- Email: [email protected]
Se precursors
Materials Letters 157 (2015) 183–187
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Control of Se layer thickness in two-step selenization and sulfurization of CuGa/In/Se precursors Jaseok Koo, Woo Kyoung Kim n School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 712-749, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 19 March 2015 Received in revised form 30 April 2015 Accepted 23 May 2015 Available online 27 May 2015
Sputter-deposited bilayer CuGa/In precursors were coated by a Se layer with a different thickness from 0.5 to 1.5 μm to yield glass/Mo/CuGa/In/Se structure. Selenization of the precursors with a 0.5 μm-thick Se layer resulted in partial selenization with a relatively uniform distribution of Ga, whereas Cu(InGa)Se2 formed from 1.0 and 1.5 μm-thick Se layers showed complete selenization but with Ga accumulation at the bottom. Partial selenization of the Se-coated metal precursors by a 0.5 μm-thick Se layer was confirmed to yield better incorporation of S and effective re-distribution of Ga to form Cu(InGa)(Se,S)2 films with homogenous depth profiles of Ga and S. & 2015 Elsevier B.V. All rights reserved.
Keywords: Electronic materials Solar energy materials Cu(InGa)Se2 Selenization Sulfurization
1. Introduction
The selenization of Cu–In–Ga metal precursors has been considered proven deposition process for fabricating high-efficiency light absorbers for Cu(InGa)Se2 (CIGS)-based thin-film solar cells [1,2]. However, typical selenization of metal precursors was known to result in Ga segregation toward the Mo back contact partly due to preferential reaction of In with Se, consequently reducing the open-circuit voltage (VOC) [3]. Additional sulfurization reaction by H2S gas was mainly adapted to compensate for VOC loss due to Ga deficiency near surface of CIGS absorber [1,2]. It was also reported that Ga depth uniformity could be controlled by adjusting selenization and sulfurization processes [4–6]. In our previous report [7], we reported that the rapid thermal process (RTP) of CuGa/In/Se precursor has a potential to reduce Ga segregation compared to conventional selenization process, by investigating detailed selenization pathways using in-situ high-temperature X-ray diffraction analysis. In this study, the thickness of Se layer in the CuGa/In/Se precursor was varied from 0.5 to 1.5 μm in order to control the degree of selenization at a given selenization time and temperature. Then, Cu(InGa)(SeS)2 (CIGSS) films formed via subsequent sulfurization by 4 mol% H2S gas n
Corresponding author. Fax: þ 82 53 810 4631. E-mail address: [email protected] (W.K. Kim).
http://dx.doi.org/10.1016/j.matlet.2015.05.108 0167-577X/& 2015 Elsevier B.V. All rights reserved.
were precisely compared.
2. Experimental
Bilayer CuGa and In thin films were deposited onto Mocoated glass by DC magnetron sputtering using CuGa (24 wt% Ga) and pure In targets. Then, Se was coated without heating sample in a vacuum evaporator [ 10 6 Torr ( 1.33 mPa)], where the Se thickness was varied from 0.5 to 1.5 μm in 0.5-μm increments. A detailed description of the metal sputtering and Se evaporation procedures was reported in our previous paper [7]. Glass/Mo/CuGa/In/Se precursors were selenized in a tube-type rapid thermal process system. The process temperature and time were fixed at 570 °C and 10 min at a ramp rate of 4 °C/s, respectively. A schematic drawing of our RTP system and details of its operation can also be found elsewhere [7]. Selenized samples were then sulfurized at 600 °C by a 4 mol% H2S/He gas mixture for 5 min in the identical RTP system. The crystal structure and cross-sectional images of the precursors and corresponding CIGSS samples were characterized by X-ray diffraction (XRD: PANalytical X’Pert PRO MPD) and fieldemission scanning electron microscopy (FE-SEM: Hitachi S-4800). The bulk atomic concentration and compositional depth profile of the samples were measured by inductively
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Table 1 Atomic composition of CIGS and CIGSS samples fabricated by two-step selenization and sulfurization (as measured by ICP-AES). Selenization Se layer thickness (μm) Cu/III Ga/III S/VI Se þ S (at%)
Fig. 1. Room-temperature XRD scans with Cu Kα1 radiation of CIGS sample selenized from glass/Mo/CuGa/In/Se precursor with Se layer thicknesses of 0.5, 1, and 1.5 μm: (a) 2θ ¼20–60° and (b) 2θ¼ 25–30°.
coupled plasma atomic emission spectroscopy (ICP-AES: Perkin-Elmer OPTIMA 8300) and an energy dispersive X-ray spectroscope (EDS) attached to a high-resolution transmission electron microscope (HR-TEM: Hitachi HF-3300).
