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Journal ArticleDOI

Transmission electron microscopy based interface analysis of the origin of the variation in surface recombination of silicon for different surface preparation methods and passivation materials

19 Jun 2017-Physica Status Solidi (a) (John Wiley & Sons, Ltd)-Vol. 214, Iss: 10, pp 1700286
TL;DR: In this article, the root cause of variation in surface recombination for Si wafers after different cleaning processes and for different passivation layers was investigated using a combination of calibrated photoluminescence (PL) imaging and transmission electron microscopy (TEM).
Abstract: In this work, the root cause of variation in surface recombination for Si wafers after different cleaning processes and for different passivation layers is investigated using a combination of calibrated photoluminescence (PL) imaging and transmission electron microscopy (TEM). The use of a HF-last or oxide-last cleaning and/or conditioning process is shown to have a strong impact on surface recombination for SiNx passivated surfaces, but little impact for Al2O3/SiNx stacks. For a SiNx passivation layer, cross-sectional TEM imaging revealed the formation of a ≈1–2 nm SiOx interlayer resulting from a controlled oxidation during the last cleaning/conditioning step. The presence of the SiOx layer reduces the interface defect density (Dit,midgap) by an order of magnitude and dramatically increases the effective carrier lifetime. However, for Al2O3/SiNx passivated surfaces, TEM studies revealed that a SiOx layer is formed at the interface between the c-Si and AlOx even for cleaning processes ending with HF-last treatment due to which the cleaning sequence has minimal impact on the effective carrier lifetime.
Citations
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Journal ArticleDOI
TL;DR: In this article, the authors investigated the effectiveness of crystalline silicon surface cleaning by a simple UV-ozone process in comparison to the industry standard RCA clean for silicon photovoltaic applications.

21 citations

Journal ArticleDOI
TL;DR: In this study, the thermal stability of a contact structure featuring hole-selective tungsten oxide and aluminum deposited onto p-type crystalline silicon was investigated using a combination of transmission line measurements (TLM) and in situ transmission electron microscopy (TEM) studies.
Abstract: In this study, the thermal stability of a contact structure featuring hole-selective tungsten oxide (WOx) and aluminum deposited onto p-type crystalline silicon (c-Si/WOx/Al) was investigated using a combination of transmission line measurements (TLM) and in situ transmission electron microscopy (TEM) studies. The TEM images provide insight into why the charge carrier transport and recombination characteristics change as a function of temperature, particularly as the samples are annealed at temperatures above 500 °C. In the as-deposited state, a ≈ 2 nm silicon oxide (SiOx) interlayer forms at the c-Si/WOx interface and a ≈ 2-3 nm aluminum oxide (AlOx) interlayer at the WOx/Al interface. When annealing above 500 °C, Al diffusion begins, and above 600 °C complete intermixing of the SiOx, WOx, AlOx and Al layers occurs. This results in a large drop in the contact resistivity, but is the likely reason surface recombination increases at these high temperatures, since a c-Si/Al contact is basically being formed. This work provides some fundamental insight that can help in the development of WOx films as hole-selective rear contacts for p-type solar cells. Furthermore, this study demonstrates that in situ TEM can provide valuable information about thermal stability of transition metal oxides functioning as carrier-selective contacts in silicon solar cells.

17 citations

Journal ArticleDOI
Abstract: It is long recognized that high-quality surface cleaning is critical for an increased performance of solar cells and semiconductor devices. In this contribution, the effectiveness of UV-ozone cleaning by comparing it against the industry standard RCA and UV-assisted deionized water (DI-O3) techniques has been demonstrated. UV-ozone cleaning results in an effective surface passivation quality that is comparable to both RCA and DI-O3 cleans, realizing a recombination current density ( J0) of 7 fA cm 2 as compared to 5 and 8 fA cm 2 for RCA and DI-O3 cleans, respectively. Repeating the UVozone clean on samples (i.e., growing of UV-ozone oxide and stripping it in HF) more than twice results in a cleaning efficiency that is nearly identical to RCA clean. Based on a high resolution transmission electron microscopy analysis, the post-annealed thickness of the UV-ozone oxide layer was found to reduce in comparison to the pre-annealed condition. This is likely due to oxygen diffusion from the UV-ozone oxide layer into the overlaying AlOx layer. Additionally, it has been found that a reduction in UV-ozone oxide deposition time to just 5min still provides a comparable cleaning efficiency to the RCA clean, and also results in good passivation quality (5–8 fA cm ) on both planar and textured samples.

9 citations

Journal ArticleDOI
TL;DR: In this paper , the authors discuss how multiscale characterization methods can be applied to a variety of module technologies that have been field exposed and have undergone accelerated age testing, including performing characterization on the module level, cell level, and finally the materials level.
Abstract: The current popularity of photovoltaic (PV) systems is due in large part to their exceptional reliability and significantly lower cost than other energy sources. Studying cell and module degradation is key to promote further development in the state of the art. Fielded oraccelerated aged modules exhibit different failure modes, of which metallization degradation (contacts and interconnections) is prevalent. In this work, we discuss how multiscale characterization methods can be applied to a variety of module technologies that have been field exposed and have undergone accelerated age testing. These methods include performing characterization on the module level, cell level, and finally the materials level. The observed performance losses from the module- and cell-level characterization can be correlated with materials properties to find out the root cause of degradation. We recommend an initial nondestructive characterization suite, including module- and cell-level current–voltage ( $I\text{--}V$ ), Suns– V OC , photoluminescence and electroluminescence imaging, quantum efficiency, ultraviolet fluorescence imaging, and thermal infrared imaging. Samples are then extracted from particularly degraded regions of the module and prepared for materials characterization techniques, such as top-down and cross-sectional scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, Raman spectroscopy, and transmission electron microscopy, allowing a deeper look into the mechanism behind the metallization degradation. This article serves as an instructional review to introduce the different multiscale characterization methods and how they can be effectively applied to perform PV degradation studies. We also share some of our examples and discuss the strengths, limitations, and best practices for each of the characterization techniques.

