Research and Development of High-Voltage CIS-Based Thin Film Solar. Cells for Industrial Technology

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1 E-04 (Registration number 2002EA007) Research and Development of High-Voltage CIS-Based Thin Film Solar Cells for Industrial Technology Research Coordinator James R. Sites Research Team Members Tokio Nakada Hans-Werner Schock Sigeru Niki Akira Yamada Miguel A. Contreras Colorado States University:USA Aoyama Gakuin University:JAPAN University of Stuttgart/Hahn-Meitner-Institute:GERMANY National Institute of Advanced Industrial Science and Technology:JAPAN Tokyo Institute of Technology:JAPAN National Renewable Energy Laboratory:USA Duration:April, 2002 March, 2005 Abstract The objective of the CIS-21 collaboration is to increase the efficiency of Cu(In,Ga)Se 2 (CIGS) solar cells improving open-circuit voltage (Voc). The two primary strategies have been researched, (1) wide-gap buffer layer which is an alternative to the traditional CdS buffer layer, (2) to improve the quality of the high-ga CIGS absorber layer. The Voc of previously reported high-efficiency CIGS thin film solar cells was less than 700mV. In this work, the efficient CIGS devices with the Voc as high as 750 mv could be achieved by the use of wide-band-gap CIGS absorber layers with a high-crystalline quality. The conventional CIGS thin film solar cells were typically fabricated using a cadmium sulfide (CdS) buffer layer. Therefore, further improvement in the short-circuit-current (Jsc) can be achieved by replacing CdS with other appropriate wider band-gap buffer materials. Moreover, the use of cadmium is undesirable from the viewpoint of environmental safety. In order to overcome this tasks, we used a wide-band-gap CBD-ZnS(O,OH) as an alternative buffer layer. As a result, we achieved a new world-record total-area conversion efficiency of 18.6% for Cd-free thin-film solar cells. Keywords: CIGS, Cu(In,Ga)Se 2, High-open-circuit voltage, Thin film solar cells, Cd-free 1. Introduction Thin-film Cu(In,Ga)Se 2 (CIGS) solar cells have major potential as a source of low-cost, high-efficiency solar electricity. One of most important issues for large-scale commercial manufacturing of such cells is the improvement of the open-circuit voltage. The objective of the CIS-21 collaboration has been to increase the

2 efficiency of Cu(In,Ga)Se 2 (CIGS) solar cells to 21% and beyond. The two primary strategies have been (1) to improve the quality of the CIGS absorber layer, especially when an increased fraction of Ga is used to enlarge the band gap, and (2) to explore the use of alternatives to the traditional chemical-bath-deposited CdS window layer that will allow increased carrier collection from short-wavelength photons and provide a better conduction-band match to higher band-gap CIGS. Light ZnO:Al ZnO Buffer CIGS Mo Soda-lime glass + Fig.1. Typical structure of a CIGS thin film solar cell 2. Experimental Five of the collaborating CIS-21 groups deposited the key Cu(In,Ga)Se 2 (CIGS) absorption layers used in the fabrication of the solar cells studied. All used variations of the three-stage process, where substrate-temperature feedback is used to optimize the CIGS quality, particularly the final surface. Collectively these groups covered a broad range of Ga/In concentrations. The window layers used for the completed cells included chemical-bath-deposition (CBD) CdS, as well as CdS deposited in an ultrasonic bath, CBD-ZnS(O,OH) CBD-InS(O,OH), and evaporated and sputtered In 2 S 3. The strategy of depositing different layers of the same solar cell at different international laboratories, each with its own expertise, was firmly established by the CIS-21 collaboration. 3. Results and discussion 3.1 CIGS Absorbers The CIS-21 team also collectively worked to improve the quality of the CIGS absorber layers using the three-stage growth process. In the course of Dr. Contreras s work on record-efficiency cells with alternative buffers, Dr. Contreras and his colleagues at NREL pushed the efficiency for the standard device structure, ZnO/CdS/CIGS/Mo, to 19.5%[2], the current record for all thin-film solar cells. Dr. Schock, using a similar absorber-deposition strategy, has fabricated cells that have exceeded 700 mv with both CdS and alternative buffer layers. To reduce contamination between the depositions of successive layers, he used a cluster system to transfer absorber layers in vacuum to the buffer-deposition chamber, and he also used selenium caps to protect his CIGS layers. Dr. Yamada attacked the high Ga problem by focusing on the changes in the deposition process that occur when the Ga ratio is increased[3,4]. His Cu(InGa)Se 2 (CIGS) thin films were also grown by the three-stage growth process, in this case using a molecular beam deposition system. He additionally developed a strategy to remove unwanted Cu 2-x Se compounds that he detected by Raman scattering. Rapid-thermal-annealing (RTA) using infrared light in a forming-gas (N 2 95% + H 2 5%) ambient under atmospheric pressure was utilized for the

