Colloidal Wurtzite Cu2SnS3 (CTS) Nanocrystals and Their

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Colloidal Wurtzite CuSnS (CTS) Nanocrystals and Their Applications in Solar Cells Uma V. Ghorpade, Mahesh P. Suryawanshi, Seung Wook Shin, Inyoung Kim, Seung Kyu Ahn, Jae Ho Yun, Chaehwan Jeong, Sanjay S Kolekar, and Jin Hyeok Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00176 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 7, 2016

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Chemistry of Materials

Colloidal Wurtzite Cu2SnS3 (CTS) Nanocrystals and Their Applications in Solar Cells Uma V. Ghorpadea, b, Mahesh P. Suryawanshia, Seung Wook Shinc, Inyoung Kima, Seung Kyu Ahnd, Jae Ho Yund, Chaehwan Jeonge, Sanjay S. Kolekarb*, Jin Hyeok Kima* a

Department of Materials Science and Engineering and Optoelectronics Convergence Research Center, Chonnam National University, 300, Yongbong-Dong, Buk-Gu, Gwangju 500-757, South Korea

b

Analytical Chemistry and Material Science Research Laboratory, Department of Chemistry, Shivaji University,

Kolhapur 416-004, India c

Department of Chemical Engineering and Materials Science, University of Minnesota, Amundson Hall, 421 Washington Ave. SE, Minneapolis, MN 55455-0132, USA

d e

Photovoltaic Laboratory, Korea Institute of Energy Research (KIER), Daejeon, 305-343, South Korea

Solar City Center, Korea Institute of Industrial Technology (KITECH), Buk-Gu, Gwangju, South Korea

ABSTRACT: In the development of low-cost, efficient, and environmentally friendly thin-film solar cells (TFSCs), the search continues for a suitable inorganic colloidal nanocrystal (NC) ink that can be easily used in scalable coating/printing processes. In this work, we first report on the colloidal synthesis of pure wurtzite (WZ) Cu2SnS3 (CTS) NCs using a polyol-mediated hot injection route, which is a non-toxic synthesis method. The synthesized material exhibits a random distribution of CTS nanoflakes with an average lateral dimension of ~ 94 + 15 nm. We also demonstrate that CTS NC ink can be used to fabricate low-cost and environmentally friendly TFSCs through an ethanol-based ink process. The annealing of as-deposited CTS films was performed under different S vapor pressures in a graphite box (volume; 12.3 cm3), at 580 ∘C for 10 min using a rapid thermal annealing (RTA) process. A comparative study on the performances of the solar cells with CTS absorber layers annealed under different S vapor pressures was conducted. The device derived from the CTS absorber annealed at 350 Torr of S vapor pressure showed the best conversion efficiency 2.77 %, which is the first notable efficiency for an CTS NCs ink-based TFSC. In addition, CTS TFSC’s performance degraded only slightly after 50 days in air atmosphere and under damp heating at 90 ∘C for 50 h, indicating their good stability. These results confirm that WZ CTS NCs may be very attractive and interesting light-absorbing materials for fabricating efficient solarharvesting devices.

INTRODUCTION Recently, ternary chalcogenide Cu2SnS3 (CTS) has emerged as an alternate to the most important group of IIV-VI semiconductors, exhibiting better economic and environmental features and retaining electronic properties equivalent to those of CuInSe2, Cu2(In,Ga)Se2 (CIGS) and Cu2ZnSnS4 (CZTS).1,2 This material has high potential for use in photovoltaic (PV) applications because of its low-cost, use of earth abundant elements, better optical absorption coefficient (~ 104 cm-1) and variable band gap (0.9-1.77 eV), and structural simplicity compared to CZTS.3,4,5 Interestingly, the band gap values of Cu-Sn-S compounds can be easily adjusted by controlling the morphology, structure, crystalline degree, defect type and concentration; this outstanding characteristic suggests that introducing rare elements, such as Ga, is not necessary in these materials.6

Pure CTS-based TFSCs with a maximum conversion efficiency of 4.63 % have been produced by annealing a co-evaporation-deposited precursor with a stacking order of stacked NaF/Cu/Sn under an Sn- and S-containing vapor atmopshere.7 Although evaporation based CTS TFSCs exhibit the best performance, their synthesis process limits their commercial use in the emerging CTS-based technologies because of the lack of large area uniformity and the requirement of additional solid Sn powder during the annealing process. In particular, it is quite difficult to precisely control the Sn vapor pressure during the annealing process because of the instability of Sn (IV).8,9 To overcome this drawback, many researchers have adopted a nanocrystal (NC)-based approach to take advantage of its low cost and simplicity. To date, various synthetic methods have been developed for the synthesis of CTS NCs.10-13 However, compared to binary chalcogenides, it is very

