Novel two-step CdS deposition strategy to improve the performance of Cu2ZnSn(S,Se)4 solar cell

Journal of Energy Chemistry 42 (2020) 77–82

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Novel two-step CdS deposition strategy to improve the performance of Cu2 ZnSn(S,Se)4 solar cell Lifang Teng a, Junye Tong a,b, Gang Wang b, Lingling Wang a,∗, Liping Chen a, Shaotong Wang a, Yinglin Wang a, Daocheng Pan b,∗, Xintong Zhang a,∗, Yichun Liu a a

Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory of UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, Jilin, China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China

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a r t i c l e

i n f o

Article history: Received 26 April 2019 Revised 31 May 2019 Accepted 2 June 2019 Available online 21 June 2019 Keywords: Kesterite Solar cell CdS Two-step deposition Heterojunction interface

a b s t r a c t Kesterite Cu2 ZnSn(S,Se)4 (CZTSSe) solar cells have drawn worldwide attention for their promising photovoltaics performance and earth-abundant element composition, yet the record efficiency of this type of device is still far lower than its theoretical conversion efficiency. Undesirable band alignment and severe non-radiative recombination at CZTSSe/CdS heterojunction interfaces are the major causes limiting the current/voltage output and overall device performance. Herein, we propose a novel two-step CdS deposition strategy to improve the quality of CZTSSe/CdS heterojunction interface and thereby improve the performance of CZTSSe solar cell. The two-step strategy includes firstly pre-deposits CdS thin layer on CZTSSe absorber layer by chemical bath deposition (CBD), followed with a mild heat treatment to facilitate element inter-diffusion, and secondly deposits an appropriate thickness of CdS layer by CBD to cover the whole surface of pre-deposited CdS and CZTSSe layers. The solar energy conversion efficiency of CZTSSe solar cells with two-step deposited CdS layer approaches to 8.76% (with an active area of about 0.19 cm2 ), which shows an encouraging improvement of over 87.98% or 30.16% compared to the devices with traditional CBD-deposited CdS layer without and with the mild annealing process, respectively. The performance enhancement by the two-step CdS deposition is attributed to the formation of more favorable band alignment at CZTSSe/ CdS interface as well as the effective decrease in interfacial recombination paths on the basis of material and device characterizations. The two-step CdS deposition strategy is simple but effective, and should have large room to improve the quality of CZTSSe/CdS heterojunction interface and further lift up the conversion efficiency of CZTSSe solar cells. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

1. Introduction Cu2 ZnSnSx Se4− x (CZTSSe) based solar cells which composed of earth-abundant and nontoxic elements, have developed rapidly within the past few years. A 12.6% power conversion efficiency (PCE) for a Se-incorporated kesterite [1] and 11% efficiency for pure sulfide Cu2 ZnSnS4 (CZTS) [2] have been achieved respectively, demonstrating substantial commercial promise. Despite the progress, the efficiency of CZTSSe solar cells is, however, still far lower than its Shockley–Queisser limit of ∼32% power efficiency [3]. The large number of lattice defects within the bulk, coexistence of secondary phases and unfavourable band alignment at ∗

Corresponding authors. E-mail addresses: [email protected] (L. Wang), [email protected] (D. Pan), [email protected] (X. Zhang).

the CZTSSe/CdS heterojunction interface are the dominant issues to limiting the current/voltage output and overall device performance [2,4–8]. Up to now, many researches have focused on improving the quality of CZTSSe absorption layer, i.e., optimizing the synthesis method, adjusting element composition of CZTSSe precursor solution, or controlling the annealing conditions for sulfuration or selenization [9–13]. Recently, partial cation substitution of Cu+ , Zn2+ and Sn4+ by Ag+ , Cd2+ and Ge4+ has shown promising effect to boost the efficiency of kesterite solar cells [14–20]. It is well known that the Cu+ , Zn2+ and Sn4+ have the similar ionic radius, so that they can easily form anti-site defects in the absorbers, and this will enhance the recombination of photo-generated electrons and holes in the CZTSSe bulk [21,22]. The introduction of these kinds of cations significantly improved the microstructure performances and reduced the anti-site defects for the lower

https://doi.org/10.1016/j.jechem.2019.06.011 2095-4956/© 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

