Mechanochemically Synthesized SnS Nanocrystals: Impact of

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Mechanochemically Synthesized SnS Nanocrystals: Impact of Nonstoichiometry on Phase Purity and Solar Cell Performance Bo-In Park, Yoon Hee Jang, Seung Yong Lee, and Doh-Kwon Lee ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02711 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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ACS Sustainable Chemistry & Engineering

Mechanochemically Synthesized SnS Nanocrystals: Impact of Nonstoichiometry on Phase Purity and Solar Cell Performance Bo-In Park,†,⊥ Yoon Hee Jang,‡,⊥ Seung Yong Lee,*,†,§ and Doh-Kwon Lee*,‡,§ †

Center for Materials Architecturing, Korea Institute of Science and Technology, Hwarang-ro

14-gil 5, Seongbuk-gu, Seoul 02792, Korea ‡

Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology,

Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Korea §

Division of Nano and Information Technology, KIST School, Korea University of Science and

Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Korea KEYWORDS:

Tin

sulfide,

Mechanochemical,

Nanocrystals,

Thin-film

solar

cells,

Nonstoichiomentry

Corresponding Author * Tel: +82-2-958-6710. E-mail: [email protected] (D.-K. Lee), Tel: +82-2-958-5381. E-mail: [email protected] (S. Y. Lee).

ACS Paragon Plus Environment

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We demonstrate non-toxic, earth-abundant light-absorbing SnS thin films fabricated by a lowcost,

environmentally

friendly

non-vacuum

process.

SnS

nanocrystals

(NCs)

are

mechanochemically synthesized from elemental powders without the use of any other additives or solvents. To investigate the effect of the Sn-to-S stoichiometric ratio on the crystalline phase of the SnS NCs, the nonstoichiometry is systematically controlled from 0.95 (Sn0.95S) to 1.05 (Sn1.05S) by adjusting the mixing ratio of the Sn and S powders. The crystallographic evolution with the milling time signifies that the formation of the SnS phase follows a mechanochemicallydriven self-propagation reaction mechanism. The as-synthesized SnS NCs with a stoichiometric composition (i.e., Sn1.00S) are found to contain a Sn2S3 impurity phase in a non-negligible amount, which can be subsequently eliminated by a post-heat treatment at 500 ºC in a reducing atmosphere. Interestingly, however, the formation of Sn2S3 during the mechanochemical synthesis process is greatly alleviated by introducing a Sn-excess composition (e.g., Sn1.05S). In addition, the solar cell with a Sn1.05S absorber exhibits a much higher efficiency as compared to the Sn0.95S- or Sn1.00S-based devices, which is likely attributed to the improved phase purity of Sn-excess SnS as well as to its better microstructure with higher crystallinity than the other compositions.

Introduction CuInS2 (CIS) and Cu(In,Ga)S2 (CIGS) thin-film solar cells have been intensively studied over the last few decades to increase their power conversion efficiency (PCE) by tailoring the materials properties and device structures as well as developing and optimizing the fabrication processes.1−5 The PCE of CIGS thin-film solar cells has recently reached 22.6% via high-vacuum evaporation techniques employing a multi-stage process.1 Despite the higher PCE level of CIGS


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solar cells than that of Si cells on a laboratory scale, their commercialization has not been as successful as that of Si-based cells, which is regarded as partly due to the higher cost per unit power generation. In this regard, the need for the development of cost-effective non-vacuum processes has been recognized.5−8 In addition, inexpensive, earth-abundant, and non-toxic new photovoltaic (PV) materials have received a considerable amount of interest as potential candidates to replace indium and gallium in CIGS. In recent years, headed by Cu2ZnSnS4 (CZTS)

and

related

quaternary

semiconducting

compounds,9

CuSbS2,10,11

FeS2,12−14

Fe2GeS4,15−16 and SnS17,18 compounds have been studied as alternative PV materials to CIGS. Among these materials, SnS has recently attracted particular interest as a promising PV absorber material as it is a simple binary compound consisting only of tin and sulfur, which are non-toxic and abundant in nature. Furthermore, SnS has suitable bandgap energies (1.1 ~ 1.5 eV) and high absorption coefficients (104 ~ 105 cm−1), enabling it to absorb sufficient light with a thickness of less than 1 µm.19,20 In the earlier stage, most studies focused on the fabrication and characterization of SnS thin films using various fabrication techniques such as RF-sputtering,21 electron beam evaporation,22 chemical vapor deposition,23 thermal evaporation,24,25 atomic layer deposition (ALD),26 sulfurization,27 chemical bath deposition,28 electrochemical deposition,29 and solution-phase deposition.30 Thus far, SnS solar cells capable of relatively high performance levels have been fabricated by high-vacuum techniques.

