Graphene As Transparent Conducting Electrodes in Organic

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Letter pubs.acs.org/NanoLett

Graphene As Transparent Conducting Electrodes in Organic Photovoltaics: Studies in Graphene Morphology, Hole Transporting Layers, and Counter Electrodes Hyesung Park,† Patrick R. Brown,‡ Vladimir Bulović,† and Jing Kong*,† †

Department of Electrical Engineering and Computer Science and ‡Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: In this work, organic photovoltaics (OPV) with graphene electrodes are constructed where the effect of graphene morphology, hole transporting layers (HTL), and counter electrodes are presented. Instead of the conventional poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) PEDOT:PSS HTL, an alternative transition metal oxide HTL (molybdenum oxide (MoO3)) is investigated to address the issue of surface immiscibility between graphene and PEDOT:PSS. Graphene films considered here are synthesized via low-pressure chemical vapor deposition (LPCVD) using a copper catalyst and experimental issues concerning the transfer of synthesized graphene onto the substrates of OPV are discussed. The morphology of the graphene electrode and HTL wettability on the graphene surface are shown to play important roles in the successful integration of graphene films into the OPV devices. The effect of various cathodes on the device performance is also studied. These factors (i.e., suitable HTL, graphene surface morphology and residues, and the choice of wellmatching counter electrodes) will provide better understanding in utilizing graphene films as transparent conducting electrodes in future solar cell applications. KEYWORDS: Graphene, CVD, organic solar cell, metal oxide, oxygen plasma

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suitable position as an alternative transparent conducting electrode material, with a potential in various applications such as solar cells, liquid crystal displays, and touch screens. One of the challenges in the integration of graphene in OPV is the incompatibility between graphene and PEDOT:PSS hole transport layer (HTL), which significantly increases device failure rate.20 When hydrophilic PEDOT:PSS is spin-coated onto graphene, it is difficult to achieve uniform and conformal coating due to the hydrophobic nature of the graphene surface. Previously, we reported20 that the compatibility of PEDOT:PSS with graphene films can be significantly improved by doping graphene films with AuCl3. Doping the graphene film also improves the overall power conversion efficiency (PCE) by increasing the conductivity of graphene. Nevertheless, the doping process introduces large Au particles (up to 100 nm in diameter) onto the graphene film, which can create shorting pathways through the device. A planarizing buffer layer with good wettability is therefore necessary for the successful integration of graphene into OPV. Shrotriya et al.30 and Godoy et al.31 reported that in both polymer and small-molecule based solar cells utilizing ITO anodes, a transition metal oxide buffer

raphene, a hexagonal arrangement of carbon atoms forming a one-atom thick planar sheet, has gained much attention since 20041 due to its superior physical properties.2−6 Apart from the micromechanical cleavage of highly ordered pyrolytic graphite (HOPG), several alternate methods have been explored to achieve reliable and repeatable synthesis of large-area graphene sheets.2,6−10 Among these, chemical vapor deposition (CVD) process has been demonstrated as an efficient way of producing continuous large area graphene. Recently the synthesis of graphene sheets up to 30 in. has been reported.11 Because of the remarkable properties of graphene, applications in various areas, such as transistors,12−14 chemical sensors,15−17 solar cells,18−21 and logic devices22 have been explored and a variety of proof-of-concept devices have been demonstrated. Similar to graphene research, solar cells based on organic materials have also drawn significant attention as an alternate source of clean energy over conventional inorganic photovoltaics due to lightweight and flexibility of organic semiconductors, and potentially low-cost, high-throughput fabrication methods.23−26 Previously, carbon nanotubes have already been demonstrated as possible transparent electrodes in organic solar cells27−29 and recently, several works have proposed to use graphene as a transparent electrode for organic photovoltaics (OPV) to replace the indium tin oxide (ITO) films.18−21 Superior flexibility, as well as abundance of source material (carbon) at lower costs compared to ITO put graphene into a © 2011 American Chemical Society

Received: September 11, 2011 Revised: November 5, 2011 Published: November 22, 2011 133

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Figure 1. Characteristics of graphene OPV device with significant amounts of PMMA residues on the graphene surface. (A) AFM image of PMMA residues on the graphene electrode. PMMA was treated by acetone for 2 h. (B) J−V response of devices made from graphene electrodes covered with large amount PMMA residues, showing poor diode characteristics.

