Electrochemical Synthesis and Photovoltaic Property of Cadmium

J. Phys. Chem. C 2008, 112, 19765–19769

19765

Electrochemical Synthesis and Photovoltaic Property of Cadmium Sulfide-Polybithiophene Interdigitated Nanohybrid Thin Films Dongjuan Xi,† Han Zhang,† Stephen Furst,‡ Bin Chen,§ and Qibing Pei*,† Department of Materials Science and Engineering, The Henry Samueli School of Engineering and Applied Science, UniVersity of California at Los Angeles, Los Angeles, California 90095, Department of Electrical Engineering, North Carolina State UniVersity, NASA Space Program, and NASA Ames Research Center, Moffett Field, California 94035 ReceiVed: September 4, 2008; ReVised Manuscript ReceiVed: October 13, 2008

Interdigitated hybrid films consisting of cadmium sulfide (CdS) nanorod arrays and interpenetrating polybithiophene have been synthesized by electrochemical depositions. The vertically aligned CdS nanorods were self-assembled on gold-coated glass substrates through a simple cathodic process of an electrolyte solution containing cadmium sulfate and potassium thiocyanate, without the use of any templates. The conjugated polymer polybithiophene was deposited into the nanorod arrays by in situ electrochemical polymerization. The resulting interdigitated nanohybrid films showed dense packing of the polymer in the nanorod arrays with a filling ratio estimated to be 76%. Raman spectroscopy and Fourier transform infrared revealed chargetransfer between the polymer and the CdS nanorods, consistent with the high filling ratio and improved polymer-CdS interface. Solar cells based on these hybrid films with vapor-deposited aluminum cathode exhibited a diode characteristic with a rectification ratio of 103 at ( 1 V. The measured open-circuit voltage was 0.84 V, and the short-circuit current was 0.52 mA/cm2. The overall power conversion efficiency was 0.38%. Introduction In recent years, hybrid films composed of inorganic semiconductor nanocrystals and conjugated polymers have attracted extensive attention in developing polymer solar cells due to short exciton diffusion length and low electron mobility in existing polymers.1-20 The discovery that power conversion efficiency can be improved by aligning the nanocrystals has triggered a broad interest in developing interdigitated bicontinuous phases consisting of a well-ordered p- or n-nanorod array and a wellinterfaced complementary layer.3,8,19,20 Many efforts have been dedicated to fabricating this idealized architecture, including infiltrating poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene (MEH-PPV) or poly(3-hexylthiophene) (P3HT) into zinc oxide (ZnO) nanorod arrays,21-23 silicon nanorod arrays, or titanium oxide (TiO2) nanoporous films.24-29 However, these efforts have met limited success due to the difficulty in synthesizing nanorod arrays with suitable semiconductor materials, nanorod geometry, and inter-rod spacing. Infiltrating the relatively nonpolar conjugated polymers into the polar inorganic semiconductor nanorod arrays also proves to be rather challenging.30 The high viscosity of the molten long-chain polymers restrains the diffusion of polymers, reducing the filling ratio of polymer inside the nanorod arrays. The low wettability may induce voids between the polymer and inorganic semiconductors and consequently impede charge transfer between the two phases.30,31 In the present paper, a new hybrid system of cadmium sulfide (CdS) nanorod arrays and in situ electropolymerized polybithiophene is described. CdS, a II-IV semiconductor with a * Corresponding author. E-mail: [email protected]. † University of California at Los Angeles. ‡ North Carolina State University. § NASA Ames Research Center.

direct bandgap of 2.4 eV, has proven to be a good acceptor in CdS/CdTe and other important inorganic solar cells. However, the direct preparation of vertically aligned CdS nanorod arrays on transparent substrates from solution process is not well studied. Template synthesis of CdS nanowires via anodized alumina templates is rather complicated,32 and uniformity of the transferred nanorods from the templates is hard to control unless special design is integrated.33 We have found that vertically aligned CdS nanorods can be grown on glass substrates by template-free electrochemical deposition. The geometry of the nanorods and the inter-rod spacing are suitable for polymer solar cell application. Moreover, we have used in situ electropolymerization to deposit conjugated polymers into the CdS nanorod arrays.34 A higher filling ratio can be achieved thanks to the greater penetration capability of the monomers than that of the molten polymers. The resulting interdigitated hybrid films have been examined by scanning electron microscopy and UV-vis absorption to study the incorporation of electropolymerized polybithiophene inside the nanorod arrays. The interface between the two phases, polybithiophene and the CdS nanorods, has been investigated by Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). Lastly, photovoltaic response of the CdS-polybithiophene interdigitated hybrid films will be described. Experimental CdS nanorods were deposited on glass substrates covered by a 1 nm thick titanium layer and a 10 nm thick gold layer. The transmission of the electrode substrate was 70% in the visible region. The electrolyte was prepared by dissolving cadmium sulfate (0.04 M) and potassium thiocyanate (0.5 M) in deionized water,35,36 with pH adjusted to 1.8 by adding hydrochloric acid. The chlorine concentration was 0.015 M. During deposition, the electrolyte was heated to 70 °C. A negative potential of

