The type-inversion to n-type at the surface of p-type Culn(1-x)Ga(x)Se2 absorber layer is taken as an important factor for the high efficiency of Culn(1-x)Ga(x)Se2 (CIGS) with low Ga content, however, the further increase of Ga content makes the n-type doping difficult and the type-inverted layer vanish, which may have a negative effect on the device performance. Previous first-principles calculation had shown that the donor density becomes lower and level deeper when Ga content increases, while it's not clear how significantly the changes in the type-inverted layer influence the device performance. Through device simulation, we show that the efficiency decreases obviously as the donor density becomes lower and the level deeper in the inversion layer, thus they are important factors responsible for the limitation of the efficiency of CIGS solar cell, i.e., the efficiency decreases as Ga content exceeds 30%. Our work gives a good example in how to combine the electronic structure calculation of materials and device simulation to explain the experimental observation.