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This study presents the synthesis of copper indium gallium (di)selenide (CIGS) films by a solvothermal method. Four factors in CIGS synthesis are considered: In/Ga ratios, hydrogen contents during thermal annealing, thermal annealing temperatures, and annealing times. Experimental results show that the optimal parameters for CIGS film synthesis are the following: proportion of Cu:In:Ga:Se = 1:0.7:0.3:2; hydrogen content during thermal annealing, 5%; thermal annealing temperature, 600 °C; and annealing time, 100 min. The largest crystal grain size of a CIGS film synthesized using these optimal parameters is about 100 nm. The crystal grain size is also found to be inversely proportional to sheet resistance. This relationship holds true because a smaller crystal indicates more grain boundaries and defects. Thus, an electron encounters more barriers in the transmission process, and electric conductivity decreases.
©2012 Optical Society of America
1. Introduction
Copper indium gallium (di)selenide (CIGS) film solar cells are characterized by high photoelectric conversion efficiency, stability of material properties, and pliability [1]. Thus, CIGS film solar cells exhibit the highest potential among all film solar cells. Common methods of CIGS film production include co-evaporation in vacuum, metal sputtering, and selenization [2]. In 2010, the Solar and Hydrogen Energy Research Institute of Germany (Zentrum fuer Sonnenenergie- und Wasserstoff-Forschung, ZSW) stated that the photoelectric conversion efficiency of CIGS solar cells by co-evaporation reached 20.3% [3]. Co-evaporation can produce a CIGS film with the highest efficiency; however, the process requires high-vacuum equipment. Thus, producing even and extensive photovoltaic products is difficult. Moreover, this method entails very high production costs [4,5]. Therefore, numerous studies have focused on widely used methods of producing lower-cost and even solar cells under non-vacuum conditions in recent years. The approach using CIGS solar cells focused on the lower cost advantage from this method as compared to that of III-V semiconductor solar cells. It is important to note that state-of-the-art results for solar cell efficiency had been reported by using multi-junction tandem cell III-V solar cells [6,7]. Recent works on on InGaAsN-containing quantum wells grown on GaAs have led to high performance and very low threshold lasers [8–10], and these advances in dilute-nitride materials had led to recent promising results with more than 43.5% solar conversion efficiency [11].
Non-vacuum process technologies mainly include powder-size coating and electrochemical-solution coating. Electrochemical-solution coating can also be used to form precursor films with stacked layer structures, of pure elements or of combinations with binary or even ternary films. Powder-size coating is a type of coating that is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form. The Electrochemical-solution coating always encounters problems such as unstable electrolyte and bubbles [12,13]. Therefore, this method yields poor-quality films and has difficulty meeting high-volume production. A series of studies on the production method of sizing agent chemical synthesis (direct liquid coating and paste coating) was conducted. Among these methods is the solvothermal method, which accelerates ionic reaction and promotes hydrolysis under solvothermal conditions with organic solvents as the menstruum. Products synthesized using this method exhibit the following advantages: superior crystal habit, stable nanomaterials, and easily synthesized powder. In 2005, Chun et al. conducted an experiment by placing copper, indium, gallium, and selenium powder in a Teflon autoclave. Anhydrous ethylenediamine was then added as the solvent. The solution was heated in the range of 140 °C to 280 °C for 36 h and then cooled to room temperature. Distilled water and absolute ethyl alcohol were used to wash the solution repeatedly. After filtration, the products were dried in vacuum for 6 h at 80 °C. Disk CIGS grains were obtained at a reaction temperature of 140 °C. Different nanoparticle shapes were produced in different solvothermal mechanisms, depending on the reaction temperature and dissolving capacity of the solvent [14]. In 2009, Jong Won Park et al. used a solvothermal method to produce a CIGS colloidal solution and then coated the colloidal solution on a base plate by the scraper method. The solution was heated at 200 °C under atmospheric pressure. It was then annealed at 450 °C in H2/Ar to avoid forming selenium oxide and obtain CIGS nanoparticles in the range of 20 nm to 30 nm [15]. Gu et al. demonstrated that the CIGS synthesis paths in solvothermal method, and synthesize single phase CIGS. CGS seed was synthesized using Cu, In, GaCl3, Se sources by keeping 12 h at 180 °C, and they were able to synthesize single phase CIGS by keeping it for 24 h at raised temperature of 230 °C [16]. In 2012, M. Guk Park et al. produced a CIGS film in a non-vacuum environment by using a precursor solution with low-cost post-selenization and direct coating. The selenized film produced consisted of a double-layer structure. The upper layer was composed of CIGS with chalcopyrite, and the bottom layer was an amorphous silicon structure mainly composed of carbon [17]. Using the scraper method for the above technique lowered the speed and required a thicker solution. Secondary thermal annealing entailed a highly complicated process. Selenization involved a long and inefficient production with high selenium consumption, steam selenium virulence, and unevenly distributed selenium. This process also required a high-cost autonomous device [18–20].
