Improvement of the cell performance in the ZnS/Cu (In, Ga) Se2 solar ...

PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. 2013; 21:217–225 Published online 6 December 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.2319

RESEARCH ARTICLE

Improvement of the cell performance in the ZnS/Cu(In,Ga) Se2 solar cells by the sputter deposition of a bilayer ZnO : Al film Dong Hyeop Shin1, Ji Hye Kim1, Young Min Shin1, Kyung Hoon Yoon2, Essam A. Al-Ammar3 and Byung Tae Ahn1* 1

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Korea 2 Solar Energy Department, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon, 305-343, Korea 3 Department of Electrical Engineering, King Saud University, PO BOX 800, Riyadh, Kingdom of Saudi Arabia, 11451

ABSTRACT ZnS is a candidate to replace CdS as the buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells for Cd-free commercial product. However, the resistance of ZnS is too large, and the photoconductivity is too small. Therefore, the thickness of the ZnS should be as thin as possible. However, a CIGS solar cell with a very thin ZnS buffer layer is vulnerable to the sputtering power of the ZnO : Al window layer deposition because of plasma damage. To improve the efficiency of CIGS solar cells with a chemical-bath-deposited ZnS buffer layer, the effect of the plasma damage by the sputter deposition of the ZnO : Al window layer should be understood. We have found that the efficiency of a CIGS solar cell consistently decreases with an increase in the sputtering power for the ZnO : Al window layer deposition onto the ZnS buffer layer because of plasma damage. To protect the ZnS/CIGS interface, a bilayer ZnO : Al film was developed. It consists of a 50-nm-thick ZnO : Al plasma protection layer deposited at a sputtering power of 50 W and a 100-nm-thick ZnO : Al conducting layer deposited at a sputtering power of 200 W. The introduction of a 50-nm-thick ZnO : Al layer deposited at 50 W prevented plasma damage by sputtering, resulting in a high open-circuit voltage, a large fill factor, and shunt resistance. The ZnS/CIGS solar cell with the bilayer ZnO : Al film yielded a cell efficiency of 14.68%. Therefore, the application of bilayer ZnO : Al film to the window layer is suitable for CIGS solar cells with a ZnS buffer layer. Copyright © 2012 John Wiley & Sons, Ltd. KEYWORDS Cu(In,Ga)Se2 solar cell; ZnS buffer; ZnO : Al window; sputtering damage *Correspondence Byung Tae Ahn, Korea Advanced Institute of Science and Technology, Department of Materials Science and Engineering, 291 Daehak-ro, Yuseong-gu, Daejeon 305–701, Korea. E-mail: [email protected]

Received 18 May 2012; Revised 4 September 2012; Accepted 16 October 2012

1. INTRODUCTION Over the last decade, the renewable energy market has shown an increase in the demand for high-efficiency Cd-free Cu(In,Ga)Se2 (CIGS) solar cells as an important component of an environmentally sustainable energy system. At present, ZnS film as a Cd-free buffer layer deposited by chemical bath deposition (CBD) is the most promising material in terms of the efficiency of CIGS solar cells. On the laboratory scale, CIGS solar cells with a ZnS buffer layer have achieved efficiency level as high as 18.6% [1–3]. Regarding their commercialized products, Copyright © 2012 John Wiley & Sons, Ltd.

the CIGS modules with a ZnS buffer layer have realized efficiency level of nearly 16% [4]. In spite of the many research papers on the subject, only a few groups have shown high efficiency in CIGS solar cells with a ZnS buffer layer. There are many factors to consider before high-efficiency CIGS solar cells can be realized. First, establishing a reliable and reproducible process for the deposition of a uniform and defect-free ZnS buffer layer on a CIGS absorber layer is complicated. The complexity of the ZnS film deposition arises from the variety of the secondary phases (Zn(OH)2, ZnO) that may grow concurrently with ZnS during the CBD process. In 217

