| 1. |
- Aghaeipour, Mahtab, et al.
(författare)
-
Optical response of wurtzite and zinc blende GaP nanowire arrays
- 2015
-
Ingår i: Optics Express. - 1094-4087. ; 23:23, s. 30177-30187
-
Tidskriftsartikel (refereegranskat)abstract
- We compare the optical response of wurtzite and zinc blende GaP nanowire arrays for varying geometry of the nanowires. We measure reflectance spectra of the arrays and extract from these measurements the absorption in the nanowires. To support our experimental findings and to allow for more detailed investigations of the optical response of the nanowire arrays than possible in experiments, we perform electromagnetic modeling. This modeling highlights the validity of the extraction of the absorptance from reflectance spectra, as well as limitations of the extraction due to anti-reflection properties of the nanowires. In our combined experimental and theoretical study, we find for both zinc blende and wurtzite nanowires an absorption resonance that can be tuned into the ultraviolet by decreasing the diameter of the nanowires. This peak stops blue-shifting with decreasing nanowire diameter at a wavelength of approximately 330 nm for zinc blende GaP. In contrast, for the wurtzite GaP nanowires, the resonance continues blue-shifting at 310 nm for the smallest diameters we succeeded in fabricating. We interpret this as a difference in refractive index between wurtzite and zinc blende GaP in this wavelength region. These results open up for optical applications through resonant absorption in the visible and ultraviolet wavelength regions with both zinc blende and wurtzite GaP nanowire arrays. Notably, zinc blende and wurtzite GaP support resonant absorption deeper into the ultraviolet region than previously found for zinc blende and wurtzite InP and InAs. (C) 2015 Optical Society of America
|
|
| 2. |
- Aghaeipour, Mahtab
(författare)
-
Tailoring the Optical Response of III-V Nanowire Arrays
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semiconductor nanowires show a great deal of promise for applications in a wide range of important fields, including photovoltaics, biomedicine, and information technology. Developing these exciting applications is strongly dependent on understanding the fundamental properties of nanowires, such as their optical resonances and absorption spectra. In this thesis we explore optical absorption spectra of arrays of vertical III-V nanowires with a special emphasis on structures optimized to enhance absorption in the solar spectrum. First, we analyze experimentally determined absorption spectra of both indium phosphide (InP) and gallium phosphide (GaP) nanowire arrays. The study provides an intuitive understanding of how the observed absorption resonances in the nanowires may be tuned as a function of their geometrical parameters and crystal structure. As a consequence, the spectral position of absorption resonances can be precisely controlled through the nanowire diameter. However, the results highlight how the blue-shift in the optical absorption resonances as the diameter of the nanowires decreases comes to a halt at low diameters. The stop point is related to the behavior of the refractive indices of the nanowires. The wavelength of the stop is different for nanowire polytypes of similar dimensions due to differences in their refractive indices. We then present a theoretical argument that it is important to consider symmetry properties when tailoring the optical modes excited in the nanowires for enhanced absorption. We show that absorption spectra may be enhanced compared to vertical nanowires at normal incidence by tilting the nanowires with normal incidence light, or by using off-normal incidence with vertical nanowires. This is because additional optical modes inside the nanowires are excited when the symmetry is broken. Looking forward to omnidirectional applications, we consider branched nanowires as a way to enhance the absorption spectra at normal incidence by taking advantage of simultaneous excitation of the spectrally different optical modes in the branches and the stems. Third, we describe in theoretical terms how integrating distributed Bragg reflectors (DBRs) with the nanowires can improve absorption spectra compared to conventional nanowires. DBRs provide a way to employ light trapping mechanisms which increases the optical path length of the excited modes and thereby improves the absorption of the excited modes. At normal incidence, DBR-nanowires improve the absorption efficiency to 78%, compared to 72% for conventional nanowires. We show that the efficiency is increased to 85% for an off-normal incident angle of 50˚. Overall, our results show that studies of optical resonances in nanowires that take the light-matter interaction into account provide opportunities to develop novel optical and optoelectronic functionalities in nanoscience and nanotechnology.
|
|
| 3. |
- Aghaeipour, Mahtab, et al.
