Rear side sphere gratings for improved light trapping in crystalline silicon single junction and silicon-based tandem solar cells

Abstract

Rear side hexagonal sphere gratings are demonstrated as diffractive structures that enhance the light path length in the near infrared, where crystalline silicon solar cells suffer from weak absorption. Moreover, the rear side sphere grating can be added behind a solar cell with flat rear surface, giving an “electrically flat but optically rough” device with high efficiency potential. Here, a thin passivating tunnel-contact layer electrically separates the sphere grating from the cell's base. Solar cells with the rear side sphere grating have obtained a VOC of up to 710 mV and a FF of up to 81.9%. External quantum efficiency measurements show a current density gain of 1.4 mA/cm2 due to the sphere grating. This leads to an overall efficiency of up to 22.1% for the solar cells with planar front side and rear side sphere grating. Estimates for perovskite–silicon and III/V-silicon tandem devices show that the efficiency of tandems can be enhanced by up to 2.4% absolute with a sphere grating on the rear side. Thus, sphere gratings could improve Si-based tandem devices that are limited due to a low current in the Si bottom cell

Wave optical simulation of the light trapping properties of black silicon surface textures

“…To further increase absorption, front-side textures and rear-side structures can be combined. Possible rear side structures could be hexagonal sphere gratings [16] nanoimprinted diffraction gratings [17] or random pyramids combined with a distributed Bragg reflector [15].…”

Hybrid dielectric light trapping designs for thin-film CdZnTe/Si tandem cells

“…To enhance light trapping in c-Si significantly further, we also would like to add a back reflector. Various designs have been suggested for a back reflector, including randomly textured [38], periodic [39], sphere gratings [40] and plasmonic back reflectors [41,42]. However, metallic grating structures have parasitic absorption associated with an undesired excitation of surface plasmon polaritons (SPPs), propagating at the metal/dielectric interface [43].…”

Efficiency increase of crystalline silicon solar cells with nanoimprinted rear side gratings for enhanced light trapping

“…Hence, a planarization of the structure with a low refractive index material is usually necessary, which increases the number of process steps and complicates the contact formation process [30,33]. Recently, two successful integrations of diffractive rear side structures (sphere gratings and binary, nanoimprinted gratings like in this work) into thick wafer based silicon solar cells have been reported [34,35]. In both works, short circuit current density gains between 1 and 1.5 mA/cm 2 250 mm, respectively.…”

“…In terms of reaching highest voltages while also reaching high current densities, this concept might be promising to reach ultimate efficiencies in the long term. In contrast to the sphere grating concept presented in [34], which follows a "bottom-up" approach for the structure fabrication, the concept presented in this paper is a "topdown" approach that requires a mastering but enables the realization of a large variety of structures including additional features, for example for contact formation as presented in [36]. In addition to the results published in [35], we also combine this diffractive rear side structure with a pyramidal front texture (Section 3.2) and investigate the effect of the grating also for thinner wafer based solar cells (150 and 100 mm cell thickness, Section 3.1), where the light trapping effect is more pronounced and where the optimum thickness for ultimately efficient silicon solar cells lies [37].…”

“…The rear side metallization has been realized via a laser based foil process (Foil-LFC) [38] that does not require any planarization step due to an optically beneficial air gap between the grating structure and the aluminium foil. Additionally, the Foil-LFC allows for a refractive index contrast between the a-Si grating and air that is higher than in the case of sphere gratings [34] and thus enables very high diffraction efficiencies. In addition to the JV-characteristics, we investigated the wavelength dependent behavior of the cells via external quantum efficiency and reflectance measurements in order to reveal the light trapping effects due to the diffractive rear side grating.…”