3. Results and discussion
Room-temperature XRD and SEM images of a cleaved sample (not shown here) confirmed that the as-deposited bilayer CuGa/In precursor was composed of intermetallic (e.g., Cu4In, CuIn, CuxGa) and pure In phases. The Se layer deposited by vacuum evaporation was identified as amorphous. The total thickness of the precursor was approximately 500 nm and the atomic composition measured by ICP-AES was Cu/III 0.94 and Ga/III 0.34, where III stands for Ga þIn. The results of the XRD analysis (Fig. 1) on the CIGS sample selenized at 570 °C for 10 min demonstrate that the samples selenized by both the 1.0 and 1.5 μm-thick Se coatings were composed of both Ga-poor (2θ E 26.75°) and Ga-rich CIGS (2θ E 27.45°). On the other hand, the sample selenized by the 0.5-μm-thick Se layer showed a single CIGS (112) peak at 2θ E26.93°, implying more uniform incorporation of Ga into
0.5 1.0 0.3 – 41
1.0 0.97 0.3 – 48
Selenization/sulfurization 1.5 0.97 0.3 – 51
0.5 1.0 0.3 0.67 45
1.0 0.97 0.3 0.12 50
1.5 0.97 0.3 0.06 50
the CIGS phase. However, it can be assumed that the 0.5 μm thickness of the Se layer may not be sufficient for complete selenization of 0.5 μm-thick CuGa/In precursors. As shown in Table 1, the ICP-AES analysis confirmed that the Se content of CIGS samples formed by the 0.5 μm-thick Se layer was approximately 41 at%, whereas that of the other two samples was 48–50 at%, which is close to the composition value for complete selenization. The TEM–EDS depth profiles in Fig. 2 show that the CIGS samples selenized by using the thin 0.5 μm-thick Se layer had a relatively uniform Ga distribution throughout the depth of the sample, whereas the Ga in the sample prepared by using the 1 μm-thick Se layer is mainly accumulated at the sample bottom, which is consistent with the XRD results. It also suggests that the Ga content slightly increased with increasing depth and that there was a thin ( 100 nm) Cu–Ga intermetallic layer near the Mo back contact. This is presumably an unreacted Cu–Ga alloy since the Se and In profiles did not reach the Mo layer or the bottom region of the Ga. On the other hand, in Fig. 2(b), Se diffused down to the Mo surface, indicating the possible completion of selenization. The cross-sectional SEM images also confirm that the sample formed by the 1 μm-thick Se layer has a cluster of small grains at the sample bottom, whereas the 0.5 μm-thick Se layer shows a relatively uniform grain size. It is believed that the small grains correspond to Ga-rich CIGS phase formed by the substitution of relatively small Ga atoms for large In atoms. In another set of experiment, precursors covered by a Se layer with different thickness (i.e., 0.5, 1.0, and 1.5 μm) were sequentially selenized and sulfurized at 570 °C for 10 min and 600 °C for 5 min, respectively. As shown in Fig. 3(a), the CIGS (112) peak shift by sulfurization of CIGS formed by the 1.0 and 1.5 μm-thick Se layers was negligible, i.e., 26.75°-26.84° ( Δ 2 θ E 0.09°) for 1 μm Se and 26.75°-26.77° ( Δ 2 θ E 0.02°) for 1.5 μm Se. This implies a negligible amount of S incorporation into the CIGS structure and nearly no re-distribution of accumulated Ga within CIGS. As summarized in Table 1, the S contents of the samples formed using precursors with 1.0 and 1.5 μmthick Se layers were measured to be approximately 6 and 3 at%, respectively. However, there was a significant change in the CIGS (112) peak position and S content of the 0.5 μm-thick Se sample, i.e., 26.93°-27.80° ( Δ 2 θ E 0.87°) and 30 at%, respectively. Cross–sectional SEM images and TEM–EDS depth profiles of CIGSS samples are compared in Fig. 3(b)–(c). The CIGSS samples formed from the precursor with a 0.5 μm-thick Se layer [Fig. 3(b)] showed a fairly uniform Ga depth profile except slight deficiency of Ga at around 100 nm-thick region near surface, and a significant amount of S incorporation through
J. Koo, W.K. Kim / Materials Letters 157 (2015) 183–187
185
Fig. 2. Cross-sectional SEM images and TEM–EDS depth profiles of CIGS samples selenized from glass/Mo/CuGa/In/Se precursor with different thicknesses of Se layer: (a) 0.5 μm and (b) 1 μm.
