Optoelectronic Properties of Strontium and Barium Copper Sulfides

Sep 18, 2017 - National Renewable Energy Laboratory, Golden, Colorado 80401, United States. Chem. Mater. , 2017, 29 (19), pp 8239–8248 ... Sulfide m...
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Optoelectronic Properties of Strontium and Barium Copper Sulfides Prepared by Combinatorial Sputtering Yanbing Han,†,‡ Sebastian Siol,‡ Qun Zhang,*,† and Andriy Zakutayev*,‡ †

Department of Materials Science, Fudan University, Shanghai 200433, China National Renewable Energy Laboratory, Golden, Colorado 80401, United States

‡

S Supporting Information *

ABSTRACT: Optically transparent materials with p-type electrical conductivity can facilitate the development of transparent electronics and improve the efficiency of photovoltaic solar cells. Sulfide materials represent an interesting alternative to oxides for these applications due to better hole transport properties. Here, transparent and conductive Ba−Cu−S thin films are prepared by combinatorial cosputtering and characterized for their composition, structure, and optoelectronic properties. The conductivity and transparency of these films are found to be strongly dependent on their chemical composition and the substrate temperature during growth. The conductivity of BaCu2S2 and BaCu4S3 can reach 53 S/cm (at 250 °C) and 74 S/cm (at 200 °C), respectively, which is higher than their solution processed/bulk counterparts. The 90% reflectance corrected transmittance is achieved in the wavelength range 600−1000 nm for BaCu2S2 and 650−1000 nm for BaCu4S3 (at 250 °C). These electrical and optical properties are comparable with other recently presented transparent p-type conductors, while the 200−350 °C processing temperature is low enough to be used in semiconductor devices with limited thermal budgets. Attempts have been made to synthesize the related Sr−Cu−S materials, following the theoretical suggestion of their potential as transparent p-type conductors, but these attempts resulted only in phase-separated SrS and CuxS phases. Alloying BaCu2S2 with Sr on the Ba site on the other hand increases the conductivity to >100 S/cm while only slightly compromising the transparency of the material. To explain the difference between the Ba and the Sr containing copper sulfides, the lower bounds on the SrCu2S2 and SrCu4S3 formation enthalpies are estimated. While the doping of the Ba−Cu−S materials presented here is too large for application in transparent electronics, it is promising for potential use as p-type contact layers in thin film solar cells.

1. INTRODUCTION

There are few known p-type transparent conductors, most of them following a design principle that was proposed 20 years ago and is supported by CuAlO2 as a prototypical example.6 This design principle states that the cation orbitals in p-type semiconductors should hybridize with anion orbitals to develop a disperse valence band, leading to higher hole mobility, while remaining in closed shell configuration to avoid excitations between different electronic orbitals that can deteriorate the transparency. Inspired by this idea, several Cu2O-based transparent p-type semiconductors have been developed. Most of these materials like CuGaO27 and CuInO28 crystallize in delafossite structure, while some of them like CuSrO29 feature a body-centered tetragonal crystal structure. However, the electrical conductivities and optical transparencies in these ternary oxides are lower than in their n-type counterparts. Replacing the more ionic oxides with more covalent sulfides facilitated the discovery of copper sulfide-based materials, like CuAlS2,10 CuxZn1−xS2,11 BaCu2S2,12,13 and related mixed-anion materials.14−16 Among these sulfide materials, BaCu2S2, with a band gap of 2.3 eV, shows a few preferable characteristics for

