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Dual function of ultrathin Ti intermediate layers in CZTS solar cells: Sulfur blocking and charge enhancement
Solar Energy Materials and Solar Cells 175 (2018) 20–28
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Dual function of ultrathin Ti intermediate layers in CZTS solar cells: Sulfur blocking and charge enhancement
MARK
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Huafei Guoa, Changhao Maa, Kezhi Zhanga, Xuguang Jiaa, Xiuqing Wanga, Ningyi Yuana, , ⁎⁎ Jianning Dinga,b, a School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China b Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang 212013, China
A R T I C L E I N F O
A B S T R A C T
Keywords: CZTS solar cell Ti blocking layer Sulfur blockage Charge increase
An ultrathin Ti film acting as both an intermediate layer and a dopant was inserted at the interface between Mo and Cu2ZnSnS4 (CZTS) to improve the performance of CZTS solar cells. Different from previously reported blocking layers, the Ti intermediate layer inhibited the formation of a MoS2 layer and voids at the interface between the Mo and CZTS absorber and also influenced the crystallinity, surface evenness, Hall mobility, and absorptivity of the CZTS absorber. When the thickness of the Ti blocking layer increased to 20 nm, the conversion efficiency of the solar cell increased by 57%, along with an increase in the open-circuit voltage of 32%. The effect of the Ti layer on the microstructure and performance of the CZTS film is discussed here in detail. These results serve as guiding principles for preparing high-quality CZTS thin films for potential applications in low-cost solar cells.
1. Introduction Kesterite copper zinc tin sulfide (Cu2ZnSnS4; CZTS) thin films are attractive light absorbing materials for low-cost, large-scale production of highly efficient thin film solar cells. These materials comprise abundant, nontoxic, and inexpensive constituent elements, and also have an ideal band gap of about 1.5 eV with predicted maximum theoretical efficiencies over 30% [1–3]. Therefore, CZTS films are the most promising alternative to replace chalcopyrite absorbers, such as copper indium gallium selenide (CIGS), which includes rare metals (indium and gallium). However, to date, the efficiencies (η) of CZTS devices (12.6%) are much lower than those of CIGS (21.7%) [4,5]. The low open-circuit voltage (Voc) is the most critical impediment to achieving higher efficiencies in CZTS devices compared to the filling factor (FF) and short-circuit current density (Jsc). A thick intermediate layer of MoS2 formed between the back contact and absorber during the growth of CZTS has been reported as the cause of the low voltage [6,7]. Two approaches have been proposed to eliminate the detrimental effect of MoS2. The first is to replace Mo with a less reactive back contact material. Compared to other contact materials, Mo remains attractive due to its low price and high stability and has so far been used in all
record-performing devices. The other approach is to use an intermediate layer to prevent the reaction between CZTS and Mo. Several materials have been used as intermediate layers, including Ag [8], ZnO [9], TiN [7], TiB2 [10], and carbon [11]. For example, Shin et al. investigated a TiN intermediate layer in CZTS solar cells and showed that it reduced the MoSe2 thickness from ~1300 to ~220 nm and the series resistance from 3.4 to 1.8 Ω cm2. Li et al. inserted a metallic Ag intermediate layer in CZTS solar cells and found that the Ag layer inhibited the formation of voids between the Mo/CZTS interface. Lopez-Marino et al. applied a ZnO blocking layer to inhibit the decomposition of CZTS near the Mo back contact and reported that the ZnO layer improved the interfacial morphology and reduced the density of voids. Zhou et al. used an ultra-thin carbon film as the intermediate layer between Mo and CZTS and found that the carbon aggregates provided a better connection between the void-containing CZTS and the Mo back contact. To date, the effect of Ti intermediate layers in CZTS solar cell on the device performance has not been investigated. In this study, we used a magnetron sputtering method to prepare thin intermediate layers of Ti between CZTS and Mo. Different from the reported blocking layers, it is expected that the Ti film could play a dual role in retarding the reaction between S and Mo, as well as influencing
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Corresponding author. Corresponding author at: School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China. E-mail addresses: [email protected] (N. Yuan), [email protected] (J. Ding). ⁎⁎
http://dx.doi.org/10.1016/j.solmat.2017.09.052 Received 20 May 2017; Received in revised form 25 September 2017; Accepted 29 September 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
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and Sn were sputtered by DC magnetron sputtering using powers of 6 W and 43 W, respectively, and ZnS was sputtered using a power of 90 W (RF magnetron) to obtain a precursor thickness of 750 nm. After sputtering, the samples were loaded into a horizontal tube furnace and annealed at 550 °C for 60 min with 50 mg of sulfur powder.
2.3. Device fabrication The Ti-incorporated CZTS solar cells were fabricated in the standard device structure of Mo/CZTS/CdS/i-ZnO/ITO/Ag grid. To fabricate this, a 60 nm CdS layer, a 50 nm intrinsic ZnO (i-ZnO) layer and a 230 nm ITO layer, were sequentially deposited by RF sputtering.
