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Co-sputtered Cu2ZnTi(S:Se)4 absorbers for thin film solar cells
Renewable Energy 145 (2020) 1672e1676
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Co-sputtered Cu2ZnTi(S:Se)4 absorbers for thin film solar cells Derya Batibay a, Yusuf Selim Ocak b, c, *, Mustafa Fatih Genisel b, c, Rasit Turan d, e a
Department of Physics, Institute of Natural &Applied Science, Dicle University, Diyarbakir, Turkey Department of Science, Faculty of Education, Dicle University, Diyarbakir, Turkey c Smart Laboratory, Dicle University, Diyarbakir, Turkey d Department of Physics, Faculty of Science, Middle East Technical University, Ankara, Turkey e Center of Solar Energy Research and Applications (GUNAM), Middle East Technical University, Ankara, Turkey b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2018 Received in revised form 31 May 2019 Accepted 16 July 2019 Available online 17 July 2019
Thin film solar cells are an exciting topic for low cost and high efficient solar cells. Owing to the high price of the indium metal in the fabrication of copper indium gallium diselenide (CIGS) solar cells, Cu2ZnSn(SSe)4 thin films are used as a new material to reduce the cost and increase the efficiency. As an alternative absorber material for solar cell production, Cu2ZnTi(S:Se)4 thin films were deposited by the co-sputtering method at various temperatures. During the deposition, Cu, ZnSe and Ti targets were used as metal sources. The Cu2ZnTi(S:Se)4 thin films were annealed in H2S:Ar (1:9) atmosphere. The morphological, structural and optical properties of Cu2ZnTi(S:Se)4 thin films was analyzed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) Raman spectroscopy and UVeViseNIR spectrometer. It was seen that the thin films had good optical absorption till the infrared region and the band gap of the Cu2ZnTi(S:Se)4 thin films were smaller than the conventional Cu2ZnSnS4 thin films. Furthermore, fabrication of a solar cell with 1.96% power conversion efficiency was reported using a Cu2ZnTi(S:Se)4 thin film as a low cost absorber layer. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Thin film Absorber Solar cell Cu2ZnTi(S:Se)4 Substrate temperature
1. Introduction The usage of energy increases exponentially with the increase of human population and the number of electronic equipment. Owing to the limitation of fossil fuel resources and pollution depend on the usage of fossil fuels, the interest in renewable energy sources increases every day. Among all clean energy sources, solar energy is the most abundant source. Reducing the materials and fabrication cost of solar cells are key parameters to use them on an industrial scale [1]. A thin film based solar cells with suitable band gap and high absorption coefficients are extensively studied [2e4]. Although high power conversion efficiency (PCE) (20%) have been obtained for Cu(In,Ga)(S,Se)2 (CIGSSe) based solar cells, alternative compounds required because of the toxic and expensive content of CIGSSe solar cells. Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) have been studied as an alternative compound for CIGS and CIGSSe solar cells [5,6]. The maximum PCE for CZTSSe solar cells was reported as 12.6% by Wang et al. [7]. They formed CZTSSe thin films by a spin coating
* Corresponding author. Smart Laboratory, Dicle University, Diyarbakir, Turkey. E-mail addresses: [email protected], [email protected] (Y.S. Ocak). https://doi.org/10.1016/j.renene.2019.07.086 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
method using hydrazine as a solvent. Many kinds of methods have been used for the deposition of CZTS or CZTSSe thin films and fabrication of solar cells including sputtering, successive ionic layer adsorption and reaction (SILAR), spin coating, pulsed laser deposition, and thermal evaporation methods [7e11]. All methods have different advantages and disadvantages. Among all methods, the sputtering method is a suitable method for deposition of ceramics and alloys [12]. Many theoretical and experimental studies have been devoted to various derivatives of CZTS and CZTSSe thin films to control their electrical and optical properties and obtain high efficient and lowcost solar cells. For instance, Cu2MSnS4 (M: Zn, Cd, Mn) thin films were deposited by chemical bath deposition (CBD) method along with a subsequent sulfurization heat treatment [13]. It was presented that the optical band gap of Cu2MSnS4 changes with the metal. Wang et al. [14] calculated electronic structure of Cu2ZnTiS4 (CZTiS) and Cu2ZnTi(S,Se)4 (CZTiSSe) compounds. They showed that the absorption coefficients of these films are higher than the conventional CZTS and CZTSSe thin films and their band gaps are lower than the conventional CZTS and CZTSSe thin films. The deposition of CZTiS thin films and CZTiS based solar cell with 0.87% power conversion efficiency have been reported previously [15,16].
D. Batibay et al. / Renewable Energy 145 (2020) 1672e1676
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Apart from these studies, here, the deposition and characterization of CZTiSSe thin films and photovoltaic properties of a CZTSSe based solar cell are presented. In this study, we deposited CZTiSSe thin films by co-sputtering of Cu, ZnSe and Ti sources and them annealed in H2S:Ar (1:9) atmosphere to show the formation of CZTiSSe thin films at various substrate temperature. Then, a 1.96% efficient solar cell was obtained using CZTiSSe as an absorber layer, and it was shown that CZTiSSe thin films could be used in the fabrication of low-cost solar cells.
