UV-ozone induced surface passivation to enhance the performance of Cu2ZnSnS4 solar cells

Solar Energy Materials and Solar Cells 200 (2019) 109892

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UV-ozone induced surface passivation to enhance the performance of Cu2ZnSnS4 solar cells

T

Shengli Zhanga, Feng Yua, Qing Yuanb, Ying Wangb, Suhuai Weic, Tuquabo Tesfamichaela, Baolai Liangb, Hongxia Wanga,∗ a

School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Brisbane, QLD, 4001, Australia College of Physics Science and Technology, Hebei University, Baoding, 071002, China c Beijing Computational Science Research Center, Beijing, 100193, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: UV-Ozone treatment Interface modification Surface passivation CZTS solar cell

Interface property has been considered one of the most critical factors affecting the performance of semiconductor devices. In this work, we demonstrate an efficient surface passivation for the interface between Cu2ZnSnS4 (CZTS) and CdS buffer layer by using UV-ozone treatment at room temperature. The passivation led to a significant enhancement of short circuit current density (Jsc) of the device from 11.70 mA/cm2 to 18.34 mA/ cm2 and thus efficiency of the CZTS solar cells from 3.18% to 5.55%. The study of surface chemistry has revealed that the UV-ozone exposure led to formation of a Sn–O rich surface on CZTS, which passivates the dangling bonds and forms an ultra-thin energy barrier layer at the interface of CZTS/CdS. The barrier is considered to be responsible for the reduction of non-radiative recombination loss in the solar cells as confirmed by photoluminescence (PL) measurement. The elongated lifetime of minority carriers in the CZTS solar cells by timeresolved PL has further verified the interface passivation effect induced by UV-ozone treatment. This work provides a fast, simple yet very effective approach for surface passivation of CZTS film to boost the performance of CZTS solar cells.

1. Introduction In the community of photovoltaics (PV), two crucial factors are known to affect the power conversion efficiency (PCE) of a PV device: the quality of bulk light absorber and the quality of interface that is formed within the p-n heterojunction, which determines the generation and extraction of charge carriers in the device, respectively [1,2]. As a promising light absorber, Cu2ZnSnS4 (CZTS) material has attracted extensive study in recent years due to its readiness of raw material composed of earth-abundant elements and its optimal bandgap with potential to achieve PCE comparable to Cu(In,Ga)Se2 (CIGS) thin film solar cells but at much lower cost [3,4]. The use of indium and gallium in CIGS raises the concern of restriction of large scale production due to their relatively high prices and limited raw material supply [5]. Although many efforts have been made to boost the performance of CZTS solar cells, such as composition tuning and alkali doping [6–9], currently the PCE of CZTS based solar cells still significantly lags behind its CIGS counterpart with PCE of only 11% for the champion CZTS, and 12.6% for CZTS containing Se, while the PCE of record CIGS cell has reached 22.9% [10].

∗

Compared to CIGS, kesterite CZTS phase is only stable at a narrow range of chemical potentials [11,12]. It means secondary phases such as CuxS, ZnS, SnxSy are readily formed in the CZTS bulk or at the surface of CZTS, which could be detrimental to the performance of the solar cell. To remove these impurities on the surface of CZTS film, methods such as selective chemical etching have been widely employed prior to the deposition of the CdS buffer layer. Hydrochloric (HCl) acid was reported to etch off ZnS secondary phase effectively to boost the performance of CZTS solar cells [13]. In addition, potassium cyanide (KCN) has been used to remove CuxS (1 < x < 2) in CZTS solar cells to create a Cu-poor surface, which is reported to be beneficial for the device performance [14–16]. However, the surface defect properties of CZTS might also be affected by the chemical etching process. For example, some dangling bonds can be left after the chemical etching, which act as the recombination centres. In addition, KCN is a toxic material. Therefore using alternative non-toxic approaches to achieve effective surface cleaning and passivation of CZTS is highly important [17,18]. Ren et al. have reported that oxidation treatment of CZTS surface can produce the same effect with KCN [18]. They found that air exposure of CZTS for 25 h led to improved quality of CdS and thus the

Corresponding author. 2 George St, Brisbane, QLD, 4001, Australia. E-mail address: [email protected] (H. Wang).

https://doi.org/10.1016/j.solmat.2019.04.014 Received 26 February 2019; Received in revised form 9 April 2019; Accepted 13 April 2019 0927-0248/ © 2019 Published by Elsevier B.V.

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solutions including 9.25 mL cadmium acetate aqueous solution (Cd (CH3COO)2, 0.05 M), 4 mL ammonia acetate (CH3COONH4, 1 M), 4 mL thiourea (0.5 M) and 0.3 mL ammonia aqueous solution (25–28%) were added into the beaker in sequence. After CdS deposition in CBD, the CZTS samples were rinsed in DI water before drying by nitrogen gas blow. The intrinsic ZnO and conductive ITO films were deposited sequentially at substrate temperature of 220 °C for 70 min in total by RF and pulsed DC sputtering, respectively [23]. Then 500 nm thick Ag grids were deposited by an e-beam evaporation system (Kurt J. Lesker) as the top contact. In the end, solar cells are defined by mechanical scribing with a designated illumination area of 0.173 cm2. To deconvolute the evolution of surface chemistry on CZTS, a 2 min UV-ozone treatment was also performed by cutting the sample into smaller pieces. The CZTS only etched by HCl solution is named as “Oz 0 min”, while the CZTS that was exposed to UV-ozone for 1 min, 2 min and 4 min after the same HCl etching procedure is named as “Oz 1 min”, “Oz 2 min” and “Oz 4 min”, respectively. To unveil the saturation condition of UV-ozone treatment, a set of 4 CZTS samples with the same precursor stacking and sulfurization process but different UV-Ozone treatment duration was also made into solar cells. The UV-ozone treatment duration for this set of samples was controlled to be 2 min, 4 min, 6 min and 8 min respectively.

performance of CZTS solar cells. This is attributed to oxidation and removal of residual Na2S on the surface of CZTS layer. The drawback of air exposure is that the procedure is tedious and time consuming. To shorten this process, annealing CZTS film in air at 290 °C or 300 °C for several minutes before CdS deposition was found to enable passivation of the surface and grain boundaries, leading to CZTS solar cells with enhanced Voc and FF [19,20]. Nevertheless, high temperature treatment, especially at above 260 °C [21], can increase Cu–Zn disorder level in CZTS, which is responsible for large Voc loss in CZTS solar cells [22,23]. Therefore, although annealing the CZTS film in air at around 300 °C can produce efficient surface passivation effect, it is still desirable to develop an alternative passivation approach on the surface of CZTS film at a low temperature to achieve high performance CZTS solar cells [23,24]. Ozone (O3) is a reactive chemical species with stronger oxidation ability than oxygen (O2). It has been used to passivate the defect states of dangling bonds in the surface of Si solar cells [25,26]. The ozone generated by ultraviolet (UV) radiation in air has also been extensively used for surface cleaning to remove organic contaminants in silicon nanowire solar cells and perovskite solar cells [26–28]. It is also reported that UV-ozone treatment is an effective method to tune the work function of a material through modifying the surface chemical composition [27]. More importantly, UV-ozone treatment can be done at room temperature. In this work, we demonstrate that UV-ozone treatment is a rapid and efficient method to passivate defects at the surface of CZTS films to achieve high quality CZTS/CdS interfaces. Compared to the previous report of slow tedious air exposure oxidation process, UV-ozone treatment is fast (less than 5 min) and is done at room temperature to avoid formation of detrimental defects of Cu–Zn disorder in the CZTS bulk.

