Fabrication and Characterization of Cu 2 ZnSnSe 4 Thin-Film Solar Cells using a Single-Stage Co-Evaporation Method: E ﬀ ects of Film Growth Temperatures on Device Performances

: Kesterite-structured Cu 2 ZnSnSe 4 (CZTSe) is considered as one of the Earth-abundant and non-toxic photovoltaic materials. CZTSe ﬁlms have been prepared using a single-step co-evaporation method at a relatively low temperature (i.e., below 500 ◦ C). Due to the volatile nature of tin-selenide, the control over substrate temperature (i.e., growth temperature) is very important in terms of the deposition of high-quality CZTSe ﬁlms. In this regard, the e ﬀ ects of growth temperatures on the CZTSe ﬁlm morphology were investigated. The suitable temperature range to deposit CZTSe ﬁlms with Cu-poor and Zn-rich compositions was 380–480 ◦ C. As the temperature increased, the surface roughness of the CZTSe ﬁlm decreased, which could improve p / n junction properties and associated device performances. Particularly, according to capacitance-voltage (C-V) and derived-level capacitance proﬁling (DLCP) measurements, the density of interfacial defects of CZTSe ﬁlm grown at 480 ◦ C showed the lowest value, of the order of ~3 × 10 15 cm − 3 . Regardless of applied growth temperatures, the formation of a MoSe 2 layer was rarely observed, since the growth temperature was not high enough to have a reaction between Mo back contact layers and CZTSe absorber layers. As a result, the photovoltaic (PV) device with CZTSe ﬁlm grown at 480 ◦ C yielded the best power conversion e ﬃ ciency of 6.47%. It is evident that the control over ﬁlm growth temperature is a critical factor for obtaining high-quality CZTSe ﬁlm prepared by one-step process.


Introduction
Climate changes induced by increased emission of greenhouse gases have become a global issue in the past decades. Currently, solar energy is recognized as one of the most promising options to address the issue [1,2]. Various photovoltaic (PV) solar cell technologies are currently being developed in three formats: (1) crystalline silicon (c-Si)-based, (2) thin film-based, and (3) emerging material-based technologies [3,4]. Even though the global PV market share of thin-film PV modules based on CdTe, Cu(In,Ga)Se 2 (CIGS), and a-Si solar cells is currently only less than 5%, while the c-Si-based market far

Experimental Details
The back electrode was prepared by depositing the molybdenum (Mo) on the 5 × 5 cm 2 soda-lime glass (SLG) with a 700 nm thickness using a direct-current magnetron spurting. A bilayer approach for the deposition of Mo on soda-lime glass (SLG) was adopted with a 280-nm-thick bottom layer at a high processing pressure for better adhesion and a 420-nm-thick top layer at a low pressure for a low resistivity.
The growth of CZTSe absorber films on Mo/SLG substrates was carried out by single-stage co-evaporation technique. CZTSe films were prepared on Mo-coated SLG substrates in a manner previously described [17]. Note that the co-evaporator was also used to deposit a pure sulfur CZTS film. CZTSe films were deposited at various growth temperatures by using four effusion cells for Cu, Zn, Sn, and Se elements. The growth temperature during film deposition was adjusted from 380 to 480 • C. A film with approximately 1 µm thickness was produced by a 90-min deposition process. When the substrate temperature was cooled down naturally, Zn, Sn, and Se elements were additionally supplied while the shutter of the Cu source was closed. The purpose of adopting the abovementioned conditions is to suppress Se vacancies in the film as well as the decomposition of the CZTSe film at relatively high temperature [18,19]. The entire temperature profile for the CZTSe film deposition is given as shown in Figure 1. The typical device structure for high-efficiency CIGS and CZTSSe PVs was employed throughout the studies [17,20]. A 60-nm-thick CdS buffer layer (n-type) was deposited on the CZTSe/Mo/SLG by using chemical bath deposition (CBD). The thickness of the CdS buffer layer was monitored via quartz crystal microbalance (QCM). For the window layer, an intrinsic zinc oxide with a 50 nm thickness and Al-doped zinc oxide with a 350 nm thickness were deposited by using a radiofrequency magnetron spurting on the CdS/CZTSe/Mo/SLG. Finally, the Al front grid was prepared by a thermal evaporator. The device area was 0.44 cm 2 .
The structural characterizations for CZTSe films were performed using an X-ray diffractometer (Rigaku D/Max-2500, Japan) and Raman spectroscopy (N8 NEOS SENTERRA, USA). To verify the film microstructure, scanning electron microscope (SEM) images were taken using a Hitachi S-4700 filed-emission SEM, Japan. Energy-dispersive X-ray spectroscopy (EDS) connected to SEM equipment was used at an accelerating voltage of 20 KeV to estimate the chemical composition of CZTSe films. Current-voltage measurements were carried out using a McScience K201-LAB 50, South Korea under AM 1.5G illumination with an intensity of 100 mW/cm 2 . The spectral responses were measured using a McScience K3100, South Korea. A calibrated Si solar cell was used as a reference for the device measurement. To estimate the carrier density of CZTSe film, both capacitance-voltage (C-V) and derived-level capacitance profiling (DLCP) were performed using a precision LCR meter (Agilent 4284A, USA) at 300 K.

