NANO LETTERS
Development of CuInSe2 Nanocrystal and Nanoring Inks for Low-Cost Solar Cells
2008 Vol. 8, No. 9 2982-2987
Qijie Guo,† Suk Jun Kim,‡ Mahaprasad Kar,† William N. Shafarman,§ Robert W. Birkmire,§ Eric A. Stach,‡ Rakesh Agrawal,*,† and Hugh W. Hillhouse*,† School of Chemical Engineering and the Energy Center, School of Materials Engineering and the Birck Nanotechnology Center, Purdue UniVersity, West Lafayette Indiana 47906, and Institute of Energy ConVersion, UniVersity of Delaware, Newark, Delaware 19716 Received July 10, 2008; Revised Manuscript Received July 17, 2008
ABSTRACT The creation of a suitable inorganic colloidal nanocrystal ink for use in a scalable coating process is a key step in the development of low-cost solar cells. Here, we present a facile solution synthesis of chalcopyrite CuInSe2 nanocrystals and demonstrate that inks based on these nanocrystals can be used to create simple solar cells, with our first cells exhibiting an efficiency of 3.2% under AM1.5 illumination. We also report the first solution synthesis of uniform hexagonal shaped single crystals CuInSe2 nanorings by altering the synthesis parameter.
Copper indium diselenide (CIS) and related materials are some of the most promising candidates for thin film photovoltaic applications due to their unique structural and electrical properties.1-3 However, the widespread utilization of CIS based solar cells has been hindered by the high costs associated with fabrication processes. The creation of a suitable inorganic colloidal nanocrystal ink for use in a scalable coating process is a key step in the development of low-cost solar cells.4,5 Notably, great progress has been made in nanocrystals synthesis in recent years.6-11 Although several methods have been reported which describe the synthesis of CIS and related nanoparticles,12-23 the materials created typically contain either binary products or additional phases, as compositional and structural control becomes difficult for ternary and quaternary compounds. Here, we present the first report of a solution synthesis of chalcopyrite CuInSe2 nanocrystals and demonstrate that inks based on these nanocrystals can be used to create simple solar cells, with our first cells exhibiting an efficiency of 3.2% under AM1.5 illumination. Surprisingly, and counter to previous reports in the literature,24-26 these R-CIS nanocrystals may be easily consolidated into large crystalline chalcopyrite domains by a brief thermal treatment which * Corresponding author. E-mail:
[email protected] (R.A.) and
[email protected] (H.H.). † School of Chemical Engineering and the Energy Center, Purdue University. ‡ School of Materials Engineering and the Birck Nanotechnology Center, Purdue University. § University of Delaware. 10.1021/nl802042g CCC: $40.75 Published on Web 08/02/2008
2008 American Chemical Society
suggests that much higher efficiency cells may be possible by this route. Near stoichiometry CuInSe2 can exist in several different crystal structures.2 In each, the selenium anion sublattice is the same (identical to the anion sublattice of the cubic zinc blende structure), but there are different possible cation orderings. For sphalerite, the Cu and In atoms are randomly distributed in the cation sites, resulting in a cubic unit cell. The chalcopyrite structure has a specific cation ordering and requires a tetragonal unit cell (space group I4j2d), with Cu occupying the (0,0,0) sites and In occupying the (1/2,1/2,0) sites of the cation sublattice. A number of other crystal structures have been reported for near-stoichiometric CIS compounds, each differing in the specific ordering of Cu and In atoms in the cation sublattice.2 In our synthesis of CIS nanocrystals, the order of addition of the Se precursor to oleylamine solution plays a major role in the resulting crystal structure of the final product (please see Supporting Information for experimental details). In the case where Se is injected into a solution of CuCl and InCl3 in oleylamine at 285 °C, sphalerite nanocrystals are obtained. This “hot injection” of Se induces abrupt supersaturation of the reaction mixture. The supersaturation results in rapid nucleation and growth of the nanocrystals which result in the formation of the sphalerite phase. A typical powder X-ray diffraction (PXRD) pattern of the as-synthesized nanocrystals along with a simulated sphalerite pattern is shown in Figure 1a. The experimental pattern agrees very well with the simulated pattern, indicating that these nanocrystals are
Figure 1. Experimental and simulated PXRD patterns of CIS nanocrystals of (a) sphalerite, (b) chalcopyrite, and (c) chalcopyrite with 50% faulting probability of ABC′D stacking as opposed to ABCD stacking. Insets of (b) and (c) are enlarged views of the lower 2θ region showing the characteristic chalcopyrite peak. (d) An example of the stacking sequence used in the simulation of stacking faults. The layers of the chalcopyrite structure are labeled A, B, C, and D. C′ is a layer in which Cu and In sit in the wrong site.