2.2. Characterizations The morphology of both top-view of CZTS film and cross-section of the device is measured by field emission scanning electron microscope (SEM, Zeiss Sigma) system, and the composition of CZTS film was determined by the integrated energy dispersive X-ray spectroscopy (EDS) at an acceleration voltage of 20 kV. X-ray photoelectron spectroscopy (XPS, Kratos AXIS) using mono Al Kα (1486.6 eV) X-rays was used to measure the chemical states of elements on the surface of CZTS with different treatments. Ultraviolet photoelectron spectroscopy (UPS) is employed to measure the work function and valence band maximum (VBM) of the CZTS after different surface treatment by using He I (21.2 eV) as excitation source. A Raman spectrometer (Renishaw) with a laser excitation wavelength of 785 nm was used to test the Raman spectra of the CZTS solar cells. The current density-voltage (J-V) curves were measured under 100 mW/cm2 irradiation (AM1.5) generated by a solar simulator (Newport). External quantum efficiency (EQE) measurement (Newport) was executed with voltage bias of 0.2, 0, −0.2 and −0.4 V in the wavelength range of 350–900 nm. The photoluminescence (PL) was excited by a 530 nm laser on the solar cells at room temperature. A PicoHarp 300 time-correlated-single-photon-counting system with a super-continuum laser at the excitation wavelength of 530 nm was used for the time-resolved photoluminescence (TRPL) measurement on the solar cells to determine the recombination process and minority carriers’ lifetime. The 530 nm wavelength laser is selected by considering the potential co-excitation of CdS and/or absorption by CdS (Eg = 2.4 eV) layer at wavelength below its absorption edge (∼520 nm). The penetration depth of the laser in CZTS is estimated to be 150 nm based on the absorption coefficient of CZTS [30]. It is sufficient to probe the region close to CZTS/CdS interface.

2. Experimental 2.1. Device fabrication Solar cells were made with a device structure of Mo/CZTS/CdS/iZnO/ITO/grid-Ag. The CZTS was fabricated by sulfurization of a magnetron sputtering deposited metal stacking precursor on Mo coated soda lime glass in the order of Mo/Zn/Sn/Cu/Zn in a rapid thermal process (RTP) furnace [29]. The thickness of each metal layer was measured by a profilometer and the thickness value of Zn (bottom), Sn, Cu and Zn (top) layer in the precursor was 101 ± 5 nm, 266 ± 53 nm, 175 ± 6 nm and 203 ± 11 nm, respectively. Due to restriction of size of sample holder in the RTP furnace, we could only put a maximum of 4 large samples (sample size: 2 × 2 cm2) each time to make sure they were made exactly under the same fabrication process except the interface treatment. In the sputtering deposition of precursor, the 4 samples were placed in a symmetric position around the centre of sample holder to minimize the composition effect. After this, they were sulfurized in the same batch under the same condition in the RTP furnace. After the CdS deposition, the same symmetric position of the 4 samples were applied in the deposition of the window layer and electrode deposition process. We believe this is critical to ensure the reliability of the results. For the 4 samples made in the same batch, one as-grown CZTS film without any surface treatment was used as reference (“Ref”). The other CZTS samples with the same composition were firstly etched by hydrochloric acid (HCl) solution (32%) for 30 s before being exposed to ozone generated by an ultraviolet radiation (UV-ozone) at room temperature for different duration of 0 min, 1 min and 4 min. The UV-ozone treatment was performed using a commercial digital UV ozone system (PSD Pro Series, Novascan) at atmospheric pressure with ambient air. After this, these CZTS films were put into the same beaker using chemical bath deposition (CBD) method to deposit CdS buffer layer at 80 °C for 4 min. The CBD process for CdS deposition was carried out in a preheated water bath. Specifically, 183 mL deionized (DI) water in a beaker was preheated to 80 °C before precursor

3. Results and discussion Top-view scanning electron microscope (SEM) image of the CZTS film is shown in Fig. 1a, where the film is composed of compact grains with size in the range of 100–1000 nm. After etching with HCl acid and even further with the UV-ozone treatment, CZTS films present a similar morphology (not shown) and no distinct change can be identified according to SEM. The cross-section morphology of the CZTS solar cell (Fig. 1b) shows that the CZTS film consists of a bilayer structure with bottom layer of nanoparticles and top layer of large grains, which is similar to our previous research [29]. A thin MoS2 layer of about 60 nm 2