Transparent conductors (TCs) play an important role in various electronic devices, such as solar cells, light emitting diodes, liquid crystal displays, sensors, and transparent circuits.1 N-type transparent conductors with high carrier concentration like indium tin oxide (ITO) have long been used as electrodes in the photovoltaic (PV) and other optoelectronic devices.2 Ntype TCs with tunable carrier concentrations such as indium gallium zinc oxide (IGZO), on the other hand, find their application in the display industry, acting as channel layers in thin film transistors (TFTs).3 In contrast to the success of ntype TCs, limited achievements have been made in their counterparts, p-type TCs. With p-type TCs that perform as well as the n-type TCs, complementary metal oxide semiconductor (CMOS) circuits could be prepared, reaching higher energy efficiency and higher complexity than today’s state-of-the-art transparent electronic devices. Also, p-type rather than n-type transparent conductors can facilitate the development of improved light emitting diode (LED) technologies, since they can provide hole injection by acting as the anodes.4 Moreover, p-type semiconductors are desirable as contact layers in photovoltaic (PV) and photoelectrochemical (PEC) solar cells.5 © 2017 American Chemical Society

Received: June 14, 2017 Revised: September 13, 2017 Published: September 18, 2017 8239

DOI: 10.1021/acs.chemmater.7b02475 Chem. Mater. 2017, 29, 8239−8248

Article

Chemistry of Materials

All thin film sample libraries reported here were prepared using combinatorial radio frequency (RF) magnetron cosputtering in a custom built UHV deposition chamber (Orion-8, AJA International), as described in our previous publications.26 The 50.8 mm diameter BaS and SrS targets (99.9% purity) were placed opposite to the 50.8 mm Cu2S target (99.99% purity) at an incident angle of 20° to create the composition gradient in Ba−Cu−S libraries and Sr−Cu−S libraries, respectively. Correspondingly, for the deposition of the Sr− Ba−Cu−S sample libraries, SrS, BaS, and Cu2S targets were distributed on three different sides of the substrate. The films were deposited on polished quartz plates of 50.8 × 50.8 × 1.6 mm3 size (GM Associates, Inc.), cleaned with detergent first, and sonicated in acetone, deionized water, and isopropanol successively before the depositions. The temperature gradients, obtained by mounting the upper quarter of the substrate on the heated substrate holder, were calibrated using Omega K-type thermocouples on the sample’s surface and an infrared thermal imaging system. Uniform temperatures were achieved by contacting the entire substrate to the heated substrate holder. Prior to the deposition, the substrate temperature was kept at 150 °C for 30 min to evaporate the solvent in the silver paint which was used for improved thermal contact and at 400 °C for 60 min to outgas organic residues. For the deposition, the substrate was cooled down or heated up to the desired depositing temperature in the 150−420 °C range. All thin films were deposited in pure argon at a process pressure of 0.2 Pa and a flow rate of 16 sccm, with a chamber base pressure of 2 × 10−4 Pa. After the deposition, the samples were cooled down to room temperature in the chamber in the same argon atmosphere as during the deposition. Each sample library was characterized at 44 points using a welldefined 4 × 11 coordinate grid. The composition and thickness were measured by X-ray fluorescence spectroscopy (XRFFischerscope XDV-SDD) with a 3 mm-diameter spot size. The typical thickness of thin films was in the range of 200 to 400 nm. The phase structure was characterized by X-ray diffraction (XRDBruker D8) in Cu Kα radiation with a spot focus size of 1 × 1 mm2 and a high resolution 2D detector. AFM images were acquired with a Bruker AFM-d3100IIIa at room temperature. The transmittance and reflectance were measured using a custom fiber optics spectrometer instrument with deuterium and halogen light sources (Ocean optics, DH-2000-BAL) and a broadband detector (Ocean optics, S2000 spectrometer) covering the wavelengths of 300 to 1100 nm. Each single point was measured 50 times for averaging. The sheet resistance was measured by a custom 4point probe instrument equipped with a current source (Keithley 6221 DC and AC current source) and voltage meter (Keithley 2000 Multimeter). The same instrument was used to collect the thermally induced voltage to estimate the Seebeck coefficients of the materials.23 All these instruments were used for combinatorial mapping as described above. Temperature-dependent Hall measurement was conducted on individual samples (0.5 × 0.5 cm2) with indium electrodes, using a Lakeshore Hall measurement instrument (model 8425) under a magnetic field of 2 T. Data analysis was performed using custom written routines in Igor Pro.

