Tunable Optical Properties in SnxSb2âyS3: A New Solar Absorber

Subscriber access provided by McMaster University Library

C: Energy Conversion and Storage; Energy and Charge Transport x

2-y

3

Tunable Optical Properties in SnSb S: A New Solar Absorber Material with an Efficiency of Near 5% Harrys Samosir, Patsorn Boon-on, Yu-En Lin, Li-Ping Chen, David J. Singh, Jen-Bin Shi, and Ming-Way Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10596 • Publication Date (Web): 10 Feb 2019 Downloaded from http://pubs.acs.org on February 10, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Tunable Optical Properties in SnxSb2-yS3: a New Solar Absorber Material with an Efficiency of Near 5% Harrys Samosir,a Patsorn Boon-on,a Yu-En Lin,a Li-Ping Chen,a David J. Singhb J. B. Shic and Ming-Way Leea,* aInstitute

of Nanoscience and Department of Physics, National Chung Hsing University, Taichung, 402, Taiwan bDepartment of Physics and Astronomy, University of Missouri, Columbia, Missouri, 65211-7010, USA cDepartment of Electronic Engineering, Feng Chia University, Taichung, 40724, Taiwan

Abstract: This work investigates the synthesis of a new ternary alloyed metal sulfide semiconductor SnxSb2-yS3 and its application in solar cells. SnxSb2-yS3 nanocrystals were synthesized by incorporating Sn+2 ions into the host binary Sb2S3 semiconductor using a two-step sequential ionic layer adsorption reaction (SILAR) process. The ternary SnxSb2-yS3 semiconductor maintains the monoclinic crystalline structure of the binary Sb2S3 host with a small expansion in lattice constants relative to that of Sb2S3. Energy dispersive X-ray analysis revealed the nominal chemical composition of the sample with eight SILAR cycles to be Sn0.52Sb1.48S3. The energy gap Eg of SnxSb2-yS3 decreases with increasing Sn content x, resulting in a tunable Eg from 620 to 800 nm

Corresponding author’s Email: [email protected] (M.-W. Lee), Tel: 886-4-

*

22852783

Fax:886-4-22862534 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(i.e. 2.0 to 1.5 eV) for x = 0 to 0.56. Liquid-junction quantum dot-sensitized solar cells were fabricated, for the first time, from the prepared SnxSb2-yS3 nanocrystals using the polyiodide electrolyte. The best cell yielded an efficiency of 2.58% with the photovoltaic parameters of JSC = 14.04 mA/cm2, VOC = 0.46 V and FF = 39.9% under 1 sun. The efficiency improved to a respectable value of 4.89% under the reduced light intensity of 0.05%. The external quantum efficiency spectrum has a maximal EQE of 71.8% at λ = 500 nm and covers the spectral range of 300-800 nm, which is significantly broader than that (300-620 nm) of the host Sb2S3. The broader optical absorption band increases light harvesting and results in a JSC ~ 64% larger than that of the host Sb2S3. The result demonstrates the tunable optical properties of SnxSb2-yS3 by controlling the cationic Sn and Sb compositions, which is a favorable property for a solar absorber.

2 ACS Paragon Plus Environment

Page 2 of 33

Page 3 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Semiconductor quantum dot-sensitized solar cells (QDSSCs) are a potential lowcost alternative to Si-based photovoltaic devices. The central component of a QDSSC consists of a mesoporous TiO2 photoanode coated with a layer of light-absorbing semiconductor sensitizer. The use of semiconductor nanoparticle sensitizer has the advantages of (a) tunable bandgap due to the quantum-size effect,1 (b) large optical absorption coefficient and (c) multiple exciton generation by a single photon.2-5 To date, the most widely studied semiconductor sensitizer materials have been the binary metal chalcogenides such as CdS, CdSe, PbS, PbSe, Sb2S3, Ag2S etc.6-11 Among these, the divalent group of CdS and CdSe are probably the most extensively investigated binary sensitizers because they are relatively easy to synthesize. CdS is one of the first materials that had been applied as a sensitizer in QDSSCs. Due to its large energy gap (Eg = 2.5 eV) and a relatively small optical absorption range,12 the power conversion efficiency (PCE) of CdS QDSSCs is low (typically ~ 1.5%).13 In contrast, CdSe has a smaller bandgap (Eg = 1.7 eV) and a broader absorption band, yielding a higher PCE of ~ 3%.14,15 This makes CdSe one of the leading sensitizer materials in QDSSC. In addition to the divalent group, the trivalent binary sulfide group, such as Sb2S3 and Bi2S3, are also potential candidates for solar absorbers. Bulk Sb2S3 has an energy gap of Eg = 1.9 eV,12 which is slightly larger but close to the 3 ACS Paragon Plus Environment


