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1 REPORT DOCUMENTATION PAGE Form Approved OMB NO The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggesstions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA, Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any oenalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 1. REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE New Reprint 4. TITLE AND SUBTITLE Nanomorphology Characteristics of Thermally AnnealedPre - Encapsulated P3HT:PCBM Thin Films UsingAtomic Force Microscopy 6. AUTHORS Gabriel Calderón Ortiz, Josee Vedrine-Pauléus, Hector M. Carrasco 5a. CONTRACT NUMBER W911NF b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER d. PROJECT NUMBER 5e. TASK NUMBER 3. DATES COVERED (From - To) - 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAMES AND ADDRESSES University of Puerto Rico at Humacao Physics and Electronics Call Box 860 Humacao, PR SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS (ES) U.S. Army Research Office P.O. Box Research Triangle Park, NC DISTRIBUTION AVAILIBILITY STATEMENT Approved for public release; distribution is unlimited. 8. PERFORMING ORGANIZATION REPORT NUMBER 10. SPONSOR/MONITOR'S ACRONYM(S) ARO 11. SPONSOR/MONITOR'S REPORT NUMBER(S) MS-REP SUPPLEMENTARY NOTES The views, opinions and/or findings contained in this report are those of the author(s) and should not contrued as an official Department of the Army position, policy or decision, unless so designated by other documentation. 14. ABSTRACT Thermal annealing of the bulk heterojunction (BHJ) has implications on device performance and probing the nanomorphology can provide insight on structural ordering in the organic polymer mix. In this study we investigated the morphology of BHJ thin films before deposition of the cathode layer using atomic force microscopy (AFM). The active region is composed of a blend of electrondonor poly(3-hexylthiophene) (P3HT), and electron acceptor phenyl-c61-butyric acid methyl ester (PCBM). The P3HT:PCBM thin films depict a network of well-dispersed nanofibrils at room temperature, 15. SUBJECT TERMS AFM, Thin-Film, Thermal Annealing, Organic Solar Cells 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF a. REPORT b. ABSTRACT c. THIS PAGE ABSTRACT UU UU UU UU 15. NUMBER OF PAGES 19a. NAME OF RESPONSIBLE PERSON Luis Rosa 19b. TELEPHONE NUMBER Standard Form 298 (Rev 8/98) Prescribed by ANSI Std. Z39.18

2 Report Title Nanomorphology Characteristics of Thermally AnnealedPre -Encapsulated P3HT:PCBM Thin Films UsingAtomic Force Microscopy ABSTRACT Thermal annealing of the bulk heterojunction (BHJ) has implications on device performance and probing the nanomorphology can provide insight on structural ordering in the organic polymer mix. In this study we investigated the morphology of BHJ thin films before deposition of the cathode layer using atomic force microscopy (AFM). The active region is composed of a blend of electrondonor poly(3-hexylthiophene) (P3HT), and electron acceptor phenyl-c61-butyric acid methyl ester (PCBM). The P3HT:PCBM thin films depict a network of well-dispersed nanofibrils at room temperature, but these networks aggregate in clusters with increasing annealing temperature. The surface roughness of films annealed at 140 "C is significantly higher than films annealed at both 100 "C and 120 "C, which demonstrate that the driving force for surface roughness has an annealing temperature threshold beyond which it is essential to achieve a considerable increase in efficiency. Although the driving force that contributes to the aggregation of the P3HT fibril network is not well understood, the contributing factors due to higher temperature annealing either before or after encapsulation of these networks have a dominant effect on overall device performance.

3 REPORT DOCUMENTATION PAGE (SF298) (Continuation Sheet) Continuation for Block 13 ARO Report Number MS-REP Nanomorphology Characteristics of Thermally A... Block 13: Supplementary Note Published in Journal of Nano Energy and Power Research, Vol. Ed. 0 3, (1) (2014), (, (1). DoD Components reserve a royalty-free, nonexclusive and irrevocable right to reproduce, publish, or otherwise use the work for Federal purposes, and to authroize others to do so (DODGARS 32.36). The views, opinions and/or findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other documentation. Approved for public release; distribution is unlimited.

