This work presents our progress in designing the rear emitter of a-Si:H / c-Si (n) heterojuncti ons solar cells (Si-HJ). We study the emitter saturation curr ent density (J 0e ) of p-type a-Si:H layers and show that it greatly depends on the layer thickness and conductivity. Di fferent p-type a-Si:H emitters are tested experimen tally on Rear Emitter (RE) as well as Interdigitated Back Contact (I BC) Si-HJ devices. A low conductivity (1.4x10 -6 S.cm -1 ) layer allows the better V oc values about 650 mV but causes resistive losses on both types of solar cells. For the IBC devices, a low emitter contact fraction induces not only fill factor (FF) losses but also a decrease o f the short circuit current (J sc ) value. By optimizing the rear side geometry an eff iciency of 12.7% is achieved for the Si-HJ IBC structure on 25 cm 2 n-type substrate.