26. Well completion is the process to ready for injection.This process includes installing tubing used to inject fluid,perforating the portion of the well in the injection zone, andcasing the well to ensure no injection fluids leakage.

27. K. E. Murray, A. A. Holland, Shale Shaker 65, 98–106(2014).

28. C. Frohlich, J. I. Walter, J. F. W. Gale, Seismol. Res. Lett. 86(2A), 492–499 (2015).

29. A. McGarr, J. Geophys. Res. 81, 1487–1494 (1976).30. A. McGarr, J. Geophys. Res. 119, 1008–1019

(2014).31. L. V. Block, C. K. Wood, W. L. Yeck, V. M. King, Seismol. Res.

Lett. 85, 609–624 (2014).32. Y. Zhang et al., Ground Water 51, 525–538

(2013).33. The volume conversion from the oil industry standard

of barrels to the metric standard of meters cubed is

~6.29 barrels per meter cubed, assuming a 42-gallonoil barrel.

34. B. Efron, R. J. Tibshirani, An Introduction to the Bootstrap (CRCpress, Boca Raton, FL, 1994).

35. Information on materials and methods is available on ScienceOnline.

36. J. Rutqvist et al., Math. Geosci. 47, 3–29 (2015).37. W. D. Mooney, M. K. Kaban, J. Geophys. Res. 115, B12424

(2010).

ACKNOWLEDGMENTS

This work was conducted as a part of the UnderstandingFluid Injection Induced Seismicity Project supportedby the John Wesley Powell Center for Analysis and Synthesis,funded by the U.S. Geological Survey (grant G13AC00023).We thank J. Hardebeck and W. Ellsworth for their thoughtfulcomments. This project was aided by injection data

contributed by A. Holland (OK), C. Eisenger (CO), T. Kropatsch(WY), J. Amrheim (IN), T. Tomastik (OH), S. Platt (PA),I. Allred (UT), M. Berry (UT), A. Wickert (TX), and I. Van-Vloten(CEUS). This project used earthquake data from theANSS Comprehensive Catalog. The well data used in thisstudy are available as supplementary materials onScience Online.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/348/6241/1336/suppl/DC1Materials and MethodsFigs. S1 to S18Tables S1 to S4References (38–46)

16 March 2015; accepted 28 May 201510.1126/science.aab1345

POLARON DYNAMICS

Long-lived photoinducedpolaron formation in conjugatedpolyelectrolyte-fullerene assembliesRachel C. Huber,1* Amy S. Ferreira,1* Robert Thompson,1 Daniel Kilbride,1

Nicholas S. Knutson,1 Lekshmi Sudha Devi,1 Daniel B. Toso,2 J. Reddy Challa,1

Z. Hong Zhou,2,3 Yves Rubin,1† Benjamin J. Schwartz,1,3† Sarah H. Tolbert1,3,4†

The efficiency of biological photosynthesis results from the exquisite organization ofphotoactive elements that promote rapid movement of charge carriers out of a criticalrecombination range. If synthetic organic photovoltaic materials could mimic thisassembly, charge separation and collection could be markedly enhanced. We showthat micelle-forming cationic semiconducting polymers can coassemble in water withcationic fullerene derivatives to create photoinduced electron-transfer cascadesthat lead to exceptionally long-lived polarons. The stability of the polarons dependson the organization of the polymer-fullerene assembly. Properly designedassemblies can produce separated polaronic charges that are stable for days orweeks in aqueous solution.

In biological photosynthetic systems, energycascade structures promote the spatial sep-aration of photogenerated charges createdat the reaction center, preventing their recom-bination. These energy cascade structures

require close proximity of the electron donorsand acceptors, on the scale of ~1 nm, and thecorresponding electron transfer (ET) processestake only a few picoseconds (1). Similarly, photo-excitation in artificial organic photovoltaic (OPV)cells generates dissociated charges at a donor-acceptor interface on subpicosecond time scales.However, OPVs suffer a large degree of recom-

bination because they rely on phase separationof the conjugated polymer donor and fullereneacceptor into domains on the length scale of10 to 20 nm to facilitate efficient exciton diffu-sion and charge transfer (2, 3). The high chargedensities present in OPVs, coupled with the lowdielectric constant of organic materials, favorcarrier recombination before the charges canbe extracted through external electrodes. IfOPVs could be designed to use ET cascadestructures that are reminiscent of photosyntheticcomplexes, it should be possible to greatly im-prove charge separation and reduce recombina-tion losses (4).Here we describe how molecular self-assembly

