Behaviors of Fe, Zn, and Ga Substitution in CuInS2 Nanoparticles

Jan 4, 2013 - Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator ... Stanford Institute for Materials and Energy Sciences, SLAC Nat...
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Behaviors of Fe, Zn, and Ga Substitution in CuInS2 Nanoparticles Probed with Anomalous X‑ray Diffraction Stephen T. Connor,†,⊥ Benjamin D. Weil,‡,⊥ Sumohan Misra,§ Yi Cui,*,‡,∥ and Michael F. Toney§,∥ †

Department of Chemistry and ‡Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States § Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States ∥ Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States S Supporting Information *

ABSTRACT: We synthesized CuInS2 nanoparticles containing up to 20% Fe, Zn, and Ga to study alloying in photovoltaic absorber materials with anomalous X-ray diffraction. The colloidal synthesis allowed for detailed analysis of complex quaternary compounds. Anomalous X-ray diffraction (AXRD) was used to clarify the elemental distribution between phases. Additionally, optical spectroscopy and X-ray diffraction were used to probe the band gap and crystal phase, respectively. Substitution of Zn into wurtzite CuInS2 produced a controllable increase in the optical band gap, whereas Ga did not substitute into wurtzite CuInS2, producing no band gap change. Secondary phase precipitation of a chalcopyrite phase was observed with Fe substitution, along with a decrease of the optical band gap. This work demonstrates progress in compositional and structural analysis of quaternary chalcogenide materials using AXRD. KEYWORDS: CIS, CIGS, elemental substitution, doping, solar cell, chalcogenide, nanoparticle, anomalous X-ray diffraction

I. INTRODUCTION

Inclusion of alloying elements, which either raise or lower the band gap, can also enable the creation of new materials with desirable band gaps but using less rare or expensive elements. One example is the case of substituting In with Sn and Zn in Cu2ZnSnS4, a similar material composed of Earth-abundant elements.6 While CuInS2 films are mostly made by inexpensive sulfurization of metal films, this technique lacks spatial control of elemental distribution due to rapid reaction kinetics.7 In contrast, coevaporation and other expensive high vacuum techniques can control elemental profiles precisely due to their easily controlled elemental fluxes.8 However, spatial control of composition is also possible by solution processing multiple layers of nanoparticles. While lateral control is ubiquitous in many forms of printing, such as newspapers and labels, elemental depth control could also be possible with careful nanoparticle ink formulation and deposition. However, to achieve this we need a better understanding of how dopants are incorporated into the nanoparticles and how these affect the optical properties. Herein, we will describe the substitution of Fe, Zn, and Ga into wurtzite CuInS2 nanoparticles synthesized using a

While extensive work has shown the benefits of alloying in CuInSe2 to the solar cell performance, the effects of similar processes are not as well understood in CuInS2. The most wellknown alloy of CuInSe2 is CuIn1−xGaxSe2 (CIGS), with x ∼ 0.3, which both raises the band gap from 1.05 to 1.3 eV and also passivates many active interfacial defects.1 The analogous substitution in CuInS2 also increases open circuit voltage,2 though segregation of the Ga to the back contact during traditional processing limits any further beneficial effects.3 It has been demonstrated that interdiffusion during the deposition of buffer layers of CdS onto CIGS films may create a buried homojunction.4 This type of junction might be a contributing factor to the higher performance of CuInSe2-based solar cells as compared to CuInS2. Therefore, a suitable means of dopant inclusion in CuInS2 may enable the creation of a similar buried homojunction. In addition to controlling semiconductor type, elemental substitution can also tune the optical band gap. If the composition and band gap of a CuInS2 thin film can be controlled as a function of depth, a multijunction solar cell could in principle be fabricated. Similar structures have been proposed for colloidal quantum dot solar cells composed of Pb(S,Se) nanoparticles with different band gaps between layers.5 © 2013 American Chemical Society

Received: August 30, 2012 Revised: December 27, 2012 Published: January 4, 2013 320

