Control of Structural and Electrical Properties of Indium Tin Oxide (ITO

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Control of Structural and Electrical Properties of Indium Tin Oxide (ITO)/Cu(In,Ga)Se2 Interface for Transparent Back-Contact Applications Yu-Seung Son,†,‡ Hyeonggeun Yu,† Jong-Keuk Park,† Won Mok Kim,† Seung-Yeop Ahn,† Wonjun Choi,† Donghwan Kim,‡ and Jeung-hyun Jeong*,† †

Korea Institute of Science and Technology (KIST), 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Korea College of Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 02841, Korea

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‡

S Supporting Information *

ABSTRACT: Development of transparent-conducting oxide (TCO) back contact for Cu(In,Ga)Se2 (CIGS) absorber is crucial for bifacial CIGS photovoltaics. However, inherent GaOx formation at the TCO/CIGS interface has hampered the photocarrier extraction. Here, by controlling the Na doping scheme, we show that the hole transporting properties at the indium−tin oxide (ITO)/CIGS back contact can be substantially improved, regardless of the GaOx formation. Na incorporation from the glass substrate during the GaOx forming phase created defective states at the interface, which allowed efficient hole extraction from CIGS, while post Na treatment after GaOx formation did not play such a role. Furthermore, we discovered that an almost GaOx-free interface could be made by reducing the underlying ITO film thickness, which revealed that ITO/CIGS junction is inherently Schottky. In the GaOx-free condition, post-Na treatment could eliminate the Schottky barrier and create ohmic junction due to generation of conducting paths at the interface, which is supported by our photoluminescence analysis.

1. INTRODUCTION Cu(In,Ga)Se2 (CIGS) thin-film solar cells have attracted much interest as a renewable energy source, because of their merits, such as low-cost production, high-efficiency performance,1,2 and strong potential for flexible,3−5 semitransparent (or bifacial),6,7 and tandem cell applications.8 Recently, CIGS solar cells with an outstanding photovoltaic conversion efficiency (PCE) exceeding 22% have been reported,1,9 spurring on research efforts to achieve even higher conversion efficiencies. In addition, semitransparent, bifacial, or tandem applications are the next promising targets in efforts to develop high-efficiency CIGS solar cells, and use of a transparent conducting oxide (TCO) as an alternative back contact is an essential ingredient to allowing CIGS solar cells to be used in these applications.7,10 Molybdenum (Mo) film has been used as a back-contact electrode for CIGS solar cells due to the ohmic contact with CIGS film. However, it is optically opaque and thus not applicable to semitransparent or bifacial applications. If CIGS solar cells are to be developed onto a TCO back contact, formation of ohmic contact at the TCO/CIGS interface will be essential. To date, transparent back contacts used for CIGS solar cells have received little attention, although several precedent studies were reported as follows. Indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and Al-doped zinc © XXXX American Chemical Society

oxide (AZO) are considered as potential TCO back contacts. AZO and ITO back contacts produced GaOx phases at the interface during CIGS growth; these phases are highly resistive n+ materials that may produce GaOx/CIGS n−p junction that blocks hole extraction from CIGS.10−12 This problem, which is related to the GaOx phase, appeared to be more significant for AZO than for ITO, and GaOx formation was found to be suppressed on ITO if the CIGS growth temperature was less than 520 °C.12 In contrast, FTO showed a lower propensity for GaOx formation, but the fluorine donors may be volatile at temperatures above 500 °C, which caused the FTO to become much more resistive and thus degraded the performance of the CIGS solar cell.12 Consequently, if the growth temperature of CIGS films is reduced below 500 °C, thus minimizing the formation of GaOx interlayer and fluorine loss, FTO or ITO back contacts could allow for a fairly good junction with CIGS films. In addition, if there is no GaOx formation, the ITO(or FTO)/CIGS junctions were shown to be ohmic, judging from good linearity in dark j−V curves measured in the ITO (or FTO)/CIGS(or CGS)/Au structure at room temperature (RT)).11 Received: November 17, 2018 Revised: January 2, 2019 Published: January 3, 2019 A

DOI: 10.1021/acs.jpcc.8b11149 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Table 1. Properties of Five Different ITO Back Contact Electrodes and Photovoltaic Performances of Corresponding ITO/ CIGS Solar Cellsa sample

back contact

A B

ITO(200) ITO(200)/Mo(10)

C D E

ITO(200) ITO(600) SiOx/ITO(600)

