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Interface Analysis of Cu(In,Ga)Se2 and ZnS Formed Using Sulfur Thermal Cracker
Interface Analysis of Cu(In,Ga)Se2 and ZnS Formed Using Sulfur Thermal Cracker
ETRI Journal. 2016. Apr, 38(2): 265-271
Copyright © 2016, Electronics and Telecommunications Research Institute (ETRI)
  • Received : July 31, 2015
  • Accepted : February 18, 2016
  • Published : April 01, 2016
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About the Authors
Dae-Hyung Cho
Woo-Jung Lee
Jae-Hyung Wi
Won Seok Han
Tae Gun Kim
Jeong Won Kim
Yong-Duck Chung

Abstract
We analyzed the interface characteristics of Zn-based thin-film buffer layers formed by a sulfur thermal cracker on a Cu(In,Ga)Se 2 (CIGS) light-absorber layer. The analyzed Zn-based thin-film buffer layers are processed by a proposed method comprising two processes — Zn-sputtering and cracker-sulfurization. The processed buffer layers are then suitable to be used in the fabrication of highly efficient CIGS solar cells. Among the various Zn-based film thicknesses, an 8 nm–thick Zn-based film shows the highest power conversion efficiency for a solar cell. The band alignment of the buffer/CIGS was investigated by measuring the band-gap energies and valence band levels across the depth direction. The conduction band difference between the near surface and interface in the buffer layer enables an efficient electron transport across the junction. We found the origin of the energy band structure by observing the chemical states. The fabricated buffer/CIGS layers have a structurally and chemically distinct interface with little elemental inter-diffusion.
Keywords
I. Introduction
Power conversion efficiencies (PCEs) of over 20% in lab-scale Cu(In,Ga)Se 2 (CIGS) thin-film solar cells have been recently achieved by several research groups [1] [4] . This excellent performance is nearly comparable to Si-based photovoltaic devices. Moreover, CIGS thin-film solar cells can be prepared on flexible substrates such as polyimide [5] , [6] , stainless steel [7] [9] , and metal foil [10] . This mechanical flexibility is very advantageous compared to other rigid wafer-type photovoltaic devices based on Si, GaAs, and organic compounds [11] .
Usually, n -type buffer layers are deposited between the CIGS light-absorber layers and the transparent oxide front contact. An n -type buffer layer forms a p-n junction with a p -type CIGS layer and is known to protect the sensitive interface during the subsequent sputtering process of the window layers [12] [14] .
High-efficiency CIGS solar cells have typically used chemical-bath-deposited CdS buffer layers [1] , [2] , [15] , [16] . However, these types of layers need to be deposited through a vacuum process for process compatibility and to be constructed from non-toxic materials (instead of toxic Cd) for environmental safety.
Thus, we used a two-step process including Zn-sputtering and sulfurization with a thermal cracker, which are dry and non-toxic processes for making Zn-based buffer films. This method was first introduced in [17] .
However, there have been few studies on depth-dependent electronic, chemical, and structural properties of buffer/CIGS solar cells. In this study, we investigated interfacial properties of Zn-based buffer layers prepared on CIGS absorber layers. This interfacial study will contribute largely to a further improvement in the ZnS/CIGS photovoltaic performances.
II. Experimental Details
To prepare the ZnS films, a Zn thin-film was first sputtered onto a CIGS absorber layer at room temperature. The dc power and working pressure were 40 W and 12 Pa, respectively. The Zn-deposited sample was successively sulfurized using a sulfur thermal cracker. Figure 1(a) shows a schematic diagram of the sulfur cracker process. The reservoir zone evaporates the solid sulfur sources (99.9% purity, Materion), and the cracking zone then heats up the vapour-phase sulfur to crack it into smaller sulfur molecules hawing higher reactivity. Here, T R-zone , T C-zone , T sub , and the duration time of the sulfurization were 150 °C, 700 °C, 550 °C, and 10 min, respectively. Figure 1(b) shows a photograph of vacuum chambers consisting of Zn sputtering, sulfurization, and a load-lock.