Figure 2. Graphene surface imaged by AFM after removal of PMMA via different routes. (A) Method (1), immersing in acetone for 24 h; (B) Method (2), first treated by acetone vapor followed by 24 h immersing in acetone; (C) Method (3), acetone vapor, 2 min acetone immersion, and 3 h of annealing; (D) Method (4), 3 h annealing. Method (3) provides the cleanest graphene surface. 134

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Figure 3. (A) Ultraviolet−visible transmittance spectra of MoO3 layer on quartz substrate with varying thicknesses (20, 30, 40 nm). UV transmittance measured at 550 nm is 93.7, 89.6, and 86.2% with increasing thicknesses. AFM images of (B) MoO 3 (10 nm) and (C) MoO3 (10 nm)/graphene (3 L) on quartz substrates. (D,E) Scanning electron micrographs (SEM) of MoO3 (20 nm) on graphene/quartz. Bright (D and inset with in-lens detector) and dark (E without in-lens detector) spots indicate graphene openings not covered by the MoO3 film. Scale bars are 20 μm for (D,E) and 200 μm for inset of (D).

devices. The films were transferred using the poly(methyl methacrylate) (PMMA) method described elsewhere.9 Before the removal of the copper foil, the graphene on one side of the foil was removed via reactive ion etching (RIE) with oxygen gas (100 W at 7 × 10−5 Torr) since the graphene growth occurs on both sides of the copper foil. If not removed, graphene pieces from the opposite side adsorb underneath the floating graphene films during the copper etching, causing problems for device fabrications afterward. The copper foil was etched using a commercially available etchant (CE-100, Transene) and the graphene films were rinsed with diluted hydrochloric acid (10%) and deionized (DI) water to remove residual iron ions left over from the copper etchant, thus preventing unintentional doping of graphene. Finally, the PMMA layer was removed via several routes that will be discussed in the following and their effects on device performance were investigated. Three layers of graphene sheets were prepared as electrodes through layer-bylayer transfers to obtain a robust film with reasonable conductivity (∼250 Ω/sq) and transmittance (∼92% at 550 nm). Detailed fabrication procedures are described in our earlier work.20 Transferred graphene films require additional patterning processes. Conventional photolithography can be used to pattern the film, but the photoresist residues are hard to remove33 and can block the charge carriers to reach the graphene electrode since the resist is not conducting. In this work, chromium (Cr) was used as a patterning mask. This method simplifies the fabrication procedure and provides a cleaner graphene surface. Cr patterns were directly deposited through a shadow mask on

layer (molybdenum trioxide, MoO3) can be used in place of PEDOT:PSS as an efficient HTL. In this work, we used MoO3 as the HTL in OPV devices using graphene electrodes and carried out further investigations in terms of the effect of graphene surface morphology on the OPV performance and the choice of the counter electrode (cathode). We confirmed that MoO3 can be successfully integrated between the graphene sheet and the subsequent organic layer as an alternative HTL.32 The use of a direct thermal evaporation of MoO3 gives a HTL on the graphene surface with better wetting compared to hydrophilic PEDOT:PSS. We then further characterized the influence of the graphene surface morphology and counter electrodes on the performance of graphene-based solar cells. Graphene Electrode OPV. Large area (5 in. × 1 in.) graphene sheets were synthesized via low-pressure chemical vapor deposition (LPCVD) using copper foil (25 μm in thickness, ALFA AESAR) as a metal catalyst. The copper foil (5 in. × 1 in.) was placed in the CVD furnace and the chamber was evacuated to a base pressure of 30−50 mTorr. The system was then heated to a growth temperature of 1000 °C under hydrogen gas (∼375 mTorr) which removes oxide layers and other contaminants on copper foil and then the copper was annealed for 30 min to initiate grain growth. Subsequently, methane gas was introduced (total pressure: ∼850 mTorr) and graphene growth was carried out for 30 min. After the completion of the synthesis, the chamber was cooled down at 45 °C/min under the hydrogen gas. As-grown graphene films require transfer to target substrates and patterning in order to be utilized as electrodes in OPV 135

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Figure 4. AFM images of graphene morphology of different number of layers: (A) three layers and (B) one layer. Cross sectional profiles of dotted regions are shown below. The rms surface roughness is 0.6 and 2.0 nm for the one layer and the three layers, respectively.