21,23,25,26

In 2014, Gordon et al.

demonstrated the promise of using SnS materials in solar cells, reporting a device with a 4.4% efficiency.26 In order to utilize the low-cost material advantage of SnS fully, however, it is necessary to develop a facile, low-cost, non-vacuum process for the fabrication of SnS films. Herein, we synthesized SnS nanocrystals (NCs) using a mechanochemical technique31,32 and applied them as precursor materials for the thin-film fabrication using a doctor-blade coating


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method. In line with using the low-cost materials and processes to realize cost-effective solar cells, we employed a CdS buffer layer deposited by a chemical bath deposition method instead of the Z(O,S) layers that have been applied for the optimized devices using a high-vacuum process, ALD. 23,25,26 It has been reported that due to the various oxidation states available to Sn, other Sn−S impurity phases such as Sn2S3 and SnS2 often form concurrently during a variety of deposition processes of SnS, rendering the formation of phase-pure SnS challenging.30 In this work, in an attempt to resolve this issue, we investigate the effect of the Sn-to-S concentration ratio on the appearance of unfavorable secondary phases in mechanochemically synthesized SnS NCs. The microstructural features as well as the phase purity were characterized for SnS NCs with different stoichiometric ratios, and their impact on the performance of SnS solar cells fabricated using the NCs was discussed. Experimental Section Synthesis of SnS nanocrystals. SnS nanocrystals (NCs) were synthesized via a mechanochemical route. The elemental Sn (Alfa Aesar, 99.9%, ~100 mesh) and S powders (Sigma Aldrich, 99.8%) were weighed as received in a glove box filled with Ar gas and loaded (10 g in total) together with ZrO2 balls (25 g of 5-mm-diameter and 25g of 10-mm-diameter balls) into a stainless steel jar (80 mL in volume). In order to investigate the effect of the stoichiometry, the mixing ratios of the constituent elements, i.e., [Sn]/[S] ≡ x (where [k] denotes the concentration of species k) were varied from 0.95 to 1.05, while the ball-to-precursor ratio was fixed at 5:1 in weight. Solvent-free mechanochemical synthesis was carried out at a speed of 500 rpm for up to 5 h in a planetary ball-mill machine (Fritsch GmbH, Pulverisette 5 classic


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line). No trace amount of impurities originating from the milling balls or container (such as Zr, Si, Fe, Al) was detected by X-ray photoelectron spectroscopy (Figure S1 in the Supporting Information (SI)). To elucidate the evolution of the crystalline phase, the milled powers were intermittently extracted and examined after every hour of the milling process. To examine if there were any phase changes at high temperatures, the as-synthesized NCs were post-heattreated at 300 °C or 500 °C for 1 h in an Ar (99%)/H2 (1%) gas atmosphere. Fabrication of SnS thin films and photovoltaic cells. SnS NC inks were prepared by dispersing the as-synthesized NCs into anhydrous ethanol as a solvent at a ratio of 160 mg/mL. In order to endow the NC ink with suitable rheological properties along with good colloidal stability, a wet-milling process was carried out using 1- and 5-mm-diameter ZrO2 balls at a ratio of 7 : 3 for 24 h with a horizontal ball-mill machine. The SnS nanocrystal layers were coated onto Mo-sputtered soda-lime glass (SLG) substrates (30 × 40 × 1 mm3) using a doctor-blade method with the prepared NC inks. In order to obtain dense and well-crystallized thin-film light absorbers, the as-coated films were annealed at 500 °C for 1 h in a sulfur-containing gas atmosphere (95% Ar/ 5% H2S at a flow rate of 100 sccm). Thin-film solar cell devices were fabricated in a configuration of Mo/SnS/CdS/ZnO/Al-doped ZnO/Ni/Al. A CdS buffer layer with a thickness of ca. 70 nm was deposited by chemical bath deposition on top of the as-annealed SnS absorber film. Subsequently, 50 nm thick ZnO and 500 nm thick Al-doped ZnO layers were deposited onto the CdS layer by means of RF sputtering. A Ni (50 nm)/Al (500 nm) grid pattern was deposited via thermal evaporation onto the Al-doped ZnO layer through a metal mask. The active area of the devices was around 0.4 cm2. Characterization of materials and devices. The mechanochemical phase evolution of the mixed powders and structural properties of the annealed NCs were analyzed by X-ray diffraction


