Formation of the front-gradient bandgap in the Ag doped CZTSe thin films and solar cells

Accepted Manuscript

Formation of the front-gradient bandgap in the Ag doped CZTSe thin films and solar cells Dongxiao Wang , Jianyu Wu , Xiyu Liu , Li Wu , Jianping Ao , Wei Liu , Yun Sun , Yi Zhang PII: DOI: Reference:

S2095-4956(19)30121-4 https://doi.org/10.1016/j.jechem.2019.03.026 JECHEM 813

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

20 January 2019 12 March 2019 18 March 2019

Please cite this article as: Dongxiao Wang , Jianyu Wu , Xiyu Liu , Li Wu , Jianping Ao , Wei Liu , Yun Sun , Yi Zhang , Formation of the front-gradient bandgap in the Ag doped CZTSe thin films and solar cells, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.03.026

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Highlights 

Front Ag-gradient bandgap ACZTSe thin films were formed by prealloying followed by selenization process.

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AgZn3, Ag3Sn, and Sn-Ag-Cu alloy phases were formed after prealloying process at 250 oC. Liquid phase Sn-Ag-Cu alloy can assist the elements distribute in the absorber layer during the selenization process.

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VOC of devices are significantly increased to above 420 mV because of the

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formation of the front Ag-gradient bandgap structure.

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Formation of the front-gradient bandgap in the Ag doped CZTSe thin films and solar cells

Dongxiao Wanga, Jianyu Wua, Xiyu Liua, Li Wub, Jianping Aoa, Wei Liua, Yun Suna,

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Yi Zhanga,*

Institute of Photoelectronic Thin Film Devices and Technology and Tianjin Key

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Laboratory of Thin Film Devices and Technology, Nankai University, Tianjin 300350, China

School of Physical Science, Nankai University, Tianjin 300071, China

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*Corresponding author. Email address: [email protected] (Y. Zhang).

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Abstract: The graded bandgap of kesterite based absorber layer is an important way to achieve high efficiency solar cells. Incorporation of Ag into CZTSSe thin films can adjust the bandgap and thus reduce the VOC-deficit and improve the quality of crystallization. However, the distribution of Ag is difficult to control due to the

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quick diffusion of Ag under the high temperature. In this study, we achieve the front Ag-gradient in kesterite structured compound films by prealloying followed by selenization process at 550 oC. AgZn3, Ag3Sn, and Sn-Ag-Cu alloy phases were

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formed during prealloying stage at 250 oC. After prealloying process, Ag tends to

distribute at the front surface of the ACZTSe thin films. Combining the results of experiment and SCAPS simulation, the significantly VOC improvement of devices is

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ascribed to the formation of the front Ag-gradient bandgap structure in the absorber layer. This facile prealloying selenization process affords a feasible method to design

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the graded bandgap structure absorber layers, which will promote the fabrication of

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high efficient graded bandgap structure solar cells. Keywords: CZTSe solar cell; Front Ag-gradient; Elements distribution; SCAPS

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simulation

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1. Introduction As a kind of direct bandgap semiconductor materials, kesterite structured Cu2ZnSn(S,Se)4 (CZTSSe) is considered as one of the high efficient solar cell materials because of its non-toxic and earth-abundant components [1–6]. In the past

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few years, many works have been made to improve the efficiency of CZTSSe thin film solar cells. However, the highest efficiency of CZTSSe solar cells has been

pinned at 12.6% for many years [7]. The main limitation of low efficiency is the large

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open circuit voltage deficit (VOC-deficit) [2,8], which was reported by many references. The main reason of Voc-deficit can be attributed to the cation disordering and the associated band tailing [4,5]. In addition, interface defects and undesirable

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energy band alignment can also lead to the large VOC-deficit [9–11]. An efficient approach to enhance the VOC of device is to appropriately enlarge the

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bandgap of the absorber layer [12,13]. However, there is a trade-off between VOC and

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short-circuit current density (JSC) [2]: a large bandgap will promote the improvement of VOC [14], but it simultaneously means a narrow spectral response range and thus

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results in a low JSC. To solve this problem, graded bandgap structure in the absorber

