| 1. |
- Abdi-Jalebi, Mojtaba, et al.
(författare)
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Dedoping of Lead Halide Perovskites Incorporating Monovalent Cations
- 2018
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 12:7, s. 7301-7311
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Tidskriftsartikel (refereegranskat)abstract
- We report significant improvements in the optoelectronic properties of lead halide perovskites with the addition of monovalent ions with ionic radii close to Pb2+. We investigate the chemical distribution and electronic structure of solution processed CH3NH3PbI3 perovskite structures containing Na+, Cu+, and Ag+, which are lower valence metal ions than Pb2+ but have similar ionic radii. Synchrotron X-ray diffraction reveals a pronounced shift in the main perovskite peaks for the monovalent cation-based films, suggesting incorporation of these cations into the perovskite lattice as well as a preferential crystal growth in Ag+ containing perovskite structures. Furthermore, the synchrotron X-ray photoelectron measurements show a significant change in the valence band position for Cu- and Ag-doped films, although the perovskite bandgap remains the same, indicating a shift in the Fermi level position toward the middle of the bandgap. Such a shift infers that incorporation of these monovalent cations dedope the n-type perovskite films when formed without added cations. This dedoping effect leads to cleaner bandgaps as reflected by the lower energetic disorder in the monovalent cation-doped perovskite thin films as compared to pristine films. We also find that in contrast to Ag+ and Cu+, Na+ locates mainly at the grain boundaries and surfaces. Our theoretical calculations confirm the observed shifts in X-ray diffraction peaks and Fermi level as well as absence of intrabandgap states upon energetically favorable doping of perovskite lattice by the monovalent cations. We also model a significant change in the local structure, chemical bonding of metal-halide, and the electronic structure in the doped perovskites. In summary, our work highlights the local chemistry and influence of monovalent cation dopants on crystallization and the electronic structure in the doped perovskite thin films.
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| 2. |
- Behrouznejad, F., et al.
(författare)
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Metal-based bracken-like single-sided dye-sensitized solar cells with horizontal separation
- 2016
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 18:7, s. 5244-5252
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Tidskriftsartikel (refereegranskat)abstract
- One of the drawbacks of typical dye-sensitized solar cells (DSCs) is their high cost and the high electrical resistance of the transparent conducting substrate. In conventional sandwich-type DSCs, only one of the FTO substrates can be replaced by a metal substrate. We investigated an all-metal-electrode single-sided DSC in which interpenetrated bracken-like Cr electrodes were created using photolithography; mesoporous TiO2 and Pt films were deposited on the laterally separated electrodes. Thermal Pt deposition and electrodeposition methods were investigated and it was found that a cyclic electrodeposition method resulted in selective Pt deposition at room temperature with a higher device performance. Cu or ZnO sacrificial layers and TiO2 or TiO2/SiO2 porous layers were used for the spacer layer that keeps the Pt electrode away from the TiO2 mesoporous layer and the optimum results were obtained when a TiO2/SiO2 layer was used. The best device had a current density of 8.47 mA cm(-2), an open circuit voltage of 0.685 V and an efficiency of 2.44%. The results of open circuit voltage decay and electrochemical impedance spectrometry showed the formation of a high-resistivity blocking layer, which was attributed to the Cr oxide formed during thermal treatment. The efficiency may be improved further by developing low-temperature fabrication processes.
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| 3. |
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| 4. |
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| 5. |
- Freitag, Marina, et al.
(författare)
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Copper Phenanthroline as a Fast and High-Performance Redox Mediator for Dye-Sensitized Solar Cells
- 2016
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:18, s. 9595-9603
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Tidskriftsartikel (refereegranskat)abstract
- The most commonly used redox mediators in dye-sensitized solar cells (DSCs), iodide/triiodide and cobalt trisbipyridine ([Co(bpy)(3)](2+/3+)), were successfully replaced by bis (2,9-dimethy1-1,10-phenanthroline) copp er (I/H) ([Cu(dmp)(2)](1+/2+)). The use of the copper complex based electrolyte led to an exceptionally high photovoltaic performance of 8.3% for LEG4-sensitized TiO2 solar cells, with a remarkably high open-circuit potential of above 1.0 V at 1000 W m(-2) under AM1.5G conditions. The copper complex based redox electrolyte has higher diffusion coefficients and is considerably faster in dye regeneration than comparable cobalt trisbipyridine based electrolytes. A driving force for dye regeneration of only 0.2 eV is sufficient to obtain unit yield, pointing to new possibilities for improvement in DSC efficiencies. The interaction of the excited dye with components of the electrolyte was monitored using steady-state emission measurements and time-correlated single-photon counting (TC-SPC). Our results indicate bimolecular reductive quenching of the excited LEG4 dye by the [Cu(dmp)(2)](2+) complex through a dynamic mechanism. Excited-state dye molecules can readily undergo bimolecular electron transfer with a suitable donor molecule. In DSCs this process can occur when the excited dye is unable to inject electrons into the TiO2. With a high electrolyte concentration the excited dye can be intercepted with an electron from the electrolyte resulting in the reduced state of the dye. Quenching of the reduced dye by the electrolyte competes with electron injection and results in a lower photocurrent. Quenching of excited LEG4 by complexes of [Cu(dmp)(2)](+), [Co(bpy)(3)](2+), and [Co(bpy)(3)](3+) followed a static mechanism, due ground-state dye-quencher binding. Inhibition of unwanted quenching processes by structural modifications may open ways to further increase the overall efficiency.
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| 6. |
- Freitag, Marina, et al.
(författare)
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High-efficiency dye-sensitized solar cells with molecular copper phenanthroline as solid hole conductor
- 2015
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Ingår i: Energy & Environmental Science. - : Royal Society of Chemistry (RSC). - 1754-5692 .- 1754-5706. ; 8:9, s. 2634-2637
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Tidskriftsartikel (refereegranskat)abstract
- Copper phenanthroline complexes in the solid phase can act as efficient molecular hole transporting material (HTM) for hybrid solar cells. We prepared solid-state dye-sensitized solar cells with the organic dye LEG4 and bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) (Cu(dmp)(2)) and achieved power conversion efficiencies of more than 8% under 1000 W m(-2) AM1.5G illumination, with open-circuit potentials of more than 1.0 V. The successful application of a copper-complex based HTM paves the way for low-cost and efficient hybrid solar cells, as well as for other opto-electronic devices.
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| 7. |
- Gao, Jiajia, et al.
(författare)
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Cation-Dependent Photostability of Co(II/III)-Mediated Dye-Sensitized Solar Cells
- 2015
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:44, s. 24704-24713
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Tidskriftsartikel (refereegranskat)abstract
- The electrolyte composition has a significant effect on the performance and stability of cobalt-based, dye-sensitized solar cells (DSSCs). The stability of DSSCs incorporating Co(II/III) tris(bipyridine) redox mediator has been investigated over 1000 h under full solar irradiation (with UV cutoff) at a temperature of 60 °C, the main focus being on monitoring the photovoltaic performance of the device and analyzing the internal charge-transfer dynamics in the presence of different cation coadditives (preferably added as tetracyanoborate salts). A clear cation-dependence is shown, not only of the early light-induced performance but also of the long-term photostability of the photovoltage of the device. These light-induced changes, which are attributed to the promotion of electron injection and less electron recombination loss, by transient spectral and electrochemical studies at the TiO2/dye/electrolyte interface, indicate that the main cation effects involve the TiO2 surface electric field and energy-state distribution. By examining the stability of adsorbed and solvated dye during aging, it has been found that the dye photodegradation is probably responsible for the decline in the photovoltage and that this is extremely dependent on the nature of the cation coadditives in the electrolyte. It is therefore suggested that optimization of the electrolyte cation composition is essential for improving the stability of cobalt-based DSSCs.
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| 8. |
- Imani, Roghayeh, et al.
