High efficiency organic photovoltaic cells based on a ...

High efficiency organic photovoltaic cells based on a vapor deposited squaraine donor Siyi Wang, Elizabeth I. Mayo, M. Dolores Perez, Laurent Griffe, Guodan Wei et al. Citation: Appl. Phys. Lett. 94, 233304 (2009); doi: 10.1063/1.3152011 View online: http://dx.doi.org/10.1063/1.3152011 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v94/i23 Published by the AIP Publishing LLC.

Additional information on Appl. Phys. Lett. Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

Downloaded 23 Sep 2013 to 110.4.12.170. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

APPLIED PHYSICS LETTERS 94, 233304 共2009兲

High efficiency organic photovoltaic cells based on a vapor deposited squaraine donor Siyi Wang,1 Elizabeth I. Mayo,2,a兲 M. Dolores Perez,1 Laurent Griffe,1 Guodan Wei,3 Peter I. Djurovich,1 Stephen R. Forrest,3,b兲 and Mark E. Thompson1,b兲 1

Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA Global Photonic Energy Corp., 20 Trading Post Way, Medford Lake, New Jersey 08055, USA 3 Departments of Electrical Engineering and Computer Science, Physics, and Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA 2

共Received 21 April 2009; accepted 15 May 2009; published online 9 June 2009兲 2,4-bis关4-共N , N-diisobutylamino兲-2,6-dihydroxyphenyl兴 squaraine 共SQ兲 is used as a donor material in vapor deposited organic heterojunction photovoltaic cells. Devices with the structure indium tin oxide/SQ 共x兲 / C60 共400 Å兲/bathocuproine 共100 Å兲 / Al 共1000 Å兲, where x = 65, 110, 150, and 200 Å were compared. Devices with x = 65 Å exhibited a power conversion efficiency of 3.1% under 1 sun, AM1.5G simulated solar irradiation, giving an open circuit voltage of 0.76⫾ 0.01 V, a short circuit current of 7.01⫾ 0.05 mA/ cm2, and a fill factor of 0.56⫾ 0.05. Thicker SQ films lead to lower short circuit currents and fill factors, giving conversion efficiencies in the range of 2.6% to 3.2%. The demonstration of sublimable SQ as a donor material opens up a family of compounds for use in small molecule based heterojunction photovoltaics. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3152011兴 Research in organic photovoltaics 共OPVs兲 has focused on the search for donor, acceptor, and blocking materials that can lead to high solar power conversion efficiencies. Active materials for efficient photovoltaics need to satisfy a number of criteria, including high optical density over the visible and near infrared solar spectral regions, high charge mobility, and a large exciton diffusion length. Here we investigate squaraine compounds as donors in vapor deposited OPVs. Squaraines are 1,3 disubstituted derivatives of squaric acid in which the electron deficient four-membered ring is linked to two aryl groups, e.g., 2,4-bis关4-共N , N-diisobutylamino兲-2,6-dihydroxyphenyl兴 squaraine 共SQ兲 in Fig. 1. By varying the amine donors or aryl groups, it is possible to design a number of symmetrical or unsymmetrical squaraines whose thin film absorptions extend into the near infrared 共to wavelengths of ␭ ⬃ 1000 nm兲, and extinction coefficients 艌105 M−1 cm−1.1 These optical properties make the squaraine family of dyes good candidates for organic electronic applications, such as dye sensitized solar cells2 and nonlinear optics.3 Recently, Silvestri et al., reported squaraine-based, solution-processed bulk heterojuction solar cells consisting of a mixture of a squaraine donor and a fullerene derivate as the acceptor phase 共PCBM, i.e., 关6,6兴-phenyl-C61-butyric acid methyl ester兲. Those devices exhibited a power conversion efficiency of 1.24%,4 enabled in part by the high extinction coefficient of the squaraine. This suggests that thin films of these materials may serve as efficient donor layers in planar OPVs. In this work, we investigate the performance of OPVs as a function of squaraine thickness in such heterojunction cells deposited by vacuum thermal evaporation.

