Electrodeposition of CdTe Thin Films for Solar Energy Water Splitting.

CdTe thin films have been prepared by electrochemical deposition. The morphological, structural, and optical properties of CdTe thin films deposited with different deposition time were investigated, and the influence of film thickness on the photoelectric characteristics of CdTe thin films was studied. At the deposition time of 1.5 h, CdTe thin films had good optical properties and the photocurrent reached 20 μAcm−2. Furthermore, the Pt/CdS/CdTe/FTO structure was prepared to improve its PEC stability and the photocurrent of 240 μAcm−2 had been achieved.


Introduction
Energy shortage has become the primary problem hindering economic development and world peace and is a focus of attention of all countries in the world. Traditional fossil energy is not only limited, but also has released a great deal of pollution to the environment. As a kind of clean and renewable energy, solar energy has attracted a lot of attention. By utilizing solar energy, the photoelectrochemical (PEC) splitting of water can directly generate hydrogen in a relatively simple process [1]. Cadmium telluride (CdTe) has a number of attractive properties as a photocathode material for PEC water splitting and absorbing materials in photovoltaic cells [2]. It has a direct band gap of 1.45 eV and high light absorption coefficient, which can reach 10 4 cm −1 in the visible light range [3][4][5].
CdTe thin films can be prepared with various methods, such as near space sublimation [6], magnetron sputtering [7], vapor transport deposition [8], and so on. Among these methods, electrochemical deposition is considered to be an ideal method for mass production of CdTe films with easy operation and high material utilization [9,10]. There are several advantages for the electrodeposition process. For example, it is easy to operate without high vacuum or a high temperature environment. Both p-type and n-type CdTe have been easily deposited and electrodeposition potential was found to be the key factor [11]. Novel morphologies, such as CdTe nanowires, can be deposited easily by electrochemical deposition method [12,13]. ZnO/CdTe core-shell nanotube arrays have also been synthesized by using a simple two-step electrochemical deposition strategy for solar energy water splitting applications [14].
Even though electrodeposited CdTe thin films and their PEC properties have been reported [15], the structure of CdTe photocathode still needs optimization for solar energy water splitting application. There are some clusters and porous structures at the surface of the deposited CdTe thin films in Figure 1a2-d2. The composition ratios of deposited CdTe thin films were estimated by EDS measurements, as shown in Table 1. Even though the CdCl2 annealing treatment can make Te in the film combine with Cd from CdCl2 to form CdTe, the ratios of Cd/Te of all the samples still indicate that Te-rich CdTe thin films were obtained in our work. There are some clusters and porous structures at the surface of the deposited CdTe thin films in Figure 1a2-d2. The composition ratios of deposited CdTe thin films were estimated by EDS measurements, as shown in Table 1. Even though the CdCl 2 annealing treatment can make Te in the film combine with Cd from CdCl 2 to form CdTe, the ratios of Cd/Te of all the samples still indicate that Te-rich CdTe thin films were obtained in our work.

Structural Properties of Deposited CdTe Thin Films
The structural properties of the CdTe films were further investigated by XRD characterizations, as shown in Figure 2a.

