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)Se₂ (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 ∼0.55 to 0.25 µm 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 (V_Cu acceptor) that can be assigned to in the CIGS layer.