International Journal of Photoenergy
Volume 2015 (2015), Article ID 483147, 8 pages
http://dx.doi.org/10.1155/2015/483147

Research Article

Improving Performance of CIGS Solar Cells by Annealing ITO Thin Films Electrodes

Chuan Lung Chuang,¹ Ming Wei Chang,¹ Nien Po Chen,¹ Chung Chiang Pan,¹ and Chung Ping Liu1,2

¹Department of Photonics Engineering, Yuan Ze University, 135 Yuan-Tung Road, Chungli 320, Taiwan
²Department of Physics, Fu Jen Catholic University, 510 Zhongzheng Road, Xinzhuang District, New Taipei 242, Taiwan

Received 19 July 2015; Revised 11 October 2015; Accepted 22 October 2015

Academic Editor: Elias Stathatos

Copyright © 2015 Chuan Lung Chuang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Indium tin oxide (ITO) thin films were grown on glass substrates by direct current (DC) reactive magnetron sputtering at room temperature. Annealing at the optimal temperature can considerably improve the composition, structure, optical properties, and electrical properties of the ITO film. An ITO sample with a favorable crystalline structure was obtained by annealing in fixed oxygen/argon ratio of 0.03 at 400°C for 30 min. The carrier concentration, mobility, resistivity, band gap, transmission in the visible-light region, and transmission in the near-IR regions of the ITO sample were  cm⁻³,  cm²/Vs,  Ohm-cm, 3.2 eV, 89.1%, and 94.7%, respectively. Thus, annealing improved the average transmissions (400–1200 nm) of the ITO film by 16.36%. Moreover, annealing a copper-indium-gallium-diselenide (CIGS) solar cell at 400°C for 30 min in air improved its efficiency by 18.75%. The characteristics of annealing ITO films importantly affect the structural, morphological, electrical, and optical properties of ITO films that are used in solar cells.

1. Introduction

Indium tin oxide (ITO) film has highly favorable properties, such as low resistivity (~10⁻⁴ Ohm-cm), high optical transmittance for visible light, and high near-infrared reflectance, making it effective as an n-type window layer, particularly in solar cells [1, 2]. Chalcopyrite compounds of copper-indium-gallium-diselenide (CIGS) and related alloys are among the most promising materials for photovoltaic applications [3]. The highest conversion efficiency of CIGS solar cells that has so far been obtained is 21.7% [4].

ITO thin films can be prepared using many techniques, including e-beam evaporation [5, 6], plasma-enhanced metal organic chemical vapor deposition [7], pulsed laser deposition [8], dip coating [9], ion beam sputtering [10], magnetron sputtering [11], and thermal evaporation [12]. Among these for forming ITO thin films, magnetron sputtering has the advantage of being able to form films of high quality at room temperature [13–16], which can be used to coat large areas [17, 18]. Postdeposition annealing at temperatures of more than 200°C has been shown to be effective in improving the grain growth and crystallinity of ITO thin films [19–21].

In this work, ITO thin films were grown on glass substrates by direct current (DC) magnetron reactive sputtering at room temperature. Annealing at the optimal temperature can greatly improve the composition and structural, optical, and electrical properties of the ITO film. ITO samples with a thickness of ~300 nm were deposited on glass. Some of the ITO film samples were deposited at room temperature using a fixed oxygen/argon ratio of 0.03 before being annealed at various temperatures (as-grown/100°C/200°C/300°C/400°C) for 30 min in air.

2. Experimental Method

ITO thin films were deposited by DC reactive magnetron sputtering from an oxide ceramic target of 90 wt% In₂O₃ and 10 wt% SnO₂ with 99.999% purity, using an experimental setup that has been used elsewhere [22–25]. Glass substrates were placed vertically in a suitable frame and moved in front of the target to deposit ITO at room temperature.

