Fluorinated Copper Phthalocyanine Nanowires for Enhancing

Nov 26, 2012 - ... [email protected]; (T.M.) [email protected]. .... Egle Puodziukynaite , Jacob John , Kenneth R. Carter , Todd Emr...
3 downloads 0 Views 3MB Size
Download PDF
Letter pubs.acs.org/NanoLett

Fluorinated Copper Phthalocyanine Nanowires for Enhancing Interfacial Electron Transport in Organic Solar Cells Seok Min Yoon,† Sylvia J. Lou,†,‡ Stephen Loser,† Jeremy Smith,† Lin X. Chen,*,†,‡,∥ Antonio Facchetti,*,†,‡,§ and Tobin Marks*,†,‡ †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States The Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208, United States § Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, United States. ∥ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois, 60439, United States ‡

S Supporting Information *

ABSTRACT: Zinc oxide is a promising candidate as an interfacial layer (IFL) in inverted organic photovoltaic (OPV) cells due to the n-type semiconducting properties as well as chemical and environmental stability. Such ZnO layers collect electrons at the transparent electrode, typically indium tin oxide (ITO). However, the significant resistivity of ZnO IFLs and an energetic mismatch between the ZnO and the ITO layers hinder optimum charge collection. Here we report that inserting nanoscopic copper hexadecafluorophthalocyanine (F16CuPc) layers, as thin films or nanowires, between the ITO anode and the ZnO IFL increases OPV performance by enhancing interfacial electron transport. In inverted P3HT:PC61BM cells, insertion of F16CuPc nanowires increases the short circuit current density (Jsc) versus cells with only ZnO layers, yielding an enhanced power conversion efficiency (PCE) of ∼3.6% vs ∼3.0% for a control without the nanowire layer. Similar effects are observed for inverted PTB7:PC71BM cells where the PCE is increased from 8.1% to 8.6%. X-ray scattering, optical, and electrical measurements indicate that the performance enhancement is ascribable to both favorable alignment of the nanowire π−π stacking axes parallel to the photocurrent flow and to the increased interfacial layer-active layer contact area. These findings identify a promising strategy to enhance inverted OPV performance by inserting anisotropic nanostructures with π−π stacking aligned in the photocurrent flow direction. KEYWORDS: Organic solar cell, interfacial layers, copper hexadecaphthalocyanine, nanowire, zinc oxide

S

acting as selective contacts for efficient charge collection. Therefore, developing new, more effective IFLs is vital for enhancing BHJ solar cell performance.15,16,18,19 The quintessential hole transport IFL, poly(3,4ethylenedioxythiophene:poly(styrene−sulfonate) (PEDOT:PSS), is moisture-sensitive, electrically/structurally inhomogeneous, and rapidly degrades ITO at elevated temperatures.21−23 Note that thin films of n-type semiconducting metal oxides such as ZnO24−26 and titanium oxides (TiOx)27 are effective interfacial electron transport layers due to their moderate charge mobilities, high chemical stability, and optical transparency. Furthermore, these oxide films on indium tin oxide (ITO) can be used to fabricate “inverted” BHJ solar cell architectures in which electrons are collected at the ITO electrode,24−27 while in conventional BHJ solar cells, holes are collected at the ITO electrode. Fundamentally, the develop-

olar cells are a promising renewable energy source to meet increasing world energy demands.1−3 Recently, organic photovoltaic (OPV) cells with up to ∼9% power conversion efficiencies (PCEs) have been reported in the peer-reviewed literature,4−7 and furthermore, OPVs offer the attraction of lowcost manufacture on lightweight, mechanically flexible substrates over large areas by printing techniques.8−10 Currently, the highest efficiency solar cells utilize a solution-processed bulk heterojunction (BHJ) structure in which a polymeric donor and fullerene acceptor molecules form nanoscale interpenetrating hole and electron transporting networks, respectively.11,12 The large BHJ interfacial contact area facilitates exciton separation into free carriers at the donor− acceptor interfaces, thereby enhancing charge generation. Nonetheless, charge recombination is a significant loss mechanism both at the BHJ donor−acceptor interfaces13,14 and also at the active layer-electrode interfaces.15−20 Regarding the latter, interfacial layers (IFLs) between the active layer and the electrodes have been successfully implemented to block unwanted charges from recombining at this interface while © 2012 American Chemical Society

Received: September 13, 2012 Revised: November 20, 2012 Published: November 26, 2012 6315

dx.doi.org/10.1021/nl303419n | Nano Lett. 2012, 12, 6315−6321

Nano Letters

Letter

Figure 1. (a) SEM image of F16CuPc nanowires grown by VCR on ITO. Cross-section SEM images of (b) F16CuPc nanowires and (c) F16CuPc nanowires after removing the ZnO layer.

