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

• Published in: Physical review materials 2018-10, Vol.2 (10)
• Main Authors: Shi, Wenqin, Theelen, Mirjam, Illiberi, Andrea, Barreau, Nicolas, van Der Sar, Stefan, Butterling, Maik, Schut, Henk, Egger, Werner, Dickmann, Marcel, Hugenschmidt, Christoph, Zeman, Miro, Brück, Ekkes, Eijt, Stephan
• Format: Article
• Language: English
• Subjects: Condensed Matter; Materials Science; Physics
• ISSN: 2475-9953
• DOI: 10.1103/PhysRevMaterials.2.105403

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.