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melting point of new alloy compounds or the larger radius of doping cation. Thereby it showed the encouraging effects to passivate defects, inhibit secondary phases, reduce recombination in the bulk, and then improve the efficiencies of CZTSSe solar cells. However, apart from the bulk recombination, suppressing the recombination at interface, especially the CZTSSe/CdS heterojunction interface, which less attention has been paid to, is equally important for improving solar cell performance. Liu et al. have found that the nanoscale microstructure and chemistry of CZTS/CdS interfaces have vital effect for the device performance [23–25]. The diffusion of Cd2+ has been observed clearly in the CZTS/CdS interfaces in the simple CBD CdS process. The benign Cd2+ diffusion could reduce interface crystalline defect and thereby reduce interface recombination. This effect is similar to the Cd2+ doping in CZTSSe bulk. Recently, some researchers adopted heat treatment to promote this interfacial ion diffusion, and obtained a significantly improved device performance [2,26– 28]. Hao’s group reported a certified 11% efficiency Cu2 ZnSnS4 solar cell by annealing the glass/Mo/CZTS/CdS samples. They demonstrated that the elemental inter-diffusion resulted in the formation of new phases near the hetero-interface and more favourable conduction band alignment was obtained, contributing to the reduced non-radiative recombination. These works reveal a good interface modification strategy to enhance device performance, but they mainly focus on the study of pure sulfide CZTS/CdS interface. The research on the more complex CZTSSe/CdS interface, which has more complex ion diffuse process, has seldom been reported. The research purpose of this work is to explore an effective interface treatment method to improve the quality of CZTSSe/CdS interface and thus to improve the performance of CZTSSe solar cell. We propose herein a novel two-step CdS deposit strategy, i.e., firstly we pre-deposits CdS thin layer on CZTSSe absorber layer by CBD, followed with a mild heat treatment; and secondly deposits an appropriate thickness of CdS layer to cover the whole surface of pre-deposited CdS and CZTSSe layers. The advantage of this strategy is that the heat treatment could enhance the ion inter-diffusion and forming a more favourable band alignment for electron injection; and then the extra secondly CdS deposition could keep a well coverage for the surface of predeposited CdS and CZTSSe layers, which effective decrease the interfacial recombination paths caused by the previous heat treatment process. The solar energy conversion efficiency of CZTSSe solar cells with two-step deposited CdS layer approaches 8.76% (with an active area of about 0.19 cm2 ), which shows an encouraging improvement of over 87.98% or 30.16% compared to the devices with traditional CBD-deposited CdS layer without and with the mild annealing process, respectively. The results suggest that the two-step CdS deposit strategy is a simple but effective interface treatment method to realize high efficiency CZTSSe solar cell.

2.2. Preparation of CZTS precursor solution The CZTS precursor solution is prepared according to the previously reported works [13,29,30]. Firstly, 0.268 g Cu2 O was dissolved in a mixed solution of 2 mL ethanol, 1.2 mL CS2 , 1.5 mL 1-butylamine under magnetic stirring at 65 °C for 30 min. Analogously, the Zn and Sn precursor solutions were prepared by dissolving 0.195 g ZnO in 2 mL ethanol, 0.75 mL CS2 , 1 mL 1butylamine and 0.269 g SnO in 2 mL ethanol, 1.75 mL CS2 , 2.5 mL 1-butylamine, 0.5 mL thioglycollic acid, respectively. Note that all of the solutions were stirred on hotplates at 60 °C until the solid completely dissolved. Subsequently, the three metal precursor solutions were cooled to the room temperature and then mixed together under magnetic stirring, and were diluted to 0.55 M with ethanol, followed by centrifugation at 12,0 0 0 rpm for 5 min prior to spin-casting. All of the procedures performed in the air.

