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Semiconducting polymer blendsthat exhibit stable charge transportat high temperatures

Aristide Gumyusenge1, Dung T. Tran1, Xuyi Luo1, Gregory M. Pitch2,Yan Zhao1, Kaelon A. Jenkins1, Tim J. Dunn3, Alexander L. Ayzner2,Brett M. Savoie4*, Jianguo Mei1*

Although high-temperature operation (i.e., beyond 150°C) is of great interest for manyelectronics applications, achieving stable carrier mobilities for organic semiconductorsat elevated temperatures is fundamentally challenging. We report a general strategyto make thermally stable high-temperature semiconducting polymer blends, composedof interpenetrating semicrystalline conjugated polymers and high glass-transitiontemperature insulating matrices. When properly engineered, such polymer blends displaya temperature-insensitive charge transport behavior with hole mobility exceeding2.0 cm2/V·s across a wide temperature range from room temperature up to 220°C inthin-film transistors.

The performance of inorganic semicon-ductors optimized for operation at am-bient temperatures degrades at elevatedtemperatures. Charge carriers are ther-mally promoted across the band gap,

which leads to increased carrier densities,junction leakages, and reduced charge carriermobility (1–3). To improve the device per-formance and lifetime in these harsh ther-mal conditions, wide-bandgap materials havebeen utilized (4, 5). Alternatively, active orpassive cooling, thermally engineered pack-aging, as well as electrical isolation betweenmultiple transistors are used to maintain theoptimal electronic performance (6). By con-trast, organic semiconductors commonlydisplay thermally activated charge transportfeatures (7, 8). Charge transport is facilitatedin organics with moderate temperature in-creases, leading to improved performance (9).However, this thermally activated chargetransport becomes counteracted by unstablemorphologies and disrupted molecular pack-ing at higher temperatures, especially in poly-mer thin films (10, 11). Although devices

such as organic field-effect transistors are nowcommon (12), their operation is normally atambient conditions. High-temperature anneal-ing effects have been explored in organicsemiconductors (13–15), but in all reports,charge-carrier mobilities have been tem-perature dependent and start to decline at>150°C.Blending semiconducting polymers with

insulating hosts has been used as a generalstrategy to improve electronic performance,processability, and mechanical and environ-mental stability in electronic devices (16–18).Preserving close intermolecular interac-tions and packing motifs at elevated tem-peratures is the key challenge, especiallyfor semiconducting polymers (10, 19). Wehypothesized that interpenetrating networksbetween semicrystalline conjugated polymersand high glass-transition (Tg) insulating poly-mers can confine conformational changesof semiconducting polymer chains at ele-vated temperatures. To test this concept, wefirst select diketopyrrolopyrrole-thiophene(DPP-T; P1), a high-performance conjugatedpolymer, and poly(N-vinyl carbazole) (PVK,Tg ~220°C) as the high-Tg host (Fig. 1A) andstudied blends from 40 to 90 weight % (wt %)of PVK in spin-cast films. The blends with be-tween 55 and 65 wt % PVK formed inter-penetrating channels between the conjugatedpolymer P1 and the rigid host PVK, as observedfrom atomic force microscopy (AFM) images(Fig. 1B and fig. S1).Testing the blend films in field-effect tran-

sistors (FETs) under ambient and inert

conditions, we observed thermally stableoperations at high temperatures up to 220°Cwith hole mobilities as high as 2.5 cm2/V·sat the blend ratios of 55 to 65 wt % PVK thatcreated a bicontinuous morphology with in-terconnected P1 domains (Fig. 1C and fig. S2).With loadings outside this range, undesiredvertical or large lateral phase segregationoccurs, which leads to the loss of thermalstability. The mobility of the pristine semi-conductor P1 decreases to 8% at 220°C.Ultraviolet-visible (UV-Vis) absorption spectra(Fig. 1D) revealed an increase in the 0-0vibronic peak intensities upon approach-ing the optimal blending ratios, indicativeof increasing ordering and p-p interactionsbetween P1 chains in the confined domains(20–22). The bottom surface morphologyanalysis confirms that the interpenetratingstructure is preserved at the gate interfacein the thermally stable high-performanceblends (fig. S3).To elucidate the observed thermal stabil-