Figure 5. Current density versus voltage characteristics of graphene and ITO OPV devices with PEDOT:PSS and MoO3 hole transporting layers under simulated AM 1.5G illumination at 100 mW/cm2. (A) Graphene electrodes patterned with 15, 25, and 40 nm thick Cr metal masks with PEDOT:PSS HTL. Using thinner Cr mask helps reducing shunting pathways as confirmed by the increased shunt resistance and better diode behavior. (B) Devices using graphene electrode with varying MoO3 HTL thicknesses (20−40 nm) under light. Graphene was patterned with thinner 15 nm Cr to ensure smoother surface.

BCP(10 nm)/Ag(100 nm)) was utilized in this work and was compared to devices made with MoO3 replacing the PEDOT:PSS HTL. The device schematic and corresponding energy levels are shown in Figure S1 in Supporting Information. Organic layers and Ag were evaporated at 1.0 and 1.5 Å/s, respectively. In general, PMMA on graphene can be removed by common solvents such as acetone or chloroform. However, solvent cleaning usually tears the graphene surface and introduces discontinuities in the film, thus reducing the conductivity of the graphene electrodes significantly. This method also suffers from

to the transferred graphene sheets by electron beam or thermal evaporation, and the graphene was patterned through RIE afterward. Subsequently, Cr was removed using a commercially available etchant (CR-7, Cyantek). The sheet resistance of the graphene electrode was slightly increased to 350−500 Ω/sq after patterning due to the processing but the transmittance remained the same. After patterning the graphene electrodes, subsequent HTL and organic materials were deposited followed by the top contacted cathode. A standard small-molecule OPV structure (Anode/PEDOT:PSS(40 nm)/CuPc(40 nm)/C60(40 nm)/ 136

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improved from the PEDOT HTL-based devices, (3 ± 11.0%). In ref 34, a combination of MoO3 and PEDOT bilayers were used as the HTL for OPV devices with graphene-based electrodes. During the MoO3 evaporation, achieving conformal coverage on the wrinkles in the graphene sheet is critical since these “peaks” can serve as potential shunt pathways. For the stacked multilayers of graphene, both the density of these wrinkles and the overall peak heights increase. Figure 4 shows a much denser concentration of wrinkles on three layered graphene films compared to a single layer graphene sheet. The surface of stacked layers is usually much rougher than the single layer sheet with a root-mean-square (rms) roughness more than 3-fold higher: 2.0 nm for 3 layers and 0.6 nm for 1 layer. Corresponding cross-sectional profiles of dotted regions are also shown below the respective AFM images. The surface homogeneity can be improved by controlling the thickness of the Cr mask during the graphene patterning process. Various thicknesses of Cr masks were considered in this work (10−40 nm). In our experiments, we have found when Cr thickness is below 15 nm, it will not form a continuous film on graphene and thus cannot be used as a mask (as shown in the Supporting Information Figure S5, cracks in the graphene sheets are observed after removing the Cr). Since E-beam deposition of Cr is directional, it can be anticipated that the wrinkles higher than the Cr thickness should be exposed. Thicker Cr will preserve the extruded wrinkles of graphene better than the thinner Cr, and these wrinkles could be the possible shunting pathways. During the RIE of graphene patterns, the sharp peaks not fully covered by the thin Cr are expected to be smoothen out, thus reducing the probability of shunting pathways. As shown in Figure 5A, devices made from thinner (15 nm) Cr mask typically shows better performance with higher shunt resistance: 0.85 ± 0.02% (PCE) for 15 nm Cr, and 0.50 ± 0.03% (PCE) and 0.36 ± 0.02% (PCE) for 25 and 40 nm Cr. This observation confirms the importance of surface morphology of graphene sheets in OPV application. Figure 5B presents J−V characteristics of graphene devices (patterned with 15 nm Cr) fabricated with varying thickness of MoO3 layers (20−40 nm) and Table 1 summarizes the key

