Solution-processed Cu₂SnS₃ thin film solar cells

Abstract

Cu₂SnS₃ as a promising candidate for the next generation of thin film solar cells still lacks of further understanding and study. In the present study, a simple solution method is described to fabricate monoclinic Cu₂SnS₃ thin films and their solar devices. With the utilization of a S + SnS mixture as an annealing atmosphere, we are able to tune the carrier concentration by over 2 orders of magnitude and achieve films with p-type doping of less than 10¹⁶ cm⁻³. For comparison, pure S and the S + Sn mixture are also listed. Cu₂SnS₃ thin films annealed with S + SnS mixture show good crystallinity, uniform surface potential and less surface defects, as well as good performance in the final solar device. The present findings may also provide significant insight in the design of efficient Cu₂SnS₃ based solar devices based on these advantages.

---

1. Introduction

Cu-based compounds have been considered the most promising material for thin film solar cells due to their impressive photoelectric conversion efficiencies,¹ such as 20.8% for CuInGaSe₂ (CIGS) thin film solar cells,² 12.6% for Cu₂ZnSnS_4−xSe_x (CZTSSe) thin film solar cells,³ and 11.4% for CuInS₂ thin film solar cells.⁴ Recently, one type of Cu-based ternary compound, namely Cu₂SnS₃ (CTS), has attracted a lot of attention for thin film solar cells owing to its appropriate photoelectric properties,^5–7 including environmentally friendly components, high absorption coefficient (>10⁴ cm⁻³) and suitable band gap energy (in the range of 0.96–1.35 eV dependent on the different crystal structures). It is shown to have a conductivity of 0.5–10 S cm⁻¹, a hole mobility of 1–80 cm² V⁻¹ s⁻¹ and a hole concentration of 10¹⁸ cm⁻³. The highest record conversion efficiencies of CTS thin film solar cells have reached 4.63% with a pure CTS absorb layer⁸ and 6.0% with Ge-doped CTS.⁹

In the Cu–Sn–S system, there are many stable phases,^10–12 including Cu₂SnS₃, Cu₄SnS₄, Cu₂Sn₃S₇, Cu₅Sn₂S₇, Cu₁₀Sn₂S₁₃, and Cu₄Sn₇S₁₆. Due to the wide stability range and lack of Fermi level pinning,¹³ Cu₂SnS₃ is recognized as the most suitable material for thin film solar cells devices when compared with other stable phases of ternary copper tin sulfides. However, over recent years, the low performance has been a common problem in almost all the published studies based on pure CTS solar cells with the structure SLG (soda-lime glass)/Mo/CTS/CdS/ZnO/AZO.^14–16 The two main reasons for this¹³ are explained as: (1) the non-optimal structure of the final solar devices with CdS buffer layer and (2) the very high hole concentration in the CTS thin film as an absorb layer, usually >10¹⁸ cm⁻³. Some attempts have been used to improve the V_oc of CTS solar cells. Q. Chen et al.¹⁷ used In₂S₃ as a buffer layer in the structure of CTS solar cells, the V_oc has increased to 320 mV. D. Tiwari et al.¹⁸ used the graphite/CTS/ZnO/ITO/SLG structure in a CTS solar cell and the highest V_oc in all published studies was obtained at 816 mV. Although Cu₂SnS₃ is the most studied phase of the Cu–Sn–S systems reported, there is still a lack of knowledge about how to decrease the hole concentration, which is critical to progress further in device development. According to the report by Lauryn L. Baranowski et al.,¹³ CTS thin films with moderate hole concentrations must be grow with both Cu-poor and S-poor. However, there are a few reports that have confirmed this by experiment and reported the influence of fluctuated hole concentration on the final devices. Thus, it appears that further efforts and understanding are necessary to improve the performance of CTS based solar cells.

