Cu₂ZnSnS₄ thin film solar cell utilizing rapid thermal process of precursors sputtered from a quaternary target: a promising application in industrial processes

Abstract

Cu₂ZnSnS₄ (CZTS) thin films have been attracting considerable attention as candidates for new photovoltaic materials. As a typical vacuum process, a sputtering stacked metallic layer followed by a conventional slow thermal process (STP) is usually used. This method is complex and time-consuming. Furthermore, the volatilization of Zn and Sn elements is significant during the STP process. To simplify the CZTS fabrication process and solve the element volatilization problem, in this work CZTS thin film was fabricated using a single quaternary target Radio-Frequency (RF) magnetron sputtering process followed by a rapid thermal process (RTP). The effects of sulfurization temperature on the properties of CZTS thin films have been studied. The compositional analysis shows that a combination of a single target sputtering process and the RTP technique can significantly reduce the volatilization of Zn and Sn elements compared to the conventional STP process. The results of X-ray diffraction (XRD) patterns and Raman scatting spectra show that the sulfurized CZTS thin films have a polycrystalline kesterite crystal structure. If the sulfurization process is performed at lower temperature, a large amount of disorder among the Cu and Zn cations exists in the CZTS thin film which is investigated by using Raman scattering spectra. At 550 °C, the CZTS thin film has high quality of crystallinity with large grain size and dense morphology, its band gap energy is found to be 1.53 eV. The solar cell fabricated with the CZTS absorber grown at an optimized sulfurization temperature of 550 °C shows a conversion efficiency of 2.85% for a 0.16 cm² area with V_oc = 412 mV, J_sc = 17.9 mA cm⁻², and FF = 40.5%. These results show that this process is suitable for the growth of kesterite CZTS solar cell absorbers.

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1. Introduction

Kesterite Cu₂ZnSnS₄ (CZTS) thin film is one of the most promising materials as an alternative low-cost absorber for thin film photovoltaic devices. It is known that CZTS thin film has a direct band gap of 1.4–1.6 eV and a high absorption coefficient above 10⁴ cm⁻¹.^1–4 Moreover, compared to other commercial CdTe and Cu(In,Ga)Se₂ absorbers, it has the advantage that all of the constituent elements in CZTS are naturally abundant and non-toxic.^1,2 The theoretical limit of power conversion efficiency (PCE) is as high as 32% based on Shockley–Queisser photo balance calculation.⁵ Nowadays, great efforts have been devoted to fabrication of CZTS thin film solar cells. Up to now the best PCE of CZTS thin film solar cell has been reached 8.4%, which is fabricated by thermal evaporation process.⁶ However, it is very difficult to apply the evaporation technique to large-scale production due to the difficulty in uniformity control on large area. Hence, it is necessary to explore an industrialization process route to fabricate CZTS thin film.

Numerous vacuum and non-vacuum based deposition techniques have been employed for the synthesis of CZTS thin films.^6–11 Among all of these techniques, magnetron sputtering is extensively used in the semiconductor industry to deposit thin films, since magnetron sputtering has obvious advantages in achieving uniform and stable deposition on large areas, which is very important for industrial production of thin film solar cells.¹² Therefore, when CZTS thin film is deposited in laboratory with this technique, it makes the processes more easily transferable to a large scale production in industrial terms. Recently, efforts have been done to fabricate CZTS thin films by magnetron sputtering of quaternary target followed by post-sulfurization process. Katagiri et al.¹³ and Xie et al.¹⁴ have reported the PCE of 6.48% and 4.04% through magnetron sputtering from CZTS compound target followed by sulfurization in a mixed gas of Ar/or N₂ and H₂S, respectively. It can be found that in two reported works H₂S gas and STP process were used during the post-sulfurization process, which might be not favored in industrial production. On the one hand, H₂S gas is usually used as the S source due to its high reactivity; however, H₂S is extremely toxic for health and environment. Therefore, the cost for safety handing on H₂S should be taken into account. On the other hand, the losses of Zn and Sn elements usually can be observed using STP process. In our previous work, we investigated the effect of post-sulfurization on the properties of CZTS thin film deposited by RF magnetron sputtering from a CZTS compound target.¹⁵ We found that the composition ratio of CZTS thin film transformed from Cu-poor state to Cu-rich state using STP process due to volatilization of Zn and Sn elements during the post-sulfurization process. To limit the losses of Zn and Sn elements in Cu₂ZnSnS₄ thin film, a short annealing process has been implemented by many research groups.^16–18 Furthermore compared with conventional STP process, RTP technology has several advantages, including short cycle time for reaction, reduced thermal exposure and more flexibility. These advantages will help reduce processing time and simplify fabrication procedure which is of critical important in an industrial process.¹⁹ Here, we report on the application of RTP sulfurization of precursors sputtered from a quaternary target. The elemental S powder is used as the S source, which is much less toxic than H₂S. This work is to show that RTP technology is applicable for the growth of kesterite CZTS solar cell absorbers sputtered from a quaternary target. Furthermore, the influence of different growth temperatures on the properties of CZTS thin films has been investigated. A CZTS thin film solar cell with 2.85% PCE has been obtained with the CZTS absorber sulfurized at an optimized sulfurization temperature.