9 citations

References
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Journal ArticleDOI
TL;DR: The state-of-the-art surface passivation of c-Si solar cells is achieved by Al2O3 films prepared by plasma-assisted atomic layer deposition, yielding effective surface recombination velocities of 2 and 13cm∕s on low resistivity n- and p-type cSi, respectively as mentioned in this paper.
Abstract: Excellent surface passivation of c-Si has been achieved by Al2O3 films prepared by plasma-assisted atomic layer deposition, yielding effective surface recombination velocities of 2 and 13cm∕s on low resistivity n- and p-type c-Si, respectively. These results obtained for ∼30nm thick Al2O3 films are comparable to state-of-the-art results when employing thermal oxide as used in record-efficiency c-Si solar cells. A 7nm thin Al2O3 film still yields an effective surface recombination velocity of 5cm∕s on n-type silicon.

697 citations

Journal ArticleDOI
TL;DR: In this paper, aluminum oxide (Al2O3) nanolayers synthesized by atomic layer deposition (ALD) have been used for the passivation of p-and n-type crystalline Si (c-Si) surfaces.
Abstract: The reduction in electronic recombination losses by the passivation of silicon surfaces is a critical enabler for high-efficiency solar cells. In 2006, aluminum oxide (Al2O3) nanolayers synthesized by atomic layer deposition (ALD) emerged as a novel solution for the passivation of p- and n-type crystalline Si (c-Si) surfaces. Today, high efficiencies have been realized by the implementation of ultrathin Al2O3 films in laboratory-type and industrial solar cells. This article reviews and summarizes recent work concerning Al2O3 thin films in the context of Si photovoltaics. Topics range from fundamental aspects related to material, interface, and passivation properties to synthesis methods and the implementation of the films in solar cells. Al2O3 uniquely features a combination of field-effect passivation by negative fixed charges, a low interface defect density, an adequate stability during processing, and the ability to use ultrathin films down to a few nanometers in thickness. Although various methods can be used to synthesize Al2O3, this review focuses on ALD—a new technology in the field of c-Si photovoltaics. The authors discuss how the unique features of ALD can be exploited for interface engineering and tailoring the properties of nanolayer surface passivation schemes while also addressing its compatibility with high-throughput manufacturing. The recent progress achieved in the field of surface passivation allows for higher efficiencies of industrial solar cells, which is critical for realizing lower-cost solar electricity in the near future.

684 citations

Journal ArticleDOI
TL;DR: A review of surface passivation methods used since the 1970s, both on laboratory-type as well as industrial cells is presented in this paper, where a p-n junction and the subsequent passivation of the resulting silicon surface with plasma silicon nitride are presented.
Abstract: In the 1980s, advances in the passivation of both cell surfaces led to the first crystalline silicon solar cells with conversion efficiencies above 20%. With today's industry trend towards thinner wafers and higher cell efficiency, the passivation of the front and rear surfaces is now also becoming vitally important for commercial silicon cells. This paper presents a review of the surface passivation methods used since the 1970s, both on laboratory-type as well as industrial cells. Given the trend towards lower-cost (but also lower-quality) Si materials such as block-cast multicrystalline Si, ribbon Si or thin-film polycrystalline Si, the most promising surface passivation methods identified to date are the fabrication of a p–n junction and the subsequent passivation of the resulting silicon surface with plasma silicon nitride as this material, besides reducing surface recombination and reflection losses, additionally provides a very efficient passivation of bulk defects. Copyright © 2000 John Wiley & Sons, Ltd.

683 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present an overview on the present status of SiN for industrial as well as laboratory-type c-Si solar cells, including the fundamental properties of Si-Si interfaces fabricated by PECVD.

411 citations


"Transmission electron microscopy ba..." refers background in this paper

  • ...The ability of SiNx to act simultaneously as a passivation layer and an anti-reflection coating (ARC) makes it the most commonly used passivation material for cSi solar cells [8]....

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Journal ArticleDOI
TL;DR: In this paper, negative charge dielectric Al2O3 was applied as surface passivation layer on high-efficiency n-type silicon solar cells, achieving a confirmed conversion efficiency of 23.2% on B-doped emitters.
Abstract: In order to utilize the full potential of solar cells fabricated on n-type silicon, it is necessary to achieve an excellent passivation on B-doped emitters. Experimental studies on test structures and theoretical considerations have shown that a negatively charged dielectric layer would be ideally suited for this purpose. Thus, in this work the negative-charge dielectric Al2O3 was applied as surface passivation layer on high-efficiency n-type silicon solar cells. With this front surface passivation layer, a confirmed conversion efficiency of 23.2% was achieved. For the open-circuit voltage Voc of 703.6mV, the upper limit for the emitter saturation current density J0e, including the metalized area, has been evaluated to be 29fA∕cm2. This clearly shows that an excellent passivation of highly doped p-type c-Si can be obtained at the device level by applying Al2O3.

343 citations