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 (TU Delft - RST/Fundamental Aspects of Materials and Energy)
Mirjam Theelen (TNO)
Andrea Illiberi (TNO)
Stefan van der Sar (TU Delft - RST/Medical Physics & Technology)
Maik Butterling (TU Delft - RST/Neutron and Photon Methods for Materials, TU Delft - RST/Fundamental Aspects of Materials and Energy)
Henk Schut (TU Delft - RST/Neutron and Photon Methods for Materials)
Miro Zeman (TU Delft - Electrical Sustainable Energy)
Ekkes Brück (TU Delft - RST/Fundamental Aspects of Materials and Energy)
Stephan Eijt (TU Delft - RST/Fundamental Aspects of Materials and Energy)
undefined More Authors (External organisation)
More Info
expand_more
Other than for strictly personal use, it is not permitted to download, forward or distribute the text or part of it, without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license such as Creative Commons.
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.