Cu(In,Ga)Se 2 FILM FORMATION FROM SELENIZATION OF MIX METAL/METAL-SELENIDE PRECURSORS Rui Kamada, William N. Shafarman, and Robert W. Birkmire Institute of Energy Conversion University of Delaware, Newark, DE 19716 ABSTRACT For Cu(In,Ga)Se 2 films made by the selenization of metallic precursors, Ga accumulation at the back contact prevents the achievement of high voltage solar cells. In this work, selenization of mixed metal/metal-selenide precursors has been studied with respect to the composition distribution and device performance. Precursors consisting of Cu-Se/Ga/In and (In,Ga)-Se/Cu were reacted in H 2Se at 450 C for 5, 15, and 90 minutes with metallic Cu 0.8Ga 0.2/In precursors as a control. Ga accumulation near the back contact in the selenized films was generally observed except for one precursor with a Cu-Se/Ga/In structure, which showed a hill-like Ga profile with the maximum Ga concentration in the middle of the film. Enhanced Ga incorporation into the Cu(In,Ga)Se 2 is shown by XRD for the precursors made from electrochemically deposited copper-selenium and changes in the bandgap were observed in the device behaviors. INTRODUCTION The formation of Cu(In,Ga)Se 2 (CIGS) thin films by the selenization of metallic precursors (Cu, In, Ga) has demonstrated the capability of producing modules with photovoltaic conversion efficiencies over 13% [1]. However, it is widely observed that Ga migrates away from the junction to the Mo back contact [2,3,4]. This backcontact Ga accumulation prevents the increase in bandgap near the junction that is needed for the achievement of high voltage solar cells. An additional reaction step with sulfur has been implemented to increase the voltage [1] because the sulfur incorporation also increases the bandgap. The back-contact Ga accumulation has been attributed to the formation of Cu-In and Cu-Ga intermetallics and their incongruent reaction rates [4]. Furthermore, the decomposition of Cu 9Ga 4 intermetallic remaining at the backside of the film is the rate-limiting step in the selenization process. Although Ga homogenization can be achieved with an additional annealing process [2] or a two-stage growth technique [5], reaction processes that directly yield uniform Ga distribution through the CIGS films are desired. Regarding Cu-In and Cu-Ga in the precursors, the elimination of either metal Cu or group III metals may prevent the intermetallic formation. In this study, a partial selenization of Cu or group III metals was applied to obtain mixed metal/metal-selenide precursors. Copper selenide films were prepared either by electrochemical deposition or co-evaporation. Annealing treatments after the electrochemical deposition yielded phase transformation of the copper selenide. These copper selenide films were then covered with metal Ga and In. For the (In,Ga)-Se/Cu precursor, stacked Ga/In metal films were first deposited then selenized. Copper covered the (In,Ga)-Se layer to form the precursor. The precursors were then selenized to study the effects of the precursor structures on the Ga distribution, selenization reaction rates, and device performances. EXPERIMENTAL For all the precursors, Mo-coated soda lime glass was used as the substrate. Copper-selenium films with a CuSe phase and excess Se were prepared by electrochemical deposition from an acidic aqueous solution containing CuCl 2 and H 2SeO 3 at room temperature. To obtain a Cu 2-xSe phase, samples were annealed for 60 minutes at 300ºC under vacuum. Cu 2-xSe films were also deposited by co-evaporation of Cu and Se with the substrate temperature of 520ºC. Ga and In were then e-beam evaporated onto the copper-selenium films to yield Cu/(In+Ga) = 0.9 in the precursors. Ga/(In+Ga) was 0.3 for the two Cu-Se/Ga/In precursors made from the electrochemical deposition and 0.2 for the Cu 2-xSe/Ga/In precursor made from the co-evaporation. For the (In,Ga)-Se/Cu precursors, Ga and In layers were sequentially deposited onto the substrate by e-beam evaporation to yield Ga/(In+Ga) = 0.2 in the precursors. The Ga/In films were selenized for 60 minutes at 350ºC at atmospheric pressure in a quartz tube reactor [6] using a 0.35 at.% H 2Se/0.0035 at.% O 2/Ar gas mixture. Cu was then e-beam evaporated onto the (In,Ga)-Se films to yield Cu/(In+Ga) = 0.9. As a control, Cu 0.8Ga 0.2/In precursors with a composition of Cu/(In+Ga) = 0.9 and Ga/(In+Ga) = 0.2 were prepared by sequential sputtering. Following the deposition, the precursors were annealed for 60 minutes at 250ºC under flowing 4%H 2/Ar. This annealing treatment of the sputtered precursors has been shown previously to improve the visible uniformity of the final selenized films. For all the precursors, thicknesses were chosen to give a final 2 µm thick CIGS film.
