| 1. |
- Franchi, Daniele, et al.
(author)
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Effect of the Ancillary Ligand on the Performance of Heteroleptic Cu(I) Diimine Complexes as Dyes in Dye-Sensitized Solar Cells
- 2022
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In: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 5:2, s. 1460-1470
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Journal article (peer-reviewed)abstract
- A series of heteroleptic Cu(I) diimine complexes with different ancillary ligands and 6,6'-dimethyl-2,2'-bipyridine-4,4'-dibenzoic acid (dbda) as the anchoring ligand were selfassembled on TiO2 surfaces and used as dyes for dye-sensitized solar cells (DSSCs). The binding to the TiO2 surface was studied by hard X-ray photoelectron spectroscopy for a brominecontaining complex, confirming the complex formation. The performance of all complexes was assessed and rationalized on the basis of their respective ancillary ligand. The DSSC photocurrent-voltage characteristics, incident photon-to-current conversion efficiency (IPCE) spectra, and calculated lowest unoccupied molecular orbital (LUMO) distributions collectively show a push-pull structural dye design, in which the ancillary ligand exhibits an electron-donating effect that can lead to improved solar cell performance. By analyzing the optical properties of the dyes and their solar cell performance, we can conclude that the presence of ancillary ligands with bulky substituents protects the Cu(I) metal center from solvent coordination constituting a critical factor in the design of efficient Cu(I)-based dyes. Moreover, we have identified some components in the I-/I-3(-)-based electrolyte that causes dissociation of the ancillary ligand, i.e., TiO2 photoelectrode bleaching. Finally, the detailed studies on one of the dyes revealed an electrolyte-dye interaction, leading to a dramatic change of the dye properties when adsorbed on the TiO2 surface.
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| 2. |
- Garcia Fernandez, Alberto, et al.
(author)
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Experimental and Theoretical Core Level and Valence Band Analysis of Clean Perovskite Single Crystal Surfaces
- 2022
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In: Small. - : Wiley. - 1613-6810 .- 1613-6829. ; 18:13
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Journal article (peer-reviewed)abstract
- A detailed understanding of the surface and interface properties of lead halide perovskites is of interest for several applications, in which these materials may be used. To develop this understanding, the study of clean crystalline surfaces can be an important stepping stone. In this work, the surface properties and electronic structure of two different perovskite single crystal compositions (MAPbI(3) and Cs(x)FA(1-)(x)PbI(3)) are investigated using synchrotron-based soft X-ray photoelectron spectroscopy (PES), molecular dynamics simulations, and density functional theory. The use of synchrotron-based soft X-ray PES enables high surface sensitivity and nondestructive depth-profiling. Core level and valence band spectra of the single crystals are presented. The authors find two carbon 1s contributions at the surface of MAPbI(3) and assign these to MA(+) ions in an MAI-terminated surface and to MA(+) ions below the surface. It is estimated that the surface is predominantly MAI-terminated but up to 30% of the surface can be PbI2-terminated. The results presented here can serve as reference spectra for photoelectron spectroscopy investigations of technologically relevant polycrystalline thin films, and the findings can be utilized to further optimize the design of device interfaces.
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| 3. |
- Potts, Nathan T. Z., et al.
(author)
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Probing the dye-semiconductor interface in dye-sensitized NiO solar cells
- 2020
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 153:18
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Journal article (peer-reviewed)abstract
- The development of p-type dye-sensitized solar cells (p-DSSCs) offers an opportunity to assemble tandem photoelectrochemical solar cells with higher efficiencies than TiO2-based photoanodes, pioneered by O'Regan and Gratzel [Nature 353, 737-740 (1991)]. This paper describes an investigation into the behavior at the interfaces in p-DSSCs, using a series of BODIPY dyes, BOD1-3. The three dyes have different structural and electronic properties, which lead to different performances in p-DSSCs. We have applied photoelectron spectroscopy and transient absorption spectroscopy to rationalize these differences. The results show that the electronic orbitals of the dyes are appropriately aligned with the valence band of the NiO semiconductor to promote light-induced charge transfer, but charge-recombination is too fast for efficient dye regeneration by the electrolyte. We attribute this fast recombination, which limits the efficiency of the solar cells, to the electronic structure of the dye and the presence of Ni3+ recombination sites at the NiO surface.
