Tuning the Se Content in Cu2ZnSn(S, Se)4 Absorber to Achieve 9.7

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Tuning the Se Content in Cu2ZnSn(S,Se)4 Absorber to Achieve 9.7% Solar Cell Efficiency from a Thiol/Amine-Based Solution Process Junjie Fu, Jie Fu, Qingwen Tian, Houlin Wang, Fengming Zhao, Jun Kong, Xiangyun Zhao, and Sixin Wu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00146 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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ACS Applied Energy Materials

Tuning the Se Content in Cu2ZnSn(S,Se)4 Absorber to Achieve 9.7% Solar Cell Efficiency from a Thiol/Amine-Based Solution Process Junjie Fu,†,‡,¶ Jie Fu,†,‡,¶ Qingwen Tian,*†,‡ Houlin Wang,†,‡ Fengming Zhao Kong†,‡ Xiangyun Zhao,†,‡ and Sixin Wu*†,‡

†,‡

, Jun

†

The Key Laboratory for Special Functional Materials of MOE, Henan University, Kaifeng, Henan, 475004, China ‡

Collaborative Innovation Center of Nano Functional Materials and Applications, Henan University, Kaifeng 475004, Henan Province, China ABSTRACT: The Se content in Cu2ZnSn(S,Se)4 absorber layer has a significant impact on the electronic properties, but it is rather challenging to control Se/(S+Se) ratio due to a complicated selenization process. Here, a low-toxic thiol/amine-based solution process was developed to tune the Se content in Cu2ZnSn(S,Se)4 absorber layer to an optimal value by ingeniously controlling the SeO2 in the precursor solution. We demonstrated that the crystal growth, and the band gap of Cu2ZnSn(S,Se)4 thin films are affected by the Se/(S+Se) ratio. By this approach, the open circuit voltage deficit (Voc, def) of device was effectively decreased, the short-circuit density (Jsc) and fill factor (FF) were remarkably improved, thus, the power conversion efficiency of the Cu2ZnSn(S,Se)4 solar cells was successfully increased from 5.6 to 9.7% for the optimal band gap (Eg = 1.13 eV). Keywords: kesterite, CZTSSe thin films, Se/(S+Se) ratio, thiol/amine, solution process, solar cells

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INTRODUCTION As an approach for sustainable energy production, photovoltaic (PV) technology can directly convert unlimited solar energy into electricity.1−4 Earth abundant Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaic materials has experienced a significant increase in power conversion efficiency (PCE), evolving from about 5% in 2004 up to certified 12.6% in 2014 with an absorber’s band gap (Eg) of 1.13 eV.5−8 However, despite these remarkable progress, the performance gap between the CZTSSe record efficiency and the well-developed Cu(In,Ga)Se2 record efficiency of 22.6% is still quite large.9,10 Particularly, the current record of 12.6% efficiency has been realized with a undesirable hydrazine solution-based process.1 Investigation on the major factors limiting CZTSSe device performance have been accurately addressed by various theoretical and experimental studies; these include the large open circuit voltage deficit (Voc,

def),

the fluctuant band gap, the high defects density, and the

narrow phase stability region.11−17 However, it is rather challenging to overcome these major limitation and intrinsic factor in CZTSSe solar cells. This triggered a world effort to further seek for environmentally-friendly processes and improve the photovoltaic performance of CZTSSe solar cells.18−22 Most of our reported solution-processed CZTSSe thin-film deposition routes employ Cu2ZnSnS4 (CZTS) precursor films as intermediate materials which were subsequently selenized to prepare CZTSSe absorber layers. Generally, the band gap of CZTSSe absorber layer was not fine-tuned to an optimal value using this common route.23−27 In particular, the existence of superfluous residual sulfur in CZTSSe thin films has a high bulk defect density and a deeper defect energy level, which leads to stronger recombination losses and damage the performance of devices.28,29 Using the first principle calculation and Density Functional Theory, Chen et al. proposed that the CZTSSe devices with high Se/(S+Se) ratio are expected to have better carrier collection efficiency than pure CZTS alloys.28,30 Therefore, it has been proposed that the device performance should be improved by preparing a high homogenous Se content CZTSSe absorber and adjusting band gap to an optimal value. 2


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To obtain a high homogenous Se content CZTSSe absorber with an optimal band gap

value.

