Photoluminescence Spectroscopy for Understanding Light Management in Solar Cells

Abstract

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 mm 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) and random pyramid textures (RAN) are experimentally evaluated with the photoluminescence technique. By fabricating a silicon wafer with RIE and RAN textures on the front and rear side respectively, we demonstrate a structure with near ideal absorption in the ultraviolet and visible spectrum and a light trapping efficiency of 55% in the near infrared region of the solar spectrum. Using an analytical model with independent angular distribution parameters on both surfaces, we carry out a quantitative analysis on the impact of front reflection, rear absorption and the angular distribution on the implied current generation of these silicon samples. With the origins of the optical loss of these light management structures revealed, the effective approach for reaching maximum photo-current for high efficiency silicon solar cells is discussed. We conclude that the non-perfect angular distribution is the main limitation for approaching Lambertian light trapping in high efficiency silicon solar cells. Dielectric based diffuse reflectors have excellent optical properties and can be applied on a wide range of solar cells. A diffuse reflector prepared by Snow Globe coating is applied on a-Si:H/mc-Si:H tandem solar cell. The reflector has close to 100% reflectance over a spectral range of 500-1300 nm which indicates much lower parasitic optical losses compared to the standard textured ZnO:Al/Ag reflector. The application of DR avoids the localized surface plasmon and propagating surface plasmon resonances existing on randomly textured ZnO:Al/Ag back contacts. Both of the resonances can couple with the incident light and introduce significant amount of parasitic absorption and thereby reducing the overall cell performance. By replacing ZnO:Al/Ag with SGC reflector on tandem thin film silicon solar cells as a rear reflector, the short circuit current of the bottom solar cell is enhanced and the overall cell efficiency is improved from 10.2% to 10.4%. Organic/inorganic hybrid perovskite material has the potential for being a lowcost and high efficiency photovoltaic technology. The knowledge of absorption coefficient of such novel absorber materials is essential in understanding the extent to which perovskite solar cells may suffer from parasitic absorption. The fundamental relationship between band-to-band absorptance and photoluminescence is used to measure the absorption coefficient absorption coefficient of organic-inorganic hybrid perovskite methylammonium lead iodide (CH3NH3PbI3) films from 675 nm to 1400 nm. Unlike other methods used to extract the absorption coefficient, photoluminescence is only affected by band-to-band absorption and is capable of detecting absorption events at very low energy levels. Absorption coefficients as low as 10e-14 cm-1 are detected at room temperature for long wavelengths, which is 14 orders of magnitude lower than reported values at shorter wavelengths. The temperature dependence of absorption coefficient is calculated from the photoluminescence spectra of CH3NH3PbI3 in the temperature range 80-360K with 10K intervals. Based on the temperature dependent absorption coefficient, a polynomial parameterization describing the product of the radiative recombination coefficient and square of the intrinsic carrier density is also presented. This thesis focuses on understanding and improving the light management of solar cells. The method of extracting band-to-band absorptance from photoluminescence spectra is used to compare a wide range of light trapping structures on silicon wafers and to extract the absorption coefficient of perovskite film at very low energy levels. Show more