Ultrafast Spatial Imaging of Charge Dynamics in Heterogeneous

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Letter pubs.acs.org/JPCL

Ultrafast Spatial Imaging of Charge Dynamics in Heterogeneous Polymer Blends Chris Tsz On Wong,† Shun Shang Lo,‡ and Libai Huang*,† †

Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, United States Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States

‡

S Supporting Information *

ABSTRACT: Proof-of-concept transient absorption microscopy (TAM) with simultaneously high spatial and temporal resolution was demonstrated to image charge generation and recombination in model systems of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends upon extended thermal annealing. Significant spatial heterogeneity in charge generation and recombination dynamics was revealed on the length scale of hundreds of nanometers near the micrometer-sized PCBM crystallites, suggesting that information obtained in ensemble measurements by integrating over microscopically inhomogeneous areas could be misleading. In contrast to previous studies, high sensitivity of our instrumentation allows us to employ low excitation intensities to minimize higher-order recombination processes. TAM provides a unique noncontact tool to probe local functionality in microscopically heterogeneous energy harvesting systems. SECTION: Kinetics, Spectroscopy

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understanding of these complex, multiscale processes and their interplay over a wide range of length and time scales. Significant advances have been made in the field of ultrafast imaging with techniques such as four-dimensional (4D) electron microscope developed to achieve impressive subdiffraction spatial resolution along with femtosecond time resolution.27 However, demanding instrumentation for ultrafast electron microscopy along with additional restrictions for samples limits its application in solar energy harvesting systems. More accessible all-optical techniques such as transient absorption microscopy (TAM) and Kerr-gated wide-field fluorescence microscopy have been developed to provide simultaneous diffractionlimited spatial resolution and femtosecond time resolution and have been mostly applied to interrogate the dynamics of nanostructures.28−31 For solar energy harvesting systems, TAM offers a distinct advantage over fluorescence-based techniques: because the signal is based on absorption, both excitons and separated charges can be monitored easily. Very recently, Grancini et al. demonstrated the first measurements of transient absorption (TA) imaging on polymer photovoltaic blends to probe charge-transfer states.32,33 However, the excitation intensity employed in refs 32 and 33 was ∼6 orders of magnitude higher than conventional TA measurements on the ensemble level due to much smaller focal volume, which could lead to complicated higher-order dynamical processes, making it difficult to compare to existing research in the field.

fficient conversion of solar energy to electricity in heterogeneous photovoltaic devices requires complex interplay between multiple functional components and processes occurring over a wide range of length and time scales. In organic solar cells, donor and acceptor materials are intimately mixed to form interconnected micro- and nanoscale domains in a thin film.1−4 Morphology, including the packing of the molecules and phase segregation of different compositions, plays a critical role in device performance of organic solar cells.5−9 The time scales of energy conversion in organic solar cells range from subpicosecond processes associated with charge separation to microsecond processes associated with charge transport and collection.10−12 Ultrafast spectroscopic techniques with subpicosecond or better time resolution have been extensively applied to study organic photovoltaics and have yielded important insights into charge generation and recombination processes.10−17 However, timeresolved studies so far have been carried out almost exclusively on the ensemble level where signal is integrated over a large area and information about the microscopic morphological changes is lost. On the other hand, microscopic methods, such as transmission electron microscopy (TEM),6,18 X-ray microscopy,19 X-ray diffraction,20,21 and scanning probe microscopy,5,22−26 have provided excellent spatial resolution on morphology but with time resolution limited to 100 ns or longer, which is not sufficient to resolve fast charge generation processes that could occur on picosecond or shorter time scales. The design and optimization of effective solar energy conversion systems requires experimental tools that provide © 2012 American Chemical Society

Received: February 12, 2012 Accepted: March 9, 2012 Published: March 10, 2012 879