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layer is proposed through engineering the elements distribution along the depth of films [15]. Theoretically, ion substitution of absorber layer materials is a feasible way to tune the bandgap, such as S/Se, Ag/Cu, and Ge/Sn substitution [2,16]. For example, Yang et al. achieved a 12.3% efficient device with front bandgap gradient using an appropriate SeS2/Se ratio [17], and Wu et al. reported a V-shaped bandgap structured absorber layer by Ag substitution, and the efficiency of device was increased to 11.2%

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[18]. As indicated above, Ag doping is a promising method to realize graded bandgap in CZTSSe solar cells, the bandgap of (CuxAg1-x)2ZnSn(S,Se)4 (ACZTSSe) can be adjusted from 1.0 eV to 2.05 eV [2,19]. Moreover, the introduction of Ag ions can

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also suppress Cu/Zn antisite defects because the radius of Ag+ (1.14 Å) is larger than that of Cu+ (0.74 Å) [2,20]. Ag doping can improve the crystallization quality of

CZTSSe thin films, increase grain size and reduce defects. However, Ag diffuses

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very fast and easily distribute uniformly into the film at high temperature, so it is difficult to control the content along the depth of ACZTSe thin films under the high

selenization temperature. Usually, the distribution of Ag can be controlled by Ag gradient concentration under the low selenization temperature. For example, the “V”

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shape structured Ag-gradient CZTSSe reported by Wu et al. was selenized at 480 oC

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[18]. However, the low selenization temperature will deteriorate the quality of

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crystallization of absorber layer.

In this work, we prepared the Ag doped CZTSe film by high temperature

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selenization process. A front Ag-gradient bandgap ACZTSe thin film through

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prealloying followed by selenization process was obtained and the VOC of the devices was improved. Simultaneously, it was found that the Ag deposited on the Mo layer can affect the elements distribution after the selenization. The incorporation of Ag can improve the uniformity of elements distribution and improve the crystal quality of the absorbers. Finally, the Voc of the ACZTSe devices was increased to above 420 mV. This method not only affords a feasible method to achieve Ag-gradient doping

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in ACZTSe film but also affords a new insight into the role of Ag doping on the growth of kesterite structured films. High efficient kesterite structured solar cells can be prepared through designing Ag-gradient ACZTSe thin films architecture. 2. Experimental

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2.1 Fabrication of Ag doping ACZTSe film The Ag films were prepared on the Mo layer through a facile chemical deposition method. 4 mL of triethanolamine and 5 mL of silver nitrate solution were added to

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the beakers, and deionized water was added to reach 100 mL. Then, the glass

deposited with Mo layer was put into the beaker at room temperature. The thickness of Ag films can be controlled with different deposited times 0 min, 1 min, 3 min,

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and 5 min which were named as Ag-0 min, Ag-1 min, Ag-3 min, Ag-5 min, respectively. the

metallic

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Mo/Ag/Sn/Cu/Zn/Sn/Cu were deposited by DC-magnetron sputtering on 4×4 cm2 Mo-coated SLG using metal targets with 99.99% purity. The compositions of

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ACZTSe metallic precursors were about Cu/(Zn+Sn)≈0.75, Zn/Sn≈1.05. The ratio

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of Ag/(Cu+Ag) was 0 (Ag-0 min), 0.13 (Ag-1 min), 0.18 (Ag-3 min) and 0.24 (Ag-5 min), respectively (Table S1). The metallic precursors were first prealloyed at 250 oC and then selenized under 550 oC to synthesize ACZTSe absorber layer. The elements composition of ACZTSe thin films is shown in Table S2. The detailed selenization process was described in Ref. [21–23].

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2.2 Fabrication of ACZTSe thin film solar cells All the ACZTSe thin film solar cells in this work were prepared under the same selenization condition. The 50 nm-thick CdS buffer layer was synthesized by chemical bath deposition method (CBD) at 78 °C for 7 min, followed by depositing

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a 50 nm-thick i-ZnO layer and a 500 nm-thick ZnO:Al layer. The Ni/Al grid contacts were deposited by electron beam evaporation. The prepared ACZTSe solar cells were mechanical scribing with active area about 0.345 cm2.

