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- Fast, Jonatan
(författare)
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Hot-carrier extraction in nanowires
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A hot-carrier solar cell aims to generate power from energetic, photoexcited, charge carriers, so called hot carriers, in order to reach higher conversion efficiencies than current solar cell technology.Creating a hot-carrier solar cell has proven challenging for two main reasons: hot carriers lose their energy very quickly, and they need to be extracted over distances of a few hundrednanometers via energy selective filters.Semiconducting III-V nanowires offer high flexibility and control in heterostructure growth, enabling the realisation of numerous types of energy filters, in combination with promising properties such as reduced thermal conductivity, increased hot-carrier temperatures, and various possibilities to tune optical absorption.This thesis aims to expand current knowledge of how to optimally design devices for hot-carrier extraction in practice.Specifically, three experimental papers (I-III) study the generation of electrical power by extracting charge carriers across energy selective filters within single semiconducting nanowires. The fourth paper (IV) reviews current literature relating to hot carriers in nanowires.The experiments are based on InAs nanowires with epitaxially defined heterostructures of InP or InAsP that form energy filters. Charge carrier extraction is studied by three different means: excitation of a non-equilibrium distribution by optical or electron-beam exposure, or the generation of an equilibrium distribution by heat. In Papers I and II, hot-carrier extraction is spatially resolved over a rectangular InP barrier. Paper I uses the high spatial resolution of an electron beam, while Paper II studies the operation of a similar devices under highly focused optical excitation. Both papers observe hot-carrier extraction around the barrier. The mechanism for extraction is better understood and valuable input for the future design of hot-carrier photovoltaic devices is extracted, such as hot-electron diffusion lengths on the order of a few hundred nanometers. Paper III studies thermoelectric power generation in a nonlinear transport regime of a ramp-shaped potential barrier, realised by gradually changing x in InAs_xP_(1-x). It is observed that fill factor, and thus maximum output power, can be tuned beyond the linear response limits. This opens up a new door of possibility for tuning the performance of both thermoelectric and hot-carrier photovoltaic systems.
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| 2. |
- Fast, Jonatan, et al.
(författare)
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Hot-carrier optoelectronic devices based on semiconductor nanowires
- 2021
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Ingår i: Applied Physics Reviews. - : AIP Publishing. - 1931-9401. ; 8:2
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Tidskriftsartikel (refereegranskat)abstract
- In optoelectronic devices such as solar cells and photodetectors, a portion of electron-hole pairs is generated as so-called hot carriers with anexcess kinetic energy that is typically lost as heat. The long-standing aim to harvest this excess energy to enhance device performance hasproven to be very challenging, largely due to the extremely short-lived nature of hot carriers. Efforts thus focus on increasing the hot carrierrelaxation time and on tailoring heterostructures that allow for hot-carrier extraction on short time and length scales. Recently, semiconductornanowires have emerged as a promising system to achieve these aims, because they offer unique opportunities for heterostructure engineeringas well as for potentially modified phononic properties that can lead to increased relaxation times. In this review we assess thecurrent state of theory and experiments relating to hot-carrier dynamics in nanowires, with a focus on hot-carrier photovoltaics. To providea foundation, we begin with a brief overview of the fundamental processes involved in hot-carrier relaxation and how these can be tailoredand characterized in nanowires. We then analyze the advantages offered by nanowires as a system for hot-carrier devices and review the statusof proof-of-principle experiments related to hot-carrier photovoltaics. To help interpret existing experiments on photocurrent extractionin nanowires we provide modeling based on non-equilibrium Green’s functions. Finally, we identify open research questions that need to beanswered in order to fully evaluate the potential nanowires offer toward achieving more efficient, hot-carrier based, optoelectronic devices.
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| 3. |
- Fast, Jonatan, et al.
(författare)
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Hot-carrier separation in heterostructure nanowires observed by electron-beam induced current
- 2020
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Ingår i: Nanotechnology. - : IOP Publishing. - 1361-6528 .- 0957-4484. ; 31:39
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Tidskriftsartikel (refereegranskat)abstract
- The separation of hot carriers in semiconductors is of interest for applications such asthermovoltaic photodetection and third-generation photovoltaics. Semiconductor nanowiresoffer several potential advantages for effective hot-carrier separation such as: a high degree ofcontrol and flexibility in heterostructure-based band engineering, increased hot-carriertemperatures compared to bulk, and a geometry well suited for local control of light absorption.Indeed, InAs nanowires with a short InP energy barrier have been observed to produce electricpower under global illumination, with an open-circuit voltage exceeding the Shockley-Queisserlimit. To understand this behaviour in more detail, it is necessary to establish control over theprecise location of electron-hole pair-generation in the nanowire. In this work we performelectron-beam induced current measurements with high spatial resolution, and demonstrate therole of the InP barrier in extracting energetic electrons.We interprete the results in terms ofhot-carrier separation, and extract estimates of the hot carriers’ mean free path.
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| 4. |
- Fast, Jonatan, et al.
(författare)
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Optical-Beam-Induced Current in InAs/InP Nanowires for Hot-Carrier Photovoltaics
- 2022
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Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 5:6, s. 7728-7734
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Tidskriftsartikel (refereegranskat)abstract
- Using the excess energy of charge carriers excited above the band edge (hot carriers) could pave the way for optoelectronic devices, such as photovoltaics exceeding the Shockley-Queisser limit or ultrafast photodetectors. Semiconducting nanowires show promise as a platform for hot-carrier extraction. Proof of principle photovoltaic devices have already been realized based on InAs nanowires, using epitaxially defined InP segments as energy filters that selectively transmit hot electrons. However, it is not yet fully understood how charge-carrier separation, relaxation, and recombination depend on device design and on the location of optical excitation. Here, we introduce the use of an optical-beam-induced current (OBIC) characterization method, employing a laser beam focused close to the diffraction limit and a high precision piezo stage, to study the optoelectric performance of the nanowire device as a function of the position of excitation. The photocurrent response agrees well with modeling based on hot-electron extraction across the InP segment via diffusion. We demonstrate that the device is capable of producing power and estimate the spatial region within which significant hot-electron extraction can take place to be on the order of 300 nm away from the barrier. When comparing to other experiments on similar nanowires, we find good qualitative agreement, confirming the interpretation of the device function, while the extracted diffusion length of hot electrons varies. Careful control of the excitation and device parameters will be important to reach the potentially high device performance theoretically available in these systems.
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