(författare)
-
Tunable absorption resonances in the ultraviolet for InP nanowire arrays
- 2014
-
Ingår i: Optics Express. - 1094-4087. ; 22:23, s. 29204-29212
-
Tidskriftsartikel (refereegranskat)abstract
- The ability to tune the photon absorptance spectrum is an attracting way of tailoring the response of devices like photodetectors and solar cells. Here, we measure the reflectance spectra of InP substrates patterned with arrays of vertically standing InP nanowires. Using the reflectance spectra, we calculate and analyze the corresponding absorptance spectra of the nanowires. We show that we can tune absorption resonances for the nanowire arrays into the ultraviolet by decreasing the diameter of the nanowires. When we compare our measurements with electromagnetic modeling, we generally find good agreement. Interestingly, the remaining differences between modeled and measured spectra are attributed to a crystal-phase dependence in the refractive index of InP. Specifically, we find indication of significant differences in the refractive index between the modeled zinc-blende InP nanowires and the measured wurtzite InP nanowires in the ultraviolet. We believe that such crystal-phase dependent differences in the refractive index affect the possibility to excite optical resonances in the large wavelength range of 345 < lambda < 390 nm. To support this claim, we investigated how resonances in nanostructures can be shifted in wavelength by geometrical tuning. We find that dispersion in the refractive index can dominate over geometrical tuning and stop the possibility for such shifting. Our results open the door for using crystal-phase engineering to optimize the absorption in InP nanowire-based solar cells and photodetectors. (C) 2014 Optical Society of America
|
|
| 4. |
- Anttu, Nicklas, et al.
(författare)
-
Absorption and transmission of light in III-V nanowire arrays for tandem solar cell applications
- 2017
-
Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 28:20
-
Tidskriftsartikel (refereegranskat)abstract
- III-V semiconductor nanowires are a platform for next-generation photovoltaics. An interesting research direction is to embed a nanowire array in a transparent polymer, either to act as a stand-alone flexible solar cell, or to be stacked on top of a conventional Si bottom cell to create a tandem structure. To optimize the tandem cell performance, high energy photons should be absorbed in the nanowires whereas low energy photons should be transmitted to and absorbed in the Si cell. Here, through optical measurements on 1.95 eV bandgap GaInP nanowire arrays embedded in a polymer membrane, we identify two mechanisms that could be detrimental for the performance of the tandem cell. First, the Au particles used in the nanowire synthesis can absorb >50% of the low-energy photons, leading to a <40% transmittance, even though the Au particles cover <15% of the surface area. The removal of the Au particles can recover the transmission of low energy photons to >80%. Second, after the removal of the Au particles, a 40% reflectance peak shows up due to resonant back-scattering of light from in-plane waveguide modes. To avoid the excitation of these optical modes in the nanowire array, we propose to limit the pitch of the nanowire array.
|
|
| 5. |
|
|
| 6. |
- Anttu, Nicklas, et al.
(författare)
-
Absorption of light in InP nanowire arrays
- 2014
-
Ingår i: Nano Reseach. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 7:6, s. 816-823
-
Tidskriftsartikel (refereegranskat)abstract
- An understanding of the absorption of light is essential for efficient photovoltaic and photodetection applications with III-V nanowire arrays. Here, we correlate experiments with modeling and verify experimentally the predicted absorption of light in InP nanowire arrays for varying nanowire diameter and length. We find that 2,000 nm long nanowires in a pitch of 400 nm can absorb 94% of the incident light with energy above the band gap and, as a consequence, light which in a simple ray-optics description would be travelling between the nanowires can be efficiently absorbed by the nanowires. Our measurements demonstrate that the absorption for long nanowires is limited by insertion reflection losses when light is coupled from the air top-region into the array. These reflection losses can be reduced by introducing a smaller diameter to the nanowire-part closest to the air top-region. For nanowire arrays with such a nanowire morphology modulation, we find that the absorptance increases monotonously with increasing diameter of the rest of the nanowire.
|
|
| 7. |
- Anttu, Nicklas, et al.
