CuInSe2 (CIS) Thin Film Solar Cells by Direct Coating and

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CuInSe2 (CIS) Thin Film Solar Cells by Direct Coating and Selenization of Solution Precursors SeJin Ahn,† ChaeWoong Kim,† Jae Ho Yun,† Jihye Gwak,† Sunho Jeong,‡ Beyong-Hwan Ryu,‡ and KyungHoon Yoon*,† PhotoVoltaic Research Center, Korea Institute of Energy Research, 71-2 JangDong, YuseongGu, DaeJeon, 305-343, Korea, and DeVice Materials Research Center, Korea Research Institute of Chemical Technology, P.O. Box 107, YuseongGu, DaeJeon, 305-600, Korea ReceiVed: January 25, 2010; ReVised Manuscript ReceiVed: March 23, 2010

CuInSe2 (CIS) absorber layer was formed by a direct nonvacuum coating and a subsequent selenization of precursor solutions of Cu(NO3)2 and InCl3 dissolved in methanol. The viscosity of precursor solutions was adjusted by adding ethyl-cellulose (EC) to be suitable for the doctor-blade coating. During the coating and drying process Cu2+ ions in the starting solution were reduced to Cu+, resulting in precursor films consisting of CuCl crystals and amorphous In compound embedded in EC matrix. Selenization of the precursor films with Se vapor at elevated temperature generated double-layered films with an upper layer of chalcopyrite CIS and a carbon residue bottom layer. Significant In loss was observed during the selenization, which was attributed to the evaporation of the In2Se binary phase, confirmed by investigating the change in the Cu/In ratio of the selenized film as a function of Se flux and substrate temperature. As a proof-of-concept, thin film solar cells were fabricated with the double-layered absorber film and the devices exhibited reproducible conversion efficiency as high as about 2%. Introduction CuInSe2 (CIS) and related chalcopyrite compounds have attracted considerable interest for thin film photovoltaic device on account of their high optical absorption coefficient and adjustable band gap, which makes it possible to achieve high conversion efficiency in CIS-type solar cells. The best CIS related thin film solar cell has reached a confirmed conversion efficiency of about 20% based on the Ga containing absorber layer (Cu(In,Ga)Se2; CIGS).1 However, in spite of the high efficiency of state-of-the-art CI(G)S solar cells, the high production cost of the conventional vacuum-based processes,2 i.e., a multistage coevaporation and a two-step process of sputtering and selenization are considered to be the main obstacles to the widespread use of CI(G)S thin films solar cells. In this regard, many efforts have been devoted to develop alternative deposition techniques for thin film CIGS solar cells based on non- or low-vacuum processes3–17 due to their inherent advantages such as no requirement of expensive vacuum equipments, less energy intensive deposition, and much better material utilization. They can be roughly divided into two categories depending on precursor types, in which the first approach uses the solution type precursors3–7 and the other uses particle-based precursors.8–17 The main difference in the two approaches is that the solution precursors with proper rheology can be directly coated on a substrate while the latter needs a complicated synthesis of nanocrystals which generally suffers from low yield, poor crystallinity, and poor uniformity in composition and phase due to the phase complexity of the compounds in nanometer scale.16 This means that we can reduce * To whom correspondence should be addressed. Phone: +82-42-8603191. Fax: +82-42-860-3739. E-mail: [email protected]. † Korea Institute of Energy Research. ‡ Korea Research Institute of Chemical Technology.

