The spatial variation of minority‐carrier diffusion length in the vicinity of a boundary surface in monocrystalline and semicrystalline silicon solar cells has been measured by the scanned monochromatic light beam induced current (MBIC) technique with a very small spot size (2&mgr;m in diameter) and 750–1050‐nm‐long wavelength light beam. The technique we developed uses a short‐arc xenon lamp source with line spectra. It was found that most of the grain boundaries in cast semicrystalline silicon materials were accompanied with crystal imperfections along them, and the effect of a boundary surface extended beyond the distance of a minority‐carrier diffusion length. Furthermore, the effective minority‐carrier diffusion length due to crystal imperfections in the vicinity of the grain boundary was estimated and it was indicated that the grain boundary where the crystal imperfections were concentrated could be characterized using the concept of the effective influence width of grain boundary.