Figure 2. Two-dimensional GIXD images: (a) a F16CuPc film, (b) F16CuPc nanowires, and (c) ZnO-coated F16CuPc nanowires (F16CuPc NW/ ZnO) on ITO. GIXD reflections of F16CuPc nanowires (blue solid line), F16CuPc NW/ZnO (blue dashed line), F16CuPc film (red solid line), and ZnO coated F16CuPc film (F16CuPc film/ZnO, red dashed line) in (d) horizontal and (e) vertical cuts from the 2-D diffraction images (f) (010) peak (2.04 Å−1) peak intensity distribution according to angle from 2-D diffraction images of F16CuPc nanowires and F16CuPc NW/ZnO. Schematic view of molecular orientations in structures of (g) vacuum-evaporated F16CuPc film, (h) F16CuPc nanowires, and (i) F16CuPc NW/ZnO film structure on ITO.

6316

dx.doi.org/10.1021/nl303419n | Nano Lett. 2012, 12, 6315−6321

Nano Letters

Letter

Figure 3. (a) Schematic of a F16CuPc nanowire (NW) interfacial layer modified organic solar cell. (b) Schematic energy level diagram of an ITO/ F16CuPc/ZnO/P3HT:PCBM/MoO3/Al inverted solar cell. (c) Representative J−V data for type A (ITO/ZnO/P3HT:PCBM/MoO3/Al, black line), type B (ITO/F16CuPc film/ZnO/P3HT:PCBM/MoO3/ Al, blue line), and type C (ITO/F16CuPc NW/ZnO/P3HT:PCBM/MoO3/Al, red line) under white light illumination with 100mW/cm2 and A.M. 1.5. (d) Typical external quantum efficiency (EQE) spectra of the type A (black line), type B (blue line) and type C (red line) solar cells. (e) Representative J−V data for type A′ (ITO/ZnO/PTB7:PC71BM/MoO3/Al, black line), type B′ (ITO/F16CuPc film/ZnO/PTB7:PC71BM/MoO3/Al, blue line), and type C′ (ITO/F16CuPc NW/ZnO/PTB7:PC71BM/MoO3/Al, red line) under white light illumination with 100mW/cm2 and A.M. 1.5. (f) Typical external quantum efficiency (EQE) spectra of the type A′ (black line), type B′ (blue line), and type C′ (red line) solar cells.

including smooth thin films,32 nanobelts,33,34 and nanowires aligned preferentially to the substrate surface.35 In this contribution, structurally well-characterized F16CuPc nanowire (NW) arrays and thin films are inserted as IFLs between the ITO and ZnO layers of archetypical inverted bulk-heterojunction OPVs. We show that the F16CuPc nanowire IFLs significantly enhance the performance of both P3HT:PC61BM and PTB7:PC71BM OPVs over that of the F16CuPc thin films and other control device structures due to more favorable electron transport arising from the face-on alignment of the πstacked nanowires along the device current flow direction and the enhanced surface area of the F16CuPc nanowires embedded in the ZnO film. In this study, two types of F16CuPc IFLs were explored: (1) thermally evaporated F16CuPc thin films and 2) vaporizationcondensation-recrystallization (VCR)33,36,37-grown F16CuPc

ment of efficient inverted OPVs not only requires understanding charge selectivity at interfacial contacts but also developing diverse OPV architectures with appropriate BHJ active materials. To further improve the performance of inverted OPV cells, modified metal oxide layers have been investigated, such as embedding single-wall carbon nanotubes or polymers within the ZnO or TiOx film. Such hybrid ZnO IFLs enhance charge collection28,29and mechanical durability,28 as well as suppress leakage currents at the electrodes.30,31 We report here the implementation of copper hexadecafluorophthalocyanine (F16CuPc) nanowires as an IFL to enhance electron transport in inverted OPV cells. Importantly, F16CuPc has excellent environmental and thermal stability, and exhibits stable n-type semiconducting properties under ambient conditions.32,33 Depending on the deposition conditions, F16CuPc coatings can be grown in various morphologies, 6317

dx.doi.org/10.1021/nl303419n | Nano Lett. 2012, 12, 6315−6321

Nano Letters

Letter

Table 1. Photovoltaic Properties of Optimized P3HT:PC61BM Inverted Organic Solar Cellsa,b inverted OPV cell type type A: ITO/ZnO/P3HT:PC61BM/MoO3/Al type B: ITO/F16CuPc film/ZnO/P3HT:PC61BM/MoO3/Al type C: ITO/F16CuPc NW/ZnO/P3HT:PC61BM/MoO3/Al a