2. Experimental details Soda-lime glass (SLG) substrates were cleaned by washing with detergent and then ultrasonically vibrating in acetone and 2propanol. Between each step, SLGs were washed with deionized water. After cleaning procedures, the SLG substrates dried under nitrogen environment. CZTiSSe thin films were deposited in an Nanovak NVTS 400 vacuum system. High purity Cu, Ti and ZnSe targets used during the deposition. Direct-current (DC) power was applied to the Cu target, whereas radio frequency (RF) power was applied to the ZnSe and Ti targets. The films were deposited at 100, 175 and 250 C substrate temperatures and called DS34, DS35, and DS36, respectively. The deposition parameters, including applied power to targets and substrate temperature, are given in Table 1. The power parameters were chosen by taking cross-sectional scanning electron microscopy (SEM) images of single Cu, ZnSe and Ti thin films and energy dispersive spectroscopy (EDS) results of co-sputtered thin films before the annealing process to obtain stoichiometric thin films. The co-sputtering processes are drawn in Fig. 1a. After sputtering procedures, the films were annealed at 550 C for 30 min in a quartz furnace system with 30 sccm H2S:Ar (1:9) flow. Before and after annealing of the samples, the system was purged with high purity N2. The gas flows and the temperature gradient of the annealing system are drawn in Fig. 1b. The morphological properties and elemental composition of thin films were analyzed by FEI Quanta 250 FEG SEM with EDS apparatuses and structural properties of thin films were executed by Bruker D8 Davinci X-ray diffraction (XRD) system and SNOM Raman Spectroscopy with 532 nm excitation wavelength. Finally, the optical properties of CZTiSSe thin films were determined by Shimadzu UV3600 spectrophotometer. Furthermore, a solar cell was fabricated using the Cu-poor, and Zn-rich CZTiSSe thin film (namely DS34) deposited on Mo-coated glass. After the formation of CZTiSSe thin film on Mo contact, 50 nm CdS, 100 nm ZnO and 300 nm Al:ZnO thin films were sputtered on CZTiSSe/Mo structure to obtain a solar cell. Ag metal was evaporated using a shadow mask to complete Ag/AZO/ZnO/ CZTiSSe/Mo solar cell structure. The photovoltaic properties of the cell were determined via current density-voltage (J-V) using Keithley 2400 sourcemeter and a solar simulator with AM1.5 global filter and 100 mW/cm2 intensity.
Table 1 Deposition parameters of Cu2ZnTi(S:Se)4 thin films. Sample Name
DS34 DS35 DS36
Applied Power (Watt) Cu
Ti
ZnSe
25 25 25
80 85 85
85 80 80
Substrate Temperature ( C)
100 175 250
Fig. 1. a) Schematic view of the co-sputtering of ZnSe, Cu and Ti targets b) gas flows and temperature gradient during the annealing of Cu2ZnTi(S:Se)4 thin films.
3. Results and discussion SEM images of the all sputtered and annealed Cu2ZnTi(S:Se)4 thin films are presented in Fig. 2. As seen from the figure, all Cu2ZnTi(S:Se)4 thin films have homogeneous surface. As known, the sputtering method allows obtaining very homogeneous films concerning other methods, and the homogeneous films are preferred to obtain high-quality thin film solar cells [12]. The elemental ratio of Cu2ZnTi(S:Se)4 thin films obtained from EDS analysis is given in Table 2. As seen from the table, although the ratio of Cu:Zn:Ti fixed to 2:1:1 for all unannealed thin films, after annealing, it was found that the Zn metal having a lower evaporation temperature compared to Cu and Ti metals exhibited lower ratio in CZTiSSe structure with the increase in temperature. The decrease in Zn atoms in the structure resulted in Cu and Ti-rich, Zinc-poor structures obtained at higher substrate temperatures. Furthermore, the Se/(S þ Se) ratio is between 0.17 and 0.26, and the (S þ Se)/metal ratio was nearly ideal for all samples. The XRD patterns of CZTiSSe thin films deposited on SLG substrates are presented in Fig. 3. The main peaks of the film deposited at 100 C are at 28.21, 28.60, 31.78, 32.79, 46.89, 47.41, and 56.27. The peaks for the film grown at 175 C are given as 28.73, 31.75, 32.81, 47.60, 46.52 and 56.27. The peaks associated with the film deposited at 250 C are determined as 28.67, 31.75, 32.80, 46.27,
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Fig. 3. The XRD patterns of Cu2ZnTi(S:Se)4 thin films deposited at a) 100 b) 175 and c) 250 C.
phase. In this study, for the film obtained at 100 C, the very close peaks at 28.21 and 28.60 can be attributed to (112) twin planes, the peaks at 31.78 and 32.79 can be attributed to (200) twin planes and the peaks at 46.89 and 47.41 can be attributed to (220) twin planes of CZTiSSe structure related to both Se and S anions. The peaks obtained for the films deposited at 175 and 250 C can be evaluated in the same way. Therefore, the results of this study confirm the formation of Cu2ZnTi(S:Se)4 thin films with very similar XRD pattern of CZTSSe phase. As it is known that the XRD patterns of Cu2ZnSnS4 and secondary phases such as Cu2S and ZnS are very similar to each other. Therefore another important analysis method Raman spectroscopy is needed to understand the structural properties of the films. Fig. 4 presents the Raman spectra of the films. As seen from the figure, all thin films have a peak at 329 cm1 related to CZTSSe and DS34 sample has a secondary peak at 470 cm1 related to CuxS [19,20]. The Raman peak positions of CZTiSSe is reported as 329 cm1. In the figure, the peaks were shifted to a lower frequency
Fig. 2. SEM images of Cu2ZnTi(S:Se)4 thin films deposited at a) 100 b) 175 and c) 250 C.