In

this

work,

we

successfully

demonstrated

a

thioglycolic

acid/butylamine-based solution approach to adjust the Se content in CZTSSe absorber layer. Low-cost metal/non-metal oxides can be used as the starting materials, and dissolved in a mixture solution of ethanol, 1-butylamine, and thioglycolic acid to prepare metal-organic precursor solution. We have investigated that tuning the SeO2 content in the precursor solution used to prepare the CZTSSe absorber layer has a significant effect on the performance of our photovoltaic devices. Meanwhile, the band gap tuning of absorber layer can be realized by adjusting the SeO2 additive amount in precursor solution. Surprisingly, the device performances are found to be remarkably improved by tuning the SeO2 additive amount in precursor solution, especially at the optimal SeO2 additive amount (x = 2.0 mmol). Experimental Materials: Copper oxide (Cu2O, 99.9%) was purchased from Alfa Aesar Co. Zinc oxide (ZnO, 99.9%), tin oxide (SnO, 99%), selenium dioxide (SeO2, 99,9%), ethanol (CH3CH2OH, AR), 1-butylamine [CH3(CH2)3NH2, 99%], cadmium sulfate (CdSO4·8/3H2O, 99%), thiourea (NH2CSNH2, 99%), and selenium (Se, 99.9%) were obtained from Aladdin Co. Thioglycolic acid (HSCH2COOH, 98%) was purchased from Acros Co., and Ammonium hydroxide (NH4OH, 25%) was obtained from Beijing chemical works. All chemicals and solvents were used as received without any further purification. Synthesis of CZTSSe precursor solution: The precursor solution was prepared by dissolving SeO2, SnO, Cu2O, and ZnO into a mixture solution of ethanol, 1-butylamine (BA), and thioglycolic acid (TGA). The schematic illustration of general dissolution procedures for CZTSSe precursor solution is shown in Figure 1. First, a certain amount of SeO2 powder (0 mmol, 0.5 mmol, 1.0 mmol, 1.5 mmol, 2.0 mmol, or 2.5 mmol) was added into a mixture solution of 4 mL of ethanol, 4 mL of 1-butylamine in a sample bottle while being 3



magnetically stirred at room temperature. Subsequently, 1.2 mL of TGA was added slowly to the above solution, yielding a homogeneous reddish brown solution. Then, 0.9 mmol of SnO, 0.792 mmol of Cu2O, and 1.08 mmol of ZnO were added in sequence [the total metal concentration: 0.38 M, Cu:(Zn+Sn)= 0.80 and Zn:Sn= 1.20]. The ratios of SeO2/(Cu+Zn+Sn) in the above precursor solutions are 0, 0.14, 0.28, 0.42, 0.56, and 0.70, respectively. The precursor solution with different SeO2 additive amount was kept under continuously stirring at 50 ℃ for 24 h until all solids had dissolved, followed by centrifugation at 10 000 rpm for 5 min to remove impurities. All the preparation processes were performed in the open air. Deposition and selenization of CZTSSe films: CZTS and CZTSSe precursor thin films were deposited by spin-coating the prepared precursor solution with different SeO2 additive amount on a 20 mm ×20 mm × 1.1 mm molybdenum-coated soda-lime glass (with ~ 700 nm thick Mo) at 2500 rpm for 20 s, followed by a sintering process on a 320 ℃ hot plate for 1 min to form CZTS (CZTSSe) nanocrystal thin film.25 This procedure was repeated five cycles for obtaining about 1.2 μm thick precursor film. All the operations described above were conducted in an argon-filled glove box. Finally, the prepared CZTS (CZTSSe) nanocrystal thin films with different SeO2 additive amount were selenized at 550 ℃ for 15 min in a round-shaped graphite box with 300 mg of selenium (Se, 99.9%). To provide sufficient selenium vapour and avoid severe decomposition of CZTS (CZTSSe) nanocrystal films, the selenization processes were conducted in a rapid thermal processing (RTP) furnace (ramp to 550 ℃ at 9.0 ℃ s−1, MTI, OTF-1200X-4-RTP) under a slight nitrogen flow (over 1 atm). The digital photographs of round graphite box and RTP furnace are shown in Figure S1. During selenization process, a piece of conventional soda-lime glass was placed on top of the CZTS (CZTSSe) precursor thin film to improve the grain growth and the hole-carrier concentration by the diffusion of sodium. Fabrication of CZTSSe photovoltaic devices: 4


