Structural and Solar Cell Properties of a Ag-Containing Cu2ZnSnS4

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Structural and Solar Cell Properties of an Ag-containing CuZnSnS Thin Film Derived from Spray Pyrolysis 2

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Thi Hiep Nguyen, Takato Kawaguchi, Jakapan Chantana, Takashi Minemoto, Takashi Harada, Shuji Nakanishi, and Shigeru Ikeda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14929 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 28, 2018

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ACS Applied Materials & Interfaces

Structural and Solar Cell Properties of an Agcontaining Cu2ZnSnS4 Thin Film Derived from Spray Pyrolysis Thi Hiep Nguyen,† Takato Kawaguchi,‡ Jakapan Chantana,§ Takashi Minemoto,§ Takashi Harada,† Shuji Nakanishi,† and Shigeru Ikeda‡,* †

Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan.

‡

Department of Chemistry Konan University, 9-1 Okamoto Higashinada, Kobe, Hyogo 6588501, Japan

Department of Electrical and Electronic Engineering, Ritsumeikan University, 1-1-1 Nojihigashi Kusatsu, Shiga 525-8577, Japan.

KEYWORDS: Cu2ZnSnS4 thin films, spray pyrolysis, Ag incorporation, structural modification, photovoltaic properties.

ACS Paragon Plus Environment

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ABSTRACT

A silver (Ag)-incorporated kesterite Cu2ZnSnS4 (CZTS) thin film was fabricated by a facile spray pyrolysis method. Crystallographic analyses indicated successful incorporation of various amounts of Ag up to an Ag / (Ag + Cu) ratio of ca. 0.1 into the crystal lattice of CZTS in a homogeneous manner without formation of other impurity compounds. From the results of morphological investigations, Ag-incorporated films had larger crystal grains than those of the CZTS film. The sample with a relatively low Ag content (Ag / (Ag + Cu) of ca. 0.02) had a compact morphology without appreciable voids and pinholes. However, an increase in the Ag content in the CZTS film (Ag / (Ag + Cu) ca. 0.10) induced formation of a large number of pinholes. As can be expected from these morphological properties, the best sunlight conversion efficiency was obtained by the solar cell based on the film with Ag / (Ag + Cu) of ca. 0.02. Electrostructural analyses of the devices suggested that the Ag-incorporated film in the device achieved reduction in the amounts of unfavorable copper on zinc antisite defects in comparison with those in the bare CZTS film. Moreover, the use of an Ag-incorporated film improved band alignment at the CdS(buffer)-CZTS interface. These alterations should also contribute to enhancement of device properties.

1. INTRODUCTION Kesterite Cu2ZnSnS4 (CZTS) is a promising material for photovoltaics because of its suitable photoabsorption properties including its band gap energy (Eg, 1.4-1.5 eV) and absorption coefficient (104 cm-1).1-4 From environmental and economic viewpoints, moreover, the use of CZTS is attractive because all of its constituent elements are less toxic and more earth-abundant


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than the constituent elements of currently used chalcopyrite Cu(In,Ga)(Se,S)2 (CIGS). Several vacuum and non-vacuum processes for obtaining a high-quality CZTS thin film that is appropriate for photovoltaic applications have been reported.5-14 Among them, we have emphasized that the use of the spray pyrolysis deposition (SPD) method is one of the most suitable methods for achieving low-cost production of CZTS-based solar cells because the method can realize a fast deposition rate and because the equipment for large-scale production is similar to that used for conventional spray coating in various industrial processes.15,16 It is known that a CZTS thin film tends to include a large number of CuZn antisite defects owing to its relatively low formation energy compared to that of the favorable Cu vacancy (VCu).17-21 Recent theoretical results reported by S. Chen et al. predicted that Ag-based kesterite (i.e., Ag2ZnSnS4 (AZTS)) has a small number of AgZn antisite defects since significant differences in ionic radiuses between Ag and Zn ions lead to relatively high formation energy of the defect.22 Based on these aspects, a partial replacement of Cu to Ag to obtain (Cu1xAgx)2ZnSnS4

(ACZTS) films has been attempted by many researchers in order to reduce the

number of CuZn antisite defects; appreciable improvements of solar cell properties by these ACZTS-based devices have been reported.23-26 When using the above-mentioned SPD method, the film composition can be easily controlled by just changing the composition of the precursor solution used. This feature is beneficial for fabricating ACZTS films with different Ag contents. In this study, therefore, we used the SPD method for fabrication of ACZTS thin films. Improvements of solar cell performances achieved by devices based on these ACZTS thin films are reported in relation to structural and electric properties of the ACZTS thin films.