2.4. Characterization Fig. 1. The XRD patterns of sulfurided Mo films with and without the Ti blocking layer.
The crystallographic analysis of the films was performed using X-ray diffraction (XRD D/MAX Ultima III, Rigaku Corporation, Tokyo, Japan). The film morphology was investigated by scanning electron microscopy (SEM; 6360LA, JEOL, Japan). The elemental composition of CZTS was determined by energy-dispersive X-ray spectroscopy (EDS). The surface roughness of the Ti-doped CZTS thin films was measured by atomic force microscopy (AFM; Nanman VS). X-ray photoelectron spectroscopy (XPS; AXIS-ULTRA DLD) was performed using a monochromatic X-ray source (Al-Kα line of 1486.6 eV energy and 350 W power). Raman spectra were collected using a Renishaw in via Raman system using a 532 nm excitation wavelength (Thermo Fisher, USA). The electrical properties were analyzed using a Hall measurement system (QT-50, Quatek, Germany). The optical transmittance was measured using a UV–Vis spectrophotometer (UV-2250, Shimadzu, Japan). The ultraviolet photoelectron spectroscopy (UPS) was conducted with a SPECS Microwave UV light source (He I = 21.2 eV). A solar simulator (500 W Xe lamp) was employed as the light source, and the light intensity was adjusted with a Si reference solar cell to approximate the AM 1.5 solar spectrum. The external quantum efficiency (EQE) was measured with a Solar Cell Scan Photovoltaic characterization system (Solar Cell Scan, China).
physical, electrical, and optical properties of the CZTS thin films. The mechanism by which the Ti layer affects these properties is discussed here in detail. 2. Experimental section 2.1. Preparation of Ti blocking layers on Mo-coated soda lime glass substrates Thin Ti films with different thicknesses (0, 5, 10, 15, 20, and 25 nm) were deposited on soda-lime glass and Mo-coated glass substrates by radio frequency (RF) magnetron sputtering using an RF power of 60 W. 2.2. Preparation of CZTS thin films The CZTS thin films were prepared on the Ti layers by a two-step process. CZTS precursors were fabricated by co-sputtering with three different targets, i.e., Cu (99.99%), Sn (99.99%), and ZnS (99.99%). Cu
Fig. 2. The surface EDS mapping images of the CZTS film with 20 nm thick Ti blocking layer.
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Fig. 3. The XPS patterns of CZTS film with 20 nm thick Ti blocking layer.
3. Results and discussion
(002) plane of monoclinic TiS3 (JCPDS#: 36-1137). These results indicate that the Ti blocking layer prevented the diffusion of S into the Mo films, and thereby prevented the formation of MoS2. In order to investigate the influence of the Ti blocking layer on the performance of physical and chemical properties of the CZTS solar cells, EDS mapping and XPS were used to determine the elemental compositions of the CZTS films and their valences with and without the Ti blocking layer. The surface EDS spectra of CZTS with a Ti blocking layer are shown in Fig. 2. Five elements (Cu, Zn, Sn, Ti, and S) were clearly detected in the spectra while no signals attributable to Mo were detected. Fig. S1 shows the cross-sectional EDS mapping of a CZTS film with a 20 nm Ti blocking layer, which clearly indicates the diffusion of
First, the effect of the Ti blocking layer on the diffusion of S into the Mo back layer was investigated. Mo films with and without the Ti layer were subjected to sulfuration using the same conditions as those used for the CZTS sulfuration. Fig. 1 shows the XRD patterns of Mo films with and without Ti layers, after sulfuration. Without the present of Ti blocking layer, obvious MoS2 peaks were observed at 2θ = 14.3° and 33.5° (JCPDS#:65-1941), in addition to the Mo peaks at 2θ = 40.4°, 58.6°, and 73.6° (JCPDS#: 65-7742). However, with the Ti blocking layer on the Mo substrate, diffraction peaks of MoS2 were not observed. A new peak was observed at 2θ = 20.38°, which may correspond to the
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Fig. 4. (a) The XRD and (b) Raman patterns of CZTS films with different thick Ti blocking layers.
Ti into the CZTS film. Fig. S2 shows elemental distribution of Cu, Zn, Sn, Ti, and S in the CZTS film with a 20 nm Ti blocking layer using SEMEDS. To confirm the location of Ti in the CZTS lattice, XPS was used to
ascertain the stoichiometry and oxidation states of the constituent elements in the CZTS film with a 20 nm Ti blocking layer; these results are shown in Fig. 3. The Cu 2p peak was split into peaks at 931.6 eV (2p3/2) and 951.4 eV (2p1/2) with a splitting energy of 19.8 eV, which Fig. 5. The surface SEM images of CZTS films with different thickness of Ti blocking layers (a) 0 nm, (b) 5 nm, (c) 10 nm, (d) 15 nm, (e) 20 nm and (f) 25 nm.
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Fig. 6. The cross-sectional image of CZTS (a) without Ti blocking layer, (b) with 20 nm thick Ti blocking layer.
Fig. 7. The AFM images of CZTS films with different thickness of Ti blocking layers (a) 0 nm, (b) 5 nm, (c) 10 nm, (d) 15 nm, (e) 20 nm and (f) 25 nm.
