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Electronic absorption band broadening and surface roughening of phthalocyanine double layers by saturated solvent vapor treatment
Materials Research Bulletin 47 (2012) 2744–2747
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Electronic absorption band broadening and surface roughening of phthalocyanine double layers by saturated solvent vapor treatment Jinhyun Kim, Sanggyu Yim * Department of Chemistry, Kookmin University, Seoul 136-702, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 26 April 2012
Variations in the electronic absorption (EA) and surface morphology of three types of phthalocyanine (Pc) thin film systems, i.e. copper phthalocyanine (CuPc) single layer, zinc phthalocyanine (ZnPc) single layer, and ZnPc on CuPc (CuPc/ZnPc) double layer film, treated with saturated acetone vapor were investigated. For the treated CuPc single layer film, the surface roughness slightly increased and bundles of nanorods were formed, while the EA varied little. In contrast, for the ZnPc single layer film, the relatively high solubility of ZnPc led to a considerable shift in the absorption bands as well as a large increase in the surface roughness and formation of long and wide nano-beams, indicating a part of the ZnPc molecules dissolved in acetone, which altered their molecular stacking. For the CuPc/ZnPc film, the saturated acetone vapor treatment resulted in morphological changes in mainly the upper ZnPc layer due to the significantly low solubility of the underlying CuPc layer. The treatment also broadened the EA band, which involved a combination of unchanged CuPc and changed ZnPc absorption. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Surfaces A. Thin films C. Atomic force microscopy
1. Introduction Phthalocyanines (Pcs) are a particularly important class of organic semiconducting material finding use in a variety of optoelectronic devices such as organic photovoltaic (OPV) cells and organic light emitting diodes (OLEDs) due to their high stability and well-ordered film growth [1–3]. Copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc), planar Pc molecules with 18p-electrons in its aromatic system, are the most widely used electron donor materials in small-molecular weight based OPV device applications [4–6]. The current power conversion efficiency (PCE) of small-molecule OPV cells is between 4 and 5% for a single cell [7,8], which is relatively low when compared to inorganic solar cells. The main factors hampering further PCE enhancement of small-molecule OPV cells are the narrow spectral absorption range of the electron donor materials [9] and the short exciton diffusion lengths with respect to the film thickness [10,11]. In polymer solar cells, efforts have been made to broaden the absorption band of polymer materials through attachment of conjugated side chains [10] or the use of blended donors [11–13]. However, such approaches have not been used for small-molecule OPV cells. In regards to exciton diffusion lengths, several attempts to fabricate controlled nanostructures with a large contact area between electron donating and accepting layer and to increase the efficiency of charge (hole and electron) separation before
* Corresponding author. Tel.: +82 2 910 4734; fax: +82 2 910 4415. E-mail address: [email protected] (S. Yim). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.04.132
excitons recombination have been made [10–13]. The bulk heterojunction (BHJ), which can be produced by co-deposition or homogeneous blends was initially introduced [13,14]. This interpenetrating network resulted in a increase in the PCE to some extent, but the charge trapping at bottlenecks and cul-de-sacs induced by the random distribution of the materials limited any further increase in efficiency [6,15]. A heterolayer nanostructure with an interdigitated morphology has therefore been proposed to be an ideal interface [16,17], and various attempts to fabricate nanostructured interfaces have been made. The proposed technologies included nanoimprinting [18–20], solvent drop treatment [21], and solvent vapor spray treatment [22]. However, the nanoimprinting methods require long and complicated nano-processes. While solvent drop and solvent vapor spray treatment are much simpler compared to the nanoimprinting method, bulk molecules as well as surface molecules in thin films are also dissolved by the solvents and bare substrates are often exposed, which is disadvantageous for device applications. In this work, we developed an alternative simple surface nanostructuring method, saturated solvent vapor treatment, and applied this method to molecular double layer thin films consisting of two Pc materials with different solubility in organic solvents. The difference in solubility was expected to lead to selective dissolution of the materials, which should broaden the electronic absorption (EA) bands, i.e. combination of absorption bands from undissolved and dissolved material. In addition, variations in the surface nanostructures and prevention of substrate exposure due to the partial dissolution are also expected.
