  Graphene has shown great application opportunities in future nanoelectronic
devices due to its outstanding electronic properties. Moreover, its impressive
optical properties have been attracting the interest of researchers, and,
recently, the photovoltaic effects of a heterojunction structure embedded with
few layer graphene (FLG) have been demonstrated. Here, we report the
photovoltaic response of graphene-semiconductor junctions and the controlled
open-circuit voltage (Voc) with varying numbers of graphene layers. After
unavoidably adsorbed contaminants were removed from the FLGs, by means of in
situ annealing, prepared by layer-by-layer transfer of the chemically grown
graphene layer, the work functions of FLGs showed a sequential increase as the
graphene layers increase, despite of random interlayer-stacking, resulting in
the modulation of photovoltaic behaviors of FLGs/Si interfaces. The surface
photovoltaic effects observed here show an electronic realignment in the depth
direction in the FLG heterojunction systems, indicating future potential toward
solar devices utilizing the excellent transparency and flexibility of FLG.
