Font Side Solutions for c-Si Solar Cells with High-Temperature Passivating Contacts

In this work, we studied the potential of using thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) for two main purposes: introducing an n-type passivating contact at the front of a TOPCon solar cell, or simplifying the fabrication of TOPCon solar cells having passivating contact only at the rear. The recombination at the metal/crystalline silicon (c-Si) interface remains a challenge in current TOPCon solar cells. To overcome this issue, we investigated the potential of nanocrystalline silicon carbide (nc-SiC) layers as transparent high-temperature passivating contact (HTPC). We obtained nc-SiC by using a high density of H-radicals in the plasma followed by thermal treatment. The conditions giving the highest crystallinity were used as a starting point to fabricate phosphorous (P) doped nc-SiC layers. We observed that increasing the dopant flow to 20 and 30 sccm during the deposition resulted in layers with reduced carbon content, which led to Si and SiC nanocrystallites (NCs) inside the layer after annealing. Conversely, in samples from lower doping fluxes, we detected only SiC NCs. Further, we investigated the influence of P-doping and the thermal budget on the performance of the nc-SiC layers as transparent passivating contact. We found that increasing the doping flow led to a reduction in contact resistance down to 10 mO.cm2 but also a significant reduction in implied open circuit voltage (iVoc). The conditions for the highest implied efficiency correspond to an iVoc of 708 mV on textured surfaces and contact resistivity close to 100 mO.cm2. Furthermore, this particular layer showed improved ultraviolet (UV) transparency compared to both amorphous silicon (a-Si) and c-Si. In a parallel effort, we studied methods to simplify the fabrication of TOPCon solar cells. Our approach relies on the fabrication of a boron diffusion source (boron-doped SiOx) at the front side and an n-type poly-Si-based passivating contact at the rear, both through PECVD. Subsequently, the B-diffused emitter at the front and the passivating contact at the rear are formed by a single annealing step. Firstly, we demonstrated the ability to tailor the boron emitter profile by adjusting the flow rates of H2 and TMB during the deposition and by varying the subsequent thermal treatment. After annealing at 900~°C for 15 minutes, we obtained surface concentrations ranging from approximately 3x1019 to 1x1020 for TMB flows of 50 and 90 sccm respectively. The depth of the dopant profiles was approximately 150 nm for the former and 350 nm for the latter. Secondly, we studied poly-Si-based rear passivating contacts that are compatible with the same annealing process used for boron emitter formation. Finally, we demonstrated the practicality of the co-annealing process in a proof-of-concept solar cell that achieved a power conversion efficiency of around 19%. Our results provide initial evidence that a single-step processing approach can produce a diffused emitter in conjunction with a rear-side passivating contact.