Electrical and Optical Properties of ZnO Processed by Atomic Layer

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Electrical and Optical Properties of ZnO Processed by Atomic Layer Deposition in Inverted Polymer Solar Cells† Hyeunseok Cheun, Canek Fuentes-Hernandez, Yinhua Zhou, William J. Potscavage, Jr., Sung-Jin Kim, Jaewon Shim, Amir Dindar, and Bernard Kippelen* Center for Organic Photonics and Electronics (COPE), School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ReceiVed: July 16, 2010; ReVised Manuscript ReceiVed: September 17, 2010

We report on the photovoltaic properties of inverted polymer solar cells where the transparent electroncollecting electrode is formed by a ZnO-modified indium-tin oxide (ITO) electrode. The ZnO layers were deposited by atomic layer deposition (ALD) with varying thicknesses from 0.1 to 100 nm. The work function, surface roughness, and morphology of ITO/ZnO were found to be independent of the ZnO thickness. However, the device performance was found to be strongly dependent on a critical ZnO thickness, around 10 nm. Below the critical thickness the device performance was degraded because of the appearance of a “kink” in the current-voltage characteristics. The kink features became less pronounced after ultraviolet (UV) exposure. This was attributed to oxygen desorption, leading to an increased conductivity of the ZnO layer. At and above this critical thickness, the device performance significantly improved and no longer depended strongly on the thickness of the ZnO layer, in agreement with optical simulations. Instead, these optical simulations showed that the thickness of the active layer plays a more important role than the thickness of the ZnO layer in optimizing the photovoltaic properties of inverted solar cells. Inverted polymer solar cells with an increased thickness of the active layer showed a power conversion efficiency (PCE) of 3.06% estimated for AM1.5G, a 100 mW cm-2 illumination. I. Introduction Polymer solar cells continue to receive considerable attention as a promising alternative to conventional silicon-based solar cells because of their unique ability to tune the electrical and optical properties of organic materials and the potential for lowcost and simplified processing.1–4 Polymer-based bulk heterojunction cells have reached power conversion efficiencies (PCEs) of up to 7.7% through the use of novel polymers and the control of the active layer morphology.5–7 Despite recent advances, further improvements in device efficiency and stability will be required for this technology to reach its full potential. In a “conventional” organic solar cell, a low work function metal is used as a top, electron-collecting electrode. The facile oxidation of the low work function metal is a major factor that contributes to limiting the environmental stability of solar cells and forces the use of encapsulation layers to improve their shelf stability.8 Recently, inverted solar cells with a transparent electroncollecting electrode have demonstrated improved air stability with comparable efficiencies to “conventional” solar cells.9–12 In solar cells with an inverted structure, a high work function metal is used as a hole-collecting electrode, while an indium-tin oxide (ITO) electrode, modified with a thin layer of TiOx or ZnO, acts as an electron-collecting electrode. Zinc oxide is especially attractive to produce inverted solar cells because its high optical transmittance in the visible range is combined with a high electrical conductivity and a low cost.13 Recently, there have been a number of studies characterizing inverted solar cells that use ITO/ZnO transparent electrodes. In these studies, many methods have been employed for producing the ZnO layers, †

Part of the “Mark A. Ratner Festschrift”. * Author to whom correspondence should be addressed. E-mail: kippelen@ ece.gatech.edu.

Figure 1. (a) Device structure of an inverted P3HT:PCBM solar cell with ZnO layer. (b) Energy level diagram of the components of the device.

including solution processed sol-gel,10 nanoparticle approaches,14 and sputtering.15 However, there have been few reports of the influence of the thickness of the metal oxide layer on the optical and electrical properties of the inverted solar cell devices.16–18 This paper reports on the performance of inverted solar cells with the general structure glass/ITO/ZnO/P3HT:PCBM/PEDOT: PSS/Ag (see Figure 1a). Here, we employed atomic layer deposition (ALD) to fabricate thin ZnO layers on top of ITO. The use of the ALD deposition method is particularly suitable for a thickness study because it uses a self-limiting process that allows layer-by-layer growth and, therefore, the growth of layers with very controlled thickness on the nanometer scale. In addition, ALD is expected to produce defect-free, uniform, and highly conformal films.19 We studied the electrical and optical properties of ZnO layers with different thicknesses in inverted polymer solar cells. The photovoltaic performance was found to be dependent on the thickness of the ZnO layer, with two

