Article pubs.acs.org/cm
Improving the Performance of Solution-Processed Cu2ZnSn(S,Se)4 Photovoltaic Materials by Cd2+ Substitution Jie Fu,†,‡ Qingwen Tian,*,†,‡ Zhengji Zhou,†,‡ Dongxing Kou,†,‡ Yuena Meng,†,‡ Wenhui Zhou,†,‡ and Sixin Wu*,†,‡ †
The Key Laboratory for Special Functional Materials of MOE, Henan University, Kaifeng, Henan 475004, China Collaborative Innovation Center of Nano Functional Materials and Applications, Henan University, Kaifeng 475004, Henan Province, China
‡
S Supporting Information *
ABSTRACT: Additional elements in the Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers can play a crucial role in improving the performance of thin film solar cells. In this paper, a significant performance enhancement of CZTSSe thin film solar cells was achieved by the partial substitution of the Zn2+ cation with Cd2+. A small amount of Cd2+ can be successfully incorporated into the host lattice of CZTSSe to form a homogeneous Cu2Zn1−xCdxSn(S,Se)4 (CZCTSSe) alloy material. We demonstrated that the crystal growth and the band gap of CZCTSSe thin films are affected by the Cd doping level. Additionally, the impact of Cd content on the space-charge density (Nc‑v) and the depletion width (Wd) of CZCTSSe solar cells was systematically investigated. By this cation substitution approach, the power conversion efficiency of the solar cells based on the CZCTSSe absorber was successfully increased from 5.41 to 8.11% for the optimal composition (x = 5%).
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INTRODUCTION Kesterite, Cu2ZnSn(S,Se)4 (CZTSSe), has been considered one of the most promising absorber materials for thin film solar cell applications.1−4 During the past decade, a remarkable progress was achieved in photovoltaic applications, with a power conversion efficiency (PCE) improving from ∼5% in 2004 to 12.7% in 2014.5 However, the CZTSSe record efficiency of 12.7% is still far lower than the value of 21.7% reported for Cu(In,Ga)(S,Se)2 (CIGSSe) chalcogenide-based solar cells.6 The major limitation and intrinsic factor in CZTSSe solar cells have been accurately addressed by various experimental and theoretical studies; these include the high antisite defect density, the fluctuating potential of valence and conduction bands, the poor crystal growth, and the narrow phase stability region.7−10 Indeed, there is an urgent need to further improve the current cell performance to a level comparable to that obtained with CIGSSe devices. To improve the microstructure and electronic properties of CZTSSe absorbers, several chemical elements have been introduced as impurities, traces, or even sizable amounts. The most noticeable studies are based on the doping with antimony (Sb) and alkali metals (Na and K) of CIGSSe and CZTSSe thin film solar cells; this has proven to be beneficial in improving the level of crystallinity and influencing the number of charge carriers in thin films.11−16 However, it has been reported that these elements often segregate to CZTSSe surfaces and grain boundaries rather than enter host lattice, because of high formation energies and unmatched ionic radii. Meanwhile, it is © 2016 American Chemical Society
demonstrated that they have a weak effect on tuning band gaps.17,18 The current champion CZTSSe and Cu(In,Ga)Se2 (CIGSe) solar cells have the same band gap (Eg) of 1.13 eV.5,6 Eg values of ∼1.20 eV were frequently obtained in solutionprocessed CZTSSe thin films deposited following a two-stage annealing route.19−22 An appealing way to reach a band gap value close to the optimum of 1.13 eV is to reduce the S/Se ratio; however, this cannot be finely controlled during the typical selenization process.23,24 Another approach to tuning the band gap and affecting the electronic properties of the CZTSSe absorber layer is by metal cation substitution. Recently, the cation substitution of LiCu and GeSn in CZTSSe thin films enhanced the band gap and improved the opencircuit voltage (Voc).25,26 These strategies are thus not suitable for reducing the Eg to the desired optimal values. Therefore, seeking a versatile cation substitution approach to reducing the band gap and alleviating the current limitation of the CZTSSe thin film is still of great importance. Su et al. used the larger Cd atom to partially replace Zn to produce a Cu2Zn1−xCdxSnS4 (CZCTS) thin film, and the device PCE increased from ∼5 to >9% at the optimal Cu2Zn0.6Cd0.4SnS4 composition.27,28 Although Cd is an outstanding substitution agent, the use of such a large Cd proportion in the Cu2Zn0.6Cd0.4SnS4 thin film is Received: May 25, 2016 Revised: July 29, 2016 Published: July 31, 2016 5821
DOI: 10.1021/acs.chemmater.6b02111 Chem. Mater. 2016, 28, 5821−5828
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Chemistry of Materials
system with a 532 nm wavelength excitation laser. The scanning electron microscope (SEM) images were collected with a Nova Nano SEM 450, which was equipped with an energy dispersive X-ray (EDX) analyzer (Nano SEM 45050/EDAX). Photocurrent density−voltage curves (J−V curves) were recorded with a Keithley 2400 source meter. The light intensity was standardized to 100 mW cm−2 by using a Newport optical power meter (model 842-PE). The external quantum efficiency (EQE) curves were measured by using the Zolix SCS100 QE system. The C−V characterization curves were measured with a HERA-DLTS system (Phys Tech). The valence states of Cu, Zn, Cd, Sn, S, and Se elements were determined by X-ray photoelectron spectroscopy (XPS) spectra that were recorded with a thermo ESCALAB 250 spectrometer using an Al Kα monochromatized source and a multidetection analyzer under a residual pressure of 10−8 Pa.