J. Kim, S. Yim / Materials Research Bulletin 47 (2012) 2744–2747
2. Experimental CuPc and ZnPc single and double layer thin films were grown by organic molecular beam deposition (OMBD) in an ultrahigh vacuum (UHV) chamber at a base pressure of 2 10 8 Torr. Commercially available CuPc (Aldrich Chemical, 97%) and ZnPc (Aldrich Chemical, 97%) powders were purified three times using temperature gradient sublimation. The purified materials were then outgassed in an OMBD chamber for 15–20 h before growth and then sublimed from miniature effusion cells onto well cleaned glass substrates held at room temperature. The cell temperature was 370 8C for both materials, which corresponded to a growth rate of 0.3 A˚/s as determined using a quartz crystal microbalance (QCM) positioned near the substrate. For saturated solvent vapor treatment, the films were transferred to another vacuum chamber (base pressure 2 10 4 Torr) that was attached to the OMBD chamber. Fig. 1 shows a schematic diagram of the system used for the treatment.
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The chamber was then saturated with acetone vapor evaporated from the liquid contained in a cylinder attached to the chamber. The pressure after acetone vapor saturation was 2.4 10 Torr. The films were maintained under saturated acetone vapor for 1 h and then transferred back to the OMBD chamber and dried at 100 8C for 1 h under ultrahigh vacuum. The surface morphology of the samples was analyzed using a tapping mode atomic force microscope (AFM) (SII SPA 400) and field emission scanning electron microscope (FE-SEM) (JEOL JSM 740F), and the EA of the films was recorded using an Ultraviolet (UV)–visible spectrophotometer (Scinco S-3100). 3. Results and discussion Three types of Pc thin films with a thickness of 400 A˚ were grown and treated with saturated acetone vapor; CuPc single layer (400 A˚), ZnPc single layer (400 A˚) and ZnPc (200 A˚) on CuPc (200 A˚) double layer film. The last double layer film was denoted as CuPc/
Fig. 1. Schematic diagram of the saturated solvent vapor treatment system used in this study.
Fig. 2. AFM images of the as-deposited (a) CuPc (400 A˚), (b) ZnPc (400 A˚) and (c) CuPc (200 A˚)/ZnPc (200 A˚) thin films, and saturated acetone vapor treated (d) CuPc (400 A˚), (e) ZnPc (400 A˚) and (f) CuPc (200 A˚)/ZnPc (200 A˚) thin films.
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J. Kim, S. Yim / Materials Research Bulletin 47 (2012) 2744–2747
Fig. 3. FE-SEM images of the saturated acetone vapor treated (a) CuPc (400 A˚), (b) ZnPc (400 A˚) and (c) CuPc (200 A˚)/ZnPc (200 A˚) thin films.