2.3. Fabrication of the CZTSSe absorber layers and corresponding solar cells In nitrogen-filled glove box, the CZTS precursor solution was spin coated on the molybdenum glass substrates (20 × 20 × 1.1 mm3 ) at 2750 rpm for 20 s followed by a sintering process on a 320 °C for 3 min on a hot plate. This procedure was repeated seven times to receive as-prepared CZTS film with a thickness of ∼1.2 μm. After that, the precursor thin films were selenized at 520 °C for 10 min in a graphite box to obtain CZTSSe thin films under a N2 atmosphere using a rapid thermal processing(RTP) furnace (MTI, OTF-1200X-4-RTP). Subsequently, CdS buffer layer was deposited on CZTSSe thin film by a modified CBD process. The CBD process was realized according to the reference method except for the deposition time [29]. Firstly, we deposited the CdS of different thickness about 10, 20, 30 and 40 nm on the CZTSSe films, and then the samples were annealed on the hot plate at 200 °C for 10 min under N2 atmosphere. After that, appropriate thickness of CdS films were deposited on the samples with the overall thickness of ∼50 nm. The thicknesses of the CdS films were controlled by changing the deposition time, and the Step Profiler was used to measure the thicknesses of CdS films on blank glass deposited in the same CBD process. For comparison, two samples that only deposited one CdS layer with the thickness of 50 nm with and without annealing were also prepared. Following, ∼ 60 nm of intrinsic ZnO (100 W, 1.0 Pa Ar, 36 min), and ∼250 nm of indium tin oxide (ITO) (100 W, 1.0 Pa Ar, 26 min) were deposited by magnetron sputtering, and 200 nm of Al grid electrode (150 A, 4 mP, 5 min) were thermally evaporated. Finally, the whole device was mechanically scribed into 9 small parts with an active area of about 0.19 cm2 . And the following devices efficiencies are all based on this active area.

2.4. Characterizations 2. Experimental 2.1. Materials All chemicals were used as received without any further purification. Copper(I) oxide (Cu2 O, 99%), cadmium sulfate (CdSO4 , 99%), carbon disulfide (CS2 , >99.9%), 1-butylamine (CH3 (CH2 )3 NH2 , >99.5%) and thioglycollic acid (HSCH2 COOH, 85%) were purchased from Aladdin. Tin (II) oxide (SnO, 99.9%) and thiourea (NH2 CSNH2 , 99%) were obtained from Alfa Aesar. Ethanol (CH3 CH2 OH, AR) and ammonium hydroxide (NH4 OH, 25%) were purchased from Beijing reagent. Zinc oxide (ZnO, 99.9%) and selenium (Se, 99%) were obtained from Sigma-Aldrich.

X-ray diffraction (XRD) patterns were determined by a Rigaku D/MAX-2500 X-ray diffractometer with Cu Kα radiation. Raman spectra were taken on a JobinYvon HR800 micro-Raman spectrometer with a wavelength of 488 nm laser from an Ar+ laser as excitation source. The morphologies of samples were observed with a FEI Quanta 250 scanning electron microscope (SEM). Steady-state PL spectrum was measured using Horiba JobinYvon system with an excitation laser beam at 637 nm. J–V curves were recorded on a Keithley 2400 source meter and a solar simulator (Abet Sun 20 0 0; AM 1.5) by a home-made probe station. The external quantum efficiency (EQE) curves were measured using the Zolix SCS100 QE system.

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Fig. 1. J-V curves of the best performances of CZTSSe solar cells based on CZTSSe/CdS(50 nm), CZTSSe/CdS(50 nm, HT), CZTSSe/CdS(10 nm(HT)/40 nm), CZTSSe/CdS(20 nm(HT)/30 nm), CZTSSe/CdS(30 nm(HT)/20 nm), and CZTSSe/ CdS(40 nm(HT)/10 nm); named S1, S2, S3, S4, S5 and S6.

