Are Chalcogenide Perovskites an Emerging Class of Semiconductors

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Perspective

Are Chalcogenide Perovskites an Emerging Class of Semiconductors for Optoelectronic Properties and Solar Cell? Abhishek Swarnkar, Wasim J. Mir, Rayan Chakraborty, Metikoti Jagadeeswararao, Tariq Sheikh, and Angshuman Nag Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04178 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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Chemistry of Materials

Are Chalcogenide Perovskites an Emerging Class of Semiconductors for Optoelectronic Properties and Solar Cell? Abhishek Swarnkar, †, ‡, * Wasim J. Mir,† Rayan Chakraborty,† Metikoti Jagadeeswararao,†,‡ Tariq Sheikh,† Angshuman Nag†,‡,* †Department

of Chemistry, and ‡Centre for Energy Science, Indian Institute of Science Education and Research (IISER), Pune, 411008, India.

Abstract: Metal chalcogenide perovskites were proposed as potential solar cell material in 2015. Theoretical maximum solar cell efficiency of some chalcogenide perovskites are ~ 30%, similar to CH3NH3PbI3 perovskites. The foreseen advantages of chalcogenide perovskites are high thermal and aqueous stability along with non-toxic elemental composition. Till date, a reasonable amount of computational and experimental work has been reported on synthesis, electronic and optical properties of chalcogenide perovskites. Major experimentally studied compounds are AZrS3 (A = Ba and Sr), Ba2Zr3S7 and LaYS3, which have direct bandgaps in the range of 1.3 to 2 eV, along with strong light absorption coefficients and small effective masses of charge carriers. There are a few more compositions with similar properties that have been suggested by computational screening. In this perspective article, we summarize both the computational and experimental progress made in designing chalcogenide perovskites for optoelectronic properties. Then we discuss the material design challenges that need to be addressed in the coming few years for successful solar cell application.

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1. Introduction: Search for efficient solar cell materials, or optoelectronic materials in general, is an old topic of research in the chemistry and physics of materials. The sought after material is a good quality semiconductor with suitable optical bandgap. There have been major successes in developing solar cell materials like Si, GaAs, CdTe, CuInxGa1-xSe2 (CIGS), and Pb-halide perovskite (Figure 1).1-4 Unfortunately, each of these materials still have problems for large scale applications. Therefore, search for good solar cell materials still remains one of the most challenging and important areas of research. In this regard, a new category of materials, namely, chalcogenide perovskite is emerging in recent years.5-10 The progress made in the design and properties of this new class of material is the topic of this perspective article.

Figure 1: Champion solar cell materials. Present status of solar cell materials showing efficiency >20%. The values are taken from NREL’s best research-cell efficiencies chart.4 Drawbacks of each material are also mentioned at the bottom of the histogram. The case of chalcogenide perovskite included here, is not based on experimental results but based on theoretical prediction by using spectroscopic limited maximum efficiency.8 Figure 1 shows that GaAs exhibits the world record solar cell efficiency of 27.6 %.1 But the epitaxial growth of GaAs layers makes these solar cells very expensive. Therefore GaAs solar cells are being used in applications such as satellites and spacecrafts,11 but not yet affordable to 2 ACS Paragon Plus Environment

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provide energy requirement of our daily life. Till date, Si based solar cells are industrially the most popular ones. This success is in spite of the fact that Si is an indirect bandgap semiconductor with poor absorption co-efficient for solar light. This demerit of Si demands for very thick (200 - 400 m)1 Si active layer to absorb enough sunlight. But then a very high quality of the material is required such that the exciton/charge carriers generated by light-absorption can travel through the thick Si layer to reach interfaces/electrodes, before recombination. This requirement of a large amount of high quality single crystals of Si increases the cost of Si solar cells. Interestingly, thin films of direct bandgap materials, such as CdTe and CIGS are already being used in commercial solar cells, but these also suffer from scarcity of elements like Te (0.00055 ppm) and In (0.14 ppm) as compared to Si availability (270000 ppm), in the earth’s crust.12 Earth abundant Cu2ZnSn(SxSe1-x)4 (CZTSSe) have been reported, but with maximum solar cell efficiency of ~13%.13 The latest sensation in solar cell materials is the organicinorganic hybrid Pb-halide perovskites, for which the efficiency reached to 22.7 % in 2018,4 starting from 3.8 % in 2009.14 Unfortunately, these hybrid perovskites also suffer from poor thermal and moisture stability, along with toxicity of Pb.15 Extensive research is presently going on across the globe on Pb-halide perovskites,16,