ZnPc. Fig. 2 shows surface AFM images of as-deposited (a–c) and saturated acetone vapor treated (d–f) CuPc (a and d), ZnPc (b and e) and CuPc/ZnPc (c and f) films. All images were taken with the same scan area of 1.5 mm 1.3 mm. The morphology of all the nontreated samples (Fig. 2a–c) was similar and characterized by tiny spherical crystallites with a root mean square (rms) roughness of 0.70 nm, 0.76 nm and 1.01 nm, respectively, which are typical of aphase planar Pc polymorph [22,23]. AFM images of treated samples (Fig. 2d–f) were significantly different from those of non-treated samples. The rms roughness increased to 2.66 nm, 9.27 nm and 5.48 nm, respectively, which were 3.8–12.2 times larger than those of non-treated films. The morphology and roughness changed most for the ZnPc (e) and least for the CuPc (d) single layer film. The change in morphology and roughness for the double layer film was between those of the two single layer films. The variation in morphological change might stem from the solubility difference between the two materials in acetone. We found that the solubility of ZnPc in acetone was 0.12 g/L, which was around two times larger than that of CuPc, 0.06 g/L, at room temperature. The change in surface morphology and formation of surface nanostructures were observed more clearly in the FE-SEM images (Fig. 3). For the saturated acetone vapor treated CuPc single layer thin film, the surface was covered with bundles of nanorods that had an average aspect ratio of 18.6 (average length and width of 670 nm and 36 nm, respectively) (Fig. 3a). In contrast, for the treated ZnPc single layer film, longer nanorods with widths ranging from 54 nm to 490 nm were observed (Fig. 3b). These long and wide surface nanostructures implied that more molecules were dissolved and rearranged due to the relatively high solubility of ZnPc in acetone. However, due to the high solubility, the substrate surface of the ZnPc single layer film was exposed through the nanorods. Beamshaped nanostructures with much smaller lengths and aspect ratios were observed for the double layer thin films. For the surface CuPc/ ZnPc thin film, the surface was covered with a mixture of nano-beam and nanorod shaped surface nanostructures (Fig. 3c). The average length, width and aspect ratio of the nano-beams were 260 nm, 160 nm and 1.63, respectively, and those of the nanorods were 160 nm, 45 nm and 3.56, respectively. Compared to the ZnPc single layer film, the significantly shortened nano-beams for the double layer thin film indicate that the less soluble CuPc molecules limited the dissolution and elongation of the ZnPc nanostructures. In the case of the double layer thin film, the substrate surface was not exposed, which was also probably due to the less soluble underlying CuPc layer. The EA spectra recorded for the as-deposited CuPc (Fig. 4a) and ZnPc (Fig. 4b) single layer thin films were indicative of the characteristic of a-phase Pc polymorph [22,24], where the absorption maxima were centered at 623 nm and 694 nm for CuPc, and 623 nm and 710 nm for ZnPc films. The EA spectrum for the as-deposited CuPc/ZnPc double layer thin film seemed to be a simple linear combination of the EA spectra from the CuPc and ZnPc single layer films (Fig. 4c). However, the EA spectra for the treated samples were largely dependent on the type of the film. For
the treated CuPc single layer film (Fig. 4d), the spectrum of the treated sample was not significantly different from the spectrum of the non-treated CuPc film (Fig. 4a), except that the shoulder peak at around 578 nm became more obvious. In contrast, remarkable changes were observed in the EA spectrum of the treated ZnPc single layer film (Fig. 4e). The intensity of the original peaks was significantly decreased and a new peak with noticeable intensity at 768 nm appeared. This peak corresponded to absorption from newly formed edge-to-edge (J-aggregates) molecular aggregation due to the dissolution and rearrangement of ZnPc molecules [22], which is in good agreement with the AFM and FE-SEM results. For the EA spectra of the treated double layer film (Fig. 4f), broadening of the absorption band ranging from 530 nm to 850 nm was clearly observed as expected. This broadening might stem from a combination of unchanged CuPc (Fig. 4d) and changed ZnPc (Fig. 4e) absorptions. In conclusion, we developed a simple saturated solvent vapor method to fabricate surface nanostructures on molecular thin films and applied this method to single and double layer CuPc and ZnPc
Fig. 4. Electronic absorption spectra recorded for the as-deposited (a) CuPc (400 A˚), (b) ZnPc (400 A˚) and (c) CuPc (200 A˚)/ZnPc (200 A˚) thin films, and saturated acetone vapor treated (d) CuPc (400 A˚), (e) ZnPc (400 A˚) and (f) CuPc (200 A˚)/ZnPc (200 A˚) thin films.
J. Kim, S. Yim / Materials Research Bulletin 47 (2012) 2744–2747
thin films. For the treated CuPc single layer film, little changes in the EA spectra were observed although bundles of nanorods were formed on the surface. In contrast, for ZnPc which has a relatively high solubility in acetone, remarkable changes in the EA spectra were observed, indicating that a part of ZnPc molecules dissolved and their molecular stacking was rearranged. The relatively high solubility of ZnPc also resulted in the formation of long and wide nanostructures on the film surface. Treatment of the double layer thin film led to several advantageous characteristics for optoelectronic device applications. First, the range of the EA was broadened by the combination of unchanged absorption of the CuPc layer and largely changed absorption of the ZnPc layer. Exposure of the substrate surface was also prevented in the CuPc/ZnPc double layer system due to the underlying CuPc layer which has significantly lower solubility in acetone. Acknowledgement This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0013057). References [1] C. Leznoff, A.B.P. Lever (Eds.), Phthalocyanines: Properties and Applications, VCH, Publishers, New York, 1996.
