8 Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy Further increase of w results in a decrease of the overall charge yield due to the growth of PC 71 BM clusters, which prevents a significant number of excitons from reaching an interface. The dependence of the yield on w for the three polymers blended with the PC 71 BM is noticeably different. The RRa-P3HT blends (fig. 3.3a) do not exhibit clearly observable exciton losses even with the highest content of PC 71 BM (green closed symbols in fig. 3.3a). High charge yields in the RRa-P3HT blends signify good intermixing between the PC 71 BM and the RRa-P3HT with mostly small PC 71 BM clusters forming up to w = 9. In contrast, the MDMO-PPV blends (fig. 3.3b) demonstrate a significant loss of exciton dissociation efficiency already at the PC 71 BM content of w~1.5. This is not surprising since MDMO-PPV blends with fullerene derivatives are known to form large fullerene domains above certain acceptor weight fraction (fig. S3.8), depending on the preparation method 7, (e.g. w = 1 to 4). The PC 71 BM:RRe-P3HT blends (squares in fig. 3.3c) exhibit a decrease of the yield at w = similarly to the MDMO-PPV blends but to a significantly smaller extent. The observed difference of charge yield between the RRa-P3HT and the RRe-P3HT was expected because the morphology of the films that are formed by these two polymers is very different: the film of RRa-P3HT is completely amorphous, but the film of RRe-P3HT forms semicrystalline domains. The molecules of RRe-P3HT form nanocrystals prior to the aggregation of PC 71 BM during solution drying process 29. Hence, most of the PC 71 BM molecules can only be dispersed in the disordered regions outside the nanocrystals of the RRe-P3HT film 30. Consequently, PC 71 BM molecules have less volume to be dispersed within and, as a result, the PC 71 BM is pushed to aggregate into the clusters. Therefore, we assign exciton losses (fig. 3.3c) in the blends of RRe-P3HT with w = to the formation of the PC 71 BM cluster sizes comparable or larger than the exciton diffusion length. Extrapolating this trend, one would expect the charge yield from the RRe-P3HT blends to further decrease above w = 1.5, however, the yield suddenly increases at w = 2.3. This unexpected and significant turn indicates an abrupt change in the nanostructure. The confirmation of this comes from the fact that linear absorption spectra demonstrate the disappearance of the red absorption shoulder of the RRe-P3HT near these blend compositions (fig. S2.2e in Chapter 2) associated with the absorption by the RRe-P3HT nanocrystals 31. Others have also observed disruption of the RRe-P3HT nanocrystal at similar donor:acceptor compositions 32, 33. Hence, the morphology of the RRe-P3HT blends becomes more similar to 73

9 that of the RRa-P3HT blends for w 2.3 because most of the nanocrystals are no longer present. Summarizing the discussion above the three PC 71 BM:polymer blends exhibit very different charge yield as a function of the blend composition, which is reflected in the amplitude of the charge-induced response. Interestingly enough, the charge yield is sensitive to subtle changes in the morphology like the disappearance of nanocrystals in RRe-P3HT. To analyze the particular characteristic size of PC 71 BM domains, the dynamics of exciton diffusion should be considered Exciton dissociation represents their diffusion The characteristic size of the PC 71 BM cluster can be estimated on the basis of excitons that dissociate via HT almost immediately after photoexcitation as they are generated at the interface with polymer (see also Chapter 2), and excitons, which are delayed because they are generated in the bulk of PC 71 BM and, therefore, have to diffuse prior to the dissociation. These two very different time-scales, namely the HT (< 1 ps 25 ) and the exciton diffusion ( ps 20, 34 ), allow calculation of the fraction of excitons, which are generated at the interface with respect to the total number of excitons, by simply comparing the transient amplitudes at the respective times. This, in turn, provides a unique opportunity to make an estimate of the characteristic PC 71 BM cluster size as the ratio between the surface (interfacial) excitons and the entirety of (surface plus bulk) excitons through the surface-to-volume ratio. This ratio is inversely proportional to the linear size of the PC 71 BM cluster. This idea is illustrated in fig The charge yield at 1 ps (red open symbols) and 100 ps (green closed symbols) delays is related to the interfacial and all harvested excitons, respectively. The difference between the two represents the bulk excitons (semi-open blue symbols in fig. 3.3). The general observation of all blends studied in this work is a decrease of the share of interfacial excitons when PC 71 BM content increases, which is assigned to a larger number of excitons harvested from the bulk of PC 71 BM clusters. RRa-P3HT blends show the increasing percentage of the dissociated bulk excitons when w increases: the increase is steep for low w = , while the increase becomes slower with larger w > 0.5. MDMO-PPV blends (triangles in fig. 3.3b) exhibit the steep increase of the bulk exciton fraction only in the range of w = when PC 71 BM content is low. These blends with further increase of PC 71 BM content w = show an almost constant share of dissociated bulk excitons while the portion of excitons created at the 74