The current study presents the most innovative solvothermal method for CIGS film synthesis. In this experiment, we placed nitrate compounds of copper, indium, and gallium, as well as selenic chloride, in water. An adhesive (ethyl cellulose) and a dispersant (alpha-terpineol) were added to adjust the viscosity and uniformity of the mixed materials. The solution was used to prepare a low-viscosity colloid by using a hot-plate stirrer. Dip coating on the glass plate was performed, and argon was fixed. We noted the changes that occurred when different hydrogen contents were introduced while annealing, synthesizing the CIGS films. We then adjusted the annealing temperature and annealing time to obtain optimal characteristics. We observed the changes in crystalline characteristic according to the peak strength measured by X-ray diffraction (XRD) and the crystal grain characteristics obtained by scanning electron microscopy (SEM). A CIGS film with a grain size of 100 nm was successfully synthesized. The optimal synthesis conditions were Cu:In:Ga:Se = 1:0.7:0.3:2; 5% hydrogen content; 600 °C annealing temperature; and 100 min annealing time. Among the advantages of this method were the non-vacuum, non-selenization, and simple production process, low costs, and high utility ratio of raw materials
2. Experimental procedure
Chun et al. were the first to present the use of a solvothermal method to produce CIGS nanoparticles. They used ethylenediamine as the solvent and placed the four elements in an autoclave to obtain nanoparticles through heating [14,21]. The general solvothermal method involves placing the four elements into an organic solution. Oleylamine is usually chosen as the dispersant or reaction solution [22]. Oleylamine is considered useful in reducing the activation energy of synthesized sulfur or selenide nanoparticles and promoting the dispersion and stability of the suspended solid of the composite materials in the liquid. However, oleylamine molecules block the transmission of the carrier in the unit, affecting efficiency. This blockage problem requires a solution. Moreover, mixing an adhesive strengthens the adhesive force of coating materials [23]. A CIGS mixed compound is then formed after heating and stirring evenly. Doctor-blading or spin-coating are techniques used to coat the mixed compound on the base plate. A CIGS film is produced by annealing and selenizing [14–17,24,25].
The proposed method involves the following procedure: copper nitrate, indium nitrate, gallium nitrate, and selenic chloride are added into an ethanol solution and stirred for 60 min until they are crystallized and dissolved. Ethyl cellulose and alpha-terpineol are then incorporated into the solution. The solution in a covered bottle is stirred evenly at room temperature. The solution is uncovered and then heated and stirred continuously under atmospheric pressure. The prepared solution is placed on a clean glass base plate for spin-coating. The precursor film is finally placed into a high-temperature tube furnace with H2/Ar, annealed at different temperatures (200 °C to 800 °C) and different times (80 min to 120 min). Under synthesized conditions, the chemical reaction is described below:
Based on the synthesis procedure, the three possible In/Ga ratios are 0.7:0.3, 0.5:0.5, and 0.3: 0.7. Compared with the general solvothermal method, the solvothermal method discussed in the present study does not require synthesis in vacuum, secondary annealing, and post-selenization. This study provides a simple and low-cost method of CIGS film production.3. Results and discussion
3.1 Different In/Ga ratios