3 removal of the Cu 2-x Se compounds[5]. The required annealing temperature was found to be about 400 o C and the heating rate should be higher than 450 o C/min, corresponding to an optimal annealing time of about 1 sec. Longer duration annealing, however, was found to cause deterioration of the surface. This strategy was successful in improving the open-circuit voltage (V OC ) of CIGS solar cells to values as high as 750 mv when the CIGS Ga content was 60% (Figs. 2 ). CuSe 2 CuSe 2 Cu(In 0.7 Ga 0.3 )Se 2 after RTA Intensity [a.u.] Cu(In 0.3 Ga 0.7 )Se 2 CuGaSe 2 Intensity [a.u.] before RTA Raman shift [cm -1 ] (a) Raman shift [ nm] [cm -1 ] (b) Fig. 2 Dependence of the Raman spectra on (a) the Ga contents of CIGS absorber and (b) the heating rate of RTA treatment. Dr. Niki also used three-stage molecular-beam deposition to form his absorber layers. He adopted the approach, known as the cadmium partial electrolyte (Cd-PE), in which sulfur is omitted from the CBD-CdS deposition process, and hence no CdS layer is formed. A key question in this study was where the Cd atoms reside. X-ray absorption fine-structure measurements (EXAFS) on Cd-PE treated layers of CuGaSe 2 indicated that Cd is found on group I (Cu) sites and in a Cd(OH) 2 surface phase. Subsequent improvement in CIGS solar cell processes led to V OC = 740 mv and conversion efficiencies as high as 17.2% (Fig. 3). A technique to characterize the role of the diffused Cd in CIGS was also developed. The location of p-n junction in the CIGS solar cells was determined by electron-beam-induced-current (EBIC) measurements. It was confirmed that Cd atoms diffused into the CIGS absorber layer from the CdS buffer layer, and formed a buried p-n junction within the CIGS. Fig. 3. J-V characteristics of CIGS thin film solar cells using 3-stage process.

4 3.2 Alternative Buffers Two members of the CIS-21 team, Drs. Contreras and Nakada, collectively produced Cu(In,Ga)Se 2 solar cells with no cadmium in the buffer layer and a certified total-area energy conversion efficiency of 18.6%[1]. These cells incorporated a ZnS(O,OH) buffer layer as an alternative to the traditional CdS, and the ZnS-based buffer layer was shown to be a highly competitive alternative to CdS. There is a clear short-wavelength advantage in current collection with the potential of increasing short-circuit current density by ~3.3 ma/cm 2 compared to CdS-based buffer layers. There is in the cells studied to date, however, a larger red falloff in the quantum efficiency near the band gap and a slightly smaller voltage in comparison to the band gap. Somewhat lower, but still very respectable efficiencies of about 15% were achieved with In x S y buffer layers deposited on similar CIGS absorbers. Dr. Nakada pioneered the use of the chemical-bath-deposited (CBD)-ZnS(O,OH) alternative to the traditional CBD-CdS buffer layer, which led to the record-efficiency non-cd cells. He subsequently improved the process through the use of ultrasonic vibration during the deposition of the Zn(O,OH) buffer-layers[6]. The fill factor (FF) and the diode quality factor of his CIGS devices improved significantly by using ultrasonic vibration during CBD ZnS(O,OH) deposition, resulting in the improved efficiency of CIGS devices. Fig. 4. The J-V characteristics and spectral response curve of record Cd-free CIGS thin film solar cell using ZnS(O,OH) buffer layer.

