Understanding the impurity gettering effect of polysilicon/oxide passivating contact structures through experiment and simulation

Liu, AnYao; Yang, Zhongshu; Feldmann, Frank; Polzin, Jana‐Isabelle; Steinhauser, Bernd; Phang, Sieu Pheng; Macdonald, Daniel

doi:10.1016/j.solmat.2021.111254

Cited by 14 publications

(49 citation statements)

References 32 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Details of the process can be found in refs. [17,59]. The resulting bulk interstitial iron concentration ([Fe i ]) was (1 ± 0.1) × 10 13 cm −3 , which was confirmed through lifetime-based bulk Fe measurements (detailed below).…”

Section: Methodsmentioning

confidence: 58%

“…The segregation coefficient k seg can be determined experimentally from the initial and final Fe concentrations in the silicon wafer bulk at steady state, as detailed in ref. [17]. The diffusivity of Fe in poly-Si can be estimated by This approximation was used in modeling phosphorus diffusion gettering of Fe in c-Si, [69] and was also found to produce simulations that agree well with the experimental gettering kinetics of poly-Si/SiO x in ref.…”

Section: Methodsmentioning

confidence: 65%

“…Simulation Details: The process of gettering Fe from the silicon bulk to the heavily doped poly-Si layer could be numerically simulated, [67] according to a 1D diffusion-limited segregation gettering model, as detailed in ref. [17]. This 1D diffusion model for simulating samples with pinholes (which involves lateral diffusion) was validated by carrying out a 3D finite-element simulation using comsol multiphysics (see Supporting Information).…”

Section: Methodsmentioning

confidence: 99%

“…[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.…”

mentioning

confidence: 99%

“…[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. [17] Understanding the blocking effects of the SiO x interlayers and the factors affecting this blocking effect are crucial for the optimization of the passivation contact fabrication process in order to take advantage of its potentially strong gettering effects, therefore enabling higher efficiency solar cells to be made.This work aims to study the blocking effects of the SiO x interlayers in the poly-Si/SiO x passivating contact structures by investigating two general types of the SiO x interlayers: the ultrathin tunneling SiO x interlayer (≈1.3 nm) with negligible pinholes, and the relatively thicker SiO x interlayer (≈2.5 nm) with pinholes created by high temperature annealing. The heavily doped poly-Si layers were prepared either by plasma enhanced chemical vapor deposition (PECVD) with in situ doping or by low pressure chemical vapor deposition (LPCVD) Polycrystalline-silicon/oxide (poly-Si/SiO x ) passivating contacts for high efficiency solar cells exhibit excellent surface passivation, carrier selectivity, and impurity gettering effects.…”

mentioning

confidence: 99%

See 4 more Smart Citations

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

Yang

Krügener

Feldmann

et al. 2022

Advanced Energy Materials

Self Cite

View full text Add to dashboard Cite

Polycrystalline‐silicon/oxide (poly‐Si/SiOx) passivating contacts for high efficiency solar cells exhibit excellent surface passivation, carrier selectivity, and impurity gettering effects. However, the ultrathin SiOx interlayer can act as a diffusion barrier for metal impurities and this potentially slows down the overall gettering rate of the poly‐Si/SiOx structures. Herein, the factors that determine the blocking effects of the SiOx interlayers are identified and investigated by examining two general types of the SiOx interlayers: 1.3 nm ultrathin tunneling SiOx with negligible pinholes and 2.5 nm SiOx with thermally created pinholes. Iron is used as tracer impurity in silicon to quantify the gettering rate. By fitting the experimental gettering kinetics by a diffusion‐limited segregation gettering model, the blocking effects of the SiOx interlayers are quantified by a transport parameter. Both the oxide stoichiometry and pinhole density affect the effective transport of iron through SiOx interlayers. The oxide stoichiometry depends strongly on the oxidation method, while the pinhole density is affected by the activation temperature, doping concentration, doping technique, and possibly the dopant type as well. To enable a fast gettering process during typical high‐temperature formation of the poly‐Si/SiOx structures, a SiOx interlayer that is less stoichiometric or with a higher pinhole density is preferred.

show abstract

Section: Methodsmentioning

confidence: 58%

Section: Methodsmentioning

confidence: 65%

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

See 3 more Smart Citations

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

Yang

Krügener

Feldmann

et al. 2022

Advanced Energy Materials

Self Cite

View full text Add to dashboard Cite

show abstract

Electronic Characteristics of Ultra‐Thin Passivation Layers for Silicon Photovoltaics