47.54 and 56.45 . The XRD patterns of all films are very similar to the XRD pattern of kesterite Cu2ZnSnS4 crystal structure [17] and imply the formation of Cu2ZnTi(S:Se)4 compound. Cu2ZnSnS4 nanoparticles were synthesized by a facile solvothermal method by Zhou et al. [18]. They showed that the XRD patterns of Cu2ZnSnS4 nanoparticles had peaks at 2q ¼ 28.52, 33.08, 47.64 and 56.60 which can be attributed the (112), (200), (220) and (312) planes (JPDS 26e0575). Singh et al. [19] deposited Cu2ZnSn(S:Se)4 thin films by sequential reactive sputtering of Zn and Cu:Sn targets in H2S flow and annealed in Se atmosphere at 550 C. They have shown that the films had (112), (200), (220) and (312) planes and the (112) planes had two close peaks with slightly differed peak position. They also reported that one peak related to the Se anion and other related to S anion and both peaks represents the CZTSSe
Fig. 4. The Raman spectra of Cu2ZnTi(S:Se)4 thin films.
Table 2 Elementel ratio of Cu2ZnTi(S:Se)4 thin films. Sample Name
Cu
Ti
Zn
S
Se
Cu/(Zn þ Ti)
Zn/Ti
Se/(S þ Se)
(S þ Se)/Metal
DS34 DS35 DS36
23.70 25.09 26.85
11.39 12.58 13.85
13.46 12.78 9.68
38.60 41.06 36.89
12.84 8.49 12.93
0.95 0.99 1.14
1.18 1.02 0.70
0.25 0.17 0.26
1.06 0.98 0.99
D. Batibay et al. / Renewable Energy 145 (2020) 1672e1676
than the CZTiS peak position in 336 cm1 [16] and peak broadening was observed around 336 cm1. This observation shows that the atomic positions of S atoms were filled with Se atoms without changing the CZTiS structure. The frequencies of the Raman peaks are inversely proportional to the vibration frequency of the bonds, and the vibration frequency is inversely proportional to the reduced mass of atoms. It is expected to reduce the vibration frequency of the S atom, which is replaced by the Se atom, in the CZTiS structure. By taking XRD patterns, it can be said that the main Raman peak has been shifted to the low frequency as the structure has both CZTiS and CZTiSSe, and it is observed that this structure is larger as a result of the vibration frequency of the two structures.
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The absorbance vs. wavelength plots of all Cu2ZnTi(S:Se)4 thin films presented in Fig. 5 has great absorbance values from the ultraviolet region to the infrared region, which is preferred for a good absorber. The optical band gap of a semiconductor can be determined by the help of the equation given as
ahn ¼ A(hn -Eg)m
(1)
where a, A and h are the absorption coefficient, a constant, and the Planck constant, respectively. The exponent m is related to the transition where m is equal to ½ for allowed direct transitions. CZTS and related semiconductors have a direct band gap, and the relationship between the optical absorbance and the absorption coefficient is given as A ¼ 0.434ad [21]. The optical coefficient values of the films were determined for each wavelength values using the film thickness obtained from SEM. The optical band gap values were calculated using Eq. (1) as 1.29, 1.25, and 1.33 eV for the films deposited at 100, 175, and 250 C, respectively. According to Shockley and Queisser relation, the band gap values around 1.1 eV are very suitable for the fabrication of heterojunction solar cells [22] and the conventional Cu2ZnSnS4 thin film has 1.51 eV band gap value [23]. Table 3 presents the deposition and annealing parameters of some Cu2ZnMS4 and Cu2ZnM(SSe)4 (M ¼ Sn, Si, Ge and Ti) thin films and their optical band gap values. As seen from the table, while the band gap values of the Cu2ZnSiS4, Cu2ZnGeS4 and Cu2ZnSiSe4 semiconductors are higher than the conventional Cu2ZnSnS4 thin films, the optical band gaps of Cu2ZnTiS4, Cu2ZnSnSe4 and Cu2ZnSn(S:Se)4 are lower. The results confirm the theoretical study performed by Wang et al. [14] on the electronic structure of Cu2ZnTiS4 and Cu2ZnTiSe4 compounds and shows the suitability of Cu2ZnTi(S:Se)4 thin films for solar cell fabrication. Studies on CZTS solar cells showed that Cu-poor and Zn-rich thin films give higher power conversion efficiency [29,30]. Therefore, a solar cell with Mo/CZTiSSe/CdS/ZnO/AZO/Ag structure was obtained using the film Cu-poor and Zn-rich CZTiSSe thin film deposited at 100 C. The current density-voltage (J-V) measurement plot of the solar cell is shown in Fig. 6. As depicted in the figure, the power conversion efficiency (h) and fill factor (FF) of the cell was determined as 1.96% and 0.40, respectively. Similarly, Jia et al. [16] deposited Cu2ZnTiS4 thin films using the sputtering method and fabricated Mo/Cu2ZnTiS4/CdS/ZnO/Al structure. They reported that the device had 0.83% efficiency with 538.6 mV open circuit voltage (VOC) and of 0.387 FF value. The photovoltaic performance in this study for the cell obtained using CZTiSSe thin film is better than the Mo/Cu2ZnTiS4/CdS/ZnO/Al solar cell. Although the power conversion efficiency of CZTiSSe based solar cell is lower than the conventional CZTSSe based device with best efficient (12.6%) [7], CZTiSSe thin films can be considered as a promising material for low cost photovoltaic applications.