the number of processing steps by using the direct coating of solution precursors. Thus it is believed that the direct solution coating processes are simpler and cheaper than the particlebased ones and more suitable for the production of low-cost thin-film solar cells. Among the solution coating approaches, the highest efficiency achieved to date is 10.3% by Mitzi et al.,5 in which hydrazine was used as solvents for dissolving metal chalcogenides to fabricate the CIGS absorber layer. Although their results showed a noticeable cell efficiency with a dense CIGS absorber layer comparable to that produced by a vacuum-based deposition, the use of highly toxic and explosive hydrazine is considered to be a hurdle for commercial utilization. Here, we report the fabrication of CIS thin films by direct coating of solution precursors and subsequent selenization, emphasizing that all the starting materials for preparation of precursor solutions are low-toxic materials of metal-nitrates, metal-halide, and alcoholic solvent. Selenization is performed with the elemental Se vapor, which is less toxic than H2Se gas. Regarding that the most important factor affecting the properties of CIS absorber film is its compositional ratio of Cu/In, we systematically investigate the change in Cu/In ratio from the starting solution to the final CIS thin film at each processing step and the effect of experimental conditions on the composition was examined and analyzed. As a proof-of-concept, thin film solar cells were fabricated with the CIS absorber layer and the devices exhibited reproducible conversion efficiency as high as about 2%. Experimental Details Materials. Copper nitrate hydrate (Cu(NO 3) 2 · 3H 2O; 99.999%) and indium chloride (InCl3; 99.999%) were purchased from Sigma Aldrich, methanol (CH3OH, 99.6%) from Junsei, elemental Se (99.999%, 3 mm shot) from Cerac,

10.1021/jp1007363  2010 American Chemical Society Published on Web 04/08/2010

CuInSe2 Thin Film Solar Cells and ethyl-cellulose (EC, 90-110 mPa · s) from Kanto Chemical Co. Cu(NO3)2 · 3H2O and InCl3 were stored in a nitrogenfilled glovebox to prevent air or humidity induced degradation. Preparation of Precursor Solutions. One gram of Cu(NO3)2 · 3H2O and 0.93 g of InCl3 were dissolved in 20 mL of methanol. The ratio of metals, i.e., Cu/In, in this solution corresponded to 1. The Cu/In ratio of the solution was able to be modified by simply changing the amount of Cu(NO3)2 · 3H2O and InCl3 dissolved in the solution. To adjust the viscosity of the precursor solution to be suitable for doctor-blade coating, 6 g of ethyl-cellulose (EC) was dissolved in 40 mL of methanol, and this viscous solution was mixed with the Cu- and Incontaining methanol solution mentioned above. The final mixture was stirred with a magnetic bar vigorously for 1 h. All the preparation processes were performed at room temperature in air. Thin Film Deposition and Selenization. Precursor films were deposited on a 1 µm thick Mo-coated sodalime glass by doctor-blade coating. The thickness of the coated films was adjusted by changing the number of pieces 3 M scotch tape that were applied to both edges of the substrates. The samples were then dried at 70 °C for 2 min in air on a hot plate to evaporate methanol. Selenization was performed in a vacuum evaporator equipped with a Knudsen-type effusion cell. Initially the chamber was evacuated to a base pressure of 5 × 10-6 Torr with a turbo molecular pump and then elemental Se was evaporated from the effusion cell. Typical substrate temperature and selenization time were 530 °C and 30 min, respectively. The flux of Se vapor was regulated by the effusion cell temperature. Solar Cell Fabrication. Solar cells were fabricated according to the conventional Mo/CIS/CdS/i-ZnO/n-ZnO/Al structure. In our baseline process,18 a 60 nm thick CdS buffer layer was deposited on CIS film by chemical bath deposition (CBD) and i-ZnO(50 nm)/Al-doped n-ZnO(500 nm) were deposited by radio frequency (rf) magnetron sputtering on the CdS layer. An Al grid 500 nm thick was deposited as a current collector, using thermal evaporation. The active area of the completed cells was 0.44 cm2. Thin Film and Device Characterization. The morphology, composition, and crystal structure of the precursor films and the selenized films were investigated by high-resolution scanning electron microscopy (HRSEM, XL30SFEG Phillips Co., Holland at 10 kV), energy dispersive spectroscopy (EDS, EDAX Genesis apex, acceleration voltage: 30 kV; collection time: 100 s with standard-less method), and X-ray diffraction (XRD, Rigaku Japan, D/MAX-2500), using Cu KR line, respectively. The depth compositional profile of the selenized film was obtained by Auger electron spectroscopy (AES, Perkin-Elmer, SAM 4300). Device performances including the conversion efficiency and the EQE (external quantum efficiency) were characterized by using a class AAA solar simulator (WXS-155S-L2, WACOM, Japan) and IPCE (incident photon conversion efficiency) measurement unit (PV measurement, Inc., USA), respectively. Results and Discussion Precursor Films. Figure 1 shows SEM images of precursor films coated with 2 stripes of 3 M tape and dried at 70 °C for 2 min in air. A uniform and crack-free film 14 µm thick was formed. No noticeable crystal grain was observed in the SEM images, implying that all the chemical constituents are embedded in the ethyl-cellulose (EC). In the XRD pattern of the same precursor film (Figure 2), peaks related to Mo back contact and CuCl crystals were detected while there was no peak related to