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Sputter deposition of a bilayer ZnO : Al film as a window layer

particular, the existence of Zn(OH)2 induces the creation of pin-holes in the film and the light-soaking effect [5–7]. Another problem is connected to the physical property of ZnS film, that is, the high resistivity of CBD–ZnS film. Reducing the resistivity of ZnS film is limited by the lack of an effective dopant [8,9]. Thus, it is extremely important to control the ZnS film thickness at the nanoscale level to achieve desirable characteristics of the heterojunction. In conventional CdS/CIGS solar cells, the structure and thickness of each layer are optimized as ZnO : Al (210 nm)/i-ZnO(50 nm)/CdS(50 nm)/CIGS/Mo while exceeding an efficiency level of 20% [10–12]. The deposition of the ZnO window layer by a sputtering process was known as a very useful method to fabricate high-efficiency CIGS solar cells. However, during the deposition of the ZnO window layer by sputtering, highenergy negative oxygen ions or neutral particles in the plasma cause damage to the ZnS/CIGS interface. This is known as plasma damage by sputtering [3,13,14]. To date, the formation of a high-quality interface is a critical issue pertaining to the technology of highefficiency CIGS solar cells with a ZnS buffer layer. The efficiency limitation of the ZnS/CIGS solar cells is related to both the high electrical resistivity of the ZnS buffer layer and plasma damage by sputtering at the ZnS/CIGS interface. To the best of our knowledge, only a few studies have investigated plasma damage by sputtering. Nakada et al. found that a non-doped ZnO thin layer known to improve VOC in CdS/CIGS solar cells reduced the performance of ZnS/CIGS solar cells. It was suggested that the observed effect could be associated with surface damage because of the bombardment of high-energy negative oxygen ions or neutral particles during the sputtering process [3,13,14]. The removal of the i-ZnO layer from the window layer results in a high efficiency of 18%. To avoid plasma damage by sputtering, Showa Shell Sekiyu group developed a metalorganic chemical vapor deposition (MOCVD) process capable of growing an n-type ZnO window. The fill factor (FF) value of CIGS solar cells fabricated with a MOCVD-ZnO : B window layer was shown to be higher than that of devices with a sputtered ZnO : Ga window layer [15,16]. It should be noted that the sputtering method is more favorable for the commercial production of CIGS module production. It is a simple and fast process. However, no detailed reports are available on the effect of plasma damage on the photovoltaic properties of CIGS solar cells with a ZnS buffer layer. The structure of the window layer and the processing parameters of its deposition are not well established. Here, we report the effect of plasma damage by sputtering on the photovoltaic properties of CIGS solar cells fabricated with a ZnS buffer layer. To reduce plasma damage by sputtering and thus achieve high-efficiency CIGS solar cells, optimization of the tradeoff relationship among the thickness of the ZnS buffer layer, the resistivity of the ZnO : Al window layer, and the processing parameters of its deposition was carried out. In addition, 218

a bilayer ZnO : Al film was proposed to reduce the plasma damage and to achieve high efficiency.