PCE (%)

Voc (V)

Jsc (cm2/mA)

FF (%)

3.09 3.04 (±0.11) 3.31 3.14 (±0.12) 3.59 3.37 (±0.15)

0.571 0.567 (±0.004) 0.572 0.569 (±0.004) 0.571 0.570 (±0.004)

9.04 9.12 (±0.29) 9.44 9.22 (±0.25) 10.74 9.82 (±0.45)

59.90 58.70 (±2.66) 61.20 59.57 (±1.73) 58.5 60.48 (±2.06)

Data and statistics based on 25 cells of each type. bNumbers in bold are the maximum recorded values.

nanowire films (Experimental Methods, Supporting Information, Figure S1). Figure 1 shows SEM images of the VCRderived F16CuPc nanowires on ITO, which are ∼15 nm in average diameter. The horizontal and vertical orientations of the nanowires reflect the random directional forces of recrystallization from the condensate.37 Thus, high Ar gas fluxes typically promote thin nanowire growth, while nanobelts or submicrometer belts grow under lower Ar flow rates of 100 sccm or 50 sccm, respectively.33 To characterize the present F16CuPc nanowire structures grown on ITO, grazing incidence X-ray diffraction (GIXD) patterns were acquired from the F16CuPc thin films (10 nm) and nanowires (Figure 2). Two features of the F16CuPc crystal structure (a = 14.61, b = 3.31 Å; γ = ∼90°)38 are assignable to the (100) (d = 14.4 ± 0.9 Å, q = 0.4 Å−1) and (010) (d = 3.2 ± 0.1 Å, q = 2.0 Å−1) reflections observed in the GIXD of both the F16CuPc thin films and nanowires (Figure 2d and e). Features at q = 1.6 Å−1 and q = 2.2 Å−1are assigned to the ITO substrates. The (100) and (010) reflections are related to the F16CuPc molecular width and intermolecular π−π stacking distance, respectively. However, in the evaporated F16CuPc films, the (010) peak is only observed in the 2D GIXD horizontal linecut (Figure 2d) and the (010) peak is only observed in the vertical linecut (Figures 2e). The horizontal linecut corresponds to the in-plane lattice spacing of the F16CuPc crystallites on the substrate plane while the vertical linecut corresponds approximately to the out-of-plane lattice spacing. Therefore, observing the (010) peak only in the horizontal linecut indicates near-exclusive edge-on π−π F16CuPc molecular stacking in the evaporated film (Figure 2g), while the prominent (100) and (010) F16CuPc nanowire reflections in both the horizontal and vertical linecuts indicate both edge-on and face-on π−π F16CuPc molecular stacking on the ITO surface (Figure 2h). These GIXD molecular orientation results on ITO are completely consistent with the SEM images (Figure 1) showing randomly oriented nanowires. To use the F16CuPc structures as IFLs in inverted OPV devices, a ZnO film is deposited on the structures by annealing of a spin-coated zinc acetate [Zn(CH3COO)2·2H2O] sol−gel solution film at 170 °C (See Experimental Methods in Supporting Information). Even after formation of the ZnO film, the structure of the F16CuPc nanowires is maintained (Figure 2c). However, the (010) peak intensity of F16CuPc nanowires is decreased at high angles close to 90° and increased at low angles closer to 0° (Figure 2f), which indicates the F16CuPc nanowires are somewhat more horizontally aligned and tilted after the ZnO coating (Figure 2i). The average thickness of the F16CuPc NW/ZnO films on ITO is ∼36 nm since the ZnO sol−gel solution spin-coating process causes the NW to tilt slightly, while the pristine ZnO film and the F16CuPc film (∼ 1 nm)/ZnO film thicknessses on ITO are approximately 23 and 24 nm, respectively, as determined by