Results and Discussion
The CZTSe films were grown at various temperatures ranging from 350 to 530 °C to determine a suitable growth temperature range for high-quality CZTSe films. As seen in Figure 2a,b, when the growth temperature was set to 530 °C, a continuous film was not formed. The CZTSe films grown below 480 °C showed compact and continuous morphologies, even if few voids were formed. Actually, there was no significant change in the grain sizes. However, as seen in Figure 2i,j, when the growth temperature was less than 350 °C, the ratio of (Zn)/(Sn) concentrations was more than 2 and that of (Cu)/((Zn)+(Sn)) was lower than 0.6, which are values outside of the optimal composition ranges (e.g., for the CZTSSe-based record device, (Cu)/((Zn)+(Sn))~0.8 and (Zn)/(Sn)~1.1 [11]. Given that high-quality films possess compact and large-grained morphologies with suitable Cu-poor and Zn-rich compositions, the required growth temperatures to enable device-quality CZTSe films prepared by one-step method seem to be in the temperature range of 380~480 °C, as depicted in Figure  2c,h. The poor film coverage with island grains and unfavorable chemical composition could be The typical device structure for high-efficiency CIGS and CZTSSe PVs was employed throughout the studies [17,20]. A 60-nm-thick CdS buffer layer (n-type) was deposited on the CZTSe/Mo/SLG by using chemical bath deposition (CBD). The thickness of the CdS buffer layer was monitored via quartz crystal microbalance (QCM). For the window layer, an intrinsic zinc oxide with a 50 nm thickness and Al-doped zinc oxide with a 350 nm thickness were deposited by using a radio-frequency magnetron spurting on the CdS/CZTSe/Mo/SLG. Finally, the Al front grid was prepared by a thermal evaporator. The device area was 0.44 cm 2 .
The structural characterizations for CZTSe films were performed using an X-ray diffractometer (Rigaku D/Max-2500, Japan) and Raman spectroscopy (N8 NEOS SENTERRA, USA). To verify the film microstructure, scanning electron microscope (SEM) images were taken using a Hitachi S-4700 filed-emission SEM, Japan. Energy-dispersive X-ray spectroscopy (EDS) connected to SEM equipment was used at an accelerating voltage of 20 KeV to estimate the chemical composition of CZTSe films. Current-voltage measurements were carried out using a McScience K201-LAB 50, South Korea under AM 1.5G illumination with an intensity of 100 mW/cm 2 . The spectral responses were measured using a McScience K3100, South Korea. A calibrated Si solar cell was used as a reference for the device measurement. To estimate the carrier density of CZTSe film, both capacitance-voltage (C-V) and derived-level capacitance profiling (DLCP) were performed using a precision LCR meter (Agilent 4284A, USA) at 300 K.

