The Work Function of TiO2

Kashiwaya, Shun; Morasch, Jan; Streibel, Verena; Toupance, Thierry; Jaegermann, Wolfram; Klein, Andreas

doi:10.3390/surfaces1010007

Cited by 184 publications

(211 citation statements)

References 76 publications

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“…Figure 3a shows the valence band (i) and secondary electron cut-off (SEC) (ii) spectra of the as-deposited TiO x layer, measured via X-ray photoelectron spectroscopy (XPS). [20] These can be combined with spectroscopic ellipsometry measurements to make an estimation of the band position relative to c-Si. A clear band tail is observed in the valence band spectrum, indicative of amorphous/nanocrystalline films (as expected from our previous studies on TiO x ), [19] but no sub-bandgap defect band is seen between the valence band and Fermi energy (E v −E F of >3 eV).…”

Section: Resultsmentioning

confidence: 99%

Dopant‐Free Partial Rear Contacts Enabling 23% Silicon Solar Cells

Bullock

Wan

Hettick

et al. 2019

Advanced Energy Materials

123

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improving the performance of this technology. For example, materials such as metal oxides, nitrides, and fluorides have been demonstrated to form electron and hole selective interfaces when applied to c-Si. [1][2][3][4][5][6][7] Such an approach has potential benefits over conventional heavily doped direct-metallization approaches, including lower processing temperatures, simpler contact formation, and the removal of fundamental limitations, such as Auger recombination and free carrier absorption. [8,9] In addition, the unique interface properties of some metal compound/c-Si interfaces have even enabled novel solar cell architectures, for example, n-type c-Si cells with undoped partial rear contacts (PRCs). [10,11] This specific architecture utilizes a near-ideal surface passivation layer, such as hydrogenated silicon nitride SiN x , [12] to cover the vast majority of the rear surface which can greatly reduce the average surface recombination factor J 0 and increase the rear reflection. Only a small percentage of the area is contacted, typically < 5%, where electrons flow to be collected. An n-type undoped PRC cell structure was not previously attainable due to the tendency of n-type c-Si to form an interface potential barrier under direct metallization, which resulted in prohibitively high contact resistivity ρ c . The first successful demonstration of this cell came after a breakthrough in low resistance interfaces to n-type c-Si with a low work function LiF x /Al electrode. This contact was used to fabricate an undoped PRC cell attaining an efficiency of 20.6% with a PRC covering only ≈1% of the rear surface. [11] The next evolutionary step in this cell structure was the integration of a passivation layer at the PRC interface. This came with the introduction of a TiO x /Ca/Al contact, [10] which was found to provide both reduced surface recombination and low contact resistivity, enabling an efficiency of 21.8%. Following on from these early developments, there exist three major avenues to easily improve the electron PRC: i) reduction in the PRC interface recombination and resistivity; ii) increase in the PRC material's stability to thermal stressors; and iii) increasing the rear surface reflectivity via appropriate choice of PRC materials.This article introduces the next in this family of carrier selective interfaces, which targets the abovementioned three issues. To address this a TiO x /LiF x /Al heterocontact is developed and integrated into a PRC cell shown in Figure 1a. The

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Section: Resultsmentioning

confidence: 99%

Dopant‐Free Partial Rear Contacts Enabling 23% Silicon Solar Cells

Bullock

Wan

Hettick

et al. 2019

Advanced Energy Materials

123

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“…To validate the potential Fermi level pinning effect observed in the CuFeO 2 /ITO interface experiment, another kind of interface experiment was performed by exposing a cleaned 2H CuFeO 2 substrate surface (Figure S7, Supporting Information) to water, also denoted as water exposure experiment. The interface with water was expected to increase the E F – E VBM of CuFeO 2 further, because of the low work functions that can be obtained after water exposure . The effect of water exposure on the Cu 2p 3/2 , Fe 2p, O 1s, and valence band XP and Cu LMM Auger spectra of CuFeO 2 is shown in Figure a.…”

Section: Resultsmentioning

confidence: 99%

“…[20,21] The anisotropic photodeposition might be caused by dissimilar band bending across the different exposed facets, as measurements on single crystals suggest. [25,26] However, highly anisotropic electrical conductivities perpendicular and parallel to the c-axis, as observed for 3R CuFeO 2 single crystals, [27] might also be the reason for the observed anisotropic photodeposition.…”

Section: Synthesis and Characterization Of Anisotropic Heterostructurmentioning

confidence: 99%

“…The interface with water was expected to increase the E F -E VBM of CuFeO 2 further, because of the low work functions that can be obtained after water exposure. [26,42] The effect of water exposure on the Cu 2p 3/2 , Fe 2p, O 1s, and valence band XP and Cu LMM Auger spectra of CuFeO 2 is shown in Figure 4a. All spectra shift toward higher binding energies upon water exposure, as would be expected from an interface experiment with a low work function material.…”