practical applications: it can be synthesized at lower substrate temperature (300 °C) compared to CuAlS2 (800 °C), and it shows higher transparency (90% at 650 nm) compared to CuxZn1−xS2 (65−75% at 650 nm). In previous works αBaCu2S2 thin films with conductivity of 17 S/cm and optical transparency of near 70% were prepared by RF sputtering from a BaCu2S2 target with substrate temperature at 300 °C.12 The conductivity of the thin films was improved to 33.6 S/cm by utilizing solution processing with transparency reaching 75% in the region of 500−800 nm.13 However, it remains unclear how the BaCu2S2 properties change with the Ba/Cu stoichiometry and growth temperatures and how the properties of BaCu4S3 thin films compare with those of BaCu2S2. In addition, since Sr has a similar atomic radius to Ba, it is of interest to investigate whether Sr can form similar compounds with copper and sulfur. Previous calculations have predicted that SrCu2S2 (2.27 eV) should have a wider theoretical bandgap than BaCu2S2 (1.62 eV)17 in the same layered crystal structure.18 Nevertheless, these related strontium copper sulfides have not been investigated experimentally. In this work, we prepared Ba−Cu−S thin films with different chemical compositions (Cu/(Cu + Ba) = 0.60−0.85) at a range of substrate temperatures (T = 150−450 °C), to investigate their electrical and optical properties. We found that the preferred BaCu2S2 synthesis temperature is 250 °C, leading to an optimized electrical conductivity (53 S/cm) and optical transparency (90%). For the samples synthesized at high substrate temperature of 380−420 °C, the conductivity strongly and nonmonotonically depends on its composition close to the BaCu2S2 stoichiometry. Compared to BaCu2S2, BaCu4S3 has a lower optical bandgap (1.8 eV) but higher conductivity (73 S/ cm). Attempts have been made to synthesize the related Sr− Cu−S materials, but no new phase-pure materials were achieved. These observations were used to put lower bounds of −0.928 eV/atom and −0.599 eV/atom on the formation enthalpies of the SrCu2S2 or SrCu4S3 materials, respectively. However, we noticed that small amounts of Sr doping in BaCu2S2 can enhance its conductivity but reduce the transparency. Overall, the results from this work provide new insights into Ba−Cu−S p-type transparent semiconductors, which are necessary to promote their potential application in optoelectronic devices.

2. METHODS To explore the material structure and properties across a wide range of chemical compositions and substrate temperatures, a high throughput experimentation (HTE) approach was used.19 This HTE approach includes combinatorial synthesis, spatially resolved characterization, and semiautomated analysis of the resulting large amounts of raw data. Through a specially designed deposition chamber, gradients of composition and substrate temperature can be achieved on one substrate, called a “sample library”.20 The composition gradients result from tilting the guns with respect to the surface of a stationary, nonrotating substrate. The temperature gradients result from partially connecting the substrate to a heated sample holder.21,22 Next the composition, structure, and properties are mapped automatically using a well-defined coordinate grid on the sample library.23 Finally, the generated data can be loaded, processed, and analyzed by customized software routines to save time. This HTE approach is much faster compared to traditional experimental procedures based on single-point samples. One application for HTE is the optimization of synthesis conditions for a previously reported material,24 like Ba−Cu−S discussed here, from a wide range of possible processing parameters. HTE can also be used to explore synthesis conditions for new materials,25 like Sr−Cu−S presented in this paper.