10 Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy interface as well as the total dissociation yield decrease. Fig. 3.3c shows the charge yield for RRe-P3HT blends (fig. 3.3c). The share of excitons created at the interface in these blends follows similar dependence on PC 71 BM load as the total yield of dissociated excitons. The dissociated bulk exciton fraction increases with the amount of PC 71 BM for w > 1 similarly to RRa-P3HT. The RRe-P3HT blends, unlike the RRa-P3HT and the MDMO-PPV, do not show discernible dissociation of bulk excitons with w < 1.5 despite the fact that the overall charge yield decreases. When excitons are lost in the diffusion process, the appearance of extra charges is limited by the lifetime of excitons. Therefore, with the decrease of overall response the rise of the response limited by the finite (singlet) exciton lifetime (~500 ps in fig. S3.4, which is also consistent with fig. 4.6 in Chapter 4 and other reports 20, 35 ) is also expected. However, the experimental data presented in fig. 3.2 clearly exhibit charge-induced response saturation within the first 100 ps, which is much shorter than the exciton lifetime. This observation poses a question: why does the response saturate much earlier than exciton lifetime limitation? This seemingly contradictory observation can be resolved in the following way. The majority of PC 71 BM clusters that produce charges are smaller than the exciton diffusion length so that the leveling-off of the experimental transients is limited by the cluster size but not the exciton lifetime. In turn, the loss of charge yield is caused by the PC 71 BM domains, which are so large that they contribute a negligible number of bulk excitons to the overall charge yield. Such a situation is only possible with a bimodal distribution of the characteristic PC 71 BM cluster sizes that are very different from each other. The bimodality can be explained by the concept of hierarchical morphology, present in the BHJ. The concept of hierarchical morphology is based on large phase separated fullerene domains that are surrounded by the polymer matrix, which contains dispersed fullerene molecules and small clusters 30, 36, 37. This argument is also applicable for the MDMO-PPV blends, which also exhibit a saturation of charge-induced response (charge yield) together with the decreasing overall amplitude. A more detailed analysis will be presented below Size of PC 71 BM aggregates Monte Carlo simulations To describe the dynamics of exciton diffusion in the PC 71 BM clusters, Monte Carlo (MC) simulations were used. The important advantage of the MC approach is that they allow 75

11 the inclusion of a non-flat energy landscape, which does not require finding an analytical solution of the diffusion equation. Exciton diffusion was simulated as a random hopping between the discrete PC 71 BM molecules in a cluster of the close-packed spheres in the hexagonal arrangement; the cluster itself was modeled as a sphere. Initially, excitons are generated randomly in the PC 71 BM molecules within the cluster. Energetic disorder of the potential energy landscape of the PC 71 BM cluster was taken into account by the Gaussian disorder model 38. Differences in energies of the PC 71 BM molecules were randomly assigned with the Gaussian distribution function with the standard deviation width of σ, a starting global fit value was 70 mev 39. Exciton hopping rate k ET between neighboring sites i and j with energies E i and E j, respectively, is described as a thermally activated process with the Boltzmann distribution: (3.1) where is the hopping rate when no thermal activation is required (exo-energetic/isoenergetic exciton hopping rate), is the Boltzmann constant, and is the temperature (fig. 3.4). The initial exciton hopping rate (no barrier) was kept as a global fit parameter with the starting value of 5 ps -1, which is consistent with the exciton diffusion length of ~5-6 nm 40, 41 in the infinitely large PC 71 BM domain (fig. S3.4). Finally, exciton dissociation into charges at the surface of PC 71 BM cluster occurs with a finite hole transfer (HT) time 25 (see also Chapter 2). The only variable parameter (per sample) associated with exciton diffusion was the number of PC 71 BM molecules, which was translated into PC 71 BM cluster size as the diameter of a sphere (fig. S3.6), whereas all other parameters, such as energy disorder and exo-energetic (or iso-energetic with a very small probability) exciton hopping rate, were kept global for all transients shown in fig The loss of excitons is caused by the presence of large PC 71 BM domains, where most of the PC 71 BM excitons should not produce charges. However when the overall response becomes small then inputs from small and large PC 71 BM clusters may become comparable, which would result in an observable rise of the response at longer than 100 ps delays, therefore experimental data would not be reproduced adequately. The experimental data can be reproduced adequately only when the response (as small as it is) is dominated by the dissociation of excitons originating from small PC 71 BM clusters (a more detailed explanation 76

12 Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy is presented in S3.2). Therefore, the simulations were run on each transient shown in fig. 3.2 with both, the small PC 71 BM clusters and with the large PC 71 BM domains, to make sure that the measured PC 71 BM domain sizes are large enough for simulations to reproduce the experimental data adequately. The average PC 71 BM domain size of each particular sample was measured with atomic force microscopy (AFM) (MDMO-PPV blends, fig. S3.8) and scanning transmission electron microscopy (STEM) (RRe-P3HT blends, fig. S3.9). a) b) Fig. 3.4 Schematic representation of the Monte-Carlo simulation. (a) a schematic representation of the geometry of the simulation, (b) energy disorder model. Excitons are randomly generated within a PC 71 BM cluster (blue-red circle), diffuse by random hopping (pink arrows) to the interface where hole transfer takes place (green straight arrow). Each PC 71 BM molecule has a predefined random energy variation within energy disorder (b); the exciton hops from one molecule to another with the thermallyactivated rate k MC simulation results The MC simulations were able to reproduce experimental data presented in fig. 3.2 (symbols) reasonably well. The simulation results shown in fig. 3.2 as solid lines demonstrate that the large PC 71 BM domains contribute negligible response (no exciton lifetime rise of the response is observed) and their surface indeed can be neglected, which means that these domains are spectroscopically invisible. The fact that the large PC 71 BM domains are spectroscopically invisible allows the detection of the volume fraction of large and small PC 71 BM clusters, which was used as a fitting parameter in order to reproduce the amplitudes of the transients in fig. 3.2 (see S3.2 for more details). The results of the simulations are summarized in fig. 3.5 as solid lines the PC 71 BM cluster sizes. The estimated PC 71 BM cluster size varies from 2 nm to 7 nm. The global fit parameters resulted in energy disorder mev (consistent with other report 39 ) and exo-energetic exciton hopping rate ps -1. Both parameters represent a statistical average value of the whole ensemble. The energy disorder σ determines the time delay position when the response starts to saturate because the further arrival of excitons becomes 77

