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Efficient flexible Mo foil-based Cu2ZnSn(S, Se)4 solar cells from In-doping technique
Solar Energy Materials & Solar Cells 209 (2020) 110434
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Efficient flexible Mo foil-based Cu2ZnSn(S, Se)4 solar cells from In-doping technique Xue Yu a, Shuying Cheng a, b, *, Qiong Yan a, d, Junjie Fu c, Hongjie Jia a, Quanzhen Sun a, Zhiyuan Yang a, Sixin Wu c, ** a
College of Physics and Information Engineering, Institute of Micro-Nano Devices and Solar Cells, Fuzhou University, Fuzhou, 350108, PR China Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou, 213164, PR China Key Laboratory for Special Functional Materials of MOE, Collaborative Innovation Center of Nano Functional Materials and Applications, Henan University, Kaifeng, 475004, PR China d Key Laboratory of Green Perovskites Application of Fujian Province Universities, Fujian Jiangxia University, Fuzhou, 350108, PR China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Flexible CZTSSe solar cells Mo foil In dopant Solution process
Cation substitution has an important impact on the power conversion efficiency (PCE) of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Herein, a significant PCE enhancement of flexible indium-doped CZTSSe solar cells has been achieved by partially substituting Sn4þ with In3þ. Systematic measurements indicate that In doping in CZTSSe film effectively improves the crystallinity and carrier concentration of the absorber layer. Meanwhile, the treatment reduces interface recombination and band tailing by passivating deep defects and increases the opencircuit voltage (Voc) of the solar cells. By physical analysis, the key parameters for the solar cell diode such as A, J0, Rs and Rsh are significantly improved after In-doping, indicating better PN junction quality. Under the optimal In-doping (x ¼ In/(Sn þ In) ¼ 9%), the flexible Cu2ZnSn1-xInx(S,Se)4 solar cell has been successfully obtained with the best efficiency of 7.19% and the Voc enhancement of 62 mV due to reduced Voc deficit and band tailing.
1. Introduction Cu2ZnSn(S,Se)4 (CZTSSe) materials have attracted much attention over recent decades due to its tunable band gap and natural abundant components [1–7]. But the efficiencies of CZTSSe solar cells on flexible substrates (the highest PCE of 10.34%) are still lower than those on rigid soda lime glass (SLG) substrates (the highest PCE of 12.6%) [8,9]. Na doping is widely used in high-efficient flexible CZTSSe solar cells since Na can efficiently increase the carrier concentration and improve grain boundary properties, thereby improving the device performance [10]. The current bottleneck that CZTSSe solar cells encounter is the large Voc deficit (Voc,def) [11–15]. The abundant point defects, defect clusters, and associated band tailing have attracted much attention recently. Except for CuZn antisite defects caused by Cu/Zn disorder, there still exist VSn, CuSn defects caused by the similar atomic radius of Cu (0.77 Å) and Sn (0.69 Å) [16], and the Sn-related deep-level defects which influence the efficiency of CZTSSe solar cells [11]. Meanwhile, the decomposition of kesterite CZTSSe in heat treatment caused by the multivalent Sn (þII
and þIV oxidation states) is another issue in the preparation of CZTSSe solar cells [17]. Various studies on Ge substitution have been conducted to investi gate band alignment and performances of the CZTSSe solar cells by replacing smaller Sn atoms. The efficiency of Cu2Zn(Sn1-xGex)Se4 solar cell with 40% Ge-substituted has been up to 12.3% by reducing defects [18], increasing minority carrier lifetime and adjusting the band gap of CZTSSe. However, Ge changes into volatile GeSe2 phase during seleni zation, leading to Ge loss and reducing the short-circuit current density (Jsc), thus deteriorating the performance of the solar cell [6,19]. The efficiency of CZTSSe solar cell prepared by double-buffer layers (CdS/In2S3) is 12.7% [20]. During the high-temperature annealing process, indium (In) diffuses into the absorption layer to form InCd and InSn defects, thereby increasing carrier concentration and reducing Voc, def. According to related reports [21–25], In doping can increase the conductivity, hole concentration and carrier mobility of the CZTSSe thin films. Therefore, the new dopant of element indium (III group) is hopeful to be attractive in the future.
* Corresponding author. College of Physics and Information Engineering, Institute of Micro-Nano Devices and Solar Cells, Fuzhou University, Fuzhou, 350108, PR China. ** Corresponding author. E-mail addresses: [email protected] (S. Cheng), [email protected] (S. Wu). https://doi.org/10.1016/j.solmat.2020.110434 Received 18 November 2019; Received in revised form 20 January 2020; Accepted 24 January 2020 Available online 10 February 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.
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Solar Energy Materials and Solar Cells 209 (2020) 110434
Herein, we adopted an elemental precursor solution method for preparing Cu2ZnSn1-xInx (S,Se)4 (CZTISSe) thin films on Mo foils by doping In with varied x values (x ¼ In/(In þ Sn)). Solar cells with a structure of Ag/ITO/i-ZnO/CdS/CZTISSe/Mo foil were fabricated and their photovoltaic properties were systematically investigated. Furthermore, the effects of In dopant on the photovoltaic performance of flexible CZTSSe solar cells were studied in detail.
increased from 0 to 12%. The EDX data of In/(In þ Sn) ratios are decreased slightly from solutions to films due to different bonding strengths of the chelating edtH2/en with different metal elements during the selenization process. The ratios of elements for the absorber layers follow the stoichiometry of Cu-poor and Zn-rich. With decreasing the content of Sn, the contents of Cu and Zn are almost unchanged, which indicates that In dopant has little influence on other metal elements. The XRD patterns of the CZTISSe thin films with x range of 0–12% are shown in Fig. 1(a). The positions of the main XRD peaks are 27.56� , 45.46� , and 53.86� , respectively corresponding to crystal planes (112), (204) and (312), which are in accordance with the kesterite phase of CZTSSe (JCPDS No. 52–0868). The strong (200) peak around 58� cor responding to Mo (foil) peak [28]. The full widths at half maxima (FWHM) at (112) of samples with In 0% -In 12% are 0.207, 0.167, 0.157, 0.151, and 0.181, respectively. The reduction in FWHM indicates the increase in crystallite size. The enlarged views of diffraction peak (112) are shown in Fig. 1(b). It is obvious that the diffraction peak (112) shifts to lower angle with increasing In content, indicating that In3þ has been successfully incorporated into the CZTSSe to form CZTISSe by replacing Sn4þ or Zn2þ or Cu1þ, since the ionic radius of In3þ (0.80 Å) is larger than that of Sn4þ (0.69 Å), Zn2þ (0.74 Å) and Cu1þ (0.77 Å) [29]. There are MoSe2 peaks in XRD patterns as shown in Fig. 1 (a). MoSe2 is helpful for forming an Ohmic contact between the CZTSSe film and Mo back electrode, but it will increase the series resistance [9,27]. The FWHM of MoSe2 peaks is almost unchanged, indicating that the thickness of MoSe2 layer remains almost unchanged within x range of 0–12%. Raman spectra were measured to identify binary or ternary phases similar to CZTSSe such as ZnSe and Cu2SnSe3. The Raman spectra of the CZTISSe films with the excitation wavelength of 532 nm are shown in Fig. 2 (a). Remarkable peaks appear around 171, 194, and 236 cm 1 and a weak peak appears in the vicinity of 336 cm 1, indicating that the S/(S þ Se) ratio is very low in the CZTISSe film, in agreement with the results of Table 1. The Raman spectra with the excitation wavelength of 325 nm are shown in Fig. 2 (b). Remarkable peaks appear around 250 cm 1 and 501 cm 1 corresponding to Zn–Se bond, and 325 cm 1 corresponding to Zn-(S,Se) bond. The reduction in peak intensity indicates that Zn2þ is replaced by In3þ [30]. Fig. 3 shows the SEM images of the CZTISSe films on the Mo foils with x range of 0–12%. Small amount of In dopant can significantly improve the surface morphology. With increasing the In content to 9%, the crystallinity of the film is enhanced with enlarged crystalline grains, meanwhile the voids are obviously reduced. Sample In 9% is the most compact with dense large grains layers and no pin holes. After that, by doping more element In in the CZTSSe layer, the crystallinity of the film is deteriorated. The excessive In dopant might change the internal stress of the film and influence the growth of the grains, which increase voids at the grain boundary and induce deep defects (see Fig. 3 (In 12%)). By observing the SEM cross sections of the films, the CZTISSe films exhibit a three-layer structure, i.e., a large grain layer on the surface, a small grain layer in the middle and a large grain layer near the Mo foil. For sample In 12%, the large crystal layer near Mo foil is shedding perhaps due to the effect of the internal stress, indicating that high In dopant has a bad influence on the overall morphology of the film. To further evaluate the effects of In dopant on the properties of the flexible CZTISSe devices, the current-voltage (J-V) curves of the CZTISSe devices are presented in Fig. 4(a). And the performance parameters of
