Improvement in Sb2Se3 Solar Cell Efficiency through Band Alignment

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Improvement in Sb2Se3 Solar Cell Efficiency through Band Alignment Engineering at the Buffer/Absorber Interface Gang Li, Zhiqiang Li, Xiaoyang Liang, Chunsheng Guo, Kai Shen, and Yaohua Mai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17611 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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ACS Applied Materials & Interfaces

Improvement in Sb2Se3 Solar Cell Efficiency through Band Alignment Engineering at the Buffer/Absorber Interface Gang Lia, Zhiqiang Lia*, Xiaoyang Lianga, Chunsheng Guoa, Kai Shenb and Yaohua Maia,b* aNational-Local

Joint Engineering Laboratory of New Energy Photoelectric Devices, College of Physics Science and Technology, Hebei University, Baoding 071002, China bInstitute of New Energy Technology, Jinan University, Guangzhou 510632, China *Corresponding

authors: [email protected], [email protected]

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Abstract Energy band alignment plays an important role in heterojunction thin film solar cells. In this work, we report the application of ternary CdxZn1−xS buffer layers in antimony selenide (Sb2Se3) thin-film solar cells. The results of our study revealed that the Cd/Zn element ratios not only affected the optical bandgap of CdxZn1−xS buffers but also modified the band alignment at the junction interface. A Sb2Se3 solar cell with an optimal conduction-band offset value (0.34 eV) exhibited an efficiency of 6.71%, which represents a relative 32.1% enhancement as compared to the reference CdS/Sb2Se3 solar cell. The results further indicated that a “spike”-like band structure suppressed the recombination rate at the interface and hence increased the device open-circuit voltage and fill factor. Electrochemical impedance spectroscopy analysis exhibited the CdxZn1−xS/Sb2Se3 solar cell had higher recombination resistance and longer carrier lifetime than the CdS/Sb2Se3 device. Keywords: Sb2Se3, band alignment, CdxZn1−xS/Sb2Se3 heterojunction, substrate configuration, electrochemical impedance spectroscopy.

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1. Introduction Solar energy is one of the core types of renewable and clean energy sources that is currently undergoing rapid development and deployment in order to meet the growth in the global energy demand, which is primarily due to its relative abundance. The exploration of low-cost, high-efficiency photovoltaic (PV) materials that demonstrate a high degree of stability continues to be the focus of current (and extensive) research efforts. Antimony selenide (Sb2Se3) has recently been proved to be an excellent PV absorber owing to its attractive optoelectronic properties, including its high absorption coefficient in the visible-light region, binary composition with a fixed orthorhombic phase, and unique one-dimensional crystalline structure.1-4 The use of Sb2Se3 as a light absorber in PV devices was first reported by Nair et al. in 2006; at that time, the conversion efficiency was only 0.66%.5-6 In 2014, Choi et al. and Tang et al. reported the fabrication of Sb2Se3-based mesoporous and planar heterojunction solar cell structures that demonstrated power conversion efficiencies of 3.21% and 2.1%, respectively.1, 7 Since then, the development of solar cell fabrication methods, advancements in device structures, and improvements in cell efficiency have significantly accelerated. In Sb2Se3-based solar cells, the junctions are typically produced by combining an Sb2Se3 absorber with different n-type partners, such as cadmium sulfide (CdS), tin oxide (SnO2), or titanium oxide (TiO2). As part of the ongoing efforts in absorber fabrication and device structure optimization, a comparison of the performances of these compounds has revealed efficiencies of 7.6% and 4.25% for CdS/Sb2Se3 structures with both superstrate and substrate configurations, respectively.8-9 One of the significant causes of photocurrent and conversion losses in CdS/Sb2Se3 solar cells is the large amount of blue light that is absorbed by the CdS layer. The photogenerated charge carriers in this layer cannot be collected because of its high defect density. One solution is to reduce this layer’s thickness or to replace the CdS buffer layer with wide-bandgap buffers fabricated with compounds such as ZnO, TiO2, SnO2, and ZnMgO.10-13 However, it has been indicated in some studies that the conversion efficiencies of cells with Cd-free buffers were lower than those with a CdS buffer.8, 12-13 Several groups have also developed CdS-based double buffers, where the CdS thickness was reduced and an additional ZnO, TiO2, or SnO2 buffer layer was introduced in order to ensure solar cell performance.14-16 However, the application of double buffers resulted in a device fabrication process that was more complex. Another approach is to widen the bandgap of the CdS-based buffer layer by replacing some of the Cd or S atoms with other elements in order to allow more photon absorption by the absorber layer. For example, CdxZn1−xS (0 ≤ x ≤ 1) was 3