Table 1 shows the reported methods used for fabrication of CTS thin film and the efficiencies of their corresponding CTS solar cells. According to a literature survey,²³ in the past few years, many groups have elaborated on the fabrication of CTS thin films using different methods, including electron beam evaporation,^6,8 sputtering,^19,24 pulsed laser deposition (PLD),¹⁵ dip-coating,¹⁸ electro-deposition,¹⁶ chemical bath deposition (CBD),^20,25 solution methods,^21,22 and nano-inks.²⁶ Among these methods, the solution method, as a splendid technical method, has been widely used in the fabrication of Cu₂ZnSnS₄ (CZTS)²⁷ thin film solar cells and an efficiency of over 8% has already been obtained. However, reflecting on the fabrication of CTS, it still stays in the thin film fabrication and no practical devices have been made and no efficiencies of CTS based solar cells by solution method have been reported. In 2013, reported by Sandra Dias et al.,²¹ a tetragonal phase of CTS thin film with a (112) preferential orientation was obtained by drop-casting the CTS precursor solution, which was prepared by dissolving CuCl₂, SnCl₂, and thiourea into 10 mL of methanol. In 2015, H. Dahman et al.²² fabricated pure cubic CTS structure with a (111) preferential orientation by spin-coating the CTS precursor solution, which was prepared by dissolving C₄H₆CuO₄·H₂O (1 mmol), SnCl₂·2H₂O (0.5 mmol) and thiourea in methanol. Although there are no actual devices and efficiencies, the light for hope was kindled.

Table 1 The methods and efficiencies of pure CTS based solar cells

Method	Voc (V)	Jsc (mA cm^−2)	Fill factor	Efficiency	Reference
Electron beam evaporation	0.283	37.30	43.90	4.63%	8
Sputtering	0.243	26.20	47.90	3.05%	19
Co-electrodeposition	0.249	29.30	39.00	2.84%	16
Dip-coating	0.816	6.14	42.00	2.10%	18
CBD	0.157	19.20	31.90	0.96%	20
PLD	0.260	11.90	24.00	0.82%	15
Solution method	—	—	—	—	21 and 22

---

In this study, an optimized solution method was introduced to fabricate CTS thin films. As mentioned above, there are two orientations that improve the performance of CTS solar cells, namely, a appropriate p–n structure of the CTS solar cell and an appropriate hole concentration in the CTS thin film. In the present study, to obtain an appropriate hole concentration, a study of designs with S-poor was taken out. As the result of using Sn-compounds (Sn, SnS) during the annealing process, the hole concentration was decreased by nearly two orders of magnitude, to around 10¹⁶ cm⁻³. In addition, we present a detailed analysis and comparison of the characteristics of the CTS thin films and solar cells using different Sn-compounds during the annealing process, including the morphologies, surface potential, phase structures and photoelectric properties. Furthermore, the CTS solar cells prepared via a solution method were also fabricated and a J_sc value of more than 15 mA cm⁻² was obtained in the present study using the CTS thin film that was annealed under a mixed atmosphere of S + SnS.

2. Experimental details

2.1 CTS thin films and devices

Fig. 1 shows a schematic of the whole process of our designed experiment. The precursor solution was prepared by dissolving Cu(NO₃)₂·H₂O (0.31 mol L⁻¹, AR) and SnCl₂·2H₂O (0.21 mol L⁻¹, AR) into 2-methoxyethanol (AR) with stirring at room temperature for 20 min to obtain a blue-white solution. Then, CH₃CSNH₂ (0.7 mol L⁻¹, AR) was added until the blue-white solution was completely converted into a brown and transparent solution. As it is an exothermic process, the solution should be maintained at low temperature and −10 °C was introduced to stable the solution for a long time (usually no more than a month). Note that while operating the spin-coating process, an ice-water was introduced to maintain the stability of the precursor solutions. All chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd.

Fig. 1 A schematic of the experimental process.

The CTS precursor solution was spin coated on a molybdenum-coated and soda-lime glass (SLG) substrate at 2800 rpm for 30 s followed by annealing at 270 °C for 2 min on a hot plate in air. This coating step was repeated 12 times to obtain thick CTS xerogel precursors. Subsequently, the prepared xerogel precursors were annealed at 350 °C in an Ar atmosphere for 30 min to remove the organics that were used in the preparation of the CTS precursor solution. Then, the pre-heated precursor films were annealed at an appropriate temperature (600 °C) in an S atmosphere for 20 min with controlled sulfur partial pressure by adding Sn-compounds to obtain the desired hole concentration and crystallinity.

