Technical Field
The present invention relates to photovoltaic devices and fabrication methods, and more particularly to a front surface field junction for interdigitated back contact devices forming a floating junction.
Description of the Related Art
Inter-digitated back-contact (IBC) solar cell structures are of interest for increasing short circuit current (Jsc) (and therefore efficiency) of the solar cells by forming both an emitter junction, a back surface field (BSF) junction and the associated metallization on a backside of the solar cell. This permits sunlight to enter from a front side with minimum shadowing loss. In the case of heterojunction (HJ) solar cells, hydrogenated amorphous silicon (a-Si:H) containing material may be employed to form a front surface field (FSF) in addition to forming the emitter and/or BSF junctions. Such a FSF junction is typically formed by using an intrinsic (i) a-Si:H layer to passivate the front surface of a crystalline silicon (c-Si) substrate, followed by a doped a-Si:H layer having the same conductivity type as that of the c-Si substrate. In this way, the intrinsic layer is sandwiched between the c-Si substrate and the doped a-Si layer, and the c-Si substrate and the doped a-Si layer have a same dopant conductivity type.
The doped a-Si layer provides an electric field to repel minority carriers generated in the c-Si substrate from the front surface of the c-Si substrate thus reducing carrier recombination at the front surface of the c-Si substrate. In some instances, the doped a-Si:H containing material may be omitted. In such cases, the presence of the intrinsic and/or doped a-Si:H containing material at the front surface results in some absorption loss reducing the short circuit current of the solar cell accordingly. This is because a-Si:H and related alloys are direct bandgap materials, and, therefore, light absorption in these materials is strong. Due to the presence of a large density of defects in these materials, diffusion length of minority carriers in these materials is small and therefore a significant portion of the photogenerated carriers in these materials is lost by recombination prior to reaching the c-Si substrate.