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2. EXPERIMENTAL SECTION Three precursor films with different amounts of Ag were fabricated by varying the molar ratio of Ag / (Cu + Zn + Sn) in aqueous precursor solutions (Ag / (Cu + Zn + Sn) = 0, 0.02, 0.05, 0.1) Sources of metallic elements were copper nitrate (Cu(NO3)2), zinc nitrate (Zn(NO3)2), tin methanesulfonate (Sn(CH3SO3)2), and silver nitrate (AgNO3); thiourea (SC(NH2)2) was used as the sulfur source. Deposition was carried out by spraying aqueous solutions containing these sources with pH adjusted to 1.5 by adding a few drops of concentrated nitric acid on Mo-coated glass substrates heated at 380 °C. Adjustment of pH is indispensable to avoid formation of precipitates in the solution during the spray deposition.15 Thus-obtained precursor films were annealed in an evacuated borosilicate glass ampoule containing 20 mg of sulfur powder at 600 °C for 30 min to form CZTS and ACZTS films. In order to complete the solar cell device, a CdS buffer layer was deposited on CZTS and ACZTS films by chemical bath deposition.15,16 Then, iZnO and ITO layers were sequentially deposited by radio frequency (RF) magnetron sputtering on CdS-covered CZTS and ACZTS films. Finally, an Al top contact was deposited by thermal evaporation. The device area (active area) of all the solar cells were fixed at 0.030 cm2. Atomic compositions of CZTS and ACZTS films were determined by inductively coupled plasma (ICP) analysis on a Perkin-Elmer OPTIMA 3000-XL ICP emission spectrometer. For this measurement, the film samples were immersed in aqua regia overnight to dissolve all of the components. The thus-obtained clear solution was then diluted to an appropriate concentration before analysis. Crystallographic structures were determined by X-ray diffraction (XRD) analyses using a Rigaku Mini Flex X-ray diffractometer (CuKα, Ni filter) and Raman spectroscopy using a JASCO NRS 3100 laser Raman spectrophotometer. The depth profile was analyzed by secondary ion mass spectroscopy (SIMS) using an ATOMIKA SIMS 4100 equipped


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with O2+ source ions (5 keV). Photoelectrochemical measurements were performed by using a conventional three-electrode setup. Three electrodes, namely Pt wire, Ag/AgCl and CZTS (or ACZTS)-based films as counter, reference and working electrodes, respectively, were inserted into a three-necked flask with a flat window containing 0.2 M Eu(NO3)3 solution with pH 4. Measurements were conducted by a linear sweep voltamonmetric mode with a negative scan direction at the scan rate of 10 mV s-1 under chopped illumination of simulated sunlight (AM1.5G) using an Asahi Spectra HAL-320 Compact Xenon Light Source. Surface morphologies were examined by using a Hitachi S-5000 field emission scanning electron microscope (SEM) at an acceleration voltage of 20 kV and a KEYENCE VK-9700 laser microscope. Current density-voltage (J-V) characteristics and external quantum efficiency (EQE) spectra of solar cell devices under simulated AM1.5G irradiation were determined with a Bunkoh-Keiki CEP-015 photovoltaic measurement system. Capacitance-voltage (C-V) characteristics of solar cell devices were examined by using an HP-HEWLETT PACKED 4284A apparatus. X-ray photoelectron (XP) spectroscopy was performed by using a Shimadzu AXIS ULTRA X-ray photoelectron spectrometer in monochromated Al Kα radiation. For surface etching, the sample was bombarded by Ar+ ions accelerated to 3.5 keV for appropriate durations.