can enable dissolved OPV materials (conjugatedpolymers and fullerenes) in aqueous solutionto mimic the ET cascade structures of biolog-ical complexes and allow us to “spatially” con-trol photogenerated charges. We demonstrateefficient long-time charge separation follow-ing photoexcitation: The ET cascade producesseparated polarons that are exceptionally sta-ble for weeks, a lifetime that is unprecedented

for OPV materials. Although long polaron life-times have been observed in covalently linkeddonor-acceptor dyads and triads (5) and micel-lar structures (6), our use of standard organicphotovoltaic materials sets this work apart. Inaddition, our use of self-assembly provides po-tential future advantages in reproducibility andscalability, both of which are major hurdlesfor conventional OPVs with kinetically controlledstructures (7–9). Finally, the photoinduced chargeseparation we achieve takes place in water, open-ing possibilities for the “green” production of ar-tificial photosynthetic devices.The particular materials used in this study are

a combination of a conjugated polyelectrolyte,poly(fluorene-alt-thiophene) (PFT) (10), andseveral regioisomers of the charged fullerenederivatives C60-N,N-dimethylpyrrolidinium iodide[C60(PI)n], where n is the number of chargedpyrrolidinium iodide groups (11) (Fig. 1, A to C).PFT is a water-soluble semiconducting polyelec-trolyte whose bis-alkylated sp3-hybridized fluo-renyl carbon forms a wedge-shaped monomerthat facilitates the assembly of the charged poly-mer into rod-like micelles (Fig. 1B); details ofhow this polymer assembles have been publishedpreviously (10). Because of the charged nature ofthe polymer, the electron acceptor(s) must alsocarry cationic charges to avoid heterocoagula-tion. The synthesis of C60(PI)n, depending on thereaction conditions, produced multiadducts withn ranging from 2 to 5, including multiple regio-isomers for each n. To avoid confusion, we willrefer to C60(PI)n with n = 3 to 5 as “higher”adducts and fullerenes with n = 2 as “mixed-bis”adducts.We achieved control over the solution-phase

aggregation of these materials by exploitingthe different solubility properties of the conju-gated polyelectrolyte and charged fullerenederivatives. Mixed-bis adducts show limitedsolubility (without PFT) in aqueous solution,whereas higher adducts are water soluble athigh concentration. This difference suggests thatthe mixed-bis adducts should coassemble in aque-ous solution with PFT, a result we confirmed bycryogenic electron microscopy (cryoEM), small-angle x-ray scattering (SAXS), and luminescencequenching studies. CryoEM images of pure PFT,PFT:mixed-bis adducts, and PFT:high adducts

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1Department of Chemistry and Biochemistry, University ofCalifornia–Los Angeles (UCLA), Los Angeles, CA 90095-1569, USA. 2Department of Microbiology, Immunology andMolecular Genetics, and the Biomedical EngineeringProgram, UCLA, Los Angeles, CA 90095, USA. 3TheCalifornia NanoSystems Institute (CNSI), UCLA, Los Angeles,CA 90095, USA. 4Department of Materials Science andEngineering, UCLA, Los Angeles, CA 90095, USA.*These authors contributed equally to this work. †Correspondingauthor. E-mail: tolbert@chem.ucla.edu (S.H.T.), schwartz@chem.ucla.edu (B.J.S.), rubin@chem.ucla.edu (Y.R.)

