Prof. Dr. Johanna Eichhorn
Academic Research Focus
- Nanoscale microscopy and spectroscopy of energy materials
- Multilayer photoelectrodes for stable and efficient energy conversion
- Multilayer photoelectrodes for stable and efficient energy conversion
Fields of Application
Photo-(electro)catalysis, Solar Cells / Photovoltaics
Publications
25.
E Sirotti, B Scaparra, S Böhm, F Pantle, L I Wagner, F Rauh, F Munnik, C-M Jiang, M Kuhl, K Müller, J Eichhorn, V Streibel, I D Sharp
Oxygen Incorporation as a Route to Nondegenerate Zinc Nitride Semiconductor Thin Films Journal Article
In: ACS Applied Materials & Interfaces, vol. 17, no. 5, pp. 7958-7968, 2025, ISSN: 1944-8244.
@article{nokey,
title = {Oxygen Incorporation as a Route to Nondegenerate Zinc Nitride Semiconductor Thin Films},
author = {E Sirotti and B Scaparra and S B\"{o}hm and F Pantle and L I Wagner and F Rauh and F Munnik and C-M Jiang and M Kuhl and K M\"{u}ller and J Eichhorn and V Streibel and I D Sharp},
url = {https://doi.org/10.1021/acsami.4c16921},
doi = {10.1021/acsami.4c16921},
issn = {1944-8244},
year = {2025},
date = {2025-02-05},
journal = {ACS Applied Materials \& Interfaces},
volume = {17},
number = {5},
pages = {7958-7968},
abstract = {Zinc nitride (Zn3N2) comprises earth-abundant elements, possesses a small direct bandgap, and is characterized by high electron mobility. While these characteristics make the material a promising compound semiconductor for various optoelectronic applications, including photovoltaics and thin-film transistors, it commonly exhibits unintentional degenerate n-type conductivity. This degenerate character has significantly impeded the development of Zn3N2 for technological applications and is commonly assumed to arise from incorporation of oxygen impurities. However, consistent understanding and control of the role of native and impurity defects on the optoelectronic properties of this otherwise promising semiconductor have not yet emerged. Here, we systematically synthesize epitaxial Zn3N2 thin films with controlled oxygen impurity concentrations of up to 20 at % by plasma-assisted molecular beam epitaxy (PA-MBE). Contrary to expectations, we find that oxygen does not lead to degenerate conductivity but instead serves as a compensating defect, the control of which can be used to achieve nondegenerate semiconducting thin films with free electron concentrations in the range of 1017 cm\textendash3, while retaining high mobilities in excess of 200 cm2 V\textendash1 s\textendash1. This understanding of the beneficial role of oxygen thus provides a route to controllably synthesize nondegenerate O-doped Zn3N2 for optoelectronic applications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Zinc nitride (Zn3N2) comprises earth-abundant elements, possesses a small direct bandgap, and is characterized by high electron mobility. While these characteristics make the material a promising compound semiconductor for various optoelectronic applications, including photovoltaics and thin-film transistors, it commonly exhibits unintentional degenerate n-type conductivity. This degenerate character has significantly impeded the development of Zn3N2 for technological applications and is commonly assumed to arise from incorporation of oxygen impurities. However, consistent understanding and control of the role of native and impurity defects on the optoelectronic properties of this otherwise promising semiconductor have not yet emerged. Here, we systematically synthesize epitaxial Zn3N2 thin films with controlled oxygen impurity concentrations of up to 20 at % by plasma-assisted molecular beam epitaxy (PA-MBE). Contrary to expectations, we find that oxygen does not lead to degenerate conductivity but instead serves as a compensating defect, the control of which can be used to achieve nondegenerate semiconducting thin films with free electron concentrations in the range of 1017 cm–3, while retaining high mobilities in excess of 200 cm2 V–1 s–1. This understanding of the beneficial role of oxygen thus provides a route to controllably synthesize nondegenerate O-doped Zn3N2 for optoelectronic applications.
24.
K Nisi, J C Thomas, S Levashov, E Mitterreiter, T Taniguchi, K Watanabe, S Aloni, T R Kuykendall, J Eichhorn, A W Holleitner, A Weber-Bargioni, C Kastl
Scanning probe spectroscopy of sulfur vacancies and MoS2 monolayers in side-contacted van der Waals heterostructures Journal Article
In: 2D Materials, vol. 12, no. 1, pp. 015023, 2024, ISSN: 2053-1583.
@article{nokey,
title = {Scanning probe spectroscopy of sulfur vacancies and MoS2 monolayers in side-contacted van der Waals heterostructures},
author = {K Nisi and J C Thomas and S Levashov and E Mitterreiter and T Taniguchi and K Watanabe and S Aloni and T R Kuykendall and J Eichhorn and A W Holleitner and A Weber-Bargioni and C Kastl},
url = {https://dx.doi.org/10.1088/2053-1583/ada046},
doi = {10.1088/2053-1583/ada046},
issn = {2053-1583},
year = {2024},
date = {2024-12-30},
journal = {2D Materials},
volume = {12},
number = {1},
pages = {015023},
abstract = {We investigate the interplay between vertical tunneling and lateral transport phenomena in electrically contacted van der Waals heterostructures made from monolayer MoS2, hBN, and graphene. We compare data taken by low-temperature scanning tunneling spectroscopy to results from room-temperature conductive atomic force spectroscopy on monolayer MoS2 with sulfur vacancies and with varying hBN layers. We show that for thick hBN barrier layers, where tunneling currents into the conductive substrate are suppressed, a side-contact still enables addressing the defect states in the scanning tunneling microscopy via the lateral current flow. Few-layer hBN realizes an intermediate regime in which the competition between vertical tunneling and lateral transport needs to be considered. The latter is relevant for device structures with both a thin tunneling barrier and a side-contact to the semiconducting layers.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We investigate the interplay between vertical tunneling and lateral transport phenomena in electrically contacted van der Waals heterostructures made from monolayer MoS2, hBN, and graphene. We compare data taken by low-temperature scanning tunneling spectroscopy to results from room-temperature conductive atomic force spectroscopy on monolayer MoS2 with sulfur vacancies and with varying hBN layers. We show that for thick hBN barrier layers, where tunneling currents into the conductive substrate are suppressed, a side-contact still enables addressing the defect states in the scanning tunneling microscopy via the lateral current flow. Few-layer hBN realizes an intermediate regime in which the competition between vertical tunneling and lateral transport needs to be considered. The latter is relevant for device structures with both a thin tunneling barrier and a side-contact to the semiconducting layers.
23.
P Moser, L M Wolz, A Henning, A Thurn, M Kuhl, P Ji, P Soubelet, M Schalk, J Eichhorn, I D Sharp, A V Stier, J J Finley
Atomically Flat Dielectric Patterns for Bandgap Engineering and Lateral Junction Formation in MoSe2 Monolayers Journal Article
In: Advanced Functional Materials, vol. n/a, no. n/a, pp. 2418528, 2024, ISSN: 1616-301X.
@article{nokey,
title = {Atomically Flat Dielectric Patterns for Bandgap Engineering and Lateral Junction Formation in MoSe2 Monolayers},
author = {P Moser and L M Wolz and A Henning and A Thurn and M Kuhl and P Ji and P Soubelet and M Schalk and J Eichhorn and I D Sharp and A V Stier and J J Finley},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202418528},
doi = {https://doi.org/10.1002/adfm.202418528},
issn = {1616-301X},
year = {2024},
date = {2024-12-23},
journal = {Advanced Functional Materials},
volume = {n/a},
number = {n/a},
pages = {2418528},
abstract = {Abstract Combining a precise sputter etching method with subsequent AlOx growth within an atomic layer deposition chamber enables the fabrication of atomically flat lateral patterns of SiO2 and AlOx. The transfer of MoSe2 monolayers onto these dielectrically modulated substrates results in the formation of lateral heterojunctions due to the interaction with alternating regions of SiO2 and AlOx, with the flat substrate topography leading to minimal strain across the junction. Kelvin probe force microscopy measurements show significant variations in the contact potential difference (CPD) across the interface, with AlOx regions inducing a 230 mV increase in CPD. Photoluminescence spectroscopy reveals shifts in spectral weight of neutral and charged exciton species across the different dielectric regions. On the AlOx side, the Fermi energy moves closer to the conduction band, leading to a higher trion-to-exciton ratio, indicating a bandgap shift consistent with CPD changes. In addition, transient reflection spectroscopy highlights the influence of the dielectric environment on carrier dynamics, with the SiO2 side exhibiting rapid carrier decay typical of neutral exciton recombination. In contrast, the AlOx side shows slower, mixed decay behavior consistent with conversion of trions back into excitons. These results demonstrate how dielectric substrate engineering can tune 2D materials, allowing scalable fabrication of advanced junctions for novel (opto)electronics applications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Combining a precise sputter etching method with subsequent AlOx growth within an atomic layer deposition chamber enables the fabrication of atomically flat lateral patterns of SiO2 and AlOx. The transfer of MoSe2 monolayers onto these dielectrically modulated substrates results in the formation of lateral heterojunctions due to the interaction with alternating regions of SiO2 and AlOx, with the flat substrate topography leading to minimal strain across the junction. Kelvin probe force microscopy measurements show significant variations in the contact potential difference (CPD) across the interface, with AlOx regions inducing a 230 mV increase in CPD. Photoluminescence spectroscopy reveals shifts in spectral weight of neutral and charged exciton species across the different dielectric regions. On the AlOx side, the Fermi energy moves closer to the conduction band, leading to a higher trion-to-exciton ratio, indicating a bandgap shift consistent with CPD changes. In addition, transient reflection spectroscopy highlights the influence of the dielectric environment on carrier dynamics, with the SiO2 side exhibiting rapid carrier decay typical of neutral exciton recombination. In contrast, the AlOx side shows slower, mixed decay behavior consistent with conversion of trions back into excitons. These results demonstrate how dielectric substrate engineering can tune 2D materials, allowing scalable fabrication of advanced junctions for novel (opto)electronics applications.
22.
E Sirotti, S Böhm, G Grötzner, M Christis, L I Wagner, L Wolz, F Munnik, J Eichhorn, M Stutzmann, V Streibel, I D Sharp
Amorphous nitride semiconductors with highly tunable optical and electronic properties: the benefits of disorder in Ca–Zn–N thin films Journal Article
In: Materials Horizons, 2024, ISSN: 2051-6347.