(författare)
-
Beyond ray optics absorption of light in CsPbBr3perovskite nanowire arrays studied experimentally and with wave optics modelling
- 2023
-
Ingår i: Nanotechnology. - 0957-4484. ; 35:9
-
Tidskriftsartikel (refereegranskat)abstract
- We study experimentally and with wave optics modelling the absorption of light in CsPbBr3perovskite nanowire arrays fabricated into periodic pores of an anodized aluminum oxide matrix, for nanowire diameters from 30 to 360 nm. First, we find that all the light that couples into the array can be absorbed by the nanowires at sufficient nanowire length. This behavior is in strong contrast to the expectation from a ray-optics description of light where, for normally incident light, only the rays that hit the cross-section of the nanowires can be absorbed. In that case, the absorption in the sample would be limited to the area fill factor of nanowires in the hexagonal array, which ranges from 13% to 58% for the samples that we study. Second, we find that the absorption saturates already at a nanowire length of 1000-2000 nm, making these perovskite nanowires promising for absorption-based applications such as solar cells and photodetectors. The absorption shows a strong diameter dependence, but for all diameters the transmission is less than 24% already at a nanowire length of 500 nm. For some diameters, the absorption exceeds that of a calculated thin film with 100% coverage. Our analysis indicates that the strong absorption in these nanowires originates from light-trapping induced by the out-of-plane disorder due to random axial position of each nanowire within its pore in the matrix.
|
|
| 8. |
- Anttu, Nicklas
(författare)
-
Connection between modeled blackbody radiation and dipole emission in large-area nanostructures
- 2016
-
Ingår i: Optics Letters. - 0146-9592. ; 41:7, s. 1494-1497
-
Tidskriftsartikel (refereegranskat)abstract
- When modeling the emission of light from nanostructures, we typically study either (1) blackbody radiation or (2) dipole emission. For effective analysis, it is important to know how results from these two types of modeling are related. Here, we use Kirchoff's reciprocity to study how interference affects the emissivity and number of emitted blackbody photons from a thin film for varying thickness. Next, we use Lorentz's reciprocity to study how interference modifies the emission rate of a dipole placed within the same film. Finally, to find the connection between these two emission types, we use Kirchoff's and Lorentz's reciprocity simultaneously for an arbitrary three-dimensional large-area nanostructure. We show analytically how the blackbody radiation can be represented as the integrated emission from homogeneously distributed dipoles in the nanostructure. In this case, the dipole moment density is determined by the refractive index of the nanostructure.
|
|
| 9. |
- Anttu, Nicklas, et al.
(författare)
-
Coupling of Light into Nanowire Arrays and Subsequent Absorption
- 2010
-
Ingår i: Journal of Nanoscience And Nanotechnology. - : American Scientific Publishers. - 1533-4899 .- 1533-4880. ; 10:11, s. 7183-7187
-
Konferensbidrag (refereegranskat)abstract
- We present a theoretical study of the absorption of light in periodic arrays of InP nanowires. The absorption in the array depends strongly on the diameter and the length of the nanowires, as well as the period of the array. Nanowires of a length of just 2 Am are able, after an appropriate choice for the other parameters, to absorb more than 90% of the incident energy of TE and TM polarized light, with photon energies almost all the way down to the band gap energy and an incidence angle up to 50 degree. This high total absorption arises from a good coupling of the incident light into the nanowire array at the top interface between air and the array and absorption inside the array before the light reaches the interface between the nanowires and the substrate. We find that for a given photon energy there exists a critical nanowire diameter above which a dramatic increase in the absorption occurs. The critical diameter decreases for increasing photon energies, and is directly related to the dispersion of waveguiding modes in single isolated nanowires. A characterization showed that the absorption characteristics of the nanowire arrays are very promising for photovoltaic applications.
|
|
| 10. |
- Anttu, Nicklas, et al.
