Fabrication of high efficiency poly-SiOx passivated c-Si FBC solar cells

Abstract

Carrier selective passivating contacts (CSPC) have proven to effectively curtail the recombination losses emerging at directly metallised contacts of crystalline Silicon (c-Si) solar cells. CSPCs enabled using an ultra-thin interfacial tunnel oxide layer (SiOx) capped by a doped polycrystalline Silicon (poly-Si) layer also referred to as Tunnel Oxide Passivating Contacts (TOPCon) have resulted in efficiencies as high as 25.8%. This thesis project addresses the development of oxygen alloyed poly-Si (poly-SiOx) in combination with an interfacial oxide layer grown by dry thermal oxidation. The limited transparency of poly-Si based contacts brought on by high free carrier absorption (FCA) can be mitigated by the use of poly-SiOx based passivating contacts owing to their wider bandgaps which induce stronger band bending.
To begin with, poly-SiOx CSPC were optimised by determining the optimum thermal budgets for tunnel oxide growth and hydrogenation scheme. Tunnel oxide layers grown at 675 ͦC 6 minutes demonstrated very good passivation for p-type polished and n-type textured CSPCs indicated by their implied Voc of 709 mV and 711 mV respectively. For the p-type textured CSPC identified as the primary limiting factor when deploying in c-Si solar cells, a tunnel oxide layer grown at 675 ͦC 3 minutes in conjunction with a two-step annealing scheme showed a crucial enhancement in passivation quality with a final implied Voc of 687 mV.
The single side textured front back contacted (FBC) solar cell fabricated using the optimised p-type polished and n-type textured poly-SiOx CSPC recorded a conversion efficiency of 20.94% on a 4 cm2 screen printed solar cell. The reported efficiency is the maximum that has been attained so far for the configuration that uses a thermally grown tunnel oxide layer with poly-SiOx CSPCs. Effective carrier transport and carrier collection was illustrated by a fill factor (FF) of 79.6%. A Jsc of 37.91 mA/cm2 was recorded for the same. A comparison with a single side textured FBC solar cell that employed a tunnel oxide layer grown by nitric acid oxidation of Silicon (NAOS) revealed a superiority in performance by the thermally grown tunnel oxide layer resulting in better passivation and carrier selectivity.
Lastly, the optimised n-type textured and p-type textured CSPCs were implemented on a double side textured FBC solar cell. The two-step annealing scheme that showed beneficial results for the p-type textured CSPC was implemented within the FBC solar cell, leading to an implied Voc of 698mV post hydrogenation. It is worth mentioning that this is the highest value achieved until now for this novel cell architecture. Implementation of screen printing resulted in a final conversion efficiency of 19.38% on a 4 cm2 solar cell with a FF and Jsc of 77.89% and 37.65 mA/cm2 respectively.