Improving the conversion efficiency of Cu2ZnSnS4 solar cell by low pressure sulfurization Kun Zhang, Zhenghua Su, Lianbo Zhao, Chang Yan, Fangyang Liu, Hongtao Cui, Xiaojing Hao, and Yexiang Liu
Citation: Appl. Phys. Lett. 104, 141101 (2014); doi: 10.1063/1.4870508 View online: https://doi.org/10.1063/1.4870508 View Table of Contents: http://aip.scitation.org/toc/apl/104/14 Published by the American Institute of Physics
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APPLIED PHYSICS LETTERS 104, 141101 (2014)
Improving the conversion efficiency of Cu2ZnSnS4 solar cell by low pressure sulfurization Kun Zhang,1 Zhenghua Su,1 Lianbo Zhao,1 Chang Yan,2 Fangyang Liu,1,2,a) Hongtao Cui,2 Xiaojing Hao,2,a) and Yexiang Liu1 1
School of Metallurgy and Environment, Central South University, Changsha 410083, China Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia 2
(Received 22 February 2014; accepted 23 March 2014; published online 7 April 2014) Cu2ZnSnS4 thin films have been prepared by the sol-gel sulfurization method on Mo-coated substrates, and the comparative studies between the atmospheric pressure sulfurization and low pressure sulfurization was carried out. The Cu2ZnSnS4 film sulfurized at low pressure exhibits larger grain size, thinner MoS2 layer, and free of SnS secondary phase, but more ZnS on surface. The device efficiency of 4.1% using Cu2ZnSnS4 absorber from atmospheric pressure sulfurization is improved to 5.7% using that from low pressure sulfurization via the boost of open-circuit and fill C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4870508] factor. V Cu2ZnSnS4 (CZTS) is an excellent semiconductor for thin-film solar cells owing to a near-optimum direct band gap of about 1.5 eV and a high absorption coefficient.1 So far, the reported highest efficiency of CZTS-based thin-film solar cells has achieved 8.5% under AM1.5G illumination.2 High quality CZTS absorber is a prerequisite for high device efficiency. Currently, CZTS thin films are usually synthesized via the deposition of the precursors followed by post-sulfurization.2–5 In such process, sulfurization plays an extremely important role in the formation of the high-quality CZTS thin film and thereby has been investigated extensively, including pre-alloying (Refs. 6 and 7), sulfur source (Refs. 8 and 9), sulfur partial pressure (Refs. 10 and 11), sulfurization time (Refs. 12 and 13), and sulfurization temperature.14,15 However, few attempts have been made on the pressure during sulfurization, which should have considerable influence on the growth process of CZTS thin film and also affects the generation of MoS2 at CZTS/Mo interface. In this Letter, we compared the properties of CZTS thin films sulfurized at atmospheric pressure and low pressure and their corresponding device performance. The results suggest that the CZTS absorber sulfurized at low pressure exhibits better properties, and thereby yields a higher device performance. The device efficiency is improve from 4.1% by atmospheric pressure sulfurized CZTS absorber to 5.7% by low pressure sulfurized one. The CZTS precursors were prepared by the sol-gel method for investigating the impact of the sulfurization pressure. More details of the preparation procedure can be found in a previous report.16 The sulfurization process is carried out under a mixed Ar and sulfur vapour atmosphere. The sulfur vapour is generated from heated sulfur powders at 200 C. Two different base pressure of sulfurization process were set for this study, i.e., atmospheric pressure (0.1 MPa) and low pressure of 0.04 MPa. In both cases, the sulfurization used for annealing CZTS precursor was maintained at 580 C for 60 min. The average compositions of the two sulfurized a)
