Introduction
Copper tin sulfide (Cu2SnS3) is a promising absorber material for thin film photovoltaic due to its suitable optoelectronic properties and attractive raw-material features. Depending on its crystal structure, Cu2SnS3 has direct band gaps ranging from 0.93-1.51 eV covering the visible and near-infrared part of solar spectrum [1,2]. It also has large absorption coefficient exceeding 104 cm-1 at energies right above the band gap, and possesses naturally p-type doping with hole mobility of 80 cm2/Vs [3]. On materials side, Cu2SnS3 is a compound composed of non-toxic, inexpensive and earth-abundant elements. Its ternary nature also contains less types of crystal defects as compared to copper zinc tin sulfoselenide (CZTSSe), reducing the complexicity of phase and defect control in CZTS system [4,5]. Consequently, thin film solar cells using Cu2SnS3 as the absorber received increased research interest in these years [3,6,7]. For example, Berg et al. produced monoclinic Cu2SnS3 film by annealing electrodeposited precursor in a sulfur and tin sulfide environment and achieved photovoltaic device efficiency of 0.54% [8]. In 2012, Koike et al. optimized the composition of electrodeposited Cu2SnS3 film and produced an impressive device efficiency of 2.84%, the highest efficiency for Cu2SnS3 solar cell reported so far [9].
In this paper, we presented the fabrication of Cu2SnS3 film using a hydrazine process. Hydrazine has strong capability to dissolve metal chalcogenide in the presence of excess chalcogens without introducing carbon and oxygen impurities and could result in device quality chalcogenide films after thermal annealing [10]. Consequently, high quality metal chalcogenide field effect transistors [11] and world record efficiency CZTSSe solar cells [12,13] were produced using hydrazine process. We thus adopted this strategy to produce high quality Cu2SnS3 film for material characterization and photovoltaic device application. Cu, Sn and S elements were directly dissolved into hydrazine to prepare the solution for spin-coating. The spin-coated film was then annealed inside a N2 filled glovebox to generate Cu2SnS3 film with phase-pure composition and large grain size. As produced Cu2SnS3 film was characterized as triclinic with an optical band gap of 0.88 eV, having p-type doping with hole mobility of 0.86 cm2/Vs. We further integrated our Cu2SnS3 film into a heterojunction solar cell achieving solar conversion efficiency of 0.78%.This study laid the foundation for the further advancement of earth abundant, low-cost Cu2SnS3 solar cells.
Experiments
Fabrication of Cu2SnS3 thin film
Cu2SnS3 thin film was fabricated from a layer-by-layer spin-coating method using hydrazine solution. First, 0.315 g Cu power and 0.238 g S flakes were weighted and then 3 mL hydrazine (containing 2% water) was quickly injected. Similarly, 0.356 g Sn powder and 0.384 g S flakes were weighted and then 3 mL hydrazine was quickly injected. After stirring for 2 days, a clear red (Cu-S) or light-green (Sn-S) hydrazine solution was obtained. These two solutions were mixed together with the addition of 10 μL Sb2S3 hydrazine stock solution and 25 μL Na2S hydrazine stock solution to prepare the precursor solution ready for spin-coating. Caution: Hydrazine is highly flammable and toxic; all operation should be done inside a glovebox with great care!
Molybdenum covered soda lime glass was carefully rinsed with ammonium solution (28%-30% concentration) and then used as the substrate for film deposition. A few drops of Cu2SnS3 precursor solution was spread onto the substrate, then spun (400 r/min, 4 s; 800 r/min, 20 s; 2100 r/min, 20 s) and baked at 80°C on a hotplate for 5 min to evaporate hydrazine. The film was further baked at 425°C for 2 min to remove excess S contained inside the film. This spin-dry-bake process was repeated for another 4 times to yield the film with desired thickness. Finally the film was baked at 600°C on a hotplate with the addition of excess sulfur for 20 min and then naturally cooled down on asbestos substrates. Extreme carefulness should be applied during the operation to minimize sudden temperature change and avoid glass breakage.
