R E S EARCH ART I C L E

MATER IALS ENG INEER ING

1Solar and Photovoltaics Engineering Research Center, Division of Physical Science andEngineering, King Abdullah University of Science and Technology, Thuwal 23955-6900,Saudi Arabia. 2Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academyof Sciences, Zhongguancun North First Street No. 2, Beijing 100190, P. R. China. 3Imagingand Characterization Laboratory, King Abdullah University of Science and Technology,Thuwal 23955-6900, Saudi Arabia. 4School of Chemistry and Chemical Engineering,Shanghai Jiao Tong University, Shanghai 200240, P. R. China.*Present address: Department of Chemistry, National University of Singapore, 3 ScienceDrive, Singapore 117543, Singapore.†Corresponding author. E-mail: osman.bakr@kaust.edu.sa

Shi et al. Sci. Adv. 2016; 2 : e1501491 15 April 2016

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Spiro-OMeTAD single crystals:Remarkably enhanced charge-carrier transportvia mesoscale ordering

Dong Shi,1* Xiang Qin,2 Yuan Li,1 Yao He,3 Cheng Zhong,1 Jun Pan,1 Huanli Dong,2 Wei Xu,3 Tao Li,4 Wenping Hu,2

Jean-Luc Brédas,1 Osman M. Bakr1†

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We report the crystal structure and hole-transport mechanism in spiro-OMeTAD [2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene], the dominant hole-transporting material in perovskite and solid-state dye-sensitized solar cells. Despite spiro-OMeTAD’s paramount role in such devices, its crystal structurewas unknown because of highly disordered solution-processed films; the hole-transport pathways remained ill-defined and the charge carrier mobilities were low, posing a major bottleneck for advancing cell efficiencies.We devised an antisolvent crystallization strategy to grow single crystals of spiro-OMeTAD, which allowed us toexperimentally elucidate its molecular packing and transport properties. Electronic structure calculations enabled usto map spiro-OMeTAD’s intermolecular charge-hopping pathways. Promisingly, single-crystal mobilities were foundto exceed their thin-film counterparts by three orders of magnitude. Our findings underscore mesoscale orderingas a key strategy to achieving breakthroughs in hole-transport material engineering of solar cells.

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INTRODUCTION

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Spiro-OMeTAD [2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene] has long been the prominent hole-transport ma-terial (HTM) in solid-state dye-sensitized solar cells (ssDSCs), eversince it provided a solid-state replacement to liquid electrolyte–basedhole transporters (1). More recently, spiro-OMeTADHTM has playeda key role in the rapid rise of perovskite solar cells (PSCs), includingrecent major breakthroughs in solar-to-electric power conversion effi-ciencies (2, 3), which now reach more than 20% (4). In addition toenergy levels that match with those of the absorber (for example, pe-rovskite or sensitizer) and the electron-transport layer (ETL; for exam-ple, mesoporous TiO2 or ZnO), spiro-OMeTAD provides remarkableglass-forming properties related to its spiro-center (1). When used inPSCs or ssDSCs with a mesoporous ETL, the spiro-center frustratescrystallization while securing sufficient infiltration of the material intothe mesoporous layer upon spin-coating processed film formation.However, the glass-forming tendency of spiro-OMeTAD has pre-cluded the understanding of its intermolecular p-p stacking andhole-transport pathways and mechanisms, which has prevented the ma-terial from reaching its ultimate performance limits. Thus, despite themany strategies to engineer this HTM, this challenge remains a majorbottleneck in the advancement of PSC and ssDSC efficiencies. Particu-larly in the case of PSCs, whose rapid efficiency rise is primarily due toimprovements in the crystallinity of the perovskite (4–11) and ETLlayers (10), spiro-OMeTAD appears to be the only device layer whosesingle-crystal structure is yet to be reported and exploited. Consequent-

ly, tremendous efforts have been made to design and synthesize neworganic hole conductors as “enhanced” hole mobility alternatives (12).