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The band gap of the alloy material is a function of the molar fractions, x and y, and the bandgaps of the pure material Eg,x and Eg,y. The nonlinear bowing parameter, b, was assumed to be zero for the purpose of predicting band gaps as a function of composition. Optical spectra from samples with 20% substitution of each element are shown in Figure S2 in the Supporting Information as representatives of the typical absorption curve shapes. XRD was performed at the SNL on a PANalytical X’Pert with Cu Kα radiation at 45 kV and 40 mA. Anomalous X-ray Diffraction. To determine whether the distribution of Fe, Zn, and Ga in CuInS2 is substitutional, interstitial, or segregated, we have used AXRD measurements near the absorption edges of the above cations. Using this technique, we perform diffraction scans at a variety of energies and plot the integrated intensity of a particular Bragg peak for specific crystallographic phases (e.g., chalcopyrite vs wurtzite). There is a decrease in the diffracted intensity as we traverse through the absorption edge if the cation is present in this crystallographic phase, while if the cation is absent then there is no such change. This is analyzed by comparing the experimental data to the simulated |F(h,k,l)|2, using the relationship between the structure factor and the X-ray energy:

solvothermal route to better understand compositional changes and optical effects pertinent to solar absorbers. These elements are chosen to allow either a decrease or an increase of the band gap and to replace the In, as has been demonstrated with Cu2ZnSnS4. Once alloy nanoparticles (NPs) are synthesized, it is essential to confirm the dopant elements are included at the desired sites within the nanoparticles (e.g., substitutional, interstitial). Several studies have described the synthesis and analysis of alloyed NPs of CuIn 1−x Ga x S 2 , 9 CuIn 1−x Ga x Se 2 , 10 and CuIn1−xZnxS2.11−15 However, all of these studies have involved hot-injection syntheses, which follow different mechanisms than the solvothermal route. A critical difference which arises is that most syntheses of CuInS2 alloys have focused on the chalcopyrite phase (JCPDS no. 047-1372); while the synthesis described here creates nanoparticle films in the related wurtzite phase. In this synthesis, the CuInS2 forms anisotropic NPs in the wurtzite structure by epitaxial overgrowth off a hexagonal Cu2S NP that forms in situ.16 Nanoparticles formed in the wurtzite phase were shown to grow controllably in the (002) direction, with sizes ranging from 5 to 200 nm, notably larger than nanoparticles formed in the chalcopyrite phase. Traditional laboratory X-ray diffraction experiments cannot distinguish between closely adjacent elements in the periodic table, such as Cu and Zn. However, a synchrotron radiation source provides high incident X-ray intensity and allows variation of the X-ray energy, which enables compositionally sensitive analysis of phases by anomalous X-ray diffraction (AXRD). This technique combines both structural (diffraction) and chemical (absorption spectroscopy) analysis techniques, thereby being sensitive to specific crystallographic phases and unique crystal sites within a particular phase. The above technique was used to determine the distribution of Fe, Zn, and Ga in CuInS2 as substitution, interstitial, or segregated.

fn = f0 (Q ) + f ′ (E) + if ″ (E) atoms

F(h , k , l) =

∑ n=1

fn (E) e(2πi)(hxn+ kyn + lzn)

(3)

Here, f 0(Q) is the atomic scattering factor, f′(E) is the real part of the anomalous scattering factor, f ′′(E) is the imaginary part of the anomalous scattering factor, and xn, yn, and zn are the atomic coordinates. AXRD of the CuInS2 nanoparticles substituted with Fe, Zn, and Ga was performed on beamline 2-1 at Stanford Synchrotron Radiation Lightsource (SSRL). The nanoparticle samples were measured in θ− 2θ geometry with 1 mm slits to define the diffracted beam acceptance (about 2 milliradians), and a Vortex Detector was used to collect the diffracted X-rays. The diffracting intensity was recorded as a function of the scattering vector, Q = (4π sin(2θ/2)/λ), where 2θ is the angle between the incident and diffracted X-rays and λ is the wavelength of the X-rays. For anomalous X-ray diffraction (AXRD), profiles were measured between Q of 1.75−3.32 Å−1 around the (100), (112), (013), (020)/(004), (220), (024) peaks at 39 energies between 6700 and 7600 eV, across the X-ray absorption edge of Fe at 7112 eV. Similar profiles were measured for both Zn and Ga samples. Data were collected around the (100), (002), and (101) peaks at 30 energies between 9500 and 9800 eV, across the X-ray absorption edge of Zn at 9659 eV, and 31 energies between 10 210 and 10 500 eV, across the Xray absorption edge of Ga at 10 367 eV. The peaks in the XRD profile were fitted to either one or two peaks (for the case of Fe which had two phases) with pseudo-Voigt function using Origin 8.0 (Origin Lab Corporation). The integrated area (Aint(h,k,l)) of each individual fitted peak were obtained at each X-ray energy. Aint(h,k,l) is directly proportional to the structure factor |F(h,k,l)|2 of the sample at that Xray energy. The experimental data were corrected for beamline attenuation resulting from air path-length, ion-chambers, and Bewindows (in the detector) but not for sample absorption because of uncertainty in film thickness.