Structural Properties of Deposited CdTe Thin Films
The structural properties of the CdTe films were further investigated by XRD characterizations， as shown in Figure 2a. By comparison with the standard card (JCPDS 15-0770), it can be seen that the deposited CdTe thin films have diffraction peaks at 2θ = 23.7°, 39.2°, 46.4°, 56.8°, 62.3°, and 71.2°, which correspond to (111), (220), (311), (400), (331), and (422) planes of CdTe, respectively. Comparing with standard PDF card (JCPDS 42-1445), it is speculated that the peaks at 2θ = 26.8°, 33.6°, 38°, 51.8°, and 65° may be caused by substrate SnO2 [19,20]. Peaks at 2θ = 26.8°, 38°, 51.8°, and 65° can also correspond to TeO2 (JCPDS 42-1365). The CdTe thin films deposited with 1.5 h have strong diffraction peaks on (111), (220), and (311) planes. Compared with XRD patterns of CdTe thin films deposited with 1 h, the miscellaneous peaks of the diffraction pattern are significantly reduced, which proves that the quality of the prepared CdTe thin films is improved. The CdTe thin films deposited with 2 h have strong diffraction peaks on the (111), (220), and (311) planes, and the diffraction intensity of the miscellaneous peak is further reduced, which proves that the quality of the prepared CdTe thin films is further improved. The XRD analysis of the thin films deposited with 2.5 h shows that the diffraction peaks of planes (111), (220), and (311) are particularly strong, which perfectly corresponds to the peak positions shown in PDF card (JCPDS 15-0770). According to the Debye Scherrer formula [21]: where K is the Scherrer constant, λ is the wavelength of X-rays used (λ = 0.15 nm), β is the full-width at half maximum (FWHM) of the diffraction peaks, and θ is Bragg's angle. The average grain sizes and lattice spacing of (111) plane can be calculated, as shown in Table 1. Compared with the card of PDF (JCPDS 15-0770), it is found that the prepared thin films are of face-centric cubic structure, and the crystalline space group is f-43 m, which is the same as the reported CdTe crystal structure. Raman diffraction patterns of CdTe thin films deposited with different deposition time are shown in Figure 2b. The Raman peaks at the position of 164 cm −1 and 327.5 cm −1 are consistent with the reported Raman peaks of CdTe thin films [22,23]. All the deposited CdTe thin films also have peaks at 139.9 cm −1 , which is a combination of TO (CdTe) and elemental Te [24]. It indicates again that Te-rich CdTe thin films have been deposited in our work.   [19,20]. Peaks at 2θ = 26.8 • , 38 • , 51.8 • , and 65 • can also correspond to TeO 2 (JCPDS 42-1365) . The CdTe thin films deposited with 1.5 h have strong diffraction peaks on (111), (220), and (311) planes. Compared with XRD patterns of CdTe thin films deposited with 1 h, the miscellaneous peaks of the diffraction pattern are significantly reduced, which proves that the quality of the prepared CdTe thin films is improved. The CdTe thin films deposited with 2 h have strong diffraction peaks on the (111), (220), and (311) planes, and the diffraction intensity of the miscellaneous peak is further reduced, which proves that the quality of the prepared CdTe thin films is further improved. The XRD analysis of the thin films deposited with 2.5 h shows that the diffraction peaks of planes (111), (220), and (311) are particularly strong, which perfectly corresponds to the peak positions shown in PDF card (JCPDS 15-0770). According to the Debye Scherrer formula [21]:

Optical Properties of Deposited CdTe Thin Films
where K is the Scherrer constant, λ is the wavelength of X-rays used (λ = 0.15 nm), β is the full-width at half maximum (FWHM) of the diffraction peaks, and θ is Bragg's angle. The average grain sizes and lattice spacing of (111) plane can be calculated, as shown in Table 1. Compared with the card of PDF (JCPDS 15-0770), it is found that the prepared thin films are of face-centric cubic structure, and the crystalline space group is f-43 m, which is the same as the reported CdTe crystal structure. Raman diffraction patterns of CdTe thin films deposited with different deposition time are shown in Figure 2b. The Raman peaks at the position of 164 cm −1 and 327.5 cm −1 are consistent with the reported Raman peaks of CdTe thin films [22,23]. All the deposited CdTe thin films also have peaks at 139.9 cm −1 , which is a combination of TO (CdTe) and elemental Te [24]. It indicates again that Te-rich CdTe thin films have been deposited in our work.  Figure 3 shows the UV-Vis diffuse-reflection spectra of CdTe thin films with deposition time of 1 h, 1.5 h, 2 h and 2.5 h, respectively. From the images, it can be seen that CdTe thin films have strong absorption of visible light.