13 PC 71 BM cluster size (nm) much less probable when these excitons are trapped. The exo-energetic exciton hopping rate k 0 determines how fast the excitons arrive at the interface, therefore, changing has a very similar effect to the change of the size of PC 71 BM cluster. The strong variation of does not make sense because PC 71 BM exciton diffusion length has to be realistic (~5-6 nm 40, 41 ). 6 RRa-P3HT MDMO-PPV RRe-P3HT PC 71 BM/polymer weight ratio (w) Fig. 3.5 Estimated PC 71 BM cluster size dependence on the blend composition. The lines are results of Monte Carlo simulations while the symbols represent estimates made from surface volume / total volume ratio. (a) represents RRa-P3HT blends, (b) MDMO-PPV blends, (c) RRe-P3HT blends. Summarizing the results, the PC 71 BM load can be ascribed to three main categories: 1) the low PC 71 BM load w 0.11; 2) the average PC 71 BM load w = ; 3) the high PC 71 BM load w 2. The low PC 71 BM load blends of RRa-P3HT and MDMO-PPV exhibit the steep growth of small PC 71 BM clusters up to the size of ~3 nm. Similar PC 71 BM content dependence should be present in RRe-P3HT blends 14, however, experimental data does not exhibit growth of small PC 71 BM clusters up to w = 1 due to limited experimental accuracy as the PC 71 BM excitation selectivity was too low (part of the absorption goes to the charge transfer state excitation as explained in Chapter 2). The growth of small PC 71 BM clusters with further PC 71 BM load appears to be rather slow in RRa-P3HT and MDMO-PPV blends as compared to low PC 71 BM loads. On the other hand, the growth of PC 71 BM clusters with increasing w of RRe-P3HT blends becomes comparable to that of RRa-P3HT. Note that the previously discussed disruption of nanocrystals of RRe-P3HT is reflected as the disappearance of the large PC 71 BM domains, shown in fig. 3.6a w = The shortcoming of using the response from dissociated PC 71 BM excitons is that MC simulations cannot extract the particular size of large PC 71 BM domains where excitons do not reach the interface. The size of large PC 71 BM domains was obtained from the AFM for the a) b) c) 78

14 Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy MDMO-PPV blends (fig. S3.8) and from STEM for the RRe-P3HT blends (fig. S3.9); the results are shown in fig. 3.6b. a) b) Fig. 3.6 Simulated volume fraction and size of large the PC 71 BM domains where most excitons do not reach the interface with a polymer. The top panel (a) represents the volume fraction while the bottom panel (b) shows the size of large PC 71 BM domains (obtained from AFM and STEM measurements). The large (> 15 nm) PC 71 BM domains start to appear in the MDMO-PPV and the RRe-P3HT blends for w > 0.5. Coincidently, the small clusters of PC 71 BM do not exhibit significant growth with an increase of w. It may seem that with increasing PC 71 BM load not only the size and density of the large (> 15 nm) PC 71 BM domains should increase but also that the small ( 7 nm) PC 71 BM clusters should grow in size substantially, especially for the MDMO-PPV blends with high PC 71 BM loads. This unanticipated behavior can be explained in the following terms: when the PC 71 BM load increases, the density of small PC 71 BM clusters should increase; hence, these clusters would start to merge at some point forming large PC 71 BM domains. Consequently, the size of small PC 71 BM clusters would not increase substantially with the addition of extra PC 71 BM but the density of large PC 71 BM domains would definitely increase. Another explanation for the slow growth of the small ( 7 nm) PC 71 BM clusters is that, in fact, these clusters are percolated with polymer chains or segments, therefore, effectively reducing their size. This effect is expected to be slightly more pronounced in the PC 71 BM:polymer blends as compared to the PC 61 BM:polymer because the PC 71 BM (unlike PC 61 BM) is a mixture of three isomers and hence, the PC 71 BM is expected to exhibit higher solubility and lower crystallinity, which consequently leads to higher spatial disorder. Alternatively, it is possible that two PC 71 BM molecules might prefer to stick to each other 79

