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Enhanced broadband absorption in nanowire arrays with integrated Bragg reflectors
Lund University, Lund, Sweden.
Halmstad University, School of Information Technology, Halmstad Embedded and Intelligent Systems Research (EIS), MPE-lab. Lund University, Lund, Sweden. (Nanovetenskap)ORCID iD: 0000-0001-5027-1456
2018 (English)In: Nanophotonics, E-ISSN 2192-8614, Vol. 7, no 5, p. 819-825Article in journal (Refereed) Published
Abstract [en]

A near-unity unselective absorption spectrum is desirable for high-performance photovoltaics. Nanowire arrays are promising candidates for efficient solar cells due to nanophotonic absorption resonances in the solar spectrum. The absorption spectra, however, display undesired dips between the resonance peaks. To achieve improved unselective broadband absorption, we propose to enclose distributed Bragg reflectors (DBRs) in the bottom and top parts of indium phosphide (InP) nanowires, respectively. We theoretically show that by enclosing only two periods of In0.56Ga0.44As/InPDBRs, an unselective 78% absorption efficiency (72% for nanowires without DBRs)is obtained at normal incidence in the spectral range from 300 nm to 920 nm. Under oblique light incidence, the absorption efficiency is enhanced up to about 85% at an incidence angle of 50º. By increasing the number of DBR periods from two to five, the absorption efficiency is further enhanced up to 95% at normal incidence. In this work we calculated optical spectra for InP nanowires, but the results are expected to be valid for other direct band gap III-V semiconductor materials. We believe that our proposed idea of integrating DBRs in nanowires offers great potential for high-performance photovoltaic applications. ©2018 Håkan Pettersson et al., published by De Gruyter, Berlin/Boston.

Place, publisher, year, edition, pages
Berlin: De Gruyter Open, 2018. Vol. 7, no 5, p. 819-825
Keywords [en]
light trapping, distributed Bragg reflectors (DBRs), nanowires, photovoltaics
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:hh:diva-35885DOI: 10.1515/nanoph-2017-0101Scopus ID: 2-s2.0-85045636459OAI: oai:DiVA.org:hh-35885DiVA, id: diva2:1166437
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research Knut and Alice Wallenberg Foundation
Note

Funding: NanoLund, the Swedish Research Council (VR), the Swedish Foundation for Strategic Research (SSF), and the Knut and Alice Wallenberg Foundation

Available from: 2017-12-14 Created: 2017-12-14 Last updated: 2018-10-29Bibliographically approved
In thesis
1. Tailoring the Optical Response of III-V Nanowire Arrays
Open this publication in new window or tab >>Tailoring the Optical Response of III-V Nanowire Arrays
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Lund: Lund University, 2017. p. 63
Keywords
III-V nanowires, absorption, optical modes, photovoltaics
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:hh:diva-36683 (URN)978-91-7753-277-4 (ISBN)978-91-7753-278-1 (ISBN)
Public defence
2017-06-02, Rydbergsalen, Fysicum, Sölvegatan 14, Lund, 13:15 (English)
Opponent
Supervisors
Available from: 2018-05-03 Created: 2018-04-25 Last updated: 2018-05-03Bibliographically approved

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