Stela Canulescu - Academia.edu (original) (raw)

Papers by Stela Canulescu

ACS Applied Materials & Interfaces, 2022

Multijunction solar cells in a tandem configuration could further lower the costs of electricity ... more Multijunction solar cells in a tandem configuration could further lower the costs of electricity if crystalline Si (c-Si) is used as bottom cell. However, for direct monolithic integration on c-Si, only a restricted number of top and bottom cell architectures are compatible, due to either epitaxy or high temperature constraints, where the interface between subcells is subject to a trade-off between transmittance, electrical interconnection, and bottom cell degradation. Using polySi/SiOx passivating contacts for Si, this degradation can be largely circumvented by tuning the polySi/SiOx stacks to promote gettering of contaminants admitted into the Si bottom cell during the top cell synthesis. Applying this concept to the low-cost top cell chalcogenides Cu2ZnSnS4 (CZTS), CuGaSe2 (CGSe) and AgInGaSe2 (AIGSe), fabricated under harsh S or Se atmospheres above 550 °C, we show that increasing the heavily-doped polySi layer thickness from 40 to up to 400 nm prevents a reduction in Si carrier lifetime by one order of magnitude, with final lifetimes above 500 µs uniformly across areas up to 20 cm 2. In all cases, the increased resilience was correlated with a 99.9% reduction in contaminant concentration in the c-Si bulk, provided by the thick polySi layer, which acts as a buried gettering layer in the tandem structure without compromising the Si passivation quality. The Si resilience decreased as AIGSe > CGSe > CZTS, in accordance with the measured Cu contamination profiles and higher annealing temperatures. An efficiency of up to 7% was achieved for a CZTS/Si tandem, where the Si bottom cell is no longer the limiting factor.

2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018

Kesterite Cu2ZnSnS4 (CZTS) can be a promising, alternative material for thin-film solar cells, be... more Kesterite Cu2ZnSnS4 (CZTS) can be a promising, alternative material for thin-film solar cells, because it is earthabundant, non-toxic and displays relevant physical properties. For future up-scaling possibilities and to decrease the environmental impact of the photovoltaic technology, solutionprocessing deposition methods are desired. This paper deals with CZTS absorber layers fabricated from CZTS nanoparticles that are functionalized with KCl, and investigates two different types of furnaces for the annealing process; a hydrogen sulfide (H 2S) flow furnace and a nitrogen atmosphere closed furnace. We find that more uniform grain growth is achieved when annealing in a closed furnace, while very large grains on the surface of the film are obtained when annealing in H 2S.

 Users may download and print one copy of any publication from the public portal for the purpose... more  Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

 Users may download and print one copy of any publication from the public portal for the purpose... more  Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Post deposition annealing is a frequently used approach in CZTS(Se) research community to boost t... more Post deposition annealing is a frequently used approach in CZTS(Se) research community to boost the efficiency of the cells. In this study, we investigate the influence of post deposition annealing in air on CZTS devices performance, produced by pulsed laser deposition. There are competing beneficial and detrimental effects depending on the temperature. We suggest that Cu-Zn ordering at 150°C and diffusion of Cd into CZTS from 250°C to higher temperature could be the main beneficial effects that increase the efficiency. A record efficiency for pulsed laser deposition of 5.4% was obtained at 150°C and 15 min post deposition annealing.

Recently, Si-based tandem solar cells have been the most compelling strategy to improve the effic... more Recently, Si-based tandem solar cells have been the most compelling strategy to improve the efficiency of the existing photovoltaic technology beyond its practical and theoretical limits. Thin-film chalcogenides have shown performance, reliability, stability and costs comparable to the Si technology, and could be suitable top-cell partners for Si. However, limited work has been done to evaluate the compatibility of these two technologies. The synthesis process of chalcogenides generally includes high temperature annealing (> 500 oC) of the precursors in a reactive atmosphere, which could lead to in-diffusion of the detrimental metallic species into the Si cell. In this work, we investigated monolithic integration of Cu2ZnSnS4 (CZTS), a promising representative from the chalcogenides family, on top of a thermally resilient silicon structure featuring polysilicon-based electron and hole selective passivating contacts. We used different Ti-based barrier layers, namely TiN, Ti(OxNy),...

