19934
J. Phys. Chem. C 2008, 112, 19934–19938
Facile Method for Fabrication of Nanostructured CuPC Thin Films To Enhance Photocurrent Generation Hongxia Xi,†,‡ Zhongming Wei,†,‡ Zhiming Duan,†,‡ Wei Xu,*,† and Daoben Zhu*,† Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, China ReceiVed: September 11, 2008; ReVised Manuscript ReceiVed: October 20, 2008
It has been demonstrated that bulk heterojunction with controlled nanostructures is needed to improve the performance of the organic solar cells. But, simple methods for production of such nanostructure are still rare. We found that when several drops of chloroform, toluene, or acetone were added on the surface of vacuum-deposited CuPC films, nanorods with diameters of about 50 nm could be obtained after the evaporation of the solvent. The size, shape, and orientation of these nanostructures changed with the solvent used. In comparison with the flat bilayer cells, significant increase in photocurrent generation was observed in the bilayer cells based on such nanostructured films with a spin-coated [6,6]-phenyl-C61-butyric acid methyl ester thin film as the acceptor materials. The nanostructures with more ordered orientation that resulted from acetone treating displayed the largest enhancement in the photocurrent generation. Introduction In the past decade, due to the increasing demand of renewable energy supply, organic solar cells (OSCs) have attracted growing interests for their potential applications with low-cost and largescale fabrication.1-3 However, despite much development of both novel materials4 and device architectures,3,5 the power conversion efficiency (ηp) of OSCs was still lower when compared with inorganic solar cells or dye-sensitized solar cells.6 For further improvement of OSCs, the current density under short-circuit condition (Jsc) was one of the crucial parameters needed to be optimized. In OSCs, one of the key processes related to photocurrent generation is the dissociation of photogenerated excitons into free charge carriers at the donor/acceptor interface. So an enhanced interface is needed for the efficient dissociation of the excitons. Currently, donor/acceptor bulk heterojunction, which was usually induced by the phase separation of the homogeneous blend, has been used for the achievement of a large donor/acceptor interface. However, such a random interpenetrating network will lead to the charge trapping at bottlenecks and cul-de-sacs in the conducting pathway to the electrodes.1 So, a bulk heterojunction with interdigitated morphology has been supposed as the ideal structure for OSCs,2a,7 where the donor and acceptor materials form continuous columns with an average scale equal or less than the excitons diffusion length laying vertically to the electrodes. This structure can ensure a large interface for efficient exciton dissociation as well as a continuous conducting pathway for rapid charge carrier transport.2a,7 Fabrication of such a well-organized nanostructure is not easy. To date, research work toward this aspect is still rare. Controlled organic vapor-phase deposition1 was employed by Forrest and co-workers, and a nanoimpriting method8 has been used by Dissanayake et al. for construction of nanostructured small molecular OSCs; a surface-directed demixing * To whom correspondence should be addressed. E-mail:
[email protected] (W.X.);
[email protected] (D.Z.). † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.
template9 and a nematic gel template10 have been used by Hany et al. and O′Neill et al. for fabrication of nanostructured polymer solar cells, respectively. Here we present a simple solvent treating method to fabricate copper phthalocyanine (CuPC) thin film with controlled nanostructure. Bulk heterojunctions were formed by spin-coating an electron-acceptor material ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM) on top of these CuPC films. Preliminarily, these nanostructured heterojunctions were subjected to the studies of photocurrent generation by using photoelectrochemical cells. The surface-enhanced films presented substantial increase in photocurrent compared with a reference photoelectrode without solvent treatment. Experimental Section Preparation of the Nanostructured Thin Film. CuPC was purchased from Aldrich Co. and purified by gradient sublimation twice. PCBM was purchased from Solenne and used without further purifying. ITO glasses were cleaned by surfactant, deionized water, ethanol, and chloroform in an ultrasonic bath before use. Under a base pressure of (4-6) × 10-4 Pa, thin films of CuPC with thickness of 50 nm were vacuum-deposited at 0.3 Å/s on ITO substrates at room temperature. The deposition rate and film thickness were monitored by ULVAC CRTM6000. In the solvent treatment experiments, CuPC thin film was put into a Petri dish. The solvent was dropped onto the film and covered the film as a thin liquid layer. After the solvent volatilized completely at room temperature, PCBM was deposited subsequently by spin-coating on top of the CuPC film using a solution in chlorobenzene (10 mg/mL). The reference electrode was fabricated by the same way without the solvent treatment. Photo-electrochemical Measurements. The photocurrent response was measured in a three-electrode photo-electrochemistry cell. ITO glasses modified with the nanostructured bilayer thin films were used as the working electrode (WE). A platinum wire was used as the counter electrode (CE), and the saturated calomel electrode was used as the reference electrode (RE). An
10.1021/jp8080673 CCC: $40.75 2008 American Chemical Society Published on Web 11/19/2008
Fabrication of Nanostructured CuPC Thin Films
Figure 1. SEM images of the CuPC films: (a) the CuPC thin film as-deposited on ITO glass; (b) the film treated by CHCl3 (the inset shows a side view of this film); (c) the film treated by toluene; (d) the film treated by acetone.
