ZnO Hybrid Nanowires

Aug 14, 2014 - *(C.-A.D.)Telephone: 886-2-3366-3051. Fax: 886-2-2362-3040. E- mail: [email protected]., *(L.W.) Telephone: 886-2-3366-5276...
1 downloads 0 Views 6MB Size
Download PDF
Article pubs.acs.org/Macromolecules

In Situ Fabrication of Poly(3-hexylthiophene)/ZnO Hybrid Nanowires with D/A Parallel-Lane Structure and Their Application in Photovoltaic Devices Yi-Huan Lee,†,‡ Yu-Ping Lee,† Chi-Ju Chiang,† Ching Shen,† Yang-Hui Chen,‡ Leeyih Wang,*,‡,§ and Chi-An Dai*,†,‡ †

Department of Chemical Engineering, ‡Institute of Polymer Science and Engineering, and §Center for Condensed Matter Sciences, National Taiwan University, No. 1, Roosevelt Rd. Sec. 4, Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: In this study, we demonstrate a facile in situ synthetic strategy to fabricate self-assembled organic/inorganic hybrid nanowires, wherein a “pre-crystallization” approach was first utilized to co-organize P3HT molecules and zinc precursors into highly elongated nanowires, followed by a thermal oxidation treatment to directly grow ZnO nanocrystals on the existing nanofibrillar template. By further thermal annealing the ZnO embossed hybrid nanowires, a unique superhighway-like architecture which composed of alternating parallel channels of P3HT nanofibrils and ZnO nanocrystals could be further obtained. This donor/acceptor (D/A) parallel-channel structure gave access to the improvements in the exciton dissociation and charge transport, thereby enhancing photoluminescence quenching, charge transport, and device performance. The photovoltaic devices with the D/A parallel-lane structure gave a high PCE of 0.61% as compared to only 0.07% from a conventional P3HT/ZnO bulk heterojuction solar cell. Our approach offers a versatile route to coassemble inorganic nanocrystals with π-conjugated polymer hosts, forming uniform one-dimensional hybrid nanochannels potentially useful in optoelectronic applications.

T

where solvent-assisted ligands were attached onto the nanocrystal surface to control their interparticle distance.14,15 However, this has led to poorer photovoltaic device performance since an isolating barrier often forms between the polymer matrix and nanocrystals due to the capping ligands.16 To overcome these drawbacks, it is necessary to remove excess capping ligands17 or replace bulky ligands with smaller ones.,4 Additionally, other routes employed to incorporate nanocrystals into conducting polymer also include deposition18 and grafting methods.19−23 An alternative strategy, which involves π-conjugated polymer matrix containing a precursor of an inorganic component required for the synthesis of organic/ inorganic nanohybrids, has recently received a great deal of attention on the fabrication of organic photovoltaic devices. This is because the so-called “in situ” formation routes possess important advantages in that they are ligand-free and do not require a separate nanoparticle synthesis step. So far, a number of inorganic semiconductors (PbS, CdS, CdSe, and etc.) directly synthesized in conjugated polymers have been reported in several studies.24−30 Despite continuous improvements in

he fabrication of organic/inorganic hybrid materials based on π-conjugated polymers and inorganic semiconductors has attracted great interests over the past decades due to its potential usage in novel applications such as light-emitting diodes, and solar cells.1−4 In particular, significant attention has been paid to the use of nanocrystals (e.g., CdSe, ZnO, and TiO2) as light absorbers and electron acceptors in solutionprocessed polymer solar cells.4−11 By organizing electron-donor and -acceptor materials, one can provide phase separated pathways that match the size of the exciton diffusion length (∼10 nm) which would allow for a continuous route facilitating charge separation and transport.12 Such a process is crucial for improving performances of hybrid devices. However, inorganic nanocrystals often suffer from macrophase separation from nonpolar conjugated polymers following increasing loading concentrations with decreasing interfacial area. This effect can be alleviated to some degree by altering the processing conditions and choice of solvents. Therefore, new hybrid nanocrystal photovoltaics will require a more controlled phase separation, increased interfacial areas, and improved optoelectronic interactions of inorganic nanoparticles and organic polymers.13 To this end, a number of methods have been pursued to achieve a favorable dispersion of nanocrystals in hybrid solar cells. Many researchers have focused on the ligand method, © XXXX American Chemical Society