Section: Interface Experiments To Determine Fermi Level Pinning In 2hmentioning

confidence: 99%

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Pinning of the Fermi Level in CuFeO₂ by Polaron Formation Limiting the Photovoltage for Photochemical Water Splitting

Hermans

Klein

Sarker

et al. 2020

Adv Funct Materials

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CuFeO2 is recognized as a potential photocathode for photo(electro)chemical water splitting. However, photocurrents with CuFeO2‐based systems are rather low so far. In order to optimize charge carrier separation and water reduction kinetics, defined CuFeO2/Pt, CuFeO2/Ag, and CuFeO2/NiOx(OH)y heterostructures are made in this work through a photodeposition procedure based on a 2H CuFeO2 hexagonal nanoplatelet shaped powder. However, water splitting performance tests in a closed batch photoreactor show that these heterostructured powders exhibit limited water reduction efficiencies. To test whether Fermi level pinning intrinsically limits the water reduction capacity of CuFeO2, the Fermi level tunability in CuFeO2 is evaluated by creating CuFeO2/ITO and CuFeO2/H2O interfaces and analyzing the electronic and chemical properties of the interfaces through photoelectron spectroscopy. The results indicate that Fermi level pinning at the Fe3+/Fe2+ electron polaron formation level may intrinsically prohibit CuFeO2 from acquiring enough photovoltage to reach the water reduction potential. This result is complemented with density functional theory calculations as well.

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“…From Figure 4a we see that energy levels of ZnS(110) are above those of TiO 2 (001), which in turns lie above those of TiO 2 (101), according to the picture reported in the Literature. [51,84,85] This already suggests an enhanced charge separation following a cascade scheme where the photoexcited electrons follow a path toward the lowest CB edge at TiO 2 (101), while holes should preferentially be hosted on the ZnS moiety, with the TiO 2 (001) component conveniently acting as a spacer between photogenerated charge carriers of opposite signs. TiO 2 anatase (001) slabs display computed band gaps usually 0.5-0.6 eV lower than that of anatase (101) surface, because of the presence of surface states due to exposed O atoms in the surface layer.…”

Section: Modelling Of the Tio 2 /Zns Compositementioning

confidence: 98%

Charge Carriers Cascade in a Ternary TiO₂/TiO₂/ZnS Heterojunction: A DFT Study

2020

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Composite materials whose band alignment induces a favorable separation of photogenerated electrons and holes often reveal a stronger photocatalytic activity compared to their separate components. As shown by experiments, titania composites display a heterojunction between the anatase (101)‐(001) surfaces, where the former stabilizes electrons and the latter the holes. In principle, an even more efficient carriers separation is achieved if a third component with high hole‐stabilizing capability, ZnS(110), is growth on anatase (001). However, even though this ternary TiO2/TiO2/ZnS composite material displays good photoactivity, it does not overperform the TiO2/TiO2 one. In this paper an explanation of this evidence is provided by means of periodic hybrid DFT calculations, showing how Coulomb forces play a role against the separation of charge carriers predicted based on the relative energy of the band edges. This highlights the necessity to explicitly account for structural and electronic junction's effects, as well as charge carriers’ localization.

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The Work Function of TiO2

Cited by 184 publications

References 76 publications

Dopant‐Free Partial Rear Contacts Enabling 23% Silicon Solar Cells

Dopant‐Free Partial Rear Contacts Enabling 23% Silicon Solar Cells

Pinning of the Fermi Level in CuFeO₂ by Polaron Formation Limiting the Photovoltage for Photochemical Water Splitting

Charge Carriers Cascade in a Ternary TiO₂/TiO₂/ZnS Heterojunction: A DFT Study

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The Work Function of TiO2

Cited by 184 publications

References 76 publications

Dopant‐Free Partial Rear Contacts Enabling 23% Silicon Solar Cells

Dopant‐Free Partial Rear Contacts Enabling 23% Silicon Solar Cells

Pinning of the Fermi Level in CuFeO2 by Polaron Formation Limiting the Photovoltage for Photochemical Water Splitting

Charge Carriers Cascade in a Ternary TiO2/TiO2/ZnS Heterojunction: A DFT Study

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Pinning of the Fermi Level in CuFeO₂ by Polaron Formation Limiting the Photovoltage for Photochemical Water Splitting

Charge Carriers Cascade in a Ternary TiO₂/TiO₂/ZnS Heterojunction: A DFT Study