2. EXPERIMENTAL To study the effect of plasma damage by sputtering on the performance of CIGS solar cells, a set of the CIGS solar cells was fabricated. The CIGS films were deposited by a three-stage co-evaporation process onto a Mo-coated soda lime glass substrate. The experimental details of the deposition procedure are described in our previous publication [17]. The composition of the CIGS film was adjusted to Cu(In0.65Ga0.35)Se2. The thickness of CIGS film was about 2.5 mm. The CIGS films showed large grains and smooth surface. Additionally, the surface of the CIGS films showed faceted and terraced morphology because of (112)-preferred orientation of the CIGS films. The conversion efficiency of the CIGS solar cell with a CdS buffer layer showed above 17% [17]. The ZnS buffer layers were grown on CIGS absorber layers by CBD [18,19]. The ZnS films were prepared from an alkaline aqueous solution of zinc sulfate (ZnSO47H2O), thiourea ((NH2)2CS), and ammonia hydroxide (NH4OH). ZnSO47H2O (99.9%), (NH2)2CS (98%), and NH4OH (28%) were Aldrich-grade chemicals. The concentrations of the ZnSO47H2O, (NH2)2CS, and NH4OH in the bath solution were 0.1, 0.6, and 7.0 M, respectively. The deposition of the ZnS film was carried out in a home-made bath equipped with a Teflon substrate holder. The samples with CIGS films were dipped in the bath at 73 C, and ZnS films were grown on them for 25 min. To remove the loosely bonded homogeneous precipitation of large Zn(OH)2 particles from the surface of the ZnS films, the ZnS films were washed in 0.5-M NH4OH solution [6,20]. The samples were then rinsed with deionized water under ultrasonic irradiation and dried in a nitrogen flow. The thickness of the ZnS film deposited by one cycle of CBD process was estimated to be 27 nm. Thicker ZnS films were obtained by repeating the CBD process. Al-doped ZnO (ZnO : Al) as a window layer was deposited on the ZnS film by radio-frequency magnetron sputtering. A radio-frequency magnetron sputtering system with a base pressure of 2 10 7 torr was used. Prior to the deposition of the ZnO : Al film, the Al (2.5 wt%)-doped ZnO target (6-inch diameter) was cleaned by pre-sputtering for 10 min. The thickness of the ZnO : Al window layer deposited on the ZnS buffer layer was estimated to be 127–155 nm. To investigate the effect of sputtering damage on the photovoltaic properties of CIGS solar cells, the sputtering power was varied in the range of 50–400 W. Finally, a front grid of Al was deposited by vacuum evaporation using resistance heating. The thickness of Al front grid was about 700 nm. After Al deposition, the active cell area was about 0.44 cm2. To be activated with good performance, light soaking was performed using a constant light solar simulator under a 1-sun (AM 1.5 and 100 mW/cm2 at 25 C) condition. Prog. Photovolt: Res. Appl. 2013; 21:217–225 © 2012 John Wiley & Sons, Ltd. DOI: 10.1002/pip

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The morphology and thickness of the films were investigated by field-emission scanning electron microscope without any conductor coating and high-resolution transmission electron microscope (HR-TEM). The lattice distortion of CISG films was characterized by Raman scattering measurement. The current–voltage (J–V) characteristics were recorded at 25 C in the dark and under illumination with a Spectra Physics Oriel 300 W Solar Simulator with an AM 1.5G filter set. A thermopile radiant power meter (Spectra Physics Oriel, model 70260) with a fused silica window was used to set the integrated intensity to 100 mW/cm2. This was kept constant throughout the measurements by means of a digital exposure controller (Spectra Physics Oriel, model 68950). Spectral quantum efficiency (QE) measurements were performed by onesource illumination (xenon lamp) combined with a monochromator. A calibrated Si cell was used as reference for the J–V as well as the QE measurements.

3. RESULTS 3.1. Growth and characterization of ZnS buffer layers on the CIGS absorber layers Figure 1 shows the scanning electron microscope (a) and transmission electron microscope (TEM) (b) crosssectional images of the ZnS buffer layers deposited on the CIGS absorbers by one cycle of the CBD process. It is known that the removal of uncontrollable precipitated Zn (OH)2 nanoparticles from the surface of the ZnS film induces the formation of the pin-holes in the ZnS films [5,6]. However, the developed growth procedure yields a dense and pin-hole-free ZnS film, and the film completely covered the CIGS absorber layer after one cycle of the CBD process (Figure 1a). The morphology of the ZnS film reveals a fine grain-accumulated structure with a grain size varying in the range of 5–15 nm [18]. The TEM image shows that the thickness of the ZnS film is estimated to be 27 nm after one cycle of the CBD process (Figure 1b). The thickness of the ZnS film after two cycles of the CBD process is about 50 nm. The ZnS buffer layer deposited by CBD process contained Zn–S, Zn–O, and Zn–OH bonds by XPS analysis of our previous publication [18] and often can be expressed as Zn(S,O) [6], Zn(O,S,OH) [7], ZnSx(OH)yOz [18], and so on. Regardless of the compositional variation, it is called as ZnS buffer layer in this study. 3.2. Effect of the ZnS buffer thickness on the performance of the CIGS solar cell To verify the effect of the resistance of the ZnS buffer layer to plasma damage by sputtering, a set of CIGS solar cells with two different thick ZnS buffer layers was fabricated. ZnO : Al window layers were deposited simultaneously on both ZnS buffer layers at a sputtering power of 400 W. Figure 2 shows the J–V curves of the CIGS solar cells fabricated with (a) 27 and (b) 50-nm-thick ZnS buffer Prog. Photovolt: Res. Appl. 2013; 21:217–225 © 2012 John Wiley & Sons, Ltd. DOI: 10.1002/pip