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regimes clearly defined by the electrical properties of layers with thicknesses below and above 10 nm. The change in the optical properties of the solar cells that results from changing the thickness of the ZnO layers was simulated. Optical simulations show that these changes are expected to have a minimal impact over the photovoltaic properties of these solar cells and also show that variations of the active layer thickness could have a bigger impact over the solar cell efficiency. Inverted solar cells with an optimized active layer and ZnO thicknesses reach a PCE of 3.06% estimated for AM1.5 G, a 100 mW cm-2 illumination. This efficiency was found to be comparable to that of reference devices with a conventional structure ITO/PEDOT: PSS/P3HT:PCBM/Al. II. Experimental Section To study the dependence of the photovoltaic performance on the thickness of the ZnO layer, inverted polymer solar cells were fabricated using blends of the donor polymer poly(3-hexylthiophene) (P3HT-4002E, Rieke Metals, Lincoln, NE) with the acceptor [6,6]-phenyl C61 butyric acid methyl ester (PCBM, Nano-C, Westwood, MA) as the active layer. ITO-coated glass (Colorado Concept Coatings LLC, Loveland, CO) with a sheet resistivity of 15 Ω sq-1 was used as substrate. The substrates were cleaned in an ultrasonic bath of detergent water, rinsed with deionized water, and then cleaned in sequential ultrasonic baths of deionized water, acetone, and isopropanol. Nitrogen was used to dry the substrates after each of the last three baths. ZnO layers of various thicknesses (0.1, 0.5, 1, 5, 10, 20, 40, and 100 nm) were deposited by ALD (Savannah 100, Cambridge NanoTech, Cambridge, MA) with pulses of H2O for 15 ms and diethylzinc for 15 ms at 200 °C on the cleaned ITO substrates. The work function of ITO modified by ZnO was measured in air by using a Kelvin probe (Besocke Delta Phi, Jülich, Germany). Using e-beam deposition (AXXIS, Kurt J. Lesker, Clairton, PA), a 300 nm thick layer of SiOx was deposited through a shadow mask on ITO/ZnO substrates to pattern the bottom electrode. Next, the substrates were ultrasonicated in isopropanol for 10 min and dried with nitrogen. After loading into a nitrogen-filled glovebox, P3HT:PCBM films were spin-coated onto the ITO/ZnO substrates for 1 min at 700 rpm from solutions where P3HT and PCBM were mixed together in chlorobenzene with a weight ratio of 1:0.7 and a total concentration of either 17 mg mL-1 for 80 nm thick layers or 34 mg mL-1 for 200 nm thick layers. The solutions were filtered through 0.2 µm pore PTFE filters prior to spin coating, and the samples were annealed at 160 °C for 10 min in the glovebox. Next, PEDOT:PSS CPP 105 DM (H.C. Starck, Newton, MA) was spin-coated on top of the active layer at 5000 rpm for 60 s in air and annealed at 120 °C for 10 min in the glovebox. The samples were then loaded into a vacuum thermal evaporation system (SPECTROS, Kurt J. Lesker, Clairton, PA) connected to the glovebox, and 150 nm of Ag was deposited through a shadow mask at a rate of 1-3 Å s-1 and a base pressure of 2 × 10-7 torr. Reference devices with the structure ITO/PEDOT:PSS(AI4083)/P3HT:PCBM/Al were also prepared using the same procedure as for the inverted devices except for the PEDOT: PSS and Al depositions. For the reference devices, cleaned, SiOx-patterned ITO was first treated with oxygen plasma for 3 min, and PEDOT:PSS Al4083 (H.C. Starck, Newton, MA) was filtered through a 0.45 µm pore PVDF filter and spin-coated in air at 5000 rpm for 60 s. The substrates were then annealed for 10 min at 140 °C. After the spin coating of P3HT:PCBM in the glovebox, 200 nm of Al was deposited through a shadow mask at a rate of 1-3 Å s-1.

Cheun et al.

Figure 2. Work function of ITO/ZnO as a function of the ZnO thickness.