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0.65 numerical aperture (NA) objective (Nikon Apo). The transmitted beams were collected by a bright-field condenser with an NA of 0.60 (Nikon), and the probe was detected with an avalanche photodiode (APD, Hamamatsu C5331-12). Pump-induced changes in the probe transmission (ΔT) were measured by modulating the pump beam at 500 kHz with an acousto-optic modulator (AOM, IntraAction Corp, AOM-40AF Series) and monitoring the output of the APD with a lock-in amplifier (Stanford Research Systems, SR844). TA traces were obtained by delaying the probe with respect to the pump with a mechanical translation stage (Newport Corp.). The timeresolution at the sample was ca. 300 fs. TA images at a fixed pump−probe delay were acquired by raster scanning the sample with a closed loop piezo-stage (PhysikInstrumente, P527.3Cl), and recording the change in probe transmission, ΔT, as a function of sample position. The spot size of the pump at the sample was ∼400 nm, and the spot size of the probe was ∼800 nm, giving an overall spatial resolution of ∼400 nm in these images. Samples were placed in a flow cell under a continuous flow of argon gas to prevent degradation. AFM height measurements were performed using a Veeco Bioscope II AFM in tapping mode. Ensemble TA spectroscopy was performed with a 1 kHz amplified Ti:sapphire laser system (Clark MXR CPA-2010). The pump and probe beams came from splitting the 150 fs, 775 nm, fundamental output of the Ti:sapphire amplifier. Ninetyfive percent of the 775 nm fundamental was doubled to create 387 nm pump pulses, while the other 5% was used for white light continuum probe pulses. A femtosecond TA spectrometer (Ultrafast System Helios) was employed for data acquisition. In order to protect samples from oxygen and moisture, all ensemble measurements were performed in evacuated optical cells. To directly compare dynamics obtained by TAM and ensemble measurements, pump fluences were set to be the same for the two methods. Please note that the pump wavelength employed for TAM was 400 nm; however, the absorption at 387 and 400 nm was very similar for all blends in this study. We choose P3HT:PCBM as a model system because micrometer-sized PCBM crystals are known to form upon extended thermal annealing,19,34−37 and the morphological changes associated with the formation of these crystals can be resolved by diffraction-limited optical techniques. We note that optimized P3HT:PCBM-based devices are free of these micrometer-sized PCBM crystals; however, the long-term stability of solar cell devices could be limited by the formation of such crystals during device operation.38 On the other hand, it was also demonstrated that careful control over the size and distribution of the PCBM crystals could benefit the efficiency of P3HT:PCBM solar cell devices.37,39 Figure 2 shows the optical micrographs and AFM images of three annealed P3HT:PCBM blends investigated in this study: a 1:3 blend annealed at 125 and 150 °C, and a 1:1 blend annealed at 150 °C. The thickness of these films was determined to be ∼80 nm by AFM. Optical micrographs of the films (Figure 2a,c,e) show needle-like structures with size ranging from a few micrometers to tens of micrometers. Previous investigation of these structures by selected area electron diffraction and X-ray microscopy concluded that they were pure single crystalline PCBM.19,34 Increasing the annealing temperature and PCBM concentration served to increase the degree of phase separation, leading to increases in both the size and density of PCBM crystals (Figure 2). AFM

