Transition metal oxides (TMOs) are abundant, non-toxic, and inexpensive. It can be deposited by thermal evaporation. Charge carriers can pass through TMO layer. It is used as permeable transport layer.

For electron transport layer, TiO₂ or ZnO is used, and for hole transport layer, MoO₃ or WO₃ is used. In the a-Si:H/n-type c-Si HIT solar cells, p-type a-Si:H is used as emitter. Here, in TMO/n- c-Si solar cells, instead of p-type a-Si:H emitter, TMO is used as hole transport layer and therefore there is no p-n junction in the TMO/c-Si solar cell. TMO role is to pull out holes from n-type c-Si.

Fermi level in MoO₃ is much lower than that of c-Si. When MoO₃ and n-type c-Si forms a junction, electrons from n-type c-Si diffuse to MoO₃, and hence MoO₃ Fermi level comes up and c-Si Fermi level comes down and Fermi levels on both sides align with each other.

Band bending occurs at c-Si side and no band bending occurs at the MoO₃ side because diffused electrons stay within few nanometers and band bending is negligible. Diffused electrons stay in a narrow range because MoO₃ consists of high-level donor states. Entire deletion layer is on the c- Si side but not on the TMO side since there is no band bending in TMO.