optimal Eg (~ 1.4 eV) for a solar absorber.16 It also has a large optical absorption coefficient of α ~ 5×104 cm-1 at the wavelength of 600 nm.17 These two features make Sb2S3 a potential solar absorber material. In addition, the two elements contained in Sb2S3 are nontoxic, earth-abundant and low cost, providing further advantages of a solar material. Much work had been performed to study the photovoltaic performance of various types of Sb2S3 QDSSCs, including liquid-state, solid-state and colloidal quantum dot solar cells.18-20 The efficiency of solid-state QDSSCs, typically employing the spiro OMeTAD electrolyte, could reach a high PCE value of 5% [19], making Sb2S3 one of the best binary metal sulfide sensitizers. However, the efficiency in liquid-junction Sb2S3 QDSSC is only 1.80% with a short-circuit current density of Jsc = 8.55 mA/cm2,21 significantly lower than that of solid-state cells. A major disadvantage of binary semiconductor sensitizers is that the bandgap of a specific semiconductor is a fixed value, i.e., which also fixes the optical absorption spectral range. Hence, only a limited number of binary semiconductors has their Egs close to that (~1.4 eV) required for an optimal solar absorber. The fixed bandgap problem can be overcome by tuning the optical properties through various approaches such as varying particle size, changing material composition, doping impurities into the semiconductor, or designing heterostructured nanoparticles.22 Among the various approaches, the composition control method has been widely used to engineer the 4 ACS Paragon Plus Environment

Page 4 of 33

Page 5 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bandgap of a binary semiconductor. There are three ways to control the composition of a binary metal sulfide: (1) cation alloy, (2) anion alloy and (3) cation-anion alloy. In a cation alloyed semiconductor, the optical properties are tunable by changing the stoichiometries of the cation constituents. For example, the optical spectral range of the cation alloyed semiconductor ZnxCd1-xS can be tuned from 474 to 391 nm by controlling the composition ratios of the two cation elements Zn and Cd.23 Here we investigate the effect of Sn-substitution into Sb2S3 on the optical properties. The bandgap tuning was motivated by that the Sn p conduction band states are generally lower than the Sb p derived conduction band states relative to the S p derived valence bands. The substitution of Sn into the host Sb2S3 lattice could produce a lower bandgap. Gassoumi and M. Kanzari reported the Eg of Sn2Sb2S5 to be ~ 1.5 – 1.6 eV,24 which is significantly lower than 1.9 eV of Sb2S3. In this work SnxSb2-yS3 nanocrystals were synthesized using the sequential ionic layer adsorption reaction (SILAR) method. The bandgap of SnxSb2-yS3 were tuned by controlling the Sn content x in the SILAR process. The SnxSb2-yS3 sample with x = 1.15 has an energy gap near that (1.4 eV) of the optimal Eg for a solar absorber, making it a potential candidate for solar material. In addition, the three elements contained in SnxSb2-yS3 are low cost, earth abundant and environmentally friendly. We investigate the photovoltaic properties of QDSSCs fabricated from the 5 ACS Paragon Plus Environment


synthesized SnxSb2-yS3 nanocrystals. The broad absorption band in the SnxSb2-yS3 cell produces a power conversion efficiency of near 5% (under 0.05 sun) and a shortcircuit current density ~ 65% higher than that of the host Sb2S3 semiconductor. 2. EXPERIMENTAL 2.1. Preparation of TiO2 Photoanode The three-layered photoanode consisted of a TiO2 blocking layer, a mesoporous TiO2 sensitizer layer and a TiO2 scattering layer. The preparation procedure for each layer is described in detail below. The TiO2 blocking layer, ~ 80 nm in thickness, was prepared by spin-coating (2000 rpm, 2 min) a 0.2 M titanium (IV) isopropoxide solution onto a pre-cleaned fluorine-doped tin oxide glass substrate (FTO, Pilkington, sheet resistance ~ 7 Ω/□). Second, an active mesoporous sensitizer TiO2 layer (mpTiO2, particle size ~ 30 nm, thickness ~ 10-12 μm, GreatCell Solar 30NR-T) was prepared using the doctor-blade technique, followed by heating at 190˚C for 10 min. Finally, a TiO2 scattering layer (GreatCell Solar WER 4-0, particle size ~ 300 nm, thickness ~ 6 μm) was coated over the mp-TiO2 layer, followed by heating at 500˚C for 30 min. 2.2. Synthesis of Sn-Sb-S Nnoparticles Ternary Sn-Sb-S nanoparticles were synthesized using a two-step SILAR process. First, binary Sn-S nanoparticles were grown on a mp-TiO2 electrode. Second, 6 ACS Paragon Plus Environment

Page 6 of 33

Page 7 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


binary Sb-S nanoparticles were grown on top of the Sn-S nanoparticles. Post annealing transformed the Sn-S/Sb-S structure into a ternary Sn-Sb-S semiconductor. A Sn-S SILAR cycle was performed by dipping a TiO2 electrode into a 0.1 M, 27˚C SnCl2 ethanol solution for 30 s, rinsed in ethanol to clean the excess Sn+2 ions. The substrate was then dipped into a 0.1 M, 27˚C Na2S methanol solution for 30 s, rinsed in methanol then dried in air. This completed one Sn-S SILAR cycle. The Sn-S SILAR cycles were repeated several times to obtain the proper amount of material on the TiO2 electrode For the Sb-S SILAR cycle, the Sn-S-coated TiO2 electrode was dipped into a 27˚C, 0.1 M SbCl3 ethanol solution for 15 s, rinsed and dried as above. The substrate was then dipped into a 0.1 M Na2S methanol solution for 30 s. This completed the Sb-S SILAR cycle. The number of Sb-S SILAR cycles was kept to be two cycles lower than that of Sn-S SILAR cycles. For example, a sample with ten SnS SILAR cycles would have eight Sb-S SILAR cycles. For simplicity, only the number of Sn-S SILAR cycles will be shown herein. That is, a Sn-Sb-S (10 cycles) represents a sample having 10 Sn-S and 8 Sb-S SILAR cycles. After finishing the SnS/Sb-S SILAR cycles, the mp-TiO2 electrode was annealed in a N2 gas-flowing tube furnace at 325˚C for 12 min. Ternary Sn-Sb-S nanoparticles formed after annealing. 2.3. Solar Cell Fabrication Liquid-junction QDSSCs were fabricated by assembling the Sn-Sb-S-coated TiO2 7 ACS Paragon Plus Environment