17

and we are hopeful that the research

community will find out a way to overcome these problems, leading to commercial applications of halide perovskites. The success of halide perovskites, and also some interesting results of oxide perovskites18-21 in photovoltaic applications, encouraged researchers to explore chalcogenide perovskite for solar cell and other optoelectronic properties. The major chalcogenide perovskites that have been studied for optoelectronics are AZrS3 (A = Ba, Sr), Ba3Zr2S7, LaYS3 and a few more related compositions. There are plenty of review articles available in literature on optoelectronic



properties of metal halide and oxide perovskites.3,

19, 22-27

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Also, properties of chalcogenide

perovskites have been mentioned in few review articles that are mainly focused on halide perovskite.28-30 We are not aware of review/perspective article with major emphasis on optoelectronic properties of chalcogenide perovskite. Therefore, we decided to write this perspective article that is focused on chalcogenide perovskite. Crystal structure, electronic band structure, synthesis and optical properties are major topics of this article. The article is further divided into the following sub-sections: (i) scope of chalcogenide perovskites compared to oxides and halides, (ii) Zr-chalcogenide perovskite, (iii) LaYS3 chalcogenide perovskite, (iv) computational screening, (v) outstanding material design challenges, and (vi) conclusions and future outlook. 2. Scope of Chalcogenide Perovskites Compared to Oxides and Halides: Perovskite name is traditionally used for calcium titanate (CaTiO3) ore. Presently, the materials that exhibit the structure of CaTiO3 with generic chemical formula of ABX3, are termed as perovskite, such as BaTiO3 (A2+B4-X2-3)31 and CsPbI3 (A+B2+X-3).32,

33

In this structure (see

Figure 2a) B-site cations form octahedral coordination with 6 X-site anions, forming [BX6]noctahedra. These octahedra then form a three dimensional framework sharing the corners through X-site ions and consequently results into a network of B-X-B-X bonds in all three directions. These B-X bonds mainly constitute the valence and conduction band edges of ABX3 perovskites. Therefore, the corner-shared octahedra control the major optical and electronic properties of ABX3 perovskite. The voids created by these corner-shared octahedra are occupied by A-site cations, which in turn is coordinated with 12 X-site ions forming a cuboctahedron. These A-site cations ensure both structural stability and charge neutrality, and also can influence the B-X octahedral network, thereby fine-tuning the electronic and optical properties.34 Structure


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of oxide, halide and also chalcogenide perovskite can largely be governed by Goldschmidt 𝑟𝐴 + 𝑟𝑋

tolerance factor defined as 𝑡 = √2(𝑟𝐵 + 𝑟𝑋), where rA, rB, and rX are the radii of A-, B- and X-site ions respectively.35,

36

The values of these radii should be taken from the list of Shannon’s

effective ionic radii.37, 38 t = 1 implies the ideal cubic perovskite structure with B-X-B bond angle 180o as shown in the Figure 2a (left panel).39 But distorted perovskite structures can be obtained with 0.71< t < 0.9.36 This flexibility in structural parameters allows incorporation of different elements at A, B and X sites, yielding a plethora of interesting properties.3, 26, 40-42 Our present discussion is restricted to semiconductor perovskites with narrow bandgaps, which are applicable for photovoltaic (PV) and other optoelectronic applications in the visible and near infrared (NIR) region. Earth abundance, thermal stability, water/air stability, and less toxicity are other desired parameters.

Figure 2: Intuitive benefits of chalcogenide perovskites. a) Ideal perovskite (ABX3) structure (left panel). Corner shared network of [BX6]n- octahedra (central panel) mainly controls 5 ACS Paragon Plus Environment


electronic and optical properties. Right panel suggests the flexibility of perovskite structure that allows multiple compositions and properties. b) Comparison of the nature of B-X bond in oxide, chalcogenide and halide perovskites on the basis of electronegativity difference of B- and X-site ions. Also, the Columbic interactions of A-site cation with the negatively charged octahedra have been compared, as a preliminary measure of moisture/water stability. Figure 2b provides a preliminary comparison of basic nature of the B-X bond in oxide, halide and chalcogenide perovskites using one example of each variety. Since B-X bond mainly controls the semiconducting electronic and optical properties, this comparison is important even though some part of it is based on intuition without having enough experimental evidence. Electronegativity is a fundamental chemical property that can describe the capability of an atom to attract the electron density in a chemical bond. In the case of BaTiO3 perovskite, the electronegativity difference between Ti (1.54) and O (3.44) is very high. Therefore, the electron density of Ti-O bonds will be more attracted towards O, giving rise to the polar nature of Ti-O bond, which is the source of the famous ferroelectricity in distorted BaTiO3 perovskite.43 In halide perovskite like MAPbI3 (MA = methylamine), the electronegativity difference between Pb and I is less forming a less polar covalent bond. Likewise, chalcogenide perovskites like SrSnS3 also exhibit a more covalent Sn-S bond. Covalent B-X bonds (or lesser electronegativity difference between B and X) are desired for better charge transport across the B-X-B-X network of ABX3 perovskites. Also, electronegativity can be related to electron affinity and ionization energies of individual atoms, which in turn influence the bandgap. Therefore, lower electronegativity difference along B-X bond can also lead to lower bandgap, which is desired for visible and NIR range optoelectronics. This lower electronegativity difference of B-X bond in both halide and chalcogenide perovskites makes them potential semiconductors for solar cell and other optoelectronic applications. Whereas, high electronegativity difference along B-X bond in oxide perovskites often makes them a wider bandgap insulator. 6 ACS Paragon Plus Environment