Received: December 25, 2013 Revised: July 7, 2014

A

dx.doi.org/10.1021/ma402637e | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

Figure 1. Reaction scheme for the formation of in situ nanohybrid containing ZnO nanoparticle decorated P3HT nanofibrils and the D/A parallellane hybrid superhighway structure.

markedly improved photovoltaic performance as compared to those of the ex situ blending and in situ discontinuous ZnO nanocrystals decorated nanowire counterparts. We believe that the current study offers a scalable and modular approach to bring dissimilar materials into close association in regular hybrid nanochannels. The fabrication of P3HT/ZnO nanohybrids is schematically illustrated in Figure 1. This method starts with a P3HT/Zn2+ complex prepared by dissolving P3HT in a toluene solution to obtain a polymer solution, followed by blending with a zinc acetate dihydrate solution in THF. For the P3HT/Zn2+ hybrid solution, the sulfur atoms along the backbone of the P3HT were used to act as anchoring sites for coordination with zinc ions. The molar ratio between 3HT units to Zn2+ was fixed to be 1:1. Figure 2a shows a bright field TEM image of P3HT micelles loaded with zinc ions prepared by drop casting the P3HT/Zn2+ solution onto a carbon-coated copper grid. From the TEM image, we observed that these micelles are dispersed with an average diameter of roughly 50 nm. Note that neither large aggregation of Zn2+ ions nor phase separation of P3HT/ Zn2+ were found, indicating that the inorganic precursors could be uniformly dispersed with P3HT chains as chelating surfactants. Subsequently, nanowire assembly of the polymeric complexes was carried out via the addition of anisole into the solution. The addition of anisole, a marginal solvent to P3HT, could induce a transition for the P3HT/Zn2+ complexes from the twisted coil-like micelle state to a more planar rod-like conformation, thereby resulting in a strong π−π interaction between P3HT, which served as the dominant driving force for molecular packing, leading to highly ordered nanowires with a micrometer-scale in length, as shown in Figure 2b of a dropcast sample. It is worth noting that the P3HT/Zn2+ nanowire structure is highly robust. This was determined following an observation that indicated the morphology of the hybrid had

this hybridization strategy, the in situ formation of organic/ inorganic hybrids remains suboptimal in the area of charge transport and exciton utilization. This is due to the disordered and kinetically dictated nature of the hybrid. It is generally believed that the formation of 1-dimensional (1-D) nanostructures with an extremely high aspect ratio, e.g., extended nanofibrils, is technically desirable for optoelectronic nanodevices as the nanostructures may exhibit a highly efficient photoinduced exciton dissociation rate and enhanced photoconductive response in a bulk heterojunction (BHJ).18,23 However, only a few reports were related to the polymer template-directed in situ synthesis of 1-D inorganic semiconductor nanocrystals.25,29 Unfortunately, these methods often restrict the obtainable size and organization of the structures. The synthesized hybrid fibrils showed to be substantially shorter than those formed from pristine P3HT alone, presumably reflecting the tendency of nanocrystals to sterically hinder the growing crystal faces of the P3HT nanowires. Herein, we report on a facile in situ method to fabricate highly elongated P3HT nanowires lining along their long fibrilaxis with continuous and highly crystalline ZnO nanocrystal pathways, where a “pre-crystallization” approach was first utilized to simultaneously organize organic P3HT molecules and inorganic zinc precursors into highly ordered nanowires with micrometer-scale lengths, followed by a thermal oxidation treatment to directly grow discrete ZnO nanocrystals on the existing nanofibrillar motif. In addition to this formed nanowire hybrid decorated with ZnO nanocrystals, alternating lanes of ZnO nanocrystals along with the P3HT fibrils positioned parallel to one another formed a unique long-range ordered “superhighway” by further thermally annealing the hybrid nanowires. The D/A parallel-lane nanowire hybrid thus formed act as efficient pathways for charge transport and collection, resulting in better optimized D/A nanostructures and thus, a B

dx.doi.org/10.1021/ma402637e | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

