Development of Silicon-rich poly-Silicon Carbide passivating contacts for solar cells

Abstract

Carrier-selective passivating contacts (CSPC) are one of the most promising silicon solar cell contact structures. These contacts enable the passivation of the silicon surface as well as a high carrier selectivity. So far excellent results have been demonstrated with the use of poly-Si in stack with SiO2. In this work, we optimize an alternative stack, based on carbon alloyed poly-Si (poly-SiCx). Alloying with carbon provides wet-chemical stability, the material becomes resilient to blistering and the bandgap can be tuned depending on the carbon content. This optimized poly-SiCx is implemented as the rear contact of a solar cell. We investigate doped poly-Si alloyed layers with different carbon contents. Structure characterization reveals that carbon incorporation hinders the crystallization of the material. In addition, alloying with carbon decreases the absorption coefficient of the material in the as-deposited state (amorphous), but following the thermal treatment higher carbon contents lead to a higher absorption coefficient. Finally, we observe that higher carbon alloying reduces the conductivity of the poly-SiCx by several orders of magnitude, but the optimized material demonstrates a dark conductivity of 0.006 S/cm, which is 3 orders of magnitude higher than the amorphous one. We optimize the passivation quality of (p)poly-SiCx in symmetrical samples by inserting an (i)a-Si interlayer between the SiO2 and the doped layer using different deposition methods (PECVD/LPCVD). With the optimization of the gas flow ratio, high-temperature annealing and hydrogenation, we obtain a high passivation quality (iVoc of 709 mV and Jo of 10.5 fA/cm2) for the optimum LPCVD deposited interlayer, while the optimum PECVD deposited interlayer demonstrates a signi cantly lower passivation quality (iVoc of 684 mV and Jo of 34.7 fA/cm2). The hydrogenation is performed with the deposition of a SiNx capping layer followed by Forming Gas Annealing at 400°C for 30 minutes. The potential of the (p)poly-SiCx passivating contact is investigated by the application on the rear side of a solar cell. We fabricate initially double-side polished cells with the champion cell achieving an active area efficiency (ηact ) of 15.68%, featuring a Voc of 692 mV, FF of 74.7% and EQE current (Jsc,EQE) of 30.35 mA/cm2. Finally, we also investigate the potential on front side textured cells, obtaining the champion cell with an ηact of 16.80%, maintaining the FF at 74.7%, a lower Voc of 677 mV and a Jsc,EQE at 33.22 mA/cm2 . For all cells the parasitic absorption at the front (n)poly-SiCx contact remains high, due to the non-optimized thickness of the aforementioned contact.