To prevent molecular decomposition on sublimation, typical of many squaraine compounds, we have used squaraines with hydroxyl groups at the 2⬘ , 6⬘-positions of the two phenyl rings.5 The squaraine used is SQ 共shown in Fig. 1 inset兲. This compound was synthesized according to literature procedures5 and purified by multiple cycles of thermal gradient sublimation in vacuum.6 The absorption spectrum of a CH2Cl2 solution of SQ shows a narrow, intense band at ␭max = 652 nm 共␧ ⬃ 4.5⫻ 105 M−1 cm−1兲, which broadens and redshifts upon film formation 共see Fig. 1兲.7,8 The broadening of the absorption band in the solid state results from a strong excitonic interaction between neighboring molecules. The peak optical density in the thin film is 2.0⫻ 105 cm−1 at a wavelength of ␭max = 700 nm. Electrochemistry of SQ in CH2Cl2 shows reversible oxidation waves at 0.49 V, and a quasireversible reduction wave at −1.23 V 共both versus ferrocene兲, respectively, corresponding to a thin film highest occupied molecular orbital 共HOMO兲 energy of 5.3 eV 共Refs. 9 and 10兲 and a lowest unoccupied molecular orbital 共LUMO兲 energy of 3.4 eV.11 These values suggest SQ serves

a兲

Present address: Applied Materials, 3535 Garret Drive, Santa Clara, CA 95052-8039. Authors to whom correspondence should be addressed. Electronic addresses: [email protected] and [email protected].

b兲

0003-6951/2009/94共23兲/233304/3/$25.00

FIG. 1. 共Color online兲 Ultraviolet-visible absorption spectra of squaraine 共SQ兲 in CH2Cl2 solution and optical density of a 40 nm SQ thin film. Inset: The chemical structural formula of SQ.

94, 233304-1

© 2009 American Institute of Physics


233304-2

Appl. Phys. Lett. 94, 233304 共2009兲

Wang et al.

TABLE I. Photovoltaic device performance under 1 sun AM 1.5G simulated solar illumination. The device structure is ITO/ donor/ C60共400 Å兲 / BCP共100 Å兲 / Al共1000 Å兲. Donor-thickness 共Donor x兲共Å兲

FIG. 2. 共Color online兲 Current density vs voltage characteristics of ITO/donor/C60共400 Å兲 / BCP共100 Å兲 / Al共1000 Å兲: donor= SQ 65 Å 共SQ65兲, 110 Å 共SQ-110兲, NPD 50 Å/SQ 65 Å 共NPD/SQ兲, and CuPc 400 Å 共CuPc兲. Dark current characteristics are shown as dashed lines.

as a donor in a heterojunction structure when used with the acceptor C60, whose HOMO and LUMO energies are 6.2 eV and 3.5 eV, respectively.12,13 Photovoltaic cells were grown on indium tin oxide 共ITO兲-coated glass substrates that were solvent cleaned and treated in UV ozone for 10 min immediately prior to loading into a high vacuum 共base pressure ⬃3 ⫻ 10−6 Torr兲 chamber. The organic materials, SQ, copper phthalocyanine 共CuPc兲共Aldrich兲, C60 共MTR limited兲, and 2,9-dimethyl-4,7diphenyl-1,10-phenanthroline 共bathocuproine, BCP兲共Aldrich兲 were purified by thermal gradient sublimation in vacuum prior to use. Squaraine samples require multiple sublimation steps to achieve the highest device efficiency, as shown previously for CuPc.14 Metal cathode materials, Al 共99.999% pure, Alfa Aesar兲 were used as received. Materials were sequentially grown by vacuum thermal evaporation at the following rates: SQ 共0.3– 0.6 Å / s兲 or CuPc 共4 Å / s兲, C60 共2 Å / s兲 and metals: 1000 Å thick Al 共3 Å / s兲. The cathode was evaporated through a shadow mask with an array of 1 mm diameter openings. Current-voltage characteristics of the cells were measured in the dark and under simulated AM 1.5G solar illumination. Incident power was adjusted to match 1 sun intensity 共100 mW/ cm2兲 using a Si photodiode, calibrated by the National Renewable Energy Laboratory, Golden, CO, and spectral response was measured using a monochromated light source. Spectral mismatch was calculated and used to correct the measured efficiencies following standard procedures.15 Errors were estimated for each of the OPV parameters by examining the variation between devices on a given substrate. Devices with the structure ITO/ SQ共x兲 / C60共400 Å兲 / BCP共100 Å兲 / Al共1000 Å兲, x = 65, 110, 150, and 200 Å exhibited performances presented in Fig. 2 and Table I. For comparison, the data are provided for devices prepared in a single fabrication run, to eliminate run-to-run variations. The highest power conversion efficiencies are observed for devices with x = 65 Å. Devices with doubly sublimed SQ fabricated on five different runs give efficiencies in the range of 3.2% ⫾ 0.3%. In contrast, a device using singly sublimed source material results in a reduced power conversion efficiency of ␩ P = 2.2%, while OPVs with triply sublimed SQ gave similar efficiencies to those with doubly sublimed SQ, indicating that purification beyond two sublimations has a diminishing affect on device performance.

