The causes of degradation of electrical characteristics, which affect the energy conversion efficiency of solar cells, were evaluated using a small p-n diode array fabricated on a multi-crystalline silicon (mc-Si) substrate. Many of the current-voltage (I-V) characteristics of diodes with grain boundaries deteriorated. However, some deteriorated diodes without grain boundaries were also found. We especially evaluated the diodes to determine the causes of degradation by electron-beam-induced current (EBIC) imaging, photoluminescence (PL) mapping, transmission electron microscopy (TEM), and electron backscatter diffractometry (EBSD). As a result, it was clarified that the electrical characteristics severely deteriorated with the existence of small-angle grain boundaries. Mc-Si solar cell efficiency was significantly affected by not only obvious grain boundaries but also small-angle grain boundaries consisting of periodically aligned dislocations and possibly metallic and oxygen impurities.