We have developed a method to improve the short circuit current density in copper phthalocyanine (CuPc)/fullerene (C60) organic solar cells by ~60% by modifying the CuPc crystal orientation through use of a molecular interlayer to maximize charge transport in the direction between the two electrodes. Powder x-ray diffraction and electronic absorption spectroscopy show that a thin 3,4,9,10-perylenetetracarboxylic dianhydride interlayer before CuPc growth templates the CuPc film structure, forcing the molecules to lie flat with respect to the substrate surface, although the intrastack orientation is unaffected. This modified stacking configuration facilitates charge transport and improves charge collection

A. J. Ferguson

El-Nahass M. M.

P. Sullivan

S. Heutz

T. S. Jones

English

Sullivan, Paul J.

Jones, T. S. (Tim S.)

Ferguson, A. J.

Heutz, S.

Warwick Research Archives Portal Repository

  University of Warwick institutional repository: http://go.warwick.ac.uk/wrapThis paper is made available online in accordance with publisher policies. Please scroll down to view the document itself. Please refer to the repository record for this item and our policy information available from the repository home page for further information.  To see the final version of this paper please visit the publisher’s website. Access to the published version may require a subscription.  Author(s): P. Sullivan, T. S. Jones, A. J. Ferguson and S. HeutzArticle Title:   Structural templating as a route to improved photovoltaic performance in copper phthalocyanine/fullerene (C60) heterojunctions Year of publication: 2007 Link to published version: http://dx.doi.org/ 10.1063/1.2821229 Publisher statement: None    Structural templating as a route to improved photovoltaic performance incopper phthalocyanine/fullerene „C60… heterojunctionsP. Sullivan and T. S. JonesaDepartment of Chemistry, University of Warwick, Coventry, CV4 7AL, United KingdomA. J. FergusonDepartment of Chemistry, Imperial College London, London SW7 2AZ, United KingdomS. HeutzLondon Centre for Nanotechnology, Department of Materials, Imperial College London, London SW7 2AZ,United KingdomReceived 9 August 2007; accepted 13 November 2007; published online 6 December 2007We have developed a method to improve the short circuit current density in copper phthalocyanineCuPc/fullerene C60 organic solar cells by 60% by modifying the CuPc crystal orientationthrough use of a molecular interlayer to maximize charge transport in the direction between the twoelectrodes. Powder x-ray diffraction and electronic absorption spectroscopy show that a thin3,4,9,10-perylenetetracarboxylic dianhydride interlayer before CuPc growth templates the CuPcfilm structure, forcing the molecules to lie flat with respect to the substrate surface, although theintrastack orientation is unaffected. This modified stacking configuration facilitates charge transportand improves charge collection. © 2007 American Institute of Physics. DOI: 10.1063/1.2821229Considerable effort is being directed at the developmentof small-molecule organic photovoltaic OPV devices usingmaterials such as copper phthalocyanine CuPc as both thelight absorbing and hole-transport layer.1–3 Fullerene C60has become the preferred electron acceptor and it has beenshown that cell structures containing mixed donor-acceptorlayers, fabricated by codeposition of CuPc and C60, showsignificant improvement in device performance over simplebilayer heterojunctions.4–6 Although CuPc /C60 OPV deviceshave reached power conversion efficiencies p of 5%,5their performance remains limited by the low exciton diffu-sion length of CuPc and poor charge transport in the mixedCuPc /C60 layers.The -polymorph of CuPc, which is formed when filmsare grown on noninteracting substrates at room temperature,displays strongly anisotropic charge mobility as a result ofthe formation of one-dimensional 1D stacks ofmolecules.7,8 The molecules in these stacks align with theirmolecular planes parallel to each other, tilted by approxi-mately 65° to the stacking axis, with the stacks oriented suchthat a “herringbone” arrangement is adopted by molecules inneighboring columns. The peak charge mobility occurs alongthe stacking axis due to the strong -coupling betweenneighboring molecules, with charge transport between col-umns limited by a hopping mechanism. The 1D stacks withinthe polycrystallites are preferentially oriented with the stack-ing axis parallel to the substrate surface, resulting in a peakcharge mobility