Herein, a method to quantify the amount of reduction in the internal quantumefficiency (IQE) of multicrystalline silicon solar cells that can be attributed to grainboundaries is presented. By correlating the IQE maps obtained via light beaminduced current (LBIC) topography with optical images of Secco-etched samples,the distribution of IQE values at the positions of grain boundaries can be comparedwith the distribution of IQE values for all other positions where other defects mayexist. The segmentation of IQE at 826 nm maps of the samples shows grainboundaries to be more detrimental compared with the combined effects of all otherdefects that may exist in other regions of the cell. The grain boundaries reduce theaverage IQE by up to 4.07% absolute for the cell from the bottom of the ingot.