Figure 1. Scanning electron microscope (a) and transmission electron microscope (b) cross-sectional images of ZnS buffer layers deposited on Cu(In,Ga)Se2 (CIGS) absorbers by one cycle of chemical-bath-deposited process.

Figure 2. J–V curves of Cu(In,Ga)Se2 solar cells fabricated with (a) 27 and (b) 50-nm-thick ZnS buffer layers.

layers. The cell efficiency of the CIGS solar cell with a 50-nm-thick ZnS buffer layer is much lower than that of the CIGS solar cell with a 27-nm-thick ZnS buffer layer. The JSC value increases sharply from 8.22 to 26.85 mA/cm2 when the thickness of the ZnS film decreases from 50 to 27 nm. The increase in the value of JSC can be clearly 219

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Table I. Photovoltaic parameters of ZnS/Cu(In,Ga)Se2 solar cells with a ZnO : Al window layer deposited at various sputtering powers. P (W)/tZnS (nm) 400/27 400/50 200/27 150/27 50/200/27

Voc (V)

Jsc (mA/cm2)

FF (%)

(%)

RS (Ω cm)

RSh (Ω cm)

0.52 0.60 0.54 0.55 0.64

26.85 8.22 31.90 33.28 35.30

45.97 38.63 51.62 59.63 64.88

6.49 1.91 8.98 10.95 14.68

3.46 9.5 2.89 1.99 1.08

178 179 201 575 820

understood when the series resistance of both CIGS solar cells are compared. The series resistance, RS, values of the CIGS solar cells with 27 and 50-nm-thick ZnS buffer layer were estimated to be of 3.46 and 9.5 Ω, respectively. As a result, the JSC value of the CIGS solar cell with a 50-nm-thick ZnS buffer layer is reduced to one-third of that of the CIGS solar cell with a 27-nm-thick ZnS buffer layer. The series resistance of the CIGS solar cell with a 50-nm-thick ZnS buffer layer is about three times larger than that of the CIGS solar cell with a 27-nm-thick ZnS buffer layer. The significantly large value of the series resistance hinders the electron transport in the cell. To obtain the values of the series and shunt resistance, an analysis procedure reported by Steven Hegedus et al. was used [21]. The VOC, JSC, FF, , RS, and RSh of the CIGS solar cells with two different ZnS buffer layer thicknesses are summarized in Table I. However, the VOC value of the CIGS solar cell with the 50-nm-thick ZnS buffer layer is higher than that of the CIGS solar cell with the 27-nm-thick ZnS buffer layer. This increase in the value of VOC can be explained in terms of the tolerance to plasma damage by sputtering at the ZnS/CIGS interface. The observed results suggest that a thicker ZnS buffer layer can provide stronger protection for the CIGS absorber layer. This results in a robust heterojunction. Figure 3 shows the dark log J–V curves of the CIGS solar cells fabricated with 27 and 50-nm-thick ZnS buffer layers. In the dark, the recombination current (J0) can be deduced from the log J–V curve. The J0 values of the CIGS solar cells with 27 and 50-nm-thick ZnS buffer layers were found to be 1.0 10 6 and 1.0 10 7 mA/cm2, respectively. The J0 value of CIGS solar cell with the 50-nm-thick

Figure 3. Dark log J–V curves of the Cu(In,Ga)Se2 solar cells fabricated with 27 and 50-nm-thick ZnS buffer layers.

