n-a-Si thin film solar cells

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Fabrication and photovoltaic properties of Cu2 ZnSnS4 /i-a-Si/n-a-Si thin film solar cells Feng Jiang a , Honglie Shen a,b,∗ a

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, PR China Key Laboratory for Intelligent Nano Materials and Devices of the Ministry of Education, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, PR China b

a r t i c l e

i n f o

Article history: Received 9 October 2012 Received in revised form 1 April 2013 Accepted 23 April 2013 Available online xxx Keywords: CZTS Sulfurization Heterojunction a-Si p–n p–i–n

a b s t r a c t Cu2 ZnSnS4 (CZTS) films were successfully prepared by sulfurization of Zn/Sn/Cu multilayers at different temperatures from 350–575 ◦ C. The film sulfurized at 500 ◦ C presents no any secondary phases and Raman peaks at 251, 288, 336 and 368 cm−1 are observed with the main Raman peak locating at 336 cm−1 . The surface of the Cu2 ZnSnS4 film is compact and the thickness of film is about 1 ␮m. The mapping and point characterization of energy dispersive spectrometer show that the composition element ratios are close to the stoichiometry of Cu2 ZnSnS4 . The absorption coefficient of Cu2 ZnSnS4 film is larger than 104 cm−1 in the visible light region of 400-800 nm and the direct band gap of the Cu2 ZnSnS4 film is estimated to be about 1.5 eV. The effect of intrinsic amorphous silicon layer is obvious and Cu2 ZnSnS4 /i-a-Si/na-Si solar cell shows much higher conversion efficiency and more obvious diode rectifying effect than Cu2 ZnSnS4 /n-a-Si solar cell. The reverse saturation current density of Cu2 ZnSnS4 /i-a-Si/n-a-Si hetero junction has a value around 1.42 × 10−3 mA/cm2 and the ideal diode factor of this junction is estimated to be about 2.85. The open circuit voltage, short circuit current density and fill factor of this Cu2 ZnSnS4 /ia-Si/n-a-Si solar cell are 562 mV, 12.3 mA/cm2 and 43.8%, respectively, and the conversion efficiency is calculated to be about 3.03%. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The technologies of Cu(In, Ga)Se2 (CIGS) thin film solar cell have demonstrated significant improvements over recent years and are recently experiencing substantial production development. CIGS solar cells have exhibited a record efficiency of 20.3% at the laboratory level [1]. Despite or even as a direct consequence of its recent and near-future success, the scarcity and high prices of In, Ga and the toxicity In, Cd and Se eventually could limit the production growth of this type of solar cells. Therefore, there is a strong desire to synthesize novel, high-efficiency, low cost solar cell absorber materials to replace CIGS. An alternative compound Cu2 ZnSnS4 (CZTS) which contains more abundant elements appears to have suitable solar cell absorber materials and several researchers have given a lot of successful works on this CZTS till now [2–15]. It is know that CZTS has a high absorption coefficient above 104 cm−1 and a direct band gap about 1.5 eV [4–15]. The kesterite type structure can be obtained

∗ Corresponding author at: College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, PR China. Tel.: +86 25 52112626; fax: +86 25 52112626. E-mail address: [email protected] (H. Shen).

from that of CuInS2 by replacing In with Zn and Sn. According to Shockley–Queisser photon balance calculations, CZTS is expected to have a theoretical efficiency of more than 30% [16]. So far, the highest experimental efficiency is 8.4% [17] and is quite low compared with the theoretical limit. Several techniques such as spray pyrolysis [18], electrodeposition [19,20], pulsed laser deposition [21], thermal evaporation [22], sol–gel sulfurization [23], and sputtering deposition [24–30], have been used for the preparation of the CZTS layer. Considering that sputtering deposition is scaled up to large-area deposition and the best conversion efficiency is obtained by the conversion of sputtered metallic films into sulfide films via sulfurization, the method of sputtering precursors and subsequent sulfurization is very promising for the fabrication of high-efficiency CZTS solar cells. In this work, Zn/Sn/Cu precursors were fabricated by sputtering Zn, Sn, and Cu targets on soda lime glass substrates. For the fabrication of CZTS thin films and research on the effect of the temperature on the properties of CZTS film, precursors were sulfurized in an atmosphere of N2 and sulfur vapor with different temperature. The purity of Zn, Sn, Cu targets and N2 , sulfur vapor are all 99.999%. Based on the fundamental work of CZTS preparation mentioned above, we fabricated a novel completely non toxic CZTS/n-a-Si