2. Experimental

CZTS thin films were deposited on glass substrates by RF magnetron sputtering process from a sintered pellet target (2 inch and 4 mm thick). The non-stoichiometric CZTS pellet was synthesized by the solid state reaction of Cu₂S, ZnS, SnS₂ and S powders mixed at 2 : 1.5 : 1:1 mol ratio. The detailed progress for the target can be found in ref. 15. The glass substrates (25 mm × 25 mm) were ultrasonically cleaned in acetone, distilled water, and ethanol, and dried in a nitrogen gas stream before being put into the vacuum chamber. The substrates were placed on a rotating heater and the distance between target and substrate was fixed as 6.5 cm. The deposition chamber was evacuated to a background pressure of 3.0 × 10⁻⁴ Pa, using a turbo molecular pump (TMP). First, the as-deposited thin films were deposited on glass from a quaternary CZTS target, using high-purity argon (20 sccm) discharged with an RF power density of 4 W cm⁻². The working pressure during the deposition was at 1.6 Pa. Before as-deposited process, the pre-sputter process has been done. The pre-sputter pressure was at 1.6 Pa, using high-purity argon (20 sccm) discharged with an RF power density of 2 W cm⁻². The pre-sputtering time was 2 min. All of the thin films were deposited for 1 h at room temperature. Then, the sulfurization process of precursors was performed by using RTP process. The background pressure in the RTP furnace was 0.2 Pa using rotary oil sealed mechanical pump. The sample was located in a graphite box with 20 mg S powders. The working pressure in the furnace was 130 Pa and the N₂ flow rate was 40 sccm. The RTP furnace was rapidly heated. The heating rate was ramped up at 20 °C per second by radiation from room temperature to final sulfurization temperature. The sulfurization temperature was kept constant during 5 min and then the furnace set to cool down naturally. The sulfurized CZTS thin films were named as S450, S500, S520 and S550 corresponding to their sulfurization temperature 450, 500, 520 and 550 °C, respectively.

The crystalline structures of the CZTS thin films were analyzed by X-ray diffraction (XRD) using Cu Kα radiation (Bruker D8 Advance) from 10° to 70°. The tube voltage and current for XRD patterns were 40 kV and 40 mA, respectively. Raman scattering experiments were performed with a micro-Raman spectrometer (Jobin-Yvon LabRAM HR 800UV). The normal-incident transmittance spectra were recorded using a double beam ultraviolet-infrared spectrophotometer (Perkin-Elmer Lambda 950). The surface micrographs and the composition of thin films were determined by a field emission scanning electron microscopy (FESEM: Philips XL30FEG) with an energy dispersive X-ray (EDX) analyzer. Current–voltage characteristics of the devices were measured under AM1.5 global spectrum with their radiance set to 1000 W m⁻² (Xelamp; Newport). External quantum efficiency (EQE) measurements were performed by a single source illumination system (halogen lamp) combined with a monochromator. All measurements were performed at room temperature.