the entire depth of CIGSS absorber. However, a Cu–Ga intermetallic still remained at the bottom, which was also observed in the sample after selenization in Fig. 2(a). It is also supported by the ICP-AES result of S þ Se¼ 45 at%, implying incomplete consumption of the precursors. On the other hand, the results of the CIGSS samples formed from the precursor with a 1.0 μm-thick Se layer [Fig. 3(c)] revealed that only part of Se in the top thin layer (200–300 nm) was replaced by S, and thus most of the Ga remained near the Mo side, even though all precursors were selenized and/or sulfurized, as evidenced by the ICP-AES result of S þSe ¼ 50 at%. Therefore, it is suggested that the optimum thickness of Se layer to obtain both reaction completeness and better Ga depth profile at the present process conditions should lie in between 0.5 and 1.0 μm. Otherwise, process conditions should be further optimized.
4. Conclusion
The degree of selenization of glass/Mo/CuGa/In/Se precursors was controlled by adjusting the thickness of the Se coating on the precursors and the reaction conditions, e.g., reaction temperature, ramp rate and duration time. Partial
selenization of the metal precursors achieved by reducing the thickness of the Se layer (0.5 μm in this report) is preferred for subsequent incorporation of S and re-distribution of Ga to complete selenization by a thicker Se layer (1– 1.5 μm in this report). It is concluded that the optimization of compositional depth profile (i.e., energy band gap profile) for high-performance solar cell absorber can be achieved by manipulating the degree of selenization and subsequent sulfurization conditions.
Acknowledgment
This research was supported by the Human Resources Development of Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant (No. 20124030200100) funded by the Korea government Ministry of Trade, Industry and Energy, and the World-Class 300 Project (Development of 5th generation batch type selenization and sulfurization system for CIGS thin-film solar cell production of 14.5% highest efficiency) funded by the Small and Medium Business Administration of Korea (SMBA).
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Fig. 3. Room-temperature XRD scans with Cu Kα1 radiation (a), cross-sectional SEM images and TEM–EDS depth profiles (b, c) of CIGSS sample formed from two-step selenization (570 °C for 10 min) and sulfurization (600 °C for 5 min) of glass/Mo/CuGa/In/Se precursor with different thicknesses of Se layer (0.5, 1, and 1.5 μm). Se layer thickness: (b) 0.5 μm and (c) 1 μm.
References [1] Kushiya K, Tanaka Y, Hakuma H, Goushi Y, Kijima S, Aramoto T, et al. Interface control to enhance the fill factor over 0.70 in a large-area CIS-based thin-film PV technology. Thin Solid Films 2009;517:2108–10. [2] Karg F. High efficiency CIGS solar modules. Energy Procedia 2012;15:275–82.
[3] Hanket GM, Shafarman WN, McCandless BE, Birkmire RW. Incongruent reaction of Cu-(InGa) intermetallic precursors in H2Se and H2S. J. Appl. Phys. 2007;102:074922. [4] Alberts V, Titus J, Birkmire RW. Material and device properties of single-phase Cu(In,Ga)(Se,S)2 alloys prepared by selenization/sulfurization of metallic alloys. Thin Solid Films 2004;451–452:207–11. [5] G.M. Hanket, R. Kamada, W.K. Kim, W.N. Shafarman, Effect of reaction
J. Koo, W.K. Kim / Materials Letters 157 (2015) 183–187
temperature on Cu(lnGa)(SeS)2 formation by a sequential H2Se/H2S precursor reaction process, in: Proceedings of the 33rd IEEE Photovoltaic Specialists Conference, San Diego, CA, May 11-16, 2008. [6] Kim WK, Hanket GM, Shafarman WN. Ga homogenization by simultaneous H2Se/H2S reaction of Cu–Ga–In precursor. Sol. Energy Mater. Sol. Cells 2011;95:235–8.