Fig. 5. a) UVeVis spectra and b) (ahn)2 vs. hn plots of Cu2ZnTi(S:Se)4 thin films deposited at a) 100 b) 175 and c) 250 C.
Table 3 The deposition and annealing parameters of some Cu2ZnMS4 and Cu2ZnM(SSe)4 (M ¼ Sn, Si, Ge and Ti) thin films and their optical band gap values. Thin Film
Cu2ZnSnS4 Cu2ZnSiS4 Cu2ZnTiS4 Cu2ZnTiS4 Cu2ZnGeS4 Cu2ZnSiSe4 Cu2ZnGeSe4 Cu2ZnSnSe4 Cu2ZnTi(S:Se)4
Deposition
Annealing
Used Targets/Metal
Method
Temp (C)
Atmosphere
Time (min)
Cu2S/ZnS/SnS2 Cu/ZnS/Si Cu/ZnS/Ti Cu/ZnS/Ti Ge/Zn/Cu Cu/Zn/Si Cu/Zn/Ge Cu/Zn/Sn Cu/ZnSe/Ti
co-sputter co-sputter co-sputter reactive-sputter sequential sputter electron beam evaporation sequential sputter sequential sputter co-sputter
400 800 580 400 550 490 460 500 550
Ar þ S2 Sulphur Sulphur Ar:H2S Sulphur H2Se/N2 H2Se/N2 Se/Ar Ar:H2S
Not given 10/20 120 60 25 15 15 30 60
Band Gap (eV)
Ref
1.51 3.09/2.71 1.42 1.11e1.28 1.85 2.20 1.50 0.90 1.25e1.33
[23] [24] [16] [15] [25] [26] [27] [28] present work
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Fig. 6. J-V plot of a solar cell obtained a Cu2ZnTi(S:Se)4 thin film.
4. Conclusion Cu2ZnTi(S:Se)4 absorber films were obtained using cosputtering of ZnSe, Cu, and Ti targets at 100, 175 and 250 C. The films were annealed in Ar:H2S mixture environment at 550 C to improve the quality of the films. The physical characterization of co-sputtered Cu2ZnTi(S:Se)4 thin films was analyzed using SEM, EDS, XRD, Raman and UVeVis spectra. It was seen that all Cu2ZnTi(S:Se)4 thin films nearly homogeneous and stoichiometric. Furthermore, the XRD and Raman analysis demonstrated the formation of Cu2ZnTi(S:Se)4 structures. It was reported that the optical band gap of the films is lower than the conventional Cu2ZnSnS4 films and a solar cell with 1.96% efficient was obtained using Cu2ZnTi(S:Se)4 as an absorber layer in the structure. The study shows the possibility to obtain low cost solar cells because of the price advantages of Ti metal with respect to Sn metal. Acknowledgment This study is supported by TUBITAK with 114F363 grand number. References [1] K. Branker, M. Pathak, J.M. Pearce, A review of solar photovoltaic levelized cost of electricity, Renew. Sustain. Energy Rev. 15 (2011) 4470e4482. [2] Y.S. Lee, T. Gershon, T.K. Todorov, W. Wang, M.T. Winkler, M. Hopstaken, O. Gunawan, J. Kim, Atomic layer deposited Aluminum Oxide for interface Passivation of Cu2ZnSn(S,Se)4 thin-film solar cells, Adv. Energy Mater. 6 (2016) 1600198. [3] M. Mezher, R. Garris, L.M. Mansfield, K. Horsley, L. Weinhardt, D.A. Duncan, €r, K. Ramanathan, Electronic structure of the M. Blum, S.G. Rosenberg, M. Ba Zn(O,S)/Cu(In, Ga)Se2 thin-film solar cell interface, Prog. Photovolt. Res. Appl. 24 (8) (2016) 1142e1148. [4] Z. Cao, S. Yang, M. Wang, X. Huang, H. Li, J. Yi, J. Zhong, Cu(In,Ga)S2 absorber layer prepared for thin film solar cell by electrodeposition of Cu-Ga precursor from deep eutectic solvent, Sol. Energy 139 (2016) 29e35. [5] R. Ma, F. Yang, S. Li, X. Zhang, X. Li, S. Cheng, Z. Liu, Fabrication of Cu2ZnSn(S, Se)4 (CZTSSe) absorber films based on solid-phase synthesis and blade coating processes, Appl. Surf. Sci. 368 (2016) 8e15.