Figure 2. TEM micrographs of P3HT/Zn2+ hybrid solution (a) before and (b) after solution assembly via anisole addition. (c) Solution assembly of P3HT/Zn2+ after 1 week of stirring. (d) After thermal oxidation in air, P3HT/ZnO-A sample reveals the embossed P3HT/ZnO nanofibril morphology. (e) Upon further annealing in N2, P3HT/ZnO-B exhibits D/A parallel-channel network structure with bicontinuous P3HT/ZnO phases. The micrographs in parts f and g show the HR-TEM image of the hybrid samples shown in parts d and e, respectively. (h) HR-TEM image focused only on the ZnO nanocrystal phase shown in part g. The inset pictures in parts f and h show the FFT results of the corresponding TEM realspace images.

behavior. High-resolution TEM (HR-TEM) analysis of the nanohybrid sample (Figure 2f) revealed discrete ZnO nanoparticles with well-pronounced (002)ZnO (distance of 0.258 nm) lattice fringe, belonging to a wurtzite structure. This hybrid sample is denoted as P3HT/ZnO-A. The inset picture in Figure 2(f) shows the corresponding Fourier transformation (FFT) of the (002)ZnO lattice fringe. Upon further annealing of the nanowire embossed with ZnO nanocrystals at 170 °C for 24 h under a nitrogen environment, an unique donor/acceptor parallel-lane nanowire network structure composed of alternating coextensive lanes of ZnO nanocrystals and P3HT

not changed after a stirring of the solution for over a week, as shown in Figure 2c. Figure 2d further shows the TEM image of the nanowire structured P3HT/Zn2+ sample after a thermal oxidation treatment at 170 °C for 30 min in air. From the TEM image, we observed that the ZnO nanocrystals were grown sporadically along the P3HT nanowire template to form a 1-D nanowire hybrid. There was no apparent change in the morphology of the polymer nanowires before and after the thermal treatment, indicating that the growth of ZnO nanoparticles did not perturb the P3HT’s self-assembly C

dx.doi.org/10.1021/ma402637e | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

Figure 3. Two-dimensional GIWAXS scattering spectrum of (a) P3HT/Zn2+, (c) P3HT/ZnO-A, and (e) P3HT/ZnO-B hybrid films. The corresponding one-dimensional GIWAXS profiles along qz, and qxy axis were extracted for the three samples, as shown in parts b, d, and f, respectively.

high-resolution image further supports structural information for the enhanced interconnection of crystalline ZnO phase. GIWAXS measurements were subsequently carried out to identify the crystalline structure and molecular orientation of the hybrid system at various synthesis stages. Shown in parts a, c, and e of Figure 3 are the two-dimensional scattering profiles of the P3HT/Zn2+, P3HT/ZnO-A, and P3HT/ZnO-B samples, respectively. The corresponding one-dimensional intensity profiles along qz and qxy axes of the three samples are also shown in parts b, d, and f of Figure 3. For the P3HT/Zn2+ sample, its GIWAXS pattern exhibited intense but broad arcs associated with the (h00)P3HT reflections along the qz (substrate normal) axis and the (020)P3HT reflections along the qxy (substrate parallel) axis, indicating a partial preference for P3HTs crystalline structure aligning normal to the film surface. Furthermore, a significant additional reflection at q = 0.48 Å−1 could be found in the GIWAXS pattern. We attributed this diffraction reflection to a metastable phase of P3HT crystal induced from complexation with zinc ions, referred as (100)′P3HT. For the P3HT/ZnO-A sample, Figure 3c clearly shows the intense arcs of the (h00) layers and the (020) crystals along the qz and qxy axes, respectively, implying that the crystallinity of P3HT is not affected by the brief thermal

nanowires was observed, as shown in Figure 2e. The formation of this structure implies that the thermal annealing treatment induced a better balance between the increased ordered packing of the π-conjugated P3HT chains and the finer nanoscale phase separation in the hybrid sample. Upon annealing, the reorganized crystal structure of P3HT yielded a high orientation of chain axes perpendicular to the fibril axis, thus causing ZnO nanocrystals to preferentially diffuse, accumulate, and grow along the edge of the crystalline P3HT nanofibrils. This resulted in a long-ranged parallel-lane superhighway hybrid structure with P3HT nanofibrils forming the electron-donor channels and an accompanying continuous ZnO nanocrystal phase forming the electron-acceptor channels. Figure 2g shows the HR-TEM micrograph of the 24-h annealed sample in N2 (designated as P3HT/ZnO-B), which gives a view of the interface of the P3HT/ZnO components. From the HRTEM image that focused only on the ZnO nanochannel (Figure 2h), it further demonstrates a highly ordered ZnO nanocrystalline phase exhibiting well-pronounced (100)ZnO (fringe distance 0.279 nm), (002)ZnO (fringe distance 0.258 nm) and (101)ZnO (fringe distance 0.244 nm) lattice fringes for P3HT/ ZnO-B, all belonging to the wurtzite structure. The corresponding FFT image is shown in the inset picture. The D

dx.doi.org/10.1021/ma402637e | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