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(a) Schematic diagram of sulfur thermal cracker for sulfurization of sputtered Zn film and (b) photograph of equipment consisting of Zn sputter, sulfurization chamber, and load-lock chamber.
The solar cell structure of the Al/Ni/ITO/ZnO/Zn-base buffer/CIGS/Mo was fabricated on soda-lime glass substrates. The CIGS film was deposited using a co-evaporation method through a multistage process. Detailed information of our solar cell baseline process can be found in previous papers [17] [20] . We prepared six 0.47 cm 2 sized cells using different buffer-layer thicknesses.
The electrical performance of the solar cells was characterized through a current density–voltage ( J–V ) analysis under a global air mass 1.5 spectrum for 1000 Wm −2 irradiance at room temperature. The energy band-gap ( E g ) of the buffer layer was observed through reflection electron-energy-loss spectra (or REELS, VG ESCALAB 210) measurements. The ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measured the electronic structure and secondary electron cut-off region of the valence band, and chemical states of each elemental atom with Ar + sputtering depth profiling (1 keV, 4 × 10 −4 Pa), respectively. The UPS measurement used a He I (hω = 21.22 eV) discharge lamp as an excitation source with a sample bias of −10 V to measure the low kinetic energy region. The XPS measurement used an Mg Kα (hω = 1248.5 eV) without a monochromator. The energy resolutions were approximately 0.1 eV and 1.0 eV for UPS and XPS, respectively. Three minutes of Ar ion etching was carried out before the measurement to remove the carbon contaminants and adsorbates on the film surfaces (0.35 keV, mild condition). We observed the film morphology using an SEM (FEI, Magellan400). An SIMS (CAMECA IMS 6f) analysis of the ZnS/CIGS films was carried out using O 2+ primary ions with an impact energy of 15 keV and a 5 nA current.
III. Results and Discussion
We fabricated CIGS solar cells by varying the thickness of the sulfurized Zn films. In this work, we refer to sulfurized Zn as “ZnS” because a sulfurized Zn film mainly includes ZnS, as shown in detail in Fig. 6 . To obtain ZnS thicknesses ( t ZnS ) of 8 nm, 15 nm, 20 nm, and 30 nm, initial Zn thin-films were prepared at thicknesses of 3 nm, 5 nm, 7 nm, and 10 nm, respectively. In our previous work [17] , [21] , we observed that the Zn thickness increases about three-fold after the cracker-sulfurization process.
The J–V curves and cell parameters of the best-efficiency cell under each condition are shown in Fig. 2 and Table 1 , respectively. The solar cell performance was largely influenced by t ZnS . The solar cell containing 8 nm–thick ZnS showed a much higher PCE (7.97%) than the cell without a ZnS layer (3.99%). However, a ZnS thicker than 8 nm degraded the cell performance, particularly in the short circuit current ( J SC ). Additionally, we obtained the best cell efficiency of 12.6% by optimizing the ZnO/ITO window layer of the 8 nm ZnS/CIGS cell. The detailed parameters are shown in Table 1 . The J SC was largely improved by reducing the ITO thickness from 150 nm to 50 nm because of the optical gain in the short wavelength range. In addition, the open circuit voltage ( V OC ) was significantly increased through a reduction in the RF power from 400 W to 100 W during the ZnO deposition, which is likely because of the lowered plasma damage.
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(a) Schematic of CIGS solar cell structure. (b) J–V curves of ZnS-buffer-employed CIGS solar cells with different tZnS.