successfully used as a CdS-based buffer layer in Cu(In,Ga)Se2 and Cu2ZnSnS4 thin-film solar cells.17-19 The bandgap of the CdxZn1−xS film, which is wider than that of the pure CdS buffer and provides a better response in the short-wavelength region, can be adjusted by controlling the Zn content in the buffer layer.17 On the other hand, band alignment represents another important optimization parameter because the formation of a suitable conduction-band offset (CBO) between the buffer and the Sb2Se3 absorber layer plays an important role in solar cell performance. The band alignment of a CdS/Sb2Se3 junction has been reported by several groups, but their conclusions were inconsistent with one another.20-21 It has been reported in some studies that the CBO value was positive (spike-like), whereas others claimed that it was negative (cliff-like). The differences in the CBO might be due to variations in the surface quality of the Sb2Se3 absorber, the use of different characterization methods, and/or the different junction formation processes for the superstrate and substrate solar cells8, 21. Moreover, although the tunable band energies of CdxZn1−xS enable the formation of optimal band alignment at the buffer/Sb2Se3 junction interface, very few attempts have been made to control and optimize this alignment at Sb2Se3-based heterojunction interfaces, especially for devices with a substrate configuration. In this study, a series of CdxZn1−xS thin films were obtained by varying the Cd and Zn source concentrations in the precursor solution. The structural, electrical, chemical, and optical properties of the CdxZn1−xS thin films were studied in detail, and a series of Sb2Se3 solar cells with different CBOs were fabricated by varying the Zn content in the CdxZn1−xS buffer layers. The device performance is discussed in terms of the CBO values, and the carrier recombination dynamics for CdxZn1−xS-based solar cells are described on the basis of our further analysis by electrochemical impedance spectroscopy (EIS) and transient photovoltage measurements.

2 Experimental Sections 2.1 Preparation of CdxZn1−xS (0 ≤ x ≤ 1) Thin Films The sources of the Cd, Zn and S ions in the CdxZn1−xS (0x < 1) films were 0.03 M zinc sulfate (ZnSO4 ·7H2O); 1 mM, 3 mM and 5 mM cadmium sulfate (3CdSO4· 8H2O); and 0.3 M thiourea (H2NCSNH2), respectively. The CdxZn1−xS (0x < 1) films were deposited onto the Sb2Se3 absorber by chemical bath deposition (CBD) in 3 M ammonia (NH4OH) at 78 °C. In contrast, the pure CdS buffer layer was deposited with a 0.015M cadmium sulfate (CdSO4), 2.0 M ammonia (NH4OH) and 1.5 M thiourea (SC(NH2)2) aqueous solution at 70 °C, without the zinc 4


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source. The thicknesses for all of the CdxZn1−xS (0 ≤ x ≤ 1) thin films were in the range of 60 to 80 nm by controlling the duration time.

2.2 Fabrication of Solar Cells Sb2Se3 solar cells were fabricated in a substrate configuration (ZnO:Al/ZnO/CdxZn1−xS/Sb2Se3/Mo/glass), as described in our previous work.9 The Mo back contact layer was prepared by 1200 W DC sputtering under a 0.5 Pa Ar atmosphere, in which the obtained Mo thickness was about 800 nm. Sb2Se3 absorbers were deposited on the prepared Mo using the closed-space sublimation (CSS) process.22-23 The Sb2Se3 powder, as the source, was placed in a graphite plate, and Mo-coated glass substrate was placed on other piece of graphite plate, which was about 10 mm above the Sb2Se3 source. When the pressure of CSS chamber was lower than 10-1 Pa, the deposition process was started. The source and substrate was heated up to 500 and 300 °C, respectively, and maintained at these temperatures for 1 min. The samples were taken out when the substrate was cooled down to about 170 °C. Then, CdxZn1−xS (0 ≤ x ≤ 1) was deposited using the above process. Finally, a 70 nm thick zinc oxide (ZnO) and a 300 nm thick aluminum zinc oxide conductive (ZnO:Al) film were deposited in a vacuum environment by radiofrequency magnetron sputtering at room temperature. The total area of the final cells was approximately 0.2–0.3 cm2, which was defined by mechanical scribing.