3. RESULTS AND DISCUSSION 3.1. Ba−Cu−S Materials Structure. There are two known chemical compositions in the Ba−Cu−S material system (BaCu2S2, BaCu4S3), each with two possible crystal structures (α and β). The structure of α-BaCu2S2 belongs to the Pnma space group, with lattice constants of a = 9.3081(4) Å, b = 4.0612(3) Å, and c = 10.4084(5) Å27 (Figure 1a). The βBaCu2S2 polymorph is stable above 700 °C and adopts the ThCr2Si2-type structure (I4mmm space group).28 Similar to αBaCu2S2, α-BaCu4S3 crystallizes with space group Pnma (Figure 1b) and transitions into β-BaCu4S3 with space group Cmcm at 640 ± 10 °C.29 As shown in Figure 1, there are some differences between the crystal structures of α-BaCu2S2 and α-BaCu4S3. Ba occupies 7fold S-coordinated sites in BaCu2S2, versus 8-fold S-coordinated 8240

DOI: 10.1021/acs.chemmater.7b02475 Chem. Mater. 2017, 29, 8239−8248

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Chemistry of Materials

Figure 1. Crystal structures of (a) α-BaCu2S2 and (b) α-BaCu4S3, showing 7-fold vs 8-fold S-coordinated Ba−S motifs (green-yellow) and tetrahedral vs chain Cu−S motifs (blue-yellow) in these two materials, respectively.

sites in BaCu4S3. The Cu atoms in α-BaCu2S2 are in the tetrahedral S environment, with the tetrahedrons spatially distributed in the crystal structure and separated by Ba-centered motifs, sharing one corner or one side. In contrast, α-BaCu4S3 contains zigzag Cu−S chains forming an interlayer between Bacentered motifs and leading to a stronger Cu−Cu interaction and shorter Cu−Cu distance (2.556 Å in α-BaCu4S3 compared to 3.2415 Å in α-BaCu2S2). Considering that lower processing temperatures are more practical for real-world semiconductor applications, we focus on the α phases of BaCu2S2 and BaCu4S3 in rest of this paper. First, we studied the phase composition of Ba−Cu−S thin films as a function of chemical composition and synthesis temperature. The Cu/(Ba + Cu) ratios are calculated from the Cu and Ba atomic contents determined from XRF. Figure 2a shows the phase map derived from the XRD patterns measured on the Ba−Cu−S thin film sample libraries. Both BaCu2S2 and BaCu4S3 start to crystallize at 200 °C and remain crystalline to 420 °C. However, BaCu2S2 remains phase pure across the whole studied temperature range, while CuxS starts to coexist with BaCu4S3 at >300 °C. It demonstrates that BaCu4S3 thin films can be deposited at temperatures as low as 200 °C, while the previously reported bulk materials require much higher processing temperatures.29,30 The resulting phase also strongly depends on the chemical composition, as illustrated in Figure 2b for the films deposited at 250 °C. The evolutions of XRD patterns as a function of composition at other temperatures can be found in Figure S1 in the Supporting Information. In the Cu/(Ba+Cu)−0.928 eV/atom) and > −4.792 eV/ f.u. (>−0.599 eV/atom), respectively. Since it is difficult to prepare the Sr−Cu−S materials compared to Ba−Cu−S materials, we investigated how much Sr can substitute the Ba without phase separation and whether such Sr-for-Ba substitution can improve the properties. For these studies, we prepared (SrxBa1−x)Cu2S2 thin film sample libraries with x = Sr/(Sr + Ba) in the range from 0.10 to 0.55 and with Cu/(Cu + Sr + Ba) fixed at 0.67 ± 0.01. The XRD color map of these compositionally graded thin films deposited at substrate temperature of 300 °C is illustrated in Figure 9a. The materials remain in the (Ba,Sr)Cu2S2 phase when x < 0.3 but transform from BaCu2S2-type to BaCu4S3-type structure when x > 0.3. The resulting electrical and optical properties of the phase pure (Ba,Sr)Cu2S2 alloys up to x = 0.3 are shown in Figure 9b. The Sr substitution increases the conductivity to over 100 S/ cm at x = 0.15, and then the conductivity drops down with further increase in the Sr content. The origin of this nonmonotonic change may be related to defects induced by the Sr substitution or the Sr related secondary phase in the

Figure 7. Phase map of Sr−Cu−S sample libraries derived from their XRD patterns. The SrS and CuxS peaks were observed at all studied compositions, with no indication of SrCu2S2 or SrCu4S3 phases.