aqueous solution of 0.5 M KCl was used as the supporting electrolyte in all measurements. Photocurrents of the films were measured on a model 660C electrochemistry analyzer (CHI660C, Chenhua Instrument Co., Shanghai, China). A halogen lamp (CMH-250, Aobodi, Beijing, China) was used as the light source and the intensities of incident beams were checked by a radiometer (FZ-A, Beijing, China). The light intensity was changed from 1 to 70 mW/cm2 during the experiments. X-ray diffraction (XRD) measurements were carried out in the reflection mode at room temperature (RT) using a 2 kW Rigaku X-ray diffractometer (Cu KR radiation, λ ) 1.54 Å). Atomic force microscopy (AFM) measurements were carried out with Multimode Nanoscope controller IIIa (Veeco Inc.) operating in tapping mode. The scanning electron microscopy (SEM) images were obtained with instrument Hitachi S-4800 SE. Results and Discussion Phthalocyanines (PCs) are important molecular materials due to their outstanding electronic and optical properties as well as chemical and thermal stabilities.11 To date, the PCs have been one of most intensively studied materials for small molecular solar cells, due to their high extinction coefficients in the visible region and their relatively high hole mobility.12 A highest power conversion efficiency of 5.5% has been achieved on the basis of a tandem cell with CuPC/C60 heterojunctions.13 It has been realized that for further improvement of the efficiency, a nanostructured donor/acceptor interface with controlled morphology is demanded.1a Due to the coplanar rigid structure, unsubsituted PCs display poor solubility in common organic solvents. Most of the CuPC thin films for device fabrication are prepared by vacuum deposition, and the nanostructures are produced by control of the deposition condition1,14 or use of a template.15 And it is difficult to grow nanostructures with a scale down to 50 nm that is required for the diffusion of exciton to reach the interface in its lifetime. Considering the low solubility of CuPC, it may be possible to fabricate nanostructures of CuPC via solution processes. On the basis of the above consideration, a series of experiments were performed. First, thin films of CuPC with a thickness of 50 nm were vacuum-deposited on ITO substrates. From SEM and AFM images (Figures 1a and 2a), it can be seen that these films are
J. Phys. Chem. C, Vol. 112, No. 50, 2008 19935 vary flat. Then several drops of solvent (such as chloroform, toluene, or acetone) were added on the surface of the asdeposited films; after evaporating the solvent under ambient condition, the surface morphology of the film was dramatically changed. Many nanorods had grown up from the underlying film that maintained continuous and unbroken. These nanorods had quite uniform shape and size. As shown in Figure 1b–d, different solvents led to different morphologies. When chloroform or toluene was used, the resulting nanorods showed similar random orientations (as shown in Figure 1b,c) with different sizes. For the films treated with chloroform, the nanorods were obtained with diameters of about 25-40 nm (calculated by SEM data) and lengths of about 200-250 nm, while, for the films treated with toluene, the nanorods showed diameters of about 40-60 nm and lengths of about 300-400 nm. Different from these randomly oriented nanorods, dropping acetone on the film resulted in mostly vertical oriented nanorods with diameters of about 50-70 nm (after solvent treatment for three times, it will be discussed later with Figure 3). According to the previous reports, the exciton diffusion length in CuPC was about 10-50 nm.1 So the dimension of these nanostructures obtained by solvent treating matched well with the exciton diffusion length. Through the measurement of AFM, we can calculate the data about the surface characters of these films. The original CuPC film had a root-mean square (rms) surface roughness of 3 nm, and the average height of peak was 4 nm. After treatment with solvents, the rms surface roughness increased to 15, 14, and 10 nm and the average height of peak was about 48, 40, and 30 nm for chloroform, toluene, and acetone treatment, respectively. These resulted in an increase of the surface area from 4 to 6, 5, and 5 µm2 for chloroform, toluene, and acetone, respectively. To find out the possible reason of the formation of the above nanostructures, other