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One major disadvantage of MAPbI3 thin film is their poor stability. The small molecule MA (methylamine) starts evaporating at ~85 oC, a temperature that an active solar cell can achieve while in operation.44 Replacing organic MA with inorganic Cs can improve thermal stability of the CsPbI3 composition to 460 oC.45 But then it comes with an additional problem that the desired corner shared cubic phase of CsPbI3 is stable only at a temperature >300 oC.39, 46 Recent works on nanostructuring (smaller grain size) of CsPbI3 yields the desired low bandgap cubic phase around room temperature, and solar cell devices with efficiencies 10-17% have been reported using cubic CsPbI3.47-50 This smaller grain size somewhat hinders the charge transport, but the major concern still remains is moisture/water instability of all these halide perovskites. Dissolution of Pb-halide perovskite in water can contaminate our environment with lead.15 We hope that a cost-effective solution to this problem will soon lead to commercialize the Pb-halide perovskite solar cells. On the other hand, oxide perovskite like BaTiO3 is stable in aqueous environment. Likewise, experimentally known chalcogenide perovskites are also stable in aqueous environment.7 The origin of high stability of chalcogenide perovskites is not yet completely understood. The competition between multiple factors such as ion-ion interaction in the solute (lattice energy) and solute-solvent interaction (solvation/hydration energy) mainly decides the solubility of a solute in a solvent. The lattice energy in chalcogenide perovskites are expected to be higher because of double the amount of charges on A2+ and [BX6]8- compared to the halide perovskites (A+ and [BX6]4-). Higher charges strengthen the Coulomb attraction between cations and anions (Figure 2b). Apart from being insoluble in water, the chalcogenide perovskites also have to be less or non-reactive towards hydroxyl formation, to remain stable in aqueous phase.



These preliminary intuitions presented in Figure 2b prompt us to ask the following question. Can chalcogenide perovskite combine both the good semiconducting properties of halides and good stability of oxides? The answer to this question is not yet known. But the subsequent subsections will shed more light on finding an answer to this question.

3. Zr-Chalcogenide Perovskite 3.1 BaZrS3 and SrZrS3:

Figure 3: Effect of substituting O with S. A comparison of projected density of states (PDOS) of a) BaZrO3 and b) BaZrS3. Inset of b) shows the distorted perovskite structure of BaZrS3. Covalency increases and bandgap decreases by substituting O with S. Reprinted from ref 51. Copyright 2009 American Physical Society. Zr-chalcogenides are the family of chalcogenide perovskites that are most studied for optoelectronic properties. Figure 3 compares electronic structures of BaZrO3 and BaZrS3, 8 ACS Paragon Plus Environment

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calculated by using first-principle density functional theory (DFT).51 Inset of Figure 3b shows the distorted perovskite structure of BaZrS3 where the [ZrS6]8- octahedra are tilted at an angel of 12.2°. The projected density of states (PDOS) in Figure 3a shows that conduction band minimum (CBM) (lowest unoccupied molecular orbital) of BaZrO3 does not have noticeable contribution from O. In difference, CBM of BaZrS3 (Figure 3b) shows a reasonably good mixing of S 3d along with major contribution from Zr 4d orbitals. This mixing suggests the increased covalency of Zr-S bond compared to Zr-O bond, consistent with lower electronegativity of S compared to O as discussed in Figure 2b. Consequently, the energy for CBM decreases lowering the bandgap of BaZrS3. The calculated bandgap of BaZrO3 is 3.9 eV, whereas that for BaZrS3 is reduced to 1.7 eV. More evidence of covalency is also observed in the broader and diffused PDOS peaks in the valence band of BaZrS3 compared to that of BaZrO3.



Figure 4: Synthesis and optical properties of AZrS3 (A = Ba and Sr). a) Schematic showing sulfurization of wide bandgap (white) BaZrO3 powder to narrow bandgap (black) BaZrS3. b) Systematic decrease in bandgap value (from 2.87 eV to 1.75 eV) on increasing the sulfur atomic ratio in BaZr(O1-xSx)3. Inset shows the corresponding UV-visible absorption spectra. c) Schematic representation of solid state synthesis of BaZrS3. d) Comparison of theoretical maximum solar cell efficiencies (spectroscopic limited maximum efficiency) of BaZrS3 and related alloys with MAPbI3 as a function of the absorber thickness. Inset shows diffused reflectance spectrum of BaZrS3 powder with bandgap 1.85 eV. e) Schematic showing a catalytic synthesis route for chalcogenide perovskite (AZrS3). f) Optical images, g) UV-visible absorption spectra (μ represents absorption coefficient), and h) PL spectra of catalytically synthesized BaZrS3 (BZS), α-SrZrS3 (α-SZS) and β -SrZrS3 (β -SZS) showing fine-tuning of bandgap. Panel a-b are adopted from ref 7. Copyright 2016 Elsevier Ltd. Panel c-d are adopted from ref 8. 10 ACS Paragon Plus Environment

