We present zinc phthalocyanine (ZnPc): C60 bulk-heterojunction top-illuminated organic solar cells using ultrathin metal layers as transparent top contacts. We show that solar cell performance sensitively depends on the interface and morphology of the cathode, which can be influenced by varying the composition and layer structure of the metal contact. We investigate various metal combinations, such as 3 nm Al/8 nm Ag and 7 nm Al/14 nm Ag, to illustrate the necessity to find a suitable combination of morphology and electrical and optical properties. Solar cells using standard materials and a 1 nm Al/14 nm Ag cathode exhibit promising efficiencies of over 2.2%. © 2009 American Institute of Physics.</em

Leo, K

Meiss, J

Riede, MK

Applied Physics Letters

We present zinc phthalocyanine (ZnPc): C60 bulk-heterojunction top-illuminated organic solar cells using ultrathin metal layers as transparent top contacts. We show that solar cell performance sensitively depends on the interface and morphology of the cathode, which can be influenced by varying the composition and layer structure of the metal contact. We investigate various metal combinations, such as 3 nm Al/8 nm Ag and 7 nm Al/14 nm Ag, to illustrate the necessity to find a suitable combination of morphology and electrical and optical properties. Solar cells using standard materials and a 1 nm Al/14 nm Ag cathode exhibit promising efficiencies of over 2.2%. © 2009 American Institute of Physics

ORA - Oxford University Research Archive

Towards efficient tin-doped indium oxide (ITO)-free inverted organic solar cells using metal cathodes

Oxford University Research Archive (ORA)