parallel to the substrate of around 4.1310−3 cm2 /V s.9 However, in a standard OPV cell, the de-sired direction of charge transport is perpendicular to thesubstrate surface and, hence, device performance is limitedby the hopping of charges between neighboring stacks. Mo-bilities around four orders of magnitude lower at 510−7 cm2 /V s have been reported for this orientation.10 Inorder to achieve maximum mobility in CuPc donor layers, itwould be desirable for the 1D stacks to be rotated so that thestacking direction lies parallel to the surface normal.It is well-known that 3,4,9,10-perylenetetracarboxylicdianhydride PTCDA molecules lie approximately parallelto the substrate surface when deposited onto noninteractingsubstrates.11,12 It has also been reported that the initialgrowth of a thin layer of PTCDA causes a subsequent free-base phthalocyanine H2Pc layer to follow the underlyingstructure rather than adopting its usual herringbonestructure,13–15 an effect referred to as structural templating.This templating effect is caused by an interaction betweenthe -orbitals of the PTCDA underlayer and the H2Pc mol-ecules, and is long range with no sign of the reappearance ofthe H2Pc herringbone structure in films as thick as380 nm.13,14 In this letter, we show that this templating effectcan also be used for CuPc films in an OPV heterojunctioncell with significant increase in the short circuit current den-sity JSC. The results demonstrate the effectiveness of tailor-ing the structure of the CuPc films for OPV applications.The structures were fabricated on commercially avail-able indium-tin-oxide ITO coated glass substrates CRLOpto after initial solvent cleaning. The organic layers weregrown by vacuum deposition in a Kurt J. Lesker Spectrossystem with a base pressure of about 810−8 mbar. Theorganic materials used in the devices CuPc Aldrich 97%,C60 MER Corp. 99.5%, bathocuproine Aldrich 98%, andPTCDA Aldrich 97% were twice purified by thermal gra-dient sublimation prior to deposition. A deposition rate of1 Å s−1 was used for all organic layers except PTCDA,which was grown at 0.1–0.2 Å s−1. The Al cathodes weredeposited in situ by evaporation through a shadow mask to athickness of 1000 Å, giving an active area of 0.16 cm2.X-ray diffraction XRD traces were obtained from a PhilipsPW1700 series automated powder diffractometer usingCu K radiation, whilst electronic absorption spectra weremeasured using a Perkin Elmer Lambda 2 spectrometer. J-Vcurves were measured with a Keithley 2400 sourcemeter un-aFAX: 44 024 7652 4112. Electronic mail: t.s.jones@warwick.ac.uk.APPLIED PHYSICS LETTERS 91, 233114 20070003-6951/2007/9123/233114/3/$23.00 © 2007 American Institute of Physics91, 233114-1Downloaded 01 Jul 2009 to 137.205.202.8. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspder AM1.5 solar illumination at 100 mW cm−2 1 sun froma Sciencetech solar simulator.Powder XRD traces for 300 Å CuPc layers grown on topof PTCDA interlayers of thicknesses ranging from 0–50 Åon ITO substrates are shown in Fig. 1. With no PTCDAinterlayer, the CuPc exhibits a single intense diffractionfeature centered at 6.8°, corresponding to diffraction fromthe 200 plane of -CuPc, and consistent with the stackingaxis lying parallel to the substrate.16,17 Inclusion of a PTCDAinterlayer results in a peak appearing at 27.6°,corresponding to diffraction from the 102 plane of PTCDA,but is also accompanied by a substantial change in the rest ofthe diffraction pattern. On increasing the PTCDA layerthickness, the intensity of the -CuPc peak at 6.8° is severelydiminished, until disappearing completely at 50 Å. Thissuggests that the underlying PTCDA layer has a considerableeffect on the structure of the CuPc layer. The disappearanceof this peak is accompanied by the gradual emergence of adifferent peak at 26.6°, corresponding to an interplanarseparation of 3.33 Å, which cannot be explained by thepresence of the PTCDA layer or by a reorientation of the bulkCuPc herringbone structure since the peak is not present for-CuPc in powder form.18 Instead, it appears that thePTCDA has forced the CuPc layer to adopt a completelydifferent structure.Electronic absorption spectra of a 300 Å CuPc layergrown directly on ITO, and 300 and 1000 Å CuPc layersgrown on a 20 Å PTCDA interlayer are shown in Fig. 2. Thenontemplated CuPc layer shows a spectrum typical of-phase CuPc, with the main absorption displaying peaks at694 and 622 nm as a result of aggregation. It is known thatthe relative intensity of the peak at 