Finally, the CTS based solar devices were completed, without any anti-reflection layers and etching steps, by CBD of CdS (50 nm), RF sputtering of i-ZnO (50 nm) and AZO (250 nm). A similar preparation process for the CTS thin film was performed. The coating step was repeated 6 times and the thickness of the CTS thin films in the final devices was about 414 nm. The typical structure of the CTS cells was Al/AZO/ZnO/CdS/CTS/Mo/soda-lime glass substrate, which is similar to our previous reports.²⁸

2.2 Characterization and analysis

The surface and cross-section morphologies of the CTS thin films were characterized by field emission SEM (FE-SEM siron 200). The roughness and surface potential were characterized by Kelvin probe force microscopy (KPFM). The crystallinity and preferred orientations of samples were investigated by X-ray diffraction (XRD) using a Bruker Advance D8 diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.5406 Å). Further identification of the phase of the CTS thin films were characterized by Raman analysis at room temperature using a LABRAM-HR micro-Raman system in the back scattering configuration with a laser source of 532 nm. The valence states in the CTS thin film were detected by X-ray photoelectron spectroscopy (XPS). Finally, CTS based solar devices are characterized using the oriel AAA solar simulator under standard AM1.5 illumination (100 mW cm⁻², 25 °C) and the external quantum efficiency (EQE, Model SPIEQ200) was measured using a single source illumination system (halogen lamp) combined with a monochromator.

3. Results and discussion

3.1 CTS thin film fabrication

The solution method has been successfully utilized in the fabrication of CZTS and CuInSe₂ thin film solar cells.^29–31 In general, in non-aqueous sol–gel processes, metal salts react with alcohols to form metal alkoxides. Then, nanoparticles are formed after alcoholysis and polycondensation between the metal salt and metal alkoxides.³² In the present study, thioacetamide (TA) was introduced to replace thiourea (Tu) as the sulfur source during the fabrication of the CTS thin films. Metal–S complexes were formed in the final precursor solution. A similar mechanism for the formation of the precursor solution has been reported.³³ Many important parameters, such as the concentration of the solution, heating rate, heating temperature, additives and substrates, are critical for this process to obtain high-quality films without cracks and cavities. Fig. 2 shows the thermogravimetric analysis (TGA) results. The first weight-loss (WL) stage from 100 °C to 300 °C indicates the thermal decomposition of the metal–TA–oxygen complexes and other organic solvents. Above 300 °C, the weight loss was relatively slower due to the decomposition of sulfides. Based on the TGA, pre-heating at 350 °C for 30 min was introduced to remove the organic solvents that used during the preparation of the precursor solution.

Fig. 2 TGA of the CTS precursor solution.

To obtain the desired thick films, the coating step was repeated 12 times. The experimental details are shown in Fig. S1 in the ESI.† After pre-heating, the thickness of the precursor film was decreased from 1.3 μm to 820 nm. Then, the pre-heated precursor films were annealed at different temperatures in an S atmosphere for 20 min. In the present study, the annealing temperature range from 500–600 °C was studied and the morphologies of the surface and cross-section of the CTS films are shown in Fig. 3. It can be obviously found that the grain size was increased with increasing annealing temperature. For this section, while the temperature was under 560 °C, the bottoms of all the CTS films comprised small grain sizes and the tops comprised some abnormal larger particles. Relatively, uniform CTS thin films were obtained, whereas the annealing temperature was above 580 °C.

Fig. 3 SEM images of the CTS thin films annealed at various temperatures from 500 to 600 °C.

Combining the literature survey^34,35 and our previous study,²⁰ it is difficult to rule out the possibility of what type of CTS phases were prepared using only XRD analysis due to the overlap of the diffraction peaks, including cubic CTS (ICDD01-089-2877), tetragonal CTS (ICDD01-089-4714), monoclinic CTS (ICDD04-010-5719) and triclinic CTS (JCPDS27-0198). On this occasion, Raman analyses at room temperature were always utilized for phase identification. Fig. 4 shows the Raman spectra of the CTS thin films annealed at various temperature ranging from 500 °C to 600 °C using laser excitation at 532 nm. The Raman peaks reported for the different CTS phases¹² are at 336 and 351 cm⁻¹ for tetragonal CTS; 303 and 355 cm⁻¹ for cubic CTS; 290 and 352 cm⁻¹ for the monoclinic CTS; and 318, 348 and 295 cm⁻¹ for orthorhombic Cu₃SnS₄. In the present study, the dominant peaks at 290 cm⁻¹ and 351 cm⁻¹ are shown in Fig. 4, which are usually attributed to the monoclinic CTS phase that is also similar to that reported by Z. Jia et al.³⁶ However, similar to the findings reported by D. M. Berg et al.,³⁷ some unexpected secondary phases are also indicated in the Raman analysis. The spectra shown in Fig. 4 reveal two weak peaks at 313 cm⁻¹ and 374 cm⁻¹, which suggests that Cu₂Sn₃S₇ is one possible candidate for the secondary phase. By combining the results from the phase structure analyses and the reports of other researchers, the monoclinic CTS thin film was obtained via the solution method with an annealing temperature of above 560 °C in this study.

