A Versatile Thin-Film Deposition Method for Multidimensional

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A Versatile Thin-Film Deposition Method for Multidimensional Semiconducting Bismuth Halides Maryam Khazaee, Kasra Sardashti, Jon-Paul Sun, Hanhan Zhou, Charlotte Clegg, Ian G. Hill, Jacob L. Jones, Doru C. Lupascu, and David B. Mitzi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01341 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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Chemistry of Materials

A Versatile Thin-Film Deposition Method for Multidimensional Semiconducting Bismuth Halides Maryam Khazaee1,2, Kasra Sardashti1, Jon-Paul Sun1,4, Hanhan Zhou3, Charlotte Clegg4, Ian G. Hill4, Jacob L. Jones3, Doru C. Lupascu2, David B. Mitzi*,1,5 1

Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, United States 2

Institute for Materials Science and Center for Nanointegration Duisburg-Essen (CENIDE), University of DuisburgEssen, Universitätsstraße 15, 45141 Essen, Germany

3

Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695

4

Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada

5

Department of Chemistry, Duke University, Durham, North Carolina 27708, United States

ABSTRACT: Despite the significant progress in fabricating hybrid organic-inorganic lead halide perovskite solar cells, their toxicity and low stability remain as major drawbacks, thereby hindering large-scale commercialization. Given the isoelectronic nature of lead(II) and bismuth(III) ions, potentially stable and non-toxic alternatives for efficient light absorption in thin-film photovoltaic (PV) devices may be found among bismuth-based halide semiconductors. However, high-quality polycrystalline films of many of these systems have not been demonstrated. Here we present a versatile and facile two-step co-evaporation approach to fabricate A3Bi2I9 (A = Cs, Rb) and AgBi2I7 polycrystalline films with smooth, pin-hole-free morphology and average grain size of > 200 nm. The process involves an initial two-source evaporation step (involving CsI, RbI or AgI, and BiI3 sources), followed by an annealing step under BiI3 vapor. The structural, optical and electrical characteristics of the resulting thin films are studied by X-ray diffraction, optical spectroscopy, X-ray/UV photoelectron spectroscopy and scanning electron microscopy.

Introduction Lead-based halide perovskites have recently attracted substantial attention due to their remarkable semiconducting properties, including a stoichiometry-tunable band gap,1-2 relatively modest charge carrier mobilities (µ ≈ 1−100 cm2 V−1 s−1),3-5 long carrier diffusion lengths,6 low carrier recombination rates,6-7 and high absorption coefficients.8 Among the perovskite systems, methylammonium lead iodide (CH3NH3PbI3) has been at the center of attention due to its notable performance in thin-film solar cells, reaching power conversion efficiencies (PCEs) beyond 22%.9 Nevertheless, toxicity of the constituting elements (i.e. Pb) 10 and chemical/electronic instability under ambient conditions 11-13 represent two major issues preventing the large-scale commercial deployment of lead-based perovskites. With the goal of boosting stability and reducing toxicity, multiple studies have pursued partial or full substitution of the monovalent methylammonium and divalent Pb with alternative organic and inorganic cations. While attempts to substitute methylammonium with formamidinium,14 Cs,15 formamidinium and Cs alloys,16 and Rb 17 yield high-performance devices with modest increase in stability, the levels of stability required for commercial applications are not yet achievable. Additionally, although Pb substitution with Sn (II) and Ge (II) has been successfully demonstrated, the presence of these ions leads to inferior stability due to their propensity to convert to tetravalent states (Ge(IV), Sn(IV)).18-23 Alternatively, non-toxic and stable semiconducting halides can be realized by replacing Pb2+ with isoelectronic Bi3+. Recently, several Bi-based compounds have been introduced as potential light absorbers for thin-film solar cells including, (CH3NH3)3Bi2I9,24-26 A3Bi2I9 (A = K, Rb, Cs),27-29 Cs2AgBiX6 (X = Cl, Br),30 and AgaBibIa+3b.31-35 However, so far, these systems have shown PCEs far below the Pb-based systems (maximum efficiency of 4.3%),35 necessitating more in-depth

