Stability of TCO Window Layers for Thin-Film CIGS ...

Stability of TCO Window Layers for Thin-Film CIGS Solar Cells upon Damp Heat Exposures – Part II R. Sundaramoorthy, F.J. Pern, C. DeHart, T. Gennett, F.Y. Meng1, M. Contreras, and T. Gessert National Center for Photovoltaics, National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401, USA 1

Solar Energy Institute, Department of Physics, Shanghai Jiao-Tong University, Shanghai, China ABSTRACT Long-term performance reliability is essential for any photovoltaic module to become established in the PV market. Reliability is characterized based on many factors, one of the most important being the capability of the module to be resistant to moisture at elevated temperatures. This work continues our efforts to search for a high-performance and high-stability transparent conducting oxide (TCO) window layer for CuInGaSe2 (CIGS) devices. In this experimental study, we compared the optical, electrical, and structural stability of various TCOs deposited on glass, including singlelayer Al-doped ZnO (AZO), bilayer intrinsic-/Al-doped ZnO (BZO), B-doped ZnO (ZnO:B), amorphous In2O3:SnO2 (ITO), and amorphous In2O3:ZnO (IZO). The samples were exposed to damp heat (DH) at 85°C and 85% relative humidity (RH) and were characterized periodically. The results showed that all ZnO-based TCOs are more sensitive to moisture with substantial electrical degradation and apparent optical changes than the ITO and IZO. The amorphous IZO showed peculiar behavior in electrical property, and exhibited structural change with the appearance of some finite crystallinity after DH >220 h. The results from this experimental series will assist in determining the best-performing TCO for CIGS solar cells. Keywords: transparent conducting oxides, Al-doped ZnO, bi-layer ZnO, boron-doped ZnO, indium zinc oxide, indium tin oxide, AZO, BZO, ZnO:B, IZO, ITO, damp heat, degradation, CIGS, thin-film solar cell.

INTRODUCTION Thin-film photovoltaic (PV) technologies such as CdTe, CIGS, and amorphous Si have shown substantial improvements in module performance in recent years. For example, CIGS fabricated on soda lime glass is promising for its high efficiency and performance. But novel applications can be envisioned for military, space, aviation, mobile electronics, portable power sources, and in BIPV areas if the modules are flexible. The feasibility of CIGS being fabricated on flexible substrates is an added advantage for this thin-film technology. Not only do modules need to exhibit high performance, they also need to exhibit long-term reliability to become better established in the markets. If the module needs to be flexible, then the more conventional glass-to-glass lamination with edge sealant must be replaced with flexible substrates such as stainless steel or polyimide films, on which the thin film is fabricated and finally encapsulated with a polymeric-type encapsulant. Encapsulation may comprise various combinations of different kinds of polymeric front sheets such as TefzelTM, encapsulants such as EVA (ethylene vinyl acetate), and back sheets such as TedlarTM—all of which have different water vapor transmission rates [1,2]. Depending on the combination of the front and back sheets and their layered construction, moisture ingress from the atmosphere may attack the device components and cause their degradation at different rates in the flexible modules. So it is essential to determine how moisture can degrade the individual component layers comprising the devices. Accelerated life testing, using damp heat (DH) conditions at 85°C and 85% RH specified by the IEC61646 standard, has been employed to evaluate the reliability of thin-film PV modules. Long-term outdoor testing of chalcopyrite-based (e.g., CIGS) modules of glass/glass laminates has demonstrated stability with an annual degradation rate in the 1%–1.3% Reliability of Photovoltaic Cells, Modules, Components, and Systems II, edited by Neelkanth G. Dhere, John H. Wohlgemuth, Dan T. Ton, Proc. of SPIE Vol. 7412, 74120J · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.826604 Proc. of SPIE Vol. 7412 74120J-1