Results and Discussion
The CZTSe films were grown at various temperatures ranging from 350 to 530 • C to determine a suitable growth temperature range for high-quality CZTSe films. As seen in Figure 2a,b, when the growth temperature was set to 530 • C, a continuous film was not formed. The CZTSe films grown below 480 • C showed compact and continuous morphologies, even if few voids were formed. Actually, there was no significant change in the grain sizes. However, as seen in Figure 2i,j, when the growth temperature was less than 350 • C, the ratio of (Zn)/(Sn) concentrations was more than 2 and that of (Cu)/((Zn)+(Sn)) was lower than 0.6, which are values outside of the optimal composition ranges (e.g., for the CZTSSe-based record device, (Cu)/((Zn)+(Sn))~0.8 and (Zn)/(Sn)~1.1 [11]. Given that high-quality films possess compact and large-grained morphologies with suitable Cu-poor and Zn-rich compositions, the required growth temperatures to enable device-quality CZTSe films prepared by one-step method seem to be in the temperature range of 380~480 • C, as depicted in Figure 2c,h. The poor film coverage with island grains and unfavorable chemical composition could be associated with the high volatility of Sn-Se compounds during CZTSe film deposition [21,22]. Moreover, it was Energies 2020, 13, 1316 4 of 10 observed that as the growth temperature was increased from 380 to 480 • C, the surface roughness of the CZTSe films was reduced. The smoother surface could lead to an improvement in p/n junction properties and associated device performance.
X-ray diffraction patterns of CZTSe films grown at various growth temperatures were measured, as shown in Figure 3. Regardless of various applied growth temperatures, all XRD peaks were assigned to the phase of CZTSe material (JCPDS No. 52-0868) and the films were observed to have (112)preferred-orientation. This indicates that no secondary phases in the CZTSe films were observed. Nevertheless, the presence of secondary phases, such as ZnSe and Cu2SnSe3, in the films cannot be ruled out [23][24][25][26]. As seen in Figure 3b, the (112) peak was shifted toward a higher XRD diffraction angle when the film was deposited at the higher growth temperature. Such a peak shift seems to be related to the incorporation of sulfur residue from the co-evaporation chamber, since the coevaporator was used to deposit pure CZTS sulfide and the vaporizing temperature of sulfur is very low. To investigate the formation of secondary phases and the single-phase nature of CZTSe films, X-ray diffraction patterns of CZTSe films grown at various growth temperatures were measured, as shown in Figure 3. Regardless of various applied growth temperatures, all XRD peaks were assigned to the phase of CZTSe material (JCPDS No. 52-0868) and the films were observed to have (112)-preferred-orientation. This indicates that no secondary phases in the CZTSe films were observed. Nevertheless, the presence of secondary phases, such as ZnSe and Cu 2 SnSe 3 , in the films cannot be ruled out [23][24][25][26]. As seen in Figure 3b, the (112) peak was shifted toward a higher XRD diffraction Energies 2020, 13, 1316 5 of 10 angle when the film was deposited at the higher growth temperature. Such a peak shift seems to be related to the incorporation of sulfur residue from the co-evaporation chamber, since the co-evaporator was used to deposit pure CZTS sulfide and the vaporizing temperature of sulfur is very low.  To identify the formation of the secondary phases and impacts of various growth temperatures on CZTSe film, Raman scattering measurement was performed at room temperature with a 532 nm laser excitation. In literature, the A1 mode Raman peak of CZTSe and CZTS is located at 194-197 cm -1 and 322-327 cm −1 , respectively [27][28][29][30][31][32]. Thus, as seen in Figure 4, the primary main peak of the films is assigned to the Al peak for CZTSe and the second main peak seems to be assigned to the A1 peak for CZTS. Furthermore, the Al peak for CZTSe films is shifted as a function of growth temperature and the intensity of peak at ~ 324 cm −1 is increased without a peak shift. This observation is consistent with the XRD result, as seen in Figure 3b. In general, high growth temperature could induce a small amount of sulfur incorporation if a small amount of sulfur exists in the chamber. Actually, the incorporation of sulfur can influence the bandgap of CZTSe films and voltage output of PV devices.  