the XRD patterns. The corresponding XRD patterns can be found in Figure S11 in the Supporting Information. Unfortunately, all the observed XRD peaks come from either SrS or CuxS (1.75 < x < 2) phases. The SrS appears to be stable up to Cu/(Sr + Cu) = 0.6−0.7 composition, whereas at higher Cu content additional CuxS peaks were observed. The absence of CuxS XRD peaks at lower Cu content may be related to the general low crystallinity of these phases. Overall, it seems much more difficult to prepare Sr−Cu−S materials compared to the related Ba−Cu−S compounds. To understand why it is more difficult to synthesize the Sr− Cu−S compared to Ba−Cu−S materials, we consider the chemical reactions that are likely to occur during the thin film deposition process: BaS + Cu 2S → BaCu 2S2

(1)

SrS + Cu 2S → SrCu 2S2

(2)

The direction of these reactions can be quantified as the formation energy or decomposition energy from the known

Table 1. Formation Energy of BaS, SrS, Cu2S, BaCu2S, BaCu4S3, SrCu2S2, and SrCu4S3 ΔH (eV/atom) ΔH (eV/f.u.)

BaS

SrS

Cu2S

BaCu2S2

BaCu4S3

SrCu2S2

SrCu4S3

−2.411 −4.822

−2.482 −4.964

−0.128 −0.384

−1.193 −5.965

−0.892 −7.316

>−0.928 >−4.640

>−0.599 >−4.792

8245

DOI: 10.1021/acs.chemmater.7b02475 Chem. Mater. 2017, 29, 8239−8248

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Chemistry of Materials

Figure 8. Ternary convex hull diagram illustrating the formation energies of materials in the (a) Ba−Cu−S system and (b) Sr−Cu−S system, with the color scale denoting the formation energy of specific material in eV/atom. Binary convex hulls for BaS−Cu2S and SrS−Cu2S are shown under the ternary diagrams. BaCu2S2 lies below the BaS−BaCu4S3 line, and BaCu4S3 lies below BaCu2S2−Cu2S line; hence, these two ternaries are stable with respect to each other. SrCu2S2 and SrCu4S3 are supposed to lie above the SrS−Cu2S line, since they were not observed during the experiments. From these plots, the lower bounds of SrCu2S2 (>−4.640 eV/f.u) and SrCu4S3 (>−4.792 eV/f.u) can be estimated.

CdTe). Among the current state of the art p-type TCs, some require high processing temperatures (like Cu1+xAl1−xS2 prepared at above 650 °C10 or BaCu(Se1−xTex)F at 600 °C14) while others need to be deposited on special substrates (like Ca-doped or Sn-doped CuInO2, on alpha-Al2O3(001) single crystal substrates8 or Mg-doped LaCuOSe on MgO (001) single-crystal substrates16). If only p-type TCs that do not require high processing temperature (>500 °C) and special substrates are considered, suitable thin films are confined to just a few groups of materials. For example, CuAlO2 is reported to be prepared on glass at 400 °C, showing a conductivity of 2.4 S/cm and transmittance around 60%.47 SrKCu2O2:K can be prepared on quartz at a lower temperature of 300 °C, also showing transmittance around 60% but with a much lower conductivity of 4.8 × 10−2 9. In this work, barium/strontium copper sulfides are prepared at moderate temperatures from 250 to 420 °C on glass, extending the list of p-type TCs that can be synthesized at low temperature on inexpensive substrates. Some attempts have been made to synthesize previously unreported strontium copper sulfide materials under similar conditions as barium copper sulfides reported in this study. However, only phase-separated CuxS and SrS films were observed in the Sr−Cu−S materials system. These results of the Sr−Cu−S synthesis experiments were analyzed in terms of thermodynamic stability of the constituent SrS and Cu2S phases. The lower bounds of the formation enthalpies of the SrCu2S2 and SrCu4S3 phases are determined to be >−4.640 eV/ f.u. and >−4.792 eV/f.u., respectively. These results may be helpful to other researchers to inform the preparation of these materials via other synthesis routes. Even though no new materials were found in Sr−Cu−S thin films, it appears that the Sr substitution of Ba sites in (SrxBa1−x)Cu2S2 alloys may lead to better conductivity with slight compromise in transmittance.