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Page 6 of 38

(XRD; Bruker D8 Advance) with Cu Kα radiation (λ = 1.5406 Å) operated at 60 kV and 300 mA in the θ−2θ scan mode and Raman spectroscopy equipped with an excitation beam of a 5 W Arion laser source having a wavelength of 514.5 nm and a spectral resolution of 0.6 cm−1 (Horiba Jobin-Yvon LabRam Aramis spectrometer). The morphology, local crystal structure, and elemental distribution of the as-synthesized NCs were investigated using a high-resolution transmission electron microscope (HR-TEM; FEI Titan 80-300, Talos F200X) equipped with a scanning transmission electron microscopy-energy dispersive X-ray spectroscopy system (STEM-EDS). The optical absorption properties of the annealed NCs (under 99% Ar/1% H2 or 95% Ar/ 5% H2S) were examined by diffuse reflectance measurements using a VARIAN Cary 5000 UV−vis−NIR spectrometer. The as-coated and the annealed (under 95% Ar/ 5% H2S) films underwent an X-ray photoelectron spectroscopy (XPS) analysis using a PHI 5000 VersaProbe spectrometer (ULVAC-PHI) equipped with a monochromated Al Kα (1486.6 eV) X-ray source operating at 24.5 W and 15 kV with a pass energy of 23.5 eV and a spot size of 0.1 × 0.1 mm2. The carbon 1s peak (at 284.6 eV) was used as the energy reference for the calibration of the measured binding energies. The cross-sectional morphologies of the SnS solar cells were examined by scanning electron microscopy (SEM; FEI Inspect F) with an acceleration voltage of 15 kV. The current density−voltage (j−V) characteristics of the SnS solar cells were measured by a class-AAA solar simulator (Yamashita Denso, YSS-50S) under standard AM 1.5G 1-sun (100 mW cm−2) illumination. The light intensity of the solar simulator was calibrated using an NRELcalibrated Si reference solar cell. Results and Discussion Mechanochemical synthesis and post-heat treatment of Sn1.00S nanocrystals. Figure 1a shows the XRD patterns of the milled powders with a [Sn]/[S] mixing ratio (i.e., x in SnxS) of


6

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1.00 as a function of the milling time. From the spectra of the powders milled for up to 2 h, one can see that the elemental Sn (denoted by , according to JCPDS card no. 04-0673) and S (, JCPDS card no. 08-0247) powders remained unreacted. A dramatic change in the XRD pattern occurred after milling for 3 h. All of the diffraction peaks detected for the sample milled for 3 h turned out to belong to Herzenbergite SnS (α-SnS) in an orthorhombic crystal structure (Pbnm, JCPDS card no. 39-0354), and no further change in the peak position or intensity was identified in the spectra for up to 5 h (see also Figure S2). According to previous reports,33−36 this sudden and rapid crystallographic change is one of the unique characteristics of the mechanochemically induced self-propagation reaction mechanism, as schematically illustrated in Figure 1b. The mechanical energy provided in the first 2 h is suggested to be utilized to pulverize and activate the elemental precursor powders. When the mechanical stress possibly exceeds the activation barrier, the chemical reaction appears to be induced to form SnS between 2 to 3 h of milling, as follows, Sn (s) + S (s) = SnS (s); ∆H f = - 107,949 kJ mol -1 ,

(1)

where ∆H f denotes the enthalpy of formation of SnS. Upon the initiation of the reaction, the heat released in association with the formation enthalpy likely accelerates and propagates the reaction, enabling its completion within 1 h. The lower bound of the average crystallite size of the SnS NCs at 5 h was estimated to be ca. 15 nm using the full width at half maximum of the (111) reflection at 31.6° according to Scherrer’s equation. In this way, the α-SnS NCs (in a relatively large amount, 10 g) were successfully synthesized in a facile and environmentally friendly manner without using any expensive and/or toxic solvents or additives. It should also be noted that the present method is easy to scale up.


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(a)

Intensity / a.u.

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

♣ 5h ♣ ♣ ♣ ♣ 4h ♣ ♣ ♣♣ ♣ 3h ♣ ♣♣ ♣

♣

Sn1.00S ♣ ♣♣

♣

♣ ♣♣

♣

♣ ♣♣

•

2h

1h

♣ SnS

•

•

•

♦ ♦♦♦♦

•

Page 8 of 38

Sn2S3 • Sn ♦S

•

• • •

♦ ♦♦♦ ♦

•

20 25 30 35 40 45 50 55 60

2θ /

o

Figure 1. Crystallographic phase evolution of the Sn and S powder mixture with a Sn-to-S mixing ratio of 1.00 during the solvent-free mechanochemical synthesis of SnS nanocrystals at 500 rpm: (a) XRD patterns of powders milled for various periods of time and (b) schematic illustration depicting the progress of the mechanochemical reaction.