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shown in Fig. S4. The grain sizes in the CZTS film increased with increasing thickness of the Ti blocking layers up to 25 nm. This indicates that Ti doping could promote grain growth in the CZTS films. The major XRD diffraction peaks of tetragonal CZTS are known to coincide with those of several other phases, such as cubic ZnS and tetragonal Cu2SnS3 [17,18]. Therefore, Raman spectroscopy was employed to further confirm the phase purity of CZTS. Fig. 4(b) shows the Raman spectra of the CZTS film with different thicknesses of the Ti blocking layer; three main peaks of CZTS are observed at approximately 284, 334, and 364 cm−1, whereas other binary and ternary compounds were not observed, indicating that all the prepared samples were pure with no miscellaneous phases [19–21]. Fig. 5(a–f) show surface SEM images of the CZTS films with different Ti blocking layer thicknesses. The CZTS film with no Ti blocking layer showed compact and well-formed grains; however, small pinholes and a rough surface were observed. After the introduction of the Ti blocking layer, these pinholes started to shrink and gradually disappeared [22–24]. The CZTS films with the Ti blocking layers exhibited improved crystallinity and densification (Fig. 5(b)). The SEM images shown in Fig. 5(c–f) indicate that the crystallinity and grain sizes increased gradually with increasing thickness of the Ti blocking layers, which is in good agreement with the XRD results. As it is known that when the grain size is increased, the density of grain boundaries will be decreased and the probability of electrons being trapped will be reduced. Fig. 6 shows cross-sectional SEM images of the CZTS films with and without Ti blocking layers. As shown in Fig. 6(a), without the Ti layer, a large number of voids formed at the back contact region, which could limit the free-carrier transportation. As shown in Fig. 6(b), with the introduction of the Ti intermediate layer, the voids formed at the back contact gradually disappeared and the crystallinity of the CZTS films increased, which is consistent with the XRD and surface SEM results. In order to confirm the change in the surface roughness of the CZTS films with different thicknesses of Ti blocking layers, AFM characterization was carried out and the obtained images are shown in Fig. 7. The surface roughness of the CZTS film increased from 51.9 to 57.2 nm with increasing Ti layer thickness from 0 to 10 nm. With a further increase in layer thickness, the surface roughness decreased from 57.2 to 45.4 nm, while the roughness of the CZTS film with the 25 nm Ti layer increased again to 58.6 nm; this trend is depicted in Fig. S5. The electrical properties of 1.0 µm CZTS films deposited on glass substrates with Ti blocking layers were measured using a Hall measurement system at 300 K. Fig. 8 presents hole concentration, hole mobility, and electrical resistivity data. As is evident from Fig. 8, the mobility increased, while the carrier concentration decreased, as the thickness of the Ti blocking layer increased from 0 to 25 nm. Table 1 summarizes the detailed electrical properties. All the CZTS films exhibited p-type conduction behaviors and Ti doping in CZTS resulted in a high mobility of 154.43 cm2 v−1 s−1. The Ti dopant reducing the hole concentration and enhancing the mobility of CZTS film. The high mobility of Ti-doped CZTS was attributed to the improved grain size, low porosity, and fewer scattering centers. Fig. 9(a) and (b) show the positions of the UPS valence band maximum (VBM) for the CZTS films with and without 20 nm Ti blocking layers. The VBM were obtained by extrapolating the leading edge of the valence band onset to the background level, below the Fermi level. The VBM of both CZTS films were determined to be 0.7 eV. Fig. 9(c) and (d) shows the energy level diagrams of CZTS films with and without the 20 nm Ti blocking layer. According to the band gap of these two CZTS films (Fig. S6 in the ESI), the location of the Fermi level was closer to VBM than the conduction band maxima (CBM), which implies that both the CZTS films were ptype semiconductors [25–27]. Comparing the energy levels of these two films indicated that the EF of the CZTS film with the 20 nm Ti blocking layer was more accessible to the CBM. This implies that the hole
Fig. 8. The electrical properties of CZTS films with different thickness of Ti blocking layers.
Table 1 The detailed electrical properties of CZTS thin film with different thickness of Ti blocking layer. Ti thickness (nm)
0 5 10 15 20 25
CZTS Resistivity (Ω cm)
2
Mobility (cm /Vs)
Carrier Density (cm−3)
62.65 87.07 115.65 159.32 220.8 340.2
2.85 7.21 42.86 76.67 109.86 154.43
7.05 2.34 8.54 5.01 2.15 4.34
× × × × × ×
Type of carrier
1016 1016 1015 1015 1015 1014
P P P P P P
is in good agreement with that of Cu(I) [12,13]. The Zn 2p peak was observed at binding energies of 1021.3 (2p3/2) and 1044.6 eV (2p1/2) with a splitting energy of 23.3 eV, indicative of Zn(II) [14]. The Sn 3d peak was split into peaks at 486.0 (2p5/2) and 494.4 eV (2p3/2) with a splitting energy of 8.5 eV, similar to the standard splitting energy of 8.4 eV for Sn 3d [15]. The S 2p spectral peaks were located at 161.4 (2p3/2) and 162.5 eV (2p1/2) with a separation of 1.1 eV, consistent with the 160–164 eV expected for S in sulfide phases. The Ti 2p binding energy was located at 473 eV, which indicates that the valence state of Ti in CZTS film was four. This suggests that Ti diffused into the CZTS film and occupied the Sn sites in the lattice. In order to further investigate the influence of Ti on the structural properties of the CZTS thin films, XRD, Raman spectroscopy, SEM, and AFM investigations were performed. Fig. 4(a) shows the XRD patterns of the CZTS films with Ti blocking layers of different thicknesses. For Ti thicknesses of 0, 5, 10, 15, 20, and 25 nm, diffraction peaks were observed at characteristic 2θ values corresponding to the kesterite structure of CZTS (JCPDS#:26-0575). Fig. S3 compares the (112) peaks of the CZTS absorbers with Ti blocking layers of different thicknesses. Obvious peak shifts were observed when the Ti blocking layer was introduced, which further confirms that Ti entered the lattice of the CZTS film. The grain sizes were calculated and averaged according to all the XRD peaks using the Debye-Scherrer equation [16]. The results are
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Fig. 9. UPS spectra of the valence band edge region for CZTS (a) without and (b)with 20 nm thick Ti blocking layer. The energy level diagram patterns of CZTS films (c) without and (d) with 20 nm thick Ti blocking layer.