@article{nokey,
title = {Amorphous nitride semiconductors with highly tunable optical and electronic properties: the benefits of disorder in Ca\textendashZn\textendashN thin films},
author = {E Sirotti and S B\"{o}hm and G Gr\"{o}tzner and M Christis and L I Wagner and L Wolz and F Munnik and J Eichhorn and M Stutzmann and V Streibel and I D Sharp},
url = {http://dx.doi.org/10.1039/D4MH01525H},
doi = {10.1039/D4MH01525H},
issn = {2051-6347},
year = {2024},
date = {2024-12-16},
journal = {Materials Horizons},
abstract = {Semiconducting ternary nitrides are a promising class of materials that have received increasing attention in recent years, but often show high free electron concentrations due to the low defect formation energies of nitrogen vacancies and substitutional oxygen, leading to degenerate n-type doping. To achieve non-degenerate behavior, we now investigate a family of amorphous calcium\textendashzinc nitride (Ca\textendashZn\textendashN) thin films. By adjusting the metal cation ratios, we demonstrate band gap tunability between 1.4 and 2.0 eV and control over the charge carrier concentration across six orders of magnitude, all while maintaining high mobilities between 5 and 70 cm2 V−1 s−1. The combination of favorable electronic properties, low synthesis temperatures, and earth-abundant elements makes amorphous Ca\textendashZn\textendashN highly promising for future sustainable electronics. Moreover, the successful synthesis of such materials, as well as their broad optical and electrical tunability, paves the way for a new class of tailored functional materials: amorphous nitride semiconductors \textendash ANSs.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Semiconducting ternary nitrides are a promising class of materials that have received increasing attention in recent years, but often show high free electron concentrations due to the low defect formation energies of nitrogen vacancies and substitutional oxygen, leading to degenerate n-type doping. To achieve non-degenerate behavior, we now investigate a family of amorphous calcium–zinc nitride (Ca–Zn–N) thin films. By adjusting the metal cation ratios, we demonstrate band gap tunability between 1.4 and 2.0 eV and control over the charge carrier concentration across six orders of magnitude, all while maintaining high mobilities between 5 and 70 cm2 V−1 s−1. The combination of favorable electronic properties, low synthesis temperatures, and earth-abundant elements makes amorphous Ca–Zn–N highly promising for future sustainable electronics. Moreover, the successful synthesis of such materials, as well as their broad optical and electrical tunability, paves the way for a new class of tailored functional materials: amorphous nitride semiconductors – ANSs.
21.
J Chen, Q Zhong, E Sirotti, G Zhou, L Wolz, V Streibel, J Dittloff, J Eichhorn, Y Ji, L Zhao, R Zhu, I D Sharp
Ligand-Tuned AgBiS2 Planar Heterojunctions Enable Efficient Ultrathin Solar Cells Journal Article
In: ACS Nano, vol. 18, no. 49, pp. 33348-33358, 2024, ISSN: 1936-0851.
@article{nokey,
title = {Ligand-Tuned AgBiS2 Planar Heterojunctions Enable Efficient Ultrathin Solar Cells},
author = {J Chen and Q Zhong and E Sirotti and G Zhou and L Wolz and V Streibel and J Dittloff and J Eichhorn and Y Ji and L Zhao and R Zhu and I D Sharp},
url = {https://doi.org/10.1021/acsnano.4c07621},
doi = {10.1021/acsnano.4c07621},
issn = {1936-0851},
year = {2024},
date = {2024-12-10},
journal = {ACS Nano},
volume = {18},
number = {49},
pages = {33348-33358},
abstract = {AgBiS2 quantum dots (ABS QDs) have emerged as highly promising candidates for photovoltaic applications due to their strong sunlight absorption, nontoxicity, and elemental availability. Nevertheless, the efficiencies of ABS solar cells currently fall far short of their thermodynamic limits due in large part to sluggish charge transport characteristics in nanocrystal-derived films. In this study, we overcome this limitation by tuning the surfaces of ABS semiconductor QDs via a solvent-induced ligand exchange (SILE) strategy and provide key insights into the role of surface composition on both n- and p-type charge transfer doping, as well as long-range charge transport. Using this approach, the electronic properties of ABS films were systematically modulated, thereby enabling the design of planar p\textendashn heterojunctions featuring favorable band alignment for solar cell applications. Carrier transport and separation are significantly enhanced by the built-in electric fields generated within the ultrathin (30 nm) ABS heterojunction absorber layers, resulting in a notable solar-cell power conversion efficiency of 7.43%. Overall, this study presents a systematic and straightforward strategy to tune not only the surfaces of ABS, but also the electronic properties of solid-state films, thereby enabling junction engineering for the development of advanced semiconductor structures tailored for photovoltaic applications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
AgBiS2 quantum dots (ABS QDs) have emerged as highly promising candidates for photovoltaic applications due to their strong sunlight absorption, nontoxicity, and elemental availability. Nevertheless, the efficiencies of ABS solar cells currently fall far short of their thermodynamic limits due in large part to sluggish charge transport characteristics in nanocrystal-derived films. In this study, we overcome this limitation by tuning the surfaces of ABS semiconductor QDs via a solvent-induced ligand exchange (SILE) strategy and provide key insights into the role of surface composition on both n- and p-type charge transfer doping, as well as long-range charge transport. Using this approach, the electronic properties of ABS films were systematically modulated, thereby enabling the design of planar p–n heterojunctions featuring favorable band alignment for solar cell applications. Carrier transport and separation are significantly enhanced by the built-in electric fields generated within the ultrathin (30 nm) ABS heterojunction absorber layers, resulting in a notable solar-cell power conversion efficiency of 7.43%. Overall, this study presents a systematic and straightforward strategy to tune not only the surfaces of ABS, but also the electronic properties of solid-state films, thereby enabling junction engineering for the development of advanced semiconductor structures tailored for photovoltaic applications.
20.
P Bootz, K Frank, J Eichhorn, M Döblinger, T Bagaria, B Nickel, J Feldmann, B Debnath
S-Scheme Interface Between K–C3N4 and FePS3 Fosters Photocatalytic H2 Evolution Journal Article
In: ACS Applied Materials & Interfaces, vol. 16, no. 47, pp. 65610-65619, 2024, ISSN: 1944-8244.
@article{nokey,
title = {S-Scheme Interface Between K\textendashC3N4 and FePS3 Fosters Photocatalytic H2 Evolution},
author = {P Bootz and K Frank and J Eichhorn and M D\"{o}blinger and T Bagaria and B Nickel and J Feldmann and B Debnath},
url = {https://doi.org/10.1021/acsami.4c15236},
doi = {10.1021/acsami.4c15236},
issn = {1944-8244},
year = {2024},
date = {2024-11-27},
journal = {ACS Applied Materials \& Interfaces},
volume = {16},
number = {47},
pages = {65610-65619},
abstract = {In photocatalysis, photogenerated charge separation is pivotal and can be achieved through various mechanisms. Building heterojunctions is a promising method to enhance charge separation, where effective contact and charge exchange between heterojunction components remains challenging. Mostly used synthesis processes for making heterostructures require high temperatures, difficult processes, or expensive materials. Herein, a heterojunction of potassium intercalated graphitic carbon nitride (K-CN) and nanoflakes of iron phosphor trisulfide (FPS) is designed via a simple mechanical grinding process to boost the hydrogen evolution by a factor of more than 25 compared to pure K-CN. This significant improvement is rarely reached by other combinations of two semiconductors without cocatalysts, such as platinum. It can be attributed to the band alignment and band bending of an S-scheme that is validated via optical and X-ray photoelectron spectroscopy. As a consequence, strong quenching of the photoluminescence and significant H2 evolution occur for this unique heterojunction. Furthermore, the excellent durability of the designed photocatalytic heterostructure is confirmed by monitoring the catalysts’ H2-evolution rate and crystal structure after 72 h under light illumination. This study opens up promising and simple pathways for constructing efficient S-scheme heterojunctions for photocatalytic water-splitting.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In photocatalysis, photogenerated charge separation is pivotal and can be achieved through various mechanisms. Building heterojunctions is a promising method to enhance charge separation, where effective contact and charge exchange between heterojunction components remains challenging. Mostly used synthesis processes for making heterostructures require high temperatures, difficult processes, or expensive materials. Herein, a heterojunction of potassium intercalated graphitic carbon nitride (K-CN) and nanoflakes of iron phosphor trisulfide (FPS) is designed via a simple mechanical grinding process to boost the hydrogen evolution by a factor of more than 25 compared to pure K-CN. This significant improvement is rarely reached by other combinations of two semiconductors without cocatalysts, such as platinum. It can be attributed to the band alignment and band bending of an S-scheme that is validated via optical and X-ray photoelectron spectroscopy. As a consequence, strong quenching of the photoluminescence and significant H2 evolution occur for this unique heterojunction. Furthermore, the excellent durability of the designed photocatalytic heterostructure is confirmed by monitoring the catalysts’ H2-evolution rate and crystal structure after 72 h under light illumination. This study opens up promising and simple pathways for constructing efficient S-scheme heterojunctions for photocatalytic water-splitting.
19.
E Sirotti, L I Wagner, C-M Jiang, J Eichhorn, F Munnik, V Streibel, M J Schilcher, B März, F S Hegner, M Kuhl, T Höldrich, K Müller-Caspary, D A Egger, I D Sharp
Beyond Cation Disorder: Site Symmetry-Tuned Optoelectronic Properties of the Ternary Nitride Photoabsorber ZrTaN3 Journal Article
In: Advanced Energy Materials, vol. 14, no. 42, pp. 2402540, 2024, ISSN: 1614-6832.