Highly pure argon and oxygen were introduced to a vacuum chamber using independent mass flow controllers after the vacuum chamber had been evacuated to a pressure of less than 1.0 × 10⁻⁵ torr. The final pressure thus reached was fixed at 1.1 × 10⁻³ torr. The applied sputtering power was 2.5 kW. All films were deposited at room temperature. The thickness of each deposited ITO film was approximately 300 nm (Figure 1). Following the deposition process, some of the samples were annealed at various temperatures (as-grown/100°C/200°C/300°C/400°C) for 30 min in air. The structural, optical, and electrical characteristics of the ITO coatings were analyzed as functions of the deposition and annealing parameters by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-VIS transmission spectroscopy, photoluminescence (PL) spectrophotometry, and the making of Hall effect measurements. Moreover, fabricate of a glass/Mo/CIGS/CdS/ZnO/ITO solar cell, and annealing at a best temperature for 30 min in air. I-V characteristics of CIGS solar cell can be obtained by using simulated standard test conditions of AM1.5.

Figure 1: Cross-sectional SEM image of as-deposited ITO film on glass.

3. Results and Discussion

3.1. XRD Measurements

Figure 2(a) presents XRD patterns of the ITO films that were deposited at room temperature using a fixed oxygen/argon ratio of 0.03 and then annealed at various temperatures (as-grown/100°C/200°C/300°C/400°C) for 30 min in air. The XRD measurements were made by scanning the diffraction angle from 20 to 65° at a grazing angle of incidence of 2°. The XRD patterns in Figure 2(a) reveal that the deposited and annealed ITO samples yielded many diffraction peaks at (211), (222), (400), (440), and (622), corresponding to various crystalline structures. As the substrate temperature is reduced, the mobility of the atoms and the particles that are deposited on the substrate falls. The probabilities of interaction are thereby also reduced. The polycrystalline films that were formed herein contained many structural defects and a nonstoichiometric composition. Postannealing can oxidize nonstoichiometric films, such as and [26], and rearrange their atoms to produce stable polycrystalline films. In air, free oxygen may react with the stable polycrystalline films thus formed, improving their crystalline structures, as is revealed by the peak intensities from the annealed films herein. The deposited ITO films in this work have a cubic bixbyite structure of indium oxide with a strong preferential orientation. No evidence of a separate tin oxide phase was obtained herein. Samples that were deposited in pure argon exhibited no preferential crystalline orientation, whereas the XRD diffraction patterns of the ITO films that were deposited at high PO₂ (10% or more) exhibited a preferred [111] orientation, as revealed by the strong (222) peak, consistent with the literature [27]. The lattice distortion calculated with reference to the ideal lattice of In₂O₃ [28] was less than 0.8%.

(a)

(b)

(c)

Figure 2: (a) XRD patterns of ITO thin films deposited using fixed oxygen/argon ratio (0.03) and annealed at various temperatures (as-grown/100°C/200°C/300°C/400°C), (b) XRD intensities of (222) and (400) planes (I ₂₂₂ and I ₄₀₀), and (c) total crystallinity obtained by summing intensities of XRD peaks (I ₂₂₂ + I ₄₀₀) and ratio of XRD peak intensities (I ₂₂₂/I ₄₀₀).

Notably, all of the as-grown ITO thin films in this work exhibited the (222) preferred orientation. Figure 2(b) shows the XRD intensities of the (222) and (400) planes (I ₂₂₂ and I ₄₀₀) and Figure 2(c) presents their total crystallinity, which was obtained by summing the intensities of the XRD peaks (I ₂₂₂ + I ₄₀₀) and by calculating the ratio of the XRD peak intensities (I ₂₂₂/I ₄₀₀). As presented in Figures 2(b) and 2(c), the XRD patterns that were obtained following deposition with a fixed oxygen/argon ratio of 0.03 and annealing at various temperatures (as-grown/100°C/200°C/300°C/400°C) for 30 min in air have the interesting features; at a fixed oxygen/argon ratio (0.03), the XRD peak intensities of the (222) and (400) planes I ₂₀₀ and I ₄₀₀ and I ₂₂₂ + I ₄₀₀ and the peak ratio I ₂₂₂/I ₄₀₀ decreased stepwise as the annealing temperature increased (as-grown/100°C/200°C/300°C/400°C). The increase in the intensity ratio I ₂₂₂/I ₄₀₀ with the substrate temperature and the increase upon postannealing treatment are strongly consistent with the literature [29]. Generally, the preferred (222) orientation of ITO thin films on glass substrates is strongly related to adatom mobility [30].