3. RESULTS AND DISCUSSION As determined by inductively coupled plasma (ICP) analyses, Ag / (Ag + Cu) ratios of thusobtained CZTS and ACZTS films were 0, 0.016, 0.055, and 0.104 (These films were labeled CZTS, A(2%)CZTS, A(5%)CZTS, and A(10%)CZTS, respectively.). It should be noted that all of the films were confirmed to have group I-poor (i.e., (Ag + Cu) / (Zn + Sn) < 1) and Zn-rich (i.e., Zn / Sn > 1) compositions, which are empirically optimal for CZTS-based solar cells.11


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Figure 1a shows XRD patterns of the films. The CZTS film showed several reflections assignable to reflections of the kesterite CZTS structure. With increase in the Ag contents of the films, those reflections gradually shifted to lower 2θ angles, as typically shown by magnified images of (112), (200)/(004), and (312)/(116) reflections in Figure 1b. Since the ionic radius of Ag ions is larger than that of Cu ions, the observed shifts of reflections (i.e., increases in lattice constants) are strong indications of the occurrence of gradual replacements of Cu to form mixed kesterite crystals of ACZTS instead of the formation of various separate phases. It should be noted that weak reflections at 2θ of 28.5° and 56.3°, assignable to (111) and (311) reflections of ZnS crystal, appeared for the A(10%)CZTS film. These results indicate presence of a small fraction of ZnS in the present CZTS and ACZTS films (see below). As shown in Figure 1c, Raman spectrum of the CZTS film exhibited intense peaks centered at 338 cm-1, which is assigned to the A1-band for kesterite CZTS; three weak peaks centered at 252, 286, and 369 cm-1 are also typically observed for kesterite CZTS.13 Raman spectra of ACZTS films indicated occurrences of slight shifts of A1-bands to lower wavenumber regions though they were not significant (e.g., 336 cm-1 for A(10%)CZTS). Since similar results were reported in the literature by several groups,23,26,27 present results were empirical evidences of successful incorporation of Ag components into the kesterite lattice. Figure 2 shows depth profiles of CZTS and ACZTS films obtained by secondary ion mass spectrometry (SIMS). At the surface region, all the film showed appreciable gradients were observed except for the sulfur (S) component. Especially, presences of large contents of Zn components were pronounced for all the films. As discussed above XRD results, these results indicate presences ZnS crystals at the surface of these kesterite films. According to the literature,28,29 such segregations of ZnS at the surface should be due to the evaporation of the Sn


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component during the high temperature heat treatment. It should also be noted that all the films showed similar depth profiles of constituent elements regardless of presences of Ag components, suggesting compositional idealities among the CZTS and ACZTS films. Regarding Ag profiles in A(2%)CZTS, A(5%)CZTS, and A(10%)CZTS films, almost no compositional grading was observed in bulk regions of these kesterite films. Based on results of the structural investigations, we can conclude that different amounts of Ag were incorporated homogeneously in the CZTS films by using the SPD method. Photoelectrochemical measurements of CZTS and ACZTS films were performed in an electrolyte solution containing an Eu3+ / Eu2+ redox couple at pH 4.27,28 Figure 3 shows typical photocurrent density-voltage (J-V) plots of these films obtained by using the lock-in technique. All of the films showed appreciable photocurrents at potentials more negative than ca. -0.1 V (vs Ag/AgCl) derived from reduction of Eu3+ ions in the solution, indicating p-type characteristics of them. Compared to the CZTS film, slight shifts of photocurrent onsets, defined as the leading potential showing 0.02 mA cm-2 of cathodic photocurrent, to positive regions were observed for the ACZTS samples, suggesting deepening of their flat band potentials (FBs) induced by Ag incorporation (see below). Figures 4 shows top-view SEM images of CZTS and ACZTS films. The CZTS film was composed of densely packed submicron-sized crystal grains. In the CZTS film, appreciable voids were also observed between them, whereas there seemed to be relatively small amounts of voids between the grains in ACZTS films. Moreover, based on the analyses of grain-size distributions obtained by measuring major axes of several tens grains in these films (Figure S1), the grains in ACZTS were larger than those in the CZTS film. Formation of relatively large grains in the ACZTS films were also confirmed by their cross-sectional SEM images as shown in Figure S2.


