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Understanding the cell-to-module efficiency gap in Cu(In,Ga)(S,Se)2 photovoltaics scale-up

Nature Energyvolume 3pages466475 (2018) | Download Citation



Cu(In,Ga)(S,Se)2 (CIGS) solar cells show record efficiencies comparable to those of crystalline Si-based technologies. Their industrial module production costs are also comparable to those of Si photovoltaics in spite of their much lower production volume. However, the competitiveness of CIGS is compromised by the difference in performance between cell and module scales, known as the cell-to-module efficiency gap, which is significantly higher than in competing industrial photovoltaic technologies. In this Review, we quantify the main cell-to-module efficiency loss mechanisms and discuss the various strategies explored in academia and industry to reduce the efficiency gap: new transparent conductive oxides, hybrid modularization approaches and the use of wide-bandgap solar absorbers in the 1.4–1.5 eV range. To implement these strategies, research gaps relating to various device layers need to be filled.

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  1. 1.

    Whitepaper for CIGS Thin Film Solar Cell Technology (CIGS, 2015).

  2. 2.

    2Q 2017 Global Market Outlook. Under pressure (Bloomberg New Energy Finance, 2017).

  3. 3.

    Global Market Outlook. For Solar Power 2017-2021 (Solar Power Europe, 2016).

  4. 4.

    17.52% efficiency-new world record for Solibro’s CIGS thin film panels Solibro (2018).

  5. 5.

    Green, M. A. et al. Solar cell efficiency tables (version 51). Prog. Photovolt. 26, 3–12 (2018).

  6. 6.

    Solar Frontier achieves world record thin-film solar cell efficiency of 22.9% Solar Frontier (2017).

  7. 7.

    Ward, J. S. et al. Cu(In,Ga)Se2 solar cells measured under low flux optical concentration. In 40th Photovoltaic Specialists Conference 2934–2937 (IEEE, 2014);

  8. 8.

    Jackson, P. et al. Effects of heavy alkali elements in Cu(In, Ga)Se2 solar cells with efficiencies up to 22.6%. Phys. Status Solidi RRL 10, 583–586 (2016).

  9. 9.

    Chirila, A. et al. Potassium induced surface modification of Cu(In, Ga)Se2 thin films for high-efficiency solar cells. Nat. Mater. 12, 1107–1111 (2013).

  10. 10.

    Solibro GmbH company presentation Solibro (2016).

  11. 11.

    Shockely, W. & Last, J. T. Statistics of the charge distribution for a localized flaw in a semiconductor. Phys. Rev. 107, 392–396 (1957).

  12. 12.

    Kurtz, S. et al. A framework for a comparative accelerated testing standard. In 39th Photovoltaic Specialists Conference 0132–0138 (IEEE, 2013);

  13. 13.

    New world record efficiency for a CIGS PV module from Solibro Solibro (2017).

  14. 14.

    Richards, M. Hanergy breaks another solar efficiency world record for CIGS modules MiaSolé (2018).

  15. 15.

    Colville, F. PERC solar cell production to exceed 15GW in 2017 PVTech (2016)

  16. 16.

    Gabor, A. Cell-to-module gains and losses in crystalline silicon PV Intersolar (2013).

  17. 17.

    Matsunaga, K., Komaro, T., Nakayama, Y., Kume, T. & Suzuki, Y. Mass-production technology for CIGS modules. Sol. Energy Mater. Sol. Cells 93, 1134–1138 (2009).

  18. 18.

    Powalla, M. et al. Large-area CIGS modules: processes and properties. Thin Solid Films 523–533, 431–432 (2003).

  19. 19.

    Lin, J. V. et al. A recombination analysis of Cu(In,Ga)Se2 solar cells with low and high Ga compositions. Sol. Energy Mater. Sol. Cells 124, 143–149 (2014).

  20. 20.

    Kushiya, K. Key near-term R&D issues for continuous improvement in CIS-based thin-film PV modules. Sol. Energy Mater. Sol. Cells 93, 1037–1041 (2009).