2. Experimental section Mo foils were potentiostatically polished in methanol and sulfuric acid mixed solution. The CZTISSe thin films were fabricated on Mo foils by green solution-process [2,16,21,27]. Cu (99.9%, Aladdin), Zn (99.9%, Aladdin), Sn (99.8%, Alfa Aesar), In (99.99%, Aladdin), S (99.9%, Aladdin) and Se (99%, Alfa Aesar) powders were used as received without further purification. To prepare CZTISSe precursor solution, 1.10 mmol Cu, 0.75 mmol Zn, 0.72 mmol Sn (or mixture of Sn & In), 0.30 mmol Se, and 2.60 mmol S were mixed into 1,2-ethanedithiol (edtH2) and 1,2-ethylenediamine (en) solution. To explore the influence of In dopant on the properties of the flexible solar cells, we prepared 5 precursor solutions with different contents of In/(Sn þ In) mole ratio, i. e., x ¼ In/(Sn þ In) ¼ 0, 3, 6, 9 and 12%, respectively. The CZTSSe and CZTISSe precursor films were prepared by spin-coating the precursor solution on the Mo foils and then selenized in a cylindrical graphite box (π � 332 � 8 mm3) with 600 mg of Se pellets (99.999%) at 550 � C for 15 min. High-purity Se pellets were used to avoid introducing possible impurities [26]. Since Na dopant in flexible CZTSSe devices is critical, the CZTSSe and CZTISSe precursors samples were covered with SLG on the top in the graphite box during selenization process to achieve Na diffusion, as reported in our previous work [27]. The samples of the films were named as In x, which means film with x concentration (x ¼ In/(Sn þ In) ¼ 0, 3, 6, 9 and 12%). CZTISSe devices with the structure of Ag/ITO/i-ZnO/CdS/CZTISSe/Mo-foil were made according to our pre vious reports [16,27]. These solar cells samples were named as C100x, with x ¼ In/(Sn þ In) ¼ 0, 3, 6, 9 and 12%, respectively. Finally, nine CZTISSe solar cells with each active area of 0.21 cm2 were scribed from one substrate (2 � 2 cm2). The scanning electron microscope (SEM) images were collected by a Nova Nano field-emission SEM 450 with an energy dispersive X-ray (EDX) analyzer. The X-ray diffraction (XRD) patterns were obtained by a Rigaku Smartlab. Raman spectra were measured via Renishaw Raman microscope with excitation wavelength of 532 nm and 325 nm, respectively. Steady-state photoluminescence spectra were measured at room temperature by using an 808 nm solid state laser with a filter of 850 nm. Current density-voltage (J-V) curves were measured by a Keithley 2400 source meter under AM1.5 illumination (100 mW cm 2). The external quantum efficiency (EQE) curves were collected by a Zolix SCS100 QE system under a 150 W xenon light source. The capacitancevoltage (C–V) curves were measured by a Keithley 4200 semiconductor characterization system. 3. Results and discussion The composition results of CZTISSe thin films with x range of 0–12% by EDX spectrometry are given in Table 1. The atomic content of element In in the CZTISSe increases (from 0 to 1.42%) when x is Table 1 Composition results (atomic ratio) of CZTISSe thin films with x from 0 to 12%. Sample
x (%)
In/(In þ Sn)
Cu (%)
Zn (%)
Sn (%)
In (%)
S (%)
Se (%)
Cu/(Zn þ Sn þ In)
Zn/(Sn þ In)
In In In In In
0 3 6 9 12
0 0.02 0.05 0.08 0.11
20.82 21.24 20.13 20.64 20.82
12.75 13.40 12.81 12.64 13.01
13.10 13.27 13.04 12.74 11.46
0 0.27 0.77 1.18 1.42
2.25 1.85 2.75 2.22 2.80
51.09 49.97 50.51 50.58 50.49
0.81 0.79 0.76 0.78 0.80
0.97 0.99 0.93 0.91 1.01
0% 3% 6% 9% 12%
2
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Solar Energy Materials and Solar Cells 209 (2020) 110434
Fig. 1. (a) XRD patterns, (b) enlarged views of diffraction peaks (112) for the CZTISSe films.
Fig. 2. Raman spectra of the CZTISSe films with the excitation wavelength of (a) 532 nm and (b) 325 nm.
Fig. 3. Typical surface and cross-section SEM images of the CZTISSe films on the Mo foils with x range of 0–12%.
the CZTISSe devices with different x are listed in Table 2. It can be seen intuitively that, under the optimum In-doped ratio of 9%, sample C9 exhibits the best efficiency of 7.19%, which is obviously higher than 4.41% of undoped sample C0. With increasing x from 0 to 12%, the efficiencies of the solar cells increase from 4.41 to 7.19% and then decrease to 3.94%. At the same time, the Voc significantly increases from 331 to 393 mV and then decreases to 351 mV. The Jsc has a similar change tendency, i.e., from 28.89 mA/cm2 upward to 32.12 mA/cm2 and then down to 26.79 mA/cm2. The improvement of carrier concen tration significantly enhances the efficiency of the flexible CZTSSe de vice [31].
Fig. 5(a) shows the EQE spectra of the CZTISSe solar cells with varied In-doped ratios. Most of the CZTISSe solar cells (especially samples C6 and C9) have higher photo-response than the CZTSSe solar cell (sample C0) in the visible and near infrared wavelength. With increasing In dopant, the low-lying absorption at 600–800 nm has been improved regularly. In can enhance the process of carrier separation, and reduce the recombination rate of photo-generated carriers due to the decrease of the defect density in the CZTISSe, which improves the photo-quantum efficiency. In the meantime, In dopant not only improves the light ab sorption in the visible region, but also enhances the spectral response in the long wavelength region. The possible reasons are: Firstly, doping In 3
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Solar Energy Materials and Solar Cells 209 (2020) 110434
Fig. 4. (a) Current-voltage curves of the CZTISSe devices with different x. The distribution of (b) PCE, (c) Voc, (d) Jsc, and (e) FF for the devices.
demonstrates that doping In can indeed improve the Jsc. The EQE IntegJsc values of the other devices with variation of In ratios are listed in Table 2. The EQE of device C12 becomes weaker compared with device C0, indicating that excessive In dopant decreases the photo-quantum efficiency. Fig. 6 (a) shows the normalized photoluminescence (PL) spectra under varied In contents in the CZTISSe films. Fig. 6(b) depicts the bandgap Eg of the CZTISSe thin film obtained from the data near the band edge by plotting [E�ln(1 EQE)]2 versus E [32], and the results are listed in Table 3. The PL peak position has a small fluctuation, which decreases initially and then increases with the amount of In dopant. With increasing In dopant from 0 to 6%, the PL peak decreases from 1.029 to 1.008 eV. Then, with the increase of In dopant from 6 to 12%, the Eg increases slightly to 1.023 eV. Because Voc,def can be expressed as Voc,def ¼ Eg/q- Voc. As listed in Table 3, the Voc,def of the CZTSSe solar cells decreases from 818 to 682 mV after partially substituting Sn4þ with In3þ, indicating that Sn-related defects have been suppressed. The trend of Eg values and room-temperature PL peak values with the variation of x (x ¼ In/(In þ Sn)) are listed in Table 3. The Eg values of CZTISSe obtained from EQE have a clear trend of decreasing initially and then increasing, i.e., from 1.149 eV (sample C0) down to 1.075 eV
Table 2 Performance parameters of CZTISSe devices with different x. Sample
x (%)
PCE (%)
Voc (mV)
Jsc (mA/ cm2)
FF (%)
Integ-Jsc (mA/ cm2)
C0 C3 C6 C9 C12
0 3 6 9 12
4.41 5.07 6.36 7.19 3.95
331 353 367 393 351
28.89 28.01 30.05 32.12 26.79
46.09 51.20 57.60 56.96 41.88
28.42 29.23 31.63 32.50 25.52
can reduce the Eg and defects in the CZTISSe bulk, thereby promoting the generation and separation of photo-generated carriers. Secondly, the recombination at the CZTSSe/CdS interface is suppressed by doping In. Finally, the EQE curve near the band edge decreases sharply, which indicates that the band tailing is reduced. It is worth mentioning that in the wavelength range of 500–1000 nm, device C9 shows a high EQE value, exceeding 80%. By verifying the high short-circuit current density obtained by J-V test, we have fitted the EQE curve by integral. As shown in Fig. 5(b), the integral current density (Integ- Jsc) value is 32.50 mA/ cm2, which is consistent with the Jsc value (32.12 mA/cm2). It
Fig. 5. (a) Relative EQE spectra of the CZTISSe solar cells, (b) EQE spectra and integral current density of device C9. 4
Solar Energy Materials and Solar Cells 209 (2020) 110434
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Fig. 6. (a) Normalized PL spectra of CZTISSe films varied with In contents, (b) Bandgap Eg of the devices calculated from EQE data.
