The solar power conversion efficiency of compositionally graded In(x)Ga(1-x)N solar cells was simulated using a finite element approach. Incorporating a compositionally graded region on the InGaN side of a p-GaN/n-In(x)Ga(1-x)N heterojunction removes a barrier for hole transport into GaN and increases the cell efficiency. The design also avoids many of the problems found to date in homojunction cells as no p-type high-In content region is required. Simulations predict 28.9% efficiency for a p-GaN/n-In(x)Ga(1-x)N/n-In(0.5)Ga(0.5)N/p-Si/n-Si tandem structure using realistic material parameters. The thickness and doping concentration of the graded region was found to substantially affect the performance of the cells. (C) 2009 Elsevier B.V. All rights reserved.