  21. 21.

    Lee, D. et al. Effects of the Cu/(Ga+In) ratio on the bulk and interface properties of Cu(In,Ga)(S,Se)2 solar cells. Solar Energy Mater. Solar Cells 149, 195–203 (2016). This paper highlights the importance of the Cu/Ga+In (CGI) ratio within the loss performance factors of Cu(In,Ga)(S,Se) 2 . Lower CGI ratios are related with reduced interface recombination.

  22. 22.

    Oliva, F. et al. Optical methodology for process monitoring of chalcopyrite photovoltaic technologies: Application to low cost Cu(In, Ga)(S, Se)2 electrodeposition based processes. Sol. Energy Mater. Sol. Cells 158, 168–183 (2016).

  23. 23.

    Kijima, S. & Nakada, T. High-temperature degradation mechanism of Cu(In, Ga)Se2-based thin film solar cells. Appl. Phys. Express 1, 075002 (2008).

  24. 24.

    Jeng, M.-J. et al. Improving efficiency of multicrystalline silicon and CIGS solar cells by incorporating metal nanoparticles. Materials (Basel) 8, 6761–6771 (2015).

  25. 25.

    Reinhold, B. et al. Monolithically interconnected lamellar Cu(In, Ga)Se2 micro solar cells under full white light concentration. Prog. Photovolt. Res. App. 23, 2611 (2015).

  26. 26.

    Bayman, A. Miasole Flexible Product and Manufacturing IW-CIGSTech (Miasole, 2016);

  27. 27.

    Yan, F., Metacarpa, D. J., Sudaramoorthy, R., Fobare, D. & Haldar, P. Evaluation of CIGS Interconnection methods. In 39th Photovoltaic Specialist Conference 2064–2067 (IEEE, 2013);

  28. 28.

    Kessler, F. & Rudmann, D. Technological aspects of flexible CIGS solar cells and modules. Solar Energy 77, 685–695 (2004).

  29. 29.

    Schoop, U. Commercial Flexible CIGS Technology IW-CIGS Tech 7 (Global Solar, 2016);

  30. 30.

    Brown, G., Stone, P., Woodruff, J., Cardozo, B. & Jackrel, D. Device characteristics of a 17.1% efficient solar cell deposited by a non-vacuum printing method on flexible foil. In 38th Photovoltaic Specialist Conference 003230–003233 (IEEE, 2012);

  31. 31.

    The US CIGS Photovoltaic Roadmap Reports, Modules and Packaging (US PVMC, 2013);

  32. 32.

    Powalla, M. et al. High-efficiency Cu(In, Ga)Se cells and modules. Sol. Energy Mater. Sol. Cells 119, 51–58 (2013).

  33. 33.

    Rekow, M. et al. CIGS P1,P2,P3 scribing processes using a pulse programmable industrial fiber laser. In 25th European PVSEC/WCPEC-5 2862–2871(NREL, 2010);

  34. 34.

    Nishiwaki, S. et al. A monolithically integrated high-efficiency Cu(In,Ga)Se2 mini-module structrured solely by laser. Prog. Photovolt. Res. App. 23, 1908–1915 (2015). The importance of this paper lies in the optimization of a full laser structuring processing for P1P2P3 modularization to minimize the dead area width, as a means of increasing module efficiency.

  35. 35.

    Kessler, F., Herrmann, D. & Powalla, M. Approaches to flexible CIGS thin-film solar cells. Thin Solid Films 480–481, 491–498 (2005).

  36. 36.

    Ehrhardt, M. et al. Fabrication of contact holes by rear side laser ablation of polyimide foils for CIGS solar modules. Appl. Surf. Sci. 278, 219–222 (2013).

  37. 37.

    Wang, X. et al. The influence of the laser parameter on the electrical shunt resistance of scribed Cu(In,Ga)Se2 solar cells by nested circular laser scribing techniques. Appl. Surf. Sci. 302, 194–197 (2014).

  38. 38.