The completed devices were transferred in a sealed container to another nitrogen-filled glovebox for electrical measurements. Current-voltage characteristics were measured using a source meter (2400, Keithley Instruments, Cleveland, OH) controlled by a LabVIEW program. When testing the solar cells under illumination, an AM1.5 G solar simulator (Orial 91160, Oriel Instruments, Stratford, CT) with an irradiance of IL ) 100 mW cm-2 was used in the glovebox. For the investigation of ultraviolet (UV) effects on devices with ZnO layers with thicknesses below 10 nm, a UV lamp (Electro-Lite Bondwand UV Flood System, Electro-Lite Corporation, Bethel, CT) was used to expose the devices through the ITO glass side for 60 min. A monochromator coupled to a 175 W xenon lamp (ASBXE-175EX, CVI Spectral Products, Putnam, CT) and a calibrated photodiode (S2386-44K, Hamamatsu, Bridgewater, NJ) were used to measure external quantum efficiency (EQE). Atomic force microscopy (AFM) images were acquired under atmospheric conditions using a commercial MultiMode AFM (Dimension 3100, Veeco, Santa Barbara, CA) equipped with a NanoScope III controller. The microscope was housed within an acoustic isolation hood and stabilized on a floating nitrogen table (Micro-g, Technical Manufacturing Corporation, Peabody, MA). The AFM piezo scanner was calibrated using a threedimensional reference silicon grating (part number 498-000026, Veeco, Santa Barbara, CA) with a 10 µm lateral pitch and a step height of 200 nm. Cantilevers (NSC35/NoAl, Mikromasch, San Jose, CA) were made from n-type silicon (phosphorus doped), utilizing a cantilever of 130 ( 5 µm in length, which has a typical probe radius of 10 nm and typical spring constants of 4.5 N m-1. III. Results and Discussion First, we studied the dependence of the work function of ITO/ ZnO as a function of the thickness of the ZnO layer. Figure 2 shows the work function of all ITO/ZnO films averaged over three measurements. The work function of cleaned ITO was measured to be 4.7 eV. After modification by ZnO layers, the work function of ITO/ZnO remained around 4.3 eV for all ZnO thicknesses. Remarkably, this indicates that one cycle of ZnO deposited by ALD is enough to change the work function of ITO, and this value remains relatively constant for any other ZnO thicknesses. Moreover, the work function of ITO/ZnO electrode is close to the lowest unoccupied molecular orbital (LUMO) energy level (4.1 eV) of PCBM as shown in Figure 1b. Therefore, in principle, ZnO layers of any thickness can be used to modify ITO, to serve as a transparent electron-collecting electrode in organic solar cells. However, the role of ITO/ZnO electrode is not only determined by its work function. It also

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Figure 3. (a) J-V characteristics for inverted devices, ITO/ZnO/P3HT: PCBM (80 nm)/PEDOT:PSS/Ag, with varying ZnO thicknesses deposited by ALD. (b) PCE (%) and FF as a function of the ZnO thickness.

can be affected by other factors such as the electrical conductivity and the density of defects of the ZnO layer. To study the dependence of the photovoltaic performance on the thickness of the ZnO layer in devices, we fabricated the inverted solar cells with the general structure: glass/ITO (150 nm)/ZnO (x nm)/ P3HT:PCBM (80 nm)/PEDOT:PSS (40 nm)/ Ag (150 nm). Figure 3a shows the J-V characteristics of such devices with varying thicknesses of ZnO from x ) 0.1-100 nm. Solar cells having ZnO layers with thicknesses below 10 nm show J-V curves with an “s-shaped” kink, resulting in poor device performance. Table 1 provides values averaged over three or four devices, of the photovoltaic performance as a function of ZnO layer thickness. All devices below 10 nm have similar open-circuit voltage values, VOC, around 300 mV and fill-factor, FF, below 0.40 resulting in a PCE below 1%. Figure 3a also shows that the kink completely disappeared from the J-V curves of devices with ZnO layers with thicknesses of 10 nm and above. In these devices, the FF and the PCE substantially increased. On the best device with a 10 nm ZnO layer, a PCE of 2.13%