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SnOx, and ZnO) and ZnS in the HCl solution, leading to the atomic ratio of CZTS closer to stoichiometric condition. The Zn/Sn ratio decreases from 1.34 to 1.06 while Cu/(Zn + Sn) increases from 0.49 to 0.78, and O/(Cu + Zn + Sn) decreases from 0.34 to 0.14 from sample “Ref” to “Oz 0 min” as shown in Fig. 2a. This confirms that the HCl solution treatment can effectively remove the excessive Zn at the surface of CZTS film. It is noted that the O content on the surface of CZTS increases as the exposure duration increases under UV-ozone environment while the content of Cu, Zn, Sn, and S decrease, as indicated by the risen trend of O/(Cu + Zn + Sn) and decline trend of S/ (Cu + Zn + Sn) in Fig. 2a. These results indicate that S is partially replaced by O during the UV-ozone exposure. In terms of impact of UVozone treatment on cations, as shown in Table 1, the decrease of Sn content is the slowest as the UV-ozone exposure duration increases because both atomic ratios of Zn/Sn and Cu/Sn are reduced and because a new Sn–O phase appears to form. In contrast, the decrease of Cu is the fastest because the ratio of Cu/Zn also decreases (Table 1). The XPS signal of an element is sensitive to its surrounding chemical environment [36,37]. High resolution XPS spectrum of Cu 2p, Zn 2p, Sn 3d, S 2p and O 1s characteristic peaks at surface of CZTS with different treatment are shown in Fig. 2b-f. The intensity of Cu 2p peak varies slightly which suggests the change of the content of Cu on CZTS surface with different treatment (Fig. 2b). In contrast, peak shoulders distinctively appear at higher binding energy side for both Zn 2p (1021.5 eV and 1044.5 eV in Fig. 2c) and Sn 3d (486.0 eV and 494.5 eV in Fig. 2d) peaks as the UV-ozone exposure time is more than 2 min comparing to the pristine CZTS. These shoulder peaks can be ascribed to the chemical environment change of the elements. Because the Zn 2p and Sn 3d bonding with oxygen have higher binding energy compared to their bonding with sulphur [36,37], these shoulders indicate that surface oxidation of CZTS occurs, resulting in Sn–O and Zn–O bonds partially replacing the Sn–S and Zn–S bonds in the pristine CZTS (Ref). This hypothesis is further supported with the observation of significantly enhanced oxygen concentration with the increase of UVozone treatment duration (Fig. 2f). The fitting of Sn 3d5/2 peak in Fig. 3 reveals the percentage of Sn–O and Sn–S bonds, which is also summarised in Table 1. The results illustrate a dramatic increase of Sn–O content from the Ref sample (< 20%) to Oz-4 min (∼70%) while the Sn–S content decreases from above 80% for Ref sample to 30% for Oz-4 min. The ratio of Sn–S to Sn–O increases a bit initially because of the removal of oxides by HCl etching and then is reduced greatly from 5.17 to 0.46 with increasing UV-ozone exposure duration from 0 min to 4 min as a result of surface oxidation as shown in Table 1. Fitting XPS of Zn 2p is not successful due to the small shift in binding energy (< 0.5 eV) and the small shoulder peak. The S in the surface of CZTS decreases as the UV-ozone treatment duration increases. Because of the strong oxidation ability of O3 generated by the UV light, even higher oxidation state of sulfur S6+ in the form of SO42− is observed with the sample experiencing 4 min UV-ozone exposure (Fig. 2e). The content of S in the form of SO42− is 2.8% in the CZTS treated by UV-ozone for 2 min (Fig. S3a), which further increases to 6.4% after a treatment of 4 min (Fig. S3b). It is worth to note the other samples without or with UV-ozone treatment less than 2 min, the content of SO42− is negligible. Clearly, the main effect of UV-ozone treatment on the CZTS film can be attributed to the surface oxidation. Based on the results above, it is reasonable to assume that an ultra-thin SnO2 layer or a Sn-rich CZTS surface layer with S partially replaced by O, namely CZT(S,O), may be formed at the surface of CZTS with UV-ozone exposure. The possible phases might be formed at the surface of CZTS by UV-ozone treatment are also summarised in Table 1.

Fig. 1. (a) Top-view morphology of CZTS film and (b) cross-sectional morphology of CZTS solar cell.

is also observed between Mo back contact layer and CZTS absorber layer as a consequence of reaction of Mo with sulfur vapour or with CZTS at high temperature [31,32]. On top of the CZTS film, CdS buffer layer, ZnO and ITO window layers with thickness of 50 nm, 50 nm and 140 nm can be seen in Fig. 1b, respectively. There is no obvious difference at the interface between CdS and CZTS for the samples with different surface treatments based on the SEM results. Raman spectra (Fig. S1) of the CZTS solar cells with and without surface treatment verify the CZTS films are kesterite phase without any secondary phases detected. However, we cannot rule out the existence of large band gap ZnS in the CZTS film because of the detection limit of laser excitation (785 nm) used in the Raman spectroscopy measurement in detecting ZnS [33,34]. 3.1. Chemical composition and chemical states of CZTS Energy dispersive X-ray spectroscopy (EDS) results of the CZTS films show that the films are slightly Zn-poor with Zn/Sn = 0.94 and Cu-poor with Cu/(Zn + Sn) = 0.76. X-ray photoelectron spectroscopy (XPS) was further employed to investigate the surface chemistry of the CZTS film after UV-ozone exposure for different durations. The XPS spectra of CZTS films with different durations of UV-ozone treatment are presented in Fig. S2a. Besides the main elements of Cu, Zn, Sn and S in CZTS, carbon (C 1s) and oxygen (O 1s) were also detected due to the chemical/physical adsorption of oxygen and hydrocarbons at the surface of CZTS in air before the samples were put into the vacuum chamber of XPS equipment. The XPS of sodium (Na 1s) peak in CZTS films with different UV-ozone treatment duration is shown in Fig. S2b. Na 1s peak is clearly observed in the surface of reference CZTS which is originated from the soda-lime glass substrate in the sulfurization process [35]. After HCl etching, the Na 1s signal disappeared due to the dissolution reaction in the aqueous environment. The atomic ratios of the main elements in CZTS together with O on the surface of CZTS based on the XPS spectra (Fig. S2a) are shown in Fig. 2a. The ratios of cations are summarised in Table 1. A high Zn/Sn ratio at the film surface is observed with the as-grown CZTS (Ref), this is ascribed to the designed precursor stacking of Zn/Sn/Cu/Zn in this work, which can form a Zn-rich surface consisting of CZTS and large band gap ZnS [29]. After HCl solution etching, the content of both Zn and O on the surface of CZTS decreases due to dissolution of the metal oxides (e.g., CuxO,

3.2. Work function and band alignment To test the effect of UV-ozone treatment on the electronic property of CZTS, we measured the surface work function of CZTS by ultraviolet photoelectron spectroscopy (UPS). As shown in Fig. 4a and inset 3

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Fig. 2. (a) Evolution of surface atomic ratio in CZTS with respect to UV-ozone exposure time after HCl etching, high resolution XPS spectra of (b) Cu 2p, (c) Zn 2p, (d) Sn 3d, (e) S 2p, and (f) O 1s at the surface of CZTS after different ozone treatment durations. Inset of (e) is enlarged S 2p (SO42−). “Ref” represents the CZTS before HCl etching.

of the treatment duration. The work function (WF), defined by WF = 21.2 eV (He I) - Φ, of the different CZTS samples were calculated. Therefore the WF of the Ref, HCl etched and UV-ozone treated surfaces are determined to be 4.75 eV, 5.15 eV, and 5.05 eV, respectively. The deviation in the valence band maximum (VBM) of these CZTS samples

(lower), a distinct cut-off shift of the binding energy (Φ) towards smaller value is observed with the CZTS (Oz 0 min) after HCl etching compared with the “Ref” CZTS. The shift is about 0.4 eV (Φ = 16.45 eV for Ref vs Φ = 16.05 eV for Oz 0 min). After UV-ozone treatment, the binding energy slightly shifts back by 0.1 eV (Φ = 16.15 eV) regardless

Table 1 Surface atomic ratio of cations and chemical bonding states percentage of Sn with potential surface phases in CZTS after different surface treatments. Sample

Zn/Sn

Cu/Sn

Cu/Zn

Sn–S

Sn–O

[Sn–S]/[Sn–O]

Surface phases

Ref Oz 0 min Oz 1 min Oz 2 min Oz 4 min

1.34 1.06 0.90 0.75 0.88

1.14 1.61 1.29 0.99 1.00

0.85 1.52 1.43 1.31 1.13

81.29% 83.78% 57.99% 37.37% 31.34%

18.71% 16.22% 42.01% 62.66% 68.66%

4.34 5.17 1.38 0.60 0.46

ZnS, CZTS, oxides CZT(S0.84O0.16) CZTS + SnO2 or CZT(S0.58O0.42) CZTS + SnO2 or CZT(S0.37O0.63) CZTS + SnO2 or CZT(S0.31O0.69)