sphalerite (δ-CIS). Furthermore, if the observed pattern is indexed with a tetragonal unit cell, the ratio of lattice constants (c/2a) is 1.000 ( 0.001. Thus, there is no tetragonal distortion of the structure, providing a strong indication that the nanocrystals are truly cubic, commensurate with sphalerite. No significant differences were observed in the PXRD patterns of nanocrystals synthesized by the same method with longer growth times. On the other hand, chalcopyrite nanocrystals were obtained by adding the Se along with all other precursors at 130 °C and then raising the temperature to the final growth temperature and holding for 1 h. It is believed that adding the Se precursors at low temperature and letting the temperature rise slowly prevents large supersaturation and allows the atoms to form the thermodynamically favored chalcopyrite phase (R-CIS). Figure 1b shows a PXRD pattern of the assynthesized chalcopyrite CIS nanocrystals. The average crystal domain size of the nanocrystals calculated using Scherrer’s equation based on the (112) peak is 45 nm. The major diffraction peaks observed at 26.65, 44.22, 52.39, 64.36, 70.90, 81.38, 87.52, and 97.63 °2θ can be indexed to the (112), (204)/(220), (116)/(312), (008)/(400), (316)/(332), (228)/(424), (336)/(512), and (408)/(440) of the tetragonal chalcopyrite crystal structure, respectively. However, these major peaks are common to both the sphalerite and the Nano Lett., Vol. 8, No. 9, 2008
chalcopyrite structures. Thus, in order to ascertain that the nanocrystals are chalcopyrite, it is critical to be able to observe the minor peaks that are unique to the chalcopyrite structure. For example, the minor peaks at 17.14°, 27.74°, and 35.55° corresponding to the (101), (103), and (211) peaks, respectively, are unique to the chalcopyrite structure and are shown as an inset of Figure 1b. The lattice constants refined from the PXRD data were a ) 5.787 ( 0.003 Å and c ) 11.617 ( 0.001 Å, with the c/2a ratio of 1.004 ( 0.001. The observed lattice parameters and c/2a ratio agree very well with the reported values for the chalcopyrite phase, R-CIS.2 The surface morphology of the CIS nanocrystals was examined using field emission scanning electron microscopy (FESEM). Figure S2a,b shows FESEM images of the as-synthesized sphalerite and chalcopyrite CIS nanocrystals, respectively. The sphalerite nanocrystals are mostly isotropic with an average diameter and standard deviation of 37 ( 11 nm. However, the chalcopyrite nanocrystals appear to be more polydisperse in both size and shape. The increased polydispersity is consistent with the nature of the reaction. Since all of the precursors were added at low temperature, followed by a gradual increase to the final growth temperature, the nucleation and growth of the nanocrystals is not expected to be uniform. The 2983
Figure 2. Large area TEM image of chalcopyrite CIS nanocrystals. (b) HRTEM of a chalcopyrite nanocrystal along the [221] zone axis showing a slight distortion due to the tetragonal chalcopyrite structure. (c) Bright filed TEM image of a single nanocrystal. (d) A TED pattern from the nanocrystal shown in (c). The zone axis of the TED pattern was determined to be [221]. The (01j2), and (11j0) spots are kinematically forbidden spots in the chalcopyrite phase that result from faulting and disorder in the cation sublattice.