II. EXPERIMENTAL SECTION In this study, all nanoparticles were made by a modified version of a solution phase synthesis of CuInS2 nanoparticles,17 which uses oleylamine and dodecanethiol as a mixed solvent system wherein metal oleates are sulfurized at moderate temperatures. Precursors. The Cu and In precursors are made by ionic replacement of Na-oleate by a corresponding metal chloride in a hexane/water biphasic solution. Further details can be found in our previous report.16 The synthesis of Ga, Fe, and Zn oleates followed the same procedure as that of Cu, and the accompanying nanoparticle syntheses replaced Cu or In oleates in proportions which retained charge balance, assuming Ga3+, Fe3+, and Zn2+. Semiquantitative values were obtained by means of energy-dispersive X-ray spectroscopy, (EDS) of single nanoparticles and of small clusters of nanoparticles. Table S1 in the Supporting Information displays the quantities of added metals found in the resultant nanoparticles versus the amount of metal added to the synthesis, with a nearly linear correlation in every case except Fe. All transmission electron microscopy (TEM) was performed on a CM-20 with standardless EDS at the Stanford Nanocharacterization Laboratory (SNL). Optical Absorption and X-ray Diffraction. Samples for optical absorption and XRD were prepared by evaporating toluene from a concentrated dispersion of nanoparticles on a glass slide. Optical absorption measurements were taken on a Cary 3000 UV−visible spectrometer in transmission mode. All band gaps were extrapolated from plots of squared absorption and wavelength. The band gap of an alloy usually follows Vegard’s law due to the dependence of the band gap on lattice constants.18

Eg = Eg, x x + Eg, yy − bxy

(2)

III. RESULTS AND DISCUSSION The basic growth mechanism of CuInS2 nanoparticles from the solvothermal decomposition of metal oleates involves epitaxial growth of CuInS2 from a Cu2S seed nanoparticle. The final CuInS2 NPs have an XRD pattern with peaks corresponding to the wurtzite crystal structure, as seen in Figure 1b. A TEM image in Figure 1a shows that the original CuInS2 NPs are both cone-shaped and single crystalline, with a (002) growth direction. Substitution of Fe, Zn, and Ga into these nanoparticles will be analyzed. All compositions mentioned

(1) 321

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The substitution of Fe into the CuInS2 nanoparticles produced radical morphological changes, and EDS also confirmed the presence of Fe in samples. At the maximum alloy concentration of 20% Fe, NPs with a size of 11 ± 2 nm are observed (Figure 3a), and HRTEM shows only lattice

Figure 1. (a) TEM image and (b) XRD pattern of pristine CuInS2 nanoparticles, indexed as the wurtzite phase.

below for the three elements are presented as loading compositions during their synthetic procedure. Optical Band Gap. The primary expected effect of elemental substitution in CuInS2 is a shift in the band gap (Eg) from its original value of 1.5 eV. Values of observed band gaps (solid lines) from UV−visible spectroscopy are shown in Figure 2, along with projected values obtained from eq 1

Figure 3. (a) TEM image of CuInS2 nanoparticles with 20% Fe substituted. (b) XRD patterns of CuInS2 samples with 5%, 10%, and 20% Fe substituted; chalcopyrite and wurtzite peaks are indicated for clarity. (c) AXRD spectra of the (112) and (100) reflections in samples with 10% Fe substitution. Vertical dashed lines mark the Fe Kedge.

Figure 2. Experimental optical band gaps of nanoparticles with substitution (solid lines) and expected values from alloying (dashed lines).

(dashed lines) for simple alloying with CuFeS2, ZnS, and CuGaS2. Representative absorption spectra, along with optical band gap fitting parameters can be seen in the Supporting Information. Assuming alloying with ZnS (Eg = 3.6 eV), Zn substitution onto Cu and In lattice sites should lead to an increase in the band gap. As can be seen, the actual increase was close and proportional to the expected amount. Fe showed a similar correlation between the experimental and the expected drop from alloying with CuFeS2 (Eg = 0.6 eV). Ga, however, showed no significant change in band gap as would be expected from alloying with CuGaS2 (Eg = 2.2 eV). To determine the origin of these spectroscopic phenomena, structural information was gathered on all samples by XRD, TEM, and AXRD. Effects of Alloying on CuInS2 Morphology and Phase. Elemental additions will be discussed separately in the following sections, so that the effect of each element on the structure of CuInS2 will be clearer. i. Substitution of Fe. Fe substitution in CuInS2 can be represented as alloying with chalcopyrite CuFeS2, which has Eg = 0.6 eV. Fe inclusion in CuInS2 in low levels has been previously studied,19 though it has not been thoroughly explored as a substitutional element. UV−visible spectroscopy showed optical band gaps which were within 5% of values expected based on eq 1; this is consistent with Fe substituting on the lattice sites in the nanoparticles.