Heterojunction solar cell preparation
Heterojunction solar cell Mo/Cu2SnS3/CdS/i-ZnO/Al-ZnO/Au was fabricated as follows: first Cu2SnS3 film was prepared on Mo substrates according to the procedure described above. Then CdS buffer layer was deposited using the standard chemical bath deposition (CdSO4, thiourea, NH3∙H2O react at 65°C for 13 min) [14]; ZnO and Al doped zinc oxide (AZO) were subsequently sputtered onto the devices kept at 200°C. Sputtering condition for ZnO was 0.2 Pa Ar pressure, 100 W power for 8 min; for AZO was 0.2 Pa Ar pressure, 300 W power for 40 min. Finally Au electrodes were e-beam evaporated to complete the device. No antireflection layer was deposited, and the device was mechanically scribed to define the device area.
Materials and device characterization
Cu2SnS3 thin film and photovoltaic device were carefully characterized. Cu2SnS3 precursor solution was naturally dried inside glovebox to yield a gel for thermo gravimetric analysis (TGA). TGA was done on Perkin Elmer Instruments (Diamond TG/DTA6300) with a heating rate of 10°C/min under N2 flow. Power X-ray diffraction (Philips, X pert pro MRD), scanning electron microscopy (SEM, FEI Nova NanoSEM450, without Pt coating) and X-ray photoelectron spectroscopy (XPS, EDAX Inc. Genesis) measurements were done directly using the Cu2SnS3 film. For XPS measurement, C 1s peak at 284.8 eV was applied for calibration. Film transmittance was carried out on Cu2SnS3 film deposited onto fluorine-doped tin oxide (FTO) using Perkin Elmer Instruments (Lambda950) with an integrating sphere. For solar cell measurement, the device was mechanically scribed into 0.40 cm2 area. A solar simulator with a Xe light source (450 W, Oriel, model 9119) and an AM1.5G filter was used to produce the simulated 100 mW/cm2 solar irradiation. Current density-voltage characteristics were measured using a Keithley 2400 source-meter in the air. No intentional temperature control or aperture was used for the efficiency measurement.
Results and discussion
Film fabrication and optimization
Hydrazine process was applied to fabricate Cu2SnS3 thin film. First, Cu and excess S (molar ratio Cu:S= 1.0:1.5), and Sn and excess S (molar ratio Sn:S= 1.0:4.0) were mixed and stirred for days to yield a clear solution. For conventional hydrazine process, Cu2S and SnS2 were used as the Cu and Sn precursors [13,15]. We preferred metal precursor over metal sulfide because commercial Cu2S and SnS2 raw materials often deviate from perfect stoichiometry and contain impurities such as Cu2-xS and SnS2-x. The batch to batch variation often jeopardizes precise composition control and device reproducibility. In practice, we adjusted the amount of sulfur added and the speed of hydrazine injection to get fully dissolved Cu and Sn solution. No difference in terms of solution appearance and film morphology was observed when use metal instead of metal sulfide as the precursors. Metal power and S flakes might react in situ and experience different reaction pathways as compared to reaction using metal sulfide precursors, but they probably evolved into the same final products when fully dissolved into hydrazine. Figure 1(a) presents digital images of Cu-S and Sn-S hydrazine solution and the mixed Cu-Sn-S precursor solution. Both Cu-S and Sn-S solution were clear without any detectable precipitates; when mixed together, the solution remained clear, suggesting the molecular level dissolution of Cu and Sn into hydrazine [10]. Trace amount of Na and Sb dopants were introduced during the mixing to promote Cu2SnS3 defects passivation and grain growth, respectively, similar to the copper indium gallium selenide (CIGS) [16,17] and CZTSSe [12] case.
TGA of the dried Cu-Sn-S precursor solution was carried out to investigate the temperature dependent hydrazine and excess sulfur loss (Fig. 1(b)). Since the boiling temperature of hydrazine and melting point of sulfur are 114°C and 115°C, respectively, we attributed the first 5% weight loss (room temperature to 160°C) to the leaving of freely presented hydrazine and sulfur and the second abrupt 8% weight loss (160°C-200°C) to decomposition of solute species such as N4H9Cu7S4 and (N2H5)4Sn2S6. As proposed by Mitzi, the decomposition products for N4H9Cu7S4 and (N2H5)4Sn2S6 were N2H4 and H2S, Cu2S and SnS2 respectively [10]. The evaporation of Cu2S and more likely SnS2, a compound known for its high vapor pressure [18], accounted for a further gradual 7% weight loss up to 600°C. At temperature higher than 600°C, no further weight loss was observed which indicated that Cu2SnS3 formation was completed.