Solar Energy Materials and Solar Cells, 2020

Abstract In this work, we have investigated Cu2ZnSnS4 (CZTS) solar cells made from oxide, oxy-sul... more Abstract In this work, we have investigated Cu2ZnSnS4 (CZTS) solar cells made from oxide, oxy-sulfide and sulfide precursors produced by pulsed laser deposition (PLD). Although sulfide precursors are widely used to fabricate CZTS solar cells, Sn loss is commonly observed due to the high volatility of SnxSy species during high temperature sulfurization. This can lead to a non-ideal absorber composition and a high density of detrimental Sn-related defects that severely affect the performance of the device. By using oxide precursors, we have shown that the Sn loss can be significantly reduced due to the higher thermal stability of SnxOy species when compared to their sulfide counterparts. However, the reaction mechanism for the oxide route results in rough CZTS films. We hypothesize that the SO2 gas that forms during the conversion from oxide to sulfide is trapped in the film during sulfurization, and can lead to grains with hollow cavities and thus increase the surface roughness. Therefore, we have developed an annealing route for the oxide precursors at lower annealing pressures, which leads to improved film morphology and device performance. As a result, we report a power conversion efficiency of 5.4% for solar cells made from oxide precursors. This is the highest value reported for a CZTS absorber produced by PLD.

Scientific Reports, 2020

Energy band alignment at the heterointerface between CdS and kesterite Cu2ZnSnS4 (CZTS) and its a... more Energy band alignment at the heterointerface between CdS and kesterite Cu2ZnSnS4 (CZTS) and its alloys plays a crucial role in determining the efficiency of the solar cells. Whereas Ag alloying of CZTS has been shown to reduce anti-site defects in the bulk and thus rise the efficiency, the electronic properties at the interface with the CdS buffer layer have not been extensively investigated. In this work, we present a detailed study on the band alignment between n-CdS and p-CZTS upon Ag alloying by depth-profiling ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Our findings indicate that core-level peaks and the valence band edge of CdS exhibit a significant shift to a lower energy (larger than 0.4 eV) upon the etching of the CdS layer, which can be assigned due to band bending and chemical shift induced by a change in the chemical composition across the interface. Using a simplified model based on charge depletion layer conservation, a sign...

ACS Applied Materials & Interfaces, 2020

In kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell research, an asymmetric crystallization profile i... more In kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell research, an asymmetric crystallization profile is often obtained after annealing, resulting in a bilayered-or double-layered-CZTSSe absorber. So far, only segregated pieces of research exist to characterize the appearance of this double layer, its formation dynamics and its effect on the performance of devices. In this work, we review the existing research on double-layered kesterites and evaluate the different mechanisms proposed. Using a cosputtering-based approach, we show that the two layers can differ significantly in morphology, composition and optoelectronic properties, and complement the results with a large statistical dataset of over 850 individual CZTS solar cells. By reducing the absorber thickness from above 1000 nm to 300 nm, we show that the double layer segregation is alleviated. In turn, we see a progressive improvement in the device performance for lower thickness, which alone would be inconsistent with the well-known case of ultrathin CIGS solar cells. We therefore attribute the improvements to the reduced double-layer occurrence, and find that the double layer limits the efficiency of our devices to below 7%. By comparing the results with CZTS grown on monocrystalline Si substrates, without a native Na supply, we show that the alkali metal supply does not determine the double layer formation, but merely reduces the threshold for its occurrence. Instead, we propose that the main formation mechanism is the early migration of Cu to the surface during annealing and formation of Cu2-xS phases, in a self-regulating process akin to the Kirkendall effect. Finally, we comment on the generality of the mechanism proposed, by comparing our results to other synthesis routes, including our own in-house results from solution processing and pulsed laser deposition of sulfide-and oxide-based targets. We find that although the double layer occurrence largely depends on the kesterite synthesis route, the common factors determining the double layer occurrence appear to be the presence of metallic Cu and/or a chalcogen deficiency in the precursor matrix. We suggest that understanding the limitations imposed by the double layer dynamics could prove useful to pave the way for breaking the 13% efficiency barrier for this technology.

RSC Advances, 2018

We present a route where organic ligand-free, KCl-functionalized Cu2ZnSnS4 nanoparticles grow int... more We present a route where organic ligand-free, KCl-functionalized Cu2ZnSnS4 nanoparticles grow into large, dense grains during annealing in nitrogen/sulfur atmosphere.