JSC VOC 共 ⫾ 0.01 V兲共 ⫾ 0.05 mA/ cm2兲共V兲共mA/ cm2兲

0.76共0.03兲 SQ-65, avea SQ-65b 0.75 0.83 SQ-110b SQ-150b 0.84 0.84 SQ-200b NPD-50/SQ-65 0.82 CuPc-400 0.42 NPD-50/CuPc-400 0.43

7.01共0.38兲 7.13 6.89 6.44 5.98 6.67 4.99 4.74

␩

FF 共 ⫾ 0.05兲

共 ⫾ 0.1% 兲共%兲

0.56共0.05兲 0.60 0.55 0.53 0.50 0.55 0.58 0.58

3.1共0.3兲 3.2 3.2 2.9 2.6 3.0 1.2 1.2

a

The values are averages for each prameter measured on five different substrates prepared in different fabrication runs. The values in parentheses are the standard deviations. b The SQ OPV data listed for ITO/ SQ/ C60 / BCP/ Al devices 共x = 65– 200 Å兲 prepared in parallel in a single fabrication run. The data are averages for several devices on each substrate.

The measured open circuit voltage 共VOC兲 for the devices employing SQ donors are VOC = 0.75– 0.84 V, or 300– 400 mV greater than that observed for a comparable CuPc device. The higher observed VOC is consistent with an increased donor-acceptor interface energy gap 共⌬EDA兲,11,16 defined as the energy difference between the HOMO of the donor and the LUMO of the acceptor. Here, ⌬EDA is 1.8 eV for the SQ/ C60 heterojunction, 100 mV larger than that of the CuPc/ C60 heterojunction, however, the increase in VOC is larger than the observed differences in ⌬EDA. The open circuit voltage of OPVs is approximated by VOC = 共nkBT / q兲ln共JSC / JS兲, where n is diode ideality factor, kB is Boltzmann’s constant, T is temperature, q is the fundamental charge, JSC is the short circuit current density, and JS is the saturation dark current density, which is given by JS = JSO exp共−⌬EDA / 2nkT兲.16–18 For the SQ based devices, JS = 10−2 – 10−3 ␮A / cm2, while analogous CuPc based devices give JS ⬵ 1 ␮A / cm2.16,19 The ideality factor, n, for devices with CuPc and SQ 共x = 110 Å兲 are 2.1⫾ 0.1 and 2.5⫾ 0.1, respectively. For the x = 110 Å device, JS = 0.02 ␮A / cm2. These values result in a calculated VOC of 0.40 and 0.79 V 共assuming ⌬EDA = 1.7 and 1.8 eV兲 for CuPc and SQ devices, respectively. These are approximately equal to the observed values of 0.42⫾ 0.01 and 0.83⫾ 0.01 indicating that differences in dark current largely account for VOC differences between CuPc and SQ based devices. The differences in JS for SQ and CuPc based devices is, therefore, largely due to their pre-exponential currents, JSO, which are 16 and 2.1 ⫻ 104 mA/ cm2, respectively. Incident photon-to-current conversion efficiency 共IPCE兲 measurements in Fig. 3 show that a significant fraction of the photocurrent is contributed by the SQ layer. The highest JSC is observed for OPV with x = 65 Å, and monotonically decreases as the SQ thickness is increased. The fill factor 共FF兲 also decreases at x ⬎ 65 Å, although FF⬎ 0.5 for the range of x 艋 200 Å studied. The trends observed for JSC and FF as functions of SQ thickness are consistent with increased resistance to hole transport in the thicker SQ films. For the thinnest layers, the SQ may not form a continuous film on the ITO surface whose root mean square roughness is approximately 1 nm. Discontinous films can lead to


233304-3

Appl. Phys. Lett. 94, 233304 共2009兲

Wang et al.