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are shown in Fig. 1, D to F. Pure PFT samplesself-assemble into branched micelles that areroughly 4 T 0.5 nm in diameter and 30 to 50 nmin length. CryoEM images of PFT assembled withmixed-bis adducts are visually similar to the purePFT, indicating association of the C60(PI)2 withthe PFT micelles. In contrast, cryoEM images ofPFT:high adducts appear blurry because thesesolutions contain separate PFT and fullereneagglomerates.This interpretation of coassembly of PFT with

mixed-bis adducts is also supported by SAXSmeasurements (Fig. 2, A and B). We radiallyaveraged the SAXS data and fit it to a powerlaw to extract the exponent a, which is relatedto the polymer fractal structure (12, 13). Valuesof a = 1, 2, and 4 correlate to rigid rod, lamellar,and spherical structures, respectively, althoughinteractions between molecules cause deviationsfrom these ideal slopes. Analysis of SAXS datafor pure PFT yielded a = 1.5 at low q (rod-like atlarge size), increasing to a = 3.7 at high q (sphere-like at small size). Deviation from a = 1 arose from

the branched network seen by cryoEM (Fig. 1D)(14). SAXS power-law slopes for C60(PI)n high-adducts correspond to a percolation networkat low q and rod-like behavior at high q, in-dicating aggregation (12, 15). SAXS data fromthe combined PFT:high adducts solution waswell approximated as the mass-scaled sum ofthe pure PFT and pure fullerene scattering,suggesting a nonassembled mixture, similar tothat seen by cryoEM. By contrast, mass-scaledSAXS from solutions of PFT and mixed-bis ad-ducts is nearly identical to the pure PFT. Theseresults provide strong evidence that C60(PI)2and PFT coassemble into a single micellaraggregate.Finally, electronic interactions in the polymer-

fullerene assemblies were confirmed with lumi-nescence quenching, which provides an indirectmeasure of the photoinduced charge transferfrom the polymer to the fullerenes (16). Solu-tions of PFT:high adducts showed relativelylittle photoluminescence (PL) quenching, pre-sumably because the donors and acceptors were

not in close physical proximity, but aqueoussolutions of PFT with the mixed-bis adductshad substantial PL quenching, indicating bothphysical and electronic contact (Fig. 3A). Thedata indicate that >75% of the PFT excitationswere quenched in the presence of the mixed-bisadducts.We determined the dynamics of charge sep-

aration in these donor-acceptor assemblies usingultrafast broadband transient absorption spec-troscopy on dilute aqueous solutions of coas-sembled PFT with mixed-bis adducts (17).Representative transient absorption spectra atdifferent probe delays following excitation at470 nm are shown in Fig. 3B. We assigned thenegative transient absorption peak near 520 nmto stimulated emission, as the spectral fea-tures and the lifetime (Fig. 3C) matched thefluorescence emission. Interestingly, the 690-nmabsorption of the PFT hole polaron (P+) ap-peared on a subpicosecond time scale afterphotoexcitation (18, 19). This ultrafast appear-ance of P+ confirmed that the C60(PI)2 adductsmust be coassembled with PFT, because othergeometries would require diffusion or otherstructural rearrangements that could not occurso quickly. Once formed, about 75% of the PFTpolarons in these dilute samples decayed backto the ground state in ~200 ps (Fig. 3C). The re-maining polarons survived past the nanosecondtime scale.To mimic biological charge-separation sys-

tems, coassembly and rapid charge separationare required, but if they are followed by rapidrecombination, the charges cannot be extracted.A fullerene acceptor that is optimized for chargeseparation thus needs to contain one class ofcompounds that can assemble intimately withthe PFT for efficient charge transfer, and a sec-ond class of compounds that can assemble moreloosely, allowing us to pull the electron away fromthe PFT and prevent recombination. Fortunate-ly, both types of compounds were already avail-able within our mixed-bis sample, and theirproperties could be examined simply by sep-arating C60(PI)2 regioisomers. Our mixed-bissamples were primarily composed of four iso-mers (10% trans-1, 39% trans-2, 44% trans-3,

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40 nm 40 nm 40 nm

Br-Br-

N+ N+

Sn

Fig. 1. PFT and charged fullerene structure and assembly. PFT structure (A); cartoon of a PFTmicelle (B); charged fullerenes (C). CryoEM images of pure PFT (D), PFT:mixed-bis adducts (E), andPFT:high adducts (F).

Fig. 2. SAXS data for PFT and PFT/fullerene mixtures. (A) Data for PFT:high-adducts are reasonably approximated by a sum of PFT + high-adducts.(B) The PFT:mixed-bis profile overlap mass-scaled PFT data. (C) Raw scattering data for all PFT and PFT:bis-fullerene samples are similar. (D) Distancedistribution functions, P(r), obtained by Fourier transformation of the data in (C) show different fullerene environments for trans-1,2 and trans-3,4, withPFT:mix-bis corresponding to the sum of the two.