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difficult to control the stoichiometry and crystal phase of ternary and quaternary NCs because of the different reactivities of the metal precursors and the complex interactions between the capping molecules and the NCs.14,15 Among the available approaches, the colloidal synthesis of NCs is particularly attractive because of its potential to produce processable, low-cost and efficient TFSCs from NC-based ink.16,17 Although many works have focused on synthesizing cubic CTS NCs, only a few reports describe systematic studies of the structural flexibility and optoelectronic properties of CTS NCs.1,6,10-13 Moreover, information on the colloidal synthesis of CTS NCs, ink formulations and their utilization in fabricating low-cost and efficient TFSCs is lacking (Table S2). Stoichiometric CTS can exist in various crystal structures, such as a cubic sphalerite-like phase, a monoclinic phase with a sphalerite superstructure, and a wurtzite (WZ) phase with a hexagonal structure.18 Zhai et al. 19 investigated the reasons underlying the structural diversity of CTS materials using first-principles total energy and band-structure calculations. This research strongly supported a close relationship between the crystal structure and the optoelectronic properties of the CTS system, indicating that exploring new synthesis strategies for CTS NCs with unusual metastable crystal phases and investigating their potential viability in TFSCs would be of great value.20 In addition, the transformation of the metastable phase was reported to require less energy.21 Recently, this advantage was found to facilitate the easy phase transformation from WZ to a most stable phase at a relatively low temperature (400 °C) because of the randomly distributed cations in the WZ crystal system.22 Thus, a colloidal synthesis of WZ CTS NCs is clearly needed. We have previously demonstrated the environmentally benign, surfactant-free, polyol-mediated hot injection (HI) route for the synthesis of WZ CZTS NCs and the NC’s viability as absorbers in TFSCs.23 This study suggested that the reaction temperature and time are key parameters in controlling the WZ phase formation. These interesting results revealed that WZ CTS NCs could be easily synthesized via an environmentally benign, surfactant-free, polyol-mediated HI route. Here, we report a versatile and non-toxic approach for the synthesis of WZ CTS NCs and the deposition of NCbased CTS thin films using low-cost and non-toxic solvents, such as ethylene glycol and ethanol. The controlled synthesis of WZ CTS NCs was achieved by simply tuning the reaction temperature and time. Pure WZ CTS NCs were formed at the relatively low temperature of 190 °C after a short reaction time of 15 min. Interestingly, WZ CTS NCs can be easily dispersed using a non-toxic and low-cost solvent, such as ethanol, and fabricated into CTS

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absorber layers through scalable coating processes. In addition, upon thermal treatment at elevated temperatures (500-600 °C) with a moderate partial pressure of S, these CTS thin films readily consolidated into largegrained polycrystalline CTS thin films. In particular, we demonstrate that ink based on WZ CTS NCs can be fabricated into eco-friendly and low-cost TFSCs, with our first cell exhibiting a notable efficiency of 2.77 %. This efficiency was certified by an external, accredited laboratory (Korea Institute of Industrial Technology, KITECH), as shown in Figure S9. EXPERIMENTAL SECTION Chemicals. Copper chloride (CuCl2, 97.00%), tin chloride (SnCl2, 98.00%), thiourea (CH4N2S, 99.99%), ethylene glycol (EG), C2H6O2, 99.99%), CdSO4, ammonia and ethanol (C2H5OH, 99.99%) of analytical grade were purchased and used as received from Sigma Aldrich (Gyeonggi-do, South Korea). Synthesis of CTS NCs. CTS NCs were prepared via an HI method based on a reaction of the glycol complexes of metallic salts with a thiourea anionic precursor using different reaction temperatures and times. This method was adapted from our previously reported synthesis method for CZTS NCs.23 Temperature-dependent experiments were performed at reaction temperatures of 130, 150, 170 and 190 °C using a fixed reaction time of 15 min. In addition, time-dependent experiments were performed with reaction times of 5, 10, 15, 20, 25 min at 190 oC. The NC precipitate was separated by centrifugation at 3000 rpm for 10 min, washed with deionized water and anhydrous ethanol, and dried at 60 °C overnight under vacuum. The resulting optimized NCs were processed into ink for further thin film formation and device fabrication. Thin film formation and device fabrication. The spincoated CTS NC-based absorber layers were further allowed to anneal in the presence of solid S powder at 58o o C for 10 min in a rapid thermal annealing (RTA) system inside a graphite box (12.3 cm3). Prior to annealing at 580 ∘C, the NC based film were subjected to soft baking at 300 ∘C for 10 min at atmospheric pressure to prevent the formation of organic residue layers at the CTS/Mo interface.24 During annealing in the RTA system, the total pressure inside the graphite box was controlled using different masses of S powder, which were calculated using an ideal gas law, and the S vapor pressure-dependent study was performed. The S vapor pressures inside the graphite box were maintained at 150, 350, 550 and 750 Torr using 0.0047, 0.0109, 0.0171 and 0.0233 g of solid S powder, respectively. The resulting films were processed into solar