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2.3 Characterization of ACZTSe film and solar cells

The elements composition of ACZTSe thin films were determined by a PANalytical MagixPW2403 X-ray fluorescent spectrometer (XRF). The ACZTSe

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absorbers have a Cu-poor and Zn-rich stoichiometry with Cu/(Zn+Sn)≈0.75 and Zn/Sn≈1.05. The crystal properties of ACZTSe thin films were investigated by a

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Philips X-pert pro X-ray diffractometer with Cu Kα as the radiation source (λKα=

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1.5416 Å). The phase composition of the selenized ACZTSe thin films was characterized by A Raman spectra (SR-500I-A) with an excitation wavelength of

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532 nm. The morphologies of ACZTSe thin films were characterized by a

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JSM-7800F scanning electron microscope (SEM). EDS line scan depth profiles and mapping images were characterized by the same SEM apparatus equipped with an EDS detector at 15 kV. The current density-voltage (J-V) characteristics of ACZTSe solar cells were measured by a solar simulator under the standard condition (AM 1.5 G, 1000 W/m2, room temperature). The external quantum efficiency (EQE) was performed with a chopped white light source (150 W halogen lamp). The

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capacitance-voltage (C-V) measurements were performed using 50 mV and 100 kHz alternating current (AC) excitation source with direct current (DC) bias from 0.5 to -1.0 V under dark condition at room temperature (HP 4284A LCR meter). 3 Results and discussion

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3.1 Characterization of ACZTSe thin films

Fig. 1. (a) X-ray diffraction spectra of the Ag doped CZTSe thin films with different thicknesses of Ag film, (b) enlarged view of (112) peak, (c) enlarged view of (220), (204), (312), and (116) peaks, (d) variations of the lattice constants a and c with the

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change of Ag thickness, (e) c/a ratio of the sample S-Ag-0, S-Ag-1, S-Ag-3 and S-Ag-5, (f) intensity of Mo (110) peak after selenization with different thicknesses Ag films. ACZTSe

thin

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selenizing

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Mo/Ag/Sn/Cu/Zn/Sn/Cu metallic stacked precursors. The thickness of Ag films deposited on Mo substrates was adjusted through chemical deposition time (Fig. S1). The ACZTSe thin films with different thicknesses of Ag films were named as

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S-Ag-0 (0 min), S-Ag-1 (1 min), S-Ag-3 (3 min) and S-Ag-5 (5 min), respectively.

Fig. 1(a) shows the XRD patterns of the Ag doped CZTSe films. All the diffraction peaks of ACZTSe thin films agree well with that of tetragonal kesterite phase

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CZTSe (JCPDS 00-52-0868). Fig. S2(a,b) shows that the intensity and the FWHM values of (112) diffraction peak were optimized with doping moderate amount of Ag.

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Ag doping can improve the crystal quality of ACZTSe thin film. However, excess

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amount of Ag incorporation deteriorated the crystal quality of the ACZTSe thin film. Fig. S3 shows that the volume of the unit cell is increased with increasing the amount

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of Ag incorporation. The incorporation of Ag enlarges the grain size of the ACZTSe

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thin films. The peak position of (112) shifts to the lower angle as the thickness of Ag films increase, indicating that Ag was successfully doped into the lattice of CZTSe (Fig. 1b) [24,25]. The peak shift results from the substitution of large Ag+ for small Cu+. Fig. 1(c) shows that the peaks (220) and (312) split after doping with Ag [25]. This is due to the change of lattice constant. The calculated lattice constant a of the ACZTSe film shows a linear relationship with increasing the Ag/(Ag+Cu) ratios,

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which follows Vegard's law as shown in Fig. 1(d) [26–28]. However, the lattice constant c is almost the same for all the samples with different Ag contents. As shown in Fig. 1(e), the c/a ratio decreases from 1.9926 to 1.9804, resulting in a separation of (220)/(204) peaks into individually resolved peaks [25,29]. Similarly,

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the peak intensity of Mo (110) decreases with increasing the Ag/(Ag+Cu) ratio (Fig.

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1f) [30].

Fig. 2. Raman spectra of S-Ag-0, S-Ag-1, S-Ag-3, and S-Ag-5, respectively. Inset is

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the enlarged pattern of Raman 196 cm-1 peak of ACZTSe thin film. Raman spectra of samples excited with 532 nm wavelength are shown in Fig. 2.