Fig. 4 The Raman spectra of the CTS thin films annealed at various temperatures from 500 to 600 °C.

3.2 Reduction of carrier concentration

To obtain an appropriate hole concentration, a study with a S-poor design was taken out, namely, adding partial Sn-compounds instead of a pure S atmosphere in the annealing process. Sn and SnS were introduced as Sn-compounds. Three types of S atmosphere were marked as “S”, “S + Sn”, and “S + SnS”, respectively. To ensure the accuracy of this study, the amount of S powder in the present study was maintained at constant values (about 500 mg) and the same molar quantity of Sn and SnS powder were added as the mixed annealing atmosphere. Based on the results obtained from the fabrication of the CTS thin films mentioned above, 600 °C for 20 min was chosen for the following studies. Table 2 shows the results of the Hall measurements. As expected, the hole concentration was continuously decreased from 10¹⁸ to 10¹⁶ cm⁻³ upon the utilization of Sn and SnS. In addition, the values obtained for the Hall mobility were continuously increased from 2.14 to 17.03 cm² V⁻¹ s⁻¹, which would increase the separation of the electron–hole pairs and increase the performance of the final solar devices. However, no continuous decrease was demonstrated while continuously increasing the amount of SnS (as shown in Table S1 of the ESI†). Two possible explanations³⁸ for the increase in hole mobility are the reduction in grain boundary scattering due to grain growth during annealing or a reduction in the ionized defect density.

Table 2 Hall measurements of the CTS films

Sample	Specific resistance	Square resistance	Hall coefficient	Hall mobility	Carrier concentration
S	2.693 × 10^0 Ω cm	6.732 × 10^4 Ω □^−1	5.762 × 10^0 cm^3 C^−1	2.140 × 10^0 cm^2 V^−1 s^−1	1.083 × 10^18 cm^−3
S + Sn	1.245 × 10^0 Ω cm	2.490 × 10^4 Ω □^−1	1.180 × 10^1 cm^3 C^−1	9.477 × 10^0 cm^2 V^−1 s^−1	5.290 × 10^17 cm^−3
S + SnS	6.490 × 10^0 Ω cm	1.298 × 10^5 Ω □^−1	1.105 × 10^2 cm^3 C^−1	1.703 × 10^1 cm^2 V^−1 s^−1	5.649 × 10^16 cm^−3

---

The surface morphologies of the CTS thin films annealed under different types of S atmosphere with Sn and SnS are shown in Fig. 5. It was apparent that all the films were almost the same, non-porous and non-cracked. For comparison, the roughness of these films was measured. By comparing the other two CTS thin films annealed with pure S and a S + Sn mixture, the CTS thin film annealed under a S + SnS mixture during the annealing process shows a uniform grain size and good surface roughness with 57.8 nm. To refine our study, we studied the properties of the grain boundaries (GBs) in the CTS thin films using Kelvin probe force microscopy (KPFM), as shown in Fig. 6. Fig. 6a–c shows the three-dimensional surface potential spatial maps of the CTS thin films annealed under a S, Sn, and SnS atmosphere. Overall, the average surface potential difference remains at a low level (about 5.08 mV) and the lowest value of 3.53 mV was obtained with the CTS thin film annealed under a mixed S + Sn atmosphere. The surface potential fluctuations are extremely small when compared to those of CIGS³⁹ and CZTS⁴⁰ films (generally >100 mV). In addition, some obvious bright lines (Fig. 6b marked by a dotted yellow circle) and points (Fig. 6a marked by a dotted yellow circle) are shown in the CTS thin films annealed under an S and mixed S + Sn atmosphere, which indicates the non-uniform surface potential and surface defects.⁴¹ Furthermore, the contact potential differences along the dashed lines drawn in Fig. 6a–c around those non-uniform areas are shown in Fig. 6d. In an illustrative line scan crossing the GBs of the CTS thin films annealed under a mixed S + SnS atmosphere, the surface potential difference between two grains is as low as 20 mV, which indicates a lack of significant band bending and surface defects in the CTS thin films. As expected, the surface potential difference between two grains of the CTS thin films annealed under a pure S and mixed S + Sn atmosphere are almost twice than that formed under a mixed S + SnS atmosphere.