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| 4. |
- Sloboda, Tamara, et al.
(author)
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A method for studying pico to microsecond time-resolved core-level spectroscopy used to investigate electron dynamics in quantum dots
- 2020
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In: Scientific Reports. - : Springer Nature. - 2045-2322. ; 10:1
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Journal article (peer-reviewed)abstract
- Time-resolved photoelectron spectroscopy can give insights into carrier dynamics and offers the possibility of element and site-specific information through the measurements of core levels. In this paper, we demonstrate that this method can access electrons dynamics in PbS quantum dots over a wide time window spanning from pico- to microseconds in a single experiment carried out at the synchrotron facility BESSY II. The method is sensitive to small changes in core level positions. Fast measurements at low pump fluences are enabled by the use of a pump laser at a lower repetition frequency than the repetition frequency of the X-ray pulses used to probe the core level electrons: Through the use of a time-resolved spectrometer, time-dependent analysis of data from all synchrotron pulses is possible. Furthermore, by picosecond control of the pump laser arrival at the sample relative to the X-ray pulses, a time-resolution limited only by the length of the X-ray pulses is achieved. Using this method, we studied the charge dynamics in thin film samples of PbS quantum dots on n-type MgZnO substrates through time-resolved measurements of the Pb 5d core level. We found a time-resolved core level shift, which we could assign to electron injection and charge accumulation at the MgZnO/PbS quantum dots interface. This assignment was confirmed through the measurement of PbS films with different thicknesses. Our results therefore give insight into the magnitude of the photovoltage generated specifically at the MgZnO/PbS interface and into the timescale of charge transport and electron injection, as well as into the timescale of charge recombination at this interface. It is a unique feature of our method that the timescale of both these processes can be accessed in a single experiment and investigated for a specific interface.
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| 5. |
- Sloboda, Tamara, et al.
(author)
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Photovoltage Generation across Different Interfaces in a PbS QuantumDot Solar Cell Investigated by Time-Resolved PhotoelectronSpectroscopy
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Other publication (other academic/artistic)abstract
- Quantum dot solar cells have not yet achieved optimal device performances and to direct development there is thereforea need to understand the device function of present solar cell structures in more detail. Understanding where photovoltage isgenerated in a device and where energy losses occur is a key aspect of this. We have previously shown that time-resolved core levelphotoelectron spectroscopy can be used to follow the photovoltage rise and decay at a specific interface from pico- to microsecondtimescales. Here, we extend this study and investigate the photovoltage generation in different parts of a PbS quantum dot solar cellthrough sample design. We show that thick absorbing quantum dot layers are required for generating a high photovoltage at theinterface between n-type PbS quantum dots and p-type quantum dots. Furthermore, we show that the full photovoltage is only generatedwhen a gold contact is deposited on the quantum dots and that the presence of this contact also leads to significantly slowercharge recombination.
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| 6. |
- Sloboda, Tamara
(author)
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Spectroscopy on the Dot : Photoelectron Spectroscopy and Time-Resolved Studies of Lead Sulfide Quantum Dots for Solar Cells
- 2022
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Doctoral thesis (other academic/artistic)abstract
- Renewable energy is an important topic as global energy consumption continues to rise. Because the sun emits an enormous amount of energy, solar energy is a promising source. However, most of the commercial solar cell technology is manufactured in an energy demanding process and there is a need for new, easily processed materials. This thesis concerns quantum dots, which are nanoparticles that can absorb light of different energies depending on their size. They can be synthesised by solution-based chemistry and turned into solid thin films to harvest sunlight. The fundamental properties of quantum dots need to be better understood before production on large scales may commence. The aim of this thesis was to investigate the fundamental properties of lead sulfide quantum dots. The methods used in this thesis are based on photoelectron spectroscopy. They allowed investigation of materials as-is, but also changes upon excitation by laser or X-rays. Using a laser, dynamics on pico- to microsecond timescales were studied by time-resolved photoelectron spectroscopy. Using a range of X-rays, the probability of charge transfer in the attosecond range was investigated. Steady-state investigation showed that different surface treatment of the quantum dots caused different resistance towards surface oxidation and X-ray damage. Different layers in the structure of solar cells can influence the photovoltage, an important parameter in achieving high power conversion efficiencies. Time-resolved photoelectron spectroscopy was developed and used to investigate the contributions of the layers to photovoltage generation. We observed photovoltage dynamics on a timescale covering six orders of magnitude. The mechanism of charge transfer in quantum dots of different sizes was studied by core-hole clock spectroscopy in the attosecond regime. Our results show that quantum confinement affects the charge transfer only at low excitation energies.