electrode with a counter electrode into a sandwich configuration. A parafilm of thickness ~ 190 µm was used as the spacer and sealant. The counter electrode material was Pt, which was prepared by dropping a 0.005 M H2PtCl6 ethanol solution onto an FTD substrate, followed by annealing at 400˚C for 15 min. A polyiodide solution, consisting of 0.03 M I2, 0.3 M LiI, 0.6 M BMII (1-Butyl-3-methylimidazilium iodine), 0.5 M 4-tert-butylpyridine in acetonitrile (2.5 mL) and valeronitrile (25 mL), was used as the electrolyte. Polyiodides have been the most commonly used electrolyte for dye-sensitized solar cells. But it does not work well for QDSSCs because many semiconductor particles would dissolve in this electrolyte. Polysulfides have instead been used as electrolytes for QDSSCs. However, polyiodides have an advantage over polysulfides because the redox level of polyiodides is lower than that of polysulfides. Polyiodides will produce a higher Voc and a higher efficiency. We had carefully tested the electrolyte to ensure that the SnxSb2-yS3 particles did not dissolve in polyiodides over a long period of several hours. 2.4. Material characterization and photovoltaic measurements The crystalline structure and morphology of the synthesized Sn-Sb-S nanoparticles were characterized with an X-ray diffractometer (XRD, Bruker, D8 SSS) and a transmission electron microscope (TEM, Joel JEM-2010). Energydispersive X-ray spectroscopy (EDS) was analyzed using the same SEM machine. 8 ACS Paragon Plus Environment

Page 8 of 33

Page 9 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Optical absorption spectra were recorded using a Hitachi U-2800A UV–Vis spectrophotometer. Photocurrent-voltage (I-V) curves were measured using a Keithley 2400 source meter under 100 mW/cm2 light illumination from an Oriel 150 W Xe lamp with an Oriel band-pass filter simulating the AM 1.5 solar spectrum. External quantum efficiency (EQE) spectra were measured using an Acton monochromator with a 250 W tungsten halogen lamp source (without white-light biasing). The active area of the cell, defined by a metal mask, was a circle 3 mm in diameter. 3. RESULTS AND DISCUSSION 3.1. XRD Figure 1(a) shows the XRD pattern of the synthesized Sn-Sb-S nanoparticles. For comparison, the spectra of the starting materials Sb2S3 and SnS are also displayed at the bottom panel. Many pronounced peaks are observed in the Sn-Sb-S sample. Several large peaks due to TiO2 and the FTO glass can also be observed. The XRD pattern of the Sn-Sb-S nanoparticles maintains the orthorhombic structure of the host material Sb2S3. However, the peak positions are shifted slightly to the left (lower angles). The plane indices (hkl) are labelled in the figure. Table S1 (Supporting Information) compares the angles of the major Sn-Sb-S and Sb2S3 peaks. The result that Sn-Sb-S maintains the host Sb2S3 orthorhombic structure indicates that the Sn+2 ions entering the host Sb2S3 lattice substitute a fraction of the cationic Sb+3 ions, 9 ACS Paragon Plus Environment


forming a cation-alloyed ternary Sn-Sb-S semiconductor. The shifts to lower angle are partly attributed to the size of the cations. The radius of Sn+2 ions is 118 pm whereas Sb+3 is 76 pm. Incorporating the large Sn+2 ions into the Sb2S3 lattice results in a lattice expansion, which in turn leads to downshifts of the XRD angles. The lattice expansion is qualitatively compatible with the assignment of Sn2+ which is larger than Sb3+, and not Sn4+ for which a contraction may be expected. The calculated lattice constants are a = 11.272, b = 11.319 and c = 3.834 Å. For comparison, the lattice constants of the host Sb2S3 are a = 11.239, b = 11.313 and c = 3.841 Å (JCPDS No. 00-042-1393). The a-axis lattice constant of the Sn-Sb-S nanoparticles are larger than that of Sb2S3 by 0.3%, which clearly shows the lattice expansion. The b- and c-axis lattice constants change by 0.05 and -0.18%, respectively, which are too small (and close to the limits of experimental error) to demonstrate the effect of lattice expansion. The Sn-Sb-S structure was further characterized with selected area electron diffraction (SAED). Fig. 1(b) shows a SAED pattern that can also be assigned to the orthorhombic structure of the host material Sb2S3 with the associated (hkl) plane indices labelled in the figure. The SAED pattern provides further support for the structure of the Sn-Sb-S nanoparticles. An additional concern with material quality is the possible formation of oxidized SnO2 during the annealing process. This issue was checked by X-ray photoelectron spectroscopy (XPS). The chemical environments of SnxSb2-yS3 10 ACS Paragon Plus Environment

Page 10 of 33

Page 11 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanocrystals could also be revealed by XPS analysis. Figure S-1(a) (Supporting Information) clearly shows the presence of Sn 3d, Sb 3d, S 2p, Ti 2p, O 1s elements in the Sn-Sb-S/mesoporous TiO2 electrode annealed at 325°C. No obvious impurities are observed in the spectrum. Figure S1(b) shows two strong peaks at 485.6 and 494.2 eV, which correspond respectively to Sn (3d5/2) and Sn (3d3/2) core levels of Sn2+ cations.25,26 This demonstrated that the Sn precursor layer transformed into the SnxSb24+ yS3 phase completely without any secondary Sn peak associated with the SnO2 phase.