5 The best results were obtained for devices fabricated with a 100-nm-thick ZnS(O,OH) layer formed by repeating the CBD process for three times. The advantages of ultrasonic (US) vibration include an enhancement of ammonia etching effects on CIGS surface, in particular the removal of secondary phases, and a removal of small particles from growing films. The deposition rate of the ZnS(O,OH) layers under the US vibration, however, was slower than that with the conventional CBD process. Dr. Schock also studied alternative sulfur-containing window layers, and he also fabricated such devices that demonstrated good performance in Cd-free solar cells. A device with an CIGS-absorber/(In,OH,S)-heterojunction fabricated at IPE, Stuttgart, and the ZnO contact window deposited at NREL showed a total area efficiency of 15.6%[7], which is the highest efficiency reached with this material combination. Additionally, Dr. Schock has deposited In x S y as a compound by both evaporative and sputter processes for the formation of heterojunctions that yielded high-voltage solar-cell devices without a wet chemical step. These experiments have been carried largely out in a newly built system with separate chambers for the deposition of the absorber and the surface layer. 3.3 Analysis and Simulation The choice of buffer layers for the record-efficiency cells was partially driven by analysis of the impact of conduction-band offset on solar-cell performance parameters. Results from Dr. Sites and his group showed that CIGS cells with E g near 1.15 ev have very nearly the optimal offset, about 0.3 ev. Higher-gap cells do not effectively shield the photoelectrons from the interfacial states that can degrade photovoltaic performance. Even small amounts of interfacial recombination will limit the voltage when the offset is negative or only slightly positive (ΔE C. 0.1 ev, which corresponds to E g / 1.3 ev when a CdS window is used) [8]. The result is that the cell s voltage will become band-gap independent at higher gaps. At lower band gaps, there is a current limitation when the band offset forms a barrier for electrons. The result is a distortion in the J-V curve, especially in the absence of blue photons, which can mitigate the effect through photoconductivity in the CdS layer[9,10]. "spike" "cliff" 1 ΔE C > 0 1 ΔE C < 0 0 CIGS 0 CIGS -1 ZnO CdS -1 ZnO CdS position [μm] position [μm] Fig. 5. Calculated voltage and efficiency for different band alignments.

6 One feature of CIGS cells is that grain boundaries do not appear to harm solar-cell performance, and perhaps actually improve it. There have been many suggestions as to why CIGS grain boundaries are benign, but no general agreement. The approach of Dr. Sites group was to consider the different possibilities for how a grain boundary might affect cell performance and to calculate the specific effects of each possibility. These possibilities included recombination centers at the grain boundary, a charge sheet at the grain boundary, and a band-gap expansion, seen primarily in the valence band, near the grain boundary. Two-dimensional calculations[11] showed that the latter possibility is the most likely. 4. Conclusions The CIS-21 group has explored a large variety of techniques to increase the efficiency of CIGS solar cells. Working together, the members achieved a world-record efficiency of 18.6% for Cd-free thin-film solar cells. They used several approaches including careful process control, increased selenium flux, and the use of rapid thermal annealing to improve the CIGS absorber layer. They also applied surface-modification techniques to further cell improvements, through the use of cadmium partial-electrolyte and sulfur surface treatments. A third area for CIGS cell improvement was the use of alternatives to chemical-bath-deposited CdS for the window or buffer layer. These alternative layers included ZnS(O,OH), In(O,OH), and In/S. In particular, it was shown that ultrasonic vibration of the deposition bath led to significant cell improvements. Finally, in the course of the experimental work, a better understanding of the basic device physics of the CIGS cell, including the role of the conduction-band offset and the grain boundaries, evolved. The CIS-21 team effort has been a complex task of simultaneously optimizing all aspects of CIGS cells, and though there are further refinements for the future, the collaboration has clearly demonstrated the value of such a group effort. References [1] M.A. Contreras, T. Nakada, M. Hongo, A.O. Pudov, and J. R. Sites, ZnO/ZnS(O,OH)/Cu(In,Ga)Se 2 /Mo Solar Cell with 18.6% Efficiency, Proc. 3 rd World Conf. on Photovoltaic Energy Conversion, (2003). [2] M.A.Contreras, K. Ramanathan, J.AbuShama, F.Hasoon, D. L. Young, B. Egaas and R. Noufi; Prog. Photovoltaics (in press). [3] H.Miyazaki, R.Mikami, A.Yamada and M. Konagai, Cu(InGa)Se2 thin film absorber with high Ga contents and its application to the solar cells, J. of Phys. and Chem. of Solids 64, 2055 (2003). [4] H.Miyazaki, R.Mikami, A.Yamada and M. Konagai, Efficiency Improvement of Cu(InGa)Se 2 Thin Film Solar Cells with a High Ga Composition, Jpn. J. Appl. Phys., in press. [5] A. Yamada, Improved Performance of Cu(InGa)Se 2 Thin Film Solar Cells with High Ga Composition Using Rapid Thermal Annealing Process, Proc. 3 rd World Conference in Photovoltaic Energy Conversion, Osaka (2003). [6] T. Nakada, Ultrasonic Chemical Bath Deposition of ZnS(O,OH) Buffer Layers and Its Application to CIGS