15 entwining with their side chains already within a solution, before the drop-casted solution dries. Consequently, as PC 71 BM concentration in the solution would increase with an increase of PC 71 BM, the probability of PC 71 BM aggregation would also increase. This increasing probability of aggregation could account for the faster growth of PC 71 BM clusters up to w = As soon as w would become larger than 0.25, the growth of the small PC 71 BM clusters would saturate, which could be caused by saturation of aggregation in the solutions used in this work (i.e. most PC 71 BM molecules are paired). Formation of larger PC 71 BM aggregates in the solution is not very likely. Therefore, further growth of PC 71 BM clusters would be driven by aggregation during solution drying process, which would not be as steep as the initial aggregation occurring already in the solution. It cannot be entirely excluded that excitons are delocalized over two or more isoenergetic PC 71 BM molecules (i.e. PC 71 BM nanocrystal 32 ) and dissociate immediately via hole transfer to the polymer. Therefore, the actual size of PC 71 BM clusters may be slightly larger than estimated. Referring to Kim et al. 32 formation of PC 61 BM nanocrystals is not very pronounced with w 1 for MDMO-PPV and for RRa/RRe-P3HT blends. As was already indicated, the PC 71 BM is expected to be less likely to form nanocrystals as compared to PC 61 BM. Therefore, exciton delocalization should not play a significant role in underestimation of PC 71 BM cluster size at least for the relatively low PC 71 BM loads of w 1. Moreover, due to the disappearance of the RRe-P3HT nanocrystals for w 1.5, RRe-P3HT becomes more amorphous, whereas PC 71 BM molecules are highly soluble in the amorphous regions of P3HT as compared to the nanocrystalline regions Short vs. long time-scale PC 71 BM exciton dissociation Estimation of the PC 71 BM cluster size can be performed using a simple intuitive method: the calculation of the ratio of interfacial excitons to the total excitons. The interface/total exciton ratio is equal to the ratio of the volume of an outer PC 71 BM layer over the total volume:, where R is the total radius of PC 71 BM cluster and r is the radius of inner sphere associated with the bulk excitons, V S and V T are surface volume and the total volume respectively, A(1 ps) and A(MAX) are charge yields at 1 ps and the maximum yield (~100 ps), respectively, and r=r-1 nm assuming the size of PC 71 BM molecule to be ~1 nm 42. Using this simple approach, the diameter of the PC 71 BM domains was calculated for all blends (fig. 3.5 symbols). The PC 71 BM cluster size obtained using the 80

16 Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy simple volume ratios is in excellent agreement with the MC simulations. This simple approach opens an opportunity to check quickly the characteristic size of PC 71 BM clusters on the fly even on the working devices Conclusions An ultrafast pump-probe technique has been implemented to gain insights into the BHJ morphology of organic donor:acceptor materials for solar cells. This approach is complementary to the one demonstrated by Westenhoff et al. 13 where the donor polymer was excited, as it allows estimating PC 71 BM cluster size. The proposed method is based on the selective photoexcitation of the acceptor material PC 71 BM and detecting the appearance of charges on the donor polymers, preceded by PC 71 BM exciton diffusion, by probing the low energy charge-induced (polaronic) transition at 0.4 ev. Unlike the excitation of polymer, which is delocalized along the polymer chain in the range of several nanometers 43-45, excitation of PC 71 BM is usually localized in a single molecule. As the (de)localization length determines the minimum attainable spatial resolution, PC 71 BM excitation appears more advantageous than excitation of polymer domains as was done using Westenhoff et al. 13 approach. The amplitdue and time dependence of the charge yield dynamics have been shown to contain information on the PC 71 BM cluster size. A detailed modeling of the system was performed using MC simulations. The BHJ morphology of the organic photovoltaic blends studied here contains small PC 71 BM clusters of the size of several nanometers (up to 7 nm diameter) as well as the large PC 71 BM domains with sizes exceeding 15 nm, for which most excitons are lost. These observations are consistent with the paradigm of hierarchical morphology 30, 36, 37. Significant differences of BHJ morphology in terms of formation of the small ( 7 nm) PC 71 BM clusters and the large (> 15 nm) PC 71 BM domains have been observed for the mixtures with donor polymers of RRa-P3HT, MDMO-PPV, and RRe-P3HT. The small PC 71 BM clusters varied from 2 to 7 nm in PC 71 BM:RRa/RRe-P3HT blends and 2-4 nm in PC 71 BM:MDMO-PPV blends. MDMO-PPV blends exhibit a steeper increase of the size of the small PC 71 BM clusters than RRa-P3HT blends with an increase of PC 71 BM load. RRa-P3HT did not show an appearance of large PC 71 BM domains within the whole range of PC 71 BM loads investigated. In contrast, the MDMO-PPV and RRe-P3HT based blends exhibit the formation of large PC 71 BM domains with PC 71 BM loads w > 0.4. An interesting 81

17 property of RRe-P3HT blends was observed: upon increasing the PC 71 BM load from w = 1.5 to w = 2 an abrupt increase of charge yield is observed, which was associated with the decrease of the volume fraction of the large PC 71 BM domains due to the disruption of the RRe-P3HT nanocrystals. It has also been demonstrated that a simple rationale of taking the ratio between the interface PC 71 BM excitons, which dissociate within the first picosecond, and the total number of PC 71 BM excitons (e.g. all dissociated excitons when there are no losses) can reveal the size of PC 71 BM clusters. The outcome of this simple approach is in excellent agreement with the results of MC simulations. The proposed method can provide a valuable feedback on how BHJ morphology should be optimized to balance between both the charge generation and the transport. The charge generation after excitation of PC 71 BM is especially important for modern solar cells involving narrow bandgap polymers, where PC 71 BM becomes the main absorber in the blue edge of the visible range. The limited number of excitons, which arrive at an interface is a direct consequence of both the PC 71 BM cluster size and the energy disorder. Therefore, optimization of BHJ morphology should involve not only tuning the size of PC 71 BM (or polymer) clusters, but also the ordering of the molecules, i.e. formation of nanocrystals 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 C 70 derivative [6,6]-Phenyl C 71 butyric acid methyl ester (a mixture of isomers 19 ) (PC 71 BM) 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 PC 71 BM content ranging from 0.02 to 9 as a PC 71 BM to polymer weight ratio w. The preparation procedure was the following: the polymer and PC 71 BM 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 PC 71 BM 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 82