Solar Energy Materials and Solar Cells, 2020

Following the recent success of monolithically integrated Perovskite/Si tandem solar cells, great... more Following the recent success of monolithically integrated Perovskite/Si tandem solar cells, great interest has been raised in searching for alternative wide bandgap top-cell materials with prospects of a fully earthabundant, stable and efficient tandem solar cell. Thin film chalcogenides (TFCs) such as the Cu 2 ZnSnS 4 (CZTS) could be suitable top-cell materials. However, TFCs have the disadvantage that generally at least one high temperature step (> 500 • C) is needed during the synthesis, which could contaminate the Si bottom cell. Here, we systematically investigate the monolithic integration of CZTS on a Si bottom solar cell. A thermally resilient double-sided Tunnel Oxide Passivated Contact (TOPCon) structure is used as bottom cell. A thin (< 25 nm) TiN layer between the top and bottom cells, doubles as diffusion barrier and recombination layer. We show that TiN successfully mitigates in-diffusion of CZTS elements into the c-Si bulk during the high temperature sulfurization process, and find no evidence of electrically active deep Si bulk defects in samples protected by just 10 nm TiN. Post-process minority carrier lifetime in Si exceeded 1.5 ms, i.e., a promising implied open-circuit voltage (i-V oc) of 715 mV after the high temperature sulfurization. Based on these results, we demonstrate a first proof-of-concept two-terminal CZTS/Si tandem device with an efficiency of 1.1% and a V oc of 900 mV. A general implication of this study is that the growth of complex semiconductors on Si using high temperature steps is technically feasible, and can potentially lead to efficient monolithically integrated two-terminal tandem solar cells.

physica status solidi (a), 2015

The influence of the coating/substrate interface on the photocatalytic behavior of Al-TiO 2 coati... more The influence of the coating/substrate interface on the photocatalytic behavior of Al-TiO 2 coatings was investigated. The TiO 2 coatings were prepared by magnetron sputtering. The nanoscale structure of the coating was analyzed using X-ray diffraction; atomic force microscopy; scanning electron microscopy; and transmission electron microscopy. The photocatalytic behavior was investigated through optical spectrophotometry studies and electrochemical experiments; as photo voltage; photocurrent; and electrochemical impedance measurements. Consistent results from both optical and electrochemical measurements showed a maximum UV-light absorption by titanium dioxide occurring slightly prior to the energy of the maximum photocurrent. The photocurrent of titanium dioxide decreases with increasing thickness of the aluminum oxide interface layer. Aluminum oxide acts as an insulator; disfavoring the electron transport between the coating and the metallic substrate. The highest photocurrents were indeed obtained when the thickness of interfacial aluminum oxide could be reduced by sputtering a thin Ti layer prior to TiO 2 coating. Photocurrent plotted for different photon energy for a TiO 2 coating on a Ti interlayer consisting of both rutile and anatase. Two photon-excitation peaks from the TiO 2 coating (anatase and rutile) are observed at a potential of 0.34 V versus SHE, while only one peak was observed at a potential 0.14 V versus SHE closer to the conduction band.

Journal of Materials Science, 2015

Theoretical calculations and experimental studies were carried out on polyurethane (PU)-TiO 2 com... more Theoretical calculations and experimental studies were carried out on polyurethane (PU)-TiO 2 composite coatings on bright and matte aluminium surfaces with an aim to understand and tailor the light scattering from particles incorporated into an anodised layer for designing the optical appearance of anodised surfaces. PU matrix was selected for its matching refractive-index (n = 1.7) with anodic alumina layer. Three different TiO 2 particle size distributions were dispersed in PU and spin coated onto bright high-gloss and matte caustic-etched aluminium substrates. The reflectance spectra of coated surfaces in the visible region were analysed using an integrating sphere-spectrophotometer. Data showed that the coated surfaces have a high diffuse reflectance due to the multiple scattering from TiO 2 particles and the coatingsubstrate interface. The diffuse reflectance spectra of the coated surfaces varied weakly with TiO 2 particle concentration and reached a steady state value at 1 wt% but were dependent on the substrate type used. Using Kubelka-Munk two-stream model, the scattering and absorption coefficient of TiO 2 in PU was predicted. The studies presented in this paper provide insight into generating bright white-anodised aluminium surfaces based on aluminium-TiO 2 composites.