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These results imply that incorporation of Ag is effective for grain growth and suppression of void formation in the present SPD system, as was discussed in the previous report for partial replacement of Cu with Ag in the Cu(In,Ga)S2 system.29 Another point worth noting is that the ACZTS film with a relatively high Ag content (i.e., A(5%)CZTS and A(10%)CZTS films) had many pinholes over the entire surface upon observation of a relatively low magnification using a laser microscope, as shown in Figures 5c and 5d, whereas no appreciable pinhole was observed in CZTS and A(2%)CZTS films (see Figure 4b). CZTS and ACZTS films were then processed into solar cells with structures of Al/indium tin oxide (ITO)/intrinsic ZnO (i-ZnO)/CdS/CZTS (or ACZTS)/Mo/glass. Figures 6a-6c shows current J-V curves, Mott-Schottky plots, and external quantum efficiency (EQE) spectra of thusobtained devices derived from CZTS and ACZTS films. Several parameters extracted from these results are also summarized in Table 1. The device made from the CZTS film exhibited power conversion efficiency (PCE) of 4.7%, which is lower than that achieved in our previous study.16 The appreciably low shunt resistance (Rsh) and high series resistance (Rs) resulted in a decrease in PCE of the present device. As shown in Figure 4a, the presence of voids in the CZTS film would be a main factor for shunts. These voids were also likely to have enhanced the formation of the MoS2 layer due to the ease of diffusion of sulfur vapor during fabrication of the CZTS film, leading to an increase in Rs. The acceptor density (NA) of the CZTS-based device obtained by capacitance-voltage (C-V) measurement tended to be high. Theoretical study in the literature claimed that formation energy of Cu on Zn antisite defect (CuZn) is much lower than other point defects such as Cu vacancy.18 Therefore, we can expect presences of large amounts of CuZn in the present CZTS film compared to the other ones: a probable explanation for the large NA value observed for the CZTS-based device is the presence of large amounts of CuZn antisite


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defects.17,21,26 The large NA affected the EQE response of the device at a relatively long wavelength region (600-800 nm) due to reduction of the width of the space charge layer (SCL). The presence of large amounts of defects should also enhance Shockley-Read-Hall (SRH) recombination at the SCL. These failures are attributed to relatively small short circuit current density (JSC) and relatively small open circuit voltage (VOC). Significant improvements of device properties were achieved by the use of AZCTS films, especially for the device based on the Ag(2%)CZTS film: the best device achieved PCE of 7.1%. As discussed in the literature,22 the Ag-based kesterite (i.e., Ag2ZnSnS4 (AZTS)) was expected to include a small number of AgZn antisite defects because significant differences in ionic radiuses between Ag and Zn ions lead to relatively high formation energy of the defect. Hence, Ag incorporation in the present system should contribute the reduction of the CuZn defect, leading to reduction of the NA value. As discussed above, the absence of appreciable voids in the Ag(2%)CZTS film is also beneficial for the solar cell device. It should be noted that the built-in potential (Vbi) of the Ag(2%)CZTS-based device was larger than that of the CZTS-based device. Based on the results of above photoelectrochemical analyses, enlargement of Vbi by incorporation of Ag into CZTS caused the positive shift of the FB potential (i.e., valence band maximum (VBM)). Thus, the relatively large VOC achieved by the Ag(2%)CZTS-based device can be explained by differences in the energy structures. However, incorporation of larger amounts of Ag was detrimental in the devices. Since the NA value of the Ag(10%)CZTS film became larger than that of the Ag(2%)CZTS film, the Ag component would have other unfavorable effects on the film quality though the effects have not been clarified. Another notable failure of the Ag(10%)CZTS-based device is the presence of pinholes in the Ag(10%)CZTS film. Indeed, induction of shunts resulted in significant lowering of VOC in the


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Ag(10%)CZTS-based

device.

