Paper: Identifying the Real Minority Carrier Lifetime in Nonideal Semiconductors: A Case Study of Kesterite Materials

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Title: Identifying the Real Minority Carrier Lifetime in Nonideal Semiconductors: A Case Study of Kesterite Materials
Authors: Charles J. Hages,* Alex Redinger, Sergiu Levcenko, Hannes Hempel, Mark J. Koeper, Rakesh Agrawal, Dieter Greiner, Christian A. Kaufmann, and Thomas Unold*
Link: Adv. Energy Mater. 2017, 1700167 (Cited by 16)

Abstract:


Time‐resolved photoluminescence (TRPL) is a powerful characterization technique to study carrier dynamics and quantify absorber quality in semiconductors. The minority carrier lifetime, which is critically important for high‐performance solar cells, is often derived from TRPL analysis. However, here it is shown that various nonideal absorber properties can dominate the TRPL signal making reliable extraction of the minority carrier lifetime not possible. Through high‐resolution intensity‐, temperature‐, voltage‐dependent, and spectrally resolved TRPL measurements on absorbers and devices it is shown that photoluminescence (PL) decay times for kesterite materials are dominated by minority carrier detrapping. Therefore, PL decay times do not correspond to the minority carrier lifetime for these materials. The lifetimes measured here are on the order of hundreds of picoseconds in contrast to the nanosecond lifetimes suggested by the decay curves. These results are supported with additional measurements, device simulation, and comparison with recombination limited PL decays measured on Cu(In,Ga)Se2. The kesterite material system is used as a case study to demonstrate the general analysis of TRPL data in the limit of various measurement conditions and nonideal absorber properties. The data indicate that the current bottleneck for kesterite solar cells is the minority carrier lifetime.

Highlights:

  • PL decay times or TRPL do not correspond to the minority carrier lifetime for CZTSe
  • Processes which influence the TRPL decay are:
    • Radiative and nonradiative recombination
    • Surface recombination
    • Carrier drift in an electric field
    • Absorber inhomogeneity
    • Material degradation
    • Minority carrier trapping  (capture and emission)
  • For kesterites, the connection between PL time decay and the assumed minority charge lifetime is not apparent. (For CdTe technology is correlated)
  • V-TRPL: In contrast to CIGSe the TRPL data of kesterites shows no dependence on voltage.

Characterization techniques:

  • Steady-state PL (photoluminescence)
  • TRPL – Time-resolved photoluminescence is used to study carrier dynamics and quantify absorber quality in semiconductors: (Minority carrier lifetime and charge carrier density)
  • Intensity-dependent TRPL
  • Voltage-dependent TRPL
  • Temperature-dependent TRPL

Notes:

  • Data presented here indicate that minority carrier lifetime is the current bottleneck for improving efficiency in CZTSe solar cells.
  • This paper indicates that if the minority carrier trapping is ignored during TRPL decay time observation, then the minority carrier lifetime can be severely overestimated.
Disclaimer: The intention of this post is to bring some personal notes of the literature review. I’m not sharing the PDF files. For that purpose, please ask the authors or follow the link to the journal. 

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