Optical Properties of Deposited CdTe Thin Films
Materials 2020, 13, x FOR PEER REVIEW 5 of 9 Figure 3 shows the UV-Vis diffuse-reflection spectra of CdTe thin films with deposition time of 1 h, 1.5 h, 2 h and 2.5 h, respectively. From the images, it can be seen that CdTe thin films have strong absorption of visible light. And the optical band gaps of the as-deposited thin films were calculated by diffuse-reflection spectra according to the following equation [25]: In the equation, α was the optical absorption coefficient, hv was the photoelectron energy, Eg was the band gap width, and K was a constant of the material. According to the above equation, CdTe band gaps were calculated as 1.66 eV, 1.48 eV, 1.35 eV, and 1.32 eV, as shown in Figure 3b. The data were consistent with the previous literature reports [12,13].

Photoelectrochemical Properties of Deposited CdTe Thin Films
AC impedance test was carried out and the results were shown in Figure 4a1-a4. The test was performed with a three-electrode configuration. Solution resistance Ru between reference electrode and working electrode, double layer capacitance Cd, and charge transfer resistance Rct can be obtained from the test. Rct and Ru of four groups of CdTe thin films in the system were calculated by following formula [26]: And the optical band gaps of the as-deposited thin films were calculated by diffuse-reflection spectra according to the following equation [25]: In the equation, α was the optical absorption coefficient, hv was the photoelectron energy, Eg was the band gap width, and K was a constant of the material. According to the above equation, CdTe band gaps were calculated as 1.66 eV, 1.48 eV, 1.35 eV, and 1.32 eV, as shown in Figure 3b. The data were consistent with the previous literature reports [12,13].

Photoelectrochemical Properties of Deposited CdTe Thin Films
AC impedance test was carried out and the results were shown in Figure 4a1-a4.
Materials 2020, 13, x FOR PEER REVIEW 5 of 9 Figure 3 shows the UV-Vis diffuse-reflection spectra of CdTe thin films with deposition time of 1 h, 1.5 h, 2 h and 2.5 h, respectively. From the images, it can be seen that CdTe thin films have strong absorption of visible light. And the optical band gaps of the as-deposited thin films were calculated by diffuse-reflection spectra according to the following equation [25]: In the equation, α was the optical absorption coefficient, hv was the photoelectron energy, Eg was the band gap width, and K was a constant of the material. According to the above equation, CdTe band gaps were calculated as 1.66 eV, 1.48 eV, 1.35 eV, and 1.32 eV, as shown in Figure 3b. The data were consistent with the previous literature reports [12,13].