(författare)
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Band edge engineering of TiO2@DNA nanohybrids and implications for capacitive energy storage devices
- 2015
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 7:23, s. 10438-10448
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Tidskriftsartikel (refereegranskat)abstract
- Novel mesoporous TiO2@DNA nanohybrid electrodes, combining covalently encoded DNA with mesoporous TiO2 microbeads using dopamine as a linker, were prepared and characterised for application in supercapacitors. Detailed information about donor density, charge transfer resistance and chemical capacitance, which have an important role in the performance of an electrochemical device, were studied by electrochemical methods. The results indicated the improvement of electrochemical performance of the TiO2 nanohybrid electrode by DNA surface functionalisation. A supercapacitor was constructed from TiO2@DNA nanohybrids with PBS as the electrolyte. From the supercapacitor experiment, it was found that the addition of DNA played an important role in improving the specific capacitance (C-s) of the TiO2 supercapacitor. The highest Cs value of 8 F g(-1) was observed for TiO2@DNA nanohybrids. The nanohybrid electrodes were shown to be stable over long-term cycling, retaining 95% of their initial specific capacitance after 1500 cycles.
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| 9. |
- Imani, Roghayeh, et al.
(författare)
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Biocompatibility of different nanostructured TiO2 scaffolds and their potential for urologic applications
- 2016
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Ingår i: Protoplasma. - : Springer Science and Business Media LLC. - 0033-183X .- 1615-6102. ; 253:6, s. 1439-1447
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Tidskriftsartikel (refereegranskat)abstract
- Despite great efforts in tissue engineering of the ureter, urinary bladder, and urethra, further research is needed in order to improve the patient's quality of life and minimize the economic burden of different lower urinary tract disorders. The nanostructured titanium dioxide (TiO2) scaffolds have a wide range of clinical applications and are already widely used in orthopedic or dental medicine. The current study was conducted to synthesize TiO2 nanotubes by the anodization method and TiO2 nanowires and nanospheres by the chemical vapor deposition method. These scaffolds were characterized with scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods. In order to test the urologic applicability of generated TiO2 scaffolds, we seeded the normal porcine urothelial (NPU) cells on TiO2 nanotubes, TiO2 nanowires, TiO2 nanospheres, and on the standard porous membrane. The viability and growth of the cells were monitored everyday, and after 3 weeks of culturing, the analysis with scanning electron microscope (SEM) was performed. Our results showed that the NPU cells were attached on all scaffolds; they were viable and formed a multilayered epithelium, i.e., urothelium. The apical plasma membrane of the majority of superficial NPU cells, grown on all three different TiO2 scaffolds and on the porous membrane, exhibited microvilli; thus, indicating that they were at a similar differentiation stage. The maximal caliper diameter measurements of superficial NPU cells revealed significant alterations, with the largest cells being observed on nanowires and the smallest ones on the porous membrane. Our findings indicate that different nanostructured TiO2 scaffolds, especially nanowires, have a great potential for tissue engineering and should be further investigated for various urologic applications.
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| 10. |
- Imani, Roghayeh, et al.
(författare)
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Combined cytotoxic effect of UV-irradiation and TiO2 microbeads in normal urothelial cells, low-grade and high-grade urothelial cancer cells
- 2015
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Ingår i: Photochemical and Photobiological Sciences. - : Springer Science and Business Media LLC. - 1474-905X .- 1474-9092. ; 14:3, s. 583-590
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Tidskriftsartikel (refereegranskat)abstract
- The differentiation of urothelial cells results in normal terminally differentiated cells or by alternative pathways in low-grade or high-grade urothelial carcinomas. Treatments with traditional surgical and chemotherapeutical approaches are still inadequate and expensive, as bladder tumours are generally highly recurrent. In such situations, alternative approaches, using irradiation of the cells and nanoparticles, are promising. The ways in which urothelial cells, at different differentiation levels, respond to UV-irradiation (photolytic treatment) or to the combination of UV-irradiation and nanoparticles (photocatalytic treatment), are unknown. Here we tested cytotoxicity of UV-irradiation on (i) normal porcine urothelial cells (NPU), (ii) human low-grade urothelial cancer cells (RT4), and (iii) human high-grade urothelial cancer cells (T24). The results have shown that 1 minute of UV-irradiation is enough to kill 90% of the cells in NPU and RT4 cultures, as determined by the live/dead viability assay. On the other hand, the majority of T24 cells survived 1 minute of UV-irradiation. Moreover, even a prolonged UV-irradiation for 30 minutes killed <50% of T24 cells. When T24 cells were pre-supplemented with mesoporous TiO2 microbeads and then UV-irradiated, the viability of these high-grade urothelial cancer cells was reduced to < 10%, which points to the highly efficient cytotoxic effects of TiO2 photocatalysis. Using electron microscopy, we confirmed that the mesoporous TiO2 microbeads were internalized into T24 cells, and that the cell's ultrastructure was heavily compromised after UV-irradiation. In conclusion, our results show major differences in the sensitivity to UV-irradiation among the urothelial cells with respect to cell differentiation. To achieve an increased cytotoxicity of urothelial cancer cells, the photocatalytic approach is recommended.
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| 11. |
- Imani, Roghayeh, et al.
(författare)
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Excited-state charge polarization and electronic structure of mixed-cation halide perovskites : the role of mixed inorganic-organic cations in CsFAPbI(3)
- 2022
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Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 12:39, s. 25415-25423
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Tidskriftsartikel (refereegranskat)abstract
- Mixed-cation perovskite materials have shown great potential for sunlight harvesting and have surpassed unmixed perovskite materials in solar cell efficiency and stability. The role of mixed monovalent cations in the enhanced optoelectronic properties and excited state response, however, are still elusive from a theoretical perspective. Herein, through time dependent density functional theory calculations of mixed cation perovskites, we report the electronic structure of Cs formamidinium (FA) mixed cationic lead iodide (Cs(0.17)FA(0.87)PbI(3)) in comparison to the corresponding single monovalent cation hybrid perovskite. The results show that the Cs(0.17)FA(0.87)PbI(3) and FAPbI(3) had negligible differences in the optical band gap, and partial and total density of states in comparison to a single cation perovskite, while the effective mass of carriers, the local atomic density of states, the directional transport, and the structural distortions were significantly different. A lattice-distortion-induced asymmetry in the ground-state charge density is found, and originates from the co-location of caesium atoms in the lattice and signifies the effect on the charge density upon cation mixing and corresponding symmetry breaking. The excited-state charge response and induced polarizabilities are quantified, and discussed in terms of their importance for effective light absorption, charge separation, and final solar cell performance. We also quantify the impact of such polarizabilities on the dynamics of the structure of the perovskites and the implications this has for hot carrier cooling. The results shed light on the mechanism and origin of the enhanced performance in mixed-cation perovskite-based devices and their merits in comparison to single cation perovskites.
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| 12. |
- Imani, Roghayeh, et al.
(författare)
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Fabrication of Microfibre-nanowire Junction Arrays of ZnO/SnO2 Composite by the Carbothermal Evaporation Method
- 2014
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Ingår i: Nanomaterials and Nanotechnology. - : SAGE Publications. - 1847-9804. ; 4, s. 21-
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Tidskriftsartikel (refereegranskat)abstract
- A cotton-like ZnO/SnO2 nanocomposite was grown by the carbothermal evaporation of a mixture of ZnO and SnO2 powders at 1100 degrees C by the vapour-liquid-solid process, in which the Sn particles produced by the reduction of SnO2 act as the catalyst. Field-emission scanning electron microscope images suggest that the composites are made of microfibre-nanowire junction arrays. The structure is formed due to the fast growth of the ZnO microfibre and the subsequent epitaxial radial growth of the ZnO nanowires with Sn particles at the tips. The photovoltaic performance of the ZnO/SnO2 nanocomposite sensitized with a D35-cpdt dye was investigated. A dye-sensitized solar cell (DSSC) with a ZnO/SnO2 nanocomposite photoanode based on a cobalt electrolyte achieved a solar-to-electricity conversion efficiency of similar to 0.34% with a short circuit current (JSC) of 0.66 mA/cm(2), an open circuit voltage (VOC) of 870 mV, and a fill factor (FF) of 59. The results show the potential of this one dimensional structure in cobalt electrolyte-based DSSCs; the further optimization which is needed to achieve higher efficiencies is also discussed.
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| 13. |
- Imani, Roghayeh, et al.