@article{nokey,
title = {Beyond Cation Disorder: Site Symmetry-Tuned Optoelectronic Properties of the Ternary Nitride Photoabsorber ZrTaN3},
author = {E Sirotti and L I Wagner and C-M Jiang and J Eichhorn and F Munnik and V Streibel and M J Schilcher and B M\"{a}rz and F S Hegner and M Kuhl and T H\"{o}ldrich and K M\"{u}ller-Caspary and D A Egger and I D Sharp},
url = {https://doi.org/10.1002/aenm.202402540},
doi = {https://doi.org/10.1002/aenm.202402540},
issn = {1614-6832},
year = {2024},
date = {2024-11-01},
journal = {Advanced Energy Materials},
volume = {14},
number = {42},
pages = {2402540},
abstract = {Abstract Ternary nitrides are rapidly emerging as promising compounds for optoelectronic and energy conversion applications, yet comparatively little of this vast composition space has been explored. Furthermore, the crystal structures of these compounds can exhibit a significant amount of disorder, the consequences of which are not yet well understood. Here, the deposition of bixbyite-type ZrTaN3 thin films is demonstrated by reactive magnetron co-sputtering and observed semiconducting character, with a strong optical absorption onset at 1.8 eV and significant photoactivity, with prospective application as functional photoanodes. It is found that Wyckoff-site occupancy of cations is a critical factor in determining these beneficial optoelectronic properties. First-principles calculations show that cation disorder leads to minor deviations in the total energy but modulates the bandgap by 0.5 eV, changing orbital hybridization of valence and conduction band states. In addition to demonstrating that ZrTaN3 is a promising visible light-absorbing semiconductor and active photoanode material, the findings provide important insights regarding the role of cation ordering on the electronic structure of ternary semiconductors. In particular, it is shown that not only cation order, but also the cationic Wyckoff site occupancy has a substantial impact on key optoelectronic properties, which can guide future design and synthesis of advanced semiconductors.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Ternary nitrides are rapidly emerging as promising compounds for optoelectronic and energy conversion applications, yet comparatively little of this vast composition space has been explored. Furthermore, the crystal structures of these compounds can exhibit a significant amount of disorder, the consequences of which are not yet well understood. Here, the deposition of bixbyite-type ZrTaN3 thin films is demonstrated by reactive magnetron co-sputtering and observed semiconducting character, with a strong optical absorption onset at 1.8 eV and significant photoactivity, with prospective application as functional photoanodes. It is found that Wyckoff-site occupancy of cations is a critical factor in determining these beneficial optoelectronic properties. First-principles calculations show that cation disorder leads to minor deviations in the total energy but modulates the bandgap by 0.5 eV, changing orbital hybridization of valence and conduction band states. In addition to demonstrating that ZrTaN3 is a promising visible light-absorbing semiconductor and active photoanode material, the findings provide important insights regarding the role of cation ordering on the electronic structure of ternary semiconductors. In particular, it is shown that not only cation order, but also the cationic Wyckoff site occupancy has a substantial impact on key optoelectronic properties, which can guide future design and synthesis of advanced semiconductors.
18.
L M Wolz, G Grötzner, T Rieth, L I Wagner, M Kuhl, J Dittloff, G Zhou, S Santra, V Streibel, F Munnik, I D Sharp, J Eichhorn
Impact of Defects and Disorder on the Stability of Ta3N5 Photoanodes Journal Article
In: Advanced Functional Materials, vol. 34, no. 40, pp. 2405532, 2024, ISSN: 1616-301X.
@article{nokey,
title = {Impact of Defects and Disorder on the Stability of Ta3N5 Photoanodes},
author = {L M Wolz and G Gr\"{o}tzner and T Rieth and L I Wagner and M Kuhl and J Dittloff and G Zhou and S Santra and V Streibel and F Munnik and I D Sharp and J Eichhorn},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202405532},
doi = {https://doi.org/10.1002/adfm.202405532},
issn = {1616-301X},
year = {2024},
date = {2024-07-10},
journal = {Advanced Functional Materials},
volume = {34},
number = {40},
pages = {2405532},
abstract = {Abstract The photoelectrochemical performance of Ta3N5 photoanodes is strongly impacted by the presence of shallow and deep defects within the bandgap. However, the role of such states in defining stability under operational conditions is not well understood. Here, a highly controllable synthesis approach is used to create homogenous Ta3N5 thin films with tailored defect concentrations to establish the relationship between atomic-scale point defects and macroscale stability. Reduced oxygen contents increase long-range structural order but lead to high concentrations of deep-level states, while higher oxygen contents result in reduced structural order but beneficially passivate deep-level defects. Despite the different defect properties, the synthesized photoelectrodes degrade similarly under water oxidation conditions due to the formation of a surface oxide layer that blocks interfacial hole injection and accelerates charge recombination. In contrast, under ferrocyanide oxidation conditions, it is found that Ta3N5 films with high oxygen concentrations exhibit long-term stability, whereas those possessing lower oxygen contents and higher deep-level defect concentrations rapidly degrade. These results indicate that deep-level defects result in rapid trapping of photocarriers and surface oxidation but that shallow oxygen donors can be introduced into Ta3N5 to enable kinetic stabilization of the interface.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract The photoelectrochemical performance of Ta3N5 photoanodes is strongly impacted by the presence of shallow and deep defects within the bandgap. However, the role of such states in defining stability under operational conditions is not well understood. Here, a highly controllable synthesis approach is used to create homogenous Ta3N5 thin films with tailored defect concentrations to establish the relationship between atomic-scale point defects and macroscale stability. Reduced oxygen contents increase long-range structural order but lead to high concentrations of deep-level states, while higher oxygen contents result in reduced structural order but beneficially passivate deep-level defects. Despite the different defect properties, the synthesized photoelectrodes degrade similarly under water oxidation conditions due to the formation of a surface oxide layer that blocks interfacial hole injection and accelerates charge recombination. In contrast, under ferrocyanide oxidation conditions, it is found that Ta3N5 films with high oxygen concentrations exhibit long-term stability, whereas those possessing lower oxygen contents and higher deep-level defect concentrations rapidly degrade. These results indicate that deep-level defects result in rapid trapping of photocarriers and surface oxidation but that shallow oxygen donors can be introduced into Ta3N5 to enable kinetic stabilization of the interface.
17.
V Streibel, J L Schönecker, L I Wagner, E Sirotti, F Munnik, M Kuhl, C-M Jiang, J Eichhorn, S Santra, I D Sharp
Zirconium Oxynitride Thin Films for Photoelectrochemical Water Splitting Journal Article
In: ACS Applied Energy Materials, vol. 7, no. 9, pp. 4004-4015, 2024.
@article{nokey,
title = {Zirconium Oxynitride Thin Films for Photoelectrochemical Water Splitting},
author = {V Streibel and J L Sch\"{o}necker and L I Wagner and E Sirotti and F Munnik and M Kuhl and C-M Jiang and J Eichhorn and S Santra and I D Sharp},
url = {https://doi.org/10.1021/acsaem.4c00303},
doi = {10.1021/acsaem.4c00303},
year = {2024},
date = {2024-05-13},
journal = {ACS Applied Energy Materials},
volume = {7},
number = {9},
pages = {4004-4015},
abstract = {Transition metal oxynitrides are a promising class of functional materials for photoelectrochemical (PEC) applications. Although these compounds are most commonly synthesized via ammonolysis of oxide precursors, such synthetic routes often lead to poorly controlled oxygen-to-nitrogen anion ratios, and the harsh nitridation conditions are incompatible with many substrates, including transparent conductive oxides. Here, we report direct reactive sputter deposition of a family of zirconium oxynitride thin films and the comprehensive characterization of their tunable structural, optical, and functional PEC properties. Systematic increases of the oxygen content in the reactive sputter gas mixture enable access to different crystalline structures within the zirconium oxynitride family. Increasing oxygen contents lead to a transition from metallic to semiconducting to insulating phases. In particular, crystalline Zr2ON2-like films have band gaps in the UV\textendashvisible range and are n-type semiconductors. These properties, together with a valence band maximum position located favorably relative to the water oxidation potential, make them viable photoanode candidates. Using chopped linear sweep voltammetry, we indeed confirm that our Zr2ON2 films are PEC-active for the oxygen evolution reaction in alkaline electrolytes. We further show that high-vacuum annealing boosts their PEC performance characteristics. Although the observed photocurrents are low compared to state-of-the-art photoanodes, these dense and planar thin films can offer a valuable platform for studying oxynitride photoelectrodes, as well as for future nanostructuring, band gap engineering, and defect engineering efforts.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Transition metal oxynitrides are a promising class of functional materials for photoelectrochemical (PEC) applications. Although these compounds are most commonly synthesized via ammonolysis of oxide precursors, such synthetic routes often lead to poorly controlled oxygen-to-nitrogen anion ratios, and the harsh nitridation conditions are incompatible with many substrates, including transparent conductive oxides. Here, we report direct reactive sputter deposition of a family of zirconium oxynitride thin films and the comprehensive characterization of their tunable structural, optical, and functional PEC properties. Systematic increases of the oxygen content in the reactive sputter gas mixture enable access to different crystalline structures within the zirconium oxynitride family. Increasing oxygen contents lead to a transition from metallic to semiconducting to insulating phases. In particular, crystalline Zr2ON2-like films have band gaps in the UV–visible range and are n-type semiconductors. These properties, together with a valence band maximum position located favorably relative to the water oxidation potential, make them viable photoanode candidates. Using chopped linear sweep voltammetry, we indeed confirm that our Zr2ON2 films are PEC-active for the oxygen evolution reaction in alkaline electrolytes. We further show that high-vacuum annealing boosts their PEC performance characteristics. Although the observed photocurrents are low compared to state-of-the-art photoanodes, these dense and planar thin films can offer a valuable platform for studying oxynitride photoelectrodes, as well as for future nanostructuring, band gap engineering, and defect engineering efforts.
16.
Y Zou, J Eichhorn, J Zhang, F C Apfelbeck, S Yin, L Wolz, C-C Chen, I D Sharp, P Müller-Buschbaum
Microstrain and Crystal Orientation Variation within Naked Triple-Cation Mixed Halide Perovskites under Heat, UV, and Visible Light Exposure Journal Article
In: ACS Energy Letters, vol. 9, no. 2, pp. 388-399, 2024.
@article{nokey,
title = {Microstrain and Crystal Orientation Variation within Naked Triple-Cation Mixed Halide Perovskites under Heat, UV, and Visible Light Exposure},
author = {Y Zou and J Eichhorn and J Zhang and F C Apfelbeck and S Yin and L Wolz and C-C Chen and I D Sharp and P M\"{u}ller-Buschbaum},
url = {https://doi.org/10.1021/acsenergylett.3c02617},
doi = {10.1021/acsenergylett.3c02617},
year = {2024},
date = {2024-02-09},
journal = {ACS Energy Letters},
volume = {9},
number = {2},
pages = {388-399},
abstract = {The instability of perovskite absorbers under various environmental stressors is the most significant obstacle to widespread commercialization of perovskite solar cells. Herein, we study the evolution of crystal structure and microstrain present in naked triple-cation mixed CsMAFA-based perovskite films under heat, UV, and visible light (1 Sun) conditions by grazing-incidence wide-angle X-ray scattering (GIWAXS). We find that the microstrain is gradient distributed along the surface normal of the films, decreasing from the upper surface to regions deeper within the film. Moreover, heat, UV, and visible light treatments do not interfere with the crystalline orientations within annealed polycrystalline films. However, when subjected to heat, the naked perovskite films exhibit a rapid component decomposition, induced by phase separation and ion migration. Conversely, under exposure to UV and 1 Sun light soaking, the naked perovskite films undergo a self-optimization structure evolution during degradation and develop into smoother films with reduced surface potential fluctuations.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The instability of perovskite absorbers under various environmental stressors is the most significant obstacle to widespread commercialization of perovskite solar cells. Herein, we study the evolution of crystal structure and microstrain present in naked triple-cation mixed CsMAFA-based perovskite films under heat, UV, and visible light (1 Sun) conditions by grazing-incidence wide-angle X-ray scattering (GIWAXS). We find that the microstrain is gradient distributed along the surface normal of the films, decreasing from the upper surface to regions deeper within the film. Moreover, heat, UV, and visible light treatments do not interfere with the crystalline orientations within annealed polycrystalline films. However, when subjected to heat, the naked perovskite films exhibit a rapid component decomposition, induced by phase separation and ion migration. Conversely, under exposure to UV and 1 Sun light soaking, the naked perovskite films undergo a self-optimization structure evolution during degradation and develop into smoother films with reduced surface potential fluctuations.