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Our CZTSSe photovoltaic devices with a structure of Soda-lime glass (SLG)/Mo/CZTSSe/CdS/i-ZnO/ITO/Ag were fabricated. About 50 nm thick cadmium sulfide (CdS) was deposited by a chemical bath deposition methods reported in our previous work; 12.0 mL of Ammonium hydroxide, 50 mL of cadmium sulfate (0.006 M), and 50 mL of thiourea (0.03 M), and 150 mL of deionized H2O were mixed in a 65 ℃ water bath for 13 min. 26, 31 Then, 50 nm i-ZnO thin films (100 W, 0.4 Pa Ar, 5 min), and 200 nm ITO thin films (90 W, 0.5 Pa Ar, 5 min) were sequentially deposited onto the SLG/Mo/CZTSSe/CdS films by RF sputtering. 25 Finally, 100 nm Ag grid electrodes, as the top contact fingers, were deposited by thermally evaporation method (thermal evaporation current of 40 A, evaporation time of 2 min) to ensure a good current collection. All CZTSSe photovoltaic devices have an active area of 0.21 cm2 (~ 91% of the total device area). Characterizations: The powder X-ray diffraction (XRD) patterns were performed by a Bruker D8 Advance X-ray diffractometer. The scanning electron microscope (SEM) images were collected by using a Nova Nano SEM 450 equipped with an energy dispersive X-ray (EDX) analyzer (Nano SEM 45050/EDX). The overall composition of each type of ion is measured by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) using Optima 2100DV with spectral region of 165~782nm. The Raman spectra were measured via Raman microscope using different excitation wavelength of 325 nm, 532 nm, 633 nm, 785 nm. Photocurrent density-voltage curves (J-V curves) were recorded by a Keithley 2400 sourcemeter. The light intensity was standardized to 100 mW/cm2 by using Newport optical power meter (model842-PE). RESULTS AND DISSCUSSION A series of molecular precursor solutions with different SeO2 additive amount were successfully prepared by dissolving SeO2, SnO, Cu2O, and ZnO into a mixture solution of ethanol, 1-butylamine (BA), and thioglycolic acid (TGA) at 50 ℃ and ambient pressure (Figure 1). With the increase of SeO2 adding amount from 0 to 2.5 5



mmol, the corresponding precursor solution become darker. The schematic illustration of general dissolution procedures for CZTSSe precursor solution is shown in Figure 1a. The possible reaction mechanism of the starting materials with 1-butylamine and thioglycolic acid is that the thiol is deprotonated due to the presence of R-NH2 groups, and thus the deprotonated organic thiol and the protonated amine are generated in this solvent mixture. The existence of RS− species can bind to metal ions and form complexes.29 In our work, it has demonstrated that carboxyl group in thioglycolic acid plays a weak role in forming precursor solutions, because ethanedithiol (HSCH2CH2SH) and ethylthioglycolate (HSCH2COOCH2CH3) can also dissolve these starting materials.23, 27

Figure 1. (a) Schematic illustration of general dissolution procedures for CZTSSe precursor solution; (b) Photographs of the CZTSSe precursor solution with different SeO2 additive amount (x).

