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In this paper, we present our latest developments in n+ poly-Si/SiO x passivated contacts elaborated on a high throughput LPCVD furnace as well as their integration in high efficiency large area n-type PERT solar cells. Both in-situ and ex-situ doping of the poly-Si layer have been investigated. Following a fine tuning of the thin oxide layer growth and of the poly-Si doping process, iVoc up to 735 mV could be reached on symmetrical samples with Cz silicon wafers. The high performances of this structure were also confirmed on advanced cast mono wafers with similar iVoc levels. Finally, a fabrication process based on high throughput industrial equipment from SEMCO SMARTECH was developed to form high efficiency polySi based PERT solar cells. Efficiency values up to 22.8% were measured on large area Cz silicon wafer using a standard 5 busbars screen printed metallization.

The noncontact crucible (NOC) method was proposed for obtaining Si single bulk crystals with a large diameter and volume using a cast furnace and solar cells with high conversion efficiency and yield. This method has several novel characteristics that originate from its key feature that ingots can be grown inside a Si melt without contact with a crucible wall. Si ingots for solar cells were grown by utilizing the merits resulting from these characteristics. Single ingots with high quality were grown by the NOC method after furnace cleaning, and the minority carrier lifetime was measured to investigate reduction of the number of impurities. A p -type ingot with a convex growth interface in the growth direction was also grown after furnace cleaning. For p -type solar cells prepared using wafers cut from the ingot, the highest and average conversion efficiencies were 19.14% and 19.0%, respectively, which were obtained using the same solar cell structure and process as those employed to obtain a conversion efficiency of 19.1% for a p -type Czochralski (CZ) wafer. Using the cast furnace, solar cells with a conversion efficiency and yield as high as those of CZ solar cells were obtained by the NOC method.

International audience

International audience; Polysilicon (poly-Si) based passivating contacts are promising to improve silicon solar cells conversion efficiency. However, the use of Transparent Conductive Oxides (TCO) has to be considered to improve lateral conductivity while maintaining good optical and surface passivation properties especially for bifacial devices. In this work different Aluminum-doped Zinc Oxide (AZO) based layers have been investigated after high temperature firing steps to contact Phosphorus-doped poly-Si layers. Contact resistivity below 100 mΩ.cm$^2$ at the AZO/n$^+$ poly-Si interface and stable implied open circuit voltage values (> 715 mV) have been obtained for firing temperatures from 550°C to 900°C. Moreover, the use of capping layers allows to maintain highly conductive AZO layers upon annealing. This novel high temperature contacting method via indium-free TCOs, is particularly interesting for the industrial integration of poly-Si based passivated contacts and provides new perspectives for advanced homojunction solar cells.

We report on the investigation of a variety of metallization processes for hole selective passivating contacts, which consist in the combination of an ultra-thin silicon oxide (SiOx) interfacial film (grown by wet treatments) and a 25 nm thick boron (B) doped polysilicon layer (poly-Si). Plasma Enhanced Chemical Vapor Deposition (PECVD) of amorphous silicon (a-Si) followed by crystallization annealing is used for poly-Si elaboration. Optimization of metallization steps is required in order to (1) reach low specific contact resistances (ρc), and (2) preserve passivation quality along with structural integrity. This study focuses on the analysis of two processes: electron-beam evaporation deposition, and Physical Vapor Deposition (PVD) by sputtering. Different metals (silver (Ag) and aluminum (Al)) and annealing conditions are studied. Additionally, we investigated the effect of an additional a-Si capping layer on both (1) and (2) items. PVD of Ag appears as the most promising technique combining relatively low ρc (90 mΩ.cm2) as deposited with limited passivation degradations, and thus seems suitable for device integration as a full area back contact.

International audience; Nowadays, compensated silicon (Si) is used in photovoltaic (PV) processes, whether it is through intentional co-doping of resistivity-adjusted Czochralski ingots for high efficiency n-type Si solar cells, as a result of alternative Si purification processes for the production of low-cost Si feedstock, or as a result of recycling end-of-life materials. Whatever the origin of the compensated Si, the doping concentrations need to be accurately and quickly characterized in order to control such processes. In this work, a rapid and highly sensitive characterization technique based on low temperature Hall Effect measurements is described in scientific details and compared to three well-established chemical methods: Glow Discharge Mass Spectrometry (GDMS), Inductively-Coupled Plasma Mass Spectrometry (ICP-MS), and Secondary Ion Mass Spectrometry (SIMS). The characterized samples were extracted from the n-type top part of a casted solar grade Si ingot. A very good agreement is observed between the dopants densities extracted from the electrical method and from the standard methods. With the advantage of a very low detection limit combined with a short measurement time, the advanced Hall Effect technique is promising for the rapid and accurate characterization of dopant concentrations in compensated Si.