range, depending on various irradiance ranges [3]. Results of the DH-exposed mini-modules from Shell Solar indicate that the dominant failure mechanism arose from the increased electrical resistance in the ZnO:Al window layer [4-6]. From the electrical point of view, the TCO layer should maintain low sheet resistance (R□) to minimize ohmic loss. To better understand, and quantify, the critical causes of degradation for the component layers, (e.g., Mo, ZnO, and CIGS absorber on the CIGS devices), a study of the individual layers forming the stack of the solar cell is essential [7-10]. In our previous work, we demonstrated that the stability trend of some transparent conducting oxides (TCOs) was in the decreasing order of SnO2:F > In2O3:SnO2 > Zn1-xMgxO:Al ≈ ZnO:Al when tested in DH conditions [8,9]. The degradation rate of ZnO:Al and Zn1-xMgxO:Al was observed to be influenced by film thickness, deposition conditions, and exposure history [8,9]. In this study, we examined further in a more quantitative manner the DH stability of ZnO:Al (AZO) with two film thicknesses, three films of ZnO:B with different thicknesses, and amorphous InZnO and ITO that were deposited on different glass substrates. The bilayer i-ZnO/ZnO:Al (BZO) was used as a reference for comparison. We included ZnO:B samples to compare the effects of dopant type and deposition method/conditions on ZnO stability. The DH stability of ITO and IZO, both fabricated in house (versus the commercial ITO studied earlier [8]), were investigated for their suitability as potential replacements for the conducting window layer (AZO) on CIGS devices [11]. This paper presents the results of optical, electrical, and structural changes on these TCOs upon exposures to the damp heat up to >1000 h.

EXPERIMENTAL TCO Samples. The TCO samples, their compositions, composite layers, ID, substrate, thickness, and deposition method and conditions are given in Table 1. More details on the deposition systems and parameters are described below. ZnO:Al (AZO) and i-ZnO/ZnO:Al (BZO): A Semicore/MRC 603 system was used to deposit Al-doped ZnO (AZO) at two different thicknesses: ~0.12 and 0.22 μm on Corning 7059TM Table 1. TCO Samples, ID, Substrate, Film Thickness, and Deposition Parameters. glass substrates. The same system Thickness Deposition Deposition was used to deposit i-ZnO/ZnO:Al TCO Type Sample ID Substrate (μm) Method Condition (BZO) on a 7.5-cm x 15-cm Eagle TM TM 0.12 RF Sputtered Ambient AZO1 ZnO:Al Corning 7059 2000 glass substrate. These TM AZO2 0.12 Corning 7059 substrates were subsequently cut AZO3 0.22 into ~2.0-cm x 2.8-cm specimens Corning 7059TM for DH exposure tests. The i-ZnO 0.22 AZO4 Corning 7059TM and ZnO:Al (2 wt% Al2O3-doped i-ZnO/ZnO:Al BZO1 0.10/0.12 RF Sputtered Ambient Eagle 2000TM ZnO) targets obtained from Plasma ZnO:B1 Fused Silica 0.75 MOCVD ZnO:B 138oC Materials are 11.88 cm x 37.18 cm. ZnO:B2 Fused Silica 1.56 158oC The target-to-substrate distance ZnO:B3 Fused Silica 1.36 178oC was 5 cm. A pressure of 5 mTorr TM In2O3:ZnO* IZO1 0.16 RF/DC Sputtered Ambient Corning 7059 was established during deposition. TM IZO2 0.16 Corning 7059 The i-ZnO was sputtered in a 1% IZO3 0.19 Eagle 2000TM concentration of O2 in Ar at 60 In2O3:SnO2 ITO1 0.11 RF Sputtered Ambient sccm, and the AZO was sputtered Corning 1737TM TM in Ar at 40 sccm. The glass 0.11 ITO2 Corning 1737 substrates were not intentionally * 70% In2O3 - 30% ZnO target (wt. %) heated. InZnO (IZO): A magnetron sputter gun, 10-cm in diameter, was oriented in a sputter-down orientation with a rotating, temperature-controlled stage positioned 7.5 cm from the target surface. The glass substrate temperature during deposition was always 1000 h was achieved. Analytical Characterization. Baseline optical, electrical, and structural measurements were performed on all the samples before exposing them to DH conditions in the AES chamber. The TCO samples were characterized at 100-h intervals. Optical measurements (transmittance and reflectance) were performed on a CARY 6000i UV-Vis-NIR spectrophotometer equipped with an integrating sphere. Sheet resistance, mobility, and carrier concentration were obtained using an Ascent Hall 500 Advance system. Structural properties were characterized using an X1 Advanced Diffraction System from Scintag.