Since a total of 20% weight loss was accompanied during the whole heating process, layer-by-layer spin-coating was applied to fabricate Cu2SnS3 film because the cracks and pinholes associated with significant weight loss in the underlying layer could be filled by the subsequent layer on top [19]. In addition, the number of deposited layers controlled film thickness. Guided by this principle and the data from TGA measurement, we spun our Cu-Sn-S precursor solution and then applied a pre-bake step at 80°C to eliminate hydrazine, a soft-bake step at 425°C to remove excess sulfur and decompose the precursors. This process was repeated for another 4 times to yield a film with desired thickness. Finally the film was subjected to a hard-bake at 600°C to complete Cu2SnS3 formation and grain growth. The film fabrication procedure was shown as a flowing chart in Fig. 1(c), and the key optimization steps would be discussed later.
To achieve Cu2SnS3 film with good film morphology, the conditions for the hard-bake was crucial. For solar cell application, compact film without any cracks and pinholes is a must to avoid shunting problems, and large grain size is preferred to minimize recombination at grain boundaries and promote carrier transport within the grain [20]. Annealing temperature and environment are key parameters to achieve optimal film morphology. We first studied the hard-bake temperature. For these experiments, films were subjected to the same pre-bake and soft-bake steps, and 5 mg sulfur was added to the annealing environment, the only difference being the hard-bake temperature. Four temperatures were investigated: 450°C, 500°C, 550°C and 600°C. As shown in Fig. 2, top-view SEM characterization revealed that when hard-baked at temperature below 600°C, the film was porous containing many cracks and pores. Also, the grain size increased as the annealing temperature increased. When hard-baked at 600°C, the film was crack- and pinhole-free and the grains reached micrometer size. Higher temperature resulted in faster ions’ thermal movement, promoting diffusion and consequently promoting grain growth and sintering.
We further investigated the amount of sulfur addition during hard-bake at 600°C. Figure 3 showed the top-view and cross-sectional SEM images of Cu2SnS3 film baked at 600°C with the addition of 0, 1 and 10 mg sulfur powder. The amount of sulfur addition, or equivalently the sulfur vapor pressure, had little influence on the film thickness because for all three samples, Cu2SnS3 film was 1.0-1.3 μm thick. However, sulfur addition played a paramount effect on film morphology. When no sulfur presented, film was compact and the grain size was large. Unfortunately, there was a thin fluffy layer on top, in addition to some scattered points which were believed to be secondary phases. Energy dispersive X-ray analysis of Cu2SnS3 film surface observed more sulfur than its stoichiometric value. When 10 mg sulfur was introduced, the film was very porous containing many pores, and the grain size was not satisfactory. When 1 mg sulfur was presented, the film was crack-, pinhole- and secondary phase-free from the top view SEM observation, and the grains were large and compact from cross-sectional SEM observation. Such a high quality Cu2SnS3 is comparable to the state-of-art CZTS film [12] and paves its way for film characterization and device application. Sulfur pressure governed the delicate equilibrium between possible Cu2SnS3 decomposition, SnS and SnS2 evaporation, and grain sintering and growth, probably in a similar fashion as in the CZTS system [18]. The detailed underlying mechanism is a subject for further investigation.
Film characterization
We applied X-ray diffraction (XRD), Raman spectroscopy and absorption measurement to study the phase and optical properties of as-obtained film. Figure 4(a) presented the XRD patterns of our film. The position and relative intensities of diffraction peaks agreed very well with the standard triclinic Cu2SnS3 (JCPDS 027-0198). The standard patterns and the indices of major diffraction peaks were included for comparison and clarity. Apart from the Mo substrate, no secondary phase such as SnS and Cu2S was observed within XRD detection. Depend on the local cation coordination around anions, Cu2SnS3 has cubic, monoclinic and triclinic phases and monoclinic and triclinic are phases stabilized at low temperatures (<775°C) [21]. Literature reported that different film fabrication procedure and heat treatment history results in Cu2SnS3 film with different phases like tetragonal or monoclinic or cubic [22,23].Our hydrazine process probably favored cation distributions as in triclinic phase.