profilometry and AFM. As further confirmation that the nanowires remain intact under the ZnO film, an aqueous 1 M NH3 solution was used to etch away the ZnO film. Following the ZnO removal, the SEM micrograph clearly shows that nanowires are still in place (Figure 1c). The F16CuPc films were next implemented as IFLs between the ITO and ZnO layers of inverted organic solar cells utilizing the structure (Figure 3a): ITO/ZnO/P3HT:PC61BM/MoO3/ Ag (OPV type A), since these are n-type organic semiconductors and able to form energetically favorable contacts with the ITO surface (Figure 3b).34,39,40 Inverted solar cells were fabricated with evaporated F16CuPc films (OPV type B: ITO/F16CuPc film/ZnO/P3HT:PC61BM/MoO3/Ag) and F16CuPc nanowire films (OPV type C: ITO/F16CuPc NW/ ZnO/P3HT:PC61BM/MoO3/Ag) (Figure 3a). Specifically, ZnO sol−gel films were spun-cast on the ITO glass substrates coated with F16CuPc films or F16CuPc nanowires and annealed at 170 °C for 5 min. Next, a blend of regioregular poly(3hexylthiophene)(P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM) was spun-cast from o-dichlorobenzene onto the ZnO film. After the P3HT:PC61BM deposition, the resulting film was slowly dried for 30 min and then annealed at 110 °C for 10 min. The P3HT:PC61BM active layer thickness in all devices was ∼210 nm. Finally, a 10 nm MoO3 film and 100 nm of Ag were deposited on the P3HT:PC61BM blend film by thermal evaporation in vacuum. Here, the MoO3 and Ag act as a hole transport layer and reflective anode, respectively. OPV response data were recorded on 25 separate specimens for each type of cell. Figure 3c shows representative OPV J−V measurements under white light illumination (100mW/cm2, AM 1.5). The power conversion efficiency (PCE) of the type A cell without F16CuPc is 3.09%, with an open circuit voltage (Voc) of 0.571 V, a short circuit current (Jsc) of 9.044 mA/cm2, and a fill factor (FF) of 59.9%. The most enhanced PCE, 3.59%, is measured for the F16CuPc NW-based type C cell (Voc = 0.571 V, Jsc = 10.74 mA/cm2, FF = 58.5%), exhibiting a 16% increase in PCE over the type A cells. Note in contrast that the type B cell (Voc = 0.572 V, Jsc = 9.441 mA/cm2, PCE = 3.31%, FF = 61.2%) incorporating a 1 nm thick vacuum-evaporated F16CuPc film marginally increases the PCE by 7% over the type A cell. Performance characteristics of the types A, B, and C inverted OPV cells and their statistics (of 25 cells of each type) are summarized in Table 1. Figure 3d shows typical external quantum efficiency (EQE) data for cell types A, B, and C. Note that the EQE response of the F16CuPc NW-based type C cell is significantly enhanced between about 350 and 650 nm. The highest measured EQE of the type C cell is 67% at 560 nm. Using thicker F16CuPc coatings results in lower PCEs and decreased Jsc (Figure S2, Supporting Information), likely a result of increased F16CuPc film optical absorption (Figure S3, Supporting Information). The optimized F16CuPc NW and 1 6318

dx.doi.org/10.1021/nl303419n | Nano Lett. 2012, 12, 6315−6321

Nano Letters

Letter

Table 2. Photovoltaic Properties of Optimized PTB7:PC71BM Inverted Organic Solar Cellsa,b inverted OPV cell type type A′: ITO/ZnO/PTB7:PC71BM/MoO3/Al type B′: ITO/F16CuPc film/ZnO/PTB7:PC71BM/MoO3/Al type C′: ITO/F16CuPc NWs/ZnO/PTB7:PC71BM/MoO3/Al a

PCE (%)

Voc (V)

Jsc (cm2/mA)

FF (%)

8.07 7.64 (±0.24) 8.26 7.99 (±0.25) 8.57 8.15 (±0.30)

0.744 0.735 (±0.005) 0.738 0.737 (±0.003) 0.736 0.735 (±0.004)

15.06 14.42 (±0.32) 15.23 14.52 (±0.34) 15.83 15.23 (±0.39)

72.04 71.95 (±1.26) 73.55 72.07 (±0.85) 73.85 72.39 (±0.76)

Data and statistics based on 25 cells of each type. bNumbers in bold are the maximum recorded values.