Thus, we were motivated to find a way to grow single crystals ofspiro-OMeTAD to address the lack of fundamental understanding oftheir intermolecular packing and transport properties, particularlytheir intrinsic upper limit of hole mobility, and to develop a clear mod-el for charge hopping and electronic structure that could be used to deviserational strategies for engineering this crucial device component (13).

RESULTS AND DISCUSSION

Using an antisolvent vapor–assisted crystallization (AVC) strategy, wehave succeeded in growing sizable high-quality spiro-OMeTAD singlecrystals (10). Figure 1A displays the setup and working mechanism ofthe two-vial–based AVC. Successful application of this method forcrystallization depends on the judicious selection of solventsaccording to the intrinsic properties of the materials under study.Considering that each spiro-OMeTAD molecule contains twop-conjugated fluorene fragments, intermolecular p-p stacking mayoccur spontaneously upon crystallization. To exclude all possible dis-ruptive intermolecular (p-p) interactions between spiro-OMeTADand the solvent molecules, we used dimethyl sulfoxide (DMSO) as thesolvent instead of the commonly used chlorobenzene. In this case, theinner vial contains the solution of spiro-OMeTAD in DMSO (1 mg/ml)and the outer vial contains methanol as the antisolvent. Slow diffusionof the antisolvent vapor from the outer vial into the inner vial at roomtemperature (RT) gradually reduces the solubility of spiro-OMeTADand eventually triggers crystallization once a supersaturation state isreached. The as-grown spiro-OMeTAD single crystals have overallcrack-free morphology and well-defined borders (Fig. 1B). The high qual-ity and macroscopic dimensions of the as-grown crystals enable theirdetailed structural and electrical characterization.

We proceeded to characterize the structure of the crystals using single-crystal x-ray diffraction (XRD). Preliminary temperature-dependent

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single-crystal XRD measurements at both RT and 140 K yield the samelattice parameters for the as-grown crystal (14), confirming that thecrystal structures at these two temperatures are identical. Therefore,the structure refined directly from the data collected at 140 K repre-sents the actual single-crystal structure of the as-grown crystals at RT.Direct single crystallography refinement on the as-collected data yieldsa low R factor of 0.0445 (14), which is indicative of the high quality ofthe as-grown crystals.

Table 1 summarizes some key parameters of the crystal structureextracted out of the completely refined data set (14). The crystal belongsto the triclinic space group, consistent with the crystal’s macroscopiclong-sheet morphology with bevel edges (Fig. 1B). The single-crystalunit cell contains two spiro-OMeTAD molecules featuring inter-molecular parallel p-p stacking, as highlighted in yellow in Fig. 1C.The vertical plane-to-plane distance between the two fluorene rings(yellow, Fig. 1C) is estimated to be ~3.8 Å, which is slightly longerthan ~3.5 Å, a distance characteristic of closely packed p-p systems[for example, in pentacene (15)], but is still liable to provide significantelectronic coupling between stacked conjugated fragments (16). Twotypes of steric hindrances which affect p-p stacking are manifestedin the crystal structure of spiro-OMeTAD (Fig. 1C). The steric hin-drance between the two molecules inside a single-crystal unit cellprevents the formation of close p-p stacking upon crystallization. Onthe other hand, the steric hindrance between the outer fragments ofeach unit cell prevents the formation of continuous p-p stacking. Thelatter type of hindrance has long been acknowledged as a result of thetwisted central spiro-carbon, which spurred tremendous efforts to de-sign and synthesize modified hole-transporting molecules that excludethe twisted central spiro-carbon or replace it with untwisted linkingfragments such as a thiophene or bisthiophene core (17). The dis-continuity in the p-p stacking hinders the delocalization of charge car-riers, and therefore charge transport occurs primarily via hopping fromone spiro-OMeTAD molecule to the next one, an observation cor-roborated by theoretical modeling (vide infra).

