Facile and Low-Cost Sodium-Doping Method for High-Efficiency

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A Facile and Low-Cost Sodium-Doping Method for High-Efficiency CuZnSnSe Thin Film Solar Cells 2

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Yanchun Yang, Xiaojiao Kang, Lijian Huang, Song Wei, and Daocheng Pan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06381 • Publication Date (Web): 21 Sep 2015 Downloaded from http://pubs.acs.org on September 26, 2015

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The Journal of Physical Chemistry

A Facile and Low-Cost Sodium-Doping Method for High-Efficiency Cu2ZnSnSe4 Thin Film Solar Cells Yanchun Yang, Xiaojiao Kang, Lijian Huang, Song Wei, Daocheng Pan* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin, 130022, China Tel & Fax: +86-431-85262941; Email: [email protected] ABSTRACT We present a simple and low-cost sodium doping method for Cu2ZnSnSe4 (CZTSe) thin film solar cells. In this method, a piece of soda-lime glass (SLG) is served as the sodium source and is placed on top of the CZTSe precursor thin film during selenization. It was observed that the grain growth and the hole-carrier concentration can be significantly improved by the diffusion of sodium from the top SLG. Through this approach, high quality CZTSe absorber layer is obtained after the selenization, and the photoelectric conversion efficiencies (PCE) of 7.51% and 6.09% are achieved for CZTSe thin film solar cells deposited on a Mo-coated SLG substrate and a Mo-coated quartz substrate, respectively. The difference in PCE on SLG and quartz substrate revealed that Na diffusion from the bottom SLG substrate and the top SLG was most effective for the high-performance of CZTSe solar cell devices. KEYWORDS: Na doping; Selenization; CZTSe; Solar cells; Solution process

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INTRODUCTION Earth abundant Cu2ZnSn(S,Se)4 (CZTSSe) semiconductor has been considered as a promising candidate for the next generation thin film solar cells due to their excellent properties, such as low material cost, low toxicity, and high photoelectric conversion efficiency (PCE).1-4 To enhance the PCE of CZTSSe thin film solar cells, various modification methods have been reported.5-15 Na doping in CZTSSe thin film is one of the most important methods that is utilized to improve the performance of CZTSSe solar cells. It was reported that the incorporation of Na can effectively increase the hole-carrier concentration of CZTSSe absorber layer, resulting in a significant increase in the solar cell efficiency.8-10 Besides, it was observed that Na doping can promote the crystal growth of CZTSSe thin film, which is beneficial for the performance of CZTSSe solar cells.10-15 Thus, Na doping plays an important role in high-performance CZTSSe thin film solar cells. Generally, Mo-coated soda-lime glass (SLG) was used as the back electrode of CZTSSe solar cells, and the Na can diffused into CZTSe absorber layer through the Mo layer from SLG substrate. However, the diffusivity of Na into the absorber layer strongly depends on the thickness and pore size of Mo electrode; in case that the Mo layer is extremely dense, the supply of Na from Mo-coated SLG might be insufficient.16,17 Therefore, a number of Na doping methods have been developed to fabricate Na-doped CZTSSe thin film solar cells, including vacuum-based and solution-based methods.8-15 Repins et al. incorporated Na into CZTSe thin films via electron-beam depositing NaF layer on top of Mo.12 In addition, Na can be also introduced into CZTSSe thin film by thermally evaporating