nm F16CuPc films do not reduce the optical transparency as significantly (Figure S3, Supporting Information). In addition, when the deposition order of the F16CuPc NW and ZnO IFLs is reversed, yielding the device structure: ITO/ZnO/F16CuPc NW/P3HT:PCBM/MoO3/Al, the cells exhibit decreased PCEs versus the type A cell (Figure S4, Supporting Information), because electron flow from the active layer to ITO is now energetically unfavorable. To determine whether the beneficial effects of the F16CuPc NW/ZnO IFLs can be applied to enhance other highly efficient OPV systems, devices utilizing a PTB7:PC71BM BHJ blend6 were also fabricated. Three kinds of OPV devices were again made: type A′ (ITO/ZnO/PTB7:PC71BM/MoO3/Ag), type B′ (ITO/F16CuPc film/ZnO/PTB7:PC71BM/MoO3/Ag), and type C′ (ITO/F16CuPc NW/ZnO/PTB7:PC71BM/MoO3/ Ag)) (Figure 3e). The PTB7:PC71BM BHJ blend films were ∼95 nm in thickness for all types of cells. Similar to P3HT:PC 61 BM cells, the inverted OPV devices using PTB7:PC71BM also show impressive improvements in device metrics, including PCE and Jsc enhancement, due to insertion of F16CuPc structures. In fact, type C′ shows the highest PCE (8.57%), whereas type B′ (PCE = 8.26%) and type A′ (PCE = 8.07%), exihibit lower device performance (Figure 3e and Table 2). The percentage increase in Jsc for these devices (∼6%) is somewhat lower than for the P3HT-based cells (∼ 16%) probably because the PTB7:PC71BM photocurrent is also generated at longer wavelengths where there is significant nanowires absorption. The Jsc increase is supported by the EQE enhancement data, and the F 16 CuPc/ZnO IFL based PTB7:PC71BM inverted solar cells show the highest EQE (76% at 620 nm, Figure 3f). Note that the PCE demonstrated here for the F16CuPc NW/ZnO IFL-based PTB7:PC71BM OPVs is among the highest values reported to date in the peerreviewed literature for a single-junction cell. The increased Jsc observed for both active layer materials makes a significant contribution to the enhanced PCE and indicates that the F16CuPc structures promote more favorable electron transport within the device. Moreover, the F16CuPc nanowire IFL OPVs exhibit significantly greater Jsc values than the vacuum-evaporated F16CuPc film OPVs. This is plausible considering the difference in F16CuPc molecular orientations, with the face-on π−π F16CuPc molecular stacking in the standing nanowires transporting the dissociated electrons from the active layer to the ITO electrode more efficiently than in edge-on π−π stacking.41,42 Because there is exclusively edge-on π−π stacking in the vacuum-evaporated F16CuPc films, electron transport to the ITO anode is less efficient. Furthermore, the nanowires can provide increased interfacial contact between the BHJ active layer and the electron transport layer by increasing the IFL surface roughness (Figure S5, Supporting Information). Optical reflectance measurements of the completed OPV devices using the PTB7:PC71BM active layer were next

obtained for all the IFL types (Figure S6, Supporting Information) in order to determine whether the observed performance differences result from optical field effect variations due to the different interlayers.43 From the reflectance measurements, it is determined that the F16CuPc NW IFL-based and the vacuum-evaporated F16CuPc film-based OPVs have nearly identical absorption characteristics. There are small differences when compared to the ZnO-only device, however, integrating the total absorbed light within the device and calculating an ideal Jsc leads to identical current densities (Figure S6, Supporting Information). Thus, these data demonstrate that the enhanced Jscs of the F16CuPc NW-based OPVs is primarily the result of favorable orientation and more efficient electron transport through the F16CuPc NW IFL. To further clarify the electron transport enhancement by the insertion of F16CuPc structures, zero-field vertical conductivities of the ZnO film, the ZnO film on the F16CuPc thin-film (F16CuPc film/ZnO), and the ZnO film on the F16CuPc NWs (F16CuPc NWs/ZnO) were measured (Figure 4). After fitting

Figure 4. Current density-electric field (J−E) plot of ZnO (black circles), F16CuPc film/ZnO (blue circles), and F16CuPc NW/ZnO (red circles). Inset image is the electrical device structure for vertical conductivity measurement.

the current density−electric field (J−E) plot to a Poole− Frenkel model44 as a function of the electric field (see Supporting Information), the vertical conductivities could then be extracted for the ZnO film ((1.8 ± 0.5) × 10−7 Sm−1), F16CuPc film/ZnO ((2.6 ± 0.5) × 10−7 Sm−1), and F16CuPc NW/ZnO ((7.7 ± 0.8) × 10−7 Sm−1). Clearly, the F16CuPc NW/ZnO film exhibits the highest vertical conductivity among the three kinds of IFL film structures, which strongly supports the fact that the F16CuPc NW/ZnO IFLs most efficiently enhances the Jsc in the inverted OPVs due to the partial π-faceon alignment in the current flow direction and the increased interfacial surface area. In summary, F16CuPc films and nanowires were grown by thermal evaporation and VCR, respectively, and inserted 6319