3. Results and discussion The devices with Glass/Mo/CZTSSe/CdS/ZnO/ITO/Al structures were fabricated by six types of CZTSSe films (CZTSSe/CdS(50 nm), CZTSSe/CdS(50 nm, heat treatment (HT)), CZTSSe/CdS(10 nm(HT)/40 nm), CZTSSe/CdS(20 nm(HT)/ 30 nm), CZTSSe/CdS(30 nm(HT)/20 nm), and CZTSSe/ CdS(40 nm(HT)/10 nm)). We named the samples as S1, S2, S3, S4, S5 and S6, respectively. Fig. 1 shows the best photovoltaic performances of S1, S2, S3, S4, S5 and S6 devices under AM 1.5 G illumination, and the corresponding performance parameters of six types of solar cells are shown in Table S1. The efficiency of CZTSSe device S1 is 4.66%, which basically agrees with previously reported works [13,29,30]. As expected, the efficiency of CZTSSe/CdS (50 nm, HT) is improved to 6.73% after the heat treatment of CZTSSe/CdS sample. For device S2, the open-circuit photovoltage (Voc ) is 0.40 V, the photocurrent density (Jsc ) is 30.60 mA/cm2 , the fill factor (FF) is 54.98%, which are observably higher than those of the device S1. This could be attributed to the beneficial Cd2+ diffusion process similar to the previously reported works [2,24]. Interestingly, when the deposition of CdS was divided into two steps, the sample with firstly deposited 20 nm CdS film showed the best device performance and its power conversion efficiency (PCE) was increased to 8.76% (Voc = 0.42 V, Jsc = 35.42 mA/cm2 , and FF = 58.92%). It shows an encouraging improvement of over 87.98% or 30.16% enhancement compared with the traditional one-step CBD CdS (device S1) or one-step CBD CdS with annealing process (device S2). When the thickness of firstly deposited CdS film is higher (S5 and S6) or lower (S3) than 20 nm, the device efficiencies declined significantly. This indicates that appropriate thickness combination of two-step deposited CdS layer is benefit for the improvement of device performance. Moreover, the statistics on PCEs distributions for the S1, S2 and S4 solar cells are shown in the Fig. S1, and demonstrate the good repeatability. To gain a deeper understanding for this dramatic efficiency improvement from two-step CdS deposition process, a series of characterization were conducted for three types of CZTSSe film, i.e. CZTSSe, CZTSSe/CdS(50 nm) and CZTSSe/CdS(20 nm, without secondly CdS deposition). Fig. 2 shows the XRD pattern of these three types of CZTSSe films before and after heat treatment to research their corresponding crystal structures. As shown in Fig. 2(a) and (c), the XRD pattern of these three samples shows the similar diffraction peaks, but some minor changes were found in the detailed XRD pattern. From the enlarged XRD pattern of before heat treatment samples (Fig. 2(b)), we could find the (112) peaks of deposited CdS samples have a slight shift towards lower angle. After the heat treatment, the shifting of (112) peaks become stronger. It

Fig. 2. (a) The normal and partial enlarged XRD pattern of three kinds of CZTSSe film: CZTSSe, CZTSSe/CdS(50 nm) and CZTSSe/CdS(20 nm) before (a, b) and after (c, d) heat treatment.