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Please cite this article in press as: F. Jiang, H. Shen, Fabrication and photovoltaic properties of Cu2 ZnSnS4 /i-a-Si/n-a-Si thin film solar cells, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.04.110

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solar cell [25]. In that work, we only fabricated p–n structure solar cell(CZTS/a-Si) and we did not research the interface and buffer layer further. In this work, we used the nontoxic intrinsic amorphous silicon/n-type amorphous silicon multi layer (i-a-Si/na-Si) and n-type amorphous silicon single layer(n-a-Si) to replace the toxic CdS layer and formed glass/Mo/CZTS/i-a-Si/n-a-Si/ITO (p–i–n) and glass/Mo/CZTS/n-a-Si/ITO (p–n) novel structure hetero junction thin film solar cells, respectively. The property and PV performance of these two structure hetero junction thin film solar cells were firstly discussed and compared, and the effect of the intrinsic amorphous silicon thin layer (i-a-Si) on the property and PV performance of prepared CZTS based solar cells was firstly investigated further in this work. 2. Experimental

Fig. 1. XRD patterns of the CZTS films with different sulfurization temperature.

2.1. Preparation 2.1.1. Preparation of CZTS film The CZTS absorber used in this study was prepared by the sulfurization of Zn/Sn/Cu precursors which were deposited on the glass substrate by sputtering, and the sputtering system contains three targets. The first layer with a thickness of 120 nm on the glass substrate is Zn, the second layer with a thickness of 140 nm is Sn and the top layer with a thickness of 100 nm is Cu. The conditions of precursor fabrication are as follows: Ar gas flow rate of 20 sccm, sputtering gas pressure of 0.1 Pa, substrate rotation of 10 rpm, room temperature without substrate heating, and sputtering powers of Zn, Sn and Cu targets of 20, 30 and 50 W, respectively. The finished metallic precursor was sulfurized at 350, 425, 500 and 575 ◦ C for 2 h in an atmosphere of N and sulfur vapor with atmospheric 2 pressure in an electric furnace. The sulfurization temperature was increased with a ramp rate of 5 ◦ C/min. After sulfurization, the sample was cooled to room temperature naturally in the furnace. 2.1.2. Preparation of CZTS/n-a-Si and CZTS/i-a-Si/n-a-Si thin film solar cell The preparation process of CZTS/i-a-Si/n-a-Si solar cells is separated into three steps: Firstly, 1-␮m-thick CZTS film was fabricated on Mo-coated glass. Secondly, a 10-nm-thick intrinsic amorphous silicon thin film (i-a-Si) and 20-nm-thick n-type amorphous silicon (n-a-Si) thin film were deposited on the prepared p-type CZTS film by hot-wire CVD immediately in order to form a CZTS/i-a-Si/n-aSi (p–i–n) heterojunction and to protect the absorber surface from ion damage during the ITO sputtering process. The n-type amorphous silicon layer was doped by phosphorus, and the preparation details are already published by a lot of researchers [31]. Finally, an Ag metal grid is deposited through an aperture mask on the single cells as a front contact by sputtering. Some details of the p–i–n heterojunction are shown in Fig. 6. The preparation process of CZTS/n-a-Si (p–n) heterojunction is in the same of above, but without deposition of intrinsic amorphous silicon thin film (i-a-Si) [25]. 2.2. Characterization X-ray diffraction (XRD) analysis was carried out using a Bruker D8 Advance diffractometer with Cu K␣ radiation ( = 0.15406 nm) over the 2 collection range of 10–70◦ . The Raman spectrum of the CZTS film was recorded using Raman T64000. The morphology and composition element of the CZTS film were characterized using a Hitachi S4800 scanning electron microscope and the energy dispersive spectrometer (EDS) analysis implemented on the scanning electron microscope. The thickness of the film was measured using an AMBIOS XP-2 Surface Profiler. Transmittance and reflectance