composition of the chalcopyrite nanocrystals was analyzed by energy dispersive X-ray spectroscopy (EDX). Spectra (Figure S1) were collected from at least 10 randomly selected areas and yielded an average composition and standard deviation of Cu0.99 ( 0.11In1.02 ( 0.07Se2. The crystal structures of individual R-CIS nanocrystals were examined using transmission electron microscopy (TEM). Figure 2a shows a large area bright-field TEM image of the chalcopyrite nanocrystals. A high-resolution brightfield TEM image of a characteristic nanoparticle (∼120 nm) from the chalcopyrite synthesis is shown in Figure 2b. The image is taken along the [221] zone axis with the (22j0), (2j04), and (02j4) crystallographic planes indicated by white lines. Spacing measurements based on 10 planes indicated d-spacings of 1.97 ( 0.02 Å for the (22j0) and 2.03 ( 0.01 Å for the (2j04) and (02j4). The measured angle between the (22j0) and (02j4) planes is 59.3 ( 0.5° and is 61.4 ( 0.7° between (2j04) and (02j4) planes. If the structure were sphalerite, the measured angles should be 60° between all three planes, and the measured d-spacings should be 2.04Å. These observed differences provide additional evidence of lattice distortion that results from cation ordering and the chalcopyrite crystal structure. Interestingly, transmitted electron diffraction (TED) patterns (Figure 2d) taken from the chalcopyrite nanocrystals (Figure 2c) show the presence of a number of extra reflections. The zone axis of the TED pattern was determined to be [221], and the pattern is typical for nanocrystals from 2984
this same synthesis procedure. A similar TED pattern was reported27 for the vacancy ordered compound β-CIS, which has stoichiometry of CuIn3Se5. However, since the CIS nanocrystals here are near stoichiometric CuInSe2, the extra spots suggest some type of disordering in the chalcopyrite crystal structure, such as InCu antisite substitution. The (1j1j4), (12j2), (02j4), and (22j0) spots are fundamental reflections while the (01j2) and (11j0) spots are kinematically forbidden reflections. However, the forbidden reflections can show up due to a change in the structure factor caused by disordering of the cation lattice. If such disorder exists, then the structure factor for these planes will not cancel as it does for perfect chalcopyrite cation ordering. This will lead to the appearance of the extra reflections observed in Figure 2d. Besides the fundamental and kinematically forbidden reflections, we also observed additional diffraction spots, circled in white in Figure 2d. These extra spots correspond to 2/3 the distance of the (1j1j4) or (12j2) fundamental reflections and have a d-spacing of 3.5 Å. Weak XRD peaks corresponding to these TED spots are observed in PXRD patterns from some batches of equivalently synthesized nanocrystals, shown as Figure 1c. This PXRD pattern is similar to that of the chalcopyrite pattern shown in Figure 1b. However, close inspection reveals a small shoulder, indicated by the * in the inset of Figure 1c, at 25.6 °2θ (corresponding to a d-spacing of 3.5 Å). Deviation from perfect chalcopyrite cation ordering can also explain these additional TED and XRD spots. In particular, if the strict Nano Lett., Vol. 8, No. 9, 2008
chalcopyrite order is disrupted by stacking faults in the cation sublattice along these directions, these spots may appear. Figure 1d shows slices of a stacking sequence along the [11j0] used to simulate chalcopyrite CIS with InCu antisite substitution. In this direction, the chalcopyrite CIS can be represented as a stacking of four different layers (ABCD) in the sequence shown in Figure 1d. A stacking fault with InCu antisite substitution can be introduced by interchanging the Cu and In ion positions in one of the layers, (C′) in this case. An example of a simulated PXRD pattern of ABCD stacking with a 50% probability for ABC′D stacking is shown in Figure 1c. Equivalent patterns are found if the disorder is introduced in the layers (A′BCD, AB′CD, or ABCD′). The close match between these simulated patterns and the