(författare)
-
Crystal Phase-Dependent Nanophotonic Resonances in InAs Nanowire Arrays
- 2014
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 14:10, s. 5650-5655
-
Tidskriftsartikel (refereegranskat)abstract
- Nanostructures have many material, electronic, and optical properties that are not found in bulk systems and that are relevant for technological applications. For example, nanowires realized from III-V semiconductors can be grown into wurtzite crystal structure. This crystal structure does not naturally exist in bulk where these materials form the zinc-blende counterpart. Being able to concomitantly grow these nanowires in the zinc-blende and/or wurtzite crystal structure prlovides an important degree of control for the design and optimization of optoelectronic applications based on these semiconductor nanostructures. However, the refractive indices of this new crystallographic phase have so far not been elucidated. This shortcoming makes it impossible to predict and utilize he full potential of these new nanostructured materials for optoelectronics applications a careful design and optimization of optical resonances by tuning the nanostrucuted geometry is needed to achieve optimal performance. Here, we report and analyze striking differeences in the optical response of nanophotonic resonances in wurtzite and zinc-blend InAs nanowire arrays. Specifically, through reflectance measurements we find that the resonance can be tuned down to lambda approximate to 380 nm in wurtzite nanowires by decreasing the nanowire diameter. In stark contrast, a similar tuning to below approximate to 500 nm is not possible in the zinc-blende nanowires. Furthermore, we find that the wurtzite nanowires can absorb twice as strongly as the zinc-blende nanowires. We attribute these strikingly large differences in resonant behavior to large differences between the refractive indices of the two crystallographic phases realized in these nanostructures. We anticipate our finding to be relevant for other III-B materials as well as for all material systems that manifest polytypism. Taken together, our results demonstrate crystal phase engineering as a potentially new design dimension for optoelectronics applications.
|
|
| 11. |
- Anttu, Nicklas, et al.
(författare)
-
Drastically increased absorption in vertical semiconductor nanowire arrays: A non-absorbing dielectric shell makes the difference
- 2012
-
Ingår i: Nano Reseach. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 5:12, s. 863-874
-
Tidskriftsartikel (refereegranskat)abstract
- Enhanced absorption of especially long wavelength light is needed to enable the full potential of semiconductor nanowire (NW) arrays for optoelectronic applications. We show both experimentally and theoretically that a transparent dielectric shell (Al2O3 coating) can drastically improve the absorption of light in InAs NW arrays. With an appropriate thickness of the Al2O3 shell, we achieve four times stronger absorption in the NWs compared to uncoated NWs and twice as good absorption as when the dielectric completely fills the space between the NWs. We provide detailed theoretical analysis from a combination of full electrodynamic modeling and intuitive electrostatic approximations. This reveals how the incident light penetrates better into the absorbing NW core with increasing thickness of the dielectric shell until a resonant shell thickness is reached. We provide a simple description of how to reach this strongly absorbing resonance condition, making our results easy to apply for a broad wavelength range and a multifold of semiconductor and dielectric coating material combinations.
|
|
| 12. |
- Anttu, Nicklas, et al.
(författare)
-
Efficient light management in vertical nanowire arrays for photovoltaics
- 2013
-
Ingår i: Optics Express. - 1094-4087. ; 21:9, s. 558-575
-
Tidskriftsartikel (refereegranskat)abstract
- Vertical arrays of direct band gap III-V semiconductor nanowires (NWs) hold the prospect of cheap and efficient next-generation photovoltaics, and guidelines for successful light-management are needed. Here, we use InP NWs as a model system and find, through electrodynamic modeling, general design principles for efficient absorption of sun light in nanowire arrays by systematically varying the nanowire diameter, the nanowire length, and the array period. Most importantly, we discover the existence of specific band-gap dependent diameters, 170 nm and 410 nm for InP, for which the absorption of sun light in the array is optimal, irrespective of the nanowire length. At these diameters, the individual InP NWs of the array absorb light strongly for photon energies just above the band gap energy due to a diameter-tunable nanophotonic resonance, which shows up also for other semiconductor materials of the NWs. Furthermore, we find that for maximized absorption of sun light, the optimal period of the array increases with nanowire length, since this decreases the insertion reflection losses. (C) 2013 Optical Society of America
|
|
| 13. |
- Anttu, Nicklas, et al.