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Copyright 2016 American Chemical Society. Panel f-h are reprinted from ref52. Copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Among various methods of syntheses,53-55 there are three major reports7,

8, 52

that discussed

synthesis along with optical properties of Zr-based chalcogenide perovskites, namely BaZrS3 and SrZrS3. We will discuss these three methods one by one. Schematics in Figure 4a shows the transformation of white colored (wide bandgap) BaZrO3 powder into black colored (narrow bandgap) BaZrS3 powder by reacting BaZrO3 with CS2 in an argon atmosphere at 1050 °C for 4 hours.7 The reaction conditions were tuned to tailor compositions of BaZr(O1-xSx)3 oxysulfide with “x” varying from 0 to 1. Consequently, the experimentally measured bandgap systematically varies from 2.87 eV for BaZrO3 (x = 0) to 1.75 eV for BaZrS3 (x = 1) as shown in Figure 4b. Inset of Figure 4b shows the corresponding absorption data of BaZr(O1-xSx)3. Meng et al reported synthesis of BaZrS3 by typical solid state reaction of BaS and ZrS2 (Figure 4c) at 800-1000 oC, after repeated heating and grinding cycles.8 Inset to Figure 4d shows that the bandgap of BaZrS3 product is 1.85 eV. Figure 4d shows the maximum theoretical power conversion efficiency (PCE) calculated using spectroscopic limited maximum efficiency method56-58. The maximum theoretical PCE of BaZrS3 film with 0.5 to 1 μm thickness is ~23%. This is an impressive number but somewhat smaller than the maximum theoretical PCE (~26%) of MAPbI3 films. It is to be noted here that the theoretical PCE is obtained by considering mainly the bandgap and the absorption coefficient of the semiconductor. However, the experimental PCE will strongly depend on additional parameters such as charge transport and device fabrication. So the experimental realization of this theoretical PCE for chalcogenide perovskites still remains a future challenge, unlike the MAPbI3 films that experimentally also shows high PCE. The maximum theoretical PCE of BaZrS3 can be improved even higher than that of



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MAPbI3 by reducing the bandgap through alloying either by substituting S with Se (BaZrSeS2 and BaZrSe2) or Zr with Ti (BaZr1-xTixS3). Unfortunately, these alloys have not been realized yet experimentally. Attempts to prepare BaZr1-xTixS3 alloys lead to the formation of binary mixture of BaZrS3 and BaTiS38 and a certain composition of the alloy (BaZr0.75Ti0.25S3) can exist in perovskite phase but at a high temperature (900 °C).59 In a different report, Niu et al.52 synthesized AZrS3 (A = Ba, Sr) by catalytic reaction of AS, Zr and S powder at temperature range of 600-1100 °C (Figure 4e). Figure 4f shows optical images of powder products. In this improved method, I2 has been used as catalyst to reduce the reaction temperature to 600 °C for synthesizing BaZrS3. It is reported that iodine acts as a transporting agent for chemical vapors required in single crystal growth.52 By forming volatile transition metal iodides, iodine increases the chemical reactivity of the constituent metal ions and makes the reaction faster (few hours to days). UV-visible absorption and photoluminescence (PL) data of AZrS3 samples are shown in Figure 4g and 4h respectively. They report a bandgap of 1.83 eV for BaZrS3, which also exhibit PL with peak value closer to band-edge energies. Overall, the experimental bandgap of BaZrS3 reported by three groups7,

8, 52

employing different synthesis

methodologies fall in the range of 1.75-1.85 eV. Furthermore, to tune the bandgap they52 replaced Ba with Sr as A-site cation. On using smaller sized Sr, two possible crystal phases exist: needle like phase of α-SrZrS3 synthesized at 850 °C and β-SrZrS3 distorted perovskite phase synthesized at 1100 °C. β- SrZrS3 shows higher bandgap (2.05 eV) than BaZrS3, while the α-SrZrS3 shows a lower bandgap value (1.52 eV), as shown in Figure 4g. This difference of physical properties of two materials with same composition but with different structures is not new and have also been found in halide perovskite, such as CsPbI3.39 Owing to the high density of states of both conduction band and valence band edges,