18 Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy obtain the variety of PC 71 BM 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 annealing 46, 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 OPO 49. NOPA was producing visible light output 30 fs, 40 μj with wavelength tunability in the range of nm. A 3- stage IR OPO 49 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 PC 71 BM:RRa/RRe-P3HT and PC 71 BM: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-PPV 24 RRa-P3HT 23 (fig. 4.7 in Chapter 4) as well as charge/charge Transfer (CT) exciton in RRe-P3HT 23, 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/cm 2 for the P3HT blends and 120 μj/cm 2 for the MDMO-PPV blends. In all cases, the absorbed photon density was below 10-3 photons/nm 3 (i.e. ~1 photon per 10 nm of length) to ensure a low probability of biexciton (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 and 83

19 simultaneously detecting two different polarizations of the signal. Parallel and perpendicular polarizations were used to recalculate isotropic component using the following relation 51 :, (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 resolution 24, 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. 84

20 (cm -1 ) Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy S3 Supplementary information S3.1. Background subtraction The background response of polymer and PC 71 BM was estimated by taking into account the absorption fractions of these two compounds. For the estimation of absorption fraction, the absorption coefficient was calculated using the following equation for each polymer:pc 71 BM 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 at 680 nm PC 71 BM:RRa-P3HT y = * x at 630 nm PC 71 BM:MDMO-PPV y = * x at 680 nm PC 71 BM:RRe-P3HT y = * x PC 71 BM fraction by weight (%) a) b) c) PC 71 BM fraction by weight (%) PC 71 BM fraction by weight (%) Fig. S3.1 Absorption coefficients (symbols) of (a) PC 71 BM:RRa-P3HT, (b) PC 71 BM:MDMO-PPV and (c) PC 71 BM: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, PC 71 BM absorbs more, by approximately a factor of 2 than it does at 680 nm. The measured IR PIA response of PC 71 BM:polymer blends in this work originally had small (negligible in most cases) background response from pristine PC 71 BM 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 PC 71 BM loads while the response from pristine PC 71 BM becomes more visible at high PC 71 BM loads. For background subtraction, the responses of pristine PC 71 BM 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 PC 71 BM in the blends (fig. S3.2, solid blue lines) was estimated by rescaling the transient of pristine PC 71 BM film response (bottom panels) using the following equation: 85

21 , (S3.2) where is the PIA response of pristine PC 71 BM film (fig. S3.2 bottom panel) with absorption A, A Total is the total absorption of the particular blend and is the fraction of absorption by PC 71 BM, which is expressed as:, (S3.3) where the nominator represents the absorption by PC 71 BM and denominator represents absorption by PC 71 BM+polymer, x is the weight fraction of PC 71 BM from 0 to 1, the remaining weight fraction (1-x) is that of a polymer and x relates to the PC 71 BM/polymer weight ratio, k is the ratio between absorption coefficients of PC 71 BM and polymer. The blends of PC 71 BM:RRa-P3HT, PC 71 BM:MDMO-PPV and PC 71 BM:RRe-P3HT exhibit k equal to 74, 25 and 18 respectively as determined from the linear fit (fig. 3.1). To address the question of whether the PC 71 BM response is inherent for PC 71 BM molecules or it originates from PC 71 BM clusters, we performed separate measurements on PC 71 BM diluted in dichlorobenzene (not shown). These experiments did not reveal any response of an amplitude, that is, at least, 2-orders of magnitude lower than from the PC 71 BM film with comparable OD. Therefore, we can neglect the background response of isolated PC 71 BM molecules, which are probably mostly contained in the blends with small amounts of PC 71 BM (w = ). Therefore, the background correction for RRa-P3HT blends with low PC 71 BM loads was not performed. On the other hand, MDMO-PPV and RRe-P3HT blends with w < 0.42 had negligibly low PC 71 BM contribution (fig. S3.2) so that the background correction does not significantly change the PIA transients. Similarly to PC 71 BM, the response of the pristine polymer in the blends (fig. S3.2 black dashed lines) was calculated by rescaling the transient of pristine polymer film response using the following equation:, (S3.4) where is the photoinduced response dynamics of pristine polymer film (fig. S3.2 top panel) with the absorption A, and is the fraction of absorption by polymer expressed as: 86