ity of the 60 wt % PVK blend versus pure P1,we use in situ temperature-dependent UV-Vis spectroscopy, AFM, and grazing incidencex-ray diffraction (GIXD), as well as moleculardynamics simulations to study the effect oftemperature on the intermolecular inter-actions of the semiconducting chains. Uponheating, the UV-Vis absorption spectra ofpristine P1 films revealed a blue shift of35 nm in the maximum absorption peak,accompanied by a decrease in both the 0-0and the 0-1 peaks (Fig. 2A). These phenome-na are consistent with the polymer chains’deaggregating and reorganizing caused bythe thermal energy disrupting the crystal-lites. For the blend films, the chain orderingand interchain interactions were less af-fected upon heating, in comparison withpristine P1 films, as evidenced by the lesspronounced decrease of the 0-0 vibronicpeak intensity. More distinctively, the 0-0vibronic peak that vanished in the P1 filmswas retained in the blended films even attemperatures up to 220°C (Fig. 2B). Thetemperature-dependent AFM analysis alsorevealed that the microscale morphologyof the pristine P1 films changes upon heat-ing, whereas the P1/PVK blend film mor-phology is not affected by heating (fig. S4).Together, these observations indicated thatthe matrix polymer effectively confined thesemiconducting polymer and limited dihe-dral twisting and larger structural reorgan-izations that were responsible for the loss ofthe carrier mobility at high temperatures.In situ temperature-dependent GIXD

studies showed that in the 60% PVK blendfilm, the p-p stacking distance of P1 was re-duced from 3.70 to 3.64 Å, relative to thepristine P1 (Fig. 2, C and D, and fig. S5). Inboth cases, the p-p stacking distance in-creased when the thin films were heatedand reached 3.79 and 3.73 Å at 200°C for thepure P1 and the PVK blend films, respectively.

RESEARCH

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1Department of Chemistry, Purdue University, 560 OvalDrive, West Lafayette, IN 47907, USA. 2Department ofPhysical & Biological Sciences-Chemistry andBiochemistry, University of California Santa Cruz, 1156High Street, Santa Cruz, CA 95064, USA. 3SLAC NationalAccelerator Laboratory, Stanford University, 2575 SandHill Road, Menlo Park, CA 94025, USA. 4Charles D.Davidson School of Chemical Engineering, 480 StadiumMall Drive, Purdue University, West Lafayette,IN 47906, USA.*Corresponding author. Email: bsavoie@purdue.edu (B.M.S.);jgmei@purdue.edu (J.M.)

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To evaluate the effect of this p-p stackingconfinement on the polymer dihedral dis-tribution, molecular dynamics modeling wasperformed. In these simulations, the p-p sep-aration of the semiconducting polymer chainswas restrained to model varying levels ofconfinement, and the resulting dihedral dis-tributions were compared to characterizethe polymer reorganization dynamics (Fig.2E). At p-p confinements of 3.0 Å, we ob-served complete conservation of the dihedraldistributions at all temperatures. Notably,the CCCN dihedral, corresponding to theDPP-T conformations, exhibited intercon-version between gauche conformers, butthere was no evidence of gauche-to-transinterconversion (i.e., the onset of chaintwisting) at any temperature. Likewise, theSCCC dihedral angle, corresponding tothe thiophene-thiophene conformations,broadened with temperature but remainedsharply peaked. By contrast, at p-p confine-ments of 5.0 Å, the SCCC dihedral dis-tribution is broadened at all temperatures,and the CCCN dihedral exhibits gauche-to-trans interconversion at all temperatures(Fig. 2, F and G). Systematic studies of thedihedral distributions under confinementsfrom 3 to 6 Å allowed us to conclude thatlarge-scale DPP-T reorganizations began rel-atively abruptly once fluctuations in the in-terchain p-p separation reach ~5 Å (figs. S7to S9). On the basis of these results, we in-

terpret the p-p confinement exhibited by thebicontinuous P1/PVK blends to play a criticalrole in restricting intrachain reorganizationand enabling temperature-insensitive mobil-ity. This mechanism suggests that this con-finement strategy should be general to othersemiconducting polymers embedded in sim-ilarly rigid matrix polymers or potentiallyto other confinement strategies like chan-nel templating. The relatively broad rangeof blending concentrations that exhibit tem-perature insensitivity (40 to 70%) impliesthat this effect is relatively insensitive tothe width of the confined semiconductingdomains.To evaluate the stability of the blend films

under prolonged thermal stress, the fabri-cated FET devices were subjected to con-stant heating at 150°C for 6 hours in air. Forinorganic semiconductors, prolonged heat-ing leads to increased charge-carrier densityand uncontrolled thermal doping (23). Fororganics, prolonged heating, especially abovethe Tg or the melting point of the semi-conductors, leads to morphology changesand device performance degradation (10, 11, 15).In contrast with the pristine P1 devices un-der the same conditions, the devices basedon the 60% PVK blends retained excellentelectronic properties (as high as 95% of theoriginal mobility) under thermal stress (Fig.3A). The FET devices made from pristineP1 showed a declining on-off current ratio