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Co-sputtered Cu2ZnTi(S:Se)4 absorbers for thin film solar cells Derya Batibay a, Yusuf Selim Ocak b, c, *, Mustafa Fatih Genisel b, c, Rasit Turan d, e a
Department of Physics, Institute of Natural &Applied Science, Dicle University, Diyarbakir, Turkey Department of Science, Faculty of Education, Dicle University, Diyarbakir, Turkey c Smart Laboratory, Dicle University, Diyarbakir, Turkey d Department of Physics, Faculty of Science, Middle East Technical University, Ankara, Turkey e Center of Solar Energy Research and Applications (GUNAM), Middle East Technical University, Ankara, Turkey b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2018 Received in revised form 31 May 2019 Accepted 16 July 2019 Available online 17 July 2019
Thin film solar cells are an exciting topic for low cost and high efficient solar cells. Owing to the high price of the indium metal in the fabrication of copper indium gallium diselenide (CIGS) solar cells, Cu2ZnSn(SSe)4 thin films are used as a new material to reduce the cost and increase the efficiency. As an alternative absorber material for solar cell production, Cu2ZnTi(S:Se)4 thin films were deposited by the co-sputtering method at various temperatures. During the deposition, Cu, ZnSe and Ti targets were used as metal sources. The Cu2ZnTi(S:Se)4 thin films were annealed in H2S:Ar (1:9) atmosphere. The morphological, structural and optical properties of Cu2ZnTi(S:Se)4 thin films was analyzed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) Raman spectroscopy and UVeViseNIR spectrometer. It was seen that the thin films had good optical absorption till the infrared region and the band gap of the Cu2ZnTi(S:Se)4 thin films were smaller than the conventional Cu2ZnSnS4 thin films. Furthermore, fabrication of a solar cell with 1.96% power conversion efficiency was reported using a Cu2ZnTi(S:Se)4 thin film as a low cost absorber layer. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Thin film Absorber Solar cell Cu2ZnTi(S:Se)4 Substrate temperature
1. Introduction The usage of energy increases exponentially with the increase of human population and the number of electronic equipment. Owing to the limitation of fossil fuel resources and pollution depend on the usage of fossil fuels, the interest in renewable energy sources increases every day. Among all clean energy sources, solar energy is the most abundant source. Reducing the materials and fabrication cost of solar cells are key parameters to use them on an industrial scale [1]. A thin film based solar cells with suitable band gap and high absorption coefficients are extensively studied [2e4]. Although high power conversion efficiency (PCE) (20%) have been obtained for Cu(In,Ga)(S,Se)2 (CIGSSe) based solar cells, alternative compounds required because of the toxic and expensive content of CIGSSe solar cells. Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) have been studied as an alternative compound for CIGS and CIGSSe solar cells [5,6]. The maximum PCE for CZTSSe solar cells was reported as 12.6% by Wang et al. [7]. They formed CZTSSe thin films by a spin coating
* Corresponding author. Smart Laboratory, Dicle University, Diyarbakir, Turkey. E-mail addresses: [email protected], [email protected] (Y.S. Ocak). https://doi.org/10.1016/j.renene.2019.07.086 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
method using hydrazine as a solvent. Many kinds of methods have been used for the deposition of CZTS or CZTSSe thin films and fabrication of solar cells including sputtering, successive ionic layer adsorption and reaction (SILAR), spin coating, pulsed laser deposition, and thermal evaporation methods [7e11]. All methods have different advantages and disadvantages. Among all methods, the sputtering method is a suitable method for deposition of ceramics and alloys [12]. Many theoretical and experimental studies have been devoted to various derivatives of CZTS and CZTSSe thin films to control their electrical and optical properties and obtain high efficient and lowcost solar cells. For instance, Cu2MSnS4 (M: Zn, Cd, Mn) thin films were deposited by chemical bath deposition (CBD) method along with a subsequent sulfurization heat treatment [13]. It was presented that the optical band gap of Cu2MSnS4 changes with the metal. Wang et al. [14] calculated electronic structure of Cu2ZnTiS4 (CZTiS) and Cu2ZnTi(S,Se)4 (CZTiSSe) compounds. They showed that the absorption coefficients of these films are higher than the conventional CZTS and CZTSSe thin films and their band gaps are lower than the conventional CZTS and CZTSSe thin films. The deposition of CZTiS thin films and CZTiS based solar cell with 0.87% power conversion efficiency have been reported previously [15,16].
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Apart from these studies, here, the deposition and characterization of CZTiSSe thin films and photovoltaic properties of a CZTSSe based solar cell are presented. In this study, we deposited CZTiSSe thin films by co-sputtering of Cu, ZnSe and Ti sources and them annealed in H2S:Ar (1:9) atmosphere to show the formation of CZTiSSe thin films at various substrate temperature. Then, a 1.96% efficient solar cell was obtained using CZTiSSe as an absorber layer, and it was shown that CZTiSSe thin films could be used in the fabrication of low-cost solar cells.