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cell devices following standard procedures, including chemical bath deposition of CdS (~65 nm), radio frequency (RF) sputtering of i-ZnO (~75 nm) and Al-ZnO (AZO) (~500 nm), and direct current (DC) sputtering of a patterned Al grid as the top electrode.25 The final devices (2.5 cm × 2.5 cm) were mechanically scribed into cells with an active area of 0.30 cm2. The details of the synthetic procedure, NC purification, characterization, ink formulation, annealing conditions and device fabrication are provided in the Electronic Supplementary Information (ESI). RESULTS AND DISCUSSION Figure 1a shows the X-ray diffraction (XRD) patterns of the CTS NCs synthesized at various reaction temperatures. A crystallographic evolution can be observed as the reaction temperature increases. The reactions performed below 190 °C contain many peaks from other binary secondary phases of Cu2-xS (JCPDS Card No. – 00-002-1772), Cu9S5 (JCPDS Card No. – 00-009-0064) and Sn2S3 (JCPDS Card No. – 00-014-0619), among others, and CTS. These additional peaks most likely originated from the intermediate states of the complex that formed with the EG at the lower reaction temperatures of 130 and 150 °C. The results at 190 °C confirm the systematic formation of a pure WZ CTS phase based on a gradual evolution of the diffraction planes to that of a single phase during the synthesis. All of the peaks are assigned to the hexagonal WZ CTS phase, which is consistent with the literature.26,27 In addition to structural analysis using XRD, Raman spectroscopy was employed to confirm the formation of a pure CTS phase. The Raman peak (Figure 1b) at 314 cm-1 of CTS NCs synthesized at 190 °C for 15 min corresponds to the single phase of WZ.26,27 The Raman spectra are typical of WZ CTS, however the peaks are slightly shifted with respect to those of as reported. The shift in the Raman peaks may be normally due to the disorder induced by minor phases or size effects, sample composition and excitation intensity.28,29 Since the Raman spectrum depends on the symmetry of the crystal structure, the strength of the chemical bonds and the masses and charges of the constituent elements. The Raman peaks of the NCs synthesized at temperatures lower than 190 °C show major peaks corresponding to Sn2S3, CTS and Cu2-xS. These findings further confirm the phase evolution from binary secondary phases of Cu2-xS, Cu9S5 and Sn2S3 to the pure WZ phase, which is consistent with the XRD results. In addition to this work, time-dependent studies were also performed to further study the pure-phase formation. Figure 1c shows the XRD patterns of CTS NCs synthesized at different reaction times: 5, 10, 15, 20 and 25 min at 190 oC. Even as early as 5 min after injection, the diffraction peaks can be observed, and they are all characteristic of the WZ phase.

Thus, the reaction process is very fast. All the other NCs extracted after 10, 15, 20 and 25 min were also in good agreement with the crystalline WZ CTS phase, exhibiting well-aligned peaks with improved peak intensity. Figure 1d shows the Raman spectra of the corresponding CTS NCs, which further verified the formation for pure-phase WZ CTS in all NCs. Time-dependent as-synthesized CTS NCs were used for the optical studies because structural variations affect the band gap of the material. Figure 2a and its inset shows the corresponding absorption spectrum and Tauc’s plot of CTS NCs synthesized at 190 oC for 15 min. The band gap energy of CTS NCs synthesized at 190 °C was estimated to be 1.06 eV. NCs synthesized at different reaction times show smaller deviations in the band gap and, hence, in the absorption edge (Figures S1a and b) because all the NCs exhibit the WZ phase with slightly improved peak intensities. The CTS NCs synthesized at 190 °C for 15 min exhibit a band gap energy of 1.06 eV. Thus, CTS NCs synthesized at 190 °C for 15 min were considered to be optimized and were used for further surface chemistry and morphological studies. The capping ligands on the surface of the CTS NCs were identified via Fourier transform infrared (FTIR) spectroscopy, as shown in Figure 2b. Pure EG exhibits major bands at 3392, 3017, 1486, 1283 and 1110 cm-1, which correspond to O-H stretching, C-H stretching, C-H bending, alcoholic O-H stretching and C-O-H stretching, respectively.30,31 In contrast, the bands corresponding to optimized CTS NCs are only slightly shifted towards lower frequencies with significantly reduced intensity, which may indicate a reduction in bond order resulting from shattering of the metal complex and the appearance of ligands on the NC surface. As shown in Scheme 1, CTS can be formed from an intermediate glycolate complex. The species with the purple tail indicate the solvent molecule i.e EG. In this process, EG can be deprotonated by its counter anion, which plays a vital role in the dissolution of the precursor salt.32 Further, cations can coordinate with the O-atoms of deprotonated EG molecules. The transient complex can be formed by ligands with different denticities and with metal-O-covalent and metal←OH coordination bonds, as shown in Scheme 1.33 The monoionic state of EG is known to be the most active form.32 However, Riveres et al. 34 recently reported the presence of an anionic moiety consisting of a bis-ethylene glycolate Cu2+ dianion in the reaction of the EG–copper chloride dehydrate–sodium hydroxide system. During this reaction process, the redox reaction occurs between Cu2+ and Sn2+ cations yielding Cu+ and Sn4+, which furnishes into nucleated Cu2SnS3 (CTS) by the reaction between Cu+, Sn4+ and