In this Letter, we demonstrate TAM as a novel tool to monitor charge generation and recombination events in heterogeneous polymer blends with subpicosecond temporal resolution, and hundreds of nanometers spatial resolution. We employed model systems of thermally annealed poly(3hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends with the formation of micrometer-sized PCBM crystallites. The TAM instrumentation described here is capable of detecting a differential transmission ΔT/T of 10−7, 3 orders of magnitude higher sensitivity than conventional TA spectroscopy, allowing for low excitation intensities to be used to minimize higher order dynamical processes. Atomic force microscopy (AFM) was applied to provide detailed morphology information. Upon thermal annealing, TAM revealed significant spatial heterogeneity in charge dynamics due to differences in local morphology and composition, which was obscured in ensemble measurements. In summary, TAM provides a means to directly visualize how charge generation and recombination processes vary across these highly heterogeneous systems. Poly(3-hexylthiophene-2,5-diyl) (Sigma Aldrich, Product: 445703, Mn = 87000 g/mol) with >98% regioregularity and phenyl-C61-butyric acid methyl ester (American Dye Source, Inc., >99%) were combined in 1:1 and 1:3 ratios to form a 20 mg/mL solution in chlorobenzene. Films were spin-cast at 1500 rpm for 30 s on glass substrates. In order to correlate TA images to AFM images, the glass substrates were patterned using photolithography. Sample preparation was performed in a glovebox purged with nitrogen. Annealing was performed at temperatures of 125 °C, or 150 °C for 30 min in a tube furnace (TF55035A, Thermo Scientific) while being flushed with argon. TA imaging was based on a Ti:sapphire oscillator (Coherent Chameleon). A pulse picker (Coherent) was employed to reduce the repetition rate of the laser to 5 MHz. A schematic drawing of the TAM set up is illustrated in Figure 1; more details can be found in our previous publications.30,31 The output from the Ti:sapphire oscillator at 800 nm was split into two beams, one of which was doubled by a 0.4 mm-thick βbarium borate crystal to provide the pump pulses at 400 nm. The polarizations of the pump and probe beams were made parallel, and the beams were focused at the sample with a 40×,

Figure 1. (a) Schematics of the TAM setup. DC: dichroic mirrors; AOM: acoustic optical modulator; NA: numerical aperture; APD: avalanche photodiode; TS: translation stage; BBO: β-barium borate crystal. 880

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Figure 2. (a,c,e) Optical micrographs for 1:3 (P3HT:PCBM) blends annealed at 125 °C and 150 °C, and a 1:1 blend (P3HT:PCBM) annealed at 150 °C, respectively. (b,d,f) Representative AFM images of areas around PCBM crystals for films as shown in panels a, c, and e. Scale bars in a, c, and e are 50 μm, and scale bars in b, d, and are 5 μm.

images (Figure 2) and line section graphs (Figure S1 of the Supporting Information) show three distinct regions for the 1:3 blends, namely, PCBM crystals, thinner regions around the PCBM crystals, and finely phase-segregated areas. For the 1:1 blend annealed at 150 °C, the PCBM-depleted areas are not as pronounced, but can be observed upon close examination (Figure S1). Representative AFM images of the finely phasesegregated areas of all three blends are also depicted in Figure S1, showing that phase-segregation in these regions are on the order of tens of nanometers. The thinner regions surrounding the PCBM crystal were determined to be P3HT-rich as a result of the depletion of the PCBM.19,34,36 The formation of PCBM crystals has been understood as softening of P3HT chains during annealing that allows the PCBM molecules to diffuse and form crystals.36 These PCBM crystals are known to form at the film/air interfaces on top of a P3HT-rich layer when the blends are spun on glass substrate.19,36,40 To interrogate the consequences of micrometer-size PCBM crystal formation on charge dynamics, we employ correlated TAM and AFM imaging on areas near these crystals. The results from the 1:3 blend annealed at 150 °C are displayed in Figure 3. To find the suitable pump excitation intensity for TAM experiments, ensemble TA measurements were performed on all three samples. Pump intensity-dependent dynamics and spectra are shown in Figure S2 in the Supporting Information. Using low pump intensities is critical for correctly characterizing charge dynamics, because both TA spectra and dynamics were shown to have strong pump intensity dependence, as illustrated in Figure S2.10,16 A pump intensity of ∼6 μJcm−2 was found to achieve good signal-to-noise while ensuring charge dynamics were in a reasonable regime where higher-order processes were not dominating (Figure S2). Previous works demonstrated that short-circuit photocurrent collection was suppressed by less than a factor of 2 as a result of minor second-order effects at an intensity of ∼6 μJ cm−2.16,41 The ensemble transient spectra taken at pump intensity of ∼6 μJ cm−2 are plotted in Figure S3, and the shape and time evolution of these spectra agree well with previous reported works.10,16 A probe wavelength of 800 nm was chosen to monitor charge dynamics in the blends in the TAM measurements. The photoinduced absorption (PA) signal at