2. Experimental details Soda-lime glass (SLG) substrates were cleaned by washing with detergent and then ultrasonically vibrating in acetone and 2propanol. Between each step, SLGs were washed with deionized water. After cleaning procedures, the SLG substrates dried under nitrogen environment. CZTiSSe thin films were deposited in an Nanovak NVTS 400 vacuum system. High purity Cu, Ti and ZnSe targets used during the deposition. Direct-current (DC) power was applied to the Cu target, whereas radio frequency (RF) power was applied to the ZnSe and Ti targets. The films were deposited at 100, 175 and 250 C substrate temperatures and called DS34, DS35, and DS36, respectively. The deposition parameters, including applied power to targets and substrate temperature, are given in Table 1. The power parameters were chosen by taking cross-sectional scanning electron microscopy (SEM) images of single Cu, ZnSe and Ti thin films and energy dispersive spectroscopy (EDS) results of co-sputtered thin films before the annealing process to obtain stoichiometric thin films. The co-sputtering processes are drawn in Fig. 1a. After sputtering procedures, the films were annealed at 550 C for 30 min in a quartz furnace system with 30 sccm H2S:Ar (1:9) flow. Before and after annealing of the samples, the system was purged with high purity N2. The gas flows and the temperature gradient of the annealing system are drawn in Fig. 1b. The morphological properties and elemental composition of thin films were analyzed by FEI Quanta 250 FEG SEM with EDS apparatuses and structural properties of thin films were executed by Bruker D8 Davinci X-ray diffraction (XRD) system and SNOM Raman Spectroscopy with 532 nm excitation wavelength. Finally, the optical properties of CZTiSSe thin films were determined by Shimadzu UV3600 spectrophotometer. Furthermore, a solar cell was fabricated using the Cu-poor, and Zn-rich CZTiSSe thin film (namely DS34) deposited on Mo-coated glass. After the formation of CZTiSSe thin film on Mo contact, 50 nm CdS, 100 nm ZnO and 300 nm Al:ZnO thin films were sputtered on CZTiSSe/Mo structure to obtain a solar cell. Ag metal was evaporated using a shadow mask to complete Ag/AZO/ZnO/ CZTiSSe/Mo solar cell structure. The photovoltaic properties of the cell were determined via current density-voltage (J-V) using Keithley 2400 sourcemeter and a solar simulator with AM1.5 global filter and 100 mW/cm2 intensity.
Table 1 Deposition parameters of Cu2ZnTi(S:Se)4 thin films. Sample Name
DS34 DS35 DS36
Applied Power (Watt) Cu
Ti
ZnSe
25 25 25
80 85 85
85 80 80
Substrate Temperature ( C)
100 175 250
Fig. 1. a) Schematic view of the co-sputtering of ZnSe, Cu and Ti targets b) gas flows and temperature gradient during the annealing of Cu2ZnTi(S:Se)4 thin films.
3. Results and discussion SEM images of the all sputtered and annealed Cu2ZnTi(S:Se)4 thin films are presented in Fig. 2. As seen from the figure, all Cu2ZnTi(S:Se)4 thin films have homogeneous surface. As known, the sputtering method allows obtaining very homogeneous films concerning other methods, and the homogeneous films are preferred to obtain high-quality thin film solar cells [12]. The elemental ratio of Cu2ZnTi(S:Se)4 thin films obtained from EDS analysis is given in Table 2. As seen from the table, although the ratio of Cu:Zn:Ti fixed to 2:1:1 for all unannealed thin films, after annealing, it was found that the Zn metal having a lower evaporation temperature compared to Cu and Ti metals exhibited lower ratio in CZTiSSe structure with the increase in temperature. The decrease in Zn atoms in the structure resulted in Cu and Ti-rich, Zinc-poor structures obtained at higher substrate temperatures. Furthermore, the Se/(S þ Se) ratio is between 0.17 and 0.26, and the (S þ Se)/metal ratio was nearly ideal for all samples. The XRD patterns of CZTiSSe thin films deposited on SLG substrates are presented in Fig. 3. The main peaks of the film deposited at 100 C are at 28.21, 28.60, 31.78, 32.79, 46.89, 47.41, and 56.27. The peaks for the film grown at 175 C are given as 28.73, 31.75, 32.81, 47.60, 46.52 and 56.27. The peaks associated with the film deposited at 250 C are determined as 28.67, 31.75, 32.80, 46.27,
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Fig. 3. The XRD patterns of Cu2ZnTi(S:Se)4 thin films deposited at a) 100 b) 175 and c) 250 C.
phase. In this study, for the film obtained at 100 C, the very close peaks at 28.21 and 28.60 can be attributed to (112) twin planes, the peaks at 31.78 and 32.79 can be attributed to (200) twin planes and the peaks at 46.89 and 47.41 can be attributed to (220) twin planes of CZTiSSe structure related to both Se and S anions. The peaks obtained for the films deposited at 175 and 250 C can be evaluated in the same way. Therefore, the results of this study confirm the formation of Cu2ZnTi(S:Se)4 thin films with very similar XRD pattern of CZTSSe phase. As it is known that the XRD patterns of Cu2ZnSnS4 and secondary phases such as Cu2S and ZnS are very similar to each other. Therefore another important analysis method Raman spectroscopy is needed to understand the structural properties of the films. Fig. 4 presents the Raman spectra of the films. As seen from the figure, all thin films have a peak at 329 cm1 related to CZTSSe and DS34 sample has a secondary peak at 470 cm1 related to CuxS [19,20]. The Raman peak positions of CZTiSSe is reported as 329 cm1. In the figure, the peaks were shifted to a lower frequency
Fig. 2. SEM images of Cu2ZnTi(S:Se)4 thin films deposited at a) 100 b) 175 and c) 250 C.