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and 7% trans-4). Structures of each of the iso-mers are shown in Fig. 4, A to D. We partiallyseparated these isomers by silica gel columnchromatography of the neutral pyrrolidineprecursors (prior to quaternization), produc-ing fractions that we refer to as trans-1,2 (29%trans-1 and 71% trans-2) and trans-3,4 (14%trans-2, 74% trans-3, and 12% trans-4). Thefull characterization of all of these materialsis found in figs. S1 to S22 of the supplementarymaterials (17). The trans-1,2 fullerenes havecharges on nearly opposite sides of thebuckyball and can be viewed as isotropicallycharged molecules that should not easilyinsert into a PFT micelle. By contrast, theangle between charges in trans-3 is ~145° andthat between charges in trans-4 is 103°, suggest-ing more amphiphilic molecules that could insertinto the PFT micelle.The coassembly of PFT with trans-1,2 and trans-

3,4 was examined via SAXS. Raw scattering datafor all of the samples looked similar to the datafor pure PFT (Fig. 2C), but Fourier analysis usingcylindrical boundary conditions showed subtlevariations. In Fig. 2D, PFT and PFT:trans-3,4showed similar probability distributions, sup-porting the model of insertion of fullerene intothe PFT micelle. PFT:trans-1,2 shows two peaks,reminiscent of a polymer micelle with a partial“shell” of fullerenes surrounding the outside.The PFT:mixed-bis data were well fit by a sim-ple linear combination of the PFT:trans-1,2 andPFT:trans-3,4 probability distributions, furthersupporting the idea that trans-3,4 assembles onthe inside of the polymer micelle, whereas trans-1,2 surrounds the outside. The relative locationsof the two different sets of fullerenes were alsoconfirmed via solvatochromic absorption mea-surements (fig. S25). These measurements showthat the ultraviolet absorption of trans-1,2fullerenes assembled with PFT matches thatof the fullerenes in pure water, indicating thattrans-1,2 sits outside of the PFT micelle. Incontrast, the absorption of the trans-3,4 full-erenes assembled with PFT matches that ofthe fullerenes in organic solvents, indicatingthat trans-3,4 sits in a lower dielectric envi-ronment like the micelle interior.Figures 4, F and G, show luminescence quench-

ing measurements that further support theidea that different isomers of C60(PI)2 assemblein different places in the PFT micelle. The lu-minescence decays shown in Fig. 4G were takenwith a Kerr-gated time-resolved fluorescencesetup using CS2 as the gate medium, providinga time resolution of ~1 ps (20). Clearly, the PFTfluorescence is quenched nearly to the instru-ment limit in concentrated solutions when as-sembled with trans-3,4 fullerenes, verifying thatthe photoinduced charge transfer to these ful-lerenes is ultrafast. In contrast, there is almostno fast quenching of the PFT emission with anequal amount of trans-1,2 fullerenes, reflectingtheir assembled position predominantly on theoutside of the micelle, out of range for fast ET.Figure 4F shows steady-state luminescencemeasurements on these same samples. Con-

sistent with the time-resolved data, assembliesof PFT with trans-1,2 fullerenes showed littleluminescence quenching, whereas PFT assem-bled with trans-3,4 fullerenes had strong quench-ing. These quenching results suggested that notonly can we selectively associate fullerenes withpolymer micelles using the number of charges,we can also control the position of the fullerenewithin the micelle by the placement of thecharges (Fig. 4E).Given this degree of control, the next step was

to examine long-lived excitations in assembliesof PFT and mixed-bis adducts containing bothintimately assembled trans-3,4 and more iso-tropically charged trans-1,2 fullerenes. Ideally,this coassembly should permit rapid photoin-duced electron transfer from PFT to the trans-3,4 fullerenes, followed by a second ET step tothe trans-1,2 fullerenes. If this type of directedET cascade occurs, electrons on the trans-1,2fullerenes would then be stabilized in the high-dielectric environment of the water surround-ing the micelle, preventing recombination with