10.1021/jp807868j CCC: $40.75  2008 American Chemical Society Published on Web 11/14/2008

19766 J. Phys. Chem. C, Vol. 112, No. 49, 2008 -750 mV was applied on the working electrode vs a platinum counter electrode. A layer of yellowish film containing CdS nanorods was observed after several hours of deposition. Before further depositing polybithiophene, substrates containing CdS nanorod arrays were first cleaned by O3 plasma and subsequently soaked in propylene carbonate containing 0.2 M 2,2′-bithiophene and 0.5 M lithium perchlorate. Polymerization was carried out on a BASi CV-50w voltammetric analyzer with Ag/AgCl as the reference electrode and a platinum sheet as a counter electrode. During polymerization, the working potential was kept constant at 660 mV. The measured current density was approximately 3 µA/cm2. The as-obtained polymer was at a doped (oxidized) state with perchlorate ions as the dopants. It was subsequently dedoped (neutralized) to release perchlorate ions. To prepare hybrid films for photovoltaic application, polybithiophene was continuously deposited until the total hybrid film thickness reached approximately 200 nm. The cross section of the hybrid film was inspected by scanning electronic microscopy (SEM), and the absorption spectrum was taken on a UV-visible spectrophotometer. Subsequently, a layer of metal film composed of 20 nm calcium and 200 nm aluminum in sequence was deposited on the hybrid films by E-beam evaporation under 10-7 Torr as top electrodes to study the photovoltaic performance of the interdigitated hybrid films. Samples with thin, neutralized polybithiophene electropolymerized on the CdS nanorod arrays were also prepared for investigation the hybrid interfaces by Raman spectroscopy under the excitation of the He:Ne laser at 633 nm and by FTIR under a reflection mode. The polymer film was 2-3 nm thick and controlled via the electropolymerization charge, to minimize the interference of polymers in the bulk on the characteristic signals resulting from charge transfer between the polymer and CdS at the interface. Pure neutralized polybithiophenes electropolymerized on flat gold substrates were also prepared for comparison.

Xi et al.

Figure 1. SEM image of a CdS nanorod array (side view). The inset is an enlarged top view of the nanorods.

Figure 2. XRD of a CdS nanorod array on gold-covered glass substrate: the bare substrate is shown by the dashed line.

Results and Discussions The cathodic electrochemical process of aqueous solutions containing cadmium sulfate and potassium thiocyanate formed vertically aligned CdS nanorods on the gold-covered glass substrates. The dimension of the nanorods was tailored by experimental conditions with diameter from 20 to 40 nm and height from 100 to 300 nm. The optical properties of these prepared CdS nanorods have been studied in other papers.37,38 On the basis of the optical absorption length of polybithiophene, nanorod arrays with 25 nm nanorod diameter and 150 nm average height, as shown in Figure 1, were used to fabricate nanohybrid solar cells. The enlarged top view of the nanorods, as shown in the inset of Figure 1, exhibits a wurtzite structure of these nanorods with a hexagonal top. All the nanorods orient along the [001] direction. The average rod-to-rod spacing is approximately 20 nm, and the estimated nanorod density is about 4 × 1010 cm2, which leaves 75% opening space in the array for the complementary polymer to interpenetrate. The X-ray spectrum is shown in Figure 2 (solid line). A single diffraction peak observed at 26.8°, in comparison with Au-glass substrates (dashed line), is assigned to the (002) plane of wurtzite CdS.39,40 Bithiophene electropolymerized uniformly onto the gold substrates coated with the CdS nanorod arrays. Figure 3 shows thecross-sectionandtopviewoftheresultingCdS-polybithiophene nanohybrid film under SEM. The bright cylindrical rods are CdS, and the dark material is polybithiophene. The hybrid film is uniform over the entire substrate. The cross-section at 100K magnification clearly shows that polybithiophene interpenetrates