(författare)
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Modelling iodine diffusion in 2D-Perovskites as a function of the length of the organic spacer molecules
- 2024
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Ingår i: Solar Energy. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0038-092X .- 1471-1257. ; 272
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional (2D) hybrid perovskites have emerged as promising materials for solar cell applications, offering increased stability and reduced ionic movements compared to the more commonly used 3D-perovskites. The atomistic mechanisms behind the lower ionic diffusion in these perovskites have remained rather underexplored. In this study, we have used density functional theory (DFT) to investigate the potential tunability of ionic movements in 2D-perovskites by varying the spacer molecules in the structure. We demonstrate that by changing the length of the spacer molecule, and thus the distance between the two-dimensional inorganic slabs of corner sharing lead halide octahedra, it is possible to significantly modulate the periodic potential distribution and long-range electrostatic interactions within these materials. This modulation has a distinct impact on the energy barriers for ionic movement. Notably, we find that longer spacer molecules decrease the energy barrier for in-plane iodine diffusion, while increasing the energy barrier for inter-layer diffusion. Furthermore, we observe a logarithmic relationship between the energy barrier for inter-layer diffusion and the inter-layer distance. These results clarify the mechanisms underlying the lower ionic movement in 2D-perovskites and open avenues for engineering ionic migration in novel 2D-perovskite structures, thus enhancing their applicability in optoelectronic technologies.
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| 14. |
- Imani, Roghayeh, et al.
(författare)
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Multifunctional Gadolinium-Doped Mesoporous TiO2 Nanobeads : Photoluminescence, Enhanced Spin Relaxation, and Reactive Oxygen Species Photogeneration, Beneficial for Cancer Diagnosis and Treatment
- 2017
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Ingår i: Small. - : Wiley. - 1613-6810 .- 1613-6829. ; 13:20
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Tidskriftsartikel (refereegranskat)abstract
- Materials with controllable multifunctional abilities for optical imaging (OI) and magnetic resonant imaging (MRI) that also can be used in photodynamic therapy are very interesting for future applications. Mesoporous TiO2 sub-micrometer particles are doped with gadolinium to improve photoluminescence functionality and spin relaxation for MRI, with the added benefit of enhanced generation of reactive oxygen species (ROS). The Gd-doped TiO2 exhibits red emission at 637 nm that is beneficial for OI and significantly improves MRI relaxation times, with a beneficial decrease in spin-lattice and spin-spin relaxation times. Density functional theory calculations show that Gd3+ ions introduce impurity energy levels inside the bandgap of anatase TiO2, and also create dipoles that are beneficial for charge separation and decreased electron-hole recombination in the doped lattice. The Gd-doped TiO2 nanobeads (NBs) show enhanced ability for ROS monitored via center dot OH radical photogeneration, in comparison with undoped TiO2 nanobeads and TiO2 P25, for Gd-doping up to 10%. Cellular internalization and biocompatibility of TiO2@xGd NBs are tested in vitro on MG-63 human osteosarcoma cells, showing full biocompatibility. After photoactivation of the particles, anticancer trace by means of ROS photogeneration is observed just after 3 min irradiation.
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| 15. |
- Imani, Roghayeh, et al.
(författare)
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Unravelling in-situ formation of highly active mixed metal oxide CuInO2 nanoparticles during CO2 electroreduction
- 2018
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Ingår i: Nano Energy. - : ELSEVIER SCIENCE BV. - 2211-2855 .- 2211-3282. ; 49, s. 40-50
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Tidskriftsartikel (refereegranskat)abstract
- Technologies and catalysts for converting carbon dioxide (CO2) to immobile products are of high interest to minimize greenhouse effects. Copper(I) is a promising catalytic active state of copper but hampered by the inherent instability in comparison to copper(II) or copper(0). Here, we report a stabilization of the catalytic active state of copper(I) by the formation of a mixed metal oxide CuInO2 nanoparticle during the CO2 electroreduction. Our result shows the incorporation of nanoporous Sn:In2O3 interlayer to Cu2O pre-catalyst system lead to the formation of CuInO2 nanoparticles with remarkably higher activity for CO2 electroreduction at lower overpotential in comparison to the conventional Cu nanoparticles derived from sole Cu2O. Operando Raman spectroelectrochemistry is employed to in-situ monitor the process of nanoparticles formation during the electrocatalytic process. The experimental data are collaborated with DFT calculations to provide insight into the electro-formation of the type of Cu-based mixed metal oxide catalyst during the CO2 electroreduction, where a formation mechanism via copper ion diffusion across the substrate is suggested.
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| 16. |
- Jacobsson, T. Jesper, et al.
(författare)
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Exploration of the compositional space for mixed lead halogen perovskites for high efficiency solar cells
- 2016
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Ingår i: Energy & Environmental Science. - : Royal Society of Chemistry (RSC). - 1754-5692 .- 1754-5706. ; 9:5, s. 1706-1724
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Tidskriftsartikel (refereegranskat)abstract
- Lead halide perovskites have attracted considerable interest as photoabsorbers in PV-applications over the last few years. The most studied perovskite material achieving high photovoltaic performance has been methyl ammonium lead iodide, CH3NH3PbI3. Recently the highest solar cell efficiencies have, however, been achieved with mixed perovskites where iodide and methyl ammonium partially have been replaced by bromide and formamidinium. In this work, the mixed perovskites were explored in a systematic way by manufacturing devices where both iodide and methyl ammonium were gradually replaced by bromide and formamidinium. The absorption and the emission behavior as well as the crystallographic properties were explored for the perovskites in this compositional space. The band gaps as well as the crystallographic structures were extracted. Small changes in the composition of the perovskite were found to have a large impact on the properties of the materials and the device performance. In the investigated compositional space, cell efficiencies, for example, vary from a few percent up to 20.7%. From the perspective of applications, exchanging iodide with bromide is especially interesting as it allows tuning of the band gap from 1.5 to 2.3 eV. This is highly beneficial for tandem applications, and an empirical expression for the band gap as a function of composition was determined. Exchanging a small amount of iodide with bromide is found to be highly beneficial, whereas a larger amount of bromide in the perovskite was found to cause intense sub band gap photoemission with detrimental results for the device performance. This could be caused by the formation of a small amount of an iodide rich phase with a lower band gap, even though such a phase was not observed in diffraction experiments. This shows that stabilizing the mixed perovskites will be an important task in order to get the bromide rich perovskites, which has a higher band gap, to reach the same high performance obtained with the best compositions.
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| 17. |
- Jacobsson, T. Jesper, et al.
(författare)
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Goldschmidt's Rules and Strontium Replacement in Lead Halogen Perovskite Solar Cells : Theory and Preliminary Experiments on CH3NH3SrI3
- 2015
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:46, s. 25673-25683
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Tidskriftsartikel (refereegranskat)abstract
- During the past few years, organic lead halogen perovskites have emerged as a class of highly promising solar cell materials, with certified solar cell efficiencies now surpassing 20%. Concerns have, however, been raised about the possible environmental and legalization problems associated with a new solar cell technology based on a water-soluble lead compound. Replacing lead in the perovskite structure: with a less toxic element, without degrading the favorable photo physical properties, would therefore be of interest. In this paper, the possibility of replacing lead with other metal ions is explored by following the replacement rules of Goldschmidt together with additional quantum mechanical considerations. This analysis provides a conceptual toolbox toward replacing lead, as well as additional insights into the photo physics of the metal halogen perovskites. This approach is exemplified by focusing on strontium in particular, which is nontoxic and relatively inexpensive. The ionic radius of Sr2+ and Pb2+ are almost identical, suggesting an exchange could be made without affecting the crystal structure. Couple cluster calculations on the metal ions and their halogen salts give the bonding patterns to be sufficiently similar and density functional theory (DFT) revealed the strontium perovskite, CH3NH3SrI3, to be a stable phase, despite the difference in electronegativity between lead and strontium. This is further supported by the existence of binary PM, and SrI2 compounds and the beneficial formation energy of the strontium perovskite. The electronic properties of both CH3NH3SrI3 and CH3NH3PbI3 were simulated and compared, revealing a higher degree of ionic interaction in the metal halogen bound in the strontium perovskite. This is a consequence of the lower electronegativity of strontium, which, together with the lack of d-orbitals in the Valence of Sr2+, results, in a higher band gap. The band gap for the strontium perovskite was estimated to 3.6 eV, which unfortunately is too high for an efficient photo absorber. Initial investigations on experimental synthesis of the strontium perovskite, using wet chemical methods, revealed it to be harder to produce than the lead perovskite This is explained as a:consequence of different bonding patterns in the metal iodine salts, which obstruct the methylammonium intercalation pathway utilized for forming the perovskite. Vapor phase methods are instead suggested as more promising synthesis routes.
