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Photoluminescence Spectroscopy for Understanding Light Management in Solar Cells

The purpose of applying a light management structure in a solar cell is to absorb the largest possible amount of incident radiation in the active layer. Independent of the material, thickness and structure of the solar cell, the fundamental approaches for light management are: A) reducing the proportion of the light being reflected out at the front surface and B) increasing the path length of the light within the absorbing substrate. Therefore, characterizing and optimizing the light management technique is essential for further improving solar cell efficiency. In this thesis, a method based on the generalized Planck’s law of radiation is developed to extract the band-to-band absorptance from the photoluminescence spectra of semiconductor materials. Unlike the traditional way of obtaining absorptance from the reflection and transmission measurement, this method is only sensitive to absorbed photons that can generate electron-hole pairs. Therefore, the parasitic losses such as free carrier absorption and the absorption in non-active layers can be automatically eliminated. With the extracted band-to-band absorptance, the implied photo-current of the sample is accurately estimated without the need of forming a p-n junction. By comparing the band-to-band absorptance of silicon wafers with and without light trapping structure, the optical enhancement is rapidly accessed. Using this method, a wide range of light trapping structures are studied on crystalline silicon wafers to improve the photo-current generation. Self-assembled plasmonic Ag particles (AgNP) together with a dielectric based diffuse reflector (DR) are applied on the rear side of silicon wafers to provide excellent optical enhancement without sacrificing the electrical performance of the device. AgNP have proven to scatter the light at wide angles, so that total internal reflection occurs and the light can be trapped in the solar cell. Dielectric based diffuse reflectors have the advantage of high reflectivity and low absorption loss compared to metal reflectors. The combination of DR and AgNP provides light trapping performance (62% of Lambertian enhancement) which is comparable with inverted pyramids (67% of Lambertian enhancement) on a 200 μm thick silicon wafer. By applying AgNP in the back of a silicon wafer with an interdigitated back contact (IBC) structure, a maximum of 53% of fraction of the Lambertian enhancement is achieved with an optimized capping layer. For a standard IBC cell with AgNP embedded in the rear side, the short circuit current density is estimated to enhance by 18% in the spectral range from 1000 nm to 1200 nm. Texturing is still the most widely applied light management technique in the c-Si solar cell industry. The textured surface produces broad-band anti-reflection properties as well as effective light trapping to the solar cell. We extend the application of the PL technique to evaluate the optical performance of textured samples. Several structures including reactive ion etched textures (RIE), metal-assisted textures (MET)