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NaF layer on the top of CZTS precursor thin film.13 However, these additional procedures will increase the manufacturing cost of CZTSSe solar cell device. Recently, Yang et al. reported a Na doping approach by preparing Na-doped CZTS nanoparticles.14 Very recently, Aydil et al. introduced a piece of SLG or NaOH into a sealed quartz tube in order to promote the crystal growth of CZTS thin film via sulfuration of Cu-Zn-Sn alloy thin film at 600 oC for 8 h.15 Interestingly, the SLG was not in direct contact with the CZTS thin film. Herein, we developed a simple Na doping method to promote the grain growth and enhance the hole concentration of CZTSe thin film, i.e. as-deposited CZTSe nanocrystal thin film was covered with a piece of Na-rich SLG, and sodium can diffuse into CZTSe thin film during the selenization. EXPERIMENTAL SECTION Deposition and selenization of CZTSe nanocrystal thin film The CZTSe precursor solution was prepared according to our previously reported method by dissolving the elemental Cu, Zn, Sn and Se powders in thioglycolic acid and ethanolamine.18 CZTSe precursor solution has a Cu/(Zn+Sn) target ratio of 0.8 and a Zn/Sn target ratio of 1.2. CZTSe nanocrystal thin films were obtained by repeating spin-coating and sintering process on Mo-coated SLG or Mo-coated quartz substrate (SiO2 content, 99.99%). Afterwards, as-prepared CZTSe nanocrystal thin films were covered with a bare piece of SLG (25×25×1 mm) and were selenized in a quasi-closed graphite box containing 0.3 g of Se powder at 540°C for 15 min, forming the selenized CZTSe thin film.

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Device Fabrication: our CZTSe solar cell devices with a structure of SLG(Quartz)/Mo/CZTSe/CdS/i-ZnO/ITO/Al were fabricated, and the detailed fabrication process has been reported in our previous papers.18-23 CdS (60 nm), i-ZnO (70 nm) and ITO (200 nm) thin films were sequentially deposited by a chemical bath deposition approach, RF-sputtering, and DC-sputtering, respectively. Al grid electrode (~2.0 μm) was made though thermal evaporation. No anti-reflection coating was applied. Finally, the CZTSe device with an active area of 0.368 cm2 was separated on a Mo-coated soda lime glass slide by a tungsten needle. Characterizations: The X-ray diffraction (XRD) patterns were taken with a Bruker D8 X-ray diffractometer. X-ray photoelectron spectra (XPS) were measured with VGESCALAB (VG Co., U.K.) by using Al Kα X-ray source (hν=1486 eV), and the binding energy was calibrated by the C1s (284.6 eV). The scanning electron microscope (SEM) images were taken using a Hitachi S-4800. The thickness of the thin film was measured by Ambios XP-100 profilometer. The energy dispersive X-ray spectrometry (EDS) spectrum and line scan were conducted by Bruker AXS XFlash detector 4010. The Raman spectra were measured by a Renishaw inVia Raman microscope using the Ar+ ion laser with the excitation wavelength of 532 nm and He-Cd laser with the excitation wavelength of 488 nm. Photocurrent density-voltage curves were recorded under the standard AM1.5 illumination (100 mW·cm-2) with a Keithley 2400 source meter. Hall effects of CZTSe thin films on quartz substrate were measured by a hall effect measurement system (ET9000, East Changing Technologies, Inc.). The Na content was measured by inductively coupled plasma-optical emission spectrometry (ICP-OES) (ICP-PLASMA 1000).

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RESULTS AND DISCUSSION The incorporation of sodium plays an important role in improving the performance of CZTSSe thin film solar cells. Generally, Na is supplied by Mo-coated SLG substrate; however Mo layer is not beneficial for the diffusion of Na from SLG substrate. Therefore, the additional Na-doping is required. In our approach, a piece of SLG was putted on the top of CZTSe precursor thin film during the selenization (See Figure S1), which was served as the sodium source. Note that CZTSe precursor thin film was in direct contact with the top SLG. To confirm the effectiveness of the Na-doping method, a series of control experiments and characterizations were conducted for the selenized CZTSe thin films which were deposited on Mo-coated SLG and Mo-coated quartz substrates with or without the cover of SLG. Figure 1a and 1b show the surface morphologies of the selenized CZTSe thin films deposited on the Mo-coated SLG substrate without and with SLG cover during the selenization. The selenized CZTSe thin film with SLG cover exhibits a dense and compact surface morphology and better crystallinity, as compared to the CZTSe thin film without SLG cover, indicating that the top SLG can dramatically promote the grain growth of the CZTSe thin film, which roots in the Na diffusion from the top SLG. As can be seen in Figure 1a, Na diffusion from Mo-coated SLG substrate was greatly suppressed due to Mo layer as a Na diffusion barrier. To further confirm the role of the top SLG, CZTSe precursor thin films were deposited on Mo-coated quartz substrate (See Figure 1c-1d). Although the high-purity quartz was used as the substrate, the selenized CZTSe thin film became compact with large grains upon it was covered with a SLG during the selenization. Our results are clearly shown that