Figure 2 displays the XRD patterns measured on the as-prepared CZTSSe films and selenized films with different SeO2 additive amount varied from x= 0 to 2.5 mmol. The preparation details of thin films can be found in the experimental section. When x= 0 mmol, the weak diffraction pattern for the CZTS film has six broad peaks at 26.86°, 28.39°, 30.32°, 47.39°, 51.43°, and 56.16°, corresponding to (100), (002), (101), (110), (103), and (112) planes of wurtzite CZTS nanocrystals, respectively (Figure 1a). Interestingly, with the introduction of SeO2, the as-prepared CZTSSe thin films (x= 0.5 – 2.5 mmol ) show three broad diffraction peaks corresponding to (112), 6


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(220)/(204), and (312)/(116) kesterite planes. The crystal structure of the as-prepared CZTSSe films (x= 0.5 – 2.5 mmol ) are converted to tetragonal phases, indicating the formation of tetragonal phase CZTSSe nanocrystals. Morever, apart from the Mo peak, all the XRD peaks of as-prepared films (x= 0.5 – 2.5 mmol ) gradually shift to lower angle for increasing SeO2 additive amount, from 2θ = 28.3°(x = 0.5 mmol) to 2θ = 27.1° (x = 2.5 mmol), as shown in Figure 2a. Surprisingly, the metastable wurtzite structure of CZTS nanocrystals (x = 0 mmol) converted into a thermodynamically stable kesterite phase via selenization at 550 ℃. When x= 0.5 – 2.5 mmol, after selenization, some invisible peaks in the as-prepared films, such as (110), (103), and (211), become observable in Figure 2b. The XRD peaks of (112), (220)/(204), and (312)/(116) become sharper, indicating that high crystallinity CZTSSe thin films were obtained. The expanded views at the dominant (112) peaks clearly show the typical peak-shift towards lower diffraction angles with an increasing SeO2 additive amount from 0 to 2.5 mmol. This is attributed to the expansion of the unit cell volume by the replacement of S with Se. The unit cell volume increased due to the larger Se radius compared to that of S, which demonstrates Se2− was introduced into the host lattice of CZTSSe by substitution of S2−.31 With the increase of SeO2 additive amount from 0 to 2.0 mmol in the precursor solution, the band gap of CZTSSe thin films decrease from 1.27 to 1.13 eV due to the shrinkage of sulfur content.32

To further determine the possible impurity phases in the selenized

CZTSSe thin films, such as ZnS, ZnSe, Cu2SnS3, and Cu2SnSe3, Raman spectra were measured using four different excitation wavelengths of 325 nm, 532 nm, 633 nm, 785 nm, as shown in Figure S2. The Raman peaks that appeared at the vicinity of 175, 197, and 234 cm−1 can be attributed to kesterite CZTSe phase, and the peak in the vicinity of 327 cm−1 corresponds to kesterite CZTS phase.1, 31 This results further demonstrated that no peaks of the impurity phases (ZnS, ZnSe, Cu2SnS3, and Cu2SnSe3) were detected in all the thin films.

7



Figure 2. X-ray diffraction spectra of the as-prepared CZTSSe films (a) and the selenized films (b) with different SeO2 additive amount (x); inset: expanded views at the (112) peaks.

The detailed elemental compositions of a series of Mo/CZTSSe thin films made with different contents of SeO2 added to the precursor solution was determined by an energy dispersive X-ray (EDX) analyzer, as shown in Figure S3 (see Supporting Information) and summarized in Table 1 and Table 2. It is clear that the Se/(S+Se) ratio varies with the SeO2 additive amount from 0 to 2.5 mmol. More importantly, the Se/(S+Se) ratios of thin films increased with the increase of the amount of SeO2 to the precursor solution. However, EDX is insufficient for accurate determination about compositions of the above CZTSSe thin films. The chemical composition should be further reconfirmed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) measurements. The tested films were dissolved in 5% nitric acid solution for total film analysis, and the results are as shown in Table S1 and S2 (Supporting Information). The obtained results are close to the measured values via EDX. It has been demonstrated that this method offers an effective leverage upon adjusting the Se/(S+Se) ratios in CZTSSe absorber layer. By employing this leverage to optimize the properties of our CZTSSe thin films and achieve high solar cell efficiency. Figure 3 displays the top-view and cross-section SEM images of the selenized CZTSSe thin films prepared from the corresponding precursor solutions with different SeO2 additive amount. The average grain sizes of the selenized CZTSSe thin films increase remarkably from ~ 400 nm to 1.2 μm when the SeO2 additive amount from 0 to 2.0 mmol, and a MoSe2 layer of about 190 nm thickness was observed. Specifically for 8