and an FF of 0.65 were obtained. Figure 3b shows that, for devices with ZnO layers from 10 to 100 nm, there is little variation in PCE and FF. In the past, device performance of inverted solar cells with metal oxides has been related to the surface quality of the films.16,20 Figure 4 shows AFM topographic images of the ZnO films with different thicknesses processed by ALD. The surface roughness (root-mean-square) values of 5, 10, and 20 nm thick ZnO films are 1.89, 2.32, and 2.41 nm, respectively. These small variations of surface roughness cannot be correlated with the strong variations in device performance observed in our solar cells. Similar grain sizes and surface morphology were observed in the ZnO films. These results suggest that, rather than the surface roughness or morphology, other factors are responsible for the difference in device performance. One possible explanation is that, for the thinnest ZnO layers, the large surface-tobulk ratio causes grain defects to play an important role in decreasing the electrical conductivity of these layers. Recently, a number of studies have related kink features with the low conductivity of the ZnO layers in organic solar cells.21,22 The low conductivity of these thin ZnO layers results from the presence of oxygen at its grain boundaries. Oxygen can act as an electron trap at these boundaries and therefore reduce the conductivity of these layers. Also, it has been documented that UV illumination can remove these kink features by increasing the electrical conductivity of the ZnO layers.21,22 The mechanism behind the increase of conductivity is attributed to the UVinduced desorption of negatively charged oxygen molecules trapped at the grain boundaries.23 This mechanism was therefore tested in device with ZnO layers with thicknesses below 10 nm. After exposing the solar cells to UV illumination for 60 min through the glass/ITO side, the kinks were found to be significantly reduced, thus improving device performance. Figure 5 shows J-V characteristics for an representative inverted device, ITO/ZnO (0.1 nm)/P3HT:PCBM (80 nm)/PEDOT:PSS/ Ag, before and after UV exposure. After UV exposure, the PCE values were significantly enhanced in devices with ZnO layers with thicknesses of 0.1, 0.5, and 1 nm of the ZnO layer and relatively less in devices with a thickness of 5 nm as shown in the inset of Figure 5. This trend can be explained because thicker ZnO layers, with a larger density of grain defects, are expected to require longer UV exposure to increase their conductivity and to completely restore the FF than in thinner layers with a lower density of grain defects. However, the sudden disappearance of the kink in J-V characteristics of solar cells with thicker ZnO layers, even without any UV exposure, also points to the existence of a critical thickness at which these effects become less dominant. This critical thickness may be related to a point where trapping at the grain boundaries becomes less dominant than the electrical conductivity in the bulk. However, further studies will be required to clarify the origin of this behavior.

TABLE 1: Averagea Device Performance of Inverted Devices, ITO/ZnO/P3HT:PCBM (80 nm)/ PEDOT:PSS/Ag, with Varying ZnO Thicknesses Deposited by ALD JSC (mA cm-2)

ZnO thickness (nm)

VOC (mV)

0.1 0.5 1 5 10 20 40 100

305 ( 41 (510 ( 10) 315 ( 36 (482 ( 9)b 298 ( 22 (457 ( 5)b 397 ( 44 (379 ( 44)b 561 ( 12 556 ( 2 559 ( 9 560 ( 7

a

b

FF

4.8 ( 0.8 (4.5 ( 0.9) 3.9 ( 0.4 (4.1 ( 0.1)b 3.2 ( 0.5 (3.6 ( 0.8)b 3.4 ( 0.6 (3.4 ( 0.6)b 5.4 ( 0.4 5.8 ( 0.3 6.0 ( 0.3 5.9 ( 0.1 b

PCE (%)

0.38 ( 0.01 (0.57 ( 0.01) 0.40 ( 0.01 (0.56 ( 0.01)b 0.32 ( 0.01 (0.53 ( 0.02)b 0.40 ( 0.03 (0.40 ( 0.03)b 0.62 ( 0.03 0.62 ( 0.01 0.62 ( 0.02 0.64 ( 0.01 b

0.56 ( 0.18 (1.32 ( 0.32)b 0.50 ( 0.17 (1.13 ( 0.14)b 0.19 ( 0.21 (0.89 ( 0.16)b 0.47 ( 0.16 (0.53 ( 0.15)b 1.88 ( 0.25 2.02 ( 0.12 2.09 ( 0.21 2.12 ( 0.12

Average was obtained over three or four devices. b Parameters in parentheses were measured after UV exposure for 60 min.

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Figure 5. J-V characteristics for an representative inverted device, ITO/ZnO (0.1 nm)/P3HT:PCBM (80 nm)/PEDOT:PSS/Ag, before and after UV exposure. Inset: PCE (%) as a function of the ZnO thickness before and after UV exposure.

Figure 6. (a) Refractive index for all of the layers, ITO, ZnO, P3HT: PCBM, and PEDOT:PSS, in the inverted solar cell structure. (b) (left) ZnO thickness (TZnO) dependence of the JSC for inverted structures with four different active layer thicknesses, 80, 150, 200, and 250 nm. (right) P3HT:PCBM thickness (TP3HT:PCBM) dependence of the JSC for inverted structures with a 20 nm thick ZnO layer.