The XPS spectrum of antimony, shown in Figure S1(c), shows two peaks at binding energy of 529.7 eV for Sb 3d5/2 and 538.9 eV for Sb 3d3/2, separated by 9.2 eV, confirming the presence of trivalent antimony Sb3+ state in SnxSb2-yS3 phase. Figure S1(d) shows a large peak at 161.5 eV and a small peak at 162.6 eV corresponding to S 2p3/2 and S 2p1/2 with a small energy difference of 1.1 eV, which is indicative of sulfur as divalent S2- anion in the SnxSb2-yS3 ternary semiconductor. The SnxSb2-yS3 particle size d was estimated from the XRD spectrum using Scherrer’s equation: d = kλ/(β∙cosθ), where k = 0.89 is the shape factor, λ is the wavelength of X ray, β is the peak width at half maximum, and θ is the diffraction angle. Analysis of the three large peaks of (120), (211) and (221) yielded an average d = 21 nm. Since the quantum size effect usually appears in nanoparticles of size smaller than 10 nm, the large size of SnxSb2-yS3 particles indicates that the quantum size effect is probably not important in SnxSb2-yS3 particles. 3.2. EDX The elemental composition of Sn-Sb-S was determined by EDX measurements.



The composition is expected to depend on the SILAR cycles n. Table S2 (Supporting Information) lists the compositions for three SnxSb2-yS3 with n = 7,8,9. Figure 1(c) displays an EDX spectrum of Sn-Sb-S nanoparticles with 8 SILAR cycles. The Sn composition increases with SILAR cycles n. The Sn/Sb ratio varies from 22/78, 26/74 to 28/72 for n = 7,8,9, respectively. Based on the EDX data, the chemical formula for the three SnxSb2-yS3 samples can be expressed as Sn0.44Sb1.56S3 (n = 7), Sn0.52Sb1.48S3 (n = 8) and Sn0.56Sb1.48S3 (n = 9), respectively. The EDX data reveals that nearly 25% of the Sb atoms in the host Sb2S3 lattice have been substituted by Sn atoms. 3.3. TEM Figure 2(a) shows a TEM image of a bare TiO2 film. The TiO2 particles are rectangular or hexagonal in shape and have sizes in the range of 20-40 nm. Figure 2(b) shows an image of SnxSb2-yS3 nanoparticles coated on a TiO2 film. An enlarged image of a small area of Figure 2(b) is shown in Figure 2(c). SnxSb2-yS3 nanoparticles can be seen to be distributed randomly over TiO2 nanoparticles without observable aggregation. The particle sizes of SnxSb2-yS3 are in the range of 10-15 nm, significantly smaller than that (20-40 nm) of TiO2 particles. Figure 2(d) shows a high-magnification TEM image of a SnxSb2-yS3 nanoparticle. The clear lattice fringes show good crystallinity of the SnxSb2yS3

particles. The fringe spacing of 0.361 nm can be assigned to the (130) plane of the

SnxSb2-yS3 phase. 12 ACS Paragon Plus Environment

Page 12 of 33

Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.4. Optical Spectra Figure 3(a) displays the transmission spectra T(λ) of three SnxSb2-yS3 samples with SILAR cycles n = 7, 8, 9. The transmission decreases with increasing n, indicating increasing light absorption as the amount of deposited material increased with SILAR cycle n. Figure 3(b) shows the absorbance spectra A(λ) = -log10T(λ). The absorbance A(λ) increases with increasing n, which is, again, due to the increasing amount of deposited materials. Figure 3(c) shows the Tauc plots (Ahν)2 vs. hν of three samples. The intercept to the x-axis yields the optical energy gap Eg. The bandgap decreases with increasing n: Eg = 1.76, 1.63 and 1.35 eV for n = 7, 8, 9, respectively. The decreasing Eg with increasing SILAR cycle can be partly attributed to the quantum size effect: a high SILAR cycle produces larger nanoparticles, which in turn leads to a lower Eg. Compared with bulk Sb2S3, the Eg of SnxSb2-yS3 (1.35 eV for the sample with n = 9) is significantly lower than that (1.9 eV) of bulk Sb2S3. The result reveals the effect of bandgap tuning by incorporating Sn into the host Sb2S3. A notable feature of the transmission spectra is the extremely small transmission (T(λ) < 1 % ) over the spectra range of 400-700 nm for the two samples with n = 8, 9. The low transmission indicates high optical absorption in Sn-Sb-S nanoparticles, which is a favorable property for a solar material. The decrease in Eg with increasing SILAR cycle can be considered to be a weak 13 ACS Paragon Plus Environment