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Figure 1. (a) Planar and (b) cross-sectional SEM images of the precursor film coated with 2 stripes of 3 M tape and dried at 70 °C for 2 min in air.

Figure 2. XRD patterns of the precursor film shown in Figure 1.

In. From EDS analysis, the Cu/In ratio of the precursor film was found to remain at 1, which is the same as that of the starting solution, demonstrating that In may exist in the amorphous state in the precursor film and there was no loss of elements during the coating and drying steps. Interestingly, Cu appeared to exist in the form of CuCl, i.e., Cu+ state, in the precursor film (Figure 2) even though we used Cu(NO3)2 as a starting material in which Cu is in the Cu2+ state. This means that the Cu2+ ion was reduced to Cu+ by accepting an electron from any reducing agent in the precursor mixture. We suggest EC is the presumable reducing agent in our experiment as it is well-known that hydrolyzed or oxidized cellulose is capable of reducing certain metallic ions to lower valence states, which is often described as a “copper number”.19

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Figure 3. (a) Planar and (b) cross-sectional SEM micrographs of the film selenized at 530 °C for 30 min with Se vapor effused at 150 °C.

20a,b

The concept was also used in fabrication of elemental copper and platinum nanoparticles20c from metal salts and cellulose as the starting materials. These results implies that the primary intention of using EC in this study is the viscosity adjustment but it also results in the reduction of Cu2+ ions in the starting solution to Cu+, which is a more suitable form for forming I-III-VI type compounds. Selenized Films. Figure 3 shows SEM micrographs of the film selenized at 530 °C for 30 min with Se vapor effused at 150 °C. SEM figures show that the total thickness of selenized film was greatly reduced to about 1.8 µm from 14 µm of the precursor film (Figure 1b), and had a double-layered structure with a 550 nm thick upper layer, which appears to be dense, flat, and well crystallized (Figure 1a), and a 1.2 µm thick amorphous-like bottom layer. From the XRD pattern and the AES depth profile of the same sample (Figure 4), it was found that the upper layer is a chalcopyrite CIS compound with a strong (112) preferred orientation, demonstrating that CuCl and amorphous In compound in the precursor film were well converted to CIS. However, the bottom layer is mainly composed of carbon, which can be attributed to binder materials remaining even after the high-temperature selenization. To understand the mechanism of a double layer formation, a viscous methanol-based solution containing only EC without Cu(NO3)2 and InCl3 was prepared and coated on a Mo/SLG glass to make a film of EC about 14 µm thick. The thickness is intentionally adjusted to be similar to that shown in Figure 1b to avoid any thickness related effect. Subsequent selenization of the EC film resulted in a complete removal of EC (see Figure S1 in the Supporting Information for SEM images of EC film before and after the selenization), revealing that EC itself can be evaporated by heating and/or Se vapor supply. Comparing this result with the double layer structure shown in Figure 3, we suggest that the CIS layer on the top of the selenized film

Figure 4. (a) XRD pattern and (b) AES depth profile of the sample shown in Figure 3.