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ZnS buffer layer is one order of magnitude lower than that of the CIGS solar cell with the 27-nm-thick ZnS buffer layer. The significant decrease in the J0 value may be attributed to the reduction of plasma damage by sputtering at the ZnS/CIGS interface. It should be also noted that the plasma damage can create additional defect states at the interface, which can act as charge recombination centers and induce additional recombination pathways. Both the low J0 value and high VOC value point out the important role of the thickness of ZnS buffer layer and indicate the robust p–n junction in the CIGS solar cell with the 50-nm-thick ZnS buffer layer. However, it is clear that the low JSC value created by the high series resistance limits the efficiency of the CIGS solar cells. Therefore, to increase the CIGS solar cell efficiency, the thickness of the ZnS film should be decreased. 3.3. Effect of the sputtering power on the performance of the ZnS/CIGS solar cell We have shown that to achieve a high JSC, the thickness of the ZnS film should be thin, that is, 27 nm. A set of CIGS solar cells with 27-nm-thick ZnS buffer layers were fabricated, and ZnO : Al window layers were deposited at various sputtering powers in the range of 150–400 W. The sputtering power was considered as a processing parameter to control the plasma damage by sputtering. Figure 4 shows the J–V curves of the CIGS solar cells fabricated with a ZnO : Al window layer deposited at

Figure 4. J–V curves of the Cu(In,Ga)Se2 solar cells fabricated with a ZnO : Al window layer deposited at sputtering powers of 150, 200, and 400 W. Prog. Photovolt: Res. Appl. 2013; 21:217–225 © 2012 John Wiley & Sons, Ltd. DOI: 10.1002/pip

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sputtering powers of 150, 200, and 400 W. As shown in Figure 4, the increase in the CIGS solar cell efficiency was consistent with the decrease of the sputtering power. Indeed, the decrease in the sputtering power from 400 to 150 W results in an increase in the efficiency from 6.49% to 10.95%. The JSC and FF values of the CIGS solar cell with a ZnO : Al window layer deposited at 150 W increased to 33.28 mA/cm2 and 59.63%, respectively. It should be noted that the JSC and FF values of the CIGS solar cell with a ZnO : Al window layer deposited at 150 W are over 19% and 22% higher than those of the CIGS solar cell with a ZnO : Al window layer deposited at 400 W. Most of all, the increase of the FF value with reduced sputtering power strongly supports the suggestion that sputtering damage is responsible for the generation of recombination centers. The series resistance, RS, values of the CIGS solar cells with a ZnO : Al window layer deposited at the sputtering powers of 150, 200, and 400 W were 1.99, 2.89, and 3.46 Ω cm, respectively. The shunt resistance, RSh, values of the cells with a ZnO : Al window layer deposited at sputtering powers of 150, 200, and 400 W were 575, 201, and 178 Ω cm, respectively. Note that RS and RSh were improved by reducing the sputtering power. The VOC, JSC, FF, , RS, and RSh of the CIGS cells with a ZnO : Al window layer deposited at various sputtering powers are also summarized in Table I. The origin of the increase in the JSC value with a lower sputtering power can be explained by examining the QE spectra. Figure 5 shows the QE spectra of CIGS solar cells fabricated with a ZnO : Al window layer deposited at sputtering powers of 150, 200, and 400 W. The figure shows that there is a pronounced difference in the longwavelength region (700–1050 nm). The CIGS solar cell with a ZnO : Al buffer layer deposited at 400 W shows the lowest spectral response in both long and shortwavelength ranges. The QE value in the long-wavelength region significantly increases as the sputtering power decreases. This change in the electrical loss can arise from the damage inside the CIGS film because of the high sputtering power. Our result indicates that sputtering can cause damage inside CIGS film and that it can be recovered by reducing