concentration of the Ti-doped CZTS film was lower than that of the pure CZTS film, which is consistent with the results of the Hall measurements. Fig. 10(a) shows the photocurrent density-voltage (J-V) curves of CZTS solar cells with Ti blocking layers of different thicknesses and Table 2 summarizes the photovoltaic parameters. The open circuit voltages increased from 407 to 541 mV with increasing Ti layer thickness from 0 to 20 nm. We believe there are three possible reasons for this increase in Voc. The Ti blocking layer can prevent S diffusion into Mo and therefore resulted in a reduction in the thickness of the MoS2 layer. The thin MoS2 layer results in a decrease in the back contact recombination and contact resistance (which result in decrease in back current and series resistance), for improving Voc and FF [28–30]. The second reason was the decrease in the surface roughness of the CZTS film, which was confirmed by AFM measurements. The CdS buffer can grow more uniformly on smoother surfaces and therefore enable the formation of a better p-n junction with the CZTS film. The third reason is that the increased crystallinity can reduce bulk recombination. However, with increasing Ti blocking layer thickness, Jsc first decreased, then increased, and then decreased again. With increasing Ti layer thickness from 5 nm to 20 nm, Jsc increased from 16.04 to 17.98 mA/cm 2, which is attributed to the increase in the mobility and light absorption (Fig. S7). The EQE is shown in Fig. 10(b). The calculated Jsc values obtained by integrating the product of the EQEs are 18.11, 15.80, 16.31, 17.34, 17.87 and 16.88 mA cm−2 which is in good agreement with the directly measured Jsc values from the J–V
characteristics (Table 2). For a Ti blocking layer with a thickness of 5 nm, a significant decrease in EQE was observed compared to the CZTS film without the Ti layer. With increasing thickness of Ti intermediate layers, the intensity of EQE slightly increased. It can be seen that the increase of Jsc with Ti thickness varying from 5 to 20 nm was mainly due to the enhancement over the entire visible region. This can be attributed to the presence of the Ti blocking layer resulting in a high quality CZTS layer (i.e., larger grains with less grain boundaries). However, the EQE decreased over the longer wavelength region from 700 to 900 nm. Because the light with long wavelength is absorbed on the back of the solar cell, while the electrons are collected at the top electrode, a low minority carrier length in p-type semiconductor results a low electron collection at the top electrode. This decay on long wavelength EQE was most likely caused by a low minority carrier diffusion length or insufficient penetration of the depletion width into the absorber [31,32]. 4. Conclusion We presented an easy and cost-effective approach to overcome limitations in CZTS solar cells related to defects in the interface between the CZTS and Mo layers by inserting an ultrathin Ti intermediate layer. Different from other reported blocking layers, the Ti layers played a dual role, acting as both a blocking layer and a dopant. During the sulfuration process, Ti was incorporated in the lattice of the CZTS film, improving crystallinity, surface evenness, mobility, and light
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Fig. 10. (a) The J-V characteristics of illuminated CZTS solar cell, (b) EQE measurements of the corresponding CZTS solar cell device.
Table 2 Photovoltaic parameters of the CZTS film solar cells with different thickness of Ti blocking layer, measured at 100 mW cm−2 (AM 1.5 G) light intensity. Ti thickness (nm)
0 5 10 15 20 25
CZTS film solar cells Voc(mV)
Jsc(mA/cm−2)
FF (%)
Eff (%)
407 446 476 510 541 460
18.26 16.04 16.71 17.67 17.98 16.76
34 36 36 38 41 35
2.52 2.57 2.86 3.62 3.98 2.7
absorption. The Ti blocking layer prevented the diffusion of S into the Mo films, thereby preventing the formation of MoS2. We demonstrated that the use of this Ti layer easily increased the efficiencies of the CZTS solar cells, particularly the Voc. This is a crucial finding in the quest to achieve improved performance of these devices.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant nos. 91648109, 51335002, 51572037, 51272033), the Priority Academic Program Development of Jiangsu Higher Education Institutions (2014), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant no. 14KJA430001).
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Dual function of ultrathin Ti intermediate layers in CZTS solar cells: Sulfur blocking and charge enhancement
MARK
⁎
Huafei Guoa, Changhao Maa, Kezhi Zhanga, Xuguang Jiaa, Xiuqing Wanga, Ningyi Yuana, , ⁎⁎ Jianning Dinga,b, a School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China b Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang 212013, China
A R T I C L E I N F O
A B S T R A C T
Keywords: CZTS solar cell Ti blocking layer Sulfur blockage Charge increase
An ultrathin Ti film acting as both an intermediate layer and a dopant was inserted at the interface between Mo and Cu2ZnSnS4 (CZTS) to improve the performance of CZTS solar cells. Different from previously reported blocking layers, the Ti intermediate layer inhibited the formation of a MoS2 layer and voids at the interface between the Mo and CZTS absorber and also influenced the crystallinity, surface evenness, Hall mobility, and absorptivity of the CZTS absorber. When the thickness of the Ti blocking layer increased to 20 nm, the conversion efficiency of the solar cell increased by 57%, along with an increase in the open-circuit voltage of 32%. The effect of the Ti layer on the microstructure and performance of the CZTS film is discussed here in detail. These results serve as guiding principles for preparing high-quality CZTS thin films for potential applications in low-cost solar cells.