spectra were recorded in the wavelength range of 200-2500 nm using a Varian Cary5000 spectrophotometer. The I–V characteristic of the CZTS/n-a-Si and CZTS/i-a-Si/n-a-Si heterojunctions were recorded by the Keithley 2400 source meter under AM1.5G spectrum (Oriel Solar simulator, 91192-1000W) 3. Results and discussion 3.1. Structural properties In order to prepare pure CZTS film with good optical and electrical property which is suitable for the application in solar cell, we sulfurized the Zn/Sn/Cu multilayer with different temperature. Fig. 1 shows the XRD patterns of the CZTS film with different sulfurization temperature. First of all, the obvious main peaks observed in all of the samples which locate at 2 = 18.3, 28.54, 29.76, 33.01, 47.39, 56.21◦ are attributed to the Kesterite CZTS phase (PDF# 260575). The three strong peaks of all the samples are correspond to the (1 1 2), (2 2 0) and (3 1 2) feature phase of CZTS, which is accord with the previously published XRD results very well [19–24]. However, there are some differences each other. The CZTS film sulfurized at 350 ◦ C contains Cu2 S and the intensity of CZTS diffraction peaks are low and the FWHM of (1 1 2) peaks is about 0.4 degree. When the sulfurization temperature increases to 425 ◦ C, the intensity of CZTS diffraction peaks is obviously higher than that sulfurized at 350 ◦ C and the FWHM of (1 1 2) peak decreased to 0.2◦ , but the secondary phase of Cu2 S is still observed. The intensity of diffraction peak and the peak-to background ratio of the CZTS film sulfured at 500 ◦ C are the highest, and the FWHM of (1 1 2) peak of 0.15◦ is the lowest. Furthermore, there are no any other phases detected in this film. But the phases of Cu2 S and SnS2 were observed when the sulfurization temperature increased to 575 ◦ C. The intensity of CZTS diffraction peaks is lower but the FWHM of (1 1 2) peak of 1.67◦ is bigger than that sulfurized at 500 ◦ C. So the best sulfurization temperature is 500 ◦ C, and the CZTS film sulfurized at 500 ◦ C is pure which is suitable for the application in CZTS based solar cell. Based on the XRD results mentioned above, we characterized the CZTS film sufurized at 500 ◦ C further in this manuscript. Fig. 2 shows the Raman peaks of all the CZTS samples, the main peaks locate at 251, 288, 336 and 368 cm−1 are attributed to the standard Kesterite CZTS phase and are similar to other previously results [32]. However, we found that the binary Cu2 S, SnS and ternary Cu2 SnS3 secondary phases are existed in the CZTS films which were sulfurized at 350, 425 and 575 ◦ C. But not any peaks from Cu2 SnS3 , ZnS, and Cux S phases were observed in the CZTS film sulfurized at 500 ◦ C. The FWHM of 350 ◦ C and 425 ◦ C sufurized CZTS film is 28 cm−1 and 25 cm−1 , but the value of FWHM of the 336 cm−1 from 500 ◦ C and 575 ◦ C sulfurized CZTS films is

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Fig. 3. Surface and cross section morphologies of CZTS film: (a) surface morphology; (b) cross section morphology.

3.2. Morphology and composition element

Fig. 2. Raman spectrum of CZTS films with different sulfurization temperature.

16 cm−1 and 17 cm−1 . It is represented that the film sulfurized at higher temperature has higher degree crystallinity. The intensity of Raman peaks and the FWHM of the main Raman peak from the 500 ◦ C sulfurized CZTS is the highest, which indicated that it may be suitable for the application in solar cell and worth to be studied further.