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covering a piece of SLG on CZTSe precursor thin film does not affect the diffusivity of Se vapor toward CZTSe thin film. According to the previous report,15 if a piece of bare SLG is placed nearby CZTS thin film, Na and K have significant effect on the grain growth in a sealed space. However, the crystal growth promotion ability was only observed in our case when the top SLG was in direct contact with the CZTSe precursor thin film, probably because our selenization was conducted in an open environment; some impurities contained in SLG may diffuse into the selenized CZTSe thin film, which may detriment or improve the efficiency of the device. To explore the diffusion of the impurities from soda-lime glass in the selenized CZTSe thin films, X-ray photoelectron spectroscopy (XPS) spectra were measured for the selenized CZTSe thin film which was deposited on the quartz and was selenized with a bare SLG. Beside the four main types of elements in the selenized CZTSe thin films (see Figure S2), XPS spectra (the detection limit of 0.1-0.5 at % ) confirmed the absence of K (Figure 2b), Ca (Figure 2c), Si (Figure 2d), and Mg (Figure 2e) which are contained in the soda-lime glass, as shown in Figure 2. As expected, Na can be obviously observed, and the Na 1s peak locates at 1071.5 eV (see Figure 2a), which is consistent with that of Na+ ions. Note that a strong peak in Figure 2b is assigned to C 1s. Besides, the strong peak in Figure 2e stands for Se 3d, instead of Mg 2p. To eliminate the influence of possible impurities on the surface of the selenized CZTSe thin film, XPS spectrum of the selenized CZTSe thin film was taken after Ar plasma etching for 30 s. As shown in Figure S3, Na 1s peak is still apparent, which demonstrates that only Na can diffuse into the selenized CZTSe thin films from the soda-lime glass.

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The energy dispersive X-ray (EDS) line scan was used to measure the depth distribution of Na in the selenized CZTSe thin film. Figure 3a shows a cross-sectional SEM image of the selenized CZTSe thin film deposited on the quartz. The selenized CZTSe thin film exhibits a bi-layer structure consisting of a large-grain CZTSe top layer and a Se-rich fine-grain CZTSe bottom layer. EDS line scans in Figure 3b using SEM mode across the back interface region reveal that Na is mainly present in the large-grain regions, but K cannot be obviously observed. According to the previous report,13 Na can induce the crystal growth of CZTSSe thin films by the formation of low melting point NaxSe during the selenization. Thus, Na is mainly found in the large-grain CZTSe top layer. Figure 4a presents XRD patterns of the selenized CZTSe thin films under the four conditions. The XRD was used to investigate the effect of the top bare SLG on the crystallinity of the selenized CZTSe thin films. All XRD patterns are similar except for the diffraction peak of 29.5º. According to the literature,24 the XRD peak at 29.5º can be indexed to the (101) plane of Se. The crystalline Se was formed in the fine-grain CZTSe bottom layer during the selenization, which is further confirmed by the EDS line scan (See Figure 3c). The XRD peak of Se became more apparent for those selenized CZTSe thin films without SLG cover due to their low crystallinity and discrete top layer. Thus, covering a piece of SLG on the top of CZTSe precursor thin film plays a crucial role in improving the crystallinity of CZTSe thin film. Note that the other crystalline impurities except for Se were not detected, including Cu2Se, ZnSe, and SnSe2. To further identify the purity of CZTSe thin film, Raman spectra