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be considered the same as the bulk CZTS, and hence the CBO of CdS to CZTS is −0.3 eV as well. The band alignment diagram of “Oz 0 min” sample with CdS is shown in Fig. 4(b2), where a high density of interface defects such as dangling bonds might be formed after the HCl etching. On the other hand, an ultrathin SnO2 or Sn-rich CZT(S,O) layer is present on the surface of UV-ozone treated CZTS, which demonstrates 0.1 eV smaller work function compared with HCl etched surface. The thickness of this layer is dependent on the treatment duration, but the work function is the same. By using the bandgap of SnO2 (∼3.7eV) [37], we are able to draw the band alignment diagram of p-n junction of the CZTS solar cells with UV-ozone treatment as illustrated in Fig. 4(b3). This ultra-thin interface layer can effectively reduce the defect density through saturation (oxidation) of the dangling bonds and sulphur vacancies at the surface of CZTS, and/or reduce interface recombination due to the band alignment transferring from cliff-like to spike-like [42]. The CBO of this surface layer is estimated to be ∼0.88 eV relative to the CZTS. 3.3. Performance of CZTS solar cells The current density-voltage (J-V) curves of the best solar cell in each sample of the main set are shown in Fig. 5a. It is clear that HCl etching only leads to a slight increase of the Jsc, from 11.70 mA/cm2 (Ref) to 13.40 mA/cm2 (Oz 0 min). This increment of Jsc can be attributed to the removal of ZnS and metal oxide at the surface of CZTS by HCl acid solution as discussed above. A dramatic enhancement of Jsc (= 17.31 mA/cm2) was achieved for 1 min UV-ozone exposure (Oz 1 min). Further increasing UV-ozone exposure duration to 4 min (Oz 4 min) leads to further increase of Jsc to 18.34 mA/cm2 and also a slight increase of Voc to 652 mV. The characteristic performance parameters of the corresponding solar cells are listed in Table 2 and the statistical distribution of Voc, Jsc, FF and PCE of the CZTS solar cells is illustrated in Fig. S4. By comparison of the trend of Jsc, Voc, FF and PCE with different treatment, apparently the PCE improvement can be mainly ascribed to the enhancement of Jsc of the device. The series resistance (Rs) and shunt resistance (Rsh) of the cells obtained in the J-V plots under light are shown in Table 2 as well. Clearly a higher FF is obtained when the device has lower Rs although larger Rsh seems also help to improve FF as well. It is worth to note that the efficiency of the solar cells in this work is relatively low compared to the champion device reported in literature [43]. This is mainly due to the low FF of the devices, which is related with the high Rs that can be caused by variable factors such as existence of a thick MoS2 layer (60 nm) between the CZTS and Mo substrate, and/or non-optimised window and its contact resistance with the electrode, resistance of CZTS absorber, etc. Furthermore, the diode ideality factor, represents by symbol A, of the solar cell was extracted from the dark J-V curves and are listed in Table 2. The ideality factor of all the devices are above 2. The value is close to the result of the reference solar cell without heat treatment reported by Yan et al. [43]. It suggests the non-ideal quality of the CZTS film is responsible for the large ideality factor. After HCl etching (Oz 0 min), the ideality factor of solar cell diode slightly increases due to a possible damage of the surface of CZTS compared with the “Ref” sample. When the UV-ozone treatment was applied, the ideality factor reduced a bit due to decreased interface recombination at heterojunction of CZTS/ CdS as a result of the passivation effect by ozone treatment. The statistical performance of the CZTS solar cells with 2 min, 4 min, 6 min and 8 min UV-ozone exposure is illustrated in Fig. S5. In this sample set, the highest PCE is achieved with a 2 min UV-ozone treatment which is ascribed to the highest Jsc of the cell. A slight drop of Jsc and thus PCE occurs for a longer treatment of 4 min. An even longer UV-ozone exposure leads to more severe reduction of Jsc and PCE due to aggregation of CdS on the surface of CZTS as confirmed by the digital image of the samples after CdS deposition (Fig. S6). Therefore, we conclude exposure of 2–4 min under UV-ozone provides the best passivation effect. Since the beneficial effect of the UV-Ozone treatment is mainly on

Fig. 3. Components fitting of high resolution XPS of Sn 3d5/2 peak at surface of CZTS for (a) Ref, (b) Oz 0 min, (c) Oz 1 min, (d) Oz 2 min and (e) Oz 4 min. Sn–O and Sn–S represent the bonding states of Sn.

with/without surface treatment is less than 0.05 eV with VBM located at binding energy of about 0.2 eV (upper inset of Fig. 4a). For the samples before HCl etching or with UV-Ozone treatment, VBM corresponding to ZnS or surface oxide layer of SnO2 or CZT(S,O) is also observed at higher binding energy, which is 1.5 eV for ZnS and 1.4 eV for the surface oxide layer (upper inset of Fig. 4a). Thus the VBM of ZnS and surface oxide layer is 1.3 eV and 1.2 eV below the VBM of CZTS, respectively. Nevertheless, it is clear the VBM of the sample is dominated by the CZTS because of either non-fully covered surface by ZnS before etching or the ultra-thin thickness of the surface metal oxide layer after UV-ozone treatment. The Fermi energy (EF) of Au measured by UPS was at binding energy of −0.3 eV. After calibrating the energy scale by taking the Au metal Fermi edge at 0 eV, the VBM of CZTS is determined to be 0.5 eV below EF. Generally, the band alignment of CdS and CZTS is reported to be a “cliff-like” with conduction band offset (CBO) of about −0.3 eV [38,39]. Here, we assume the same CBO between CdS and CZTS in our samples for ease of discussion. All the CZTS absorbers possess the same bandgap, which is 1.62 ± 0.01 eV as extracted from the EQE results in the following section. For the as-grown (Ref) CZTS without any surface treatment, the surface of CZTS contains ZnS and metal oxide phases according to the composition shown in Fig. 2a. ZnS has a larger bandgap (∼3.7 eV) [40] and its CBO to CZTS is 0.78 eV, which is similar to the value reported by Li et al. [41] The band alignment diagram of CZTS with CdS is schematically presented in Fig. 4(b1). Since the ZnS phase is only covering part of the areas on the surface of CZTS based on our previous reports [29], the CBM and VBM of both CZTS and ZnS are overlapped at the surface region. The surface work function should also be the combined effect of all these phases, which is 4.75 eV (Ref), 0.4 eV smaller than the work function of pure CZTS (5.15 eV, HCl etched sample) as obtained from Fig. 4a, thus a smaller band bending. As discussed above, the composition of the HCl etched surface of CZTS is close to stoichiometry of kesterite CZTS (Fig. 2a). As a result, the electron affinity at the surface of CZTS etched by HCl acid solution can

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Fig. 4. (a) UPS of CZTS with different surface treatments, schematic band diagrams of (b1) As-grown (Ref), (b2) HCl etched (Oz 0 min), and (b3) UV-ozone treated (Oz 1 min, Oz 2 min, Oz 4 min) CZTS films to CdS. Insets of (a) are the zoomed-in regions in the dash squares.

