Article pubs.acs.org/JPCC
Interface Formation Between ZnO Nanorod Arrays and Polymers (PCBM and P3HT) for Organic Solar Cells Jung Han Lee,†,# Jeong-Ho Shin,‡ Jae Yong Song,‡,§ Wenfeng Wang,∥ Rudy Schlaf,∥ Kyung Joong Kim,†,§ and Yeonjin Yi*,⊥ †
Department of Nano and Biosurface Science, University of Science and Technology, 217 Gajeong-ro, Yuseong-Gu, Daejeon 305-350, Republic of Korea ‡ Department of Nanomaterials Science and Engineering, University of Science and Technology, 217 Gajeong-ro, Yuseong-Gu, Daejeon 305-350, Republic of Korea § Division of Industrial Metrology, Korea Research Institute of Standards and Science, 209 Gajeong-ro, Yuseong-gu, Daejeon 305-600, Republic of Korea ∥ Department of Electrical Engineering, University of South Florida, Tampa, Florida 33620, United States ⊥ Department of Physics, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea S Supporting Information *
ABSTRACT: We investigated the interface formation between a ZnO nanorod array and active layers of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly[3-hexylthiophene] (P3HT) in organic solar cells (OSC). We measured the interfacial electronic structures with in situ photoemission spectroscopy combined with an electrospray deposition system. Different interfacial electronic structures were observed on the ZnO nanorod array, which were compared to those of a two-dimensional ZnO film. Comparing the interfacial orbital line-ups of the active layers on the nanorod array and the film, PCBM shows Fermi level pinning behavior, but P3HT does not. These induce nearly identical orbital line-ups at the interfaces of PCBM/film and PCBM/nanorod but different line-ups at the interfaces of P3HT/film and P3HT/nanorod. These differences are understood with the integer charge transfer model with the different thresholds of Fermi level pinning of PCBM and P3HT. These results give insight into the design not only of OSCs but also of any organic electronic devices with nanostructures: changes in electronic structure due to the nanostructure formation should be considered thoroughly.
1. INTRODUCTION ZnO has attracted much attention as an oxide semiconductor with various applications for several decades because of its large piezoelectric coefficient, large exciton binding energy, low material costs, nontoxicity, and abundance. The most promising application is as a transparent electrode due to its wide band gap (3.3−3.4 eV).1 Therefore, ZnO has been studied widely as a prospective transparent conducting oxide (TCO) material for various optoelectronic devices. Another distinct property of ZnO is its preferential crystal growth along the (0001) axis of wurtzite structure. As a result, ZnO crystal has three main surfaces of Zn- or O-terminated polar surface (0001 or 0001̅) and mixed-terminated nonpolar surface (101̅0). Since the Zn(0001) surface has higher surface energy compared to other surfaces,2 ZnO can be grown in rod shape. With the help of this property, various shapes of ZnO nanorods have been grown and applied to various fields.3−6 In organic solar cells (OSCs), ZnO has conventionally been used with a 2-dimensional (2D) film as a TCO or a hole blocking layer (HBL). Recently, the application of nano© 2012 American Chemical Society
structured ZnO has increased considerably since its shape and large surface area suggest ideal interdigitated architecture for OSCs.7−11 Because the exciton diffusion length is a few nanometers, nanoscale mixtures of donor and acceptor, socalled bulk heterojunctions (BHJ), are desired for efficient charge separation. For efficient charge transport, a continuous current path is needed without breaking by counterpart material. Therefore, the ideal device architecture is believed to be comb-like interdigitated structures at the donor−acceptor interface. A ZnO nanorod array would be an excellent template to form such structures. In order to use this nanostructured ZnO, its electronic structures should be understood since the device characteristics are highly dependent on the electronic structures of ZnO and the organic active layers. Charge injection/transport efficiency is understood with the interfacial electronic structures, and Received: August 16, 2012 Revised: November 19, 2012 Published: November 19, 2012 26342
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Figure 1. UPS results and SEM image of a ZnO nanorod array. (a) Normalized secondary electron cutoff and (b) valence region with various aspect ratios. Work functions and aspect ratios are indicated at the left and right side of each spectrum, respectively. (c) The SEM image of ZnO nanorod array. (d) The ionization energies (triangles) and work functions (circles) of ZnO substrates as a function of the aspect ratios of the nanorods.