Fig. 5 The morphology and roughness of the CTS thin films annealed under different atmospheres: (a–d) pure S atmosphere, (b–e) mixed S + Sn atmosphere, and (c–f) mixed S + SnS atmosphere.

Fig. 6 The three-dimensional surface potential spatial maps for the CTS thin films annealed under an (a) S, (b) S + Sn, and (c) S + SnS atmosphere. (d) The contact potential difference along the dash lines tested by KPFM.

After the utilization of different valence states of Sn-compounds during the annealing process, the phase structure, UV-vis and XPS analyses were investigate to study their influence on the CTS thin films. Fig. 7 shows the Raman, XRD and UV-vis spectra of the CTS thin films. No other secondary phases were found in both the XRD and Raman spectra, except Cu₂Sn₃S₇, which always coexists with Cu₂SnS₃, while the ratio of Cu/Sn deviated from the stoichiometry.¹⁰ The half width at half maximum (FWHM) of the CTS thin film annealed under a mixture S + SnS and S + Sn show low values, which indicates the good crystallinity when compared with that obtained under a pure S atmosphere. It is in good accordance with the results obtained from the Hall measurements that demonstrate the increasing hole mobility. The influence of the amount of SnS powder on the CTS was also presented and more information on the residue obtained after the annealing process is shown in Fig. S2 and 3 of the ESI.† The same Sn-compounds, i.e., SnS₂, were formed. Sn₃S₄ coexisted with SnS₂ while excessive SnS was added. The absorption profiles of the CTS thin films under various annealing atmospheres were investigated in the UV-visible region shown in Fig. 7c and d. It is clear that the sample absorbs radiation spanning the whole range of the visible spectrum and all the samples show a relatively high absorbance. An obvious difference was demonstrated in the long wavelength region around 1000 nm, as shown in Fig. 7c, and the film annealed under a S + SnS mixture shows the highest absorbance. In addition, calculation of the band gap was also carried out from the UV-vis spectra. Calculation of the band gap of the films, which was determined by the differential of the UV-vis spectra (as shown in Fig. 7d), gives the values of E_g for all the samples at around 0.94 eV, which is in good agreement with the values reported in the literature.⁴²

Fig. 7 (a) Raman, (b) XRD, and (c and d) UV-vis spectra of CTS annealed under different atmospheres: S, S + Sn, and S + SnS.

As mentioned above, the different valence states of the Sn-compounds used in the annealing process may affect the CTS thin films and further studies on the valence state of all the elements in the CTS films were performed and are shown in Fig. 8. The binding energies obtained from the XPS analysis were standardized for specimen charging using C (1s) as the reference at 284.8 eV. The Cu(I) 2p core splits into 2p3/2 (931.4 eV) and 2p1/2 (951.2 eV) peaks, which is consistent with the standard separation of 19.8 eV.⁴³ No 942 eV peak was found in Fig. 8b, indicating no existence of the Cu 2p3/2 satellite peak of Cu(II). Sn(IV) was confirmed by a peak splitting of 8.4 eV for the two peaks at 486 and 494.4 eV. The S 2p core level spectrum shows two peaks for 2p3/2 and 2p1/2 at 161.2 and 162.2 eV, respectively, with a peak splitting about 1 eV, which are consistent with the 160–164 eV range expected for S in sulfide phases. Almost no differences were demonstrated in all the films. When combined with the results obtained from XRD and Raman spectroscopy, we can infer that tiny influences can be observed in the phase structure upon adding different valence states of Sn-compounds during the annealing process.

Fig. 8 The XPS analyses of the CTS thin films annealed under different atmospheres: S, S + Sn, and S + SnS.