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| 18. |
- Micah, Angela E., et al.
(författare)
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Tracking development assistance for health and for COVID-19 : a review of development assistance, government, out-of-pocket, and other private spending on health for 204 countries and territories, 1990-2050
- 2021
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Ingår i: The Lancet. - : Elsevier. - 0140-6736 .- 1474-547X. ; 398:10308, s. 1317-1343
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Forskningsöversikt (refereegranskat)abstract
- Background The rapid spread of COVID-19 renewed the focus on how health systems across the globe are financed, especially during public health emergencies. Development assistance is an important source of health financing in many low-income countries, yet little is known about how much of this funding was disbursed for COVID-19. We aimed to put development assistance for health for COVID-19 in the context of broader trends in global health financing, and to estimate total health spending from 1995 to 2050 and development assistance for COVID-19 in 2020. Methods We estimated domestic health spending and development assistance for health to generate total health-sector spending estimates for 204 countries and territories. We leveraged data from the WHO Global Health Expenditure Database to produce estimates of domestic health spending. To generate estimates for development assistance for health, we relied on project-level disbursement data from the major international development agencies' online databases and annual financial statements and reports for information on income sources. To adjust our estimates for 2020 to include disbursements related to COVID-19, we extracted project data on commitments and disbursements from a broader set of databases (because not all of the data sources used to estimate the historical series extend to 2020), including the UN Office of Humanitarian Assistance Financial Tracking Service and the International Aid Transparency Initiative. We reported all the historic and future spending estimates in inflation-adjusted 2020 US$, 2020 US$ per capita, purchasing-power parity-adjusted US$ per capita, and as a proportion of gross domestic product. We used various models to generate future health spending to 2050. Findings In 2019, health spending globally reached $8. 8 trillion (95% uncertainty interval [UI] 8.7-8.8) or $1132 (1119-1143) per person. Spending on health varied within and across income groups and geographical regions. Of this total, $40.4 billion (0.5%, 95% UI 0.5-0.5) was development assistance for health provided to low-income and middle-income countries, which made up 24.6% (UI 24.0-25.1) of total spending in low-income countries. We estimate that $54.8 billion in development assistance for health was disbursed in 2020. Of this, $13.7 billion was targeted toward the COVID-19 health response. $12.3 billion was newly committed and $1.4 billion was repurposed from existing health projects. $3.1 billion (22.4%) of the funds focused on country-level coordination and $2.4 billion (17.9%) was for supply chain and logistics. Only $714.4 million (7.7%) of COVID-19 development assistance for health went to Latin America, despite this region reporting 34.3% of total recorded COVID-19 deaths in low-income or middle-income countries in 2020. Spending on health is expected to rise to $1519 (1448-1591) per person in 2050, although spending across countries is expected to remain varied. Interpretation Global health spending is expected to continue to grow, but remain unequally distributed between countries. We estimate that development organisations substantially increased the amount of development assistance for health provided in 2020. Continued efforts are needed to raise sufficient resources to mitigate the pandemic for the most vulnerable, and to help curtail the pandemic for all. Copyright (C) 2021 The Author(s). Published by Elsevier Ltd.
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| 19. |
- Park, Byung-wook, et al.
(författare)
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Understanding Interfacial Charge Transfer between Metallic PEDOT Counter Electrodes and a Cobalt Redox Shuttle in Dye-Sensitized Solar Cells
- 2014
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 6:3, s. 2074-2079
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Tidskriftsartikel (refereegranskat)abstract
- Conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with iron(111) tris-p-toluenesulfonate (PEDOT:Tos) having metallic conductivity was coated onto fluorine-doped tin oxide (FTO) glass and plain glass substrates and used as a counter electrode (CE) in a dye-sensitized solar cell (DSC) with a [Co(bpy)(3)](3+/2+) complex redox shuttle. DSCs with PEDOT:Tos/glass CE yielded power conversion efficiencies (PCE) of 6.3%, similar to that of DSCs with platinized FTO glass CE (6.1%). The PEDOT:Tos-based counter electrodes had 5 to 10 times lower charge-transfer resistance than the Pt/FTO CE in DSCs, as analyzed by impedance spectroscopy. More detailed studies in symmetrical CE-CE cells showed that the PEDOT:Tos layers are nanoporous. Not all internal area can be used catalytically under solar cell conditions and effective charge-transfer resistance was similar to that of Pt/FTO.
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| 20. |
- Patra, Hirak Kumar, et al.
(författare)
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On/off-switchable anti-neoplastic nanoarchitecture
- 2015
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Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 5:14571, s. 1-9
-
Tidskriftsartikel (refereegranskat)abstract
- Throughout the world, there are increasing demands for alternate approaches to advanced cancer therapeutics. Numerous potentially chemotherapeutic compounds are developed every year for clinical trial and some of them are considered as potential drug candidates. Nanotechnology-based approaches have accelerated the discovery process, but the key challenge still remains to develop therapeutically viable and physiologically safe materials suitable for cancer therapy. Here, we report a high turnover, on/off-switchable functionally popping reactive oxygen species (ROS) generator using a smart mesoporous titanium dioxide popcorn (TiO2 Pops) nanoarchitecture. The resulting TiO2 Pops, unlike TiO2 nanoparticles (TiO2 NPs), are exceptionally biocompatible with normal cells. Under identical conditions, TiO2 Pops show very high photocatalytic activity compared to TiO2 NPs. Upon on/off-switchable photo activation, the TiO2 Pops can trigger the generation of high-turnover flash ROS and can deliver their potential anticancer effect by enhancing the intracellular ROS level until it crosses the threshold to open the death gate, thus reducing the survival of cancer cells by at least six times in comparison with TiO2 NPs without affecting the normal cells.
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| 21. |
- Pazoki, Meysam, et al.
(författare)
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Ab initio study of electronic effects in the ZnO/TiO2 core/shell interface : application in dye sensitized solar cells
- 2014
-
Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 4:1, s. 301-307
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Tidskriftsartikel (refereegranskat)abstract
- Core/shell structure of ZnO nanowires coated with a monolayer of TiO2 is investigated using Density Functional Theory (DFT). The electronic states of the semiconductor is calculated and compared before and after coating of the TiO2 monolayer on a ZnO [10 (1) over bar 0] surface. The effect of TiO2 coating induce surface states changes and shifts the conduction and valence band edges to higher energies. Our results, in qualitative agreement with the experimental work of Matt Law et al. (J. Phys. Chem. B, 110, 22652), show an increase in open circuit voltage and a decrease in short circuit current in ZnO/TiO2 core shell dye sensitized solar cells. Regarding the semiconductor density of states (DOS), TiO2 coated ZnO have more conduction band acceptor states and lower electronic back recombination in agreement with experimental results. Surface dipoles are attributed to changes of the local density of states of the surface. This method can be used for more investigation of starting effects of semiconductor interface and helps the study of surface states and their physical origin in dye sensitized solar cells.
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| 22. |
- Pazoki, Meysam, et al.
(författare)
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Bismuth Iodide Perovskite Materials for Solar Cell Applications : Electronic Structure, Optical Transitions and Directional Charge Transport
- 2016
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120, s. 29039-29046
-
Tidskriftsartikel (refereegranskat)abstract
- Cesium and methylammonium bismuth iodides (Cs3Bi2I9 and MA(3)Bi(2)I(9)) are new low-toxic and air stable compounds in the perovskite solar cell family with promising characteristics. Here, the electronic structure and the nature of their optical transitions, dielectric constant, and charge carrier properties are assessed for photovoltaic applications with density functional theory (DFT) calculations and experiments. The calculated direct and indirect band gap values for Cs3Bi2I9 (2.17 and 2.0 eV) and MA(3)Bi(2)I(9) (2.17 and 1.97 eV) are found to be in good agreement with the experimental optical band gaps (2.2, 2.0 eV and 2.4, 2.1 eV for Cs3Bi2I9 and MA(3)Bi(2)I(9), respectively) estimated for solution-processed films. There is an error cancelation in the DFT calculated band gap similar to that for lead perovskites. However, fully relativistic DFT calculations indicate that the size of the spin orbit coupling (SOC) error cancelation for bismuth perovskite (0.5 eV) is less than for lead perovskite (1 eV), and other factors are therefore also important. Band structure calculations show high effective masses of the charge carriers along the c-axis but on the other hand lower electron effective mass in the a-b planes, revealing the interesting possibility for a directional charge transport. Calculations of dielectric constants, absorption coefficients, carrier effective masses, and exciton binding energies emphasize the fundamental differences between the lead and bismuth iodide perovskites and clarify the reasons behind the lower power conversion efficiency of bismuth iodide perovskite solar cells. Also the calculations show that the orientational disorder of the MA dipoles in the lattice has meaningful impacts on the near valence and conduction band edge of the electronic structure.