(ION/IOFF) and increased threshold voltagesunder constant heating, consistent with ear-lier observation of unstable morphologies athigh temperatures. The thermally stabilizedblend-based FET devices retain an on-offcurrent ratio higher than 103 and thresh-old voltages below 3 V after thermal stress-ing (Fig. 3, B to D).To demonstrate the generality of our blend-

ing strategy, we explore the FET thermalstability of P1 blended with four other high-Tg matrices i.e., polycarbonate (PC), poly-acenaphthylene (PAC), polyetherimide (PEI),and Matrimid 5218 (MI) (Fig. 4A). We firstoptimized the blending ratios to attain in-terpenetrating morphologies (fig. S10A). FETdevices based on these optimized blendsexhibited thermally stable charge trans-port and the P1/PAC blend pair could reachhole mobilities as high as 2.0 cm2/V·s thatwere stable up to 220°C in open air (Fig. 4B).The optimized P1/PC blend only providedthermally stable operation up to 180°C, whichis near the Tg of the host. We also tested otherdonor-acceptor semiconductors based onhigh-performance DPP (P2) (20) and iso-indigo (P3) (24) (Fig. 4C) and studied thethermal stability of their blend films withthe champion high-Tg matrices, i.e., PVKand PAC. After optimizing the blend ratiosto obtain an interpenetrating morphology(fig. S10B), FET devices with excellent ther-mal stability up to 220°C were also achieved

Gumyusenge et al., Science 362, 1131–1134 (2018) 7 December 2018 2 of 4

Fig. 1. Morphology stabilization: Interdigitation andthermal stability can be tuned with blending ratiocontrol. (A) Chemical structures of the semiconductingpolymer (P1) and the insulating matrix polymer (PVK).(B) AFM phase images of the spin-cast thin films of thepolymer blends with various amounts of PVK. An optimalamount of the semiconductor was required to establishinterpenetrated morphology. Beyond this threshold, lateralphase separation occurs, and the interconnectivity waslost. (C) Temperature (temp.)–dependent charge carriermobility in FET devices based on P1 blends containingvarious ratios of PVK. The data points represent theaverage hole mobility measured from 10 different devices for each blending ratio, and the error bars represent the standarddeviation from the average. The pure P1 data are also plotted. (D) Normalized UV-Vis absorption spectra (Norm. abs.) of P1-PVKblends with varying amounts of the matrix polymer. The confinement of the semiconductor within rigid domains of the hostled to increased ordering of the conjugated polymer chains. a.u., arbitrary units.

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Gumyusenge et al., Science 362, 1131–1134 (2018) 7 December 2018 3 of 4

Fig. 2. Thermally stable packing behavior inPVK blends. (A and B) Temperature-dependentUV-Vis absorption spectra of pristine P1 filmsand the corresponding PVK blend. (C andD) Evolution of p-p d-spacing and lamellarstacking in thin films treated at different temper-atures. The presence of PVK in the films notonly induced closer packing of the conjugatedpolymer chains, but it also reduced theirfreedom to thermally expand and rearrange.p-p stacking distance as close as 3.72 Å can beretained at 200°C in the case of the PVKblend film. The peak position was extracted fromGaussian fits to the one-dimensional I(q) versusq plots. (E) Simulated dihedral distributions atdifferent temperatures when p-p separations areconfined to 3 Å and 5 Å. With the p-p restraintof 5 Å, the distribution broadens, indicative ofchains twisting at all temperatures. (F andG) Packing behavior of DPP-T chains whenconfined to 3 Å and 5 Å, respectively.

Fig. 3. Effect of thermal stress on FETdevices and thermal stabilityof PVK blends. (A) Measured hole mobilities under constant thermalstress for 6 hours. A sudden decline in mobility was observed for pure P1when the FET device is heated. The blend can retain its original mobilityafter 6 hours. (B) Characteristic transfer curves of FET devices based onP1 with and without PVK after 1 hour of heating. SQRT, square root.

(C) Impact of heating on the ION/IOFF. After 1 hour of heating, the ratio fell tonearly 10 for the devices based on P1 while remaining > 104 for the blenddevices. (D) Threshold voltages (Vth) for FETdevices based on the 60% PVKblends (below 3 V) and pure P1 (exceeding 20 V) upon prolonged heating.The data points represent the average values measured from 10 differentdevices, and the error bars represent the standard deviation from the average.