To determine the hole mobility of single-crystal spiro-OMeTAD,we prepared a field-effect transistor (FET) using the as-grown singlecrystals. The device was constructed on the basis of the well-laminated,long, sheet-like crystals on a bottom-gated configuration. Belt-like goldelectrodes (20 nm thick), thermally evaporated on silicon substrate,were peeled off from the substrate and then pasted on top of the pre-laminated crystals. The as-fabricated FET device of spiro-OMeTADsingle crystals has a channel length (L) of 23 mm and a channel width(W) of 3.15 mm (fig. S1). For comparison, FET devices based on

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solution-processed, highly disordered spiro-OMeTAD thin films werealso prepared with the same architecture (details are given in Materialsand Methods). The spiro-OMeTAD thin films were prepared usinga spin-coating method normally used in solar cell fabrication (2, 3).The top Au contact source and drain electrodes, with a channel length(L) of 20 mm and a width (W) of 160 mm, were thermally evaporateddirectly on the thin films (fig. S2). Both the single crystals and thethin films used for device fabrication were freshly prepared. Trans-fer characterizations were performed at the moment the devices werefabricated.

Figure 2 displays the transfer characteristics (Fig. 2, A and C) andoutput curves (Fig. 2, B and D) of the FET devices of single-crystaland thin-film spiro-OMeTAD. Both transfer and output characteris-tics show excellent scaling with the gate voltage, indicating the absenceof any severe contact issues. Typical hole transport is exhibited in thetransfer characteristics (Fig. 2, A and C). The threshold voltages(VT) are ~60 and ~90 V for the spiro-OMeTAD single crystal–basedand thin film–based FET devices, respectively. One possible reason forthe positive shift of VT could be the unintentional oxygen-inducedhole-doping from the bottom dielectric surface, which underwent ox-ygen plasma treatment before deposition of the spiro-OMeTAD layer.Because the transfer characteristics are more sensitive to the top thanto the bottom surface of the semiconducting layer of spiro-OMeTAD,the positive hole-doping effect on the VT shift is less apparent in thesingle crystal–based device because of its larger thickness. We have toremark that without oxygen plasma treatment, we could not depositspiro-OMeTAD with full coverage and good electric contact. We cansee that both transistors work in the saturation region when −VDS >VG − VT, as a result of depletion of charges in the channel. By fittingthe square root of the source-drain current (IDS)

1/2 plot versus gatevoltage (VG) (black line in Fig. 2, A and C) in the saturation region,the slope (msat) was estimated as noted in Fig. 2 (A and C). The holemobility was then calculated using the following equation: m ¼2m2

satðL=WÞð1=CiÞ; where L/W is the ratio between channel length

Fig. 1. Crystal growth, shape, and crystallography. (A) Schematic diagram of the crystallization process. (B) Confocal optical microscopy imageof a spiro-OMeTAD single crystal. (C) Unit cell of the single-crystal structure of spiro-OMeTAD (the fluorene fragments are highlighted in yellow).

Table 1. Key lattice parameters of the spiro-OMeTAD single crystal.

Space group

Cell length (Å) Cell angle (°)

a

b c a b g

Triclinic P−1 (no. 2)

13.66 14.72 17.28 86.23 68.98 80.01

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and width, which is equal to 7.302 for the single crystal–based deviceand 0.125 for the thin film–based device (inset values in Fig. 2, A andC, and figs. S1 and S2), and Ci is the gate capacitance per unit area.The hole mobility of m ~ 1.69 × 10−6 cm2/Vs (inset in Fig. 2C) esti-mated for spiro-OMeTAD thin films is consistent with previousexperimental results (18, 19). The hole mobility of the single-crystalspiro-OMeTAD is estimated to be m ~ 1.30 × 10−3 cm2/Vs (Fig. 2A);remarkably, this value represents an increase of nearly three ordersof magnitude over that of the spiro-OMeTAD thin film. This findingunderlines that spiro-OMeTAD has still much untapped potential tooffer in PSCs, and the future of device engineering of the HTM liesin enhancing its crystallinity. In other words, at least in the shortterm, it may not be necessary to design and synthesize radicallynew organic hole conductors as replacements to spiro-OMeTAD inPSCs.