3. Results and discussion

3.1 Compositional analyses

The compositional analysis of the samples was performed using the EDX technique. Table 1 shows the element composition values of precursors and sulfurized films. These average element compositions were measured at two different areas. As shown in Table 1, excess metals and deficient sulfur are observed in precursors. Moreover, all precursors deposited at room temperatures are in Cu-poor and Zn-rich states (i.e. Cu/(Zn + Sn) < 1 and Zn/Sn > 1). It is notable that the CZTS thin films with highest PCE are usually of Cu-poor and Zn-rich states.¹² Hence, we expect that the sulfur content can be enhanced by post-sulfurization process; meanwhile, the compositional ratios of metals in sulfurized films can be consistent with those in precursors after sulfurization process. As expected, the compositional ratios of sulfur significantly increase in all sulfurized CZTS thin films which is beneficial to decreasing the amounts of sulfur vacancies and improving the crystalline quality of CZTS thin films. Meanwhile, the compositional ratios of metal in sulfurized films have changed very slightly. Only a little amount of Zn element volatilize in sulfurized thin film. In our previous work, Cu-poor state in precursors was changed into Cu-rich state during the STP process due to the much more volatilization of Zn and Sn. Here, the compositional analysis shows that RTP process can reduce effectively the volatilization of Zn and Sn elements.

Table 1 Chemical composition of as-deposited and post-sulfurization thin films

Sample ID	Precursors			Post-sulfurization films
	Cu/(Zn + Sn)	Zn/Sn	Metals/S	Cu/(Zn + Sn)	Zn/Sn	Metals/S
S450	0.82	1.13	1.42	0.83	1.12	1.01
S500	0.83	1.10	1.45	0.85	1.09	0.99
S520	0.81	1.12	1.41	0.85	1.10	0.99
S550	0.82	1.15	1.43	0.86	1.11	1.02

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3.2 Microstructure characterization

Fig. 1 shows the XRD patterns of CZTS thin films sulfurized at different temperature. The XRD patterns of all CZTS thin films match the kesterite structure of CZTS (JCPDS 26-0575).⁶ No impurity phases are observed in all samples. In addition, Fig. 1 inset presents the full width at half maximum (FWHM) of (112) diffraction peak. As shown in Fig. 1 inset, the FWHM of (112) diffraction peak decreases with increasing sulfurization temperature, which means high sulfurization temperature can improve the crystalline quality of CZTS thin films. Based on Scherrer's formula the average crystallite size is inversely proportional to FWHM values, the values of average crystallite size for four samples (i.e. S450, S550, S520 and S550) are 42, 48, 56 and 62 nm, respectively. Thus, the sulfurized CZTS thin films sulfurized at high temperature may have bigger grain sizes. Comparing the peak intensity ratios of different patterns, significant changes of textures at different sulfurization temperature can be observed. Sample S450 showed a very strong (112) orientation preference with much weaker (101), (200), (211), (220)/(204), (312) and (224) peaks. However, all of the weak peaks become more and more intense until sulfurization temperature increased to 550 °C. The XRD results suggest that low sulfurization temperature is more likely to form [112]-preferred CZTS thin films, while high sulfurization temperature is advantageous to form polycrystalline thin films with no distinct texture. The reason for this is related with the crystallographic characteristic of the (112) plane of CZTS thin film, which is reported to have the lowest surface energy among all existing planes.^20,21 At lower sulfurization temperature, the energy provided is just high enough for the atoms to migrate to the lowest energy position, that is, to form the (112) plane. However, once the atoms come to the lowest energy position, they are confined by the stable crystal structure. Further migration and diffusion of the atoms need more energy, which can be supplied by further increasing sulfurization temperature. Therefore, a higher sulfurization temperature enhances the diffusion of Cu, Zn, Sn and S atoms and promotes a random atomic arrangement, resulting in the decline of the [112] preference and the rise of polycrystalline feature of the thin film.

