Improving the Thermal Stability of CIGS Photovoltaic Devices

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.