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samples tested by energy-dispersive X-ray spectroscopy (EDS) are very close, with the Cu/(Zn þ Sn) ratio of 0.8 and Zn/Sn ratio of 1.2, though the sulfurization process causes a slight loss in Sn content. They coincide with those previously reported for high-efficiency CZTS devices.8,17 Figs. 1(a) and 1(b) show the surface scanning electron microscope (SEM) images of the CZTS thin films from atmospheric pressure and low pressure sulfurization, respectively. Both of them show compact morphology, composed of isolated grains with large uniform size and well-defined boundaries. It is well-known that low pressure results in the decrease in melting point of metal sulfides, especially for copper chalcogenides, which acts as a flux during the growth of CnInS2 and Cu(In, Ga)Se2.18,19 Similarly, during the growth of CZTS film, liquid Cu2xS was reported to promote the reaction process, leading to accelerating formation of high quality CZTS.20,21 Besides, it seems to serve as a catalyst in the grain growth process and facilitates the growth of large CZTS grains.22 It is very important for us to acquire the absorber with large grain size and less grain boundaries for high device efficiency because smaller grains with excess grain boundaries will lead to recombination, which generates loss in Voc and cause a reduction in current without sufficient passivation.23 The grain size of the CZTS thin film from low pressure sulfurization (about 1.0 lm) is considerably larger than that from atmospheric pressure sulfurization (about 0.5 lm), which indicates an enhanced grain growth during sulfurization at low pressure. It is worth noting that extremely Zn-rich particles exist on the surface of CZTS thin film from low pressure sulfurization confirmed by the EDS micro-area analysis, which can be assigned to ZnS phase and often observed by other reports.16,24 Figs. 1(c) and 1(d) show the cross-section SEM micrographs of the devices from the CZTS thin films sulfurized at different pressure. As shown in Figs. 1(c) and 1(d), both CZTS thin films exhibit the thickness of around 1 lm and have some grains extending from the bottom to the top of the CZTS layer without large number of voids, relevant to the slight Sn loss during sulfurization process, which leads to the voids at the grain boundary and/or CZTS/Mo interface.25 For MoS2 layer—which may
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FIG. 1. (a) Surface morphology of CZTS thin film sulfurized at atmospheric pressure, (b) surface morphology of CZTS thin film sulfurized at low pressure, (c) cross-sectional morphology of CZTS device from atmospheric pressure sulfurization, and (d) cross-sectional morphology of CZTS device from low pressure sulfurization.
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facilitate an electrical quasi-ohmic contact and improve the adhesion of absorber to Mo back contact, but leads to high series resistance and therefore deteriorates the device efficiency if not thin enough,26 similar to the case of CIGSe solar cells27—its thickness decreases from 230 nm for sample from atmospheric pressure sulfurization to 110 nm for that from low pressure sulfurization. We have reported previously that introducing a TiB2 intermediate layer between absorber and back contact can significantly inhibit the formation of MoS2 layer at absorber/back contact interface region,28 which however degrades the crystallinity of absorber. By contrast, in this work, above results reveal that low pressure sulfurization can reduce the formation of MoS2 layer and in the meantime ensure better crystal quality. Fig. 2(a) shows the X-ray diffraction (XRD) spectra of the CZTS thin films from atmospheric pressure and low pressure sulfurization. Both films are polycrystalline and exhibit the characteristic peaks in agree with that of kesterite CZTS (JCPDS 26-0575). Compared with other peaks of CZTS, the relative intensity of (112) peak is much higher revealing strong preferential orientation of (112) plane.29 Enlarged (112) peak region in the 2h of 27.9 –28.7 is displayed in the inset for more details. As is known, the (112) peak of CZTS (2hCZTS ¼ 28.53 ) is very close to (111) peak of cubic-ZnS (2hZnS ¼ 28.50 ) and if co-existed in the film, it is difficult to distinguish one from the other. However, the existence of ZnS will make the diffraction peak from (112) plane of
FIG. 2. (a) XRD patterns of CZTS thin films from atmospheric and low pressure sulfurization. (b) Raman spectra (488 nm excitation) of CZTS thin films from atmospheric and low pressure sulfurization. (c) Raman spectra (325 nm excitation) of CZTS thin films from atmospheric and low pressure sulfurization.