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| 7. |
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| 8. |
- Sloboda, Tamara, et al.
(author)
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Unravelling the ultrafast charge dynamics in PbS quantum dots through resonant Auger mapping of the sulfur K-edge
- 2022
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In: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 12:49, s. 31671-31679
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Journal article (peer-reviewed)abstract
- There is a great fundamental interest in charge dynamics of PbS quantum dots, as they are promising for application in photovoltaics and other optoelectronic devices. The ultrafast charge transport is intriguing, offering insight into the mechanism of electron tunneling processes within the material. In this study, we investigated the charge transfer times of PbS quantum dots of different sizes and non-quantized PbS reference materials by comparing the propensity of localized or delocalized decays of sulfur 1s core hole states excited by X-rays. We show that charge transfer times in PbS quantum dots decrease with excitation energy and are similar at high excitation energy for quantum dots and non-quantized PbS. However, at low excitation energies a distinct difference in charge transfer time is observed with the fastest charge transfer in non-quantized PbS and the slowest in the smallest quantum dots. Our observations can be explained by iodide ligands on the quantum dots creating a barrier for charge transfer, which reduces the probability of interparticle transfer at low excitation energies. The probability of intraparticle charge transfer is limited by the density of available states which we describe according to a wave function in a quantum well model. The stronger quantum confinement effect in smaller PbS quantum dots is manifested as longer charge transfer times relative to the larger quantum dots at low excitation energies.
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| 9. |
- Sloboda, Tamara, et al.
(author)
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Unravelling the ultrafast charge dynamics in PbS quantum dotsthrough resonant Auger mapping of the sulfur K-edge
-
Other publication (other academic/artistic)abstract
- There is a great fundamental interest in charge dynamics of PbS quantum dots, as they arepromising for application in photovoltaics and other optoelectronic devices. The ultrafastcharge transport is intriguing, offering insight into the mechanism of electron tunnelingprocesses within the material. In this study we investigated the charge transfer times of PbSquantum dots of different sizes and non-quantized PbS reference materials by comparing thepropensity of localized or delocalized decays of sulfur 1s core hole states excited by X-rays.We show that charge transfer times in PbS quantum dots decrease with excitation energy andare similar at high excitation energy for quantum dots and non-quantized PbS. However, atlow excitation energies a distinct difference in charge transfer time is observed with thefastest charge transfer in non-quantized PbS and the slowest in the smallest quantum dots.Our observations can be explained by iodide ligands on the quantum dots creating a barrierfor charge transfer, which reduces the probability of interparticle transfer at low excitationenergies. The probability of intraparticle charge transfer is limited by the density of availablestates which we describe according to a wavefunction in a quantum well model. The strongerquantum confinement effect in smaller PbS quantum dots is manifested as longer chargetransfer times relative to the larger quantum dots at low excitation energies.
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| 10. |
- Svanström, Sebastian, et al.
(author)
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Direct Measurements of Interfacial Photovoltage and Band Alignment in Perovskite Solar Cells Using Hard X-ray Photoelectron Spectroscopy
- 2023
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In: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 15:9, s. 12485-12494
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Journal article (peer-reviewed)abstract
- A heterojunction is the key junction for charge extraction in many thin film solar cell technologies. However, the structure and band alignment of the heterojunction in the operating device are often difficult to predict from calculations and, due to the complexity and narrow thickness of the interface, are difficult to measure directly. In this study, we demonstrate a technique for direct measurement of the band alignment and interfacial electric field variations of a fully functional lead halide perovskite solar cell structure under operating conditions using hard X-ray photoelectron spectroscopy (HAXPES). We describe the design considerations required in both the solar cell devices and the measurement setup and show results for the perovskite, hole transport, and gold layers at the back contact of the solar cell. For the investigated design, the HAXPES measurements suggest that 70% of the photovoltage was generated at this back contact, distributed rather equally between the hole transport material/gold interface and the perovskite/hole transport material interface. In addition, we were also able to reconstruct the band alignment at the back contact at equilibrium in the dark and at open circuit under illumination.
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