alloys, but determining its exact origin would require further studies. Absorption of (Sr,Ba)−Cu−S thin films as a function of the x value in (SrxBa1−x)Cu2S2 is summarized in Figure 9c. The optical absorption increases linearly with the addition of Sr, consistent with the higher likelihood of precipitation of the BaCu4S3-type phases at higher Sr and lower Ba compositions. Similar attempts have been made to alloy BaCu4S3 with Sr, but the secondary SrS phase was observed.

4. SUMMARY AND CONCLUSIONS A series of Ba−Cu−S, Sr−Cu−S, and (Sr,Ba)−Cu−S thin films with different compositions were prepared at a broad range of substrate temperatures, in order to compare the BaCu2S2 and BaCu4S3 compounds, and to contrast the barium copper sulfides vs strontium copper sulfides. The highest transparency (90% in 600−1000 nm range) and conductivity (31−53 S/cm) are achieved from 250 to 350 °C substrate temperature in 350 nm BaCu2S2 films. The conductivity of BaCu4S3 (56−74 S/cm) is almost independent of temperature from 200 to 380 °C, while it drops down somewhat at 420 °C where CuxS impurities may be present. The conductivity of Ba−Cu−S thin films is not sensitive to compositional changes at lower temperatures but changes nonmonotonically at higher temperatures as the Cu/(Ba + Cu) ratio changes. Comparing the two different compounds, BaCu2S2 shows a larger bandgap and is also more transparent than BaCu4S3. On the other hand, BaCu4S3 has higher conductivity than BaCu2S2, especially at higher synthesis temperatures. Overall, the high doping of these materials makes Ba−Cu−S compounds promising candidates for transparent p-type contacts in solar cells and light emitting devices but unlikely candidates for thin film transistors and other electronic applications where the field modulation of carrier density is required. The simple low-temperature processing used to fabricate Ba−Cu−S thin films paves the way for them to be utilized in real-world devices with limited thermal budgets, such as in chalcogenide solar cells in superstrate configuration (e.g., 8246

DOI: 10.1021/acs.chemmater.7b02475 Chem. Mater. 2017, 29, 8239−8248

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Qun Zhang). *E-mail: [email protected] (Andriy Zakutayev). ORCID

Yanbing Han: 0000-0003-1159-1005 Sebastian Siol: 0000-0002-0907-6525 Andriy Zakutayev: 0000-0002-3054-5525 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by U.S. Department of Energy, under Contract No. DE-AC36-08GO28308 to NREL. Andriy Zakutayev acknowledges support from the Office of Energy Efficiency and Renewable Energy, Solar Energy Technology Program. Sebastian Siol acknowledges support from the Office of Science, Basic Energy Sciences, as part of the Energy Frontier Research Center “Center for Next Generation of Materials by Design”. Qun Zhang is supported by the National Natural Science Foundation of China (No. 61471126) and a grant from Science and Technology Commission of Shanghai Municipality (No. 16JC1400603). Yanbing Han acknowledges fellowship support from the Chinese Scholarship Council. The authors would like to thank Kevin Talley for help with the combinatorial synthesis and characterization instruments, Rachel Woods-Robinson for the grammar editing, and Bobby To for assistance with AFM measurements.