Fig. 5. (a) J-V curves and (b) EQE of the best CZTS solar cells in the main set with different CZTS surface treatment processes. (c) Derivative of EQE with respect to hν, d(EQE)/d(hν), vs energy plot and the energy of peak position represents to the bandgap of the absorber. 6

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Table 2 Characteristic data of the best CZTS solar cells in each sample of the main set with different UV-ozone treatments. JEQE is current density calculated from EQE, and A represents ideality factor extracted from dark J-V curves. Sample ID

Voc (V)

Jsc (mA/cm2)

Rs (Ω⋅cm2)

Rsh (Ω⋅cm2)

FF (%)

PCE (%)

JEQE (mA/cm2)

A

Ref Oz 0 min Oz 1 min Oz 4 min

0.622 0.630 0.623 0.652

11.70 13.40 17.31 18.34

13 17 11 11

157 135 145 182

43.70 40.73 44.49 46.38

3.18 3.43 4.80 5.55

11.44 14.27 16.72 17.95

2.47 2.73 2.64 2.66

JEQE are slightly lower than the corresponding Jsc measured by solar simulator except Oz 0 min with only HCl etching treatment. The slightly larger JEQE vs Jsc could be due to overestimate active area of the cell, leading to a smaller Jsc (underestimated) in the I–V measurement. The band gap of all CZTS absorber films are estimated to be 1.62 ± 0.01 eV by taking the energy at the peak of d(EQE)/d(hν) of the corresponding solar cells (Fig. 5c) [44]. Furthermore, the effect of bias voltages (−0.2 V, −0.4 V and 0.2 V) on the EQE was measured to investigate the minority-carrier collection and recombination behaviours of the devices. It is clear that the depletion region (also called the space charge region) in the solar cells’ p-n junctions narrows down at positive bias conditions and expands at negative bias conditions [45]. A stronger voltage-dependent EQE at longer wavelengths indicates poor minority-carrier collection from the bulk of the absorber. The wavelength independence of normalised EQE with bias voltage suggests a loss mechanism that equally affects all carriers regardless of where they are generated in the device. Such a mechanism can be attributed to the interface of the heterojunction, either interface recombination or a heterojunction band-offset induced barrier [45]. The EQE spectrum of the sample with high surface/interface defect density is more sensitive to the voltage bias. The EQE spectra with different bias voltages and normalised EQE relative to the EQE at 0 bias of all devices are shown in Fig. 6. The EQE of “Ref” device measured at bias voltage of −0.4 V showed strong fluctuation, which is therefore discarded. It is found that the EQE of the devices show different extent of increase/decrease at negative/positive bias voltage compared to their performance at zero bias irrelevant to the wavelength. In the case of the reference (Ref) sample, the effect of bias on EQE is the largest. The average ratio of EQE-0.2V/EQE0V and EQE0.2V/ EQE0V is approximate 1.2 and 0.8, respectively (Fig. 6a). In contrast, these average values of normalised EQE are 1.1 and 0.9 for sample “Oz 0 min” at bias voltage of −0.2 V and 0.2 V. They are much closer to unit 1, indicating smaller bias voltage effect than the “Ref” sample. In the meantime, the ratio is slightly below 1.2 at a bias voltage of −0.4 V (Fig. 6b). Furthermore, the “Oz 1min” sample shows that the normalised value of EQE are around 1.1, 1.05, and 0.92 at bias voltage of −0.4 V, −0.2 V and 0.2 V (Fig. 6c), which is closer to 1 compared to “Oz 0 min”. The smallest effect of bias voltage on the EQE is obtained with the sample “Oz 4 min” (Fig. 6d), where the ratio is 1.05, 1.03 and 0.94 under bias voltage of −0.4 V, −0.2 V and 0.2 V, respectively. A relatively large value of normalised EQE is observed on the un-treated samples. The largely constant EQE of the devices with UV-ozone treatment indicate the interface quality between CZTS and CdS with the CZTS surface treated by UV-ozone exposure have been significantly improved. It is known that recombination of a solar cell is proportional to the defect density. Apparently, the surface passivation by UV-ozone treatment helps to reduce the recombination centres at the surface of CZTS absorber film, leading to higher separation efficiency of the photo-generated electron-hole pairs at the CZTS/CdS heterojunction.

improving Jsc, we believe the observed phenomenon in this work should also be valid for high efficiency device, which however need to be investigated in the future. The interface modification effect on the performance of CZTS solar cells can be easily understood based on the surface composition and band alignments discussed before. The worst performance was obtained with the CZTS without any treatment which can be ascribed to blocking effect by the high CBM of the ZnS phase at surface of the CZTS film. When the ZnS is removed by HCl etching, it creates a surface of CZTS with favourable stoichiometry composition. However, such cliff band alignment between CZTS and CdS can shift the interface defects such as dangling bonds, wrong bonds or surface adsorbed [H+] after the acid treatment deep inside the gap, which may become interface recombination centres. Therefore, the improvement of Jsc is not significant. A further UV-ozone treatment effectively passivated these defects and created an ultrathin oxide barrier layer (Fig. 4b3) without blocking the transport of electrons at the interface of CZTS/CdS due to possible tunnelling effect. Longer UV-ozone exposure duration can passivate more surface defects as verified by higher oxygen content on the material surface, leading to higher PCE, and it saturates before 4 min exposure. The effect of UV-ozone treatment on device performance observed in this work is in good agreement with the effect of long duration of air exposure (25 h) on CZTS performance reported by Ren et al. [18]. Nevertheless, the UV-ozone treatment method demonstrated in this work is much simpler, quicker and effective and the dose of O3 can be accurately controlled by tuning the exposure time to optimise the output performance of CZTS solar cells. The oxidised surface of CZTS effectively helps reducing the recombination loss at the interface of CZTS/CdS and thus boosts the PCE. SnO2 layer inserted between CZTS and CdS has been reported to benefit CZTS solar cells [37], but mainly improving the Voc and FF of the device. Herein, only an increase of FF is clearly observed as the UV-ozone treatment duration increased from 0 min to 4 min owing to the drop of Rs. However, the Voc does not show the same monotonous trend although the highest Voc is achieved with a 4 min UV-ozone treatment. Hence, the main increment of Jsc can be attributed to an interface layer of Sn–O rich CZT(S,O) rather than SnO2 at the initial stage of short UV-ozone exposure period, which passivates the deep level defects of dangling bonds, wrong bonds or surface adsorbed [H+] first. A prolonged exposure further transforms the surface into thicker SnO2 layer, which degrades the heterojunction through inducing aggregation of CdS, and thus leading to the drop of PCE of the solar cells. Alternatively the formation of Cd(S,O) layer at the interface to CdS buffer in the chemical deposition process may also occur, which further modify the band offsets at the heterojunction with a proper content ratio of oxygen. The interface layer effectively reduces the deep level defect density, which behaves as non-radiative recombination centre of the photo-generated carriers, leading to improved charge extraction. Fig. 5b shows external quantum efficiency (EQE) of the best solar cells on each sample in the main set in wavelength range of 350–900 nm without bias voltage. A clear rise of the EQE can be seen with the CZTS solar cells with UV-ozone treatment in the range of 350–750 nm, which indicates a decrease in recombination associated with surface modification for the CZTS. The integrated current density of the cell from EQE (JEQE) is summarised in Table 2 as well. All of the