(författare)
-
Excitations of surface plasmon polaritons in double layer metal grating structures
- 2012
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:9
-
Tidskriftsartikel (refereegranskat)abstract
- We study the light scattering properties of double layer gratings (DLGs) made from Au on SiO2 substrates. It is found that surface plasmon polaritons (SPPs) can be excited in the DLGs for a separation of up to 150 nm between the two Au grating layers and the collective reflectance spectra exhibit a strong resonant peak and a closely lying dip as a result of the surface plasmon polariton excitations. It is also found that the angle-resolved specular reflectance spectra show a dip-peak pair structure, while the angle-resolved reflectance spectra of higher diffracted orders show a complementary peak-dip pair structure. Finally, operation of the DLGs for efficient wavelength demultiplexing is proposed and discussed in light of these results. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3690947]
|
|
| 14. |
- Anttu, Nicklas
(författare)
-
Geometrical optics, electrostatics, and nanophotonic resonances in absorbing nanowire arrays
- 2013
-
Ingår i: Optics Letters. - 0146-9592. ; 38:5, s. 730-732
-
Tidskriftsartikel (refereegranskat)abstract
- Semiconductor nanowire arrays have shown promise for next-generation photovoltaics and photodetection, but enhanced understanding of the light-nanowire interaction is still needed. Here, we study theoretically the absorption of light in an array of vertical InP nanowires by moving continuously, first from the electrostatic limit to the nanophotonic regime and then to the geometrical optics limit. We show how the absorption per volume of semiconductor material in the array can be varied by a factor of 200, ranging from 10 times weaker to 20 times stronger than in a bulk semiconductor sample. (c) 2013 Optical Society of America
|
|
| 15. |
- Anttu, Nicklas, et al.
(författare)
-
Light scattering and plasmon resonances in a metal film with sub-wavelength nano-holes
- 2008
-
Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6596 .- 1742-6588. ; 100, s. 052037-052037
-
Konferensbidrag (refereegranskat)abstract
- We report on a theoretical study of optical extinction in a metal film of 15-230 nm in thickness patterned periodically with sub-wavelength nano-holes of 140 nm in diameter. The gold plate was on a thick SiO2 wafer and the nano-holes as well as the top side of the metal plate were filled with water or solvent. Light was sent in toward the plate from the SiO2 side. The simulations were performed by solving the Maxwell equations using the scattering matrix method. It was seen that the extinction can, depending on the periodicity of the hole array, show one or several peaks in the visible wavelength range. The positions of the peaks were redshifted when the thickness of the gold plate was decreased. It was found that the peak positions for a thick plate can be identified from a simple surface plasmon dispersion relation.
|
|
| 16. |
- Anttu, Nicklas
(författare)
-
Modifying the emission of light from a semiconductor nanowire array
- 2016
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 120:4
-
Tidskriftsartikel (refereegranskat)abstract
- Semiconductor nanowire arrays have been identified as a promising platform for future light emitting diodes (LEDs), for example, due to the materials science freedom of combining lattice-mismatched materials in them. Furthermore, the emission of light from nanowires can be tailored by designing their geometry. Such tailoring could optimize the emission of light to the top side as well as enhance the emission rate through the Purcell effect. However, the possibility for enhanced light extraction from III-V nanowire arrays over a conventional bulk-like LED has not been investigated systematically. Here, we use electromagnetic modeling to study the emission of light from nanowire arrays. We vary both the diameter of the nanowires and the array period to show the benefit of moving from a bulk-like LED to a nanowire array LED. We study the fraction of light emitted to the top air side and to the substrate at wavelength λ. We find several diameter-dependent resonant peaks for which the emission to the top side is maximized. For the strongest such peak, by increasing the array period, the fraction of emitted light that is extracted at the top air side can be enhanced by a factor of 30 compared to that in a planar bulk LED. By modeling a single nanowire, we confirm that it is beneficial to place the nanowires further apart to enhance the emission to the top side. Furthermore, we predict that for a nanowire diameter D > λ/2, a majority of the emitted power ends up in the substrate. Our results offer direction for the design and optimization of nanowire-array based light emitting diodes.
