Electron-selective atomic-layer-deposited TiOx layers: Impact of post-deposition annealing and implementation into n-type silicon solar cells

Titova, Valeriya; Startsev, Dimitrij; Schmidt, Jan

doi:10.1063/1.5049285

“…The fit gives a diffusion constant of 1.2±0.1 cm 2 /s, close to the 0.4 cm 2 /s value estimated in an annealed thin film of amorphous TiO2 nanoparticles 38 . The surface recombination or injection velocity from the fit is 200±50 cm/s, which is also similar to previous measurements of >200 cm/s 39,40 . The fit surface recombination velocity should only be taken as qualitative since the scan time range of 200 ps is too short for significant recombination to occur.…”

Section: Discussionsupporting

confidence: 89%

Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO ₂ -Si photoanode

Cushing

¹

,

Porter

²

,

R.

³

et al. 2020

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Metal-oxide-semiconductor junctions are central to most electronic and optoelectronic devices. Here, the element-specificity of broadband extreme ultraviolet (XUV) ultrafast pulses is used to measure the charge transport and recombination kinetics in each layer of a Ni-TiO 2 -Si junction. After photoexcitation of silicon, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in spectral shifts in the Ni M 2,3 XUV edge that are characteristic of holes and the absence of holes initially in TiO 2 . Meanwhile, the electrons are observed to remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiO 2 , shifting the Ti spectrum to higher oxidation state, followed by electron-hole recombination at the Si-TiO 2 interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO 2 and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously.

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“…The aim of this study is to elucidate how composition changes on a nanoscale affect the electronic properties of atomic layer deposition (ALD) grown, n-Si/SiO /TiO /Al carrier selective contacts with native SiO layers. As was previously shown 20 , 26 – 28 , solar cells with such contacts exhibit quite poor interface passivation and high series resistance in the as-deposited state. After a low temperature annealing step though, both properties improve considerably, leading to efficiencies of up to 20.3% for the batch of solar cells investigated here 26 and 22.1% for the current record solar cells with SiO /TiO based contacts 1 .…”

Section: Introductionmentioning

confidence: 70%

“…As was previously shown 20 , 26 – 28 , solar cells with such contacts exhibit quite poor interface passivation and high series resistance in the as-deposited state. After a low temperature annealing step though, both properties improve considerably, leading to efficiencies of up to 20.3% for the batch of solar cells investigated here 26 and 22.1% for the current record solar cells with SiO /TiO based contacts 1 . However, the detailed reasons for these findings are not unequivocally clear yet.…”

Section: Introductionmentioning

confidence: 70%

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Composition and electronic structure of $${\rm SiO}_{\rm x}$$/$${\rm TiO}_{\rm y}$$/Al passivating carrier selective contacts on n-type silicon solar cells

Flathmann

¹

,

Meyer

²

,

Titova

³

et al. 2023

Sci Rep

Self Cite

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Carrier-selective and passivating SiO$$_{\rm x}$$ x /TiO$$_{\rm y}$$ y heterocontacts are an attractive alternative to conventional contacts due to their high efficiency potentials combined with relatively simple processing schemes. It is widely accepted that post deposition annealing is necessary to obtain high photovoltaic efficiencies, especially for full area aluminum metallized contacts. Despite some previous high-level electron microscopy studies, the picture of atomic-scale processes underlying this improvement seems to be incomplete. In this work, we apply nanoscale electron microscopy techniques to macroscopically well-characterized solar cells with SiO$$_{\rm x}$$ x /TiO$$_{\rm y}$$ y /Al rear contacts on n-type silicon. Macroscopically, annealed solar cells show a tremendous decrease of series resistance and improved interface passivation. Analyzing the microscopic composition and electronic structure of the contacts, we find that partial intermixing of the SiO$$_{\rm x}$$ x and TiO$$_{\rm y}$$ y layers occurs due to annealing, leading to an apparent thickness reduction of the passivating SiO$$_{\rm x}$$ x . However, the electronic structure of the layers remains clearly distinct. Hence, we conclude that the key to obtain highly efficient SiO$$_{\rm x}$$ x /TiO$$_{\rm y}$$ y /Al contacts is to tailor the processing such that the excellent chemical interface passivation of a SiO$$_{\rm x}$$ x layer is achieved for a layer thin enough to allow efficient tunneling through the layer. Furthermore, we discuss the impact of aluminum metallization on the above mentioned processes.

