In this work, we focus on the comprehension and optimization of the rear side of n-type amorphous/crystalline (a-Si:H/c-Si) heterojunction solar cells. The back amorphous stack and the role of the heterointerface have been investigated in order to elucidate the basic mechanisms governing device performance. Thus, thin amorphous silicon films with different phosphorous contents have been deposited by PECVD and extensively characterised (optical, electrical, and structural properties have been studied). Moreover, the passivation and the interface properties of such different a-Si:H thin films on crystalline silicon surface have been also investigated by Quasi-Steady-State Photoconductance (QSSPC) measurements. The effect of the doping content of the amorphous back-surface-field was deeply analysed and simulated in order to explain the observed tendencies on solar cell precursors. The influence of defect density and activation energy on the carrier transport properties varying with the doping of thin a-Si:H layers has been widely investigated. Furthermore, optimized heterojunction solar cells have been fabricated and an efficiency of 20.5% on n-type 148 cm2 silicon wafer has been achieved.