Figure 4. (a) UV−vis and (b) PL spectra of P3HT/Zn2+, the embossed P3HT/ZnO-A, and the D/A parallel-channel P3HT/ZnO-B samples. (c) Schematic illustration showing the architecture of photovoltaic device. (d) Current density−voltage (J−V) characteristics of the P3HT/ZnO solar cells. The solid and dotted arrows in part a correspond to UV absorption for ZnO and P3HT, respectively.

oxidation in air and these films have highly ordered edge-on hexyl side chains and that the π-conjugated planes of P3HT block are parallel with respect to the substrate. The rest of the diffraction peaks in higher qxy can be assigned to (100), (002), and (101) planes corresponding to the wurtzite structure of ZnO, confirming the presence of crystallinity of the nanoparticles. Notably, for P3HT/ZnO-B, a significant enhancement in the scattering intensity of the reflections of both P3HT and ZnO (shown in Figure 3e) demonstrated that the additional thermal annealing process in N2 resulted in even better molecular rearrangements of P3HT and the growth of ZnO crystallites in the nanohybrids, which was consistent with the results from the TEM measurements. On the basis of the combined TEM and GIWAXS results, it can be said that the current in situ methodology provided an effective strategy to directly coassemble ZnO nanocrystals with highly ordered and long-ranged P3HT nanofibrils, providing large D/A interfaces, thereby increasing the probability of exciton generation, dissociation and charge transport. A detailed study of the optoelectronic properties and device performance of the cooperative self-assembly P3HT/ZnO system is described in the following sections. Figure 4a shows the UV−vis absorption spectra of all the hybrid samples. For the P3HT/Zn2+sample, its UV−vis profile showed three absorption peaks located at 519, 555, and 603 nm, which correspond to the spectral features of highly ordered P3HT. After the oxidation treatment, the UV−vis spectrum of the P3HT/ZnO-A hybrid showed that the relative contribution of the ZnO absorbance (at 350 nm) increases, compared to the π−π* band of P3HT in the 400−650 nm region. Simultaneously, a significant enhancement of the vibronic structure was

observed (∼610 nm), indicating an improvement in P3HT molecular packing. Upon further annealing of the P3HT/ZnO nanohybrid at 170 °C for 24 h, it could be observed that the UV−vis spectrum of the annealed P3HT/ZnO-B sample exhibited the highest absorption intensity of spectral features as compared to all of the other samples. It could be gathered from these absorption features that the structure and property of the P3HT/ZnO hybrid could be synergistically improved via thermal annealing, which would result in highly ordered, interchain π−π stacking and crystalline domains in the hybrid. We now turn our attention to the fluorescent properties of the P3HT/ZnO hybrid systems. The photoluminescence (PL) spectra of the hybrid samples are shown in Figure 4(b). A broad emission band present around 600−750 nm occurred as a result of the P3HT/Zn2+ film’s optical excitation at 550 nm. Upon introducing ZnO nanoparticles to the nanohybrid by thermal oxidation, a significant PL quenching effect on the P3HT emission was observed. This substantial reduction in the photoluminescence could be ascribed to the efficient charge transfer from P3HT chains to the nearby ZnO nanoparticles embossed around the nanowire structure, resulting in a fast quenching process as compared with the photoluminescence of P3HT/Zn2+ sample. Upon further annealing for 24 h, the PL intensity was further quenched to only about 20% of the initial emission intensity of the P3HT/Zn2+ sample, indicating that the D/A parallel-lane nanowires of the P3HT/ZnO-B sample provided efficient superhighways for exciton dissociation and charge transport, and thus acted as better optimized D/A parallel-lane superhighway nanostructure than that of the embossed sample. E

dx.doi.org/10.1021/ma402637e | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