Fig. 1 XRD patterns of CZTS thin films sulfurized at different sulfurization temperature. Inset: the FWHM of (112) diffraction peak.

The texture coefficients, TC_hkl, give the degree of enhancement of a particular set of hkl planes with respect to a reference sample:²²

where TC is the texture coefficient of the (hkl) plane, I_(hkl) is the measured intensity of the (hkl) plane, I_0(hkl) is the intensity of the same peak in the reference spectrum which is considered a completely randomly oriented sample⁴ and n is the reflection number. The deviation of texture coefficient from unity implies the preferred orientation. Fig. 2 shows the variation of the texture coefficient for (112) and (220) planes as the dependence of the sulfurization temperature. It can be observed that TC₍₁₁₂₎ value gradually decreases from 4.568 for S450 to 2.511 for S550, whereas TC₍₂₂₀₎ value increases from 0.007 for S450 to 0.392 for S550. It can be concluded that [220]-orientation degree increases gradually with the enhancement in sulfurization temperature, accompanied with the decrease in [112]-orientation degree. For CuInSe₂ or Cu(In,Ga)Se₂ thin films, [220]-orientation can be manipulated by controlling growth temperature,²³ and (220)-textured Cu(In,Ga)Se₂ thin films have some special features such as lower density of non-radiative recombination centers, more open structure and stronger inactive grain boundary, in contrast with [112] texture thin films.²⁴ CZTS has the crystal structural descendant from CuInSe₂ by the substitution of Zn, Sn for In and S for Se, so we expect that CZTS thin films inherit the feature of Cu(In,Ga)Se₂ thin films and CZTS thin films with more [220]-orientation degree have better optoelectronic properties in thin film solar cell applications.

Fig. 2 Texture coefficients for (112) and (220) planes.

Raman spectroscopy is also useful to analyze the structure and the phase purity of CZTS samples besides XRD analysis. Fig. 3 shows the Raman spectra of CZTS thin films sulfurized at different sulfurization temperature taken using the excitation laser wavelength of 633 nm. It is worthy to note that during the Raman measurement, two different zones in each film were measured and their spectra were perfectly overlapping, indicating excellent phase homogeneity of the thin films. In order to obtain the exact peaks, which represent the presence of Raman active mode, the measured spectra were fitted, and the fitted curves were decomposed into individual Lorentzian components. As shown in Fig. 3, the intense peak at 337 cm⁻¹ can be assigned to the A₁ mode which is the strongest mode generally observed in the Raman spectra of kesterite compounds.^25,26 The sharpness of A₁ mode peak reflect the crystalline quality of CZTS thin films. Hence, the effect of sulfurization temperature on crystalline quality can be better understood by comparing the FWHM values of A₁ mode peaks. Fig. 4 presents the FWHM values of A₁ mode peaks for all samples. The same results with the XRD analysis, with the increase of sulfurization temperature, the FWHM values of A₁ mode peaks decrease from 18.3 cm⁻¹ to 16.5 cm⁻¹ indicating the better crystalline quality of CZTS thin film deposited at higher sulfurization temperature. Furthermore, the peaks at 289 and 305 cm⁻¹ can be assigned to A₂ and A₃ mode, respectively.^26,28 Moreover, several weaker peaks locate at about 252, 268, 358 and 372 cm⁻¹ which are identified with B/E, B/E, B and B mode, respectively.^25,26 Also no significant indication of phase separation for Cu₂SnS₃ at 318 cm⁻¹, ZnS at 355 cm⁻¹ and Cu_2−xS at 475 cm⁻¹ can be found in Fig. 3 implying that all thin films are single phase.²⁷ Recently, J. Scragg et al. reported that the disorder of Cu and Zn in the crystal structure was studied using Raman scatting spectra.²⁸ Here, we also use this method to study the relationship between order-disorder transition and post-sulfurization temperature. According to the ref. 28, the quantity Q = I(mA₂)/I(mA₃) is defined, where I(m) is the intensity of the indicated Raman mode based on full spectrum fitting. With this definition, Q is larger for more ordered samples. The values of Q are plotted in Fig. 4. Q is equal to 2.76, 2.07, 1.13 and 1.08 for S550, S520, S500 and S450, respectively. With the sulfurization temperature increasing, the values of Q are enhanced which indicating that the Cu and Zn cations in CZTS films become more ordered. For the low temperature sulfurization process, the thermal energy is insufficient for the inter-diffusion of Cu and Zn cations. It may be lead large disorder among the Cu and Zn cations in the CZTS lattice.