dx.doi.org/10.1021/nl303419n | Nano Lett. 2012, 12, 6315−6321

Nano Letters

Letter

(6) Liang, Y. Y.; Xu, Z.; Xia, J. B.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright FutureBulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%. Adv. Mater. 2010, 22, E135−E138. (7) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Polymer Solar Cells with Enhanced Open-Circuit Voltage and Efficiency. Nat. Photonics 2009, 3, 649−653. (8) Facchetti, A. π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications. Chem. Mater. 2011, 23, 733−758. (9) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated PolymerBased Organic Solar Cells. Chem. Rev. 2007, 107, 1324−1338. (10) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Plastic Solar Cells. Adv. Funct. Mater. 2001, 11, 15−26. (11) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. High-Efficiency Solution Processable Polymer Photovoltaic Cells by Self-Organization of Polymer Blends. Nat. Mater. 2005, 4, 864−868. (12) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789− 1791. (13) Servaites, J. D.; Ratner, M. A.; Marks, T. J. Bulk Heterojunction Organic Solar Cells: A New Look at Traditional Models. Energy Environ. Sci. 2011, 4, 4410−4422. (14) Pal, S. K.; et al. Geminate Charge Recombination in Polymer/ Fullerene Bulk Heterojunction Films and Implications for Solar Cell Function. J. Am. Chem. Soc. 2010, 132, 12440−12451. (15) Irwin, M. D.; et al. Structural and Electrical Functionality of NiO Interfacial Films in Bulk Heterojunction Organic Solar Cells. Chem. Mater. 2011, 23, 2218−2226 and references therein. (16) Hains, A. W.; Ramanan, C.; Irwin, M. D.; Liu, J.; Wasielewski, M. R.; Marks, T. J. Designed Bithiophene-Based Interfacial Layer for High-Efficiency Bulk-Heterojunction Organic Photovoltaic Cells. Importance of Interfacial Energy Level Matching. ACS Appl. Mater. Interfaces 2010, 2, 175−185 and references therein. (17) Hains, A.; Liu, J.; Martinson, A. B. F.; Irwin, M. D.; Marks, T. J. Anode Interfacial Tuning via Electron-Blocking/Hole-Transport Layers and Indium Tin Oxide Surface Treatment in BulkHeterojunction Organic Photovoltaic Cells. Adv. Funct. Mater. 2010, 20, 595−606. (18) Gomez, E. D.; Loo, Y. L. Engineering the Organic Semiconductor-Electrode Interface in Polymer Solar Cells. J. Mater. Chem. 2010, 20, 6604−6611. (19) Irwin, M. D.; Liu, J.; Leever, B. J.; Hersam, M. C.; Marks, T. J. Consequences of Anode Interfacial Layer Deletion. Modifications and P3HT:PCBM-Based Bulk-Heterojunction Organic Photovoltaic Device Performance. Langmuir 2010, 26, 2584−2591. (20) Irwin, M. D.; Buchholz, B.; Hains, A. W.; Chang, R. P. H.; Marks, T. J. p-Type Semiconducting Nickel Oxide as an EfficiencyEnhancing Anode Interfacial Layer in Polymer Bulk-Heterojunction Solar Cells. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2783−2787. (21) Kawano, K.; Pacios, R.; Poplavskyy, D.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R. Improvement of a-Si Solar Cell Properties by Using SnO2:F TCO Films Coated with an Ultra-Thin TiO2 Layer Prepared by APCVD. Sol. Energy Mater. Sol. Cells 2006, 90, 3514− 3520. (22) Veinot, J. G. C.; Marks, T. J. Toward the Ideal Organic LightEmitting Diode: The Versatility and Utility of Interfacial Tailoring by Siloxane Self-Assembly. Acc. Chem. Res. 2005, 38, 632−643. (23) Yan, H.; Lee, P.; Armstrong, N. R.; Graham, A.; Evmenenko, G. A.; Dutta, P.; Marks, T. J. High-Performance Hole-Transport Layers for Polymer Light-Emitting Diodes. Implementation of Organosiloxane Cross-Linking Chemistry in Polymeric Electroluminescent Devices. J. Am. Chem. Soc. 2005, 127, 3172−3183. (24) Sun, Y.; Seo, J. H.; Takacs, C. J.; Seifer, J.; Heeger, A. J. Inverted Polymer Solar Cells Integrated with a Low-Temperature-Annealed Sol-Gel-Derived ZnO Film as an Electron Transport Layer. Adv. Mater. 2011, 23, 1679−1683.