PMMA residues left on the graphene surface. For instance, Figure 1 demonstrates the effect of PMMA residues on the graphene electrode solar cell. In this device, graphene on quartz substrate with PMMA on top was immersed in acetone for 2 h and Figure 1A shows significant amount of PMMA residues on the graphene surface. The current density versus voltage (J−V) characteristics (Figure 1B) of a device made from graphene electrode with large amounts of PMMA residues illustrates considerably poor diode behavior. In fact, all the devices made from these graphene electrodes showed either similar behavior as Figure 1B or no photoresponse at all. There are several factors that could contribute to this. Since PMMA is insulating, the residues will prevent the charge carriers to reach the graphene electrode; the large chunks of PMMA (>100 nm) cause a rough surface which could be detrimental for the OPV device (since each layer in the device is only tens of nanometers in thickness). Last but most importantly, PMMA residues appear to worsen the wetting of PEDOT on graphene surface even further, causing device failures. Therefore, we considered several approaches to remove PMMA on graphene surface: (1) Immersing in acetone for 24 h. (2) Remove most of the PMMA with acetone vapor to minimize tearing of graphene by direct immersion in acetone solution, then soaking in acetone for 24 h. (3) Acetone vapor, brief acetone dipping for 2 min, followed by 3 h of annealing. (4) Annealing for 3 h at 500 °C under the protecting gas mixtures of hydrogen (700 sccm, standard cubic centimeters per minute) and argon (400 sccm). AFM images show that the residues are almost removed for soaking in acetone for 24 h (Figure 2A,B). Additional annealing after acetone treatment greatly improved in removing PMMA residues (Figure 2C). Annealing alone was also enough to remove most of the PMMA from the graphene surface while minimizing tearing (Figure 2D). Raman spectroscopy was also performed to investigate the defect density and the quality of graphene. The collected signal showed pronounced D peaks (∼1350 cm−1) for acetone assisted cleaning samples (method (1), (2)) as illustrated in Figure S2 in Supporting Information. Since method (1) and (2) takes 24 h, not very suitable for fabricating a large number of devices and minimal differences in the device performances from methods (3) and (4) were observed, in our experiments we have mostly used method (4). Representative device characteristics from methods (3) and (4) are described in the Supporting Information Figure S3. Effects of Graphene Morphology and MoO3 HTL. OPV devices employing a graphene anode and transition metal oxide HTL were constructed with a device structure of graphene/MoO3/CuPc(40 nm)/C60(40 nm)/BCP(10 nm)/ Ag(100 nm). The MoO3 layer with its wide band gap of 3 eV is relatively transparent with transmittance ranging from 85−95% for 20−40 nm films at a wavelength of 550 nm (Figure 3A). AFM images of MoO3 (10 nm) surfaces on bare quartz and graphene/quartz substrates are displayed in Figure 3B,C, illustrating the smooth surface profile of the MoO3 layer (Figure 3B, rms roughness of ∼0.4 nm) and conformal coverage of thin layer of MoO3 on the graphene surface (Figure 3C). Further SEM characterization of the MoO3 layer deposited on graphene (Figure 3D,E) shows in fact there is still not 100% wettability of MoO3 on graphene (as indicated by the holes in Figure 3D,E). Nevertheless, this wetting behavior is much better than the PEDOT case (further illustrated in the Supporting Information Figure S4) and the device yield with MoO3 HTL (43 ± 14.1%) was significantly

Table 1. Summary of Photovoltaic Parameters with PEDOT:PSS and MoO3 HTLs for Devices in Figure 5B anode

HTL

JSC (mA/cm2)

VOC (V)

FF

PCE (%)

graphene graphene graphene graphene

PEDOT:PSS MoO3 (20 nm) MoO3 (30 nm) MoO3 (40 nm)

4.43 3.93 3.41 2.47

0.53 0.49 0.54 0.50

0.36 0.37 0.40 0.25

0.85 0.71 0.75 0.31

photovoltaic parameters (short circuit current density (JSC), open circuit voltage (VOC), fill factor (FF), and power conversion efficiency (PCE)). As the MoO3 thickness increases, the device becomes more resistive and the performance decreases given the insulating nature of the MoO3 layer. The PCEs are 0.71 ± 0.01% and 0.75 ± 0.01% for 20 and 30 nm MoO3 and then decreases to 0.31 ± 0.01% for 40 nm of MoO3. The efficiency of the PEDOT:PSS reference cell was 0.85 ± 0.03%. MoO 3-based devices perform ∼86 ± 2% of the PEDOT:PSS reference cell. Similar relation was observed with devices using ITO electrodes, where these devices with MoO3 HTLs performed ∼88 ± 6% of those with PEDOT:PSS 137