We also investigated the effects of doping with lithium salts andoxidation (two commonly used strategies in device engineering ofthe HTM) on the mobility of spiro-OMeTAD. Our findings confirmthat lithium salts only have a negligible effect on the hole mobility ofthe bulk of spiro-OMeTAD (fig. S3), which is consistent with the view thatlithium salts are mainly present at the interface between spiro-OMeTAD

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and other layers in practical solar cell devices (20). Furthermore, wealso found a negligible effect on the hole mobility of spiro-OMeTADby oxidation, as shown in fig. S4. These results suggest that moreattention should be directed toward studying the role of oxidationand lithium salts as potential interfacial additives.

To elucidate the charge-transport processes at the molecular level,we performed density functional theory (DFT) calculations for spiro-OMeTAD molecules extracted from the XRD-refined single-crystalstructure. The calculations were performed with the tuned long-rangecorrected wB97X-D functional and 6-31G** basis set, using the Gaussian09 package (21). As a result of the twisted spiro structure of the in-dividual molecules, the two highest occupied molecular orbitals(HOMO and HOMO-1), which are responsible for hole transport,are nearly degenerate in energy and their wave functions are localizedon one of the two fluorene rings. The values of the transfer integrals,the most critical parameter governing charge transport (15), stronglydepend on the relative orientations of adjacent fluorene moieties alongthe p-p stacking direction (a direction) of the spiro-OMeTAD crystal.The transfer integral between the two molecules within a unit cell iscalculated to be different from that between adjacent unit cells. As aresult, to map out the charge-transport channel in the spiro-OMeTAD

Fig. 2. Comparison of transistor characteristics between spiro-OMeTAD single crystals and thin films. (A) Transfer characteristics in thesaturated regime for the single-crystal FET. (B) Output curve of the single-crystal FET. (C) Transfer characteristics in the saturated regime for thethin-film FET. (D) Output curve of the thin-film FET.

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Fig. 3. Modeling of intermolecular charge transport. (A) DFT-wB97X-D/6-31G** energies and wave functions for the HOMO (left) and HOMO-1 (right)levels of a molecular trimer extracted from the spiro-OMeTAD single crystal along the a direction. Lower panels: Illustration of the transfer integralsestimated for adjacent molecules in the trimer; the blue squares highlight the fluorene rings for which wave-function overlap has a major contributionto intermolecular electronic couplings (transfer integrals). (B) DFT-B3LYP/6-31G** valence band structure and density of electronic states (DOS) of the spiro-OMeTAD single crystal; the crystallographic coordinates of high-symmetry points in the first Brillouin zone correspond to G = (0,0,0), X = (0.5,0,0), Y = (0,0.5,0),Z = (0,0,0.5), M = (0.5,0.5,0), and R = (0.5,0.5,0.5).