parameters and Voc. Compared with undoped sample C0, sample C9 has smaller Gsh, Rs, J0 and A. The Gsh decreases from 4.41 to 3.51 mS/cm2 due to the reduction of carrier recombination and the improvement of crystallization quality. The Rs reduces from 1.71 to 0.64 Ω cm2 after doping In. Meanwhile parameter A is improved from 2.03 to 1.58, indicating that In dopant can effectively alleviate the serious recombi nation occurred at the interface [34,35]. The reduction of J0 demon strates that defects and carrier recombination are suppressed, which will increase the Voc and FF. The depletion region width (Wd) and charge carrier density (Nc-v) of the samples were measured by a C–V method as shown in Fig. 7. The Nc-v and Wd are based on the following equations:
Table 3 Detailed performance parameters of CZTISSe solar cells with varied In dopant levels. Sample
Eg(eV)
Eg/q-Voc (mV)
PL (eV)
Δ(Eg-PL) (eV)
Wd(μm)
Nc-v(cm
C0
1.149
818
1.029
0.12
0.291
C3
1.093
740
1.011
0.082
0.278
C6
1.077
710
1.008
0.069
0.275
C9
1.075
682
1.016
0.059
0.247
C12
1.116
765
1.023
0.093
0.219
7.53 � 1015 1.08 � 1016 1.18 � 1016 1.39 � 1016 1.54 � 1016
3
)
Nc
(sample C9) and then upward to 1.116 eV (sample C12). When the In content x raised from 0% to 9%, the energy difference Δ(Eg -PL) be tween the Eg and the PL peak position is diminished gradually from 120 to 59 meV, which indicates that the band tailing can be effectively reduced when x is 9%, and then the Δ(Eg -PL) is increased abruptly to 93 meV with increasing x from 9% to 12%. This indicates that doping In indeed reduces the density of defects, such as VSn, CuSn and SnCu, SnZn deep-level defects, and suppress the band tailing. However, excessive In dopant can aggravate the band tailing. According to our previous work [16], the shunt conductance (Gsh), shunt resistance (Rsh), series resistance (Rs), diode ideality factor (A), and reverse saturation current density (J0) can be obtained from the standard light J-V curves in Fig. 4(b). The Gsh was obtained from the y-axis intercept at dJ/dV vs. V curves. The Rs and A were obtained from the y-axis intercept and slope (AkT/q) at dV/dJ vs. (J þ Jsc) 1 curves. The J0 was obtained from the y-axis intercept at J þ Jsc–GV vs. V–RsJ curves. Table 4 gives the detailed data from samples C0 and C9. The Voc is based on the following equation [33]: Gsh Voc ¼ Jsc
J0 ðexp
qVoc AkT
¼
Wd ¼
C3 dC ð Þ qS2 ε0 εr dV
Sε0 εr C
1
(2) (3)
where C, q and S stand for the measured capacitance, electron charge and active device area, respectively. The ε0 and εr are vacuum dielectric constant (~8.854 � 10 12 F/m) and relative dielectric constant of CZTSSe (8 � 106). The detailed values of Wd and Nc-v for the CZTISSe solar cells with different x are listed in Table 3. When x is increased from 0 to 12%, the Nc-v is enhanced from 7.53 � 1015 to 1.54 � 1016 cm 3, but the Wd is reduced from 0.291 to 0.219 μm. This indicates that the incorporation of In dopant will greatly increase the Nc-v of the absorber and decrease the Wd of the CZTISSe solar cell. It is well known that a narrow depletion region width can influence the charge separation and collection, especially for the long wavelength region close to the band edge, and finally affect the PCE of solar cells. According to the result, we can find that device C9 has the best performance, and the Wd of device C9 is not wider than that of device C0. Partially replacing Sn4þ with In3þ in CZTSSe solar cell can significantly increase the Nc-v value under the optimal In content (x ¼ 9%). As a result, the Voc and Jsc are successfully enhanced from 331 to 393 mV and 28.89 to 32.12 mA/cm2, respectively. The above results prove that both of the Nc-v and Wd are very sensitive to the relative content of In3þ, and the optimal incorporation of In (x ¼ 9%) can significantly raise the efficiency of the CZTSSe solar cell.
(1)
1Þ
v
The Voc can be can be enhanced by reducing Gsh and J0. In addition, Jsc and FF follow a similar trend as the relationship between these
4. Conclusions Table 4 Performance parameters of CZTISSe solar cells for samples C0 and C9. Sample
x (%)
Gsh（mS/ cm2）
Rsh (Ω. cm2)
Rs (Ω. cm2)
Jo (mA/ cm2)
A
C0
0
4.41
226
1.71
2.03
C9
9
3.51
284
0.64
7.17 � 10 2 2.23 � 10 2
An elemental precursor solution method has been successfully adopted to prepare CZTISSe thin films on Mo foils by partially substituting Sn4þ with In3þ. Doping In can improve the crystallinity and semiconducting properties of the films. Partially replacing Sn with In can produce shallow level defects of InSn and reduce Sn-related defects such as SnCu and SnZn, thus improving the Voc and carrier transport characteristics (inhibiting the carrier recombination and band tailing in
1.58
5
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Solar Energy Materials and Solar Cells 209 (2020) 110434
Fig. 7. (a) Capacitance-voltage (C–V) curves of flexible CZTISSe solar cells with variation of x. (b) Space-charge density and depletion width derived from C–V curves.
CZTSSe). Meanwhile the substitution of Sn4þ with In3þ can inhibit the decomposition of CZTSSe in heat treatment caused by the multivalent Sn (þII and þIV oxidation states)，thus reducing the Voc,def by increasing the crystallizability and inhibiting the Sn-related deep level defects. Finally, a PCE up to 7.19% has been achieved for the flexible CZTISSe solar cell with the In-doped content of x ¼ 9%, which is 2.78% higher than that of the undoped solar cell. The Voc is increased by 62 mV due to lower Voc,def and defects. The In doping technique and analysis method are helpful for improving the performances of flexible CZTSSe solar cells and promoting the development of them.