|
|
| 17. |
- Anttu, Nicklas
(författare)
-
Nanophotonics in absorbing III-V nanowire arrays
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We have studied the interaction of light with an array of vertically oriented III-V semiconductor nanowires both theoretically and experimentally. For the theoretical studies, electromagnetic modeling has been employed. This modeling shows that with proper tuning of the nanowire diameter, the absorption per volume semiconductor material can be 20 times higher in the nanowires than in a corresponding bulk semiconductor sample. This enhancement occurs when nanophotonic resonances show up in the nanowires. We have shown that the optical response of a nanowire array can be described by electrostatics for small-diameter nanowires and by geometrical optics for large-diameter nanowires. None of these two limit cases showed resonances, motivating the interest for the intermediate nanophotonic regime where the diameter of the nanowires is comparable to the wavelength of the incident light. Supported by theoretical analysis, we have shown experimentally a resonant photodetection response in an InAsSb nanowire array in the infrared region, which is of potential interest for thermal imaging and chemical analysis. Furthermore, we have demonstrated a solar cell based on InP nanowires. The nanowire-array solar cell showed an efficiency of 13.8 % and converted more than 70 % of the above-bandgap photons into electron-hole pairs that contributed to the short-circuit current, even though the nanowires covered only 12 % of the surface. By combining the electromagnetic modeling with reflectance measurements, we have developed an optical method for simultaneously measuring both the diameter and the length of nanowires in large-area arrays. The accuracy of the method is comparable to that of scanning electron microscopy. Furthermore, we have developed tools for studying the crystal-phase dependent optical response of III-V materials. The studies showed that a tuning of the crystal phase, which is possible in the nanowire geometry, can be used for enabling and disabling strongly absorbing nanophotonic resonances in nanowire arrays.
|
|
| 18. |
- Anttu, Nicklas, et al.
(författare)
-
Optical Far-Field Method with Subwavelength Accuracy for the Determination of Nanostructure Dimensions in Large-Area Samples.
- 2013
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 13:6, s. 2662-2667
-
Tidskriftsartikel (refereegranskat)abstract
- The physical, chemical, and biological properties of nanostructures depend strongly on their geometrical dimensions. Here we present a fast, noninvasive, simple-to-perform, purely optical method that is capable of characterizing nanostructure dimensions over large areas with an accuracy comparable to that of scanning electron microscopy. This far-field method is based on the analysis of unique fingerprints in experimentally measured reflectance spectra using full three-dimensional optical modeling. We demonstrate the strength of our method on large-area (millimeter-sized) arrays of vertical InP nanowires, for which we simultaneously determine the diameter and length as well as cross-sample morphological variations thereof. Explicitly, the diameter is determined with an accuracy better than 10 nm and the length with an accuracy better than 30 nm. The method is versatile and robust, and we believe that it will provide a powerful and standardized measurement technique for large-area nanostructure arrays suitable for both research and industrial applications.
|
|
| 19. |
- Anttu, Nicklas, et al.