3.3 Photoelectric performance

Furthermore, the preliminary CTS based solar cells were also produced with the films mentioned above annealed under different S atmospheres. Fig. 9 shows the (a) light and (b) dark J–V characteristics of the CTS based solar cells annealed under different atmospheres: S, S + Sn, and S + SnS. More detailed results are shown in Table S2 of the ESI.† Although the whole performance of the CTS based thin film solar cells are relatively low, as expected, the best device performance was obtained with the sample annealed under a mixture of S + SnS vapor; the highest J_sc value obtained was over 15 mA cm⁻². This can be attributed to the grain size partially contributing to the performance of the final devices reported in the present study. Large grains generally benefit device performance because of the lower opportunity for recombination of the photo-generated carriers at the grain boundaries.⁴⁴ In addition, the highest absorbance of the CTS thin film was in the range of the visible spectrum with the utilization of a S + SnS mixture also contributing to the highest J_sc value. Moreover, the highest value for the Hall mobility was 17.03 cm² s⁻¹ V⁻¹ and was demonstrated by the CTS thin film annealed under the mixed S + SnS atmosphere mentioned above, which would increase the separation of electron–hole pairs and increase the performance of final solar devices. The dark J–V characteristics (shown in Fig. 9b) show a relatively high leakage current in all the CTS based solar cells and the poor p–n junction formed between the CTS and CdS layers. The leakage current is a result of carrier recombination in the depletion layer on either side of the junction. The small V_oc value observed in our CTS based solar cells relative to the planar cell is thought to originate from the large forward current formed under dark conditions. The low value for the V_oc (about 153 mV in our study) when compared to the E_g (about 0.94 eV) indicates that the main recombination was band-to-band along with a combination at the deep traps within the band gap. The poor p–n junction formed between the CTS thin films and CdS layers and the numerous recombination at the interface of CdS/CTS, which can be inferred by the dark J–V measurements of our studies may also contribute to the low value for the V_oc. In addition, the unexpected secondary phases (e.g. Cu₂Sn₃S₇), mentioned above in the analyses of the Raman spectra, are favorable for the recombination of current in the p–n junction diode.

Fig. 9 (a) Light and (b) dark J–V characteristics of the CTS based solar cells annealed under different atmospheres: S, S + Sn, and S + SnS.

The external quantum efficiency (EQE) curves for the CTS thin film solar cells annealed under different types of annealing atmosphere are presented in Fig. 10a. The EQE starts slowly at about 400 nm due to the cut-off of the CdS buffer layer and the ZnO window layer. Again, as expected, the film annealed under a mixed S + SnS atmosphere shows the highest value and the maximum quantum efficiency of CTS reached the value of 37% at around 520 nm. Fig. 10b shows the derivative of EQE as a function of wavelength, which was used to estimate the effective band gap of the absorbers. According to the inverse correlation between photon energy and wavelength, the band gap of all the CTS absorbers can be calculated to be around 0.94 eV from the peak at around 1320 nm, which is ideal for applications as an absorber layer in solar energy devices as mentioned above. Similar to the results obtained from the UV-vis analysis, all the post annealing CTS thin films show the same band gaps. However, there are satellite peaks observed in both the UV-vis spectra (Fig. 7c and d marked with a green circle) and EQE spectra (Fig. 10a and b marked with ①) at around 1180 nm, which can indicate a secondary band gap with a value of 1.05 eV. No similar results have been reported in other Cu-based solar devices.^45–48 In the CTS based solar cells, it is a unique characteristic that has frequently and deceptively arisen.^12,14,19,20 As mentioned in previous reports,^6,14,20 it may be attributed to the secondary phase and other phase structures of CTS itself, because of the fluctuant band gaps in a range of 0.95–1.35 eV, which are dependent on the different crystal structures and the coexistence of a secondary phase in CTS. The future study is under way to make this clear.

Fig. 10 (a) The EQE characteristic curve for the CTS thin film solar cells and (b) the band gaps of the CTS thin films calculated by the derivative of the EQE as a function of wavelength.

4. Conclusions

In summary, CTS thin films and based devices were successfully prepared via a solution and spin-coating method. In the present study, the monoclinic CTS thin films were obtained using this solution method with an annealing temperature of above 560 °C. To tune the hole concentration to an appropriate value, Sn-compounds were introduced as a mixed annealing atmosphere during the annealing process. When comparing a pure S and S + Sn mixture, the CTS thin films annealed with a S + SnS mixture showed good crystallinity, the most uniform surface potential and the least surface defects, as well as the best performance in the final solar device. By controlling the amount of SnS, we were able to tune the carrier concentration at over 2 orders of magnitude and achieve films with p-type doping of less than 10¹⁶ cm⁻³. By further study, no negative influences in the phase structure and valence state of all elements in the CTS films were observed using the different valence states of Sn-compounds during the annealing process. Finally, as expected, a preliminary CTS based solar cell annealed under a mixed S + SnS atmosphere shows the best performance with the highest J_sc value of over 15 mA cm⁻². However, the low V_oc value was still a common problem in the CTS based solar cells with SGL/Mo/CTS/CdS/ZnO/AZO structure. As mentioned above, future study is under way to find an appropriate n-type buffer layer to improve the performance of the p–n structure based on CTS layers, which may provide an important breakthrough in the realization of high performance CTS based solar cells.