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the SeO2 additive amount of 2.5 mmol, the average grain sizes increase to ~ 2.0 μm. Unfortunately, the selenized CZTSSe sample exhibits an obvious porous morphology. As expected, the CZTSSe devices made from these selenized films have a worse performance. In addition, the top-view SEM images of the selenized Cu2ZnSn(S,Se)4 thin films (x= 2.0 mmol) prepared from different sintering temperature are shown in Figure S4 (Supporting Information). Figure 4 shows the absorption spectra of as-fabricated CZTSSe thin films with different SeO2 additive amount (x), and their calculated band gaps are 1.27, 1.22, 1.17, 1.16, and 1.13 eV respectively. Table 1. Chemical Compositions of the As-Prepared Cu2ZnSn(S,Se)4 Thin Films with Different SeO2 Additive Amount. SeO2

Se%

Cu/(Zn+Sn)

Zn/Sn

Se/(S+Se)

0

0.83

1.36

0

7.24

0.83

1.28

0.13

1.0 mmol 20.85 14.27 10.35 37.59 16.84

0.84

1.38

0.31

1.5 mmol 20.72 13.75 10.56 30.59 24.43

0.85

1.30

0.44

2.0 mmol 20.64 13.72 10.70 24.56 30.37

0.84

1.28

0.55

2.5 mmol 20.24 13.35

0.87

1.34

0.68

0

Cu% Zn%

Sn%

S%

mmol 21.36 14.70 10.82 53.12

0.5 mmol 20.62 13.95 10.86 47.34

9.94

17.90 38.57

Table 2. Chemical Compositions of the Selenized Cu2ZnSn(S,Se)4 Thin Films with Different SeO2 Additive Amount. SeO2

Cu%

Zn%

Sn%

S%

Se%

Cu/(Zn+Sn)

Zn/Sn

Se/(S+Se)

mmol

20.82

12.36

9.58

25.76

31.48

0.95

1.29

0.55

0.5 mmol

20.89

12.18

9.59

14.34

43.00

0.96

1.27

0.75

1.0 mmol

21.02

12.65

9.73

8.51

48.09

0.94

1.30

0.84

1.5 mmol

20.95

12.51

10.01

8.48

48.05

0.93

1.25

0.85

2.0 mmol

19.83

11.77

9.34

7.68

51.38

0.94

1.26

0.87

2.5 mmol

20.90

12.79

9.92

6.77

49.62

0.92

1.29

0.88

0

9



Figure 3. Top-view and cross-section SEM images of the selenized Cu2ZnSn(S,Se)4 thin films prepared from the corresponding precursor solutions with different SeO2 additive amount (x): (a) x = 0 mmol, (b) x=0.5 mmol, (c) x = 1.0 mmol, (d) x = 1.5 mmol, (e) x = 2.0 mmol, and (f) x = 2.5 mmol.

Figure 4. (a) The absorption spectra of the as-fabricated CZTSSe thin films with different SeO2 additive amount, and their corresponding plots of (ahv)2 vs hv (b).