observed patterns suggest InCu antisite substitution is present. This disorder will also lead to additional weak spots observed in TED. Notably, different nanocrystals show different relative intensities in these extra spots (see Supporting Information Figure S3), indicating that the degree of InCu antisite substitution varies from one nanocrystal to the next. Furthermore, the shape of the CIS nanocrystals can be controlled utilizing a secondary ligand in the nanocrystal synthesis, in which the precursors are dissolved in trioctylphosphine (TOP) and hot injected into oleylamine to form the single crystal CIS nanorings (please refer to appendix for synthesis details). The nanorings have very well-defined hexagonal facets with an average inner and outer diameter of ∼25 nm and ∼5 nm, respectively; see Figure 3a. The thickness of the nanorings are ∼5 nm as shown in Figure 3b, where the samples are prepared by drop casting from a concentrated solution in which the nanorings pack together face-to-face because of van der Waals force. HRTEM image of a single nanoring is shown in Figure 3c showing a very well-packed hexagonal packing of the atoms. The measured d-spacing and contained angles are 3.49 ( 0.03 Å and 60.0 ( 0.3°, respectively. These CIS nanorings are particularly interesting because in recent years solid state nanorings have attracted a strong interest due to their unique properties associated with the ring structure.28-31 Of particular importance is the Aharonov-Bohm effect which appears naturally in systems with ring geometry in the presence of magnetic field.29,30,32 However, there are limited reports in the literature on the synthesis of solid state nanorings, and the most commonly used technique is molecular beam epitaxy.33-35 To the best of the authors’ knowledge, this is the first reported solution synthesis of single crystal nanorings. The observed formation of the nanoring structure might be unique to the ternary system presented here and might be applicable to other ternary materials. However, details in the formation mechanism and properties of the nanorings are yet to be fully understood and are under further investigation. The primary technological advantage of creating nanocrystals by this solution based method is the capability to easily form an ink that is compatible with a large variety of scalable film formation or printing processes. Any of the CIS nanocrystal samples may be used to prepare an ink by precipitation and redispersion in toluene (or other organic Nano Lett., Vol. 8, No. 9, 2008
Figure 3. TEM images of the CIS nanorings (a) laying flat on the TEM grid, (b) self-assembled rings laying on the edge, and (c) HR-TEM image of a single nanoring. The measured d-spacing and contained angles are 3.49 ( 0.03 Å and 60.0 ( 0.3° respectively.
solvents). The inks are stable suspensions of nanocrystals and appear jet-black because of their strong absorption. An example of an R-CIS nanocrystal ink of ∼45 nm is shown in Figure 4 along with its optical absorption spectrum. The band gap of the nanocrystals was determined to be 1.06 ( 0.02 eV, which is in good agreement with the reported value of 1.04 eV for bulk R-CIS, as the particles in the sample shown are too large to show significant blue-shift due to quantum confinement. Here, we demonstrate the utility of these inks in solar cell fabrication (Figure 5). The absorber layer may be formed simply by drop-casting the ink on molybdenum coated sodalime glass substrate. The films are annealed under flow of Ar at 500 °C for 1 h to remove the organic capping molecules, and then under Se vapor at temperature range from 450-550 °C for 30 min. Figure 5a,5b is FE-SEM cross sectional images of thin films of sphalerite CIS nanocrystals 2985
Figure 4. UV-vis absorption spectrum of the as-synthesized CIS nanocrystals. The band gap of the nanocrystals is approximated using the direct band gap method by plotting the absorbance squared versus energy, and extrapolating to zero as shown in the inset. Also shown, inset is a photograph of the undiluted nanocrystals ink.