ACS applied materials & interfaces, Jan 24, 2014

The photocatalytic behavior of magnetron sputtered anatase TiO2 coatings on copper, nickel, and g... more The photocatalytic behavior of magnetron sputtered anatase TiO2 coatings on copper, nickel, and gold was investigated with the aim of understanding the effect of the metallic substrate and coating-substrate interface structure. Stoichiometry and nanoscale structure of the coating were investigated using X-ray diffraction, Raman spectroscopy, atomic force microscope, and scanning and transmission electron microscopy. Photocatalytic behavior of the coating was explored by using optical spectrophotometry and electrochemical methods via photovoltage, photocurrent, and scanning kelvin probe microscopy measurements. The nature of the metal substrate and coating-substrate interface had profound influence on the photocatalytic behavior. Less photon energy was required for TiO2 excitation on a nickel substrate, whereas TiO2 coating on copper showed a higher band gap attributed to quantum confinement. However, the TiO2 coating on gold exhibited behavior typical of facile transfer of electrons...

Applied Surface Science, 2014

ABSTRACT The mechanism of interaction of light with the microstructure of anodized layer giving s... more ABSTRACT The mechanism of interaction of light with the microstructure of anodized layer giving specific optical appearance is investigated using Al–Zr sputter deposited coating as a model system on an AA6060 substrate. Differences in the oxidative nature of various microstructural components result in the evolution of typical features in the anodized layer, which are investigated as a function of microstructure and correlated with its optical appearance. The Zr concentration in the coating was varied from 6 wt.% to 23 wt.%. Heat treatment of the coated samples was carried out at 550 °C for 4 h in order to evolve Al–Zr based second phase precipitates in the microstructure. Anodizing was performed using 20 wt.% sulphuric acid at 18 °C with an intention to study the effect of anodizing on the Al–Zr based precipitates in the coating. Detailed microstructural characterization of the coating and anodized layer was carried out using high resolution scanning and transmission electron microscopy, grazing incidence X-ray diffraction analysis, glow discharge optical emission spectroscopy, and optical appearance using spectrophotometry. The evolution of microstructure in the anodized layer as a function of anodizing parameters and their influence on the interaction of light is investigated and the results in general are applicable to discolouration of anodized layer on recycled aluminium alloys due to intermetallics.

RSC Advances, 2013

ABSTRACT The change in the surface potential of TiO2 coatings upon UV-illumination was investigat... more ABSTRACT The change in the surface potential of TiO2 coatings upon UV-illumination was investigated on the nanoscale using Scanning Kelvin Probe Force microscopy and on the micro-scale using photo-electrochemical measurements. A good correlation between the two techniques was obtained. The changes in the surface potential of TiO2 coatings upon UV-illumination are closely correlated to the band gap and thickness of the coatings. The inhomogeneity surface potential distribution of a 100 nm TiO2 film indicates a heterogeneous coating. Transition to a homogeneous surface potential distribution was observed with increasing thickness of the TiO2 coating.

Surface and Coatings Technology, 2014

ABSTRACT Effect of alloy composition and prior surface finish on the optical appearance of the an... more ABSTRACT Effect of alloy composition and prior surface finish on the optical appearance of the anodised layer on aluminium alloys was investigated. Four commercial alloys namely AA1050, Peraluman 706, AA5754, and AA6082 were used for the investigation. Microstructure and surface morphology of the substrate prior to anodising were analysed using scanning electron microscopy and atomic force microscopy. The optical appearance of the anodised surface with and without sealing was investigated using a photography setup, photospectrometry and bidirectional reflectance distribution function. It was found that the roughness of the as-etched surface increases with the degree of alloying due to second phase particles making the reflection more diffused, and that the as-etched surface morphology is similar to the oxide–substrate interface after anodising. Proper polishing is achieved on hard alloys and the glossy appearance was kept for alloys of high purity. Sealing made the specular reflection of the mechanically polished specimens more distinct.