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CZTS broader and more asymmetric, and accordingly the evaluation of the crystallinity by full width high maximum (FWHM) uncertain.30 In fact, the presence of the ZnS phase is evidenced from bright features of SEM images and EDS micro-analysis for both samples and more in low pressure sulfurization one. This contributes to inconsistent observation of similar FWHM from XRD but larger grain size from SEM for CZTS thin film by low pressure sulfurization. Besides, it is as expected that the (112) peak of the sample from low pressure sulfurization is more asymmetric. Raman scattering with 488 nm excitation wavelengths have been employed to detect the presence of secondary phases, as shown in Fig. 2(b). Both films exhibit a major peak at 336 cm1 attributed to the CZTS A1 mode (Ref. 31) along with two weak peaks at 284 cm1 and 370 cm1, which are consistent with previously reported characteristic CZTS Raman modes.32,33 Raman spectra also reveal the presence of MoS2 with characteristic mode at 402 cm1 in the film by atmospheric pressure sulfurization, in accordance with the SEM results that it has a much thicker MoS2 layer. Besides, the band at 164 cm1 assigned to SnS for the film from atmospheric pressure sulfurization is observed,34 which would lead the decrease in open-circuit voltage due to the low energy gap and high interfacial recombination velocity due to the large lattice mismatch with CZTS.35 Raman spectroscopy with excitation wavelengths of 325 nm was also used to further identify the existence of ZnS, as shown in Fig. 2(c). The peak at 347 cm1,693 cm1, and 1040 cm1 can be assigned to the R1LO, R2LO, R3LO vibrational mode of ZnS,36 and the peak at 620 cm1 shown in the film sulfurized at low pressure are attributed to the 2TO vibrational mode of ZnS.37 Comparing Raman spectra of two samples, it is obvious that the intensity of the ZnS Raman peaks for the sample from low pressure sulfurization is much stronger than that from atmospheric pressure sulfurization. This agrees with SEM and EDS results that more ZnS phase exist at the top surface region of the sample from low pressure sulfurization, which leads to very weak the CZTS Raman characteristic peaks. In contrast, sample from atmospheric pressure sulfurization can display a second order CZTS peak at 663 cm1 spectral region38 due to less Zn-rich phase covered surface. Devices based on both CZTS absorbers were fabricated according to the conventional configuration of AZO/ i-ZnO/CdS/CZTS/Mo/glass without antireflection layer. The current densityvoltage (JV) curves for these two solar cells measured under AM1.5 and 100 mW/cm2 illumination are shown in Fig. 3(a). The efficiency of device with CZTS absorber from low pressure sulfurization is 5.7%, compared to 4.1% efficiency of device with CZTS absorber from atmospheric pressure sulfurization. The improvement originates from the boost of open-circuit (VOC) from 598 mV to 664 mV and fill factor (FF) from 0.45 to and 0.58. The enhancement of Voc and FF mainly comes from the larger CZTS grains and free of SnS phase. It should be noticed that the short-circuit current (Jsc) does not change obviously for both sulfurization processes. This is due to the counteraction between thinner MoS2 at back contact region and more ZnS phase on the surface in low pressure sulfurized sample. Chemical etching to remove superficial ZnS is an effective
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FIG. 3. Current densityvoltage (a) and external quantum efficiency (b) characteristics of CZTS solar cells based on atmospheric and low pressure sulfurization.
way to increase Jsc and has been employed in our previous work,16 but the efficiency can be only improved slightly due to the drop of Voc. Therefore, further optimization for surface ZnS phase should be performed. Fig. 3(b) shows the external quantum efficiency (EQE) spectra of the two cells in wavelengths ranging from 350 to 1000 nm. The integrated EQE with AM 1.5 solar spectrum values are 15.10 and 14.87 mA/cm2 for CZTS devices based on atmospheric and low pressure sulfurization process, respectively, coincident with the Jsc values in J-V tests. Although the integrated current densities are nearly the same, the characteristic response in wavelength differs significantly. In the blue region, device from atmospheric pressure sulfurized CZTS absorber shows higher EQE value. This may be attributed to less ZnS on the surface and accordingly larger CdS/CZTS hetero-junction area and promoted inter-diffusion of CdS into CZTS, which reduces the effective thickness of CdS buffer layer and lowers the absorption by CdS. On the contrary, in the wavelength region from 530 to 1000 nm, the EQE value of the device from low pressure sulfurization CZTS absorber is higher than that from atmospheric pressure sulfurization. This can be explained by the fact that the absorber from low pressure sulfurization has better CZTS quality: larger grains with less grain boundaries
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and free of detrimental SnS secondary phase. Besides, the band-gap for the CZTS films sulfurized at atmospheric pressure and low pressure are estimated to be about 1.42 and 1.39 eV, respectively. This suggests that the difference in sulfurization does not remarkably change the band gap of the prepared CZTS absorbers. Moreover, the fast drop after the peak EQE value for both devices indicates very short minority life time and/or limited depletion width, suggesting large room for the improvement of CZTS absorber quality. In summary, a 5.7%-efficiency CZTS solar cell fabricated by sulfurization of sol-gel precursor under low pressure has been presented, with improvements in Voc and FF relative to an analogous 4.1% device from atmospheric pressure sulfurization. Sulfurization at low pressure shows advantages of acquiring larger grains with less grain boundaries and free of detrimental SnS secondary phase, which leads to significant improvement to VOC and FF. In addition, the thickness of MoS2 at back contact can be reduced by low pressure sulfurization, while more ZnS phase was found on the top surface of CZTS. The counter impacts from ZnS and MoS2 on Jsc result in that Jsc is not affected by the sulfurization pressure. J-V and EQE analysis indicate that further optimization of absorber surface phase is needed for higher efficiency. This work was supported by the National Natural Science Foundation of China (Grand No. 51204214), Hunan Provincial Innovation Foundation for Postgraduate (Grant No. CX2012B04), and the Fundamental Research Funds for the Central Universities of Central South University (Grant No. 2013zzts027).
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