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investigations into the factors limiting their performance. One aspect that could strongly limit PV performance is the poor film morphology (i.e., high pinhole densities and small grains) of the active bismuth-iodide-based absorbers, particularly for films prepared by solution-based deposition methods. Therefore, it is crucial to develop robust methods for growing high-quality thin films with high crystallinity and minimal pinhole densities. One potential approach to achieve this is to use coevaporation, which has successfully led to growth of high-quality lead halide perovskite materials.36-39 In the present study, we introduce a facile and reproducible two-step fabrication approach for obtaining smooth and pinhole-free Cs3Bi2I9, Rb3Bi2I9, and AgBi2I7 films with grain sizes in the range of 100-200 nm for Cs3Bi2I9 and > 2 µm for Rb3Bi2I9 and AgBi2I7. This is an interesting set of representative compounds, because the electronically-relevant framework of bismuth iodide octahedra can adopt different connectivity – i.e., from effectively zero-dimensional dimers of edge-sharing bismuth iodide octahedra in Cs3Bi2I9, to two-dimensional (2D) sheets of corner-sharing bismuth iodide octahedra in Rb3Bi2I9 and finally a three-dimensional (3D) network of edge-sharing alternating bismuth/silver iodide octahedra in AgBi2I7. Both structural and electronic dimensionalities play essential roles in optical/electronic properties of semiconductor materials.40 Lower electronic dimensionality (generally arising from the lower dimensionality of the crystal structure) leads to a lower VBM and higher CBM and, therefore, a larger semiconductor band gap. Low electronic dimensionality may also promote larger and more anisotropic effective masses for the carriers, as well as deeper point defects, which may act as electron/hole traps and/or recombination centers. As shown in Figure 1, the two-step deposition approach consists of coevaporation of metal halide (MI, with M = Cs, Rb, Ag) and bismuth triiodide (BiI3) followed by a post-deposition anneal under BiI3 vapor. An analogous approach has been successfully demonstrated for fabrication of high-quality films of Cs3Sb2I9,41 Cs2SnI6,22 and most recently Cs2TiBr6.42 By optimizing parameters such as film thickness, deposition rate ratio between the two precursors and annealing conditions, we acquire smooth and compact films. The direct band gap values of 2.16, 2.10, and 1.83 eV obtained for the films of Cs3Bi2I9, Rb3Bi2I9, and AgBi2I7, respectively, agree with the literature values for analogous solution-deposited films. 24, 27 Photoelectron spectroscopy (PES) of these films demonstrates deep valence band positions, highlighting the necessity for employing hole transport materials with deep highest occupied molecular orbital (HOMO) levels for prospective PV device design. Application of this facile thinfilm growth technique should facilitate realization of Bi-based halide optoelectronic devices with superior performance levels.