Since

the

A(5%)CZTS

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film

also

had

similar

structural/morphological failures, the solar cell based on the film showed relatively low solar cell properties in comparison with those of the cell composed of the A(2%)CZTS film. Thus, the device based on the A(2%)CZTS film is likely to be close to the optimum though further optimizations were not performed at present. For further evaluation of device properties, electric structures of CdS-CZTS and CdSACZTS heterojunctions were evaluated by XP spectroscopy.28,30 Figure 7 shows XP spectra of CdS/CZTS and CdS/ACZTS films (i.e., CZTS and ACZTS films covered by a CdS layer without processing coverage of Al, ITO, and i-ZnO layers) at a shallow binding energy (BE) region (< 25 eV). Firstly, all of the samples were bombarded by Ar+ for 5 min to expose clean CdS surfaces. As a result, all of the samples showed an intense peak derived from the Cd4d core in addition to a broad spectrum at the valence band region. Assuming that onset BEs of valence band spectra were VBM of CdS, we determined energy differences (∆Es) between the Cd4d core level and VBM of CdS in CdS/CZTS and CdS/ACZTS films After the surfaces of CdS/CZTS and CdS/ACZTS had been bombarded by Ar+ for ca. 40 min to be etched to some extent, Zn3d core level peaks appeared in addition to Cd4d peaks, indicating appearances of CdS/CZTS and CdS/ACZTS heterointerfaces. After deconvolution of these spectra to separate Cd4d and Zn3d peaks, ∆Es between the Cd4d core and the Zn3d core were estimated. When further Ar+ ion etching for ca. 80 min was carried out, the Cd4d peak disappeared and the XP spectra showed surface structures of CZTS and ACZTS layers. As shown in our previous study, the Eg value of the surface-covered CdS was determined to be 2.44 eV.28 Regarding Egs of CZTS and ACZTS films, transformation of EQE spectra (Figure 4c) into incident photon energy (hν)-vs-(hν×ln(1EQE))2 plots was applied. From intersects of linear portions of these plots, Egs of CZTS,


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A(2%)CZTS, A(5%)CZTS, and A(10%)CZTS films were determined to be 1.48 eV, 1.47 eV, 1.47 eV, and 1.48 eV, respectively: a slight Eg bowing appeared, as expected from our previous study on CZTS and ACZTS powder systems.31 Based on these values and parameters, schematic energy diagrams of CdS/CZTS and CdS/ACZTS heterointerfaces were obtained, as shown in Figure 8. Significant results of these analyses are differences in the conduction band alignments, i.e., ∆E values between conduction band minimum (CBM) of CdS and those of CZTS (or ACZTS). The ∆E value between CdS and CZTS is largely negative (-0.25 eV), indicating cliff-type conduction band alignment (Figure 8a), in agreement with many results in the literature.3,10,32,33 Although cliff-type conduction band alignments were also observed for CdS-ACZTS heterojunctions (Figures 8b-8d), ∆E values became lower than that of the CdS-CZTS heterointerface. As discussed above, these differences in ∆E are likely to originate from positive shifts of FB potentials induced by incorporation of Ag in the CZTS crystalline lattice, leading to increases in Vbis as well as efficient suppression of interface recombination.

4. CONCLUSIONS We have demonstrated fabrication of Ag-containing CZTS thin films applicable for photovoltaics: the best PCE achieved in this study was 7.1% using a device based on the Ag(2%)CZTS film. The value is one of the best among the devices based on Ag-incorporated pure sulfide kesterites,26 while much higher PCEs were achieved by using the devices composed of selenide containing kesterites (i.e., Ag-incorporated Cu2ZnSn(S,Se)4).24,25 Various analyses suggested that PCE values in the present system depend strongly on film morphologies (e.g., presence of voids and pinholes), defect disparities, and band alignments toward the CdS layer.


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Since these properties would not have trade-off relations, the strategy for Ag incorporation is promising for further improvements of PCE of CZTS-based solar cells.

TABLES Table 1. Solar cell, electric, and optical parameters of CZTS- and ACZTS-based devices.