not compatible with the need to develop an environmentally friendly route. Here, we propose a low-cost solution-based method for depositing CZTSSe thin films with a partial cation substitution of Zn2+ with Cd2+. A small amount of Cd2+ was successfully incorporated into the host lattice of CZTSSe thin films to form the Cu2Zn1−xCdxSn(S,Se)4 (CZCTSSe, with an optimal doping level of x = 5%) semiconductor materials. The surface morphology and crystal growth of the CZTSSe thin films are found to be remarkably improved by the Cd2+ substitution, especially at the optimal Cd/(Cd + Zn) ratio. Changes in the band gap (Eg), the space-charge density (Nc‑v), and the depletion width (Wd) with Cd content in the CZCTSSe thin films were systematically investigated. The low-energy potential barrier for the formation of CuZn and ZnCu antisite defects has been attributed to the similar covalent radii of Cu and Zn.26 For this reason, the partial substitution of Zn2+ with larger Cd2+ ions possibly inhibits the formation of CuZn and ZnCu antisite defects.28−31
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RESULTS AND DISCUSSION Characterization of Materials. In this paper, we employed a low-cost solution-based approach to fabricate the CZCTSSe thin films. CuO, ZnCl2 (CdCl2·2.5H2O), and SnCl2·2H2O were used as starting materials and dissolved in a solution of ethanol, 1-butylamine, and CS2, forming a series of CZCTSSe precursor solutions with different Cd/(Cd + Zn) ratios. The representative Cu2Zn1−xCdxSnS4 (x = 5%) precursor solution was selected to perform TGA (see Figure S1). TGA shows that the CZCTS precursor is almost completely decomposed at ∼320 °C. Thus, the annealing temperature of the precursor film was fixed at 320 °C. Because of the similar metal−sulfur bond enthalpies, the TGA curve of the CZCTS precursor corresponds well to those reported in the literature for other metal sulfide precursors.20,27,32−35 XRD characterizations of CZCTSSe thin films with x ranging from 0 to 0.2 are displayed in Figure 1a. Apart from the Mo
EXPERIMENTAL SECTION
Materials. Copper oxide (CuO, 99.99%) was purchased from ZhongNuo Advanced Material Technology Co. Zinc chloride (ZnCl2, 99.99%) and tin chloride (SnCl2·2H2O, 98%) were obtained from Alfa Aesar Chemical Co. Cadmium chloride (CdCl2·2.5H2O, 99%), carbon disulfide (CS2, 99.9%), 1-butylamine [CH3(CH3)3NH2, 99%], cadmium sulfate (CdSO4·8/3H2O, 99%), thiourea (NH2CSNH2, 99%), selenium (Se, 99.9%), and ethanol (CH3CH2OH, AR) were purchased from Aladdin Co. Synthesis of a CZCTS Precursor Solution. To prepare a typical CZCTS precursor solution, 5 mL of ethanol, 3 mL of 1-butylamine, and 2 mL of CS2 were mixed in a sample bottle while being magnetically stirred at room temperature. Afterward, 1.6 mmol of CuO (∼0.127 g), 1 mmol of ZnCl2 and CdCl2·2.5H2O, and 1.1 mmol of SnCl2·2H2O (∼0.237 g) were added in sequence and