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crystal, a model consisting of at least three molecules (trimer) has tobe taken into account, as shown in Fig. 3A. As in the single-moleculecase, the HOMO and HOMO-1 levels of the molecular trimer are en-ergetically nearly degenerate and their wave functions are distributedover two different pairs of the fluorene rings. This is related to the twolarge transfer integrals obtained for the corresponding fluorene pairsas a result of the coupling of the HOMO and HOMO-1 of the indi-vidual molecules (Fig. 3A). The magnitude of the transfer integrals issubstantial, on the order of nearly 40 meV, even though it is lower by afactor of 2.0 to 2.5 with respect to the transfer integrals calculated forhigh-mobility rubrene (~100 meV) and pentacene (~85 meV) singlecrystals (22). Such transfer integrals open up an efficient charge-transport channel within the spiro-OMeTAD single crystal, which ex-plains the remarkably improved hole mobility as compared with thatof the less ordered thin-film counterpart. The calculations also showthat the hole geometry relaxation energy (polaron binding energy) ison the order of 145 meV, a value larger than that of the transfer in-tegrals; this finding indicates that charge carriers tend to localize onone spiro-OMeTAD molecule (or a few molecules) to form small po-larons (15). For the sake of completeness, we have also evaluated theelectronic band structure of the spiro-OMeTAD crystal at the DFTB3LYP-6-31G** level with the CRYSTAL14 package (23) (see Fig.3B); the upper valence subband (which comes from the interactionsamong the HOMO and HOMO-1 levels of the two inequivalentmolecules per unit cell) is calculated to be narrow. Therefore, alltheoretical results point to the fact that hole transport in the spiro-OMeTAD crystal is expected to take place in the hopping regime,which is consistent with the measured hole mobility on the order of10−3 cm2/Vs.

In summary, for the first time, single crystals of spiro-OMeTADhave been grown, and their crystal structure has been determinedand shown to be contrary to the conventional wisdom that was builtupon the material’s glass-forming properties (1). Currently, most re-search efforts in the field are directed toward designing and synthesiz-ing new HTMs, a much more time-consuming process of uncertainoutcome. Our work not only elucidates a straightforward and assuredstrategy for creating a vastly improved hole transporter through im-proving the crystallinity of spiro-OMeTAD, one of the most widelyused commercially available HTMs for photovoltaic and opto-electronic devices, but also highlights mesoscale molecular orderingas the key to promoting the material’s charge transport pathways. Al-though our method, which was developed to make structure-analysisquality single crystals, is not directly implementable in its current formin large-scale device fabrication, we foresee that the use of an antisol-vent to trigger crystallization, as we did in this study, could poten-tially be adopted to enhance the crystallinity of the spiro-OMeTADlayer by simply immersing the as-deposited “wet” layer from solu-tion into an antisolvent. Indeed, a similar strategy has been success-fully used in preparing a hybrid perovskite layer with enhancedcrystallinity and performance for PSCs, but it remains to be testedin spiro-OMeTAD (24). In particular, PSCs with planar hetero-junction TiO2/perovskite/HTM architecture, which are competingstrongly with mesoporous-type solar cells in terms of efficiencybut have the advantage of not requiring the infiltration of the HTMinside the perovskite layer, stand to benefit most from enhancingthe hole mobility in the HTM layer by using a single-crystallinespiro-OMeTAD layer. More broadly, the issue of hysteresis in PSCsmay be alleviated by the use of single-crystalline spiro-OMeTAD

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HTM because recent studies have found a strong correlation be-tween reduced current density–voltage (J-V) hysteresis and improvedelectron and hole transport in PSCs (25).

MATERIALS AND METHODS

Materials and solventsAll chemicals were used as received, unless otherwise stated. Spiro-OMeTAD (sublimated grade) was purchased from Borun NewMaterialTechnology Ltd. Bis(trifluoromethane)sulfonimide lithium salt (productnumber: 449504-50G), DMSO (product number: 276855-250 ml),methanol (product number: 34860-1L-R), and chlorobenzene (productnumber: 270644-1L) were purchased from Sigma-Aldrich.

Crystallization of spiro-OMeTADSpiro-OMeTAD (5 mg) was dissolved in DMSO (5 ml) under stirringin a closed vial (7 ml) at RT, yielding completely transparent solutionwith light yellowish color. The solution was further cleaned using asyringe filter through a Millipore membrane (100 mm). Then, the fil-trated solution was equally divided into five vials (5 ml), and each vialwas transferred inside a big vial (20 ml) containing methanol (8 ml).The outer vial was closed tightly, and the system was then transferredinto a nitrogen-filled glove box. Slow evaporation of the antisolventmethanol in the solution of spiro-OMeTAD in DMSO (1 mg/ml)triggered the crystallization.