Na doping

η (%)

open-circuit voltage, VOC (V)

short-circuit current, JSC (mA/cm2)

fill factor, FF (%)

nm, R = 8.4 Ω/sq nm (Mo: 10 nm),

no NaF PDT no NaF PDT

7.5 8.5

0.540 0.532

32.1 31.4

43.1 51.1

nm, R = 8.4 Ω/sq nm, R = 3 Ω/sq nm (SiOx: 1 μm),

NaF PDT NaF PDT NaF PDT

14.6 7.8 0.1

0.653 0.508 0.344

33.5 32.9 1.2

66.7 46.9 17.5

ITO property thickness = 200 thickness = 200 R = 7.9 Ω/sq thickness = 200 thickness = 600 thickness = 600 R = 3.1 Ω/sq

a

Anti-reflective coating (ARC) was not applied.

Figure 1. Varying ITO back-contact structures on sodalime glass used to deposit CIGS films at 450 °C: (a) device A: ITO (200 nm) and (b) device B: ITO (200 nm)/Mo (10 nm), which are subjected to no NaF PDT; (c) device C: ITO (200 nm), (d) device D: ITO (600 nm), and (e) device E: SiOx/ITO (600 nm), which are subjected to NaF PDT after CIGS deposition.

insight on how to electrically improve the ITO/CIGS junction characteristic to lead to high-efficiency solar cells.

In contrast, it was reported recently that the TCO/CIGS interface could form ohmic contact even though the CIGS absorber was grown on TCO back contact at a temperature of >550 °C.13−15 We have also seen ohmic-like j−V characteristics in our ITO/CIGS cells fabricated at a temperature of >550 °C. They are possibly the cases that the GaOx properties were modified in a way that the interfacial barrier was reduced. In fact, Heinemann et al. speculated in their study, using a superstrate cell structure consisting of glass/AZO/i-ZnO/ CIGS/Au such that not only the donor states but also the acceptor states might be possibly generated within the GaOx phase by incorporation of Sn, Zn, Cu, In, Se, and their mutual compensation would change the semiconducting properties.16 On the other hand, a beneficial role of Na doping was speculatively proposed that Na may be involved in forming an ohmic-like ITO/CIGS interface or in changing the electrical properties of GaOx to favor hole transport, but there has been no solid evidence for it.13,14 Consequently, although previous studies have provided some useful knowledge on TCO/CIGS interfaces, many aspects of the interfaces have yet to be elucidated to further improve TCO/CIGS solar cells. For example, it remains unclear what would be the condition for TCO/CIGS ohmic contact, what drives the interfacial GaOx formation, how to control their electronic nature, and so on. Here, we unravel the structural and electrical properties of ITO/CIGS interface associated with interfacial GaOx formation and demonstrate improved junction characteristics for efficient photovoltaic performance. We show that ITO back contact thickness and Na doping scheme are crucial factors to control the junction property. Particularly, the roles of Na incorporation at the interface are investigated with electrical, structural, and photophysical analysis. To avoid severe GaOx formation at the ITO/CIGS interface, we employed a reduced process temperature of 450 °C when growing a CIGS absorber layer on the ITO back contact, followed by NaF postdeposition treatment (PDT). At the end, we will provide

2. EXPERIMENT 2.1. ITO Film Preparation. ITO thin films were deposited on soda-lime glass (SLG) by radio-frequency magnetron sputtering at elevated temperature (working pressure of 1 mTorr, substrate temperature (Tsub) of 270 °C, O2/(O2 + Ar) of 0.75% in a flow rate). The thickness of the ITO was changed from 200 nm to 600 nm to obtain a lower series resistance for the solar cell. Details are listed in Table 1. Some of ITO films were deposited on SLG/SiOx where SiOx prevents Na diffusion from SLG during CIGS growth. The SiOx layer was deposited via plasma-enhanced chemical vapor deposition (PECVD) and is a fairly effective barrier against Na diffusion.17 ITO thin films were annealed under Se vapor for 1 h at 550 °C to check its stability during CIGS deposition. It turned out that its electrical resistivity was slightly decreased by the increase of carrier density or mobility, proving its high-temperature stability (see Table S1 in the Supporting Information). 2.2. CIGS Film Deposition and Device Fabrication. With the aim of suppressing formation of GaOx at the ITO/ CIGS interface, the growth temperature of CIGS films was reduced to 450 °C, 100 °C below the normal temperature of CIGS deposition of 550 °C. The low-temperature process was based on the three-stage co-evaporation process. However, it was modified to alleviate the severe nonuniformity of Ga depth distribution, because of the slower diffusion rate of the Ga atom at 450 °C, compared with the 550 °C process, similar to the process suggested by Tiwari et al.3 The Cu excess point beyond stoichiometric composition was detected by an inhouse infrared detector that gauges thermal emission from the sample during CIGS fabrication. When the process reached the Cu excess point, thermal emissivity was changed, because of the formation of Cu−Se secondary phase, which will be captured by the detector. After a Cu-poor composition was achieved at the final step of CIGS growth, NaF post-deposition B