P3HT/ZnO samples were prepared by heating the P3HT/Zn2+ film samples at 170 °C for 30 min under ambient air condition with the sample designation as P3HT/ZnO-A. The thermally oxidized P3HT/ ZnO-A sample was further annealed at 170 °C for 24 h under an ultrapurified nitrogen (99.998%) condition to prepare the P3HT/ ZnO-B samples. Transmission Electron Microscopy (TEM). Bright field TEM was performed on a JEOL 1230EX operating at 120 kV with a Gaten Dual Vision CCD Camera. High-resolution (HR) images were taken using a JEOL 2100F operating at 200 kV. Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS). GIWAXS experiments were performed at the 17A1 endstation of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The WAXS experiments were performed using a 1.3314 Å X-ray beam and the incidence beam angle was set at 0.2° in order to increase the scattering intensity. Two-dimensional XRD data were recorded on a Mar 345-image plate detector, and the corresponding one-dimensional diffraction profiles along the qxy and qz axis were also plotted as the scattering intensity versus the scattering vector. Optical Property Analysis. The UV−vis spectra were acquired using a JSACO V-570 UV/vis/NIR spectrophotometer, and the photoluminescence (PL) measurements were conducted using a JSACO FP-6300 spectrophotometer operated with an excitation source at the wavelength of 550 nm. In order to ensure the same film thickness for the optical measurements, the solution concentration for preparing all hybrid films was kept the same (0.25 wt %). Photovoltaic Device Fabrication and Characterization. Before use, ITO glass substrates were cleaned by a procedure involving sequential ultrasonication in detergent, deionized water, acetone, and methanol, followed by an oxygen plasma treatment for 5 min. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was then spin-coated onto the ITO glass substrates at 3500 rpm and annealed in air at 140 °C for 10 min. The active layer of in situ synthesized P3HT/ZnO nanohybrid films was prepared by using the procedures described above. A cathode was further prepared by depositing a ∼100 nm thick gold layer onto the hybrid layer. The solar cell performance of the in situ synthesized devices was also made to compare with that of an ex situ synthesized device that was prepared with presynthesized ZnO nanoparticles and P3HT solution. Power efficiency measurements under AM 1.5G conditions (100 mW/cm2) were performed using a Newport 91160 W, a xenon-based lamp solar simulator. A Keithley 2400 electrometer was then used to evaluate the J−V characteristics of the solar cells. Fabrication of Ex-Situ Device. Additionally, a photovoltaic device based on a P3HT/ZnO blend was prepared and characterized using the same method as the in situ samples. The ex situ premade ZnO nanoparticles were prepared by using an improved procedure originated from literature.31 Zinc acetate dihydrate (0.27 g) was dissolved in 125 mL methanol then heated at 60 °C for 30 min. A solution of 0.11 g KOH dissolved in 65 mL of methanol was added dropwise to the zinc acetate/methanol solution. The reaction mixture was further stirred for 2 h at 60 °C. White precipitates were filtered and washed by fresh methanol several times then dried by using rotary evaporator. The ex situ ZnO/P3HT devices were prepared similarly as the in situ P3HT/ZnO nanohybrid devices except that presynthesized ZnO nanoparticles were used in the ex situ sample and znic acetate dihydrate was used in the in situ P3HT/ZnO nanohybrid film. The ex situ ZnO/P3HT samples were annealed at 170 °C for 10 min under an inert atmosphere (N2) for the solar cell performance evaluation.