Figure 2 shows the detailed morphological and crystallographic characteristics of the mechanochemically synthesized SnS NCs (for 5 h) as investigated by TEM. The as-synthesized Sn1.00S powders were found to consist of agglomerates of non-uniform sizes ranging from 50 to 200 nm in diameter, as exemplified in Figure 2a. The primary particle size was estimated to be a few tens of nanometers, which is more easily observable in Figure S3 and is consistent with the aforementioned XRD analysis. Each primary nanoparticle was crystalline in nature, as shown in


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the HR-TEM image (Figure 2b), although some of the nanoparticle edges were found to be amorphous (see Figure S3c). In Figure 2b, the crystalline lattice fringes were clearly observed with interplanar distances of 0.200 ± 0.005 and 0.231 ± 0.004 nm, which can be indexed to the (002) and (131) planes of orthorhombic α-SnS according to JCPDS card no. 39-0354, respectively. As shown in Figure 2c, the selected area electron diffraction (SAED) pattern for the region marked in Figure 2a consistently indicates the {002} and {131} planes of α-SnS with the zone axis along the [3 1 0] direction. Both constituent elements (Sn and S) were homogenously distributed throughout the agglomerates of single-crystalline NCs, as demonstrated by the STEM image and the corresponding EDS mapping analysis (Figure 2d).

Figure 2. TEM analysis of the as-synthesized Sn1.00S nanocrystals: (a) TEM image, (b and c) HR-TEM image and SAED pattern for the marked area in part (a), and (d) STEM-EDS elemental mapping of the constituent elements (Sn and S).


9


It should be noted here that the possible existence of secondary phases in the synthesized SnS NCs cannot be ruled out with the XRD analysis alone, as trace amounts of Sn2S3 and SnS2 phases may not be detected by XRD.37−40 Thus, Raman spectroscopy was employed to provide more decisive information on the phase purity of the Sn1.00S NCs prepared by milling for 5 h. This result is shown in Figure 3. While most of the detectable Raman signals can be assigned to the orthorhombic α-SnS phase, i.e., at 93, 180, and 218 cm−1 (Ag mode) and 160 cm−1 (B3g mode),41 a characteristic Raman peak for Sn2S3 was also observed at 305 cm−1, indicating that the as-synthesized SnS NCs coexist with the Sn2S3 impurity phase. The Sn2S3 phase has been reported to have its strongest Raman band at 305−308 cm−1 (Ag mode).37,42−44 On the other hand, as shown in Figure 3, no trace amounts of SnS2, of which the most intense Raman scattering is known to be at 312−316 cm−1 (A1g mode), were detected.37,44−46

As-synthesized NCs (5 h) Intensity / a.u.

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

Page 10 of 38

B3g 160

Ag 180

Sn1.00S Ag 218 cm

−1

Sn2S3 (Ag) 305 cm

Ag −1 93 cm

−1

100 150 200 250 300 350 400 450 500 -1

Raman shift / cm

Figure 3. Raman spectra of the Sn1.00S nanocrystals synthesized by ball-milling for 5 h.

The secondary phase Sn2S3 may have an adverse effect on the performance of solar cells based on a p-type SnS absorber,40,47−49 as Sn2S3 is known as an n-type semiconductor.50,51 Therefore, the presence of the n-type Sn2S3 phase in SnS absorbers should be suppressed to achieve high-


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efficiency SnS solar cells. Steinmann et al. reported that Sn2S3 in a SnS powder sample can be removed by post-annealing under a vacuum (15 mTorr) at 500 °C for 1 h. Similarly, in an attempt to remove the Sn2S3 phase, the as-synthesized Sn1.00S NCs were post-heat-treated at 300 or 500 °C, but in an Ar (99%)/H2 (1%) gas atmosphere at ambient pressure. The XRD and Raman spectroscopy results are shown in Figure 4. Compared to the as-synthesized Sn1.00S NCs (Figure 1a), the intensity of the diffraction peaks was significantly augmented as the temperature of the post-heat treatment increases, enabling the clear identification of most of the characteristic diffraction peaks for α-SnS, as indexed in Figure 4a. On the other hand, it was also found that the secondary phase Sn2S3 still remained in the sample annealed at 300 °C, as indicated by the arrows in Figure 4c. The Sn2S3 impurity phase was finally eliminated at 500 °C, as shown in Figure 4b. This change can be more evidently observed in the Raman spectra (Figure 4d) from the disappearance of the 309 cm−1 peak corresponding to Sn2S3 at 500 °C. It may be worth noting here that the most intense vibration mode for SnS was blue-shifted as the annealing temperature increased, approaching its standard value of 192 cm−1.41 The shift of the Raman peaks could be induced by differences in the crystal growth conditions, chemical compositions, and surface roughness levels.52,53


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(b),(c)

20 25 30 35 40 45 50 55 60

2θ /

o

(d)

500 ° C

Post heat-treated Sn1.00S NCs 192 cm

300

−1

500 °C

200

300 °C

100

165

0

30

35

(c) 120

2θ /

o

40

45

300 °C 90 60

Sn2S3

Sn2S3

(212)

(015)

Intensity / a.u.

Intensity / a.u.

(231)/(061) (042) (250)

(200)

(131) (041) (210) (141) (002) (211) (112)/(151) (122)/(160)

(120) (021) (101) (040) (110)

500 °C 300 °C

(b) 400

Intensity / a.u.