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these Zr-based chalcogenide (except the α-SrZrS3 which show weaker absorption onset) perovskites exhibit high value of absorption coefficient (~105 cm-1) in their photon absorption range.52 Such a higher value of absorption coefficient allows thin layer (~100 nm) of material to absorb maximum amount of incident light (>95%).52 A thin layer of material as an active layer of solar cell is desired to improve collection of charges at electrodes with less contribution from recombination losses. PL spectra (Figure 4h) of all the three Zr-chalcogenide samples also suggest the direct bandgap nature of synthesized samples. Apart from these photophysical properties, it has been found that these Zr-chalcogenide perovskite materials are thermally stable (~550 °C)60. Moreover, on keeping the sample in ambient conditions or in desiccators for one year, the sample maintains its properties.52 Perera et al reported that the BaZrS3 remains stable on repeated washing of the sample in deionized water.7 This thermal and moisture/water stability of chalcogenide perovskite is a major advantage over Pb-halide perovskites.

3.2 Ba3Zr2S7 Layered Ruddlesden-Popper (RP) Perovskite Recently, Niu et al also reported synthesis and properties of Ba3Zr2S7 2D layered RP perovskite.61 Such 2D perovskite structures are well known in both oxide62,

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and halide64-66

perovskites. In Ba3Zr2S7 RP structure, rock salt BaS (AX) layer is sandwiched between alternating perovskite (BaZrS3)2 ((ABX3)n with n = 2 in the present case) layers, as shown in Figure 5a. A generic formula of such layered RP perovskite is An+1BnX3n+1 for the same A cation in both perovskite and rock salt phase. Perovskite phase in RP structures can have different octahedral rotations and distortion compared to their 3D perovskite counterpart, and therefore, can exhibit new electronic, optical, and ferroelectric properties. Electronic band structure of



Ba3Zr2S7 shown in Figure 5b exhibits an indirect bandgap of 1.25 eV with CBM at  point and valence band maximum (VBM) at M point. Interestingly, the direct bandgap at  point is 1.35 eV, and is suitable for single-junction solar cell.

Figure 5: Ba3Zr2S7 Ruddlesden-Popper layered perovskite. a) Schematics showing the crystal structure and b) electronic band structure of Ba3Zr2S7. c) Schematic showing the high temperature synthesis setup (salt flux crystal growth) for Ba3Zr2S7. d) Comparison of steady state PL spectra of synthesized Ba3Zr2S7, InP and GaAs wafers. e) Quantitative emission (left axis) with the corresponding open circuit voltage (VOC) (right axis) as a function of incident power density. Reprinted from ref61. Copyright 2018 American Chemical Society.


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Single crystals of Ba3Zr2S7 were prepared by using BaCl2 flux (Figure 5c). PL data (Figure 5d) shows a peak at 1.28 eV, which agrees with the calculated 1.35 eV direct bandgap. The PL intensity of Ba3Zr2S7 is reasonably good considering this is the first report of this material, and is about an order of magnitude smaller compared to state-of-the-art single crystalline InP and GaAs wafers. External luminescence efficiency (ηext) is the ratio of output to input photon flux, and is found to be ~0.1% to 0.15% for Ba3Zr2S7 with incident power in the range of 103 to 106 W m-2, as shown in Figure 5e. Further they estimated the open circuit voltage (VOC) (Figure 5e) using ηext at different illumination power (Figure 5d) following prior reports.67, 68 Overall the synthesis and optical properties of Ba3Zr2S7 RP perovskite are encouraging for solar cell applications, but charge transport studies are required before judging its potential.

4. LaYS3 Thin film LaYS3 is different than the traditional A2+B4+S32- perovskites. Here the oxidation states of both A- and B- sites are 3+. The structure of LaYS3 is shown in Figure 6a. It is made up of two dimensional (2D) [Y3S9]9- layers extended in the [bc] plane of the crystal and separated by larger sized La3+ ions. These layers are constructed by edge-shared double chain of [YS7] monocapped trigonal prism (A) and double chain of [YS6] octahedra (B) which is further connected with single chain of [YS6] octahedra (C). This makes an arrangement of ACBCA, in which the C chain is corner shared with A and B chains.5 Kuhar et al first computationally found that LaYS3 has the desired electronic and optical properties for solar energy applications, and therefore prepared the films of LaYS3. La and Y are first sputter deposited (La/Y atomic ratio 1.01), and then the resultant film is sulfurized (Figure 6b). Experimentally obtained X-ray diffraction data and elemental analysis suggest the formation of LaYS3 in CeTmS3 structure, similar to prior



report of LaYS3 powder.69 Thickness of LaYS3 film is ~550 nm. This is a rare (if not only) example of film deposition of an optoelectronically active chalcogenide perovskite. To achieve solar cell and other optoelectronic devices, fabrication of good quality film is mandatory.