Fig.4 Characterization of Cu2SnS3 film. (a) XRD pattern of Cu2SnS3 film on Mo substrate. The major diffraction peaks of (2,1,1), (2,0,10) and (3,2,10) and the standard triclinic Cu2SnS3 diffraction peaks (JCPDS 027-0198) were included; (b) Raman spectrum of Cu2SnS3 film with the position of 4 peaks included; (c) absorption spectrum and corresponding (αhv)2 vs. (hv) fitting to extract the optical band gap of Cu2SnS3 film |
Raman spectrum is a standard tool to analyze the composition and phase of target substance. We applied Raman analysis to check our Cu2SnS3 film. Two dominant Raman peaks centered at 289 and 352 cm-1, characteristic of a′vibration symmetry of Cu2SnS3, was observed in Fig. 4(b). The two additional peaks with reduced intensities at 314 and 371 cm-1 were also originated from Cu2SnS3 phase, in concordance with previous literature report [21]. In a word, both XRD and Raman results confirmed our film was pure Cu2SnS3.
The value of band gap is crucial for a potential photovoltaic absorber material. Absorption measurement was applied to measure the absorption onset of Cu2SnS3 film. As shown in Fig. 4(c), Cu2SnS3 showed a strong absorption starting from ~1200 nm (black line). By plotting (αhv)0.5 vs. (hv) and fitting the linear part, we obtained the intersect with x-axis as 0.88 V, the direct band gap of Cu2SnS3. We could not get a nice linear zone when plot (αhv)2 vs. (hv), thus ruling out the possibility of Cu2SnS3being an indirect band gap material. This measured value supported previous theoretical simulation of Cu2SnS3 where a direct band gap of 0.8-0.9 eV was predicted [1]. This value, however, it is relatively small and deviated from the optimal value, 1.0-1.5 eV for thin film solar cells, suggesting that alloying with oxygen or germanium [24] to increase its band gap might be needed for high efficiency Cu2SnS3 based photovoltaic devices.
XPS was employed to investigate the film’s chemistry nature. Figure 5 presented the XPS spectra of Cu, Sn and S elements of fresh Cu2SnS3 sample. The binding energy of Cu 2p1/2 and 2p3/2 was 952.5 and 932.8 eV, respectively. Both peaks were characteristic of Cu+ [25]. Similarly, the binding energy of Sn 3d3/2 and 3d5/2, S 2p1/2 and 2p3/2 was 495.2 and 486.7 e V, 163.1 and 162.0 eV, in good agreement with Sn4+ and S2-, respectively. The perfect Gaussian-Lorentzian peak fitting for all these three peaks excluded the presence of Cu2+, Sn2+ and S in the sample within XPS detection limit. The right stoichiometry in combination with the desired element valences further confirmed our film was Cu2SnS3.
Hall effect measurement was further employed to study the electrical properties of our Cu2SnS3 film. Samples were prepared by depositing Cu2SnS3 film on insulating substrates with four Au electrodes at the corner as contacts. The measured Hall coefficient RH was 1.30 cm3/C confirming our Cu2SnS3film was p-type. Based on the equation that RH = 1/(pq), where p is hole concentration and q is elemental charge, we estimated hole concentration as 4.8 × 1018 cm-3. The measured film conductivity was 0.66 Ω-1 cm-1 and hole mobility was estimated to be 0.86 cm2/Vs. These values were similar to Cu2SnS3 film derived from direct liquid coating, where hole concentration of 3.71 × 1018 cm-1 and mobility of 0.93 cm2/Vs was obtained [6].