Pain

Khorani

Niewelt

et al. 2022

Adv Materials Inter

View full text Add to dashboard Cite

on PERC technologies and get even closer to the theoretical single-junction efficiency limit, electrical losses in the contacted regions must be reduced. [3][4][5] Passivating contacts can help alleviate such losses by simultaneously suppressing the current of non-collected carriers to the contact, and by reducing recombination sites at the interface. Introducing a passivating interlayer between the metal/silicon interface provides a route to reducing the recombination current density, J 0 , [6,7] thereby increasing device voltage. [3] Passivating contacts have achieved some success to date, with the strongest candidates being polysilicon on top of thin silicon oxide layers (e.g., tunnel oxide passivating contacts (TOPCon) or poly silicon on oxide (POLO)) and amorphous silicon (a-Si) heterojunctions. [3,7,8] TOPCon is an efficient electron-selective contact but has a high thermal budget with temperatures around 900 °C needed to reduce the contact resistivity to acceptable levels. [9] An efficient hole-selective layer that can match or exceed the performance of the current electron-selective materials would be of considerable interest. The use of SiO 2 -based hole-selective contacts has so far failed to reach equivalent levels. [10,11] The most promising hole-selective contacting materials are p-type a-Si and siliconrich SiC, but conventional high-temperature Ag screen printing methods are not necessarily compatible with such contacts. [10] Surface passivating thin films are crucial for limiting the electrical losses during charge carrier collection in silicon photovoltaic devices. Certain dielectric coatings of more than 10 nm provide excellent surface passivation, and ultra-thin (<2 nm) dielectric layers can serve as interlayers in passivating contacts. Here, ultra-thin passivating films of SiO 2 , Al 2 O 3 , and HfO 2 are created via plasma-enhanced atomic layer deposition and annealing. It is found that thin negatively charged HfO 2 layers exhibit excellent passivation properties-exceeding those of SiO 2 and Al 2 O 3 -with 0.9 nm HfO 2 annealed at 450 °C providing a surface recombination velocity of 18.6 cm s −1 . The passivation quality is dependent on annealing temperature and layer thickness, and optimum passivation is achieved with HfO 2 layers annealed at 450 °C measured to be 2.2-3.3 nm thick which give surface recombination velocities ≤2.5 cm s −1 and J 0 values of ≈14 fA cm −2 . The superior passivation quality of HfO 2 nanolayers makes them a promising candidate for future passivating contacts in high-efficiency silicon solar cells.

show abstract

Comparing the Gettering Effect of Heavily Doped Polysilicon Films and Its Implications for Tunnel Oxide‐Passivated Contact Solar Cells

et al. 2022

Self Cite

View full text Add to dashboard Cite

In addition to excellent surface passivation and carrier selectivity, the structure based on the heavily doped polysilicon layer on an ultrathin silicon oxide interlayer also demonstrates strong impurity gettering effects. Herein, the gettering strength of a range of phosphorus‐ or boron‐doped polysilicon films from different fabrication techniques is assessed and compared. Iron, one of the most common metallic impurities in silicon, is used as a tracer impurity to quantify the gettering strength (segregation coefficient). A comparison of the experimental results to the literature, combined with measurements of the electrically active and inactive dopant concentrations, enables us to suggest the main gettering mechanisms in different polysilicon films. The differences in the segregation coefficients of the phosphorus‐doped polysilicon films for iron are within one order of magnitude, in spite of their different combinations of gettering mechanisms. On the other hand, boron‐doped polysilicon films show a large variation in their gettering effects, although the predominant gettering mechanisms are all attributed to electrically inactive boron, according to the current understanding of the gettering mechanisms from the literature. Finally, the impact of different polysilicon gettering effects on the efficiency of tunnel oxide‐passivated contact (TOPCon) cells is simulated and discussed.

show abstract

Understanding the impurity gettering effect of polysilicon/oxide passivating contact structures through experiment and simulation

Cited by 14 publications

References 32 publications

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

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

Electronic Characteristics of Ultra‐Thin Passivation Layers for Silicon Photovoltaics

Comparing the Gettering Effect of Heavily Doped Polysilicon Films and Its Implications for Tunnel Oxide‐Passivated Contact Solar Cells

Contact Info

Product

Resources

About