Figure 3. Cross-sectional SEM image of a CdS-polybithiophene hybrid film. The inset shows the hybrid at 100K magnification.

inside the CdS nanorod array, embraces the nanorods from the root to top, and occupies nearly the entire space surrounding the nanorods. The polythiophene also deposits above the nanorods, forming a solid film approximately 50 nm thick which can potentially prevent electron back transfer and reduce dark (leakage) current. This formation of solid polybithiophene film is contrary to the electropolymerization in nanoporous templates such as anodized alumina, where nanotubes are formed on the walls of the deep and narrow nanowells.41,42 The formation of the continuous polymer phase in the nanorod arrays could increase the absorption efficiency of the hybrid films. The absorption spectrum of the CdS-polybithiophene nanohybrid film, with gold-covered glass substrate as a reference, is

CdS-Polybithiophene Nanohybrid Thin Films

Figure 4. UV-vis absorption spectrum of CdS-polybithiophene nanohybrid film (solid line). The two fitting curves are the absorption spectra of CdS nanorods (dashed line) and polybithiophene (dotted line), respectively.

displayed in Figure 4. The absorbance spectrum has a peak at 505 nm, and it can be decomposed into two spectra peaked at 400 nm for pure CdS and 520 nm for polybithiophene, respectively.43,44 Since the absorption coefficient of polybithiophene is 5 × 104 cm at 520 nm,45,46 the absorption intensity of polybithiophene at 520 nm, 0.68, suggests that the polybithiophene in the hybrid film is equivalent to a 136 nm thick solid flat film. Considering that there is a 50 nm polybithiophene overcoat atop the nanorods and that the open spacing in the nanorod array is 75%, the filling ratio of polybithiophene interpenetrating in the nanorod array is calculated to be 76%. This filling ratio is a significant improvement over that of melting-infiltrated hydrid films, wherein the infiltration ratio could only be increased to around 22% after several special treatments being done on the nanoporous networks.30,31 During electropolymerization, bithiophene molecules diffuse in the electrolyte solution onto the exposed gold-coated glass substrate where the monomers are oxidatively polymerized, forming doped, conducting polythiophene which serves as an electrode for subsequent electropolymerization. The polythiophene therefore grows from the roots of the nanorod array. These charged polythiophene chains could electrostatically interact with CdS nanorod surfaces, resulting in close packing of the polymer on the nanorods and the formation of a solid polymer phase with high filling ratio in the nanorod array.41,42 The subsequent reduction of the polymer from the charged state to neutral involves expulsion of the perchlorate ions, the dopants, and explains the reduced filling ratio of 76%. The interaction between the neutral polybithiophene and CdS remains strong, as ultrasonic treatment of the hybrid films in an aqueous bath for hours did not cause any delamination. The polymer layer could not be peeled off from the nanorod array. The interface between polybithiophene and CdS nanorods was further studied by comparing (i) the Raman spectrum of pure neutral polybithiophene with (ii) that of polybithiophene, 2-3 nm thick, coated on the CdS nanorods (see Figure 5). The characteristic peaks at 703, 1049, 1223, 1376, 1458, and 1503 cm-1 for C-S-C ring deformation, C-H symmetric in-plane bending, CR-CR′ stretching, Cβ-Cβ′ ring stretching, CRdCβ ring stretching, and CRdCβ stretching (anti), respectively, are present in both spectra.47,48 Nonetheless, the two spectra differ significantly in the relative ratio of peak intensities. By normalizing the highest peak at 1458 cm-1 for both spectra, the relative intensity of the other characteristic peaks is listed in Table 1. For polybithiophene formed on CdS nanorods, the apparent

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19767

Figure 5. Raman spectrum of pure neutral polybithiophene (i) and neutral polybithiophene on CdS nanorods (ii).

