Heavy Alkali Treatment of Cu(In,Ga)Se2 Solar Cells: Surface versus Bulk Effects

Susanne Siebentritt*, Enrico Avancini, Marcus Bär, Jakob Bombsch, Emilie Bourgeois, Stephan Buecheler, Romain Carron, Celia Castro, Sebastien Duguay, Roberto Félix, Evelyn Handick, Dimitrios Hariskos, Ville Havu, Philip Jackson, Hannu Pekka Komsa, Thomas Kunze, Maria Malitckaya, Roberto Menozzi, Milos Nesladek, Nicoleta NicoaraMartti Puska, Mohit Raghuwanshi, Philippe Pareige, Sascha Sadewasser, Giovanna Sozzi, Ayodhya Nath Tiwari, Shigenori Ueda, Arantxa Vilalta-Clemente, Thomas Paul Weiss, Florian Werner, Regan G. Wilks, Wolfram Witte, Max Hilaire Wolter

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

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Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post-deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open-circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single-phase Cu-alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries.

Original languageEnglish
Article number1903752
Pages (from-to)1-15
Number of pages15
JournalAdvanced Energy Materials
Issue number8
Early online date1 Jan 2020
Publication statusPublished - Feb 2020
MoE publication typeA1 Journal article-refereed


  • alkali treatment
  • bulk
  • chalcopyrite solar cells
  • grain boundaries
  • recombination
  • surface


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