Device performance and stability
Last we checked the photovoltaic device performance using Cu2SnS3 as the absorber. Device configuration is schematically shown in Fig. 6(a). Mo coated soda lime glass was used as the substrates and back contact; CdS derived from chemical bath deposition was applied as the buffer layer to form the heterojunction with Cu2SnS3 absorber; ZnO was sputtered to increase device shunt resistance; AZO was also sputtered on top to serve as the transparent conductive oxide (TCO), and Au electrodes were e-beam evaporated to work as the top electrodes. Light was shed from the AZO side, passing from the AZO, ZnO and CdS layer and being absorbed by the Cu2SnS3 layer. Photogenerated carriers were separated at the Cu2SnS3/CdS interface; electrons injected into CdS layer and finally collected by the top Au electrodes, and holes travelled through the Cu2SnS3 layer and were finally collected by the bottom Mo electrodes.
The typical current density-voltage (J-V) curves of our Cu2SnS3 solar cells in the dark and fewer than 100 mW/cm2 simulated AM1.5G illumination were presented in Fig. 6(b). This device showed pretty poor rectification in the dark, suggesting the quality of Cu2SnS3/CdS heterojunction interface needs further improvement. Under light irradiation, the best device demonstrated an open circuit voltage (Voc) of 0.199 V, a short circuit current density (Jsc) of 14.2 mA/cm2, and a fill factor (FF) of 27.4%, corresponding to an energy conversation efficiency of 0.78%. Aside from this champion device, multiple devices with efficiency exceeding 0.6% have been successfully fabricated. The champion device showed a small shunt resistance of only 45.4 Ω and a large series resistance of 29.4 Ω, giving rise to the low FF. Our device characteristics echoed previous Cu2SnS3 solar cells where large Jsc yet low Voc were generally reported [8,9,23]. For example, Chino et al. presented Cu2SnS3 solar cell with champion device [23] showing Voc of 0.211 V and Jsc of 28 mA/cm2. Such a low Voc severely limited overall device performance. The underlying reason was unclear at present, and solving the low Voc associated with Cu2SnS3 photovoltaic devices should be the major thrust for the future research.
Finally, we checked the storage stability of our Cu2SnS3 device. Three representative devices A, B, and C were stored in ambient laboratory environment and their device performance was periodically measured. For all three checked devices, device efficiencies monotonically increased during ambient storage (Fig. 6(c)). The largest efficiency augment was observed in device A where the initial device efficiency was 0.35% and after 80 days ambient storage, device efficiency soared up to 0.72%. We compared in detail the device characteristics of solar cell B (shown in Fig. 6(d)): fresh device showed a Voc of 0.218 V, Jsc of 9.6 mA/cm2 and FF of 25.3%, corresponding to an efficiency of 0.53%; after 80 days laboratory storage the aged device showed a Voc of 0.229 V, Jsc of 12.1 mA/cm2 and FF of 25.5%, corresponding to an efficiency of 0.70%. Device efficiency improvement during ambient storage was also encountered in CIGS and CZTS solar cells, and this might be resulted from the improved interface quality such as the Cu2SnS3/CdS junction interface or the CdS/ZnO interface during storage. Mechanism study will be done to uncover the reason behind.
Fig.6 Solar cell configuration, performance and stability. (a) Schematic demonstration of the device configuration of Cu2SnS3 solar cell; (b) dark and light J-V curves of Cu2SnS3 solar cell. Light was simulated AM1.5G irradiation at an intensity of 100 mW/cm2; (c) efficiency evolution of three representative unencapsulated devices A, B, C when stored in lab environment for 0, 40, 55 and 80 days; (d) comparison of device characteristics Voc, Jsc, FF and η between fresh (0 day storage) and aged (80 days ambient storage) of Cu2SnS3 photovoltaic device B |
Conclusions
In summary, we have demonstrated fabrication of high quality Cu2SnS3 film from hydrazine process using Cu, Sn and S element as the precursors. By optimizing the amount of sulfur addition and annealing temperature, triclinic Cu2SnS3 film with high crystallinity and large grain size was obtained. The film has a direct band gap of 0.88 eV estimated from optical absorption measurement, and hall measurement showed the film was p-type with hole mobility of 0.86 cm2/Vs. Finally, Cu2SnS3/CdS heterojunction solar cells were constructed and achieved a solar conversion efficiency of 0.78%. Further research should focus on the increase of band gap through alloying and consequently Voc improvement to guarantee its promising application for photovoltaic devices.