Figure 6: LaYS3 chalcogenide perovskite. a) Schematic showing crystal structure of LaYS3 exhibiting CeTmS3 prototype of perovskite structure. b) Schematics showing the fabrication of LaYS3 thin film following two-step approach in which first step includes the simultaneous sputter deposition of La and Y (1:1) and in second step the deposited film is sulfurized to make LaYS3 thin film. c) Spectrum showing absorption coefficient (α) of synthesized film of LaYS3 based on spectroscopic ellipsometry. Inset plot showing a direct bandgap of 2.0 eV. d) Comparison of steady state PL spectra of LaYS3 thin film with Cu2ZnSnS4 thin film. Solid and dashed lines are used to show the energy difference of bandgap and PL maximum of LaYS3 and Cu2ZnSnS4 respectively. Panel c-d are reprinted from ref 10. Copyright 2017 The Royal Society of Chemistry. LaYS3 films show a bandgap of 2 eV (Figure 6c), which is near to the theoretically calculated bandgap of 1.79 eV.10 Also, the absorption coefficient value is high in the range of 104-105 cm-1. 16 ACS Paragon Plus Environment

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The bandgap is slightly higher than the ideal requirement for a single-junction solar cell, but the LaYS3 film is still a promising solar light harvester. Figure 6d shows 100 meV red-shift (Stoke’s shift) of PL peak position from the bandgap of the LaYS3 film. Such broad and red-shifted PL indicates contribution from defect states in the PL.70 As a comparison, PL from well-stabilized high efficiency (PCE > 10%) solar cell material Cu2ZnSnS4 is also shown in Figure 6d, in which the Stoke’s shift is even more (100-200 meV). This lower value of 100 meV Stoke’s shift in PL of LaYS3 suggests lower density of deep defects in the as-fabricated polycrystalline thin film of LaYS3. As a first report, films of LaYS3 exhibit reasonably good optical properties, but further fine tuning (reduction) bandgap towards 1.5 eV, and narrower band-edge PL with higher intensity is desired for optoelectronic applications.

5. Computational Screening Computational data has been discussed in previous sub-sections as well but in conjunction with experimental results. In this sub-section, we will discuss the computational predictions of potential materials for which significant experimental results are not yet available. Computational predictions of suitable compositions have been a major driving force for the development of chalcogenide perovskites for solar cell. Sun et al, in 2015, proposed that the electronic and optical properties of CaTiS3, BaZrS3, CaZrSe3, and CaHfSe3 with distorted perovskite structure are suitable for single-junction solar cells.6 Out of which, BaZrS3 has been studied experimentally as discussed above, but other three compositions with desired structure and properties have not been realized experimentally. Computational study of Meng et al suggested compositions like BaZrSe3 and BaZr1-xTixS3 exhibit optimal bandgap with theoretical



maximum PCE ~30%, but such compositions with desired bandgap have not been yet realised experimentally.8

Figure 7: Tin based chalcogenide perovskite. a) Absorption coefficient data of different perovskite materials (chalcogenide and lead halide) and Si based on theoretical calculations. Schematic in the inset shows the distorted perovskite structure of tin-based (ASnX3) perovskite with corner-shared octahedra. b) Histogram comparing the bandgap (Eg) of SrSnS3 and SrSnSe3 with MAPbI3. Bandgap and excitonic binding energy (Eb) value for MAPbI3 have been taken from ref 71. The value of effective masses for MAPbI3 charge carriers are taken from ref 72. Inset lists effective masses of charge carriers of SrSnS3 and SrSnSe3 in different directions taken from ref 9. Panel-a is adopted from ref 9. Copyright 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Ju et al screened a series of ASnX3 (X = S, Se) compounds with distorted perovskite structure (inset of Figure 7a), and suggested SrSnS3, SrSnSe3 and their alloys are potential solar cell materials.9 The bandgap of SrSnS3 is 1.56 eV, and that of SrSnSe3 is 1 eV with absorption coefficients as high as MAPbI3 and significantly higher than Si, as shown in Figure 7. Not only optimal bandgap, the excitonic binding energies and the effective masses of electron and hole (Figure 7b) are small for both SrSnS3 and SrSnSe3, suggesting the possibility of efficient exciton dissociation and high carrier mobility respectively. In short, the calculated electronic and optical 18 ACS Paragon Plus Environment

