Indium Tin Oxide (ITO) layers in silicon hetero junction solar cells change their electrical and optical properties when exposed to temperature treatments. Hydrogen which effuses from underlying amorphous silicon layers is identified to dope the ITO layer. This leads to an additional increase in conductivity. In this way an almost isolating ITO can become degenerately doped through temperature treatments. The resulting carrier density in the range of 10(20) cm(-3) leads to a substantial increase in free carrier absorption, which in turn leads to an increased parasitic absorption in the cell device. Thus hydrogen effusion in silicon hetero-junction (SHJ) solar cells does not only affect the degradation of amorphous silicon (a-Si:H) passivation of crystalline silicon (c-Si), but also the electrical and optical properties of both front and back ITO layers. This leads to the further design rule for SHJ solar cells, meaning that ITO properties have to be optimized in the state after modification during temperature treatment. (C) 2016 Elsevier B.V. All rights reserved.