quantum-size effect. As indicated in the XRD spectra, the crystallographic structure of SnxSb2-yS3 is the same as that of the host Sb2S3. Hence, the Bohr radius aB of SnxSb2yS3

should also be close to that of the host Sb2S3, which is 0.9 nm according to the

literature.27 The SnxSb2-yS3 particle size, estimated from TEM and XRD, is ~ 15-20 nm, much larger than the Bohr radius. Hence, the quantum size effect in SnxSb2-yS3 should be small. To observe a large quantum size effect in SnxSb2-yS3, the particle size should be near the Bohr radius of 1 nm. The decrease in Eg with increasing SILAR cycle, shown in Figure 3, can also be investigated by XPS. Previous work on CdS indicated that the cation Cd could form Cd-Cd levels slightly above the valence band, leading to Eg narrowing.28,29 This problem is especially significant for particles prepared by SILAR. Therefore one would ask if the same effect would appear in the present SnxSb2-yS3. That is, would the cation in SnxSb2-yS3 (Sb herein) forms Sb-Sb levels, which would appear as a weak, higherenergy side peak near the XPS Sb main peak. Deconvolution of the Sb peaks in Figure S1(c) with Gaussian fitting does not show any clear side peak next to the main peaks. Thus, the possibility of Eg narrowing in SnxSb2-yS3 caused by Sb-Sb level can be ruled out. 3.5. Photovoltaic Properties The photovoltaic performance of a SILAR-prepared QDSSC by depends 14 ACS Paragon Plus Environment

Page 14 of 33

Page 15 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


strongly on the number of SILAR cycle n. An insufficient (low n) or excess amount (high n) of semiconductor material both leads to low performance. Figure 4(a) shows the I-V curves of SnxSb2-yS3 QDSSC with SILAR cycles n = 7, 8, 9. Table 1 lists the photovoltaic parameters. The power conversion efficiency (PCE = 0.91%) was low for the sample with low SILAR cycle (n = 7, sample No. 1). It then increased to a maximal value of 1.63% at n = 8 (sample No. 2). After that, further increase in the SILAR cycles (n = 9, sample No. 3) resulted in a lower PCE of 0.85%. These are typical photovoltaic results for QDSSCs prepared by SILAR. A low SILAR cycle produced an insufficient amount of semiconductor, leading to insufficient solar light harvesting. A SILAR cycle too high overloaded the TiO2 electrode, which reduced the porous spaces among the TiO2 particles and hampered the flow of the liquid electrolyte, also leading to a decreased efficiency. After finding the optimal SILAR cycle, the optimal sample (No.2) was treated with a passivation layer to suppress carrier recombination. Two types of passivation layer, ZnS replaced ZnSe, were used. A ZnS passivation layer increased the PCE from 1.63% (untreated, No. 2) to 2.11% (No. 4). A ZnSe coating further increased the PCE to 2.58% (No. 5), which is 58% higher than the untreated sample (No. 2). Nanoparticles prepared by SILAR contain a large number of surface defects acting as recombination centers. The ZnS coating forms a potential barrier between the conduction bands (CB) of SnxSb2-yS3 and the 15 ACS Paragon Plus Environment


electrolyte, which prevents the CB electrons of SnxSb2-yS3 from recombining with holes in the electrolyte. ZnSe is a better passivation material because of more proper band alignment, as reported in the literature.30 Finally, we compare the performance of the SnxSb2-yS3 QDSSC with that of the host binary Sb2S3 reported recently in the literature.21 As seen in Table 1, the PCE (2.58%) of SnxSb2-yS3 is larger than that (1.80%) of Sb2S3 in Ref. 21. The enhanced PCE is mainly due to the higher Jsc (14.04 mA/cm2 in SnxSb2-yS3) relative to that (8.55 mA/cm2) of Sb2S3. The higher Jsc in SnxSb2-yS3 is the consequence of a broader optical absorption band. The effect will be explained clearly in the discussion of the EQE result below. Photovoltaic performance of a QDSSC could be improved by measuring the I-V curves under low-light intensities. Figure 4(b) displays the I-V curves of the best SnxSb2-yS3 cell under various sun intensities. Table 2 lists the photovoltaic parameters. The performance increased with reducing light intensity. The PCE increased from 2.58% (1 sun) to 4.89% (0.05 sun), a significant increase of 90%. The improved PCE arises mainly from Jsc. At the light intensity of 1 sun, Jsc = 14.04 mA/cm2. For most semiconductors Jsc is linearly dependent on the light intensity I0. Assuming SnxSb2-yS3 obeys the linear dependence, then Jsc at 0.05 sun should be equal to 14.04 mA/cm2× 0.05 sun/1 sun = 0.70 mA/cm2. However, Jsc = 1.36 mA/cm2 (Table 2), which is nearly double that (0.70 mA/cm2) expected from linear response. The improved 16 ACS Paragon Plus Environment

