Impurity Gettering by Boron‐ and Phosphorus‐Doped Polysilicon Passivating Contacts for High‐Efficiency Multicrystalline Silicon Solar Cells

Abstract

Highly doped polysilicon (poly-Si) on ultra-thin oxide layers are highlighted as they allow both carrier collection efficiency with a low contact resistivity and an excellent surface passivation. Their integration at the rear surface of a highquality single-crystalline silicon solar cell allows to achieve a record conversion efficiency of 25.7% for a double-side contacted device. However, so far, only a very few studies investigate the interactions between poly-Si passivating contacts and low-quality cheaper silicon wafers. Thus, this study focuses on the external gettering response of both boron (B) and phosphorus (P) in situ doped poly-Si passivating contacts on high-performance multicrystalline silicon. Wafers are extracted from five ingot heights and experience P-and B-doped poly-Si passivating contact fabrication processes. Subsequently, the bulk carrier lifetime and interstitial iron (Fe i ) concentration are characterized and compared with conventional POCl 3 and BCl 3 thermal diffusion steps, and as-cut references. The P-doped poly-Si contact fabrication process results in gettering more than 99% of the Fe i , which leads to an increase in the bulk carrier lifetime. Interestingly, the B-doped poly-Si contact also develops a substantial external gettering action, and allows removing 96% of the Fe i from the bulk.

Gettering in PolySi/SiOx Passivating Contacts Enables Si-Based Tandem Solar Cells with High Thermal and Contamination Resilience

“…Gettering in Si has been studied for decades, and several individual gettering mechanisms have been identified, namely, in the work by Myers, Seibt, and Schröter, which reviews metal-silicide precipitation, segregation into binary phases (e.g., Al–Si or B–Si), atomic trapping by defects, electronic dopant interactions (electrostatic and Fermi-level shifting), nonequilibrium processes associated with defect fluxes and, in particular, phosphorous-diffusion gettering, which is known to have a very high gettering efficiency. Similarly, the use of polycrystalline Si as a gettering agent has been long studied, − and the gettering in the specific polySi/SiO x structures discussed in this work has received considerable attention recently, precisely due to their important role as passivating contacts in high-efficiency Si solar cells. − , Although the classification of the different gettering mechanisms outlined above has been a topic of debate, they can generally be labeled as either a segregation or a relaxation process. The difference between the two is that segregation gettering results from an equilibrium reached at a given annealing temperature, whereas relaxation results from the nonequilibrium induced in the system during the cooling stage of the annealing .…”

“…Additionally, under certain deposition conditions, it has been shown that heavily phosphorous-doped and boron-doped polySi layers can getter preexisting contaminants from the mono-Si base. For a sufficiently thin silicon oxide (<1.3 nm), the polySi gettering efficiency is comparable to that of POCl 3 - and BBr 3 -diffused Si (i.e., without PolySi, conventional phosphorous and boron diffusion gettering, PDG and BDG, respectively). , However, polySi/SiO x contacts achieve this gettering without compromising the quality of the interface passivation, leading to an overall improvement in the Si carrier lifetime τ eff without the need for extra removal steps, such as etching of the gettering layer(s). , This property is of particular interest for a tandem application because at least one of the polySi layers is effectively buried in the tandem structure, and therefore cannot be removed after the processing. In the above-mentioned studies, the key parameters to control the polySi barrier and gettering effects were found to be the polySi deposition method, thickness, doping density, doping profile, and annealing temperature.…”

Recent advancements in poly-Si/SiOxpassivating contacts for high-efficiency silicon solar cells: technology review and perspectives

“…135 The Fe concentrations in poly-Si/SiO x structures may be reduced by many orders of magnitude following heavy P doping into the TOPCon structures, either through ex-situ thermal diffusion from the poly-Si surface or in situ doping during the poly-Si (or later a-Si) layer deposition period. [136][137][138] Structures formed via the ion implantation and thermal treatment of poly-Si/SiO x materials have also been shown to increase the lifetimes of minority carriers in the bulk of oat-zone, epitaxial, and cast-mono Si substrates. [139][140][141] In the standard phosphorus diffusion process, for example, various gettering processes may be in play; therefore, it can be assumed that the doping strategy will simultaneously alter the gettering effects.…”

Impurity Gettering in Polycrystalline‐Silicon Based Passivating Contacts—The Role of Oxide Stoichiometry and Pinholes

“…[8][9][10][11] By relocating metals to a region of the device where they have less impact on the overall device performance, gettering improves the resulting cell efficiency.The high-temperature fabrication process of poly-Si/SiO x passivating contacts, which provides sufficient thermal budget for many metals to diffuse across the Si wafer thickness, has previously been found to result in an effective gettering of metal impurities by the heavily doped poly-Si/SiO x structures. [12][13][14][15][16][17] The main gettering region of the poly-Si/SiO x passivating contact structure is the highly doped poly-Si layer. [14] The ultra-thin SiO x interlayer can act as a diffusion barrier for metals to reach the poly-Si gettering layer from the silicon wafer bulk, thus potentially reducing the gettering rate of the poly-Si/SiO x structure.…”