Three peaks are centered at around 172 cm−1, 196 cm−1, and 238 cm−1, which are close to the Raman characteristic peaks of kesterite structured CZTSe [21,23]. The peak of A mode shifts to a smaller wave number with increasing the content of Ag, as shown in the inset of Fig.2 [20]. The Raman modes intensity at around 172 cm-1 with

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respect to that at 196 cm-1 decrease after the incorporation of Ag in CZTSe thin film, which indicates the concentration of [VCu+ZnCu] defect clusters were increased [31,32]. Furthermore, the decrease of Raman intensity at about 172 cm-1 also indicates that the Cu content was reduced [33–35]. The effective analysis depth of

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532 nm laser is about 100 nm, so the Raman spectra only show the surface information of CZTSe film [36]. Therefore, the surface of ACZTSe absorber is more Cu-poor compared with the un-doped CZTSe thin film (Fig. S4). The formation

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energy of CuZn anti-site defect is relatively low compared with other defects,

resulting in that CuZn is always the dominant defect in CZTSe film [4–6]. The concentration of VCu defect is increased and CuZn anti-site defect decreased

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dramatically due to the formation energy of VCu defect is reduced after the surface of ACZTSe film is more Cu-poor. Besides, the peak intensity of Raman spectra

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centered at around 238 cm−1 was also decreased after doped with Ag, indicating that

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the density of detrimental [2CuZn+SnZn] defect cluster was decreased [31]. As a consequence, the intrinsic [CuZn+ZnCu] and [VCu+ZnCu] defect clusters in ACZTSe

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films surface were optimized after doped with Ag.

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Fig. 3. Surface and cross-sectional SEM images of ACZTSe films. The surface SEM

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images are: (a-1) sample S-Ag-0, (b-1) sample S-Ag-1, (c-1) sample S-Ag-3, (d-1) sample S-Ag-5. Cross-sectional SEM images are: (a-2) sample S-Ag-0, (b-2) sample

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S-Ag-1, (c-2) sample S-Ag-3, (d-2) sample S-Ag-5. The surface and cross-sectional SEM morphologies of Ag doped CZTSe thin

films are shown in Fig. 3. Fig. 3(a-1) shows that the sample S-Ag-0 possesses a relatively rough surface and exists many voids, while the content of void at the surface of sample S-Ag-1 is reduced after doped with Ag (Fig. 3b-1). The grain size of sample S-Ag-3 and S-Ag-5 is increased significantly, leading to a smooth and

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compact surface (Fig. 3c-1 and 3d-1). Besides, the cross-sectional image of Fig. 3(b-2) shows that the voids in the sample S-Ag-1 are eliminated and with larger grains compared with the sample S-Ag-0. This morphological change can be explained by the low melting point of Ag-alloyed compounds [37]. During the

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prealloying process, Ag was trend to form the Cu-Ag-Sn alloy with a low melting temperature (217 °C) [38]. The melting points of Ag-Se compounds are lower than the selenization temperature (550 °C) under Se-rich atmosphere [39,40]. In the

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process of selenization, the liquid Cu-Ag-Sn alloy and metal-Se compounds can assist the grain growth.

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3.2 The effect on Se and Na distribution with Ag-gradient doping

Fig. 4. The EDS line scan of ACZTSe thin films: (a) Ag distribution of S-Ag-1, S-Ag-3 and S-Ag-5. (b) Na and Se distribution of S-Ag-0 and S-Ag-1.

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Fig. 4(a) shows the EDS depth profiles of Ag in the sample S-Ag-1. Interestingly, the content of Ag at the surface of S-Ag-1 film is larger than that at the back region after high selenization temperature (550 °C) although the Ag films were deposited on the Mo layer. Thus, the EDS line scan of Ag indicates the front

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Ag-gradient in ACZTSe absorber layer was achieved. Similar to the distribution trend of Ag, the content of Se and Na at the surface region of S-Ag-1 is larger than

that of sample S-Na-0 (Fig. 4b). The distribution of Na and Se changed obviously

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after the incorporation of Ag in the CZTSe thin film. The Ag-related liquid phase promotes the distribution of Na element. Na atoms and Se atoms are easily reacted each other to form binary compounds Na2Sex [41]. The melting points of Na-Se

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binary compounds are below the selenization temperature at the Se-rich region. Therefore, the liquid phase Na2Sex compounds will also promote the growth of

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ACZTSe films and optimize the Se distribution.