22 Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy. (S3.5) Fig. S3.2 PIA transients with different weight ratios of PC 71 BM to polymer: (a) RRa-P3HT (b) MDMO-PPV (c) RRe-P3HT. Symbols represent measured dynamics of the PIA response. Solid blue lines and dashed black lines represent the PC 71 BM and the pristine polymer contributions, respectively, calculated from their relative absorption. Accurate background subtraction relies on accurate measurements of the real absorption of the film. This is especially important for the films with low PC 71 BM content, where the absorption of a pristine polymer is extremely low and comparable to the surface reflection and scattering effects. RRa-P3HT and MDMO-PPV blends were assumed to reflect and scatter 15% of the incoming light referring to Lee et al. 54. In addition, for Fresnel losses, blends of pristine RRe-P3HT exhibited substantial light scattering, readily observable by the naked eye. An addition of extra PC 71 BM resulted in a noticeable change of the transmitted light, resembling the clear (i.e. without scattering) colored glass. The film scattering effects were verified using a laser pointer at the wavelength of 660 nm as a light source and a light detector placed just behind the sample to minimize the scattering effects. It was confirmed that dependence of optical density at 680 nm and 660 nm 87

23 nm on w (fig. S3.3) is exactly the same (with a small offset in OD ~0.1 due to the different wavelength) except for pristine polymer and w = 0.02 blend. Therefore, scattering-corrected (laser measurement) values of RRe-P3HT were used when calculating the actual absorption of pristine RRe-P3HT and blend with w = OD Absorption spectrometer = 680 nm 0.2 OD-0.05 Laser = 660 nm PC 71 BM fraction by weight (%) Fig. S3.3 Linear absorption of PC 71 BM:RRe-P3HT as measured by a standard absorption spectrometer (full squares and a solid line) and a laser pointer operating at 660 nm (open squares and dotted line). The latter dependence was shifted by OD=0.1 to compensate for differences in absorption between 680 and 660 nm. Note that OD does not scale linearly with PC 71 BM weight because of different film thicknesses (for details, see fig. 2 in S2 of Chapter 2) S3.2. Monte Carlo simulations The Monte Carlo (MC) simulations require several parameters for modeling the data: 1) PC 71 BM exciton lifetime; 2) energy disorder of excitons on different PC 71 BM molecules (which results as energy differences E i - E j in Eq. 3.1); 3) initial (exo-energetic) exciton hop rate between PC 71 BM molecules (k 0 in Eq. 3.1); 4) number of PC 71 BM molecules in the cluster sphere (PC 71 BM cluster size). Exciton lifetime was derived from photoluminescence measurements with streak camera (excitation wavelength 525 nm, apparatus function σ = 10 ps) shown in fig. S3.3. The estimated lifetime of 0.5 ns is comparable to the reported values 20, 35. The starting energy disorder value of 70 mev was taken from reference 39 as a global fit parameter. The starting, exo-energetic exciton hop rate was chosen 5 ps -1 to be consistent with the PC 71 BM exciton diffusion length of 5-6 nm 40, 41 as shown in fig. S3.5 as a global fit parameter. fig. S3.5 demonstrates how energy disorder affects exciton diffusion: the increase of the distance traveled by the exciton slows down with time due to decreasing average exciton hopping rate. With the fixed exciton lifetime, the global energy disorder and 88

24 Exciton hopping rate (ps -1 ) Distance (nm) Photoluminescence (arb. u.) Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy the global exo-energetic exciton hop rate in our simulations, the only variable parameter per sample remaining is the number of PC 71 BM molecules in the cluster = 500 ps Delay (ps) Fig. S3.4 PC 71 BM photoluminescence measured with a streak camera. The symbols are the measured data. The line depicts the exponential fit with 500 ps decay Delay (ps) Fig. S3.5 Exciton hopping rate and diffusion distance dependencies on time after excitons are generated. Energy disorder is 70 mev, initial hopping rate is 5 ps -1 and it gradually decreases due to loss of energy as excitons hop from one PC 71 BM molecule to another. 89

25 Probability (arb. u.) MC simulations have revealed that the simple approach of close-packed PC 71 BM molecules, approximated as spheres, was able to reproduce both amplitude and dynamics of all shown in fig. 3.2 with the cluster size shown in fig Naturally, simulation with only small ( 7 nm) PC 71 BM clusters was not sufficient to reproduce blends involving excitonic losses due to finite exciton lifetime (fig. S3.6a). Therefore, in addition to the small ( 7 nm) PC 71 BM clusters, which perfectly reproduce the exciton dissociation dynamics, we added the large (> 15 nm) PC 71 BM domains to account for excitonic losses. This bimodal PC 71 BM size distribution is called the hierarchical morphology 29, 33, 34. The approach of the hierarchical morphology is also in strong agreement with the estimated PC 71 BM cluster size by PC 71 BM surface excitons to the total excitons ratio (fig. 3.5). a) b) c) Log-normal distr. = 15 = 10 Normal distribution = 50 = 30 d) 1.0 Normal distribution = 50 = = 36 = 50 Log-normal distribution PC 71 BM domain size Fig. S3.6 An example of simulations on PC 71 BM:MDMO-PPV blend with w=1.5. (a) the cluster size of 4 nm (dashed green line) reproduces the dynamics very well but totally misses the amplitude (the solid black line is obtained by dividing the amplitude of the dashed green line by a factor of 4.5). On the other hand, 15 nm cluster size (dotted blue line) does not reproduce the (initial) amplitude but fails in reproducing the dynamics. Normal (dashed green line) (b) and log-normal (solid black line) (c) distributions of PC 71 BM cluster sizes cannot reproduce the dynamics. (d) Shows the normal (red dashed lines) and log-normal (black solid lines) distribution of PC 71 BM domain sizes. 90