Post heat-treated Sn1.00S NCs (111)

(a)

Intensity / a.u.

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

Page 12 of 38

219 cm

95 cm

−1

−1

188 cm

161

−1

221 cm

−1

Sn2S3 309 cm

30

96 cm

−1

−1

0

30

35

2θ /

o

40

45

100 150 200 250 300 350 400

Raman shift / cm

-1

Figure 4. XRD and Raman investigation on the post-heat-treated Sn1.00S NCs at 300 and 500 °C under 99% Ar/1% H2: (a) XRD patterns, magnified XRD patterns of the samples annealed at (b) 300 °C and (c) 500 °C, and (d) corresponding Raman spectra.

Effect of the Sn-to-S ratio on the formation of the Sn2S3 secondary phase. We further investigated the effect of nonstoichiometry, i.e., the Sn-to-S concentration ratio ([Sn]/[S] ≡ x), on the formation of the Sn2S3 secondary phase. Both Sn-deficit (x = 0.95) and Sn-excess (x = 1.05) samples were additionally prepared in a manner identical to how the stoichiometric Sn1.00S NCs were prepared (see Figure S4). Figure 5a,b shows the XRD patterns and Raman spectra of the as-synthesized Sn0.95S, Sn1.00S, and Sn1.05S NCs. It is readily recognized that, irrespective of the mixing ratios, α-SnS was formed as a majority phase after milling for 5 h. The XRD patterns rendered lattice parameters of the synthesized NCs (a = 4.587(12), b = 10.02(2), and c = 4.044(15) Å for Sn0.95S; a = 4.315(8), b = 11.196(7), and c = 4.007(9) Å for Sn1.00S; a = 4.299(6), b = 11.241(5), and c = 3.999(6) Å for Sn1.05S). The evaluated values for Sn1.00S and Sn1.05S were in good agreement with the reported (a = 4.25−4.33, b = 11.18−11.23, and c =


12

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3.98−4.02),44,54−56 whereas the a and b values for Sn0.95S were found to deviate significantly. Indeed, the XRD pattern of the only Sn0.95S NCs (Figure 5a) unambiguously shows the presence of the Sn2S3 impurity phase at 21.3, 23.6, 28.7, and 40.0°, which can correspondingly be assigned to the (103), (202), (112), and (015) planes of orthorhombic Sn2S3 (Pnma, JCPDS card no. 30-1379), indicating that the amount of the impurity phase in Sn0.95S is significantly larger than that in the Sn1.00S samples. Likewise, the Raman spectrum of the Sn0.95S NCs (Figure 5b) also indicates that the characteristic Raman peak for Sn2S3 (at 306 cm−1) was more pronounced, as compared to the Sn1.00S sample. In contrast, no peaks indicative of Sn2S3 were detected in either XRD or Raman spectra of the Sn1.05S sample. It is emphasized here that SnS NCs having no detectable secondary phases (Sn2S3 or SnS2) were successfully synthesized simply by controlling the Sn-to-S concentration ratio without a high-temperature annealing step. The assynthesized SnxS NCs with different x values were further heat-treated at 500 °C for 1 h in an Ar (99%)/H2 (1%) gas atmosphere. As shown in the XRD patterns (Figure 5c) and by the Raman spectra (Figure 5d), a significant amount of the Sn2S3 phase remained in the Sn0.95S sample even after the heat treatment. With regard to Sn1.05S, no secondary phases emerged due to this annealing process. We note here that a heat treatment in a sulfur-containing (95% Ar/5% H2S) atmosphere had a similar effect on the phase purity of SnxS NCs, as shown in Figure S5,S6. However, since the sulfur-containing atmosphere is known to have beneficial effects on the microstructural development during sintering as well as on the electrical properties of SnS films,23,26,57 a 95% Ar/5% H2S atmosphere was employed in the sintering process for the solar cell absorbers. The measured Sn-to-S concentration ratios ([Sn]/[S] ≡ x) using TEM-EDS on the SnxS NCs after the heat-treatment under a 95% Ar/5% H2S gas atmosphere were summarized in Figure S7 and Table S1 along with those of the as-synthesized NCs. All of the measured


13


compositions were in good agreement with the initial compositions within the experimental error bound. (b)

♣♣ ♣

♣

(116) /(314)

∇

(015)

(212)

∇

∇

∇

220

160

Sn1.05S

♣ ♣ ♣♣ Sn1.00S

∇∇

As-syn. NCs

94 187

Intensity / a.u.

♣

As-syn. NCs

(112)

Intensity / a.u.