the PFT. Indeed, we found that photoexcitationof aqueous PFT:mixed-bis adduct solutions causeda dramatic color change from yellow to darkgreen over time (Fig. 3D); once the solution wasexposed to light, the color change was essen-tially permanent, lasting days to weeks. Dilutesolutions, like those used to collect the data inFig. 3, B and C, required extensive light exposure(fig. S24), but when higher concentrations wereused, the color change took place in just a fewseconds under room lights, indicating that thequantum yield for long-lived charge separationis much higher than the ~25% in dilute solu-tions (compare Fig. 3B).PFT is a blue-absorbing polymer with an ab-

sorption maximum at 430 nm in water and littleto no absorbance above 550 nm (10). The colorchange from yellow to green was confirmed toarise from the appearance of the PFT hole po-laron (P+) by comparing steady-state data (Fig.3D) with transient absorption data (Fig. 3B) andabsorption from PFT oxidized with iodine, bothof which show absorption in the sub-gap region

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Fig. 3. Formation of P+ and N– polarons requires intimate assembly of the polymer and fullerene.(A) PL of PFT, PFT:high adducts, and PFT:mixed-bis; (B) pump-probe spectroscopy for dilute PFT:mixed-bis solutions excited at 470 nm showing the rapid formation of both excitons and polarons; (C)time decays for the stimulated emission and the polaronic absorption from (B). Absorption from a greenconcentrated PFT/fullerene solution showing both P+ and N- polarons (D). EPR from a similar greensolution, again showing both P+ and N– polarons (E). PL for various concentrated PFT:mixed-bis solutionsshowing that polarons quench luminescence, but the addition of THF, which, destroys the PFT/fullereneassembly, restores PL intensity (F).

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at ~690 nm (fig. S23) (21). The Fig. 3D inset showsthat simultaneously, a substantially weaker neg-ative (N –) polaron absorption peak caused bythe C60(PI)2 molecular anion was observed at1180 nm. The low intensity of the N– absorp-tion has several origins: (i) The absorption cross-section of fullerene anions is much smaller thanthat of the polymer polarons (22); (ii) the weak N–

absorption peak sits on top of a broad scatteringbackground from the coassembled micelles insolution; and (iii) the N– polaron might reactwith impurities in the water, despite our bestefforts to deoxygenate the solutions by freeze-pump-thaw techniques (23).To further confirm the formation of stable,

separated N– and P+ polarons after exposure tolight, we performed electron paramagneticresonance (EPR) experiments. Figure 3E showsthe EPR signal from the green PFT:mixed-bisadducts solution; the g-factors for the N– andP+ polarons are 2.0004 and 2.0040, respec-tively, in good agreement with reported valuesfor many other polymer-fullerene systems (24).The N– polaron line width that we observedis broader than that in other polymer-fullerenesystems, both because of the interaction ofthe water dipoles with the polarons and be-cause of different spin-relaxation times for theelectron and hole (25, 26). The critical differ-ence between our coassembled system andprevious systems, however, is that the previ-ous EPR work required active photoexcita-tion (light-induced EPR) in order to observethe polaron signals. In contrast, once ex-posed to light, the polarons created in ourPFT:C60(PI)2 solutions remained stable essen-tially indefinitely.

Final confirmation that the long-lived sep-arated charges resulted from a self-assembledET cascade comes by examining the details ofabsorption and luminescence for a range ofsamples in different solvents. As discussed above,aqueous solutions of PFT and trans-1,2 full-erenes show little PL quenching (Fig. 4, F andG), but they did briefly turn green during thecourse of the dissolution, indicating polaronformation (possibly from disordered polymerthat transiently allowed the fullerene to partlyinsert into the micelle). By contrast, despite theefficient luminescence quenching in solutionsof PFT coassembled with the trans-3,4 full-erenes (Fig. 4, F and G), the solutions did notturn green and ultrafast experiments (datanot shown) indicate that polarons are formedon subpicosecond time scales (as in Fig. 3B),but recombine with 100% yield over the nextfew hundred picoseconds. These results indi-cate that controlling the spatial position of thefullerenes can dramatically affect carrier dy-namics. Moreover, photoexcitation of green-colored PFT:mixed-bis fullerene solutions resultsin increased luminescence quenching becausePFT excitons are further quenched by P+-polarons (Fig. 3F). However, when tetrahydro-furan (THF), which is known to disassemblethe polymer micelles (27), was added to the co-assembled green system, the luminescence sig-nal regained its intensity, indicating a fullyreversible system (Fig. 3F). These results fur-ther support the idea that intimate assemblieswith well-controlled molecular positions arerequired to facilitate a charge transfer cascadeand avoid recombination. When nanoscale ar-chitecture is optimized, the result is stable

polarons that could potentially be applied toimprove organic photovoltaic cells via sup-pression of charge recombination.