26 Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy We could assume that all excitons are simply lost in these large PC 71 BM domains > 15 nm. Although most PC 71 BM excitons originating from large domains decay prior to reaching interface with a polymer, some of them are created close to the interface and therefore successfully dissociate. If the large (> 15 nm) PC 71 BM clusters are dominating in the blend this situation might lead to a very low but comparable response from large (> 15 nm) PC 71 BM domains (slow rise of the charge yield, limited by the exciton lifetime) and small ( 7 nm) PC 71 BM clusters (fast rise of the charge yield, not limited by exciton lifetime). Therefore, in order to make sure that exciton dissociation dynamics, which is not limited by exciton lifetime (the majority of charges are produced by small PC 71 BM clusters), is correctly modeled we involved the simulation of large PC 71 BM domains as well. As simulations of the large PC 71 BM domains required long computational time because of the large number of molecules involved, the simulations were sped-up by simplifying a large PC 71 BM cluster to uniform sphere representing a PC 71 BM domain, where excitons are randomly generated within a sphere at any location and have freedom to move to any direction a distance of 1 nm. The simplification is possible because the difference between close-packed spheres of PC 71 BM molecules and continuous spheres is negligible for a domain size above 15 nm diameter (fig. S3.7). The close-packed sphere model is based on adding PC 71 BM molecules in such a way that the distance to the center of the cluster would be minimized. The size of PC 71 BM clusters composed of molecules was calculated as the distance from the last PC 71 BM molecule, added to the simulation, to the center of the cluster (fig. S3.7 red circles). Note that the size of PC 71 BM cluster with a small number of molecules (e.g. < 4) is slightly overestimated due to the imperfect shape of the cluster (fig. S3.7 red circles): size of the cluster consisting of 2-3 molecules has the length scale of exciton diffusion of 2 nm in some directions and 1 nm in the other directions. The equivalent diameter of the uniform sphere (fig. S3.7 blue triangles) was recalculated from the following relation: (S3.6) where R is the radius of an equivalent uniform sphere, r = 0.5 nm is the approximated radius of PC 71 BM molecule and n is the number of PC 71 BM molecules. Naturally, the two methods of estimation of PC 71 BM domain size have some mismatch caused by the fact that a uniform sphere assumes the complete filling of the three-dimensional space within a sphere, whereas 91

27 PC 71 BM Cluster size (nm) the cluster composed of molecules does not fill the whole space. The mismatch was not corrected because essentially the difference of ~2 nm does not play any significant role when the PC 71 BM domain size is equal or larger than 30 nm as was used in our simulations. Gaps Distance from the last molecule to the center of the cluster 10 Equivalent size of uniform sphere # of PC 71 BM molecules Fig. S3.7 Relation between the number of PC 71 BM molecules and the size of PC 71 BM cluster under the approximation of hexagonal close packing of 1 nm sized spheres (red circles) and equivalent for a uniform sphere (blue triangles). The pictorial diagram inside the graph represents a sphere composed of spherical PC 71 BM molecules, which leave gaps between them, and, therefore, the sphere is not completely filled. The MC simulations were also tested with a number of other assumptions in order to verify if there were alternative explanations for the observed exciton dynamics in fig For instance, we attempted to tune the PC 71 BM cluster size alone without introducing large clusters; however this approach fails to reproduce the amplitude either during the first picosecond or at tens of picoseconds (fig. S3.6a). Next, instead of a fixed (averaged) PC 71 BM cluster size, we attempted PC 71 BM domain size distribution: normal and log-normal 55 (fig. S3.6b-c). This approach also failed to reproduce the dynamics adequately. This outcome was perhaps not surprising since the atomic force microscopy images, shown in fig. S3.9b-c, do not reveal the whole distribution of PC 71 BM domain sizes, but most of them range from several hundred (> 250 nm) of nanometer size up to micrometers. Such large PC 71 BM 92

28 Charge yield (arb. u.) Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy domains should produce virtually no dissociated excitons whereas the estimated yield is ~20% for w = 1.5 in MDMO-PPV based blends. The MC simulations also require taking into account the finite hole transfer (HT) time. Because the HT time is not a constant but depends on PC 71 BM:polymer composition (see Chapter 2), two HT times were used, t 1 = 50 fs and t 2 = 200 fs, where the share of excitons dissociating with time t 1 and time t 2 was used as a fitting parameter. The rising slope of the charge yield is delayed by the HT time. Nevertheless, does exciton diffusion have any influence? To answer this question, simulations were done by tuning two parameters: a number of PC 71 BM molecules in the cluster and switching on/off the exciton diffusion as shown in fig. S3.8. A response with the diffusion on or off shows only minor change as observed in fig. S3.8 between the blue dotted line and cyan dash-dotted line or green dashed line. The number of molecules affects only the overall amplitude but not the shift of the rising slope, whereas diffusion effect shows up only for delays longer than the hole transfer time. Whereas the change in the hole transfer time results in an immediate shift of the rising slope, which is observed in the transients of fig hole transfer time (HT) = 30 fs number of molecules 1 1 (amplitude x 0.617) diffusion 10 (HT = 60 fs) Delay (ps) Fig. S3.8 Simulation of the transient rise time. Solid red line: one PC 71 BM molecule with 50 fs hole transfer (HT) time, dashed green line: one PC 71 BM molecule, HT time of 50 fs with amplitude multiplied by to match the amplitude of the response with 10 PC 71 BM molecules, dotted blue line: 10 PC 71 BM molecules, HT = 50 fs, dashed-dotted cyan line: 10 PC 71 BM molecules, HT = 50 fs with exciton diffusion, short dashed orange line: 10 PC 71 BM molecules, HT = 200 fs. 93