♣ SnS ♣ ∇ Sn2S3

(103) (202)

(a)

Sn2S3 Sn1.05S

93 180 160 218

305 cm

183

220

158

306 cm

♣ SnS ∇ Sn2S3

Post-heattreated NCs

100 150 200 250 300 350 400 −1

Raman shift / cm

(d)

Post-heat-treated NCs 191

♣

♣

Sn1.05S

♣ ♣ ♣ ♣ ♣♣♣

(212)

(015)

(116) /(314)

Sn1.00S

∇∇

∇

∇

∇

∇ Sn0.95S

20 25 30 35 40 45 50 55

2θ / °

Intensity / a.u.

95

♣♣ ♣

(112)

Intensity / a.u.

♣♣

-1

Sn0.95S

20 25 30 35 40 45 50 55 2θ / °

(c)

-1

Sn1.00S

93

Sn0.95S

(103) (202)

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

Page 14 of 38

Sn2S3

163 219

Sn1.05S

192 95

95

165

219

Sn1.00S

186 158

220 236

308 cm

−1

Sn0.95S 100 150 200 250 300 350 400

Raman shift / cm

−1

Figure 5. (a,c) XRD patterns and (b,d) Raman spectra of SnxS nanocrystals with x values of 0.95, 1.00, and 1.05 for (a,b) as-synthesized NCs by ball-milling for 5 h and (c,d) post-heattreated NCs at 500 °C for 1 h under a 99% Ar/1% H2 gas atmosphere.

SnS has been known to accommodate Sn-deficiency to a certain degree,58 which is regarded as responsible for its intrinsic p-type conductivity via the formation of Sn vacancies.47 On the other hand, studies on the Sn-excess stoichiometry are relatively scarce in the literature. Several research groups have reported that phase-pure SnS could be obtained with off-stoichiometric [Sn]/[S] ratios of 1.02−1.15.40,44,49,53,59 However, the Sn-excess compositions in most previous works were not deliberately controlled but were instead a consequence of volatile sulfur loss


14

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during high-temperature annealing and/or vacuum deposition processes. Huang et al. claimed that Sn-excess SnS films ([Sn]/[S] = 1.11) also exhibited a p-type behavior with a higher hole concentration than a Sn-deficit sample ([Sn]/[S] = 0.96).60 In the present work, in addition to the improved phase purity of Sn-excess SnS, slightly higher diffraction peak intensities of the Sn1.05S samples were recognized (Figure 5a and c), implying their better crystallinity than the other compositions. Optical and photovoltaic properties of SnxS annealed in a 5% H2S atmosphere. As solar cell absorbers, the doctor-blade-coated SnxS films (with x = 0.95, 1.00, and 1.05) were employed after annealing at 500 °C for 1 h in a 95% Ar/5% H2S atmosphere instead of the heat treatment in a Ar/H2 (1%) atmosphere to achieve a better microstructure. First, to evaluate the optical bandgap (Eg) values, the diffuse reflectance spectra of the SnxS NCs annealed in an identical manner were measured, with which Kubelka−Munk transformations were conducted according to the relationship of F(R) = (1 − R)2/2R. Here, F(R) and R denote the Kubelka−Munk function and the diffuse reflectance, respectively. The calculated F(R) function was analyzed using the equation, [F(R)hν]n ∝ hν − Eg, near the band edge regime, where hν is the photon energy and the n values are 2 or 1/2 corresponding to the direct and indirect allowed transition models.59 In order to examine the type of band transition of the present samples, both models were employed to extract Eg values by extrapolating the linear regime of the plots to the abscissa, where [F(R)hν]n = 0. The results are as shown in Figure 6. The evaluated direct and indirect bandgaps ( E gdir and Egind ) of the SnxS NCs were 1.19 eV and 1.08~1.09 eV, respectively, for all of the samples. The composition dependence of the optical bandgap was found to be negligible. The nature of the band-to-band transition of SnS has been controversial in the literature, which we summarize in a comprehensive manner in Table S2. Despite such controversy, there have been


15


several attempts to extract both types of bandgaps from a single optical measurement.30,44,57,60,61 The evaluated bandgaps in the present work are in fairly good agreement with the reported values (1.26 < E gdir /eV < 1.34 and 1.00 < Egind /eV < 1.17). Nonetheless, it is worth noting that, as can be recognized in Figure 6a,b, the direct transition model yielded a better R-squared value (> 0.999 in comparison with ~0.99 for the indirect transition model), possibly suggesting the direct transition characteristic of the SnxS NCs prepared in this work. Furthermore, we note that a slightly smaller E gdir value for the present samples is presumably attributed to the hightemperature annealing process according to Malaquias et al.44 They found that the E gdir values of SnS films sulfurized at 520 °C were reduced to 1.26 eV from 1.45 eV for such films sulfurized at 300 °C. In passing, it is noted that the E gdir and Egind values of SnxS NCs annealed in a 99% Ar/1% H2 atmosphere were also measured and found to be 1.24 and 1.08 eV, respectively, regardless of the composition (Figure S8). The reason for the slight difference in E gdir between the two groups of samples is yet to be elucidated. (b)

1.0

/ a.u.