Materials Science and Engineering: B, 2004

BaTiO3 (Barium titanate, BTO) thin films were grown on Pt coated Si substrates by radiofrequency ... more BaTiO3 (Barium titanate, BTO) thin films were grown on Pt coated Si substrates by radiofrequency discharge assisted pulsed laser deposition (RF-PLD). A standard experimental set-up consisting in a pulsed YAG-Nd laser working at wavelength of 355nm and incident fluence in the range of 2–3J/cm2 and assisted by a radio frequency plasma discharge (power RF of about 50–200W in a O2

Materials Chemistry and Physics, 2011

ACS Applied Materials & Interfaces, 2022

Multijunction solar cells in a tandem configuration could further lower the costs of electricity ... more Multijunction solar cells in a tandem configuration could further lower the costs of electricity if crystalline Si (c-Si) is used as bottom cell. However, for direct monolithic integration on c-Si, only a restricted number of top and bottom cell architectures are compatible, due to either epitaxy or high temperature constraints, where the interface between subcells is subject to a trade-off between transmittance, electrical interconnection, and bottom cell degradation. Using polySi/SiOx passivating contacts for Si, this degradation can be largely circumvented by tuning the polySi/SiOx stacks to promote gettering of contaminants admitted into the Si bottom cell during the top cell synthesis. Applying this concept to the low-cost top cell chalcogenides Cu2ZnSnS4 (CZTS), CuGaSe2 (CGSe) and AgInGaSe2 (AIGSe), fabricated under harsh S or Se atmospheres above 550 °C, we show that increasing the heavily-doped polySi layer thickness from 40 to up to 400 nm prevents a reduction in Si carrier lifetime by one order of magnitude, with final lifetimes above 500 µs uniformly across areas up to 20 cm 2. In all cases, the increased resilience was correlated with a 99.9% reduction in contaminant concentration in the c-Si bulk, provided by the thick polySi layer, which acts as a buried gettering layer in the tandem structure without compromising the Si passivation quality. The Si resilience decreased as AIGSe > CGSe > CZTS, in accordance with the measured Cu contamination profiles and higher annealing temperatures. An efficiency of up to 7% was achieved for a CZTS/Si tandem, where the Si bottom cell is no longer the limiting factor.

2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018

Kesterite Cu2ZnSnS4 (CZTS) can be a promising, alternative material for thin-film solar cells, be... more Kesterite Cu2ZnSnS4 (CZTS) can be a promising, alternative material for thin-film solar cells, because it is earthabundant, non-toxic and displays relevant physical properties. For future up-scaling possibilities and to decrease the environmental impact of the photovoltaic technology, solutionprocessing deposition methods are desired. This paper deals with CZTS absorber layers fabricated from CZTS nanoparticles that are functionalized with KCl, and investigates two different types of furnaces for the annealing process; a hydrogen sulfide (H 2S) flow furnace and a nitrogen atmosphere closed furnace. We find that more uniform grain growth is achieved when annealing in a closed furnace, while very large grains on the surface of the film are obtained when annealing in H 2S.

 Users may download and print one copy of any publication from the public portal for the purpose... more  Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

 Users may download and print one copy of any publication from the public portal for the purpose... more  Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Post deposition annealing is a frequently used approach in CZTS(Se) research community to boost t... more Post deposition annealing is a frequently used approach in CZTS(Se) research community to boost the efficiency of the cells. In this study, we investigate the influence of post deposition annealing in air on CZTS devices performance, produced by pulsed laser deposition. There are competing beneficial and detrimental effects depending on the temperature. We suggest that Cu-Zn ordering at 150°C and diffusion of Cd into CZTS from 250°C to higher temperature could be the main beneficial effects that increase the efficiency. A record efficiency for pulsed laser deposition of 5.4% was obtained at 150°C and 15 min post deposition annealing.

Recently, Si-based tandem solar cells have been the most compelling strategy to improve the effic... more Recently, Si-based tandem solar cells have been the most compelling strategy to improve the efficiency of the existing photovoltaic technology beyond its practical and theoretical limits. Thin-film chalcogenides have shown performance, reliability, stability and costs comparable to the Si technology, and could be suitable top-cell partners for Si. However, limited work has been done to evaluate the compatibility of these two technologies. The synthesis process of chalcogenides generally includes high temperature annealing (> 500 oC) of the precursors in a reactive atmosphere, which could lead to in-diffusion of the detrimental metallic species into the Si cell. In this work, we investigated monolithic integration of Cu2ZnSnS4 (CZTS), a promising representative from the chalcogenides family, on top of a thermally resilient silicon structure featuring polysilicon-based electron and hole selective passivating contacts. We used different Ti-based barrier layers, namely TiN, Ti(OxNy),...

