2, Mechanical Engineering, University of Nevada, Las Vegas, Las Vegas, Nevada, United States
3, Physics and Astronomy, Bowling Green State University, Bowling Green, Ohio, United States
4, Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg, Stuttgart, , Germany
Cu(In,Ga)Se2 (CIGS)-based solar cell have yielded the highest light-to-electrical energy conversion efficiency of all thin-film solar cells. To achieve a high conversion efficiency, incorporation of alkali metal (AM) impurities in CIGS has become critical. In addition, light induced and voltage-bias induced metastabilities could further enhance the performance of p-type CIGS by increasing hole concentration. However, the relationship between AM impurities and positive light/voltage-bias induced metastabilities is not well understood and remains largely unexplored. It has been experimentally observed that a large amount of AM impurities segregate at grain boundaries and the absorber/buffer interface, but they can diffuse into the grain depending on the growth conditions or over long aging times. Therefore, analysis of AM impurities both in the grain and at the grain boundaries are important to elucidate their role on metastabilities.
In this work, we performed first principles calculations with a hybrid functional to explore effects of AM impurities on electronic properties of CuInSe2(CIS), particularly on light-induced metastabilities. First, we calculated binding interaction between AM and divacancy complexes (VSe–VCu), which has been speculated to exhibit metastable properties in CIGS. We found that formation of (VSe–AMCu) complexes, in which AM is located at the VCu site, is energetically favorable. The binding energy of the (VSe–AMCu) complexes with respect to the (VSe–VCu) and AMCu increases in order of increasing atomic size of AM (Na < K < Rb). Similar to the (VSe–VCu) complex, the (VSe–AMCu) complexes also have different donor and acceptor configurations, and conversion between these two stable configurations can occur by overcoming energy barriers accompanied by accepting electrons/releasing holes. Our calculations predict that the required energy for the forward reaction (donor to acceptor conversion) is 0.16, 0.42, and 0.60 eV eV for the (VSe–NaCu), (VSe–KCu), and (VSe–RbCu), respectively, which is larger than 0.05 eV for the (VSe–VCu). This suggests that formation of the metastable acceptor configuration would be largely suppressed for the (VSe–AMCu) complexes with heavier AM.
Second, we examined geometries, energetics, and electronic structures of AMCu and its associated defect complexes at grain boundaries. Among many possible grain boundaries, we constructed (112)Σ3 twin boundaries (TB), which are commonly observed in CIGS. Three different (112)Σ3 twin boundaries were found with different Se local coordination environment at the boundary; for the most stable TB, all Se atoms at the TB are regularly coordinated with 2 Cu and 2 In, while for the least stable TB, Se atoms at the TB are coordinated with either 3 Cu and 1 In or 1 Cu and 3 In. Our calculations demonstrate that the formation energy of AMCu decreases at all three TB considered in this work with respect to that in the bulk, while the amount of decrease is larger for the TB containing more irregularly coordinated Se atoms at the boundary. Also, we found that heavier AM has stronger tendency (Na < K < Rb) to segregate at the grain boundaries, which agree well with the experimental observations demonstrating that K or Rb locates at the grain boundaries by kicking out Na at the grain boundaries. Furthermore, AMCu interaction with other point defects and defect complexes will be discussed to understand more comprehensive effects of AM impurities at the grain boundaries.