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with the excitation source of 532 nm were measured and shown in Figure 4b. All the selenized CZTSe thin films under different conditions have similar Raman spectra, and their Raman peaks at 172 cm-1, 196 cm-1 and 234 cm-1 are ascribed to kesterite CZTSe.25 Because ZnSe and CZTSe have the same characteristic peak at 251 cm-1,25, 26

in order to identify the peak at 251 cm-1, Raman spectra with the excitation

wavelength of 488 nm were taken and shown in Figure S4. According to the literature report,27 under the excitation of 488 nm, the Raman peaks of CZTSe should be located at 171 cm-1, 196 cm-1, and 238 cm-1, and the peak for ZnSe is at 250 cm-1. When the CZTSe thin films were excited by 488 nm, all of the Raman peaks at 251 cm-1 disappear, confirming that the peaks located at 251 cm-1 in Raman spectra with the 532 nm excitation should belong to CZTSe, instead of ZnSe. To confirm the effectiveness of our Na-doping method, the four types of CZTSe devices were fabricated. The photograph of CZTSe device is shown in the inset of Figure S5. The active area of the device is 0.368 cm2. Their current-voltage characteristics were measured under AM 1.5 solar illumination at 100 mW/cm2. Figure S5 shows two J-V curves of CZTSe devices which were deposited on SLG and quartz substrates and selenized without SLG covering. No matter the substrate is the SLG or quartz, their PCEs are almost 0%. Because the selenized CZTSe thin films without the cover of SLG are not compact and continuous, leading to a large number of shunting paths, which is severely detrimental to the performance of device. In addition, the J-V curves of CZTSe devices which were covered with SLG are shown in Figure 5. The PCE of CZTSe solar cell with a SLG substrate is 7.51%, which is

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significantly higher than 6.09% for CZTSe solar cell with a quartz substrate (see Figure 5a and 5b). The apparent improvement in PCE is mainly due to an increase in open circuit voltage (Voc) and fill factor (FF). As the selenized CZTSe thin films on SLG substrate and quartz substrate have almost the same chemical composition (see Figure S6) and thickness (see Figure S7), the enhancement in Voc and FF for the CZTSe device on SLG substrate should be ascribed to the diffusion of Na from the SLG substrate, which leads to the higher efficiency than that of on quartz substrate. To confirm the reproducibility of CZTSe thin film solar cells, we tested randomly 40 CZTSe devices which were fabricated on the quartz and SLG substrates, respectively, and their PCE distributions are shown in Figure S8. For the devices deposited on SLG substrate, the power conversion efficiencies are mainly in the range of 6-7% (~51%), which are higher than 5-6% (~42%) for those deposited on the quartz substrate. Previous studies show that the incorporation of Na can result in marked improvement in the hole concentration and therefore an increase in Voc and FF.10,

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Table S1

presents the effective hole concentrations for four types of the selenized CZTSe thin films grown on the SLG and quartz substrates and selenized with and without SLG covering by Hall effect measurements. When the CZTSe thin film was deposited on the quartz substrate and covered by a piece of SLG during the selenization, a massive increase in the net hole concentration was observed from 9.6×1015 cm-3 to 6.6×1017 cm-3. When SLG was used the substrate, CZTSe thin film covered by SLG had a greater improvement in the hole concentration. The respective Na content in the CZTSe thin films which were deposited on the Mo-coated SLG substrate and