[5] D.B. Mitzi, O. Gunawan, T.K. Todorov, K. Wang, S. Guha, The path towards a highperformance solution-processed kesterite solar cell, Sol. Energy Mater. Sol. Cell. 95 (2011) 1421–1436. [6] J.J. Li, D.X. Wang, X.L. Li, Y. Zeng, Y. Zhang, Cation substitution in earth-abundant kesterite photovoltaic materials, Avd. Sci. 5 (2018), 1700744. [7] B. Vermang, G. Brammertz, M. Meuris, T. Schnabel, E. Ahlswede, L. Choubrac, S. Harel, C. Cardinaud, L. Arzel, N. Barreau, J. van Deelen, P.-J. Bolt, P. Bras, Y. Ren, E. Jaremalm, S. Khelifi, S. Yang, J. Lauwaert, M. Batuk, J. Hadermann, X. Kozina, E. Handick, C. Hartmann, D. Gerlach, A. Matsuda, S. Ueda, T. Chikyow, R. F� elix, Y. Zhang, R.G. Wilks, M. B€ ar, Wide band gap kesterite absorbers for thin film solar cells: potential and challenges for their deployment in tandem devices, Sustain. Energy. Fuels. 3 (2019) 2246–2259. [8] W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency, Adv. Energy. Mater. 4 (2014), 1301465. [9] K.J. Yang, S. Kim, S.Y. Kim, K. Ahn, D.H. Son, S.H. Kim, S.J. Lee, Y.I. Kim, S. N. Park, S.J. Sung, D.H. Kim, T. Enkhbat, J. Kim, C.W. Jeon, J.K. Kang, Flexible Cu2ZnSn(S,Se)4 solar cells with over 10% efficiency and methods of enlarging the cell area, Nat. Commun. 10 (2019) 2959. [10] K.-J. Yang, S. Kim, J.-H. Sim, D.-H. Son, D.-H. Kim, J. Kim, W. Jo, H. Yoo, J. Kim, J.-K. Kang, The alterations of carrier separation in kesterite solar cells, Nanomater. Energy 52 (2018) 38–53. [11] S.Y. Chen, A. Walsh, X.G. Gong, S.H. Wei, Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth-abundant solar cell absorbers, Adv. Mater. 25 (2013) 1522–1539. [12] Q. Tian, H. Lu, Y. Du, J. Fu, X. Zhao, S. Wu, S.F. Liu, Green Atmospheric Aqueous Solution Deposition for High Performance Cu2ZnSn(S,Se)4 Thin Film Solar Cells, Solar RRL, 2018, 1800233. [13] T. Gershon, Y.S. Lee, P. Antunez, R. Mankad, S. Singh, D. Bishop, O. Gunawan, M. Hopstaken, R. Haight, Photovoltaic materials and devices based on the alloyed kesterite absorber (AgxCu1-x)2ZnSnSe4, Adv. Energy. Mater. 6 (2016) 1502468. [14] D. Shin, B. Saparov, D.B. Mitzi, Defect engineering in multinary earth-abundant chalcogenide photovoltaic materials, Adv. Energy. Mater. 7 (2017), 1602366. [15] Z.K. Yuan, S.Y. Chen, H.J. Xiang, X.G. Gong, A. Walsh, J.S. Park, I. Repins, S. H. Wei, Engineering solar cell absorbers by exploring the band Alignment and defect disparity: the case of Cu- and Ag-based kesterite compounds, Adv. Funct. Mater. 25 (2015) 6733–6743. [16] X. Yu, S. Cheng, Q. Yan, J. Yu, W. Qiu, Z. Zhou, Q. Zheng, S. Wu, Efficient (Cu1 xAgx)2ZnSn(S,Se)4 solar cells on flexible Mo foils, RSC Adv. 8 (2018) 27686–27694. [17] J.J. Scragg, T. Ericson, T. Kubart, M. Edoff, C. Platzer-Bj€ orkman, Chemical insights into the instability of Cu2ZnSnS4 films during annealing, Chem. Mater. 23 (2011) 4625–4633. [18] S. Kim, K.M. Kim, H. Tampo, H. Shibata, S. Niki, Improvement of voltage deficit of Ge-incorporated kesterite solar cell with 12.3% conversion efficiency, APEX 9 (2016), 102301. [19] S. Giraldo, Z. Jehl, M. Placidi, V. Izquierdo-Roca, A. Perez-Rodriguez, E. Saucedo, Progress and perspectives of thin film kesterite photovoltaic technology: a critical review, Adv. Mater. 31 (2019), 1806692. [20] J. Kim, H. Hiroi, T.K. Todorov, O. Gunawan, M. Kuwahara, T. Gokmen, D. Nair, M. Hopstaken, B. Shin, Y.S. Lee, High efficiency Cu2ZnSn (S, Se) 4 solar cells by applying a double In2S3/CdS emitter, Adv. Mater. 26 (2014) 7427–7431. [21] Y.L. Pei, J. Guo, D.X. Kou, W.H. Zhou, Z.J. Zhou, Q.W. Tian, Y.N. Meng, S.X. Wu, Precise-tuning the in content to achieve high fill factor in hybrid buffer structured Cu2ZnSn(S, Se)4 solar cells, Sol. Energy 148 (2017) 157–163. [22] D. Sharath, A.P. Gaikwad, S. Choudhury, N. Gupta, R. Sasikala, C.A. Betty, Effect of Indium doping on the photoelectrochemical and photocatalytic properties of zinc sulphide, Mater. Sci. Eng. B. Adv. Funct. Solid. State. Mater. 226 (2017) 57–63. [23] R. Chetty, M. Falmbigl, P. Rogl, P. Heinrich, E. Royanian, E. Bauer, S. Suwas, R. C. Mallik, The effect of multisubstitution on the thermoelectric properties of
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Xue Yu: Writing - original draft. Shuying Cheng: Supervision. Qiong Yan: Writing - review & editing. Junjie Fu: Investigation. Hongjie Jia: Investigation. Quanzhen Sun: Data curation. Zhiyuan Yang: Data curation. Sixin Wu: Supervision. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 61574038). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2020.110434. References [1] D. Tiwari, T. Koehler, X.Z. Lin, R. Harniman, I. Griffiths, L. Wang, D. Cherns, R. Klenk, D.J. Fermin, Cu2ZnSnS4 thin films generated from a single solution based precursor: the effect of Na and Sb doping, Chem. Mater. 28 (2016) 4991–4997. [2] Y.C. Yang, G. Wang, W.G. Zhao, Q.W. Tian, L.J. Huang, D.C. Pan, SolutionProcessed highly efficient Cu2ZnSnSe4 thin film solar cells by dissolution of elemental Cu, Zn, Sn, and Se powders, ACS Appl. Mater. Interfaces 7 (2015) 460–464. [3] D.B. Mitzi, O. Gunawan, T.K. Todorov, D.A.R. Barkhouse, Prospects and Performance Limitations for Cu-Zn-Sn-S-Se Photovoltaic Technology, vol.371, Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences, 2013, p. 20110432. [4] F. Jiang, S. Ikeda, T. Harada, M. Matsumura, Pure sulfide Cu2ZnSnS4 thin film solar cells fabricated by preheating an electrodeposited metallic stack, Adv. Energy. Mater. 4 (2014), 1301381.
6
X. Yu et al.
[24] [25] [26] [27] [28] [29]
[30]
Solar Energy Materials and Solar Cells 209 (2020) 110434
chalcogenide-based Cu2.1Zn0.9Sn1-xInxSe4 (0 <¼ x <¼ 0.1), Phys. Status Solidi. App. Mater. Sci. 210 (2013) 2471–2478. D.H. Kuo, M. Tsega, Electrical conduction and mobility enhancement in p-type Indoped Cu2ZnSnSe4 bulks, Jpn. J. Appl. Phys. 53 (2014), 035801. X.Y. Shi, F.Q. Huang, M.L. Liu, L.D. Chen, Thermoelectric properties of tetrahedrally bonded wide-gap stannite compounds Cu2ZnSn1-xInxSe4, Appl. Phys. Lett. 94 (2009), 122103. E. Toda, A. Hioki, Determination of impurities in high-purity selenium by inductively coupled plasma mass spectrometry after matrix separation with thiourea, Anal. Sci. 11 (1995) 115–118. Q. Yan, S. Cheng, H. Li, X. Yu, J. Fu, Q. Tian, H. Jia, S. Wu, High flexible Cu2ZnSn (S,Se)4 solar cells by green solution-process, Sol. Energy 177 (2019) 508–516. Y. Zhang, Q. Ye, J. Liu, H. Chen, X. He, C. Liao, J. Han, H. Wang, J. Mei, W. Lau, Earth-abundant and low-cost CZTS solar cell on flexible molybdenum foil, RSC Adv. 4 (2014) 23666–23669. Z. Xiao, H. Luan, R. Liu, B. Yao, Y. Li, Z. Ding, G. Yang, R. Deng, G. Wang, Z. Zhang, L. Zhang, H. Zhao, Behavior of indium alloying with Cu2ZnSn(S,Se)4 and its effect on performances of Cu2ZnSn(S,Se)4-based solar cell, J. Alloys Compd. 767 (2018) 439–447. M. Dimitrievska, H. Xie, A.J. Jackson, X. Fontan�e, M. Espíndola-Rodríguez, E. Saucedo, A. P�erez-Rodríguez, A. Walsh, V. Izquierdo-Roca, Resonant Raman
[31] [32] [33] [34]
[35]
7
scattering of ZnSxSe1 x solid solutions: the role of S and Se electronic states, Phys. Chem. Chem. Phys. 18 (2016) 7632–7640. J. Li, S. Kim, D. Nam, X. Liu, J. Kim, H. Cheong, W. Liu, H. Li, Y. Sun, Y. Zhang, Tailoring the defects and carrier density for beyond 10% efficient CZTSe thin film solar cells, Sol. Energy Mater. Sol. Cell. 159 (2017) 447–455. A. Niemegeers, M. Burgelman, R. Herberholz, U. Rau, D. Hariskos, H.W. Schock, Model for electronic transport in Cu(In,Ga)Se2 solar cells, Prog. Photovoltaics Res. Appl. 6 (1998) 407–421. G. Yang, Y.-F. Li, B. Yao, Z.-H. Ding, R. Deng, H.-F. Zhao, L.-G. Zhang, Z.-Z. Zhang, Improvement of the photovoltaic performance of Cu2ZnSn(SxSe1 x)4 solar cells by adding polymer in the precursor solution, J. Phys. Appl. Phys. 51 (2018), 105103. G. Brammertz, M. Buffiere, S. Oueslati, H. ElAnzeery, K. Ben Messaoud, S. Sahayaraj, C. Koble, M. Meuris, J. Poortmans, Characterization of defects in 9.7% efficient Cu2ZnSnSe4-CdS-ZnO solar cells, Appl. Phys. Lett. 103 (2013), 163904. H.Y. Park, D.G. Moon, J.H. Yun, S.K. Ahn, K.H. Yoon, S. Ahn, Efficiency limiting factors in Cu(In,Ga)Se2 thin film solar cells prepared by Se-free rapid thermal annealing of sputter-deposited Cu-In-Ga-Se precursors, Appl. Phys. Lett. 103 (2013), 263903.
