Understanding the effect of Na in improving the performance of Cu(InGa)Se2-based photovoltaics

The incorporation of Na into Cu(InGa)Se₂ (CIGS) based absorber layers is critical for fabricating high efficiency photovoltaic devices. In the present study we show that Na transport through Mo and CIGS from soda-lime glass substrates takes place through the grain boundaries with oxygen playing a critical role. Device performance drops as the Na level in CIGS is reduced but recovers by mild air heat treatment implying that Na plays a major role at the junction and at CIGS grain boundaries within the depletion region. Finally, we present results on the use of NaF as a post-deposition Na source.