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Using Flexible Polyimide as a Substrate to Deposit ZnO:Ga Thin Films and Fabricate p-i-n -Si:H Thin-Film Solar Cells
The GZO thin films were deposited on the polyimide (PI) substrates to investigate their properties for the possibly flexible applications. The effects of substrate temperature (from room temperature to 200°C) on the surface and cross-session morphologies, X-ray diffraction pattern, optical transmission spectrum, carrier concentration, carrier mobility, and resistivity of the GZO thin films on PI substrates were studied. The measured results showed that the substrate temperature had large effect on the characteristics of the GZO thin films. The cross-section observations really indicated that the GZO thin films deposited at 200°C and below had different crystalline structures. The value variations in the films’ optical band gap () of the GZO thin films were evaluated from plots of, revealing that the measured values increased with increasing deposition temperature. Finally, the prepared GZO thin films were also used as the transparent electrodes to fabricate the -Si amorphous silicon thin-film solar cells on the flexible PI substrates, and the properties of which were also measured. We would also prove that substrate temperature of the GZO thin films had large effect on the characteristics of the fabricated -Si amorphous silicon thin-film solar cells.
Tin doped indium oxide (ITO) thin films are widely used as a transparent conducting oxide (TCO) electrode in optoelectronic devices because they have high performance in the visible region, including very low resistivity and high transmittance. However, the price of indium is increasing due to the high demand of ITO in the rapid development of flat panel displays (FPDs) industry. In addition, the toxic nature and high cost due to the scarcity of indium have led researchers to seek an alternative candidate for ITO. Recently, impurity doped zinc oxide is a possible alternative to ITO due to its unique electrical and optical properties. Zinc oxide (ZnO) is a novel II–VI compound n-type oxide semiconductor with various electrical, optical, acoustic, and chemical properties because of its wide direct band gap () value of 3.37 eV at room temperature (RT) . Because of the good properties of thin films, ZnO-based thin films could be used as a transparent electrode in optoelectronic applications such as solar cells , and undoped ZnO thin films could be used as a buffer layer of solar cells  and the active layer of transparent thin films transistor . Alternatively, the effects of different dopant of In2O3 , Al2O3 , and Ga2O3  in ZnO films have also been explored and are also regarded as the ideal candidates for replacing ITO as TCO electrodes owing to their promising optical and electrical properties as well as its low cost, nontoxicity, and long-term environmental stability.
Many researchers have reported about the Ga2O3-doped ZnO thin films according to a different doping concentrations of Ga2O3 . The highly conductive and transparent Ga2O3-doped ZnO thin films had been deposited at high growth rates by radio frequency magnetron sputtering . The thin films processed at room temperature on soda lime glass substrates using a ceramic Ga2O3-doped ZnO target; a low resistivity of Ω-cm was obtained. In the present paper, a compound of ZnO with 3 wt% Ga2O3 (ZnO : Ga2O3 = 97 : 3 in wt%, GZO) was prepared by solid-state reaction method. For the possibly flexible applications, the polyimides (PI) were used as substrates and the GZO thin films were deposited on them to investigate their physical, optical, and electrical properties. However, there is a still controversy about the effects of substrate temperature of the Ga2O3-doped ZnO thin films depositing on PI substrates. For that, the GZO thin films were deposited on PI substrates by changing substrate temperature from RT to 200°C. We would show that substrate temperature played an important role in nucleation, relatively diffraction intensity of orientations, lattice constant, crystalline size, optical value, carrier mobility, carrier concentration, and resistivity of the GZO thin films. In the past, GZO thin films were used as the electrodes of the p-i-n -Si:H thin-film solar cells , produced in a modified single chamber reactor, and the fabricated solar cells exhibited very good electrical characteristics . In the past, the GZO thin films were also deposited on the polyethylene naphthalate (PEN) substrates by RF magnetron sputtering at room temperature . In this paper, the GZO thin films were deposited on polyimide (PI) substrates by RF magnetron sputtering under different substrate temperatures (from RT to 200°C) to investigate their characteristics for the possible applications in flexible photo devices. Finally, the investigated GZO thin films on PI substrates were also used as the transparent electrodes to fabricate the -Si thin-film solar cells. The current-voltage characteristics were measured to determine the effects that substrate temperature of the GZO thin films had on the fabricated -Si solar cells.