indicates that the heat treatment promotes the diffusion of ions, and lead to the minor change of crystal structure. According to the Mitzi and Hao’s works, the Cu+ , Zn2+ , Cd2+ , Sn4+ , S2− and Se2− can diffuse between the CZTSSe and CdS layers, so the diffusion process is complicated [17,18]. As we known, the Cd2+ diffusion could cause the lower angle shift of (112) peak, but the S2− could make it shift to the larger angle, so the Cd2+ diffusion plays the key role in the XRD change. In addition, it is possible that a trace amount of oxygen and hydroxide will exist in the CdS layer as residual substances of CBD, and it also could diffuse in the heat treatment process. But comparing to the Cd2+ , S2− , Se2− and other constituent elements in the CdS and CZTSSe layers, the amount of oxygen ion is so little that we can ignore its diffusion effect in the heat treatment. It worth noting that the characteristic peaks of CdS crystal were not detected in the XRD pattern. This can be due to its relatively low content and weak crystallinity. Fig. 3 shows the Raman spectra of CZTSSe and heat treated CZTSSe/CdS(50 nm, HT), CZTSSe/CdS(20 nm, HT) films. The major Raman peaks of CZTSSe film (black line in Fig. 3(a)) at 171, 196, and 229 cm−1 are presented, which reveal a typical kesterite structure. Some possible binary or ternary impurity phases are not observed in Raman spectra [31,32]. The kesterite characteristic peaks can also be seen in the other two heat treatment samples with deposited CdS layers. From the partially enlarged spectra, we can see that the peaks at ∼196 cm−1 of deposited CdS samples exhibit an obvious shift toward lower wavenumbers, indicating the ion diffusion has occurred in the film, and this is consistent with the results of XRD pattern. But it is interesting to note that, the peak at 296 cm−1 which assigned to CdS compound is only found in sample of CZTSSe/CdS(50 nm, HT)(blue line in Fig. 3(a)) [33]. For the relatively thin CdS layer deposited sample (CZTSSe/CdS(20 nm, HT), red line in Fig. 3(a)), we could hardly see this signal. This further indicates the existing of the diffusion of Cd2+ , but in this mild heat treatment process, the amount of Cd2+ diffusion is certain. The surface morphologies of CZTSSe, CZTSSe/CdS (50 nm, HT) and CZTSSe/CdS (20 nm, HT) films are shown in Fig. 4. In Fig. 4(a), dense CZTSSe absorber layer cover the whole substrates and show

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Fig. 3. Large-scale (a) and detailed (b) Raman spectra of the samples CZTSSe, CZTSSe/CdS(50 nm, HT) and CZTSSe/CdS(20 nm, HT).

Fig. 4. Plane-views of the samples CZTSSe (a), CZTSSe/CdS (50 nm, HT) (b) and CZTSSe/CdS (20 nm, HT) (c).

the smooth grain surface morphology. When the 50 or 20 nm CdS film were deposited on the surface of CZTSSe and then annealed at 200 °C for 10 min, the larger sizes of CZTSSe grains with relatively more compact and dense morphologies compared with that of the pure CZTSSe thin films are obtained (Fig. 4(b) and (c)). This morphology change of CZTSSe films could be attributed to the Cd2+ diffusion caused by CdS annealing, the similar phenomenon has been extensively reported in previous Cd2+ substitution works [17,34]. Moreover, after CdS deposition and heat treatment, the surfaces of CZTSSe grains become rough, which indicates that the CdS has been deposited on the surfaces of CZTSSe grains. But a bit of voids were also be seen on the surface of CdS and CZTSSe layers (as shown in Fig. 4(b) and (c)), which can be due to the shrinkage of CdS crystal during annealing. These open voids may act as shunting paths between CZTSSe and ZnO and thus lead to the degradation of efficiency for solar cell [35]. Therefore the secondly deposition of CdS layer is needful. In addition, the surface morphology of CZTSSe, CZTSSe/CdS(50 nm, HT) and CZTSSe/CdS(20 nm, HT) films with a magnification of 13,0 0 0 are shown in Fig. S2. It is hard to see the voids formed by shrinkage of CdS during the annealing process from these large area images, but we can still see that the CZTSSe/CdS film with annealing treatment can promote the growth of CZTSSe crystals. Fig. 5(a) shows the external quantum efficiency (EQE) curves of device S1, S2 and S4, which corresponding to the traditional CBD CdS without and with heat treatment, and the novel two-step CdS deposition treated devices, respectively. Through fitting the EQE data, the integrated Jsc of CZTSSe solar cell was obtained. The device of S1 has a little bit higher integrated Jsc (30.34 mA/cm2 ) than the Jsc value measured under the solar simulator (30.30 mA/cm2 ). The integrated Jsc of S2 solar cell is 30.69 mA/cm2 , and the value is similar to the S1. This suggests that the traditional CBD CdS with heat treatment process could not effective improve the Jsc of solar cell, and it is consistent with the previously reported work [2]. Remarkably, the integrated Jsc of S4 solar cell is 35.55 mA/cm2 , which is significantly higher than those of the S1 and S2 solar cells. Com-