15.
M Aust, M I Schönherr, D P Halter, L Schröck, T Pickl, S N Deger, M Z Hussain, A Jentys, R Bühler, Z Zhang, K Meyer, M Kuhl, J Eichhorn, D D Medina, A Pöthig, R A Fischer
Benzene-1,4-Di(dithiocarboxylate) Linker-Based Coordination Polymers of Mn2+, Zn2+, and Mixed-Valence Fe2+/3+ Journal Article
In: Inorganic Chemistry, vol. 63, no. 1, pp. 129-140, 2024, ISSN: 0020-1669.
@article{nokey,
title = {Benzene-1,4-Di(dithiocarboxylate) Linker-Based Coordination Polymers of Mn2+, Zn2+, and Mixed-Valence Fe2+/3+},
author = {M Aust and M I Sch\"{o}nherr and D P Halter and L Schr\"{o}ck and T Pickl and S N Deger and M Z Hussain and A Jentys and R B\"{u}hler and Z Zhang and K Meyer and M Kuhl and J Eichhorn and D D Medina and A P\"{o}thig and R A Fischer},
url = {https://doi.org/10.1021/acs.inorgchem.3c02471},
doi = {10.1021/acs.inorgchem.3c02471},
issn = {0020-1669},
year = {2024},
date = {2024-01-08},
journal = {Inorganic Chemistry},
volume = {63},
number = {1},
pages = {129-140},
abstract = {Three new coordination polymers (CPs) constructed from the linker 1,4-di(dithiocarboxylate) (BDDTC2\textendash)─the sulfur-analog of 1,4-benzenedicarboxylate (BDC2\textendash)─together with Mn-, Zn-, and Fe-based inorganic SBUs are reported with description of their structural and electronic properties. Single-crystal X-ray diffraction revealed structural diversity ranging from one-dimensional chains in [Mn(BDDTC)(DMF)2] (1) to two-dimensional (2D) honeycomb sheets observed for [Zn2(BDDTC)3][Zn(DMF)5(H2O)] (2). Gas adsorption experiments confirmed a 3D porous structure for the mixed-valent material [Fe2(BDDTC)2(OH)] (3). 3 contains a 1:1 ratio of Fe2+/3+ ions, as evidenced by 57Fe M\"{o}ssbauer, X-band EPR, and X-ray absorption spectroscopy. Its empirical formula was established by elemental analysis, thermal gravimetric analysis, infrared vibrational spectroscopy, and X-ray absorption spectroscopy in lieu of elusive single-crystal X-ray diffraction data. In contrast to the Mn- and Zn-based compounds 1 and 2, the Fe2+/3+ CP 3 showed remarkably high electrical conductivity of 5 × 10\textendash3 S cm\textendash1 (according to van der Pauw measurements), which is within the range of semiconducting materials. Overall, our study confirms that sulfur derivatives of typical carboxylate linkers (e.g., BDC) are suitable for the construction of electrically conducting CPs, due to acceptedly higher covalency in metal\textendashligand bonding compared to the electrically insulating carboxylate CPs or metal-organic frameworks. At the same time, the direct comparison between insulating CPs 1 and 2 with CP 3 emphasizes that the electronic structure of the metal is likewise a crucial aspect to construct electrically conductive materials.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Three new coordination polymers (CPs) constructed from the linker 1,4-di(dithiocarboxylate) (BDDTC2–)─the sulfur-analog of 1,4-benzenedicarboxylate (BDC2–)─together with Mn-, Zn-, and Fe-based inorganic SBUs are reported with description of their structural and electronic properties. Single-crystal X-ray diffraction revealed structural diversity ranging from one-dimensional chains in [Mn(BDDTC)(DMF)2] (1) to two-dimensional (2D) honeycomb sheets observed for [Zn2(BDDTC)3][Zn(DMF)5(H2O)] (2). Gas adsorption experiments confirmed a 3D porous structure for the mixed-valent material [Fe2(BDDTC)2(OH)] (3). 3 contains a 1:1 ratio of Fe2+/3+ ions, as evidenced by 57Fe Mössbauer, X-band EPR, and X-ray absorption spectroscopy. Its empirical formula was established by elemental analysis, thermal gravimetric analysis, infrared vibrational spectroscopy, and X-ray absorption spectroscopy in lieu of elusive single-crystal X-ray diffraction data. In contrast to the Mn- and Zn-based compounds 1 and 2, the Fe2+/3+ CP 3 showed remarkably high electrical conductivity of 5 × 10–3 S cm–1 (according to van der Pauw measurements), which is within the range of semiconducting materials. Overall, our study confirms that sulfur derivatives of typical carboxylate linkers (e.g., BDC) are suitable for the construction of electrically conducting CPs, due to acceptedly higher covalency in metal–ligand bonding compared to the electrically insulating carboxylate CPs or metal-organic frameworks. At the same time, the direct comparison between insulating CPs 1 and 2 with CP 3 emphasizes that the electronic structure of the metal is likewise a crucial aspect to construct electrically conductive materials.
14.
T Tian, S Tu, A Xu, S Yin, A L Oechsle, T Xiao, A Vagias, J Eichhorn, J Suo, Z Yang, S Bernstorff, P Müller-Buschbaum
Unraveling the Morphology-Function Correlation of Mesoporous ZnO Films upon Water Exposure Journal Article
In: Advanced Functional Materials, vol. n/a, no. n/a, pp. 2311793, 2023, ISSN: 1616-301X.
@article{nokey,
title = {Unraveling the Morphology-Function Correlation of Mesoporous ZnO Films upon Water Exposure},
author = {T Tian and S Tu and A Xu and S Yin and A L Oechsle and T Xiao and A Vagias and J Eichhorn and J Suo and Z Yang and S Bernstorff and P M\"{u}ller-Buschbaum},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202311793},
doi = {https://doi.org/10.1002/adfm.202311793},
issn = {1616-301X},
year = {2023},
date = {2023-11-05},
journal = {Advanced Functional Materials},
volume = {n/a},
number = {n/a},
pages = {2311793},
abstract = {Abstract Ubiquitous moisture in synthetic conditions and ambient environments can strongly influence the conductivity of ZnO semiconductors via the chemisorption and physisorption of water molecules on the ZnO surface. Such an intrinsically water-sensitive nature will become more evident in mesoporous ZnO films where a large surface area and active sites are created simultaneously. However, fundamental insights underlying water-mediated ZnO surface chemistry and electrical conductivity and the factors affecting them remain ambiguous due to the complexity of ZnO surfaces and the difficulties of in situ characterizations at multi-dimensions. Here, self-assembling diblock copolymers are exploited as structure-directing agents to achieve mesoporous ZnO thin films with highly tailorable structural characteristics ranging from nanomorphologies, over crystalline levels, to defect contents. As verified by theoretical calculations, the presence of oxygen vacancy will facilitate favorable water adsorption and subsequent dissociation on the polar ZnO surfaces. Upon humidity exposure with progressively increased levels, mesoporous ZnO films are revealed to follow an almost positive relationship between adsorption and electrical conductivity but show superior morphological stability. This work not only elucidates the water-governed ZnO surface chemistry but may also promote a comprehensive understanding of the morphology-function relationship on ZnO-based electronics.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Ubiquitous moisture in synthetic conditions and ambient environments can strongly influence the conductivity of ZnO semiconductors via the chemisorption and physisorption of water molecules on the ZnO surface. Such an intrinsically water-sensitive nature will become more evident in mesoporous ZnO films where a large surface area and active sites are created simultaneously. However, fundamental insights underlying water-mediated ZnO surface chemistry and electrical conductivity and the factors affecting them remain ambiguous due to the complexity of ZnO surfaces and the difficulties of in situ characterizations at multi-dimensions. Here, self-assembling diblock copolymers are exploited as structure-directing agents to achieve mesoporous ZnO thin films with highly tailorable structural characteristics ranging from nanomorphologies, over crystalline levels, to defect contents. As verified by theoretical calculations, the presence of oxygen vacancy will facilitate favorable water adsorption and subsequent dissociation on the polar ZnO surfaces. Upon humidity exposure with progressively increased levels, mesoporous ZnO films are revealed to follow an almost positive relationship between adsorption and electrical conductivity but show superior morphological stability. This work not only elucidates the water-governed ZnO surface chemistry but may also promote a comprehensive understanding of the morphology-function relationship on ZnO-based electronics.
13.
L I Wagner, E Sirotti, O Brune, G Grötzner, J Eichhorn, S Santra, F Munnik, L Olivi, S Pollastri, V Streibel, I D Sharp
Defect Engineering of Ta3N5 Photoanodes: Enhancing Charge Transport and Photoconversion Efficiencies via Ti Doping Journal Article
In: Advanced Functional Materials, vol. 34, pp. 2306539, 2023, ISSN: 1616-301X.