47.54 and 56.45 . The XRD patterns of all films are very similar to the XRD pattern of kesterite Cu2ZnSnS4 crystal structure [17] and imply the formation of Cu2ZnTi(S:Se)4 compound. Cu2ZnSnS4 nanoparticles were synthesized by a facile solvothermal method by Zhou et al. [18]. They showed that the XRD patterns of Cu2ZnSnS4 nanoparticles had peaks at 2q ¼ 28.52, 33.08, 47.64 and 56.60 which can be attributed the (112), (200), (220) and (312) planes (JPDS 26e0575). Singh et al. [19] deposited Cu2ZnSn(S:Se)4 thin films by sequential reactive sputtering of Zn and Cu:Sn targets in H2S flow and annealed in Se atmosphere at 550 C. They have shown that the films had (112), (200), (220) and (312) planes and the (112) planes had two close peaks with slightly differed peak position. They also reported that one peak related to the Se anion and other related to S anion and both peaks represents the CZTSSe
Fig. 4. The Raman spectra of Cu2ZnTi(S:Se)4 thin films.
Table 2 Elementel ratio of Cu2ZnTi(S:Se)4 thin films. Sample Name
Cu
Ti
Zn
S
Se
Cu/(Zn þ Ti)
Zn/Ti
Se/(S þ Se)
(S þ Se)/Metal
DS34 DS35 DS36
23.70 25.09 26.85
11.39 12.58 13.85
13.46 12.78 9.68
38.60 41.06 36.89
12.84 8.49 12.93
0.95 0.99 1.14
1.18 1.02 0.70
0.25 0.17 0.26
1.06 0.98 0.99
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than the CZTiS peak position in 336 cm1 [16] and peak broadening was observed around 336 cm1. This observation shows that the atomic positions of S atoms were filled with Se atoms without changing the CZTiS structure. The frequencies of the Raman peaks are inversely proportional to the vibration frequency of the bonds, and the vibration frequency is inversely proportional to the reduced mass of atoms. It is expected to reduce the vibration frequency of the S atom, which is replaced by the Se atom, in the CZTiS structure. By taking XRD patterns, it can be said that the main Raman peak has been shifted to the low frequency as the structure has both CZTiS and CZTiSSe, and it is observed that this structure is larger as a result of the vibration frequency of the two structures.
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The absorbance vs. wavelength plots of all Cu2ZnTi(S:Se)4 thin films presented in Fig. 5 has great absorbance values from the ultraviolet region to the infrared region, which is preferred for a good absorber. The optical band gap of a semiconductor can be determined by the help of the equation given as
ahn ¼ A(hn -Eg)m
(1)
where a, A and h are the absorption coefficient, a constant, and the Planck constant, respectively. The exponent m is related to the transition where m is equal to ½ for allowed direct transitions. CZTS and related semiconductors have a direct band gap, and the relationship between the optical absorbance and the absorption coefficient is given as A ¼ 0.434ad [21]. The optical coefficient values of the films were determined for each wavelength values using the film thickness obtained from SEM. The optical band gap values were calculated using Eq. (1) as 1.29, 1.25, and 1.33 eV for the films deposited at 100, 175, and 250 C, respectively. According to Shockley and Queisser relation, the band gap values around 1.1 eV are very suitable for the fabrication of heterojunction solar cells [22] and the conventional Cu2ZnSnS4 thin film has 1.51 eV band gap value [23]. Table 3 presents the deposition and annealing parameters of some Cu2ZnMS4 and Cu2ZnM(SSe)4 (M ¼ Sn, Si, Ge and Ti) thin films and their optical band gap values. As seen from the table, while the band gap values of the Cu2ZnSiS4, Cu2ZnGeS4 and Cu2ZnSiSe4 semiconductors are higher than the conventional Cu2ZnSnS4 thin films, the optical band gaps of Cu2ZnTiS4, Cu2ZnSnSe4 and Cu2ZnSn(S:Se)4 are lower. The results confirm the theoretical study performed by Wang et al. [14] on the electronic structure of Cu2ZnTiS4 and Cu2ZnTiSe4 compounds and shows the suitability of Cu2ZnTi(S:Se)4 thin films for solar cell fabrication. Studies on CZTS solar cells showed that Cu-poor and Zn-rich thin films give higher power conversion efficiency [29,30]. Therefore, a solar cell with Mo/CZTiSSe/CdS/ZnO/AZO/Ag structure was obtained using the film Cu-poor and Zn-rich CZTiSSe thin film deposited at 100 C. The current density-voltage (J-V) measurement plot of the solar cell is shown in Fig. 6. As depicted in the figure, the power conversion efficiency (h) and fill factor (FF) of the cell was determined as 1.96% and 0.40, respectively. Similarly, Jia et al. [16] deposited Cu2ZnTiS4 thin films using the sputtering method and fabricated Mo/Cu2ZnTiS4/CdS/ZnO/Al structure. They reported that the device had 0.83% efficiency with 538.6 mV open circuit voltage (VOC) and of 0.387 FF value. The photovoltaic performance in this study for the cell obtained using CZTiSSe thin film is better than the Mo/Cu2ZnTiS4/CdS/ZnO/Al solar cell. Although the power conversion efficiency of CZTiSSe based solar cell is lower than the conventional CZTSSe based device with best efficient (12.6%) [7], CZTiSSe thin films can be considered as a promising material for low cost photovoltaic applications.