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Figure 7. (A) J−V characteristics of graphene/PEDOT:PSS/CuPc(40 nm)/C60(40 nm)/BCP(10 nm)/metals(100 nm) devices under light with various metal electrodes. (B) JSC, VOC, and PCE of devices with different cathodes. Al was coated over Mg and Ca electrodes to prevent oxidization.

performance is not optimized (e.g., the thicknesses of the CuPc/C60 active layers), at this stage no further work is carried out for the optimization of the MoO3 HTL thickness. Nonetheless, the device yield with MoO3 (∼43%) HTLs was much improved from the PEDOT:PSS (∼3%) cases due to the improved wetting. Previously, MoO3 layer was used to improve the charge transfer from CuPc to the anode by reducing the effective energy barrier between the anode and the highest occupied molecular orbital (HOMO) of CuPc, and a work function of 5.2−5.3 eV has been commonly reported.35−37 However, Kroger et al.38 reported deep-lying electronic states of MoO3 where the HOMO and LUMO (lowest unoccupied molecular orbital) levels are shifted down ∼4.3 eV with a work function of ∼6.7 eV. In contrast to previously published interpretations of MoO3 induced enhancement of hole injection, they argue that hole injection occurs via electron extraction from the HOMO of the donor through the MoO3 conduction band. Irfan et al.39 also observed a similarly high work function of ∼6.75 eV for MoO3 and attributed the low previously reported values of ∼5.3 eV to air/oxygen exposure of the MoO3 surface during the measurement. We thus investigated the effect of oxygen exposure on MoO3 and the resulting device performance. After evaporation of MoO3 onto graphene surface, the graphene/MoO3 electrode was removed from the evaporation chamber and briefly exposed to O2 plasma (8 s). Interestingly, the plasma treatment on MoO3 did not result in any significant difference in the

Figure 6. Current density versus voltage characteristics of graphene and ITO OPV devices with MoO3 hole transporting layers with and without O2 plasma under simulated AM 1.5G illumination at 100 mW/cm2. (A) Graphene anode patterned by thinner (15 nm) Cr mask with MoO3 (20 nm) HTL. (B) ITO anode with MoO3 (20 nm) along with PEDOT:PSS reference under light. For both (A) and (B) the effect of O2 plasma on the anode/MoO3 surface appear to be minimal. (C) Graphene anode patterned by thicker (25 nm) Cr mask with 20 nm of MoO3 HTL. It is observed that O2 plasma on these rougher surfaces help to improve the device performance by planarizing the surface.

HTLs. These numbers are not optimized but are rather intended to suggest a guideline for the graphene−metal oxide OPV structure. A certain thickness of HTL is required to guarantee complete coverage on the graphene surface, however too thick of a MoO3 layer degrades the device performance as a result of the increased bulk series resistance. Since the overall device 138

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Table 2. Summary of Photovoltaic Parameters of Various Metal Cathodes Devices anode

HTL

cathode

work function (eV)

JSC (mA/cm2)

VOC (V)

FF

PCE (%)

graphene graphene graphene graphene graphene graphene

PEDOT PEDOT PEDOT PEDOT PEDOT MoO3 (20 nm)