    Brecl, K., Topic, M. & Smole, F. A detailed study of monolithic contacts and electrical losses in large area thin-film module. Prog. Photovolt. 13, 297–310 (2005). This paper described the large impact of the distributed series resistance of transparent conducting oxide in monolithic devices and how it plays a crucial impact in final module performance.

  39. 39.

    Fortunato, E., Ginley, D., Honoso, H. & Paine, D. C. Transparent conducting oxides for photovoltaics. MRS Bull. 32, 242 (2007).

  40. 40.

    Klein, A. et al. Transparent conducting oxides for photovoltaics: Manipulation of Fermi level, work function and energy band alignment. Materials (Basel) 3, 4892–4914 (2010).

  41. 41.

    Escoubas, L., Simon, J., Le Rouzo, J. & Bermudez, V. Optical Thin Films and coatings: from materials to applications (eds Piegari, A. & Flory, F.) Ch. 16 (Woodhead Publishing, Oxford, 2013)..

  42. 42.

    Koida, T., Fujiwara, H. & Kondo, M. Hydrogen-doped In2O3 as high-mobility transparent conductive oxide. Jpn J. Appl. Phys. 46, 25–28 (2007).

  43. 43.

    Coutts, T. J., Youg, D. L. & Li, X. Characterization of transparent conducting oxides. MRS Bull. 25, 58–65 (2000).

  44. 44.

    Abou-Ras, D. et al. Innovation highway: Breakthrough milestones and key developments in chalcopyrite photovoltaics from a retrospective viewpoint. Thin Solid Films 633, 2–12 (2017). The paper is a useful review highlighting the main key developments in Cu(In,Ga)(S,Se)2 solar cells and modules, providing a historical perspective of the technology.

  45. 45.

    Nomoto, J., Konagai, M., Miyata, T. & Minami, T. Resistivity characteristics of transparent conducting impurity-doped ZnO films for use in oxidizing environments at high temperatures. J. Vac. Sci. Tech. A 28, 861 (2010).

  46. 46.

    Zhang, K., Zhu, F., Huan, C. H. A. & Wee, A. T. S. Effect of hydrogen partial pressure on optoelectronic properties of indium tin oxide thin films deposited by radio frequency magnetron sputtering method. J. Appl. Phys. 86, 974 (1999).

  47. 47.

    Zhou, Z. et al. Electrical conductivity of Cu-doped ZnO and its change with hydrogen implantation. J. Eur. Ceram. Soc. 24, 139 (2004).

  48. 48.

    Theys, B. et al. Effects of intentionally introduced hydrogen on the electrical properties of ZnO layers grown by metallorganic chemical vapor deposition. J. Appl. Phys. 91, 3922 (1997).

  49. 49.

    Delahoy, A. E. & Guo, S. Handbook of Photovoltic Science and Engineering (eds Luque, A. & Hegedus, S.) Ch. 17 (John Wiley & Sons Ltd, Hoboken, NJ, 2011).

  50. 50.

    Yoshida, Y., Wood, D. M., Gessert, T. A. & Coutts, T. J. High-mobility, sputtered films of indium oxide doped with molybdenum. Appl. Phys. Lett. 84, 2097 (2004).

  51. 51.

    Delahoy, A. E., Chen, L., Akhtar, M., Sang, B. & Guo, S. New technologies for CIGS photovoltaics. Solar Energy 77, 785 (2004).

  52. 52.

    Delahoy, A. E. & Guo, S. Y. Reactive-environment, hollow cathde sputtering: Basic characteristics and application to Al2O3, doped ZnO, and In2O3:Mo. J. Vac. Sci. Technol. 23, 1215 (2005).

  53. 53.

    van Hest, M. F. A. M., Dabney, M. S., Perkins, J. D., Ginley, D. S. & Taylor, M. P. Titanium-doped indium oxide: A high-mobility transparent conductor. Appl. Phys. Lett. 87, 032111 (2005). This paper highlights the potential of titanium-doped indium oxide for future transparent conducting oxides with high carrier mobility and transparency for a wide range of titanium doping levels.