Figure 1. Schematic illustration of two-step deposition method


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Experimental Section Thin film growth. All chemicals were purchased in powder form from Alfa Aesar and used as supplied, including BiI3 (99.999%), CsI (99.999%), RbI (99.8%), and AgI (99.9%). Thin films of Cs3Bi2I9, Rb3Bi2I9, and AgBi2I7 were grown using a dual-source coevaporation approach with Radak sources in an Angstrom Engineering (EvoVac) thermal evaporator. For this purpose, one of the sources was filled with BiI3 and the other one with the metal halide of interest (i.e. CsI, RbI, or AgI). During the coevaporation, the deposition rate ratio (r) between MI and BiI3 were maintained at 1.2 for CsI/BiI3, 0.7 for RbI/BiI3, and 0.6 for AgI/BiI3. The deposition rate for each source was monitored in-situ by quartz crystal microbalances, with the base pressure during the evaporation held below 4 × 10−6 Torr. All the films were deposited with no intentional substrate heating. The deposited films were subjected to post-deposition annealing under BiI3 vapor (under a quartz cover inside a nitrogen-filled glovebox) followed by quenching of the films to room temperature. For each of these compounds, annealing temperatures ranging from 150-320 °C and annealing times ranging from 10-20 min were optimized for achieving polycrystalline thin films with large grain size and high crystallinity. Film thicknesses were varied in the range of 350-450 nm. Thin Film Characterization. X-ray diffraction (XRD) measurements on the deposited bismuth-iodide-based films were performed under ambient conditions over the 2θ angle range of 10 – 50° using Cu Kα radiation (PANalytical Empyrean X-ray diffractometer). Surface morphology of the films was investigated using a FEI XL30 scanning electron microscope (SEM). Compositional analysis of the grown films was carried out using SEM energy-dispersive X-ray spectroscopy (EDX) (FEITM Verios 460L). In order to study the optical properties, UV-Vis absorption measurements were performed using a Shimadzu UV3600 spectrometer. Subsequently, direct and indirect band gaps were derived from the linear fit of the Tauc plots of the absorption data. X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES) were carried out under ultra-high vacuum (10-10 mbar) for thin films deposited on indium tin oxide (ITO)-coated glass substrates. For UPS and XPS studies, the analysis chamber was equipped with a hemispherical energy analyzer (Specs Phoibos 150). For XPS, Al Kα (1486.6 eV) and Mg Kα (1253.6 eV) sources were used. UPS measurements were carried out using a He I & II (hν = 21.22 eV, 40.8 eV) source. IPES was performed in the isochromat mode, with a spectrometer utilizing a band-pass photon detector consisting of an electron multiplier/KCl photocathode coupled with a SrF2 window. The films were analyzed as-loaded and sputtered (i.e., Ar ion source with an extractor voltage of 3 kV and a beam current of 5 µA, rastered over a 10 mm × 10 mm area). Surface sputtering was employed to test the effect of removing surface contaminants. Results and discussion Film Crystal Structure: XRD patterns for annealed films were collected to determine the associated crystal structures and to establish phase purity (Figure 2). Comparison of the collected patterns for the Cs3Bi2I9, Rb3Bi2I9, and AgBi2I7 films with the respective reference patterns confirms successful growth of single-phase thin films of the three compounds. Cs3Bi2I9, Rb3Bi2I9, and AgBi2I7 patterns were indexed using hexagonal (space group P63/mmc, no. 194), monoclinic (P21/n, no. 14), and cubic (Fd3തm, no. 227) crystal systems, respectively. Depending on the composition and preparation conditions, two possible structures can be achieved for silver bismuth iodide, the rhombohedral layered CdCl2-type (R3തm) and cubic defect-spinel (Fd3തm) structures 33 shown in Figures S1a and b, respectively. The 3D network of AgBi2I7 consists of edge-sharing octahedra in which each octahedron is occupied by either a Ag+ or Bi3+ cation [(Ag/Bi)I6]. This partially occupied AgBi2I7 octahedral-coordination framework can yield either a rhombohedral CdCl2-layered structure (R3തm space group) or a cubic defect-spinel structure (Fd3തm space group) similar to AgBiI4. 33, 40, 43 Fitting the current experimental XRD data for as-deposited and annealed AgBi2I7 yields the rhombohedral structure with lattice parameters a = b = 4.3593(2) Å, c = 20.6815(9) Å (Figure S2) and the cubic structure with lattice parameters a = b = c = 12.2043(3) Å (Figure 2a), respectively, consistent with previously published reports.33, 44 Given the relatively stable composition of the silver bismuth iodide films during the post-deposition anneal (Table S1), this phase transition from rhombohedral to cubic symmetry for the AgBi2I7 films during annealing may result from a higher thermodynamic stability of the cubic defect-spinel structure versus the CdCl2-type structure, at least at elevated temperature.33 Further investigation is necessary to unravel the detailed nature of this structural transition. The XRD pattern of the Rb3Bi2I9 film indexes to a monoclinic unit cell with lattice parameters a = 14.6451(1) Å, b = 8.1787(6) Å, c = 20.8940(2) Å, and β = 90.4283(1)°, in the space group P21/n (Figure 2b). Figure S1c shows the crystal structure of the monoclinic 2D layered modification of Rb3Bi2I9, which consists of distorted corner-connected BiI6 octahedra in which only two-thirds of the octahedral Bi sites are occupied and one-third remain vacant.27 This structure can be thought of as being derived from the hypothetical (not charge balanced) 3D perovskite RbBiI3 by removing every third Bi layer along to yield a layered perovskite structure. On the other hand, the XRD pattern for the Cs3Bi2I9 films indexes to a hexagonal structure (space group P63/mmc) with lattice parameters a = b = 8.3855(3) Å and c=



