Imperial College London

Nicholas Hylton

Faculty of Engineering

Research Strategy Manager
 
 
 
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Contact

 

+44 (0)20 7594 9659n.hylton

 
 
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Location

 

Electrical EngineeringSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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42 results found

Pusch A, Hylton NP, Ekins-Daukes NJ, 2018, Comparison of possible realizations of quantum ratchet intermediate band solar cells, Pages: 1841-1844

Three fundamentally different approaches for the realization of a quantum ratchet intermediate band solar cell are compared. The quantum ratchet is a mechanism by which an energy loss of excited electrons is turned into an improvement in the efficiency of intermediate band solar cells by reducing carrier recombination in the system. The main requirement for this is to engineer forbidden transitions which can be achieved through vanishing spatial overlap of the wave-functions, spinselection rules or a momentum mismatch.

Conference paper

Richards RD, Harun F, Cheong JS, Mellor A, Hylton NP, Wilson T, Thomas T, Ekins-Daukes NJ, David JPRet al., 2018, GaAsBi: an alternative to InGaAs based multiple quantum well photovoltaics, Photovoltaic Specialists Conference (PVSC), Publisher: IEEE, Pages: 1135-1137

A series of GaAsBi/GaAs multiple quantum well p-i-n diodes are characterized using IV, photocurrent and illuminated IV measurements. The results are compared to an InGaAs/GaAsP multiple quantum well control device of a design that has demonstrated excellent performance in triple junction photovoltaics. The extended absorption of the GaAsBi/GaAs devices, compared to that of the InGaAs/GaAsP device, suggests that GaAsBi/GaAs could present a viable alternative to InGaAs/GaAsP for quad junction photovoltaics.

Conference paper

Wilson T, Hylton NP, Harada Y, Pearce PM, Alonso-Alvarez D, Mellor A, Richards RD, David JP, Ekins-Daukes NJet al., 2018, Assessing the nature of the distribution of localised states in bulk GaAsBi, Scientific Reports, Vol: 8, ISSN: 2045-2322

A comprehensive assessment of the nature of the distribution of sub band-gap energy states in bulk GaAsBi is presented usingpower and temperature dependent photoluminescence spectroscopy. The observation of a characteristic red-blue-red shift inthe peak luminescence energy indicates the presence of short-range alloy disorder in the material. A decrease in the carrierlocalisation energy demonstrates the strong excitation power dependence of localised state behaviour and is attributed to thefilling of energy states furthest from the valence band edge. Analysis of the photoluminescence lineshape at low temperaturepresents strong evidence for a Gaussian distribution of localised states that extends from the valence band edge. Furthermore,a rate model is employed to understand the non-uniform thermal quenching of the photoluminescence and indicates thepresence of two Gaussian-like distributions making up the density of localised states. These components are attributed to thepresence of microscopic fluctuations in Bi content, due to short-range alloy disorder across the GaAsBi layer, and the formationof Bi related point defects, resulting from low temperature growth.

Journal article

Vaquero-Stainer A, Yoshida M, Hylton NP, Pusch A, Curtin O, Frogley M, Wilson T, Clarke E, Kennedy K, Ekins-Daukes NJ, Hess O, Phillips CCet al., 2018, Semiconductor nanostructure quantum ratchet for high efficiency solar cells, Communications Physics, Vol: 1, ISSN: 2399-3650

Conventional solar cell efficiencies are capped by the ~31% Shockley–Queisser limit because, even with an optimally chosen bandgap, some red photons will go unabsorbed and the excess energy of the blue photons is wasted as heat. Here we demonstrate a “quantum ratchet” device that avoids this limitation by inserting a pair of linked states that form a metastable photoelectron trap in the bandgap. It is designed both to reduce non-radiative recombination, and to break the Shockley–Queisser limit by introducing an additional “sequential two photon absorption” (STPA) excitation channel across the bandgap. We realise the quantum ratchet concept with a semiconductor nanostructure. It raises the electron lifetime in the metastable trap by ~104, and gives a STPA channel that increases the photocurrent by a factor of ~50%. This result illustrates a new paradigm for designing ultra-efficient photovoltaic devices.

Journal article

Richards RD, Mellor A, Harun F, Cheong JS, Hylton NP, Wilson T, Thomas T, Roberts JS, Ekins-Daukes NJ, David JPRet al., 2017, Photovoltaic characterisation of GaAsBi/GaAs multiple quantum well devices, Solar Energy Materials and Solar Cells, Vol: 172, Pages: 238-243, ISSN: 0927-0248

A series of strained GaAsBi/GaAs multiple quantum well diodes are characterised to assess the potential of GaAsBi for photovoltaic applications. The devices are compared with strained and strain-balanced InGaAs based devices.The dark currents of the GaAsBi based devices are around 20 times higher than those of the InGaAs based devices. The GaAsBi devices that have undergone significant strain relaxation have dark currents that are a further 10–20 times higher.Quantum efficiency measurements show the GaAsBi devices have a lower energy absorption edge and stronger absorption than the strained InGaAs devices. These measurements also indicate incomplete carrier extraction from the GaAsBi based devices at short circuit, despite the devices having a relatively low background doping. This is attributed to hole trapping within the quantum wells, due to the large valence band offset of GaAsBi.