2.2. Organic Material Deposition. For the in situ stepwise analysis, we used electrospray,15,16 which can deposit polymer in vacuum by direct injection of polymer/macromolecules from a solution. This method is proper for the precise stepwise X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) analysis since this can control the amount of depositing material finely and minimize the effect of subsequent contamination from air exposure. PCBM (>99.9%, Sigma-Aldrich Co.) and P3HT (>98%, Sigma-Aldrich Co.) were deposited in a deposition chamber (base pressure of 1 × 10−8 Torr) linked directly to the photoemission analysis chamber for in situ measurements. PCBM and P3HT were dissolved in toluene (1 mg/mL) and stirred for 1 day in dark conditions to avoid light exposure. For P3HT, the solution was heated to 40 °C during the stirring to increase solubility in toluene. The solutions were injected into the deposition chamber at a rate of 4 μL/s. To produce the fine spray, the injection capillary was biased at −3500 V relative to the grounded chamber. All injection processes were conducted in dry N2 ambient to minimize contaminations. 2.3. Measurements. We measured the interfacial electronic structures during the interface formation between the two polymers and the ZnO substrate with two different structures of nanorod array and film. To investigate the interfacial electronic structures, we measured XPS and UPS spectra before and after each deposition step. The volume (cumulative) for each step was 0.01 (only for PCBM), 0.03, 0.06 (0.09 for P3HT), 0.12 (only for PCBM), 0.25, 0.50, and 1.00 mL on the 2D film substrate and 0.03, 0.10 (0.09 for P3HT), 0.25, 0.50, 1.00, 2.00, and 4.00 mL on the nanorod array substrate. We needed a larger amount of solution to cover the nanorod due to its high aspect ratio. The analysis system consisted of a hemispherical energy analyzer (SPECS Phoibos 100), dual Xray sources (Al Kα and Mg Kα), and ultraviolet light source. The base pressure of the photoemission analysis chamber was 1 × 10−10 Torr. For this study, we used Mg Kα (1253.6 eV) for XPS and He I (21.22 eV) for UPS, respectively. The UPS spectra were obtained with a sample bias of −10 V for secondary electron cutoffs (SECs) and the valence region. All spectra were plotted with respect to the Fermi level (EF).
furthermore, the relative position of the energy levels at the donor−acceptor interface determines the open circuit voltage (Voc).12 However, surprisingly, there have been few reports that focus on the interfacial electronic structures between nanostructured ZnO and organic materials. Although some reports tried to explain the electronic structures of nanostructured ZnO with its bulk electronic structures, it is well-known that the nanosized material could have quite different properties. Therefore, a direct study on the interfacial electronic structure between nanostructured ZnO and organic materials is essential to understand/optimize the OSC based on nanostructured ZnO. In this article, we measured the interfacial electronic structures of widely used OSC materials of [6,6]-phenyl-C61butyric acid methyl ester (PCBM) and poly[3-hexylthiophene] (P3HT) deposited on a ZnO nanorod array and 2D film. Different electronic structures between the nanorod array and 2D film result in different interface formation with PCBM and P3HT.