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Fig. 5. Cross-sectional EDS mappings of ACZTSe films: (a) sample S-Ag-0, (b) sample S-Ag-1.

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Fig. 5 shows the EDS mappings of the cross-sectional images in samples

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S-Ag-0 and S-Ag-1. The corresponding SEM images of EDS mappings are shown in Fig. S5. It is obvious that all the elements in the sample S-Ag-1 distribute uniformly compared with that in the sample S-Ag-0. The elements distribution in the ACZTSe film were optimized after incorporation with Ag film. The ACZTSe film was prepared through selenized metallic precursor with a stacking order of Cu/Sn/Zn/Cu/Sn/Ag on Mo substrate. The selenization process of Ag doping CZTSe

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film can be shown as below [42]: Prealloying (250 °C): Ag + 3Zn → AgZn3

(1)

3Ag + Sn → Ag 3 Sn

(2) (3)

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Ag + Cu + Sn → Ag − Cu − Sn

The metallic precursors undergo prealloying process for 10 min under 250 oC

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annealing temperature without Se atmosphere.

Fig. 6. The XRD pattern of the ACZTSe precursors after the prealloying process.

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Fig. 6 shows the XRD pattern of the ACZTSe precursors after the prealloying

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process. After prealloying process at 250 °C for 10 min without Se atmosphere, the metals turned into alloy phase, such as Ag3Sn, Cu6Sn5, Cu5Zn8 AgZn, CuZn.

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Especially the Ag3Sn phase and eutectic for the Cu-Ag-Sn alloy with a low melting temperature (217 °C) [43]. The Ag film diffused to the front region of precursors and reacted with Sn and Zn to form AgZn3 and Ag3Sn phase. The liquid Ag-related phase and eutectic of the Cu-Ag-Sn alloy will assist the grain growth and affect element distribution in the absorber during the selenization process.

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Fig. 7. Schematic illustrations of (a) deposited ACZTS precursor, (b) alloy

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formation in the ACZTS precursor after annealing at 250 oC, (c) ACZTS thin films formation after selenization.

Fig. 7(a,b) shows the simulation diagram of Ag diffuse from back region on the

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Mo surface to the front of the absorber layers by form Ag-related phase under 250 o

C during the prealloying process. Consequently, ACZTSe thin films were

synthesized after the selenization annealing at 550 oC. Hao et al. also deposited 20

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nm Ag layer on the Mo substrate by thermal evaporation and then prepared metallic

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precursors stacked with the sequence of Mo/Ag/Zn/Cu/Sn by sputtering metal targets Zn, Cu and Sn. The metallic precursors were then sulfurized at 570 oC for 30

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min in a sulfur (S) atmosphere. Their EDS line scan indicated that Ag was distribute

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uniform in the ACZTS absorber [42]. However, compared with Hao’s work, a 10 min prealloying stage at 250 oC was added before selenization process, so the Ag is

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alloyed with other metal elements firstly. Furthermore, Fig. 4 indicates that the content of Ag and Se increased significantly at the surface of ACZTSe absorbers. Due to the Se elements diffuses from the surface of the metallic precursors to the back, the Ag alloying phases preferentially react with Se during the selenization process. The liquid Ag-related phase promoted grain growth, resulting in compact surface of the absorbers. The upper dense grain layer blocked Se diffusion to the

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bottom, resulting in Ag aggregated at the surface region. The ACZTSe thin films possessed a higher Ag content at the surface than that at the back after prealloying selenization process. Then, Ag-gradient distribution in the ACZTSe films can be achieved by prealloying selenization process. The prealloying selenization process

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avoided the low quality crystallization of ACZTSe under low selenization temperature which usually applied in Ag doped CZTSe film preparation [18]. The prealloying of metallic precursors contain Ag provides a feasible method to prepare

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Ag-graded distributed absorber by selenization process in a relatively high

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3.3 The performance of front Ag-gradient ACZTSe thin film solar cells

Fig. 8. The statistic photovoltaic performances of the efficiencies, Voc, Fill factor,

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Jsc, Rs and Rsh of C-Ag-0, C-Ag-1 and C-Ag-3, respectively.

