Nucleation and Growth Behavior of Quaternary-Sputtered Copper Indium Gallium Diselenide Thin Films
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Nucleation and Growth Behavior of Quaternary-Sputtered Copper Indium Gallium Diselenide Thin Films
Jason D. Myers1, Jesse A. Frantz1, Robel Y. Bekele1,3, Vinh Q. Nguyen1, Allan Bruce2, Sergey V. Frolov2, Michael Cyrus2, and Jas S. Sanghera1 1
Optical Sciences Division, U.S. Naval Research Laboratory, Washington, DC
2
Sunlight Photonics, South Plainfield, NJ
3
University Research Foundation, Greenbelt, MD
Abstract In the past two decades, the growing global demand for solar energy has spurred scientific interest in alternative technologies to conventional silicon. In particular, CuIn1-xGaxSe2 (CIGS) has emerged as a competitor. We have developed a scalable deposition technique using RF magnetron sputtering of quaternary CIGS. Notably, the resulting films do not require postselenization, reducing processing time and cost. We have fabricated devices above 10% efficiency using this approach, showing its promise as a production method for highperformance CIGS photovoltaics. However, the morphology of the sputtered CIGS layer is markedly different from conventional evaporated films; grain sizes vary through the thickness of the film, with numerous small grains dominating at the Mo/CIGS interface that then either terminate or grow in an inverted-pyramid fashion to form large, columnar grains at the CIGS/CdS interface. To better understand the origin of this morphology, we have studied the growth behavior of the CIGS layer using a combination of atomic force microscopy and electron microscopy to observe initial nucleation and grain growth behavior of quaternary-sputtered CIGS. We also discuss the effects of interfacial layers at the Mo/CIGS interface, demonstrating a novel wetting layer that conformally coats the Mo surface.
Introduction CuIn1-xGaxSe2 (CIGS) has become a promising thin-film photovoltaic material, with laboratory efficiencies of ~20% [1,2]. However, the highest efficiency devices are produced using thermal co-evaporation of the individual constituents in a so-called “three-stage” process [3] to optimize morphology and device performance. While this has been effective in the laboratory it poses
great challenges for commercial production in terms of capital expense, reproducibility, and film uniformity. We have therefore been developing an alternative deposition method, quaternary sputtering, where pre-batched bulk CIGS is sputtered onto a substrate in a one-step process that, unlike other sputtering techniques [4–6], does not require further selenization treatment, further reducing cost and increasing manufacturability. We have previously reported devices with power conversion efficiencies of approximately 10% [7]. However, the morphology of the active layer in those devices is distinct from that of co-evaporated CIGS. Instead of uniform, µm-sized grains we observe an inverted pyramid structure with numerous small grains (< 100 nm diameter) near the CIGS/Mo interface. Some of the small grains terminate, while others grow and dominate the film; grain sizes at the top of the active layer are on the or
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