2. EXPERIMENTAL SECTION 2.1. ZnO Substrates Preparation. A ZnO nanorod array was grown on a pregrown ZnO film by a vapor transport process in a vacuous quartz tube. We adjusted growth time and flow rate to control the shape of the ZnO nanorod. The identical stoichiometry and crystal structure of all ZnO substrates were verified by energy dispersive spectroscopy, Xray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy. (Detailed ZnO nanorod growth is explained elsewhere.13) ZnO nanorods with a diameter of about 50 nm have a single crystalline structure with the c-axis in the longitudinal direction. We cleaned the ZnO substrates chemically to remove residual contaminants on the surface. The substrates were rinsed with acetone, toluene, and dimethyl sulfoxide and blown with dry N2 between each rinsing step. We did not use conventional Ar+ sputter cleaning because the preferential sputtering of oxygen in ZnO has been reported,14 which would cause significant changes of the surface electronic structure. Furthermore, a sputter cleaned surface is quite different from the real surface for OSC fabrication. 26343
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Additional low intensity XPS (LIXPS) were conducted to cross-check if sample charging occurred.17
3. RESULTS AND DISCUSSION 3.1. Characterization of the ZnO Nanorod Array. First, the electronic structures of ZnO nanorod arrays were studied to understand the effect of the aspect ratio on their electronic structure. Figure 1C shows a SEM image of a typical ZnO nanorod array substrate used in this study. The ZnO nanorod arrays were grown vertically to the surface, and their diameters were about 50 nm. The nanorod arrays were categorized according to the aspect ratio [length (l) divided by diameter (d)] for comparison. The aspect ratios of prepared nanorod substrates are 0 (film), 8.5, 15.6, and 22.7. Figure 1a,b shows the measured UPS spectra of the SEC and valence region of the ZnO nanorod substrates. The SECs (a) were normalized with their maximum values for clear comparison. The black solid line at the bottom represents the ZnO film substrate, and the colored lines above it represent nanorod arrays with various aspect ratios as indicated at the right side of each spectrum. The SEC shifts toward lower binding energies as the aspect ratio increases. The SEC shifts saturate at the aspect ratio of 15.6, where the total shift is 0.2 eV. The work functions (WFs) of each film and nanorod array are also indicated in the left side of each spectrum. In Figure 1b, the valence band maximum (VBM) shifts toward higher binding energies as the aspect ratio increases. The shifts saturate at the aspect ratio of 15.6, and the total shift is 0.15 eV. The ionization energies (Eion = WF + VBM) and WFs of each substrate versus the aspect ratio are plotted in Figure 1d. Eion increases from 7.3 to 7.7 eV and the WF from 3.8 to 4.0 eV as the aspect ratio increases. Both show a saturated behavior at the aspect ratio of 15.6. These changes in Eion between the film and the nanorod array originate from the different electronic structures of different surfaces of ZnO crystal such as (0001), (0001)̅ , and (1010̅ ). Considering the preferred growth in [0001] direction, the exposed surface of the film is mostly the polar surface (0001), while the ZnO nanorod with a high aspect ratio contains large portions of mixed-terminated nonpolar surface (1010̅ ) at its side wall. Although it is not trivial to measure the Eion of each crystal surface due to the very sensitive surface of ZnO,1,2 the Eion difference between (0001) and (101̅0) surfaces is suggested as approximately 0.4 eV in previous studies.2,18,19 This value is very close to current results [7.7 − 7.3 = 0.4 (eV)], suggesting the increment of the portion of (101̅0) surface with nanorod formation. 3.2. Interfacial Electronic Structures between ZnO Substrates and PCBM and P3HT. To find the interfacial electronic structures of the PCBM and P3HT overlayers on the ZnO substrates, we measured in situ UPS and XPS spectra during the formation of four interfaces with the PCBM and P3HT overlayers on the ZnO film and the nanorod array substrates. ZnO nanorod arrays with the aspect ratio of 15.6 were used for these measurements because the largest difference in Eion is obtained with this aspect ratio compared to that of the ZnO film, thus showing distinctive differences in the interfacial electronic structures. Figure 2 shows the measured UPS spectra of the normalized SEC (left panel) and the background-removed valence region (right panel) during the step-by-step deposition of PCBM on (a) the ZnO film and (b) the ZnO nanorod array. In Figure 2a, the bottom spectra (black) show the emission features from the ZnO film in the SEC and valence region, indicating a WF of
Figure 2. UPS spectra of the normalized secondary electron cutoff (left panel) and valence (right panel) region measured during the stepby-step deposition of PCBM on ZnO (a) film and (b) nanorod array. Initial and final work function and deposition volume are indicated in the cutoff and valence regions, respectively.