187
[7] Koo J, Jeon S, Oh M, Cho H, Son C, Kim WK. Optimization of Se layer thickness in Mo/CuGa/In/Se precursor for the formation of Cu(InGa)Se2 by rapid thermal annealing. Thin Solid Films 2013;535:148–53.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Control of Se layer thickness in two-step selenization and sulfurization of CuGa/In/Se precursors Jaseok Koo, Woo Kyoung Kim n School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 712-749, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 19 March 2015 Received in revised form 30 April 2015 Accepted 23 May 2015 Available online 27 May 2015
Sputter-deposited bilayer CuGa/In precursors were coated by a Se layer with a different thickness from 0.5 to 1.5 μm to yield glass/Mo/CuGa/In/Se structure. Selenization of the precursors with a 0.5 μm-thick Se layer resulted in partial selenization with a relatively uniform distribution of Ga, whereas Cu(InGa)Se2 formed from 1.0 and 1.5 μm-thick Se layers showed complete selenization but with Ga accumulation at the bottom. Partial selenization of the Se-coated metal precursors by a 0.5 μm-thick Se layer was confirmed to yield better incorporation of S and effective re-distribution of Ga to form Cu(InGa)(Se,S)2 films with homogenous depth profiles of Ga and S. & 2015 Elsevier B.V. All rights reserved.
Keywords: Electronic materials Solar energy materials Cu(InGa)Se2 Selenization Sulfurization
1. Introduction
The selenization of Cu–In–Ga metal precursors has been considered proven deposition process for fabricating high-efficiency light absorbers for Cu(InGa)Se2 (CIGS)-based thin-film solar cells [1,2]. However, typical selenization of metal precursors was known to result in Ga segregation toward the Mo back contact partly due to preferential reaction of In with Se, consequently reducing the open-circuit voltage (VOC) [3]. Additional sulfurization reaction by H2S gas was mainly adapted to compensate for VOC loss due to Ga deficiency near surface of CIGS absorber [1,2]. It was also reported that Ga depth uniformity could be controlled by adjusting selenization and sulfurization processes [4–6]. In our previous report [7], we reported that the rapid thermal process (RTP) of CuGa/In/Se precursor has a potential to reduce Ga segregation compared to conventional selenization process, by investigating detailed selenization pathways using in-situ high-temperature X-ray diffraction analysis. In this study, the thickness of Se layer in the CuGa/In/Se precursor was varied from 0.5 to 1.5 μm in order to control the degree of selenization at a given selenization time and temperature. Then, Cu(InGa)(SeS)2 (CIGSS) films formed via subsequent sulfurization by 4 mol% H2S gas n
Corresponding author. Fax: þ 82 53 810 4631. E-mail address: [email protected] (W.K. Kim).
http://dx.doi.org/10.1016/j.matlet.2015.05.108 0167-577X/& 2015 Elsevier B.V. All rights reserved.
were precisely compared.
2. Experimental
Bilayer CuGa and In thin films were deposited onto Mocoated glass by DC magnetron sputtering using CuGa (24 wt% Ga) and pure In targets. Then, Se was coated without heating sample in a vacuum evaporator [ 10 6 Torr ( 1.33 mPa)], where the Se thickness was varied from 0.5 to 1.5 μm in 0.5-μm increments. A detailed description of the metal sputtering and Se evaporation procedures was reported in our previous paper [7]. Glass/Mo/CuGa/In/Se precursors were selenized in a tube-type rapid thermal process system. The process temperature and time were fixed at 570 °C and 10 min at a ramp rate of 4 °C/s, respectively. A schematic drawing of our RTP system and details of its operation can also be found elsewhere [7]. Selenized samples were then sulfurized at 600 °C by a 4 mol% H2S/He gas mixture for 5 min in the identical RTP system. The crystal structure and cross-sectional images of the precursors and corresponding CIGSS samples were characterized by X-ray diffraction (XRD: PANalytical X’Pert PRO MPD) and fieldemission scanning electron microscopy (FE-SEM: Hitachi S-4800). The bulk atomic concentration and compositional depth profile of the samples were measured by inductively
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Table 1 Atomic composition of CIGS and CIGSS samples fabricated by two-step selenization and sulfurization (as measured by ICP-AES). Selenization Se layer thickness (μm) Cu/III Ga/III S/VI Se þ S (at%)
Fig. 1. Room-temperature XRD scans with Cu Kα1 radiation of CIGS sample selenized from glass/Mo/CuGa/In/Se precursor with Se layer thicknesses of 0.5, 1, and 1.5 μm: (a) 2θ ¼20–60° and (b) 2θ¼ 25–30°.
coupled plasma atomic emission spectroscopy (ICP-AES: Perkin-Elmer OPTIMA 8300) and an energy dispersive X-ray spectroscope (EDS) attached to a high-resolution transmission electron microscope (HR-TEM: Hitachi HF-3300).