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S2-. Thus, we identify the plausible mechanism as responsible for the formation of CTS NCs. Figure 3a and its inset show a transmission electron micrograph (TEM) of the CTS NCs synthesized at 190 °C for 15 min. The sample exhibits a nanoflake-like morphology with random orientations. The average lateral dimension is ~ 94 + 15 nm, as shown in the histogram in Figure S2. A high-resolution TEM (HR-TEM) analysis of the CTS NCs (Figures 3b-c) shows clear lattice fringes with interplanar spacings of 0.337 nm and 0.342 nm, which are ascribed to the (1 1 0) and (1 0 0) planes, respectively, and indicate WZ structural characteristics.35,36 This finding was further confirmed based on a Fast-Fourier transform (FFT) image of the CTS NCs, as shown in Figure 3d.37,38 To further verify the presence of all three constituent elements in each individual NC, a chemical analysis was conducted. A lowresolution high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) and elemental mapping of the WZ CTS NCs in Figure S3 revealed homogenous distributions of Cu, Sn and S elements in the optimized CTS NCs. The chemical states of Cu, Sn, and S in the CTS were determined by X-ray photoelectron spectroscopy (XPS) (Figure S4). The Cu 2p core splits into 2p3/2 (932.0 eV) and 2p1/2 (952.2 eV) peaks, and the Sn 3d splits into two peaks at 2d5/2 (486.7 eV) and 2d3/2 (495.1 eV). The two peaks located at 161.5 and 162.7 eV, which exhibit a peak splitting of 1.2 eV, are assigned to S2-. These results indicate that the valence states of Cu, Sn and S in the CTS NCs are 1+, 4+ and 2-, respectively, and all of these values are in good agreement with the literature.39,40 The Cu/Sn and S/metal compositional ratios of the CTS NCs synthesized at 190 °C for 15 min are 1.41 and 0.87, respectively, as shown in Table S1. One advantage of the colloidal synthesis of CTS NC ink is its compatibility with a variety of scalable coating/printing processes. The as-synthesized CTS NCs can be easily dispersed into a non-toxic solvent (ethanol), fabricated into CTS films using a suitable spin-coating method, and annealed to form polycrystalline films with large-sized grains. The details of the ink and thin film formation and the annealing conditions can be found in the ESI. The as-deposited CTS films were subjected to annealing at 580 °C for 10 min using an RTA system in a graphite box at different S vapor pressures. The S vapor pressures inside the graphite box were maintained at 150, 350, 550 and 750 Torr using 0.0047, 0.0109, 0.0171 and 0.0233 g of S powder, respectively. The resulting films were characterized with regard to their structural, morphological, optical and optoelectronic properties. The films were designated as CTS-1, CTS-2, CTS-3 and CTS-4

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when grown at S vapor pressures of 150, 350, 550 and 750 Torr, respectively. The crystallinity and orientation of the films were investigated using XRD. Figure S5a shows the XRD patterns of the CTS thin films with different S vapor pressures and reveals that the as-deposited WZ CTS NC thin film undergoes a phase transformation from a WZ to a monoclinic structure. All of the films show several main peaks at 2θ values of 28.43 o, 33.01 o, 47.33 o and 56.12 o. In addition, a secondary phase of SnS is observed in CTS-3 and CTS-4 films annealed under higher S vapor pressures of 550 and 750 Torr (JCPDS card No. – 00-001-0984 (SnS)), respectively. Further, the Raman spectroscopy was employed to confirm the pure phase formation and the phase transformation in the annealed CTS films. Figure S5b shows the Raman spectra of annealed CTS films under different S vapor pressures. All the films exhibit major peaks at 292 and 351 cm−1, which are identified as the main vibrational symmetry modes of monoclinic phase. 41,42 Raman spectra also reveal the presence of SnS, with a characteristic mode at 221 cm−1, in the films annealed under the pressures of 550 and 750 Torr.43 UV-Vis absorption of the annealed CTS films by using a glass substrate has been carried out for the detailed interpretation. The band gap energy at higher sulfur pressure further shifts the optical absorption edge towards higher energy. The corresponding spectra of films are shown in Figure S6. The elemental composition of the annealed CTS films was characterized by X-ray fluorescence (XRF). The observed atomic percentages and atomic ratios of Cu, Sn, and S are shown in Table S1. As the S vapor pressure increased, the S doping concentration also increased. All the films showed similar compositions, except for that (CTS-4) annealed under a high S vapor pressure of 750 Torr. The surface and cross-sectional morphologies of the film were characterized using a field emission scanning electron microscope (FE-SEM). Figure 4 shows the FESEM images of the as-deposited films and those annealed under various S vapor pressures. It is clear that the S vapor pressure significantly influences the microstructure and grain growth of CTS absorber layers. The asdeposited film shows a crack-free and compact microstructure without interlayer delamination and a thickness of ~ 1 µm. All the annealed films show relatively dense microstructures without voids/cracks/pores and have thicknesses of approximately 0.9 µm, except for CTS-4. The CTS-1 film annealed under a lower S vapor pressure of 150 Torr exhibits uniform growth of smaller grains and a compact cross section extending through the thickness of the film, as shown in Figures 4c and d. The CTS-2 film annealed under a relatively high S vapor pressure of 350 Torr has comparatively large grains without any voids at