magnetically stirred at 50 °C for 2 h until all the solids had dissolved. Finally, the precursor solution was centrifuged at 10000 rpm for 5 min to remove impurities. All preparation processes were performed in air. Deposition and Selenization of CZCTS Films. CZTS and CZCTS precursor thin films were prepared by spin-coating the asprepared precursor solution on 20 mm × 20 mm × 1.1 mm molybdenum-coated soda lime glasses (SLG) at 2500 rpm for 20 s. After spin-coating each layer, the coated substrate was sintered on a hot plate at 320 °C for 1 min. This procedure was repeated six times to reach the desired film thickness (∼1.3 μm). All the operations described above were performed in an argon-filled glovebox. CZTS and CZCTS precursor thin films were then selenized in a round graphite box with 300 mg of selenium at 550 °C for 15 min (see Figure S7). Fabrication of CZCTSSe Photovoltaic Devices. CZTSSe and CZCTSSe photovoltaic devices were fabricated according to the structure glass/Mo/CZCTSSe/CdS/i-ZnO/ITO/Ag. A 60 nm thick cadmium sulfide (CdS) layer was deposited onto the SLG/Mo/ CZCTSSe films by an improved chemical bath deposition method; 50 mL of cadmium sulfate (0.006 M), 12 mL of NH3·H2O, and 50 mL of thiourea (0.03 M) were added to 150 mL of deionized H2O in a 65 °C water bath for 12 min.20 Then, 50 nm intrinsic ZnO (i-ZnO) and 200 nm indium tin oxide (ITO) were deposited by magnetron sputtering. Finally, 1 μm Ag grid electrodes, as the front contact, were thermally evaporated through a shadow mask. All solar devices have an active area of 0.21 cm2 (∼91% of the total device area, 0.23 cm2). No antireflection layer was added. Characterizations. Thermogravimetric analysis (TGA) was conducted with a model TGA/SDTA 851E instrument from Mettler-Toledo. The powder X-ray diffraction (XRD) patterns were determined by a Bruker D8 Advance X-ray diffractometer. The Raman spectra were recorded through a Renishaw inVia Raman microscope
Figure 1. (a) X-ray diffraction spectra, (b) enlarged view of the (112) peaks, and (c) Raman spectra as obtained for Cu2Zn1−xCdxSn(S,Se)4 thin films with different Cd/(Cd + Zn) ratios.
peak (110, 2θ = 40.53°, JCPDS Card No. 42-1120), all XRD peaks can be indexed according to the kesterite phase of CZTSSe. It was reported that a transition from I4 kesterite to I42m stannite structure occurs in CZCTSSe thin films when more than 60% of Zn2+ is replaced with Cd2+ (i.e., for x ≥ 0.6).28,36 In our work, because x was kept below 0.2, the phase change of the CZCTSSe thin films did not occur. The increase in the x value is found to shift the (112) XRD reflection from 5822
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Figure 2. Top-view and cross-section SEM images of the Cu2Zn1−xCdxSn(S,Se)4 thin films with different Cd/(Cd + Zn) ratios (x): (a) x = 0%, (b) x = 3%, (c) x = 5%, (d) x = 7%, (e) x = 10%, and (f) x = 20%.