bined with Fig. 1, the two-step CBD CdS process could remarkedly improve the Jsc , Voc and FF of the CZTSSe solar cells. The band gaps derived from the EQE data of the CZTSSe devices are shown in Fig. 5(b). The bandgaps of S2 and S4 are slightly lower than the S1, and this demonstrates that the heat treatment could enhance the ion diffusion process and then the bandgaps become reduced. Compared with the CZTS/CdS interface, the CZTSSe/CdS interface may lead to a more complex diffusion process for the additional S2− and Se2− diffusion, so the decrease of bandgaps is not only attributed by the Cd2± diffusion process, but Cd2± plays a major role in the heat treatment process. In order to further investigate the influence of the different CdS treated methods on CZTSSe solar cells based on CZTSSe/CdS (50 nm) (S1), CZTSSe/CdS (50 nm, HT) (S2) and CZTSSe/CdS (20 nm(HT)/30 nm) (S4) heterojunction, the steady-state photoluminescence (PL) spectra were tested (see Fig. 6). Usually, the PL emission is related to the recombination channel concerning the trap state and the bandgap. As shown in Fig. 6, the PL intensities of the sample S2 and S4 are significantly higher than that of the sample S1, and the peak position of S4 has a little redshift compared with the sample S2. The PL intensity enhancement is due to the decreased non-radiative recombination of the sample S2 and S4 [36,37], and the redshift is mainly due to the decrement of bandgap, which is consistent with the Fig. 5(b). The sample of S4 shows better charge transfer property at the interfaces than the S1 and S2, which demonstrates that the two-step CBD CdS deposition process is obviously superior to the traditional CBD CdS without and with heat treatment methods. According to the previously reported works by Mitzi and Hao’s groups, CZTSSe/CdS heterojunction heat treatment may lead to a complex ion diffusion [2,23]. Both of the Cu+ , Na+ , Zn2+ , Cd2+ , Sn4+ , S2− and Se2− ions could diffuse between the CZTSSe and CdS layers, so it may produce some new compounds near the CZTSSe/CdS heterojunction. The diffused concentration and depth of ions are influenced by the heat treatment temperature and time, so it is hard to clearly characterize the CZTSSe/CdS heterojunc-

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Fig. 5. (a) External quantum efficiency (EQE) and integrated Jsc of device S1, S2 and S4. (b) The bandgap plots of device S1, S2 and S4.

Fig. 6. Steady-state photoluminescence spectra of device CZTSSe/CdS (50 nm) (S1), CZTSSe/CdS (50 nm, HT) (S2) and CZTSSe/CdS (20 nm (HT)/30 nm) (S4).

tion after heat treatment [8]. However, the probable bandgaps, ECB and EVB of newly generated compounds are clearly. Therefore, we could rationally speculate the band energy alignment of the twostep CdS treatment CZTSSe solar cells. In the heat treatment, Cd2+ and S2− of CdS layer could diffuse into the CZTSSe and then form the Cu2 Cdx Zn1 −x Sn(S, Se)4 (CCZTSSe) compound [8]. Meanwhile, Cd2+ diffusion could decline the bandgap, but the S2− diffusion can increase the bandgap, so the final bandgap depends on the diffused concentration of Cd2+ and S2− . As above discussion in EQE