Solar Energy Materials and Solar Cells, 2020

Abstract In this work, we have investigated Cu2ZnSnS4 (CZTS) solar cells made from oxide, oxy-sul... more Abstract In this work, we have investigated Cu2ZnSnS4 (CZTS) solar cells made from oxide, oxy-sulfide and sulfide precursors produced by pulsed laser deposition (PLD). Although sulfide precursors are widely used to fabricate CZTS solar cells, Sn loss is commonly observed due to the high volatility of SnxSy species during high temperature sulfurization. This can lead to a non-ideal absorber composition and a high density of detrimental Sn-related defects that severely affect the performance of the device. By using oxide precursors, we have shown that the Sn loss can be significantly reduced due to the higher thermal stability of SnxOy species when compared to their sulfide counterparts. However, the reaction mechanism for the oxide route results in rough CZTS films. We hypothesize that the SO2 gas that forms during the conversion from oxide to sulfide is trapped in the film during sulfurization, and can lead to grains with hollow cavities and thus increase the surface roughness. Therefore, we have developed an annealing route for the oxide precursors at lower annealing pressures, which leads to improved film morphology and device performance. As a result, we report a power conversion efficiency of 5.4% for solar cells made from oxide precursors. This is the highest value reported for a CZTS absorber produced by PLD.

Scientific Reports, 2020

Energy band alignment at the heterointerface between CdS and kesterite Cu2ZnSnS4 (CZTS) and its a... more Energy band alignment at the heterointerface between CdS and kesterite Cu2ZnSnS4 (CZTS) and its alloys plays a crucial role in determining the efficiency of the solar cells. Whereas Ag alloying of CZTS has been shown to reduce anti-site defects in the bulk and thus rise the efficiency, the electronic properties at the interface with the CdS buffer layer have not been extensively investigated. In this work, we present a detailed study on the band alignment between n-CdS and p-CZTS upon Ag alloying by depth-profiling ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Our findings indicate that core-level peaks and the valence band edge of CdS exhibit a significant shift to a lower energy (larger than 0.4 eV) upon the etching of the CdS layer, which can be assigned due to band bending and chemical shift induced by a change in the chemical composition across the interface. Using a simplified model based on charge depletion layer conservation, a sign...

ACS Applied Materials & Interfaces, 2020

In kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell research, an asymmetric crystallization profile i... more In kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell research, an asymmetric crystallization profile is often obtained after annealing, resulting in a bilayered-or double-layered-CZTSSe absorber. So far, only segregated pieces of research exist to characterize the appearance of this double layer, its formation dynamics and its effect on the performance of devices. In this work, we review the existing research on double-layered kesterites and evaluate the different mechanisms proposed. Using a cosputtering-based approach, we show that the two layers can differ significantly in morphology, composition and optoelectronic properties, and complement the results with a large statistical dataset of over 850 individual CZTS solar cells. By reducing the absorber thickness from above 1000 nm to 300 nm, we show that the double layer segregation is alleviated. In turn, we see a progressive improvement in the device performance for lower thickness, which alone would be inconsistent with the well-known case of ultrathin CIGS solar cells. We therefore attribute the improvements to the reduced double-layer occurrence, and find that the double layer limits the efficiency of our devices to below 7%. By comparing the results with CZTS grown on monocrystalline Si substrates, without a native Na supply, we show that the alkali metal supply does not determine the double layer formation, but merely reduces the threshold for its occurrence. Instead, we propose that the main formation mechanism is the early migration of Cu to the surface during annealing and formation of Cu2-xS phases, in a self-regulating process akin to the Kirkendall effect. Finally, we comment on the generality of the mechanism proposed, by comparing our results to other synthesis routes, including our own in-house results from solution processing and pulsed laser deposition of sulfide-and oxide-based targets. We find that although the double layer occurrence largely depends on the kesterite synthesis route, the common factors determining the double layer occurrence appear to be the presence of metallic Cu and/or a chalcogen deficiency in the precursor matrix. We suggest that understanding the limitations imposed by the double layer dynamics could prove useful to pave the way for breaking the 13% efficiency barrier for this technology.

RSC Advances, 2018

We present a route where organic ligand-free, KCl-functionalized Cu2ZnSnS4 nanoparticles grow int... more We present a route where organic ligand-free, KCl-functionalized Cu2ZnSnS4 nanoparticles grow into large, dense grains during annealing in nitrogen/sulfur atmosphere.