fringes with 3.2 and 2 Å spacings. The absence of the wurtzite (100) reflection and the prominence of the chalcopyrite (112) reflection in XRD (Figure 3b) confirms that the sample is entirely in the chalcopyrite phase. Scherrer broadening of the (112) peak gives an estimate of ∼17 nm for average nanoparticle diameter, consistent with the TEM results. At lower Fe concentrations of 5% and 10%, the synthesis yields NPs with diameters of 17 nm ± 4 nm and 32 ± 5 nm, respectively. XRD patterns obtained on 5% and 10% samples (Figure 3b) show an apparent mixture of chalcopyrite and wurtzite phases. High intensity at the (112) reflections indicates a majority of the chalcopyrite phase, while intensity in the (100) peak indicates the presence of some wurtzite phase material. Anomalous XRD of a 10% Fe sample is shown in Figure 3c, which compares the Fe signal of the minority wurtzite versus majority chalcopyrite reflections. Similarly, a drop in peak area intensity near the Fe K-edge were observed for both peaks unique to wurtzite, the (100), and the prominent chalcopyrite peak, (112); this indicates that Fe is present at similar levels in both phases, instead of favoring the chalcopyrite phase. To gain more insight into the microstructure of the nanoparticles, HRTEM and SAED were performed on many individual nanoparticles from a 10% Fe sample. A representa322

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tive nanoparticle is shown in Figure 4, with two grains of indeterminate phase. Both grains have lattice spacings of 3.2

Figure 4. HRTEM image of a CuInS2 nanoparticle with 10% Fe. Lattice spacings are marked on both sides of the grain boundary for phase determination.

Figure 5. (a) TEM image of CuInS2 nanoparticles with 20% Zn substituted. (b) XRD pattern of CuInS2 samples with 20% Zn substitution, with pure CuInS2 inset for comparison. (c) AXRD spectra of the (101) and (100) reflections in samples with 5%, 10%, and 20% Zn substitution. Vertical dashed lines mark the Zn K-edge.

and 2.0 Å, corresponding to the either the (211) and (024) chalcopyrite reflections or the (002) and (110) wurtzite reflections. However, the right grain can be positively identified as wurtzite due to the (102) and (001) planes, with spacings of 2.3 and 6.4 Å. This indicates that each nanoparticle is likely biphasic, with an epitaxial stack of wurtzite-CuIn1−xFexS2 and chalcopyrite-CuIn1−xFexS2. This also may explain the discrepancy between the expected and actual band gap for the particles. By combining the results of TEM, XRD, and AXRD, we conclude that the chalcopyrite phase of CuIn1−xFexS2 grows off of the small seeds of the wurtzite CuIn1−xFexS2. The preferential formation of chalcopyrite CuIn1−xFexS2 is not surprising given the instability at room temperature of the wurtzite phase of CuFeS2 in the bulk.20 The low temperature conversion of this alloy to the chalcopyrite phase might be advantageous for controlling the phase of nanoparticles, though further studies are required. Furthermore, the addition of Fe is shown to be an effective means of lowering the band gap of CuInS2. ii. Substitution of Zn. UV−visible spectroscopy shows that there is a strong increase in optical band gap with increasing Zn substitution. Assuming that the sample is the alloyed phase of wurtzite CuInS2 (Eg = 1.5 eV) and wurtzite ZnS (Eg = 3.6 eV), the measured and expected values of optical band gap compare favorably, as seen in Figure 2. The general increase in band gap indicates likely Zn substitutes for Cu and/or In in the wurtzite CuInS2 lattice, but the values are lower than estimates by 4− 8%, possibly due to incomplete substitution of Zn. TEM (Figure 5a) shows that Zn substitution results in little change to the overall morphology of the CuInS2 nanoparticles. The average size and shape matches well with that of unalloyed CuInS2 nanoparticles (Figure 1a), implying that Zn substitution does not change the growth mechanism substantially. However,