Equation (1) indicates that CIGS synthesis can be performed using three ratios. We prepared a series of samples to determine the best material characteristics among the different In/Ga ratios, as shown in Table 1 . The In/Ga ratios represented by Samples A, B, and C were 0.7:0.3, 0.5:0.5, and 0.3:0.7, respectively. The thermal annealing temperature was increased from 450 °C to 600 °C. Figure 1(a) presents the XRD maps obtained by thermal annealing of the film at 450, 500, 550, and 600 °C; the film is obtained using the Sample A series (In:Ga = 0.7:0.3) by spin-coating on the base plate; the films are labeled as Samples A1, A2, A3, and A4. Figure 1(a) shows that annealing at 450 °C produces diffraction peaks in the (112) and (220)/(204) directions, although these peaks are not obvious. Gradually increasing the annealing temperature enhances the (112) diffraction peak. Sample A4 exhibits the best main crystallographic preferred (112) orientation, which indicates that increased annealing temperature leads to enhanced crystallinity. Figure 1(b) represents the XRD maps obtained by annealing the film at 450, 500, 550, and 600 °C. The film is obtained using the Sample B series (In:Ga = 0.5:0.5) by spin-coating on the base plate; the films are labeled Samples B1, B2, B3, and B4. Figure 1(b) indicates that gradually increasing the annealing temperature increases the diffraction peak intensity. This behavior is similar to that observed in the XRD maps of Samples A1, A2, A3, and A4. At 550 °C annealing temperature, the strongest (112) diffraction peak is obtained. Figure 1(c) shows the XRD maps obtained by annealing the film at 450, 500, 550, and 600 °C; the films are obtained using the Sample C series (In:Ga = 0.3:0.7) by spin-coating on the base plate. The films are labeled as Samples C1, C2, C3, and C4. These four figures are similar to Samples A1 to A4 and Samples B1 to B4. The most suitable annealing temperature is 550 °C. Figure 1 shows that when the Ga/(In + Ga) ratio increases, the optimal annealing temperature decreases. The reason for this behavior is that the melting point of Ga (29.76 °C) is lower than that of In (156.6 °C), as described in the study by Chun [14]. Therefore, when the Ga ratio increases, the materials are more easily synthesized, and the optimal annealing temperature decreases. We also sorted out the 2θ positions of the (112) orientations in Figs. 1(a) to 1(c), as shown in Table 1. After annealing the films produced using the three solutions with different In/Ga ratios at different temperatures, the 2θ position of their main peaks (112) shifted to lower values as the temperature increased. Bragg's formula can then be used to obtain the space length d of the reflecting surface (hkl) in reverse. The crystallographic lattice constant a of all films can be obtained from the relational expressions of the chalcopyrite lattice structure (Eqs. (2) and (3)) such that c = 2a [26], as shown in Table 1.
Table 1. Parameter list of CIGS samples synthesized in different In/Ga ratios, position of diffraction peak (112) obtained after annealing, and lattice constant tablea
Fig. 1 XRD map of the CIGS samples synthesized with (a) In:Ga = 0.7:0.3 (b) In:Ga = 0.5:0.5, and (c) In:Ga = 0.3:0.7 after annealing in the range of 450 °C to 600 °C.
This relationship clearly indicates that the crystallographic lattice constant increases with the increase in annealing temperature. A number of studies [24,27–29] have demonstrated this behavior; that is, when the Ga/(Ga + In) ratio increases, the crystallographic lattice constant a decreases. This relationship holds true because the atomic radius of Ga (r = 1.22 Å) is smaller than that of In (1.62 Å). A larger crystallographic lattice constant is obtained when the place of the group 3 elements in the structure is occupied by In. Therefore, when the sintering temperature is increased, some Ga elements are displaced. Figure 1 shows the results of Samples A, B, and C after annealing at 600 °C. The crystal orientation (112) of the sample with In:Ga = 0.7:0.3 exhibits the strongest diffraction peak value. Therefore, In:Ga = 0.7:0.3 is the most suitable ratio for this method. We subsequently prepared atomic percentages of Cu, In, Ga, and Se in 1:0.7:0.3:2 ratio and then added different changing factors for improved results.