  54. 54.

    Hashimoto, R., Abe, Y. & Nakada, T. High mobility titanium-doped In2O3 thin films prepared by sputtering/post-annealing technique. Appl. Phys. Express 1, 015002 (2008).

  55. 55.

    Gupta, R. K., Ghosh, K., Mishra, S. R. & Kahol, P. K. High mobility Ti-doped In2O3 transparent conductive thin films. Mater. Lett. 62, 1033–1035 (2008).

  56. 56.

    Calnan, S. et al. Application of high mobility transparent conductors to enhance long wavelength transparency on the intermediate solar cell in multi-junction solar cells. Thin Solid Films 92, 2340–2343 (2009).

  57. 57.

    Shisegato, Y., Takaki, S. & Haranoh, T. Electrical and structural properties of low resistivity tin-doped indium oxide films. J. Appl. Phys. 71, 3356 (1992).

  58. 58.

    Suzuki, A., Matsushita, T., Aoki, T., Yoneyama, Y. & Okuda, M. Pulsed laser deposition of transparent conducting indium tin oxide films in magnetic field perpendicular to plume. Jpn J. Appl. Phys. 40, 401 (2001).

  59. 59.

    Agura, H. Low resistivity transparent conducting Al-doped ZnO films prepared by pulsed laser deposition. Thin Solid Films 445, 263–267 (2003).

  60. 60.

    Song, P. K., Watanabe, M., Kon, M., Mitsui, A. & Shigesato, Y. Electrical and optical properties of gallium-doped zinc oxide films deposited by dc magnetron sputtering. Thin Solid Films 411, 82–86 (2002).

  61. 61.

    Assuncao, V., Fortunato, E. & Marques, A. Influence of the deposition pressure on the properties of transparent and conductive ZnO:Ga thin film produced by r.f. sputtering at room temperature. Thin Solid Films 427, 401–405 (2003).

  62. 62.

    Kykyneshi, R., Zeng, J. & Cann, D. P. in Handbook of Transparent Conductors (eds Ginley, D. S. et al.) Ch. 6 (Springer, New York, 2011).

  63. 63.

    Hosono, H., Yusukawa, M. & Kawazoe, H. Novel oxide amorphous semiconductors: transparent conducting amorphous oxides. J. Non-Cryst. Solids 203, 334–344 (1996).

  64. 64.

    Hosono, H. in Handbook of Transparent Conductors (eds Ginley, D. S. et al.) Ch. 13 (Springer, New York, 2011).

  65. 65.

    Ginley, D. S. & Perkins, J. D. in Handbook of Transparent Conductors (eds Ginley, D. S. et al.) Ch. 1 (Springer, New York, 2011).

  66. 66.

    Hitosugi, T., Furubayashi, Y. & Ueda, A. Ta-doped anatase TiO2 epitaxial film as transparent conducting oxide. Jpn J. Appl. Sci. 44, L1063–L1065 (2005).

  67. 67.

    Hoye, R. L. Z., Musselman, K. P. & MacManus-Criscoll, J. L. Research update: Doping ZnO and TiO2 for solar cells. Appl. Phys. Lett. Materi. 1, 060701 (2013).

  68. 68.

    Mason, T. O., Harvey, S. P. & Poeppelmeier, K. R. in Handbook of Transparent Conductors (eds Ginley, D. S. et al.) Ch. 8 (Springer, New York, 2011).

  69. 69.

    Ellmer, K. Past achievements and future challenges in the development of optically transparent electrodes. Nat. Photon. 6, 809–817 (2012).

  70. 70.

    Minami, T. New n-type transparent conducting oxides. MRS Bull. 25, 38–44 (2000).

  71. 71.

    Granqvist, C. G. Transparent conductors as solar energy materials: A panoramic view. Sol. Energy Mater. Sol. Cells 91, 1529–1598 (2007).

  72. 72.