Ca/Al Mg/Al Al Ag Au Mg/Al

2.9 3.7 4.2 4.3 5.3 3.7

0.99 5.01 4.42 3.85 2.64 6.42

0.47 0.48 0.42 0.39 0.18 0.41

0.25 0.23 0.26 0.25 0.26 0.28

0.12 0.56 0.47 0.37 0.12 0.72

and Mg top electrodes showed moderate behavior with similar performances: PCEs of 0.37 ± 0.01, 0.47 ± 0.01, and 0.56 ± 0.02%, respectively. On the other hand, Au- and Ca-based devices showed considerably reduced efficiency where PCE’s of both devices are only 0.12 ± 0.03%. The high work function of Au likely forms a Schottky type contact and limit charge extraction in which case holes are injected back to the active layer that causes recombination of charge carriers thus lowering the device performance.41,42,45 Ca with its low work function should favor an ohmic contact to the adjacent organic layer. However, Ca cathode device showed significantly reduced JSC and FF with PCE of only 0.12 ± 0.03%. Eo et al. argued that Ca with a work function lower than the LUMO of C60 forms a Schottky type contact as well as nonspontaneous electron extraction process that lowers the device performance.40,41 Nonetheless, the exact reason is still not clear at this point. These observations highlight the importance of appropriate choices of counter electrodes for successful integration of graphene electrodes in organic solar cells. In summary, OPV devices using small molecules as active materials and a transition metal oxide (MoO3) as HTL were fabricated with graphene anodes. By utilizing the thermally evaporated MoO3 HTL, the wetting of HTL on graphene surface can be improved compared to the conventional PEDOT:PSS HTL. The effects of the surface morphology of graphene, MoO3 thickness, and air/oxygen exposure on MoO3/ graphene surface on the OPV performance were investigated. Finally, we demonstrated how different metal cathodes with varying work functions affect the performance of solar cells constructed with graphene anodes. The intent of this work is to provide a better understanding of graphene-based solar cell fabrication, facilitating the progress toward the realization of graphene integration even beyond PVs into areas such as OLEDs and flexible displays.

device performance for both ITO electrode and graphene electrode patterned with thinner (15 nm) Cr mask, as shown in Figure 6A,B. We suspect this could be due to the possible contamination of our evaporation chamber with moisture, pinning the work function of MoO3 with a lower value although the exact reason is still under investigation. On the other hand, for graphene electrodes patterned with thicker (25 nm) Cr that may have more potential shorting pathways, the plasma treatment seemed to smoothen out rough regions of the graphene/MoO3 surface. As a result, the overall PV performance was improved as shown in Figure 6C. For 20 nm of MoO3, the PCE increased from 0.42 ± 0.01 to 1.03 ± 0.01% on average (∼145% improvement). The key parameters of previous devices are summarized in Tables S1 and S2 in Supporting Information. Supporting Information Figure S6B illustrates external quantum efficiency (EQE) of the graphene device presented in Figure 6C with 20 nm of MoO3. One can clearly see an improvement in EQE with O2 plasma treatment. There was less improvement for thicker MoO3 since a thicker MoO3 layer results in fewer possible shunt paths and thus less effect from the O2 plasma. For 30 nm of MoO3, the PCE increased from 0.73 ± 0.05% to 0.96 ± 0.03% (∼32% improvement) and for 40 nm of MoO3, the PCE increased from 0.73 ± 0.01 to 0.97 ± 0.01% (∼33% improvement) on average. The J−V characteristics are illustrated in the Supporting Information Figure S7. Effect of the Cathode Work Function. Built-in potential in OPV, one of the key parameters represented by the VOC, is mainly determined by the energy level difference between the HOMO of donor and the LUMO of acceptor. On the other hand, the choice of metal contact is very important for facilitating the extraction of charge carriers. Metals forming ohmic contact were shown to enhance the PCE over the Schottky type contact, because injection barriers at the Schottky contacts can lead to accumulation of charge carriers causing diffusion current that must be balanced by drift current at open circuit.40 Previously, various studies were conducted on how metal cathodes affect the overall OPV device performance with ITO anodes.40−43 Here, we performed similar studies on how metal cathodes with varying work functions affect graphene anode based solar cells. The metals considered in this work are those with moderate work functions compared with graphene, that is, magnesium (Mg, ∼3.7 eV), aluminum (Al, ∼4.2 eV), and silver (Ag, ∼4.4 eV), and those with much lower and higher values of work functions than graphene, that is, calcium (Ca, ∼2.9 eV) and gold (Au, ∼5.3 eV). Work function values are referred from the literature.44 The work function of graphene generally ranges from 4.0−4.5 eV depending on the synthesis conditions. Solar cells with these cathodes were fabricated and tested under the same condition. Figure 7 illustrates J−V responses from various metal cathodes and Table 2 summarizes the key PV parameters. Similar to the previous works with ITO anodes, devices made from Ag, Al,

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ACKNOWLEDGMENTS

* Supporting Information Additional information, figures, and tables. This material is available free of charge via the Internet at http://pubs.acs.org. S

Corresponding Author *E-mail: [email protected].

The authors gratefully acknowledge financial support for this work from Eni S.p.A. under the Eni-MIT Alliance Solar Frontiers Center. P.R.B. also gratefully acknowledges support from the Fannie and John Hertz Foundation and the National Science Foundation. 139

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dx.doi.org/10.1021/nl2029859 | Nano Lett. 2012, 12, 133−140