Figure 4. AFM images of the (a) 5, (b) 10, and (c) 20 nm thick ZnO films processed by ALD.

The presence of the ZnO layer is expected to have an influence over the distribution of the optical field within a solar cell. To investigate the optical properties of solar cells with ZnO layers of various thicknesses, the refractive index of 100 nm thick ZnO and 80 nm thick P3HT:PCBM (after annealing) layers were measured using a Woollam M-2000 spectroscopic ellipsometer operating in reflection mode. Si substrates were used to deposit these layers for ellipsometric characterization. The optical constants for ITO, PEDOT:PSS, and Ag were taken from the literature.24,25 Figure 6a shows the refractive index used for all of the layers in the inverted solar cell structure. Simulations

of the optical properties of each solar cell structure were implemented using the transfer matrix method (TMM). The absorption spectrum calculated through the TMM was then related to the photovoltaic performance by considering that the short circuit current is given by the equation26

JSC ) e

∫AM1.5 A(λ)ηIQE(λ)N(λ) dλ

(1)

where A(λ) is the absorption spectrum of the active layer, ηIQE(λ) the internal quantum efficiency, and N(λ) the spectral photon flux density under AM1.5G illumination. Equation 1 was used to simulate JSC by assuming ηIQE (λ) ) 1 and numerically

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TABLE 2: Averagea Device Performance of Inverted Devices, ITO/ZnO (20 nm)/P3HT:PCBM (200 nm)/ PEDOT:PSS/Ag with Increased P3HT:PCBM Thickness

inverted reference deviceb

VOC (mV)

JSC (mA cm-2)

FF

PCE (%)

corrected PCE (%)c

588 ( 13 583 ( 3

8.7 ( 0.3 7.9 ( 0.2

0.64 ( 0.01 0.54 ( 0.01

3.23 ( 0.14 2.77 ( 0.06

3.07 ( 0.13 2.63 ( 0.06

a

Average was obtained over four or five devices. b The reference device has the structure ITO/PEDOT:PSS/P3HT:PCBM (200 nm)/ Al. c A 0.95 correction factor was applied to PCE on the basis of the spectral mismatch.

evaluating the integral from 350 to 670 nm. The lower wavelength was determined by the cutoff wavelength due to the drop in transmittance produced by the glass substrate, and the longer wavelength was determined by the cutoff wavelength (i.e., where EQE ) 0) for the P3HT:PCBM active layer. The left panel of Figure 6b shows the ZnO-thickness dependence of the JSC for inverted solar cell structures with an 80 nm active layer. As it is clear, variations of the ZnO layer thickness produce only very small variations in the JSC. Since the FF and VOC can be assumed to be independent of the ZnO layer thickness, the PCE is expected to be linearly dependent on the JSC through the proportionality constant FF × VOC × Pin-1. Hence, the left panel of Figure 6b can be considered to be proportional to the PCE expected in these solar cells. With this in mind, for a solar cell with an active layer thickness of 80 nm, the simulations clearly correlate, qualitatively, with the trend followed by the PCE (Figure 3b) measured for devices with ZnO layer thicknesses above 10 nm. The optical interference effects produced by the ZnO layer are rather small, compared with those observed due to the electrical properties of the ZnO layers. Hence, in these inverted solar cells with 80 nm thick active layers, the ZnO layer does not act as an effective optical spacer to increase the absorption within the active layer. Instead, the thickness of the active layer is expected to play a more significant role on solar cell performance. The left panel of Figure 6b also shows the dependence of the JSC with respect to the ZnO layer thickness for solar cells having an active layer of 150, 200, and 250 nm. As shown from these simulations, in none of these solar cells, the thickness of the ZnO layer plays a significant role to increase the absorption and consequently the current density JSC expected in these solar cells. Instead, differences in active layer thicknesses have a stronger effect. For instance, the right panel of Figure 6b shows that for a 20 nm thick ZnO layer, if the morphology of the active layer remains similar to that of an 80 nm thick layer, an increase of the thickness of the active layer to around 200 nm could provide at least a 22% increase in cell efficiency due to the increased absorption within the active layer. The right panel of Figure 6b also shows that the use of thicker layers cannot be expected to produce any further improvement in absorption and therefore in solar cell performance. To confirm the results obtained through these simulations, inverted solar cells with a 20 nm thick ZnO layer, thick enough to prevent contact effects, and around 200 nm thick P3HT: PCBM active layers were fabricated. Table 2 shows the photovoltaic performance of inverted solar cells with 200 nm thick P3HT:PCBM. These solar cell devices, with a 200 nm thick layer of P3HT:PCBM, show comparable values of FF ) 0.64 ( 0.01 and VOC ) 588 ( 13, but increased values of JSC ) 8.7 ( 0.3 mA cm-2 resulting in PCE ) 3.23 ( 0.14%, compared to the previous devices with 80 nm thick active layers. Although these improvements are higher than those expected