after drop-casting and after thermal treatment, respectively. Surprisingly, the nanocrystals sinter to form large crystal grains on the length scale of the thickness of the film, a morphological feature of the highest efficiency CIS and CIGS solar cells. This facile recrystallization is unexpected as previous syntheses of casted CIS, binary selenide, or oxide nanoparticles have not yielded large grains after seleniza-
tion.24-26 We fabricated the first batch of solar cells in our laboratory using these films following chemical bath deposition of CdS layer, and RF sputtering of 50 nm intrinsic ZnO and 300 nm of ITO layers. A picture of the finished device is shown in Figure 5c, and the corresponding I-V characteristic of the solar cell is shown in Figure 5d. The devices had an efficiency and fill factor of 2.8% and 39% respectively under standard AM1.5 illumination. Small area devices of ∼0.12 cm2 were also fabricated with silver paint as top contact. The small area cells had a cell efficiency of 2.7% and active area efficiency of 3.2% by excluding the shadowed area of the silver paint. The initial device performance is relatively poor partly because of the large series resistance as indicated by the broad knee of the IV curve. The high series resistance is likely a result of the formation of a thick MoSe2 layer after Se vapor annealing and further studies are underway to improve the device performance. A rapid and simple solution phase synthesis of crystalline CIS nanoparticles has been developed. The structure of the CIS nanocrystals can be controlled by the synthesis conditions to yield the sphalerite or chalcopyrite structure. Some deviation from perfect ordering of the cation sublattice is observed and can be successfully modeled as stacking faults resulting from Cu in In sites or vice versa. The first batch of solar cells fabricated using the nanocrystals ink is also demonstrated. The efficiency and fill factor of the solar cells are 3.2% and 39%, respectively. However, the surprisingly facile nanocrystal sintering to form micrometer-scale crystal grains is very promising for the development of much higher
Figure 5. FESEM images of (a) CIS nanocrystals thin film prepared on Mo coated soda lime glass by drop casting of the nanocrystals ink and (b) after thermal treatment in a Se/Ar atmosphere. The thickness of the film is ∼1.5 µm and is uniform across large areas. Note the significant recrystallization to yield large domains. (c) Photograph of the completed solar cells after chemical bath deposition of CdS, sputtering of i-ZnO and ITO, and evaporation of metal contacts, using the film shown in (b). (d) The I-V characteristic of the finished device shown in (c). The efficiency of the cell is 2.8% under standard AM1.5 illumination. 2986
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efficiencies with these new inks. Such solar cells have the potential to dramatically reduce the cost of solar electricity. Acknowledgment. We would like to thank the Energy Center at Purdue University for the seed funding and the National Renewable Energy Laboratory (NREL) for providing calibrated CIGS solar cells used in the calibration of our efficiency measurements. We also hank Professor Chelsey D. Baertsch for use of the Cary 5000 UV-vis-NIR spectrometer, Dr. Kannan Ramanathan for helpful discussions, and Professor Carol Handwerker for financial support of S.J.K.. Supporting Information Available: Experimental details, EDX analysis, and TED of another disordered CIS nanocrystal. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Guillemoles, J.; Rau, U.; Kronik, L.; Schock, H.; Cahen, D. AdV. Mater. 1999, 11 (11), 957. (2) Stanbery, B. J. Critical ReV. Solid State Mater.s Sci. 2002, 27 (2), 73–117. (3) Ramanathan, K.; Contreras, M. A.; Perkins, C. L.; Asher, S.; Hasoon, F. S.; Keane, J.; Young, D.; Romero, M.; Metzger, W.; Noufi, R.; Ward, J.; Duda, A. Progress in PhotoVoltaics 2003, 11 (4), 225–230. (4) Alivisatos, A. J. Phys. Chem. 1996, 100 (31), 13226–13239. (5) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Science 2005, 310 (5747), 462–465. (6) Murray, C.; Norris, D.; Bawendi, M. J. Am. Chem. Soc. 1993, 115 (19), 8706–8715. (7) Alivisatos, A. Science 1996, 271 (5251), 933–937. (8) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287 (5460), 1989–1992. (9) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545–610. (10) Trindade, T.; O’Brien, P.; Pickett, N. L. Chem. Mater. 2001, 13 (11), 3843–3858. (11) Joo, J.; Na, H. B.; Yu, T.; Yu, J. H.; Kim, Y. W.; Wu, F. X.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125 (36), 11100–11105. (12) Schulz, D. L.; Curtis, C. J.; Flitton, R. A.; Wiesner, H.; Keane, J.; Matson, R. J.; Jones, K. M.; Parilla, P. A.; Noufi, R.; Ginley, D. S. J. Electron. Mater. 1998, 27 (5), 433–437. (13) Malik, M. A.; O’Brien, P.; Revaprasadu, N. AdV. Mater. 1999, 11 (17), 1441–1444.
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