Defect-Mediated Metastability and Carrier Lifetimes in Polycrystalline (Ag,Cu)(In,Ga)Se2 Absorber Materials

Andrew Ferguson, Rouin Farshchi, Pran Paul, Pat Dippo, Jeff Bailey, Dmitry Poplavskyy, Afrina Khanam, Filip Tuomisto, Aaron Arehart, Darius Kuciauskas

Research output: Contribution to journalArticlepeer-review

15 Scopus Citations

Abstract

Using a combination of optical and electrical measurements, we develop a model for metastable defects in Ag-alloyed Cu(In,Ga)Se2, one of the leading thin film photovoltaic materials. By controlling the pre-selenization conditions of the back contact prior to the growth of polycrystalline (Ag,Cu)(In,Ga)Se2 absorbers and subsequently exposing them to various stresses (light soaking and dark-heat), we explore the nature and role of metastable defects on the electro-optical and photovoltaic performance of high-efficiency solar cell materials and devices. Positron annihilation spectroscopy indicates that dark-heat exposure results in an increase in the concentration of the selenium-copper divacancy complex (VSe-VCu), attributed to depassivation of donor defects. Deep-level optical spectroscopy finds a corresponding increase of a defect at Ev + 0.98 eV, and deep-level transient spectroscopy suggests that this increase is accompanied by a decrease in the concentration of mid-bandgap recombination centers. Time-resolved photoluminescence excitation spectroscopy data are consistent with the presence of the VSe-VCu divacancy complex, which may act as a shallow trap for the minority carriers. Light-soaking experiments are consistent with the VSe-VCu optical cycle proposed by Lany and Zunger, resulting in the conversion of shallow traps into recombination states that limit the effective minority carrier recombination time (and the associated carrier diffusion length) and an increase in the doping density that limits carrier extraction in photovoltaic devices.

Original languageAmerican English
Article number215702
Number of pages10
JournalJournal of Applied Physics
Volume127
Issue number21
DOIs
StatePublished - 7 Jun 2020

Bibliographical note

Publisher Copyright:
© 2020 Author(s).

NREL Publication Number

  • NREL/JA-5900-75301

Keywords

  • Ag-alloyed Cu(In,Ga)Se2
  • metastable defects
  • model
  • thin film photovoltaic materials

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