1. Introduction Kesterite copper zinc tin sulfide (Cu2ZnSnS4; CZTS) thin films are attractive light absorbing materials for low-cost, large-scale production of highly efficient thin film solar cells. These materials comprise abundant, nontoxic, and inexpensive constituent elements, and also have an ideal band gap of about 1.5 eV with predicted maximum theoretical efficiencies over 30% [1–3]. Therefore, CZTS films are the most promising alternative to replace chalcopyrite absorbers, such as copper indium gallium selenide (CIGS), which includes rare metals (indium and gallium). However, to date, the efficiencies (η) of CZTS devices (12.6%) are much lower than those of CIGS (21.7%) [4,5]. The low open-circuit voltage (Voc) is the most critical impediment to achieving higher efficiencies in CZTS devices compared to the filling factor (FF) and short-circuit current density (Jsc). A thick intermediate layer of MoS2 formed between the back contact and absorber during the growth of CZTS has been reported as the cause of the low voltage [6,7]. Two approaches have been proposed to eliminate the detrimental effect of MoS2. The first is to replace Mo with a less reactive back contact material. Compared to other contact materials, Mo remains attractive due to its low price and high stability and has so far been used in all
record-performing devices. The other approach is to use an intermediate layer to prevent the reaction between CZTS and Mo. Several materials have been used as intermediate layers, including Ag [8], ZnO [9], TiN [7], TiB2 [10], and carbon [11]. For example, Shin et al. investigated a TiN intermediate layer in CZTS solar cells and showed that it reduced the MoSe2 thickness from ~1300 to ~220 nm and the series resistance from 3.4 to 1.8 Ω cm2. Li et al. inserted a metallic Ag intermediate layer in CZTS solar cells and found that the Ag layer inhibited the formation of voids between the Mo/CZTS interface. Lopez-Marino et al. applied a ZnO blocking layer to inhibit the decomposition of CZTS near the Mo back contact and reported that the ZnO layer improved the interfacial morphology and reduced the density of voids. Zhou et al. used an ultra-thin carbon film as the intermediate layer between Mo and CZTS and found that the carbon aggregates provided a better connection between the void-containing CZTS and the Mo back contact. To date, the effect of Ti intermediate layers in CZTS solar cell on the device performance has not been investigated. In this study, we used a magnetron sputtering method to prepare thin intermediate layers of Ti between CZTS and Mo. Different from the reported blocking layers, it is expected that the Ti film could play a dual role in retarding the reaction between S and Mo, as well as influencing
⁎
Corresponding author. Corresponding author at: School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China. E-mail addresses: [email protected] (N. Yuan), [email protected] (J. Ding). ⁎⁎
http://dx.doi.org/10.1016/j.solmat.2017.09.052 Received 20 May 2017; Received in revised form 25 September 2017; Accepted 29 September 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
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and Sn were sputtered by DC magnetron sputtering using powers of 6 W and 43 W, respectively, and ZnS was sputtered using a power of 90 W (RF magnetron) to obtain a precursor thickness of 750 nm. After sputtering, the samples were loaded into a horizontal tube furnace and annealed at 550 °C for 60 min with 50 mg of sulfur powder.
2.3. Device fabrication The Ti-incorporated CZTS solar cells were fabricated in the standard device structure of Mo/CZTS/CdS/i-ZnO/ITO/Ag grid. To fabricate this, a 60 nm CdS layer, a 50 nm intrinsic ZnO (i-ZnO) layer and a 230 nm ITO layer, were sequentially deposited by RF sputtering.
2.4. Characterization Fig. 1. The XRD patterns of sulfurided Mo films with and without the Ti blocking layer.
The crystallographic analysis of the films was performed using X-ray diffraction (XRD D/MAX Ultima III, Rigaku Corporation, Tokyo, Japan). The film morphology was investigated by scanning electron microscopy (SEM; 6360LA, JEOL, Japan). The elemental composition of CZTS was determined by energy-dispersive X-ray spectroscopy (EDS). The surface roughness of the Ti-doped CZTS thin films was measured by atomic force microscopy (AFM; Nanman VS). X-ray photoelectron spectroscopy (XPS; AXIS-ULTRA DLD) was performed using a monochromatic X-ray source (Al-Kα line of 1486.6 eV energy and 350 W power). Raman spectra were collected using a Renishaw in via Raman system using a 532 nm excitation wavelength (Thermo Fisher, USA). The electrical properties were analyzed using a Hall measurement system (QT-50, Quatek, Germany). The optical transmittance was measured using a UV–Vis spectrophotometer (UV-2250, Shimadzu, Japan). The ultraviolet photoelectron spectroscopy (UPS) was conducted with a SPECS Microwave UV light source (He I = 21.2 eV). A solar simulator (500 W Xe lamp) was employed as the light source, and the light intensity was adjusted with a Si reference solar cell to approximate the AM 1.5 solar spectrum. The external quantum efficiency (EQE) was measured with a Solar Cell Scan Photovoltaic characterization system (Solar Cell Scan, China).
physical, electrical, and optical properties of the CZTS thin films. The mechanism by which the Ti layer affects these properties is discussed here in detail. 2. Experimental section 2.1. Preparation of Ti blocking layers on Mo-coated soda lime glass substrates Thin Ti films with different thicknesses (0, 5, 10, 15, 20, and 25 nm) were deposited on soda-lime glass and Mo-coated glass substrates by radio frequency (RF) magnetron sputtering using an RF power of 60 W. 2.2. Preparation of CZTS thin films The CZTS thin films were prepared on the Ti layers by a two-step process. CZTS precursors were fabricated by co-sputtering with three different targets, i.e., Cu (99.99%), Sn (99.99%), and ZnS (99.99%). Cu
Fig. 2. The surface EDS mapping images of the CZTS film with 20 nm thick Ti blocking layer.