2.3. Measurement and Characterization In order to measure the device performance, current density–voltage (J–V) measurements were performed using an AM1.5 solar simulator equipped with a 300 W xenon lamp (Model No. XES-100S1; SAN-EI Electric Co., Ltd.). The external quantum efficiency (EQE) was measured using an Enlitech QER3011 system (Enlitech) equipped with a 150 W xenon light source. EIS measurements were performed on an electrochemical workstation (Zahner Zennium) at frequencies ranging from 1 Hz to 1 MHz under dark conditions with an amplitude of 10 mV. The transient photovoltage curves of the devices were measured using the Dyenamo toolbox. X-ray photoelectron spectroscopy (XPS) measurements were conducted using an X-ray photoelectron spectrometer (ESCALab 250Xi; Thermo Scientific), and the crystal structures were characterized by X-ray diffraction (XRD) with Cu Kα (1.54056 Å) radiation (D8 ADVANCE; Bruker). The lattice vibrations of the CdxZn1−xS layers were studied using Raman spectroscopy at room temperature (LabRAM HR800, 532 nm excitation wavelength; HORIBA Jobin Yvon,). The surface morphology images of the thin films were obtained using scanning electron 5



microscopy (SEM) (Nova NanoSEM 450, FEI), and the optical properties of the thin films were measured using a spectrophotometer equipped with an integrating sphere (LAMBDA 950, PerkinElmer).

3 Results and Discussion CBD CdxZn1−xS thin films with different Cd/Zn ratios (Cd0.82Zn0.18S, Cd0.75Zn0.25S, and Cd0.62Zn0.38S) were obtained from an aqueous solution containing CdSO4, ZnSO4, thiourea, and ammonia. Figure 1 shows the surface SEM images of the different CdxZn1−xS layers deposited on the Sb2Se3 thin films. All of the CdxZn1−xS films were dense and completely covered the Sb2Se3 absorber layer after the CBD process. The grain size of the CdxZn1−xS (x ≠ 1) layer was smaller compared to that of the pure CdS layer, which could be attributed to the differences in the supersaturation degree with respect to hydroxide and the reaction kinetics of the Zn and Cd systems, respectively, in the ammonium-based solution.24-26 The optical properties and crystal structure were further examined by ultraviolet– visible–near-infrared (UV–Vis–NIR) spectroscopy and XRD, respectively. Figure 2(a) shows the transmittance spectra of the CdxZn1−xS thin films in the wavelength range of 300–1200 nm. The transmittance properties of the films were clearly influenced by the Zn/Cd ratio. In the short-wavelength region, the transmittance increased with the Zn content. By contrast, all of the thin films exhibited high transmittance in the long-wavelength region (800–1100 nm). Using the Tauc relation, the optical bandgaps of the CdxZn1−xS thin films were calculated to be 2.43, 2.47, 2.52, and 2.60 eV for CdS, Cd0.82Zn0.18S, Cd0.75Zn0.25S, and Cd0.62Zn0.38S, respectively. These values for CdxZn1−xS were between those of CdS and ZnS thin films, which are consistent with previously reported values for such films.27-28 The XRD patterns of the CdxZn1−xS layers grown on the Sb2Se3 absorbers are shown in Fig. 2(b). For the CdS/Sb2Se3 sample, in addition to the strong peaks associated with Sb2Se3, only one other peak was observed at 28.18°, which corresponded to the (101) diffraction peak of hexagonal CdS (JCPDS no. 41-1049). By contrast, only the diffraction peaks corresponding to the Sb2Se3 absorbers were detected for the CdxZn1−xS/Sb2Se3 (x ≠ 1) samples, and similar results were observed for CdxZn1−xS grown on bare glass substrates, which suggested that the crystallinity of the CdS layers was higher than that of the ternary CdxZn1−xS (x ≠ 1) layers. This conclusion is consistent with the SEM results, in which the grains of CdZnS were finer than those of the CdS thin film. The Raman measurement (Fig. S2) also revealed characteristics similar to those of the XRD patterns. The Raman spectra of all the CdxZn1−xS films displayed peaks at 300 and 600 cm−1, corresponding to the 1LO and 2LO vibration 6


