| 1. |
- Fondell, Mattis, et al.
(författare)
-
Optical quantum confinement in low dimensional hematite
- 2014
-
Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488. ; 2:10, s. 3352-3363
-
Tidskriftsartikel (refereegranskat)abstract
- Hematite is considered to be a promising material for various applications, including for example photoelectrochemical cells for solar hydrogen production. Due to limitations in the charge transport properties hematite needs to be in the form of low-dimensional particles or thin films in several of these applications. This may however affect the optical properties, introducing additional complications for efficient design of photo-active devices. In this paper the optical absorption is analyzed in detail as a function of film thickness for 35 thin films of hematite ranging between 2 and 70 nm. Hematite was deposited by atomic layer deposition on FTO-substrates using Fe(CO)(5) and O-2 as precursors. It was found that for film thicknesses below 20 nm the optical properties are severely affected as a consequence of quantum confinement. One of the more marked effects is a blue shift of up to 0.3 eV for thinner films of both the indirect and direct transitions, as well as a 0.2 eV shift of the absorption maximum. The data show a difference in quantum confinement for the indirect and the direct transitions, where the probability for the indirect transition decreases markedly and essentially disappears for the thinnest films. Raman measurements showed no peak shift or change in relative intensity for vibrations for the thinnest films indicating that the decrease in indirect transition probability could not be assigned to depression of any specific phonon but instead seems to be a consequence of isotropic phonon confinement. The onset of the indirect transition is found at 1.75 eV for the thickest films and shifted to 2.0 eV for the thinner films. Two direct transitions are found at 2.15 eV and 2.45 eV, which are blue shifted 0.3 and 0.45 eV respectively, when decreasing the film thickness from 20 to 4 nm. Low dimensional hematite, with dimensions small enough for efficient charge transport, thus has a substantially lower absorption in the visible region than expected from bulk values. This knowledge of the intrinsic optical behavior of low dimensional hematite will be of importance in the design of efficient photo-active devices.
|
|
| 2. |
- Jacobsson, Daniel, et al.
(författare)
-
Particle-assisted GaxIn1-xP nanowire growth for designed bandgap structures
- 2012
-
Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 23:24
-
Tidskriftsartikel (refereegranskat)abstract
- Non-tapered vertically straight GaxIn1-xP nanowires were grown in a compositional range from Ga0.2In0.8P to pure GaP in particle-assisted mode by controlling the trimethylindium, trimethylgallium and hydrogen chloride flows in metal-organic vapor phase epitaxy. X-ray energy dispersive spectroscopy in transmission electron microscopy revealed homogeneous radial material composition in single nanowires, whereas variations in the material composition were found along the nanowires. High-resolution x-ray diffraction indicates a variation of the material composition on the order of about 19% measuring an entire sample area, i.e., including edge effects during growth. The non-capped nanowires emit room temperature photoluminescence strongly in the energy range of 1.43-2.16 eV, correlated with the bandgap expected from the material composition.
|
|
| 3. |
- Jacobsson, Jesper T., et al.
(författare)
-
A monolithic device for solar water splitting based on series interconnected thin film absorbers reaching over 10% solar-to-hydrogen efficiency
- 2013
-
Ingår i: Energy & Environmental Science. - : Royal Society of Chemistry (RSC). - 1754-5692 .- 1754-5706. ; 6:12, s. 3676-3683
-
Tidskriftsartikel (refereegranskat)abstract
- Efficient production of hydrogen from solar energy is anticipated to be an important component in a future sustainable post-carbon energy system. Here we demonstrate that series interconnected absorbers in a PV-electrolysis configuration based on the compound semiconductor CIGS, CuInxGa1-xSe2, are a highly interesting concept for solar water splitting applications. The band gap energy of CIGS can be adjusted to a value close to optimum for efficient absorption of the solar spectrum, but is too low to drive overall water splitting. Therefore we connect three cells in series, into a monolithic device, which provides sufficient driving force for the full reaction. Integrated with a catalyst this forms a stable PV/photo-electrochemical device, which when immersed in water reaches over 10% solar-to-hydrogen efficiency for unassisted water splitting. The results show that series interconnected device concepts, which enable use of a substantial part of the solar spectrum, provide a simple route towards highly efficient water splitting and could be used also for other solar absorbers with similar electro-optical properties. We discuss how the efficiency could be increased for this particular device, as well as the general applicability of the concepts used in this work. We also briefly discuss advantages and disadvantages of photo-electrochemical cells in relation to PV-electrolysis with respect to our results.
|
|
| 4. |
- Jacobsson, Jesper T, et al.
