The benefits of gallium (Ga) grading on Cu(In,Ga)Se2 (CIGS) solar cell performance are demonstrated by comparing with ungraded CIGS cells. Using drive-level capacitance profiling (DLCP) and admittance spectroscopy (AS) analyses, we show the influence of Ga grading on the spatial variation of deep defects, free-carrier densities in the CIGS absorber, and their impact on the cell's open-circuit voltage Voc. The parameter most constraining the cell's Voc is found to be the deep-defect density close to the space charge region (SCR). In ungraded devices, high deep-defect concentrations (4.2 × 1016cm-3) were observed near the SCR, offering a source for Shockley-Read-Hall recombination, reducing the cell's Voc. In graded devices, the deep-defect densities near the SCR decreased by one order of magnitude (2.5 × 1015 cm-3) for back surface graded devices, and almost two orders of magnitude (8.6 × 1014 cm-3) for double surface graded devices, enhancing the cell's Voc. In compositionally graded devices, the free-carrier density in the absorber's bulk decreased in tandem with the ratio of gallium to gallium plus indium ratio GGI = Ga/(Ga + In), increasing the activation energy, hindering the ionization of the defect states at room temperature and enhancing their role as recombination centers within the energy band. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]