Fig. 3 shows the FE-SEM images of the CZTS film sulfurized at 500 ◦ C, where (a) is the surface morphology and (b) is the cross section morphology. As it can be seen from Fig. 3(a), the surface of CZTS film is compact and uniform, and the average grain size of CZTS is about 1 ␮m. Fig. 3(b) indicates that the cross section of CZTS film is compact and the thickness is about 1 ␮m which is close to the test results of step profiler. The corresponding AFM images of CZTS film are shown in Fig. 4. From Fig. 4(a), we know that the root mean square (Rms) roughness of the CZTS film is 153 nm. The surface height fluctuations are detected by line scan by AFM system, which is shown in Fig. 4(b). The average surface height fluctuation of CZTS film is about 100 nm. In order to analyze the composition element ratio of the CZTS film mentioned above, we tested the film’s element by EDS. Fig. 3 also shows the EDS test of surface and cross section, where (a) is the

Fig. 4. The AFM images of CZTS film: (a) Surface morphology of CZTS, (b) line scan data of CZTS film.

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Table 1 The EDS test’s results of surface and cross section of CZTS film. Type of EDS test

Cu ratio (%)

Zn ratio (%)

Sn ratio (%)

S ratio (%)

Mapping characterization Point characterization A Point characterization B Point characterization C Point characterization D Point characterization E Point characterization F

24.12 26.23 25.26 25.46 24.21 25.32 27.56

13.01 12.78 13.63 13.53 12.42 12.81 12.07

11.95 13.05 12.21 12.40 13.89 11.48 11.34

50.92 47.94 48.90 48.61 49.48 50.39 49.03

EDS test of surface and (b) is the EDS test of cross section. In this study, we used the EDS Mapping characterization and EDS point characterization. The red pane in Fig. 3(a) is the region of EDS Mapping characterization, while points A, B and C shown in Fig. 3(a) and points D, E and F shown in Fig. 3(b) are for the region of EDS point characterization. The results are shown in Table 1. The surface and point characterization of EDS in surface and cross section show that the composition element ratios are close to the stoichiometric value of Cu2 ZnSnS4 . The Zn content is relatively higher than Sn. We supposed that maybe some Sn was slightly lost in the sulfurization process because of the high temperature. 3.3. Optical properties As it is can be seen from Fig. 5(a), the reflectance of the CZTS film is below 15% in the wavelength region of 200-2500 nm. Also the transmittance of the CZTS film is almost 0% in the visible light region of 400-700 nm, when the wavelength is longer than about 700 nm, the transmittance increases rapidly. The thickness of the CZTS film here is about 1000 nm, and the transmittance and reflectance of CZTS in this work are similar to the previous results [25]. Between the transmittance (T), total reflectance (R) and absorption coefficient (˛) of the CZTS film, there is the following relationship [33]: T = (1 − R)e−˛d ,

(1)

here d is the thickness of the film. If d, T, and R are given, the absorption coefficient ˛ can be calculated from the Eq. (1). From the calculation based on Eq. (1), the absorption coefficient of CZTS film can be calculated. Fig. 5(b) shows the absorption coefficient of the prepared CZTS film in the wavelength region of 200-2500 nm. The absorption coefficient of CZTS film is higher than 2.7 × 104 cm−1 in the visible light region of 400-800 nm. When the wavelength is longer than about 900 nm, the absorption coefficient decreases to about 2.7 × 104 cm−1 . In order to determine the fundamental

Fig. 6. The sketch of CZTS/i-a-Si/n-a-Si thin film solar cell.

absorption edge, we need to calculate the optical band gap Eg by the following equation [33]: 2

