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1 Microstructure formation in molecular and polymer semiconductors assisted by nucleation agents Neil D. Treat, 1,2,3 Jennifer A. Nekuda Malik, 2,3 Obadiah Reid, 4 Liyang Yu, 2,3 Christopher G. Shuttle, 1 Garry Rumbles, 4 Craig J. Hawker, 1 Michael L. Chabinyc, 1* Paul Smith, 3,5 Natalie Stingelin 2,3,5* 1 Materials Department and Materials Research Laboratory, University of California Santa Barbara, Santa Barbara, CA Department of Materials, Imperial College London, London SW7 2AZ, UK 3 Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, UK 4 Chemical and Materials Science Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, USA 5 Department of Materials, Eidgenössische Technische Hochschule (ETH) Zürich, CH-8093 Zürich, Switzerland Nucleation of P3DDT with BTA and P3HT with DMDBS The increase in the onset of the crystallization temperature, T c, upon addition of the alternate nucleation agent to poly(3-dodecylthiophene) (P3DDT) and poly(3-hexylthiophene) (P3HT), was also observed with differential scanning calorimetry (DSC) cooling thermograms. For P3DDT, processed from the melt, we observe an increase in T c of 5 C (from 134 C to 139 C) with the addition of 20% wt BTA (Figure S1A). For P3HT solidified from solution (6 wt% p-xylene), the T c was raised by 2 C (from 37 C to 39 C) with the addition of 20% DMDBS (Figure S1B). It is likely that the reduction in the nucleating agent efficiency of DMDBS in solution relative to BTA in solution is because DMDBS is approximately 5 times more soluble in p-xylene than BTA is (i.e. 0.5 mg/ml and 0.1 mg/ml respectively). This increased miscibility with p-xylene limits the selfassembly of the nucleating agent in solution and therefore should reduce the nucleating agent efficiency. These data indicate that both the BTA and DMDBS can be used to control the onset of crystallization of semiconducting polymers both from the melt and solution. Additionally, our data reveal that DMDBS is most effective from the melt and BTA from solution (due to solubility effects). Figure S1. Cooling differential scanning calorimetry theromograms of a, a P3DDT neat (dashed) and P3DDT with 20% wt BTA (solid) showing an increase T c of 5 C, and b, a neat P3HT solution (6% wt Xylene, dashed) and 20% wt DMDBS (solid) showing an increase in the T c of 2 C, which indicates nucleation. NATURE MATERIALS 1

2 Measuring the Degree of Crystallinity of P3DDT With and Without DMDBS Enthalpies of fusion collected from differential scanning calorimetry (DSC) cooling thermograms reveal that the addition of DMDBS does not change the degree of crystallinity of P3DDT, which has been wellestablished by the commodity polymer community. 1, 2 These DSC experiments were conducted under N 2 atmosphere at a scan rate of 20 C min -1 with a Mettler Toledo STARe system DSC 1. Standard Mettler aluminum crucibles were used for polymer powder and powder mixtures comprising the nucleation agent (~ 5 to 7 mg sample weight). The enthalpy of fusion from cooling differential scanning calorimetry thermograms of a P3DDT directly relates to the heat generated during the crystallization of a molecule and thus to the degree of crystallinity. In these experiments, P3DDT was cooled from the melt and the increase in the onset of T c of P3DDT in the presence of DMDBS revealed an increased rate of crystallization (i.e. decrease in the kinetic energy required for crystallite nucleation). We find that the heat of fusion (taken from the T c peak area) is nearly indistinguishable with and without DMDBS. Therefore, these experiments reveal that the cooling rate of 20 C min -1 does not kinetically limit the crystallization within neat P3DDT and, more importantly, that the nucleation agent does not change the degree of crystallinity. Figure S2. Cooling differential scanning calorimetry theromograms of a P3DDT neat (black) and P3DDT with 1% wt DMDBS (orange) showing an increase T c of 24 C and nearly distinguishable enthalpies of fusion indicating identical degrees of crsytallinity. Next, we probed the relative degree of crystallinity of P3DDT films with 2D grazing-incidence wideangle X-ray scattering (2D GIWAXS) with and without the addition of DMDBS. These P3DDT films were drop-cast from p-xylene at a solution and substrate temperature of 100 C. They were tested after annealing at 200 C (i.e. above the melting point of P3DDT) and cooled at a rate of 20 C/min. 2D GIWAXS experiments were performed at the Stanford Synchrotron Radiation laboratory on beamline 11-3 with an area detector, 2 NATURE MATERIALS