3.77 eV and VBM of 3.54 eV. The SEC shifts toward lower binding energies as the injection volume of the PCBM layer increases. The shifts saturate to 0.38 eV at an injection volume of 1.00 mL, and the WF of the final PCBM layer is 4.15 eV. The right panel shows the valence band of the ZnO film (bottom) with emerging spectral features of the highest occupied molecular orbital (HOMO) of PCBM as a function of injection volume. The onset of the HOMO of PCBM at the final deposition step is 1.85 eV, which is 1.69 eV lower than the VBM of the ZnO film. Figure 2b shows the spectral changes during the deposition of PCBM on the ZnO nanorod array. The bottom spectra show the different electronic structure of the ZnO nanorod array compared to that of ZnO film, indicating a WF of 4.02 eV and a VBM of 3.62 eV. The SEC (left panel) shifts toward lower binding energy by 0.16 eV during the PCBM deposition, and the WF of the final PCBM layer is 4.18 eV. The shifts saturate when the injection volume is 4 mL, which is larger than the case of the ZnO film due to the large surface area of the nanorod array. The onset of the PCBM HOMO is 1.87 eV at the final deposition, which is 1.75 eV lower than the VBM of the ZnO nanorod array. In addition, the HOMO shows 0.20 eV shift from its first deposition step, indicating a band bending (Vb). Although the ZnO nanorod array has larger WF (4.02 eV) and VBM (3.62 eV) than those of the ZnO film (WF = 3.77 eV; VBM = 3.54 eV), the final values of WF (4.15 vs 4.18 eV) and HOMO (1.85 vs 1.87 eV) are almost the same, indicating different interface dipole 26344
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formation on each interface. (Details will be indicated with energy level diagrams.) Figure 3 shows the C 1s core level spectra to cross-check the Vb of PCBM at the interface. We fitted these spectra with two
Figure 3. C 1s core levels measured during the step-by-step deposition of PCBM on ZnO (a) film and (b) nanorod array. Measured (dots) and fitted data (red lines) with two components (pink, PCBM; blue, surface carbon) are shown.
peaks for surface carbon (blue component, ambient contamination on substrates) and PCBM (pink component) to trace the PCBM component. The PCBM component appears at 285.34 eV and shifts toward lower binding energies by 0.2 eV with the deposition of PCBM on both the film and nanorod substrates. This shift of 0.2 eV accords well with the HOMO shifts in the UPS results in Figure 2. Collectively, the Vb in the PCBM layer is the same regardless of the structures (film and nanorod) of the ZnO substrate. In the same manner, we investigated the spectral changes during the deposition of P3HT on the two ZnO substrates. Figure 4 shows the measured UPS spectra of the P3HT layers deposited on (a) the ZnO film and (b) the ZnO nanorod array as a function of injection volume. In panel b, the magnified spectra measured from the final film are presented in the insets due to the relatively low emission features of the P3HT HOMO. In Figure 4a, the bottom spectra (black lines) show the emission features of ZnO film in the SEC and valence regions. The WF and VBM of the ZnO film are 3.80 and 3.50 eV, which is nearly identical to the ZnO film used for the PCMB measurements. In the left panel, the SEC shifts toward lower binding energies as the P3HT is deposited. The shifts saturate when the injection volume is 1.00 mL total shift being 0.13 eV, and the final WF of P3HT layer is 3.93 eV. The right panel shows the valence band spectra of the ZnO film (bottom) with emerging spectral features of the HOMO of P3HT during P3HT deposition. The final deposition step shows the onset of the HOMO at 1.13 eV (inset), which is 2.37 eV lower than the VBM of the ZnO film. Figure 4b shows the spectral changes during the deposition of P3HT on the ZnO nanorod array. The bottom spectra of the nanorod array also show similar electronic structures to those for the PCBM measurements in Figure 2b, indicating a WF of 4.00 eV and a VBM of 3.65 eV. The SEC shifts toward lower binding energy by 0.11 eV after
Figure 4. UPS spectra of the normalized secondary electron cutoff (left panel) and valence (right panel) region measured during the stepby-step deposition of P3HT on ZnO (a) film and (b) nanorod array. Initial and final work function and the deposition volume are indicated in the cutoff and valence regions, respectively.