@article{nokey,
title = {Defect Engineering of Ta3N5 Photoanodes: Enhancing Charge Transport and Photoconversion Efficiencies via Ti Doping},
author = {L I Wagner and E Sirotti and O Brune and G Gr\"{o}tzner and J Eichhorn and S Santra and F Munnik and L Olivi and S Pollastri and V Streibel and I D Sharp},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202306539},
doi = {https://doi.org/10.1002/adfm.202306539},
issn = {1616-301X},
year = {2023},
date = {2023-10-19},
urldate = {2023-10-19},
journal = {Advanced Functional Materials},
volume = {34},
pages = {2306539},
abstract = {Abstract While Ta3N5 shows excellent potential as a semiconductor photoanode for solar water splitting, its performance is hindered by poor charge carrier transport and trapping due to native defects that introduce electronic states deep within its bandgap. Here, it is demonstrated that controlled Ti doping of Ta3N5 can dramatically reduce the concentration of deep-level defects and enhance its photoelectrochemical performance, yielding a sevenfold increase in photocurrent density and a 300 mV cathodic shift in photocurrent onset potential compared to undoped material. Comprehensive characterization reveals that Ti4+ ions substitute Ta5+ lattice sites, thereby introducing compensating acceptor states, reducing the concentrations of deleterious nitrogen vacancies and reducing Ta3+ states, and thereby suppressing trapping and recombination. Owing to the similar ionic radii of Ti4+ and Ta5+, substitutional doping does not introduce lattice strain or significantly affect the underlying electronic structure of the host semiconductor. Furthermore, Ti can be incorporated without increasing the oxygen donor content, thereby enabling the electrical conductivity to be tuned by over seven orders of magnitude. Thus, Ti doping of Ta3N5 provides a powerful basis for precisely engineering its optoelectronic characteristics and to substantially improve its functional characteristics as an advanced photoelectrode for solar fuels applications.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract While Ta3N5 shows excellent potential as a semiconductor photoanode for solar water splitting, its performance is hindered by poor charge carrier transport and trapping due to native defects that introduce electronic states deep within its bandgap. Here, it is demonstrated that controlled Ti doping of Ta3N5 can dramatically reduce the concentration of deep-level defects and enhance its photoelectrochemical performance, yielding a sevenfold increase in photocurrent density and a 300 mV cathodic shift in photocurrent onset potential compared to undoped material. Comprehensive characterization reveals that Ti4+ ions substitute Ta5+ lattice sites, thereby introducing compensating acceptor states, reducing the concentrations of deleterious nitrogen vacancies and reducing Ta3+ states, and thereby suppressing trapping and recombination. Owing to the similar ionic radii of Ti4+ and Ta5+, substitutional doping does not introduce lattice strain or significantly affect the underlying electronic structure of the host semiconductor. Furthermore, Ti can be incorporated without increasing the oxygen donor content, thereby enabling the electrical conductivity to be tuned by over seven orders of magnitude. Thus, Ti doping of Ta3N5 provides a powerful basis for precisely engineering its optoelectronic characteristics and to substantially improve its functional characteristics as an advanced photoelectrode for solar fuels applications.
12.
Y Zou, J Eichhorn, S Rieger, Y Zheng, S Yuan, L Wolz, L V Spanier, J E Heger, S Yin, C R Everett, L Dai, M Schwartzkopf, C Mu, S V Roth, I D Sharp, C-C Chen, J Feldmann, S D Stranks, P Müller-Buschbaum
Ionic liquids tailoring crystal orientation and electronic properties for stable perovskite solar cells Journal Article
In: Nano Energy, vol. 112, pp. 108449, 2023, ISSN: 2211-2855.
@article{nokey,
title = {Ionic liquids tailoring crystal orientation and electronic properties for stable perovskite solar cells},
author = {Y Zou and J Eichhorn and S Rieger and Y Zheng and S Yuan and L Wolz and L V Spanier and J E Heger and S Yin and C R Everett and L Dai and M Schwartzkopf and C Mu and S V Roth and I D Sharp and C-C Chen and J Feldmann and S D Stranks and P M\"{u}ller-Buschbaum},
url = {https://www.sciencedirect.com/science/article/pii/S2211285523002860},
doi = {https://doi.org/10.1016/j.nanoen.2023.108449},
issn = {2211-2855},
year = {2023},
date = {2023-04-21},
journal = {Nano Energy},
volume = {112},
pages = {108449},
abstract = {The crystallization behavior of perovskite films has a profound influence on the resulting defect densities, charge carrier dynamics and photovoltaic performance. Herein, we introduce ionic liquids into the perovskite component to tailor the crystal growth of perovskite films from a disordered to a preferential corner-up orientation and accordingly increase the charge carrier mobility to accelerate electron transport and extraction. Using time-resolved measurements, we probe the charge carrier generation, transport and recombination behavior in these films and related devices. We find the ionic liquid-containing samples exhibit lower defects, faster charge carrier transport and suppressed non-radiative recombination, contributing to higher efficiency and fill factor. Via operando grazing-incidence small- and wide-angle X-ray scattering measurements, we observe a light-induced lattice compression and grain fragmentation in the control devices, whereas the ionic liquid-containing devices exhibit a slight light-induced crystal reconstitution and stronger tolerance against illumination. Under ambient conditions, the non-encapsulated device with the pyrrolidinium-based ionic compound (Pyr14BF4) maintains 97% of its initial efficiency after 4368 h.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The crystallization behavior of perovskite films has a profound influence on the resulting defect densities, charge carrier dynamics and photovoltaic performance. Herein, we introduce ionic liquids into the perovskite component to tailor the crystal growth of perovskite films from a disordered to a preferential corner-up orientation and accordingly increase the charge carrier mobility to accelerate electron transport and extraction. Using time-resolved measurements, we probe the charge carrier generation, transport and recombination behavior in these films and related devices. We find the ionic liquid-containing samples exhibit lower defects, faster charge carrier transport and suppressed non-radiative recombination, contributing to higher efficiency and fill factor. Via operando grazing-incidence small- and wide-angle X-ray scattering measurements, we observe a light-induced lattice compression and grain fragmentation in the control devices, whereas the ionic liquid-containing devices exhibit a slight light-induced crystal reconstitution and stronger tolerance against illumination. Under ambient conditions, the non-encapsulated device with the pyrrolidinium-based ionic compound (Pyr14BF4) maintains 97% of its initial efficiency after 4368 h.
11.
Y Zou, J Eichhorn, S Rieger, Y Zheng, S Yuan, L Wolz, L V Spanier, J E Heger, S Yin, C R Everett, L Dai, M Schwartzkopf, C Mu, S V Roth, I D Sharp, C-C Chen, J Feldmann, S D Stranks, P Müller-Buschbaum
Ionic liquids tailoring crystal orientation and electronic properties for stable perovskite solar cells Journal Article
In: Nano Energy, vol. 112, pp. 108449, 2023, ISSN: 2211-2855.
@article{nokey,
title = {Ionic liquids tailoring crystal orientation and electronic properties for stable perovskite solar cells},
author = {Y Zou and J Eichhorn and S Rieger and Y Zheng and S Yuan and L Wolz and L V Spanier and J E Heger and S Yin and C R Everett and L Dai and M Schwartzkopf and C Mu and S V Roth and I D Sharp and C-C Chen and J Feldmann and S D Stranks and P M\"{u}ller-Buschbaum},
url = {https://www.sciencedirect.com/science/article/pii/S2211285523002860},
doi = {https://doi.org/10.1016/j.nanoen.2023.108449},
issn = {2211-2855},
year = {2023},
date = {2023-04-14},
journal = {Nano Energy},
volume = {112},
pages = {108449},
abstract = {The crystallization behavior of perovskite films has a profound influence on the resulting defect densities, charge carrier dynamics and photovoltaic performance. Herein, we introduce ionic liquids into the perovskite component to tailor the crystal growth of perovskite films from a disordered to a preferential corner-up orientation and accordingly increase the charge carrier mobility to accelerate electron transport and extraction. Using time-resolved measurements, we probe the charge carrier generation, transport and recombination behavior in these films and related devices. We find the ionic liquid-containing samples exhibit lower defects, faster charge carrier transport and suppressed non-radiative recombination, contributing to higher efficiency and fill factor. Via operando grazing-incidence small- and wide-angle X-ray scattering measurements, we observe a light-induced lattice compression and grain fragmentation in the control devices, whereas the ionic liquid-containing devices exhibit a slight light-induced crystal reconstitution and stronger tolerance against illumination. Under ambient conditions, the non-encapsulated device with the pyrrolidinium-based ionic compound (Pyr14BF4) maintains 97% of its initial efficiency after 4368 h.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The crystallization behavior of perovskite films has a profound influence on the resulting defect densities, charge carrier dynamics and photovoltaic performance. Herein, we introduce ionic liquids into the perovskite component to tailor the crystal growth of perovskite films from a disordered to a preferential corner-up orientation and accordingly increase the charge carrier mobility to accelerate electron transport and extraction. Using time-resolved measurements, we probe the charge carrier generation, transport and recombination behavior in these films and related devices. We find the ionic liquid-containing samples exhibit lower defects, faster charge carrier transport and suppressed non-radiative recombination, contributing to higher efficiency and fill factor. Via operando grazing-incidence small- and wide-angle X-ray scattering measurements, we observe a light-induced lattice compression and grain fragmentation in the control devices, whereas the ionic liquid-containing devices exhibit a slight light-induced crystal reconstitution and stronger tolerance against illumination. Under ambient conditions, the non-encapsulated device with the pyrrolidinium-based ionic compound (Pyr14BF4) maintains 97% of its initial efficiency after 4368 h.
10.
A Henning, J D Bartl, L Wolz, M Christis, F Rauh, M Bissolo, T Grünleitner, J Eichhorn, P Zeller, M Amati, L Gregoratti, J J Finley, B Rieger, M Stutzmann, I D Sharp
Spatially-Modulated Silicon Interface Energetics Via Hydrogen Plasma-Assisted Atomic Layer Deposition of Ultrathin Alumina Journal Article
In: Advanced Materials Interfaces, vol. 10, iss. 6, pp. 2202166, 2022, ISSN: 2196-7350.
@article{nokey,
title = {Spatially-Modulated Silicon Interface Energetics Via Hydrogen Plasma-Assisted Atomic Layer Deposition of Ultrathin Alumina},
author = {A Henning and J D Bartl and L Wolz and M Christis and F Rauh and M Bissolo and T Gr\"{u}nleitner and J Eichhorn and P Zeller and M Amati and L Gregoratti and J J Finley and B Rieger and M Stutzmann and I D Sharp},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/admi.202202166},
doi = {https://doi.org/10.1002/admi.202202166},
issn = {2196-7350},
year = {2022},
date = {2022-12-16},
urldate = {2022-12-16},
journal = {Advanced Materials Interfaces},
volume = {10},
issue = {6},
pages = {2202166},
abstract = {Abstract Atomic layer deposition (ALD) is a key technique for the continued scaling of semiconductor devices, which increasingly relies on scalable processes for interface manipulation of structured surfaces on the atomic level. While ALD allows the synthesis of conformal films with utmost control over the thickness, atomically-defined closed coatings and surface modifications are challenging to achieve because of 3D growth during nucleation. Here, a route is presented toward the sub-nanometer thin and continuous aluminum oxide (AlOx) coatings on silicon substrates for the spatial control of the surface charge density and interface energetics. Trimethylaluminum in combination with remote hydrogen plasma is used instead of a gas-phase oxidant for the transformation of silicon dioxide (SiO2) into alumina. Depending on the number of ALD cycles, the SiO2 can be partially or fully transformed, which is exploited to deposit ultrathin AlOx layers in selected regions defined by lithographic patterning. The resulting patterned surfaces are characterized by lateral AlOx/SiO2 interfaces possessing 0.3 nm step heights and surface potential steps exceeding 0.4 V. In addition, the introduction of fixed negative charges of 9 × 1012 cm−2 enables modulation of the surface band bending, which is relevant to the field-effect passivation of silicon and low-impedance charge transfer across contact interfaces.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Atomic layer deposition (ALD) is a key technique for the continued scaling of semiconductor devices, which increasingly relies on scalable processes for interface manipulation of structured surfaces on the atomic level. While ALD allows the synthesis of conformal films with utmost control over the thickness, atomically-defined closed coatings and surface modifications are challenging to achieve because of 3D growth during nucleation. Here, a route is presented toward the sub-nanometer thin and continuous aluminum oxide (AlOx) coatings on silicon substrates for the spatial control of the surface charge density and interface energetics. Trimethylaluminum in combination with remote hydrogen plasma is used instead of a gas-phase oxidant for the transformation of silicon dioxide (SiO2) into alumina. Depending on the number of ALD cycles, the SiO2 can be partially or fully transformed, which is exploited to deposit ultrathin AlOx layers in selected regions defined by lithographic patterning. The resulting patterned surfaces are characterized by lateral AlOx/SiO2 interfaces possessing 0.3 nm step heights and surface potential steps exceeding 0.4 V. In addition, the introduction of fixed negative charges of 9 × 1012 cm−2 enables modulation of the surface band bending, which is relevant to the field-effect passivation of silicon and low-impedance charge transfer across contact interfaces.
