Polymer-Based Rear Side Light Trapping Structures for Silicon-Based Tandem Solar Cells

In this study, we investigated potential improvements for polymeric rear side light trapping structures in silicon-based tandem solar cells. The focus is on process variations and a drastic simplification for the realization. We earlier demonstrated an enhancement of the near IR response of a silicon bottom solar cell by such a rear side structure leading to an efficiency gain of 1.9 % absolute to a 33.3 % efficient device. This structure was realized using nanoimprint lithography for the patterning of a diffraction grating together with plasma etching to remove polymeric residues prior to the silver metallization. Now, we investigated the possibility of substituting the plasma etching process by a wet chemical etching process. While external quantum efficiency (EQE) measurements showed the same excellent light trapping, plasma induced damages can be avoided this way. Furthermore, we investigated gold and aluminum as potential alternative to silver as rear side metallization. Among these, silver was found to be the most promising material. Besides these process variations, we investigated a self-organization process leading to a pseudo-periodic structure with a well-defined and narrow size pitch distribution of polymeric features (self-organized photonic contact). This is based on the phase separation of two immiscible polymers in a solution that is spin coated. After the spin coating and the simultaneous phase separation, one polymer can selectively be removed. EQE measurements of solar cells with such structures showed that very similar results can be expected for this bottom-up process as for the structures realized via NIL. Thus, a photonic light trapping structure can be realized without applying any lithography.