694 nm is indicative ofthe level of monomer character of the film.4,19 The only sig-nificant change in the absorption features on inclusion of a20 Å PTCDA interlayer is the relative intensity of the694 nm peak, suggesting that the dominant electronic inter-actions between CuPc molecules are unaffected by the struc-tural templating effect imparted by the PTCDA underlayer,and that the 1D stacks remain intact in the modified struc-ture. The 694 nm peak shows a decrease in relative intensityon inclusion of the 20 Å PTCDA interlayer and a furthersignificant decrease with an increase in the thickness of thetemplated CuPc layer. This suggests that a higher proportionof the CuPc molecules are able to form aggregate stacks inthe templated layers, and is evidence of increased order im-parted in the structure by the underlying PTCDA layer. Thiseffect is increasingly apparent at higher CuPc thicknesses,implying that the templating effect is long range.A structure in which the 1D stacks remain intact assuggested by electronic absorption spectroscopy, but theparallel plane separation is reduced to around 3.3 Å as sug-gested by XRD is proposed in Fig. 3. Here the initial layerof CuPc molecules are forced to lie flat by the strong -interactions with the flat lying PTCDA interlayer, with sub-sequent layers also forced into the same orientation. The lat-eral displacement of the subsequent layers remains equal tothat in the 1D stacks in the -phase herringbone structure,resulting in a tilting of the stacking axis by around 26.5°from the surface normal. In this orientation the diffractionplane parallel to the surface has an interplanar separationequal to the perpendicular separation between molecules in astack, in this case 3.4 Å, which correlates well to the po-sition of the observed XRD peak. This structure is consistentwith that seen for PTCDA /H2Pc.14 A series of multilayerOPV devices were grown of structure ITO /x ÅPTCDA /300 Å CuPc /400 Å C60 /100 Å BCP/Al, with 0 Åx50 Å. CuPc and C60 layer thicknesses were selected asthose optimized previously.6 Figure 4a shows the J-Vcurves for this series of OPV cells, while the inset at Fig.4a displays an energy level diagram for these devices. Fig-FIG. 1. Powder XRD patterns for a 300 Å CuPc layer grown on PTCDAinterlayers of thicknesses between 0 and 50 Å. The peak at 30.2° is due tothe ITO coating and is used as a reference for peak position and height,while the sloped background is due to the glass substrate.FIG. 2. Electronic absorption spectra of a 300 Å CuPc layer grown directlyon ITO crosses and on a 20 Å PTCDA interlayer empty circles, and a1000 Å CuPc layer grown on a 20 Å PTCDA interlayer empty triangles.FIG. 3. A schematic showing the structure of a an -CuPc film and b atemplated CuPc film with a PTCDA underlayer. The intrastack geometry,highlighted by the dashed rectangle, remains the same, but the stacking axisis rotated through approximately 90°.233114-2 Sullivan et al. Appl. Phys. Lett. 91, 233114 2007Downloaded 01 Jul 2009 to 137.205.202.8. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspure 4b shows the trend of short circuit current density JSC,open circuit voltage VOC, and power conversion efficiencyp obtained under AM1.5 solar illumination as a functionof PTCDA interlayer thickness.In the absence of the PTCDA interlayer, the standardnonoptimized heterojunction CuPc /C60 device affords ashort circuit current density of JSC=2.56 mA cm−2 and anopen-circuit voltage of VOC=0.5 V, leading to an overallpower conversion efficiency of p=0.73%. Inclusion of a50 Å thick PTCDA layer prior to the CuPc /C60 heterostruc-ture gives rise to a significant increase in JSC to2.93 mA cm−2, which we believe is due to the improvedtransport in the templated CuPc layer as a consequence ofthe modified stacking direction. Unfortunately, the improvedcurrent density is accompanied by a 40% fall in VOC, consis-tent with appreciable charge buildup at the PTCDA/CuPcinterface, resulting in an overall drop in the power conver-sion efficiency to p=0.43%. The offset in highest occupiedmolecular orbital levels results in a barrier for hole extractionof 1.5 eV, two orders of magnitude greater than thermalenergy at room temperature. Hole transport across thePTCDA layer would, therefore, only be expected throughpinholes and defects in the structure or by tunneling throughareas of low film thickness due to the roughness of the layer.Reduction of the PTCDA layer thickness x results in agradual increase in JSC and VOC and, hence, p, with