Solar Energy Materials and Solar Cells, 2020

Following the recent success of monolithically integrated Perovskite/Si tandem solar cells, great... more Following the recent success of monolithically integrated Perovskite/Si tandem solar cells, great interest has been raised in searching for alternative wide bandgap top-cell materials with prospects of a fully earthabundant, stable and efficient tandem solar cell. Thin film chalcogenides (TFCs) such as the Cu 2 ZnSnS 4 (CZTS) could be suitable top-cell materials. However, TFCs have the disadvantage that generally at least one high temperature step (> 500 • C) is needed during the synthesis, which could contaminate the Si bottom cell. Here, we systematically investigate the monolithic integration of CZTS on a Si bottom solar cell. A thermally resilient double-sided Tunnel Oxide Passivated Contact (TOPCon) structure is used as bottom cell. A thin (< 25 nm) TiN layer between the top and bottom cells, doubles as diffusion barrier and recombination layer. We show that TiN successfully mitigates in-diffusion of CZTS elements into the c-Si bulk during the high temperature sulfurization process, and find no evidence of electrically active deep Si bulk defects in samples protected by just 10 nm TiN. Post-process minority carrier lifetime in Si exceeded 1.5 ms, i.e., a promising implied open-circuit voltage (i-V oc) of 715 mV after the high temperature sulfurization. Based on these results, we demonstrate a first proof-of-concept two-terminal CZTS/Si tandem device with an efficiency of 1.1% and a V oc of 900 mV. A general implication of this study is that the growth of complex semiconductors on Si using high temperature steps is technically feasible, and can potentially lead to efficient monolithically integrated two-terminal tandem solar cells.

physica status solidi (a), 2015

The influence of the coating/substrate interface on the photocatalytic behavior of Al-TiO 2 coati... more The influence of the coating/substrate interface on the photocatalytic behavior of Al-TiO 2 coatings was investigated. The TiO 2 coatings were prepared by magnetron sputtering. The nanoscale structure of the coating was analyzed using X-ray diffraction; atomic force microscopy; scanning electron microscopy; and transmission electron microscopy. The photocatalytic behavior was investigated through optical spectrophotometry studies and electrochemical experiments; as photo voltage; photocurrent; and electrochemical impedance measurements. Consistent results from both optical and electrochemical measurements showed a maximum UV-light absorption by titanium dioxide occurring slightly prior to the energy of the maximum photocurrent. The photocurrent of titanium dioxide decreases with increasing thickness of the aluminum oxide interface layer. Aluminum oxide acts as an insulator; disfavoring the electron transport between the coating and the metallic substrate. The highest photocurrents were indeed obtained when the thickness of interfacial aluminum oxide could be reduced by sputtering a thin Ti layer prior to TiO 2 coating. Photocurrent plotted for different photon energy for a TiO 2 coating on a Ti interlayer consisting of both rutile and anatase. Two photon-excitation peaks from the TiO 2 coating (anatase and rutile) are observed at a potential of 0.34 V versus SHE, while only one peak was observed at a potential 0.14 V versus SHE closer to the conduction band.

Journal of Materials Science, 2015

Theoretical calculations and experimental studies were carried out on polyurethane (PU)-TiO 2 com... more Theoretical calculations and experimental studies were carried out on polyurethane (PU)-TiO 2 composite coatings on bright and matte aluminium surfaces with an aim to understand and tailor the light scattering from particles incorporated into an anodised layer for designing the optical appearance of anodised surfaces. PU matrix was selected for its matching refractive-index (n = 1.7) with anodic alumina layer. Three different TiO 2 particle size distributions were dispersed in PU and spin coated onto bright high-gloss and matte caustic-etched aluminium substrates. The reflectance spectra of coated surfaces in the visible region were analysed using an integrating sphere-spectrophotometer. Data showed that the coated surfaces have a high diffuse reflectance due to the multiple scattering from TiO 2 particles and the coatingsubstrate interface. The diffuse reflectance spectra of the coated surfaces varied weakly with TiO 2 particle concentration and reached a steady state value at 1 wt% but were dependent on the substrate type used. Using Kubelka-Munk two-stream model, the scattering and absorption coefficient of TiO 2 in PU was predicted. The studies presented in this paper provide insight into generating bright white-anodised aluminium surfaces based on aluminium-TiO 2 composites.