the final P3HT deposition and the final P3HT show a WF of 4.09 eV. The onset of the P3HT HOMO is seen at 1.02 eV (inset), which is located 2.63 eV lower than the VBM. In contrast to the case of PCBM, the final WF and HOMO show different values depending on the substrate electronic structures. (Detailed discussion will be given below.) We estimated the Vb in the P3HT layer with the C 1s core level spectra due to the relatively small HOMO signal in the UPS spectra. We fitted these spectra again with two peaks for residual carbon (blue component) and PCBM (pink component) to trace the P3HT component. Figure 5 shows the C 1s peaks during the deposition of P3HT on each substrate. At the beginning of deposition, the C 1s peak appears at 285.48 eV and shifts toward lower binding energies by 0.14 eV during the deposition of P3HT on the ZnO film as shown in Figure 5a. In the case of the deposition on the ZnO nanorod array, the C 1s peak appears at 285.51 eV and shows an identical shift of 0.14 eV as shown in Figure 5b. We also confirmed the shift with the changes of S 2p spectra (see Figure S1 of the Supporting Information). Thus, the Vb in the P3HT layer is the same regardless of the structures of the ZnO substrates. Combining all spectral changes, we drew the energy level diagrams in Figures 6 and 7. We evaluated the interface dipole (eD) by subtracting the Vb from the total SEC shift.20 Vb in ZnO side was determined from the Zn 2p core level shifts during the deposition (data not shown). We used the band gap energies reported (3.40 eV for ZnO,1 2.10 eV for PCBM,21 and 26345
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2.40 eV for P3HT21) to determine the position of conduction band minimum (CBM) of ZnO and the lowest unoccupied molecular level (LUMO) of PCBM and P3HT. With this estimation, the CBM of ZnO is located below the EF like a degenerated semiconductor, which agrees with recent reports.22,23 Figure 6 shows the energy level alignments at the interfaces between PCBM and the two types of ZnO substrates. The estimated eDs at the interfaces are (a) 0.07 eV (essentially zero) for PCBM/ZnO film and (b) 0.29 eV for PCBM/ZnO nanorod. The eD at the PCBM/ZnO nanorod interface pushes all the energy levels of PCBM down and thus creates nearly the same orbital line-ups compared to those on the ZnO film. It is quite interesting to compare this eD with the WF difference between two substrates. The WF difference between the ZnO film and nanorod is 0.25 eV, which is very close to the eD (0.29 eV) induced on the nanorod. As a result, the orbital line-ups are almost the same at the two interfaces regardless of the electronic structures (mainly the WF) of the ZnO substrate. In other words, it shows Fermi level pinning behavior in these contacts. However, P3HT shows unpinned behavior. Figure 7 shows the energy level alignments at the interfaces between P3HT and the two ZnO substrates. The estimated eDs at the interfaces are (a) −0.01 eV for P3HT/ZnO film and (b) 0.02 eV for P3HT/ZnO nanorod. In this case, essentially no eD (no pinning) is induced at both interfaces having a different final WF; thus, it shows different orbital line-ups at the two interfaces. Interestingly, the differences in hole/electron barriers are close to the WF difference between two substrates [4.09 − 3.93 = 0.16 (eV)], indicating unpinned behavior again. These different orbital line-ups of PCBM and P3HT can be explained by the integer charge transfer model.24 According to the model, under weak interaction conditions, the energy level alignment of organic material is governed by the WFs of the contact materials. The WF of organic material varies in proportion to the WF of contact material within the limited range of WF; slope parameter of S = 1. However, out of such range, the WF of organic material is not changed anymore (S = 0), meaning Fermi level pinning. The range of the WF of contact material inducing Fermi level pinning depends on the electronic structures of the organic material of interest. Xu et al. reported Fermi level pinning for PCBM and P3HT.25 PCBM shows pinned behavior in contact with a substrate having WF < 4.3 eV, while P3HT shows pinned behavior with a substrate having WF > 3.9 eV. Figure 8 shows the summary of our experiments with the indication of the pinning range from Xu et al. As expected, PCBM shows almost pinned behavior (S = 0.12) for both substrates, while P3HT shows a slope parameter close to unity (0.89). This explains well the two different behaviors for PCBM and P3HT on ZnO film and nanorods. For PCBM, the two substrates have the WFs in the range of Fermi level pinning and thus show nearly identical orbital lineups, i.e., nearly the same HOMO and LUMO position with respect to the pinned Fermi level. However, P3HT shows different line-ups for the two substrates due to the smaller substrate WF than the pinning threshold. However, the quantitative value of the induced interface dipole is not perfectly matched with the work function difference for the PCBM/ZnO interface. The pinning threshold is 4.3 eV for PCBM, thus one might expect the interface dipole of 0.0/0.3/ 0.5 eV with the ZnO having the work function of 4.3/4.0/3.8 eV, respectively. The results show, however, that the interface dipoles of 0.3 (0.29, evaluated value in parentheses) and 0.1
Figure 5. C 1s core level spectra measured during the step-by-step deposition of P3HT on ZnO (a) film and (b) nanorod array. Measured (dots) and fitted data (lines) are shown.