To identify the formation of the secondary phases and impacts of various growth temperatures on CZTSe film, Raman scattering measurement was performed at room temperature with a 532 nm laser excitation. In literature, the A1 mode Raman peak of CZTSe and CZTS is located at 194-197 cm −1 and 322-327 cm −1 , respectively [27][28][29][30][31][32]. Thus, as seen in Figure 4, the primary main peak of the films is assigned to the Al peak for CZTSe and the second main peak seems to be assigned to the A1 peak for CZTS. Furthermore, the Al peak for CZTSe films is shifted as a function of growth temperature and the intensity of peak at~324 cm −1 is increased without a peak shift. This observation is consistent with the XRD result, as seen in Figure 3b. In general, high growth temperature could induce a small amount of sulfur incorporation if a small amount of sulfur exists in the chamber. Actually, the incorporation of sulfur can influence the bandgap of CZTSe films and voltage output of PV devices.  To identify the formation of the secondary phases and impacts of various growth temperatures on CZTSe film, Raman scattering measurement was performed at room temperature with a 532 nm laser excitation. In literature, the A1 mode Raman peak of CZTSe and CZTS is located at 194-197 cm -1 and 322-327 cm −1 , respectively [27][28][29][30][31][32]. Thus, as seen in Figure 4, the primary main peak of the films is assigned to the Al peak for CZTSe and the second main peak seems to be assigned to the A1 peak for CZTS. Furthermore, the Al peak for CZTSe films is shifted as a function of growth temperature and the intensity of peak at ~ 324 cm −1 is increased without a peak shift. This observation is consistent with the XRD result, as seen in Figure 3b. In general, high growth temperature could induce a small amount of sulfur incorporation if a small amount of sulfur exists in the chamber. Actually, the incorporation of sulfur can influence the bandgap of CZTSe films and voltage output of PV devices.  To evaluate the PV device performance with CZTSe films grown at various temperatures, the CZTSe film is incorporated as an absorber in the conventional structure for CIGS solar cells, for instance, ITO/i-ZnO window/CdS buffer/absorber/Mo back contact. The current density-voltage (J-V) characteristics of PV devices with a CZTSe film grown at various growth temperatures are shown in Figure 5a. In addition, the photovoltaic parameters measured from the J-V curves are listed in Table 1. Basically, all PV parameters and device efficiencies improved gradually with an increase in the growth temperature for CZTSe film. The device efficiency prepared at the higher growth temperature of 480 • C increased from 1.36% to 6.08% because of the improvement in both shunt resistance and saturated recombination current density (J 0 ), resulting in reduced V OC deficit (Table 1). Such improvements seem to be associated with the formation of a sturdy p/n junction by the reduction in the film surface roughness, as seen in Figure 2. In addition, the incorporated sulfur can affect device performance, since the cutoff of EQE spectra in the long-wavelength (Figure 5b) is decreased as a function of growth temperature. However, the current best cell is still lower than the highest value of 9.15% for a CZTSe film prepared by one-step method [13].
Energies 2020, 13, x FOR PEER REVIEW 6 of 9 To evaluate the PV device performance with CZTSe films grown at various temperatures, the CZTSe film is incorporated as an absorber in the conventional structure for CIGS solar cells, for instance, ITO/i-ZnO window/CdS buffer/absorber/Mo back contact. The current density-voltage (J-V) characteristics of PV devices with a CZTSe film grown at various growth temperatures are shown in Figure 5a. In addition, the photovoltaic parameters measured from the J-V curves are listed in Table. 1. Basically, all PV parameters and device efficiencies improved gradually with an increase in the growth temperature for CZTSe film. The device efficiency prepared at the higher growth temperature of 480 °C increased from 1.36% to 6.08% because of the improvement in both shunt resistance and saturated recombination current density (J0), resulting in reduced VOC deficit (Table 1). Such improvements seem to be associated with the formation of a sturdy p/n junction by the reduction in the film surface roughness, as seen in Figure 2. In addition, the incorporated sulfur can affect device performance, since the cutoff of EQE spectra in the long-wavelength (Figure 5b) is decreased as a function of growth temperature. However, the current best cell is still lower than the highest value of 9.15% for a CZTSe film prepared by one-step method [13].  Table 1. Summary of photovoltaic parameters of photovoltaic (PV) devices based on the CZTSe films prepared at various growth temperatures. The Voc deficit of Eg q -Voc was calculated with an estimated bandgap (Eg) from the EQE spectra fitting, as seen in Figure S1. The estimated bandgap (Eg) increased from 1.14 to 1.23 eV as the growth temperature of CZTSe film increased from 380 to 480 °C.