Oxford University Research Archive

Towards efficient tin-doped indium oxide „ITO…-free inverted organic solarcells using metal cathodesJ. Meiss,a M. K. Riede, and K. LeoInstitut für Angewandte Photophysik, Technische Universität Dresden, D-01062 Dresden, GermanyReceived 24 October 2008; accepted 9 December 2008; published online 7 January 2009We present zinc phthalocyanine ZnPc :C60 bulk-heterojunction top-illuminated organic solar cellsusing ultrathin metal layers as transparent top contacts. We show that solar cell performancesensitively depends on the interface and morphology of the cathode, which can be influenced byvarying the composition and layer structure of the metal contact. We investigate various metalcombinations, such as 3 nm Al/8 nm Ag and 7 nm Al/14 nm Ag, to illustrate the necessity to finda suitable combination of morphology and electrical and optical properties. Solar cells usingstandard materials and a 1 nm Al/14 nm Ag cathode exhibit promising efficiencies of over 2.2%.© 2009 American Institute of Physics. DOI: 10.1063/1.3059552As a potential alternative to expensive silicon solar cells,organic solar cells OSCs got into the focus of researchinterest in the last years.1,2 OSCs are currently particularlyinteresting for niche markets where cost-efficient, light-weight, and flexible solutions are advantageous.A widely recognized issue in the context of thin-filmoptoelectronic devices is the standard transparent conductivesubstrate, indium tin oxide ITO. While this material pro-vides an excellent combination of transparency and conduc-tivity, it is also brittle3 inhibiting its use in flexible devicesand expensive. If inverted devices are to be fabricated, ITOis not suitable since sputtering onto organic layers damagesthe layers underneath. The same issue arises when replacingITO with other transparent conductive oxides TCOs. Gen-erally, increasing effort is being given to find alternativetransparent conductive materials, e.g., carbon nanotubes forpolymer-based solar cells,4 conductive polymers,5,6 metals,7,8or solution-processed nanowire mesh arrays.9In this paper, TCO-free top-illuminated small-moleculeOSCs deposited on a nonopaque anode are presented. Assemitransparent contacts, they employ different thin-filmmetal systems. It is demonstrated that multilayer metal con-tacts are a feasible alternative electrode with good processi-bility. Solar cells using the ZnPc:C60 absorber system and a1 nm Al/14 nm Ag cathode exhibit promising efficiencies ofover 2.2%.The solar cells are fabricated in a custom-made vacuumsystem K.J. Lesker, UK at a base pressure of 10−6 mbarusing shadow masks. Float glass cleaned with organic sol-vents is used as substrate. A metal back contact of100 nm Al is deposited, followed by 1 nm of a proprietaryp-type dopant Novaled AG, Dresden, Germany.10 Ashole transporting material, 30 nm of 10 wt % p-doped 4,4 ,4-tris1-naphthylphenylamino-triphenylamineis used. For light absorption, a layer of zinc phthalocyanineZnPc 10 nm, followed by a layer of coevaporatedZnPc:C60 25 nm, ratio of 1:1 are deposited. After an addi-tional absorber and transport layer of C60 40 nm, 7 nm of4,7-diphenyl-1,10-phenanthroline BPhen is used as excitonblocker. Different thicknesses and combinations of Al/Ag9–21 nm total thickness are used as transparent top contact.A 60 nm capping layer of tris8-hydroxy-quinolinato-aluminum Alq3 is used for increased light outcoupling outof the solar cell.8 All organic materials have been purified atleast twice by vacuum gradient sublimation.A total of 16 samples are made on the same substrate inone run. This way, the only variations are for the top contactwhile keeping the deposition conditions for all other layershole transport layer, absorber, electron transport layer, andcapping layer constant. Typical solar cell areas are around6.7 mm2, measured using a light microscope.The completed solar cells are encapsulated in a nitrogenglovebox attached to the vacuum deposition chamber, thenstored at ambient conditions. JV-characteristics are re-corded using a source measurement unit 236 SMU Kei-thley under an AM 1.5G sun simulator Hoenle AG, moni-tored with a silicon photodiode with respect to whichintensities are given. The JV-characteristics are not cor-rected for spectral mismatch. External quantum efficiencyEQE is measured in the same setup using color filters.Measurements with different illumination intensities are per-formed with neutral density filters. Reflection and transmis-sion are measured on a Lambda 900 UV/visible/near infraredspectrometer PerkinElmer.The obtained solar cell characteristics for different metalcontacts consisting of variations in aluminum 1–7 nm andsilver 8–14 nm in different combinations are summarizedin Table I. All short-circuit currents given are normalized toan incident light intensity of 100 mW /cm2. Values of OSCwith 5 nm Al cathodes are omitted due to space constraints.While there are some deviations due to experimental scatter,it can be seen that the composition and layer thickness of thetransparent top contact have significant influence on theoverall device performance and exhibit clear trends. Threemain factors can be distinguished: the thickness of Al, thethickness of Ag, and the overall metal thickness of Al/Agcombined.The thickness of the Al layer is varied between 1 and7 nm. As found in a previous study,8 the addition of Al to thecathode alone can lead to a significant improvement, presum-ably due to surfactant effects that lead to more closed Aglayers and prevent cluster formation compared to stand-alonepure Ag layers. Previous studies11 showed that less reactiveaElectronic mail: jan.meiss@iapp.de.APPLIED PHYSICS LETTERS 94, 013303 20090003-6951/2009/941/013303/3/$23.00 © 2009 American Institute of Physics94, 013303-1Downloaded 10 Jan 2010 to 141.30.25.195. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspmetals such as Ni and Cu can form metallic precipitateswithin polyimide layers without reacting, while Cr reactedwith the organic material and was bound to its surface form-ing continuous layers. This suggests that the Al layers pre-sented in this paper can also react with BPhen and form asmooth surface on which the Ag can then adhere in closedflat layers. Noble metals alone, such as Ag, Au, and Pt, onthe contrary, tend to diffuse and form clusters even on crys-talline organic materials at room temperature.12As can be seen from the data shown here, an increase inAl thickness leads to a clear decrease in the short-circuitcurrent Jsc, which can be explained by higher reflection of Alin the ranges of 400–500 nm and 600–700 nm, decreasingthe photon density in the absorber and inhibiting excitongeneration. This is illustrated in the reflection measurementsshown in the inset of Fig. 1, where the lowest reflectionvalues are obtained for a metal electrode having only 1 nmAl and a total thickness of 15 nm.Note that the reflection is high, especially in the 400–600 nm range. The values shown in Fig. 1, however, charac-terize a sample without Alq3 capping layer, which consider-ably reduces reflection and at the same time utilizes themetal reflection to create a microcavity effect within theOSC for improved performance.8The electrode with 3 nm Al and a total thickness of only11 nm exhibits higher reflection, showing that the main con-tribution toward reflection stems from the Al content. Thesefindings are supported by EQE measurements shown in Fig.1, where in particular the EQE in the 600–700 nm rangedrops from 35% to almost 20% upon increasing the Al thick-ness. This coincides with the main ZnPc absorption range,leading to lower photocurrents with increasing Al thickness.Highest currents are obtained with 1 nm Al with currents ofup to 7.9 mA /cm2, which drops to 6–6.6 mA /cm2 for 3 nmAl, 5.5–5.7 mA /cm2 for 5 nm Al, and reaches a minimumfor 7 nm Al with 4.6–4.9 mA /cm2. Thicker Al layers seemto slightly reduce the open-circuit voltage Voc from 0.52 Vfor 1 nm Al to 0.51 V for 7 nm Al with decreasing voltagefor increasing Al thickness. It is currently not clear if thiseffect is caused by different work functions of Ag 4.26 eVand Al 4.28 eV values reported by Michaelson13, or pos-sibly the diffusion of Al atoms into adjacent organic layers ofBPhen and C60, which might lead to unintentional doping.These negative effects are partially compensated by an im-provement of the fill factor FF for thicker layers. However,since an increase in FF can also be observed for increasingAg layer thickness, this superposition of influences makes itdifficult to evaluate the proportions of the contributions ofboth materials to FF. Generally it can be seen that thicker Allayers are disadvantageous to solar cell performance, mainlydue to negative optical properties.In the current work, silver has been used as the mainconductive component for the metal electrodes due to itsadvantageous optical properties. Ultrathin Ag films are verysensitive to deposition conditions, and for thin metal films itis assumed that uniform films are found only at thicknessesabove a certain coalescence threshold, depending on evapo-ration rate, substrate, pressure, etc.14 With an Al base layer,the morphological features of the Ag layer less likely inhibitan optimal contact because the Al seems to act as surfactant,mediating smooth Ag morphology.For the solar cells with 1 nm Al/8 nm Ag, it is assumedthat the amount of silver is too small for a closed layer,despite the 1 nm Al deposition. While a small photovoltageis observed, the measured efficiency is in the low 10−3%range and is considered negligible.Examples of selected JV-characteristics under illumi-nation and in the dark are shown in Fig. 2. Fully operationalsolar cells are obtained with 3 nm Al/8 nm Ag, having Jsc=6.1 mA /cm2 and Voc=0.52 V, which is in the same rangeTABLE I. Solar cell characteristics.Metal contactJscmA /cm2VocVFA%%1 nm Al, 8 nm Ag 0.01 0.525 N/A N/A1 nm Al, 10 nm Ag 3.26 0.503 13.45 0.221 nm Al, 12 nm Ag 7.42 0.520 52.17 2.011 nm Al, 14 nm Ag 7.90 0.519 53.86 2.213 nm Al, 8 nm Ag 6.14 0.515 29.53 0.933 nm Al, 10 nm Ag 5.95 0.516 52.14 1.713 nm Al, 12 nm Ag 6.49 0.512 52.64 1.493 nm Al, 14 nm Ag 6.57 0.514 52.52 1.927 nm Al, 8 nm Ag 4.55 0.503 57.56 1.437 nm Al, 10 nm Ag 4.73 0.508 55.69 1.347 nm Al, 12 nm Ag 4.74 0.516 57.50 1.407 nm Al, 14 nm Ag 4.92 0.504 60.59 1.39FIG. 1. Color online EQE measurements. Inset: reflection measurements.Filled squares: 1 nm Al/14 nm Ag. Empty circles: 3 nm Al/8 nm Ag. Emptytriangles: 7 nm Al/14 nm Ag.FIG. 2. Color online Current-voltage curves with AM 1.5G and withoutillumination. Filled symbols: under illumination. Empty symbols: in thedark. Squares: 1 nm Al/14 nm Ag. Circles: 3 nm Al/8 nm Ag. Triangles:7 nm Al/14 nm Ag.013303-2 Meiss, Riede, and Leo Appl. Phys. Lett. 94, 013303 2009Downloaded 10 Jan 2010 to 141.30.25.195. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspas the characteristics of other solar cells, albeit having alower FF below 30%. The creation of an operational solarcell suggests that well-defined closed layers are indeed pos-sible even with thicknesses of individual metal films below10 nm.The solar cell with 7 nm Al/14 nm Ag has the thickestmetal contact of all devices presented in this study. Due to itslower transmission especially caused by the 7 nm Al, thephotocurrent is low with 4.9 mA /cm2. This is more thancompensated for by the high FF of over 60%, leading to anoverall increase in efficiency to 1.39% from 0.93%, which isfound for the 3 nm Al/8 nm Ag contact, despite considerablylower quantum efficiency. This is expected to originate froma superior electrical contact between the organic/metal inter-face and an increased number of charge carrier percolationpathways within the metal contact. This is reflected in alower series resistance for voltages Voc as can be observedin the slope of the JV-curves under illumination and in thedark. In both cases, the sample having 7 nm Al/14 nm Ag hasthe lowest series resistance and, as suggested in the satura-tion behavior, the highest parallel resistance, indicating goodelectrical contact and low leakage current. In contrast, thethin metal contact of 3 nm Al/8 nm Ag exhibits the highestseries and lowest parallel resistance. The low FF and thevisible “S-kink” hint at issues in charge extraction, or highrecombination. This can be caused by the Ag layer, which isthick enough for electrical contact, but still has some isolatedclusters, islands, or hillocks that act as charge carrier trapsfor electrons, leading to unbalanced charge carrier extractionand creating a counterfield. It cannot be excluded that thenoncontinuous Ag layer allows residual oxygen to penetrateAl, leading to Al2O3.The best solar cell of the current study has a combinationof 1 nm Al, preserving high transmission while at the sametime acting as a surface-mediating layer, and 14 nm Ag for aclosed layer with only few clusters. This configuration yieldsthe best compromise of Jsc=7.90 mA /cm2 and FF=54%,leading to an overall efficiency of over 2.2%. It is expectedthat this metal contact can be used in an optimized solar cellstack with different transport materials or absorbers toachieve considerably higher performance.Neutral density filter measurements are performed to fur-ther investigate the effects of the metal contact for differentillumination intensities. The effect of incident light intensityon FF and Voc is shown in Fig. 3, where the thinnest 3 nmAl/8 nm Ag and thickest 7 nm Al/14 nm Ag solar cells arechosen. Voc shows an exponential increase for increasing il-lumination for both contact types, as is expected when thequasi-Fermi–Niveau splitting becomes more pronounced dueto higher charge carrier generation.A clear difference is visible for the FF. While the solarcell with the thick metal contact shows an exponential in-crease with a saturationlike behavior, the thin metal electrodeleads to a peak at 0.025 suns, followed by decreasing FFvalues. This suggests an increasing influence of the seriesresistance at higher photogenerated currents and higher re-combination due to hindered charge carrier extractionthrough an insufficiently formed percolation network.In summary, ITO-free inverted organic bulk heterojunc-tion solar cells with cathodes from thermally evaporatedcombinations of ultrathin Al and Ag films are presented. It isshown that despite better optical properties, too thin metalcathodes are inferior under operation due to isolated clustersand remaining islands that lower FF. The optimal solar cellstructure employs a combination of 1 nm Al for improvedmorphology of the metal contact and 14 nm Ag for improvedelectrical and optical properties, and reaches promising effi-ciencies of over 2.2%, which is expected to increase furtherby optimization of solar cell stack and used materials.The current work is supported by the Bundesministeriumfür Bildung und Forschung Project No. 03IP602.1C. W. Tang, Appl. Phys. Lett. 48, 183 1986.2J. Xue, S. Uchida, B. P. Rand, and S. R. Forrest, Appl. Phys. Lett. 85,5757 2004.3Z. Chen, B. Cotterell, W. Wang, E. Guenther, and S.-J. Chua, Thin SolidFilms 394, 201 2001.4J. van de Lagemaat, T. M. Barnes, G. Rumbles, S. E. Shaneen, T. J.Coutts, C. Weeks, I. Levitsky, J. Peltola, and P. Glatkowsky, Appl. Phys.Lett. 88, 233503 2006.5J. Meiss, C. L. Uhrich, K. Fehse, S. Pfuetzner, M. K. Riede, and K. Leo,Proc. SPIE 7002, 700210 2008.6B. Zimmermann, M. Glatthaar, M. Niggemann, M. K. Riede, A. Hinsch,and A. Gombert, Sol. Energy Mater. Sol. Cells 91, 374 2007.7T. Oyamada, Y. Sugawara, Y. Terao, H. Sasabe, and C. Adachi, Jpn. J.Appl. Phys., Part 1 46, 1734 2007.8J. Meiss, N. Allinger, M. K. Riede, and K. Leo, Appl. Phys. Lett. 93,103311 2008.9J.-Y. Lee, S. T. Connor, Y. Cui, and P. Peumans, Nano Lett. 8, 689 2008.10Used for better processibility, comparable in performance to the com-monly available 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane.11F. K. LeGoues, B. D. Silverman, and P. S. Ho, J. Vac. Sci. Technol. A 6,2200 1988.12A. E. Duerr, F. Schreiber, M. Kelsch, H. D. Carstanjen, and H. Dosch,Adv. Mater. Weinheim, Ger. 14, 961 2002.13H. B. Michaelson, J. Appl. Phys. 48, 4729 1977.14R. S. Sennett and G. D. Scott, J. Opt. Soc. Am. 40, 203 1950.FIG. 3. Color online Intensity-dependent FF and open-circuit voltageVoc. Right scale: Voc of solar cells with 3 nm Al/8 nm Ag filled circlesand 7 nm Al/14 nm Ag filled triangles contacts. Left scale: FF of solar cellswith 3 nm Al/8 nm Ag empty circles and 7 nm Al/14 nm Ag emptytriangles contacts. The lines are guide to the eyes.013303-3 Meiss, Riede, and Leo Appl. Phys. Lett. 94, 013303 2009Downloaded 10 Jan 2010 to 141.30.25.195. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Towards efficient tin-doped indium oxide (ITO)-free inverted organic solar cells using metal cathodes

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