the intensity ratios of the diffraction peaks in the two samples are different which suggests that they have different texturing. Substitution of Zn is challenging to confirm with XRD alone; the pattern of even the highest substitution amounts (Figure 4b) show little variation from the original wurtzite CuInS2 NPs’ pattern. ZnS easily forms solid solutions with wurtzite CuInS2 with small changes in the lattice constants. To further confirm the alloying of CuInS2 and ZnS, AXRD measurements were performed on a representative sample. In the AXRD scan, the (101) or (100) peak intensity was measured as a function of Xray energy, and Zn substitution in either Cu or In sites of the wurtzite phase reduces the peak intensity near the Zn K-edge (9659 eV). Figure 5c shows the peak intensity as a function of incident X-ray energy for the three alloying amounts. The characteristic drop near 9659 eV is present in all samples, with a larger intensity drop with higher Zn concentrations and a shift in the location of the drop due to ionization into Zn2+. While not quantified, this shows that Zn is a feasible substitution alloy to controllably increase the band gap of CuInS2. iii. Substitution of Ga. Ga inclusion in CuInSe2 solar cells has been instrumental in reaching high efficiencies. The most important effect of Ga substitution is an increase in the band gap, which allows a higher voltage in the solar cell. Interestingly, the substitution of Ga in the CuInS2 nanoparticles resulted in no changes in the optical band gap, XRD pattern, or morphology (Figure 6a,b). This shows that Ga does not go into the crystal structure. While there were no measurable changes in the position of the X-ray reflections for Ga alloying, Ga was still detected by EDS inside the TEM. While substitution of Fe and Zn in CuInS2 results in expected changes in the optical band gap, the apparently contradictory finding that Ga does not change the optical band gap and crystal structure is interesting and was explored in greater depth with AXRD. 323

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phase. Thus, Ga is present in the nanoparticle film and not substituting for Cu or In in the lattice, but its exact location is not clear. AXRD of the Fe, Zn, and Ga substituted nanoparticles shows the different nature of incorporation of the dopants into the CuInS2 structure. The substitution of Fe into CuInS2 results in formation of a secondary chalcopyrite phase. It is believed that phase and morphology are determined largely by the relative stability of the chalcopyrite and wurtzite phases of the alloyed materials. Furthermore, the optical band gap could be effectively tuned over a wide range through the use of Fe and Zn substitution. On the other hand the substitution of Ga contradicted expected band gap changes and AXRD was an effective technique to observe the unusual distribution of Ga in the CuInS2 crystal. We speculate that it was unfavorable for the Ga precursor to form a stable substituted wurtzite phase under our reaction conditions.

V. CONCLUSION In conclusion, substitution of Fe, Zn, and Ga in CuInS2 nanoparticles have been analyzed by optical band gap, TEM, and AXRD, and the nature of their incorporations were determined. AXRD revealed that Ga incorporation was different from that of Zn and Fe, which were both found to incorporate into the lattice. Further exploration of the state of gallium in the structure may explain its role in improving chalcogenide solar cells. Future studies correlating elemental distribution with electrical performance are forthcoming.

Figure 6. (a) TEM image of CuInS2 nanoparticles with 20% Ga substituted. (b) XRD pattern of CuInS2 samples with 20% Ga substitution, with pure CuInS2 inset for comparison. (c) AXRD spectra of the (101) and (100) reflections in samples with 5%, 10%, and 20% Ga substitution. Vertical dashed lines mark the Ga K-edge.

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AXRD was performed to clarify the substitution of Ga in the CuInS2 lattice. Similar to the Zn alloying case, AXRD of a wurtzite peak will show an intensity dependence on X-ray energy near the Ga edge (for peaks sensitive to In lattice sites). As can be seen in Figure 6c, no dip is observed when scanning the X-ray energy through the Ga edge. This shows that the Ga is not substitutionally incorporated in the wurtzite crystal lattice at the level of a few percent; however, it does not exclude the presence of Ga on the NP surface or in interstitial sites. HRTEM in Figure 6a shows no visible amorphous shell, which would need to be >3 nm to contain the observed Ga content. To confirm the Ga presence in the nanoparticles, fluorescence spectroscopy was done on Ga-containing samples. Figure 7 shows a jump near the Ga-edge, approximately proportionate to the alloyed Ga amount. This shows that Ga is present in the matrix of the nanoparticles yet is not in the wurtzite crystal

ASSOCIATED CONTENT

S Supporting Information *

Average stoichiometries of nanoparticles from energy dispersive X-ray spectrometry and absorption curves for various doped CuInS2 nanoparticles showing the optical band gap (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ⊥

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.C. acknowledges support from the U.S. Department of Energy under Award DE FG36-08GO18005 and from the Stanford Global Climate and Energy Project. S.T.C. acknowledges support from a National Science Foundation Graduate Fellowship. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource (SSRL), a National User Facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences.

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Figure 7. Fluorescence spectra of CuInS2 samples with 0%, 5%, 10%, and 20% Ga substitution. Vertical dashed lines mark the Ga K-edge. 324

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