Photoelectrochemical Properties of Deposited CdTe Thin Films
AC impedance test was carried out and the results were shown in Figure 4a1-a4. The test was performed with a three-electrode configuration. Solution resistance Ru between reference electrode and working electrode, double layer capacitance Cd, and charge transfer resistance Rct can be obtained from the test. Rct and Ru of four groups of CdTe thin films in the system were calculated by following formula [26]: The test was performed with a three-electrode configuration. Solution resistance R u between reference electrode and working electrode, double layer capacitance C d , and charge transfer resistance Materials 2020, 13, 1536 6 of 9 R ct can be obtained from the test. R ct and R u of four groups of CdTe thin films in the system were calculated by following formula [26]: where ω was the frequency, Z' was the real part of the impedance, Z" was the real part of the impedance. As shown in Table 2, the change of R u is not obvious. Generally speaking, the test solution does not change, nor does R u . However, there are errors in the measurements, such as the distance between electrodes and the samples in our experiments, the direction of the sample according to the counter electrode, and so on. As deposition time increases, R ct decreases, which may be caused by film thickening and grain size enlargement. Under chopped AM 1.5 G light illumination (Newport, Oriel Instruments, optical density = 100 mWcm −2 ), the PEC properties of the films were measured in 0.5 mol/L Na 2 SO 4 solution by an electrochemical work station (CHI 660B). The tests were measured from −0.3 V to 0.3 V and the scanning rate was 0.01 V/s. Finally, the relationship between current and voltage (I-V curves) was presented in Figure 5a.
where ω was the frequency, Z' was the real part of the impedance, Z'' was the real part of the impedance. As shown in Table 2, the change of Ru is not obvious. Generally speaking, the test solution does not change, nor does Ru. However, there are errors in the measurements, such as the distance between electrodes and the samples in our experiments, the direction of the sample according to the counter electrode, and so on. As deposition time increases, Rct decreases, which may be caused by film thickening and grain size enlargement. Under chopped AM 1.5 G light illumination (Newport, Oriel Instruments, optical density = 100 mWcm −2 ), the PEC properties of the films were measured in 0.5 mol/L Na2SO4 solution by an electrochemical work station (CHI 660B). The tests were measured from −0.3 V to 0.3 V and the scanning rate was 0.01 V/s. Finally, the relationship between current and voltage (I-V curves) was presented in Figure 5a. It can be seen from the I-V curves that the maximum current difference appears at −0.3 V and this potential is negative, which proves that the prepared CdTe film is p-type material. In Figure 5b, with the increase of deposition time, the photocurrent tends to stabilize at around 25 µ Acm −2 . By comparing the difference of photo and dark current in Table 2, it can be found that the true photo response current first increases and then decreases with the increasing of deposition time. It can be concluded that maximum current can be achieved when the deposition time is 1.5 h and 2 h. The photocurrents are first increased and then decreased, rather than increasing with the deposition time. As known, when the light is irradiated on the semiconductor film, electron and hole pairs are excited inside the semiconductor film. If the film is too thick, the electron-hole pairs will be more likely to recombine before they move to the surface of the films, which will decrease the PEC It can be seen from the I-V curves that the maximum current difference appears at −0.3 V and this potential is negative, which proves that the prepared CdTe film is p-type material. In Figure 5b, with the increase of deposition time, the photocurrent tends to stabilize at around 25 µAcm −2 . By comparing the difference of photo and dark current in Table 2, it can be found that the true photo response current first increases and then decreases with the increasing of deposition time. It can be concluded that maximum current can be achieved when the deposition time is 1.5 h and 2 h. The photocurrents are first increased and then decreased, rather than increasing with the deposition time. As known, when the light is irradiated on the semiconductor film, electron and hole pairs are excited inside the semiconductor film. If the film is too thick, the electron-hole pairs will be more likely to recombine before they move to the surface of the films, which will decrease the PEC performance of deposited CdTe thin films [27]. 15 nm CdS film was grown on CdTe film by chemical bath deposition and a 5 nm Pt layer was sputtered on the CdS surface using a sputtering method. The PEC properties of the Pt/CdS/CdTe/FTO structure were characterized, as shown in Figure 6.
Materials 2020, 13, x FOR PEER REVIEW 7 of 9 performance of deposited CdTe thin films [27]. 15 nm CdS film was grown on CdTe film by chemical bath deposition and a 5 nm Pt layer was sputtered on the CdS surface using a sputtering method. The PEC properties of the Pt/CdS/CdTe/FTO structure were characterized, as shown in Figure 6. The photocurrents are enhanced greatly and the difference of the light-dark current is increased to 240 µ Acm −2 . The biggest difference of the light-dark current of our deposited CdTe thin film is 20.1 µ Acm −2 . The difference of the light-dark current is enhanced about 12 times after fabrication of the Pt/CdS/CdTe/FTO structure, which demonstrates that this structure can improve the PEC performance of CdTe thin films greatly.

Conclusions
Te-rich CdTe films have been deposited with the electrodeposition method. SEM studies have shown that the film thickness increases with the deposition time, and the deposition time should be controlled at about 1.5 h-2 h to obtain films with good morphology. The annealed CdTe thin films deposited at 1.5 h-2 h have the largest photocurrent and have good PEC performance. The Pt/CdS/CdTe/FTO structure can improve the PEC properties greatly, and the highest photocurrent of 240 µ Acm −2 has been achieved.