3 MAR345 image plate, at grazing and specular incidences with an incident energy of 12.7 kev. The samples were kept under a helium atmosphere during irradiation to minimize X-ray beam damage. Films were typically exposed for 60 s at an incidence angle of The resulting patterns were carefully normalized for sample dimensions, exposure time, and incident beam intensity, which enables direct comparison of the diffraction patterns between samples (patterns presented in Figure S3). For ease of interpretation, integrated cake slices along the meridian of the 2D diffraction patterns are plotted in Figure S4. It was found that the P3DDT films with and without 1% wt DMDBS had very similar relative populations of P3HT crystallites, estimated from the total diffracted intensity (within 5%). Therefore, both the DSC and 2D GIWAXS data reveal that DMDBS does not influence the degree of crystallinity of P3DDT films. These films were identical to those investigate with transient microwave conductivity (TRMC) experiments and again highlights that the addition of DMDBS does not change the relative degree of crystallinity. Therefore, changes in the charge yield measured via TRMC must be due to changes in the crystalline/amorphous domain interfacial area, or more simply the crystallite shape. Figure S3. 2D grazing-incidence wide-angle X-ray scattering (2D GIWAXS) images of thin films of a, neat P3DDT and b, with 1% wt DMDBS after at 200 C for 15 min and cooling to room temperature at a rate of 20 C min -1. NATURE MATERIALS 3

4 Figure S4. Cake slices collected along the meridian of a 2D GIWAXS diffraction patterns of thin films of P3DDT (black) and with 1% wt DMDBS (orange) (dotted) annealed at 200 C for 15 min. Transient Microwave Conductivity (TRMC) We utilized transient microwave conductivity (TRMC) to measure the photo-generated charges in organic semiconducting materials with and without nucleating agents. We measured the transient microwave absorption of solid thin films in response to a 4-ns laser pulse (OPO Continuum Panther pumped with a Nd:YAG laser: Continuum Powerlite). The origin of microwave absorption is mobile photogenerated charge interacting non-resonantly with the microwave electric field. Samples are mounted at one electric-field maximum of an X-band TE102 microwave resonance cavity tuned to ~9 GHz, which enhances the signal to noise ratio. Due to the differences in the molecular orbital energy levels, we expect an increased carrier yield upon the creation of a larger interfacial area between crystalline and amorphous domains. It is possible that this increase in charge carriers is due to either (i) increased amount of amorphous/crystalline interfaces resulting in more rapid exciton generation and/or (ii) faster separation of geminate pairs (coulomb-bound charges originating from the same excitation) at the increased number of interfaces. Note that all samples with and without nucleating agents were prepared so that they contained indistinguishable degrees of crystallinity (Figure S4 and S6), which allows for the direct correlation of the change in mobile charge yield to the amorphous/crystalline interfacial area (i.e. crystallite shape). We observe that upon the addition of 1% wt DMDBS to P3DDT, there is an increase in the TRMC signal and carrier yield of up to 50% (Figure S5). Since there is no change in the P3DDT degree of crystallinity (see Figure S1-S3), this must be due to the change in the interfacial area between the crystalline and amorphous domains. Additionally, our data reveals that BTA, the less efficient nucleating agent for the melt processing of P3DDT relative to DMDBS, produces an indistinguishable change in the carrier yield relative to the neat P3DDT. Even though the direct mechanism of these observations is still a subject of investigation, these data reveal a positive correlation between the nucleating agent efficiency and the change in the shape of crystalline 4 NATURE MATERIALS