RESULTS AND DISCUSSION I. Electrical Degradation Sheet Resistance. Figure 1 shows the sheet resistance (R□) of different TCO layers as a function of DH exposure time. Figure 1a compares the R□ of AZO films having two different thicknesses (0.12 and 0.22 μm, respectively) along with BZO1 (0.22 μm). The R□ of the 0.12-μm AZO1 film increased by one order of magnitude while the sheet resistance of another 0.12-μm film (AZO2) increased by about three orders of magnitude within an exposure time of 220 h. This suggests that the quality of the AZO films may be different from one batch to another. Similar variations in film quality have been observed previously [8,9]. The thicker 0.22-μm AZO films (AZO3 and AZO4) showed an order of magnitude increase in the sheet resistance within an exposure time of 200 h, but became highly resistive thereafter. It is interesting to note that fewer variations are seen in terms of the sheet resistance among the thicker films. However, the R□ of the 0.22-μm BZO film increased by four orders of magnitude compared to the thicker AZO3 and AZO4 films, indicating that the BZO films tend to degrade faster than AZO films of similar thicknesses. Previous work [8,9] shows the sheet resistance of the AZO films were within 500 Ω/□ of each other after an exposure of 200 h in DH conditions while the BZO films were within ~2000 Ω/□ after similar DH exposure. The large variation in the degradation extent observed for different samples prepared at different times from the same Semicore system and exposed to the same DH conditions suggests that there is a variation in the quality of the sputtered films. The cause of such variations in the film stability, or quality, is not yet clear and will be investigated. Figure 1b compares R□ changes of B-doped ZnO films grown at different substrate temperatures. The B-doped ZnO films grown at 138°–178°C degraded during DH, similar to the AZO and BZO films sputtered at ambient temperature. The thickness of the films played a vital role in the degradation process. The thinner ZnO:B1 film prepared at 138°C had the highest initial sheet resistance (~34,000 Ω/□) and became more resistive after 200 h of DH exposure. The ZnO:B3 film prepared at 178°C had a sheet resistance of ~1500 Ω/□ before exposure. It was slightly thicker than ZnO:B1 and was able to withstand the DH condition for about 500 h. The ZnO:B2 film was the thickest among the three films studied and had the lowest initial sheet resistance (~15 Ω/□). The sheet resistance of ZnO:B2 film increased gradually to about three orders of magnitude upon exposure to 1000 h of DH.

Proc. of SPIE Vol. 7412 74120J-3

Figure 1c compares the R□ changes of amorphous IZO films. The R□ of (0.16 μm) amorphous IZO films (IZO1 and IZO2) initially decreased for the first 100 h of exposure, but the sheet resistance increased to ~55 Ω/□ and later became fairly stable up to 1000 h. The thicker IZO film (IZO3) showed a similar trend. Figure 1d compares the R□ of the amorphous ITO films (0.11 μm), which became more conductive with DH exposure. A peculiar electrical behavior was observed for the amorphous In2O3:ZnO (IZO) films. While performing the Hall measurements on these IZO films, we observed that the current-voltage (I-V) curve obtained was not linear during the first Hall measurement but eventually became linear as the number of measurements increased (figures not shown). To explain this, we hypothesize that there may be some randomly oriented micro-crystallite domains in the original IZO films, which were not detectable in the XRD scan, but pose an obstacle for current during Hall measurements. As shown in Fig. 5, the IZO films exhibited structural changes after a DH exposure of about 300 h. We hypothesize that the original micro-crystallites might be very small—below the detection limit of the Scintag XRD. But as the sample was exposed (“annealed”) to DH at 85oC for ~300 h, the micro-crystallites might have grown in size, thus enabling us to observe the changes in the XRD. The presence of these micro-crystallites might be one of the reasons for the decrease in the mobility. Thus, the presence of these micro-crystallites may also influence current flow during initial Hall measurements. However, it is unclear at the moment how this structural change may affect the accuracy of measured Hall parameters. R□ of ZnO:B as a Function of DH Exposure Time

R□ of AZO and BZO as a Function of DH Exposure Time

1000000

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Fig. 1. Changes in sheet resistance of different TCOs as a function of DH exposure time: (a) in-house sputterdeposited AZO and BZO, (b) MOCVD ZnO:B, (c) amorphous IZO, and (d) amorphous ITO.