Sn1.00S Sn1.05S

1.1

1.2

Annealed NCs (in 5% H2S)

1/2

2

Annealed NCs (in 5% H2S) Sn0.95S

(F(R)hν)

(a) (F(R)hν) / a.u.

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

Page 16 of 38

1.3

1.4 1.0

hν / eV

Sn0.95S Sn1.00S Sn1.05S 1.1

1.2

1.3

1.4

hν / eV

Figure 6. Tauc’s plots of the annealed SnxS NCs under 95% Ar/5% H2S with x = 0.95, 1.00, and 1.05: (a) [F(R)hν]2 vs. hν and (b) [F(R)hν]1/2 vs. hν plots to extract Eg for direct and indirect transitions, respectively.


16

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To demonstrate their potential as absorber materials, the SnxS films fabricated from the mechanochemically synthesized NCs were integrated into solar cells, as depicted schematically in Figure 7a. Figure 7b−d shows cross-sectional SEM images of the completed devices with Sn0.95S, Sn1.00S, and Sn1.05S, respectively. One can immediately recognize that as the [Sn]/[S] ratio increases, the microstructure of the SnxS absorber films gradually improves (in terms of the grain size and porosity). This trend of variation in the microstructure appears to be coincident with the variation in the amount of elemental Sn existing in the as-coated films (see Figure S9). One may surmise that elemental Sn with a low melting point (231.9 °C) may act as a sintering agent during the annealing step. However, further studies are required to clarify the correlation between the elemental Sn content and the morphological development of SnS films. Figure 7e shows the j−V characteristics of SnS solar cells under AM 1.5G 1 sun illumination and in the dark. The corresponding photovoltaic parameters are summarized in Table 1. The performance of the solar cells with Sn0.95S and Sn1.00S absorber layers was quite low, while the device with a Sn1.05S absorber exhibited a power conversion efficiency (η) of 1.71% with a short-circuit current density (jSC) of 18.14 mA cm−2, an open-circuit voltage (VOC) of 0.215 V, and a fill factor (FF) of 0.438 (see Figure S10 for the PV performance of six devices with Sn1.05S absorbers prepared in a single batch). The significantly enhanced performance of the Sn1.05S-based solar cell compared with Sn0.95S- and Sn1.00S-based devices could be attributed to the improved microstructure, crystallinity, and possibly to the suppressed formation of Sn2S3 impurity phase (Figure S5,S6), achieved by introducing a Sn-excess composition of the starting materials.


17


Page 18 of 38

Figure 7. Characterization of SnS thin-film solar cells fabricated using mechanochemically synthesized SnxS nanocrystals: (a) Schematic of a SnS solar cell; cross-sectional SEM images of completed devices with (b) Sn0.95S, (c) Sn1.00S, (d) Sn1.05S absorbers; and (e) j–V curves measured under AM 1.5G 1 sun illumination (closed circle) and in the dark (dashed lines).

Table

1.

Photovoltaic

parameters

of

SnS

thin-film

solar

cells

fabricated

using

mechanochemically synthesized SnxS nanocrystals Absorber

VOC / V

jSC / mA cm−2

FF

η/%

Sn0.95S

0.085

2.82

0.358

0.09

Sn1.00S

0.047

5.75

0.317

0.09


18

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Sn1.05S

0.215

18.14

0.438

1.71

In this study, we have demonstrated a simple, environmentally friendly, up-scalable route for synthesizing phase-pure SnS nanocrystals (NCs) and SnS-based thin-film solar cells via a solvent-free mechanochemical method. The α-SnS phase was abruptly formed after 2 to 3 h of milling, which is indicative of a mechanochemically induced self-propagating reaction mechanism. The as-synthesized SnS NCs prepared with compositions of Sn1.00S, Sn0.95S were found to coexist with a secondary phase of Sn2S3 in a non-negligible amount, which may deteriorate the solar cell performance. Meanwhile, the Sn-excess composition led to the formation of phase-pure SnS NCs without an additional thermal process. In addition, with an increase in the Sn content, the crystallinity and morphology of the annealed films were found to have been improved. As a consequence, the solar cells with a Sn1.05S absorber layer exhibited a much higher PCE (1.7%) than the other compositions. From this work, it is suggested that preventing the formation of unfavorable secondary phases as well as achieving a wellcrystallized microstructure are major challenges to achieve highly efficient SnS thin-film solar cells.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng. XPS spectra of as-coated and as-annealed SnxS, comparison of XRD patterns of Sn1.00S powders milled for 3 to 5 h, TEM images of Sn1.00S powders milled for 5 h, phase evolution of Sn0.95S


19


Page 20 of 38

powders, XRD, Raman spectra, and STEM-EDS mapping results with chemical composition of SnxS NCs annealed under a 95% Ar/5% H2S gas atmosphere, comparison of the reported values for optical bandgap of SnS, Tauc’s plots for post heat-treated SnxS NCs under an Ar/H2 gas mixture, XPS spectra of SnxS thin films, comparison of binding energies of Sn 3d5/2 and their assignments (PDF)

AUTHOR INFORMATION Corresponding Author * Tel: +82-2-958-6710. E-mail: [email protected] (D.-K. Lee), Tel: +82-2-958-5381. E-mail: [email protected] (S. Y. Lee). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ⊥These authors contributed equally. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This

work

was

financially

supported

by

the

KIST-UNIST

partnership

program

(1.160097.01/2.160482.01), by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) (20163010012580, 20163010012450), and by the KIST institutional program.