controlled experiments were carried out. When we immersed the as-deposited CuPC film into the chloroform for several minutes, if this film was taken out and put into a dish with the CuPC film facing up and the remnant solvent was allowed to dissipate naturally, similar nanostructures could be observed. However, if the remnant solvent on the surface was sucked with a piece of filter paper immediately after the film was taken out from the solvent, just some increase in roughness of the surface but no nanostructures could be observed (see the Supporting Information). These indicated that the nanostructure formed during the progress of the volatilization of the solvent. When the solvent was dropped on the film, a thin layer of the liquid covered the surface of the film, CuPC began to dissolve slowly from the surface of the film. Due to the poor solubility of CuPC and limited volume of solvent, the dissolved CuPC was very little. If this solution was taken away by sucking with a filter paper, the surface morphology was changed by dissolution but not so significantly. While the solvent volatilized, the dissolved CuPC recrystallized on the surface and resulted in the formation of those nanostructures. The difference in polarity of the solvents should lead to different interactions with the CuPC molecules. So the resulting nanostructures displayed different shapes, size, and orientation. In addition, the different solubility and boiling point of the solvents should also contribute to the variation of the nanostructures. For the example of acetone, the solubility of CuPC in acetone is ultrasmall, and the boiling point of acetone is lower than that of CHCl3 and toluene. So, the morphology changed a little after being treated with actone once. However, the treatment can be carried out time after time, and the roughness and the surface area will be increased consequently. As shown in Figure 3, the rms and the surface area increase linearly with
19936 J. Phys. Chem. C, Vol. 112, No. 50, 2008
Xi et al.
Figure 2. 3D height AFM images of the CuPC films: (a) the as-deposited CuPC thin film on ITO glass; (b) the film treated with CHCl3; (c) the film treated with toluene; (d) the film treated with acetone.
Figure 3. Relationship of the roughness (rms) and surface area of CuPC films with the times of acetone treating.
the times of treatments (the AFM images of these film are presented in the Supporting Information). In this work, the film was treated three times before deposition of PCBM. The X-ray diffraction patterns (XRD) of 50 nm thick CuPC films are shown in Figure 4. To ensure the high degree of crystallization of the deposition film, the rate of the deposition was very slow. The diffraction peak at 2θ ) 6.7° confirms the existence of R-phase CuPC.16 After treatment with solvent, the diffraction intensity has changed little except that the films
Figure 4. X-ray diffraction patterns of CuPC films grow on ITO treated by different solvents. “None” presents the original film without solvent treatment. The films thicknesses are 50 nm.
treated with toluene showed a small increase. In a previous report, the metal phthalocyanines changed from one phase to another phase or crystallinity enhanced when putting them in some solvents’ vapor for 24 h17 or in solvents for hours.18 In our experiments, the solvent treatment was in a few minutes and the diffraction peak has changed little. This indicated that the short-time treatment did not destroy the stacking pattern of
Fabrication of Nanostructured CuPC Thin Films
J. Phys. Chem. C, Vol. 112, No. 50, 2008 19937
Figure 5. Photocurrent responses of the CuPC/PCBM films upon switching the light on and off. The supporting electrolyte was an aqueous solution containing 0.5 M KCl and saturated with O2, without bias. (a) Responses of the original film and film treated by three solvents. Input power: 40 mW/cm2. The arrows illustrate the determination of the photocurrent values (Jph) from the photocurrent transients. (b) Response dependence on the input power.
TABLE 1: Increase of the Surface Area and the Photocurrent of the Films Treated by Different Solvents
solvent none chloroform toluene acetone
rms surface roughnessa areaa increment photocurrent increment (nm) (µm2) (%) (µA/cm2) (%) 3 15 14 10
4 6 5 5
0 50 25 25
56.1 69.1 134.8 185.3
0 23 140 230
a The rms roughness and surface area were calculated by the software of AFM.