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Fig. 3. The XPS patterns of CZTS film with 20 nm thick Ti blocking layer.
3. Results and discussion
(002) plane of monoclinic TiS3 (JCPDS#: 36-1137). These results indicate that the Ti blocking layer prevented the diffusion of S into the Mo films, and thereby prevented the formation of MoS2. In order to investigate the influence of the Ti blocking layer on the performance of physical and chemical properties of the CZTS solar cells, EDS mapping and XPS were used to determine the elemental compositions of the CZTS films and their valences with and without the Ti blocking layer. The surface EDS spectra of CZTS with a Ti blocking layer are shown in Fig. 2. Five elements (Cu, Zn, Sn, Ti, and S) were clearly detected in the spectra while no signals attributable to Mo were detected. Fig. S1 shows the cross-sectional EDS mapping of a CZTS film with a 20 nm Ti blocking layer, which clearly indicates the diffusion of
First, the effect of the Ti blocking layer on the diffusion of S into the Mo back layer was investigated. Mo films with and without the Ti layer were subjected to sulfuration using the same conditions as those used for the CZTS sulfuration. Fig. 1 shows the XRD patterns of Mo films with and without Ti layers, after sulfuration. Without the present of Ti blocking layer, obvious MoS2 peaks were observed at 2θ = 14.3° and 33.5° (JCPDS#:65-1941), in addition to the Mo peaks at 2θ = 40.4°, 58.6°, and 73.6° (JCPDS#: 65-7742). However, with the Ti blocking layer on the Mo substrate, diffraction peaks of MoS2 were not observed. A new peak was observed at 2θ = 20.38°, which may correspond to the
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Fig. 4. (a) The XRD and (b) Raman patterns of CZTS films with different thick Ti blocking layers.
Ti into the CZTS film. Fig. S2 shows elemental distribution of Cu, Zn, Sn, Ti, and S in the CZTS film with a 20 nm Ti blocking layer using SEMEDS. To confirm the location of Ti in the CZTS lattice, XPS was used to
ascertain the stoichiometry and oxidation states of the constituent elements in the CZTS film with a 20 nm Ti blocking layer; these results are shown in Fig. 3. The Cu 2p peak was split into peaks at 931.6 eV (2p3/2) and 951.4 eV (2p1/2) with a splitting energy of 19.8 eV, which Fig. 5. The surface SEM images of CZTS films with different thickness of Ti blocking layers (a) 0 nm, (b) 5 nm, (c) 10 nm, (d) 15 nm, (e) 20 nm and (f) 25 nm.
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Fig. 6. The cross-sectional image of CZTS (a) without Ti blocking layer, (b) with 20 nm thick Ti blocking layer.
Fig. 7. The AFM images of CZTS films with different thickness of Ti blocking layers (a) 0 nm, (b) 5 nm, (c) 10 nm, (d) 15 nm, (e) 20 nm and (f) 25 nm.
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shown in Fig. S4. The grain sizes in the CZTS film increased with increasing thickness of the Ti blocking layers up to 25 nm. This indicates that Ti doping could promote grain growth in the CZTS films. The major XRD diffraction peaks of tetragonal CZTS are known to coincide with those of several other phases, such as cubic ZnS and tetragonal Cu2SnS3 [17,18]. Therefore, Raman spectroscopy was employed to further confirm the phase purity of CZTS. Fig. 4(b) shows the Raman spectra of the CZTS film with different thicknesses of the Ti blocking layer; three main peaks of CZTS are observed at approximately 284, 334, and 364 cm−1, whereas other binary and ternary compounds were not observed, indicating that all the prepared samples were pure with no miscellaneous phases [19–21]. Fig. 5(a–f) show surface SEM images of the CZTS films with different Ti blocking layer thicknesses. The CZTS film with no Ti blocking layer showed compact and well-formed grains; however, small pinholes and a rough surface were observed. After the introduction of the Ti blocking layer, these pinholes started to shrink and gradually disappeared [22–24]. The CZTS films with the Ti blocking layers exhibited improved crystallinity and densification (Fig. 5(b)). The SEM images shown in Fig. 5(c–f) indicate that the crystallinity and grain sizes increased gradually with increasing thickness of the Ti blocking layers, which is in good agreement with the XRD results. As it is known that when the grain size is increased, the density of grain boundaries will be decreased and the probability of electrons being trapped will be reduced. Fig. 6 shows cross-sectional SEM images of the CZTS films with and without Ti blocking layers. As shown in Fig. 6(a), without the Ti layer, a large number of voids formed at the back contact region, which could limit the free-carrier transportation. As shown in Fig. 6(b), with the