Fig. 3 Raman spectra of CZTS thin films annealed at different sulfurization temperature. Note: measured Raman scattering spectra (open circles), fitted spectra (red solid line) and decomposed active modes (blue solid lines).

Fig. 4 The FWHM and the quantity Q for all samples.

In order to investigate the effect of sulfurization temperature on surface morphological features of CZTS thin films, SEM observation was carried out on CZTS thin films. Fig. 5 shows the SEM images of CZTS thin films sulfurized at different sulfurization temperature. Micrograph of the film S450 shows uniform surface morphology with smaller grains. After increasing sulfurization temperature to 500 °C, the micrograph of the film S500 shows distinct grains about ∼1 μm size. However, plenty of grain boundaries exist in the film S500, which is negative for the photocurrent collection in the cell. After further increase of sulfurization temperature, the film morphology is evidently improved, exhibiting visible edges and facets as well as satisfactory compactness and surface flatness. The average grain size of films S520 and S550 reach positively ∼2 μm and ∼2.5 μm, respectively. The evolution of grain size indicates that high temperature can promote the grain growth. In terms of a high PCE CZTS thin film solar cell, CZTS thin films usually have large gain size. The decrease in the amount of grain boundaries by large grains diminishes recombination, which increases the effective diffusion length of minority carriers and thereby causes high short circuit photocurrent.⁶ Hence, for photovoltaic application, the sample S550 is more appropriate than the other films judging from the grain size.

Fig. 5 SEM surface micrographs for CZTS thin films annealed at different sulfurization temperature.

3.3 Optical properties

Fig. 6 shows the transmittance spectra of all CZTS thin films in the wavelength range from 250 to 2500 nm (0.5 to 4.96 eV). The spectra can be roughly divided into three specific regions: a transparent oscillating one (labeled “I”), a low transmittance one (“II”), and a strong absorption one (“III”) at lower wavelength (i.e. higher photo energy). Note that the absorption edge is near the end of the interference oscillations and located in the region “II”. As shown in Fig. 6, it can be clearly observed that the absorption edge shift toward the higher wavelength side with increasing the sulfurization temperature. The optical band gap (E_g) is determined by extrapolating straight line of (αhν)² versus photon energy curve to the intercept on horizontal photon energy axis.¹⁵ As shown in the inset of Fig. 6, these determined E_g values of four CZTS films (i.e. S550 S520, S500, and S450) are 1.53, 1.60, 1.64, and 1.80 eV, respectively. With increase in the growth temperatures, the band gap values shift to lower energies. The band gap energy of the CZTS thin film is dependent on the composition. Tanaka et al.²⁹ investigated the band gap energy of CZTS thin film as a function of the chemical composition of Cu/(Zn + Sn). They found that the band gap energy of CZTS shifts to lower energies as the ratio of Cu/(Zn + Sn) increases. In CZTS compound, the Valance Band Maximum (VBM) is due to an antibonding state of the anion S–p and Cu–d orbitals, whereas the Conduction Band Minimum (CBM) is due to an antibonding state of the anion S–s and Sn–s orbitals.³⁰ As discussed in compositional analysis, Cu content is slight enhanced with the increasing of the sulfurization temperature. Therefore, the band gap energy shifts of CZTS thin films may be also attributed to the increase of Cu content which results in the change of the degree of p–d hybridization between the Cu d-levels and S p-levels. Therefore, when the sulfurization temperature is at 550 °C, not only the CZTS thin film have the well crystalline quality and large grain size, but also the band gap energy (i.e. 1.53 eV) is very close to the optimum band gap energy of semiconductor used for photovoltaic conversion. Thus, in the future work the CZTS thin film solar cells will be prepared with CZTS thin films sulfurized at 550 °C.

