Evolution and role of vacancy clusters at grain boundaries of ZnO:Al during accelerated degradation of Cu(In, Ga)Se2 solar cells revealed by positron annihilation

Wenqin Shi, Mirjam Theelen, Andrea Illiberi, Stefan van der Sar, Maik Butterling, Henk Schut, Miro Zeman, Ekkes Brück, Stephan Eijt, More Authors

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Abstract

Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening positron annihilation spectroscopy (DB-PAS) depth profiling demonstrate pronounced growth of vacancy clusters at the grain boundaries of as-deposited Al-doped ZnO films deposited as transparent conductive oxide (TCO) on Cu(In,Ga)Se2 (CIGS) solar cells upon accelerated degradation at 85∘C/85% relative humidity. Quantitative fractions of positrons trapped either in the vacancy clusters at the grain boundaries or in Zn monovacancies inside the grains of ZnO:Al were obtained by detailed analysis of the PALS data using a positron trapping model. The time and depth dependence of the positron Doppler depth profiles can be accurately described using a planar diffusion model, with an extracted diffusion coefficient of 35nm2/hour characteristic for in-diffusion of molecules such as H2O and CO2 into ZnO:Al TCO films via the grain boundaries, where they react with the ZnO:Al. This leads to increased open volume at the grain boundaries that imposes additional transport barriers and may lead to charge carrier trapping and nonradiative recombination. Simultaneously, a pronounced increase in series resistance and a strong reduction in efficiency of the ZnO:Al capped CIGS solar cells is observed on a remarkably similar timescale. This strongly indicates that these atomic-scale processes of molecular in-diffusion and creation of open volume at the grain boundaries play a key role in the degradation of the solar cells.
Original languageEnglish
Article number105403
Pages (from-to)1-18
JournalPhysical Review Materials
Volume2
Publication statusPublished - 2018

Keywords

  • Solar Cells
  • Positron Annihilation Spectroscopy
  • Grain Boundaries
  • Vacancies
  • Thin Films
  • Diffusion
  • Electrical properties
  • Solid State Chemistry
  • Optoelectronics
  • PhySH:

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