7 Thin Film Solar Cells, Proc. 14 th International Photovoltaic Science and Engineering Conference, Bangkok (2004). [7] Q. Nguyen, K. Orgassa, I.M. Kotshau, U. Rau, and H.W. Schock, Influence of Heterointerfaces on the Performance of Cu(In,Ga)Se 2 Solar Cells with CdS and In(OH x S y) Buffer Layers, Thin Solid Films , 330 (2003). [8] M. Gloeckler and J.R. Sites, Efficiency Limitations for Wide-Band-Gap Chalcopyrite Solar Cells, Thin Solid Films, , (2005). [9] A.O. Pudov, M.A. Contreras, T. Nakada, H.-W. Schock, and J.R. Sites, CIGS J-V Distortions in the Absence of Blue Photons, Thin Solid Films, , (2005). [10] A.O. Pudov, F.S. Hasoon, A. Kanevce, H. Al-Thani, and J.R. Sites, Secondary Barriers in CdS/CuIn 1-x Ga x Se 2 Solar Cells, J. Appl. Physics, 97, (2005). [11] M. Glockler, J.R. Sites, and W.K. Metzger, Grain-Boundary Recombination in Cu(In,Ga)Se 2 Solar Cells, submitted to J. Appl. Phys. The list of the most important papers and patents from the project Papers (Total 73) [1] A.O. Pudov, J.R. Sites, M.A. Contreras, T. Nakada and H.-W. Schock, CIGS J-V Distortions in the Absence of Blue Photons, Thin Solid Films, , (2005). [2] H. Miyazaki, R. Mikami, A.Yamada and M. Konagai, Efficiency Improvement of Cu(InGa)Se 2 Thin Film Solar Cells with a High Ga Composition Using Rapid Thermal Annealing, Jpn. J. of Applied Physics, 43, (2004). [3] Q. Nguyen, K. Orgassa, I. Koetschau, U. Rau and H.W. Schock, Influence of heterointerfaces on the performance of Cu(In,Ga)Se 2 solar cells with CdS and In(OHx,Sy) buffer layers, Thin Solid Films, , (2003). [4] T. Nakada, M. Hongo, and E. Hayashi, Band Offset of High Refficiency CBD-ZnS/CIGS Thin Film Solar Cells, Thin Solid Films, , (2003). [5] K. Sakurai, R. Hunger, N. Tsuchimochi, T. Baba, K. Matsubara, P. Fons, A. Yamada, T. Kojima, T. Deguchi, H. Nakanishi and S. Niki, Properties of CuInGaSe 2 solar cells based upon an improved three-stage process, Thin Solid Films , 6-10 (2003). Presentations (Total 99) [1] H.W.Schock et al., Electrical and structural investigations of indium sulfide buffer layers in Cu(In,Ga)Se 2 solar cells, International Conference on Ternary and Multinary Compounds, France, (2002). [2] M.A. Contreras, T. Nakada, M. Hongo, A.O. Pudov and J.R. Sites, ZnO/ZnS(O,OH)/Cu(In,Ga)Se 2 /Mo Solar Cell with 18.6% efficiency, 3 rd World Conference in Photovoltaic Energy Conversion, Japan, 2LN-C-08 (2003). [3] A. Yamada, H. Miyazaki, R. Mikami and M. Konagai, Improved Performance of Cu(InGa)Se 2 Thin Film

8 Solar Cells with High Ga Composition Using Rapid Thermal Annealing Process, 3 rd World Conference in Photovoltaic Energy Conversion, Japan, S4O-B12-03 (2003). [4] S. Niki et al., Improved open circuil voltage in wide-gap Cu(In1-xGax)Se 2 thin film solar cells, 19th European Photovoltaic Solar Energy Conference and Exhibition, France, (2004). Patents (Total 1) [1] M. Konagai et. :Japanese Patent Production method of CIGS thin film solar cells (Resistration number 2002 EA007).