Next, the device performance of the hybrid systems for solar cell application was investigated. A structure of glass/ITO/ PEDOT:PSS/active layer/Al was referenced to produce photovoltaic devices, as shown in Figure 4c. Figure 4d shows the current−voltage characteristics of the P3HT/ZnO hybrid solar devices using an ex situ ZnO blending, P3HT/ZnO-A, and P3HT/ZnO-B films as a photoactive layer, whereas Table S1 shows a summary of the photovoltaic parameters of these devices. Our results show that the device with embossed nanofibril active layer exhibited an open-circuit voltage (Voc) of 0.43 V, a short-circuit current density (Jsc) of 1.38 mA/cm2, a fill factor (FF) of 38.01%, and gave a power conversion efficiency (PCE) of 0.22% higher than that of the device with ex situ blending film (∼0.07%). Furthermore, a significant improvement in the photovoltaic parameters was achieved in the hybrid devices fabricated using parallel-lane nanowire structure as the photoactive layer, revealing an improved Voc of 0.44 V, Jsc of 3.47 mA/cm2 and FF of 39.99%. These parameters could be translated to a power conversion efficiency of up to 0.61%, approximately 180% higher than that of the device based on P3HT/ZnO hybrid with embossed nanofibrils. Considering the significant quenching effect observed from our fluorescence studies and improved charge transport due to substantially lower series resistances from device performance measurements (shown in Table S1), it is not surprising that the parallel-lane hybrid network system exhibited a synergy of enhanced performance. The superhighway-like architecture of the π-conjugated polymer and the inorganic semiconductor nanocrystals served to facilitate not only charge separation, but also charge carrier transport and therefore reduced recombination, leading to the superior Jsc, FF, and PCE of the device. In conclusion, we have presented a facile and effective strategy to directly coassemble ZnO nanocrystals into highly ordered and elongated P3HT nanofibril hybrids. Thermal treatments can be used to control the organization of ZnO into two types of hierarchical structures, including embossed and parallel-lane nanowires, and to synergistically enhance the crystallinity of P3HT nanofibrils while the brief thermal oxidation process does not appear to have an adverse effect on the physical and chemical properties of P3HT (shown in Figure 3 and Figure S1). In particular, the parallel-lane superhighway nanowires with bicontinuous electron donors, P3HT/acceptors, ZnO phases acted as efficient superhighways for exciton dissociation and charge transport, thereby enhancing the optoelectronic properties and performance of the device. We believe that this novel approach provides a feasible way to enhance photovoltaic performance and shows potential for use in future optoeletronic applications.

■

EXPERIMENTAL SECTION

Synthesis of P3HT/ZnO Nanohybrids. In the current study, the P3HT/ZnO nanohybrids were prepared by using the follow procedures. First, 20.0 mg P3HT was added in 5 mL toluene to prepare a 4 mg mL−1 solution, and the solution (in flask A) was stirred at 50 °C for 8 h. Meanwhile, 26.4 mg of Zinc acetate dehydrate (Zn(CH3COO)2·2H2O) were dissolved in 2 mL THF to obtain a precursor solution (in flask B) followed by stirring it at 50 °C for 8 h. Subsequently, compounds in flask A and B were then mixed and stirred at 50 °C to prepare a P3HT/Zn2+ solution with a molar ratio of 3-hexylthiophene monomer unit to Zn2+ (3HT/Zn2+) of 1:1. After mixing for 48 h, anisole was added to the solution (toluene/THF/ anisole (0.5/0.2/0.3, v/v/v)), followed by stirring for 4 days. Subsequently, the prepared precursor solution was drop cast on a substrate, followed by drying the coated substrate in air for 3 h.

■

ASSOCIATED CONTENT

S Supporting Information *

Detailed synthesis procedures and characterization data of polymer samples. This material is available free of charge via the Internet at http://pubs.acs.org. F

dx.doi.org/10.1021/ma402637e | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

■

Article

(24) Watt, A.; Thomsen, E.; Meredith, P.; Rubinsztein-Dunlop, H. Chem. Commun. 2004, 20, 2334−2335. (25) Liao, H.-C.; Chen, S.-Y.; Liu, D.-M. Macromolecules 2009, 42, 6558−6563. (26) Dayal, S.; Kopidakis, N.; Olson, D. C.; Ginley, D. S.; Rumbles, G. J. Am. Chem. Soc. 2009, 131, 17726−17727. (27) Oosterhout, S. D.; Wienk, M. M.; van Bavel, S. S.; Thiedmann, R.; Jan Anton Koster, L.; Gilot, J.; Loos, J.; Schmidt, V.; Janssen, R. A. J. Nat. Mater. 2009, 8, 818−824. (28) Leventis, H. C.; King, S. P.; Sudlow, A.; Hill, M. S.; Molloy, K. C.; Haque, S. A. Nano Lett. 2010, 10, 1253−1258. (29) Liao, H.-C.; Lin, C.-C.; Chen, Y.-W.; Liu, T.-C.; Chen, S.-Y. J. Mater. Chem. 2010, 20, 5429−5435. (30) Dowland, S.; Lutz, T.; Ward, A.; King, S. P.; Sudlow, A.; Hill, M. S.; Molloy, K. C.; Haque, S. A. Adv. Mater. 2011, 23, 2739−2744. (31) Pachoski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188−1191.