Fig. 6 Transmittance spectra of CZTS thin films sulfurized at different sulfurization temperature. Inset: plot of (αhν)² versus hν for CZTS thin films sulfurized at different sulfurization temperature.

3.4 Device characterization

In order to examine the quality of Cu₂ZnSnS₄ thin films utilizing rapid thermal processing sulfurization of precursors sputtered from a single quaternary target for the application of CZTS thin film solar cells, glass/Mo/CZTS/CdS/i-ZnO/AZO, were fabricated. CdS buffer layer was deposited by chemical bath deposition (CBD) using CdSO₄, ammonia, (NH₂)₂SC aqueous solution at 70 °C. After the deposition of CdS layer, i-ZnO layer was prepared by RF magnetron sputtering of a pure ZnO target at 2.25 W cm⁻² RF power density and 0.4 Pa working pressure. Then, the AZO film was deposited by RF magnetron sputtering of a ceramic ZnO:Al₂O₃ (2 wt%) target at 6 W cm⁻² RF power density and 0.4 Pa working pressure. The deposition temperature of ZnO and AZO thin film were at room temperature. Fig. 7 and its inset show the J–V characteristics for CZTS thin film solar cell prepared with an optimized CZTS thin films sulfurized at 550 °C and the cross-sectional FESEM image of the fabricated CZTS cell, respectively. The best cell processed achieved an efficiency of 2.85% with an open circuit voltage (V_oc) of 412 mV, a short circuit density (J_sc) of 17.9 mA cm⁻², a fill factor (FF) of 40.5%, series resistance (R_s) of 22.5 Ωcm² and shunt resistant (R_sh) of 155.1 Ωcm². Compared to the highest PCE of 8.4% for CZTS thin film solar cells obtained by Shin et al., our cell has low fill factor due to higher R_s and lower R_sh. The high R_s can reduce V_oc and the low R_sh lead to decrease in J_sc. In terms of high efficiency CZTS thin film solar cell R_s of less than 1 Ωcm² and R_sh of over 10³ Ωcm² are expected.⁶ Bad electrode ohmic contact or poor carrier transportation in the absorber layers can result in high R_s and the existence of pinholes or the insufficient thickness of the absorber layer can result in low R_sh. Note that no metallic grid electrode were prepared on the top of the our cell, it may result in high R_s. And as shown in the inset of Fig. 7, the thickness of CZTS absorber is about 800 nm, thickness of the absorber layer is insufficient compared to high efficiency cell which can result in low R_sh. In the further work, the thickness of CZTS absorber will be enhanced by appropriately increasing the sputtering time. Meanwhile, one striking feature in the cell is to exhibit a crossover behavior in the dark and light J–V characteristics. This behavior can be attributed to a Schottky barrier at the back contact or an electrical barrier in the buffer/absorber interface.³¹ The external quantum efficiency (EQE) curve of the best solar cell is shown in Fig. 8, where a maximum EQE of 69% at 520 nm is obtained. The band gap is estimated to be 1.52 eV from the plot of [hν × ln(1 − EQE)]² vs. hν, where hν is the photo energy.³¹ It is consistent with the band gap extracted from the Transmittance spectra measurement (Fig. 6). Based on the EQE curve, one can conclude that the major energy loss of short circuit current is a result of carrier collection efficiency in the range of 520 nm to 750 nm which may be attributed to severe backside recombination, short minority carrier lifetime and the insufficient thickness of the absorber layer.^32,33 Continued optimization of absorber fabrication for solar cell application is underway. Higher efficiency can be expected for CZTS thin film solar cells by further improving this low cost and large scale technique.

Fig. 7 J–V characteristics for CZTS thin film solar cell. Inset: the cross-sectional FESEM image of the fabricated CZTS cell.

Fig. 8 External quantum efficiency of CZTS solar cell at 0 V bias. Inset: the band gap of CZTS thin film obtained from EQE data.