Mobility. The changes in the mobility of different TCOs upon exposure to DH are illustrated in Fig. 2. The corresponding changes in the mobility and sheet carrier concentration are shown in Figs. 2 and 3. The increase in the sheet resistance of the AZO, ZnO:B, and IZO films from Fig. 1 indicates that either the mobility or the carrier concentration, or both, must decrease. In Fig. 2a the mobility of the AZO1 and AZO2 films of the same thickness showed different behavior after exposure for 100 h and were not measurable after ~200 h. The difference in the mobility of AZO1 and AZO2 of the same thickness is an evidence for the variation in the quality of the films. The mobility of the thicker AZO films, AZO3 and AZO4, decreased relatively more slowly. The mobility of the ZnO:B films is shown in Fig. 2b. The mobility of the thickest ZnO:B2 film decreased drastically in the first 100 h of exposure followed by a gradual decrease to minimal values. The initial mobility of ZnO:B3 deposited at 178°C was very low and maintained its low mobility until 400 h; it was not measurable thereafter. The mobility of the thinnest ZnO:B1 film was very low and was not measurable after 300 h of DH exposure. The mobilities of the IZO films are shown in Fig. 2c. Of particular interest, the IZO1 and IZO2 of the same thickness showed an initial increase in the mobility for the first 100 h followed by a drop in the mobility to ~20 cm2/V-s before becoming fairly stable up to 1000 h of DH exposure. The relatively thick IZO3 had a low initial mobility of 20 cm2/V-s and showed a gradual decrease to about 15 cm2/V-s at 1000 h of DH exposure. The mobility of the ITO films in Fig. 2d showed some initial changes in the first 400 h and then remained fairly constant afterward up to 1000 h DH exposure. Mobility of ZnO:B as a Function of DH Exposure Time

Mobility of AZO & BZO as a Function of DH Exposure Time 16

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Mobility of ITO as a Function of DH Exposure Time

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Fig. 2. Changes in mobility of different TCOs as a function of DH exposure time: (a) in-house sputter deposited AZO and BZO, (b) MOCVD ZnO:B, (c) amorphous IZO, and (d) amorphous ITO.


Carrier Concentration. The changes in the sheet carrier concentration of different TCOs upon exposure to DH are illustrated in Fig. 3. The carrier concentration of AZO1 and AZO2 films showed significant reduction to unmeasurable values after the first 200 h. While the thicker AZO3 and AZO4 films demonstrated higher initial carrier concentration, the sheet carrier concentration also decreased to unmeasurable values after 200 h as seen in Fig. 3a. The three MOCVDdeposited ZnO:B films at 138°, 158°, and 178°C are compared in Fig. 3b. The trend shows that the thick ZnO:B2 film deposited at 158°C exhibited slower degradation as opposed to the thinner ZnO:B films. Fig. 3c compares the amorphous IZO films of different thickness. The IZO exhibited essentially stable sheet carrier concentration over the 1000 h exposure time, which was also substantiated by the stable optical properties shown by IZO in Fig. 4e below. Figure 3d compares the amorphous ITO films (ITO1 and ITO2). Although the ITO films demonstrate an initial reduction of the carrier concentration after the first 300 h of DH exposure, they demonstrate an increase in the carrier concentration and stabilized over the rest of the exposure time. [Ns] of ZnO:B as a Function of DH Exposure Time

[Ns] of AZO & BZO as a Function of DH Exposure Time

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[Ns] of IZO as a Function of DH Exposure Time -2.5E+16

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IZO3 (0.19 μm)