Single-crystal XRDThis characterization was performed at 140 K on a Bruker D8 Venturediffractometer equipped with a PHOTON 100 CMOS detector andCu Ka monochromated radiation (l = 1.54178 Å). The unit cell wasdetermined using 9123 reflections. Preliminary temperature-dependentsingle-crystal XRD measurements at RT and 140 K yielded the samelattice parameters for the as-grown crystal [triclinic, space group P−1(no. 2), a = 13.66 Å, b = 14.72 Å, c = 17.28 Å, a = 86.23°, b =68.98°, g = 80.01°], confirming no crystal structural changes in thistemperature range. The crystal structure was refined using SHELXTLsoftware. Detailed refinement parameters are given in the affiliated sin-gle crystal .cif file.

Silicon substrate cleaningThe silicon substrates used for FET devices were treated in a stepwisemanner using the following procedures: (i) immersion in sulfuric acid(98%) at 100°C for 2 hours, (ii) thorough cleaning with Milli-Q water,and (iii) treatment with oxygen plasma for 30 min.

Single crystal–based FET device fabricationThe as-grown long sheet–like spiro-OMeTAD single crystals werewashed thoroughly with methanol and then transferred directly ontothe pretreated silicon substrate. The clustered crystals were separatedand laminated on top of the substrate using tweezers. The prelami-nated crystal samples were dried under vacuum at 60°C for 4 hours.Finally, predeposited belt-like gold electrodes on the silicon sub-strate were peeled off and transferred on top of the crystal samplesusing microprobes. Figure S1 displays a real photograph of thesingle-crystal FET device taken under an optical microscope. The chan-nel width and channel length were estimated to be 3.15 and 23 mm,respectively.

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Thin film–based FET device fabricationThin filmswerepreparedusing the spin-coatingmethod. For thenondopedspiro-OMeTAD thin-filmFETdevice, a drop of spiro-OMeTAD in chlo-robenzene (0.2mg/ml)was applied onto the as-prepared silicon substratethrough a syringe filter equipped with a Millipore membrane (100 mm).After spinning (6000 rpm, 30 s) the excessive solution, a uniform thin filmof spiro-OMeTADon the silicon substratewas obtained and further driedundervacuumat60°Cfor4hours.Goldelectrodes (thickness, 20nm;widthof the gap between the two electrodes, 20 mm; length of each electrode,160 mm)were thermally evaporated onto the thin film through a patternedmask. Figure S2 displays a real photograph of the thin-film FET devicetaken under an optical microscope. For the lithium salt–doped spiro-OMeTADthin-filmFETdevice, a solutionof spiro-OMeTAD(0.2mg/ml)and bis(trifluoromethane)sulfonimide lithium salt (saturated) in chlo-robenzene was prepared. Then, the devices were prepared using exactlythe same method as detailed for nondoped thin-film devices.

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SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/4/e1501491/DC1Single-crystal data file.fig. S1. Optical microscope image of the spiro-OMeTAD single-crystal FET device.fig. S2. Optical microscope image of the spiro-OMeTAD thin-film FET device.fig. S3. Transfer characteristic of lithium salt–doped spiro-OMeTAD thin films.fig. S4. Transfer characteristics of oxidized spiro-OMeTAD single crystals.

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REFERENCES AND NOTES1. U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissörtel, J. Salbeck, H. Spreitzer, M. Grätzel,

Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron con-version efficiencies. Nature 395, 583−585 (1998).

2. J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Grätzel,Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature499, 316–319 (2013).

3. M. Liu, M. B. Johnston, H. J. Snaith, Efficient planar heterojunction perovskite solar cells byvapour deposition. Nature 501, 395–398 (2013).

4. W. S. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, High-performancephotovoltaic perovskite layers fabricated through intramolecular exchange. Science 348,1234–1237 (2015).

5. S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, T. C. Sum, Y. M. Lam, Theorigin of high efficiency in low-temperature solution-processable bilayer organometalhalide hybrid solar cells. Energy Environ. Sci. 7, 399–407 (2014).