3. Results and discussion
Room-temperature XRD and SEM images of a cleaved sample (not shown here) confirmed that the as-deposited bilayer CuGa/In precursor was composed of intermetallic (e.g., Cu4In, CuIn, CuxGa) and pure In phases. The Se layer deposited by vacuum evaporation was identified as amorphous. The total thickness of the precursor was approximately 500 nm and the atomic composition measured by ICP-AES was Cu/III 0.94 and Ga/III 0.34, where III stands for Ga þIn. The results of the XRD analysis (Fig. 1) on the CIGS sample selenized at 570 °C for 10 min demonstrate that the samples selenized by both the 1.0 and 1.5 μm-thick Se coatings were composed of both Ga-poor (2θ E 26.75°) and Ga-rich CIGS (2θ E 27.45°). On the other hand, the sample selenized by the 0.5-μm-thick Se layer showed a single CIGS (112) peak at 2θ E26.93°, implying more uniform incorporation of Ga into
0.5 1.0 0.3 – 41
1.0 0.97 0.3 – 48
Selenization/sulfurization 1.5 0.97 0.3 – 51
0.5 1.0 0.3 0.67 45
1.0 0.97 0.3 0.12 50
1.5 0.97 0.3 0.06 50
the CIGS phase. However, it can be assumed that the 0.5 μm thickness of the Se layer may not be sufficient for complete selenization of 0.5 μm-thick CuGa/In precursors. As shown in Table 1, the ICP-AES analysis confirmed that the Se content of CIGS samples formed by the 0.5 μm-thick Se layer was approximately 41 at%, whereas that of the other two samples was 48–50 at%, which is close to the composition value for complete selenization. The TEM–EDS depth profiles in Fig. 2 show that the CIGS samples selenized by using the thin 0.5 μm-thick Se layer had a relatively uniform Ga distribution throughout the depth of the sample, whereas the Ga in the sample prepared by using the 1 μm-thick Se layer is mainly accumulated at the sample bottom, which is consistent with the XRD results. It also suggests that the Ga content slightly increased with increasing depth and that there was a thin ( 100 nm) Cu–Ga intermetallic layer near the Mo back contact. This is presumably an unreacted Cu–Ga alloy since the Se and In profiles did not reach the Mo layer or the bottom region of the Ga. On the other hand, in Fig. 2(b), Se diffused down to the Mo surface, indicating the possible completion of selenization. The cross-sectional SEM images also confirm that the sample formed by the 1 μm-thick Se layer has a cluster of small grains at the sample bottom, whereas the 0.5 μm-thick Se layer shows a relatively uniform grain size. It is believed that the small grains correspond to Ga-rich CIGS phase formed by the substitution of relatively small Ga atoms for large In atoms. In another set of experiment, precursors covered by a Se layer with different thickness (i.e., 0.5, 1.0, and 1.5 μm) were sequentially selenized and sulfurized at 570 °C for 10 min and 600 °C for 5 min, respectively. As shown in Fig. 3(a), the CIGS (112) peak shift by sulfurization of CIGS formed by the 1.0 and 1.5 μm-thick Se layers was negligible, i.e., 26.75°-26.84° ( Δ 2 θ E 0.09°) for 1 μm Se and 26.75°-26.77° ( Δ 2 θ E 0.02°) for 1.5 μm Se. This implies a negligible amount of S incorporation into the CIGS structure and nearly no re-distribution of accumulated Ga within CIGS. As summarized in Table 1, the S contents of the samples formed using precursors with 1.0 and 1.5 μmthick Se layers were measured to be approximately 6 and 3 at%, respectively. However, there was a significant change in the CIGS (112) peak position and S content of the 0.5 μm-thick Se sample, i.e., 26.93°-27.80° ( Δ 2 θ E 0.87°) and 30 at%, respectively. Cross–sectional SEM images and TEM–EDS depth profiles of CIGSS samples are compared in Fig. 3(b)–(c). The CIGSS samples formed from the precursor with a 0.5 μm-thick Se layer [Fig. 3(b)] showed a fairly uniform Ga depth profile except slight deficiency of Ga at around 100 nm-thick region near surface, and a significant amount of S incorporation through
J. Koo, W.K. Kim / Materials Letters 157 (2015) 183–187
185
Fig. 2. Cross-sectional SEM images and TEM–EDS depth profiles of CIGS samples selenized from glass/Mo/CuGa/In/Se precursor with different thicknesses of Se layer: (a) 0.5 μm and (b) 1 μm.