The samples herein exhibited (222)-orientated crystalline growth following annealing above 100°C, with extensive structural distortion (Figures 2(a)–2(c)). As the annealing temperature is increased, the lattices in the grains relaxed and some oxygen entered penetrated the grain boundaries [31].

3.2. Transmission Measurements

Figure 3(a) presents the total transmission spectra (400–1200 nm), and Figure 3(b) presents the visible (400–800 nm) and near-IR (800–1200 nm) transmission spectra of the ITO films that were deposited at room temperature using a fixed oxygen/argon ratio of 0.03 and then annealed at various temperatures (as-grown/100°C/200°C/300°C/400°C) in air. And Table 1 provided the transmission measurements data. Annealing (400°C) increased the average transmissions (400–1200 nm) of the ITO film by 16.36%. Figure 3(a) reveals that the fundamental absorption edge of the ITO film shifted to shorter wavelengths as the annealing temperature increased. Meng et al. reported [32] an accompanying increase in the carrier concentration, which is known as the Burstein-Moss shift. The Fermi level of a heavily doped n-type semiconductor is such that the bottom of the conduction band bottom is filled, causing the absorption edge to shift to higher energies with an increase in doping.

Table 1: Data concerning the ITO films annealing before and after annealing.

Samples
Parameters

Carrier concentration
(cm⁻³)
Mobility
(cm²/Vs)
Resistivity
(Ohm-cm)
Transmission
(400–1200 nm)
(%)
Transmission
(400–800 nm)
(%)
Transmission
(800–1200 nm)
(%)

As-grown
−3.73E + 20
2.29E + 01
7.32E − 04
79.04
77.14
80.96

After annealing (100°C)
−3.61E + 20
2.38E + 01
7.27E − 04
79.38
77.81
80.96

After annealing (200°C)
−5.55E + 20
2.26E + 01
4.99E − 04
75.39
83.79
67.01

After annealing (300°C)
−5.93E + 20
2.48E + 01
4.25E − 04
85.11
87.84
82.37

After annealing (400°C)
−1.63E + 20
2.71E + 01
1.42E − 03
91.97
89.16
94.77

(a)

(b)

Figure 3: (a) Transmission spectra in visible and near-IR regions for ITO films deposited at room temperature at fixed oxygen/argon ratio (0.03) and annealed at various temperatures (as-grown/100°C/200°C/300°C/400°C) and (b) average visible-light (400–800 nm) and near-IR (800–1200 nm) transmissions of films deposited using a fixed oxygen/argon ratio of 0.03 and annealed at various temperatures (as-grown/100°C/200°C/300°C/400°C).

Table 1 and Figures 3(a) and 3(b) clearly reveal that the ITO films are deposited at room temperature with a fixed oxygen/argon ratio of 0.03; after that, the ITO films annealing at various temperatures (as-grown/100°C/200°C/300°C/400°C) improves their transmission of visible and near-IR lights. In the visible-light (400–800 nm) and near-IR (800–1200 nm) regions, the average transmissions of the as-grown ITO film that was deposited in an oxygen/argon ratio of 0.03 were approximately 77.1% and 80.9%, respectively; these values increased to more than 89.1% and 94.7% for the films that were annealed at 400°C in air. Annealing therefore improved the average transmissions of visible (400–800 nm) and near-IR (800–1200 nm) lights by 15.58% and 17.10%, respectively. The effects of annealing on crystallization reveal that the transmission of visible light through ITO films is closely related to their structure. The surface morphology of films also influences their transmission. The two scattering mechanisms in polycrystalline ITO films are ionized impurity scattering and grain boundary scattering; the dominant mechanism is the latter, as was discussed by Wu and Chiou [33]. As the sizes of the surface grain increase, the fewer grain boundaries scatter less light, increasing transmission in the visible region. Postannealing also favors oxidation of the ITO film compounds with a lower valence state [26]. These effects improve the transmission of visible light in the films. However, carrier absorption importantly affects the transmission of near-IR light through ITO films. The film that was annealed at 400°C in air exhibited the highest transmission. Therefore, most of the scattering of light in the films was Rayleigh scattering [34].

