Applications of Spectrally-Resolved Photoluminescence for Silicon Solar Cells
Creators: name: Wu, Huiting
Publisher: name: The Australian National University
Year: 2021

description: This thesis aims to improve our understanding of the photoluminescence (PL) spectra from the heavily doped region in crystalline silicon materials in solar cells. We demonstrate in detail the correlations of the PL spectral shapes with doping profiles in thermal diffusions under different solar cell fabrication structures. The passivation properties of amorphous silicon film on top of crystalline silicon material are also examined. The advantages offered by PL spectroscopy, such as non-destructive nature of measurements, micron-scale mapping capabilities, as well as time-resolved features are demonstrated abundantly in this thesis. Following are the break downs. Firstly, when measured at 79 K, the PL spectra from silicon wafers containing a diffused heavily doped layer exhibit a second peak due to band-gap narrowing in the diffused region. The work aims to decompose this peak into components arising from the various doping concentrations within the diffused layer. Whilst the peak position of silicon band to band PL spectra changes significantly with the doping concentration in silicon, the shape of the spectra also varies strongly with doping concentrations due to broadening effects of band-filling and band-tail state. By measuring PL spectra on a range of uniformly heavily doped wafers, we show that these changes in spectral position and shape can be accurately modelled for heavy dopant concentrations using simple parameterisations, with minimal impact of variations in excitation intensity or injection level. This allows the PL spectra for a range of arbitrary doping concentrations to be reconstructed. In the following project, microPL spectroscopy is applied to characterise the shallow phosphorus diffusions occurring below polysilicon passivating contacts for silicon solar cells, courtesy of the bandgap narrowing effect in the heavily doped region. We present the PL spectra from samples of various diffusion profiles, achieved by changing the phosphorus diffusion temperature and oxide interlayer thickness. The PL peak from the shallow phosphorus diffusion region, when normalised against the substrate crystalline silicon peak, changes in intensity in accordance with the dopant profiles. Micron-scale line scanning across localised polysilicon structures demonstrates that the method can identify the presence of the underlying diffused regions with a spatial resolution of several microns. The next work examines the PL peaks from thermally boron diffused crystalline silicon wafers and explores the link between the intensities of the diffusion peak and the sample dopant profiles. It is observed that when normalised against the crystalline silicon substrate peak, the diffusion peak intensities are well correlated with the integrated dopant profile of the samples. The application is further developed by reducing the integration by a product of surface dopant concentrations and a depth factor where the qualities of fits remain uncompromised. And lastly, we examine correlations between the recombination lifetime and hydrogen content of hydrogenated amorphous silicon films (a-Si:H), and the surface passivation afforded by such films when deposited on crystalline silicon wafers, during annealing at 350-500 degrees Celsius. Our results show that, as the annealing duration increases, both the a-Si:H recombination lifetime and the surface recombination velocity evolve at a similar rate to the hydrogen concentration. This suggests the loss of hydrogen during annealing is the direct cause of the reduction in the a-Si:H film lifetime. We calculated the activation energy of the surface de-passivation reaction during annealing to be 0.62 eV, which suggests the de-passivation reaction is limited by the migration of hydrogen within the film, without significant hydrogen trapping. Secondary ion mass spectrometry further demonstrates the loss of hydrogen across the film thickness during annealing.