3.2 Different hydrogen contents during thermal annealing
Yamada demonstrated in his study that hydrogen reacts with Cu2-xSe compounds to reduce the effect of Cu2-xSe on the properties of the cells. However, appropriate amounts of hydrogen should be used. The reaction of excessive hydrogen with selenium produces a large quantity of hydrogen selenide. On the other hand, an insufficient amount of hydrogen cannot remove the Cu2-xSe compound [30,31]. Table 2 presents the synthesis parameters with different hydrogen contents at the thermal annealing stage. Samples H0, H5, H8, H13, and H26 correspond to 0%, 5%, 8%, 13%, and 26% of airing amounts, respectively. These parameters differ from the synthesized parameters in Table 1 in that the amounts of ethyl alcohol and adhesive (ethyl cellulose) in the latter were reduced and their usage levels should be 0.25 and 0.05 g, respectively. In addition, 1 mL of dispersant (alpha-terpineol) should be added to induce even dispersion of the suspended particles in the solution and influence the preferred crystal growth orientation. Figures 2(a) to 2(e) present the high-magnification top view SEM images of H0, H5, H8, H13, and H26. Each group of the parameter surface exhibits a high-density crystal structure. We used the ImageJ software to measure the grain diameter [32]. We randomly selected crystal sizes of 100 particles from each table and then calculated the average size to evaluate the effect of various hydrogen contents on the crystal size. The average particle sizes of H0, H5, H8, H13, and H26 were 32.63, 38.06, 36.10, 35.90, and 33.43 nm, respectively. A large particle diameter can be obtained when 5% hydrogen is fed during annealing. Figure 2(f) shows the measurement results for Samples H0, H5, H8, H13, and H26 by using XRD. The results show that optimal crystallinity was obtained at a hydrogen content of 5%. Figure 3 presents the SEM end views of the H0, H5, H8, H13, and H26 films.
Table 2. Data sheet obtained by feeding different hydrogen contents and different annealing temperatures, position of diffraction peak (112) after annealing, and lattice constant tablea
Fig. 2 High-magnification top view SEM images of Samples (a) H0, (b) H5, (c) H8, (d) H13, and (e) H26; (f) XRD results of Samples H0, H5, H8, H13, and H26.
3.3 Different annealing temperatures
Table 3 shows the CIGS synthesis parameters at different annealing temperatures. Samples T200, T400, T500, T600, and T800 have annealing temperatures of 200, 400, 500, 600, and 800 °C, respectively. Figure 4(a) reveals the XRD patterns of Samples T200, T400, T500, T600, and T800. As indicated in the figure, the diffraction strength is enhanced with an increase in temperature. After annealing at 600 °C, Sample T600 exhibits the chalcopyrite structure with preferred (112) orientation. At temperatures above 600 °C, the diffraction peak of (112) decreases. This behavior is due to the extremely high temperature, which damages the sample and destroys the crystal structure of the film. When the temperature increases, the 2θ position of the main peak (112) shifts to a lower value. Using Bragg's formula and the relationship expressions of the lattice structure, we obtained the crystallographic lattice constant a of all film materials. The crystallographic lattice constant increases with increasing temperature. This trend is the same as those exhibited by Samples A, B, and C. Figures 4(b) to 4(f) show the SEM top views of Samples T200, T400, T500, T600, and T800 films, respectively. Figures 5(a) to 5(e) reveal the SEM end views of Samples T200, T400, T500, T600, and T800 film, respectively. The figures reveal an indistinct structure on the partial surface of Sample T200, which may be due to the incomplete, low-temperature evaporation of the solvent. The crystal density also increases with the increase in thermal annealing temperature. Figure 6 represents the distribution map of the grain size measured using ImageJ. The average grain diameters of Samples T200, T400, T500, T600, and T800 are 27.2, 28.9, 30.8, 38.06, and 26.7 nm, respectively. A larger grain diameter is distributed at an annealing temperature of 600 °C. This result corresponds with the trend of XRD diffraction strength.
Table 3. Data sheet of the samples at different annealing temperatures, position of diffraction peak (112) after annealing, and lattice constant table
Fig. 4 (a) XRD patterns of Samples T200, T400, T500, T600, and T800. SEM top views of Samples (b) T200, (c) T400, (d) T500, (e) T600, and (f) T800.
Fig. 6 Distribution map (a), (b), (c), (d), and (e) of the grain sizes of Samples T200, T400, T500, T600, and T800 measured using ImageJ.
3.4 Different annealing times
Table 4 shows the parameters for CIGS synthesis with different annealing times. Samples t80, t90, t100, t110, and t120 have corresponding annealing times of 80, 90, 100, 110, and 120 min, respectively. Figure 7(a) shows the XRD measurement results of Samples t80, t90, t100, t110, and t120. The figure suggests that the annealing time should not be too long. More desirable main crystal preferred orientations are observed from 90 min to 100 min. With increased annealing time, the peak strength of the main crystal orientation (112) decreases. Figures 7(b) to 7(f) show the high-magnification top view SEM images of Samples t80, t90, t100, t110, and t120 films. Figure 7(g) shows the top view SEM image of Sample t110 at 150,000 × magnification and 500 nm in size. The surface grain size evidently increased with annealing times in the range of 80 min to 100 min. Figures 8(a) to 8(e) show the SEM end views of Samples t80, t90, t100, t110, and t120 films. The upper-right figure on each map represents the cross-sectional images of the samples. From the figure, with an 80 min annealing time, the surface appears indistinct. This appearance may be due to the insufficient annealing time, which retains some of the adhesive. An obvious grain distribution is observed with annealing times of 90 or 100 min. The grain size decreases, and the film hole increases with an annealing time longer than 100 min. Figure 9 shows the distribution map of the grain size measured using ImageJ. The average grain diameters of Samples t80, t90, t100, t110, and t120 are 34.1, 38.06, 51.7, 33.1, and 32.9 nm, respectively. With an annealing time of 100 min, more 100 nm grains appear. This method represents a major breakthrough in CIGS film synthesis.