9.
T Grünleitner, A Henning, M Bissolo, M Zengerle, L Gregoratti, M Amati, P Zeller, J Eichhorn, A V Stier, A W Holleitner, J J Finley, I D Sharp
In: ACS Nano, 2022, ISSN: 1936-0851.
@article{nokey,
title = {Real-Time Investigation of Sulfur Vacancy Generation and Passivation in Monolayer Molybdenum Disulfide via in situ X-ray Photoelectron Spectromicroscopy},
author = {T Gr\"{u}nleitner and A Henning and M Bissolo and M Zengerle and L Gregoratti and M Amati and P Zeller and J Eichhorn and A V Stier and A W Holleitner and J J Finley and I D Sharp},
url = {https://doi.org/10.1021/acsnano.2c06317},
doi = {10.1021/acsnano.2c06317},
issn = {1936-0851},
year = {2022},
date = {2022-12-14},
journal = {ACS Nano},
abstract = {Understanding the chemical and electronic properties of point defects in two-dimensional materials, as well as their generation and passivation, is essential for the development of functional systems, spanning from next-generation optoelectronic devices to advanced catalysis. Here, we use synchrotron-based X-ray photoelectron spectroscopy (XPS) with submicron spatial resolution to create sulfur vacancies (SVs) in monolayer MoS2 and monitor their chemical and electronic properties in situ during the defect creation process. X-ray irradiation leads to the emergence of a distinct Mo 3d spectral feature associated with undercoordinated Mo atoms. Real-time analysis of the evolution of this feature, along with the decrease of S content, reveals predominant monosulfur vacancy generation at low doses and preferential disulfur vacancy generation at high doses. Formation of these defects leads to a shift of the Fermi level toward the valence band (VB) edge, introduction of electronic states within the VB, and formation of lateral pn junctions. These findings are consistent with theoretical predictions that SVs serve as deep acceptors and are not responsible for the ubiquitous n-type conductivity of MoS2. In addition, we find that these defects are metastable upon short-term exposure to ambient air. By contrast, in situ oxygen exposure during XPS measurements enables passivation of SVs, resulting in partial elimination of undercoordinated Mo sites and reduction of SV-related states near the VB edge. Correlative Raman spectroscopy and photoluminescence measurements confirm our findings of localized SV generation and passivation, thereby demonstrating the connection between chemical, structural, and optoelectronic properties of SVs in MoS2.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Understanding the chemical and electronic properties of point defects in two-dimensional materials, as well as their generation and passivation, is essential for the development of functional systems, spanning from next-generation optoelectronic devices to advanced catalysis. Here, we use synchrotron-based X-ray photoelectron spectroscopy (XPS) with submicron spatial resolution to create sulfur vacancies (SVs) in monolayer MoS2 and monitor their chemical and electronic properties in situ during the defect creation process. X-ray irradiation leads to the emergence of a distinct Mo 3d spectral feature associated with undercoordinated Mo atoms. Real-time analysis of the evolution of this feature, along with the decrease of S content, reveals predominant monosulfur vacancy generation at low doses and preferential disulfur vacancy generation at high doses. Formation of these defects leads to a shift of the Fermi level toward the valence band (VB) edge, introduction of electronic states within the VB, and formation of lateral pn junctions. These findings are consistent with theoretical predictions that SVs serve as deep acceptors and are not responsible for the ubiquitous n-type conductivity of MoS2. In addition, we find that these defects are metastable upon short-term exposure to ambient air. By contrast, in situ oxygen exposure during XPS measurements enables passivation of SVs, resulting in partial elimination of undercoordinated Mo sites and reduction of SV-related states near the VB edge. Correlative Raman spectroscopy and photoluminescence measurements confirm our findings of localized SV generation and passivation, thereby demonstrating the connection between chemical, structural, and optoelectronic properties of SVs in MoS2.
8.
M Kuhl, A Henning, L Haller, L I Wagner, C-M Jiang, V Streibel, I D Sharp, J Eichhorn
Designing Multifunctional Cobalt Oxide Layers for Efficient and Stable Electrochemical Oxygen Evolution Journal Article
In: Advanced Materials Interfaces, vol. 9, no. 21, pp. 2200582, 2022, ISSN: 2196-7350.
@article{nokey,
title = {Designing Multifunctional Cobalt Oxide Layers for Efficient and Stable Electrochemical Oxygen Evolution},
author = {M Kuhl and A Henning and L Haller and L I Wagner and C-M Jiang and V Streibel and I D Sharp and J Eichhorn},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/admi.202200582},
doi = {https://doi.org/10.1002/admi.202200582},
issn = {2196-7350},
year = {2022},
date = {2022-06-24},
journal = {Advanced Materials Interfaces},
volume = {9},
number = {21},
pages = {2200582},
abstract = {Abstract Disordered and porous metal oxides are promising earth-abundant and cost-effective alternatives to noble-metal electrocatalysts. Herein, nonsaturated oxidation in plasma-enhanced atomic layer deposition is leveraged to tune the structural, mechanical, and optical properties of biphasic cobalt hydroxide films, thereby tailoring their catalytic activities and chemical stabilities. Short oxygen plasma exposure times and low plasma powers incompletely oxidize the cobaltocene precursor to Co(OH)2 and result in carbon impurity incorporation. These Co(OH)2 films are highly porous and catalytically active, but their electrochemical stability is impacted by poor substrate adhesion. In contrast, long exposure times and high powers completely oxidize the precursor to Co3O4, reduce the carbon incorporation, and improve the crystallinity. While the Co3O4 films have high electrochemical stability, they are characterized by low oxygen evolution reaction activity. To overcome these competing properties, the established relation between deposition parameters and functional film properties is applied to design bilayer films exhibiting simultaneously improved electrochemical performance and chemical stability. The bilayer films combine a highly active Co(OH)2 surface with a stable Co3O4 interface layer. These coatings exhibit minimal light absorption, thus making them suitable as protective catalytic layers on semiconductor light absorbers for application in photoelectrochemical devices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract Disordered and porous metal oxides are promising earth-abundant and cost-effective alternatives to noble-metal electrocatalysts. Herein, nonsaturated oxidation in plasma-enhanced atomic layer deposition is leveraged to tune the structural, mechanical, and optical properties of biphasic cobalt hydroxide films, thereby tailoring their catalytic activities and chemical stabilities. Short oxygen plasma exposure times and low plasma powers incompletely oxidize the cobaltocene precursor to Co(OH)2 and result in carbon impurity incorporation. These Co(OH)2 films are highly porous and catalytically active, but their electrochemical stability is impacted by poor substrate adhesion. In contrast, long exposure times and high powers completely oxidize the precursor to Co3O4, reduce the carbon incorporation, and improve the crystallinity. While the Co3O4 films have high electrochemical stability, they are characterized by low oxygen evolution reaction activity. To overcome these competing properties, the established relation between deposition parameters and functional film properties is applied to design bilayer films exhibiting simultaneously improved electrochemical performance and chemical stability. The bilayer films combine a highly active Co(OH)2 surface with a stable Co3O4 interface layer. These coatings exhibit minimal light absorption, thus making them suitable as protective catalytic layers on semiconductor light absorbers for application in photoelectrochemical devices.
7.
V F Kunzelmann, C-M Jiang, I Ihrke, E Sirotti, T Rieth, A Henning, J Eichhorn, I D Sharp
Solution-based synthesis of wafer-scale epitaxial BiVO4 thin films exhibiting high structural and optoelectronic quality Journal Article
In: Journal of Materials Chemistry A, 2022, ISSN: 2050-7488.
@article{nokey,
title = {Solution-based synthesis of wafer-scale epitaxial BiVO4 thin films exhibiting high structural and optoelectronic quality},
author = {V F Kunzelmann and C-M Jiang and I Ihrke and E Sirotti and T Rieth and A Henning and J Eichhorn and I D Sharp},
url = {http://dx.doi.org/10.1039/D1TA10732A},
doi = {10.1039/D1TA10732A},
issn = {2050-7488},
year = {2022},
date = {2022-04-22},
journal = {Journal of Materials Chemistry A},
abstract = {We demonstrate a facile approach to solution-based synthesis of wafer-scale epitaxial bismuth vanadate (BiVO4) thin films by spin-coating on yttria-stabilized zirconia. Epitaxial growth proceeds via solid-state transformation of initially formed polycrystalline films, driven by interface energy minimization. The (010)-oriented BiVO4 films are smooth and compact, possessing remarkably high structural quality across complete 2′′ wafers. Optical absorption is characterized by a sharp onset with a low sub-band gap response, confirming that the structural order of the films results in correspondingly high optoelectronic quality. This combination of structural and optoelectronic quality enables measurements that reveal a strong optical anisotropy of BiVO4, which leads to significantly increased in-plane optical constants near the fundamental band edge that are of particular importance for maximizing light harvesting in semiconductor photoanodes. Temperature-dependent transport measurements confirm a thermally activated hopping barrier of ∼570 meV, consistent with small electron polaron conduction. This simple approach for synthesis of high-quality epitaxial BiVO4, without the need for complex deposition equipment, enables a broadly accessible materials base to accelerate research aimed at understanding and optimizing photoelectrochemical energy conversion mechanisms.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We demonstrate a facile approach to solution-based synthesis of wafer-scale epitaxial bismuth vanadate (BiVO4) thin films by spin-coating on yttria-stabilized zirconia. Epitaxial growth proceeds via solid-state transformation of initially formed polycrystalline films, driven by interface energy minimization. The (010)-oriented BiVO4 films are smooth and compact, possessing remarkably high structural quality across complete 2′′ wafers. Optical absorption is characterized by a sharp onset with a low sub-band gap response, confirming that the structural order of the films results in correspondingly high optoelectronic quality. This combination of structural and optoelectronic quality enables measurements that reveal a strong optical anisotropy of BiVO4, which leads to significantly increased in-plane optical constants near the fundamental band edge that are of particular importance for maximizing light harvesting in semiconductor photoanodes. Temperature-dependent transport measurements confirm a thermally activated hopping barrier of ∼570 meV, consistent with small electron polaron conduction. This simple approach for synthesis of high-quality epitaxial BiVO4, without the need for complex deposition equipment, enables a broadly accessible materials base to accelerate research aimed at understanding and optimizing photoelectrochemical energy conversion mechanisms.