    Lee, J.-Y., Connor, S. T., Cui, Y. & Peumans, P. Solution-processes metals nanowire mesh transparent electrodes. Nano Lett. 8, 680–692 (2008).

  73. 73.

    Barnes, T. M., Reese, M. O. & Bergeson, J. D. Comparing the fundamental physics and device performanec of transparent conductive nanostructured networks with conventional transparent conducting oxides. Adv. Energy Mater. 2, 353–360 (2012).

  74. 74.

    Krantz, J., Richter, M., Spallek, S., Spiecker, E. & Brabec, C. J. Solution-processed metallic nanowire electrodes as indium tin oxide replacement for thin-gilm solar cells. Adv. Funct. Mater. 21, 4784–4787 (2011).

  75. 75.

    Bae, S., Kim, H. & Lee, Y. Roll-to-roll production of 30-inch graphene films for transparent electrodes. Nat. Nanotech. 5, 574–578 (2010).

  76. 76.

    Gongoni, P. Nanostructured Transparent Conducting Oxides for Advanced Photovoltaic Applications. PhD Thesis, Politecnico di Milano (2013).

  77. 77.

    Sittinger, V., Dewald, W., Werner, W., Szyszka, B. & Ruske, F. Transparent conducting oxide deposition techniques for thin-film photovoltaics. Photovolt. Int. 10, 108–115 (2011).

  78. 78.

    Pern, F. J. & Noufi, R. Stability of CIGS Solar Cells and Component Materials Evaluated by a Step-Stress Accelerated Degradation Test Method SPIE Optics + Photonics (NREL, 2012);

  79. 79.

    Solibro company and product presentation Solibro (2013).

  80. 80.

    Solibro’s productive technology surpasses 16% efficiency Solibro (2018).

  81. 81.

    Dongaonkar, S. & Alam, M. A. In-line post-process scribing for reducing cell to module efficiency gap in monolithic thin-film photovoltaics. IEEE J. Photovolt. 4, 324–332 (2014).

  82. 82.

    Olaisen, B. R. et al. CIGS mini-modules with screen-printed front contacts. In 15th International Photovoltaic Science and Engineering Conference (PVSEC-15) 612–613 (2005);

  83. 83.

    Xie, L. et al. Electrical performance and reliability evaluation of inkjet-printed Ag interconnections on paper substrates. Mater. Lett. 88, 68–72 (2012).

  84. 84.

    Investors express confidence in ultrafast ALD Solaytec

  85. 85.

    Miikkulainen, V., Leskela, M., Ritala, M. & Puurunen, R. L. Crystallinity of inorganic films grown by atomic layer deposition: Overview and genera trends. J. Appl. Phys. 113, 021301 (2013).

  86. 86.

    Bermudez, V. Economical and operational issues for CIGS in the future PV panorama. Solar Energy 146, 85–93 (2017).

  87. 87.

    Contreras, M. et al. Graded band-gap Cu(In, Ga)Se2 thin-film solar cell absorber with enhanced open circuit voltage. Appl. Phys. Lett. 63, 198–200 (1993).This paper demonstrates the achievement of a high voltage device thanks to the control of In/Ga depth profile composition through the absorber with a computer controlled physical vapor deposition approach..

  88. 88.

    Siebentritt, S. Wide gap chalcopyrites: materials properties and solar cells. Thin Solid Films 403–404, 1–8 (2002).

  89. 89.

    Siebentritt, S., Rau, U. (eds). Wide-Gap Chalcopyrites (Springer, Berlin, 2006).

  90. 90.

    Dongaonkar, S. & Alam, M. A. In-line post-process scribing for reducing cell to module efficiency gap in monolithic thin-film photovoltaics. IEEE J. Photovolt. 4, 324–332 (2014). This paper demonstrates that it is possible to recover up to 50% of power losses through the reduction of cell to module gap via the minimization of parasitic shunts with state of the art laser scribing processes.

  91. 91.