Page 16 of 33

Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


performance under low-light intensities is attributed to reduced carrier recombination. The recombination mechanism can be understood by analysing the Jsc vs. light intensity I0 data in Table 2, which yields a sublinear power law Jsc ∝ 𝐼0.73 0 . Nelson had explained the sublinear power law using the multiple drops model for carrier recombination.31 Surface traps formed during the SILAR growth process act as recombination centers. A sublinear power law predicts a much reduced recombination rates under low light intensities. The suppression of recombination results in an improved Jsc and cell efficiency. Reducing light intensity has another effect on Voc. Table 2 clearly shows that Voc decreases with reducing light intensity from 0.46 (1sun) to 0.39 V (0.05 sun). This can be attributed to the effect due to the reduction in electron density under low light. The theoretical upper limit of Voc for a QDSSC is Voc =

1 𝑒

(EF - Eredox), where e is the

elementary charge, EF is the quasi-Fermi level of TiO2 and Eredox is the redox level of electrolyte. EF is proportional to the electron density nCB in the conduction band (CB) of TiO2 according to EF = kBT∙ln nCB.32 A reduced light intensity generates a lower electron density nCB, which lowers EF and, hence, produces a lower Voc, as revealed in Table 2. Quantum efficiency is an important photovoltaic parameter that reveals the ability of a solar material to convert a photon into an electron. Figure 5 displays the 17 ACS Paragon Plus Environment


EQE spectra for the best SnxSb2-yS3 samples with and without ZnSe coating. The sample without ZnSe coating has a maximal EQE value of 40.4% at λ = 350 nm. The ZnSe treatment significantly improves the EQE response to a maximal EQE value of 71.8% at λ = 500 nm. The ZnSe coating improves the EQE response by reducing carrier recombination. The EQE spectrum covers the range of 300-800 nm. The cuton wavelength at λ = 800 nm represents the energy that a valence band-to-conduction band transition occurs, which corresponds to the energy gap Eg. This EQE-derived Eg of 1.5 eV (800 nm) is consistent with the optical Eg of 1.35 eV shown in Figure 5. For comparison the EQE spectrum of pure Sb2S3 is also displayed in the figure. It can be seen that the Sb2S3 has an Eg ≅ 2.00 eV (λ = 620 nm), in good agreement with the Eg of 1.9 eV for bulk Sb2S3. Based on the EQE spectra, the Eg of SnxSb2-yS3 (1.5 eV) is lower than that (2.0 eV) of the host Sb2S3 nanoparticles, which provides strong evidence that the Sn-substitution lowers the bandgap of the host Sb2S3 semiconductor. This result is compatible with expectations for substitution of Sn2+ since the Sn p conduction band states are generally lower than the Sb p derived conduction band states relative to the S p derived valence bands. The area under the EQE curve corresponds to the photocurrent 𝐽ph generated by the SnxSb2-yS3 cell according to: 𝐽ph = e∫EQE(λ)Φ(λ)d𝜆, (1) 18 ACS Paragon Plus Environment

Page 18 of 33

Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where Φ(λ) is the photon flux. Integrating Eq. 1 yields Jph = 14 mA/cm2. This Jph is consistent with the experimental Jsc = 14.04 mA/cm2 observed under 1 sun, as shown in Table 2. 4. CONCLUSION We demonstrated the synthesis of ternary metal sulfide semiconductor SnxSb2-yS3 by substituting a fraction of the Sb atoms in Sb2S3 with cationic Sn atoms. The optical properties of SnxSb2-yS3 are tuneable from 1.9 to 1.5 eV by controlling the content of the Sn constituent. The PCE of the SnxSb2-yS3 QDSSCs reached 2.58% under 1 sun and improved to near 5% under 0.05%. The improved performance in SnxSb2-yS3 cell is due to an enhanced optical absorption band, which generated a Jsc ~ 60% larger than in the case of Sb2S3. Supporting Information Additional data including XRD angles, XPS spectra and EDX table. ACKNOWLEDGEMENTS The authors are grateful to the financial support from the Ministry of Science and Technology of the Republic of China under grant No. MOST 107-2112-M-005-007.



REFERENCES (1) Kamat, V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737-18753. (2) Tian, J.; Cao, G. Control of Nanostructures and Interfaces of Metal Oxide Semiconductors for Quantum-Dots-Sensitized Solar Cells. J. Phys. Chem. Lett. 2015, 6, 1859-1869. (3) González-Pedro, V.; Xu, X.; Mora-Seró, I.; Bisquert, J. Modeling HighEfficiency Quantum Dot Sensitized Solar Cells. ACS Nano 2010, 4, 5783-5790. (4) Tian, J.; Shen, T.; Liu, X.; Fei, C.; Lv, L.; Cao, G. Enhanced Performance of PbS-Quantum-Dot-Sensitized Solar Cells via Optimizing Precursor Solution and Electrolytes. Sci. Rep. 2016, 6, 23094-23102. (5) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots. Nano Lett. 2005, 5, 865-871. (6) Guijarro, N.; Lana-Villarreal, T.; Mora-Seró, I.; Bisquert, J.; Gómez, R. Improving The Performance of Colloidal Quantum-Dot-Sensitized Solar Cells. J. Phys. Chem. C 2009, 113, 4208-4214. (7) Itzhaik, Y.; Niitsoo, O.; Page, M.; Hodes, G. High Open Circuit Voltage in Sb2S3/Metal Oxide-Based Solar Cells. J. Phys. Chem. C 2009, 113, 4254-4256. 20 ACS Paragon Plus Environment