6.
M Kuhl, A Henning, L Haller, L Wagner, C-M Jiang, V Streibel, I D Sharp, J Eichhorn
Designing multifunctional CoOx layers for efficient and stable electrochemical energy conversion Journal Article
In: Cambridge: Cambridge Open Engage, 2022.
@article{nokey,
title = {Designing multifunctional CoOx layers for efficient and stable electrochemical energy conversion},
author = {M Kuhl and A Henning and L Haller and L Wagner and C-M Jiang and V Streibel and I D Sharp and J Eichhorn},
doi = {10.26434/chemrxiv-2022-23ck4},
year = {2022},
date = {2022-01-13},
urldate = {2022-01-13},
journal = {Cambridge: Cambridge Open Engage},
abstract = {Disordered and porous metal oxides are promising as earth-abundant and cost-effective alternatives to noble-metal electrocatalysts. Herein, we leverage non-saturated oxidation in plasma-enhanced atomic layer deposition to tune structural, mechanical, and optical properties of biphasic CoOx thin films, thereby tailoring their catalytic activities and chemical stabilities. To optimize the resulting film properties, we systematically vary the oxygen plasma power and exposure time in the deposition process. We find that short exposure times and low plasma powers incompletely oxidize the cobaltocene precursor to Co(OH)2 and result in the incorporation of carbon impurities. These Co(OH)2 films are highly porous and catalytically active, but their electrochemical stability is impacted by poor adhesion to the substrate. In contrast, long exposure times and high plasma powers completely oxidize the precursor to form Co3O4, reduce the carbon impurity incorporation, and improve the film crystallinity. While the resulting Co3O4 films are highly stable under electrochemical conditions, they are characterized by low oxygen evolution reaction activities. To overcome these competing properties, we applied the established relation between deposition parameters and functional film properties to design bilayer films exhibiting simultaneously improved electrochemical performance and chemical stability. The resulting biphasic films combine a highly active Co(OH)2 surface with a stable Co3O4 interface layer. In addition, these coatings exhibit minimal light absorption, thus rendering them well suited as protective catalytic layers on semiconductor light absorbers for application in photoelectrochemical devices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Disordered and porous metal oxides are promising as earth-abundant and cost-effective alternatives to noble-metal electrocatalysts. Herein, we leverage non-saturated oxidation in plasma-enhanced atomic layer deposition to tune structural, mechanical, and optical properties of biphasic CoOx thin films, thereby tailoring their catalytic activities and chemical stabilities. To optimize the resulting film properties, we systematically vary the oxygen plasma power and exposure time in the deposition process. We find that short exposure times and low plasma powers incompletely oxidize the cobaltocene precursor to Co(OH)2 and result in the incorporation of carbon impurities. These Co(OH)2 films are highly porous and catalytically active, but their electrochemical stability is impacted by poor adhesion to the substrate. In contrast, long exposure times and high plasma powers completely oxidize the precursor to form Co3O4, reduce the carbon impurity incorporation, and improve the film crystallinity. While the resulting Co3O4 films are highly stable under electrochemical conditions, they are characterized by low oxygen evolution reaction activities. To overcome these competing properties, we applied the established relation between deposition parameters and functional film properties to design bilayer films exhibiting simultaneously improved electrochemical performance and chemical stability. The resulting biphasic films combine a highly active Co(OH)2 surface with a stable Co3O4 interface layer. In addition, these coatings exhibit minimal light absorption, thus rendering them well suited as protective catalytic layers on semiconductor light absorbers for application in photoelectrochemical devices.
5.
Y Zou, S Yuan, A Buyruk, J Eichhorn, S Yin, M A Reus, T Xiao, S Pratap, S Liang, C L Weindl, W Chen, C Mu, I D Sharp, T Ameri, M Schwartzkopf, S V Roth, P Müller-Buschbaum
The Influence of CsBr on Crystal Orientation and Optoelectronic Properties of MAPbI3-Based Solar Cells Journal Article
In: ACS Applied Materials & Interfaces, vol. 14, pp. 2958, 2022, ISSN: 1944-8244.
@article{nokey,
title = {The Influence of CsBr on Crystal Orientation and Optoelectronic Properties of MAPbI3-Based Solar Cells},
author = {Y Zou and S Yuan and A Buyruk and J Eichhorn and S Yin and M A Reus and T Xiao and S Pratap and S Liang and C L Weindl and W Chen and C Mu and I D Sharp and T Ameri and M Schwartzkopf and S V Roth and P M\"{u}ller-Buschbaum},
url = {https://doi.org/10.1021/acsami.1c22184},
doi = {10.1021/acsami.1c22184},
issn = {1944-8244},
year = {2022},
date = {2022-01-06},
urldate = {2022-01-06},
journal = {ACS Applied Materials \& Interfaces},
volume = {14},
pages = {2958},
abstract = {Crystal orientations are closely related to the behavior of photogenerated charge carriers and are vital for controlling the optoelectronic properties of perovskite solar cells. Herein, we propose a facile approach to reveal the effect of lattice plane orientation distribution on the charge carrier kinetics via constructing CsBr-doped mixed cation perovskite phases. With grazing-incidence wide-angle X-ray scattering measurements, we investigate the crystallographic properties of mixed perovskite films at the microscopic scale and reveal the effect of the extrinsic CsBr doping on the stacking behavior of the lattice planes. Combined with transient photocurrent, transient photovoltage, and space-charge-limited current measurements, the transport dynamics and recombination of the photogenerated charge carriers are characterized. It is demonstrated that CsBr compositional engineering can significantly affect the perovskite crystal structure in terms of the orientation distribution of crystal planes and passivation of trap-state densities, as well as simultaneously facilitate the photogenerated charge carrier transport across the absorber and its interfaces. This strategy provides unique insight into the underlying relationship between the stacking pattern of crystal planes, photogenerated charge carrier transport, and optoelectronic properties of solar cells.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Crystal orientations are closely related to the behavior of photogenerated charge carriers and are vital for controlling the optoelectronic properties of perovskite solar cells. Herein, we propose a facile approach to reveal the effect of lattice plane orientation distribution on the charge carrier kinetics via constructing CsBr-doped mixed cation perovskite phases. With grazing-incidence wide-angle X-ray scattering measurements, we investigate the crystallographic properties of mixed perovskite films at the microscopic scale and reveal the effect of the extrinsic CsBr doping on the stacking behavior of the lattice planes. Combined with transient photocurrent, transient photovoltage, and space-charge-limited current measurements, the transport dynamics and recombination of the photogenerated charge carriers are characterized. It is demonstrated that CsBr compositional engineering can significantly affect the perovskite crystal structure in terms of the orientation distribution of crystal planes and passivation of trap-state densities, as well as simultaneously facilitate the photogenerated charge carrier transport across the absorber and its interfaces. This strategy provides unique insight into the underlying relationship between the stacking pattern of crystal planes, photogenerated charge carrier transport, and optoelectronic properties of solar cells.
4.
J Eichhorn, S P Lechner, C-M Jiang, G Folchi Heunecke, F Munnik, I D Sharp
Indirect bandgap, optoelectronic properties, and photoelectrochemical characteristics of high-purity Ta3N5 photoelectrodes Journal Article
In: Journal of Materials Chemistry A, vol. 9, pp. 20653, 2021, ISSN: 2050-7488.
@article{nokey,
title = {Indirect bandgap, optoelectronic properties, and photoelectrochemical characteristics of high-purity Ta3N5 photoelectrodes},
author = {J Eichhorn and S P Lechner and C-M Jiang and G Folchi Heunecke and F Munnik and I D Sharp},
url = {http://dx.doi.org/10.1039/D1TA05282A},
doi = {10.1039/D1TA05282A},
issn = {2050-7488},
year = {2021},
date = {2021-08-26},
urldate = {2021-08-26},
journal = {Journal of Materials Chemistry A},
volume = {9},
pages = {20653},
abstract = {The (opto)electronic properties of Ta3N5 photoelectrodes are often dominated by defects, such as oxygen impurities, nitrogen vacancies, and low-valent Ta cations, impeding fundamental studies of its electronic structure, chemical stability, and photocarrier transport. Here, we explore the role of ammonia annealing following direct reactive magnetron sputtering of tantalum nitride thin films, achieving near-ideal stoichiometry, with significantly reduced native defect and oxygen impurity concentrations. By analyzing structural, optical, and photoelectrochemical properties as a function of ammonia annealing temperature, we provide new insights into the basic semiconductor properties of Ta3N5, as well as the role of defects on its optoelectronic characteristics. Both the crystallinity and material quality improve up to 940 °C, due to elimination of oxygen impurities. Even higher annealing temperatures cause material decomposition and introduce additional disorder within the Ta3N5 lattice, leading to reduced photoelectrochemical performance. Overall, the high material quality enables us to unambiguously identify the nature of the Ta3N5 bandgap as indirect, thereby resolving a long-standing controversy regarding the most fundamental characteristic of this material as a semiconductor. The compact morphology, low defect content, and high optoelectronic quality of these films provide a basis for further optimization of photoanodes and may open up further application opportunities beyond photoelectrochemical energy conversion.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The (opto)electronic properties of Ta3N5 photoelectrodes are often dominated by defects, such as oxygen impurities, nitrogen vacancies, and low-valent Ta cations, impeding fundamental studies of its electronic structure, chemical stability, and photocarrier transport. Here, we explore the role of ammonia annealing following direct reactive magnetron sputtering of tantalum nitride thin films, achieving near-ideal stoichiometry, with significantly reduced native defect and oxygen impurity concentrations. By analyzing structural, optical, and photoelectrochemical properties as a function of ammonia annealing temperature, we provide new insights into the basic semiconductor properties of Ta3N5, as well as the role of defects on its optoelectronic characteristics. Both the crystallinity and material quality improve up to 940 °C, due to elimination of oxygen impurities. Even higher annealing temperatures cause material decomposition and introduce additional disorder within the Ta3N5 lattice, leading to reduced photoelectrochemical performance. Overall, the high material quality enables us to unambiguously identify the nature of the Ta3N5 bandgap as indirect, thereby resolving a long-standing controversy regarding the most fundamental characteristic of this material as a semiconductor. The compact morphology, low defect content, and high optoelectronic quality of these films provide a basis for further optimization of photoanodes and may open up further application opportunities beyond photoelectrochemical energy conversion.