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modes of crystalline CdS, respectively.29-30 The band alignment at the CdxZn1−xS/Sb2Se3 heterojunction interface was quantitatively measured using XPS, and the valence-band offset (VBO) at the CdxZn1−xS/Sb2Se3 junction can be obtained using the following equation:31-32 (1) VBO = 𝐸𝑏𝑉𝐵 ― 𝐸𝑎𝑉𝐵 + 𝑉𝑏𝑏, 𝑏 𝑎 where 𝐸𝑉𝐵 and 𝐸𝑉𝐵 are the energy positions of the valence-band edges of the CdxZn1−xS buffer layer and Sb2Se3 absorber layers, respectively, and 𝑉𝑏𝑏 is the band bending at the buffer/absorber junction interface, which can be obtained from the core level energy shift between the bulk area and the interface and can be calculated using the following equation: V𝑏𝑏 = (𝐸𝑎𝐶𝐿 ― 𝐸𝑎𝐶𝐿(𝑖)) + (𝐸𝑏𝐶𝐿(𝑖) ― 𝐸𝑏𝐶𝐿),

(2)

where 𝐸𝑎𝐶𝐿 and 𝐸𝑏𝐶𝐿 are the core level energies of two selected elements in the CdxZn1−xS and Sb2Se3 bulk areas, respectively, and 𝐸𝑎𝐶𝐿(𝑖) and 𝐸𝑏𝐶𝐿(𝑖) represent the core level energy positions of those elements at the CdxZn1−xS/Sb2Se3 interface. In this work, six sets of element pairs were employed because Sb or Se was chosen for Sb2Se3 and Cd, Zn, or S was chosen for CdxZn1−xS. Figure S3 in the supporting material shows the band bending values for the six sets calculated using Cd 3d, Zn 2p, and S 2p1 core level energies for the CdxZn1−xS buffer, whereas Sb 3d and Se 3d core level energies were used for the Sb2Se3 absorber. By averaging the values of the six sets, the final band bending value (𝑉𝑏𝑏) was estimated to be −0.12, 0.16, 0.19, and 0.29 eV for the CdS/Sb2Se3, Cd0.82Zn0.18S/Sb2Se3, Cd0.75Zn0.25S/Sb2Se3, and Cd0.62Zn0.38S/Sb2Se3 junction interfaces, respectively. The valence-band maximum (VBM) values (𝐸𝑉𝐵) for the Sb2Se3, CdS, Cd0.82Zn0.18S, Cd0.75Zn0.25S, and Cd0.62Zn0.38S films, which were obtained by a linear extrapolation of the corresponding valence-band data in the XPS spectra (Fig. S4), were 0.26, 1.62, 1.22, 1.01, and 0.78 eV, respectively. Employing Eq. (1), the VBO values for CdS/Sb2Se3, Cd0.82Zn0.18S/Sb2Se3, Cd0.75Zn0.25S/Sb2Se3, and Cd0.62Zn0.38S/Sb2Se3 were estimated to be 1.28, 1.12, 0.94, and 0.81 eV, respectively. Furthermore, it was possible to calculate the CBO values from the VBO value and the difference between the bandgaps of the absorber and buffer: 𝐶𝐵𝑂 = 𝐸𝑎𝑔 ― 𝐸𝑏𝑔 + 𝑉𝐵𝑂. The CBO values for CdS/Sb2Se3, Cd0.82Zn0.18S/Sb2Se3, Cd0.75Zn0.25S/Sb2Se3, and Cd0.62Zn0.38S/Sb2Se3 were calculated to be −0.09, 0.11, 0.34, and 0.55 eV, respectively. A schematic of the estimated band alignment between the Sb2Se3 absorber and the different buffers is plotted in Fig. 3 using the bandgap, CBO, and VBO values. As shown in Fig. 3, the band energy structure of the CdxZn1−xS buffer 7



