The quality and lifetime of solar cells critically depend on minimization of both bulk and surface imperfections. When a solar cell is reverse-biased with low voltage, weak emission from imperfections (pre-breakdown sites) appears. We report on the local measurement of such breakdown using a scanning near-field optical microscope. Due to the very weak signals, we use a cooled photomultiplier in the photon counting regime to achieve high sensitivity light detection. This technique allows non-destructive detection, localization and high spatial resolution of light-emitting centers originating from different imperfections in the cell. We have found that the emission from these pre-breakdown sites exhibits unique thermal characteristics. As a consequence, the thermal characteristics can distinguish among several kinds of imperfections in monocrystalline silicon solar cells. (C) 2011 Elsevier B.V. All rights reserved.