Light Trapping in High-Efficiency Crystalline Silicon Solar Cells

Abstract

Silicon solar cells require light trapping structures, because silicon as indirect semiconductor only weakly absorbs light, especially in the near infrared (NIR) close to the bandgap of silicon. In this work, such light trapping structures are theoretically investigated, experimentally developed and fabricated for both, the front of solar cells, where they have to fulfill also antireflective properties, and the rear. To increase the overall efficiency of a complete device, in both cases not only the optical but also the electrical properties were considered. The matrix-based simulation formalism Optical Properties of Textured Optical Sheets (OPTOS) was co-developed within this work. OPTOS allows for an efficient coupling of different simulation techniques for front and rear textures, especially the combination of structures operating in different optical regimes. Using OPTOS, the combination of various front and rear textures was investigated, including random and inverted pyramids at the front and diffractive gratings at the rear. Both, theoretically and experimentally, planar multilayer antireflection coatings and black silicon were investigated for the front and diffuse reflectors and diffractive gratings for the rear. With two types of gratings – binary, nanoimprinted crossed gratings and hexagonal sphere gratings – the short circuit current gains predicted by simulations were demonstrated in high-efficiency crystalline silicon solar cells. Due to binary gratings with a period of 1 µm, a short circuit current density gain between 1.2 and 1.8 mA/cm2 was measured on p-type solar cells with thicknesses between 250 and 100 µm and planar front surfaces. Due to sphere gratings, a gain of 1.4 mA/cm2 was realized for 200 µm thick n-type solar cells with planar front surface. The combination of the passivated contact TOPCon with the hexagonal sphere gratings allowed for an open circuit voltage of 710 mV and an overall efficiency of 22.1 % for cells with planar front and sphere grating rear. Especially for silicon-based tandem solar cells, these light trapping structures, which lead to efficient light trapping in combination with planar front surfaces, are very promising.