the CuPC molecules in solid state and only changed the surface morphology. Such changes in the surface morphology will enhance the photocurrent generation from donor/accepter bilayers based on these films. Bilayer structures were formed by spin-coating PCBM on top of these CuPC films. The SEM images of the cross-section of these bilayer films showed that the two materials combined each other very well for the films treated or without treated by solvent (see the Supporting Information). Then these CuPC/ PCBM films on ITO were put in a three-electrode photoelectrochemistry cell to measure the photocurrent. The results are shown in Figure 5. The photocurrents of these solvent-treated electrodes increased from 56.1 µA/cm2 of the reference electrode to 69.1, 134.8, and 185.3 µA/cm2 for the films treated by chloroform, toluene, and acetone, respectively. As summarized in Table 1, the photocurrent increment did not fit with the surface area increment, and the photocurrents had different increments for the films containing different nanostructures. When CuPC films were treated with toluene or acetone, the increment of the surface area was less than 15%, but the increment of photocurrent was more than 1- or 2-fold. This indicated that the nanostructure may not only enhance the exciton dissociation efficiency but also accelerate the charge carrier transport, since most of the nanostructures had diameters within 50 nm, which matched well with the exciton diffusion length in CuPC. And the vertical oriented structure should fast the charge carrier transport and suppress the recombination; such effect was especially obvious for the film treated with acetone. To provide further information about the recombination losses in the films, the films were subjected to light intensity dependence measurement. The performances are shown in
Figure 4b. The linear fits exhibit slopes of 0.66 for the reference electrode and 0.71, 0.82, and 0.85 for the electrode treated with chloroform, toluene, and acetone, respectively. The scaling exponent close to 1 suggests that both electron and hole transport are comparably efficient (no space-charge limiting) and bimolecular recombination is not significant.19,20 The materials forming the electrode are the same and the XRD data have no significant diferences, so for these electrodes the carrier loss due to the space-charge limiting should be at the same level. As a result, the difference in the slope is mainly due to the difference in the bimolecular recombination. As shown in Figure 4b, the carrier loss in the reference film (34%) was reduced significantly after being treated with toluene (18%) and acetone (15%). This indicated that the nanostructures indeed reduces the bimolecular recombination. For the films treated with toluene and acetone, they showed similar bimolecular recombination loss and interface area enhancement, but a large difference in the increase of photocurrent, and the orientation of the resulted nanorods is different. Compared with the randomly oriented nanorods in the film treated by toluene, the vertical nanorods in the film treated with acetone should be more favorable for the efficient carrier transport. It is quite strange that for the chloroform treated film, which displayed fewer enhancements in the photocurrent, although the surface area increment was quite large. One possible reason is that the larger surface roughness may block the proper contact of donor and acceptor materials and resulted in larger series resistance. Conclusion The extremely low solubility of CuPC in common organic solvents has been employed for the fabrication of nanostrucutured CuPC thin films. Nanorods with diameters of about 40-60 nm and lengths of about 200-400 nm were resulted from a dissolution-crystallization process when drops of solvent were casted and evaporated on the surface of the CuPC thin films prepared by vacuum deposition. Nanorods with random orientations were fabricated by treatment with chloroform and toluene, while vertical oriented nanorods were fabricated by acetone treatment. The treatment can be repeated if further increase of the surface area is needed; that is especially important for treating with acetone the solvent with ultralow solubility
19938 J. Phys. Chem. C, Vol. 112, No. 50, 2008 SCHEME 1: Schematic Diagram Describing the Fabrication Process of an Interface-Enhanced Bilayer Film
Xi et al. CuPC/PCBM films.This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
