Photovoltaic devices based on the copper-indium-gallium-selenium (CIGS) materials system are limited to thermal cycles below 350 °C. This is an obstacle for making higher performance devices and a significant barrier to building multi-junction CIGS devices. High anneal temperatures lead to the diffusion of Cd from the CdS buffer layer into the CIGS absorber. Cd counterdopes the absorber causing the space charge region to extend to the back contact, dramatically reducing the power conversion efficiency. This article studies the effect of a silicon oxynitride diffusion barrier placed between the absorber and buffer layers. The oxynitride film was deposited by plasma-enhanced atomic layer deposition. Capacitance-voltage and drive-level-capacitance methods were used to extract the carrier profile in the absorber as a function of anneal condition and barrier layer thickness. Both measurement techniques showed that the barrier significantly retards diffusion, leading to intact devices at anneal temperatures as high as 450 °C. The barrier conducts current through the Poole-Frankel mechanism and did not increase the device series resistance if it was 4.4 nm or thinner. The deposition of the barrier layer, however, remains a significant obstacle. Exposure of the CIGS absorber to a plasma introduced moderate damage, but exposure to a hydrogen-containing plasma led to severe loss of Se near the absorber surface. Such devices were nearly ohmic, suggesting a high concentration of deep states in the space charge region.
Bibliographical noteFunding Information:
Manuscript received August 19, 2019; revised October 14, 2019; accepted October 14, 2019. Date of publication November 5, 2019; date of current version December 23, 2019. This work was supported in part by the National Renewable Energy Laboratory operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy under Contract DE-AC36-08GO28308 and in part by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Solar Energy Technologies Office under Agreements 30296 and 34352. (Corresponding author: Stephen A. Campbell.) T. G. Bontrager and B. T. Benton were with the University of Minnesota, Minneapolis, MN 55455 USA. They are now with SkyWater Technology, Bloomington, MN 55425 USA (e-mail: firstname.lastname@example.org; email@example.com).
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- Copper-indium-gallium-selenium (CIGS) thin-films solar cells
- thermal stability
- thin-film photo-voltaics (PV)