Hydrogen thermal activation of defects enabling firing stable Poly-Si based passivating contacts for TOPCon solar cells

Passivating contacts based on poly-Si/SiOx also referred to as TOPCon (tunnel oxide passivated contacts) have substantially improved the performance of crystalline silicon (c-Si) solar cells. Hydrogenation in TOPCon has the utmost importance for achieving high quality surface passivation and enhanced solar cell performance. In this work, the hydrogenation mechanism and high-temperature fast firing behavior of phosphorus-doped TOPCon structures, on textured crystalline Si; coated with ALD–AlOx, PECVD–SiNx, and AlOx/SiNx stacks, are investigated. Using hot plate annealing series, our results show that thermal activation for hydrogenation is required for TOPCon/AlOx, while the hydrogenation is already activated for TOPCon/SiNx. For AlOx, activation energies (EA) are calculated in the 0.28 – 0.52 eV range, implying that hydrogenation is reaction limited rather than bulk diffusion of hydrogen atoms. The effect of TOPCon layers (SiOx and poly-Si thickness, ex-situ phosphorus diffusion, AlOx/SiNx) is explored. Among all, SiOx is the most critical factor affecting the firing stability. The firing stability is achieved for TOPCon/1.2 nm SiOx with iVOC of 720.6 mV and J0S=3.03 fA/cm2 while excellent passivation with iVOC of 735.1 mV and J0S=2.73 fA/cm2 are not maintained in TOPCon/1.6 nm SiOx after fast firing. The reason for this stability difference is explained by the fact that higher number of interfacial defects in 1.2 nm SiOx is beneficial for preventing blister formation during fast firing.