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(Fig. 4D). Temperature-dependent UV-Visabsorption analyses on the blend films ofP2 and P3 confirmed that those blend pairswith thermally insensitive charge transportproperties exhibited a similar behavior asthe P1/PVK blend at high temperatures (figs.S11 to S14). One exception was the P2/PVKpair, which did not present a thermally stablecharge transport behavior (Fig. 4D). Likewise,this pair did not preserve the characteristicintermolecular interaction vibronic peak uponheating, indicating that this was a necessaryfeature in stable blends (fig. S12). We alsonoticed that the pristine P3 film exhibited anearly thermally stable operation across thetested temperature range. Consistent withthe thermally stable blends previously dis-cussed, the P3 film itself also exhibited strongintermolecular interactions even at high tem-peratures (fig. S13), which suggested the or-ganization of P3 is unusually robust amongthe studied semiconductors. Lastly, the blend-ing strategy was tested for n-type semi-conducting systems, and the blend filmsthat form interpenetrating morphologiesexhibited thermally stable operation incomparison with the pristine thin films(fig. S20).To design high-temperature semiconduct-

ing polymer blends, a few requirements ap-pear to be essential: (i) a host matrix with aTg higher than the desired operating tem-perature; (ii) a semicrystalline semiconduct-ing polymer; (iii) the interpenetration of thesemiconducting component into the hostmatrix; and (iv) improved intermolecularp-p stacking within semiconducting chan-nels that can be retained at high temperature.The use of high-Tg matrices is demonstrated

to be a general strategy to attain these prop-erties by minimizing spatial rearrangementswithin the polymer films at elevated temper-atures. We hypothesized that the superiorordering in the confined channels enhancedcharge transport by reduced activation energyand trap density.

REFERENCES AND NOTES

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ACKNOWLEDGMENTS

We thank the Stanford Synchrotron Radiation Laboratory(a national user facility operated by Stanford University onbehalf of the U.S. Department of Energy, Office of BasicEnergy Sciences, under contract no. DE-AC02-76SF00515)for providing the equipment for the GIXD measurements.Funding: This work was supported by the Office ofNaval Research Young Investigator Program (ONR YIPAward, no. N00014-16-1-2551) and the National ScienceFoundation (NSF CAREER Award, no. 1653909).Author contributions: A.G. and J.M. designed theexperiments; A.G. processed the thin films, did themorphology characterizations, and fabricated andcharacterized the transistor devices; D.T.T. and Y.Z. helpedwith the measurements under nitrogen. X.L. synthesizedthe conjugated polymers; D.T.T. performed the DSCmeasurements; K.A.J. helped with the UV-Vis measurements.G.M.P., T.J.D, and A.L.A. designed and performed the GIXDmeasurements. B.M.S. designed and performed all thecomputational work. A.G., B.M.S., and J.M. organizedthe data and wrote the manuscript, and all authorscontributed to the editing of the manuscript. J.M. conceivedand directed the project. Competing interests: A.G. andJ.M. are inventors on a patent application no. 62677648submitted by the Office of Technology CommercializationPurdue Research Foundation. Data and materialsavailability: All data needed to evaluate the conclusions inthe manuscript are provided in the manuscript or thesupplementary materials.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/362/6419/1131/suppl/DC1Materials and MethodsTables S1 to S3Figs. S1 to S20References (25–33)

1 August 2018; accepted 29 October 201810.1126/science.aau0759

Gumyusenge et al., Science 362, 1131–1134 (2018) 7 December 2018 4 of 4

Fig. 4. Attaining universal thermalstability in semiconducting polymerblends. (A) Molecular structures andglass-transition temperatures of therepresentative matrix polymerstested for high-temperature chargetransport. Matrimid 5218; PEI, polye-therimide. (B) Hole mobilities of FETdevices based on the optimized blendsof P1 in four different matricesmeasured in open air. (C) Molecularstructures of additional semi-conducting polymers studied forthermally stable blends. (D) MeasuredFETmobilities from the blend filmsof P2 and P3 with PVK and PACused as the host matrices. The blendcombinations that had stable closepacking exhibited hole mobilitiesstable up to 220°C. The data pointsrepresent the average mobility valuesmeasured from 10 different devices,and the error bars represent the stan-dard deviation from the average.

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Semiconducting polymer blends that exhibit stable charge transport at high temperatures

Ayzner, Brett M. Savoie and Jianguo MeiAristide Gumyusenge, Dung T. Tran, Xuyi Luo, Gregory M. Pitch, Yan Zhao, Kaelon A. Jenkins, Tim J. Dunn, Alexander L.

DOI: 10.1126/science.aau0759 (6419), 1131-1134.362Science 

, this issue p. 1131Sciencematerials up to 220°C.morphology that stabilizes a network of semiconductor channels. High charge conductivity was maintained in thesesemicrystalline conjugated polymer with an insulating polymer that has a high glass-transition temperature creates a

now show that appropriate blending of aet al.contacts are disrupted, which limits potential applications. Gumyusenge be more conductive at higher temperatures. In practice, conductivity drops at high temperatures because interchain

Charge carriers move through semiconductor polymers by hopping transport. In principle, these polymers shouldBeating the heat by blending

ARTICLE TOOLS http://science.sciencemag.org/content/362/6419/1131

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2018/12/05/362.6419.1131.DC1

REFERENCES

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