2θ = 27.7° to 2θ = 27.2°. As Zn2+ is replaced with Cd2+, XRD peaks gradually shift to lower 2θ values, as shown in Figure 1b, indicating an increase in the CZCTSSe crystal lattice parameters. The unit cell volume increased due to the larger Cd2+ covalent radius (1.48 Å) compared to that of Zn2+ (1.25 Å), which demonstrates Cd2+ was introduced into the host lattice of CZTSSe by substitution of Zn2+.28 Some possible binary or ternary impurity (i.e., CdSe, SnSe, and Cu2SnSe3) has not been detected in the XRD spectra, confirming that pure kesterite CZCTSSe thin films were obtained. However, some secondary phases like ZnS and ZnSe cannot be effectively identified by XRD analysis. Raman scattering was thus performed to detect this possibly coexisting secondary phase as an auxiliary method, as shown in Figure 1c. The obtained Raman spectra from the selenized CZCTSSe thin films show a main peak at ∼200 cm−1 and two weaker broad peaks at 175 and 245 cm−1, which correspond to the literature values of CZTSe.37 The minor peaks at ∼328 cm−1 are compatible with the two-mode behavior of pure phase CZTSSe.37 Possible impurity phases (CdSe, ZnSe, SnSe, SnSe2, and Cu2SnSe3) have not been found in the series of Raman spectra of the selenized CZCTSSe thin films. The influence of incorporation of antimony (Sb) and alkali metals (Na and K) on the grain size of CZTSSe and CIGSSe has been extensively reported.11−16 Cd doping in CZTSSerelated thin films has been investigated by only a few groups.28,38 Figure 2 displays top-view and cross-section SEM images of the CZCTSSe thin films deposited on Mo-coated glasses with different Cd/(Cd + Zn) ratios. The introduction of Cd2+ is found to be favorable for promoting the grain growth for a selenization performed at 550 °C for 15 min and a Cd/ (Cd + Zn) ratio from 3 to 10%. Under these conditions, all of the selenized CZCTSSe thin films exhibit a compact and dense morphology compared to that of the pure CZTSSe thin films. The thickness of the large-grained layer also varied with Cd content, as reported in Table S1. However, when the level of doping increased to Cd/(Cd + Zn) ratios ranging from 20 to 40%, films show a multihole morphology (see Figure S2). The deterioration of the crystallization with higher Cd content is attributed to the fact that the theoretical crystallization temperature of Cu2CdSn(S,Se)4 (CCTSSe) is higher than
that of CZTSSe. There has been speculation that the used selenization temperature becomes insufficient for such a high level of Cd doping. To validate this speculation, we increased the selenization temperature in the 40% Cd-doped CZCTSSe thin film to 575 and 600 °C. The SEM images show that a better crystallization is obtained at a higher selenization temperature (see Figures S2 and S3). However, the PCE of CZCTSSe (x > 30%) solar cells is still far lower than that obtained in low-Cd content devices. The CZCTSSe valence states and chemical composition strongly affect solar cell performance. XPS and EDX spectra were thus recorded. Figure 3a−d shows XPS spectra of the
Figure 3. XPS spectra from Cu2ZnSn(S,Se)4 and from the representative Cu2Zn1−xCdxSn(S,Se)4 (x = 5%) thin films.
CZTSSe thin film and of a representative CZCTSSe (x = 5%) thin film. From the XPS results, one can see that there were no peak shifts for the Cu, Zn, and Sn peaks after the introduction of Cd into CZTSSe thin films. The peak positons of Cu 2p, Zn 2p (Cd 3d), and Sn 3d correspond well to +1, +2, and +4 oxidation states, respectively.27 Furthermore, a shoulder peak occurs near the Sn 3d peak in the CZTSSe thin films, which can 5823
DOI: 10.1021/acs.chemmater.6b02111 Chem. Mater. 2016, 28, 5821−5828
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Chemistry of Materials be ascribed to the Zn-Auger peak L3M45M45 and becomes weak in the CZCTSSe (x = 5%) thin films.39 The wide-scan XPS spectra of CZTSSe and CZCTSSe films are shown in Figure S4. From the wide-scan results, one can see that a distinct O 1s peak was located at 531 eV. Oxygen is incorporated mainly from the ambient atmosphere during thin film fabrication. The energy barrier induced by the oxidation for hole carriers can relieve the electron−hole recombination by blocking hole transport.40 As for the chemical composition of the absorber layer, it is commonly accepted that the Cu-poor compositions are beneficial for high-efficiency solar cells.20 Figure S5 and Table S2 display the detailed elemental compositions of CZCTSSe thin films as measured by EDX. The measured compositions were found to be very close to the ratios of the starting materials added to the precursor solution. Cell Characterization. To further evaluate the influence of the Cd content on device performance, a series of glass/Mo/ Cu2Zn1−xCdxSn(S,Se)4/CdS/i-ZnO/ITO/Ag solar cells were fabricated. The digital photograph and the cross-sectional SEM images of the CZCTSSe solar cell with 5% Cd/(Cd + Zn) are shown in Figure S6. The best device performances of CZCTSSe solar cells with different Cd contents are displayed in Figure 4. With the increase in Cd content from 0 to 20%, the
Figure 5. Variation of typical parameters of Cu2Zn1−xCdxSn(S,Se)4 solar cells with different Cd/(Cd + Zn) ratios: (a) PCE, (b) Voc, (c) Jsc, and (d) FF.