Figure 3. (a) AFM height image an area near a PCBM crystal from the 1:3 blend annealed at 150 °C for 30 min. Panels c and d are TA images of the same sample area taken at times of 5 and 500 ps, respectively. Probed wavelength is at 800 nm, and pump wavelength is at 400 nm with ∼6 μJ cm−2 pump fluence. Panel b shows a line section of AFM and TAM at 5 and 500 ps images from the same sample position as marked in panels a, c, and d. Scale bars in a, b, and c are 5 μm.

800 nm contains overlapping contribution from multiple transient species, namely, free P3HT+ polarons resulting from charge separation in the blend, singlet exciton of P3HT, and the PCBM− anion.11,42 P3HT+ polarons have been observed to have a broad absorption band at 700−1000 nm, while the PCBM− anion has an absorption maximum around 1030 nm.10,43 The maximum absorption coefficient of the PCBM− anion is only about 10% of that of P3HT cation, therefore, the majority of the PA signal at 800 nm reflects contribution from P3HT.10 Contributions from free P3HT+ polarons and singlet exciton of P3HT can be differentiated by the large difference in lifetime, specifically ∼100 ps for singlet excitons, while the charge-separated P3HT+ can survive up to microseconds.10,11,43 Figure 3a shows the AFM image of the area to be interrogated in the 1:3 blend annealed at 150 °C. Figure 3c,d illustrates TAM images at 5 and 500 ps, respectively. Figure 2b compares the AFM and TAM line sections from the same sample position. A PCBM crystal with a length of ∼15 μm and a height of ∼300 nm is seen in the center of the image. A PCBM-depleted area is evident in the AFM image (Figure 3a) and AFM line section graph (Figure 3b) with ∼25 nm decrease in height. The TAM images at different delay times show features similar to those observed in the AFM image, with three distinct regions clearly visible. Immediately after charge separation (5 ps image), the largest TA signal occurs in the finely phase-segregated region, and the PCBM-depleted region exhibits much lower TA signal. This can be explained by the high density of interfaces available for P3HT excitons to dissociate in the finely phase-segregated region and the lack of interfaces in the PCBM-depleted region. Interestingly, a signal 881

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Figure 4. Upper graphs plot the relative change between TA images taken at 5 and 500 ps defined as Δ (ΔT) = [ΔT(5 ps) − ΔT(500 ps)]/ΔT(5 ps). Lower graphs are the decay curves taken with different positions along with the ensemble kinetics at 800 nm. The positions where the decay curves are obtained are labeled in the upper graphs. Panels a, b, and c are for a 1:3 blend with annealing temperatures of 150 °C and 125 °C, and 1:1 blend annealed at150 °C, respectively. Scale bars are 5 μm.

Fitting of this feature yields a time constant of 33 ± 8 ps, which is similar to the fast decay time of 24 ± 2 ps observed in a neat P3HT film under the same experimental conditions (Figure S6). We tentatively assign this fast decay to the singlet P3HT excitons that do not undergo charge separation. This assignment is consistent with the high fluorescence intensity observed in PCBM depleted regions by Swinnen et al.34 The decay time of ∼30 ps is faster than the measured fluorescence lifetime of ∼100 ps, which is due to the fact that the dynamics is not totally free of higher order processes such as exciton− exciton annihilation, even at a pump fluence as low as 6 μJ cm−2.16,41 Conclusive assignment of the fast decay time requires further experiments and detailed modeling of experimental data, both of which are currently underway. The slow decay (∼437 ps) in the PCBM-depleted position 2 is slower than that of ∼170 ps in the neat P3HT film. This difference can be accounted for by the fact that the depletion of PCBM is not complete at position 2. Therefore, there is some fraction of P3HT excitons undergoing charge separation, which is responsible for the longer slow decay time constants compared to the neat P3HT film. Furthermore, there is a gradient in relative change of the TA signal in the PCBM depleted region, with the largest change (fastest decay) occurring right on the edge of the crystal, presumably due to a more complete depletion of PCBM next to the crystal.19 For the 1:3 film annealed at 125 °C (Figure 4b), similar contrast is observed with the slow dynamics occurring at the center of the crystal, but the contrast between different regions is less pronounced compared to when annealed at 150 °C (Figure 4a). Please note that the scale is different for Figure 4a and 4b. The finely phase-segregated area in the 1:3 blend annealed at 125 °C has a faster decay than the blend annealed at 150 °C. This can be understood as a larger phase segregation in the finely phase-segregated areas of the blend annealed at 150 °C compared to those areas of the blend annealed at 125 °C (Figure S1), which in turn suppresses geminate recombination. Remarkably, the decay curves measured for the 1:3 blends annealed at 125 and 150 °C are almost identical