(författare)
-
Reflection measurements to reveal the absorption in nanowire arrays
- 2013
-
Ingår i: Optics Letters. - 0146-9592. ; 38:9, s. 1449-1451
-
Tidskriftsartikel (refereegranskat)abstract
- The absorption of light is at the core of photovoltaic applications. For many nanostructure-based devices, an assessment of the absorption in the nanostructures is complicated by a thick, opaque substrate that prohibits transmission measurements. Here, we show how a single reflection measurement can be used for approximating the amount of light absorbed in vertical semiconductor nanowire arrays. (C) 2013 Optical Society of America
|
|
| 20. |
- Anttu, Nicklas, et al.
(författare)
-
Scattering matrix method for optical excitation of surface plasmons in metal films with periodic arrays of subwavelength holes
- 2011
-
Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 83:16
-
Tidskriftsartikel (refereegranskat)abstract
- We present the formulation of a scattering matrix method for the study of light-scattering properties of metal films. The method is employed for the study of the optical excitation of surface plasmons in a gold film of 15-230 nm thickness, patterned periodically with subwavelength nanoholes. The gold film is placed on a thick SiO2 wafer, and the nanoholes as well as the top side of the gold film are filled with H2O. Light is incident on the gold film from either the SiO2 or the H2O side. The extinction and reflectance spectra of the system, as well as the electromagnetic field distributions at certain characteristic wavelengths, are calculated. The extinction spectra show, depending on system parameters, one or several peaks in the visible wavelength range. The extinction peaks are found to be caused by surface plasmons. A simple model based on the dispersion relation for surface plasmons in an unperforated gold film is shown to predict the peak positions of the extinction for thick perforated films very well. Even for thin films, this simple model, which includes coupling of surface plasmons on both surfaces of the film, predicts peak positions of the extinction well if the hole diameter is small enough. As the hole diameter increases, the extinction peaks of thin films show redshifts. Extinction peaks caused by surface plasmons at the SiO2/Au interface in thick films exhibit strong redshifts when the film thickness is decreased. However, the extinction peaks caused by surface plasmons at the H2O/Au interface in thick films show a completely different behavior. In this case, the extinction peaks do not move noticeably when the film thickness is decreased. Instead, they are weakened and finally disappear. It is also found that each extinction peak is accompanied by an extinction dip and that a reflectance dip is located in the wavelength between the extinction peak and the dip. This arrangement of an extinction peak, a reflectance dip, and an extinction dip is a general property of the surface-plasmon excitation. The calculated electromagnetic field distributions in both thick and thin films show clearly the signature of the excitation of surface plasmons at the extinction peaks, the extinction dips, and the reflectance dips. In thick films with small holes, the electric-field strength in the vicinity of the holes is weak at wavelengths for which surface plasmons are excited. In contrast to this, for thin films at the surface-plasmon excitations, a much stronger electric field is seen in the vicinity of the holes.
|
|
| 21. |
- Anttu, Nicklas
(författare)
-
Shockley-Queisser Detailed Balance Efficiency Limit for Nanowire Solar Cells
- 2015
-
Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 2:3, s. 446-453
-
Tidskriftsartikel (refereegranskat)abstract
- III-V semiconductor nanowire arrays show promise as a platform for next-generation solar cells. However, the theoretical efficiency limit for converting the energy of sunlight into electrical energy in such solar cells is unknown. Here, we calculate through electromagnetic modeling the Shockley-Queisser efficiency limit for an InP nanowire array solar cell. In this analysis, we calculate first from the absorption of sunlight the short-circuit current. Next, we calculate the voltage-dependent emission characteristics of the nanowire array. From these processes, we identify how much current we can extract at a given voltage. Finally, after constructing this current-voltage (IV) curve of the nanowire solar cell, we identify from the maximum power output the maximum efficiency. We compare this efficiency of the nanowire array with the 31.0% efficiency limit of the conventional InP bulk solar cell with an inactive substrate underneath. We consider a nanowire array of 400 nm in period, which shows a high short-circuit current. We optimize both the nanowire length and diameter in our analysis. For example, nanowires of 4 mu m in length and 170 nm in diameter produce 96% of the short-circuit current obtainable in the perfectly absorbing InP bulk cell. However, the nanowire solar cell emits fewer photons than the bulk cell at thermal equilibrium, especially into the substrate. This weaker emission allows for a higher open circuit-voltage for the nanowire cell. As an end result, nanowires longer than 4 mu m can actually show, despite producing a lower short-circuit current, a higher efficiency limit, of up to 32.5%, than the bulk cell.
|
|
| 22. |
- Anttu, Nicklas, et al.