The precursors were selenized for 5, 15 or 90 minutes at 450ºC at atmospheric pressure in a quartz tube reactor [6] using a 0.35 at.% H 2Se/0.0035 at.% O 2/Ar gas mixture. The precursors and selenized films were characterized by scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (S), and x-ray diffraction (XRD) using CuKα1 incident radiation from the front surface of the films. S measurements were done with 20kV accelerating voltage with sampling depth ~ 1µm. The backside of the selenized films and Mo surface were also analyzed after peeling the films at the interface between CIGS and Mo. The CIGS films made from the three Cu-Se/Ga/In precursors and the control precursor were additionally characterized by Auger electron spectroscopy (AES) composition depth profiling. Solar cells with a device structure of soda lime glass/mo/cigs/cds/zno:ito/ni:al grid were fabricated. Current density-voltage (J-V) characteristics and external quantum efficiency (EQE) curves of the solar cells were measured. Precursors RESULTS Copper-selenium films with a CuSe phase and excess Se were obtained by the electrochemical deposition (). The films have a needle-like surface structure and the films are porous. S results show that the composition ratio Se/Cu is 1.9. Since the only crystalline phase observed by XRD is CuSe, excess Se is considered to be amorphous. Fig. 1(a) shows the XRD plot of the Cu-Se film. After Ga and In deposition, metal In peaks and one GaSe peak are observed in addition to CuSe peaks. Numerous indium nodules with the width of 1-3 µm are observed on the Cu-Se/Ga film by SEM-S. After the vacuum annealing of the Cu-Se films, the crystalline phase was completely transformed to a Cu 2-xSe, as shown in Fig. 1(b). The needle-like structure was lost but the surface of the film is still rough and porous. The composition ratio Se/Cu measured by S is 0.7, consistent with the Cu 2-xSe phase. After Ga and In deposition, metal In peaks are also observed by XRD but no gallium or indium selenide peaks are observed. Indium nodules with the width of 1-3 µm are observed on the Cu 2-xSe/Ga film surface by SEM-S. The Cu 2-xSe films deposited by co-evaporation (CE) are composed of the mixture of Cu 2-xSe and Cu 2Se phases (Fig. 1(c)). The majority of the films are Cu 2-xSe. The surface of the film is smoother compared to the Cu-Se precursors. The composition ratio Se/Cu measured by S is 0.6, consistent with the Cu 2-xSe phase. After Ga and In deposition, metal In peaks are observed by XRD in addition to the Cu 2-xSe peaks. The Cu 2Se peak is no longer observable. Indium nodules with several µm dimension are observed on the Cu 2-xSe/Ga film by SEM and S. The indium nodules are distributed more sparsely compared to the former two precursors. The Ga/In films after e-beam evaporation are apparently comprised of In and amorphous Ga because only metal In peaks are observed by XRD. Indium islands are observed on the Ga layer by SEM-S. After the Fig.1. XRD spectra of (a) Cu-Se film, (b) Cu 2-xSe film made by annealing after, (c) Cu 2-xSe film made by CE, (d) (In,Ga)-Se film, and (e) control precursor.
