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We have studied the hole-injection mechanisms of two types of Si anodes, polycrystalline Si (poly-Si) film and crystal Si (c-Si) wafer anodes, in organic light emitting diodes (OLEDs) and those of OLEDs with indium tin oxide anodes. It was found that the hole-injection mechanisms of these devices are very different from each other. The hole injection of the c-Si anode obeys the classic Richardson thermionic emission model and that of the poly-Si film anode agrees well with the modified thermionic emission model (Scott and Malliaras 1999 Chem. Phys. Lett. 299 115-9). When the c-Si anode is covered with a thin SiO2 layer, its hole injection changes to obey the modified thermionic emission model.