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For Table of Contents Use Only TOC/Abstract Graphic

SnS Nanocrystals

Sn0.95S Sn1.00S Sn1.05S

SYNOPSIS As a sustainable synthetic route of nanocrystals, a dry mechenochemical method provides phasepure SnS nanocrystals, resulting in photovoltaic devices with a higher efficiency than those with impurity phases.


30

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SnS Nanocrystals

Sn0.95S Sn1.00S Sn1.05S


TOC/Abstract Graphic


(a)

Intensity / a.u.

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

 5h     4h     3h   



2h



Sn1.00S  



 



   SnS







 



1h

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

 

  

Sn2S3  Sn S

 

20 25 30 35 40 45 50 55 60

2/

o


Fig. 1

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Fig. 2


As-synthesized NCs (5 h)

Intensity / a.u.

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

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B3g A g 160 180

Sn1.00S Ag

1

218 cm

Sn2S3 (Ag) 1

305 cm

Ag 1 93 cm

100 150 200 250 300 350 400 450 500

Raman shift / cm

-1


Fig. 3

(b),(c)

20 25 30 35 40 45 50 55 60

2 /

o

Intensity / a.u.

(231)/(061) (042) (250)

(200)

(131) (041) (210) (141) (002) (211) (112)/(151) (122)/(160)

(120) (021) (101) (040) (110)

500 C 300 C

(b)

400

(d)

500 C

Post heat-treated Sn1.00S NCs 1

192 cm

300

500 C

200

300 C

100

165

0

30

35

(c) 120 Intensity / a.u.

Post heat-treated Sn1.00S NCs (111)

(a)

Intensity / a.u.

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


2 /

o

40

45

300 C

90

Sn2S3

Sn2S3

60

Intensity / a.u.

Page 35 of 38

95 cm

1

1

188 cm

161

1

221 cm

Sn2S3

1

309 cm

(015)

(212)

1

219 cm

30

1

96 cm

0

30

35

2 /

o

40


45

100 150 200 250 300 350 400

Raman shift / cm

-1

Fig. 4




As-syn. NCs

 



(b) 94

  



(116) /(314)



(015)



(212)

(112)

Sn1.00S



As-syn. NCs

187

220

160

Sn1.05S

Intensity / a.u.

 SnS   Sn2S3

(103) (202)

Intensity / a.u.

(a)

Sn2S3 Sn1.05S

93 180 160 218

305 cm

158



183

220

306 cm

 SnS  Sn2S3

Post-heattreated NCs





Sn1.05S

    



(116) /(314)



(015)



(212)

(112)

(103) (202)

Sn1.00S



Raman shift / cm

(d)

Post-heat-treated NCs 163

95

95

165

Sn1.05S 219

Sn1.00S

186 158

220 236

308 cm

1

Sn0.95S

Sn0.95S

2

Sn2S3 219

192



20 25 30 35 40 45 50 55

1

191

Intensity / a.u.



100 150 200 250 300 350 400

95

 

-1

Sn0.95S

20 25 30 35 40 45 50 55 2

(c)

-1

Sn1.00S

93

Sn0.95S

Intensity / a.u.

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

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100 150 200 250 300 350 400

Raman shift / cm ACS Paragon Plus Environment

1

Fig. 5

Page 37 of 38

(b)

1.0

/ a.u.

Sn1.00S Sn1.05S

1.1

1.2

h / eV

Annealed NCs (in 5% H2S)

1/2

2

Annealed NCs (in Ar/5% H2S) Sn0.95S

(F(R)h)

(a) (F(R)h) / a.u.

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


1.3

1.4 1.0

Sn0.95S Sn1.00S Sn1.05S 1.1

1.2

1.3

1.4

h / eV


Fig. 6

(b) Sn0.95S


(a)

2㎛ (c) Sn1.00S

(d) Sn1.05S AZO CdS SnS Mo

(e)

Sn0.95S Sn1.00S Sn1.05S

2

20

j / mA cm

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

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10 0 -10 -20

0.0ACS Paragon0.1 0.2 Plus Environment

V/V

0.3

Fig. 7

Mechanochemically Synthesized SnS Nanocrystals: Impact of

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