4 Copyright 2015 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nano Energy and Power Research Vol. 3, 1 4, 2015 Nanomorphology Characteristics of Thermally Annealed Pre -Encapsulated P3HT:PCBM Thin Films Using Atomic Force Microscopy Gabriel Calderón Ortiz 1,JoseeVedrine-Pauléus 1 2,andHectorM.Carrasco 1 1 Department of Physics and Electronics, University of Puerto Rico at Humacao Call Box 860, Humacao, Puerto Rico 00792, USA 2 Institute of Functional Nanomaterials (IFN), Resource Center for Science and Engineering, University of Puerto Rico, San Juan, Puerto Rico , USA Thermal annealing of the bulk heterojunction (BHJ) has implications on device performance and probing the nanomorphology can provide insight on structural ordering in the organic polymer mix. In this study we investigated the morphology of BHJ thin films before deposition of the cathode layer using atomic force microscopy (AFM). The active region is composed of a blend of electrondonor poly(3-hexylthiophene) (P3HT), and electron acceptor phenyl-c61-butyric acid methyl ester (PCBM). The P3HT:PCBM thin films depict a network of well-dispersed nanofibrils at room temperature, but these networks aggregate in clusters with increasing annealing temperature. The surface roughness of films annealed at 140 Cissignificantlyhigherthanfilmsannealedatboth100 Cand 120 C, which demonstrate that the driving force for surface roughness has an annealing temperature threshold beyond which it is essential to achieve a considerable increase in efficiency. Although the driving force that contributes to the aggregation of the P3HT fibril network is not well understood, the contributing factors due to higher temperature annealing either before or after encapsulation of these networks have a dominant effect on overall device performance. Keywords: AFM, Thin-Film, Thermal Annealing, Organic Solar Cells. 1. INTRODUCTION Organic polymers are studied as alternative materials to construct photovoltaic (PV) cells because they have conductive properties that are tunable based on their chemistries. 1 3 In particular, the bulk heterojunction (BHJ) structure is studied extensively because it provides enhanced properties for efficient charge transport between donor and acceptor molecular interaction as the diffusion length scale is reduced down to 10 s of nm. 4 7 Processing conditions and composition of the organic polymer active layer, such as annealing time, annealing temperature, solvent type, solvent to polymer ratio, and polymer donor-toacceptor blend ratio are studied rigorously to characterize PV device performance. 8 Improved efficiency of annealed BHJ films has been shown to result from increase ordering of P3HT domain, and an increase in the RMS roughness or coarsening that suppresses charge recombination; 9 10 furthermore, inducing thermal treatment to the BHJ blend facilitates charge transport. Annealing of P3HT:PBCM blends even at 3 min revealed polycrystalline ordering of the P3HT, with an amorphous PCBM. As annealing time Author to whom correspondence should be addressed. increases, the crystal ordering of both P3HT and PCBM are assumed to be present. Hole transport increases with crystalline ordering of the P3HT although PCBM tends to form large crystallites or PCBM aggregation (on the scale 1 mm)between20mintoseveralhoursofannealing. 11 Various publications have documented improved photovoltaic efficiency due to thermal annealing of the BHJ, but the majority of the literature is focused on post-processing encapsulation to enhance device performance When comparing device performance between pre- and postencapsulation annealing, Ntwaeaborwa et al. revealed that post-annealed solar cells were nearly two times more efficient than devices annealed before depositing the top aluminum electrode; the authors argued that the improved efficiency was attributed to increase roughness at the interface between the photoactive layer and the aluminum top contact. While this interaction at the interface leads to improved light harvesting in the BHJ layer and helps prevent shunt paths from forming, no analysis of the morphological contribution was performed with respect to device efficiency. 15 However, AFM probing of annealed films before encapsulation can provide further information on film behavior when not confined to a top metal contact. J. Nano Energy Power Res. 2015, Vol. 3, No /2015/3/001/004 doi: /jnepr

5 Nanomorphology Characteristics of Thermally Annealed Pre-Encapsulated P3HT:PCBM Thin Films Using AFM Ortiz et al. Li et al. depicts the AFM images P3HT/PCBM composite films when annealed, but the distinct morphology or structural composition does not show network variations with respect to annealing time for the film. 16 In this work, the aggregation of P3HT fibrils in the PCBM networks, the orientation distribution of fibrils, and the contribution of a much rougher surface at increased annealing temperatures are shown to be the dominant factors towards enhanced electron transfer properties and better photovoltaic device performance, irrespective of encapsulation. Kim and co-workers demonstrated that a cathode layer deposited on P3HT:PCBM thin film lessens the preferential orientation and total crystallinity of the P3HT crystals during thermal annealing, aiding in the formation of interpenetrating networks. 