between the ZnO layer and ITO electrode of inverted BHJ OPVs to reduce the energetic loss associated with unfavorable electron transport continuity and to increase the interfacial area. Among the types of inverted organic solar cells investigated, type A or A′ (ITO/ZnO/Active layer/MoO3/Al), type B or B′ (ITO/F16CuPc film/ZnO/Active layer/MoO3/Al) and type C or C′ (ITO/F16CuPc NW/ZnO/Active layer/MoO3/Al), the F16CuPc NW/ZnO based cells (type C or C′) consistently exhibit the highest performance metrics of this study with PCE = 3.59% and EQE = 67% for P3HT:PC61BM, and PCE = 8.57% and EQE = 76% for PTB7:PC71BM. The F16CuPc NWs IFLs are proposed to more efficiently collect and transport electrons from the ZnO layer due to their face-on π−π stacking orientation parallel to the current flow and the increased surface area of the ZnO film. These n-type semiconducting organic structures can therefore be applied as buffer layers to reduce OPV energy losses, illustrating how organic materials can take advantage of controlled molecular structure and morphology to efficiently collect electrons.

■

ASSOCIATED CONTENT

S Supporting Information *

Description of the synthesis of materials, characterization of F16CuPc structures and device fabrication, and vertical conductivity measurement on ZnO films. This material is available free of charge via the Internet at http://pubs.acs.org

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: (L.X.C.) [email protected]; (A.F.) a-facchetti@ northwestern.edu; (T.M.) [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by the ANSER Center, 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-SC0001059 (S.J.L.), by the U.S. Department of Energy, Basic Energy Sciences under grant no. DE-FG02-08ER46536 (S.M.Y., J.S.), and by the NSF Center for Layered Polymeric Systems (CLiPS) under Grant DMR0423914 (S.C.L.). The Institute for Sustainability and Energy at Northwestern (ISEN) provided partial equipment funding. S.M.Y. thanks the Creative Research Initiatives Program (Functional X-ray Imaging) of MEST/KOSEF for partial fellowship support. The authors also thank S. Lubin for assistance with the reflectance measurements.

■

REFERENCES

(1) Espinosa, N.; Hösel, M.; Angmo, D.; Krebs, F. C. Solar Cells with One-Day Energy Payback for the Factories of the Future. Energy Environ. Sci. 2012, 5, 5117−5132. (2) Morton, O. Solar Energy: A New Day Dawning? Silicon Valley Sunrise. Nature 2006, 443, 19−22. (3) Sun, S. S.; Sariciftci, N. S. In Organic Photovoltaics, CRC press. 2005 by Taylor and Francis group, LLC. (4) Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.; Tsang, S. W.; Lai, T. H.; Reynolds, J. R.; So, F. High-efficiency inverted dithienogermole−thienopyrrolodione-based polymer solar cells. Nat. Photonics 2012, 6, 115−120. (5) Service, R. F. Outlook Brightens for Plastic Solar Cells. Science 2011, 332, 293. 6320