introduction of the Ti intermediate layer, the voids formed at the back contact gradually disappeared and the crystallinity of the CZTS films increased, which is consistent with the XRD and surface SEM results. In order to confirm the change in the surface roughness of the CZTS films with different thicknesses of Ti blocking layers, AFM characterization was carried out and the obtained images are shown in Fig. 7. The surface roughness of the CZTS film increased from 51.9 to 57.2 nm with increasing Ti layer thickness from 0 to 10 nm. With a further increase in layer thickness, the surface roughness decreased from 57.2 to 45.4 nm, while the roughness of the CZTS film with the 25 nm Ti layer increased again to 58.6 nm; this trend is depicted in Fig. S5. The electrical properties of 1.0 µm CZTS films deposited on glass substrates with Ti blocking layers were measured using a Hall measurement system at 300 K. Fig. 8 presents hole concentration, hole mobility, and electrical resistivity data. As is evident from Fig. 8, the mobility increased, while the carrier concentration decreased, as the thickness of the Ti blocking layer increased from 0 to 25 nm. Table 1 summarizes the detailed electrical properties. All the CZTS films exhibited p-type conduction behaviors and Ti doping in CZTS resulted in a high mobility of 154.43 cm2 v−1 s−1. The Ti dopant reducing the hole concentration and enhancing the mobility of CZTS film. The high mobility of Ti-doped CZTS was attributed to the improved grain size, low porosity, and fewer scattering centers. Fig. 9(a) and (b) show the positions of the UPS valence band maximum (VBM) for the CZTS films with and without 20 nm Ti blocking layers. The VBM were obtained by extrapolating the leading edge of the valence band onset to the background level, below the Fermi level. The VBM of both CZTS films were determined to be 0.7 eV. Fig. 9(c) and (d) shows the energy level diagrams of CZTS films with and without the 20 nm Ti blocking layer. According to the band gap of these two CZTS films (Fig. S6 in the ESI), the location of the Fermi level was closer to VBM than the conduction band maxima (CBM), which implies that both the CZTS films were ptype semiconductors [25–27]. Comparing the energy levels of these two films indicated that the EF of the CZTS film with the 20 nm Ti blocking layer was more accessible to the CBM. This implies that the hole
Fig. 8. The electrical properties of CZTS films with different thickness of Ti blocking layers.
Table 1 The detailed electrical properties of CZTS thin film with different thickness of Ti blocking layer. Ti thickness (nm)
0 5 10 15 20 25
CZTS Resistivity (Ω cm)
2
Mobility (cm /Vs)
Carrier Density (cm−3)
62.65 87.07 115.65 159.32 220.8 340.2
2.85 7.21 42.86 76.67 109.86 154.43
7.05 2.34 8.54 5.01 2.15 4.34
× × × × × ×
Type of carrier
1016 1016 1015 1015 1015 1014
P P P P P P
is in good agreement with that of Cu(I) [12,13]. The Zn 2p peak was observed at binding energies of 1021.3 (2p3/2) and 1044.6 eV (2p1/2) with a splitting energy of 23.3 eV, indicative of Zn(II) [14]. The Sn 3d peak was split into peaks at 486.0 (2p5/2) and 494.4 eV (2p3/2) with a splitting energy of 8.5 eV, similar to the standard splitting energy of 8.4 eV for Sn 3d [15]. The S 2p spectral peaks were located at 161.4 (2p3/2) and 162.5 eV (2p1/2) with a separation of 1.1 eV, consistent with the 160–164 eV expected for S in sulfide phases. The Ti 2p binding energy was located at 473 eV, which indicates that the valence state of Ti in CZTS film was four. This suggests that Ti diffused into the CZTS film and occupied the Sn sites in the lattice. In order to further investigate the influence of Ti on the structural properties of the CZTS thin films, XRD, Raman spectroscopy, SEM, and AFM investigations were performed. Fig. 4(a) shows the XRD patterns of the CZTS films with Ti blocking layers of different thicknesses. For Ti thicknesses of 0, 5, 10, 15, 20, and 25 nm, diffraction peaks were observed at characteristic 2θ values corresponding to the kesterite structure of CZTS (JCPDS#:26-0575). Fig. S3 compares the (112) peaks of the CZTS absorbers with Ti blocking layers of different thicknesses. Obvious peak shifts were observed when the Ti blocking layer was introduced, which further confirms that Ti entered the lattice of the CZTS film. The grain sizes were calculated and averaged according to all the XRD peaks using the Debye-Scherrer equation [16]. The results are
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Fig. 9. UPS spectra of the valence band edge region for CZTS (a) without and (b)with 20 nm thick Ti blocking layer. The energy level diagram patterns of CZTS films (c) without and (d) with 20 nm thick Ti blocking layer.