DOI: 10.1021/acs.jpcc.8b11149 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 2. Light j−V characteristics of CIGS solar cells fabricated on various ITO back contacts: (a) the comparison of ITO(200) and ITO(200)/ Mo(10) without NaF PDT, and ITO(200) with NaF PDT, and (b) the comparison of ITO(200), ITO(600), and SiOx/ITO(600) with NaF PDT.

treatment (PDT) was applied at 450 °C under a flux of 0.2 Å/s for 20 min for Na doping. CdS buffer layers were deposited using chemical bath deposition (CBD), then 50-nm-thick iZnO and 500-nm-thick AZO were deposited by radio frequency (rf) sputtering. A Ni−Al grid was formed using ebeam evaporation. An antireflective coating was not applied. CIGS solar cells of five different conditions (denoted as A, B, C, D, and E) were fabricated using three back-contact structures (SLG/ITO, SLG/ITO/Mo, and SLG/SiOx/ITO), where ITO thickness and NaF PDT were applied differently, as schematically illustrated in Figure 1. Sample A of SLG/ ITO(200 nm) and sample B of SLG/ITO(200 nm)/Mo(10 nm) were not exposed to NaF PDT, but sample C of SLG/ ITO(200 nm), sample D of SLG/ITO(600 nm), and sample E of SLG/SiOx/ITO(600 nm) were exposed to NaF PDT. Here, the back-contact structures were each named ITO(200), ITO(200)/Mo(10), ITO(600), SiOx/ITO(600) to indicate the layer thickness with the constituent layers, as listed in Table 1. Even when NaF PDT was not applied, Na diffusion from SLG occurred to the CIGS thin films in ITO/CIGS system at 450 °C, but the amount was much lower, by an order of magnitude. The performances of five CIGS solar cells were compared to each other and the effects of ITO thickness, NaF PDT, and SiOx interlayer were discussed. In addition, some ITO (200 nm) films were exposed to O2 microwave plasma prior to CIGS deposition to investigate the effect of ITO surface treatment alone on the electronic nature of the ITO/ CIGS interface. For this, the NaF PDT was not applied. 2.3. Characterization. Photovoltaic performance of the ITO/CIGS solar cells was characterized by current−voltage (j−V) measurements under air mass (AM) 1.5 illumination, and by the external quantum efficiency (EQE). Their temperature-dependent current−voltage characteristics were measured using a liquid nitrogen (LN)-cooled cryostat (LTS420E-P, Linkam, U.K.) and a source meter (Model 2400, Keithley Instruments, USA). Drive-level capacitance profiling (DLCP) method was performed using a Model HP4284 impedance analyzer to evaluate the carrier density. The structure and composition of the ITO/CIGS interface were observed using transmission electron microscopy (TEM) (Talos, Model F200X); the samples for the TEM analysis were prepared using dual-beam focused-ion-beam (FEI) equipment (Helios Nano-Lab 600). The depth profile of elements in the CIGS cells was measured by secondary ion mass spectroscopy (SIMS) (Cameca, Model IMS-4FE7), where the samples were sputtered with Cs+ ions. X-ray photoemission spectroscopy

(XPS) and ultraviolet photoemission spectroscopy (UPS) were measured using a Model PHI 5000 VersaProbe (Ulvac-PHI, Japan) system. The valence band depth profile was acquired by conducting XPS measurement while etching the CIGS layer toward the ITO/CIGS interface by Ar-ion sputtering. Photoluminescence (PL) was measured using 532 nm diodepumped solid-state (DPSS) laser source with the PL signal detection by a monochromator (Model SpectraPro 2300I, ACTON Research Corporation). The sample was mounted in a vacuum chamber connected to a cryostat, which kept the temperature inside the chamber at 50 K.