(˛h) = A(h − Eg ),

(2)

here A is a constant and this means that a plot of (˛h)2 versus h should be a straight line with an intercept on the h axis equal to Eg . The insert in Fig. 5(b) shows the optical band gap we calculated using Eq. (2). The band gap was approximated by plotting (˛h)2 versus the energy in eV and extrapolating the linear part of the spectrum, (˛h)2 = f(h), to zero. Thus the direct band gap of the CZTS film was estimated to be about 1.5 eV, which is the optimal band gap for the absorber of solar cells. 3.4. CZTS solar cells Based on the fundamental work on CZTS described above, we introduced the nontoxic ultrathin amorphous silicon (a-Si) film to replace the toxic CdS in order to fabricate a novel nontoxic CZTS/aSi solar cell. In this study, we prepared CZTS/a-Si thin film solar cells with p–n and p–i–n structure: CZTS/n-a-Si and CZTS/i-a-Si/na-Si. We compared the properties of these two different structure CZTS based solar cells, and found some phenomenon which is useful for the preparation of CZTS based solar cells. In order to avoid the repeated description, we defined CZTS/n-a-Si and CZTS/i-a-Si/n-aSi solar cells as Cell-A and Cell-B, respectively. Fig. 7 shows the cross section image of Cell-B. The 1-␮mthickness CZTS film is the main absorber layer, while the ultrathin n-type a-Si (n-a-Si) acts as the emitter layer role. The i-a-Si buffer layer may passivate the surface of CZTS and decrease the surface defect, which reduce the recombination of photo excited carriers.

Fig. 5. Optical properties of CZTS thin film: (a) transmittance and reflectance of prepared CZTS film; (b) absorption coefficient and energy gap of prepared CZTS film.

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Table 2 Photovoltaic performance data of Cell-A and Cell-B. Structure

Voc (mV)

Jsc (mA/cm2 )

FF (%)

 (%)

n

I0 (mA/cm2 )

Cell-A Cell-B

503 562

8.15 12.3

31.1 43.8

1.27 3.03

3.86 2.85

3.37 × 10−2 1.42 × 10−3

Fig. 7. The cross section SEM image of Cell-B.

And the space charge width is increased by inserting of i-a-Si layer. Also, the lattice mismatch of the interface between CZTS and ITO is banished by inserting an ultrathin a-Si buffer layer. The optical band gap energy of a-Si we prepared in this work is about 1.8 eV and the Raman peak of this a-Si film is locating at 487 cm−1 . From Fig. 7, we found that the Mo layer divided into tow layers. It is supposed that the surface of Mo layer may be sulfured into MoS2 . In order to characterize the electrical property of Cell-A and CellB, we tested I vs. V curves of these junctions. Fig. 8 shows I vs. V behavior for Cell-A and Cell-B in dark. It can be seen that typical rectification behaviors are presented under dark conditions of these two different CZTS based solar cell. Since the electronic transport in heterojunction device may slowly vary as a function of temperature and applied voltage as it is predicted by the Anderson model, [18] it is possible to suppose that the relationship between I and V could be used to know the junction electrical parameters. That relationship can be expressed as:



I = I0 exp

 qV  nkT



−1 ,

(3)

In Eq. (3), I0 is the reverse saturation current, V the applied voltage, n the diode quality factor, k de Boltzmann constant and T the absolute temperature. According Eq. (3), diode quality factor n can be expressed as: n=

q kT

 dV  d (ln I)

(4)

Fig. 9. I–V characteristic of the Cell-A and Cell-B under AM 1.5G.