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Fig. 3. Changes in sheet carrier concentration, [Ns], of different TCOs as a function of DH exposure time: (a) inhouse sputter-deposited AZO and BZO, (b) MOCVD ZnO:B, (c) amorphous IZO, and (d) amorphous ITO. Table 2 summarizes the electrical degradation rates of different TCOs. Assuming a linear degradation, which is an approximate assumption for the experimental data obtained, the degradation rate is calculated using (DTf-DT0)/(Tf-T0) where DT0 stands for the corresponding value of electrical property obtained before the film was exposed to DH (T0) and DTf stands for the corresponding value of electrical property obtained after the last available DH exposure (Tf). Degradation rates of the thicker (0.22 μm) AZO films show similar trends, but there is a large variation in the degradation rate observed for the thin (0.12 μm) AZO films. The BZO films exhibited a large degradation in the sheet resistance, indicating that the film became more resistive when compared with the thicker AZO films. The ZnO:B2 film


shows the lowest degradation rate among the three, while ZnO:B1 and ZnO:B3 show quicker degradation. The IZO films show 1%–2% degradation in the sheet resistance, which we attribute to the small change in the carrier concentration and mobility. Finally, the ITO films show a negative degradation rate, indicating that the sheet resistance of the film decreased after exposure, while the mobility showed little change.

II. Optical Degradation

T a ble 2. D H - in duc ed D eg rad a tion R ate of T C O S am p les.

(cm 2 /V -s)/h

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M ob i lity -2 .4 4E -2

Figure 4 shows the transmittance and reflectance 0.16 102 7 2 .0 2E -2 -4 .9 2E -3 IZO 1 spectra as a function of DH exposure obtained for the 0.16 102 7 1 .9 7E -2 -7 .3 5E -3 IZO 2 samples of different TCOs, namely (a) AZO2 (0.12 0.19 102 7 6 .9 6E -3 -3 .8 5E -3 IZO 3 μm), (b) AZO4 (0.22 μm), (c) BZO1 (0.22 μm), (d) 0.11 102 7 -1 .5 7E -2 -1 .1 6E -3 IT O 1 ZnO:B2 (1.56 μm), (e) IZO2 (0.16 μm), and (f) ITO2 0.11 102 7 -1 .2 5E -2 4.66E -3 IT O 2 (0.11 μm). The Semicore-deposited AZO and BZO films shown in Fig. 4(a-c) became highly transparent after 330 h of DH exposure. The films exhibited a slight increase in the transmittance in the ~850–1500 nm range, which is due to a decrease in free-carrier absorption during the first 200 h of exposure. The decrease in the free carrier absorption in the films suggests that most of the free carriers were lost and the film became more transparent in the infrared regime after 200 h of exposure and thus more resistive (as shown earlier in Fig. 1 a). The results are consistent with previous work [8,9]. The ZnO:B2 films lost some free-carrier absorption in the infrared regime during the first 330 h as shown in Fig. 4d and remained stable up to 1000 h of DH exposure. For the amorphous IZO2 and ITO2 shown in Fig. 4(e-f), the films show slight improvement in the transmittance and reflectance after the first 200 h of exposure and then become stable from 300 h to 1000 h in DH. More discussions will be given in Section IV below to correlate the optical changes with the structural and electrical changes observed for the various TCO samples. III. Structural Degradation The XRD patterns of AZO and BZO films in Fig. 5(a-c) show a highly preferred (002) ZnO orientation at 34.5° (2θ). The observed changes in the diffractograms are similar to that reported earlier [9]. The films exhibited a red shift in the peak position after exposure to DH, indicating lattice change. The (002) peak intensity increased up to an exposure of 330 h, after which the intensity decreased considerably depending on the thickness of the film. For the thinner AZO2 film (Fig. 5a), the initial peak intensity of the unexposed film was ~600 cps and dropped to ~100 cps after 1027 h DH exposure, while the peak intensity for the thicker 0.22 μm AZO4 film (Fig. 5b) was ~13,000 cps at 0 h and 1207 h with an increase in between. For the BZO1 film (Fig. 5c), which had an 0.10 μm i-ZnO underlayer and a 0.12 μm AZO top layer, the peak intensity was lower at ~1800 cps before and after 1027-h DH exposure, with an intermittent increase similar to that seen in AZO4 (Fig. 5c vs. 5b). The large difference in the peak intensity between the AZO4 and BZO films, both having the same thickness at 0.22-μm, apparently arose from the difference in the doped-layer thickness. This suggests that the 0.22-μm AZO4 film possessed a considerably greater crystallinity from a continuous growth than the 0.22-μm BZO1, which featured a 0.12-μm AZO layer that was grown over the 0.10-μm i-ZnO underlayer. An interesting observation is the peak at 32.95° seen in the thicker AZO4 (Fig. 5b inset) upon DH exposure for about 430 h. This small peak could be due to (112) orthorhombic Zn(OH)2 per JCPDS # 89-0138, probably formed from reaction of ZnO with moisture that saturated the film. This peak was observed until 777 h and disappeared at 1027 h. It turned out that a delay in XRD measurements by a few days was responsible for the change, during which the film was stored in a N2-filled dry box and dried out losing the water and thus the Zn(OH)2. In addition, the peak observed at 31.6° (Fig. 5c) on the BZO1 film was likely due to the hexagonal (100) ZnO (JCPDS #89-1397). The ZnO:B films deposited at various temperatures showed slightly different crystalline structures from those of Aldoped ZnO deposited at ambient temperature. The lowest substrate temperature of 138°C, used to deposit ZnO:B,


