A new mechanism responsible for the hole concentration increase in the CIGS thin films after Na doping is proposed. At high temperature, a high concentration of Na is doped into the grains. After cooling and water rinsing, the solubility of Na becomes lower, so Na diffuses out of the grains with high concentration of Cu vacancies and hole carriers formed.

CuSbSe2 appears to be a promising absorber material for thin‐film solar cells due to its attractive optical and electrical properties, as well as earth‐abundant, low‐cost, and low‐toxic constituent elements. However, no systematic study on the fundamental properties of CuSbSe2 has been reported, such as defect physics, material, optical, and electrical properties, which are highly relevant for photovoltaic application. First, using density functional theory calculations, CuSbSe2 is shown to have benign defect properties, i.e., free of recombination‐center defects, and flexible defect and carrier concentration which can be tuned through the control of growth condition. Next, systematic material, optical, and electrical characterizations uncover many unexplored fundamental properties of CuSbSe2 including band position, temperature‐dependent band gap energy, Raman spectrum, and so on, thus providing a solid foundation for further photovoltaic research. Finally, a prototype CuSbSe2‐based thin film solar cell is fabricated by a hydrazine solution process. The systematic theoretical and experimental investigation, combined with the preliminary efficiency, confirms the great potential of CuSbSe2 for thin‐film solar cell applications.

By combining the electron−phonon coupling effect and the static coupling formalism, we calculate, through the firstprinciples methods, the carrier capture cross sections of the three possible nonradiative recombination center (NRRC) defects in Cu 2 ZnSnS 4 . These values are currently unavailable but critical for understanding the limiting factors of the minority carrier lifetime and simulating the photovoltaic devices. We show that the cross sections for Sn Zn 2+ capturing one electron (a (+2/+1) transition) and for [Cu Zn −Sn Zn ] + capturing one electron (a (+1/0) transition) are both very large, whereas for Sn Zn + capturing one electron (a (+1/0) transition) is much smaller by several orders of magnitude. The minority carrier lifetime will be limited to below 1 ns if the concentrations of Sn Zn 2+ and [Cu Zn −Sn Zn ] + are higher than 10 15 cm −3 , so they are effective NRRCs, whereas the lifetime can be as long as 10 μs with the same concentration of Sn Zn + , so Sn Zn + is a noneffective NRRC. The phonon mode analysis shows that the cross section is strongly correlated with the vibration mode of Sn−S bonds around the defects and its coupling with the localized wavefunction on the defect state. Sn Zn 2+ and [Cu Zn −Sn Zn ] + have a short and strong Sn−S bond with a highfrequency vibration mode, and these two defects undergo a large structural distortion after capturing an electron, which decreases the barrier for carrier capture and thus produces a large cross section. In contrast, Sn Zn + has a softer Sn−S vibration mode and thus much higher barrier for electron capture. Our calculations not only identify two effective NRRCs, which provide the mechanism behind why the Cu-poor, Zn-rich, Sn-poor growth condition, were widely adopted for fabricating high-efficiency Cu 2 ZnSnS 4 solar cells but also show that a very large difference can exist in the carrier capture cross sections for the same defect in different charge states (Sn Zn 2+ vs Sn Zn + ). We propose that the deep-level defects may have large carrier capture cross sections if they are surrounded by strong bonds and undergo considerable structural relaxations after capturing a carrier, which can be used as an empirical criterion for the quick identification of effective NRRCs.