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the interface (Figures 4e and f). However, further increasing the S vapor pressure resulted in the formation of cracks, which extended from top to bottom (Figure 4 h) in the CTS-3 film annealed under an S vapor pressure of 550 Torr. Surprisingly, large voids are formed at the CTS/Mo interface in the CTS-4 film annealed under a high S vapor pressure of 750 Torr, as clearly seen in Figure 4j. As a result, the film became less adherent to the Mo substrate and peeled off (Figures 4i and j). Note that the annealed film does not show a bilayer structure, as is typically observed in organic ligand-capped NC-based absorbers, which contain fine grain layers of carbon impurities resulting from the decomposition of organic compounds during high-temperature annealing.22,23,31 However, a layer of MoS2 is observed in the annealed film, which is confirmed by the presence of weak Raman peak at 409 cm-1 (Figure S5b)42 and its thickness increased from 35 nm to 240 nm as the S vapor pressure increased. For better understanding, the formed MoS2 layer at the interface between CTS/Mo is marked by the red brown color in Figure 4. Under the lowest S vapor pressure, the thickness of the MoS2 layer is zero or very thin, whereas a thick MoS2 layer is formed in the CTS-4 film annealed under higher S vapor pressure. The high S vapor pressure causes the diffusion of S atoms through the CTS layer and results in the formation of the SnS secondary phase and the thick MoS2 layer at the CTS/Mo interface.44 The formation of a thick MoS2 layer during annealing of the CTS-4 film results in poor adhesion of the absorber to the substrate and high series resistance in the solar cell.45 Solar cell performance As a proof of concept, CTS NC-based thin films annealed under different S vapor pressures were tested as absorber layers in a solar cell device fabricated with a SLG/Mo/CTS/CdS/i-ZnO/Al-ZnO/Al configuration and illuminated in the gap between Al contacts on the top. The CTS absorber layers were ~ 0.9 µm thick, and the CdS/n-ZnO layers were ~ 640 nm thick. The measurements of the current density-voltage (J-ܸ) characteristics were performed using corresponding devices under AM 1.5 G one-sun illumination (100 mW/cm2). To minimize possible Jsc overestimation from current collecting outside the device active area, the isolation process was applied.46 The solar cell devices are isolated via scribing to define the active area, which is typically 0.30 cm2. Figure S7 shows photo images of the active area measured individually for each cell. The J–V characteristics of the corresponding devices are shown in Figure 5a and summarized in Table 1. The CTS-1, CTS-2, CTS-3 and CTS-4 devices exhibited power conversion efficiency (PCE) values of 1.04, 2.77, 1.67 and 0.79 %,

respectively. The magnitudes of the short circuit current density (Jsc) were found to be 15.13, 20.12, 17.92, and 14.17 mA/cm2 for the CTS-1, CTS-2, CTS-3 and CTS-4 devices, respectively. The corresponding open circuit voltages (Voc) were found to be 0.238, 0.327, 0.289, and 0.207 V for the CTS-1, CTS-2, CTS-3 and CTS-4 devices, respectively. The solar cells based on CTS thin films derived using lowest S vapor pressures clearly show relatively poor performance. This poor performance may be attributable to the formation of atomic-level defects in the absorber lattice caused by S deficiency.47 Surprisingly, the FF of the CTS-2 device derived from the CTS absorber annealed under a S vapor pressure of 350 Torr is significantly higher than those of the other devices. This result may result from the void- and crack-free, compact and dense microstructure with larger grains, which would reduce the minority charge recombination at grain boundaries.24 The PCE values of the CTS devices were found to increase as the S vapor pressure was increased to 350 Torr and then decreased as the S vapor pressure was further increased. The deterioration in the device parameters of the CTS-3 and CTS-4 devices derived from absorbers annealed under higher S vapor pressures (550 and 750 Torr) mainly originates from the presence of voids and SnS secondary phases in the CTS, which decrease the values of Voc and FF.48 In addition, excessive formation of MoS2 and a wide gap at the interface led to higher series resistance in the resulting CTS-4 device. Thus, the CTS-2 device derived from the absorber annealed at a moderate S vapor pressure of 350 Torr showed the highest Voc, Jsc and FF (0.327 V, 20.12 mA/cm2 and 42 %, respectively), resulting in the highest PCE (2.77 %) for the CTS NC-based solar cells tested. Similar Voc and Jsc values have been reported in the literature.49,50 The external quantum efficiency (EQE) of the most efficient CTS-2 device had a maximum value of ~ 69 % in the vicinity of 555 nm, as shown in Figure 5b. At shorter wavelengths, the EQE decreases because of strong absorption by the CdS layer (Eg of 2.4 eV and absorption onset of 516 nm). The curve gradually decreases at longer wavelengths, suggesting a shorter carrier diffusion length and lower collection efficiency in the bulk of the CTS absorber.7,49 The major loss of EQE between 600 nm and 1300 nm is a result of the insufficient generation and poor collection of carriers at the back side, revealing a large potential for further improvements in Jsc and efficiency by achieving a homogenously well-developed microstructure throughout the CTS absorber’s thickness.7,46,49 The band gap of the absorber layer was estimated from the EQE and found to be 1.06 eV. In addition, the low Voc (Voc deficit) value of 0.327 V in the high-efficiency CTS absorber was strongly related to the high carrier recombination in the