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Efficient flexible Mo foil-based Cu2ZnSn(S, Se)4 solar cells from In-doping technique Xue Yu a, Shuying Cheng a, b, *, Qiong Yan a, d, Junjie Fu c, Hongjie Jia a, Quanzhen Sun a, Zhiyuan Yang a, Sixin Wu c, ** a
College of Physics and Information Engineering, Institute of Micro-Nano Devices and Solar Cells, Fuzhou University, Fuzhou, 350108, PR China Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou, 213164, PR China Key Laboratory for Special Functional Materials of MOE, Collaborative Innovation Center of Nano Functional Materials and Applications, Henan University, Kaifeng, 475004, PR China d Key Laboratory of Green Perovskites Application of Fujian Province Universities, Fujian Jiangxia University, Fuzhou, 350108, PR China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Flexible CZTSSe solar cells Mo foil In dopant Solution process
Cation substitution has an important impact on the power conversion efficiency (PCE) of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Herein, a significant PCE enhancement of flexible indium-doped CZTSSe solar cells has been achieved by partially substituting Sn4þ with In3þ. Systematic measurements indicate that In doping in CZTSSe film effectively improves the crystallinity and carrier concentration of the absorber layer. Meanwhile, the treatment reduces interface recombination and band tailing by passivating deep defects and increases the opencircuit voltage (Voc) of the solar cells. By physical analysis, the key parameters for the solar cell diode such as A, J0, Rs and Rsh are significantly improved after In-doping, indicating better PN junction quality. Under the optimal In-doping (x ¼ In/(Sn þ In) ¼ 9%), the flexible Cu2ZnSn1-xInx(S,Se)4 solar cell has been successfully obtained with the best efficiency of 7.19% and the Voc enhancement of 62 mV due to reduced Voc deficit and band tailing.
1. Introduction Cu2ZnSn(S,Se)4 (CZTSSe) materials have attracted much attention over recent decades due to its tunable band gap and natural abundant components [1–7]. But the efficiencies of CZTSSe solar cells on flexible substrates (the highest PCE of 10.34%) are still lower than those on rigid soda lime glass (SLG) substrates (the highest PCE of 12.6%) [8,9]. Na doping is widely used in high-efficient flexible CZTSSe solar cells since Na can efficiently increase the carrier concentration and improve grain boundary properties, thereby improving the device performance [10]. The current bottleneck that CZTSSe solar cells encounter is the large Voc deficit (Voc,def) [11–15]. The abundant point defects, defect clusters, and associated band tailing have attracted much attention recently. Except for CuZn antisite defects caused by Cu/Zn disorder, there still exist VSn, CuSn defects caused by the similar atomic radius of Cu (0.77 Å) and Sn (0.69 Å) [16], and the Sn-related deep-level defects which influence the efficiency of CZTSSe solar cells [11]. Meanwhile, the decomposition of kesterite CZTSSe in heat treatment caused by the multivalent Sn (þII
and þIV oxidation states) is another issue in the preparation of CZTSSe solar cells [17]. Various studies on Ge substitution have been conducted to investi gate band alignment and performances of the CZTSSe solar cells by replacing smaller Sn atoms. The efficiency of Cu2Zn(Sn1-xGex)Se4 solar cell with 40% Ge-substituted has been up to 12.3% by reducing defects [18], increasing minority carrier lifetime and adjusting the band gap of CZTSSe. However, Ge changes into volatile GeSe2 phase during seleni zation, leading to Ge loss and reducing the short-circuit current density (Jsc), thus deteriorating the performance of the solar cell [6,19]. The efficiency of CZTSSe solar cell prepared by double-buffer layers (CdS/In2S3) is 12.7% [20]. During the high-temperature annealing process, indium (In) diffuses into the absorption layer to form InCd and InSn defects, thereby increasing carrier concentration and reducing Voc, def. According to related reports [21–25], In doping can increase the conductivity, hole concentration and carrier mobility of the CZTSSe thin films. Therefore, the new dopant of element indium (III group) is hopeful to be attractive in the future.
* Corresponding author. College of Physics and Information Engineering, Institute of Micro-Nano Devices and Solar Cells, Fuzhou University, Fuzhou, 350108, PR China. ** Corresponding author. E-mail addresses: [email protected] (S. Cheng), [email protected] (S. Wu). https://doi.org/10.1016/j.solmat.2020.110434 Received 18 November 2019; Received in revised form 20 January 2020; Accepted 24 January 2020 Available online 10 February 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved.
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Herein, we adopted an elemental precursor solution method for preparing Cu2ZnSn1-xInx (S,Se)4 (CZTISSe) thin films on Mo foils by doping In with varied x values (x ¼ In/(In þ Sn)). Solar cells with a structure of Ag/ITO/i-ZnO/CdS/CZTISSe/Mo foil were fabricated and their photovoltaic properties were systematically investigated. Furthermore, the effects of In dopant on the photovoltaic performance of flexible CZTSSe solar cells were studied in detail.
increased from 0 to 12%. The EDX data of In/(In þ Sn) ratios are decreased slightly from solutions to films due to different bonding strengths of the chelating edtH2/en with different metal elements during the selenization process. The ratios of elements for the absorber layers follow the stoichiometry of Cu-poor and Zn-rich. With decreasing the content of Sn, the contents of Cu and Zn are almost unchanged, which indicates that In dopant has little influence on other metal elements. The XRD patterns of the CZTISSe thin films with x range of 0–12% are shown in Fig. 1(a). The positions of the main XRD peaks are 27.56� , 45.46� , and 53.86� , respectively corresponding to crystal planes (112), (204) and (312), which are in accordance with the kesterite phase of CZTSSe (JCPDS No. 52–0868). The strong (200) peak around 58� cor responding to Mo (foil) peak [28]. The full widths at half maxima (FWHM) at (112) of samples with In 0% -In 12% are 0.207, 0.167, 0.157, 0.151, and 0.181, respectively. The reduction in FWHM indicates the increase in crystallite size. The enlarged views of diffraction peak (112) are shown in Fig. 1(b). It is obvious that the diffraction peak (112) shifts to lower angle with increasing In content, indicating that In3þ has been successfully incorporated into the CZTSSe to form CZTISSe by replacing Sn4þ or Zn2þ or Cu1þ, since the ionic radius of In3þ (0.80 Å) is larger than that of Sn4þ (0.69 Å), Zn2þ (0.74 Å) and Cu1þ (0.77 Å) [29]. There are MoSe2 peaks in XRD patterns as shown in Fig. 1 (a). MoSe2 is helpful for forming an Ohmic contact between the CZTSSe film and Mo back electrode, but it will increase the series resistance [9,27]. The FWHM of MoSe2 peaks is almost unchanged, indicating that the thickness of MoSe2 layer remains almost unchanged within x range of 0–12%. Raman spectra were measured to identify binary or ternary phases similar to CZTSSe such as ZnSe and Cu2SnSe3. The Raman spectra of the CZTISSe films with the excitation wavelength of 532 nm are shown in Fig. 2 (a). Remarkable peaks appear around 171, 194, and 236 cm 1 and a weak peak appears in the vicinity of 336 cm 1, indicating that the S/(S þ Se) ratio is very low in the CZTISSe film, in agreement with the results of Table 1. The Raman spectra with the excitation wavelength of 325 nm are shown in Fig. 2 (b). Remarkable peaks appear around 250 cm 1 and 501 cm 1 corresponding to Zn–Se bond, and 325 cm 1 corresponding to Zn-(S,Se) bond. The reduction in peak intensity indicates that Zn2þ is replaced by In3þ [30]. Fig. 3 shows the SEM images of the CZTISSe films on the Mo foils with x range of 0–12%. Small amount of In dopant can significantly improve the surface morphology. With increasing the In content to 9%, the crystallinity of the film is enhanced with enlarged crystalline grains, meanwhile the voids are obviously reduced. Sample In 9% is the most compact with dense large grains layers and no pin holes. After that, by doping more element In in the CZTSSe layer, the crystallinity of the film is deteriorated. The excessive In dopant might change the internal stress of the film and influence the growth of the grains, which increase voids at the grain boundary and induce deep defects (see Fig. 3 (In 12%)). By observing the SEM cross sections of the films, the CZTISSe films exhibit a three-layer structure, i.e., a large grain layer on the surface, a small grain layer in the middle and a large grain layer near the Mo foil. For sample In 12%, the large crystal layer near Mo foil is shedding perhaps due to the effect of the internal stress, indicating that high In dopant has a bad influence on the overall morphology of the film. To further evaluate the effects of In dopant on the properties of the flexible CZTISSe devices, the current-voltage (J-V) curves of the CZTISSe devices are presented in Fig. 4(a). And the performance parameters of