Figure 6. Energy level diagrams for PCBM on ZnO (a) film and (b) nanorod array. Each symbol represents Ψ the work function, Eion the ionization energy, Eg the band gap, eD the interface dipole, Vb the band bending, Φe the electron barrier, and Φh the hole barrier, respectively.
Figure 7. Energy level diagrams for P3HT on ZnO (a) film and (b) nanorod array. Each symbol represents Ψ the work function, Eion the ionization energy, Eg the band gap, eD the interface dipole, Vb the band bending, Φe the electron barrier, and Φh the hole barrier, respectively.
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ASSOCIATED CONTENT
S Supporting Information *
S 2p core level shifts during the interface formation of P3HT/ ZnO film and P3HT/ZnO nanorod. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address
# Samsung Electronics Corp., Ltd., Hwasung, Gyeonggi-Do 445701, Republic of Korea.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a research project of the UST overseas training program, a research project of the National Research Foundation of Korea (NRF, grant No. 20120002303), and the Korea Research Council of Fundamental Science and Technology (KRCF) through a basic research project managed by the Korea Research Institute of Standards and Science (KRISS).
Figure 8. Substrate work function versus overlayer work function. Threshold value of substrate work function for Fermi level pinning (from ref 25) and slop parameters (S) of current work are indicated.
(0.07) eV for ZnO have work functions of 4.0 (3.77) and 4.00 (4.02) eV, respectively. This would be originated from the notperfect pinning in combination with the error margin of measurement. Lastly, we suggest the possibility of electronic structure changes with nanostructure formation, which can alter device performance. Nanostructures not only change the physical contact (e.g., interdigitated contact) but also modify the orbital line-up with organic materials. These modified charge injection/blocking barriers could enhance or reduce the charge transport and the donor−acceptor gap determining the Voc. In our study, for example, the hole injection barrier from P3HT to ZnO increases with the introduction of nanorods, which would enhance the hole blocking properties and also the current in the OSC. This is demonstrated practically with PCBM:P3HT BHJ OSCs using the ZnO nanorod array, showing the enhancement in short circuit current and fill factor.7−10 Therefore, the changes in electronic structure by introducing nanostructure should be considered thoroughly before designing efficient OSCs.