Figure 5. EDX compositional profiles show the distribution of elements in cross-section of CZTSSe thin films with different SeO2 additive amount (x): (a) x = 0 mmol, (b) x = 1.0 mmol, (c) x = 2.0 mmol. 10


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To investigate the distribution of Cu, Zn, Sn, S, and Se elements in the cross-section of selenized CZTSSe thin film, EDX-line scans analysis on several CZTSSe thin film slices were performed, as shown in Figure 5. As is well-known, S and Se have only a slight chemical and size mismatch. By using the special quasi-random structure approximation, Chen et al. have presented that the isovalent CZTSSe photovoltaic materials has a better performance and can be introduced by the regular solid solution model.33 Just as our present work in Figure 5a, we have not introduced Se in the precursor solution (x= 0 mmol). It is found that the absorber near molybdenum electrodes remain higher in S concentration. The S/(S + Se) ratio is not spatially uniform across the CZTSSe selenized films. The inhomogeneous spatial distribution of Se and S anions can be attributed to the slow solid-state diffusion process, a higher concentration of Se could occur at the exterior of CZTSSe absorber, while the interior of the absorber remain higher in S concentration. Compared to Figure 5a and b, the CZTSSe absorber in Figure 5c has a better homogeneous spatial distribution of S and Se anions from the top surfaces to the CZTSSe/Mo interface. Therefore, by adding SeO2 in the precursor solution is beneficial to obtain the homogeneous distribution of S and Se anions during the selenization process.

Figure 6. (a) J−V curves of the champion Cu2ZnSn(S,Se)4 solar cells. (b) J−V curves of the Cu2ZnSn(S,Se)4 solar cells with different SeO2 additive amount under AM 1.5G illumination.

To further evaluate the influence of different SeO2 additive amount on device performance, the photovoltaic properties and the J−V characteristics studies are carried out. The highest device efficiency was achieved on a CZTSSe film fabricated with a 2 mmol SeO2 solution. Figure 6a shows the dark and light J−V curves of the 11



“champion” CZTSSe solar cells (Eg = 1.13 eV) with a power conversion efficiency (PCE) of 9.7%, yielded a high short-circuit density (Jsc) of 35.07 mA/cm2, an acceptable open-circuit voltage (Voc) of 427 mV, and a superior fill factor (FF) of 64.74 % based on an active device area of 0.21 cm2 under standard AM1.5G illumination. The best J-V curves and device parameters for fabricated CZTSSe solar cells made from the corresponding annealed films are reported in Figure 6b and Table 3. The corresponding device parameters as a function of SeO2 additive amount are displayed in Figure 7. It is clear that the device performance varies with SeO2 additive amount and the two device parameters of Jsc and FF achieve a maximum for an added SeO2 of 2.0 mmol versus the standard (0 mmol). The high sulfur content device (x= 0.5 mmol) showed a higher Voc of 442 mV, but a larger Voc deficit of 778 mV was obtained. The large Voc deficit can be explained by higher bulk defect energy level as well as higher bulk defect density leads to recombination losses compared with the low sulfur content device.28 Duan et al. found that variations in the Se and S ratio greatly changes both the concentration of the dominant defects and the energy level, and ultimately leads to the larger losses of open circuit voltage and poor carrier collection efficiency due to the increasing numbers of recombination centers.28 Dennler et al. attributed this larger Voc deficit to a cliff-like conduction band offset (CBO) at CdS/CZTSSe interface of a S-rich CZTSSe device.34 In contrast, the “champion” device fabricated from the optimal Se-rich absorber (x= 2.0 mmol) exhibited the best performance (9.7%) as a result of the relatively low bulk defect density and low activation energy.28 When x= 2.5 mmol, the average grain sizes increase to ~ 2.0 μm. Unfortunately, the selenized CZTSSe sample exhibits an obvious porous morphology. The performances of the devices show a sharp decline and we cannot even provide a normal efficiency parameter (see Figure S5). We speculated that the porous morphology lead to the direct contact between the CdS and back contact, and the corresponding devices are short-circuited. The best device parameters of CZTSSe solar cells versus different SeO2 additive amount cannot display a meaningful conclusion on the solar cells performance without adequate parallel statistics on the reliability of the data.35 Table 4 lists the average values of the 12