Table 4. Parameter of CIGS synthesis with different annealing times for Samples t80, t90, t100, t110, and t120 of 80, 90, 100, 110, and 120 min
Fig. 7 (a) XRD measured results of Samples t80, t90, t100, t110, and t120. High-magnification top view SEM images of Samples (b) t80, (c) t90, (d) t100, (e) t110, and (f) t120; (g) Top view SEM image of Sample t110 magnified 150,000 × and 500 nm in size.
Fig. 8 SEM end views of Samples (a) t80, (b) t90, (c) t100, (d) t110, and (e) t120. The upper-right map in each figure represents the cross-sectional images of the samples.
Fig. 9 Distribution map (a), (b), (c), (d), and (e) of the grain sizes of Samples t80, t90, t100, t110, and t120 measured using ImageJ.
3.5 Four-Point Probe
Table 5 shows the measured results of Samples t80, t90, t100, t110, and t120 using a four-point probe. An SEM end view (Fig. 8) is used to obtain the thickness of the film multiplied by the sheet resistance of the sample to obtain the electric resistivity of the film using Eq. (4).
where Rsheet is the sheet resistance (Ω/□); Tf denotes the film thickness; and ρ represents the resistivity in Ω·cm.Table 5. Measured result of Samples t80, t90, t100, t110, and t120 using a four-point prober
The change in resistivity is due to the difference in the grain size of the film. A smaller crystal indicates greater grain boundary and defect. Thus, an electron encounters an excessive barrier during transmission, which results in poor conductivity. Jo et al. proposed that the increase in crystal size reduces grain boundary scattering [32]. This result explains the decrease in measured resistivity. The average grain diameter measured by previous SEM and resistivity are mapped, as shown in Fig. 10 . This trend is in accordance with the results of the aforementioned studies. Therefore, the change in resistivity is caused by the grain diameter of the film. In the experimental result, the resistivity obtained using the optimal parameters is 80.96 Ω·cm. The CIGS resistivity obtained in the previous studies are as follows: Jo et al., from 0.1 Ω·cm to 100 Ω·cm [32]; Zhang et al., from 2 Ω·cm to 225 Ω·cm [33]; Islam et al., from 4 Ω·cm to 80,000 Ω·cm [34]; and Zhang et al., from 2 Ω·cm to 100 Ω·cm [35]. The resistivity of our samples matched these results, thus, the electric conductivity obtained is considered excellent.
4. Conclusion
We used a solvothermal method to synthesize a CIGS film. The advantages of this method include a non-vacuum, non-selenization, and simple production process, low costs, and high utility ratio of raw materials. In this study, four factors were considered. By comparing the different In/Ga ratios, we determined Cu:In:Ga:Se = 1:0.7:0.3:2 as the optimal ratio. The optimal hydrogen content was 5%; however, the hydrogen content should be appropriate. A large amount of hydrogen selenide is produced when excessive hydrogen reacts with selenium. Very little hydrogen did not remove the Cu2-xSe compound. The optimal annealing temperature was 600 °C. The lattice parameter increased with the increase in annealing temperature, and the 2θ position of the main peak (112) shifted to a lower position. At 800 °C, the 2θ position of the main peak (112) moved to a higher position. Under this condition, the materials were damaged. Among the different annealing times, 100 min was considered the optimal duration. Given this annealing time, 100 nm crystal grains were generated. This method represents a major breakthrough, as supported by the results presented.
Acknowledgment
This research was supported by National Science Council, The Republic of China, under the grants of NSC 100-3113-E-110-004, NSC 99-2112-M-390-002-MY3, NSC 100-2221-E-194-043, and NSC 101-2221-E-194-049.
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