21.146(1) Å (Figure 2c), corresponding to a 0D dimer configuration, with isolated pairs of face-sharing BiI6 octahedra surrounded by Cs cations (Figure S1d).27 The Pawley-refined unit cell parameters for Cs3Bi2I9 and Rb3Bi2I9 are in good agreement with previously reported values (Table S2).27, 45-46 In both Cs3Bi2I9 and Rb3Bi2I9, there are strong intralayer covalent/ionic bonds in the close-packed layers of Bi2I93- and weaker ionic interactions between the anionic framework and Cs+ or Rb+, respectively.29

Figure 2. Experimental XRD patterns for AgBi2I7 (a), Rb3Bi2I9 (b), and Cs3Bi2I9 (c) thin films, deposited using the two-step coevaporation approach with anneals of 180 °C for 20 min, 225 °C for 10 min, and 300 °C for 10 min, respectively. In each case, all peaks index to the known structure type reported in the literature (as described in the text). Selected strong diffraction peaks are labeled. Table S2 shows the Pawley fitting results for each film.

Film morphology and composition: Figure 3 displays top-down SEM micrographs for as-deposited and annealed thin films of Cs3Bi2I9 (a-b), Rb3Bi2I9 (c-d), and AgBi2I7 (e-f). After testing various annealing conditions for each material, annealing as-deposited films of Cs3Bi2I9 at 300 °C for 10 min, Rb3Bi2I9 at 225 °C for 10 min, and AgBi2I7 at 180 °C for 20 min, under excess BiI3 vapor, were determined to be the optimal conditions to achieve smooth and pinhole-free films with large grain sizes. For all three compounds, the as-deposited films consist of very small grains with maximum width of 100 nm. However, upon annealing under BiI3 vapor, significant grain growth occurs leading to grains as wide as 200 nm for Cs3Bi2I9 and > 2 µm for Rb3Bi2I9 and AgBi2I7. In addition to large grains, negligible pinhole densities are observed for the annealed films. Table S1 shows the SEM EDX compositional analysis of the three types of films. The as-deposited films (Figure 3 a, c, and e) were found to have average compositions of Cs3.0Bi1.9I8.8, Rb3.0Bi2.3I9.0, and AgBi2.0I7.0, respectively. Annealing of the films with an excess of BiI3 resulted in films with average compositions of Cs3.0Bi2.1I9.1, AgBi1.8I6.9 and Rb3.0Bi1.8I7.4. Considering the error margins for compositional analysis via EDX, these results are consistent with successful formation of Cs3Bi2I9, AgBi2I7, and Rb3Bi2I9 (perhaps with slight Bi and I deficiencies for the latter compound). Since the asdeposited rubidium bismuth iodide film is Bi- and I-rich (r = 0.7), it is susceptible to loss of Bi and I during the postannealing process (225 °C, 10 min), owing to the high volatility of BiI3 (even despite added BiI3 during the annealing process). It should be noted that annealing Cs3Bi2I9 and Rb3Bi2I9 films at identical temperatures under N2 (no BiI3) leads to partial evaporation of BiI3, leaving behind CsI and RbI secondary phases. In the case of silver bismuth iodide, while annealing under N2 at temperatures of as high as 180 °C for 10 min leads to BiI3 loss, smooth and single-phase films with cubic structure (a = b = c = 12.2079(2) Å) were realized (Figure S3) with large grain size (>3 µm), in part due to the stability of the cubic phase over a relatively wide composition range.33 The annealed film (no BiI3 vapor during the anneal) was found to have Ag1.16Bi1.04I4.00 stoichiometry, as measured by SEM EDX. The composition is Bi and I deficient compared to the as-deposited film (AgBi2.0I7.0), which verifies loss of BiI3 from the film during annealing.