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| 23. |
- Pazoki, Meysam, et al.
(författare)
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Characterization techniques for dye-sensitized solar cells
- 2017
-
Ingår i: Energy & Environmental Science. - : Royal Society of Chemistry. - 1754-5692 .- 1754-5706. ; 10:3, s. 672-709
-
Tidskriftsartikel (refereegranskat)abstract
- Dye-sensitized solar cells (DSCs) have been widely studied in the last two decades and start to be commercialized in the photovoltaic market. Comprehensive characterization is needed to fully understand and optimize the device performance and stability. In this review, we summarize different characterization methods for dye-sensitized solar cells with liquid redox electrolytes or solid state hole transporting materials, most of which can also be used for similar devices such as perovskite based thin film solar cells. Limitations and advantages of relevant methods for studying the energy levels and time scales involved in charge transfer processes as well as charge transport related characteristic lengths are discussed. A summary of recent developments in DSCs and the importance of measured parameters for the device optimization procedure are mentioned at the end.
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| 24. |
- Pazoki, Meysam, et al.
(författare)
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CVD-grown TiO2 particles as light scattering structures in dye-sensitized solar cells
- 2012
-
Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 2:32, s. 12278-12285
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Tidskriftsartikel (refereegranskat)abstract
- Chemical vapour deposition (CVD) at atmospheric pressure, using TiCl 4 as a precursor, was used to grow nanostructured TiO2 films on glass substrates. At relatively low temperatures (∼245 °C) and using relatively high reactant concentrations, different nano-morphologies of TiO2 were formed simultaneously, such as spheres, nanowires and mesoporous structures. The TiO2 spheres were successfully applied as light-scattering particles in dye-sensitized solar cells, either by direct deposition onto electrodes in the reactor, or by preparation of a printing paste from the deposited particles. For dye-sensitized solar cells using the organic dye D35 as sensitizer and a cobalt-complex based redox electrolyte, a solar cell efficiency of 4.4% was obtained using a 5 μm transparent mesoporous TiO 2 layer. Addition of a 5 μm light-scattering CVD-particle film increased the efficiency by 22% to 5.4%, which was similar to the result obtained with an equally thick commercially available light-scattering film (5.3%). Longer electron lifetime was found using CVD-based films, which is attributed to the presence of more traps in the bulk of the material.
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| 25. |
- Pazoki, Meysam, et al.
(författare)
-
Density Functional Tight Binding Theory Approach for the CO2 Reduction Reaction Paths on Anatase TiO2 Surfaces
- 2020
-
Ingår i: ACS Omega. - : AMER CHEMICAL SOC. - 2470-1343. ; 5:40, s. 25819-25823
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Tidskriftsartikel (refereegranskat)abstract
- Herein, we have investigated the CO2 reduction paths on the (101) anatase TiO2 surface using an approach based on the density functional tight binding (DFTB) theory. We analyzed the reaction paths for the conversion of carbon dioxide to methane by performing a large number of calculations with intermediates placed in various orientations and locations at the surface. Our results show that the least stable intermediate is CO2 H and therefore a key bottleneck is the reduction of CO2 to formic acid. Hydrogen adsorption is also weak and would also be a limiting factor, unless very high pressures of hydrogen are used. The results from our DFTB approach are in good agreement with the hybrid functional based density functional theory calculations presented in the literature.
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| 26. |
- Pazoki, Meysam, et al.
(författare)
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Effect of metal cation replacement on the electronic structure of metalorganic halide perovskites : Replacement of lead with alkaline-earth metals
- 2016
-
Ingår i: PHYSICAL REVIEW B. - 2469-9950. ; 93:14
-
Tidskriftsartikel (refereegranskat)abstract
- Organic and inorganic lead halogen perovskites, and in particular, CH3NH3PbI3, have during the last years emerged as a class of highly efficient solar cell materials. Herein we introduce metalorganic halogen perovskite materials for energy-relevant applications based on alkaline-earth metals. Based on the classical notion of Goldschmidt's rules and quantum mechanical considerations, the three alkaline-earth metals, Ca, Sr, and Ba, are shown to be able to exchange lead in the perovskite structure. The three alkaline-earth perovskites, CH3NH3CaI3, CH3NH3SrI3, and CH3NH3BaI3, as well as the reference compound, CH3NH3PbI3, are in this paper investigated with density functional theory (DFT) calculations, which predict these compounds to exist as stable perovskite materials, and their electronic properties are explored. A detailed analysis of the projected molecular orbital density of states and electronic band structure from DFT calculations were used for interpretation of the band-gap variations in these materials and for estimation of the effective masses of the electrons and holes. Neglecting spin-orbit effects, the band gap of MACaI(3), MASrI(3), and MABaI(3) were estimated to be 2.95, 3.6, and 3.3 eV, respectively, showing the relative change expected for metal cation exchange. The shifts in the conduction band (CB) edges for the alkaline-earth perovskites were quantified using scalar relativistic DFT calculations and tight-binding analysis, and were compared to the situation in the more extensively studied lead halide perovskite, CH3NH3PbI3, where the change in the work function of the metal is the single most important factor in tuning the CB edge and band gap. The results show that alkaline-earth-based organometallic perovskites will not work as an efficient light absorber in photovoltaic applications but instead could be applicable as charge-selective contact materials. The rather high CB edge and the wide band gap together with the large difference of the electron and hole effective masses make them good candidates for n-type selective layers in hot carrier injection solar cell devices together with some light absorber candidates. The fact that they have similar lattice constants as the lead perovskite and suitable positions of the valence band edges open up the possibility to use them also as thin epitaxial p-type hole selective contacts in combination with the lead halogen perovskite materials. This can lead to both charge selectivity as well as to superior crystal growth of lead perovskite with less contact stress, which is interesting for further investigations.
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| 27. |
- Pazoki, Meysam, et al.
(författare)
-
Electronic structure of 2D hybrid perovskites : Rashba spin-orbit coupling and impact of interlayer spacing
- 2022
-
Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry. - 2050-7488 .- 2050-7496. ; 10:39, s. 20896-20904
-
Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional (2D) lead perovskite materials are of interest and under investigation in the solar cell and light-emitting device research community stemming from their high stability and intriguing anisotropic properties. Here we report electronic properties with and without spin–orbit coupling (SOC) together with the influence of van der Waals interaction. Particular attention is given to Rashba SOC, anisotropic band structure effects, and the impact of the electronic structure as a function of interlayer spacing with successively longer organic cations. The results show that larger cations, with a series from butyl-, hexyl-, octyl-, and decyl-diammonium, decrease the electrostatic interaction between the PbI4 planes in the 2D layered perovskites. SOC splitting of the conduction band states lowers the bandgap from 2.21 eV to 1.43 eV in the butyl-diammonium layered perovskite and results in a bandgap of about 1.5 eV in the analogs with longer cation chains. The k-dependent SOC effects (Rashba and Dresselhaus SOC) in the 2D and 3D structures are smaller than the k-independent SOC and are compared to Rashba SOC in III–V semiconductors, SrTiO3, and other 2D hybrid perovskites with respect to symmetry and I–Pb–I angles. The symmetry of the p-orbitals and the bandgap shifts were utilized to perform an analysis of the SOC coupling parameter in the structures in comparison with relativistic effects of isolated Pb. We also report that spacing directly affects the curvature of the bands and the charge carrier mobility perpendicular to the inorganic planes and thus affects the directional charge transport in the 2D perovskite. A distance of 6 nm is the maximum length between the 2D layers to retain a similar effective mass of holes (3m0) in-plane as out-of-plane to allow effective hole charge carrier transport perpendicular to the inorganic layer.