(författare)
-
A size dependent discontinuous decay rate for the exciton emission in ZnO quantum dots
- 2014
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - 1463-9076 .- 1463-9084. ; 16:27, s. 13849-13857
-
Tidskriftsartikel (refereegranskat)abstract
- The time resolved UV-fluorescence in ZnO quantum dots has been investigated using femtosecond laser spectroscopy. The measurements were performed as a function of particle size for particles between 3 and 7 nm in diameter, which are in the quantum confined regime. A red shift in the fluorescence maximum is seen while increasing the particle size, which correlates with the shift in band gap due to quantum confinement. The energy difference between the UV-fluorescence and the band gap does, however, increase for the smaller particles. For 3.7 nm particles the fluorescence energy is 100 meV smaller than the band gap energy, whereas it is only 20 meV smaller for the largest particles. This indicates a stabilization of the excitons in the smallest particles. The lifetime of the UV fluorescence is in the picosecond time scale and interestingly, it is discontinuous with respect to particle size. For the smallest particles, the exciton emission life time reaches 30 ps, which is three times longer than that for the largest particles. This demonstrates a transition between two different mechanisms for the UV-fluorescence. We suggest that this is an effect of surface trapping and stabilization of the excitons occurring in the smallest particles but not in the larger ones. We also discuss the time scale limit for slowed hot carrier dynamics in ensembles of quantum confined ZnO particles.
|
|
| 5. |
- Jacobsson, Jesper T., et al.
(författare)
-
CuInxGa1-xSe2 as an efficient photocathode for solar hydrogen generation
- 2013
-
Ingår i: International journal of hydrogen energy. - : Elsevier BV. - 0360-3199 .- 1879-3487. ; 38:35, s. 15027-15035
-
Tidskriftsartikel (refereegranskat)abstract
- Utilizing the energy in the sun to efficiently split water into hydrogen and oxygen can have a huge beneficial impact on a future post-carbon energy system. There is still, however, some way to go before this concept will be fully competitive. At the heart of the problem is finding and designing materials that can drive the photoreaction in an efficient and stable way. In this work we demonstrate how CIGS (CuInxGa1-xSe2), can be used for photo reduction of water into hydrogen. CIGS, which is a proven good solar cell material, does not in itself have the appropriate energetics to drive the reaction to any larger extent. Here we show that by utilizing a solid state pn-junction for charge separation and a catalyst deposited on the surface, the efficiency is significantly improved and photocurrents of 6 mA/cm(2) are demonstrated for the reduction reaction in the configuration of a photo-electrochemical cell. The stability of CIGS in water under illumination turns out to be a problem. In our present set-up, we demonstrate that separation between the charge carrier generation, which takes place in the solar cell, from the catalysis, which takes place in the electrolyte leads to improved stability, while keeping the essential functions of the processes. By incorporating appropriate charge separation layers and optimizing the catalytic conditions at the surface of the electrodes, photocurrents in excess of 20 mA/cm2 are reached for the reduction half reaction, demonstrating how essentially the full potential of GIGS as an efficient absorber material can be utilized in photocatalytic reduction of water into hydrogen.
|
|
| 6. |
- Jacobsson, T. Jesper, et al.
(författare)
-
A Spectroelectrochemical Method for Locating Fluorescence Trap States in Nanoparticles and Quantum Dots
- 2013
-
Ingår i: The Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 117:10, s. 5497-5504
-
Tidskriftsartikel (refereegranskat)abstract
- We here devise an electrochemical method for determining the absolute energetic position of trap levels involved in fluorescence. The method utilizes potentiostatic control of the Fermi level in the material, and thereby also the electronic population of the energy states involved in the fluorescence. The method is especially useful for nanoparticle semiconductor electrodes. Here we exemplify the method by determining the position of the trap levels involved in the green fluorescence in thin films of ZnO quantum dots. The exact mechanism and the absolute positions of these states have been debated in the literature. Here we show that the visible fluorescence is caused by a transition from energy levels slightly below the conduction band edge to a deep trap within the band gap. We further pinpoint the location of the upper trap level to be at 0.35 +/- 0.03 eV below the conduction band edge. Particles between 5 and 8 nm in diameter have been analyzed, which is in the quantum confined region of ZnO. We also show that the position of the upper trap level shifts with the size of the quantum dots in the same way as the conduction band.
|
|
| 7. |
- Jacobsson, T. Jesper, et al.
(författare)
-
Absorption and Fluorescence Spectroscopy of Growing ZnO Quantum Dots : Size and Band Gap Correlation and Evidence of Mobile Trap States
- 2011
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 50:19, s. 9578-9586
-
Tidskriftsartikel (refereegranskat)abstract
- ZnO nanoparticles constitute a convenient model system for fundamental studies with many possible technical applications in, for example, sensors and the field of catalysis and optoelectronics. A large set of ZnO quantum dots in the size range 2.5-7 nm have been synthesized and analyzed in detail. Time resolved in situ UV-vis absorption measurements were used to monitor the growth of these particles in solution by correlating the optical band gap to particle size given from X-ray diffraction (XRD) measurements. The particles formed were isotropic in shape, but small initial deviations gave indications of a transition from thermodynamic to kinetically controlled growth for particles around 4 nm in diameter. On the basis of this, the behavior and mechanisms for the particle growth are discussed. The fluorescence dependence on particle size was investigated by combining fluorescence and UV-vis measurements on growing particles. This revealed that the positions of the fluorescence trap states are mobile toward the conduction- and valence band. A broadening of the trap states was also found, and a surface dependent mechanism of the trap state shift and broadening is proposed.