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Figure 3. Top-view SEM micrographs of as-deposited and annealed thin films of Cs3Bi2I9 (a-b), Rb3Bi2I9 (c-d), and AgBi2I7 (ef), deposited using the two-step coevaporation approach on glass substrates. Annealing was performed under BiI3 vapor as described in the text.

Optical properties: The optical absorption spectra of the various bismuth(III)-iodide-based semiconductor films were collected and linear extrapolation of the Tauc plots ((αhν)n versus hν) provide the direct (n = 2) optical band gaps (Figure 4), where α is the absorption coefficient, h is Planck’s constant and ν is the excitation frequency. The estimated direct band gaps (Eg) for Cs3Bi2I9, Rb3Bi2I9, and AgBi2I7 are 2.16, 2.10 eV and 1.83 eV (estimated error = ± 0.05), respectively. The layered modification of Rb3Bi2I9 is red in color as opposed to the orange color of the dimer modification of Cs3Bi2I9, consistent with the lower measured Eg for the layered structure. Due to uncertainties in the directness or indirectness of the band gaps for these compounds, the indirect band gap (n = 1/2) values of 2.07 ± 0.005, 2.06 ± 0.01, and 1.73 ± 0.01 eV have also been extracted for Cs3Bi2I9, Rb3Bi2I9, and AgBi2I7, respectively (Figure S4). The measured Eg values for these materials are in good agreement with previous reports for solution-processed films.24, 27-28, 32-33 Among the three compounds, only the AgBi2I7 band gap of 1.83 eV approaches suitability for single-junction solar cells, while Cs3Bi2I9 and Rb3Bi2I9 could be more applicable in a tandem configuration in combination with narrow band gap absorbers. In addition, these systems may be considered promising candidates for other optoelectronic applications, including transistors, photoelectrochemical cells and light-emitting devices (LEDs).

Figure 4. Tauc plots showing direct band gaps for Cs3Bi2I9 (red), Rb3Bi2I9 (black), and AgBi2I7 (blue) films, deposited using the two-step coevaporation approach. The spectra were vertically offset (7% relative to each other) for the clarity of the figure.