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REFERENCES
(1) Ellmer, K.; Klein, A.; Rech, B. Transparent Conductive Zinc Oxide; Springer: Berlin, Germany, 2008. (2) Wöll, C. Prog. Surf. Sci. 2007, 82, 55−120. (3) Tornow, J.; Schwarzburg, K. J. Phys. Chem. C 2007, 111, 8692− 8698. (4) Pal, U.; Serrano, J. G.; Santiago, P.; Xiong, G.; Ucer, K. B.; Williams, R. T. Opt. Mater. 2006, 29, 65−69. (5) Djurišić, A. B.; Leung, Y. H. Small 2006, 2, 944−961. (6) Zhang, Q.; Dandeneau, C. S.; Zhou, X.; Cao, G. Adv. Mater. 2009, 21, 4087−4108. (7) Olson, D. C.; Piris, J.; Collins, R. T.; Shaheen, S. E.; Ginley, D. S. Thin Solid Films 2006, 496, 26−29. (8) Ravirajan, P.; Peiró, A. M.; Nazeeruddin, M. K.; Graetzel, M.; Bradley, D. D. C.; Durrant, J. R.; Nelson, J. J. Phys. Chem. B 2006, 110, 7635−7639. (9) Peiro, A. M.; Ravirajan, P.; Govender, K.; Boyle, D. S.; O’Brien, P.; Bradley, D. D. C.; Nelson, J.; Durrant, J. R. J. Mater. Chem. 2006, 16, 2088−2096. (10) Takanezawa, K.; Hirota, K.; Wei, Q.-S.; Tajima, K.; Hashimoto, K. J. Phys. Chem. C 2007, 111, 7218−7223. (11) Olson, D. C.; Lee, Y.-J.; White, M. S.; Kopidakis, N.; Shaheen, S. E.; Ginley, D. S.; Voigt, J. A.; Hsu, J. W. P. J. Phys. Chem. C 2007, 111, 16640−16645. (12) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323−1338. (13) Shin, J.; Song, J.; Park, H. Mater. Lett. 2009, 63 (1), 145−147. (14) Park, S. H.; Kim, H. J.; Cho, M.-H.; Yi, Y.; Cho, S. W.; Yang, J.; Kim, H. Appl. Phys. Lett. 2011, 98, 082111. (15) Lyon, J. E.; Cascio, A. J.; Beerbom, M. M.; Schlaf, R.; Zhu, Y.; Jenekhe, S. A. Appl. Phys. Lett. 2006, 88, 222109. (16) Bökman, C. F. Analytical Aspects of Atmospheric Pressure Ionisation in Mass Spectrometry; Acta Universitatis Upsaliensis: Uppsala, Sweden, 2002. (17) Yi, Y.; Lyon, J. E.; Beerbom, M. M.; Schlaf, R. J. Appl. Phys. 2006, 100, 093719. (18) Jacobi, K.; Zwicker, G.; Gutmann, A. Surf. Sci. 1984, 141, 109− 125. (19) Blumstengel, S.; Glowatzki, H.; Sadofev, S.; Koch, N.; Kowarik, S.; Rabe, J. P.; Henneberger, F. Phys. Chem. Chem. Phys. 2010, 12, 11642−11646.
4. CONCLUSIONS We investigated the interfacial electronic structures of ZnO film and nanorod arrays with PCBM and P3HT by UPS and XPS measurements. The WF and VBM of the ZnO varies with the changes of the structures of ZnO substrate from film to nanorod array with different aspect ratios. This changes the orbital line-ups in contact with typical organic solar cell materials, PCBM and P3HT. Fermi level pinning is observed with PCBM and not with P3HT. This can be understood with the integer charge transfer model, giving different pinning thresholds for each material. Nanorods give rise to the desired contact structure but can also change the electronic structures of the original bulk materials, implying that quite different device performance can be achieved, despite contrary expectations. This aspect may not be limited to the nanorod structures shown here but may be valid for any nanostructured materials. Therefore, to design efficient organic electronic devices with nanostructured materials, the changes in electronic structures due to nanostructure formation should be considered thoroughly. 26347
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(20) Lüth, H. Solid Surfaces, Interfaces and Thin Films; Springer: Berlin, Germany, 2010. (21) Guan, Z.-L.; Kim, J. B.; Wang, H.; Jaye, C.; Fischer, D. A.; Loo, Y.-L.; Kahn, A. Org. Electron. 2010, 11, 1779−1785. (22) Piper, L. F. J.; Preston, A. R. H.; Fedorov, A.; Cho, S. W.; DeMasi, A.; Smith, K. E. Phys. Rev. B 2010, 81, 233305. (23) Lee, J. H.; Shin, J. H.; Song, J. Y.; Yi, Y. Appl. Phys. Lett. 2010, 97, 263302. (24) Braun, S.; Salaneck, W. R.; Fahlman, M. Adv. Mater. 2009, 21, 1450−1472. (25) Xu, Z.; Chen, L.-M.; Chen, M.-H.; Li, G.; Yang, Y. Appl. Phys. Lett. 2009, 95, 013301.
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