Solar-cell parameters of CIGS solar cells with various ZnS thicknesses[17].
tZnS (nm) Process condition of ZnO/ITO PCE (%) VOC (V) JSC (mA/cm−2) Fill factor (%)
0 (w/o) Normal 3.99 0.369 28.81 37.4
8 Normal 7.97 0.435 32.89 55.7
Optimized 12.60 0.504 37.40 66.8
15 Normal 3.62 0.419 17.51 49.4
49.4 Normal 2.36 0.420 10.97 51.3
30 Normal 0.19 0.119 4.12 39.2
To verify the origin of the thickness-dependent property of the ZnS-buffered solar cell, we investigated the interfacial properties of the best-performance cell (8 nm ZnS/CIGS sample). Figure 3(a) shows E g of a thin ZnS film prepared on a CIGS as observed using a REELS measurement [17] . The incident primary electron beam energy ( E 0 ) is known to be proportional to the penetration depth of the incident beam. This means that a lower E 0 provides more surface-sensitive information. The value of E g taken near the surface of the ZnS film ( E g = 3.40 eV) was slightly lower than that at the interface of the film ( E g = 3.56 eV).
The UPS measurements with the sputtering process revealed the distinguishable differences between the Fermi level ( E F ) and E VB ( E F E VB ) and work function (WF) as a function of depth of the ZnS/CIGS layers. As shown in Fig. 3(b) , E F E VB were 2.05 eV, 2.25 eV, and 2.53 eV at sputtering times of 3 min, 5 min, and 10 min, respectively; whereas, the WF decreased from 4.80 eV to 4.54 eV. At a sputtering time of 40 min, E F E VB and WF were 0.85 eV and 5.22 eV, respectively, which indicates that the near-interface CIGS had a lower E F E VB and higher WF than the ZnS film. The value of E F E VB of the CIGS layer was comparable to the value in [22] .
A band alignment of the ZnS and CIGS layers is displayed in Fig. 4 using values obtained from the UPS results in Fig. 3 . The measured values of E g , E VB , and WF were taken into account to illustrate the band structure at the ZnS surface, ZnS interface, and CIGS interface. The conduction band minimum energy ( E CB )– E F near the interface slightly shifted toward E F compared with near the surface, which indicates that the n -type property was more enhanced at the interface of the ZnS film.
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(a) REELS results at different incident electron energies of 500 eV and 1,780 eV and (b) UPS results measured at different sputtering times of 3 min, 5 min, 10 min, and 40 min in ZnS/CIGS layers. EFEVB and WF were obtained by extrapolating curves.
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Band diagram of fabricated ZnS/CIGS layers. Evac, ECB, EF, and EVB were displayed at different depths: near-buffer-surface, near-buffer-interface, and near-CIGS-interface. Values of Eg, EVB, and Evac were determined through REELS and UPS measurements, as shown in Fig. 3.
This band structure seems to positively influence the photovoltaic performance of the ZnS/CIGS solar cell. Electron-hole pairs are generated at the CIGS surface by the incident light. The generated electrons (minority carrier) move toward the n -type transparent conducting oxide. Thus, the electrons should go over the barrier of difference between E CB, ZnS and E CB, CIGS . We believe that the generated electrons can easily move through the ZnS layer because of the lower E CB at the interface. Although the CIGS bulk showed a p -type property [23] , the diagram indicates that the near-interface CIGS shows an n -type property [24] . The Zn contained in the ZnS film diffuses slightly into the CIGS layer during the deposition process of the ITO window layer [24] , [25] . The diffused interstitial Zn atoms in the CIGS become donors and induce a charge compensation of a p -type CIGS [26] .
Thus, the t ZnS -dependent photovoltaic performance can be explained through the band alignment shown in Fig. 4 . The E CB values measured from the ZnS interface and ZnS surface in contact with CIGS appear to be 0.68 eV and 1.00 eV, respectively. This means that the thicker ZnS film causes a wider E CB barrier against the electrons. The drop of the J SC of the solar cell with the thicker ZnS film, as shown in Fig. 2 and Table 1 , agrees well with the E CB alignment through the ZnS/CIGS layers.
To investigate the origin of the change in the band alignment of ZnS/CIGS, XPS measurements were carried out as shown in Fig. 5 . The Ar + sputtering time was varied from 3 min to 25 min to find the depth-dependent chemical states. The binding energy spectra of Zn 2p 3/2 , O 1s, and S 2p (Se 3p) were taken, as shown in Fig. 5 .