Absorber layer

CZTS

A(2%)CZTS

A(5%)CZTS

A(10%)CZTS

JSC / mA cm-2

13.9

18.5

17.0

16.6

VOC / mV

650

669

645

625

FF

0.524

0.576

0.554

0.571

PCE (%)

4.73

7.14

6.08

5.92

Rs / Ω cm2

4.40

2.90

3.8

2.00

Rsh / Ω cm2

197

240

200

215

NA / cm-3

6.02×1016

2.24×1016

-a

7.32×1016

Vbi / V

0.803

0.887

-a

0.940

Eg / eV

1.48

1.47

1.47

1.48

a

Not measured

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT One of authors (Thi Hiep Nguyen) would like to acknowledge MEXT, Japan for providing a scholarship during the study. This work was supported by JSPS KAKENHI Grant Number JP15H00875 in Scientific Research on Innovative Areas “Artificial Photosynthesis (AnApple)”, JSPS KAKENHI Grant Number JP15H03850 in Scientific Research (B) from MEXT Japan, and the Matching Planner (MP) program from JST Japan.

SUPPORTING INFOMATION Size distributions of grains in CZTS, A(2%)CZTS, , A(5%)CZTS and A(10%)CZTS films and cross-sectional SEM images of CZTS, A(2%)CZTS, , A(5%)CZTS and A(10%)CZTS films

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Enhancement of Open-Circuit Voltage of Solution-Processed

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SYNOPSIS


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Figure1. (a) XRD patterns of CZTS and ACZTS films and (b) their magnified images at 2θ of 28°-29°, 32.5°-33.5°, and 55.5°-65.5°, (c) Raman spectra of CZTS and ACZTS films. 120x93mm (200 x 200 DPI)


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Figure 2. Depth profiles of (a) CZTS, (b) A(2%)CZTS, (c) A(5%)CZTS, and (d) A(10%)CZTS films obtained by SIMS analyses. 99x79mm (300 x 300 DPI)



Figure 3. J-V curves of (a) CZTS, (b) A(2%)CZTS, (c) A(5%)CZTS, and (d) A(10%)CZTS films obtained from an aqueous 0.2 M Eu(NO3)3 solution (pH 4) under chopped illumination of simulated sunlight (AM1.5G). Arrows indicate photocurrent onsets, defined as the leading potential showing 0.02 mA cm-2 of cathodic photocurrent. 99x91mm (300 x 300 DPI)


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Figure 4. Top-view SEM images of (a) CZTS, (b) A(2%)CZTS, (c) A(5%)CZTS, and (d) A(10%)CZTS films. 99x77mm (300 x 300 DPI)



Figure 5. Laser microscopy images of (a) CZTS, (b) A(2%)CZTS, (c) A(5%)CZTS, and (d) A(10%)CZTS films. 99x75mm (300 x 300 DPI)


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Figure 6. (a) J-V curves of CZTS- and ACZTS- based devices under illumination (AM1.5G), (b) C-V characteristics of CZTS- and ACZTS-based devices, (c) EQE spectra of CZTS- and ACZTS-based devices measured in a short circuit condition, and (d) photon energy (hν) vs (hν×ln(1-EQE))2 plots of EQE results to determine Eg of CZTS and ACZTS films. 99x79mm (300 x 300 DPI)



Figure 7. XP spectra of (a) CdS/CZTS, (b) CdS/A(2%)CZTS, (c) CdS/A(5%)CZTS, and (d) CdS/A(10%)CZTS heterostructures obtained after Ar+ ion etching for 5 min (upper), 39.5 min (middle), and 85 min (bottom). 134x92mm (300 x 300 DPI)


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Figure 8. Energy diagrams of (a) CdS/CZTS, (b) CdS/A(2%)CZTS, (c) CdS/A(5%)CZTS, and (d) CdS/A(10%)CZTS heterojunctions estimated from XP spectroscopy and EQE data. The Eg value of the CdS layer was obtained from reference 31. 99x109mm (300 x 300 DPI)


Structural and Solar Cell Properties of a Ag-Containing Cu2ZnSnS4

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