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properties are ideal for solar cell applications. The bandgap could be continuously tuned between 0.9 to 1.56 eV by varying the composition of SrSnS1-xSex perovskite and also ASn0.5Ge0.5X3 (A = Ca, Sr, Ba; X = S, Se) double perovskite.9 However, a separate computational work finds that SrSnS3 stabilizes in a different structure with bandgap only 0.1 eV, and therefore not suitable candidate for solar cell.10 There is an old report of synthesis of SrSnS3 at high pressure.73 This experimental report suggests needle-like phase (NH4CdCl3 structure) of SrSnS3, but no optoelectronic property has been reported for this phase. Table 1: Different compositions of ABS3 chalcogenide perovskite screened on the basis of stability, bandgaps (Eg), eﬀective masses of holes (mh*) and electrons (me*), and preliminary vacancy defect tolerance. Unit for Eg is in eV while the units for mh* and me* are in electron mass. The table has been adopted from the ref. 10 ― SC ― SC ABS3 me* mh* Prototype 𝐸GLLB 𝐸GLLB g g(direct) BaHfS3 1.31 1.31 0.94 -0.35 NH4CdCl3/Sn2S3 BaZrS3 2.25 2.25 0.43 -0.75 GdFeO3 BiTlS3 1.36 1.98 0.31 -0.64 FePS3 CaHfS3 0.99 0.99 0.76 -0.34 NH4CdCl3/Sn2S3 CaSnS3 1.58 1.93 0.94 -0.61 NH4CdCl3/Sn2S3 CaZrS3 1.36 1.36 0.88 -0.76 NH4CdCl3/Sn2S3 SrHfS3 1.12 1.12 0.81 -0.33 NH4CdCl3/Sn2S3 HfPbS3 1.12 1.63 0.23 -0.27 BaNiO3 LaYS3 1.79 1.79 0.49 -0.67 CeTmS3 TaLiS3 1.98 2.00 0.98 -0.75 FePS3 MgZrS3

2.21

2.32

0.78

-0.72

Distorted

SbYS3 SrZrS3 TaTlS3 ZrZnS3

2.03 1.46 1.15 1.91

2.09 1.46 1.15 1.97

0.48 3.11 0.24 0.42

-0.37 -0.64 -0.30 -0.62

NH4CdCl3/Sn2S3 NH4CdCl3/Sn2S3 Distorted FePS3

Kuhar et al computationally screened a large number of ABS3 compounds on the basis of structure (not restricted by Goldschmidt tolerance factor), stability, bandgap (0.5 eV to 2.5 eV), effective masses ( m*e,h 30% according to Shockley–Queisser limit.77 Chalcogenide perovskites with bandgap close to the ideal bandgap range have been identified computationally (Table 1) and also experimental results suggest Ba3Zr2S7 layered perovskite has a bandgap ~1.3 eV.61 While efforts to synthesis newer compositions and structures should continue, urgent efforts are required to study the charge transport properties of existing Zrchalcogenides and LaYS3. Such charge transport properties of chalcogenide perovskites are largely absent now. Higher carrier mobility and diffusion length are two critical parameters for successful optoelectronic applications.


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Figure 8: Schematically showing the major material design challenges in the field of chalcogenide perovskites that needs to be addressed at the earliest possible, to achieve experimentally efficient solar cells. Computational screenings have mainly focused on the nature of bandgap and carrier effective masses. Another equally important parameter is defect tolerance. There are good numbers of semiconductors with desired bandgap and carrier masses, but exhibit poor efficiencies for optoelectronic processes, because of defect states lying deep within the bandgap. Meng et al studied the nature of such defects in BaZrS3,8 and Kuhar et al carried out a preliminary screening of vacancy defects.10 Therefore, it is need of the hour to carry out detailed computational studies exploring the nature of defects for all those compositions that have suitable bandgap and effective masses. Experimentally, PL is a good technique to study such defects. Intense bandedge emission with narrow spectral shape indicates less abundance of deep defects.78, 79 Carrier dynamics studies by other ultrafast spectroscopic techniques also provide useful insights about defect states. Both oxides and halide perovskites exhibit rich temperature and pressure dependent phase transitions. Different phases can exhibit altogether different optoelectronic properties. More 21 ACS Paragon Plus Environment


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detailed studies of such phase transitions and its influence on electronic and optical properties are needed to be explored. Ultimately device fabrication depends on film properties. Therefore, making good quality films, and studying their optoelectronic properties is an urgent requirement. One important reason behind the rapid progress of halide perovskite based optoelectronics is the availability of easy solution processed film growth technique. At present, the possibility of solution processed film growth of chalcogenide perovskite is gloomy based on the present synthesis methodology. It will be a major progress if solution processed synthesis of chalcogenide perovskite can be made possible. In the absence of such solution processed methods, other film fabrication methods such as sputtering needs to be explored. LaYS3 films were made by such sputtering (Figure 6b). Maintaining exact stoichiometry can become an issue in such sputtering techniques, so rigorous characterizations of such films are required. Exploration of unusual properties like ferroelectric PV leading to bulk PV effect was demonstrated in oxide perovskites.42, 80, 81 But the wide bandgap of oxides often makes them poor absorber of solar light.82 Computational studies suggest certain RP perovskites such as Ca3Zr2S7 and related compounds can combine narrow bandgaps ( < 2.2 eV) with room temperature ferroelectricity.83,84 These computational studies can guide us to develop ferroelectric chalcogenide perovskite experimentally. Other newer approaches such as tandem cell,85 hot carrier solar cells,86 and multiple exciton generation87 need to be explored using chalcogenide perovskites where a lower bandgap material compared to single junction cell will be desired. Also, other properties such as thermoelectricity can be explored in lower bandgap chalcogenide perovskites.88 Recent interesting works show optical anisotropy in mid-wave and long-wave infrared region using BaTiS3 and Sr1+xTiS3 quasi-1D perovskite.89,