2. Experimental Details
In this work, GZO with the composition of ZnO = 97.0 wt% (99.999% purity) and Ga2O3 = 3.0 wt% (99.99% purity) was used to prepare 2-inch (in diameter) target. The weighted composition was mixed, ground, calcined at 1000°C for 5 h, and sintered at 1400°C for 4 h to prepare the GZO ceramic target for sputtering process. RF (13.56 MHz) magnetron sputtering process was used, and the used substrate was 33 mm × 33 mm × 2 mm polyimide (abbreviated as PI). Before deposition process was started, base chamber pressure was pumped to less than Torr; then deposition pressure was controlled at Torr and the GZO thin films were deposited at different temperatures (from RT to ~200°C). In case of the GZO thin films, RF power was controlled as 50 W and substrate temperatures as from room temperature (RT) to 200°C, and distance between the target and polyimide substrate was 8 cm. Before deposition process was started, base chamber pressure was pumped to Torr (detected by using MKS Baratron gauge) and substrate temperature was changed from RT to 200°C; then deposition pressure was controlled at Torr. During deposition process, only argon was introduced in the chamber; the flow rate of pure argon (99.999%) was 20 sccm. Film thickness of the GZO thin films was determined by averaging five data obtained by FESEM, and deposition rates were calculated from measured thickness of the deposited GZO thin films. Film thickness was measured using a Nano-View SEMF-10 ellipsometer and confirmed by field emission scanning electron microscopy (FESEM); the films’ thicknesses were about 260 nm by controlling the deposition time. While the crystalline structure of the GZO thin films was identified by X-ray diffraction (XRD) patterns and the Hall-effect coefficient was measured using a Bio-Rad Hall set-up, the optical transmission spectrum was recorded using a Hitachi U-3300 UV-Vis spectrophotometer in the 300–1400 nm wavelength range. Superstrate p-i-n -Si:H thin film solar cells were fabricated using a single-chamber plasma-enhanced chemical vapor deposition unit at 200°C, with a working pressure of Torr and a deposition power of 20 W. The p-type -Si thin films (thickness was about 20 nm) were deposited by controlling the gas flowing rates for H2 = 100 sccm, SiH4 = 20 sccm, CH4 = 10 sccm, and B2H6 = 40 sccm; the i-type -Si thin films (400 nm) were deposited by using H2 = 100 sccm and SiH4 = 10 sccm; and the p-type -Si (50 nm) thin films were deposited by using H2 = 100 sccm, SiH4 = 20 sccm, and PH3 = 20 sccm, respectively. The current-voltage characteristic of the fabricated solar cells was measured under an illumination intensity of 300 mW/cm2 and an AM 1.5 G spectrum.
3. Results and Discussion
XRD patterns of the GZO thin films as a function of substrate temperature are shown in Figure 1. Even though all the GZO thin films exhibited the (002) peak, they really had different diffraction results. The RT- and 100°C-deposited GZO thin films exhibited only a weak (002) peak, and the diffraction intensity of (002) peak had no apparent increase as substrate temperature increased from RT to 100°C. The 200°C-deposited GZO thin films exhibited a strong (002) peak and a weak (004) peak, which indicate that the -axis is predominantly oriented parallel to the substrate normal. Those results prove that the GZO thin films have different morphologies in the cross-section and they have differently crystalline orientation. The absence of additional peaks in the XRD patterns excludes the possibility of any extra phases and/or large-size precipitates in the GZO thin films. The results in the XRD patterns match the variation in the morphology of the SEM observations that the different morphologies having in the surface (Figure 3) and cross section (Figure 4) of the GZO thin films will lead to different crystallization results. The (002) peaks of the GZO thin films prepared at substrate temperature = RT, 100°C, and 200°C were situated at , 34.18°, and 34.36°, respectively. The lattice constant was calculated by using the 2θ value; the calculated lattice constants () were 0.5249, 0.5243, and 0.5221 as substrate temperatures were RT, 100°C, and 200°C, respectively. All the lattice constant of the GZO thin films being smaller than that of the ZnO thin films is considerable, because the radius of Ga3+ ions (62 pm) is smaller than that of Zn2+ ions (72 pm). As substrate temperature is raised from RT to 200°C, the more Ga3+ ions will substitute the sites of Zn2+ ions, because the radius of Ga3+ ions is shorter than that of Zn2+ ions; for that the lattice constant of the GZO thin films becomes shorter as substrate temperature is raised.
