Study of the Degradation of p–n Diode Characteristics Caused by Small-Angle Grain Boundaries in Multi-Crystalline Silicon Substrate for Solar Cells

Abstract

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.