level as high as the finely phase-segregated region is observed at the center of the PCBM crystal, while the edge of the crystal shows an extremely low value (Figure 3b). When comparing the TAM image at 5 ps (Figure 3c) to the image at 500 ps (Figure 3d), there is very little change in the finely phasesegregated region and in the center PCBM crystal, but a more noticeable change is observed in the PCBM depleted region (note that the scales in the two images are the same). This implies that the TA signal decays much more rapidly in the PCBM-depleted region. The spatial heterogeneity in dynamics is better depicted in Figure 4. The relative change in TA signal [ΔT(5 ps) − ΔT(500 ps)]/ΔT(5 ps) is plotted in the upper panels. Figure 4a illustrates the same sample (1:3 blend) as shown in Figure 3, while Figure 4b,c shows results from the 1:3 blend annealed at 125 °C and the 1:1 blend annealed at 150 °C, respectively. TAM images at 5 and 500 ps for the same samples in Figure 4b,c are illustrated in Figure S6. In the lower panels, decay traces from several sample positions are plotted along with the decay trace obtained from the ensemble measurement of the same film obtained by conventional TA spectroscopy at the same pump excitation density. For a control, dynamics obtained by TAM and ensemble TA spectroscopy are compared for a neat PCBM film with negligible spatial heterogeneity. The decay traces for a neat PCBM film obtained by these two methods are almost identical (Figure S5), confirming that the dynamics obtained by TAM and ensemble TA spectroscopy are comparable. The darker regions in Figure 4 represent larger relative change and hence faster decay of the TA signal. The dynamics exhibit significant spatial heterogeneity, with fast decays in the PCBM-depleted regions and very slow decays in the finely phase-segregated areas and the center of the crystals. For the 1:3 blend annealed at 150 °C (Figure 4a), the signal level at the center of the PCBM crystal (position 1) and in the finely phase-segregated area (position 4) decays less than 5% over a 500 ps time window. For the PCBM-depleted region (positions 2 and 3 in Figure 4a), an additional fast decay is observed. 882

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crystalline PCBM domains exhibited a higher electron affinity than that of the finely phase-segregated areas.39 This difference in electron affinity can provide the energetic driving force for spatial separation of photogenerated electrons and holes and lead to much slower charge recombination.39 To further discern energetic differences corresponding to differences in crystallinity of PCBM domains, investigation of kinetics over time scales up to microseconds is required. We are currently modifying our instrumentation to allow for TAM imaging over time scales ranging from subpicoseconds to a few microseconds. In conclusion, we have mapped morphology-dependent charge dynamics in model P3HT−PCBM blends with simultaneous hundreds of nanometers spatial resolution and subpicosecond temporal resolution by an all-optical TAM. Our measurements reveal significant spatial heterogeneity in charge generation and recombination dynamics that is obscured in ensemble experiments. By directly comparing spatially resolved dynamics to ensemble dynamics, our results demonstrate that the apparent lifetimes measured by ensemble measurements are averaged over microscopically different areas and, therefore, can be misleading. One major limitation for TAM is the diffractionlimited spatial resolution, which prevents probing spatial heterogeneity on the nanoscales. Further improvement of spatial resolution is highly desirable because events such as exciton diffusion occur at length scales shorter than the diffraction limit. Current experimental efforts are underway to develop subdiffraction TAM.

