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In this paper, we quantitatively evaluate the relative importance of three mechanisms that are proffered by various groups to interpret the effects of sodium (Na) incorporation in copper indium gallium diselenide solar cells. The suggested mechanisms are 1) increase in the carrier density due to defect passivation; 2) spatial redistribution of gallium (Ga); and 3) change in the crystal orientation. The simulation framework which is developed for this purpose indicates that, among these three coexisting effects, the increase in the carrier density with Na incorporation is likely to be most important. If the grain boundaries (GBs) initially contain donor-like traps that are subsequently passivated by Na, the increase in carrier density can improve the cell efficiency significantly. On the other hand, we find that the effects of Ga redistribution and change in crystal orientation are limited.