Figure 7. (a) J-V characteristics and (b) EQE (%) for inverted devices, ITO/ZnO/P3HT:PCBM (200 nm)/PEDOT:PSS/Ag. The reference device has the structure ITO/PEDOT:PSS/P3HT:PCBM (200 nm)/Al.

from optical simulations, they could be explained by differences in active layer morphology and batch-to-batch variations. The performance of these inverted devices was also compared with reference devices having transparent hole-collecting electrodes. Figure 7a shows the J-V characteristics for inverted solar cell (transparent electron-collecting electrode) devices with the structure: ITO/ZnO (20 nm)/P3HT:PCBM (200 nm)/ PEDOT:PSS/Ag. When compared with reference solar cells with the conventional structure ITO/PEDOT:PSS (AI4083)/P3HT: PCBM (200 nm)/Al, inverted solar cells with ZnO layers showed a slightly better performance. Figure 7b shows a comparison of the measured EQE data. An increase of EQE in the range between 455 and 580 nm was observed in the inverted solar cell. Below 455 nm and above 580 nm, EQE was found to be smaller in the inverted solar cell compared to that of the reference solar cell. These small variations can be attributed to differences in absorption produced by the different geometry of the cells and metal electrode. On the basis of the EQE spectra shown in Figure 7b, a spectral mismatch factor of 0.95 was calculated for our devices illuminated with our solar simulator.12 Corrected PCE values for AM1.5 illumination based on corresponding adjustments made to the photocurrent are summarized in Table 2. In summary, these results indicate that optimized inverted solar cells, with potentially better air stability than conventional devices,9,12 can be fabricated without loss of performance. IV. Conclusion We reported the photovoltaic properties of inverted polymer solar cells with ITO/ZnO transparent electrodes. The device performance of solar cells with varying thicknesses of ZnO deposited by ALD was investigated. It was found that the work function, surface roughness, and morphology of ITO/ZnO are independent of the ZnO thickness. However, the photovoltaic performance was found to be strongly dependent with respect

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to a critical ZnO thickness, around 10 nm. Solar cells with ZnO layers below 10 nm yield J-V characteristics containing a parasitic “s-shaped” kink which strongly degrades photovoltaic performance. The appearance of the kink is attributed to a poor conductivity due to oxygen trapping at the grain boundaries of ZnO. For very thin layers, these grain boundaries severely limit bulk conduction. Oxygen desorption can be induced by UV exposure, causing an increased conductivity of the ZnO layers and a less pronounced kink in the J-V characteristics. Solar cells with a ZnO layer of 10 nm or thicker showed no kink features and significantly improved photovoltaic behavior. Also, these devices showed little variations with respect to the thickness of the ZnO layer, in good agreement with simulations of the optical properties. Instead, these optical simulations showed that the thickness of the active layer plays a more important role in optimizing the photovoltaic properties of inverted solar cells. Inverted solar cells with a 200 nm thick active layer can reach a high PCE of 3.06% estimated for AM 1.5G, a 100 mW cm-2 illumination, comparable with reference devices with the structure, ITO/PEDOT:PSS/P3HT:PCBM/Al. Acknowledgment. This material was funded in part through the Center for Interface Science: Solar Electric Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001084, by the STC Program of the National Science Foundation under Agreement No. DMR0120967 and by the Office of Naval Research (Grant No. N00014-04-1-0120). This work was performed in part at the Microelectronics Research Center at Georgia Institute of Technology, a member of the National Nanotechnology Infrastructure Network, which is supported by NSF (Grant No. ECS03-35765). The authors would like to thank Prof. Seth Marder for providing access to his AFM system. References and Notes (1) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15.

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