selenization reactions, the films are comprised of γ-in 2Se 3, InSe and GaSe (Fig. 1(d)). The composition ratio Se/(In+Ga) of the selenized films measured by S is 1.4, consistent with the phases mainly comprised of In 2Se 3. After Cu deposition, metal Cu peaks are observed by XRD in addition to the indium and gallium selenide phases. No copper selenide peaks are observed. The control Cu 0.8Ga 0.2/In precursor after the annealing treatment contains metal In and γ1-cu 9(In 0.6Ga 0.4) 4 alloy (Fig. 1(e)). The Ga/(In+Ga) ratio of 0.4 is estimated by application of Vegard s law to Cu 9In 4-Cu 9Ga 4 pseudobinary system. Nodules with several µm dimension cover the surface of the precursors. The nodules are Cu and Ga rich compared to the overall composition, while the background is predominantly In. The similar precursor structure was previously reported [4]. Selenized films Phases identified by XRD measurements after 5, 15, and 90 minute selenization reactions are listed in Table 1. All the precursors form single phase CIGS after the 90 minute reactions. After the 15 minute reaction, Cu 9Ga 4 was identified for the Cu 0.8Ga 0.2/In precursor along with CIGS while only CIGS was seen for the three Cu-Se/Ga/In precursors. For the (In,Ga)-Se/Cu precursor, InSe and GaSe were also identified after the 15 minute reaction. The selenization reaction rate for the (In,Ga)-Se/Cu precursors is apparently slower than the other precursors and the 5 minute reactions were not conducted on them. After the 5 minute reactions, various alloy and selenide phases were identified. Fig. 2. Cu(In,Ga)Se 2 (112) peaks from the XRD plots after the 90 minute selenization reactions of the precursors. Peak positions of CuInSe 2 and CuGaSe 2 are also indicated as vertical lines. Table 1. Phases in the selenized films identified by XRD. 1 st layer deposition Precursor structure reaction time (min) Identified phase Cu-Se/Ga/In 5 InSe, CIGS 15 CIGS 90 CIGS, MoSe 2 5 InSe, CIGS Cu 2-xSe/Ga/In 15 CIGS 90 CIGS CE Cu 2-xSe/Ga/In 5 15 CIGS 90 CIGS Cu 2-xSe, InSe, In 6Se 7 E-beam evaporation (In,Ga)-Se/Cu 15 InSe, GaSe, CIGS 90 CIGS, MoSe 2 Sputtering Cu 0.8Ga 0.2/In 5 15 InSe, CIGS, Cu 3Ga, γ1-cu 9(In,Ga) 4 CIGS, γ1-cu 9(In,Ga) 4 90 CIGS, MoSe 2 Fig. 3. Ga/(In+Ga) ratios measured by SEM-S from the front and back of the selenized films with different selenization reaction times. Dashed lines show Ga/(In+Ga) ratios in the precursors.
Although no intermediate phase was observed after the 15 minute reactions for the three Cu-Se/Ga/In precursors, SEM-S characterization of the Mo back contact surfaces after peeling the films revealed that there were nodules comprised of In-Se, Ga-Se, In-Ga-Se and Cu-Ga-Se at the interface between CIGS and Mo. Therefore, the selenization reactions were not completed in 15 minutes for all the precursors. Although more data with intermediate reaction times are needed to compare the reaction rates, it can be said that the reaction rates are similar for all the precursors. CIGS (112) peaks from the XRD plots of the films after the 90 minute selenization reactions indicate that the Ga distribution differs for different precursors (Fig. 2). The broader peak shape indicates a broader Ga distribution in the CIGS chalcopyrite structure for the films made from the two Cu-Se/Ga/In precursors, compared to the control with Cu 0.8Ga 0.2/In precursors. These peaks are also shifted to higher 2θ suggesting greater Ga incorporation. The peak shapes observed in the films made from the CE Cu 2-xSe/Ga/In and (In,Ga)-Se/Cu precursors are similar to that for the control. Ga/(In+Ga) ratios were measured by S from the front and back of the films after the 5, 15, and 90 minute selenization reactions (Fig. 3). The films made from the Cu 2-xSe/Ga/In and (In,Ga)-Se/Cu precursors show more homogeneous Ga distribution than the control. After the 90 minute reaction, the Ga/(In+Ga) ratios measured from the front of the films are 0.1-0.2 for the two precursors while it is less than 0.05 for the control. For the films made from the co-evaporated Cu 2-xSe/Ga/In and control precursors, Ga accumulates toward the back as the reaction time increases. In contrast, the films made from the Cu-Se/Ga/In precursors show higher Ga/(In+Ga) ratios measured from the front than those measured from the back regardless of the reaction time (Fig. 3(a)). The AES composition depth profiles of selected films are shown in Fig. 4. Ga accumulation near the back is observed for all the selenized films except for the one made from the Cu-Se/Ga/In precursor. Regarding the film made from the Cu 2-xSe/Ga/In precursor, the steeper rise in Ga concentration near the front surface qualitatively corresponds to the higher Ga/(In+Ga) ratios measured by S from the front and the shifted XRD peak. In contrast, Ga shows a hill-like profile for the film made from the Cu-Se/Ga/In precursor. Device performance The best device results of the solar cells made from the precursors with 15 and 90 minute selenization treatments are shown in Table 2. After the 15 minute selenizations, the device performances are inferior to those after the 90 minute selenizations. After the 90 minute selenizations, similar conversion efficiencies are obtained for the devices made from the three Cu-Se/Ga/In precursors compared to the control Cu 0.8Ga 0.2/In and no significant improvement in V OC is seen. Fig. 5 shows the EQE curves of four devices. These devices were made from the precursors with the 90 minute selenization treatments. The shifts of the long wavelength edge of the EQE curve toward shorter wavelengths imply wider Fig. 4. AES composition depth profiles of the films after the 15 minute selenization reactions. Only the film made from the Cu-Se/Ga/In precursor shows a hill-like Ga distribution with the peak in the middle of the film. All other films show higher Ga composition near the back of the film.