Page 20 of 33

Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8) Larramona, G.; Choné, C.; Jacob, A.; Sakakura, D.; Delatouche, B.; Péré, D.; Cieren, X.; Nagino, M.; Bayón, R. Nanostructured Photovoltaic Cell of the Type Titanium Dioxide, Cadmium Sulfide Thin Coating, and Copper Thiocyanate Showing High Quantum Efficiency. Chem. Mater. 2006, 18, 1688-1696. (9) Choi, Y. C.; Mandal, T. N.; Yang, W. S.; Lee, Y. H.; Im, S. H.; Noh, J. H.; Seok, S. I. Sb2Se3‐Sensitized Inorganic–Organic Heterojunction Solar Cells Fabricated Using a Single‐Source Precursor. Angew. Chem. 2014, 126, 1353-1357. (10) Lin, Y.-C.; Lee, M.-W. Bi2S3 Liquid-Junction Semiconductor-Sensitized SnO2 Solar Cells. J. Electrochem. Soc. 2014, 161, H1-H5. (11) Tubtimtae, A.; Wu, K.-L.; Tung, H.-Y.; Lee, M.-W.; Wang, G. J. Ag2S Quantum Dot-Sensitized WO3 Photoelectrodes for Solar Cells. Electrochem. Commun. 2010, 12, 1158-1160. (12) Madelung, O. Semiconductor Data Handbook; Springer, Berlin, 2004. (13) Zhang, D. W.; Chen, S.; Li, X. D.; Wang, Z. A.; Shi, J. H.; Sun, Z.; Yin X. J.; Huang, S. M. Cadmium Sulfide Quantum Dots Grown by Chemical Bath Deposition for Sensitized Solar Cell Applications. Proc. Of SPIE 2009, 7518, 751804-7518012. (14) Lee, Y.-L.; Lo, Y.-S. Highly Efficient Quantum‐Dot‐Sensitized Solar Cell Based 21 ACS Paragon Plus Environment


on Co‐Sensitization of CdS/CdSe. Adv. Funct. Mater. 2009, 19, 604-609. (15) Jun, H. K.; Careem, M. A.; Arof, A. K. Efficiency Improvement of CdS and CdSe Quantum Dot-Sensitized Solar Cells by TiO2 Surface Treatment. J. Renew. Sustain. Energy 2014, 6, 023107-023114. (16) Zdanowicza, T.; Rodziewiczb, T.; Zabkowska, W. Theoretical Analysis of the Optimum Energy Band Gap of Semiconductors for Fabrication of Solar Cells for Applications in Higher Latitudes Locations. Sol. Energy. Mater. Sol. Cells. 2005, 8743, 757-769. (17) Tigãu, N. Structural Characterization and Optical Properties of Annealed Sb2S3 Thin Film. Rom. Journ. Phys. 2008, 53, 209–215. (18) Salunkhe, D. B.; Gargote, S. S.; Dubal, D. P.; Kim, W. B.; Sankapal, B. R. Sb2S3 Nanoparticles Through Solution Chemistry on Mesoporous TiO2 for Solar Cell Application. Chem Phys Lett. 2012, 554, 150–154. (19) Moon, S.-J.; Itzhaik, Y.; Yum, J.-H.; Zakeeruddin, S. M.; Hodes, G.; Grätzel, M. Sb2S3-Based Mesoscopic Solar Cell Using an Organic Hole Conductor. J. Phys. Chem. Lett. 2010, 1, 1524–1527. (20) Abulikemu, M.; Gobbo, S. D.; Anjum, D. H.; Malik, M. A.; Bakr, O. M. Colloidal Sb2S3 Nanocrystals: Synthesis, Characterization and Fabrication of Solid-State Semiconductor Sensitized Solar Cells. J. Mater. Chem. A 2016, 4, 22 ACS Paragon Plus Environment

Page 22 of 33

Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6809-6814 . (21) Ye, Q.; Xu, Y.; Chen, W.; Yang, S.; Zhu, J.; Weng, J. Enhanced Photovoltaic Performance of Sb2S3-Sensitized Solar Cells Through Surface Treatments. Appl. Surf. Sci. 2018, 440, 294–299. (22) Shen, S.; Wang, Q. Rational Tuning the Optical Properties of Metal Sulfide Nanocrystals and Their Applications. Chem. Mater. 2013, 25, 1166−1178. (23) Zhong, X.; Feng, Y.; Knoll, W.; Han, M. Alloyed ZnxCd1-xS Nanocrystals with Highly Narrow Luminescence Spectral Width. J. Am. Chem. Soc. 2003, 125, 13559−13563. (24) Gassoumi, A.; Kanzari, M. Optical Structural and Electrical Properties of The New Absorber Sn2Sb2S5 Thin Films. Chalc. Lett. 2009, 6, 163-170. (25) Vasudeva, R.M.R.; Sreedevi, G.; Chinho, P.; Miles, R.W.; Ramakrishna R.K.T. Development of Sulphurized SnS Thin Film Solar Cells. Curr. Appl. Phys. 2015, 15, 588-598. (26) Shiga,Y.; Umezawa, N.; Srinivasan, N.; Koyasu, S.; Sakai. E.; Miyauchi, M. A Metal Sulfide Photocatalyst Composed of Ubiquitous Elements for Solar Hydrogen Production. Chem. Comm. 2016, 52, 7470-7473. (27) Validžić, I. L.; Mitrić, M.; Abazović, N. D.; Jokić, B. M.; Milošević, A. S.; Popović, Z. S.; Vukajlović, F. R. Structural Analysis, Electronic and Optical 23 ACS Paragon Plus Environment