on the ensemble level (Figure S7), despite the fact that the dynamics are significantly different at the microscopic level (Figure 4a,b). This observation points to a significant advantage of TAM. Figure 4c shows the results from the 1:1 blend annealed at 150 °C. Similar to the 1:3 blend annealed at 150 and 125 °C, the slowest decays occur at the center of the crystal and in the finely phase-segregated area. As seen in Figure 4 and Figure S4, the PCBM-depleted region in the 1:1 blend is not as distinguishable as in the 1:3 blend, consistent with the AFM results (Figure 2). On the ensemble level, the decay in 1:3 blend annealed at 150 °C is faster than that in the 1:1 blend annealed at 150 °C (Figure S7), which could be explained by the higher density of PCBM-depleted area in the 1:3 blend. Interestingly, when comparing the localized decays in the finely phase-segregated areas, decay in the 1:3 blend (black curve in Figure 4a) is slower than that in the 1:1 blend (black curve in Figure 4c), contrary to the ensemble results. This observation can be attributed to a larger phase segregation in these regions in the 1:3 blend compare to the 1:1 blend (Figure S1), which leads to slower recombination. This again demonstrates that ensemble averaging could wash out important information at the microscopic level. As discussed above, dynamics obtained by ensemble measurements represent an average over microscopically inhomogeneous areas, implying that the apparent lifetimes obtained from ensemble measurements can be misleading. For example, fitting of the ensemble decay in the 1:3 blend annealed at 150 °C results in a decay time of ∼400 ps (Figure S8). Recombination on this time scale was previously ascribed to geminate recombination.10 However, this apparent recombination time is an average of a slow decay time on the order of nanoseconds in the finely phase-segregated area and the center of PCBM crystal with a fast decay time on the order of tens of picoseconds in the PCBM-depleted region. As discussed earlier, the fast process observed in the PCBM depleted region could be the result of P3HT singlet exciton decay. Therefore, charge recombination in the annealed blends could be much slower than the value inferred from the ensemble measurements. Another notable observation is the long-lived signal observed at the center of the PCBM crystals, which is unexpected because these crystals were thought to be inefficient for exciton dissociation.44 The signal is unlikely to come from PCBM excitons, because PCBM has minimal absorption at 400 nm, and the decay of PCBM excitons is faster, on the order of 100 ps (Figure S5). It is also unlikely to be attributable to PCBM− anions because their absorption band centers around 1030 nm and has negligible contribution at 800 nm. Therefore, the signal observed in the center of the crystals is most likely due to P3HT+ polarons generated in the layer underneath the crystal. The amount of P3HT underneath the PCBM crystals should be similar to the area next to the crystals since P3HT is largely immobile during annealing.36 However, the dynamics for the area underneath the crystals are significantly slower than the area around the crystals. The absence of fast decay corresponding to the decay of singlet exciton at the center of the crystals indicates that all excitons created in the P3HT-rich layer underneath these crystals have undergone charge separation. It is possible that the dynamics reflect differences in the PCBM content for the P3HT-rich layer underneath the crystals and around the crystals. An alternative explanation is that the pure crystalline PCBM domains are energetic sinks for electrons, as recently proposed by Jamieson et al.39 The

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.T.O.W. and L.H. were supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FC02-04ER15533. S.S.L. was supported by the Strategic Research Investment Program of the University of Notre Dame. Part of the instrumentation was funded by the Strategic Research Investment Program of the University of Notre Dame and the National Science Foundation through Grant CHE-0946447. We thank Prof. Greg Hartland for his generous help with setting up the experiments. This publication is contribution No. NDRL 4902 from the Notre Dame Radiation Laboratory.

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