3.
J Eichhorn, C-M Jiang, J K Cooper, I D Sharp, F M Toma
Nanoscale Heterogeneities and Composition–Reactivity Relationships in Copper Vanadate Photoanodes Journal Article
In: ACS Applied Materials & Interfaces, vol. 13, no. 20, pp. 23575-23583, 2021, ISSN: 1944-8244.
@article{,
title = {Nanoscale Heterogeneities and Composition\textendashReactivity Relationships in Copper Vanadate Photoanodes},
author = {J Eichhorn and C-M Jiang and J K Cooper and I D Sharp and F M Toma},
url = {https://doi.org/10.1021/acsami.1c01848},
doi = {10.1021/acsami.1c01848},
issn = {1944-8244},
year = {2021},
date = {2021-05-17},
urldate = {2021-05-17},
journal = {ACS Applied Materials \& Interfaces},
volume = {13},
number = {20},
pages = {23575-23583},
abstract = {The photoelectrochemical performance of thin film photoelectrodes can be impacted by deviations from the stoichiometric composition, both at the macroscale and at the nanoscale. This issue is especially pronounced for the class of ternary compounds that are currently investigated for simultaneously achieving the optoelectronic characteristics and chemical stability required for solar fuel generation. Here, we combine macroscopic photoelectrochemical testing with atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM) to reveal relationships between photoelectrochemical activity, nanoscale morphology, and local chemical composition in copper vanadate (CVO) thin films as a model system. For films with varying Cu/(Cu + V) ratios around the ideal stoichiometry of stoiberite Cu5V2O10, AFM resolves submicrometer morphology variations, which correlate with variations of the Cu content resolved by STXM. Both stoichiometric and Cu-deficient films exhibit a clear photoresponse, which indicates electronic tolerance to reduced Cu content. While both films exhibit homogeneous O and V content, they are also characterized by local regions of Cu enrichment and depletion that extend beyond individual grains. By contrast, Cu-rich photoelectrodes exhibit a tendency toward CuO secondary phase formation and a significantly reduced photoelectrochemical activity, indicating a significantly poor electronic tolerance to Cu-enrichment. These findings highlight that the average film composition at the macroscale is insufficient for defining structure\textendashfunction relationships in complex ternary compounds. Rather, correlating microscopic variations in chemical composition to macroscopic photoelectrochemical performance provides insights into photocatalytic activity and stability that are otherwise not apparent from pure macroscopic characterization.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The photoelectrochemical performance of thin film photoelectrodes can be impacted by deviations from the stoichiometric composition, both at the macroscale and at the nanoscale. This issue is especially pronounced for the class of ternary compounds that are currently investigated for simultaneously achieving the optoelectronic characteristics and chemical stability required for solar fuel generation. Here, we combine macroscopic photoelectrochemical testing with atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM) to reveal relationships between photoelectrochemical activity, nanoscale morphology, and local chemical composition in copper vanadate (CVO) thin films as a model system. For films with varying Cu/(Cu + V) ratios around the ideal stoichiometry of stoiberite Cu5V2O10, AFM resolves submicrometer morphology variations, which correlate with variations of the Cu content resolved by STXM. Both stoichiometric and Cu-deficient films exhibit a clear photoresponse, which indicates electronic tolerance to reduced Cu content. While both films exhibit homogeneous O and V content, they are also characterized by local regions of Cu enrichment and depletion that extend beyond individual grains. By contrast, Cu-rich photoelectrodes exhibit a tendency toward CuO secondary phase formation and a significantly reduced photoelectrochemical activity, indicating a significantly poor electronic tolerance to Cu-enrichment. These findings highlight that the average film composition at the macroscale is insufficient for defining structure–function relationships in complex ternary compounds. Rather, correlating microscopic variations in chemical composition to macroscopic photoelectrochemical performance provides insights into photocatalytic activity and stability that are otherwise not apparent from pure macroscopic characterization.
2.
C-M Jiang, L I Wagner, M K Horton, J Eichhorn, T Rieth, V F Kunzelmann, M Kraut, Y Li, K A Persson, I D Sharp
Metastable Ta2N3 with highly tunable electrical conductivity via oxygen incorporation Journal Article
In: Materials Horizons, vol. 8, no. 6, pp. 1744-1755, 2021, ISSN: 2051-6347.
@article{nokey,
title = {Metastable Ta2N3 with highly tunable electrical conductivity via oxygen incorporation},
author = {C-M Jiang and L I Wagner and M K Horton and J Eichhorn and T Rieth and V F Kunzelmann and M Kraut and Y Li and K A Persson and I D Sharp},
url = {http://dx.doi.org/10.1039/D1MH00017A},
doi = {10.1039/D1MH00017A},
issn = {2051-6347},
year = {2021},
date = {2021-04-01},
journal = {Materials Horizons},
volume = {8},
number = {6},
pages = {1744-1755},
abstract = {The binary Ta\textendashN chemical system includes several compounds with notable prospects in microelectronics, solar energy harvesting, and catalysis. Among these, metallic TaN and semiconducting Ta3N5 have garnered significant interest, in part due to their synthetic accessibility. However, tantalum sesquinitride (Ta2N3) possesses an intermediate composition and largely unknown physical properties owing to its metastable nature. Herein, Ta2N3 is directly deposited by reactive magnetron sputtering and its optoelectronic properties are characterized. Combining these results with density functional theory provides insights into the critical role of oxygen in both synthesis and electronic structure. While the inclusion of oxygen in the process gas is critical to Ta2N3 formation, the resulting oxygen incorporation in structural vacancies drastically modifies the free electron concentration in the as-grown material, thus leading to a semiconducting character with a 1.9 eV bandgap. Reducing the oxygen impurity concentration via post-synthetic ammonia annealing increases the conductivity by seven orders of magnitude and yields the metallic characteristics of a degenerate semiconductor, consistent with theoretical predictions. Thus, this inverse oxygen doping approach \textendash by which the carrier concentration is reduced by the oxygen impurity \textendash offers a unique opportunity to tailor the optoelectronic properties of Ta2N3 for applications ranging from photochemical energy conversion to advanced photonics.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The binary Ta–N chemical system includes several compounds with notable prospects in microelectronics, solar energy harvesting, and catalysis. Among these, metallic TaN and semiconducting Ta3N5 have garnered significant interest, in part due to their synthetic accessibility. However, tantalum sesquinitride (Ta2N3) possesses an intermediate composition and largely unknown physical properties owing to its metastable nature. Herein, Ta2N3 is directly deposited by reactive magnetron sputtering and its optoelectronic properties are characterized. Combining these results with density functional theory provides insights into the critical role of oxygen in both synthesis and electronic structure. While the inclusion of oxygen in the process gas is critical to Ta2N3 formation, the resulting oxygen incorporation in structural vacancies drastically modifies the free electron concentration in the as-grown material, thus leading to a semiconducting character with a 1.9 eV bandgap. Reducing the oxygen impurity concentration via post-synthetic ammonia annealing increases the conductivity by seven orders of magnitude and yields the metallic characteristics of a degenerate semiconductor, consistent with theoretical predictions. Thus, this inverse oxygen doping approach – by which the carrier concentration is reduced by the oxygen impurity – offers a unique opportunity to tailor the optoelectronic properties of Ta2N3 for applications ranging from photochemical energy conversion to advanced photonics.
1.
J Eichhorn, S E Reyes-Lillo, S Roychoudhury, S Sallis, J Weis, D M Larson, J K Cooper, I D Sharp, D Prendergast, F M Toma
Revealing Nanoscale Chemical Heterogeneities in Polycrystalline Mo-BiVO(4)Thin Films Journal Article
In: Small, vol. 16, pp. 2001600, 2020, ISSN: 1613-6810.
@article{,
title = {Revealing Nanoscale Chemical Heterogeneities in Polycrystalline Mo-BiVO(4)Thin Films},
author = {J Eichhorn and S E Reyes-Lillo and S Roychoudhury and S Sallis and J Weis and D M Larson and J K Cooper and I D Sharp and D Prendergast and F M Toma},
url = {\<Go to ISI\>://WOS:000555446200001},
doi = {10.1002/smll.202001600},
issn = {1613-6810},
year = {2020},
date = {2020-08-01},
urldate = {2020-08-01},
journal = {Small},
volume = {16},
pages = {2001600},
abstract = {The activity of polycrystalline thin film photoelectrodes is impacted by local variations of the material properties due to the exposure of different crystal facets and the presence of grain/domain boundaries. Here a multi-modal approach is applied to correlate nanoscale heterogeneities in chemical composition and electronic structure with nanoscale morphology in polycrystalline Mo-BiVO4. By using scanning transmission X-ray microscopy, the characteristic structure of polycrystalline film is used to disentangle the different X-ray absorption spectra corresponding to grain centers and grain boundaries. Comparing both spectra reveals phase segregation of V(2)O(5)at grain boundaries of Mo-BiVO(4)thin films, which is further supported by X-ray photoelectron spectroscopy and many-body density functional theory calculations. Theoretical calculations also enable to predict the X-ray absorption spectral fingerprint of polarons in Mo-BiVO4. After photo-electrochemical operation, the degraded Mo-BiVO(4)films show similar grain center and grain boundary spectra indicating V(2)O(5)dissolution in the course of the reaction. Overall, these findings provide valuable insights into the degradation mechanism and the impact of material heterogeneities on the material performance and stability of polycrystalline photoelectrodes.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The activity of polycrystalline thin film photoelectrodes is impacted by local variations of the material properties due to the exposure of different crystal facets and the presence of grain/domain boundaries. Here a multi-modal approach is applied to correlate nanoscale heterogeneities in chemical composition and electronic structure with nanoscale morphology in polycrystalline Mo-BiVO4. By using scanning transmission X-ray microscopy, the characteristic structure of polycrystalline film is used to disentangle the different X-ray absorption spectra corresponding to grain centers and grain boundaries. Comparing both spectra reveals phase segregation of V(2)O(5)at grain boundaries of Mo-BiVO(4)thin films, which is further supported by X-ray photoelectron spectroscopy and many-body density functional theory calculations. Theoretical calculations also enable to predict the X-ray absorption spectral fingerprint of polarons in Mo-BiVO4. After photo-electrochemical operation, the degraded Mo-BiVO(4)films show similar grain center and grain boundary spectra indicating V(2)O(5)dissolution in the course of the reaction. Overall, these findings provide valuable insights into the degradation mechanism and the impact of material heterogeneities on the material performance and stability of polycrystalline photoelectrodes.