acts as a barrier layer for the evaporation of EC and hence hinders the continuous reaction of Se vapor with CuCl and Inrelated compounds. A possible mechanism based on this postulation is as follows: (1) At the initial stage of selenization, EC on the surface of the sample is evaporated by heat and/or Se vapor. (2) Some CuCl and In-related compounds are exposed by removal of EC and react with Se vapor to produce CIS on the top of the sample. This stage continues until the CIS layer is thick enough to hinder further evaporation of EC. (3) After stage 2, there is no more exposed CuCl and In compound between the CIS layer and the EC layer and hence the formation of CIS ceases, resulting in a double layered structure with a top CIS layer and a bottom carbon layer (with embedded CuCl and In compounds). Apart from the double-layer formation, the mechanism of conversion of precursor materials to CIS was investigated by XRD analysis of the samples selenized at various substrate temperatures (Figure 5). Other selenization conditions were the same as those adapted for producing the CIS layer shown in Figure 3. In Figure 5, the intensity of CuCl peaks significantly decreased and CIS began to form at a low temperature of 270 °C without showing peaks related to any intermediate binary phase such as Cu-Se and In-Se. From this result it can be inferred that CIS is formed via a direct route of reaction of CuCl and In compounds with Se vapor (possibly by eq 1) in this study rather than via a well-known route of the reaction between binary selenides.

2CuCl(s) + 2In-(Cl and/or NO3)(s) + 4Se(g) f 2CuInSe2(s) + Cl2(g) v and/or NOx(g)v

(1)

Regarding the composition of the selenized film, EDS analysis revealed two interesting points. First, there is no Cl residual in the selenized film within the resolution of EDS (see Figure S2

CuInSe2 Thin Film Solar Cells

Figure 5. XRD patterns of the samples selenized for 30 min at various substrate temperatures. tHE Se effusion cell temperature was 150 °C. Inset: Enlarged XRD pattern of the sample selenized at 270 °C.

in the Supporting Information for EDS results of the samples before and after the selenization). We can understand the removal of Cl by eq 1 suggesting that Cl- originally in the CuCl crystals are oxidized to form Cl2 gas and evaporated from the sample. Second, the Cu/In ratio of the selenized film is dramatically increased to 2 from 1 of the precursor layer, reflecting that there was significant In loss during the selenization. The increase of Cu/In ratio can also be confirmed by integrating the area of Cu and In signals in the AES data (Figure 4b). The average Cu/In value obtained by integrating the AES signals of each element up to the sputtering time of 150 min in Figure 4b was calculated to be 2.2, which is consistent with the EDS result. Similar In loss phenomena were also observed in the literature.12,21–23 Previously we observed significant loss of In when CIGS nanoparticles were annealed above 200 °C in nitrogen atmosphere,12 and Parretta et al.21 reported In loss during the selenization of RF-sputtered Cu-In alloys by the close-spaced vapor transport (CSVT) method with low Se pressure. Other researchers also mentioned the occurrence of In loss when Se supply was insufficient during the selenization.22,23 The common feature among those experiments is that the selenization processes were preformed under a condition of low Se partial pressure. On the other hand, the mass spectrograph-Knudsen cell experiments on In-Se liquid (50 atom % Se) showed that the largest partial pressure to be that of In2Se, with that of Se2 100 times smaller and that of In2Se2 1000 times smaller at 1030 K,24 suggesting that the evaporation of the In2Se binary phase, which only forms under the low Se partial pressure, may be the reason for the In loss observed in the literatures. If this is also the case in this study, the lower substrate temperature and the higher Se flux supplied to the sample during the selenization should result in less In loss because both of them will suppress the evaporation of In2Se. The next section is devoted to confirming this postulation. Effects of Substrate Temperature and Se Flux. In this section, results from a compositional study on the CIS film selenized with different substrate temperatures and Se evaporation temperatures are presented. If the evaporation of In2Se is the main reason for the In loss observed in this study, the In loss should be reduced by decreasing the substrate temperature and increasing the Se partial pressure (by increasing the temperature of the Se effusion cell) during the selenization. Figure 6 shows the composition of the films selenized at various substrate temperatures for 30 min with Se vapor effused at 150 °C. It is clear that as the substrate temperature decreased

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Figure 6. Effects of substrate temperature on the Cu/In ratio of the selenized films. Selenization was performed for 30 min with Se vapor effused at 150 °C.