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The ACZTSe solar cells prepared with different conent of Ag are named as C-Ag-0, C-Ag-1, C-Ag-3 and C-Ag-5, respectively. Due to the excessive Ag doping, the sample C-Ag-5 device parameters are very poor and not listed. The statistical parameters of ACZTSe solar cells are shown in Fig. 8. The VOC of the Ag doped devices increased significantly compared with the blank samples, leading to increase of efficiency. However, the Rs of the Ag doped devices are increased due to

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the poor conductivity of the ACZTSe films. The deterioration of FF and Rsh results from the high concentration dope of Ag. The solar cells prepared with S-Ag-1 thin films possess the highest efficiency, VOC and JSC. As a consequence, the VOC and efficiency of the devices are improved significantly through froint Ag-gradient

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doping of ACZTSe absorbers.

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Fig. 9. Simulated energy band diagrams of thin films: (a) A flat bandgap of CZTSe thin film with bandgap 1.05 eV, (b) the bandgap of ACZTSe thin film linear increased from 1.05 eV to 1.15 eV. Table 1. List of the simulated photovoltaic parameters of ACZTSSe solar cells with different bandgap structures.

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Sample

C-1

C-2

C-3

Eff (%)

8.9

11.2

10.5

VOC (mV)

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619

600

JSC (mA/cm2)

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28.1

27.6

FF (%)

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64.5

63.1

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To disclose the effect of front grade on the performance of ACZTSe solar cell, the performance of front Ag-gradient ACZTSe thin film solar cells was simulated by SCAPS software. The material parameters of the simulated ACZTSe devices are

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listed in Table S3 [44–46]. The largest bandgap of ACZTSe film was set as 1.15 eV.

Fig. 9(a,b) shows the bandgap structure of the devices with different bandgap structures. CZTSe thin film was designed with a flat bandgap and ACZTSe thin film

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was designed with a linear increased bandgap, named as C-1 and C-2, respectively. In addition, ACZTSe thin film with a 1.15 eV flat bandgap was also designed and

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named as C-3. Table 1 shows that the Voc of devices are improved significantly

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from 507 mV to 619 mV when the bandgap is changed to front band gradient. However, the Voc of C-3 was 600 mV which lower than sample C-1 619 mV

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although the bandgap increased to 1.15 eV. SCAPS simulation results show that the

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Jsc of the devices is reduced slightly with a front gradient bandgap due to the larger bandgap. The large band gap leads to a narrow spectral response range and decrease the Jsc. Compared with C-3, the front band gradient structure of C-2 shows better device parameters. Based on the simulation by SCAPS, the efficiencies of devices are increased from 8.9% to 11.22% through tuning the front bandgap.

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Fig. 10. (a) Simulated current density-voltage (J-V) curves of samples C-1, C-2 and C-3 with different bandgap structures. (b) J-V curves of C-Ag-0, C-Ag-1, and

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C-Ag-3. (c) Simulated EQE response of samples C-1, C-2 and C-3 with different

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bandgap structures, respectively. (d) EQE response of C-Ag-0, C-Ag-1, and C-Ag-3. Inset is the bandgap of the ACZTSe film deduced from EQE data.

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Fig. 10(a,b) shows that the simulated and experimental test data are in good

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agreement. The Voc of the devices is enhanced obviously through doped with Ag. External quantum efficiencies (EQE) of the devices with different bandgap structures were shown in Fig. 10(c). The simulation EQE responses of devices at wavelengths below 900 nm are enhanced as the front bandgap value increased. The EQE results of the prepared devices show similar trend with simulation SCAPS results. Sample C-Ag-1 indicates that the response of EQE below 1100 nm is

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enhanced and the excessive Ag doping deteriorates the devices lead low quality of crystallization and EQE response. Fig. 10(c,d) indicates that with increasing the concentration of Ag doping, the band tailing is significantly reduced. The improvement of EQE can be attributed to the optimization of band alignment

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between CZTSe and CdS heterojunction [15,19,47]. Many reports confirmed that with appropriate Ag doping, the concentration of CuZn defects can be significantly reduced [2]. Admittance spectra (AS) was utilized to investigate the influence of Ag

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doping on the defect energy level of ACZTSe thin film solar cells. As shown in Fig. S6(a,b), the defect energy of C-Ag-0 and C-Ag-1 is about 110.26 meV and 68.31 meV, respectively. According to the first principle calculation, the defect energy of 110.26