fabricated through the same procedure, which previously led to the best efficiency of 8.11%. Figure 6 displays the device performance distribution of CZCTSSe solar cells prepared with different Cd/(Cd + Zn) ratios. The detailed data are listed in Table S4, and the statistics were from the study of 10 solar cells. A clear correlation is found between device performance and Cd content. The external quantum efficiency (EQE) spectra of the series of CZCTSSe solar cells with different Cd contents are presented in Figure 7a. It is clear that the photoresponse range varies with the Cd/(Cd + Zn) ratio, and the device with a Cd content of 5% achieves the highest value beyond 80% in the visible light, near-infrared range (at 698−844 nm). The decay below 530 nm is mainly attributed to CdS buffer layer absorption. The band gap of CZCTSSe thin films was graphically determined by plotting [E × ln(1 − EQE)]2 versus E, where E = hc/λ (E stands for photon energy, λ the wavelength of light, h Planck’s constant, and c the speed of light), as shown in panels b and c of Figure 7.44,45 It was observed that the band gap of CZCTSSe thin films nonlinearly decreases from 1.182 to 1.079 eV with an increasing Cd/(Cd + Zn) ratio from 0 to 20%. As expected, the band gap of the best PCE solar cells (1.124 eV) is very close to the value of 1.13 eV corresponding to the band gap in record CZTSSe and CIGSe solar cells.5,6 Band gap values of quaternary CZTSSe films were determined by the valence band maximum (VBM) and the conduction band minimum (CBM) (i.e., CBM = VBM + Eg).25 The CBM is mainly affected by the antibonding component of the s−s and s−p hybridization between the Sn4+ and Se2− (S2−), while the VBM is governed by the antibonding component of the p−d hybridization between Se2− (S2−) and Cu+.46 Therefore, the partial substitution of Zn2+ with Cd2+ should not affect the band gap value, which is not consistent with our observations. Wong et al. attributed this fact to the expansion of the CZTSSe unit cell volume that would weaken the antibonding component of the s−s and s−p hybridization between the Sn4+ and Se2− (S2−), leading to the CBM decrease.28 Capacitance−voltage (C−V) profiling is a key electrical characterization method for studying CZTSSe thin film solar cells. It is also a useful technique for investigating the spacecharge density and the depletion width. Thus, it was performed
Figure 4. J−V curves of Cu2Zn1−xCdxSn(S,Se)4 solar cells at different Cd/(Cd + Zn) ratios under AM 1.5G illumination.
power conversion efficiency (PCE) first increases from 5.41 to 8.11% and then decreases to 4.61%. The corresponding solar cell parameters as a function of Cd content are shown in Figure 5. It is clear that the two device parameters of short-circuit (Jsc) and open-circuit (Voc) voltage achieve a maximum for an added Cd of +5% versus the standard (+0%), which produces the best PCE of 8.11% with Jsc, Voc, and FF values of 29.66 mA cm−2, 460 mV, and 60.41%, respectively. The detailed parameters of each solar cell are listed in Table S3. In our CZTSSe devices, Voc is thought to be mainly limited by the absorber band tailing due to the high Cu/Zn antisite defect density.41 Via replacement of Zn with Cd, which has a covalent radius ∼18% larger than those of Zn and Cu, the formation of Cu/Zn antisite defects may be suppressed. This would thus significantly decrease the open-circuit voltage deficit (Voc,def). The enhancement of Jsc is attributed to the improvement in crystal growth and the attenuation of the fine-grain layer in the absorber layer with a suitable Cd/(Cd + Zn) ratio (x = 5%). The best device performances of CZCTSSe solar cells versus Cd content cannot provide a meaningful conclusion about the device efficiency without adequate parallel statistics on the reliability of the data.42,43 Table 1 shows average values of the standard deviation obtained from 10 devices. Each sample was 5824
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Table 1. Summary of Device Parameters for Cu2Zn1−xCdxSn(S,Se)4 Solar Cells Prepared with Different Cd/(Cd + Zn) Ratios, in Which Each Sample Contained 10 Solar Cells Cd content (%) 0 3 5 7 10 20
Voc (mV)
efficiency (%) 5.13 6.74 7.86 6.81 5.21 4.17
± ± ± ± ± ±
0.46 0.38 0.25 0.21 0.33 0.44
427.0 421.0 437.6 411.3 399.7 333.6
± ± ± ± ± ±
19 25 22 26 12 20
Jsc (mA/cm2) 23.19 29.46 32.46 29.49 25.20 26.95
± ± ± ± ± ±
2.5 1.6 2.8 5.0 1.6 2.5
FF (%) 51.72 54.48 55.36 56.70 51.75 46.39
± ± ± ± ± ±
5.5 5.7 5.0 4.3 2.9 5.6
Eg (eV) 1.182 1.162 1.124 1.089 1.084 1.079
Eg/q-Voc (mV) 752.6 739.0 694.0 678.7 680.3 736.4
± ± ± ± ± ±
19 25 30 25 17 21
Figure 6. Device performances of the Cu2Zn1−xCdxSn(S,Se)4 solar cells as a function of Cd content: (a) PCE, (b) Jsc, (c) Voc, and (d) FF. Detailed data are listed in Tables S4−S8. Results are extrapolated from the analysis of 10 solar cells.
