The impact of grain boundary defects on the current-voltage characteristics of Cu(In,Ga)Se2- based solar cells

The work presents two-dimensional simulations of the impact of donor defects at grain boundaries (GB) on potential barriers at the GBs in Cu(In,Ga)Se₂ (CIGS) thin films as well as on the photovoltaic parameters of corresponding CIGS solar cells. The essential relationships between the height of the potential barriers at the grain boundaries and the concentration of defects, the doping level of the material, and the grain sizes are shown. The barriers cause a downward bending of the energy band, resulting in a depleted region around the grain boundaries. However, the electrons accumulate at the GBs during the illumination, and grain boundaries become channels for the electron flow. We show that despite this charge carrier separating effect, grain boundaries can be a place of significant recombination, lowering the photovoltaic parameters of the cell. To illustrate this, we compare the simulations with experimental results for CIGS cells with different sodium concentrations. The dependencies of open-circuit voltage (V_OC) on the activation energy of conductivity are shown, which, within the model, we identify as the height of the potential barriers at the grain boundaries. The comparison with experimental data gave a good agreement, showing that donor defects located at grain boundaries can create potential barriers, which determine the CIGS conductivity and may also lead to significant recombination, lowering the solar cell efficiency.