2. Experimental section Mo foils were potentiostatically polished in methanol and sulfuric acid mixed solution. The CZTISSe thin films were fabricated on Mo foils by green solution-process [2,16,21,27]. Cu (99.9%, Aladdin), Zn (99.9%, Aladdin), Sn (99.8%, Alfa Aesar), In (99.99%, Aladdin), S (99.9%, Aladdin) and Se (99%, Alfa Aesar) powders were used as received without further purification. To prepare CZTISSe precursor solution, 1.10 mmol Cu, 0.75 mmol Zn, 0.72 mmol Sn (or mixture of Sn & In), 0.30 mmol Se, and 2.60 mmol S were mixed into 1,2-ethanedithiol (edtH2) and 1,2-ethylenediamine (en) solution. To explore the influence of In dopant on the properties of the flexible solar cells, we prepared 5 precursor solutions with different contents of In/(Sn þ In) mole ratio, i. e., x ¼ In/(Sn þ In) ¼ 0, 3, 6, 9 and 12%, respectively. The CZTSSe and CZTISSe precursor films were prepared by spin-coating the precursor solution on the Mo foils and then selenized in a cylindrical graphite box (π � 332 � 8 mm3) with 600 mg of Se pellets (99.999%) at 550 � C for 15 min. High-purity Se pellets were used to avoid introducing possible impurities [26]. Since Na dopant in flexible CZTSSe devices is critical, the CZTSSe and CZTISSe precursors samples were covered with SLG on the top in the graphite box during selenization process to achieve Na diffusion, as reported in our previous work [27]. The samples of the films were named as In x, which means film with x concentration (x ¼ In/(Sn þ In) ¼ 0, 3, 6, 9 and 12%). CZTISSe devices with the structure of Ag/ITO/i-ZnO/CdS/CZTISSe/Mo-foil were made according to our pre vious reports [16,27]. These solar cells samples were named as C100x, with x ¼ In/(Sn þ In) ¼ 0, 3, 6, 9 and 12%, respectively. Finally, nine CZTISSe solar cells with each active area of 0.21 cm2 were scribed from one substrate (2 � 2 cm2). The scanning electron microscope (SEM) images were collected by a Nova Nano field-emission SEM 450 with an energy dispersive X-ray (EDX) analyzer. The X-ray diffraction (XRD) patterns were obtained by a Rigaku Smartlab. Raman spectra were measured via Renishaw Raman microscope with excitation wavelength of 532 nm and 325 nm, respectively. Steady-state photoluminescence spectra were measured at room temperature by using an 808 nm solid state laser with a filter of 850 nm. Current density-voltage (J-V) curves were measured by a Keithley 2400 source meter under AM1.5 illumination (100 mW cm 2). The external quantum efficiency (EQE) curves were collected by a Zolix SCS100 QE system under a 150 W xenon light source. The capacitancevoltage (C–V) curves were measured by a Keithley 4200 semiconductor characterization system. 3. Results and discussion The composition results of CZTISSe thin films with x range of 0–12% by EDX spectrometry are given in Table 1. The atomic content of element In in the CZTISSe increases (from 0 to 1.42%) when x is Table 1 Composition results (atomic ratio) of CZTISSe thin films with x from 0 to 12%. Sample
x (%)
In/(In þ Sn)
Cu (%)
Zn (%)
Sn (%)
In (%)
S (%)
Se (%)
Cu/(Zn þ Sn þ In)
Zn/(Sn þ In)
In In In In In
0 3 6 9 12
0 0.02 0.05 0.08 0.11
20.82 21.24 20.13 20.64 20.82
12.75 13.40 12.81 12.64 13.01
13.10 13.27 13.04 12.74 11.46
0 0.27 0.77 1.18 1.42
2.25 1.85 2.75 2.22 2.80
51.09 49.97 50.51 50.58 50.49
0.81 0.79 0.76 0.78 0.80
0.97 0.99 0.93 0.91 1.01
0% 3% 6% 9% 12%
2
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Fig. 1. (a) XRD patterns, (b) enlarged views of diffraction peaks (112) for the CZTISSe films.
Fig. 2. Raman spectra of the CZTISSe films with the excitation wavelength of (a) 532 nm and (b) 325 nm.
Fig. 3. Typical surface and cross-section SEM images of the CZTISSe films on the Mo foils with x range of 0–12%.
the CZTISSe devices with different x are listed in Table 2. It can be seen intuitively that, under the optimum In-doped ratio of 9%, sample C9 exhibits the best efficiency of 7.19%, which is obviously higher than 4.41% of undoped sample C0. With increasing x from 0 to 12%, the efficiencies of the solar cells increase from 4.41 to 7.19% and then decrease to 3.94%. At the same time, the Voc significantly increases from 331 to 393 mV and then decreases to 351 mV. The Jsc has a similar change tendency, i.e., from 28.89 mA/cm2 upward to 32.12 mA/cm2 and then down to 26.79 mA/cm2. The improvement of carrier concen tration significantly enhances the efficiency of the flexible CZTSSe de vice [31].
Fig. 5(a) shows the EQE spectra of the CZTISSe solar cells with varied In-doped ratios. Most of the CZTISSe solar cells (especially samples C6 and C9) have higher photo-response than the CZTSSe solar cell (sample C0) in the visible and near infrared wavelength. With increasing In dopant, the low-lying absorption at 600–800 nm has been improved regularly. In can enhance the process of carrier separation, and reduce the recombination rate of photo-generated carriers due to the decrease of the defect density in the CZTISSe, which improves the photo-quantum efficiency. In the meantime, In dopant not only improves the light ab sorption in the visible region, but also enhances the spectral response in the long wavelength region. The possible reasons are: Firstly, doping In 3
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Fig. 4. (a) Current-voltage curves of the CZTISSe devices with different x. The distribution of (b) PCE, (c) Voc, (d) Jsc, and (e) FF for the devices.
demonstrates that doping In can indeed improve the Jsc. The EQE IntegJsc values of the other devices with variation of In ratios are listed in Table 2. The EQE of device C12 becomes weaker compared with device C0, indicating that excessive In dopant decreases the photo-quantum efficiency. Fig. 6 (a) shows the normalized photoluminescence (PL) spectra under varied In contents in the CZTISSe films. Fig. 6(b) depicts the bandgap Eg of the CZTISSe thin film obtained from the data near the band edge by plotting [E�ln(1 EQE)]2 versus E [32], and the results are listed in Table 3. The PL peak position has a small fluctuation, which decreases initially and then increases with the amount of In dopant. With increasing In dopant from 0 to 6%, the PL peak decreases from 1.029 to 1.008 eV. Then, with the increase of In dopant from 6 to 12%, the Eg increases slightly to 1.023 eV. Because Voc,def can be expressed as Voc,def ¼ Eg/q- Voc. As listed in Table 3, the Voc,def of the CZTSSe solar cells decreases from 818 to 682 mV after partially substituting Sn4þ with In3þ, indicating that Sn-related defects have been suppressed. The trend of Eg values and room-temperature PL peak values with the variation of x (x ¼ In/(In þ Sn)) are listed in Table 3. The Eg values of CZTISSe obtained from EQE have a clear trend of decreasing initially and then increasing, i.e., from 1.149 eV (sample C0) down to 1.075 eV
Table 2 Performance parameters of CZTISSe devices with different x. Sample
x (%)
PCE (%)
Voc (mV)
Jsc (mA/ cm2)
FF (%)
Integ-Jsc (mA/ cm2)
C0 C3 C6 C9 C12
0 3 6 9 12
4.41 5.07 6.36 7.19 3.95
331 353 367 393 351
28.89 28.01 30.05 32.12 26.79
46.09 51.20 57.60 56.96 41.88
28.42 29.23 31.63 32.50 25.52
can reduce the Eg and defects in the CZTISSe bulk, thereby promoting the generation and separation of photo-generated carriers. Secondly, the recombination at the CZTSSe/CdS interface is suppressed by doping In. Finally, the EQE curve near the band edge decreases sharply, which indicates that the band tailing is reduced. It is worth mentioning that in the wavelength range of 500–1000 nm, device C9 shows a high EQE value, exceeding 80%. By verifying the high short-circuit current density obtained by J-V test, we have fitted the EQE curve by integral. As shown in Fig. 5(b), the integral current density (Integ- Jsc) value is 32.50 mA/ cm2, which is consistent with the Jsc value (32.12 mA/cm2). It
Fig. 5. (a) Relative EQE spectra of the CZTISSe solar cells, (b) EQE spectra and integral current density of device C9. 4
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Fig. 6. (a) Normalized PL spectra of CZTISSe films varied with In contents, (b) Bandgap Eg of the devices calculated from EQE data.