29 S3.3. Atomic Force Microscopy The size of large PC 71 BM domains in MDMO-PPV blends was measured using Atomic force microscope (AFM). fig. S3.9a-c shows images of surface roughness obtained with AFM in tapping mode for the PC 71 BM:MDMO-PPV blends that were used for ultrafast spectroscopy measurements. The images, shown in fig. S3.9a-c, were used to estimate characteristic PC 71 BM domain size by performing two-dimensional autocorrelation function (2D-AC) using the Gwyddion software (shown in fig. S3.9d-f). 2D-AC functions were fitted with two Gaussians, along the vertical and horizontal cross-sections of 2D-AC, and averaged to obtain the characteristic size of PC 71 BM domains (fig. 3.6b). The characteristic size of PC 71 BM domains was used in MC simulations. These domains, with sizes exceeding 30 nm naturally caused the observed excitonic losses. a) b) c) d) e) f) Fig. S3.9 Atomic force microscope images in tapping mode of surface roughness (a, b and c) and their respective two-dimensional autocorrelation functions (d, e and f) for blends of PC 71 BM:MDMO-PPV with PC 71 BM/polymer weight ratios: (a) and (d) w = 1, (b) and (e) w = 1.5, (c) and (f) w = 2.3. The bright colored spherical shapes in b and c are PC 71 BM domains. S3.4. Scanning Transmission Electron Microscopy The size of large PC 71 BM domains in RRe-P3HT blends was measured using Scanning Transmission Electron Microscopy (STEM). For STEM measurements, free-standing thin films were prepared in a clean room using the procedures described below 56, 57. RRe-P3HT and PC 71 BM were dissolved separately in ortho-dichlorobenzene at concentrations of 5 g/l and mixed with appropriate volumes to obtain w of 0.11, 0.42, 1, 2.3 and 9. Glass substrates were consequently cleaned with 1) soap and demineralized water, 2) sonicated in 94

30 Bulk Heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy demineralized water, 3) sonicated in acetone, 4) sonicated in isopropanol, 5) sonicated in demineralized water, 6) dried on the centrifuge, 7) exposed to UV light. The PEDOT:PSS solution in water was spin coated on the cleaned glass substrates at 1000 rounds per minute (RPM) for 1 min and then baked in the oven at 100 C for 20 min to dry. Mixed solutions of PC 71 BM:RRe-P3HT were spin coated at 2000 RPM with the lid closed for 5 seconds and then at 1000 RPM for 1 min with the open lid. The prepared thin films were sliced into small rectangles and immersed into demineralized water in order to dissolve the PEDOT:PSS layer and lift the films from the substrate. The copper grids (3 mm sized with square mesh) were used to pick up the freestanding films from water and put into the STEM device. Just before performing the STEM measurement, all PC 71 BM:RRe-P3HT films were stained with iodine vapor for several minutes, a procedure known to improve contrast for some PC 71 BM:polymer blends 10. The iodine vapor was obtained by dissolving solid iodine into 99.5% purity methanol and keeping copper grid with a thin film above the solution for a few minutes. Fig. S3.10 shows STEM images of PC 71 BM:RRe-P3HT blends with PC 71 BM/polymer weight ratios w = With an increase of PC 71 BM content, the bright features become more pronounced and abundant signifying that the bright features are related to the higher density of PC 71 BM molecules. In order to enhance the contrast between the PC 71 BM rich areas and the RRe-P3HT rich areas, the mask for image brightness > 50% was applied for the pictures shown in fig. S3.10c-e, the results are shown in fig. S3.10f-h. These masks were used to calculate two-dimensional Fourier transform (2D-FFT) using Gwyddion software, the results are shown in fig. S3.10i-k (the processed images of fig. S3.10 a-b are not shown because the contrast was too low). The 2D-FFT processed images were fitted with two Gaussians, along the vertical and horizontal cross-sections to obtain the characteristic PC 71 BM domain size. 95

31 a) b) c) d) e) f) g) h) i) j) k) Fig. S3.10 Scanning transmission electron microscope images of PC 71 BM:RRe-P3HT with PC 71 BM/polymer weight ratios: (a) w = 0.11, (b) w = 0.42, (c) w = 1, (d) w = 2.3 and (e) w = 9. Brighter areas are PC 71 BM enriched, darker areas are RRe-P3HT enriched. For the PC 71 BM cluster size analysis, the data mask (f-h) was applied (above ~50% of brightness) (f) w = 1, (g) w = 2.3 and (h) w = 9. The blends with lower PC 71 BM load did not show observable evidence of large PC 71 BM domains neither in STEM data nor in pump-probe data therefore further analysis was not performed. Two-dimensional Fourier transform (2D FFT) was performed on the mask (i) w = 1, (j) w = 2.3 and (k) w = 9. The Gaussian fits of the 2D FFT images gives PC 71 BM cluster sizes of nm. 96