Figure 7. Effects of Se effusion cell temperature on the Cu/In ratio of the selenized films. The samples were selenized at 530 °C for 30 min.

from 530 to 270 °C, the Cu/In ratio of the selenized films gradually decreased from 2 to 1.1. In the same manner, Figure 7 represents the effects of the Se effusion cell temperature on the composition of the selenized films. The samples were selenized at a substrate temperature of 530 °C for 30 min. Interestingly, the Cu/In ratio of the selenized films showed a dramatic decrease from 2 to 1.1 as the Se effusion cell temperature increased from 150 to 180 °C, and remained almost unchanged for further increases in temperature. From these results, the low substrate temperature and the high amount of Se vapor supplied to the samples were found to reduce the In loss presumably by suppressing the evaporation of In2Se gas during the selenization process, which is consistent with the postulation suggested in the previous section. It is noticeable that the Se evaporation temperature of 180 °C is high enough to inhibit the evolution of In2Se, which is the optimum condition for Se effusion. Effects of Solution Composition. It is well-known that the high conversion efficiency of CI(G)S-type solar cells can be obtained only from the CI(G)S layer with Cu-poor composition.25 As all the selenized films presented in Figure 7 showed Cu-rich composition (Cu/In > 1), the final step is to adjust the Cu/In ratio of selenized film to be less than 1 simply by changing the Cu/In ratio of the starting solutions. Consequently, effects of the Cu/In ratios of the starting solutions on those of the selenized films were investigated and presented in Figure 8. Selenization was performed at 530 °C for 30 min with Se vapor effused at 180 °C. All the selenized films generated from the starting solutions with Cu/In ratio ranging from 1 to 0.55 were confirmed to be chalcopyrite CIS by the XRD analysis (see Figure S3 in the Supporting Information for the XRD patterns).

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Figure 8. Effects of the Cu/In ratios of the starting solutions on those of the selenized films. Selenization was performed at a substrate temperature of 530 °C for 30 min with Se vapor effused at 180 °C.

Figure 9. Light and dark J-V characteristics of the best cell. Light illuminated J-V was measured by using a class AAA solar simulator (AM 1.5G, 100 mW cm-2) at 25 °C.

Figure 8 demonstrates that as we add InCl3 into the solution until the Cu/In ratio of the solution reaches 0.8, the Cu/In ratio of the selenized films gradually decreased to 0.91. However, further increase of In in the solution did not affect the composition of selenized films any more. This result demonstrates that the lowest value of the Cu/In ratio of the selenized film we can obtain is limited to about 0.9 in this experimental system, and the extra In is completely consumed by evaporation in the form of In2Se. Device Performance. As a proof-of-concept, the double layered CIS/carbon film with a Cu/In ratio of 0.9 was tested as an absorber layer for thin film solar cells. The solar cell has a conventional SLG/Mo/absorber layer/CdS/i-ZnO/n-ZnO/Al structure. Light illuminated and dark current density-voltage (J-V) characteristics are presented in Figure 9 for the best cell obtained, yielding a power conversion efficiency (Eff) of 1.93% with an open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor of 0.33 V, 20.9 mA cm-2, and 0.28, respectively. The uncertainty of the measured Eff is estimated to be (0.09% (absolute) in which the largest uncertain factor is JSC due to the inherent uncertainty of the reference cell calibration. The efficiency is limited particularly by low FF, indicating that the cell has a high shunt conductance (Gsh) and/ or a high series resistance (Rs). Unfortunately, it was not possible to distinguish the effects of Gsh and Rs on FF separately because the shape of the J-V curve shown in Figure 9 is far from the ideal one. Usually very high Rs and very high Gsh may result in a similar final J-V shape. However, partly from the fact that Figure 9 shows a crossover of the dark and light J-V curves, which is a typical characteristic of high Rs, and partly from the existence of the amorphous carbon layer in the cell configuration, we attribute the low FF mainly to high Rs rather than high Gsh. Subsequently, to extract the Rs of the cell, the light