Electronic properties: In order to study the electronic properties of the vacuum-deposited films, PES measurements were performed. The surface compositions of the annealed films were characterized by analyzing XPS core-level spectra (Figure S5), with results compiled in Table S3. Apart from carbon (285 eV) and oxygen (530 eV), no trace of other contaminants were observed in both as-loaded and sputtered (5 s) samples. Table S3 shows that the surface composition of cesium bismuth iodide and rubidium bismuth iodide (as-loaded and 5s-sputtered films) are either Bi-rich and/or I-deficient compared to the bulk compositional analysis using SEM EDX, most likely due to the volatility of BiI3 and the possibility of reducing Bi3+ to Bi0, e.g., in an X-ray beam under ultra-high vacuum conditions or during processing. Increasing sputtering time from 5 s to 10 s and 70 s results in the removal of volatile BiI3 from the film and the increase in the relative concentration of rubidium (Table S4). Moreover, a trace of metallic Bi (Bi0) appears in the Al Kα scan of the Bi 4f line for rubidium bismuth iodide films, in the form of two small shoulders at binding energies of ~162 eV and 157 eV for the as-loaded film, which become more distinguishable after sputtering (Figure S6). No trace of metallic Bi was realized for cesium and silver bismuth iodides (Figures S7a and b). In order to address the origin of the metallic Bi, XPS measurements were performed on as-deposited and annealed films of BiI3 (180 °C, 10 min under N2). Figure S8a shows the Al Kα close up scans of the Bi4f peaks for these films. The as-deposited BiI3 film does not exhibit any metallic Bi. However, a small amount of Bi0 is detected for the annealed BiI3 film (marked by “*”). Furthermore, bismuth spectra of as-deposited and post-deposition annealed films of rubidium bismuth iodide (annealing under excess of BiI3 vapor) were collected (Figure S8b). The two small shoulders characteristic of Bi0 are again realized only for the annealed film. In our previous XPS study on the analogous Sb-based layered structure,22 Cs3Sb2I9, a trace of metallic antimony (Sb0) was observed after 5s sputtering of the annealed film of Cs3Sb2I9, further highlighting another possible route of inducing Bi0/Sb0 on the surface of Bi- and Sb-based semiconductors. It should be noted that, if present at the interface between the absorber and the other PV device layers, metallic Bi0/Sb0 could serve as a recombination site or facilitate Fermi level pinning, which might degrade device performance. Further studies are needed to fully decipher the formation mechanism and impact of the Bi0/Sb0 in these studies. Figures S9b and c show UPS spectra of the as-loaded and 5 s sputtered film of AgBi2I7. The sputtering not only shifts the secondary electron onset to lower binding energy by ∼ 0.3 eV, consistent with removal of surface contaminants, but also a double onset feature emerges, which indicates sputtering-induced surface inhomogeneity (Figure S9 b). The XPS core peaks show little change with sputtering, except for the Bi 4f spectra, which shows a low binding energy shoulder emerging (Figure S9 a). Based on these observations, which suggest a detrimental impact of sputter cleaning, XPS, UPS, and IPES spectra of the as-loaded thin films were considered the more reliable data and are the focus of the current study. Figure 5 demonstrates UPS He I secondary electron onset, valence band states, and IPES spectra for as-loaded thin films for each of the semiconductors considered. The positions of the secondary electron onsets, valence band maxima (VBM), and conduction band minima (CBM) were determined via the intersection of tangent line fits to the features of interest and the background. Furthermore, ionization energies (IE = hν – (Eonset – EVBM)) were determined from UPS measurements of the films. Consequently, band gaps (Eg) of the fabricated films were derived as Eg = CBM – VBM. The slowly varying background in the IPES spectra makes the determination of the conduction band edge difficult and consequently results in a large uncertainty in the band gap calculation from UPS/IPES. However, within error the results are consistent with the calculated optical band gaps. Table 1 shows the derived parameters from UPS and IPES measurements. The determined ionization energies of Cs3Bi2I9 (5.8 eV), Rb3Bi2I9 (6.1 eV), and AgBi2I7 (6.0 eV) are consistent with previously reported values of 5.7 eV, 6 eV, and 6.2 eV for solution-processed films, respectively.27, 32 A schematic illustration of the calculated band positions is shown in Figure 5d, confirming that all of these materials behave as n-doped semiconductors. The observed deep VBMs for these bismuth-based semiconductors compared to lead-based halides (e.g., −5.4 eV for CH3NH3PbI3)47-48 verify the necessity of utilizing appropriate hole transport materials with deep VBMs in order to favor high prospective optoelectronic device performance.27, 32


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Figure 5. Combined UPS He I secondary electron onset and scan of valence band states, and IPES (blue) spectra for as-loaded post-deposition annealed a) Cs3Bi2I9, b) Rb3Bi2I9, and c) AgBi2I7 films. d) Band positions for the thin films of Cs3Bi2I9, Rb3Bi2I9, and AgBi2I7 derived from UPS/IPES measurements.