theoptimum PTCDA thickness found to be 10 Å. At this thick-ness, VOC approaches 0.4 V, still considerably lower than thestandard device suggesting that, as expected, charge buildupis still significant even at such low thicknesses due to thedifficulty of charges crossing the PTCDA layer. However,JSC reaches 4.06 mA cm−2, an increase of almost 60% overthe standard device and sufficient to afford an improvementto p=0.81%. Further reduction of the PTCDA layer thick-ness to 5 Å leads to slightly lower JSC and VOC values thanthe non-templated device suggesting that x10 Å is thecritical PTCDA thickness to provide the necessary templat-ing effect to the CuPc layer.The results presented here show how, in principle,charge collection can be improved by aligning the stackingaxis of molecular columns with the desired direction ofcharge transport. While PTCDA is clearly not the ideal sur-face modification in terms of energy level alignment for holeinjection, the effect of templating on charge transportthrough the CuPc layer is clear to see. A 60% improvementin JSC, despite sufficient charge buildup to cause a 20% re-duction in VOC, suggests the potential for a significant in-crease in OPV performance with a more suitable surfacemodification layer. Future work using templating layers withmore appropriate energy levels will lead to significant im-provements in device performance.1C. W. Tang, Appl. Phys. Lett. 48, 183 1986.2S. R. Forrest, L. Y. Leu, F. F. So, and W. Y. Yoon, J. Appl. Phys. 66, 59081989.3P. Peumans, V. Bulovic, and S. R. Forrest, Appl. Phys. Lett. 76, 26502000.4P. Sullivan, S. Heutz, S. M. Schultes, and T. S. Jones, Appl. Phys. Lett.84, 1210 2004.5J. G. Xue, B. P. Rand, S. Uchida, and S. R. Forrest, Adv. Mater. Wein-heim, Ger. 17, 66 2005.6S. Heutz, P. Sullivan, B. M. Sanderson, S. M. Schultes, and T. S. Jones,Sol. Energy Mater. Sol. Cells 83, 229 2004.7S. Nespurek, H. Podlesak, and C. Hamann, Thin Solid Films 249, 2301994.8S. Heutz, S. M. Bayliss, R. L. Middleton, G. Rumbles, and T. S. Jones, J.Phys. Chem. B 104, 7124 2000.9K. Xiao, Y. Q. Liu, G. Yu, and D. B. Zhu, Synth. Met. 137, 991 2003.10R. D. Gould, Thin Solid Films 125, 63 1985.11A. J. Lovinger, S. R. Forrest, M. L. Kaplan, P. H. Schmidt, and T.Venkatesan, J. Appl. Phys. 55, 476 1984.12S. Heutz, A. J. Ferguson, G. Rumbles, and T. S. Jones, Org. Electron. 3,119 2002.13S. Heutz, R. Cloots, and T. S. Jones, Appl. Phys. Lett. 77, 3938 2000.14S. Heutz and T. S. Jones, J. Appl. Phys. 92, 3039 2002.15S. Yim, S. Heutz, and T. S. Jones, Phys. Rev. B 67, 165308 2003.16A. A. Ebert and H. B. Gottlieb, J. Am. Chem. Soc. 74, 2806 1952.17Rudiono, F. Kaneko, and M. Takeuchi, Appl. Surf. Sci. 142, 598 1999.18M. M. El-Nahass, F. S. Bahabri, A. A. AL Ghamdi, and S. R. Al-Harbi,Egypt. J. Solids 25, 307 2002.19B. P. Rand, J. Xue, S. Uchida, and S. R. Forrest, J. Appl. Phys. 98, 1249022005.FIG. 4. a J-V curves for a series of ITO /x Å PTCDA /300 Å CuPc /400 ÅC60 /100 Å BCP/Al devices, x=0 Å crosses, 10 Å circles, 20 Åsquares, and 50 Å triangles, a, inset an energy level diagram for thedevices, and b device parameters for the series; VOC squares, JSC tri-angles and p circles.233114-3 Sullivan et al. Appl. Phys. Lett. 91, 233114 2007Downloaded 01 Jul 2009 to 137.205.202.8. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Structural templating as a route to improved photovoltaic performance in copper phthalocyanine/fullerene (C60) heterojunctions

We have developed a method to improve the short circuit current density in copper phthalocyanine (CuPc)/fullerene (C-60) organic solar cells by similar to 60% by modifying the CuPc crystal orientation through use of a molecular interlayer to maximize charge transport in the direction between the two electrodes. Powder x-ray diffraction and electronic absorption spectroscopy show that a thin 3,4,9,10-perylenetetracarboxylic dianhydride interlayer before CuPc growth templates the CuPc film structure, forcing the molecules to lie flat with respect to the substrate surface, although the intrastack orientation is unaffected. This modified stacking configuration facilitates charge transport and improves charge collection

Structural templating as a route to improved photovoltaic performance in copper phthalocyanine/fullerene (C-60) heterojunctions

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Structural templating as a route to improved photovoltaic performance in copper phthalocyanine/fullerene (C60) heterojunctions

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