|
|
| 8. |
- Jacobsson, T. Jesper, 1984-
(författare)
-
Highly Efficient CIGS Based Devices for Solar Hydrogen Production and Size Dependent Properties of ZnO Quantum Dots
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Materials and device concepts for renewable solar hydrogen production, and size dependent properties of ZnO quantum dots are the two main themes of this thesis.ZnO particles with diameters less than 10 nm, which are small enough for electronic quantum confinement, were synthesized by hydrolysis in alkaline zinc acetate solutions. Properties investigated include: the band gap - particle size relation, phonon quantum confinement, visible and UV-fluorescence as well as photocatalytic performance. In order to determine the absolute energetic position of the band edges and the position of trap levels involved in the visible fluorescence, methods based on combining linear sweep voltammetry and optical measurements were developed.The large band gap of ZnO prevents absorption of visible light, and in order to construct devices capable of utilizing a larger part of the solar spectrum, other materials were also investigated, like hematite , Fe2O3, and CIGS, CuIn1-xGaxSe2.The optical properties of hematite were investigated as a function of film thickness on films deposited by ALD. For films thinner than 20 nm, a blue shift was observed for both the absorption maximum, the indirect band gap as well as for the direct transitions. The probability for the indirect transition decreased substantially for thinner films due to a suppressed photon/phonon coupling. These effects decrease the visible absorption for films thin enough for effective charge transport in photocatalytic applications.CIGS was demonstrated to be a highly interesting material for solar hydrogen production. CIGS based photocathodes demonstrated high photocurrents for the hydrogen evolution half reaction. The electrode stability was problematic, but was solved by introducing a modular approach based on spatial separation of the basic functionalities in the device. To construct devices capable of driving the full reaction, the possibility to use cells interconnected in series as an alternative to tandem devices were investigated. A stable, monolithic device based on three CIGS cells interconnected in series, reaching beyond 10 % STH-efficiency, was finally demonstrated. With experimental support from the CIGS-devices, the entire process of solar hydrogen production was reviewed with respect to the underlying physical processes, with special focus on the similarities and differences between various device concepts.
|
|
| 9. |
- Jacobsson, T. Jesper, et al.
(författare)
-
Photoelectrochemical Determination of the Absolute Band Edge Positions as a Function of Particle Size for ZnO Quantum Dots
- 2012
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:29, s. 15692-15701
-
Tidskriftsartikel (refereegranskat)abstract
- The absolute position of the conduction and the valence band edges of ZnO quantum dots (Qdots) has been determined as a function of particle size with potential dependent absorption spectroscopy. The absolute position of the band edges are vital for which catalytic reactions that can occur at the surface. They are also crucial parameters for charge injection and extraction in nanoparticular solar cells and other optoelectronic devices based on nanoparticles. The position of the conduction band edge was determined by potentiostatic population of the conduction band states and monitoring the resulting increase in the optical band gap. This was performed for ZnO particles in the quantum confined region with diameters ranging between 4 and 9 nm. The particles were deposited into thin films giving an ensemble of particles for which the analysis could be performed. The relevant equations were derived and their validity in terms of applied potential and kinetic considerations was quantified. We find that essentially all of the quantum size effect of increased band gap is occurring by a shift of the conduction band edge. The extent of the validity of the parabolic approximation, which is one of the assumptions in the analysis, is investigated, both experimentally and with density functional theory calculations of bulk ZnO Here, we find that the parabolic approximation only is valid in an energy range of slightly less than 0.1 eV from the conduction band edge but in that regime constitutes an excellent approximation. We also demonstrate that the validity of the parabolic approximation follows a rising Fermi level into the conduction band energy levels.
|
|
| 10. |
- Kriegner, D., et al.
(författare)
-
Structural investigation of GaInP nanowires using X-ray diffraction
- 2013
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090. ; 543, s. 100-105
-
Tidskriftsartikel (refereegranskat)abstract
- In this work the structure of ternary GaxIn1-xP nanowires is investigated with respect to the chemical composition and homogeneity. The nanowires were grown by metal-organic vapor-phase epitaxy. For the investigation of ensemble fluctuations on several lateral length scales, X-ray diffraction reciprocal space maps have been analyzed. The data reveal a complicated varying materials composition across the sample and in the nanowires on the order of 20%. The use of modern synchrotron sources, where beam-sizes in the order of several 10 mu m are available, enables us to investigate compositional gradients along the sample by recording diffraction patterns at different positions. In addition, compositional variations were found also within single nanowires in X-ray energy dispersive spectroscopy measurements. (C) 2013 Elsevier B.V. All rights reserved.
|
|