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“…However, from an examination of the literature, it is clear that passivation performance is not the same for all of these films, and the best results in passivation have been achieved using titanium tetrachloride (TiCl 4 ) as the Ti precursor, as shown in Figure . Figure depicts the surface recombination current density prefactor J 0 of TiO x -based passivating contacts deposited using three commonly used Ti precursors: tetrakis(dimethyl amido)titanium (TDMAT), , titanium(IV) isopropoxide (TTIP), , and TiCl 4 . ,, Data are taken from the literature and include only thin layers (≤5 nm) relevant for contact applications. Interestingly, cells with a SiO x /TiO x stack fabricated using TiCl 4 also demonstrated the highest efficiency among solar cells with full-area contacts on the rear without an a-Si:H interlayer, as depicted in Figure .…”

Section: Introductionmentioning

confidence: 99%

Dramatic Reduction of Silicon Surface Recombination by ALD TiO_x Capping Layer from TiCl₄ and H₂O: The Role of Chlorine

Shehata,

Macdonald,

Black

2023

ACS Appl. Mater. Interfaces

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Amorphous thin-film TiO x prepared via atomic layer deposition (ALD) has been identified as one of the most promising materials for use in transparent passivating contacts in highefficiency and low-cost crystalline silicon (c-Si) solar cells. As highlighted in this work, the passivation performance of ALD TiO x layers strongly depends on the metal precursor used, with films prepared using TiCl 4 recently showing the best results. However, a full understanding of how such films achieve their high level of surface passivation has not yet been demonstrated. This study provides a clear demonstration that a key part of this passivation mechanism is due to chlorine (Cl) accumulation at the Si surface. This mechanism is demonstrated to be quite general in nature by showing how 2 nm of ALD TiO x (TiCl 4 + H 2 O) can be applied as a capping layer for either ZnO or Al 2 O 3 interlayers to dramatically reduce silicon surface recombination. Cl depth profiles obtained using secondary ion mass spectrometry confirm the presence of Cl extending through the depth of the interlayers with a peak at the silicon interface. Remarkably, this diffusion of Cl is observed following low-temperature (75 °C) deposition of the TiO x capping layer, without any subsequent thermal treatment. Contrary to earlier studies that treated residual Cl in ALD films as a general contamination issue, these findings reveal unequivocally that chlorine plays a crucial role in Si surface passivation and can be classed as an effective passivation element, similar to hydrogen in its ability to passivate Si dangling bonds. The outcomes of this research emphasize the importance of residual chlorine in enhancing the passivation of buried interfaces and provide additional motivation for employing metal chloride precursors for silicon surface passivation applications.

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Electron-selective atomic-layer-deposited TiOx layers: Impact of post-deposition annealing and implementation into n-type silicon solar cells

Cited by 4 publications

References 6 publications

Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO ₂ -Si photoanode

Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO ₂ -Si photoanode

Composition and electronic structure of $${\rm SiO}_{\rm x}$$/$${\rm TiO}_{\rm y}$$/Al passivating carrier selective contacts on n-type silicon solar cells

Dramatic Reduction of Silicon Surface Recombination by ALD TiO_x Capping Layer from TiCl₄ and H₂O: The Role of Chlorine

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Electron-selective atomic-layer-deposited TiOx layers: Impact of post-deposition annealing and implementation into n-type silicon solar cells

Cited by 4 publications

References 6 publications

Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO 2 -Si photoanode

Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO 2 -Si photoanode

Composition and electronic structure of $${\rm SiO}_{\rm x}$$/$${\rm TiO}_{\rm y}$$/Al passivating carrier selective contacts on n-type silicon solar cells

Dramatic Reduction of Silicon Surface Recombination by ALD TiOx Capping Layer from TiCl4 and H2O: The Role of Chlorine

Contact Info

Product

Resources

About

Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO ₂ -Si photoanode

Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO ₂ -Si photoanode

Dramatic Reduction of Silicon Surface Recombination by ALD TiO_x Capping Layer from TiCl₄ and H₂O: The Role of Chlorine