Applications of Spectrally-Resolved Photoluminescence in Silicon Photovoltaics

Abstract

In broad terms, this thesis is devoted to measuring and interpreting the photoluminescence spectra emitted from different structures in crystalline silicon wafers and solar cells. Based on the knowledge accumulated, it also establishes a variety of applications of photoluminescence spectroscopy in silicon photovoltaics. The thesis may be divided into 3 main categories: band-to-band luminescence from wafers, deep-level luminescence from defects and impurities, and composite luminescence from different structures and layers in solar cells.

First, this thesis utilizes band-to-band photoluminescence spectra emitted from planar silicon wafers to determine the values of the band-to-band absorption coefficient and the radiative recombination coefficient as a function of temperature with high precision. Parameterizations of these two coefficients are established to allow convenient calculations. Based on the newly established temperature data, the impacts of surface geometries and excess carrier profiles on luminescence spectra emitted from various silicon wafers are investigated via both modeling and experiments as a function of temperature. The results suggest that, the accuracy of many photoluminescence-based techniques, established mainly at room temperature in the literature, can be further improved by performing the measurements at higher temperatures due to the increasing impacts of surface reflectivities and excess carrier profiles on luminescence spectra with rising temperatures. These applications highlight the significance of the established data of the two coefficients for spectral fitting techniques.

Next, the thesis investigates the deep-level luminescence from defects and impurities distributed around sub-grain boundaries in multicrystalline silicon wafers. The thesis shows that, the dislocations at sub-grain boundaries and the defects and impurities trapped around the dislocations emit separate luminescence peaks at low temperatures. The luminescence intensity of the trapped defects and impurities is found to be altered significantly after phosphorus gettering, whereas the dislocation luminescence is not changed throughout different solar cell processing steps. Also, the trapped defects and impurities are found to be preferentially distributed on one side of the sub-grain boundaries due to the asymmetric distribution of their luminescence intensity across the sub-grain boundaries. In addition, the thesis also demonstrates that the damage induced by laser doping is related to dislocations, since its deep-level luminescence spectrum has similar properties to those emitted from dislocations in multicrystalline silicon wafers. The interface between the laser-doped and un-doped regions is found to contain more damage than the laser-doped regions.

Furthermore, the thesis reports a new photoluminescence-based method to separate the luminescence signatures from different layers and structures in a single silicon substrate, courtesy of the well-resolved luminescence peaks at low temperatures from different layers. In particular, the technique is applied to characterize the doping level of both locally-diffused and laser-doped regions on various silicon solar cells and cell precursors, utilizing band-gap narrowing effects in heavily-doped silicon. The results show that, the interface between the laser-doped and un-doped regions is much more heavily-doped that the doped regions. In addition, the technique is also applied to evaluate and the parasitic absorption of different surface passivation films on finished solar cells, due to the correlation between the sub band-gap luminescence intensity from these passivation films and the optical absorption in the films. The technique is contactless and nondestructive, requires minimal sample preparation, and provides micron-scale spatial resolutions.

Finally, the thesis combines the advantages of spectrally-resolved photoluminescence (PLS) and photoluminescence excitation spectroscopy (PLE) to develop a PLS-PLE-combined technique for characterizing wafers and solar cells. In particular, the entire photoluminescence spectrum from a silicon wafer or solar cell is captured and monitored while the excitation energy is varied. This technique allows us to quantitatively evaluate both the doping level and the junction depth of various diffused silicon wafers, the defects induced by the post-diffusion thermal treatment at different depths below the wafer surface, and the enhanced diffusion at grain boundaries and sub-grain boundaries in multicrystalline silicon wafers. The results show that, the enhanced diffusion happens at both grain boundaries and sub-grain boundaries.