Fig. 5. a) UVeVis spectra and b) (ahn)2 vs. hn plots of Cu2ZnTi(S:Se)4 thin films deposited at a) 100 b) 175 and c) 250 C.
Table 3 The deposition and annealing parameters of some Cu2ZnMS4 and Cu2ZnM(SSe)4 (M ¼ Sn, Si, Ge and Ti) thin films and their optical band gap values. Thin Film
Cu2ZnSnS4 Cu2ZnSiS4 Cu2ZnTiS4 Cu2ZnTiS4 Cu2ZnGeS4 Cu2ZnSiSe4 Cu2ZnGeSe4 Cu2ZnSnSe4 Cu2ZnTi(S:Se)4
Deposition
Annealing
Used Targets/Metal
Method
Temp (C)
Atmosphere
Time (min)
Cu2S/ZnS/SnS2 Cu/ZnS/Si Cu/ZnS/Ti Cu/ZnS/Ti Ge/Zn/Cu Cu/Zn/Si Cu/Zn/Ge Cu/Zn/Sn Cu/ZnSe/Ti
co-sputter co-sputter co-sputter reactive-sputter sequential sputter electron beam evaporation sequential sputter sequential sputter co-sputter
400 800 580 400 550 490 460 500 550
Ar þ S2 Sulphur Sulphur Ar:H2S Sulphur H2Se/N2 H2Se/N2 Se/Ar Ar:H2S
Not given 10/20 120 60 25 15 15 30 60
Band Gap (eV)
Ref
1.51 3.09/2.71 1.42 1.11e1.28 1.85 2.20 1.50 0.90 1.25e1.33
[23] [24] [16] [15] [25] [26] [27] [28] present work
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Fig. 6. J-V plot of a solar cell obtained a Cu2ZnTi(S:Se)4 thin film.
4. Conclusion Cu2ZnTi(S:Se)4 absorber films were obtained using cosputtering of ZnSe, Cu, and Ti targets at 100, 175 and 250 C. The films were annealed in Ar:H2S mixture environment at 550 C to improve the quality of the films. The physical characterization of co-sputtered Cu2ZnTi(S:Se)4 thin films was analyzed using SEM, EDS, XRD, Raman and UVeVis spectra. It was seen that all Cu2ZnTi(S:Se)4 thin films nearly homogeneous and stoichiometric. Furthermore, the XRD and Raman analysis demonstrated the formation of Cu2ZnTi(S:Se)4 structures. It was reported that the optical band gap of the films is lower than the conventional Cu2ZnSnS4 films and a solar cell with 1.96% efficient was obtained using Cu2ZnTi(S:Se)4 as an absorber layer in the structure. The study shows the possibility to obtain low cost solar cells because of the price advantages of Ti metal with respect to Sn metal. Acknowledgment This study is supported by TUBITAK with 114F363 grand number. References [1] K. Branker, M. Pathak, J.M. Pearce, A review of solar photovoltaic levelized cost of electricity, Renew. Sustain. Energy Rev. 15 (2011) 4470e4482. [2] Y.S. Lee, T. Gershon, T.K. Todorov, W. Wang, M.T. Winkler, M. Hopstaken, O. Gunawan, J. Kim, Atomic layer deposited Aluminum Oxide for interface Passivation of Cu2ZnSn(S,Se)4 thin-film solar cells, Adv. Energy Mater. 6 (2016) 1600198. [3] M. Mezher, R. Garris, L.M. Mansfield, K. Horsley, L. Weinhardt, D.A. Duncan, €r, K. Ramanathan, Electronic structure of the M. Blum, S.G. Rosenberg, M. Ba Zn(O,S)/Cu(In, Ga)Se2 thin-film solar cell interface, Prog. Photovolt. Res. Appl. 24 (8) (2016) 1142e1148. [4] Z. Cao, S. Yang, M. Wang, X. Huang, H. Li, J. Yi, J. Zhong, Cu(In,Ga)S2 absorber layer prepared for thin film solar cell by electrodeposition of Cu-Ga precursor from deep eutectic solvent, Sol. Energy 139 (2016) 29e35. [5] R. Ma, F. Yang, S. Li, X. Zhang, X. Li, S. Cheng, Z. Liu, Fabrication of Cu2ZnSn(S, Se)4 (CZTSSe) absorber films based on solid-phase synthesis and blade coating processes, Appl. Surf. Sci. 368 (2016) 8e15.
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