Sample
Effi. To understand defect properties of CZTSe PV devices, CV and DLCP measurements were performed. By fitting the CV and DLCP data ( Figure S2a,c), various information such as free carriers, interface, and bulk defect densities were estimated and summarized, as seen in Table 2. They indicate that the density of interfacial defect was decreased, showing less than <10 16 cm 3 as the growth temperature increased, while the density of bulk defects for all PV devices remained at a similar level. Additionally, the depletion width of the CZTSe film grown at 480 °C was relatively larger than one of films grown at the lower temperatures. Consequently, this means that interface recombination was more dominant compared to bulk recombination and may be related to surface roughness.  Table 1. Summary of photovoltaic parameters of photovoltaic (PV) devices based on the CZTSe films prepared at various growth temperatures. The Voc deficit of Eg q -Voc was calculated with an estimated bandgap (E g ) from the EQE spectra fitting, as seen in Figure S1. The estimated bandgap (E g ) increased from 1.14 to 1.23 eV as the growth temperature of CZTSe film increased from 380 to 480 • C.

Sample
Effi. To understand defect properties of CZTSe PV devices, CV and DLCP measurements were performed. By fitting the CV and DLCP data ( Figure S2a,c), various information such as free carriers, interface, and bulk defect densities were estimated and summarized, as seen in Table 2. They indicate that the density of interfacial defect was decreased, showing less than <10 16 cm 3 as the growth temperature increased, while the density of bulk defects for all PV devices remained at a similar level. Additionally, the depletion width of the CZTSe film grown at 480 • C was relatively larger than one of films grown at the lower temperatures. Consequently, this means that interface recombination was more dominant compared to bulk recombination and may be related to surface roughness.  Figure 6 shows that when MgF 2 anti-reflection coating with a 100 nm thickness was applied, the device efficiency was slightly increased from 6.04% to 6.47% due to an increase in the spectral response at long wavelength, as shown in Figure 6b. Further studies may be needed to gain deeper insights into defect passivation by post-deposition treatment of alkali elements (e.g., Na, K, Rb, and Cs) [33][34][35]. Additionally, in order to increase the spectral response at the short wavelength region, a Zn(O,S) buffer layer can be considered as a wide bandgap and Cd-free material [36][37][38].
Energies 2020, 13, x FOR PEER REVIEW 7 of 9 70.5 Figure 6 shows that when MgF2 anti-reflection coating with a 100 nm thickness was applied, the device efficiency was slightly increased from 6.04% to 6.47% due to an increase in the spectral response at long wavelength, as shown in Figure 6b. Further studies may be needed to gain deeper insights into defect passivation by post-deposition treatment of alkali elements (e.g., Na, K, Rb, and Cs) [33][34][35]. Additionally, in order to increase the spectral response at the short wavelength region, a Zn(O,S) buffer layer can be considered as a wide bandgap and Cd-free material [36][37][38].

Conclusions
Earth-abundant Cu2ZnSnSe4 films were deposited on Mo-coated SLG substrates by using a single-stage co-evaporation method. The films were grown at various growth temperatures to determine a suitable growth temperature range for high-quality CZTSe films. When the films were grown at 380~480 °C, XRD and Raman data indicate that all films adopted a kesterite structure with I-4 space group without observation of secondary phase formation. Most of all, the surface roughness of the CZTSe film grown at the higher temperature of 480 °C was significantly decreased. As a result, the PV device efficiency was increased up to 6.47%, since both shunt resistance and saturated recombination current density were improved, leading to a reduction in the Voc deficit. According to C-V and DLCP measurements, the increase in the open circuit voltage of the device is consistent with a reduction in the density of interfacial defects. Overall, our results suggest that control over film growth temperature is a critical factor for obtaining high-quality CZTSe film prepared by onestep process. Moreover, alkali post-deposition treatment may enable improved device efficiency through defect passivation. By using a wider bandgap buffer layer (e.g., Zn(O,S)), spectral responses in the short wavelength region could be enhanced. Overall, such technologies may offer a pathway to higher device efficiency.

Conclusions
Earth-abundant Cu 2 ZnSnSe 4 films were deposited on Mo-coated SLG substrates by using a single-stage co-evaporation method. The films were grown at various growth temperatures to determine a suitable growth temperature range for high-quality CZTSe films. When the films were grown at 380~480 • C, XRD and Raman data indicate that all films adopted a kesterite structure with I-4 space group without observation of secondary phase formation. Most of all, the surface roughness of the CZTSe film grown at the higher temperature of 480 • C was significantly decreased. As a result, the PV device efficiency was increased up to 6.47%, since both shunt resistance and saturated recombination current density were improved, leading to a reduction in the Voc deficit. According to C-V and DLCP measurements, the increase in the open circuit voltage of the device is consistent with a reduction in the density of interfacial defects. Overall, our results suggest that control over film growth temperature is a critical factor for obtaining high-quality CZTSe film prepared by one-step process. Moreover, alkali post-deposition treatment may enable improved device efficiency through defect passivation. By using a wider bandgap buffer layer (e.g., Zn(O,S)), spectral responses in the short Energies 2020, 13, 1316 8 of 10 wavelength region could be enhanced. Overall, such technologies may offer a pathway to higher device efficiency.