Table 1. Key parameters derived from UPS and IPES measurements on as-loaded Cs3Bi2I9, Rb3Bi2I9, and AgBi2I7 films. The energy values are referenced to the Fermi level. Phase

Eonset (±0.02) (eV)

Cs3Bi2I9 AgBi2I7

17.07 16.74 Eonset (±0.05) (eV)

Rb3Bi2I9

17.05

EVBM (±0.1) (eV) 1.6 1.5 EVBM (±0.1) (eV) 1.9

IE (±0.1) (eV)

ECBM (±0.3) (eV)

Eg (± 0.3) (eV)

5.8 6.0 IE (±0.07) (eV) 6.1

-0.4 -0.2 ECBM (±0.25) (eV)

2.0 1.7 Eg (±0.25) (eV) 2.3

-0.4

Conclusion In summary, we have introduced a versatile two-step approach for growing high-quality and pinhole-free multidimensional bismuth halide semiconducting thin films of Cs3Bi2I9, Rb3Bi2I9 and AgBi2I7. This two-step approach includes coevaporation of metal halides followed by annealing under BiI3 vapor. Structural and optical characterizations performed using XRD and UV-Vis spectroscopy confirm formation of single-phase 0D dimer, 2D layer, and 3D structures for Cs3Bi2I9, Rb3Bi2I9, and AgBi2I7, respectively, with optical direct band gaps of 2.16, 2.10, and 1.83 eV. The VBM positions relative to the vacuum level were determined by ultraviolet photoelectron spectroscopy, showing relatively deep VBM energy levels. XPS data also suggest the sensitivity of some of the film surfaces (most notably Rb3Bi2I9) to Bi3+ to Bi0 reduction. This work is expected to facilitate realizing high-performance Pb-free halide semiconductor systems, through the prospect of a robust approach for producing high-quality films of controlled thickness and morphology.



Associated Content Supporting Information Crystal structures of 2D/3D Ag-Bi-I, 2D Rb-Bi-I, and 0D Cs-Bi-I, XRD analysis of AgBi2I7 and AgBiI4 films, SEM/EDX analysis of AgBi2I7, Rb3Bi2I9, and Cs3Bi2I9 films, crystallographic refinement table, Tauc plots showing indirect band gaps, and XPS analysis (as-loaded/ sputtered thin films) of Cs3Bi2I9, Rb3Bi2I9, and AgBi2I7, XPS analysis of as-deposited and annealed thin films of BiI3 and Rb3Bi2I9, XPS/UPS analysis of as-loaded/sputtered films of AgBi2I7

AUTHOR INFORMATION Corresponding Author *E-mail, [email protected]

Acknowledgments This work was funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Number DE-EE0006712, and also by the National Science Foundation under Grant No. DMR1709294. M.K. and D.C.L. acknowledge financial support through the European Union in the Leitmarktwettbewerb NRW: Neue Werkstoffe, project EFRE-0800120; NW-1-1-040h. I.G.H. acknowledges NSERC CREATE DREAMS and NSERC (RGPIN 298170-2014). J.S. acknowledges Killam Trusts, NSERC and NSERC CREATE DREAMS for financial support. The work was performed in part at the Duke University Shared Materials Instrumentation Facility (SMIF), the Chapel Hill Analytical and Nanofabrication Laboratory (CHANL), and the Analytical Instrumentation Facility (AIF), members of the North Carolina Research Triangle Nanotechnology Network (RTNN), which are supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI). We gratefully acknowledge the generous assistance of Catherine G. Mckenas and Carrie L. Donley to perform XPS measurements and analysis. The authors also thank Tianyang Li for assistance in Pawley refinement analysis and helpful discussion. All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of the DOE or NSF. See the Supporting Information section of this article for more information.

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A Versatile Thin-Film Deposition Method for Multidimensional

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