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layer varied with the Cd/Zn ratio, which is consistent with previous reports.17, 33 The CBO of CdS/Sb2Se3 was cliff-like (−0.09 eV), indicating that the conduction band of the buffer layer was lower than that of the Sb2Se3 absorber. The CBO at the Cd0.82Zn0.18S/Sb2Se3 junction interface was positive, and the CBO increased with the Zn content in the CdxZn1−xS buffer, suggesting that the conduction band of the buffer was higher than that of the Sb2Se3 absorber. The CBO at the junction interface was sensitive and varied from slightly cliff-like to highly spike-like with increasing Zn content from 0 to 38%. In order to investigate the effects of the CBO on the performance of solar cells, the substrate of the CdxZn1−xS/Sb2Se3 heterojunction solar cells was finished with different CdxZn1−xS buffer layers, and the device configuration was the same as that used in our previous work.9 The statistics related to solar cell performance using the average values and standard deviations are presented in Fig. 4. The solar cells based on the CdS buffer prepared in this study exhibited an average open-circuit voltage (Voc) of 363 ± 23 mV, a short-circuit current density (Jsc) of 22.50 ± 1.94 mA/cm2, and a fill factor (FF) of 54.08 ± 3.74, with a corresponding average conversion of 4.78 ± 0.34%. An enhanced Voc was observed in the solar cells with CdxZn1−xS (Cd0.82Zn0.18S, Cd0.25Zn0.75S, and Cd0.62Zn0.38S) buffers owing to an upshift in the ternary CdxZn1−xS layer after the incorporation of Zn and as a result of suitable CBO values at the CdxZn1−xS/Sb2Se3 junction interfaces. The Jsc values gradually increased as the Zn content in the CdxZn1−xS buffer layer increased from 0 to 0.25(x value), which could be attributed to the increase in the optical bandgap of the CdxZn1−xS layers. The larger bandgaps allowed more photons to reach the Sb2Se3 absorber and to generate more carriers. However, the Jsc and FF values both decreased significantly for devices with the Cd0.62Zn0.38S buffer, which could be attributed to an increase in the positive CBO. When the conduction band of the buffer layer was higher than that of Sb2Se3, a notch formed at the buffer/Sb2Se3 interface, preventing the recombination of photogenerated carriers. If the conduction band of the buffer is too high, it induces the formation of a barrier that obstructs the collection of photoinduced electrons and leads to decreases in the Jsc and FF values. The J–V characteristics of the CdS/Sb2Se3 and Cd0.75Zn0.25S/Sb2Se3 heterojunction solar cells were analyzed under AM 1.5G illumination with a light intensity of 100 mW/cm2 at 25°C. The corresponding J–V curve parameters are summarized in Table 1. It is well known that the J–V behavior of a thin-film solar cell can be described by a nonideal photodiode equation, as follows:34-35 𝐽 = 𝐽0exp

[

𝑞 𝐴𝑘𝑇

(𝑉 ― 𝑅s𝐽)] + 𝐺𝑉 ― 𝐽L, 8


(3)

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where 𝐽0, 𝑅s, 𝐽L, G, and A are the reverse saturation current density, series resistance, light current density, shunt conductance, and diode quality factor, respectively. Differentiation with respect to the voltage bias V yields the following equation: d𝐽

𝑞

d𝑉 = 𝐴𝑘𝑇𝐽0exp

[

d𝐽L

( ) . 𝐴𝑘𝑇 𝑉 ― 𝑅s𝐽 ] ― d𝑉 + 𝐺 𝑞

(4)

A plot of the derivative d𝐽 d𝑉 against 𝑉 near JSC is presented in Fig. 5(b). The derivative of the diode term was negligible, 𝐽L was nearly constant in the reverse bias, and the value of G could be obtained from the plateau in the plot. In this work, the values of G for the CdS/Sb2Se3 and Cd0.75Zn0.25S/Sb2Se3 heterojunctions were estimated to be 6.72 and 3.28 mS/cm2, respectively. By rearranging and differentiating the equation, the derivative d𝑉 d𝐽 can be expressed as a function of

(𝐽 + 𝐽SC) ―1: d𝑉 d𝐽

𝑞

d𝑉

= 𝑅s + 𝐴𝑘𝑇(1 ― 𝐺 d𝐽 )/(𝐽 + 𝐽L ― 𝐺𝑉).

