AlkaCIGS - Effects of Alkali elements in Cu(In,Ga)Se2 thin films and solar cells

• Applicant: Dr. Roland Würz
• Subject Area: Synthesis and Properties of Functional Materials
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 381204229

• Final Report Year: 2023

Final Report Abstract

In this project, the positive alkali-effect of sodium (Na) and potassium (K) in Cu(In,Ga)Se2 thinfilms and solar cells (CIGS) was investigated and compared to each other. This was realized by production of alkali-free CIGS thin-films and solar cells as references and defined doping with Na and K via a post deposition process (PDT) after the growth of the CIGS layer. The electrical measurements showed that even a low Na concentration is sufficient to passivate deep defects in the CIGS layer and thereby to increase cell efficiency. Many results of the different electrical measurements at CIGS thin films as well as at solar cells could be explained by a barrier model at grain boundaries (GBs). In the model, donor like defects at the grain boundaries trap holes of the grain volume and hence lead to the formation of electrical barriers at the GBs for the transport of holes. In alkali free CIGS layers the height of the barrier at the GBs amounts to EB = 340 meV. When Na is implemented into the CIGS layers, these donor type defects at the GBs are passivated and the barrier height is reduced down to values of 40 meV so that the holes can easily surmount the barrier. With increasing Na concentration the barrier height decreases which leads to an increase of mobility and hence the conductivity as well as the charge carrier density in the CIGS layer. The increase of open circuit voltage with increasing Na concentration cannot be explained solely by an increase of charge carrier density but can only be explained by a surplus passivation of defects in the CIGS layer by Na. In principal, Na and K show similar effects: With increasing alkali concentration in the CIGS layer the efficiency of solar cells, the net charge carrier density as well as the conductivity of the CIGS thin films increases. However, Na is more doping efficient compared to K, as a 4 times lower Na concentration compared to K leads to the same electrical effects. For doping with Na, the cell efficiency monotonously increases with the temperature during the PDT (TTPD). In contrast, for KF PDT a strong loss in cell efficiency is observed for TPDT lower than 150°C. This effect could be explained by the different reactivity of Na compared to K. During a NaF PDT no secondary phases formed at the CIGS surface with the PDT parameters used here. Whereas during a KF PDT the formation of secondary phases at the CIGS surface was observed. For a KF PDT at 340°C K-In-Se bonds are formed at the CIGS surface, which lead to a positive electrical effect. However, during a KF PDT at 100°C the formation of Cu-S bonds was observed at the CIGS surface leading to the strong loss in cell efficieny for TPDT lower than 150°C. K diffuses into the GBs as well as into the volume of the grain in polycrystalline CIGS layers similar to Na. Even at TPDT = 100 °C the diffusion into the volume of the grain was observed. By fitting the K diffusion profiles, the diffusion coefficients for diffusion into the grain volume and along the GBs could be determined as well as their corresponding activation energies of EA(volume) = 0.39 eV and EA(GB) = 0.32 eV. The comparison of diffusion of K with that of Na shows that K diffused less into the volume and along the GBs under the same diffusion conditions but the concentration of K in the CIGS layer is much higher compared to Na. The reason for this is possibly the lower lattice enthalpy of KF in comparison to NaF. Hence, K is much more easily released from KF during a PDT as Na from NaF, so that at the same TPDT more K can diffuse into the CIGS layer compared to Na during a NaF PDT.

Publications

• Dependence of the Magnitude of Persistent Photoconductivity on Sodium Content in Cu(In,Ga)Se2 Solar Cells and Thin Films. IEEE Journal of Photovoltaics, 10(6), 1926-1930.
  Czudek, Aniela; Urbaniak, Aleksander; Eslam, Alexander; Wuerz, Roland & Igalson, Malgorzata
  
• Evolution of the electrical characteristics of Cu(In,Ga)Se2 devices with sodium content. Journal of Applied Physics, 128(17).
  Czudek, A.; Eslam, A.; Urbaniak, A.; Zabierowski, P.; Wuerz, R. & Igalson, M.
  
• Explaining the efficiency-boosting effects of sodium on CIGS solar cells. Scilight, 2020(45).
  Kim, Meeri
  
• A simulation study on the effect of sodium on grain boundary passivation in CIGS thin-film solar cells. 2021 IEEE 48th Photovoltaic Specialists Conference (PVSC), 0187-0191. IEEE.
  Sozzi, Giovanna; Cojocaru-Miredin, Oana & Wuerz, Roland
  
• Grain Boundaries in Cu(In, Ga)Se2: A Review of Composition–Electronic Property Relationships by Atom Probe Tomography and Correlative Microscopy. Advanced Functional Materials, 31(41).
  Cojocaru‐Mirédin, Oana; Raghuwanshi, Mohit; Wuerz, Roland & Sadewasser, Sascha
  
• Impact of substrate temperature during NaF and KF post-deposition treatments on chemical and optoelectronic properties of alkali-free Cu(In,Ga)Se2 thin film solar cell absorbers. Thin Solid Films, 739, 138979.
  Eslam, Alexander; Wuerz, Roland; Hauschild, Dirk; Weinhardt, Lothar; Hempel, Wolfram; Powalla, Michael & Heske, Clemens
  
• Potassium versus Sodium in Cu(In,Ga)Se2—Similarities and Differences in the Electrical Characteristics of Solar Cells and Thin Films after NaF or KF Postdeposition Treatment. physica status solidi (RRL) – Rapid Research Letters, 16(1).
  Czudek, Aniela; Urbaniak, Aleksander; Eslam, Alexander; Wuerz, Roland & Igalson, Małgorzata
  
• “Chemical and Electrical Impact of Alkali Fluoride Post Deposition Treatments on Cu(In,Ga)Se2 Solar Cells“ Dissertation, Karlsruher Institut für Technologie
  Alexander Eslam
  
• Consequences of grain boundary barriers on electrical characteristics of CIGS solar cells. Solar Energy Materials and Solar Cells, 253, 112252.
  Urbaniak, A.; Czudek, A.; Eslam, A.; Wuerz, R. & Igalson, M.