Discussion of Some 'Trap Signatures' Observed by Admittance Spectroscopy in CdTe Thin-Film Solar Cells

Jian V. Li, Steve W. Johnston, Xiaonan Li, David S. Albin, Timothy A. Gessert, Dean H. Levi

Research output: Contribution to journalArticlepeer-review

28 Scopus Citations


Considerable ambiguity and controversy exist concerning the defect signatures (H1, H2, and H3) frequently observed in admittance spectroscopy of thin-film CdTe solar cells. We prove that the commonly labeled H1 defects, observed in all devices in this study, are actually due to the freeze-out of the majority carriers in the neutral CdTe absorber. This freeze-out is evident in the temperature dependencies of capacitance, carrier concentration, and depletion region width. Contrary to intuitive expectation, the activation energy of freeze-out is less than, not identical to, that of the conductivity. In some other cases, H2 or H3 are observed and attributed to the back-contact potential barrier, rather than to the carrier emission from the traps. We extract the back-contact barrier height from the activation energy of the saturation current determined from the temperature-dependent current-voltage curves using the back-to-back diode model. The back-contact barrier height agrees well with the H2 or H3 energy determined by admittance spectroscopy. We present a more comprehensive and realistic equivalent circuit that includes the admittances from both the back-contact and the neutral absorber.

Original languageAmerican English
Article numberArticle No. 064501
Number of pages5
JournalJournal of Applied Physics
Issue number6
StatePublished - 15 Sep 2010

NREL Publication Number

  • NREL/JA-520-48113


  • defects
  • solar cells
  • spectroscopy


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