As Figure 2 shows, the full widths at half maximum (FWHM) values for the (002) peak of the GZO thin films were 0.412, 0.360, and 0.296 for substrate temperature were RT, 100°C, and 200°C, respectively. These results suggest that the GZO thin films deposited at higher temperature have the better crystalline structure. The morphologies of the GZO thin films deposited at different substrate temperatures are shown in Figure 3, which indicates that as deposition temperature is changed, the surface morphologies apparently changed as well. As deposited at room temperature, morphology of the GZO thin films exhibited a roughness surface, which showed the nanocrystalline structure of the GZO grains. As Figure 3 shows, the grain size distributions were around the range of 8~40 nm, 10~43 nm, and 15~55 nm. However, the variations of average crystallization sizes are dependent on substrate temperature and are not easily calculated from the surface observation. We will illustrate the variations of grain sizes from the XRD patterns and the following , and the results are also shown in Figure 2: As Figure 2 shows, the average crystallization sizes were 20.2 nm, 23.1 nm, and 28.1 nm as substrate temperatures were RT, 100°C, and 200°C, respectively.
Even the substrate temperature is raised from RT to 100°C, only a little increase in the active energy of plasma GZO molecules to improve the crystallinity and grain growth, for that the diffraction intensity of (002) peak has no apparent change and the crystallization size increases slightly; as substrate temperature is raised from 100°C to 200°C, the plasma GZO molecules will have enough active energy during deposition process and the chance for growth of the GZO crystallization sizes increases; for that the diffraction intensity of the (002) peak and the crystallization size increases. The number of thin film defects also will decrease and the crystallization of the GZO thin films will be improved; then the crystallization of the GZO thin films is also improved and the FWHM value decreases. From the results in Figures 1–3, as substrate temperature is raised from RT to 200°C, a smoother surface morphology and a more uniform -axis orientation were obtained in the 200°C-deposited GZO thin films. Thus, better crystallinity resulting from a stronger -axis orientation and smoother surface morphology could be achieved in the 200°C-deposited GZO thin films prepared on PI substrates.
Figure 4 shows the cross-section observations of the GZO thin films deposited at different substrate temperatures. Calculating the results in Figure 4, thicknesses of both 100°C- and 200°C-deposited GZO thin films were about 260 nm. The cross-section observation of the RT-deposited GZO thin films was not shown here because it had the same thickness and the same cross-section morphology as those of the 100°C-deposited GZO thin films. As the cross-session micrographs shown in Figure 4 are compared, there are different results as the substrate temperature is changed. As RT (not shown here) and 100°C were used as substrate temperatures, the GZO thin films grew like an irregular plate with no specific direction. But depositing at 200°C, the plate-shaped growths were transformed into the nanobar along the up direction. These results prove that the 200°C-deposited GZO thin films and the 100°C-deposited GZO thin films have no the highly oriented parallel to the substrate normal.
Figure 5(a) shows the transmission ratios of the GZO thin films plotted against wavelengths in the region of 300–1400 nm, with substrate temperature as the parameter. As substrate temperature was raised from RT to 200°C, the transmittance ratios of the GZO thin films were almost unchanged. The optical transmission ratios in the visible light region (400–700 nm) were more than 94% for all GZO thin films, regardless of substrate temperature. For the transmission spectra shown in Figure 5(a), as substrate temperature was raised, the optical band edge shifted to a shorter wavelength was observable and a greater sharpness was noticeable in the curves of the absorption edge, which suggest an increase in the values. Also, the GZO thin films deposited on PI substrates had high transmittance ration of over 89.6% in the near-infrared region (750 nm1400 nm).
In the past, determination of the optical band gap () was often necessary to develop the electronic band structure of the thin-film materials for investigating them as a transparent electrode of the thin-film solar cells. However, using extrapolation methods, the values of thin films can be determined from the absorption edge for direct interband transition, which can be calculated using the relation in the following equation: where is the optical absorption coefficient, is the constant for direct transition, is Planck’s constant, and is the frequency of the incident photon . Figure 5(b) plots against (photo energy) in accordance with (2), and the values can be found by extrapolating a straight line at ; the calculated values of the GZO thin films are shown. The linear dependence of on indicates that GZO thin films are direct transition type semiconductors. As the substrate temperature increases from RT to 200°C, the values increase from 3.524 eV to 3.595 eV. Many factors will affect the transmission spectra of the GZO thin films. The increase in the optical band is caused by the increase in carrier concentration because of the decreasing of the lattice defects. The improvement in the electrical properties of the GZO thin films (will be shown in Figure 6) will prove the results. Because the Eg values of the GZO thin films are larger than the energy of visible light, the high tansmittance ratio in the visible light region is expected.