3.4. Photoluminescence and time-resolved photoluminescence of CZTS solar cells To further investigate the recombination mechanism of the CZTS solar cell devices with different surface treatment, photoluminescence 7

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Fig. 6. EQE and normalised EQE relative to EQE at bias = 0 V (EQE/EQE0V) of the best CZTS solar cells of (a) Ref, (b) Oz 0 min, (c) Oz 1 min, and (d) Oz 4 min under different bias voltage.

work, the PL intensity of the device with HCl etching is comparable with the reference sample without treatment, which is consistent to the similar Jsc of both devices. The slight improvement of Jsc for the Oz 0 min sample can be ascribed to the secondary phase (ZnS) removal by HCl etching, where more electron-hole pairs can be generated due to more effective absorption by the CZTS. For the CZTS treated by UVozone, the PL intensity increases as the treatment time is prolonged, where the PL intensity of Oz 4 min is over twice of the sample of Oz 0 min. Consequently, a great drop of the non-radiative recombination can be expected at the interface of heterojunction with UV-ozone treatment [46], where Jsc of the devices are also improved with the increase of UV-ozone treatment duration. This drop of non-radiative recombination verifies the decrease of defect density at the interface of

(PL) and time-resolved photoluminescence (TRPL) were performed. In a solar cell, the photo-induced electron-hole pairs in the absorber will be partially separated at the heterojunction to generate current as charge extraction process and partially be consumed through radiative and non-radiative recombination processes. The radiative recombination generates photons emitting as PL, while the non-radiative recombination occurs in the bulk absorber and interface of the heterojunction generating phonons through deep level defects which is known as Shockley-Read-Hall (RSH) recombination process. Fig. 7a shows the PL spectra of the CZTS solar cells with different surface treatment before deposition of CdS. The strong PL intensity indicates a high radiative recombination due to either low carrier separation efficiency and/or low non-radiative recombination. In this 8

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contradictory to the trend of PCE. Actually, the performance of a solar cell is dependent on both a lifetime of minority carriers and radiative versus non-radiative recombination differences, which can be more directly reflected by the PL intensity [46,49]. As shown in Fig. 7a, the PL intensity of Oz 4 min is higher than Oz 1 min, indicating lower nonradiative recombination for sample Oz 4 min. Therefore, we can propose that the gain from the reduction of non-radiative recombination surpasses the loss induced by the slightly lower lifetime for the sample Oz 4 min compared with Oz 1 min, and thus the highest PCE is achieved by UV-ozone treatment for 4 min. More detailed studies of the treatment duration by the UV-ozone treatment could further enhance the gain through balancing the lifetime drop and non-radiative recombination loss. 4. Conclusions In this study, a simple and more effective room temperature surface modification method on CZTS layer based on UV-ozone treatment is developed to improve efficiency of CZTS solar cells. UV-ozone treatment led to partial replacement of S by O at the surface region of CZTS, forming a Sn–O rich CZT(S,O) surface layer on the CZTS. This ultrathin oxide surface layer performs as an energy barrier on the interface of CZTS/CdS junction. It effectively passivated the interface defects and thus help reduce non-radiative recombination without blocking the transport of electrons. As a result, the solar cell performance is enhanced from 3.18% to 5.55% with 4 min UV-ozone treatment, which is mainly attributed to the improvement of Jsc from 11.70 mA/cm2 to 18.34 mA/cm2, and a slight increase of Voc and FF. The enhanced Jsc can be ascribed to the reduced deep level interface defects density leading to a reduction of non-radiative recombination and surface passivation effect as confirmed by PL and TRPL. While the improvement of Voc and FF can be ascribed to the improved band alignment and reduced series resistance. A UV-ozone exposure duration of 2–4 min was found to be optimal to saturate the defects at the heterojunction interface through UV-ozone treatment passivation. Acknowledgements Fig. 7. (a) PL and (b) lifetime of minority carriers in the CZTS solar cell with different UV-ozone treatment duration on the CZTS surface compared to solar cell with as-grown CZTS “Ref”.

The authors would like to thank the technical support for the characterization by Central Analytical Research Facility (CARF) in Institute for Future Environments, Queensland University of Technology (QUT). Especially, the technical support for the operation of XPS and data analysis by Dr. Josh Lipton-Duffin is acknowledged. S. Zhang also thanks the Postgraduate Research Award (PRA) scholarship funded by QUT. This work is funded by the Australian Research Council Future Fellowship (FT120100674), Australia. The work at Hebei University is supported by the National Natural Science Foundation of China under Grant No. 61774053. The work at Beijing CSRC is supported by the National Key Research and Development Program of China under Grant No. 2016YFB0700700, and the National Natural Science Foundation of China under Grant No. 51672023; 11634003; U1530401.

CZTS/CdS with UV-ozone treatment, which acts as RSH recombination centres. As a result, UV-ozone treatment effectively passivated the interface of CZTS/CdS and enhanced the extraction of charge carriers in the device. TRPL measured at the peak of PL (∼890 nm) exhibits a multi-exponential decay feature (Fig. S7). It was fitted by a bi-exponential decay function as reported in the literature using the high injection model [47], which is composed of a fast decay (initial section, τ1) and a slow decay (final section, τ2). The fast decay component can be ascribed to the band-to-band transition in the high injection regime and the slow decay is correlated with the recombination of minority carriers [48]. The fitted lifetime in the samples with different surface treatment are shown in Fig. 7b. The fast decay lifetime is in the range of 0.2–0.4 ns, while the slow decay lifetime locates from ∼1.3 to 2 ns. In regards of recombination of minority carriers, lifetime of slow decay components is normally discussed. There is no distinct change of the lifetime for the CZTS with only HCl etching process compared with the un-etched sample (Ref), Fig. 7b. However, a clear rise of the lifetime is observed with the device after a treatment by UV-ozone with either 1 min or 4 min, ascribing to a decreased recombination rate of the minority carriers. The longest lifetime achieved in this study is 1.94 ns with the CZTS solar cell with 1 min UV-ozone treatment rather than a longer ozone treatment period of 4 min (lifetime = 1.50 ns), which seems

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solmat.2019.04.014. References [1] J. Li, D. Wang, X. Li, Y. Zeng, Y. Zhang, Cation substitution in earth-abundant kesterite photovoltaic materials, Adv. Sci. 5 (2018) 1700744. [2] S. Gao, Z. Jiang, L. Wu, J. Ao, Y. Zeng, Y. Sun, Y. Zhang, Interfaces of high-efficiency kesterite Cu2ZnSnS(e)4 thin film solar cells, Chin. Phys. B 27 (2018) 018803. [3] M. Yamaguchi, K.-H. Lee, K. Araki, N. Kojima, H. Yamada, Y. Katsumata, Analysis for efficiency potential of high-efficiency and next-generation solar cells, Prog. Photovoltaics Res. Appl. 26 (2018) 543–552.