The values of I0 and n can be obtained by extrapolation of ln I vs. V curve to V = 0. A typical plot of ln I vs. V for the fabricated diodes is shown in Fig. 8(b). From this graph, the reverse saturation current of Cell-A and Cell-B were estimated to be around 3.37 × 10−2 mA/cm2 and 1.42 × 10−3 mA/cm2 . Further more, the estimated diode factors of Cell-A and Cell-B junctions were 3.86 and 2.85 according Eq. (4). The reverse saturation current and diode factor are decrease obviously by inserting intrinsic a-Si film. These data were arranged in Table 2. Fig. 9 shows the I–V curves of Cell-A and Cell-B with the active area of 40 mm2 under AM 1.5G. The open circuit voltage, short circuit current density and fill factor of Cell-A without intrinsic a-Si film are 503 mV, 8.15 mA/cm2 and 31.1%, respectively. The conversion efficiency of this solar cell is estimated to be about 1.27%. But the Cell-B shows obvious more high photovoltaic effect. The open circuit voltage, short circuit current density and fill factor of this solar cell are 562 mV, 12.3 mA/cm2 and 43.8%, respectively. The conversion efficiency of this solar cell is estimated to be about 3.03%. The open circuit voltage, short circuit current density and fill factor of this solar cell are all increase obviously by inserting intrinsic a-Si film (i-a-Si), and the conversion efficiency was increased about 139% by inserting i-a-Si film. It is supposed that the intrinsic

Fig. 8. the I–V analyze of Cell-A and Cell-B: (a) I–V curve under dark; (b) ln (I)–V curve under dark.

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a-Si film may compensate the surface defects, which can improve the interface condition and decrease the composition ratio of carriers. The n-type a-Si film may have more defects because of the doping, so that more carriers will be recombined at the surface between p- type and n-type layer. The inserting of i-a-Si between ptype CZTS and n-type a-Si films may improves the surface condition (surface passivation) and is benefit for the collection of carriers. The data of Cell-A and Cell-B thin film solar cells are shown in Table 2. Compared with CZTS/CdS solar cells that reported by other authors previously, the Cell-A and Cell-B we prepared in this work are more environmental friendly. If the efficiencies of Cell-A and Cell-B are improved, these solar cells will be more suitable for the application and industrialization. It is found that the effect of i-aSi layer is very obvious (the conversion efficiency was increased about 139% by inserting i-a-Si film) by the comparison of the performance of Cell-A and Cell-B. It is believed that if the sulfurization process and the interface treatment are optimized, the conversion efficiency of Cell-A and Cell-B will be enhanced greatly. 4. Conclusions CZTS films were prepared by sulfurization of Zn/Sn/Cu multilayer with different temperature. The film sulfured at 500 ◦ C has no any secondary phases like Cu2 S, Cu2 SnS3 , ZnS and SnS2 . Raman peaks at 251, 288, 336 and 368 cm−1 were observed with the main peak locating at 336 cm−1 . The surface of CZTS film is compact and the average grain size is about 1 ␮m. The surface and point characterization of EDS at surface and cross section of CZTS film show that the composition element ratios are close to the stoichiometry of Cu2 ZnSnS4 . The absorption coefficient of CZTS film is larger than 104 cm−1 in the visible light region of 400-800 nm and the direct band gap of the CZTS film was estimated to be about 1.5 eV. The reverse saturation current of Cell-A has a value of around 3.37 × 10−2 mA/cm2 and the diode factor of this junction was estimated to be about 3.86, but the reverse saturation current and the ideal diode factor were changed to 1.42 × 10−3 mA/cm2 and 2.85 by inserting intrinsic a-Si film. The open circuit voltage, short circuit current density and fill factor of Cell-A are 503 mV, 8.15 mA/cm2 and 31.1%, respectively. The conversion efficiency of this Cell-A is estimated to be about 1.27%. The open circuit voltage, short circuit current density and fill factor of Cell-B are 562 mV, 12.3 mA/cm2 and 43.8%, respectively. The conversion efficiency of Cell-B is calculated to be about 3.03%. The open circuit voltage, short circuit current density and fill factor of this solar cell are all increase obviously by inserting intrinsic a-Si film (i-a-Si), and the conversion efficiency was increased about 139% by inserting i-a-Si film. Acknowledgements This work is supported by the National Nature Science Foundation of China (61176062), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Please cite this article in press as: F. Jiang, H. Shen, Fabrication and photovoltaic properties of Cu2 ZnSnS4 /i-a-Si/n-a-Si thin film solar cells, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.04.110