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p-n junction resulting from the high defect density, which can be indirectly calculated by (Eg/q)-Voc.23 The value of (Eg/q)-Voc was found to be 733 mV, which is relatively high, indicating that many defects exist in the CTS, which could lead to the high recombination rate and thereby decrease the PCE of CTS devices.23 To obtain information on the loss mechanism in the CTS device, the diode parameters, such as the shunt conductance (Gsh), series resistance (Rs), ideality factor (nd) and saturation current density (Jo), were further extracted using a lumped circuit model from the J-V curves in the dark shown in Figures 6a-c and Table 1.51 Gsh values of 2.11, 1.20, 1.52 and 3.8 mS/cm2 for CTS-1, CTS-2, CTS-3 and CTS-4, respectively, were calculated from the derivative dV/dJ against V near Jsc based on the J–V curve (Figure 6a). The enhancement (decreased value) of Gsh particularly implies that the microstructure of the CTS film was improved and showed lower porosity.52 Rs values of 6.14, 2.45, 4.80 and 9.87 Ω cm2; nd of 1.48, 1.06, 1.08 and 2.67; and Jo of 7.02 × 10-5, 1.68 × 10-6, 1.56 x 10-5, and 2.95 × 10-4 mA/cm2 were also determined for CTS-1, CTS-2, CTS-3 and CTS-4 devices, respectively, using sets of plots (Figures 6b-c) according to the diode equations. The high Rs of the devices can be attributed to the formation of MoS2 at the interface between the CTS absorber and the Mo substrate and the presence of a secondary phase in the CTS absorber. The thickness of the MoS2 layers increased with the S partial pressure (Fig. 4), and the XRD and Raman results (Figure S5) revealed the formation of a SnS secondary phase (CTS-3 and CTS-4). These results support the high Rs values. The ideality factor plays an important role in determining the quality of the absorber layer. The deviation in the nd values corresponds to the recombination in the space charge region caused by the poor quality of the absorber.53 The relationship between the device quality and the diode parameters obtained above was further studied as a function of S vapor pressure (Figures 7a and b). Figure 7a shows the parallel relationship among Gsh, nd, and J0. The CTS-2 device shows significantly improved performance in these parameters compared to the other tested devices. The correlations of Rs with Jsc and FF are also shown in Figure 7b. FF and Jsc exhibit contrasting relationships with Rs, indicating that higher Rs deteriorates the device performance by reducing the FF and Jsc. Additionally, smaller grain sizes increase the number of grain boundaries that act as higher-level recombination sites, yielding lower device performance.54 Lower device Voc may be attributable to the Jo value extracted from the intercept curve in Figure 6c. In addition, the absorber annealed at a high S vapor pressure exhibited lower Jsc and increased Jo, accounting for the Voc deficit because Voc

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is strongly correlated with J0 and the ideality factor as follows: Voc = (ndkT / q) ln (Jsc /Jo)

------- (1)