6. M. Saliba, K. W. Tan, H. Sai, D. T. Moore, T. Scott, W. Zhang, L. A. Estroff, U. Wiesner,H. J. Snaith, Influence of thermal processing protocol upon the crystallization and photo-voltaic performance of organic–inorganic lead trihalide perovskites. J. Phys. Chem. C 118,17171–17177 (2014).

7. Y. Zhou, A. L. Vasiliev, W. Wu, M. Yang, S. Pang, K. Zhu, N. P. Padture, Crystal morphologiesof organolead trihalide in mesoscopic/planar perovskite solar cells. J. Phys. Chem. Lett. 6,2292–2297 (2015).

8. A. Fakharuddin, F. D. Giacomob, I. Ahmeda, Q. Walia, T. M. Brown, R. Jose, Role of morphologyand crystallinity of nanorod and planar electron transport layers on the performance and longterm durability of perovskite solar cells. J. Power Sources 283, 61–67 (2015).

9. F. Hao, C. C. Stoumpos, Z. Liu, R. P. H. Chang, M. G. Kanatzidis, Controllable perovskitecrystallization at a gas–solid interface for hole conductor-free solar cells with steady powerconversion efficiency over 10%. J. Am. Chem. Soc. 136, 16411–16419 (2014).

10. D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland,A. Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed,E. H. Sargent, O. M. Bakr, Low trap-state density and long carrier diffusion in organoleadtrihalide perovskite single crystals. Science 347, 519–522 (2015).

11. D. W. de Quilettes, S. M. Vorpahl, S. D. Stranks, H. Nagaoka, G. E. Eperon, M. E. Ziffer,H. J. Snaith, D. S. Ginger, Impact of microstructure on local carrier lifetime in perovskitesolar cells. Science 348, 683–686 (2015).

Shi et al. Sci. Adv. 2016; 2 : e1501491 15 April 2016

12. E. J. W. Crossland, N. Noel, V. Sivaram, T. Leijtens, J. A. Alexander-Webber, H. J. Snaith,Mesoporous TiO2 single crystals delivering enhanced mobility and optoelectronic deviceperformance. Nature 495, 215–219 (2013).

13. Z. Yu, L. Sun, Recent progress on hole-transporting materials for emerging organometalhalide perovskite solar cells. Adv. Energy Mater. 5, 1500213 (2015).

14. See supplementary materials on Science Advances online.15. V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier, R. Silbey, J.-L. Brédas, Charge

transport in organic semiconductors. Chem. Rev. 107, 926–952 (2007).16. J. L. Brédas, J. P. Calbert, D. A. da Silva Filho, J. Cornil, Organic semiconductors: A theoretical

characterization of the basic parameters governing charge transport. Proc. Natl. Acad. Sci. U.S.A.99, 5804–5809 (2002).

17. H. Li, K. Fu, P. P. Boix, L. H. Wong, A. Hagfeldt, M. Grätzel, S. G. Mhaisalkar, A. C. Grimsdale,Hole-transporting small molecules based on thiophene cores for high efficiency perovskitesolar cells. ChemSusChem 7, 3420–3425 (2014).

18. T. Leijtens, I.-K. Ding, T. Giovenzana, J. T. Bloking, M. D. McGehee, A. Sellinger, Holetransport materials with low glass transition temperatures and high solubility forapplication in solid-state dye-sensitized solar cells. ACS Nano 6, 1455–1462 (2012).

19. J. Mei, D. H. Kim, A. L. Ayzner, M. F. Toney, Z. Bao, Siloxane-terminated solubilizing sidechains: Bringing conjugated polymer backbones closer and boosting hole mobilities inthin-film transistors. J. Am. Chem. Soc. 133, 20130–20133 (2011).