5 domains of P3DDT. Thus, these data reveal that nucleating agents can be utilized to influence the crystallite dimensions in organic semiconductors. Additionally, similar effects on the charge yield upon the addition of the DMDBS were also observed in P3HT (Table S1). φσµ (cm 2 V -1 s -1 ) nominal photon flux (1/cm 2 ) Figure S5. TRMC measurements of yield-mobility product of P3DDT, neat (orange squares) and comprising DMDBS (black circles) or BTA (blue triangles). The lower solubility of BTA in P3DDT likely reduced the nucleation surface area and thus the nucleating efficiency (as indicated by the DSC thermograms in Figure S1A). Table S1. Yield-mobility product at t = 0 obtained from the sum of the pre-exponential factors in the fits to the TRMC transients for samples of P3DDT and P3HT with and without DMDBS. Consistently, higher yieldmobility products were measured for the nucleated samples. We attribute the variation in the yield-mobility product recorded for the nucleated P3DDT film to a processing-dependent phenomena (e.g. limited miscibility of the nucleating agents in P3DDT). Yield-mobility product of: Neat (cm 2 V -1 s -1 ) + 1% wt DMDBS (cm 2 V -1 s -1 ) P3DDT / / P3HT / / High-resolution X-ray Diffraction (XRD) of Drop-cast P3HT Films Analogous to TOF Samples Powder of P3HT (48 kg/mol) was dissolved to make a 3 % wt chlorobenzene stock solution with and without 1% wt BTA in a N 2 filled glove box and stirred overnight at 90 C. The 40 µl of the stock solution was then drop-cast on clean Si substrates in a N 2 filled glove box and allowed to dry overnight. High-resolution specular X-ray diffraction experiments were performed at the Stanford Synchrotron Radiation laboratory on NATURE MATERIALS 5

6 beamline 2-1 with a point detector and an incident energy of 8 kev. The diffracted beam was collimated with two 1 mm slits for specular diffraction. High-resolution XRD was utilized to probe the molecular ordering and relative degree of crystallinity of drop-cast P3HT films with and without the addition of 1% wt BTA after annealing 150 C for 30 min. These experiments correlate the change in the charge carrier mobilities of P3HT (or rather lack thereof) upon the addition of 1% wt BTA to the molecular ordering of P3HT. Note that these XRD samples were fabricated from the same solutions and processing procedures as the TOF samples, thus enabling direct comparison of the TOF mobility with the relative degree of crystallinity. In Figure S6 is plotted the high-resolution diffractograms of drop-cast P3HT films with (orange) and without (black) 1% wt BTA after annealing 150 C for 30 min. Reassuringly, these data reveal a strikingly similar molecular ordering with and without BTA. Specifically, we observed three orders of diffraction corresponding to the a-axis of P3HT crystallites (packing along the side chains) found at q z = 0.39, 0.77, 1.15 Å -1, indicating that the addition of the BTA does not change the molecular ordering of P3HT. Additionally, we observe that the total diffracted intensity remains unchanged, indicating similar degrees of crystallinity. Interestingly, we do not observe a change in the full-width at half-maximum for the three orders of diffraction peaks of P3HT, indicating similar crystallite correlation lengths along the a-axis with and without the nucleating agent. These finding enable the elucidation of the effects that the nucleating agents may have on the charge transport within the organic semiconductors. Importantly, we find that the inclusion of nucleating agents do not disrupt charge transport in organic semiconductors, which is an essential property for scientific and industrial utilization of this class of material. Figure S6. High-resolution X-ray diffraction studies of drop-cast P3HT (black) and with 1% wt BTA (orange) after annealing at 150 C for 30 min analogous to the samples used for time-of-flight mobility measurements. Grazing-incidence Wide-angle X-ray Scattering of PCBM All samples were fabricated using the same procedure described in the main text. A. 2D GIWAXS patterns of as-cast PCBM films with and without nucleating agents indicate disordered PCBM films. B. Quasi-specular 6 NATURE MATERIALS

