Impact of Na diffusion on Cu(In, Ga)Se2 solar cells: Unveiling the role of active defects using thermal admittance spectroscopy

In this work, Cu(In, Ga)Se-2 (CIGS) based thin-film solar cells have been treated with the alkali element via incorporating Mo:Na layer into the device structure. A 600 nm thick Mo:Na layer behaved as a source for Na diffusion throughout the CIGS layer. The improvements have resulted in reduced depletion barrier width from similar to 0.55 to 0.25 mu m and increased p-type conductivity that improved the overall device performance from 7.06 to 12.06% for CIGS/Mo/Mo:Na in comparison with CIGS/Mo based solar cells. Thermal admittance spectroscopy has been analyzed to unveil the consequence of Na on shallow/deep-level defects and correlate with the CIGS/ Mo junction properties for both the CIGS solar cells. The defect distribution at the CIGS/buffer interface has been altered, the activation energy reduced from 260 meV to 63 meV assigned to deep level defect and 22 meV to 12 meV assigned shallow defects (VCu acceptor) that can be assigned to in the CIGS layer.