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| 28. |
- Pazoki, Meysam, et al.
(författare)
-
Electronic structure of organic–inorganic lanthanide iodide perovskite solar cell materials
- 2017
-
Ingår i: Journal of Materials Chemistry A. - 2050-7488. ; 5, s. 23131-23138
-
Tidskriftsartikel (refereegranskat)abstract
- The emergence of highly efficient lead halide perovskite solar cell materials makes the exploration and engineering of new lead free compounds very interesting both from a fundamental perspective as well as for potential use as new materials in solar cell devices. Herein we present the electronic structure of several lanthanide (La) based materials in the metalorganic halide perovskite family not explored before. Our estimated bandgaps for the lanthanide (Eu, Dy, Tm, Yb) perovskite compounds are in the range of 2.0–3.2 eV showing the possibility for implementation as photo-absorbers in tandem solar cell configurations or charge separating materials. We have estimated the typical effective masses of the electrons and holes for MALaI3 (La= Eu, Dy, Tm, Yb) to be in the range of 0.3–0.5 and 0.97–4.0 units of the free electron mass, respectively. We have shown that the localized f-electrons within our DFT+U approach, make the dominant electronic contribution to the states at the top of the valence band and thus have a strong impact on the photo-physical properties of the lanthanide perovskites. Therefore, the main valence to conduction band electronic transition for MAEuI3 is based on inner shell f-electron localized states within a periodic framework of perovskite crystal by which the optical absorption onset would be rather inert with respect to quantum confinement effects. The very similar crystal structure and lattice constant of the lanthanide perovskites to the widely studied CH3NH3PbI3 perovskite, are prominent advantages for implementation of these compounds in tandem or charge selective contacts in PV applications together with lead iodide perovskite devices
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| 29. |
- Pazoki, Meysam, et al.
(författare)
-
Mesoporous TiO2 Microbead Electrodes for Cobalt-Mediator-Based Dye-Sensitized Solar Cells
- 2014
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:30, s. 16472-16478
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Tidskriftsartikel (refereegranskat)abstract
- Light scattering, porosity, surface area, and morphology of TiO2 working electrode can affect the power conversion efficiency of dye -sensitized solar cells dramatically. Here mesoporous TiO2 microbeads were tested as working electrode in dye-sensitized solar cells based on cobalt tris-bipyridine electrolyte. Power conversion efficiencies up to 6.4% were obtained with D35 dye adsorbed onto the light-scattering microbeads. Electron transport, studied using small light perturbation methods, was found to be significantly faster in the microbead films than in standard mesoporous TiO2 films. This was attributed to the favorable assembly of nanocrystals in the microbeads, which can increase the electron diffusion coefficient in the conduction band. Electron lifetimes were similar in both types of film. While solar cell performance of microbead films was comparable to that of standard mesoporous films in acetonitrile-based electrolytes, a significant improvement was found when the more viscous 3-methoxypropionitrile was used as solvent for electrolyte.
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| 30. |
- Pazoki, Meysam, et al.
(författare)
-
Mesoporous TiO2 microbead electrodes for solid state dye-sensitized solar cells
- 2014
-
Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 4:91, s. 50295-50300
-
Tidskriftsartikel (refereegranskat)abstract
- Mesoporous TiO2 microbead films have been investigated as working electrodes for solid state dye sensitized solar cells and 3.5% efficiency was achieved for 4 micrometer thick films under 1 sun illumination. Compared to conventional mesoporous solar cells, microbead films have higher porosity, increased open circuit voltage, lower fill factor and current density, faster transport time and lower electron lifetime. Cross sectional scanning electron microscopy results show that the pore filling of a solid hole conductor (spiro-OMeTAD) inside the entire mesoporous bead film is very good even for 4 micrometer thick films. The high porosity of the microbead film allows good penetration of spiro in thick films, while its high surface area ensures good dye coverage. X-ray photoelectron spectroscopy data reveals a lower density of intra-bandgap trap states for microbead films compared to conventional mesoporous TiO2 films, which could be in part responsible for faster transport of electrons and higher voltage in microbead films. Optimization of microbead films for solid state dye sensitized solar cells can be an interesting possibility for highly efficient and relatively thick film solid state solar cells.
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| 31. |
- Pazoki, Meysam, et al.
(författare)
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Metal replacement in perovskite solar cell materials : chemical bonding effects and optoelectronic properties
- 2018
-
Ingår i: Sustainable Energy & Fuels. - : Royal Society of Chemistry. - 2398-4902. ; 2:7, s. 1430-1445
-
Forskningsöversikt (refereegranskat)abstract
- The composition of lead halide perovskite materials has been explored extensively over the last few years and as a consequence, different materials have been introduced into the perovskite solar cell family with diverse physical properties. Herein, we present recent progress within the framework of lead replacement that has led to new solar cell compounds by partial exchange or full substitution of lead with other metals. Lead replacement with divalent metals, tin and germanium perovskites as well as alkaline earth metals, and lanthanides are reviewed and discussed with respect to the chemical bonding effects and their relationship with the optoelectronic and charge mobility properties. The physical properties of the materials and the related device performances are also discussed with respect to the metal cation bonding within the perovskite lattice using transition metals and monovalent and trivalent metals.
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| 32. |
- Pazoki, Meysam, et al.
(författare)
-
Metal replacement in perovskite solar cell materials
- 2018
-
Konferensbidrag (refereegranskat)abstract
- The composition of lead halide perovskite materials has been explored extensively over the last few years and as a consequence, different materials have been introduced into the perovskite solar cell family with diverse physical properties. Concerns with possible environmental and legalization problems associated with the lead content and the water solubility of the resulting metal halide compound have motivated the search for alternative materials without lead. Herein, we present recent progress within lead replacement that has led to new solar cell compounds by partial exchange or full substitution of lead with other metals. Goldschmidt´s rules and quantum mechanical considerations provide a course selection of possible replacement elements where more details on the specific chemical bonding situations are needed to determine if the material is suitable as a photo-absorber or charge selective material in solar cells or other optoelectronic applications. Our recent reports on Bismuth, Lead, Lanthanide-, Alkaline earth- metal iodide perovskites and their applications in solar cell devices will be highlighted as well as the level of the implemented theory.
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| 33. |
- Pazoki, Meysam, et al.
(författare)
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Nature of the excited state in lead iodide perovskite materials : Time-dependent charge density response and the role of the monovalent cation
- 2019
-
Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 100:4
-
Tidskriftsartikel (refereegranskat)abstract
- Charge density response is responsible for the excited-state properties of lead iodide perovskites and is related to both the light absorption properties as well as subsequent electronic and lattice relaxation in the system, important for the working conditions of the material in solar cell applications. Here we investigate the nature of the excited state and its relation to pathways for electronic and lattice relaxations by performing time-dependent density-functional theory (TDDFT). Charge density response upon photoexcitation close to the band edge and deeper into the absorption spectra are investigated for three lead perovskite compounds with different A-site monovalent cations CsPbI3, CH2(NH2)(2)PbI3 (FAPbI(3)), and CH3NH3PbI3 (MAPbI(3)). The carrier cooling mechanism is analyzed and shows that the initial force acting on the nuclei follows the symmetry of the ground-state electronic structure upon photoexcitation with a force parallel to the polarization of the incoming light. This effect is investigated for the three different compounds and shows an initial force for induced ionic movement that depends on both the underlying symmetry of the inorganic lattice as well as on the type and orientation of the organic cation. The excess energy after thermalization under blue-light illumination is large enough for overcoming the activation energy for iodide migration and can thus trigger vacancy formation. Iodide vacancies are seen to be dipole-field compensated by the organic cation, with a shielding of the local field, and thus form an explanation for the defect tolerance found in these systems under photovoltaic operation. A partial charge transfer from the inorganic cage to the monovalent organic cation is predicted with TDDFT calculations for blue- and UV-light illumination with a population of antibinding orbitals in the N-H bond in both CH3NH3 (MA) and CH2(NH2)(2 )(FA), where the implication for this is discussed in terms of the intrinsic photo stability of organic cation containing lead perovskites. The results show the importance of a fundamental understanding of the excited-state properties of perovskite material to reveal the underlying mechanism for the defect tolerance and thus high photovoltaic performance when using organic dipolar cations as well as a rationale for using mixed halide perovskites to decrease the halide migration, effect of vacancy formation, and stability issues under blueand UV-light illumination.
