We study the effect on various properties of varying the intrinsic layer (i-layer) thickness of amorphous/ crystalline silicon heterojunction (SHJ) solar cells. Double-side monocrystalline silicon (c-Si) heterojunction solar cells are made using hot-wire chemical vapor deposition on high-lifetime n-type Czochralski wafers. We fabricate a series of SHJ solar cells with the amorphous silicon (a-Si:H) i-layer thickness at the front emitter varying from 3.2 nm (0.8xi) to similar to 96 nm (24xi). Our optimized i-layer thickness is about 4 nm (1xi). Our reference cell (1xi) performance has an efficiency of 17.1% with open-circuit voltage (V(oc)) of 684 mV, fill factor (FF) of 76%, and short-circuit current density (J(sc)) of 33.1 mA/cm(2). With an increase of i-layer thickness, V(oc) changes little, whereas the FF falls significantly after 12 nm (3xi) of i-layer. Transient capacitance measurements are used to probe the effect of the potential barrier at the n-type c-Si/a-Si interface on minority-carrier collection. We show that hole transport through the i-layer is field-driven transport rather than tunneling. (C) 2011 Published by Elsevier B.V.