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(1) Facchetti, A.; Marks, T. J. Transparent Electronics: From Synthesis to Applications; Wiley: 2010. (2) Helander, M. G.; Wang, Z. B.; Qiu, J.; Greiner, M. T.; Puzzo, D. P.; Liu, Z. W.; Lu, Z. H. Chlorinated Indium Tin Oxide Electrodes with High Work Function for Organic Device Compatibility. Science (Washington, DC, U. S.) 2011, 332, 944−947. (3) Nomura, K.; Ohta, H.; Takagi, A.; Kamiya, T.; Hirano, M.; Hosono, H. Room-Temperature Fabrication of Transparent Flexible Thin-Film Transistors Using Amorphous Oxide Semiconductors. Nature 2004, 432, 488−492. (4) Fortunato, E.; Barquinha, P.; Martins, R. Oxide Semiconductor Thin-Film Transistors: A Review of Recent Advances. Adv. Mater. 2012, 24, 2945−2986. (5) Sullivan, I.; Zoellner, B.; Maggard, P. A. Copper(I)-Based P-Type Oxides for Photoelectrochemical and Photovoltaic Solar Energy Conversion. Chem. Mater. 2016, 28, 5999−6016. (6) Kawazoe, H.; Yasukawa, M.; Hyodo, H.; Kurita, M.; Yanagi, H.; Hosono, H. P-Type Electrical Conduction in Transparent Thin Films of CuAlO2. Nature 1997, 389, 939−942. (7) Yanagi, H.; Kawazoe, H.; Kudo, A.; Yasukawa, M.; Hosono, H. Chemical Design and Thin Film Preparation of P-Type Conductive Transparent Oxides. J. Electroceram. 2000, 4, 407−414. (8) Yanagi, H.; Hase, T.; Ibuki, S.; Ueda, K.; Hosono, H. Bipolarity in Electrical Conduction of Transparent Oxide Semiconductor CuInO2 with Delafossite Structure. Appl. Phys. Lett. 2001, 78, 1583−1585. (9) Kudo, A.; Yanagi, H.; Hosono, H.; Kawazoe, H. SrCu2O2: A PType Conductive Oxide with Wide Band Gap. Appl. Phys. Lett. 1998, 73, 220−222. (10) Huang, F.-Q.; Liu, M.-L.; Yang, C. Highly Enhanced P-Type Electrical Conduction in Wide Band Gap Cu1+x Al1−x S2 Polycrystals. Sol. Energy Mater. Sol. Cells 2011, 95, 2924−2927. (11) Woods-Robinson, R.; Cooper, J. K.; Xu, X.; Schelhas, L. T.; Pool, V. L.; Faghaninia, A.; Lo, C. S.; Toney, M. F.; Sharp, I. D.; Ager, J. W. P-Type Transparent Cu-Alloyed ZnS Deposited at Room Temperature. Adv. Electron. Mater. 2016, 2, 1500396−1500404.

Figure 9. (a) XRD color map as a function of Sr/(Sr + Ba) ratio for the (Sr,Ba)−Cu−S sample libraries deposited at a substrate temperature of 300 °C. The BaCu2S2 type structures appear to persist as a pure phase up to Sr/(Sr + Ba) = 0.3 ratio. (b) Electrical conductivity and optical absorption (in the 2.3 to 2.4 eV range) of (Sr,Ba)−Cu−S thin films as a function of Sr/(Sr + Ba) ratio with Cu/(Cu + Sr + Ba) fixed at 0.67 ± 0.01. (c) Absorption of (Sr,Ba)−Cu−S thin films as a function of the x value in (SrxBa1−x)Cu2S2. The conductivity is increased and the optical absorption remains low up to a composition of Sr/(Sr + Ba) = 0.2.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02475. XRD maps of Ba−Cu−S libraries at 420, 380, 350, and 300 °C; BaS detector image; XRD patterns/references from different phases; AFM images of BaCu4S3; transmittance of Ba−Cu−S library at 250 °C; absorption and transmittance of BaCu2S2 and BaCu4S3 as a function of temperatures; XRD reference numbers; and XRD patterns from Sr−Cu−S (PDF) 8247

DOI: 10.1021/acs.chemmater.7b02475 Chem. Mater. 2017, 29, 8239−8248

Article

Chemistry of Materials

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DOI: 10.1021/acs.chemmater.7b02475 Chem. Mater. 2017, 29, 8239−8248