the sputtering power. The longer absorption tail of the CIGS solar cell with a ZnO : Al window layer deposited at 200 W is originated from smaller Ga/(In + Ga) ratio of CIGS film. With different sputtering conditions, the spectral responses of the CIGS solar cells are dramatically changed. The origin of the QE change was discussed in the Discussion section with the optical transmittance and Raman spectroscopic analysis. However, in the short-wavelength region (400–700 nm), the CIGS solar cell showed a poor spectral response of 60–70% regardless of the sputtering power. The considerable drop in the QE spectrum in the short-wavelength region indicates that the sputtering process causes plasma damage at the ZnS film surface and ZnS/CIGS interface regardless of any change in the sputtering power. In fact, the spectral response in the short-wavelength part of the QE spectrum is closely associated with the quality of the ZnS buffer layer and the surface of the CIGS film. A much lower sputtering power is necessary to minimize or avoid plasma damage at the ZnS film. However, a very long deposition time is required, and this route leads to high resistance of the ZnO : Al window layer. It is known that the resistivity of ZnO : Al film deposited at a low sputtering power is higher than that of ZnO : Al film deposited at a high sputtering power [22,23]. 3.4. Development of a bilayer ZnO : Al film for improvement of the cell performance The ZnO : Al films were deposited on glass substrates at a sputtering power in the range of 150–400 W. The resistivity values and thicknesses of the ZnO : Al films are summarized in Table II. As the sputtering power decreases from 400 to 150 W, the resistivity of the ZnO : Al film increases from 4.76 10 4 to 1.95 10 3 Ω cm. This result indicates that the sputtering power cannot be too low, as the resistivity of the ZnO : Al film increases. To overcome the high resistivity of ZnO : Al film deposited at a low sputtering power, we proposed a bilayer ZnO : Al film to obtain low resistivity in the conventional window layer used for CIGS solar cells. The bilayer ZnO : Al film consists of 50-nm-thick ZnO : Al deposited at 50 W as a sputtering protection layer and 100-nm-thick ZnO : Al deposited at 200 W as a conducting layer. The resistivity of the bilayer is 1.31 10 3 Ω cm and is a feasible value to be used for CIGS solar cells (Table II). Indeed, the resistivity value is comparable to that of the conventional window layer used for CIGS solar cells [10,11]. Table II. Resistivity values and thicknesses of ZnO : Al films deposited at various sputtering powers. Sputtering power (W)

Figure 5. Quantum efficiency (QE) spectra of Cu(In,Ga)Se2 solar cells fabricated with a ZnO : Al window layer deposited at sputtering powers of 150, 200, and 400 W. Prog. Photovolt: Res. Appl. 2013; 21:217–225 © 2012 John Wiley & Sons, Ltd. DOI: 10.1002/pip

150 200 400 50 + 200

Thickness (nm) 127 139 155 150

Resistivity (Ω cm) 1.95 10 1.38 10 4.76 10 1.31 10

3 3 4 3

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Figure 6 shows the J–V curves of CIGS solar cells fabricated with a single layer of ZnO : Al film deposited at 150 W and a bilayer of ZnO : Al film deposited at 50 and 200 W, in succession. The efficiency of the CIGS solar cell with the bilayer of ZnO : Al film was found to be 14.68% (VOC = 0.64 V, JSC = 35.3 mA/cm2, FF = 64.88%). The photovoltaic parameters of the CIGS solar cell with the bilayer of ZnO : Al film showed a noticeable improvement compared with a cell with a single layer of ZnO : Al deposited at 150 W. The series resistance, RS, values of the CIGS solar cells with single and bilayer ZnO : Al film were 1.99 and 1.08 Ω cm, respectively, and the shunt resistance, RSh, values of the CIGS solar cells with single and bilayer ZnO : Al film were 575 and 820 Ω cm, respectively. Most importantly, the increase in the values of VOC and FF was significant. This result indicates that the 50-nm-thick ZnO : Al layer deposited at 50 W acts as a protection layer from plasma damage by sputtering. Therefore, the application of a bilayer of ZnO : Al film allows us to increase the efficiency of a CIGS solar cell to 14.68% by minimizing the plasma damage caused by sputtering. Figure 7 shows the dark log J–V curves of CIGS solar cells fabricated with a single layer ZnO : Al deposited at 150 W and a bilayer ZnO : Al deposited at 50 and 200 W, in succession. The J0 values for the CIGS solar cells with a single layer and with a bilayer of ZnO : Al were found to be 4.0 10 7 and 1.5 10 7 mA/cm2, respectively. The J0 value of the CIGS solar cell with a bilayer of ZnO : Al is about one-third of that of the CIGS solar cell with a single layer of ZnO : Al deposited at 150 W. This low J0 value is attributed to the low amount of plasma damage at the ZnS/CIGS interface. Thus, the decrease in the J0 value is consistent with a significant increase of VOC of the CIGS solar cell with a bilayer ZnO : Al. Additionally, the losses in the FF values are related to shunt leakage. The increase in the FF value from 59.53 to 64.88% indicates a reduction in the amount of shunt leakage. Thus, the aforementioned results suggest that the increase in the efficiency of the CIGS solar cell with a bilayer of ZnO :