(5)

The plots of d𝑉 d𝐽 against (𝐽 + 𝐽L ― 𝐺𝑉) ―1 are shown in Fig. 5(c), where 𝐽L was replaced by the short-circuit current density 𝐽𝑆𝐶. The value of Rs was obtained by linearly fitting to the data, whereas the value of A was calculated from the slope of 𝑞 𝐴𝑘𝑇.34, 36 For the CdS/Sb2Se3 heterojunction, the values of Rs and A were 2.45 Ω/cm2 and 2.25, respectively, whereas they were estimated to be 1.98 Ω/cm2 and 1.79, respectively, for the Cd0.75Zn0.25S/Sb2Se3 device. Figure 5(d) displays the semilogarithmic plot of 𝐽 + 𝐽L against V–RJ using the values of R obtained from Fig. 5(c). Applying a linear fit to the data provided the values of A, which were consistent with the values obtained from Fig. 5(c), and the values of J0 were obtained from the intercepts. The values of J0 of the CdS/Sb2Se3 and Cd0.75Zn0.25S/Sb2Se3 devices were estimated to be 0.028 and 0.0053 mA/cm2, respectively. Based on the J–V analysis, the Cd0.75Zn0.25S/Sb2Se3 device exhibited a larger shunt resistance, smaller resistance, and higher diode quality factor than those of CdS/Sb2Se3. Additionally, the photo-generated current density calculated from the EQE curves (Figure S5) of the solar cells with pure CdS or Cd0.75Zn0.25S buffer were 23.33 and 24.49 mA/cm2, respectively, both of which are ~1 mA/cm2 smaller than the JSC values of the corresponding solar cells obtained by J-V measurement. Moreover, the integrated current density in the range of 350 to 550 nm for pure CdS and Cd0.75Zn0.25S solar cells were 3.1 and 4.4 mA/cm2, respectively. It is obviously that the increased current density for Cd0.75Zn0.25S mainly comes from the short wavelength region response. 9



EIS measurements were also performed to obtain further insight into the interfacial properties. Figure 6a shows the Nyquist plots of the impedance spectra for the Sb2Se3 solar cells with the CdS and Cd0.75Zn0.25S buffer layers. The measured data could be fitted by two semicircles in the high-frequency region and low-frequency region, respectively. The impedance spectroscopy data of our devices were analyzed based on an equivalent circuit model, as shown in Fig. 6b. The Rs obtained from the EIS analysis for Cd0.75Zn0.25S and CdS solar cells were 8.342 and 3.328 Ω, respectively. This has the same trend with the Rs obtained from the J-V curves in Figure 5a. Normally, the semicircle at the high frequency (R1-C1 element) is associated with the TCO/buffer interface, and the second one (R2-CPE element) could be ascribed to the recombination and chemical capacitance at the CdxZn1-xS buffer/Sb2Se3 absorber heterojunction interface37-40. The radii of main semicircles at low frequency for the Cd0.75Zn0.25S or CdS solar cell were different from each other, indicating that the incorporation of Zn into the buffer influenced the junction interface and the device since the other layers were deposited following the same process. Moreover, for a given R-C circuit, the real and imaginary parts represented the resistance and capacitance of impedance, respectively, and both of them were frequency dependent. To further study the resistive and capacitive properties of the Cd0.75Zn0.25S and CdS solar cells, the frequency-dependent real and imaginary parts of impedance for the devices were displayed in Figure 6c and Figure 6d, respectively. As can be seen, the plateau value in the low frequency range for Cd0.75Zn0.25S and CdS solar cell were 2.7×103 Ω and 1.1×103 Ω, respectively. The carrier lifetime for Cd0.75Zn0.25S and CdS solar cells were calculated to be 5.24×10-5 s and 3.32×10-5 s, respectively. The higher resistance and longer carrier lifetime indicated that the Cd0.75Zn0.25S solar cell has better junction, and this echoed its higher FF and VOC.