3.3. Electrical Measurements

The transmission of light in the near-IR light region (800–1200 nm) was related to resistivity, carrier concentration, and mobility, as presented in Figures 4(a)–4(c) and Table 1. In Figure 4(a), the high average transmission (800–1200 nm) and high resistivity of the film that was deposited at a fixed oxygen/argon ratio of 0.03 and annealed at 400°C are approximately 94.7% and  Ohm-cm, respectively, and the film that was annealed at 300°C had a lower resistivity of  Ohm-cm and an average transmission (800–1200 nm) of 87.8%. In Figure 4(b), the high average transmission (800–1200 nm) and low carrier concentration in the film that was deposited at a fixed oxygen/argon ratio of 0.03 and annealed at 400°C were approximately 94.7% and  cm⁻³, respectively. The corresponding values for the film that was annealed at 300°C were 82.3% and  cm⁻³. In Figure 4(c), the average transmission (800–1200 nm) and mobility for the film that was deposited at a fixed oxygen/argon ratio of 0.03 and annealed at 400°C are approximately 94.7% and  cm²/Vs, respectively, and those for the film that was annealed at 300°C are 82.3% and  cm²/Vs, respectively. According to Figures 4(b) and 4(c), the carrier concentration and mobility are strongly related to the structure of the film, a result which is consistent with the findings of Meng and dos Santos [35]. Some electrons in polycrystalline films are bound within a small volume of crystals grains in their nonuniform net structure. When such films become crystalline, these electrons are released from these bounded volumes, increasing the carrier concentration. We assert that close to the polycrystalline-crystalline transition point, the lattice transition strongly influences carrier mobility. Since indium in In₂O₃ has a valence of three, the presence of SnO₂ therein would result in n-doping of the lattice because the dopant would add electrons to the conduction band. In contrast, the presence of SnO would reduce the electron density in the conduction band. Some authors believe that, at low substrate temperature, SnO reduces carrier density and annealing transforms SnO into SnO₂, forming an n-type semiconductor with high carrier density and low resistivity [36–39]. According to Hu et al. [21], high-transmission films have a low carrier concentration, consistent by Hall effect measurements, as discussed below. In this work, ITO films were deposited at room temperature using a fixed oxygen/argon ratio of 0.03 and annealed at various temperatures in air such that the free oxygen in the air reacted with them. This reaction reduced the number of oxygen vacancies and the carrier concentration, increasing the resistivity of the films, which is strongly influenced by DC sputtering deposition using a fixed oxygen/argon ratio of 0.03 and the temperature of annealing. At a fixed oxygen/argon ratio, as the annealing temperature increased, the resistivity increased, the carrier concentration declined, and the transmission increased.

(a)

(b)

(c)

Figure 4: (a) Average transmission (800–1200 nm) and resistivity of films deposited at a fixed oxygen/argon ratio of 0.03 and annealed at various temperatures (as-grown/100°C/200°C/300°C/400°C), (b) average transmission (800–1200 nm) and carrier concentration of films deposited at a fixed oxygen/argon ratio of 0.03 and annealed at various temperatures (as-grown/100°C/200°C/300°C/400°C), and (c) average transmission (800–1200 nm) and mobility of films deposited at a fixed oxygen/argon ratio of 0.03 and annealed at various temperatures (as-grown/100°C/200°C/300°C/400°C).