The authors thank Brian E. Lassiter for helpful discussion and gratefully acknowledge the Air Force Office of Scientific Research and Global Photonic Energy Corporation for financial support, and Thin Film Devices, Inc. for providing ITO-coated glass substrates. K. Y. Law, J. Phys. Chem. 91, 5184 共1987兲. A. Burke, L. Schmidt-Mende, S. Ito, and M. Gratzel, Chem. Commun. 2007, 234. 3 S. J. Chung, S. J. Zheng, T. Odani, L. Beverina, J. Fu, L. A. Padilha, A. Biesso, J. M. Hales, X. W. Zhan, K. Schmidt, A. J. Ye, E. Zojer, S. Barlow, D. J. Hagan, E. W. Van Stryland, Y. P. Yi, Z. G. Shuai, G. A. Pagani, J. L. Bredas, J. W. Perry, and S. R. Marder, J. Am. Chem. Soc. 128, 14444 共2006兲. 4 F. Silvestri, M. D. Irwin, L. Beverina, A. Facchetti, G. A. Pagani, and T. J. Marks, J. Am. Chem. Soc. 130, 17640 共2008兲. 5 M. Q. Tian, M. Furuki, I. Iwasa, Y. Sato, L. S. Pu, and S. Tatsuura, J. Phys. Chem. B 106, 4370 共2002兲. 6 S. R. Forrest, Chem. Rev. 97, 1793 共1997兲. 7 H. J. Chen, K. Y. Law, and D. G. Whitten, J. Phys. Chem. 100, 5949 共1996兲. 8 J. Cornil, D. Beljonne, J. P. Calbert, and J. L. Brédas, Adv. Mater. 13, 1053 共2001兲. 9 J. L. Bredas, R. Silbey, D. S. Boudreaux, and R. R. Chance, J. Am. Chem. Soc. 105, 6555 共1983兲. 10 B. W. D’Andrade, S. Datta, S. R. Forrest, P. Djurovich, E. Polikarpov, and M. E. Thompson, Org. Electron. 6, 11 共2005兲. 11 K. L. Mutolo, E. I. Mayo, B. P. Rand, S. R. Forrest, and M. E. Thompson, J. Am. Chem. Soc. 128, 8108 共2006兲. 12 N. Sato, Y. Saito, and H. Shinohara, Chem. Phys. 162, 433 共1992兲. 13 R. Schwedhelm, L. Kipp, A. Dallmeyer, and M. Skibowski, Phys. Rev. B 58, 13176 共1998兲. 14 R. F. Salzman, J. G. Xue, B. P. Rand, A. Alexander, M. E. Thompson, and S. R. Forrest, Org. Electron. 6, 242 共2005兲. 15 V. Shrotriya, G. Li, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, Adv. Funct. Mater. 16, 2016 共2006兲. 16 B. P. Rand, D. P. Burk, and S. R. Forrest, Phys. Rev. B 75, 115327 共2007兲. 17 N. Li, B. E. Lassiter, R. R. Lunt, G. Wei, and S. R. Forrest, Appl. Phys. Lett. 94, 023307 共2009兲. 18 S. M. Sze, Physics of Semiconductor Devices 共Wiley Interscience, New York, 1981兲, p. 878. 19 M. D. Perez, C. Borek, S. R. Forrest, and M. E. Thompson, “Molecular and morphological influences on the open circuit voltages of organic photovoltaic devices,” J. Am. Chem. Soc. 共to be published兲. 20 M. Westphalen, U. Kreibig, J. Rostalski, H. Luth, and D. Meissner, Sol. Energy Mater. Sol. Cells 61, 97 共2000兲. 21 S. Naka, H. Okada, H. Onnagawa, Y. Yamaguchi, and T. Tsutsui, Synth. Met. 111, 331 共2000兲. 22 Y. Masumoto and T. Mori, Thin Solid Films 516, 3350 共2008兲. 1 2

FIG. 3. 共Color online兲 IPCE characteristics of ITO/donor/C60共400 Å兲 / BCP共100 Å兲 / Al共1000 Å兲 devices: donor= SQ 65 Å 共SQ-65兲, NPD 50 Å/SQ 65 Å 共NPD/SQ兲, and CuPc 400 Å 共CuPc兲.

direct contact between the C60 layer and the anode. The C60 / ITO contact is expected to form a Schottky junction,20 which could act as a competing interface for exciton dissociation, and thereby introduce current shunts. To reduce the effects of a discontinuous SQ layer, a 50 Å thick layer of the wide energy gap, hole transporting material, N , N⬘-di-1-naphthyl-N , N⬘-diphenyl-benzidine 共NPD兲 was inserted between the ITO anode and the donor film. Here, we expect NPD to form a continuous film on ITO, while maintaining a high hole conductivity.21,22 The addition of the NPD layer into the structure ITO/ NPD共50 Å兲 / SQ共65 Å兲 / C60 / BCP/ Al, gives VOC comparable to that observed for thicker 共x 艌 100 Å兲 SQ films, and JSC comparable to a device with x = 65 Å as shown in Fig. 2 and Table I. This suggests that the lower VOC for x = 65 Å may indeed be due to partial contact of C60 with the ITO anode. Introduction of a 50 Å thick NPD layer in CuPc共400 Å兲 / C60 control cells shows no change in device performance 共Table I兲. In conclusion, we have shown a power conversion efficiency of 3.2% for a SQ/ C60 based heterojunction photovoltaic cell fabricated by vacuum thermal evaporation. It is noteworthy that this power conversion efficiency and an IPCE⬎ 25% was achieved for a planar heterojunction device with only a 65 Å thick layer of SQ. The combination of low dark current and a high interface energy gap for the SQ donor based OPVs contribute to the high open circuit voltage of 0.73 V.