Acknowledgements

This study was supported by the National Basic Research Program of China (973 Program)-2012CB922001, the Fundamental Research Funds for the Central Universities, No. WK2060140022, and the National Science Foundation for Young Scholars of China (KJ2060140010).

References

1. M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog. Photovoltaics, 2015, 23, 1–9.
2. M. Powalla, P. Jackson, D. Hariskos, S. Paetel, W. Witte, R. Würz, E. Lotter, R. Menner and W. Wischmann, 29th European Photovoltaic Solar Energy Conference, 2014.
3. W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu and D. B. Mitzi, Adv. Energy Mater., 2014, 4, 1301465.
4. D. Braunger, D. Hariskos, T. Walter and H. Schock, Sol. Energy Mater. Sol. Cells, 1996, 40, 97–102.
5. T. A. Kuku and O. A. Fakolujo, Sol. Energy Mater., 1987, 16, 199–204.
6. N. Aihara, H. Araki, A. Takeuchi, K. Jimbo and H. Katagiri, Phys. Status Solidi C, 2013, 10, 1086–1092.
7. S. Dias and S. Krupanidhi, AIP Adv., 2014, 4, 037121.
8. M. Nakashima, J. Fujimoto, T. Yamaguchi and M. Izaki, Appl. Phys. Express, 2015, 8, 042303.
9. M. Umehara, Y. Takeda, T. Motohiro, T. Sakai, H. Awano and R. Maekawa, Appl. Phys. Express, 2013, 6, 045501.
10. S. Fiechter, M. Martinez, G. Schmidt, W. Henrion and Y. Tomm, J. Phys. Chem. Solids, 2003, 64, 1859–1862.
11. Z. Su, K. Sun, Z. Han, F. Liu, Y. Lai, J. Li and Y. Liu, J. Mater. Chem., 2012, 22, 16346.
12. N. Mathews, J. T. Benítez, F. Paraguay-Delgado, M. Pal and L. Huerta, J. Mater. Sci.: Mater. Electron., 2013, 24, 4060–4067.
13. L. L. Baranowski, P. Zawadzki, S. Christensen, D. Nordlund, S. Lany, A. C. Tamboli, L. Gedvilas, D. S. Ginley, W. Tumas and E. S. Toberer, Chem. Mater., 2014, 26, 4951–4959.
14. D. M. Berg, R. Djemour, L. Gütay, G. Zoppi, S. Siebentritt and P. J. Dale, Thin Solid Films, 2012, 520, 6291–6294.
15. S. Vanalakar, G. Agawane, A. Kamble, C. Hong, P. Patil and J. Kim, Sol. Energy Mater. Sol. Cells, 2015, 138, 1–8.
16. J. Koike, K. Chino, N. Aihara, H. Araki, R. Nakamura, K. Jimbo and H. Katagiri, Jpn. J. Appl. Phys., 2012, 51, 10NC34.
17. Q. Chen, X. Dou, Y. Ni, S. Cheng and S. Zhuang, J. Colloid Interface Sci., 2012, 376, 327–330.
18. D. Tiwari, T. K. Chaudhuri, T. Shripathi, U. Deshpande and R. Rawat, Sol. Energy Mater. Sol. Cells, 2013, 113, 165–170.
19. R. Chierchia, F. Pigna, M. Valentini, C. Malerba, E. Salza, P. Mangiapane, T. Polichetti and A. Mittiga, Phys. Status Solidi C, 2016, 1, 35–39.
20. J. Li, C. Xue, Y. Wang, G. Jiang, W. Liu and C. Zhu, Sol. Energy Mater. Sol. Cells, 2016, 144, 281–288.
21. S. Dias, M. Banavoth and S. Krupanidhi, Proceeding of international conference on recent trends in applied physics and material science: RAM, 2013, pp. 525–526.
22. H. Dahman and L. El Mir, J. Mater. Sci.: Mater. Electron., 2015, 26, 6032–6039.
23. A. Lokhande, K. Gurav, E. Jo, C. Lokhande and J. H. Kim, J. Alloys Compd., 2016, 656, 295–310.
24. P. Fernandes, P. Salomé and A. Da Cunha, Phys. Status Solidi C, 2010, 7, 901–904.