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Figure 10. R-J curve of the cell with a 1.93% efficiency produced in this study together with that of the CIGS device with a conversion efficiency of 16% fabricated by a 3-stage coevaporation process.

illuminated J-V curve is redrawn into the R-J form (Figure 10). For a comparison, the R-J curve of a CIGS device with a conversion efficiency of 16% fabricated by the 3-stage coevaporation technique in our laboratory is shown together. The Rs value of the 1.93% efficient device was found to be almost 30 times larger than that of the 16% device, from which it can be confirmed that the nonvacuum processed cell suffers from large Rs. As pointed out above, we believe that the high Rs results from the thick carbon layer placed between the CIS and Mo layers. An interesting point is that the films prepared by coating of pure EC/methanol solution show electrically insulating characteristics. Accordingly, the carbon layer shown in Figure 3b seems to have different properties from pure EC films, and the resistive (high Rs and hence low FF) but not insulating properties (demonstrating a working device) of the carbon layer in the 1.93% cell may be attributed to paths for the electrical flow existing inside it, possibly formed by a connection of some conductive compounds, i.e., Se, In, or CuCl. Figure 11 shows (a) the EQE curve and (b) the bandgap of the CIS layer determined from the long-wavelength region of the EQE curve. Figure 11a exhibits a long end tail with a gradually decreasing EQE from 550 nm, which is attributed to the short minority carrier (electron) diffusion length and the incomplete light absorption due to the very thin absorber layer (500 nm thick, Figure 3b). The bandgap of the CIS layer was determined from the [hν × ln(1 - EQE)]2 against hν relationship,26 showing a value of 1.03 eV that is consistent with previous reports.27 Conclusions A preparation method for CIS thin films and solar cells by a direct coating and a subsequent selenization of solution precursors is described in this work. The precursor solutions were prepared by dissolving Cu(NO3)2 and InCl3 in methanol, and the viscosity of the solutions was adjusted by adding ethylcellulose (EC) to be suitable for the doctor-blade coating. During the coating and drying process Cu2+ ions in the starting solution were reduced to Cu+, resulting in the precursor films consisting of CuCl crystals and amorphous In compound embedded in the EC matrix. Selenization of the precursor films with Se vapor at elevated temperature generated double-layered films with an upper layer of chalcopyrite CIS and a carbon residue bottom layer. Significant In loss was observed during the selenization, which was attributed to the evaporation of the In2Se binary phase, confirmed by investigating the change in the Cu/In ratio of the selenized film as a function of Se flux and substrate

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Figure 11. (a) EQE curve of the 1.93% device and (b) bandgap of the CIS layer determined from the [hν ln(1 - EQE)]2 against hν relation.

temperature. As a proof-of-concept, thin film solar cells were fabricated with the double-layered absorber films with the Cu/ In ratio of 0.9 and the devices exhibited reproducible conversion efficiency as high as about 1.93((0.09)%. The device was found to have a high series resistance that presumably resulted from the residual carbon layer, implying that the development of new fabrication processes for the carbon-less or carbon-free CIS layer is essential for performance improvement. Acknowledgment. This study was partly supported by a grant from the cooperative R&D Program (B551179-08-03-00) funded by the Korea Research Council Industrial Science and Technology, Republic of Korea. This research was also partly supported by the Converging Research Center Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 20090081909). Supporting Information Available: SEM images of pure EC coated films before and after selenization, EDS results of the precursor and selenized samples, and XRD patterns of the selenized films made from the precursor solutions with different

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