where nd, kT/q and Jo are the ideality factor, thermal voltage and reverse saturation current, respectively. Thus, the improved film morphology of a particular device significantly reduces the recombination losses, dramatically enhancing device performance. Because neither the CTS absorber layer nor the device fabrication process is fully optimized, the low performance may be attributed to reflection losses at the front contacts, unwanted absorption in the window layer and high charge carrier recombination in the absorber layer. Furthermore, the stability of the high-efficiency CTS-2 device performance was investigated under various environments, as shown in Figure 8. When kept in air at room temperature, the device exhibited a slight decrease in the PCE (2.72 %) after 50 days. In addition, damp heat testing at 90 °C with 50 % humidity did not result in a large deviation after up to 50 h. The amount of decay observed in our device is considerably lower than those of many nextgeneration solar cells, such as pervoskite and polymer solar cells. To ensure uniform absorber layer formation, the device performance of six individual cells in the CTS-2 device was also checked and is summarized in Figure S8. Table S2 presents the device performances of pure CTS TFSCs based on various methods reported to date. Although the efficiency of our solar cell is the highest of any pure CTS NC-based device prepared using a green, polyolbased synthetic approach, the device has a low efficiency compared with the highest PCE of CTS devices because it has not been fully optimized. Indeed, the device reported here represented an initial attempt to construct a solar cell device using an NC-based CTS absorber layer. Further studies are in progress to improve the device performance by elucidating the formation mechanism of CTS NCs and their properties and optimizing the absorber composition, annealing conditions and device fabrication process. CONCLUSIONS In summary, we demonstrated a facile, low-cost polyolbased approach for the synthesis of CTS NCs at low temperature and over a short reaction time. The phase transformation of WZ-derived CTS NCs to monoclinic CTS grains was observed after annealing in S vapor at 580 °C for 10 min. The influence of the S vapor pressure on the structural, morphological and photovoltaic properties of the CTS absorber was studied. We also reported an initial PCE of 2.77 % (Voc = 327 mV, Jsc = 20.12 mA/cm2, and FF = 42 %) without any antireflection coatings using a CTS


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device derived from a CTS absorber annealed under a moderate S vapor pressure of 350 Torr. In addition, the CTS-based solar cell showed good stability under air atmosphere for 50 days and damp heating at 90 °C upto 50 h. Collectively, these results support this method as a successful, relatively green approach for the synthesis of colloidal CTS NC inks as a simple and reliable route to the formation of high-quality NC-based absorber layers. This synthesis has immense potential as a means of low-cost, large-area CTS thin film processing for next-generation high-efficiency solar cells. Supporting Information Available: Experimental details of the CTS NC synthesis and washing, CTS NC ink formation, CTS NC ink-based absorber and TFSCs fabrication, characterization, reaction time-dependent absorption spectra and Tauc’s plots, histogram plots, HAADF STEM images, elemental mapping and XPS spectra of WZ-CTS NCs synthesized at 190 °C for 15 min, XRD patterns, Raman spectra and UV-Vis absorption spectra of CTS absorbers annealed in a graphite box under various S vapor pressures at 580 °C for 10 min using an RTA system, photographs of device fabrication steps employing Al deposition masking and mechanical scribing, stability and average photovoltaic performances of the devices, tables describing the composition of NCs and NC-based CTS absorbers annealed under various S vapor pressures, and some representative reports of CTS-based TFSCs prepared using various methods. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work is supported by a Human Resources Development grant (No. 20124010203180) from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korean Government Ministry of Trade, Industry and Energy and partially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF2015R1A2A2A01006856)). REFERENCES (1) Liu, X.; Wang, X.; Swihart, M. T.

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∆ Cu

S

ο Cu9S5  Sn2S3

o

190 C o

170 C o

150 C 

ο

∆ ∆

∆ ∆



∆ ∆

o

∆

ο

130 C

Intensity (arb. unit)

2-x

(1 1 0) (1 0 3) (1 1 2) (2 0 2)

(1 0 2)

(1 0 0) (0 0 2) (1 0 1)


(a)

314 cm-1

Figures

(b)

∗ Cu2-xS # Sn2S3

o

190 C o

170 C

∗ #

o

150 C o

130 C

Wurtzite CTS

20

30

40

50

60

70

80

200

300

500

600

25 min 20 min 15 min 10 min

-1

(d)

314 cm


Raman shift (cm )

(1 1 0) (1 0 3) (1 1 2) (2 0 2)

(1 0 2)


(1 0 0) (0 0 2) (1 0 1)

(c)

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-1

2θ (Degree)

25 min 20 min 15 min 10 min

5 min

5 min

Wurtzite CTS

20

30

40

50

60

70

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80

300

400

500

600

-1

Raman shift (cm )

2θ (Degree)

Figure 1. (a) and (c) XRD patterns, (b) and (d) Raman spectra of CTS NCs synthesized at various reaction temperatures (130, 150, 170 and 190°C) and reaction times (5, 10, 15, 20 and 25 min). The NCs synthesized at 190 °C for 15 min exhibit the formation of a p ure WZ phase, and the remaining patterns show evidence binary phases in addition to the main peaks. The corresponding stimulated XRD pattern of wurtzite CTS is shown in Figure (a) and (c).

250

(a)

o

190 C

2

8

1.4

Transmittance (%)

2

(α hν)2 x 10 8 (eV/cm )

Absorbance (arb. unit)

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

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1.3

0.5

1.0

1.5

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2.5

3.0

3.5

Photon Energy (eV)

1.2

(b) CTS NCs

200

Ethylene Glycol

150

100

3017 2953 3392

1486

o

190 C

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500

1169

50

600

700

800

900

1283 675 949 592 1110

4000 3500 3000 2500 2000 1500 1000

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500

-1

Wavenumber (cm ) o

Figure 2. (a) Room-temperature UV–visible absorption spectrum of CTS NCs synthesized at 190 C. The inset shows the (αhν) 2 versus (hν) plot of the corresponding CTS NCs. (b) FTIR spectra of pure ethylene glycol and the as-prepared CTS NCs synthesized at 190 °C for 15 min.