The results of the carrier mobility, carrier concentration, and resistivity shown Figure 6 indicate that the electrical properties of the GZO thin films were dependent on substrate temperature. When plasma molecules are deposited on a glass substrate, many defects result and inhibit electron movement. As different substrate temperatures are used during the deposition process, two factors are believed to cause an increase in the carrier mobility of the GZO thin films. First, higher substrate temperature provides more energy and thus enhances the motion of plasma molecules, which will improve the crystallization and grain size growth of the GZO thin films; also the defects in the thin films will be decreased. Second, as substrate temperature was raised, the defects in the GZO thin films decrease, and that will cause the decrease in the inhibiting of the barriers electron transportation . As Figure 7 shows, both the carrier concentration and carrier mobility of the GZO thin films linearly increased with raising substrate temperature and reached the maximum concentration and carrier mobility at 200°C. The carrier mobility increased from 8.74 cm2/V-s to 11.6 cm2/V-s and the carrier concentration increased from cm−3 to cm−3, respectively, as substrate temperature was raised from RT to 200°C. The resistivity of the TCO thin films is proportional to the reciprocal of the product of carrier concentration and mobility : Both the carrier concentration and the carrier mobility contribute to the conductivity. As substrate temperature was raised from RT to 200°C, the resistivity decreased from Ω-cm to Ω-cm. The minimum resistivity of the GZO thin films at a substrate temperature of 200°C is mainly caused by the carrier concentration and mobility being at their maximum.
Superstrate p-i-n -Si:H thin-film solar cells were fabricated using a single-chamber plasma-enhanced chemical vapor deposition unit at 200°C to demonstrate the opto-electrical properties of the GZO thin film. Figure 7 shows the structure of the fabricated -Si amorphous silicon thin-film solar cells; no antireflective coatings were deposited on the cells. Figure 8 shows the measured current-voltage characteristics of the solar cells under illumination. Table 1 lists the values of open-circuit voltage (), short-circuit current density (), fill factor (F.F.), and efficiency () for the devices using the developed GZO thin films as the front transparent conductive thin films. However, the value had no apparent change and the value apparently increased as substrate temperature of the GZO thin films increased from 100°C to 200°C. The efficiency of the solar cells increased from 4.15% to 4.20% as substrate temperature of the GZO thin films increased from RT to 100°C. Moreover, the efficiency was raised to 4.65% as the GZO thin film was deposited at 200°C. The efficiency of the fabricated solar cells apparently depends on substrate temperature of the GZO thin films. Many reasons will cause the improvement in the efficiency of the -Si:H thin-film solar cells. In this study, the greater efficiency is mainly ascribable to the results in Figures 2, 5, and 6.
From the cross-section (Figure 2) of the GZO thin films, the nanobar along the up direction will have the more optical waveguide function than the irregular plate with no specific direction, which may scatter the photons in any direction. As 200°C-deposited GZO thin films are used as the electrodes, more photons will reach the solar cells’ optical absorption layer to generate more electron-hole pairs and improve the efficiency of the solar cells. As the GZO thin films have the higher transmittance ratio (Figure 5), also the more photons will improve the efficiency of the solar cells. The third reason from Figure 6 is that the smaller value of resistivity of the contact electrodes (GZO thin films) can cause the fabricated solar cells having more short-circuit current density; then the efficiency of the fabricated solar cells increases. This is proved by the - curves shown in Figure 8 and Table 1, where the value increases with decreasing in the resistivity of the GZO thin films.
In this study, the GZO thin films were deposited on flexible PI substrates and their properties were well developed. As substrate temperatures increased from RT to 200°C, the FWHM values for the (002) peak of the GZO thin films decreased from 0.412 to 0.296; the crystallization sizes increased from 20.2 nm to 28.1 nm; the carrier mobility 8.74 cm2/V-s to 11.6 cm2/V-s; the carrier concentration increased from cm−3 to cm−3; and the resistivity decreased from Ω-cm to Ω-cm, respectively. The Burstein-Moss shift theorem was used to prove that as substrate temperatures increased from RT to 200°C, the value shifting from 3.524 eV to 3.599 eV was caused by the increase of carrier concentration increasing from cm−3 to cm−3. Finally, the -Si thin-film solar cells were successfully fabricated on the GZO-deposited PI substrates. As substrate temperatures increased from RT to 200°C, the value increased from 9.11 mA/cm2 to 10.13 mA/cm2 and the value increased from 4.15 to 4.65, respectively.
The authors acknowledge financial supports of NSC 102-2622-E-390-002-CC3 and NSC 102-2221-E-390-027.
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