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selenized with and without SLG covering is 2.56% and 1.72% (at, relative to total metal content), as determined by ICP-OES. When the CZTSe thin film was deposited on the Mo-coated quartz substrate and covered by a piece of SLG during the selenization, the Na content is 1.98%. Therefore, the diffusion of Na from both two sides is most beneficial to the performance of CZTSe solar cells. CONCLUSIONS In summary, we have demonstrated a simple and low-cost Na doping method to enhance the photoelectric conversion efficiency of CZTSSe thin film solar cells. In this method, a piece of SLG was placed on the as-deposited CZTSe nanocrystal thin film during the selenization process. Na out-diffusion from the top SLG can significantly promote the crystal growth and improve the hole-carrier concentration of CZTSe absorber layer, which leads to an apparent enhancement in Voc and FF. In our case, a PCE of 7.51% was obtained for CZTSe precursor thin film which was deposited on Mo-coated SLG substrate and was selenized with a bare SLG, demonstrating that Na diffusion from both two sides is the most effective for CZTSe thin film solar cells. Our cost-effective Na doping method could have the high potential to fabricate highly efficient Cu-based thin film solar cells.

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Figure Captions:

Figure 1. The morphologies of the selenized CZTSe thin films which were deposited on Mo-coated SLG (a and b) and Mo-coated Quartz (c and d) substrates. Note that the samples of b and d were covered with a piece of SLG during the selenization. Figure 2. XPS spectra of some possible impurities in the selenized CZTSe thin film resulting from the out-diffusion of the top SLG during the selenization. (a) Na; (b) K; (c) Ca; (d) Si; (e) Mg. Figure 3. Cross-sectional SEM and EDS line scans of the selenized CZTSe thin film which was deposited on the quartz substrate and was covered with a SLG during the selenization. (a) Cross-sectional SEM; (b and c) EDS line scans. Figure 4. X-ray diffraction patterns (a) and Raman spectra of 532nm (b) for the selenized CZTSe thin films on two kinds of the substrates (SLG and Quartz) with and without SLG cover.

Figure 5. J-V curves of CZTSe solar cells on different substrates with SLG covering during the selenization. (a) SLG; (b) Quartz.

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b

a

3.0 µm

c

3.0 µm

d

3.0 µm

3.0 µm

Figure 1. The morphologies of the selenized CZTSe thin films which were deposited on Mo-coated SLG (a and b) and Mo-coated Quartz (c and d) substrates. Note that the samples of b and d were covered with a piece of SLG during the selenization.

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a

b

Intensity (a.u.)

Intensity (a.u.)

Na 1s

1065

1070

1075

1080

280

K 2p

282

284

286

288

290

292

294

Binding Energy (eV)

Binding Energy (eV)

c

d Intensity (a.u.)

Intensity (a.u.)

Ca 2p

345

350

355

360

Si 2p

100

Binding Energy (eV)

105

110

115

Binding Energy (eV)

e Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mg 2p

44

46

48

50

52

54

56

58

60

Binding Energy (eV)

Figure 2. XPS spectra of some possible impurities in the selenized CZTSe thin film resulting from the out-diffusion of the top SLG during the selenization. (a) Na; (b) K; (c) Ca; (d) Si; (e) Mg.

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a

1.0 µm

b Counts (a.u.)

K Na

0

300

600

900

1200

n

1500

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Counts (a.u.)

c Cu Zn Sn Se

0

300

600

900

1200

n

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1500

Estimated Distance ( m) Figure 3. Cross-sectional SEM and EDS line scans of the selenized CZTSe thin film which was deposited on the quartz substrate and was covered by a SLG during the selenization. (a) Cross-sectional SEM; (b and c) EDS line scans.

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20

30

40

50

60

(316)

Mo

Quartz Quartz+SLG SLG SLG+SLG (400)/(008)

(116)/(312)

(220)/(204)

(103)

Se (101)

(110)

Intensity (a.u.)

(112)

a

70

2 Theta (degree)

b Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


196 172 234 251

200

300

Quartz Quartz+SLG SLG SLG+SLG

400 -1

500

Raman Shift (cm ) Figure 4. X-ray diffraction patterns (a) and Raman spectra of 532nm (b) for the selenized CZTSe thin films on two kinds of the substrates (SLG and Quartz) with and without SLG cover.