Al can be explained by reducing the sputtering damage to the ZnS film and the ZnS/CIGS interface. Figure 8 shows the QE spectra of CIGS solar cells with a single layer of ZnO : Al deposited at 150 W and a bilayer of ZnO : Al deposited at 50 and 200 W, in succession. The comparison is provided to illustrate the effect of plasma damage on the photocurrent generation in the shortwavelength region. The QE spectra show a considerable difference in the wavelength range of 400–700 nm. When the CIGS solar cell was fabricated with a bilayer of ZnO : Al, short-wavelength spectral response was considerably recovered, reaching a high QE of up to 85–90%. This result indicates that the 50-nm-thick ZnO : Al layer deposited at a sputtering power of 50 W strongly protects the ZnS/CIGS interface from plasma damage by 200 W of sputtering power. The existence of the 50-nm-thick ZnO : Al layer protects against plasma damage by the high sputtering power and maintains a good heterojunction. Thus, the application of the developed bilayer structure of ZnO : Al is suitable for CIGS solar cells with a ZnS buffer layer, which is known as plasma damage-sensitive buffer layer.

Figure 6. J–V curves of Cu(In,Ga)Se2 solar cells fabricated with a ZnO : Al window layer deposited at 150 and 50/200 W sputtering powers.

Figure 8. Quantum efficiency (QE) spectra of Cu(In,Ga)Se2 solar cells with a ZnO : Al window layer deposited at 150 and 50/ 200 W sputtering powers.

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Figure 7. Dark log J–V curves of Cu(In,Ga)Se2 solar cells fabricated with a ZnO : Al window layer deposited at 150 and 50/ 200 W sputtering powers.

Prog. Photovolt: Res. Appl. 2013; 21:217–225 © 2012 John Wiley & Sons, Ltd. DOI: 10.1002/pip

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4. DISCUSSION Because the sputtering power for ZnO : Al window layer deposition was a processing parameter, the optical transmittances of ZnO : Al window layers deposited at various sputtering powers were examined. Figure 9 shows the optical transmittance spectra of ZnO : Al films deposited at various sputtering powers: 150, 200, 400, and 50/200 W. The transmittances of the ZnO : Al films in the longwavelength region (700–1050 nm) show similar values regardless of decreasing sputtering power. Therefore, it is considered that the transmittance of the ZnO : Al film is not the factor that significantly reduces the QE values in the long-wavelength region. In the short-wavelength region (400–600 nm), the transmittance of the ZnO : Al film slightly increases as the sputtering power decreases from 400 to 150 W. In short-wavelength region, the variation of QE values is not significant to consider the effect of transmittance change. The influence of the optical transmittance of the ZnO : Al window layer is negligible as seen before. Actually, the QE change in the long-wavelength region strongly indicates that the quality of the CIGS absorber layer is significantly degraded, resulting in the loss of carriers from deep within the CIGS absorber layer. During sputtering process for ZnO : Al window layer deposition, the self-bias voltage is estimated as from 210 to 560 V as the sputtering power varies from 150 to 400 W [24]. With the self-bias voltage, the O ions sputtered from the ZnO : Al target are accelerated and hit to ZnS/CIGS film. But the acceleration energy of the O ions is not large enough for ion implantation. However, as seen in the billiard collision, the O ion collision at the ZnS/CIGS surface can transfer the momentum to the lattice of CIGS film. With the momentum transfer, the atoms such as Cu, In, and Ga can be easily displaced through Cu vacancy sites and can be located in the interstitial sites. As a result, there might be lattice distortion, and the vibration intensity of the CIGS film can be changed.