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| 34. |
- Pazoki, Meysam, et al.
(författare)
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Photoinduced Stark Effects and Mechanism of Ion Displacement in Perovskite Solar Cell Materials
- 2017
-
Ingår i: ACS Nano. - : AMER CHEMICAL SOC. - 1936-0851 .- 1936-086X. ; 11:3, s. 2823-2834
-
Tidskriftsartikel (refereegranskat)abstract
- Organometallic halide perovskites (OMHPs) have recently emerged as a promising class of materials in photovoltaic technology. Here, we present an in-depth investigation of the physics in these systems by measuring the photoinduced absorption (PIA) in OMHPs as a function of materials composition, excitation wavelength, and modulation frequency. We report a photoinduced Stark effect that depends on the excitation wavelength and on the dipole strength of the monovalent cations in the A position of the ABX(3) perovskite. The results presented are corroborated by density functional theory calculations and provide fundamental information about the photoinduced local electric field change under blue and red excitation as well as insights into the mechanism of light induced ion displacement in OMHPs. For optimized perovskite solar cell devices beyond 19% efficiency, we show that excess thermalization energy of blue photons plays a role in overcoming the activation energy for ion diffusion.
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| 35. |
- Pazoki, Meysam, et al.
(författare)
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Photon Energy-Dependent Hysteresis Effects in Lead Halide Perovskite Materials
- 2017
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:47, s. 26180-26187
-
Tidskriftsartikel (refereegranskat)abstract
- Lead halide perovskites have a range of spectacular properties and interesting phenomena and are a serious candidate for the next generation of photovoltaics with high efficiencies and low fabrication costs. An interesting phenomenon is the anomalous hysteresis often seen in current-voltage scans, which complicates accurate performance measurements but has also been explored to obtain a more comprehensive understanding of the device physics. Herein, we demonstrate a wavelength and illumination intensity dependency of the hysteresis in state-of-the-art perovskite solar cells with 18% power conversion efficiency (PCE), which gives new insights into ion migration. The perovskite devices show lower hysteresis under illumination with near band edge (red) wavelengths compared to more energetic (blue) excitation. This can be rationalized with thermalization-assisted ion movement or thermalization-assisted vacancy generation. These explanations are supported by the dependency of the photovoltage decay with illumination time and excitation wavelength, as well as by impedance spectroscopy. The suggested mechanism is that high-energy photons create hot charge carriers that either through thermalization can create additional vacancies or by release of more energetic phonons play a role in overcoming the activation energy for ion movement. The excitation wavelength dependency of the hysteresis presented here gives valuable insights into the photophysics of the lead halide perovskite solar cells.
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| 36. |
- Pazoki, Meysam, et al.
(författare)
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Stark effects in D35-sensitized mesoporous TiO2 : influence of dye coverage and electrolyte composition
- 2015
-
Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 179, s. 174-178
-
Tidskriftsartikel (refereegranskat)abstract
- Strong Stark effects are visible in the spectra of the organic donor-pi-acceptor dye D35 when it is adsorbed onto mesoporous TiO2, both under steady state conditions and under modulated light conditions. The addition of lithium cations to the electrolyte results in a significant red shift of the D35 absorption spectrum, which is attributed to adsorption of Li+ at the TiO2 surface, resulting in a change of the electric field across the adsorbed dye molecules. The dye molecules must therefore be located inside the Helmholtz double layer at the TiO2/electrolyte interface. In photoinduced absorption (PIA) spectroscopy, modulated light is used to excite dye molecules. A significant Stark bleach is found in PIA spectra, which corresponds to a blue shift of the dye absorption spectrum upon addition of electrons to TiO2, due to an electric field across the dye monolayer. The observed bleach is reduced when the concentration of supporting electrolyte is increased, indicating local charge compensation of the electrons in TiO2 by adsorbed cations. Transient absorption studies reveal that screening of the interfacial electric field is faster at lower dye coverage compared to full monolayer coverage. Analysis of the Stark effect in dye-sensitized solar cell gives valuable information on the mechanism of charge compensation of electrons in mesoporous electrodes.
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| 37. |
- Pazoki, Meysam, et al.
(författare)
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The effect of dye coverage on the performance of dye-sensitized solar cells with a cobalt-based electrolyte
- 2014
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 16:18, s. 8503-8508
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of dye coverage of the mesoporous TiO2 electrode on the performance of dye-sensitized solar cells based on the cobalt tris(bipyridine) electrolyte and the D35 dye was studied in detail. The dye coverage was controlled by using a dye bath with different dye concentrations and containing an inert salt, LiClO4, which was found to promote equilibrium conditions in the dye adsorption process. The amount of adsorbed D35 dye on mesoporous TiO2 was reasonably fit using the Langmuir adsorption isotherm, with a binding constant of 55 000 M-1. Upon increasing the dye coverage on the TiO2 electrode, the electron lifetime in the dye-sensitized solar cell increased remarkably, demonstrating the blocking behavior of the D35 dye at the TiO2-electrolyte interface. Consequently, the solar cell efficiency increased dramatically with the D35 dye coverage.
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| 38. |
- Pazoki, Meysam, et al.
(författare)
-
Time resolved photo-induced optical spectroscopy
- 2019. - 1
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Ingår i: Characterization Techniques for Perovskite Solar Cell Materials. - Amsterdam : Elsevier. - 012814727X - 9780128147276 - 9780128147283 ; , s. 139-160
-
Bokkapitel (refereegranskat)abstract
- The perovskite layer is the photoactive material within the hybrid perovskite solar cell (HPSC) device by which the incoming photon energies are absorbed and transformed into charge carriers, and after the charge separation, the corresponding charge carriers are transported via the selective contacts for the photovoltaic operation. Optical fingerprints of these physical processes i.e. the spectral response during light absorption, charge separation, transport, recombination as well as other important phenomena such as Stark effects, electron-phonon interactions, ionic movement and Frenkel defect annihilation can be studied within the scope of time resolved photo-induced optical spectroscopy. The time scales of the main fundamental processes within perovskite solar cells directly affect the device performance, and can differ significantly from conventional solar cell technology devices. The processes varies from femtosecond to several seconds and many are strongly dependent on the chemical composition and crystal quality of the material. Full characterization of the physical properties of the device, implies a careful attention to the lifetime and amplitudes of the processes, as well as experimental design for distinguishing the processes by considering the appropriate method and/or variation in the chemical composition of the materials. Here we briefly review the relevant underlying physical processes occurring in the system, their fingerprints and how they can be detected by different spectroscopic tools, together with the methodological scopes and limitations.
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| 39. |
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| 40. |
- Pazoki, Meysam, et al.
(författare)
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Vacancy dipole interactions and the correlation with monovalent cation dependent ion movement in lead halide perovskite solar cell materials
- 2017
-
Ingår i: Nano Energy. - : Elsevier BV. - 2211-2855 .- 2211-3282. ; 38, s. 537-543
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Tidskriftsartikel (refereegranskat)abstract
- Ion migration has recently been suggested to play critical roles in the operation of lead halide perovskite solar cells. However, so far there has been no systematic investigation of how the monovalent cation affects the vacancy formation, ion migration and the associated hysteresis effect. Here, we present density functional theory calculations on all possible ion migration barriers in the perovskite materials with different cations i.e. CH3NH3PbI3, CH(NH2)(2)PbI3 and CsPbI3 in the tetragonal phase and investigate vacancy monovalent-cation interactions within the framework of the possible ion migrations. The most relevant ion movement (iodide) is investigated in greater detail and corresponding local structural changes, the relationships with the local ionic dielectric response, Stark effect and current-voltage hysteresis are discussed. We observe a correlation between the energy barrier for iodine migration and the magnitude of the dipole of the monovalent cation. From the data, we suggest a vacancy-dipole interaction mechanism by which the larger dipole of the monovalent cation can respond to and screen the local electric fields more effectively. The stronger response of the high dipolar monovalent cation to the vacancy electrostatic potential in turn leads to a lower local structural changes within the neighbouring octahedra. The presented data reveal a detailed picture of the ion movement, vacancy dipole interactions and the consequent local structural changes, which contain fundamental information about the photo-physics, and dielectric response of the material.
