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Crystalline silicon solar cells with enhanced light trapping via rear side diffraction grating
Effective light trapping concepts are increasingly relevant for thinner crystalline silicon solar cells. Diffractive rear side structures can be designed to enhance optical light path lengths in the weakly absorbed infrared wavelength range. However, because of the rather difficult passivation of these nano-scale structures and possible plasma induced damages, the optical gain has so far not been shown on final device level for wafer-based crystalline silicon solar cells in combination with a high open circuit voltage.
In this work, we fabricated and characterized silicon solar cells with an optically active layer at the rear side which is electrically decoupled from the silicon bulk by a thin, planar passivation layer. A binary diffraction grating was realized on the rear side via Nanoimprint Lithography (NIL) and plasma etching. For the rear side metallization a laser based foil process and photolithographically opened point contacts with evaporated aluminum were used as two alternative process routes. The best cell shows an EQE increase of up to 29 % absolute at 1100 nm compared to planar reference cells. This corresponds to a total Jsc gain of 1.2 mA/cm2 and a best cell efficiency of 19.4 % (Voc: 683.1 mV, Jsc: 35.4 mA/cm2, FF: 81.0 %) with planar front side and SiO2 antireflection coating.
Optical simulations, which fit well to the experimental data, show that cell efficiencies of 20.8 % and 21.3 % can be reached with a planar front by using an optimized single layer (SiNxOy) or double layer (MgF, SiNx) ARC, respectively. By calculating Jsc for different cell thicknesses it is shown that a 100 μm thick solar cell incorporating the fabricated diffractive grating is expected to exhibit a higher Jsc than a planar cell with 250 μm thickness.