Towards a Better Understanding of Silicon Heterojunction Solar Cells

1. Introduction Semiconducting heterostructures increasingly attract attention for electronic junction formation in crystalline silicon (c-Si) wafer-based solar cells. A key point of such a device is the displacement of highly recombination-active (ohmic) contacts from the crystalline surface by insertion of a film with wide bandgap. To reach the full device potential, the heterointerface state density should be minimal. Practically, hydrogenated amorphous silicon (a-Si:H) films of only a few nanometer thin are appealing candidates for this: Their bandgap is wider than that of c-Si and, when intrinsic, such films can reduce the c-Si surface state density by hydrogenation. In addition, these films can be doped relatively easily, either nor p-type, allowing for the fabrication of electronically abrupt p-n and low-high heterojunctions (HJ). For such films, however, to simultaneously fulfill both the surface-passivation and the doping requirements has been proven to be challenging. Hence, typically, a few-nanometer thin intrinsic buffer layer is inserted between the c-Si surface and the doped a-Si:H films for device fabrication. For HJ solar cells featuring such stacked film structures, impressive large-area (> 100 cm 2) energy-conversion efficiencies (~23%) have been reported by Sanyo, Japan [1]. Despite this result, the fundamental origin of the poor passivation of the doped a-Si:H/c-Si interface is not yet fully understood. In this article, we focus on the role that such buffer layer plays for the interface passivation. For this, we show that post-deposition annealing offers in a straightforward way a single parameter to vary both electronic and material properties of the samples under study.