concentration of the Ti-doped CZTS film was lower than that of the pure CZTS film, which is consistent with the results of the Hall measurements. Fig. 10(a) shows the photocurrent density-voltage (J-V) curves of CZTS solar cells with Ti blocking layers of different thicknesses and Table 2 summarizes the photovoltaic parameters. The open circuit voltages increased from 407 to 541 mV with increasing Ti layer thickness from 0 to 20 nm. We believe there are three possible reasons for this increase in Voc. The Ti blocking layer can prevent S diffusion into Mo and therefore resulted in a reduction in the thickness of the MoS2 layer. The thin MoS2 layer results in a decrease in the back contact recombination and contact resistance (which result in decrease in back current and series resistance), for improving Voc and FF [28–30]. The second reason was the decrease in the surface roughness of the CZTS film, which was confirmed by AFM measurements. The CdS buffer can grow more uniformly on smoother surfaces and therefore enable the formation of a better p-n junction with the CZTS film. The third reason is that the increased crystallinity can reduce bulk recombination. However, with increasing Ti blocking layer thickness, Jsc first decreased, then increased, and then decreased again. With increasing Ti layer thickness from 5 nm to 20 nm, Jsc increased from 16.04 to 17.98 mA/cm 2, which is attributed to the increase in the mobility and light absorption (Fig. S7). The EQE is shown in Fig. 10(b). The calculated Jsc values obtained by integrating the product of the EQEs are 18.11, 15.80, 16.31, 17.34, 17.87 and 16.88 mA cm−2 which is in good agreement with the directly measured Jsc values from the J–V
characteristics (Table 2). For a Ti blocking layer with a thickness of 5 nm, a significant decrease in EQE was observed compared to the CZTS film without the Ti layer. With increasing thickness of Ti intermediate layers, the intensity of EQE slightly increased. It can be seen that the increase of Jsc with Ti thickness varying from 5 to 20 nm was mainly due to the enhancement over the entire visible region. This can be attributed to the presence of the Ti blocking layer resulting in a high quality CZTS layer (i.e., larger grains with less grain boundaries). However, the EQE decreased over the longer wavelength region from 700 to 900 nm. Because the light with long wavelength is absorbed on the back of the solar cell, while the electrons are collected at the top electrode, a low minority carrier length in p-type semiconductor results a low electron collection at the top electrode. This decay on long wavelength EQE was most likely caused by a low minority carrier diffusion length or insufficient penetration of the depletion width into the absorber [31,32]. 4. Conclusion We presented an easy and cost-effective approach to overcome limitations in CZTS solar cells related to defects in the interface between the CZTS and Mo layers by inserting an ultrathin Ti intermediate layer. Different from other reported blocking layers, the Ti layers played a dual role, acting as both a blocking layer and a dopant. During the sulfuration process, Ti was incorporated in the lattice of the CZTS film, improving crystallinity, surface evenness, mobility, and light
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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2017.09.052. References [1] 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. Cells 95 (2011) 1421–1436. [2] D.B. Mitzi, O. Gunawan, T.K. Todorov, Barkhouse, Prospects and performance limitations for Cu–Zn–Sn–S–Se photovoltaic technology, Trans. R. Soc. A 371 (2013) 20110432. [3] C.S. Teinhagen, M.G. Panthani, V. Akhavan, B. Goodfellow, B. Koo, B.A. Korgel, Synthesis of Cu2ZnSnS4 nanocrystals for use in low-cost photovoltaics, J. Am. Chem. Soc. 131 (2009) 12554–12555. [4] W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, K.T. Todorov, Y. Zhu, D.B. Mitzi, Device Characteristics of CZTSSe thin-film solar cells with 12.6% efficiency, Adv. Energy Mater. 4 (2014) 1301465. [5] K. Kim, W. 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Chung, D.B. Dougherty, J.L. Jones, P.A. Maggard, CuNb1−xTaxO3 (x≤0.25) solid solutions: impact of Ta(V) substitution and Cu(I) deficiency on their structure, photocatalytic, and photo electrochemical properties, J. Mater. Chem. A 4 (2016) 3115–3126. [19] W.S. Enloe, J.C. Parker, J. Vespoli, T.H. Myers, R.L. Harper, J.F. Schetzina, An electroreflectance study of CdTe, J. Appl. Phys. 61 (1987) 2005–2010. (20) Z. Su, J.M.R. Tan, X. Li, X. Zeng, S.K. Batabyal, L.H. Wong, Cation substitution of solution-processed Cu2ZnSnS4 thin film solar cell with over 9% efficiency, Adv. Energy Mater. 5 (2015) 1500682. [21] W.C. Hsu, H. Zhou, S. Luo, T.B. Song, Y.T. Hsieh, H.S. Duan, S. Ye, W. Yang, C.J. Hsu, C. Jiang, B. Bob, Y. Yang, Spatial element distribution control in a fully solution-processed nanocrystals-based 8.6% Cu2ZnSn(S,Se)4 Device, ACS Nano 8 (2014) 9164–9172. [22] F. Liu, C. Yan, J. Huang, K. Sun, F. Zhou, J.A. Stride, M.A. Green, X. 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Fig. 10. (a) The J-V characteristics of illuminated CZTS solar cell, (b) EQE measurements of the corresponding CZTS solar cell device.
Table 2 Photovoltaic parameters of the CZTS film solar cells with different thickness of Ti blocking layer, measured at 100 mW cm−2 (AM 1.5 G) light intensity. Ti thickness (nm)
0 5 10 15 20 25
CZTS film solar cells Voc(mV)
Jsc(mA/cm−2)
FF (%)
Eff (%)
407 446 476 510 541 460
18.26 16.04 16.71 17.67 17.98 16.76
34 36 36 38 41 35
2.52 2.57 2.86 3.62 3.98 2.7
absorption. The Ti blocking layer prevented the diffusion of S into the Mo films, thereby preventing the formation of MoS2. We demonstrated that the use of this Ti layer easily increased the efficiencies of the CZTS solar cells, particularly the Voc. This is a crucial finding in the quest to achieve improved performance of these devices.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant nos. 91648109, 51335002, 51572037, 51272033), the Priority Academic Program Development of Jiangsu Higher Education Institutions (2014), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant no. 14KJA430001).
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