Properties of the Synthesized Sb2S3 Nanowires with Small Band Gap. Semicond. Sci. Technol. 2014, 29, 035007. (28) Gualdrón-Reyes, A. F.; Meléndez, A. M.; Tirado, J.; Mejia-Escobar, M. A.; Jaramillo, .F. N.G.; Martha, E. Hidden Energy Levels? Carrier Transport Ability of CdS/CdS1−xSex Quantum Dot Solar Cells Impacted by Cd–Cd Level Formation. Nanoscale. 2019, 11, 762–774 (29) Tong, H.; Umezawa N.; Ye, J.; Ohno, T. Electronic Coupling Assembly of Semiconductor Nanocrystals: Self-Narrowed Band Gap to Promise Solar Energy Utilization. Energy Environ. Sci., 2011, 4, 1684-1689. (30) Huang, F.; Zhang, Q.; Xu, B.; Hou, J.; Wang, Y.; Masse, R. C.; Peng, S.; Liu, J.; Cao, G. A. Comparison of ZnS and ZnSe Passivation Layers on CdS/CdSe CoSensitized Quantum Dot Solar Cells. J. Mater. Chem. A 2016, 4, 14773–14780. (31) Nelson, J.; Haque, S. A.; Klug, D. R.; Durrant, J. R. Trap-Limited Recombination in Dye-Sensitized Nanocrystalline Metal Oxide Electrodes. Phys. Rev. B 2001, 63, 205321-205329. (32) Nelson, J. The Physics of Solar Cells. London: Imperial College Press, 2003.


Page 24 of 33

Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Captions: Figure 1. (a) XRD pattern of Sn-Sb-S nanoparticles, (b) SAED pattern of Sn-Sb-S nanoparticles and (c) EDX spectrum of Sn-Sb-S nanoparticles. Figure 2. (a) TEM image of a bare TiO2 film, (b) TEM image of Sn-Sb-S nanoparticles coated on a TiO2 film, (c) an enlarged TEM image and (d) lattice fringes corresponding to the (130) plane of the Sn-Sb-S phase. Figure 3. Optical spectra of SnxSb 2 -yS3 nanoparticles with various SILAR cycles n: (a) transmission, (b) absorbance and (c) (Ahv)2 vs. hv Tauc plots. Figure 4. 1-V curves of SnxSb 2 -yS3 QDSSCs (a) with various SILAR cycles n, ZnS and ZnSe passivation coatings and (b) under various sun intensities. Figure 5. EQE spectrum of a SnxSb 2 -yS3 QDSSC.



Page 26 of 33

TABLE 1. Photovoltaic performance with various SILAR cycles n and passivation coatings. Electrolyte: Polyiodide.

Sample No.

SILAR Cycles

1

7

Sn-Sb-S

5.68

0.41

39.1

0.91

2

8

Sn-Sb-S

9.91

0.44

37.4

1.63

3

9

Sn-Sb-S

6.30

0.38

35.7

0.85

4

9

Sn-Sb-S/ZnS

12.76

0.44

37.5

2.11

5

9

Sn-Sb-S/ZnSe

14.04

0.46

39.9

2.58

8.55

0.40

52.6

1.80

Ref. 21

Electrode

JSC (mA/cm2)

Sb2S3


VOC (V) FF (%)

PCE (%)

Page 27 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Photovoltaic parameters under various reduced sun intensities. The Jsc in bracket represents the current density after normalized to 1 sun.

Sun power

Jsc (mA/cm2)

Voc (V)

FF (%) PCE (%)

100%

14.04

0.46

39.9

2.58

50%

7.22

0.44

46.0

2.92

24%

4.57

0.45

48.6

3.99

11%

2.18

0.41

48.9

4.36

5%

1.36

0.39

46.2

4.89



TOC Graphic


Page 28 of 33

Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (a) XRD pattern of Sn-Sb-S nanoparticles, (b) SAED pattern of Sn-Sb-S nanoparticles and (c) EDX spectrum of Sn-Sb-S nanoparticles.

ACS Paragon Plus Environment


Figure 2. (a) TEM image of a bare TiO2 film, (b) TEM image of Sn-Sb-S nanoparticles coated on a TiO2 film, (c) an enlarged TEM image and (d) lattice fringes corresponding to the (130) plane of the Sn-Sb-S phase.


Page 30 of 33

Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 3. Optical spectra of SnxSb2-yS3 nanoparticles with various SILAR cycles n: (a) transmission, (b) absorbance and (c) (Ahv)2 vs. hv Tauc plots. 289x223mm (300 x 300 DPI)



Fig. 4. 1-V curves of SnxSb2-yS3 QDSSCs (a) with various SILAR cycles n, ZnS and ZnSe passivation coatings and (b) under various sun intensities.


Page 32 of 33

Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 5. EQE spectrum of a SnxSb2-yS3 QDSSC. 289x223mm (300 x 300 DPI)


Tunable Optical Properties in SnxSb2âyS3: A New Solar Absorber

Recommend Documents

Tunable Optical Properties in SnxSb2âyS3: A New Solar Absorber