Transmittance and Reflectance spectra of AZO2 upon DH exposure

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Transmittance and Reflectance Spectra of ZnO:B2 upon DH 100

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Transmittance and Reflectance Spectra of IZO2 upon DH Exposure 100 330h to 1027 h (e) 90

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Transmittance and Reflectance spectra of BZO1 upon DH exposure 100 1027 h 330 h (c) 90 220 h 80 110 h 0h 70

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Transmittance and Reflectance spectra of AZO4 upon DH exposure 100 1027 h (b) 330 h 90 110 h 80 220 h

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Transmittance and Reflectance Spectra of ITO2 upon DH Exposure 90 1027 h (f) 330 h to 777 h 80 70

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Fig. 4. Changes in transmittance and reflectance spectra obtained for the TCO samples as a function of DH exposure time: (a) AZO2, (b) AZO4, (c) BZO1, (d) ZnO:B2, (e) IZO2, and (f) ITO2. yielded a highly intense crystalline peak with the preferred (002) orientation at 34.5° (2θ), while the peak at 31.95° may correspond to hexagonal (100) ZnO (JCPDS #89-0510) and the peak at 36.37° to the orthorhombic (013) Zn(OH)2 (JCPDS # 89-0138) or cubic (111) ZnO (JCPDS# 77-0191). The insets in Fig. 5d show the ZnO:B deposited at 158° and 178°C, respectively, to the left and right of the main figure. The crystalline structures are similar for both. The preferred orientation at higher temperatures may be hexagonal (100) ZnO (JCPDS #89-0510), whereas the orthorhombic Zn(OH)2 or cubic ZnO are relatively small.


The amorphous nature of the InZnO (IZO2 films) was illustrated in Fig. 5e before DH exposure. But after the IZO film was exposed to DH for ~330 h, a new peak appeared at 2θ 22.3°. This peak is possibly from the (012) plane of rhombohedral In2O3 (JCPDS #72-0683). The small peak persisted to 1027 h of DH exposure. In a similar way, Fig. 5f shows the amorphous nature of the In2O3:SnO2 (ITO2 films) before exposure to DH. After exposure to DH for ~455 h very minimal structural changes were observed in ITO films until 1027 h. The peaks at 2θ 30.5° and 35.4° shown in Fig. 5f are tentatively attributed to the formation of micro-crystallites of hexagonal (101) In2O6 (JCPDS #20-1439) and hexagonal (108) In2O6 (JCPDS# 20-1439) or hexagonal (100) In2O5 (JCPDS# 20-1442). 800

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110 h

700 0h Intensity (cps)

Intensity (cps)

600 500 400

433 h

300

1027 h

0h

400 300 200

10000

100 0

777 h

200

15000

5000

32

33

34

35

36

100 1027 h 0

0

32.5

33.5

34.5 2θ (degree)

36.5

32

10000

2500

(c)

777 h

100 1000 0 31

500

33

33.5 34 34.5 2θ (degree)