    Illiberi, A. et al. Industrial high-rate (~14nm/s) deposition of low resistive and transparent ZnOx:Al films on glass. Sol. Energy Mater. Sol. Cells 95, 1955–1959 (2011).

  92. 92.

    van Deelen, J., Rendering, H., het Mannetje, H. & Hovestad, A. Grids on TCO for highly transparent materials with a resistivity below 1 Ohm/sq. In 37th Photovoltaic Specialist Conference 000768–000770 (IEEE, 2011);

  93. 93.

    Abou-Ras, D. et al. Impact of Ga concentration on the micrstructure of CuInGaSe2. Phys. Stat. Sol. (RRL) 2, 135–137 (2008).

  94. 94.

    Hahn, T., Cieslak, J. & Metzner, H. Metastability of CuInS2 and its implications on thin film growth. Appl. Phys. Lett. 88, 171915 (2006).

  95. 95.

    Hanket, G. M., Thompson, C. P. Larsen, J., Eser, E. & Shafarman, W. N. Control of Ga profiles in (AgCu)(InGa)Se2 absorber layers deposited on polyimide substrates. In 38th Photovoltaic Specialist Conference 000662-000667 (IEEE, 2012);

  96. 96.

    Hiroi, H., Iwata, Y., Sugimoto, H. & Yamada, A. Progress toward 1000-mV open-circuit voltage on chalcopyrite solar cells. IEEE J. Photovolt. 6, 1630–1634 (2016).

  97. 97.

    Lordi, V. Rational Design of Wide Band Gap Buffer Layers for high Efficiency Thin-Film Photovoltaics (Lawrence Livermore National Laborator, 2016).

  98. 98.

    Chen, L., Lee, J. W. & Shafarman, W. N. The comparison of (Ag,Cu)(In,Ga)Se2 and Cu(In,Ga)Se2 thin films deposited by three-stage coevaporation. IEEE J. Photovolt. 4, 447–451 (2014).

  99. 99.

    Salome, P. M. P., Rodriguez-Alvarez, H. & Sadewasser, S. Incorporation of alkali metals in chalcogenide solar cells. Sol. Energy Mater. Sol. Cells 143, 9–20 (2015).

  100. 100.

    Hirai, Y., Kurakawa, Y. & Yamada, A. Numerical study of Cu(In,Ga)Se2 solar cell performance towards 23% conversion efficiency. Jpn J. Appl. Phys. 53, 012301 (2014).This work analyses the effect of minimum bandgap and its position for materials with wide average bandgap around 1.4 eV. The paper shows the importance of valence band off set between the buffer and the absorber for wide bandgap devices..

  101. 101.

    Burgelman, M. & Niemegeers, A. Calculation of CIS and CdTe module efficiencies. Sol. Energy Mater. Sol. Cells 51, 129–143 (1998).

  102. 102.

    Kessler, J.., Wennerberg, J.., Bodegard, M.., & Stolt, L.. Highly efficient Cu(In, Ga)Se2 mini-modules. Sol. Energy Mater. Sol. Cells 75, 35–46 (2003).

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A.P.-R. acknowledges financial supports from the Spanish MINECO (Ministerio de Economía, Industria y Competitividad) under the WINCOST project (ENE2016-80788-C5-1-R), the European Regional Development Founds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013) and the CERCA Programme from the Generalitat de Catalunya.

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    • Veronica Bermudez

    Present address: Solar Frontier KK, Atsugi Research Center, Atsugi-shi, Kanagawa, Japan


  1. EFESE, EDF R&D, Chatou, France

    • Veronica Bermudez
  2. Solar Energy Materials and Systems Group, Catalonia Institute for Energy Research (IREC), Barcelona, Spain

    • Alejandro Perez-Rodriguez
  3. IN2UB, Department of Electronic and Biomedical Engineering, University of Barcelona, Barcelona, Spain

    • Alejandro Perez-Rodriguez


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V.B. is currently working at the Atsugi Research Center of Solar Frontier K.K.

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Correspondence to Veronica Bermudez.

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