dx.doi.org/10.1021/nl303419n | Nano Lett. 2012, 12, 6315−6321

Nano Letters

Letter

(25) Kyaw, A. K. K.; Sun, X. W.; Jiang, C. Y.; Lo, G. Q.; Zhao, D. W.; Kwong, D. L. An Inverted Organic Solar Cell Employing a Sol-Gel Derived ZnO Electron Selective Layer and Thermal Evaporated MoO3 Hole Selective Layer. Appl. Phys. Lett. 2008, 93, 221107. (26) White, M. S.; Olson, D. C.; Shaheen, S. E.; Kopidakis, N.; Ginley, D. S. Inverted Bulk-Heterojunction Organic Photovoltaic Device Using a Solution-Derived ZnO Underlayer. Appl. Phys. Lett. 2006, 89, 143517. (27) Waldauf, C.; Morana, M.; Denk, P.; Schilinsky, P.; Coakley, K.; Choulis, S. A.; Brabec, C. J. Highly Efficient Inverted Organic Photovoltaics Using Solution Based Titanium Oxide as Electron Selective Contact. Appl. Phys. Lett. 2006, 89, 233517. (28) Shim, W. H.; Park, S. Y; Park, M. Y.; Seo, H. O.; Kim, K. D.; Kim, Y. T.; Kim, Y. D.; Kang, J. W.; Lee, K. H.; Jeong, Y.; Kim, Y. D.; Lim, D. C. Multifunctional SWCNT-ZnO Nanocomposites for Enhancing Performance and Stability of Organic Solar Cells. Adv. Mater. 2011, 23, 519−522. (29) Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.; Tsang, S. W.; Lai, T. H.; Reynolds, J. R.; So, F. High-efficiency inverted dithienogermole−thienopyrrolodione-based polymer solar cells. Nat. Photon. 2012, 6, 115−120. (30) Steim, R.; Choulis, S. A.; Schilinsky, P.; Brabec, C. J. Interface Modification for Highly Efficient Organic Photovoltaics. Appl. Phys. Lett. 2008, 92, 093303. (31) Motiei, L.; Yao, Y.; Choudhury, J.; Yan, H.; Marks, T. J.; van der Boom, M. E.; Facchetti, A. Self-Propagating Molecular Assemblies as Interlayers for Efficient Inverted Bulk-Heterojunction Solar Cells. J. Am. Chem. Soc. 2010, 132, 12528−12530. (32) Bao, Z.; Lovinger, A. J.; Brown, J. New Air-Stable n-Channel Organic Thin Film Transistors. J. Am. Chem. Soc. 1998, 120, 207−208. (33) Yoon, S. M.; Song, H. J.; Hwang, I. C.; Kim, K. S.; Choi, H. C. Single Crystal Structure of Copper Hexadecafluorophthalocyanine (F16CuPc) Ribbon. Chem. Commun. 2010, 46, 231−233. (34) Tang, Q.; Li, H.; Liu, Y.; Hu, W. High-Performance Air-Stable n-Type Transistors with an Asymmetrical Device Configuration Based on Organic Single-Crystalline Submicrometer/Nanometer Ribbons. J. Am. Chem. Soc. 2006, 128, 14634−14639. (35) Mbenkum, B. N.; Barrena, E.; Kelsch, M.; Dosch, H. Selective Growth of Organic 1-D Structures on Au Nanoparticle Arrays. Nano Lett. 2006, 6, 2852−2855. (36) Yoon, S. M.; Hwang, I. C.; Kim, K. S.; Choi, H. C. Synthesis of Single-Crystal Tetra(4-pyridyl)porphyrin Rectangular Nanotubes in the Vapor Phase. Angew. Chem., Int. Ed. 2009, 48, 2506−2509. (37) Yoon, S. M.; Hwang, I. C.; Shin, N.; Ahn, D.; Lee, S. J.; Lee, J. Y.; Choi, H. C. Vaporization-Condensation-Recrystallization ProcessMediated Synthesis of Helical m-Aminobenzoic Acid Nanobelts. Langmuir 2007, 23, 11875−11882. (38) de Oteyza, D. G.; Barrena, E.; Ossó, O.; Sellner, S.; Dosch, H. Thickness-Dependent Structural Transitions in Fluorinated Copperphthalocyanine (F16CuPc) Films. J. Am. Chem. Soc. 2006, 128, 15052− 15053. (39) Szarko, J. M.; Guo, J.; Liang, Y.; Lee, B.; Rolczynski, B. S.; Strzalka, J.; Xu, T.; Loser, S.; Marks, T. J.; Yu, L.; Chen, L. X. When Function Follows Form: Effects of Side-Chains in Conjugated Copolymers on Film Morphology and BHJ Solar Cell Performance. Adv. Mater. 2010, 22, 5468−5472. (40) Shen, C.; Kahn, A. Electronic Structure, Diffusion, and pDoping at the Au/F16CuPc Interface. J. Appl. Phys. 2001, 90, 4549− 4554. (41) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Bréas, J. L. Charge Transport in Organic Semiconductors. Chem. Rev. 2007, 107, 926−952. (42) Marks, T. J. Interfaces Between Molecular and Polymeric “Metals”. Electrically Conductive, Structure-Enforced Assemblies of Metallo macrocycles. Angew. Chem., Int. Ed. 1990, 29, 857−9007 and references therein. (43) Kim, J. Y.; Kim, S. H.; Lee, H.-H.; Lee, K.; Ma, W.; Gong, X.; Heeger, A. J. New Architecture for High-Efficiency Polymer

Photovoltaic Cells Using Solution-Based Titanium Oxide as an Optical Spacer. Adv. Mater. 2006, 18, 572−576. (44) Sze, S. M. Physics of Semiconductor Devices; Wiley: New York, 1981.

6321

dx.doi.org/10.1021/nl303419n | Nano Lett. 2012, 12, 6315−6321