In this work, we present the results of the electrical characterization and the simulations of heterojunctions between transparent conductive oxide (TCO) and hydrogenated amorphous silicon (a-Si: H), with the aim to improve the performance of pin diode a-Si: H thin film solar cells. In an a-Si: H solar cell the optical and electrical performance of the TCO play an important role. For example, the effect due to the distributed resistance of the TCO causes the performance degradation of the solar cell [1, 2]. One of the most critical parts of a homojunction a-Si: H solar cells is the interface between TCO and p-type a-Si: H. Structures composed by molybdenum (Mo), p-type a-Si: H and SnO2: F were fabricated by STMicroelectronics (Catania, Italy) in order to study the properties of this heterojunction. Capacitance-voltage (CV) and current-voltage measurements (IV) at different temperatures were performed and a capacitance model was proposed. Firstly, we checked the reproducibility and the uniformity of the investigated heterostructures. The IV curves are slightly asymmetric and show two regions: a linear region (ohmic) up to voltages| V|= 0.1 V and a superlinear region (power-law) for| V|> 0.1 V [2]. Transport mechanisms such as diffusion, recombination and SCLC were tested without success in high voltage region. Besides, the Mo/p-type a-Si: H contact can not be considered ohmic because the symmetry in the IV suggested that the transport mechanism is the same for both positive and negative voltages. Hence, the current is a reverse current of two diode back to back connected. We also simulate the transport mechanism by using the 1D simulator …