20. R. Schölin, M. H. Karlsson, S. K. Eriksson, H. Siegbahn, E. M. J. Johansson, H. Rensmo, Energylevel shifts in spiro-OMeTAD molecular thin films when adding Li-TFSI. J. Phys. Chem. C116, 26300–26305 (2012).

21. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani,V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian,A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr.,J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov,R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi,M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich,A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09, RevisionD.01 (Gaussian Inc., Wallingford, CT, 2010).

22. V. Coropceanu, Y. Li, Y. Yi, L. Zhu, J.-L. Brédas, Intrinsic charge transport in single crystalsof organic molecular semiconductors: A theoretical perspective. MRS Bull. 38, 57–64 (2013).

23. R. Dovesi, V. R. Saunders, C. Roetti, R. Orlando, C. M. Zicovich-Wilson, F. Pascale, B. Civalleri,K. Doll, N. M. Harrison, I. J. Bush, P. D’Arco, M. Llunell, M. Causà, Y. Noël, CRYSTAL14 User’sManual (University of Torino, Torino, Italy, 2014).

24. M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach,Y.-B. Cheng, L. Spiccia, A fast deposition-crystallization procedure for highly efficient leadiodide perovskite thin-film solar cells. Angew. Chem. Int. Ed. 53, 9898–9903 (2014).

25. J. H. Heo, H. J. Han, D. Kim, T. K. Ahn, S. K. Im, Hysteresis-less inverted CH3NH3PbI3 planarperovskite hybrid solar cells with 18.1% power conversion efficiency. Energy Environ. Sci. 8,1602–1608 (2015).

AcknowledgmentsFunding: O.M.B. and J.-L.B acknowledges the financial support of King Abdullah University ofScience and Technology Grant URF/1/2268-01-01. J.-L.B. also acknowledges support from ONRGlobal through Grant N62909-15-1-2003. H.D. thanks the National Natural Science Foundationof China (91433115). Author contributions: D.S. conceived the idea. O.M.B. crafted the overallexperimental plan and directed the research. D.S. optimized the crystallization. D.S. and W.X.performed the confocal optical microscope imaging. D.S. and Y.H. performed single-crystalXRD and data analysis. D.S., X.Q., H.D., T.L., and W.H. planned and performed the mobilitymeasurements and analyzed the data. Y.L., C.Z., and J.-L.B. planned and performed the theoreticalcalculations. Y.L., C.Z., and J.-L.B. analyzed the data of the theoretical part. J.P. assisted D.S. in theexperiments. D.S., Y.L., J.-L.B., and O.M.B. wrote the manuscript. All authors discussed and com-mented on the manuscript. Competing interests: The authors declare that they have nocompeting interests. Data and materials availability: All data needed to evaluate the conclusionsin the paper are present in the paper and/or the Supplementary Materials. Additional data relatedto this paper may be requested from the authors.

Submitted 20 October 2015Accepted 15 March 2016Published 15 April 201610.1126/sciadv.1501491

Citation: D. Shi, X. Qin, Y. Li, Y. He, C. Zhong, J. Pan, H. Dong, W. Xu, T. Li, W. Hu, J.-L. Brédas,O. M. Bakr, Spiro-OMeTAD single crystals: Remarkably enhanced charge-carrier transport viamesoscale ordering. Sci. Adv. 2, e1501491 (2016).

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orderingSpiro-OMeTAD single crystals: Remarkably enhanced charge-carrier transport via mesoscale

and Osman M. BakrDong Shi, Xiang Qin, Yuan Li, Yao He, Cheng Zhong, Jun Pan, Huanli Dong, Wei Xu, Tao Li, Wenping Hu, Jean-Luc Brédas

DOI: 10.1126/sciadv.1501491 (4), e1501491.2Sci Adv 

ARTICLE TOOLS http://advances.sciencemag.org/content/2/4/e1501491

MATERIALSSUPPLEMENTARY http://advances.sciencemag.org/content/suppl/2016/04/11/2.4.e1501491.DC1

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