3.4. PL Measurements

PL measurements of the ITO films were made at room temperature. Figure 5 presents a typical PL spectrum of the ITO thin films that were deposited at a fixed oxygen/argon ratio of 0.03 and annealed at various temperatures (as-grown/100°C/200°C/300°C/400°C). Two bands in the visible region are observed; these are more intense than the band excitation edge, indicating poor crystallinity of the ITO film. The less intense band is in the violet-blue region and includes three broad peaks at 387 nm (3.20 eV), 396 nm (3.13 eV), and 406 nm (3.05 eV). In each case, the peak at ~400 nm may be related to the emission of free excitons [40, 41], which are not trapped in defect centers. If the excitons are bound with the phonons, then many inflexions may appear in the broad emission band owing to the exciton-phonon interaction [42]. The second more intense band in the other part of the visible region (from blue to green) has a shoulder at 496 nm (2.50 eV) and a broad peak at 536–540 nm (2.30~2.31 eV). The broad peak at 536–540 nm may originate from strong photo emissions that are caused by some defects. Therefore, many defect levels may lie between the valence band and the conduction band, so most of the emitted photons are in the visible region.

Figure 5: PL spectra of ITO thin films deposited using a fixed oxygen/argon ratio of 0.03 and annealed at various temperatures (as-grown/100°C/200°C/300°C/400°C).

3.5. Applications of CIGS Solar Cell

A glass/Mo/CIGS/CdS/ZnO/ITO solar cell was fabricated. This CIGS solar cell was annealed at 400°C for 30 min in air to improve its performance. I-V characteristics of CIGS solar cell can be obtained by using simulated standard test conditions of AM1.5. Table 2 presents data concerning the CIGS solar cell annealing before and after annealing. Figure 6 plots the illuminated I-V characteristics of the nonannealed and annealed solar cell. Annealing increased the efficiency of the CIGS solar cell by 18.75% (from 3.318% to 4.016%). The main improved parameters are open circuit voltage (), which is improved by 0.27% (from 0.367 to 0.368 V), current density (), which is improved by 5.63% (from 30.19 to 31.89%), and fill factor (FF), which is improved by 12.09% (from 30.51 to 34.20%). Therefore, the efficiency, which is the average transmission (400–1200 nm) of the ITO film, was improved by 16.36%. Additionally, annealing (400°C) improved the average transmissions of visible (400–800 nm) and near-IR (800–1200 nm) lights by the ITO films by 15.58% and 17.10%, respectively. Chen et al. [43] found that the transmission of short wavelengths (λ < 800 nm) is strongly affected by surface recombination, whereas that of long wavelengths (λ > 850 nm) is strongly influenced by bulk recombination. Liu et al. [44] found that the reflection of these wavelengths was related to surface recombination and bulk recombination, respectively. The solar cell absorbs light with wavelengths from 350 to 1200 nm, so increasing the transmission of light of these wavelengths can effectively increase the efficiency of solar cells [44–46]. Even if some points addressed on high-quality CIGS solar cells had been reported [47, 48], we would still like to verify them because our current study aims to enhance the methodology of research in this paper.

Table 2: Data concerning the CIGS solar cell annealing before and after annealing.

Samples
Parameters

Area
Efficiency

FF

(cm²)
(%)
(V)
(mA/cm²)
(%)

Before annealing
0.35
3.382
0.367
30.19
30.51

After annealing
0.35
4.016
0.368
31.89
34.20

Figure 6: The illuminated I-V characteristics of CIGS solar cell annealing before and after annealing.

4. Conclusions

ITO thin films were grown on glass substrates by DC magnetron reactive sputtering at room temperature. An ITO sample with a highly crystalline structure was obtained by deposition (at OR) using an oxygen/argon ratio of 0.03, followed by annealing at 400°C for 30 min. Annealing increased the average transmission (400–1200 nm) of the ITO film by 16.36% and the efficiency of a CIGS device that included such a film by 18.75%. These results are very encouraging for the future fabrication of CIGS solar cells with ITO films that are deposited using DC magnetron sputtering at room temperature and postdeposition annealing.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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