35

0h

300

6000

200 4000

0h

200 1027 h

36

ZnO:B3 1027 h

100

100

0

0 30

35

300

ZnO:B2

33

36

30

39

33

36

39

2000 1027 h

0h 0

0 31

300

32

33

34 2θ (degree)

35

36

31

600

(e)

32

33

(f)

IZO2 (0.16 μm)

250

34 35 2θ (degree)

36

1027 h

200

332 h 150 100 0h

37

455 h to 1027 h 200

400

wide range scan amorphous ITO 0 h, 455 h, 1027 h

100

300 0 20

200

25

30

35

40

220 h

50

0h

100

0

38

ITO2 (0.11 µm)

500

Intensity (cps)

Intensity (cps)

35.5

ZnO:B1

400

200

1500

33

8000

1027 h

Intensity (cps)

0h

32.5

(d)

332 h

BZO1 (0.22 μm)

2000 Intensity (cps)

35.5

0

21

21.5

22

22.5 2θ (degree)

23

23.5

24

28

29

30

31

32 33 34 2θ (degree)

35

36

37

38

Fig. 5. Structural changes observed for the TCO samples as a function of DH exposure time: (a) AZO2, (b) AZO4, (c) BZO1, (d) ZnO:B, (e) amorphous IZO2, and (f) amorphous ITO2.


IV. Correlation of Optical Changes with Structural and Electrical Changes Because the optical, electrical, and structural properties of TCO films are interdependent, it should be possible to find a way to link the changes between all the three properties induced by external stress. In this regard, we computed the net change in absorption for the TCO samples over the course of DH exposure. The absorption was obtained by subtracting the sum of transmittance (T%) and reflectance (R%) from 100%. The net change in absorption was then computed by subtracting the absorption spectrum at Tn from the initial at T0 = 0 h DH exposure. In this way, the spectral change in the range of ~250 to ~600 nm may be attributed or correlated to the changes in bandgap (Ebg) and crystallinity, and the spectral change in the ~850–1500-nm range to the change in the free-carrier absorption. Figure 6 shows the computed results for samples of AZO2 (0.12 μm), AZO4 (0.22 μm), BZO1 (0.22 μm), IZO2 (0.16 μm), and ITO2 (0.11 μm). Calculations for ZnO:B were not conducted because of the poor spectral quality of reflectance in the 800–900-nm range. As seen, all the curves in the figures display a net absorption peak in the 250–600-nm range, which may be either negative (loss, e.g., AZO4) or positive (gain, e.g., BZO1, IZO2, ITO2) or both (e.g., AZO2). By carefully comparing the peak change and its negative or positive direction with the peak changes—peak shift, intensity change, and appearance of new peak(s)—in the XRD diffractograms (e.g., compare Figs. 4a, 5a, and 6a for AZO2), it becomes clear that when the AZO2 layer had not degraded significantly structurally, its net optical change of absorption is negative, e.g., at DH < 300 h; but the net absorption change becomes positive after the AZO2 has degraded further at DH > 330 h. For the thicker AZO4 film (compare Figs. 4b, 5b, 6b), the net absorption change is mainly negative while its XRD pattern shows fairly small shift in peak position and some change in peak intensity but without new peak. Similarly, the appearance of small, new XRD peaks in DH-exposed BZO1 film in Fig. 5c can be correlated with the positive net absorption change seen in Fig. 6c. Likewise, the small new XRD peak at 22.3o observed for the IZO2 films seen in Fig. 5d after 200 h of DH exposure can be correlated with the positive net absorption change as seen in Fig. 6d. Meanwhile the ITO2 film shows small positive net absorption change (Fig. 6e) after 400 h of DH exposure time which can be correlated with the appearance of very small XRD peaks at 30.5o and 35.4o shown in Fig. 5f. Because the TCO films studied here typically have Ebg in the ~3.2 – ~3.4-eV range, corresponding to the ~360 – ~390-nm range, any structural change induced by DH exposure can also be reflected in their optical properties in that spectral range. As supporting evidence, in Fig. 6, the negative peaks for AZO are at 340–350 nm; the positive peaks for BZO are at 380 nm, for IZO at 367 nm, and for ITO at 350 nm. Thus, the differential method we employed here i.e., (net optical absorption change) is therefore sensitive to the structural change. In addition, a positive net absorption change observed in the 850–1500-nm range can be due to the obvious loss of bulk free carrier concentration in the TCO films, which can be correlated by comparing Figs. (3a, 4b and 6b) with Figs. (3a, 4a and 6a). A larger net absorption change is seen for the thicker 0.22-μm AZO4 than for the thinner 0.12-μm AZO2. Since the AZO4 film is about 1.8 times thicker than the AZO2 film, its loss in the bulk free carrier absorption was about 1.8 times higher than the AZO2. The variation in sheet carrier concentration upon DH exposure between the two AZO films is also clearly seen in Fig. 3a. Compared to the AZO films, the BZO1 film showed less free carrier absorption even though the degradation rate of the mobility of AZO and BZO films are about the same. For the IZO2 and ITO2 films, the net absorption change in the 850–1500-nm range are mostly flat (i.e., little change), indicating little change in free-carrier concentration, consistent with results given in Figs. 3(c and d) and Figs. 4(e and f). One implication here is that it is possible to conduct only optical measurements and then use only the optical data to extract electrical and structural data, provided a more systematic study is performed to establish a quantitative correlation.