25. R. Becerra, J. Correa, H. Suarez and G. Gordillo, J. Phys.: Conf. Ser., 2014, 012008.
26. S. Vanalakar, G. Agawane, S. Shin, H. Yang, P. Patil, J. Kim and J. Kim, Acta Mater., 2015, 85, 314–321.
27. F. Liu, F. Zeng, N. Song, L. Jiang, Z. Han, Z. Su, C. Yan, X. Wen, X. Hao and Y. Liu, ACS Appl. Mater. Interfaces, 2015, 7, 14376–14383.
28. J. Li, Y. Zhang, Y. Wang, C. Xue, J. Liang, G. Jiang, W. Liu and C. Zhu, Sol. Energy, 2016, 129, 1–9.
29. K. Tanaka, N. Moritake and H. Uchiki, Sol. Energy Mater. Sol. Cells, 2007, 91, 1199–1201.
30. K. Tanaka, Y. Fukui, N. Moritake and H. Uchiki, Sol. Energy Mater. Sol. Cells, 2011, 95, 838–842.
31. P. Luo, Z. Liu, Y. Ding and J. Cheng, J. Power Sources, 2015, 274, 22–28.
32. Z. Su, K. Sun, Z. Han, H. Cui, F. Liu, Y. Lai, J. Li, X. Hao, Y. Liu and M. A. Green, J. Mater. Chem. A, 2014, 2, 500–509.
33. Y. C. Choi, E. J. Yeom, T. K. Ahn and S. I. Seok, Angew. Chem., Int. Ed., 2015, 54, 4005–4009.
34. P. Zawadzki, L. L. Baranowski, H. Peng, E. S. Toberer, D. S. Ginley, W. Tumas, A. Zakutayev and S. Lany, Appl. Phys. Lett., 2013, 103, 253902.
35. U. Chalapathi, Y. Jayasree, S. Uthanna and V. Sundara Raja, Phys. Status Solidi A, 2013, 210, 2384–2390.
36. Z. Jia, Q. Chen, J. Chen, T. Wang, Z. Li and X. Dou, RSC Adv., 2015, 5, 28885–28891.
37. D. M. Berg, R. Djemour, L. Gütay, S. Siebentritt, P. J. Dale, X. Fontane, V. Izquierdo-Roca and A. Pérez-Rodriguez, Appl. Phys. Lett., 2012, 100, 192103.
38. L. L. Baranowski, K. McLaughlin, P. Zawadzki, S. Lany, A. Norman, H. Hempel, R. Eichberger, T. Unold, E. S. Toberer and A. Zakutayev, Phys. Rev. Appl., 2015, 4, 044017.
39. C.-S. Jiang, R. Noufi, J. AbuShama, K. Ramanathan, H. Moutinho, J. Pankow and M. Al-Jassim, Appl. Phys. Lett., 2004, 84, 3477–3479.
40. J. B. Li, V. Chawla and B. M. Clemens, Adv. Mater., 2012, 24, 720–723.
41. Y. Zhou, L. Wang, S. Chen, S. Qin, X. Liu, J. Chen, D.-J. Xue, M. Luo, Y. Cao, Y. Cheng, E. H. Sargent and J. Tang, Nat. Photonics, 2015, 9, 409–415.
42. K. Chino, J. Koike, S. Eguchi, H. Araki, R. Nakamura, K. Jimbo and H. Katagiri, Jpn. J. Appl. Phys., 2012, 51, 10NC35.
43. J. F. Moulder, J. Chastain and R. C. King, Handbook of X-ray photoelectron spectroscopy: a reference book of standard spectra for identification and interpretation of XPS data, Perkin-Elmer, Eden Prairie, MN, 1992.
44. T. K. Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu and D. B. Mitzi, Adv. Energy Mater., 2013, 3, 34–38.
45. Q. Guo, G. M. Ford, W.-C. Yang, B. C. Walker, E. A. Stach, H. W. Hillhouse and R. Agrawal, J. Am. Chem. Soc., 2010, 132, 17384–17386.
46. P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann and M. Powalla, Prog. Photovoltaics, 2011, 19, 894–897.
47. S. Ahn, T. H. Son, A. Cho, J. Gwak, J. H. Yun, K. Shin, S. K. Ahn, S. H. Park and K. Yoon, ChemSusChem, 2012, 5, 1773–1777.
48. A. Welch, L. Baranowski, P. Zawadzki, S. Lany, C. Wolden and A. Zakutayev, Appl. Phys. Express, 2015, 8, 082301.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09389b

---