and PL, the bandgap of CZTSSe is narrower after heat treatment, it means the Cd2+ diffusion is stronger than the S2− diffusion. Based on the Mitzi and Hao’s works, Se2− diffusion could form Cd(S, Se) compound, which band gap is smaller than CdS, and thus it would form a type I band energy alignment with CdS layer. Fig. 7 shows the speculative schematic diagram of band alignment at the p-n junction for two-step CdS deposition (CZTSSe/CdS(20 nm (HT)/30 nm)) and traditional CBD CdS (CZTSSe/CdS(50 nm)) treated devices. It is obviously that, comparing with the traditional CBD CdS device (CZTSSe/CdS(50 nm)), the CBO between CZTSSe and CdS for two-step CdS deposition sample was divided into several small parts. This stepwise increased ECB among CZTSSe and CdS could decrease the energy barrier for electron transportation, and thus present the superior band energy alignment, which may effectively reduce the hetero-interface recombination and enhance the device performance. This agrees well with the aforementioned steadystate PL analysis. The improved band energy alignment also exists in traditional CdS deposition followed heat treatment device. However, comparing with the two-step CdS deposition sample, the Voc , Jsc , FF and even to the overall efficiency of traditional CdS deposition with heat treatment device are all far lower. This indicates that there are other factors, which have a strong impact on the device performances. As we discussed in Fig. 4, the heat treatment will lead to the shrinkage of CdS layer in the first deposited CdS with annealing process, and leave a number of voids on the surface of CZTSSe,

Fig. 7. Speculative schematic diagram of band alignment at the p–n junction for two-step CdS deposition (CZTSSe/CdS(20 nm (HT)/30 nm)) (a) and traditional CBD CdS (CZTSSe/CdS(50 nm)) (b) treated devices.

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which act as a shunting path and thereby increase the recombination of electrons and holes within CZTSSe solar cells. The secondly deposition of CdS layer could effective resolve this problem. As shown in Fig. S3(b), the open voids on the surface of CZTSSe film were completely covered by the second-step CBD CdS layer. It is benefits to the device performance. The remarkable improvement also demonstrated by the shunt resistance (Rsh ) analysis, which is determined using the Sites’ method calculated from the corresponding dark-current curves (Fig. S4) [38]. The Rsh of the devices prepared by two-step CdS deposition and traditional CdS with heat treatment are 813  cm2 and 223  cm2 , respectively. The results indicate that the two-step CdS deposition can greatly reduce the current loss compared to traditional CBD CdS with annealing ones. Therefore, the two-step CdS deposition is a more conducive interface modified strategy to improve the performance of the CZTSSe solar cells. 4. Conclusions In summary, we reported an efficient two-step CdS deposition strategy to improve the performance of Cu2 ZnSn(S, Se)4 solar cells. The firstly deposited CdS layer with heat treatment promoted the ion inter-diffusion and formed a favorable band alignment at hetero-junction; and the next deposited CdS layer covered the voids at CZTSSe/CdS surface caused by the former annealing process, and thus reduced the shunt losses in CZTSSe device. Comparing with the traditional CBD CdS without and with annealing treated devices, the Voc , Jsc , FF, and even to the overall conversion efficiency of this novel two-step CdS deposition treated sample all improved obviously. The best device efficiency, which up to 8.76% (Voc is 0.42 V, Jsc is 35.42 mA/cm2 , and the FF is 58.92%) (with an active area of about 0.19 cm2 ), was achieved by this method. This significantly enhancement could be ascribed to the improved quality of CZTSSe/CdS heterojunction interface. This work indicates that the two-step CdS deposition strategy is simple but effective, and should has large room to improve the quality of CZTSSe/CdS heterojunction interface and further lift up the conversion efficiency of CZTSSe solar cells.

Conflicts of interest There are no conflicts to declare.

Acknowledgments The work was supported by the National Natural Science Foundation of China (91833303, 51872044, 51372036, 51202025 and 51602047), the Key Project of Chinese Ministry of Education (113020A), the 111 project (B13013), the Jilin Province Science and Technology Development Project (20180101175JC and 20140520096JH), the Fundamental Research Funds for the Central Universities (2412019FZ043), and the Open Project of Key Laboratory for UV Emitting Materials and Technology of Ministry of Education (130028857). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2019.06.011.

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