REFERENCES AND NOTES

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(2012).14. Y.-C. Li et al., Langmuir 22, 11009 (2006).15. U. Jeng et al., Nucl. Instrum. Methods Phys. Res. A 600, 294

(2009).16. Y. Park et al., Polymer (Guildf.) 55, 855 (2014).17. See supplementary materials on Science Online for details.18. R. Österbacka, C. P. An, X. M. Jiang, Z. V. Vardeny, Science

287, 839 (2000).19. F. Paquin et al., Phys. Rev. Lett. 106, 197401

(2011).20. S. Arzhantsev, M. Maroncelli, Appl. Spectrosc. 59, 206

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(1977).22. T. Kato et al., Chem. Phys. Lett. 186, 35 (1991).23. D. M. Guldi, M. Prato, Acc. Chem. Res. 33, 695

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ACKNOWLEDGMENTS

This work was supported by the NSF under grant CHE-1112569 and by the Center for Molecularly Engineered EnergyMaterials, an Energy Frontier Research Center fundedby the U.S. Department of Energy (DOE) under ContractDE-AC06-76RLO-1830. A.S.F acknowledges support from TheClean Green IGERT (NSF DGE-0903720). This work madeuse of facilities support by the NSF under equipment grant CHE-1048804. Portions of this work were conducted at the StanfordSynchrotron Radiation Lightsource (SSRL), which is supportedunder DOE Contract DE-AC02-76SF00515. The SSRL StructuralMolecular Biology Program is supported by the DOE Officeof Biological and Environmental Research and by the NationalInstitutes of Health, National Institute of General MedicalSciences (including P41GM103393). We acknowledge the useof instruments at the Electron Imaging Center for NanoMachinessupported by the NIH (1S10RR23057 to Z.H.Z.) and CNSIat UCLA.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/348/6241/1340/suppl/DC1Materials and MethodsFigs. S1 to S25References (28–31)

14 January 2015; accepted 21 May 201510.1126/science.aaa6850

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Fig. 4. Spectroscopic evidence for long-lived charged species in solution. Chemical structures oftrans-1 bis (A), trans-2 bis (B), trans-3 bis (C), and trans-4 bis (D) fullerene derivatives with color emphasizingthe hydrophobic regions. Cartoon depicting the assembly of trans-1,2 (orange) and trans-3,4 (purple) bisfullerenes with PFT (red). Photoexcitation of the PFT backbone leads to charge transfer first to trans-3,4 andthen to trans-1,2, where the electron remains due to stabilization by the reorganization of water. The hole(green, h+) remains on PFT (E). PL of PFT, PFT:mixed-bis, PFT:trans-3,4 bis, and PFT:trans-1,2 bis (F). Time-resolved luminescence for assembled concentrated PFT:trans-3,4 bis and PFT:trans-1,2 bis samples (G).

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assembliesLong-lived photoinduced polaron formation in conjugated polyelectrolyte-fullerene

Toso, J. Reddy Challa, Z. Hong Zhou, Yves Rubin, Benjamin J. Schwartz and Sarah H. TolbertRachel C. Huber, Amy S. Ferreira, Robert Thompson, Daniel Kilbride, Nicholas S. Knutson, Lekshmi Sudha Devi, Daniel B.

DOI: 10.1126/science.aaa6850 (6241), 1340-1343.348Science 

, this issue p. 1340Sciencewith lifetimes of several days.−−stable pairs of separated charges−−formation of polarons

polyelectrolyte charge donors with fullerene acceptors at a much smaller interface. They observed the photoinduced created aqueous micelles that pair conjugated et al.acceptor and donor structures are much larger. Huber

chargeHowever, unproductive charge recombination often occurs in the human-made system. This is in part because the Photosynthetic complexes and organic photovoltaics can rapidly create separated charges upon photoexcitation.

Photoinduction of long-lived polarons

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