9

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S. Zhang, et al. [4] H. Wang, Progress in thin film solar cells based on Cu2ZnSnS4, Int. J. Photoenergy 2011 (2011) 801292. [5] S. Siebentritt, High voltage, please!, Nat. Energy 2 (2017) 840–841. [6] S. Zhuk, A. Kushwaha, T.K.S. Wong, S. Masudy-Panah, A. Smirnov, G.K. Dalapati, Critical review on sputter-deposited Cu2ZnSnS4 (CZTS) based thin film photovoltaic technology focusing on device architecture and absorber quality on the solar cells performance, Sol. Energy Mater. Sol. Cells 171 (2017) 239–252. [7] K. Yu, E.A. Carter, Determining and controlling the stoichiometry of Cu2ZnSnS4 photovoltaics: the physics and its implications, Chem. Mater. 28 (2016) 4415–4420. [8] X. Liu, Y. Feng, H. Cui, F. Liu, X. Hao, G. Conibeer, D.B. Mitzi, M. Green, The current status and future prospects of kesterite solar cells: a brief review, Prog. Photovoltaics Res. Appl. 24 (2016) 879–898. [9] K. Kaur, N. Kumar, M. Kumar, Strategic review of interface carrier recombination in earth abundant Cu–Zn–Sn–S-Se solar cells: current challenges and future prospects, J. Mater. Chem. 5 (2017) 3069–3090. [10] M.A. Green, Y. Hishikawa, E.D. Dunlop, D.H. Levi, J. Hohl-Ebinger, A.W.Y. HoBaillie, Solar cell efficiency tables (version 52), Prog. Photovoltaics Res. Appl. 26 (2018) 427–436. [11] A. Nagoya, R. Asahi, R. Wahl, G. Kresse, Defect formation and phase stability of Cu2ZnSnS4 photovoltaic material, Phys. Rev. B 81 (2010) 113202. [12] S. Chen, J.-H. Yang, X.G. Gong, A. Walsh, S.-H. Wei, Intrinsic point defects and complexes in the quaternary kesterite semiconductorCu2ZnSnS4, Phys. Rev. B 81 (2010) 245204. [13] A. Fairbrother, E. Garcia-Hemme, V. Izquierdo-Roca, X. Fontane, F.A. PulgarinAgudelo, O. Vigil-Galan, A. Perez-Rodriguez, E. Saucedo, Development of a selective chemical etch to improve the conversion efficiency of Zn-rich Cu2ZnSnS4 solar cells, J. Am. Chem. Soc. 134 (2012) 8018–8021. [14] M. Buffiere, G. Brammertz, S. Sahayaraj, M. Batuk, S. Khelifi, D. Mangin, A.A. El Mel, L. Arzel, J. Hadermann, M. Meuris, J. Poortmans, KCN chemical etch for interface engineering in Cu2ZnSnSe4 solar cells, ACS Appl. Mater. Interfaces 7 (2015) 14690–14698. [15] M.E. Erkan, V. Chawla, I. Repins, M.A. Scarpulla, Interplay between surface preparation and device performance in CZTSSe solar cells: effects of KCN and NH4OH etching, Sol. Energy Mater. Sol. Cells 136 (2015) 78–85. [16] G.Y. Kim, W. Jo, K.D. Lee, H.S. Choi, J.Y. Kim, H.Y. Shin, T.T.T. Nguyen, S. Yoon, B.S. Joo, M. Gu, M. Han, Optical and surface probe investigation of secondary phases in Cu2ZnSnS4 films grown by electrochemical deposition, Sol. Energy Mater. Sol. Cells 139 (2015) 10–18. [17] A. Chavda, M. Patel, I. Mukhopadhyay, A. Ray, Facile, noncyanide based etching of spray deposited Cu2ZnSnS4 thin films for secondary phase removal, ACS Sustain. Chem. Eng. 4 (2016) 2302–2308. [18] Y. Ren, J.J.S. Scragg, M. Edoff, J.K. Larsen, C. Platzer-Bjorkman, Evolution of Na-S (-O) compounds on the Cu2ZnSnS4 absorber surface and their effects on CdS thin film growth, ACS Appl. Mater. Interfaces 8 (2016) 18600–18607. [19] J.H. Kim, S.-Y. Choi, M. Choi, T. Gershon, Y.S. Lee, W. Wang, B. Shin, S.-Y. Chung, Atomic-scale observation of oxygen substitution and its correlation with holetransport barriers in Cu2ZnSnSe4 thin-film solar cells, Adv. Energy Mater. 6 (2016) 1501902. [20] F.Y. Liu, C. Yan, J.L. Huang, K.W. Sun, F.Z. Zhou, J.A. Stride, M.A. Green, X.J. Hao, Nanoscale microstructure and chemistry of Cu2ZnSnS4/CdS interface in kesterite Cu2ZnSnS4 solar cells, Adv. Energy Mater. 6 (2016) 1600706. [21] J.J.S. Scragg, L. Choubrac, A. Lafond, T. Ericson, C. Platzer-Björkman, A low-temperature order-disorder transition in Cu2ZnSnS4 thin films, Appl. Phys. Lett. 104 (2014) 041911. [22] S. Bourdais, C. Choné, B. Delatouche, A. Jacob, G. Larramona, C. Moisan, A. Lafond, F. Donatini, G. Rey, S. Siebentritt, A. Walsh, G. Dennler, Is the Cu/Zn disorder the main culprit for the voltage deficit in kesterite solar cells? Adv. Energy Mater. 6 (2016) 1502276. [23] S. Zhang, N.D. Pham, T. Tesfamichael, J. Bell, H. Wang, Thermal effect on CZTS solar cells in different process of ZnO/ITO window layer fabrication, Sustainable Mater. Technol. 18 (2018) e00078. [24] S. Zhang, T. Wang, S. Lin, Y. Zhang, T. Tesfamichael, J. Bell, H. Wang, Effect of different thermo-treatment at relatively low temperatures on the properties of indium tin-oxide thin films, Thin Solid Films 636 (2017) 702–709. [25] M. Dutta, N. Fukata, Low-temperature UV ozone-treated high efficiency radial p-n junction solar cells: N-Si NW arrays embedded in a p-Si matrix, Nano Energy 11 (2015) 219–225. [26] S. Bakhshi, N. Zin, H. Ali, M. Wilson, D. Chanda, K.O. Davis, W.V. Schoenfeld, Simple and versatile UV-ozone oxide for silicon solar cell applications, Sol. Energy