the entire depth of CIGSS absorber. However, a Cu–Ga intermetallic still remained at the bottom, which was also observed in the sample after selenization in Fig. 2(a). It is also supported by the ICP-AES result of S þ Se¼ 45 at%, implying incomplete consumption of the precursors. On the other hand, the results of the CIGSS samples formed from the precursor with a 1.0 μm-thick Se layer [Fig. 3(c)] revealed that only part of Se in the top thin layer (200–300 nm) was replaced by S, and thus most of the Ga remained near the Mo side, even though all precursors were selenized and/or sulfurized, as evidenced by the ICP-AES result of S þSe ¼ 50 at%. Therefore, it is suggested that the optimum thickness of Se layer to obtain both reaction completeness and better Ga depth profile at the present process conditions should lie in between 0.5 and 1.0 μm. Otherwise, process conditions should be further optimized.
4. Conclusion
The degree of selenization of glass/Mo/CuGa/In/Se precursors was controlled by adjusting the thickness of the Se coating on the precursors and the reaction conditions, e.g., reaction temperature, ramp rate and duration time. Partial
selenization of the metal precursors achieved by reducing the thickness of the Se layer (0.5 μm in this report) is preferred for subsequent incorporation of S and re-distribution of Ga to complete selenization by a thicker Se layer (1– 1.5 μm in this report). It is concluded that the optimization of compositional depth profile (i.e., energy band gap profile) for high-performance solar cell absorber can be achieved by manipulating the degree of selenization and subsequent sulfurization conditions.
Acknowledgment
This research was supported by the Human Resources Development of Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant (No. 20124030200100) funded by the Korea government Ministry of Trade, Industry and Energy, and the World-Class 300 Project (Development of 5th generation batch type selenization and sulfurization system for CIGS thin-film solar cell production of 14.5% highest efficiency) funded by the Small and Medium Business Administration of Korea (SMBA).
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Fig. 3. Room-temperature XRD scans with Cu Kα1 radiation (a), cross-sectional SEM images and TEM–EDS depth profiles (b, c) of CIGSS sample formed from two-step selenization (570 °C for 10 min) and sulfurization (600 °C for 5 min) of glass/Mo/CuGa/In/Se precursor with different thicknesses of Se layer (0.5, 1, and 1.5 μm). Se layer thickness: (b) 0.5 μm and (c) 1 μm.
References [1] Kushiya K, Tanaka Y, Hakuma H, Goushi Y, Kijima S, Aramoto T, et al. Interface control to enhance the fill factor over 0.70 in a large-area CIS-based thin-film PV technology. Thin Solid Films 2009;517:2108–10. [2] Karg F. High efficiency CIGS solar modules. Energy Procedia 2012;15:275–82.
[3] Hanket GM, Shafarman WN, McCandless BE, Birkmire RW. Incongruent reaction of Cu-(InGa) intermetallic precursors in H2Se and H2S. J. Appl. Phys. 2007;102:074922. [4] Alberts V, Titus J, Birkmire RW. Material and device properties of single-phase Cu(In,Ga)(Se,S)2 alloys prepared by selenization/sulfurization of metallic alloys. Thin Solid Films 2004;451–452:207–11. [5] G.M. Hanket, R. Kamada, W.K. Kim, W.N. Shafarman, Effect of reaction
J. Koo, W.K. Kim / Materials Letters 157 (2015) 183–187
temperature on Cu(lnGa)(SeS)2 formation by a sequential H2Se/H2S precursor reaction process, in: Proceedings of the 33rd IEEE Photovoltaic Specialists Conference, San Diego, CA, May 11-16, 2008. [6] Kim WK, Hanket GM, Shafarman WN. Ga homogenization by simultaneous H2Se/H2S reaction of Cu–Ga–In precursor. Sol. Energy Mater. Sol. Cells 2011;95:235–8.
187
[7] Koo J, Jeon S, Oh M, Cho H, Son C, Kim WK. Optimization of Se layer thickness in Mo/CuGa/In/Se precursor for the formation of Cu(InGa)Se2 by rapid thermal annealing. Thin Solid Films 2013;535:148–53.