3. EXPERIMENTAL RESULTS 3.1. Current−Voltage Characteristics of ITO/CIGS Solar Cells, with Respect to Varying Back Contacts, Combined with NaF PDT. Table 1 summarizes the photovoltaic characteristics of the ITO/CIGS solar cells fabricated for five different conditions (see Figure 1), employing different thickness of ITO back contacts, a junction control layer (Mo capping layer), and different Na doping strategies, such as additional NaF PDT or the use of a Na diffusion barrier (SiOx). Their performance is substantially influenced by the thickness of the ITO back contact, the existence of a diffusion barrier (SiOx), and NaF PDT. The PCE is low for the case of ITO(200) without NaF PDT (device A), because of the low open-circuit voltage (VOC) and fill factor (FF). The use of the thin Mo layer between ITO and CIGS (device B) improves the FF substantially, while the VOC is not enhanced. The application of NaF PDT at the end of the CIGS deposition process (device C) then dramatically improves VOC and FF, and the short-circuit current (JSC) improves slightly. However, the use of thicker ITO (600 nm) as a back contact substantially decreases VOC, JSC, and particularly FF (device D), although a positive gain in FF should be expected from the decreased ITO resistance. More interestingly, the addition of SiOx layer between SLG and ITO(600 nm) back contact almost disables the performance of the solar cell (device E). Figure 2 shows the corresponding current−voltage (j−V) characteristics of the ITO/CIGS solar cells, which were measured under illumination of AM 1.5G 1 sun condition. As shown in Figure 2a, the j−V characteristics of devices A and B not subjected to NaF PDT manifest very poor p−n junction diode behavior, indicating that overall recombination of photocarriers is accelerated in their space-charge region and quasi-neutral region within the CIGS layer. On the other hand, C

DOI: 10.1021/acs.jpcc.8b11149 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C the j−V characteristics of devices A and B in a forward bias region (the fourth quadrant) exhibited distinguished behavior. The ITO(200)/Mo(10)/CIGS cell (device B) acts like a single junction diode, while the ITO(200)/CIGS cell (device A) behaves like having an additional diode opposing the behavior of primary CIGS/CdS p−n junction. This result indicates that the difference in j−V characteristics between device A and device B is originated from their back-junction properties distinguished from each other. Mo/CIGS interface is widely accepted as an ohmic contact. Employing the thin Mo layer between ITO and CIGS might form ohmic contact at the interface. In contrast, the j−V characteristics of ITO(200)/ CIGS cell (device A) indicate that the hole injection from the ITO back contact to the CIGS layer is suppressed substantially. Tentatively, it suggests that the ITO/CIGS interface takes on Schottky contact properties. We believe that the degree of distortion of the forward bias region (the fourth quadrant) in the j−V curve may reflect the Schottky barrier height at the ITO/CIGS interface. When NaF PDT is applied to the ITO(200)/CIGS system (device C), the distortion that was observed in device A is removed completely. These findings suggest that the ITO/ CIGS interface inherently forming Schottky contact can be switched to an ohmic-like contact when the level of Na doping is sufficient. As shown in Figure 2b, however, the ITO(600)/ CIGS cell (device D) shows distortion in the first and fourth quadrant of the j−V curve, although it was exposed to the NaF PDT, suggesting the existence of substantial blocking of carrier transport. Furthermore, the SiOx/ITO(600)/CIGS cell (device E) shows a huge degree of j−V distortion, such that most of the photogenerated current is blocked under shortcircuit conditions. From the results, it is obvious that the performance of the ITO/CIGS cells is very sensitive to the ITO film thickness or how Na doping is performed during CIGS deposition. 3.2. Na Concentration and Carrier Density in CIGS Layer Deposited at 450 °C on ITO Back Contacts. The use of ITO(200) and ITO(200)/Mo(10) back contacts without NaF PDT (devices A and B) led to very low cell efficiency, particularly as a result of reduced VOC and FF (Table 1). The deteriorated performance is typical of CIGS solar cells when CIGS film is not sufficiently Na-doped. Na doping has been required to achieve a high-efficiency CIGS solar cell, because it increases the carrier density in CIGS film or passivates its deep-level defects.18 SIMS depth profiles in Figure 3a shows that the Na contents in the CIGS layer for the cases of ITO/CIGS (device A) and ITO/Mo/CIGS (device B) without NaF PDT are reduced by almost an order of magnitude, compared to the case with NaF PDT (device C). Figure 3b compares the carrier density of non-NaF-PDT cell (ITO/Mo, device B) with that of the NaFPDT cell (device C), as measured via the drive-level capacitance profiling (DLCP) method. Here, device B (ITO(200)/Mo(10)) was selected to measure the carrier density of non-NaF-PDT cell instead of device A (ITO(200)), because the back-junction barrier in device A makes the DLCP measurement inaccurate. It is shown that the non-NaF-PDT cell (device B) has a very low carrier density, almost close to that of a Na-free CIGS solar cell. Such a reduced carrier density can account for the reduced VOC and FF. This means that the Na diffusion through ITO from SLG at 450 °C is fairly suppressed below the level required for achieving a high-performance CIGS solar cell. In the case of