TABLE 1: Relative Intensity of Peaks of the Raman Spectra in Figure 5 peak position (cm-1) 703 1049 1223 1376 1458 1503 pure neutral 0.033 0.12 0.043 0.072 1 0.03 polybithiophene 1 0.046 neutral polybithiophene 0.037 0.08 0.068 0.054 formed on CdS nanorods

Figure 6. FTIR spectrum of (i) CdS nanorod array, (ii) CdS nanorods and polybithiophene hybrid film, and (iii) pure neutral polybithiophene. The inset on the upper left is the spectra blown-up from 700 to 1400 cm-1.

enhancement of peaks at 1223 and 1503 cm-1 at the expense of the peaks at 1376 and 1049 cm-1 matches better with the Raman spectrum of doped polybithiophene chains with positive polarons,49 indicating that there is electron transfer from the polybithiophene to the CdS nanorods. This charge transfer is confirmed by reflective FTIR spectroscopy. Figure 6 shows the FTIR of the nanohybrid films, with the polybithiophene layer being only 2-3 nm thick. The spectrum is compared with those of a bare CdS nanorod array and a pure neutral polybithiophene film. The reflective FTIR signals are not resolutive in much of the wavenumber range, but the infrared active vibrations (IRAV) in the range of 13501450 cm-1 are quite characteristic. The absorptions at 1359 and 1446 cm-1 are related to the C-C stretching out-of-plane and

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Xi et al. Conclusion

Figure 7. Current-voltage response of a Au/CdS-polybithiophene/ Ca/Al hybrid solar cell. The inset is the current-voltage curve in dark.

CdC symmetric vibration, respectively. The absorption bands of the polymer from 1030 to 1200 cm-1 are obscured by the strong and broad absorption of CdS around 1250 cm-1. The characteristic peak around 800 cm-1, enlarged in the inset, is assigned to out-of-plane deformation of adjacent C-H on thiophene rings. For the pure neutral polybithiophene film, only one peak at 793 cm-1 is observed. For polybithiophene on the CdS nanorods, the emergence of an additional peak at a higher frequency, 800 cm-1, implies that a portion of the polymer chains has stronger C-H bonds and nondegenerated thiophene rings, which is consistent with charge transfer from the thiophene rings to the CdS nanorods.50,51 Photovoltaic cells were fabricated by evaporating thin layers of Ca and Al onto the CdS-polybithiophene nanohybrid films. The photovoltaic performance of cells was characterized in a nitrogen environment with a white light source illuminated from the back side through the gold-coated glass substrate. The light source had a power of approximately 35 mW/cm2 calibrated by a silicon photometer. Figure 7 shows the current-voltage response of the prepared solar cells. In dark (inset), the solar cell exhibits a typical diode behavior, with the forward-bias current at +1 V being 3 orders of magnitude larger than the reverse-bias current at -1 V. Under illumination, the open circuit voltage (Voc) is 0.84 V, significantly larger than 0.44 V obtained from P3HT and ZnO interdigitated hybrid films,23 consistent with the fact that the conduction band of CdS is -4.2 eV, whereas that of ZnO is -4.5 eV.52 The short-circuit current is 0.52 mA/cm2. The fill factor is calculated to be 31%, and the total power conversion efficiency (PCE) is therefore 0.38%. Both the short-circuit current and fill factor of the nanohybrid solar cells are relatively low, likely because electropolymerized polybithiophene generally has carrier mobilities orders of magnitude lower than regioregular poly(3-hexylthiophene) prepared by well-controlled polymerization and rigorous purification. Nonetheless, the electropolymerization improves the filling ratio achieved by strong interaction at the inorganic-polymer interfaces. The nanohybrid architecture demonstrates improved Voc. More importantly, the electrochemical approach reported here represents a simple solution-based fabrication process that is compatible with low-cost roll-to-roll manufacturing in ambient environment and may lead to low-cost polymer solar cells. Further improvement in the PCE could be achieved by optimizing the electropolymerization conditions and by using monomers that yield polymers with small bandgap and high structural regularity.