ACS applied materials & interfaces, Jan 24, 2014

The photocatalytic behavior of magnetron sputtered anatase TiO2 coatings on copper, nickel, and g... more The photocatalytic behavior of magnetron sputtered anatase TiO2 coatings on copper, nickel, and gold was investigated with the aim of understanding the effect of the metallic substrate and coating-substrate interface structure. Stoichiometry and nanoscale structure of the coating were investigated using X-ray diffraction, Raman spectroscopy, atomic force microscope, and scanning and transmission electron microscopy. Photocatalytic behavior of the coating was explored by using optical spectrophotometry and electrochemical methods via photovoltage, photocurrent, and scanning kelvin probe microscopy measurements. The nature of the metal substrate and coating-substrate interface had profound influence on the photocatalytic behavior. Less photon energy was required for TiO2 excitation on a nickel substrate, whereas TiO2 coating on copper showed a higher band gap attributed to quantum confinement. However, the TiO2 coating on gold exhibited behavior typical of facile transfer of electrons...

Applied Surface Science, 2014

ABSTRACT The mechanism of interaction of light with the microstructure of anodized layer giving s... more ABSTRACT The mechanism of interaction of light with the microstructure of anodized layer giving specific optical appearance is investigated using Al–Zr sputter deposited coating as a model system on an AA6060 substrate. Differences in the oxidative nature of various microstructural components result in the evolution of typical features in the anodized layer, which are investigated as a function of microstructure and correlated with its optical appearance. The Zr concentration in the coating was varied from 6 wt.% to 23 wt.%. Heat treatment of the coated samples was carried out at 550 °C for 4 h in order to evolve Al–Zr based second phase precipitates in the microstructure. Anodizing was performed using 20 wt.% sulphuric acid at 18 °C with an intention to study the effect of anodizing on the Al–Zr based precipitates in the coating. Detailed microstructural characterization of the coating and anodized layer was carried out using high resolution scanning and transmission electron microscopy, grazing incidence X-ray diffraction analysis, glow discharge optical emission spectroscopy, and optical appearance using spectrophotometry. The evolution of microstructure in the anodized layer as a function of anodizing parameters and their influence on the interaction of light is investigated and the results in general are applicable to discolouration of anodized layer on recycled aluminium alloys due to intermetallics.

RSC Advances, 2013

ABSTRACT The change in the surface potential of TiO2 coatings upon UV-illumination was investigat... more ABSTRACT The change in the surface potential of TiO2 coatings upon UV-illumination was investigated on the nanoscale using Scanning Kelvin Probe Force microscopy and on the micro-scale using photo-electrochemical measurements. A good correlation between the two techniques was obtained. The changes in the surface potential of TiO2 coatings upon UV-illumination are closely correlated to the band gap and thickness of the coatings. The inhomogeneity surface potential distribution of a 100 nm TiO2 film indicates a heterogeneous coating. Transition to a homogeneous surface potential distribution was observed with increasing thickness of the TiO2 coating.

Surface and Coatings Technology, 2014

ABSTRACT Effect of alloy composition and prior surface finish on the optical appearance of the an... more ABSTRACT Effect of alloy composition and prior surface finish on the optical appearance of the anodised layer on aluminium alloys was investigated. Four commercial alloys namely AA1050, Peraluman 706, AA5754, and AA6082 were used for the investigation. Microstructure and surface morphology of the substrate prior to anodising were analysed using scanning electron microscopy and atomic force microscopy. The optical appearance of the anodised surface with and without sealing was investigated using a photography setup, photospectrometry and bidirectional reflectance distribution function. It was found that the roughness of the as-etched surface increases with the degree of alloying due to second phase particles making the reflection more diffused, and that the as-etched surface morphology is similar to the oxide–substrate interface after anodising. Proper polishing is achieved on hard alloys and the glossy appearance was kept for alloys of high purity. Sealing made the specular reflection of the mechanically polished specimens more distinct.

Materials Science and Engineering: B, 2004

BaTiO3 (Barium titanate, BTO) thin films were grown on Pt coated Si substrates by radiofrequency ... more BaTiO3 (Barium titanate, BTO) thin films were grown on Pt coated Si substrates by radiofrequency discharge assisted pulsed laser deposition (RF-PLD). A standard experimental set-up consisting in a pulsed YAG-Nd laser working at wavelength of 355nm and incident fluence in the range of 2–3J/cm2 and assisted by a radio frequency plasma discharge (power RF of about 50–200W in a O2

Materials Chemistry and Physics, 2011