bandgap for the devices made from the Cu-Se/Ga/In and Cu 2-xSe/Ga/In precursors. The wider bandgaps are consistent with the increased Ga incorporation near the front of the CIGS for the films made from the two precursors. Table 2. Device J-V parameters of the solar cells made from the selenized precursors. Reaction J Precursor time SC V OC FF η (ma/cm 2 ) (mv) (%) (%) (min) Cu-Se/Ga/In 15 30.8 460 44.8 6.4 90 28.9 498 61.8 8.9 15 26.7 411 42.5 4.7 Cu 2-xSe/Ga/In 90 30.4 487 62.8 9.3 CE 15 35.6 440 48.7 7.6 Cu 2-xSe/Ga/In 90 37.2 439 62.4 10.2 (In,Ga)-Se/Cu 90 32.9 332 38.1 4.2 Cu 0.8Ga 0.2/In 90 36.8 474 50.5 8.8 Fig. 5. EQE curves of the devices made from the precursors after the 90 minute selenization treatments. The shifts of the long wavelength edge indicate wider bandgap in the active region of the devices made from the Cu-Se/Ga/In and Cu 2-xSe/Ga/In precursors. Discussion The Ga distribution in the selenized films can be explained by the reaction preference of In with Se over Ga with Se. For the Cu 2-xSe/Ga/In and (In,Ga)-Se/Cu precursors, selenium is supplied from the gas phase for the selenization reaction of the metals in the precursors. Therefore, the reaction interface is at the front surface of the precursor. Because indium is preferentially pulled toward the reaction interface, the front side of the film is In-rich and Ga is left behind at the back of the selenized film. For the (In,Ga)-Se/Cu precursor, the reaction preference can lead to Ga accumulation toward the back in the (In,Ga)-Se layer during the first selenization process. Since Cu will stay at the front of the precursor to form copper selenide or chalcopyrite, Ga distribution in the (In,Ga)-Se layer is preserved in the final selenized film. In this way, even without any intermetallic phases, the backcontact Ga accumulation can be formed in the selenized films. In contrast, for the Cu-Se/Ga/In precursor, there is excess Se in the copper-selenium layer. Because the excess Se can serve as the Se source for the selenization reactions, there are two reaction interfaces for this precursor; one at the front of the precursor, and one at the interface between the Cu-Se and metal (Ga,In) layers. Due to the In-Se reaction preference, indium can be extracted toward both front and back. As a result, Ga is left in the middle of the film and the hill-like Ga profile can be formed for the Cu-Se/Ga/In precursor. CONCLUSIONS The Ga distribution in the CIGS films can be altered by changing the precursor structure. Although the formation of Cu-In and Cu-Ga intermetallics can be prevented with the mixed metal/metal-selenide precursors, the back-contact Ga accumulation is still generally observed. Therefore, the Cu 9Ga 4 intermetallic remaining at the backside of the film is not the sole mechanism for the Ga distribution. The Cu 2-xSe/Ga/In precursor made from the electrochemical deposition and (In,Ga)-Se/Cu precursor yield a more homogeneous Ga distribution compared to the control Cu 0.8Ga 0.2/In precursor; and the Cu-Se/Ga/In precursor resulted in a hill-like Ga profile with the highest Ga concentration in the middle of the film which is attributed to the excess Se in the Cu-Se layer. These results suggest that homogeneous Ga distribution through selenized films may be obtained with uniformly distributed excess Se in precursors. The three Cu-Se/Ga/In precursors may require a shorter selenization time than that of Cu 0.8Ga 0.2/In precursor although further investigation is needed. The devices from the three Cu-Se/Ga/In precursors show similar conversion efficiencies compared to the control precursor after the 90 minute selenizations. The increased bandgap due to the enhanced Ga incorporation into the CIGS for the electrochemically deposited copper-selenium precursors were indicated by the long wavelength response in EQE measurements. ACKNOWLGEMENT The authors would like to acknowledge Joel Pankow at NREL for AES depth profile measurements, Kevin Hart, Kevin Dobson, Joshua Cadoret and the CIGS research group at IEC for sample preparation and measurement. This work was supported by NREL (contract # ADJ-1-30630-12.
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