parameters and Voc. Compared with undoped sample C0, sample C9 has smaller Gsh, Rs, J0 and A. The Gsh decreases from 4.41 to 3.51 mS/cm2 due to the reduction of carrier recombination and the improvement of crystallization quality. The Rs reduces from 1.71 to 0.64 Ω cm2 after doping In. Meanwhile parameter A is improved from 2.03 to 1.58, indicating that In dopant can effectively alleviate the serious recombi nation occurred at the interface [34,35]. The reduction of J0 demon strates that defects and carrier recombination are suppressed, which will increase the Voc and FF. The depletion region width (Wd) and charge carrier density (Nc-v) of the samples were measured by a C–V method as shown in Fig. 7. The Nc-v and Wd are based on the following equations:
Table 3 Detailed performance parameters of CZTISSe solar cells with varied In dopant levels. Sample
Eg(eV)
Eg/q-Voc (mV)
PL (eV)
Δ(Eg-PL) (eV)
Wd(μm)
Nc-v(cm
C0
1.149
818
1.029
0.12
0.291
C3
1.093
740
1.011
0.082
0.278
C6
1.077
710
1.008
0.069
0.275
C9
1.075
682
1.016
0.059
0.247
C12
1.116
765
1.023
0.093
0.219
7.53 � 1015 1.08 � 1016 1.18 � 1016 1.39 � 1016 1.54 � 1016
3
)
Nc
(sample C9) and then upward to 1.116 eV (sample C12). When the In content x raised from 0% to 9%, the energy difference Δ(Eg -PL) be tween the Eg and the PL peak position is diminished gradually from 120 to 59 meV, which indicates that the band tailing can be effectively reduced when x is 9%, and then the Δ(Eg -PL) is increased abruptly to 93 meV with increasing x from 9% to 12%. This indicates that doping In indeed reduces the density of defects, such as VSn, CuSn and SnCu, SnZn deep-level defects, and suppress the band tailing. However, excessive In dopant can aggravate the band tailing. According to our previous work [16], the shunt conductance (Gsh), shunt resistance (Rsh), series resistance (Rs), diode ideality factor (A), and reverse saturation current density (J0) can be obtained from the standard light J-V curves in Fig. 4(b). The Gsh was obtained from the y-axis intercept at dJ/dV vs. V curves. The Rs and A were obtained from the y-axis intercept and slope (AkT/q) at dV/dJ vs. (J þ Jsc) 1 curves. The J0 was obtained from the y-axis intercept at J þ Jsc–GV vs. V–RsJ curves. Table 4 gives the detailed data from samples C0 and C9. The Voc is based on the following equation [33]: Gsh Voc ¼ Jsc
J0 ðexp
qVoc AkT
¼
Wd ¼
C3 dC ð Þ qS2 ε0 εr dV
Sε0 εr C
1
(2) (3)
where C, q and S stand for the measured capacitance, electron charge and active device area, respectively. The ε0 and εr are vacuum dielectric constant (~8.854 � 10 12 F/m) and relative dielectric constant of CZTSSe (8 � 106). The detailed values of Wd and Nc-v for the CZTISSe solar cells with different x are listed in Table 3. When x is increased from 0 to 12%, the Nc-v is enhanced from 7.53 � 1015 to 1.54 � 1016 cm 3, but the Wd is reduced from 0.291 to 0.219 μm. This indicates that the incorporation of In dopant will greatly increase the Nc-v of the absorber and decrease the Wd of the CZTISSe solar cell. It is well known that a narrow depletion region width can influence the charge separation and collection, especially for the long wavelength region close to the band edge, and finally affect the PCE of solar cells. According to the result, we can find that device C9 has the best performance, and the Wd of device C9 is not wider than that of device C0. Partially replacing Sn4þ with In3þ in CZTSSe solar cell can significantly increase the Nc-v value under the optimal In content (x ¼ 9%). As a result, the Voc and Jsc are successfully enhanced from 331 to 393 mV and 28.89 to 32.12 mA/cm2, respectively. The above results prove that both of the Nc-v and Wd are very sensitive to the relative content of In3þ, and the optimal incorporation of In (x ¼ 9%) can significantly raise the efficiency of the CZTSSe solar cell.
(1)
1Þ
v
The Voc can be can be enhanced by reducing Gsh and J0. In addition, Jsc and FF follow a similar trend as the relationship between these
4. Conclusions Table 4 Performance parameters of CZTISSe solar cells for samples C0 and C9. Sample
x (%)
Gsh（mS/ cm2）
Rsh (Ω. cm2)
Rs (Ω. cm2)
Jo (mA/ cm2)
A
C0
0
4.41
226
1.71
2.03
C9
9
3.51
284
0.64
7.17 � 10 2 2.23 � 10 2
An elemental precursor solution method has been successfully adopted to prepare CZTISSe thin films on Mo foils by partially substituting Sn4þ with In3þ. Doping In can improve the crystallinity and semiconducting properties of the films. Partially replacing Sn with In can produce shallow level defects of InSn and reduce Sn-related defects such as SnCu and SnZn, thus improving the Voc and carrier transport characteristics (inhibiting the carrier recombination and band tailing in
1.58
5
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Solar Energy Materials and Solar Cells 209 (2020) 110434
Fig. 7. (a) Capacitance-voltage (C–V) curves of flexible CZTISSe solar cells with variation of x. (b) Space-charge density and depletion width derived from C–V curves.
CZTSSe). Meanwhile the substitution of Sn4þ with In3þ can inhibit the decomposition of CZTSSe in heat treatment caused by the multivalent Sn (þII and þIV oxidation states)，thus reducing the Voc,def by increasing the crystallizability and inhibiting the Sn-related deep level defects. Finally, a PCE up to 7.19% has been achieved for the flexible CZTISSe solar cell with the In-doped content of x ¼ 9%, which is 2.78% higher than that of the undoped solar cell. The Voc is increased by 62 mV due to lower Voc,def and defects. The In doping technique and analysis method are helpful for improving the performances of flexible CZTSSe solar cells and promoting the development of them.
