Improved Light Incoupling in Planar Solar Cells via Improved Texture Morphology of PDMS Scattering Layer

However, the front (sunward-facing) surface of a solar cell is a source of reflectance loss due to refractive-index mismatch between the absorber material and air, or the absorber material and the encapsulant material in the case of modules. This problem is remedied by texturing the absorber material for better in-coupling of light. For example, monocrystalline silicon solar cells are textured in an alkaline bath to etch random pyramids in their surfaces, while multicrystalline silicon solar cells are etched in an acid bath to etch "spherical caps" in their surfaces. Polydimethylsiloxane (PDMS) scattering layers have been shown to be effective in reducing front-surface reflectance when applied to the front of planar silicon and perovskite solar cells. However, these layers do not greatly enhance the infrared response of the planar silicon cells, indicating that they provide little trapping of escaping light. Light trapping is a phenomenon caused by surface textures, and generally, solar absorbers are double-side textured with similar texture at the front and rear. PDMS scattering layers applied to planar absorbers, however, open this degree of freedom and their texture can be tuned to better reflect escaping light back into the absorber. Optical simulations were performed to probe the dependence of the performance of planar silicon solar cells on the morphology of PDMS scattering layers applied to their front surfaces. The simulations were conducted using the Module Ray Tracer from PV Lighthouse, which combines Monte-Carlo-based ray tracing with thin-film optics. The software calculates the front-surface reflectance, escape reflectance, and absorptance in the cell absorber, as well as the absorptance in all other layers of the device. The complex refractive indices for the doped and intrinsic amorphous silicon layers, as well as for the front and rear ITO layers, were characterized using spectroscopic ellipsometry and taken from the literature. The results of the simulations show that the base angle of the pyramids in the PDMS scattering layer has a significant influence on the performance of the planar silicon solar cells. The simulations were conducted for a Flat/Flat device with three values of pyramid base angle: 40, 46, and 56°. The results show that 46° pyramids have very slightly higher absorptance at long wavelengths (1000-1200 nm), but the effect is minute. However, there is an appreciable decrease in absorptance for 40° pyramids between 500 and 950 nm, and a smaller decrease for 56° pyramids below 600 nm. The short-circuit current density (Jsc) for both the PDMS/Flat/Flat and PDMS/Flat/Tex devices shows that 0.2 mA/cm2 or 0.3 mA/cm2 can be gained, respectively, by reducing the pyramid base angle to 46° from the typical case for silicon. The results of the simulations suggest that the PDMS pyramid base angle has little influence on light trapping within the wafer. However, the simulations also show that the base angle of the pyramids in the PDMS scattering layer has a significant influence on the performance of the planar silicon solar cells. The results indicate that a pyramid base angle of 46° gives the highest short-circuit current density, and that reducing the pyramid base angle to 46° from the typical case for silicon can gain 0.2 mA/cm2 or 0.3 mA/cm2, respectively.