Mater. Sol. Cells 185 (2018) 505–510. [27] Z. Wang, J. Fang, Y. Mi, X. Zhu, H. Ren, X. Liu, Y. Yan, Enhanced performance of perovskite solar cells by ultraviolet-ozone treatment of mesoporous TiO2, Appl. Surf. Sci. 436 (2018) 596–602. [28] V.T. Tiong, N.D. Pham, T. Wang, T. Zhu, X. Zhao, Y. Zhang, Q. Shen, J. Bell, L. Hu, S. Dai, H. Wang, Octadecylamine-functionalized single-walled carbon nanotubes for facilitating the formation of a monolithic perovskite layer and stable solar cells, Adv. Funct. Mater. 28 (2018) 1705545. [29] S. Zhang, H.D. Hadi, Y. Wang, B. Liang, V.T. Tiong, F. Ali, Y. Zhang, T. Tesfamichael, L.H. Wong, H. Wang, A precursor stacking strategy to boost opencircuit voltage of Cu2ZnSnS4 thin-film solar cells, IEEE Journal of Photovoltaics 8 (2018) 856–863. [30] P.A. Fernandes, P.M.P. Salomé, A.F. da Cunha, Study of polycrystalline Cu2ZnSnS4 films by Raman scattering, J. Alloy. Comp. 509 (2011) 7600–7606. [31] V.T. Tiong, Y. Zhang, J. Bell, H. Wang, Carbon concentration dependent grain growth of Cu2ZnSnS4 thin films, RSC Adv. 5 (2015) 20178–20185. [32] J.J. Scragg, T. Kubart, J.T. Wätjen, T. Ericson, M.K. Linnarsson, C. PlatzerBjörkman, Effects of back contact instability on Cu2ZnSnS4 devices and processes, Chem. Mater. 25 (2013) 3162–3171. [33] A. Fairbrother, X. Fontané, V. Izquierdo-Roca, M. Espíndola-Rodríguez, S. LópezMarino, M. Placidi, L. Calvo-Barrio, A. Pérez-Rodríguez, E. Saucedo, On the formation mechanisms of Zn-rich Cu2ZnSnS4 films prepared by sulfurization of metallic stacks, Sol. Energy Mater. Sol. Cells 112 (2013) 97–105. [34] M. Dimitrievska, A. Fairbrother, X. Fontané, T. Jawhari, V. Izquierdo-Roca, E. Saucedo, A. Pérez-Rodríguez, Multiwavelength excitation Raman scattering study of polycrystalline kesterite Cu2ZnSnS4 thin films, Appl. Phys. Lett. 104 (2014) 021901. [35] P.M.P. Salomé, H. Rodriguez-Alvarez, S. Sadewasser, Incorporation of alkali metals in chalcogenide solar cells, Sol. Energy Mater. Sol. Cells 143 (2015) 9–20. [36] X. Li, X. Li, B. Zhu, J. Wang, H. Lan, X. Chen, Synthesis of porous ZnS, ZnO and ZnS/ ZnO nanosheets and their photocatalytic properties, RSC Adv. 7 (2017) 30956–30962. [37] H. Sun, K. Sun, J. Huang, C. Yan, F. Liu, J. Park, A. Pu, J.A. Stride, M.A. Green, X. Hao, Efficiency enhancement of kesterite Cu2ZnSnS4 solar cells via solutionprocessed ultrathin tin oxide intermediate layer at absorber/buffer interface, ACS Appl. Energy Mater. 1 (2017) 154–160. [38] A. Crovetto, O. Hansen, What is the band alignment of Cu2ZnSn(S,Se)4 solar cells? Sol. Energy Mater. Sol. Cells 169 (2017) 177–194. [39] M. Bar, B.A. Schubert, B. Marsen, R.G. Wilks, S. Pookpanratana, M. Blum, S. Krause, T. Unold, W. Yang, L. Weinhardt, C. Heske, H.W. Schock, Cliff-like conduction band offset and KCN-induced recombination barrier enhancement at the CdS/Cu2ZnSnS4 thin-film solar cell heterojunction, Appl. Phys. Lett. 99 (2011) 222105. [40] G. Altamura, J. Vidal, Impact of minor phases on the performances of CZTSSe thinfilm solar cells, Chem. Mater. 28 (2016) 3540–3563. [41] W. Li, J. Chen, C. Yan, X. Hao, The effect of ZnS segregation on Zn-rich CZTS thin film solar cells, J. Alloy. Comp. 632 (2015) 178–184. [42] R. Klenk, Characterisation and modelling of chalcopyrite solar cells, Thin Solid Films 387 (2001) 135. [43] C. Yan, J. Huang, K. Sun, S. Johnston, Y. Zhang, H. Sun, A. Pu, M. He, F. Liu, K. Eder, L. Yang, J.M. Cairney, N.J. Ekins-Daukes, Z. Hameiri, J.A. Stride, S. Chen, M.A. Green, X. Hao, Cu2ZnSnS4 solar cells with over 10% power conversion efficiency enabled by heterojunction heat treatment, Nature Energy 3 (2018) 764–772. [44] A. Guchhait, Z. Su, Y.F. Tay, S. Shukla, W. Li, S.W. Leow, J.M.R. Tan, S. Lie, O. Gunawan, L.H. Wong, Enhancement of open-circuit voltage of solution-processed Cu2ZnSnS4 solar cells with 7.2% efficiency by incorporation of silver, ACS Energy Lett 1 (2016) 1256–1261. [45] S.S. Hegedus, W.N. Shafarman, Thin-film solar cells: device measurements and analysis, Prog. Photovoltaics Res. Appl. 12 (2004) 155–176. [46] Y.S. Lee, T. Gershon, T.K. Todorov, W. Wang, M.T. Winkler, M. Hopstaken, O. Gunawan, J. Kim, Atomic layer deposited aluminum oxide for interface passivation of Cu2ZnSn(S,Se)4 thin-film solar cells, Adv. Energy Mater. 6 (2016) 1600198. [47] W.K. Metzger, I.L. Repins, M.A. Contreras, Long lifetimes in high-efficiency Cu (In,Ga)Se2 solar cells, Appl. Phys. Lett. 93 (2008) 022110. [48] S. Shirakata, T. Nakada, Time-resolved photoluminescence in Cu(In,Ga)Se2 thin films and solar cells, Thin Solid Films 515 (2007) 6151–6154. [49] S. Rühle, Tabulated values of the Shockley–Queisser limit for single junction solar cells, Sol. Energy 130 (2016) 139–147.

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