7 scattering as a function of the nominal diffraction angle (q radial ) were plotted by integrating the 2D diffraction patterns with a resolution of Δq radial = Å -1. These patterns indicate that the molecular ordering of PCBM is not significantly affected by the addition of either BTA or DMDBS. It should be noted that in this case, even though the crystallite size is reduced upon the addition of the nucleating agents, there is no observed increase in the peak width at half-maximum. This is expected since the PCBM crystallite size is on the order of the dimensions of the X-ray beam. Note that in all cases, the processing was selected as to yield indistinguishable relative degrees of crystallinity, which is indicated by the total diffracted intensity of PCBM crystallites. Again, this enables the investigation of the effects that nucleating agents might have on the mobile charge carrier transport within PCBM. Figure S7. A. 2D GIWAXS patterns of as cast films of PCBM with and without nucleating agent shows an isotropic scattering ring which corresponds to a correlation length of two PCBM molecules. B. The radial integration as a function of scattering vector (q radial ) from 2D GIWAXS patterns of neat PCBM films (black) as cast (black dashed) and annealed at 180 C for 30 min (black solid), PCBM with 0.1% wt BTA as cast (orange dashed) and annealed at 180 C for 30 min (orange solid), and PCBM with 0.1% DMDBS as cast (blue dashed) and annealed at 180 C for 30 min (blue solid). Temperature dependent UV-Vis spectroscopy of P3DDT Samples were prepared on clean glass slides by drop casting 15 µl of 2 mg/ml P3DDT in xylenes with and without 1% wt DMDBS. A. UV-Vis spectra collected at 30 second intervals with an 8 scan average and 2 box car average (scan acquisition time was ~1 sec) using a heated stage equipped with a HR2000+ Ocean Optics highresolution spectrometer and a DH-2000 UV-Vis-NIR light source. B. Intensity at nm as a function of time NATURE MATERIALS 7

8 collected at 135 C. This graph was normalized to the intensity of the same film heated to 135 C from room temperature after melt cooling and baseline was collected from the value at 750 nm. The addition of 1% wt DMDBS roughly doubled the rate of ordering of P3DDT at 135 C from the melt and resulted in a film with a larger degree of ordering. This demonstrates that nucleating agents can be used to improve the crystalline ordering of materials while also decreasing the processing time. Figure S8. UV-Vis spectra measured at 135 C as a function of time for A. neat P3DDT films and B. P3DDT films with 1% wt DMDBS cast on glass substrates. C. Intensity at nm as a function of annealing time at 135 C after melting. Ink-jet printing TIPS pentacene 8 NATURE MATERIALS

9 Fig. S9 Wide-angle X-ray diffractogram of TIPS pentacene thin films and structures comprising the nucleation agent, BTA. Fig. S10 Optical microscopy A. and corresponding transistor characteristic B. of drop-cast TIPS pentacene films. Left panels: neat TIPS pentacene structures without nucleation agent. Note that the SAM-treated substrate remained uncovered and only on the gold electrode pad is a TIPS pentacene film formed. Right panels: nucleated TIPS pentacene structures, resulting in films covering the entire substrate area and thus to significantly improved device characteristics. References NATURE MATERIALS 9

10 1. Wunderlich, B., Macromolecular Physics: Crystal nucleation, growth, annealing. Academic Press: Gornick, F.; Hoffman, J. D. Industrial and Engineering Chemistry 1966, 58, (2), 41-&. Additional References 1. Cahn, R. W. & Yamaguchi, M. Metals and alloys - Editorial overview. Current Opinion in Solid State & Materials Science 2, , doi: /s (97) (1997). 2. Bart, J. C. J. Additives in polymers: industrial analysis and applications. (John Wiley, 2005). 3. Murphy, J. Additives for Plastics Handbook. (Elsevier Advanced Technology, 2001). 4. Hans Zweifel, R. D. M., Michael Schiller. Plastics Additives Handbook, 5 th ed., (2001). 5. Bernland, K. M. Nucleating and Clarifying Polymers Ph.D. thesis, Eidgenössische Technische Hochschule (ETH), Zurich, (2010). 6. Kurja, J. & Mehl, N. A. in Plastic Additives Handbook (eds Hans Zweifel, Ralph D. Maier, & Michael Schiller) (Hanser, 2001). 10 NATURE MATERIALS