17 We analyzed the film morphology of P3HT:PCBM BHJ thin after annealing at three temperatures for 20 min and before deposition of aluminum layer. The I V characteristics of fabricated solar cells were reported. 18 The solar cells were fabricated with a blend of P3HT:PCBM; as the electron donor we used poly(3-hexylthiophene) (P3HT), and phenyl-c61-butyric acid methyl ester (PCBM) as the electron acceptor. This blend was deposited on an indium tin oxide (ITO) coated-glass substrate as the anode, and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as a passivation, and hole transport layer. We analyzed the effect of annealing at different temperatures while keeping the time of anneal constant. Using atomic force microscopy (AFM), we analyzed the assembly of interpenetration network of P3HT fibrils and PCBM networks when annealed for 20 min before depositing a cathode layer. AFM analysis provides a morphological aspect of the polymer network arrangement and profile when annealed before cathode confinement. 2. MATERIAL AND METHODS Indium tin oxide (ITO) coated glass (R s = 10 /sq, Nanocs, Inc.) were etched and patterned into multiple active areas. The ITO substrates were etched using a solution of HCl and zinc powder around the areas covered with adhesive tape. 19 The tape was removed. The substrates were then sonicated in acetone, followed by ethanol, both for 30 min at room temperature, and dried with N 2. AfilmofPEDOT:PSS(Sigma-Aldrich,Inc.)highconductivity grade was spin-coated on ITO substrates at 5000 rpm for 60 s; substrate were placed on a hot plate for 5 min. at 65 Cinambientatmospheretodry.Solarcelldevices were fabricated using P3HT-001-EE, regioregular (Rieke Metals, Inc.) and PCBM (Sigma-Aldrich, Inc.) in a using a 1:1 (w/w) mixture and dissolved in 1 ml of dichlorobenzene solvent. The solution was stirred for 60 min at room temperature for the polymers to dissolve. To form the active layer, the polymer mixture was spin coated on the substrate at 600 rpm for 60 s ( 200 nm film thickness measure by AFM). The substrates were annealed at temperatures of 100 C, 120 C, and 140 Cfor20min in nitrogen atmosphere. The controls for each annealed sample were also prepared in parallel for comparison and for AFM analysis. The top contact of Al 90 nm was evaporated on sample substrates through a shadow masks inside a vacuum thermal evaporator with a chamber pressure 10 6.TheITOcoatedglasswasetched,patterned, and cut to fit 0.5 and 1.0 cm 2 device active areas. The current density voltage (J V properties of the solar devices were analyzed using a semiconductor analyzer characterization system (Keithley 4200-SMU) under a simulated solar light source (Newport, Inc.), under Air Mass (AM) 1.5 solar reference spectrum at room temperature. 3. RESULTS AND DISCUSSION Two sets of substrates were annealed simultaneously for each thermal treatment at 100 C, 120 C, and 140 C before deposition of Al electrode. The surface morphology of controlled substrates was characterized with AFM at each temperature, while the second set of substrates were used to fabricate solar cell devices. UV-vis spectra was performed to examine theopticalabsorptionof the active layer. Figure 1 depicts the UV-Vis absorbance spectra of P3HT:PCBM composite thin films deposited on ITO/PEDOT:PSS substrates before, and after annealing at the respective temperatures. The UV-Vis reveals three dominant peaks at 515, 550, 600 nm. The peak at 515 nm depicts the transitions on the conjugated backbone. The peaks at 550 and 60 nm correspond to the two vibronic shoulder peaks from intermolecular stacking between thiophene rings, and are associated with P3HT aggregation from thermal annealing, 20 where peaks below 510 nm corresponds to PCBM, and peaks around 550 and 600 nm to that of P3HT. The maximum absorption is achieved at 140 C, and we see an enhancement in absorption withincreasingannealing Absorbance (a.u) Wavelength (nm) 140 ºC 120 ºC 100 ºC Not annealed Fig. 1. UV-Vis absorbance spectra of P3HT:PCBM thermally annealed films; room temperature (not annealed), 100 Canneal,120 Canneal, 140 Canneal,allfor20mininsideambientatmosphere,before aluminum cathode deposition. 2 J. Nano Energy Power Res. 3, 1 4, 2015

6 Ortiz et al. Nanomorphology Characteristics of Thermally Annealed Pre-Encapsulated P3HT:PCBM Thin Films Using AFM Fig. 2. AFM images of the surface morphologies of P3HT:PCMB bulk heterojunction thin films: (a) room temperature, not annealed; (b) 100 C anneal; (c) 120 Canneal;(d)140 Canneal,allfor20mininsideambientatmosphere,before aluminum deposition. temperature. The enhancement inabsorptionforannealed films was shown to be due to a reduction in reflection at the interface between PEDOT:PSS and P3HT:PCBM. As aresultmorelightcantransmit,ifthereflectionatthe interface is reduced, and the index of refraction of the P3HT:PCBM composite films decreases as annealing temperature increases. As a result, absorption is increased with ordering of the P3HT nanofibrils, and enhances absorption in the composite layer, and reducesreflectanceatthe interface. 