to explore the effect of partial substitution of Zn2+ with Cd2+. C−V curves were acquired from 0 to −1 V reverse bias, as shown in Figure 8a. Interestingly, C−V curves corresponding to different Cd contents present similar trends. Namely, capacitances decrease gradually with the increase in reverse bias, demonstrating that the bias voltage can expand the depletion width and shrink the capacitance in CZCTSSe devices.28 Figure 8b shows the space-charge density and depletion region width of the CZCTSSe device as derived from C−V curves. The detailed charge density and depletion region width of the CZCTSSe devices with different Cd contents are listed in Table S5. According to the formula Wd =
Aεoεs C
C 3 ⎛⎜ dc ⎟⎞ qA2 εoεs ⎝ dv ⎠
−1
NC − V =
εs =
Figure 8. (a) C−V curves of Cu2Zn1−xCdxSn(S,Se)4 solar cells with different Cd/(Cd + Zn) ratios. (b) Depletion width and space-charge density derived from C−V curves. These data were taken under 0 to −1 V reverse bias at 300 K.
where C, A, εo, q, S, d, and εs stand for the measured capacitance, the active device area, the vacuum dielectric constant (∼8.854 × 10−12 F/m), the electron charge (∼1.602 × 10−19 C), the total area of metal contact, the thickness of thin film, and the relative dielectric constant, respectively. Therein, the relative dielectric constant, also called the relative permittivity, indicates how easily a material can become polarized by imposition of an electric field. The depletion
cd εos
Figure 7. (a) EQE spectra of Cu2Zn1−xCdxSn(S,Se)4 solar cells with different Cd/(Cd + Zn) ratios. (b) Band gaps determined by the [E × ln(1 − EQE)]2 vs E curves. (c) Band gap variation as a function of Cd content. 5825
DOI: 10.1021/acs.chemmater.6b02111 Chem. Mater. 2016, 28, 5821−5828
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Chemistry of Materials width (Wd) of the CZCTSSe device can be determined by the intercept with the x-axis at zero bias, as shown in Figure 8b. With the increase in the Cd/(Cd + Zn) ratio from 0 to 20%, the charge density values increase from 6.38 × 1016 cm−3 to a maximum of 9.01 × 1016 cm−3 (x = 5%) and then decrease to a minimum of 5.54 × 1016 cm−3 while the depletion width (Wd) decreases from 0.29 μm (x = 0) to a minimum of 0.20 μm (x = 7%) and then increases to 0.26 μm (x = 20%). The depletion region width of semiconductor materials reflects the charge separation ability. Compared to that of high-performance CIGSSe solar cells, the Wd of CZTSSe is shallower and is regarded as a key issue for achieving further improvement.47,48 Our results indicate that the introduction of Cd element (x < 10%) into the CZTSSe absorber layer is not an effective strategy for improving charge separation. Therefore, the enhancement of the performance of CZCTSSe devices compared with that of CZTSSe devices is mainly attributed to the increase in charge density (from 6.38 × 1016 to 9.01 × 1016 cm−3). Although the charge separation ability is improved for Cd contents of >10%, the device efficiency decreases due to the poorer charge collection ability and a lower charge density. Because of this trade-off in our case, the Cd2+ content was limited to values of