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[2] G. Guillaud, J. Simon, P. Germain, Coord. Chem. Rev. 178–180 (1998) 1433. [3] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [4] M. Riede, T. Mueller, W. Tress, R. Schueppel, K. Leo, Nanotechnology 19 (2008) 424001. [5] B.P. Rand, J. Genoe, P. Heremans, J. Poortmans, Prog. Photovolt.: Res. Appl. 15 (2007) 659. [6] F. Yang, M. Shtein, S.R. Forrest, Nat. Mater. 4 (2005) 37. [7] J. Xue, B.P. Rand, S. Uchida, S.R. Forrest, Adv. Mater. 17 (2005) 66. [8] P. Peumans, S.R. Forrest, Appl. Phys. Lett. 79 (2001) 126. [9] M.C. Scharber, D. Muhlbacher, M. Koppe, P. Denk, C. Waldauf, A.J. Heeger, C.J. Brabec, Adv. Mater. 18 (2006) 789. [10] D. Cheyns, K. Vasseur, C. Rolin, J. Genoe, J. Poortmans, P. Heremans, Nanotechnology 19 (2008) 424016. [11] D.H. Wang, D.-G. Choi, K.-J. Lee, J.-H. Jeong, S.H. Jeon, O.O. Park, J.H. Park, Org. Electron. 11 (2010) 285. [12] D.M. Nanditha, M. Dissanayake, A.A.D.T. Adikaari, R.J. Curry, R.A. Hatton, S.R.P. Silva, Appl. Phys. Lett. 90 (2007) 253502. [13] P. Peumans, S. Uchida, S.R. Forrest, Nature 425 (2003) 158. [14] J. Rostalski, D. Meissner, Sol. Energy Mater. Sol. Cells 61 (2000) 87. [15] F. Yang, M. Shtein, S.R. Forrest, J. Appl. Phys. 98 (2005) 014906. [16] G. Gu¨nes, H. Neugebauer, N.S. Sariciftci, Chem. Rev. 107 (2007) 1324. [17] S. Sun, Z. Fan, Y. Wang, J. Haliburton, J. Mater. Sci. 40 (2005) 1429. [18] F.A. Castro, H. Benmansour, C.F.O. Graeff, F. Nesch, E. Tutis, R. Hany, Chem. Mater. 18 (2006) 5504. [19] M.C. Orozco, W.C. Tsoi, M. O’Neil, M.P. Aldred, P. Vlachos, S.M. Kelly, Adv. Mater. 18 (2006) 1754. [20] X. Liang, T. Chen, Y.-S. Jung, Y. Miyamoto, G. Han, S. Cabrini, B. Ma, D.L. Olynick, ACS Nano 4 (2010) 2627. [21] H. Xi, Z. Wei, Z. Duan, W. Xu, D. Zhu, J. Phys. Chem. C 112 (2008) 19934. [22] J. Kim, C.R. Park, S. Yim, RSC Adv. 2 (2012) 963. [23] S. Senthilarasu, S.-J. Baek, S.D. Chavhan, J. Lee, S.-H. Lee, J. Nanosci. Nanotechnol. 8 (2008) 5414. [24] G. Park, I. Heo, I. Ryu, S. Yim, Bull. Korean Chem. Soc. 32 (2011) 943.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Electronic absorption band broadening and surface roughening of phthalocyanine double layers by saturated solvent vapor treatment Jinhyun Kim, Sanggyu Yim * Department of Chemistry, Kookmin University, Seoul 136-702, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 26 April 2012
Variations in the electronic absorption (EA) and surface morphology of three types of phthalocyanine (Pc) thin film systems, i.e. copper phthalocyanine (CuPc) single layer, zinc phthalocyanine (ZnPc) single layer, and ZnPc on CuPc (CuPc/ZnPc) double layer film, treated with saturated acetone vapor were investigated. For the treated CuPc single layer film, the surface roughness slightly increased and bundles of nanorods were formed, while the EA varied little. In contrast, for the ZnPc single layer film, the relatively high solubility of ZnPc led to a considerable shift in the absorption bands as well as a large increase in the surface roughness and formation of long and wide nano-beams, indicating a part of the ZnPc molecules dissolved in acetone, which altered their molecular stacking. For the CuPc/ZnPc film, the saturated acetone vapor treatment resulted in morphological changes in mainly the upper ZnPc layer due to the significantly low solubility of the underlying CuPc layer. The treatment also broadened the EA band, which involved a combination of unchanged CuPc and changed ZnPc absorption. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Surfaces A. Thin films C. Atomic force microscopy