AUTHOR INFORMATION

Corresponding Authors

*(C.-A.D.)Telephone: 886-2-3366-3051. Fax: 886-2-23623040. E- mail: [email protected]. *(L.W.) Telephone: 886-2-3366-5276. Fax: 886-2-2369-6221. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The financial support from the Ministry of Science and Technology of Taiwan is greatly appreciated. The X-ray measurements were conducted at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The authors would also like to thank Dr. Jey-Jau Lee for the help during the X-ray experiments.

■

REFERENCES

(1) Tessler, N.; Medvedev, V.; Kazes, M.; Kan, S.; Banin, U. Science 2002, 295, 1506−1508. (2) Campbell, I. H.; Crone, B. K. Adv. Mater. 2006, 18, 77−79. (3) Tokmoldin, N.; Griffiths, N.; Bradley, D. D. C.; Haque, S. A. Adv. Mater. 2009, 21, 3475−3478. (4) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425−2427. (5) Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J. Adv. Funct. Mater. 2006, 16, 1112−1116. (6) Wu, M.-C.; Chang, C.-H.; Lo, H.-H.; Lin, Y.-S.; Lin, Y.-Y.; Yen, W.-C.; Su, W.-F.; Chen, Y.-F.; Chen, C.-W. J. Mater. Chem. 2008, 18, 4097−4102. (7) Zhang, S.; Pelligra, C. I.; Keskar, G.; Jiang, J.; Majewski, P. W.; Taylor, A. D.; Ismail-Beigi, S.; Pfefferle, L. D.; Osuji, C. O. Adv. Mater. 2012, 24, 82−87. (8) Pelligra, C. I.; Majewski, P. W.; Osuji, C. O. Nanoscale 2013, 5, 10511−10517. (9) Moghaddam, R. S.; Huettner, S.; Vaynzof, Y.; Ducati, C.; Divitini, G.; Lohwasser, R. H.; Musselman, K. P.; Sepe, A.; Scherer, M. R.; Thelakkat, M.; Steiner, U.; Friend, R. H. Nano Lett. 2013, 13, 4499− 4504. (10) Darling, S. B.; You, F. RSC Adv. 2013, 3, 17633−17648. (11) Chen, W.; Nikiforov, M. P.; Darling, S. B. Energy Envion. Sci. 2012, 5, 8045−8074. (12) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324−1338. (13) Tomczak, N.; Jańczewski, D.; Han, M.; Vancso, G. J. Prog. Polym. Sci. 2009, 34, 393−430. (14) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545−610. (15) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 644−670. (16) Sun, X. W. ChemSusChem 2010, 3, 679−680. (17) Zhou, Y.; Riehle, F. S.; Yuan, Y.; Schleiermacher, H.-F.; Niggemann, M.; Urban, G. A.; Krüger, M. Appl. Phys. Lett. 2010, 96, 013304. (18) Xu, J.; Hu, J.; Liu, X.; Qiu, X.; Wei, Z. Macromol. Rapid Commun. 2009, 30, 1419−1423. (19) Bokel, F. A.; Sudeep, P. K.; Pentzer, E.; Emrick, T.; Hayward, R. C. Macromolecules 2011, 44, 1768−1770. (20) Briseno, A. L.; Holcombe, T. W.; Boukai, A. I.; Garnett, E. C.; Shelton, S. W.; Fréchet, J. J. M.; Yang, P. Nano Lett. 2010, 10, 334− 340. (21) Ren, S.; Chang, L.-Y.; Lim, S.-K.; Zhao, J.; Smith, M.; Zhao, N.; Bulović, V.; Bawendi, M.; Gradečak, S. Nano Lett. 2011, 11, 3998− 4002. (22) Strong, V.; Uribe-Romo, F. J.; Battson, M.; Kaner, R. Small 2012, 8, 1191−1196. (23) Pentzer, E. B.; Bokel, F. A.; Hayward, R. C.; Emrick, T. Adv. Mater. 2012, 24, 2254−2258. G

dx.doi.org/10.1021/ma402637e | Macromolecules XXXX, XXX, XXX−XXX