4 Conclusions In summary, we introduced a buffer material that is very suitable for the substrate configuration of Sb2Se3 thin-film solar cells. By controlling the element contents of Zn and Cd, we obtained a solar cell with a Cd0.75Zn0.25S buffer that displayed an optimal conversion efficiency of 6.71%, which was a 32.1% enhancement relative to that of the CdS/Sb2Se3 control device. In our opinion, the appropriate CBO in Cd0.75Zn0.25S/Sb2Se3 is attributed to the increase of Voc and FF. The “spike-like” structure reduced the carrier recombination rate of the solar cell with the Cd0.75Zn0.25S buffer. Moreover, an enhancement in Jsc was also observed in the Cd0.75Zn0.25S-based solar cell because of the enhanced transmittance of the CdxZn1−xS buffer in the short-wavelength region. The results indicated that a CdxZn1−xS buffer layer with an 10


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appropriate Zn content is more suitable for use with Sb2Se3 solar cells and that band alignment requires more fine-tuning in our future research.

Acknowledgments This work was supported by the Advanced Talents Incubation Program at Hebei University (801260201001), the Scientific Research Foundation for Returned Overseas Chinese Scholars (CG2015003004), the National Natural Science Foundation of China (NSFC No. 61804040), and the Natural Science Foundation of Hebei Province (No. E2016201028). .

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Figures

Figure 1 SEM images of the CdxZn1−xS buffers deposited on the Sb2Se3 absorbers: (a) CdS, (b) Cd0.82Zn018S, (c) Cd0.75Zn0.25S, and (d) Cd0.62Zn0.38S.

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Figure 2 Optical transmission spectra (a) and XRD patterns (b) of the CdxZn1−xS thin films with different Zn contents.

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Figure 3 Schema of the band alignment of the Sb2Se3 absorber and the different buffers: (a) CdS/Sb2Se3, (b) Cd0.82Zn018S/Sb2Se3, (c) Cd0.75Zn0.25S/Sb2Se3, and (d) Cd0.62Zn0.38S/Sb2Se3. The VBO, CBO, and Eg values are indicated.

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Figure 4 Performance of the Sb2Se3 solar cells with the CdxZn1−xS buffer layers: (a) Voc, (b) Jsc, (c) FF, and (d) efficiency.

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Figure 5 Light J–V characteristics of the solar cells with the CdS and Cd0.75Zn0.25S buffers: (a) J–V curves under standard AM1.5 illumination, (b) characterization of the shunt conductance (G), (c) dV/dJ with the fit used to determine R and A, and (d) ln(J + Jsc − GV) with the fit used to determine A and J0.

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Figure 6 (a) Nyquist plots of the CdS and Cd0.75Zn0.25S based solar cells solar cells, (b) an equivalent circuit model employed to fit the impedance curves. (c) The frequency-dependent real parts and (d) the frequency-dependent imaginary parts in the impedance spectra of the CdS and Cd0.75Zn0.25S based solar cells solar cells.

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Table 1 Solar cell performance parameters Cd0.75Zn0.25S/Sb2Se3 thin-film solar cells.

of

the

CdS/Sb2Se3

and

Buffera

Voc (V)

Jsc (mA/cm2)

FF (%)

Eff. (%)

J0 (mA/cm2)

A

CdS

0.383

24.33

54.52

5.08

2.8E-2

2.25

Cd0.75Zn0.25S

0.403

25.69

64.78

6.71

5.3E-3

1.79

aJ , sc

Voc, FF, and η were obtained from light J–V curves. Serial resistance (Rs), shunt resistance (Rsh), ideality factor (A), and J0 were calculated using light J–V curve data.

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TOC

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Improvement in Sb2Se3 Solar Cell Efficiency through Band Alignment

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