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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.

International audience; Polysilicon (poly-Si) based passivating contacts are promising to improve silicon solar cells conversion efficiency. Thin poly-Si layers usually feature high sheet resistances compared to conventional diffused junctions. Therefore the use of Transparent Conductive Oxides (TCO) has to be considered to improve lateral conductivity while maintaining good optical and surface passivation properties. Standard sputtered Indium Tin Oxide (ITO) can alter the poly-Si passivation properties. In this work different TCO-based contacts have been investigated (various materials and deposition techniques) in order to contact phosphorus-doped poly-Si layers (n+ poly-Si). The TCO-deposited samples experienced both low and high temperature annealing steps. Implied open circuit voltage above 730 mV and contact resistivity below 50 mΩ.cm2 at the TCO/n+ poly-Si interface have been obtained for both high and low temperature approaches. Thus such novel structures are very promising to contact poly-Si layers.

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.

International audience; This work explores the possibility to use the mechanism of hopping conduction – and particularly the transition temperature between band and hopping conduction – on low temperature resistivity measurements, for the control of dopants densities in p‐type compensated silicon. This work first establishes a parametric study of the hopping conductivity: the impact of the majority dopant density and of the compensation ratio is investigated. In the range of majority dopant concentration studied (5×1016 cm–3–5×1017 cm–3), a linear relation seems to appear between the majority dopant concentration and the transition temperature, and this, apparently whatever the compensation impurity type or the crystalline structure. It was then shown that both minority and majority dopant densities can be estimated from a single resistivity versus temperature curve. To our knowledge, this work presents the first experimental study of the feasibility of using such mechanisms to collect relevant information on the compensated Si composition.

We studied the influence of copper (Cu) on the performances of conventional photovoltaic (PV) solar cells by growing two multicrystalline (mc) boron-doped silicon (Si) ingots from ultra-pure feedstocks, one of these feedstocks being deliberately contaminated with 90 ppm wt of Cu. Industrial-like solar cells were fabricated and the associated external gettering and hydrogenation effects were studied. An originality of our approach consisted in evaluating the forward but also reverse current-voltage (I-V) characteristics of the fabricated cells. Furthermore we assessed the stability under illumination of the PV parameters as Cu is known to be responsible for light-induced degradations (LID) of the carrier lifetime. On the one hand we unexpectedly showed that the PV conversion efficiency (η) was not affected by the initially large Cu concentrations. We demonstrated that it was due to the complementary actions of the external gettering effect developed by the phosphorus-diffusion and the bulk hydrogenation. The Cu-addition slightly enhanced the pn junction hard breakdown, however the extracted junction breakdown voltages fulfilled the common industrial requirements for this parameter. On the other hand we highlighted significant decreases under illumination of the PV performances for the Cu-contaminated solar cells fabricated from wafers coming from the upper part of the ingot (i.e., samples with the highest Cu concentration). These decreases could be explained by the previously proposed mechanisms in the literature, which argue that the excess charge carriers could reduce the electrostatic repulsion between interstitial Cu ions and Cu precipitates, this effect enhancing the Cu precipitation. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper a new characterization technique, the OXYMAP technology, allowing measurement of the interstitial oxygen concentration and the oxygen related defects in Czochralski grown silicon, is presented. We applied this technique to 8 inch industrial-like p-type ingots. Relevant information regarding the material quality (compositional and electrical properties) were extracted, and we demonstrated the ability of the developed technic to predict the impact of harmful oxygen-related defects on the solar cells performances.