1. Introduction Phthalocyanines (Pcs) are a particularly important class of organic semiconducting material finding use in a variety of optoelectronic devices such as organic photovoltaic (OPV) cells and organic light emitting diodes (OLEDs) due to their high stability and well-ordered film growth [1–3]. Copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc), planar Pc molecules with 18p-electrons in its aromatic system, are the most widely used electron donor materials in small-molecular weight based OPV device applications [4–6]. The current power conversion efficiency (PCE) of small-molecule OPV cells is between 4 and 5% for a single cell [7,8], which is relatively low when compared to inorganic solar cells. The main factors hampering further PCE enhancement of small-molecule OPV cells are the narrow spectral absorption range of the electron donor materials [9] and the short exciton diffusion lengths with respect to the film thickness [10,11]. In polymer solar cells, efforts have been made to broaden the absorption band of polymer materials through attachment of conjugated side chains [10] or the use of blended donors [11–13]. However, such approaches have not been used for small-molecule OPV cells. In regards to exciton diffusion lengths, several attempts to fabricate controlled nanostructures with a large contact area between electron donating and accepting layer and to increase the efficiency of charge (hole and electron) separation before
* Corresponding author. Tel.: +82 2 910 4734; fax: +82 2 910 4415. E-mail address: [email protected] (S. Yim). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.04.132
excitons recombination have been made [10–13]. The bulk heterojunction (BHJ), which can be produced by co-deposition or homogeneous blends was initially introduced [13,14]. This interpenetrating network resulted in a increase in the PCE to some extent, but the charge trapping at bottlenecks and cul-de-sacs induced by the random distribution of the materials limited any further increase in efficiency [6,15]. A heterolayer nanostructure with an interdigitated morphology has therefore been proposed to be an ideal interface [16,17], and various attempts to fabricate nanostructured interfaces have been made. The proposed technologies included nanoimprinting [18–20], solvent drop treatment [21], and solvent vapor spray treatment [22]. However, the nanoimprinting methods require long and complicated nano-processes. While solvent drop and solvent vapor spray treatment are much simpler compared to the nanoimprinting method, bulk molecules as well as surface molecules in thin films are also dissolved by the solvents and bare substrates are often exposed, which is disadvantageous for device applications. In this work, we developed an alternative simple surface nanostructuring method, saturated solvent vapor treatment, and applied this method to molecular double layer thin films consisting of two Pc materials with different solubility in organic solvents. The difference in solubility was expected to lead to selective dissolution of the materials, which should broaden the electronic absorption (EA) bands, i.e. combination of absorption bands from undissolved and dissolved material. In addition, variations in the surface nanostructures and prevention of substrate exposure due to the partial dissolution are also expected.