CONCLUSIONS We have shown the relative stability of various TCOs, namely AZO (ZnO:Al), BZO (i-ZnO/ZnO:Al), ZnO:B, IZO (In2O3:ZnO), and, ITO (In2O3:SnO2) films deposited on glass substrates in damp heat conditions at 85oC at 85% RH that were characterized by Hall, optical, and XRD measurements. Generally the degradation rates of the TCOs, calculated by using sheet resistance with an assumption of a linear degradation, are in the decreasing order of ITO ~ IZO > ZnO:B > BZO > AZO. We also observed further the variation in the quality (and so the DH stability) of AZO films, although the exact causes are unclear at present. The in-house-fabricated amorphous ITO and IZO are relatively stable in DH exposure. The rapid degradation of 0.12-μm ZnO:Al within 300 h of DH exposure presents a huge challenge to mitigate


the degradation of the CIGS solar cells that use the bi-layer ZnO (BZO). The current record-efficiency device is fabricated with BZO and hence it is essential to find out the threshold of the degradation of the device based on the performance of the TCO so as to improve the performance of the modules. There are various approaches currently being investigated, one of which is to find an alternative TCO to replace the standard ZnO:Al widely used for the CIGS devices both in the industry and at NREL. Another two approaches are to mitigate the degradation of ZnO:Al by applying a protective barrier layer and to find an effective surface treatment or coating by chemical methods. More experiments are warranted to evaluate the performance of TCOs with different thicknesses, varying sputtering parameters, and combinations of TCOs to determine the most stable TCO in DH for thin-film devices. 30

(a) 1027 h

20

433 h

10

1027 h

330 h 110 h

0 -10 110 h

-20

(b)

AZO2 (0.12 μm) Net Change in Absorption (%)

Net Change in Absorption (%)

30

AZO4 (0.22 μm)

20

1027 h 330 h

10

220 h 110 h

0 -10 -20

220 h -30

-30

250

1250

1500

BZO1 (0.22 μm)

20 1027 h

10 0

0h

-10 -20

30

250

30

(c)

-30 250





30

500

500

(d)


1250

1500

IZO2 (0.16 μm)

20 10 0 -10

0h 1027 h

-20 -30

500

(e)


1250

1500

250

500


1250

1500

ITO2 (0.11 µm)

20 10 1027 h

Fig. 6. Net change in absorption spectra calculated from the T% and R% data for different TCO samples as a function of DH exposure time: (a) AZO2, (b) AZO4, (c) BZO1, (d) IZO2, and (e) ITO2.

0 0h

-10 -20 -30 250

500

750 1000 Wavelength ( nm)

1250

1500


ACKNOWLEDGMENTS This work was performed at the National Center for Photovoltaics under DOE contract number DE-AC36-08GO28308 with the National Renewable Energy Laboratory. This paper is subject to government rights.

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