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meV should be CuZn defect, and the defect energy of 68.31 meV is VCu defect [48–50]. Therefore, the dominant defects of C-Ag-0 are CuZn anti-site defects. The dominant

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deep defect CuZn anti-site in C-Ag-0 is changed into shallow defect VCu by Ag

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doping in C-Ag-1. The change of defect energy level in ACZTSe thin film solar cells can be attributed to the more Cu-poor and Zn-rich absorber surface after treated with

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Ag. The AS results are consistent with Raman data in Fig. 2 and EDS-line scan of Cu

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in Fig. S4. Chen et al. reported that the conduction band edge and valence band edge can be lowered through doped with a high concentration of Ag according to the results of first principle calculation [15]. The optimized bandgap structure of the front Ag-gradient will facilitate the extraction of electrons and repelled the holes away from the heterojunction interface, promoting the separation of electron-hole pairs and reducing interface recombination. In addition, a reverse potential field

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near the interface betwwen Mo and ACZTSe will generate when Ag doped at the back region of ACZTSe. This reverse potential field will assist the photogenerated electrons to the Mo layer and repelling holes away from the Mo layer, thus leading to the increasement of recombination at the back interface. The high concentration

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of Ag doped at the back of ACZTSe thin film possess a poor p-type conductivity and hinder the collection of holes, leading to large series resistance of devices [51]. As the Ag content increased, the bandgap of synthesized ACZTSe thin film was

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increased from 1.05 eV to 1.10 eV deduced from EQE data [31]. The incorporation

of Ag enlarge the bandgap of ACZTSe thin films and suitable for increasing the Voc of solar cells. The band gap of C-Ag-0 and C-Ag-1 which deduced from EQE data is

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1.05 eV and 1.07 eV, respectively. The VOC of CZTSe thin film solar cells was increased after doped with Ag. Then, the Voc deficit of the devices was decreased

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from about 700 mV to 645 mV for the best performance devices. Based on above

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SCAPS simulation, high performance devices can be achieved through designing

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front Ag-graded ACZTSe films architecture.

Fig. 11. Plot of the Ncv and Xd of C-Ag-0, C-Ag-1 and C-Ag-3, respectively.

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The carrier density and depletion region width of the ACZTSe devices were characterized by Capacitance-voltage (C-V) measurements. Fig. 11 shows the space charge density (Ncv) and the distance to the junction interface (Xd), which are derived from C-V data. The calculation formula was in supporting information.

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With increasing the concentration of Ag doping in ACZTSe thin films, the Ncv decreased from 8.10×1015 to 8.0×1015, while the Xd increased from 177 nm to 247

nm. The free carrier concentration of ACZTSe was decreased after doped with Ag.

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This is due to the compensation of the acceptor and donor defects and the electrical conductivity of p-type thin film is poor [15,24,42,52]. 4 Conclusions

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In this work, we prepared front Ag-gradient ACZTSe solar cells through prealloying followed by selenization process. The metallic precursors containing Ag

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film was deposited on the Mo layer. By prealloying the metal precursors under 250 o

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C temperature, the precursors was changed into AgZn3, Ag3Sn, and Ag-Sn-Zn

alloy. The Ag-Sn-Zn alloy system is liquid phase at 217 oC and assisted the

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distribution of elements. The ACZTSe films were synthized by selenization precess

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at high temperature about 550 oC. Ag is successfully incorporated into CZTSe thin films with a front Ag-gradient distribution. The liquid Ag-related phase can facilitate the growth of large and compact ACZTSe grains. The VOC of ACZTSe devices is increased significantly to above 420 mV by optimizing the thickness of Ag films and improved the performance of the devices. This work provides a different insight into preparing the graded bandgap structure solar cells by Ag

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doping. High efficient solar cell devices can be expected through the more delicate surface Ag-gradient engineering. Acknowledgments This work was supported by the National Natural Science Foundation of China

Project

(16JCZDJC30700,

18JCZDJC31200),

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(51572132, 61674082, 61774089), Tianjin Natural Science Foundation of Key YangFan

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and

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Entrepreneurial Research Team Project (2014YT02N037), 111 Project (B16027).

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Graphical abstract Front Ag-gradient bandgap was formed in the Ag doped CZTSe thin films by prealloying followed by selenization process. The VOC of devices was increased

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significantly through the Ag-gradient bandgap structure.