J. Kim, S. Yim / Materials Research Bulletin 47 (2012) 2744–2747
2. Experimental CuPc and ZnPc single and double layer thin films were grown by organic molecular beam deposition (OMBD) in an ultrahigh vacuum (UHV) chamber at a base pressure of 2 10 8 Torr. Commercially available CuPc (Aldrich Chemical, 97%) and ZnPc (Aldrich Chemical, 97%) powders were purified three times using temperature gradient sublimation. The purified materials were then outgassed in an OMBD chamber for 15–20 h before growth and then sublimed from miniature effusion cells onto well cleaned glass substrates held at room temperature. The cell temperature was 370 8C for both materials, which corresponded to a growth rate of 0.3 A˚/s as determined using a quartz crystal microbalance (QCM) positioned near the substrate. For saturated solvent vapor treatment, the films were transferred to another vacuum chamber (base pressure 2 10 4 Torr) that was attached to the OMBD chamber. Fig. 1 shows a schematic diagram of the system used for the treatment.
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The chamber was then saturated with acetone vapor evaporated from the liquid contained in a cylinder attached to the chamber. The pressure after acetone vapor saturation was 2.4 10 Torr. The films were maintained under saturated acetone vapor for 1 h and then transferred back to the OMBD chamber and dried at 100 8C for 1 h under ultrahigh vacuum. The surface morphology of the samples was analyzed using a tapping mode atomic force microscope (AFM) (SII SPA 400) and field emission scanning electron microscope (FE-SEM) (JEOL JSM 740F), and the EA of the films was recorded using an Ultraviolet (UV)–visible spectrophotometer (Scinco S-3100). 3. Results and discussion Three types of Pc thin films with a thickness of 400 A˚ were grown and treated with saturated acetone vapor; CuPc single layer (400 A˚), ZnPc single layer (400 A˚) and ZnPc (200 A˚) on CuPc (200 A˚) double layer film. The last double layer film was denoted as CuPc/
Fig. 1. Schematic diagram of the saturated solvent vapor treatment system used in this study.
Fig. 2. AFM images of the as-deposited (a) CuPc (400 A˚), (b) ZnPc (400 A˚) and (c) CuPc (200 A˚)/ZnPc (200 A˚) thin films, and saturated acetone vapor treated (d) CuPc (400 A˚), (e) ZnPc (400 A˚) and (f) CuPc (200 A˚)/ZnPc (200 A˚) thin films.
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J. Kim, S. Yim / Materials Research Bulletin 47 (2012) 2744–2747
Fig. 3. FE-SEM images of the saturated acetone vapor treated (a) CuPc (400 A˚), (b) ZnPc (400 A˚) and (c) CuPc (200 A˚)/ZnPc (200 A˚) thin films.
ZnPc. Fig. 2 shows surface AFM images of as-deposited (a–c) and saturated acetone vapor treated (d–f) CuPc (a and d), ZnPc (b and e) and CuPc/ZnPc (c and f) films. All images were taken with the same scan area of 1.5 mm 1.3 mm. The morphology of all the nontreated samples (Fig. 2a–c) was similar and characterized by tiny spherical crystallites with a root mean square (rms) roughness of 0.70 nm, 0.76 nm and 1.01 nm, respectively, which are typical of aphase planar Pc polymorph [22,23]. AFM images of treated samples (Fig. 2d–f) were significantly different from those of non-treated samples. The rms roughness increased to 2.66 nm, 9.27 nm and 5.48 nm, respectively, which were 3.8–12.2 times larger than those of non-treated films. The morphology and roughness changed most for the ZnPc (e) and least for the CuPc (d) single layer film. The change in morphology and roughness for the double layer film was between those of the two single layer films. The variation in morphological change might stem from the solubility difference between the two materials in acetone. We found that the solubility of ZnPc in acetone was 0.12 g/L, which was around two times larger than that of CuPc, 0.06 g/L, at room temperature. The change in surface morphology and formation of surface nanostructures were observed more clearly in the FE-SEM images (Fig. 3). For the saturated acetone vapor treated CuPc single layer thin film, the surface was covered with bundles of nanorods that had an average aspect ratio of 18.6 (average length and width of 670 nm and 36 nm, respectively) (Fig. 3a). In contrast, for the treated ZnPc single layer film, longer nanorods with widths ranging from 54 nm to 490 nm were observed (Fig. 3b). These long and wide surface