[5] D.B. Mitzi, O. Gunawan, T.K. Todorov, K. Wang, S. Guha, The path towards a highperformance solution-processed kesterite solar cell, Sol. Energy Mater. Sol. Cell. 95 (2011) 1421–1436. [6] J.J. Li, D.X. Wang, X.L. Li, Y. Zeng, Y. Zhang, Cation substitution in earth-abundant kesterite photovoltaic materials, Avd. Sci. 5 (2018), 1700744. [7] B. Vermang, G. Brammertz, M. Meuris, T. Schnabel, E. Ahlswede, L. Choubrac, S. Harel, C. Cardinaud, L. Arzel, N. Barreau, J. van Deelen, P.-J. Bolt, P. Bras, Y. Ren, E. Jaremalm, S. Khelifi, S. Yang, J. Lauwaert, M. Batuk, J. Hadermann, X. Kozina, E. Handick, C. Hartmann, D. Gerlach, A. Matsuda, S. Ueda, T. Chikyow, R. F� elix, Y. Zhang, R.G. Wilks, M. B€ ar, Wide band gap kesterite absorbers for thin film solar cells: potential and challenges for their deployment in tandem devices, Sustain. Energy. Fuels. 3 (2019) 2246–2259. [8] W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency, Adv. Energy. Mater. 4 (2014), 1301465. [9] K.J. Yang, S. Kim, S.Y. Kim, K. Ahn, D.H. Son, S.H. Kim, S.J. Lee, Y.I. Kim, S. N. Park, S.J. Sung, D.H. Kim, T. Enkhbat, J. Kim, C.W. Jeon, J.K. Kang, Flexible Cu2ZnSn(S,Se)4 solar cells with over 10% efficiency and methods of enlarging the cell area, Nat. Commun. 10 (2019) 2959. [10] K.-J. Yang, S. Kim, J.-H. Sim, D.-H. Son, D.-H. Kim, J. Kim, W. Jo, H. Yoo, J. Kim, J.-K. Kang, The alterations of carrier separation in kesterite solar cells, Nanomater. Energy 52 (2018) 38–53. [11] S.Y. Chen, A. Walsh, X.G. Gong, S.H. Wei, Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth-abundant solar cell absorbers, Adv. Mater. 25 (2013) 1522–1539. [12] Q. Tian, H. Lu, Y. Du, J. Fu, X. Zhao, S. Wu, S.F. Liu, Green Atmospheric Aqueous Solution Deposition for High Performance Cu2ZnSn(S,Se)4 Thin Film Solar Cells, Solar RRL, 2018, 1800233. [13] T. Gershon, Y.S. Lee, P. Antunez, R. Mankad, S. Singh, D. Bishop, O. Gunawan, M. Hopstaken, R. Haight, Photovoltaic materials and devices based on the alloyed kesterite absorber (AgxCu1-x)2ZnSnSe4, Adv. Energy. Mater. 6 (2016) 1502468. [14] D. Shin, B. Saparov, D.B. Mitzi, Defect engineering in multinary earth-abundant chalcogenide photovoltaic materials, Adv. Energy. Mater. 7 (2017), 1602366. [15] Z.K. Yuan, S.Y. Chen, H.J. Xiang, X.G. Gong, A. Walsh, J.S. Park, I. Repins, S. H. Wei, Engineering solar cell absorbers by exploring the band Alignment and defect disparity: the case of Cu- and Ag-based kesterite compounds, Adv. Funct. Mater. 25 (2015) 6733–6743. [16] X. Yu, S. Cheng, Q. Yan, J. Yu, W. Qiu, Z. Zhou, Q. Zheng, S. Wu, Efficient (Cu1 xAgx)2ZnSn(S,Se)4 solar cells on flexible Mo foils, RSC Adv. 8 (2018) 27686–27694. [17] J.J. Scragg, T. Ericson, T. Kubart, M. Edoff, C. Platzer-Bj€ orkman, Chemical insights into the instability of Cu2ZnSnS4 films during annealing, Chem. Mater. 23 (2011) 4625–4633. [18] S. Kim, K.M. Kim, H. Tampo, H. Shibata, S. Niki, Improvement of voltage deficit of Ge-incorporated kesterite solar cell with 12.3% conversion efficiency, APEX 9 (2016), 102301. [19] S. Giraldo, Z. Jehl, M. Placidi, V. Izquierdo-Roca, A. Perez-Rodriguez, E. Saucedo, Progress and perspectives of thin film kesterite photovoltaic technology: a critical review, Adv. Mater. 31 (2019), 1806692. [20] J. Kim, H. Hiroi, T.K. Todorov, O. Gunawan, M. Kuwahara, T. Gokmen, D. Nair, M. Hopstaken, B. Shin, Y.S. Lee, High efficiency Cu2ZnSn (S, Se) 4 solar cells by applying a double In2S3/CdS emitter, Adv. Mater. 26 (2014) 7427–7431. [21] Y.L. Pei, J. Guo, D.X. Kou, W.H. Zhou, Z.J. Zhou, Q.W. Tian, Y.N. Meng, S.X. Wu, Precise-tuning the in content to achieve high fill factor in hybrid buffer structured Cu2ZnSn(S, Se)4 solar cells, Sol. Energy 148 (2017) 157–163. [22] D. Sharath, A.P. Gaikwad, S. Choudhury, N. Gupta, R. Sasikala, C.A. Betty, Effect of Indium doping on the photoelectrochemical and photocatalytic properties of zinc sulphide, Mater. Sci. Eng. B. Adv. Funct. Solid. State. Mater. 226 (2017) 57–63. [23] R. Chetty, M. Falmbigl, P. Rogl, P. Heinrich, E. Royanian, E. Bauer, S. Suwas, R. C. Mallik, The effect of multisubstitution on the thermoelectric properties of
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Xue Yu: Writing - original draft. Shuying Cheng: Supervision. Qiong Yan: Writing - review & editing. Junjie Fu: Investigation. Hongjie Jia: Investigation. Quanzhen Sun: Data curation. Zhiyuan Yang: Data curation. Sixin Wu: Supervision. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 61574038). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2020.110434. References [1] D. Tiwari, T. Koehler, X.Z. Lin, R. Harniman, I. Griffiths, L. Wang, D. Cherns, R. Klenk, D.J. Fermin, Cu2ZnSnS4 thin films generated from a single solution based precursor: the effect of Na and Sb doping, Chem. Mater. 28 (2016) 4991–4997. [2] Y.C. Yang, G. Wang, W.G. Zhao, Q.W. Tian, L.J. Huang, D.C. Pan, SolutionProcessed highly efficient Cu2ZnSnSe4 thin film solar cells by dissolution of elemental Cu, Zn, Sn, and Se powders, ACS Appl. Mater. Interfaces 7 (2015) 460–464. [3] D.B. Mitzi, O. Gunawan, T.K. Todorov, D.A.R. Barkhouse, Prospects and Performance Limitations for Cu-Zn-Sn-S-Se Photovoltaic Technology, vol.371, Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences, 2013, p. 20110432. [4] F. Jiang, S. Ikeda, T. Harada, M. Matsumura, Pure sulfide Cu2ZnSnS4 thin film solar cells fabricated by preheating an electrodeposited metallic stack, Adv. Energy. Mater. 4 (2014), 1301381.
6
X. Yu et al.
[24] [25] [26] [27] [28] [29]
[30]
Solar Energy Materials and Solar Cells 209 (2020) 110434
chalcogenide-based Cu2.1Zn0.9Sn1-xInxSe4 (0 <¼ x <¼ 0.1), Phys. Status Solidi. App. Mater. Sci. 210 (2013) 2471–2478. D.H. Kuo, M. Tsega, Electrical conduction and mobility enhancement in p-type Indoped Cu2ZnSnSe4 bulks, Jpn. J. Appl. Phys. 53 (2014), 035801. X.Y. Shi, F.Q. Huang, M.L. Liu, L.D. Chen, Thermoelectric properties of tetrahedrally bonded wide-gap stannite compounds Cu2ZnSn1-xInxSe4, Appl. Phys. Lett. 94 (2009), 122103. E. Toda, A. Hioki, Determination of impurities in high-purity selenium by inductively coupled plasma mass spectrometry after matrix separation with thiourea, Anal. Sci. 11 (1995) 115–118. Q. Yan, S. Cheng, H. Li, X. Yu, J. Fu, Q. Tian, H. Jia, S. Wu, High flexible Cu2ZnSn (S,Se)4 solar cells by green solution-process, Sol. Energy 177 (2019) 508–516. Y. Zhang, Q. Ye, J. Liu, H. Chen, X. He, C. Liao, J. Han, H. Wang, J. Mei, W. Lau, Earth-abundant and low-cost CZTS solar cell on flexible molybdenum foil, RSC Adv. 4 (2014) 23666–23669. Z. Xiao, H. Luan, R. Liu, B. Yao, Y. Li, Z. Ding, G. Yang, R. Deng, G. Wang, Z. Zhang, L. Zhang, H. Zhao, Behavior of indium alloying with Cu2ZnSn(S,Se)4 and its effect on performances of Cu2ZnSn(S,Se)4-based solar cell, J. Alloys Compd. 767 (2018) 439–447. M. Dimitrievska, H. Xie, A.J. Jackson, X. Fontan�e, M. Espíndola-Rodríguez, E. Saucedo, A. P�erez-Rodríguez, A. Walsh, V. Izquierdo-Roca, Resonant Raman
[31] [32] [33] [34]
[35]
7
scattering of ZnSxSe1 x solid solutions: the role of S and Se electronic states, Phys. Chem. Chem. Phys. 18 (2016) 7632–7640. J. Li, S. Kim, D. Nam, X. Liu, J. Kim, H. Cheong, W. Liu, H. Li, Y. Sun, Y. Zhang, Tailoring the defects and carrier density for beyond 10% efficient CZTSe thin film solar cells, Sol. Energy Mater. Sol. Cell. 159 (2017) 447–455. A. Niemegeers, M. Burgelman, R. Herberholz, U. Rau, D. Hariskos, H.W. Schock, Model for electronic transport in Cu(In,Ga)Se2 solar cells, Prog. Photovoltaics Res. Appl. 6 (1998) 407–421. G. Yang, Y.-F. Li, B. Yao, Z.-H. Ding, R. Deng, H.-F. Zhao, L.-G. Zhang, Z.-Z. Zhang, Improvement of the photovoltaic performance of Cu2ZnSn(SxSe1 x)4 solar cells by adding polymer in the precursor solution, J. Phys. Appl. Phys. 51 (2018), 105103. G. Brammertz, M. Buffiere, S. Oueslati, H. ElAnzeery, K. Ben Messaoud, S. Sahayaraj, C. Koble, M. Meuris, J. Poortmans, Characterization of defects in 9.7% efficient Cu2ZnSnSe4-CdS-ZnO solar cells, Appl. Phys. Lett. 103 (2013), 163904. H.Y. Park, D.G. Moon, J.H. Yun, S.K. Ahn, K.H. Yoon, S. Ahn, Efficiency limiting factors in Cu(In,Ga)Se2 thin film solar cells prepared by Se-free rapid thermal annealing of sputter-deposited Cu-In-Ga-Se precursors, Appl. Phys. Lett. 103 (2013), 263903.