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XPS spectra measured at various sputter times of 3 min to 25 min near binding energies of Zn 2p3/2, O 1s, and S 2p (Se 3p).
The Se 3p peaks, with binding energies of 166.5 eV and 161.0 eV, appeared after sputtering for 15 min; thus, the spectra measured during minutes 3 through 10 are of the ZnS film. In the ZnS film, the binding energies of Zn 2p 3/2 and S 2p were 1022.6 eV and 162.4 eV, respectively, without a binding energy variation over the depth. On the other hand, from the O 1s spectra, we discovered that the undesired oxygen (O) and hydroxide (OH) were incorporated in ZnS films originating from ZnO and Zn(OH) 2 , respectively [27] . The Zn(OH) 2 was relatively dominant near the surface, whereas ZnO was dominant at a deeper position. The Zn(OH) 2 /ZnO content ratios estimated through an XPS quantification in the 3 min and 5 min sputtered samples were 1.56 and 0.88, respectively. The dominant ZnO phase near the interface was believed to enlarge E g and enhance the n -type property of the film compared with the Zn(OH) 2 or Zn(O, S) phase near the surface. The phase formation of ZnO and Zn(OH) 2 is likely due to the air exposure after the Zn deposition because the Zn ions can easily react with H 2 O and O 2 . Because the heat of formation of Zn(OH) 2 (−153.66 kcal mole −1 ) is lower than that of ZnO (−83.36 kcal mole −1 ) [28] , the remaining metallic Zn even after a sulfur reaction seems to be dominantly formed with the air exposure. The Zn, detected in the CIGS film, appeared to be an artifact induced by preferential sputtering during the measurement rather than an actual presence in the CIGS film.
We observed the SEM and SIMS to characterize the structural property and depth-dependent elemental composition across the ZnS/CIGS layers. As shown in Figs. 6(a) through 6(d) , a ZnS layer with small grains was observed, which was clearly distinguishable from the CIGS layer. Moreover, a ZnS thin-film was uniformly formed on the indented CIGS surface, although ZnS is likely to form islands with a Volmer–Weber mode growth on the CIGS surface because the surface energy of the ZnS film ((002); that is, approximately 0.90 J·m −2 ) is larger than that of the CIGS substrate (0.32 − 0.71 J·m −2 ) [29] .
The SIMS result, illustrated in Fig. 6(e) , shows that both the Zn and S contents rapidly decreased over the depth. The Zn content decreased by 90% and 99% at a depth of 30 nm and 70 nm, respectively. On the other hand, the S content decreased more sharply by about 90% and 99% at a depth of 25 nm and 50 nm, respectively. Thus, the diffusion of Zn and S into the CIGS layer seemed to be negligible during the Zn sputtering and sulfurization processes [18] , [25] . The O content maintained a certain level at a 35 nm depth, whereas it decreased by two-thirds and showed a constant level throughout the CIGS layer. The high O content seemed to have originated from the ZnO and Zn(OH) 2 phases observed in Fig. 5 .
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Cross-sectional and top SEM images of ((a), (b) and (c), (d), respectively) of and ZnS/CIGS ((b) and (d)) layers, and (e) SIMS depth profiles of Zn, O, and S elements (colored lines). Cu, In, Ga, and Se profiles are drawn in black lines.
IV. Conclusion
We used cracker-sulfurized ZnS thin-films as a buffer layer in CIGS photovoltaic devices. Among the various values of t ZnS , a CIGS solar cell with 8 nm–thick ZnS showed the highest PCE of 12.6%. The energy levels, chemical states, and physical properties of the 8 nm ZnS/CIGS interfaces were investigated. The E CB value of ZnS was much higher than that of CIGS; however, the ZnO phases near the interface lower the value of E CB , thus allowing the electrons to more easily move from the CIGS through the ZnS layer. The ZnS/CIGS bilayer had a sharp interface without a notable elemental inter-diffusion. This study suggests the possibility of PCE enhancement of CIGS solar cells through an in-depth interface analysis.