4. Conclusions

CZTS thin films have been successfully deposited using RF magnetron sputtering process from a quaternary Cu–Zn–Sn–S target followed by rapid thermal process sulfurization at different sulfurization temperatures ranging from 450 to 550 °C for 5 min. The effects of sulfurization temperature on compositional, microstructural, morphological and optical properties of CZTS thin films have been studied. The compositional analysis shows that all the CZTS thin films are in Cu-poor and Zn-rich state and RTP technique can remarkably reduce the volatilization of Zn and Sn elements compared to the conventional STP process. XRD data and Raman spectra studies reveal that the sulfurized CZTS thin films have polycrystalline kesterite crystal structure and high sulfurization temperature can improve the crystalline quality of CZTS thin films. Furthermore, a higher sulfurization temperature enhances the diffusion of Cu, Zn, Sn and S atoms and promotes a random atomic arrangement, resulting in the diminishing of the (112) preference and polycrystalline feature of the film. Meanwhile, the relationship between order-disorder transition and post-sulfurization has been investigated using Raman scattering spectra. Sulfurization process at low temperature, large disorder among the Cu and Zn cations exist in CZTS thin film. At 550 °C, the CZTS thin film has large grain size and dense morphology, its band gap energy is found to be 1.53 eV determined by transmission spectra. The solar cell fabricated with the CZTS absorber grown at an optimized sulfurization temperature of 550 °C shows a conversion efficiency of 2.85% for a 0.16 cm² area with V_oc = 412 mV, J_sc = 17.9 mA cm⁻², and FF = 40.5%. These results show that this method is an alternative, environmental friendly, low elemental loss and simple process to fabricate CZTS thin films for solar cell applications.

Acknowledgements

The authors are grateful to the measurement of Professor Zhigao Hu group in Raman scattering spectra and transmittance spectra. This project was financed by the National Science Foundation of China (61106064, 61376129 and 61474045), the State Key Basic Research Program of China (2013CB922300). One of authors (Jun He) thanks the project from ECNU (Grant no. XRZZ201325).