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2

on SLG

20

0

-20

-40

-0.4

Voc=408 mV Jsc=31.6 mA/cm

2

FF=58.3% Eff=7.51% -0.2

0.0

a 0.2

0.4

40

on Quartz

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40

Current Density (mA/cm )

Current Density (mA/cm )

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0.6

Voltage (V)

20

0

-20

-40

-0.4

Voc=359 mV Jsc=32.9 mA/cm

2

FF=51.5% Eff=6.09% -0.2

0.0

b 0.2

Voltage (V)

0.4

0.6

Figure 5. J-V curves of CZTSe solar cells on different substrates with SLG covering during the selenization. (a) SLG; (b) Quartz.

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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author [email protected] ACKNOWLEDGMENT This study was financially supported by the National Natural Science Foundation of China (No: 91333108; 51302258; 51172229; 51202241). Supporting Information The selenization set-up for Na doping; XPS spectra of the selenized CZTSe thin film; XPS spectrum of Na in the selenized CZTSe thin film after Ar plasma etching for 30 s; EDS spectra of the selenized CZTSe thin films deposited on SLG and Quartz; cross-sectional SEM of completed CZTSe devices deposited on SLG and Quartz; Raman spectra with 488 nm excitation wavelength; J-V curves of CZTSe solar cells on different substrates without SLG covering during the selenization.; Inset: the picture of the CZTSe device; The efficiency distribution of as-fabricated 40 CZTSe solar cell devices deposited on the different substrates; The effective hole concentration of four types of CZTSe thin films; the complete author lists of Ref. 3, 8 and 16. This information is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES (1) Mitzi, D.; Gunawan, O.; Todorov, T.; Wang, K.; Guha, S.; The Path Towards a High-Performance Solution-Processed Kesterite Solar Cell. Sol. Energy Mater. Sol. Cells 2011, 95, 1421-1436. (2) Wang, W.; Winkler, M.; Gunawan, O.; Gokmen, T.; Todorov, T.; Zhu, Y.; Mitzi, D. Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency. Adv. Energy Mater. 2014, 4, 1301465. (3) Repins, I.; Beall, C.; Vora, N.; Dehart, C.; Kuciaukas, D.; Dippo, P.; To, B.; Mann, J.; Hus, W.; Goodrich, A. et al. Co-Evaporated Cu2ZnSnSe4 Films and Devices. Sol. Energy Mater. Sol. Cells 2012, 101, 154-159. (4) Miskin, C.; Yang, W.; Hages, C.; Carter, N.; Joglekar, C.; Stach, E.; Agrawal, R. 9.0% Efficient Cu2ZnSn(S,Se)4 Solar Cells from Selenized Nanopartcle Inks. Prog. Photovolt: Res. Appl. 2014, 22, 35-43. (5) Scragg, J.; Kubart, T.; Wätjen, J.; Ericson, T.; Linnarsson, M.; Björkman, C. Effects of Back Contact Instability on Cu2ZnSnS4 Devices and Processes. Chem. Mater. 2013, 25, 3162-3171. (6) Redinger, A.; Berg, D.; Dale, P.; Siebentritt, S. The Consequences of Kesterite Equilibria for Efficient Solar Cell. J. Am. Chem. Soc. 2011, 133, 3320-3323. (7) Guo, Q.; Ford, G.; Yang, W.; Hages, C.; Hillhouse, H.; Agrawal, R. Enhancing the Performance of CZTSSe Solar Cells with Ge Alloying. Sol. Energy Mater. Sol. Cells 2012, 105, 132-136.

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Table of contents Figure:

SLG Se

Na

Na

Na

Na

Se

CZTSe Mo SLG

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Facile and Low-Cost Sodium-Doping Method for High-Efficiency

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