90

Doping and alloying of

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chalcogenide perovskites and extracting new electronic and optical properties is another direction that can be explored more.41, 91-93

Conclusions and Future Outlook: Computational studies propose a series of chalcogenide perovskites that exhibit suitable bandgap, strong absorption coefficient and low effective masses of charge carriers. Therefore, theoretical maximum PCE of some of these materials are close to 30%. Guided by these theoretical predictions, there have been attempts to experimentally synthesize chalcogenide perovskites, and achieve the desired electronic and optical properties. Zr-chalcogenides like BaZrS3, SrZrS3 and layered perovskite Ba3Zr2S7 have been successfully synthesized. Experimentally obtained direct bandgaps of BaZrS3 (~1.8 eV), and -SrZrS3 (~2.1) are slightly higher, but that of Ba3Zr2S7 (~1.3 eV) are near ideal for single junction solar cell applications. All these materials exhibit strong absorption coefficient and near band-edge PL. In our understanding, Zr-chalcogenides are promising solar energy harvesters, but to realize its efficacy in solar cell performance, we need to make good quality films and optimize the charge transport processes. Somewhat different kind of perovskite is LaYS3. Interestingly, ~550 nm thick films of LaYS3 with bandgap 2.0 eV have been made by sputtering techniques. There are some theoretically predicted chalcogenide perovskite compositions, which have not been achieved experimentally. At present, synthesis and processing of chalcogenide perovskites are relatively difficult compared to halide perovskites. Computational and experimental material design challenges that need to be addressed in the near future are discussed in the previous sub-sections. We believe that a few more of these chalcogenide perovskites with desired optoelectronic properties will be realized experimentally within the next few years. The research on



chalcogenide perovskite should focus on any breakthrough optical, electronic and optoelectronic properties, instead of limiting it to solar cell alone. Note that many of the chalcogenide perovskites have the advantages of earth abundance, environmentally benign, and good thermal and aqueous stability.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: A.N. [email protected],

A.S. [email protected] ORCID Angshuman Nag: 0000-0003-2308-334X Abhishek Swarnkar: 0000-0001-7074-2729 Notes The authors declare no competing financial interest. Biographies: Abhishek Swarnkar is currently an integrated MS-PhD student at IISER Pune. He got his Bachelor of Science degree in Chemistry from Vinoba Bhave University, India. He worked as a Bhaskara Advanced Solar Energy (BASE) research intern in NREL USA. His major research interest is semiconductor perovskite for different kinds of optoelectronic applications. Wasim J. Mir is currently a PhD student at IISER Pune. He received his Bachelor of Science and Master of Science degrees in Chemistry from University of Kashmir, India. He has worked as Raman Charpak Fellow research internship at INSP (UPMC), Paris. His major research interest is in doping optically active metal ions in lead halide perovskite semiconductors. Rayan Chakraborty is currently an integrated MS-PhD student at IISER Pune. He obtained his Bachelor of Science degree in Chemistry from the University of Calcutta, India. His research focuses 24 ACS Paragon Plus Environment

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on the narrow band-gap semiconductors for optoelectronic applications employing both experimental methods and theoretical calculations. Metikoti Jagadeeswararao is currently working as a research associate at IISER Pune after completing his PhD (2018) from the same institute. He completed his Bachelor of Science and Master of Science degrees in Chemistry from Andhra University, India. His major research interest is on designing semiconducting materials and studying their optical properties. Tariq Sheikh is currently a PhD student at IISER Pune. He got his his Bachelor of Science and Master of Science degrees in Chemistry from University of Kashmir, India. His major research interest is the study of optical and electronic properties of 2D lead halide perovskites. Angshuman Nag received his Master of Science (2003) from IIT Guwahati and then PhD (2009) from SSCU at IISc Bangalore, India. He then completed two terms as a postdoctoral researcher at IISc Bangalore and at University of Chicago working. In 2012, Angshuman Joined at IISER Pune as a Ramanujan Fellow, and then from 2015 onward, he is continuing as an Assistant Professor in Chemistry. His current research is on developing novel semiconductors for optoelectronic applications. Angshuman has published ~70 research papers. He is an Associate of Indian Academy of Science (IASc), Bangalore, and also a recipient of the Young Scientist Platinum Jubilee Award from The National Academy of Sciences (NASI), Allahabad, India. Acknowledgement: Authors acknowledge Science & Engineering Research Board (SERB, EMR/2017/001397) Govt. of India.

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Are Chalcogenide Perovskites an Emerging Class of Semiconductors

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