Acknowledgements
This work was supported by the “Development of 25% Efficiency Grade Tandem CIGS Thin Film Solar Cell Core Technology” project of the Ministry of Science, ICT and Future Planning (MSIP), and the Korea Research Council for Industrial Science and Technology (ISTK) of the Republic of Korea (Grant B551179-12-01-00). The authors would like to express their appreciation for the financial support provided by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20153010011990 and No. 20153000000030).
BIO
Corresponding Author dhcho@etri.re.kr
Dae-Hyung Cho received his BS and MS degrees in electrical engineering from Korea University, Seoul, Rep. of Korea, in 2007 and 2009, respectively. In February 2009, he joined ETRI, where he is currently a senior researcher. His research interests include the fabrication and characterization of Cu(In,Ga)Se2 thin-film solar cells.
mirujoa@etri.re.kr
Woo-Jung Lee received her PhD degree in physics from Yonsei University, Seoul, Rep. of Korea, in 2012. Since 2013, she has been working for the IT Components and Materials Industry Technology Department, ETRI. Her main research interest is the analysis of electrical properties of p-n junctions in CIGS-based solar cells.
klnaru32@etri.re.kr
Jae-Hyung Wi received his BS degree in electro-physics from Dankook University, Cheonan, Rep. of Korea, in 2009 and his MS degree in renewable energy from Chung-Ang University, Seoul, Rep. of Korea, in 2011. Since 2012, he has been working for the IT Advanced Materials Research Section, ETRI. His research interests include advanced engineering with characterization and application of thin-film solar cells.
wshan@etri.re.kr
Won Seok Han received his BS, MS, and PhD degrees in physics from Chungnam National University, Daejeon, Rep. of Korea, in 1993, 1995, and 2000, respectively. Since 2001, he has been with ETRI and is currently a principal member of the engineering staff. His research interests include growth of III-V compound semiconductors for lasers, solar cells, and related optoelectronic devices.
taegun0629@gmail.com
Tae Gun Kim received his BE degree in polymer engineering from Kumoh National Institute of Technology, Gumi, Rep. of Korea, in 2011. He is currently working toward his MS and PhD degrees in nano-science at Korea University of Science and Technology, Daejeon, Rep. of Korea. His research interests include the characterization of thin-film solar cells.
jeongwonk@kriss.re.kr
Jeong Won Kim received his PhD degree in chemistry from the Korea Advanced Institute of Science and Technology, Daejeon, Rep. of Korea, in 1997. He worked as a research associate at Yonsei University, Seoul, Rep. of Korea and Fritz-Haber Institute, Berlin, Germany, for each of two years. In 2004, he joined the Korea Research Institute of Standards and Science, Daejeon, Rep. of Korea, as a staff scientist. His research interests include the measurement of interfacial electronic structures and carrier dynamics on thin-films.
ydchung@etri.re.kr
Yong-Duck Chung received his BS, MS, and PhD degrees in physics from Yonsei University, Seoul, Rep. of Korea, in 1995, 1997, and 2002, respectively. In 2002, he joined ETRI, where he is currently a principal researcher. Since 2010, he has also been with the Department of Advanced Devices Engineering, Korea University of Science and Technology, Daejeon, Rep. of Korea, where he is a professor. From December 2008 to January 2009, he was a visiting scientist at the University of Delaware, Network, DE, USA, where he studied thin-film solar cell technologies. He is currently involved in a project on the development of a fabrication process for a light-weight flexible Cu(In,Ga)Se2 thin-film photovoltaics module and tandem solar cell technologies. His research interests include the fabrication and characterization of thin-film photovoltaics devices. He is the author or co-author of more than 70 peer-reviewed papers and 220 conference presentations related to thin films and solar cells. He has been a program committee member of several international conferences and a reviewer of leading journals on solar cells and thin films.
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