(författare)
-
Tailored emission to boost open-circuit voltage in solar cells
- 2019
-
Ingår i: Journal of Physics Communications. - : IOP Publishing. - 2399-6528. ; 3:5
-
Tidskriftsartikel (refereegranskat)abstract
- Recently, a lot of research focus has been on how to make solar cells more efficient. One direction is to enhance the open-circuit voltage Voc by optimizing the emission of photons in the cell, where emission is a necessary loss process due to the reciprocity between absorption and emission of light. Here, we performed a Shockley-Queisser detailed balance analysis to predict the benefit of managing emitted photons in a single-junction solar cell. First, at low internal luminescence efficiency ηint, non-radiative recombination dominates, and management of emitted photons plays negligible role for Voc . Similarly, for an external luminescence efficiency ηext<10%, externally emitted photons play negligible role, and Voc is set either by non-radiative recombination; or parasitic absorption of internally emitted photons. For higher ηext, the Voc can be boosted, maximally by 15%, by restricting the external emission to match the incidence cone of the AM1.5D sun light spectrum. Such emission restriction corresponds to lower escape probability of internally emitted photons, enhances photon recycling, drops ηext, and actually makes the solar cell into a worse LED. Finally, for partly diffuse incident light, by restricting the angular emission for photons in a 130 nm wavelength range around the bandgap, we predict a maximum 14% relative boost in solar cell efficiency. The results of this paper are intended to serve as a general guideline on how to utilize emission-tuning possibilities to develop highly efficient photovoltaic devices.
|
|
| 23. |
|
|
| 24. |
- Chen, Yang, et al.
(författare)
-
Design for strong absorption in a nanowire array tandem solar cell
- 2016
-
Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
-
Tidskriftsartikel (refereegranskat)abstract
- Semiconductor nanowires are a promising candidate for next-generation solar cells. However, the optical response of nanowires is, due to diffraction effects, complicated to optimize. Here, we optimize through optical modeling the absorption in a dual-junction nanowire-array solar cell in terms of the Shockley-Quessier detailed balance efficiency limit. We identify efficiency maxima that originate from resonant absorption of photons through the HE11 and the HE12 waveguide modes in the top cell. An efficiency limit above 40% is reached in the band gap optimized Al0.10Ga0.90As/In0.34Ga0.66As system when we allow for different diameter for the top and the bottom nanowire subcell. However, for experiments, equal diameter for the top and the bottom cell might be easier to realize. In this case, we find in our modeling a modest 1–2% drop in the efficiency limit. In the Ga0.51In0.49P/InP system, an efficiency limit of η = 37.3% could be reached. These efficiencies, which include reflection losses and sub-optimal absorption, are well above the 31.0% limit of a perfectly-absorbing, idealized single-junction bulk cell, and close to the 42.0% limit of the idealized dual-junction bulk cell. Our results offer guidance in the choice of materials and dimensions for nanowires with potential for high efficiency tandem solar cells.
|
|
| 25. |
- Chen, Yang, et al.
(författare)
-
One-dimensional electrical modeling of axial p-i-n junction InP nanowire array solar cells
- 2017
-
Ingår i: 17th International Conference on Numerical Simulation of Optoelectronic Devices, NUSOD 2017. - 9781509053230 ; , s. 23-24
-
Konferensbidrag (refereegranskat)abstract
- We demonstrate one-dimensional (1D) electrical modeling of InP nanowire array solar cells. This 1D modeling gives accurate description of the current voltage response even at high surface recombination velocity. The 1D electrical model decreases the simulation time by 3 orders of magnitude compared to a full three-dimensional (3D) model.
|
|