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| 41. |
- Phuyal, Dibya, 1985-, et al.
(författare)
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Electronic Structure of Two-Dimensional Lead(II) Iodide Perovskites : An Experimental and Theoretical Study
- 2018
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 30:15, s. 4959-4967
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Tidskriftsartikel (refereegranskat)abstract
- Layered two-dimensional (2D) hybrid organic-inorganic perovskites (HOP) are promising materials for light-harvesting applications because of their chemical stability, wide flexibility in composition and dimensionality, and increases in photovoltaic power conversion efficiencies. Three 2D lead iodide perovskites were studied through various X-ray spectroscopic techniques to derive detailed electronic structures and band energetics profiles at a titania interface. Core-level and valence band photoelectron spectra of HOP were analyzed to resolve the electronic structure changes due to the reduced dimensionality of inorganic layers. The results show orbital narrowing when comparing the HOP, the layered precursor PbI2, and the conventional 3D (CH3NH3)PbI3 such that different localizations of band edge states and narrow band states are unambiguously due to the decrease in dimensionality of the layered HOPs. Support from density functional theory calculations provide further details on the interaction and band gap variations of the electronic structure. We observed an interlayer distance dependent dispersion in the near band edge electronic states. The results show how tuning the interlayer distance between the inorganic layers affects the electronic properties and provides important design principles for control of the interlayer charge transport properties, such as the change in effective charge masses as a function of the organic cation length. The results of these findings can be used to tune layered materials for optimal functionality and new applications.
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| 42. |
- Phuyal, Dibya, et al.
(författare)
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The electronic structure and band interface of cesium bismuth iodide on a titania heterostructure using hard X-ray spectroscopy
- 2018
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 6:20, s. 9498-9505
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Tidskriftsartikel (refereegranskat)abstract
- Bismuth halide compounds as a non-toxic alternative are increasingly investigated because of their potential in optoelectronic devices and their rich structural chemistry. Hard X-ray spectroscopy was applied to the ternary bismuth halide Cs3Bi2I9 and its related precursors BiI3 and CsI to understand its electronic structure at an atomic level. We specifically investigated the core levels and valence band using X-ray photoemission spectroscopy (PES), high-resolution X-ray absorption (HERFD-XAS), and resonant inelastic X-ray scattering (RIXS) to get insight into the chemistry and the band edge properties of the two bismuth compounds. Using these element specific X-ray techniques, our experimental electronic structures show that the primary differences between the two bismuth samples are the position of the iodine states in the valence and conduction bands and the degree of hybridization with bismuth lone pair (6s(2)) states. The crystal structure of the two layered quasi-perovskite compounds plays a minor role in modifying the overall electronic structure, with variations in bismuth lone pair states and iodine band edge states. Density Functional Theory (DFT) calculations are used to compare with experimental data. The results demonstrate the effectiveness of hard X-ray spectroscopies to identify element specific bulk electronic structures and their use in optoelectronic devices.
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| 43. |
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| 44. |
- Sveinbjörnsson, Kári, et al.
(författare)
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Probing Photocurrent Generation, Charge Transport, and Recombination Mechanisms in Mesostructured Hybrid Perovskite through Photoconductivity Measurements
- 2015
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 6:21, s. 4259-4264
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Tidskriftsartikel (refereegranskat)abstract
- Conductivity of methylammonium lead triiodide (MAPbI(3)) perovskite was measured on different mesoporous metal oxide scaffolds: TiO2, Al2O3, and ZrO2, as a function of incident light irradiation and temperature. It was found that MAPbI(3) exhibits intrinsic charge separation, and its conductivity stems from a majority of free charge carriers. The crystal morphology of the MAPbI(3) was found to significantly affect the photoconductivity, whereas in the dark the conductivity is governed by the perovskite in the pores of the mesoporous scaffold. The temperature-dependent conductivity measurements also indicate the presence of states within the band gap of the perovskite. Despite a relatively large amount of crystal defects in the measured material, the main recombination mechanism of the photogenerated charges is bimolecular (band-to-band), which suggests that the defect states are rather inactive in the recombination. This may explain the remarkable efficiencies obtained for perovskite solar cells prepared with wetchemical methods.
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| 45. |
- Yang, Wenxing, et al.
(författare)
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A key discovery at the TiO2/dye/electrolyte interface : slow local charge compensation and a reversible electric field
- 2015
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 17:26, s. 16744-16751
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Tidskriftsartikel (refereegranskat)abstract
- Dye-sensitized mesoporous TiO2 films have been widely applied in energy and environmental science related research fields. The interaction between accumulated electrons inside TiO2 and cations in the surrounding electrolyte at the TiO2/dye/electrolyte interface is, however, still poorly understood. This interaction is undoubtedly important for both device performance and fundamental understanding. In the present study, Stark effects of an organic dye, LEG4, adsorbed on TiO2 were well characterized and used as a probe to monitor the local electric field at the TiO2/dye/electrolyte interface. By using time-resolved photo- and potential-induced absorption techniques, we found evidence for a slow (t > 0.1 s) local charge compensation mechanism, which follows electron accumulation inside the mesoporous TiO2. This slow local compensation was attributed to the penetration of cations from the electrolyte into the adsorbed dye layer, leading to a more localized charge compensation of the electrons inside TiO2. Importantly, when the electrons inside TiO2 were extracted, a remarkable reversal of the surface electric field was observed for the first time, which is attributed to the penetrated and/or adsorbed cations now being charge compensated by anions in the bulk electrolyte. A cation electrosorption model is developed to account for the overall process. These findings give new insights into the mesoporous TiO2/dye/electrolyte interface and the electron-cation interaction mechanism. Electrosorbed cations are proposed to act as electrostatic trap states for electrons in the mesoporous TiO2 electrode.
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| 46. |
- Zhang, Jinbao, et al.
(författare)
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The effect of mesoporous TiO2 pore size on the performance of solid-state dye sensitized solar cells based on photoelectrochemically polymerized Poly(3,4-ethylenedioxythiophene) hole conductor
- 2016
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Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 210, s. 23-31
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Tidskriftsartikel (refereegranskat)abstract
- Photoelectrochemical polymerization of poly(3,4-ethylenedioxythiphene) (PEDOT) has recently been introduced and widely investigated for fabrication of the hole transporting material (HTM) in highly efficient solid state dye sensitized solar cells (sDSCs). In this work, the effects of the surface area and pore size of TiO2 film were for the first time investigated in the sDSCs employing the in-situ polymerizated PEDOT HTM. Three different varieties of mesoporous TiO2 particles with controllable surface area and pore size were synthesized through the basic route in order to study the corresponding sDSC photovoltaic performances. It was found that the pore size plays an important role in the kinetics of the photoelectrochemical polymerization (PEP) process and the formation of the PEDOT capping layer. Larger pore sizes provided a more favourable pathway for the precursor diffusion through the mesoporous pores during the PEP process, which contributed towards a more efficient PEP. However, the interfacial contact area between the formed polymer and the dyes on the surface of TiO2 particle would be lower in the case of larger pore sizes, which consequently caused a less efficient dye regeneration process. Electronic diffusion on the other hand was improved for larger particle sizes. Employing an organic dye LEG4 and the self-made TiO2 with an optimal pore size of 25 nm and particle size of 24 nm, the sDSCs showed a promising power conversion efficiency (PCE) of 5.2%, higher than 4.5% for the commercial TiO2 Dyesol DSL-30. By measuring the dye regeneration yield and the kinetics through photoinduced absorption, it was observed that the homemade TiO2 based device had more efficient dye regeneration compared to the Dyesol based device, which could result from the better interfacial contact between the PEDOT and the dye. This work provides important information on the effect of meso-pore size on sDSCs and points to the necessity of further photoanode optimization toward the enhancement of the PCE of polymeric hole conductor-based DSCs.
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