nanostructures implied that more molecules were dissolved and rearranged due to the relatively high solubility of ZnPc in acetone. However, due to the high solubility, the substrate surface of the ZnPc single layer film was exposed through the nanorods. Beamshaped nanostructures with much smaller lengths and aspect ratios were observed for the double layer thin films. For the surface CuPc/ ZnPc thin film, the surface was covered with a mixture of nano-beam and nanorod shaped surface nanostructures (Fig. 3c). The average length, width and aspect ratio of the nano-beams were 260 nm, 160 nm and 1.63, respectively, and those of the nanorods were 160 nm, 45 nm and 3.56, respectively. Compared to the ZnPc single layer film, the significantly shortened nano-beams for the double layer thin film indicate that the less soluble CuPc molecules limited the dissolution and elongation of the ZnPc nanostructures. In the case of the double layer thin film, the substrate surface was not exposed, which was also probably due to the less soluble underlying CuPc layer. The EA spectra recorded for the as-deposited CuPc (Fig. 4a) and ZnPc (Fig. 4b) single layer thin films were indicative of the characteristic of a-phase Pc polymorph [22,24], where the absorption maxima were centered at 623 nm and 694 nm for CuPc, and 623 nm and 710 nm for ZnPc films. The EA spectrum for the as-deposited CuPc/ZnPc double layer thin film seemed to be a simple linear combination of the EA spectra from the CuPc and ZnPc single layer films (Fig. 4c). However, the EA spectra for the treated samples were largely dependent on the type of the film. For
the treated CuPc single layer film (Fig. 4d), the spectrum of the treated sample was not significantly different from the spectrum of the non-treated CuPc film (Fig. 4a), except that the shoulder peak at around 578 nm became more obvious. In contrast, remarkable changes were observed in the EA spectrum of the treated ZnPc single layer film (Fig. 4e). The intensity of the original peaks was significantly decreased and a new peak with noticeable intensity at 768 nm appeared. This peak corresponded to absorption from newly formed edge-to-edge (J-aggregates) molecular aggregation due to the dissolution and rearrangement of ZnPc molecules [22], which is in good agreement with the AFM and FE-SEM results. For the EA spectra of the treated double layer film (Fig. 4f), broadening of the absorption band ranging from 530 nm to 850 nm was clearly observed as expected. This broadening might stem from a combination of unchanged CuPc (Fig. 4d) and changed ZnPc (Fig. 4e) absorptions. In conclusion, we developed a simple saturated solvent vapor method to fabricate surface nanostructures on molecular thin films and applied this method to single and double layer CuPc and ZnPc
Fig. 4. Electronic absorption spectra recorded for the as-deposited (a) CuPc (400 A˚), (b) ZnPc (400 A˚) and (c) CuPc (200 A˚)/ZnPc (200 A˚) thin films, and saturated acetone vapor treated (d) CuPc (400 A˚), (e) ZnPc (400 A˚) and (f) CuPc (200 A˚)/ZnPc (200 A˚) thin films.
J. Kim, S. Yim / Materials Research Bulletin 47 (2012) 2744–2747
thin films. For the treated CuPc single layer film, little changes in the EA spectra were observed although bundles of nanorods were formed on the surface. In contrast, for ZnPc which has a relatively high solubility in acetone, remarkable changes in the EA spectra were observed, indicating that a part of ZnPc molecules dissolved and their molecular stacking was rearranged. The relatively high solubility of ZnPc also resulted in the formation of long and wide nanostructures on the film surface. Treatment of the double layer thin film led to several advantageous characteristics for optoelectronic device applications. First, the range of the EA was broadened by the combination of unchanged absorption of the CuPc layer and largely changed absorption of the ZnPc layer. Exposure of the substrate surface was also prevented in the CuPc/ZnPc double layer system due to the underlying CuPc layer which has significantly lower solubility in acetone. Acknowledgement This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0013057). References [1] C. Leznoff, A.B.P. Lever (Eds.), Phthalocyanines: Properties and Applications, VCH, Publishers, New York, 1996.
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