21 The morphology of the P3HT:PCBM films was examined using atomic force microscopy (AFM). The AFM images allow us to gain further insight of the thermally induced phase segregation of the BHJ network. The images represented in Figure 2 depicts the variation in surface topography for unannealed films Figure 2(a), and films annealed for 20 min before aluminum deposition at 100 CinFigure2(b);120 CinFigure2(c);and140 C in Figure 2(d). The resulting surface roughness for each film as a function of thermal treatment was measured. BHJ films annealed at 100 C, and 120 C show surface roughness R q (root mean square (rms)) of 3.31 nm and 3.94 nm, respectively. We observed that increasing the annealing temperature beyond 120 C resulted in a substantial increase in the measured surface roughness as depicted in the AFM image analysis.filmsannealed at 140 C resulted in a substantial increase in surface roughness rms = 21 7 nm.thesubstantialincreaseinsurface roughness suggests that high temperature annealing at 140 C or slightly higher improves device performance. The work by Kingsley and co-workers revealed that annealing films from 140 Cto160 Cfor60min after deposition of the top Al layer provides an optimum efficiency of 4.4% at a temperature of 150 C. 3 Huang and coworkers revealed that efficiency improved with increased annealing time; annealing for a 30 min resulted in 2.308% efficiency, but increasing the time to 60 min resulted in areductionindeviceefficiencybyathird. 22 Overall, the contributing factors to improved device performance is directed to the aggregation of fibrilar networks when the BHJ film is confined between metallic layers, as well as the contribution of a rougher surface which acts to enhance electron transfer properties of the photovoltaic device. 23 In our analysis of the surface roughness properties for annealed BHJ films before Al deposition, we demonstrate that annealing at or above 120 Ciscriticaltoimproving device performance. Further increase in temperature from 120 Cto140 Cresultedinoverfivetimesincreasein surface roughness. In addition, it appears that P3HT fibrils aggregate in bundles with the fullerene as the annealing temperature increases. This bundle formation or grain features could indicate a reduction in exciton recombination, as a direct result of increased surface roughness. 28 Overall, improvement in device performance at 140 C is attributed primarily to the increase in R q.ourresults are similar to those reported in Refs. [24 27]. The large grain features augment as the temperature increased, especially for 140 C as depicted in Figure 2(d). Device J. Nano Energy Power Res. 3, 1 4,

7 Nanomorphology Characteristics of Thermally Annealed Pre-Encapsulated P3HT:PCBM Thin Films Using AFM Ortiz et al. efficiency has been shown to improve up to 150 C, 29 but PCE decreases slightly beyond that critical temperature because larger grain features produce longer conduction paths, much longer than the diffusion lengths of charge carriers ( 10 nm). Past work on these fullerene networks have shown that trap distribution in devices is broader for devices annealed before Al deposition, indicating that annealing after the Al is deposited eliminates some charge traps. Ayzner and co-workers haveshownthattheelimination of traps at the organic-cathode interface is what enables additional charge carriers to contribute to increase photocurrent and device efficiency. 30 Films annealed at 140 C clearly depict a phase segregation and formation of large PCBM crystallites; thus, increasing the average distance between polymer and fullerene molecules. The size of crystallites perhaps reach an upper limit in the range of 150 C; beyond this temperature the effective average distance between the polymer and fullerene molecules becomes larger than the percolation paths of the fullerene in the BHJ network. This percolation length slightly decreases when the top cathode is deposited before annealing, allowing more charge carriers to contribute to the current generated, and enhancing device efficiency. 4. CONCLUSIONS In this work we used AFM technique to analyze the effect of thermal annealing on the nanoscale morphology of the bulk heterojunction P3HT:PCBM layer annealed at different temperatures while maintaining the annealing time constant before we deposit the aluminum cathode layer. We conclude that controlling the size of crystallites, and relative amount and distribution of P3HT fibrils in the network is to be within the limit of the percolation pathways; this is critical to engineering better polymer-based BHJ solar cells. An optimal size of crystallites appears to be predefined when thermal annealing is induced preencapsulated or after cathode deposition. Therefore, it is crucial to balance the blend between the P3HT fibril formations, which contribute to hole mobility in the device, and PCBM that contribute to the formation of large grain features that hamper or limit the contribution of charge carriers to photocurrent for enhanced device performance. Acknowledgments: This work was funded by the National Science Foundation PENN-UPR Partnership for Research and Education in Materials under Grant No. NSF-DMR ; and by NSF-EPS (IFN). The authors would like to thank Dr. C. Kagan and Mr. Earl (E.D.) Goodwin at UPENN for additional help with device analysis. Group member Jorge Marcano for training on UV-Vis; and Dr. L. G. Rosa and Ms. Kety Jimenez at UPRH for use of the evaporator instrument. References and Notes 1. G. Dennler, M. C. Scharber, and C. J. Brabec, Adv. Mater. 21, 1323 (2009). 2. S. Gunes, H. 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Tolbert, andb. Schwartz, The Journal of Physical Chemistry 11248, (2008). Received: 27 October Accepted: 23 October J. Nano Energy Power Res. 3, 1 4, 2015