Densely packed CdS nanorod-polybithiophene interdigitated hybrid films are prepared by electrochemical processes and used for fabricating solar cells. In situ polymerization of the p-type conjugated polymer from monomers in solution significantly improves the interpenetration of the polymer into the nanorod arrays with a high filling ratio. The Raman spectrum and FTIR demonstrate the occurrence of charge transfer between polybithiopehene and CdS. The interaction between the polymer and CdS seems to be strong. The nanohybrid films with the Ca/Al cathode and Au anode have a rectification ratio as high as 103, a large open-circuit voltage of 0.84 V, and a relatively low short circuit current under white light illumination. The electrochemical approach reported here should pave the way for the fabrication of low-cost polymer solar cells in ambient environment. Acknowledgment. The authors would like to thank Dr. Haizheng Zhong for valuable discussions. This work is financially supported by the National Science Foundation under Grant No. 0507294 and the Defense Threat Reduction Agency Grant No. HDTRA1-07-1-0028. References and Notes (1) Hoppe, H.; Sariciftci, N. S. J. Mater. Res. 2004, 19, 1924. (2) Manna, L.; Milliron, D. J.; Meisel, A; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382. (3) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (4) Haizheng, Z.; Yi, Z.; Yi, Y.; Chunhe, Y.; Yongfang, L. J. Phys. Chem. C 2007, 111, 6538. (5) Mcdonald, S. A.; Konstantatos, G.; Zhang, S. G.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Nat. Mater. 2005, 4, 138. (6) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Science 2005, 310, 462. (7) Gur, I.; Fromer, N. A.; Chen, C. P.; Kanaras, A. G.; Alivisatos, A. P. Nano Lett. 2007, 7, 409. (8) Yang, F.; Shtein, M.; Forrest, S. R. Nat. Mater. 2005, 4, 37. (9) Han, L. L.; Qin, D. H.; Jiang, X.; Liu, Y. S.; Wang, L.; Chen, J. W.; Cao, Y. Nanotechology 2006, 17, 4736. (10) Lin, Y. Y.; Chen, C. W.; Chang, J.; Lin, T. Y.; Liu, I. S.; Su, W. F. Nanotechology 2006, 17, 1260. (11) Kumar, S.; Nann, T. J. Mater. Res. 2004, 19, 1990. (12) Gur, I.; Fromer, N. A.; Chen, C. P.; Kanaras, A. G.; Alivisatos, A. P. J. Phys. Chem. B. 2006, 110, 25543. (13) Zhou, Y.; Li, Y. C.; Zhong, H. Z.; Hou, J. H.; Ding, Y. Q.; Yang, C. H.; Li, Y. F. Nanotechnology 2006, 17, 4041–4047. (14) Padinger, F.; Rittberger, R. S.; Sariciftci, N S AdV. Funct. Mater. 2003, 13, 85. (15) Arici, E.; Hoppe, H.; Schaffler, F.; Meissner, D.; Malik, M. A.; Sariciftci, N. S. Thin Solid Film 2004, 451-452, 612. (16) Beek, W. J. E.; Wienk, M. M.; Kemerink, M.; Yang, X. N.; Janssen, R. A. J. J. Phys. Chem. B 2005, 109, 9505. (17) Vanhal, P. A.; Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Slooff, H.; Gennip, W. J. H.; Jonkheijm, P.; Janssen, R. A. J. AdV. Mater. 2003, 15, 118. (18) Zeng, T. W.; Lin, Y. Y.; Lo, H. H.; Chen, C. W.; Chen, C. H.; Liou, S. C.; Huang, H. Y.; Su, W. F. Nanotechnology 2006, 17, 5387. (19) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533. (20) Claude, L. C.; Ramon, T. Z.; Margaret, A. R.; Abou, K.; Gary, H. AdV. Mater. 2004, 17, 1512. (21) Shaheen, S. E.; Ginley, D. S. Encyclopedia of Nanoscience and Nanotechnology; Marcel Dekker, Inc.: New York, 2004; p 2879. (22) White, M. S.; Olson, D. C.; Shaheen, S. E.; Kopidakis, N.; Ginley, D. S. Appl. Phys. Lett. 2006, 89, 143517. (23) Olson, D. C.; Piris, J.; Collins, R. T.; Shaheen, S. E.; Ginley, D. S. Thin Solid Films 2006, 496, 26. (24) Arango, A. C.; Carter, S. A.; Brock, P. J. Appl. Phys. Lett. 1999, 74, 1698. (25) Coakley, K. M.; Liu, Y.; McGehee, M. D.; Frindel, K. L.; Stucky, G. D. AdV. Funct. Mater. 2003, 13, 301. (26) Coakley, K. M.; Liu, Y.; Goh, C.; McGehee, M. D. MRS Bull. 2005, 30, 37. (27) Qiao, Q.; McLeskey, J. T. Appl. Phys. Lett. 2005, 86, 153501. (28) Oey, C. C.; Djurisic, A. B.; Wang, H.; Man, K. K. Y.; Chan, W. K. Nanotechnology 2006, 17, 706.

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