Figure 3. (a) SIMS Na profiles of CIGS solar cells fabricated using the 450 °C CIGS growth process: comparison between devices A, B, and C; and (b) their carrier density profiles measured by the drivelevel capacitance profiling (DLCP) method.

conventional CIGS solar cell consisting of SLG/Mo/CIGS deposited at a temperature above 550 °C, Na diffusion from SLG through Mo sufficiently occurs to achieve a highefficiency CIGS solar cell. From this fact, it is assumed that Na diffusion from SLG to CIGS through the ITO film may be more difficult, compared to the diffusion through the Mo film, although the lower substrate temperature could also reduce the Na diffusivity. 3.3. ITO/CIGS Interface Structures: The Effect of ITO Thickness and Na Doping on GaOx Formation. Figure 2b shows that device performances are influenced by varying ITO thickness and the use of SiOx diffusion barrier. The findings raise the following question: what factors in the ITO back contacts are responsible for the large difference? Since the device fabrication process for the three devices (C, D, and E) were identical, except for the use of different back contacts, such as ITO(200), ITO(600), and SiOx/ITO(600), respectively, the differences in their cell performance should originate from the structure and properties of the respective ITO/CIGS interface. We examined the ITO/CIGS interfaces in devices C, D, and E by bright-field (BF) transmission electron microscopy (TEM), as shown in Figures 4a−c. Their corresponding compositional line profiles across the ITO/ CIGS interface, as measured by energy-dispersive spectrometery (EDS), are also compared in Figures 4d−f. Because the EDS equipment was not exactly calibrated for the materials used in this study, the composition data should be used for only relative comparison. The BF TEM images and EDS line profiles show that Garich phases exist between ITO and CIGS in all devices (C, D, and E), although their thickness and composition may be dependent on the ITO back-contact structure. The intensities of Cu, In, Se, and Sn are dramatically reduced in the Ga-rich region, while oxygen exists as a major element with the Ga in the region. Thus, the Ga-rich phase is recognized as GaOx, where the material structure is known as amorphous.16 The results in Figure 4 demonstrate two interesting points. First, the GaOx phase can form at the ITO/CIGS interface, even though the CIGS layer was grown at a reduced temperature of 450 °C, unlike a previous report that the Ga−O reaction at the ITO/CIGS interface was suppressed below 520 °C.12 Second, the thickness and composition of the GaOx phase formed at such a low Tsub vary, depending on the ITO back-contact structures, as shown in Figure 4. D

DOI: 10.1021/acs.jpcc.8b11149 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. Bright-field (BF) transmission electron microscopy (TEM) images of cross sections around the ITO/CIGS interface of (a) device C ITO(200), (b) device D - ITO(600), and (c) device E - SiOx/ITO(600); panels (d)−(f) show their corresponding EDS line profiles.

CIGS interfaces of devices with or without NaF PDT (fabricated in a separate experiment). Only when Na is present at the ITO/CIGS interface during CIGS deposition, i.e., via diffusion from SLG substrate, it can be engaged in the Ga−O reaction. In this regard, the SiOx layer between SLG and ITO prevents the Na diffusion from SLG to ITO/CIGS and, as a result, slightly reduces the thickness of GaOx. However, the difference in GaOx thickness is not significant, implying that such a catalytic effect, which is due to the Na diffusion, is very limited in this experiment. Consequently, the catalytic effect of Na cannot account for such a difference between ITO(200) and ITO(600). Another possible explanation for the thicker GaOx formation with thicker ITO electrode may be due to the higher absorption of infrared (IR) light by thicker ITO films. As shown in Figure S2 in the Supporting Information, as the thickness of ITO film increases from 183 nm to 550 nm, its IR absorption increases from