References

1. K. Ito and T. Nakazawa, Jpn. J. Appl. Phys., Part 1, 1998, 27, 2094–2097.
2. T. K. Todorov, K. B. Reuter and D. B. Mitzi, Adv. Mater., 2010, 22, 1–4.
3. S. Chen, A. Walsh, J. Yang, X. G. Gong, L. Sun, P. X. Yang, J. H. Chu and S. H. Wei, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 83, 125201.
4. J. He, L. Sun, S. Chen, Y. Chen, P. X. Yang and J. H. Chu, J. Alloys Compd., 2012, 511, 129–132.
5. W. Shockley and H. Queisser, Jpn. J. Appl. Phys., 1961, 32, 510–519.
6. B. Shin, O. Gunawan, Y. Zhu, N. A. Bojarczuk, S. J. Chey and S. Guha, Prog. Photovolt. Res. Appl., 2011, 21, 72–76.
7. H. Katagiri, K. Saitoh, T. Washio, H. Shinohara, T. Kurumadani and S. Miyajima, Sol. Energy Mater. Sol. Cells, 2001, 65, 141–148.
8. S. W. Shin, S. M. Pawar, C. Y. Park, J. H. Yun, J. H. Moon, J. H. Kim and J. Y. Lee, Sol. Energy Mater. Sol. Cells, 2011, 95, 3202–3206.
9. S. M. Pawar, A. V. Moholkar, I. K. Kim, S. W. Shin, J. H. Moon, J. I. Rhee and J. H. Kim, Curr.Appl. Phys., 2010, 10, 565–569.
10. H. Yoo and J. Kim, Sol. Energy Mater. Sol. Cells, 2011, 95, 239–244.
11. C. P. Chan, H. Lam and C. Surya, Sol. Energy Mater. Sol. Cells, 2010, 94, 207–211.
12. D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang and S. Guha, Sol. Energy Mater. Sol. Cells, 2011, 95, 1421–1436.
13. H. Katagiri and K. Jimbo, Proceeding of the 37th IEEE Photovoltaic Specialists Confrence, 2011, pp. 3516–3521.
14. M. Xie, D. Zhuang, M. Zhao, Z. Zhuang, L. Ouyang, X. Li and J. Song, Int. J. Photoenergy, 2013, 2013, 929454.
15. J. He, L. Sun, K. Z. Zhang, W. J. Wang, J. C. Jiang, Y. Chen, P. X. Yang and J. H. Chu, Appl. Surf. Sci., 2013, 264, 133–138.
16. S. Ahmed, K. B. Reuter, O. Gunawan, L. Guo, L. T. Romankiw and H. Deligianni, Adv. Energy Mater., 2012, 2, 253–259.
17. R. Lechner, S. Jost, J. Palm, M. Gowtham, F. Sorin, B. Louis, H. Yoo, R. A. Wibowo and R. Hock, Thin Solid Films, 2013, 535, 5–9.
18. S. M. Pawar, A. I. Inamdar, B. S. Pawar, K. V. Gurav, S. W. Shin, X. Yanjun, S. S. Kolekar, J.-H. Lee, J. H. Kim and H. Im, Mater. Lett., 2014, 118, 76–79.
19. J. He, J. H. Tao, X. K. Meng, Y. C. Dong, K. Z. Zhang, L. Sun, P. X. Yang and J. H. Chu, Mater. Lett., 2014, 126, 1–4.
20. T. Schlenker, V. Laptev, H. W. Schock and J. H. Werner, Thin Solid Films, 2005, 480–481, 29–32.
21. J. Wang, S. Li, J. Cai, B. Shen, Y. Ren and G. Qin, J. Alloys Compd., 2013, 552, 418–422.
22. A. V. Moholkar, S. S. Shinde, A. R. Babar, K. U. Sim, Y. B. Kwon, K. Y. Rajpure, P. S. Patil, C. H. Bhosale and J. H. Kim, Sol. Energy, 2011, 85, 1354–1363.
23. M. A. Contreras, B. Egaas, D. King, A. Swartzlander and T. Dullwever, Thin Solid Films, 2000, 361–362, 167–171.
24. L. Sun, J. He, H. Kong, F. Yue, P. Yang and J. Chu, Sol. Energy Mater. Sol. Cells, 2011, 95, 2907–2913.
25. A. Khare, B. Himmetoglu, M. Cococcioni and E. S. Aydil, J. Appl. Phys., 2012, 111, 123704.
26. X. Fontané, L. Calvo-Barrio, V. Lzquierdo-Roca, E. Saucedo, A. Pérez-Rodiguez, J. R. Morante, D. M. Berg, P. J. Dale and S. Siebentritt, Appl. Phys. Lett., 2011, 98, 181905.
27. K. Wang, B. Shin, K. B. Reuler, T. Todoror, D. B. Mitzi and S. Guha, Appl. Phys. Lett., 2011, 98, 051912.
28. J. J. Scragg, L. Choubrac, A. Lafond, T. Ericson and C. Platzer-Björkman, Appl. Phys. Lett., 2014, 104, 041911.
29. K. Tanaka, Y. Noriko, N. Moritake and H. Uchiki, Sol. Energy Mater. Sol. Cells, 2011, 95, 838–842.
30. S. Chen, J. H. Yang, X. G. Gong, A. Walsh and S. H. Wei, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 81, 245204.
31. J. Ge, J. C. Jiang, P. X. Yang, C. Peng, Z. P. Huang, S. H. Zuo, L. H. Yang and J. H. Chu, Sol. Energy Mater. Sol. Cells, 2014, 125, 20–26.
32. R. B. V. Chalapathy, G. S. Jung and B. T. Ahn, Sol. Energy Mater. Sol. Cells, 2011, 95, 3216–3221.
33. W. Wang, S. Y. Han, S. J. Sung, D. H. Kim and C. H. Chang, Phys. Chem. Chem. Phys., 2012, 14, 11154–11159.

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