Explaining the efficiency-boosting effects of sodium on CIGS solar cells

More than 25 years ago, researchers noticed the presence of sodium in the absorber of Cu(In,Ga)Se2 (CIGS) solar cells boosts their efficiency. It came as a surprise, since sodium is usually detrimental for electronic devices. However, the mechanisms behind this beneficial effect on solar cell performance remain under dispute.

Czudek et al. decided to take a more thorough approach to solving the alkali puzzle by systematically studying CIGS solar cells and thin films while varying their sodium content. Their findings reveal sodium acts on the devices in multiple ways, and grain boundaries play a more important role in performance than expected. Even a small amount of sodium introduced during post-deposition treatment with NaF at low temperature (room temperature up to 340 degrees Celvin) already has a large impact on the transport mechanism and secondary barriers in the device.

The researchers started by preparing nearly alkali-free CIGS layers and doping them with different amounts of sodium via post-deposition treatment. They used very sensitive methods to evaluate sodium concentration in each layer, followed by a range of electrical measurements to study the samples. The various techniques included capacitance methods for evaluation of doping level, and deep defect spectra, current-voltage characterization and conductivity measurements of thin films.

They found higher levels of sodium increased open-circuit voltage, fill factor, hole density and mobility but decreased secondary barriers responsible for distortion of current-voltage characteristics. Interestingly, the transport mechanism and defect spectra abruptly change at certain concentration levels. Czudek et al. observed the influence of grain boundaries on mobility. Sodium-containing samples had 50-fold increased hole mobility compared to their sodium-free counterparts, which is most likely explained by the sodium’s effect on potential barriers at grain boundaries.

Source: “Evolution of the electrical characteristics of Cu(In,Ga)Se₂ devices with sodium content,” by A. Czudek, A. Eslam, A. Urbaniak, P. Zabierowski, R. Wuerz, and M. Igalson, Journal of Applied Physics (2020). The article can be accessed at http://doi.org/10.1063/5.0025183 .