Figure 10. Raman spectra of Cu(In,Ga)Se2 solar cells with a ZnO : Al window layer deposited at 400 and 50/200 W sputtering powers.

The Raman spectra of the CIGS films after ZnO : Al window layer deposition with two different sputtering powers are seen in Figure 10. As seen in Raman spectra, the peak intensity of A1 vibration mode of the CIGS film with ZnO : Al window layer deposited at 400 W is greatly reduced by the distortion of CIGS lattice. The full width at half maximum value of the A1 vibration mode for the CIGS film with ZnO : Al window layer deposited at 400 W is larger than that of the A1 vibration mode for the CIGS film with ZnO : Al window layer deposited at 50/200 W. From the observation of the QE spectra and Raman spectra, the CIGS film is damaged by the momentum transfer of mostly O ions. The origin of lattice distortion could be the displacement of cation atoms through Cu vacancy sites. The temperature dependent VOC measurement might be a useful method to analyze the interface recombination of CIGS/buffer by sputtering damage [21]. Also, the lowtemperature photoluminescence measurement might be useful to identify the defects in the sputter-damaged CIGS film, which is not a trivial work to explain photoluminescence spectra. We will continue to pinpoint the defects in the sputter-damaged CIGS film.

5. CONCLUSIONS

Figure 9. Optical transmittances of ZnO : Al films deposited at various sputtering powers. Prog. Photovolt: Res. Appl. 2013; 21:217–225 © 2012 John Wiley & Sons, Ltd. DOI: 10.1002/pip

CIGS solar cells with a ZnS buffer layer deteriorate because of plasma damage during the sputter deposition process of the ZnO : Al window layer. To resist the plasma damage, a thick ZnS buffer layer is necessary. However, the high series resistance of the thick ZnS film hinders the electron transport. To achieve a high JSC value in CIGS solar cells, the thickness of the ZnS film should be relatively thin (27 nm). However, CIGS solar cells with a thin ZnS buffer layer are strongly affected by plasma damage. By reducing the sputtering power from 400 to 150 W, the JSC and FF values were greatly improved because of the reduced plasma damage in the CIGS bulk film. As a result, the QE value in the long-wavelength region 223

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(700–1050 nm) was greatly improved. However, the QE value was still low in the short-wavelength region (400–700 nm) because of plasma damage on the ZnS film and ZnS/CIGS interface. Therefore, a bilayer ZnO : Al film that consists of a 50-nm-thick ZnO : Al layer deposited at 50 W and 100-nm-thick ZnO : Al film deposited at 200 W was suggested to avoid an increase in the resistivity of the ZnO : Al film and to protect against plasma damage by sputtering. The introduction of a 50-nm-thick ZnO : Al layer creates a sturdy heterojunction that shows a high VOC and a low J0 in a CIGS solar cell with a bilayer of ZnO : Al film. Moreover, the QE value in the short-wavelength region (400–700 nm) was greatly improved. Finally, we accomplished efficiency of 14.68% in a CIGS solar cell with a bilayer ZnO : Al film, thus minimizing the plasma damage by sputtering (VOC = 0.64 V, JSC = 35.3 mA/cm2, FF = 64.88%). Thus, the bilayer structure of ZnO : Al as a window layer is suitable for CIGS solar cells with a ZnS buffer layer that are susceptible to damage caused by the sputtering process.

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ACKNOWLEDGEMENTS This work was supported by the Center for Inorganic Photovoltaic Materials (No. 2012–0001167), the KAIST EEWS Initiative program (EEWS-2012-N01120013), and the Priority Research Center Program (2011–0031407) funded by the Korean Ministry of Education, Science and Technology.

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