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standard deviation calculated from 10 devices. Figure 8 shows the photovoltaic parameters distribution of CZTSSe solar cells obtained from the study of 10 CZTSSe solar cells and the detailed photovoltaic parameters are displayed in Table S3 (see Supporting Information). A distinct correlation is found between device parameters and SeO2 additive amount. Table 3. Summary of the Champion Device Parameters for Cu2ZnSn(S,Se)4 Solar Cells Prepared with Different SeO2 Additive Amount. Efficiency [%]

Voc [mV]

Jsc [mA/cm2]

FF [%]

Eg [eV]

Eg/q-Voc [mV]

mmol

5.57

354

30.19

52.00

1.27

916

0.5 mmol

6.91

442

27.88

56.03

1.22

778

1.0 mmol

7.47

412

31.09

58.36

1.17

758

1.5 mmol

7.94

403

33.24

59.27

1.16

757

2.0 mmol

9.69

427

35.07

64.74

1.13

703

SeO2 0

Table 4. Summary of Device Parameters for Cu2ZnSn(S,Se)4 Solar Cells Prepared with Different SeO2 Additive Amount, in Which Each Sample Contained 10 Solar Cells. (The data are the average values calculated from 10 devices with standard deviation) SeO2 0

Efficiency [%]

Voc [mV]

Jsc [mA/cm2]

FF [%]

Eg [eV]

Eg/q-Voc [mV]

mmol

4.67±0.9

348.9±36.9 28.60±2.7 46.94±5.1 1.27 921.1±36.9

0.5 mmol

6.45±0.7

431.2±13.2 28.28±2.0 52.84±5.3 1.22 788.8±13.2

1.0 mmol

7.08±0.5

404.8±17.8 32.07±3.2 54.62±5.2 1.17 765.2±17.8

1.5 mmol

7.67±0.3

409.0±15.0 32.64±3.2 57.58±4.0 1.16

751.0±5.0

2.0 mmol

9.30±0.7

424.4±8.6

705.6±8.6

34.70±1.1 63.02±1.8 1.13

13



Figure 7. Variation of typical parameters of Cu2ZnSn(S,Se)4 solar cells with different SeO2 additive amount: (a) PCE, (b) Voc, (c) Jsc, and (d) FF.

Figure 8. Device performances of the Cu2ZnSn(S,Se)4 solar cells as a function of SeO2 additive amount: (a)Voc, (b) Jsc, (c) FF, and (d) PCE. Results are extrapolated from the analysis of 10 solar cells.

CONCLUSIONS In conclusion, we have presented a low-toxic solution processed route to deposit CZTSSe absorber layers. Low-cost metal/non-metal oxides can be used as the starting materials, and dissolved in a mixture solution of ethanol, 1-butylamine, and thioglycolic acid to prepare metal-organic precursor solution. We have investigated 14


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that tuning the SeO2 content in the precursor solution used to prepare the CZTSSe absorber layer has a significant effect on the performance of our photovoltaic devices. The band gap tuning of absorber layer can be achieved by controlling the SeO 2 additive amount in precursor solution. The device performances are found to be remarkably improved by tuning the SeO2 additive amount in precursor solution, especially at the optimal SeO2 additive amount (x = 2.0 mmol). The best device efficiency of 9.7% has been achieved due to a decreased Voc deficit, an optimal band gap, and the homogeneous spatial distribution of anions. ASSOCIATED CONTENT Supporting Information EDX spectra of the as-prepared Cu2ZnSn(S,Se)4 thin films with different SeO2 additive amount. The detailed photovoltaic parameters of Cu2ZnSn(S,Se)4 solar cells made from different SeO2 additive amount. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Author Contributions ¶

Junjie Fu and Jie Fu contributed equally to the work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work has been financially supported by the Joint Talent Cultivation Funds of NSFC-HN (U1604138 and U1704151), the National Natural Science Foundation of China (21603058 and 51702085), the Innovation Research Team of Science and Technology in Henan province (17IRTSTHN028), the Science and Technology

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Table of Contents Figure:

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Tuning the Se Content in Cu2ZnSn(S, Se)4 Absorber to Achieve 9.7

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