for CuPC. The XRD patterns of the thin films before and after treatment showed that there are no phase transitions during the solvent treating. These nanostructures were used for the fabrication of interface enhanced donor/acceptor bilayer junctions with spin-coated PCBM film on its surface. In photo-electrochemical studies, with an incident photointensity of 40 mW/cm2, a 230% increment in photocurrent was observed on the basis of acetonetreated CuPC films with a very limited increment of the surface area (less than 15%). For the cases of chloroform and toluene treatment, although the surface area increased not less than that of acetone treating, the photocurrent enhancements are much smaller (23% increment for chloroform and 140% increment for toluene). This indicated that besides the increase of surface area, the vertical orientation of the nanostructures played a more important role in the enhancement of photocurrent generation, as for a continuous pathway is essential for the efficient charge transport. The above studies demonstrated that CuPC nanostructures fabricated by a simple solvent treating method can be used for the construction of interface-enhanced donor/ acceptor bilayer junctions for efficient photocurrent generation. And it may find applications for the improvement of the performance of photovoltaic cells. Acknowledgment. This work was supported by National Natural Science Foundation of China (Grants 20572113 and 20721061),StateKeyBasicResearchProgram(Grant2006CB9321001), and Chinese Academy of Sciences. Supporting Information Available: More AFM images of the CuPC films and SEM images of the cross-section of the
(1) (a) Yang, F.; Shtein, M.; Forrest, S. R. Nat. Mater. 2005, 4, 37. (b) Yang, F.; Shtein, M.; Forrest, S. R. J. App. Phy. 2005, 98, 014906. (2) (a) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324. (b) Thompson, B. C.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (3) Kim, J. Y.; Kim, S. H.; Lee, H. H.; Lee, K.; Ma, W.; Gong, X.; Heeger, A. J. AdV. Mater. 2006, 18, 572. (4) (a) Scharber, M. C.; Mu¨hlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. AdV. Mater. 2006, 18, 789. (b) Wong, W. Y.; Wang, X. Z.; He, Z.; Djurisˇiæ, A. B.; Yip, C. T.; Cheung, K. Y.; Wang, H.; Mak, C. S. K.; Chan, W. K. Nat. Mater. 2007, 6, 521. (c) Blouin, N.; Michaud, A.; Leclerc, M. AdV. Mater. 2007, 19, 2295. (d) Mu¨hlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C. AdV. Mater. 2006, 18, 2884. (5) (a) Huang, J.; Li, G.; Yang, Y. AdV. Mater. 2008, 20, 415. (b) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222. (6) (a) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (b) Ma, T.; Akiyama, M.; Abe, E.; Imai, I. Nano Lett. 2005, 5, 2543. (c) Hwang, S.; Lee, J. H.; Park, C.; Lee, H.; Kim, C.; Park, C.; Lee, M. H.; Lee, W.; Park, J.; Kim, K.; Park, N. G.; Kim, C. Chem. Commun. (Cambridge) 2007, 4887. (7) Sun, S.; Fan, Z.; Wang, Y.; Haliburton, J. J. Mater. Sci. 2005, 40, 1429. (8) Dissanayake, D. M. N. M.; Adikaari, A. A. D. T.; Curry, R. J.; Hatton, R. A.; Silva, S. R. P. Appl. Phys. Lett. 2007, 90, 253502. (9) Castro, F. A.; Benmansour, H.; Graeff, C. F. O.; Nesch, F.; Tutis, E.; Hany, R. Chem. Mater. 2006, 18, 5504. (10) Orozco, M. C.; Tsoi, W. C.; O′Neil, M.; Aldred, M. P.; Vlachos, P.; Kelly, S. M. AdV. Mater. 2006, 18, 1754. (11) (a) Torre, G.; Claessens, C. G.; Torres, T. Chem. Commun. (Cambridge) 2007, 2000. (b) Elemans, J. A. A. W.; Hameren, R.; Nolte, R. J. M.; Rowan, A. E. AdV. Mater. 2006, 18, 1251. (12) (a) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (b) Wo¨hrle, D.; Meissner, D. AdV. Mater. 1991, 3, 129. (c) Xue, J. G.; Uchida, S.; Rand, B. P.; Forrest, S. R. Appl. Phys. Lett. 2004, 84, 3013. (d) Koeppe, R.; Sariciftci, N. S.; Troshin, P. A.; Lyubovskaya, R. N. Appl. Phys. Lett. 2005, 87, 244102. (e) Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2006, 128, 8108. (13) Xue, J. G.; Uchida, S.; Rand, B. P.; Forrest, S. R. Appl. Phys. Lett. 2004, 85, 5757. (14) (a) Tong, W. Y.; Djurisˇiæ, A. B.; Xie, M. H.; Ng, A. C. M.; Cheung, K. Y.; Chan, W. K.; Leung, Y. H.; Lin, H. W.; Gwo, S. J. Phys. Chem. B 2006, 110, 17406. (b) Karan, S.; Mallik, B. J. Phys. Chem. C 2008, 112, 2436. (c) Xiao, K.; Liu, Y.; Yu, G.; Zhu, D. Appl. Phys. A: Mater. Sci. Process. 2003, 77, 367. (15) McLachlan, M. A.; McComb, D. W.; Berhanu, S.; Jones, T. S. J. Mater. Chem. 2007, 3773. (16) Berger, O.; Fischer, W. J.; Adolphi, B.; Tierbach, S. J. Mater. Sci. 2000, 11, 331. (17) Iwatsu, F.; Kobayashi, T.; Uyeda, N. J. Phys. Chem. 1980, 84, 3223. (18) Suito, E.; Uyeda, N. Kolloid Z. Z. Polym 1963, 193, 7. (19) Gregg, B. A. J. Phys. Chem. 1996, 100, 852. (20) Hoth, C. N.; Choulis, S. A.; Schilinsky, P.; Brabec, C. J. AdV. Mater. 2007, 19, 3973.
JP8080673