10

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Scheme 1. Schematic representation of the proposed reaction mechanism protocol for the synthesis of CTS NCs. The species with purple tail indicates the solvent molecule.

Figure 3. (a) Bright-field TEM image of the CTS NCs synthesized at 190 °C for 15 min. (b) Low-magnification HR-TEM image of the region shown in (a). (c) High-magnification HR-TEM image of the region shown in (b). (d) FFT of the image shown in the square region in (b) with a WZ-derived CTS NC, which matches well with the peaks indexed to the zone axis of the (0001) direction for WZ-derived CTS NCs synthesized at 190 °C for 15 min.


11


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Figure 4. The surface and cross-sectional FE-SEM images of as-deposited films (a), (b) and those annealed under different S vapor pressures: 150 Torr (c), (d), 350 Torr (e), (f), 550 Torr (g), (h), and 750 Torr (i), (j). The as-deposited thin film shows a dense, crackand void-free microstructure without interlayer delamination and a thickness of ~1 µm. The thin films annealed under different S vapor pressures show notable grain growth compared to the WZ-derived CTS NC precursor thin film. The annealed films have thicknesses of ~0.9 µm, which is sufficient to produce the best solar cell performance. Formed MoS2 layer at the interface between CTS/Mo is marked by the red brown color. All scale bars are 1 μm.


12


(a) 20

(b)

80

CTS-1 CTS-2 CTS-3 CTS-4

CTS-2

70 60

EQE (%)

2

15 10

50 40 30 20

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10 0

0 0.00

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0.32

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Voltage (V)

Figure 5. (a) J–V curves for the devices derived from CTS absorbers annealed under different S vapor pressures measured under AM 1.5 illumination. (b) EQE of the best CTS NC-based TFSCs (CTS-2).

30

20

CTS-1 CTS-2 CTS-3 CTS-4

2

2

40

(a) dV/dJ (Ω cm )

40

dJ/dV (mS/cm )

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Current density (mA/cm )

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0.01 CTS-1 CTS-2 CTS-3 CTS-4

1E-3

1E-4

1E-5 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

V-RJ (V) Figure 6. (a) Plots of dJ/dV versus V redrawn from the standard dark J–V curve, where the Gsh value was determined from the plateau value, assuming that an ohmic (shunt) current dominates the diode current in this regime. (b) Plots of dV/dJ versus (J + -1 Jsc) , where the Rs value was extracted from the y-intercepts. (c) Semi-logarithmic plots of J + Jsc - GV versus V-RJ, where the Jo and nd values were extracted from the intercept and slope of the linear region of the curve, respectively.


13


12

0.3

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Gsh

0.2

2

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8

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6 4

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-2

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-5

η, Gsh (mS cm )

4

(a)

Jo x10 (mA/cm )

15 10

Pressure (Torr)

Figure 7. Device parameters of the CTS solar cells as a function of S vapor pressure. (a) Gsh, nd and J0 parameters; (b) Rs, Jsc, and FF parameters. The diode parameters were extracted from the J-V curve collected in the dark. The corresponding symbols show the error bars.

3.0 Hour

Days

Efficiency (%)

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 16

2.8 2.6 2.4

Room temperature

Humidity= 50% o

at 90 C

2.2 2.0

0

10 20 30 40 50

0 10 20 30 40 50

Average time Figure 8. Stability of the high-efficiency CTS-2 device stored in air at room temperature and under damp heating at 90 °C in normal laboratory conditions.


14

Page 15 of 16

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Table 1. Device characteristics obtained for CTS NC-based TFSCs derived from CTS absorbers annealed under different S vapor pressures.

Jsc

Device

Voc (V)

(mA/cm )

FF (%)

Efficiency (%)

Gsh 2 (mS/cm )

Rs 2 (Ω cm )

nd

J0 2 (mA/cm )

CTS-1

0.238 ± 0.02

15.13 ± 0.38

29 ± 3

1.04 ± 0.52

2.11

6.14

1.48

7.02 x 10

CTS-2

0.327 ± 0.02

20.12 ± 0.46

42 ± 2

2.77 ± 0.39

1.20

2.45

1.06

1.68 x 10

CTS-3

0.289± 0.02

17.92 ± 0.57

32 ± 3

1.67 ± 0.42

1.52

4.80

1.08

1.56 x 10

CTS-4

0.207 ± 0.02

14.72 ± 0.49

26 ± 4

0.79 ± 0.76

3.8

9.87

2.67

2.95 x 10

2


-5

-6 -5 -4

15


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Page 16 of 16

Table of Contents (TOC)


16

Colloidal Wurtzite Cu2SnS3 (CTS) Nanocrystals and Their

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