Sub-Bandgap Luminescence from Doped Polycrystalline and Amorphous Silicon Films and Its Application to Understanding Passivating-Contact Solar Cells

We report luminescence phenomena from doped polycrystalline silicon (poly-Si) films and their applications to study carrier transport properties in passivating-contact solar cells. Low-temperature luminescence spectra emitted from doped poly-Si layers are found to be very broad and stretched from the crystalline silicon (c-Si) luminescence peak to significantly lower energies. This suggests that these layers contain radiative defect levels whose energies are continuously distributed from the band edges to deep levels in the poly-Si bandgap. Moreover, photoinduced carriers inside poly-Si layers are found to be completely blocked by an ultrathin SiOx interlayer (∼1.3 nm). This demonstrates that there is no free-carrier coupling from poly-Si layers in practical passivating-contact solar cells. Finally, we demonstrate that the same principle can be applied to study carrier transport properties in hydrogenated amorphous silicon films.This work has been supported by the Australian Renewable Energy Agency (ARENA) through Research Grant RND017 and the U.S. Department of Energy under Contract DE-AC36-08GO28308 with the National Renewable Energy Laboratory (NREL). The authors acknowledge the facility and technical support support from the Australian National Fabrication Facility (ANFF) ACT Node and the Centre for Advanced Microscopy (CAM) at the ANU. H.T.N. acknowledges the fellowship support from the Australian Centre for Advanced Photovoltaics (ACAP) and the ACAP Collaboration Grant. M.T. acknowledges the support from the Australian Government Research Training Program (RTP) Scholarship