Journal article

Lee K-H, Barnham KWJ, Roberts JS, Alonso Alvarez D, Hylton NP, Fuhrer M, Ekins-Daukes NJet al., 2017, Investigation of carrier recombination dynamics of InGaP/InGaAsP multiple quantum wells for solar cells via photoluminescence, IEEE Journal of Photovoltaics, Vol: 7, Pages: 817-821, ISSN: 2156-3381

The carrier recombination dynamics of InGaP/InGaAsP quantum wells is reported for the first time. By studying the photoluminescence (PL) and time-resolved PL decay of InGaP/InGaAsP multiple-quantum-well (MQW) heterostructure samples, it is demonstrated that InGaP/InGaAsP MQWs have very low nonradiative recombination rate and high radiative efficiency compared with the control InGaP sample. Along with the analyses of PL emission spectrum and external quantum efficiencies, it suggests that this is due to small confinement potentials in the conduction band but high confinement potentials in the valence band. These results explain several features found in InGaP/InGaAsP MQW solar cells previously.

Journal article

Mellor A, Hylton NP, Wellens C, Thomas T, Al-Saleh Y, Giannini V, Braun A, Hauser H, Maier SA, Ekins-Daukes NJet al., 2017, Improving the radiation hardness of space solar cells via nanophotonic light trapping, Pages: 1-4

We show that the radiation-hardness of space solar cells can be significantly improved by employing nanophotonic light trapping. Two light-trapping structures are investigated in this work. In the first, an array of Al nanoparticles is embedded within the anti-reflection coating of a GaInP/InGaAs/Ge solar cell. A combined experimental and simulation study shows that this structure is unlikely to lead to an improvement in radiation hardness. In the second, a diffractive structure is positioned between the middle cell and the bottom cell. Computational results, obtained using an experimentally validated electro-optical simulation tool, show that a properly designed light-trapping structure in this position can lead to a relative 10% improvement in the middle-cell photocurrent at end-of-life.

Conference paper

Pusch A, Yoshida M, Hylton NP, Mellor A, Vaquero-Steiner A, Phillips CC, Hess O, Ekins-Daukes NJet al., 2017, The purpose of a photon ratchet in intermediate band solar cells, Pages: 2536-2537

The intermediate band solar cell (IBSC) concept aims to improve upon the Shockley-Queisser limit for single bandgap solar cells by also making use of below bandgap photons through sequential absorption processes via an intermediate band (IB). In order for this concept to be translated into more efficient solar cells there are still challenges to overcome; one of the most important is the increased recombination (radiative as well as non-radiative) associated with the additional states in the bandgap. A proposal to mitigate those recombination losses is the introduction of a photon ratchet into the IBSC, which effectively trades some of the energy of the excited electrons against these recombination losses. We show here that this can lead to substantial improvements even in the radiative limiting efficiency, where no non-radiative recombination is taken into account and that this advantage is especially prominent for IBSCs in which the transitions into and out of the IB are not very absorptive, a case commonly encountered for current IBSC proposals.

Conference paper

Mellor AV, Hylton N, Wellens C, Thomas T, Al-Saleh Y, Giannini V, Braun A, Hauser H, Maier S, Ekins-Daukeset al., 2016, Improving the radiation hardness of space solar cells via nanophotonic light trapping, 43rd IEEE Photovoltaic Specialists Conference, Publisher: IEEE

We show that the radiation-hardness of space solarcells can be significantly improved by employing nanophotoniclight trapping. Two light-trapping structures are investigated inthis work. In the first, an array of Al nanoparticles is embeddedwithin the anti-reflection coating of a GaInP/InGaAs/Ge solar cell.A combined experimental and simulation study shows that thisstructure is unlikely to lead to an improvement in radiationhardness. In the second, a diffractive structure is positionedbetween the middle cell and the bottom cell. Computationalresults, obtained using an experimentally validated electro-opticalsimulation tool, show that a properly designed light-trappingstructure in this position can lead to a relative 10% improvementin the middle-cell photocurrent at end-of-life.

Conference paper

Mellor AV, Hylton N, Maier S, Ekins-Daukes Net al., 2016, Interstitial light-trapping design for multi-junction solar cells, Solar Energy Materials and Solar Cells, Vol: 159, Pages: 212-218, ISSN: 0927-0248

We present a light-trapping design capable of significantly enhancing the photon absorption inany subcell of a multi-junction solar cell. The design works by coupling incident light intowaveguide modes in one of the subcells via a diffraction grating, and preventing these modesfrom leaking into lower subcells via a low-index layer and a distributed Bragg reflector, whichtogether form an omnidirectional mirror. This allows the thickness of the target subcell to bereduced without compromising photon absorption, which improves carrier collection, andtherefore photocurrent. The paper focuses on using the composite structure to improve theradiation hardness of a InGaP/Ga(In)As/Ge space solar cell. In this context, it is shown viasimulation that the Ga(In)As middle-cell thickness can be reduced from 3500 to 700 nm,whilst maintaining strong photon absorption, and that this leads to a significantly improvedend-of-life photocurrent in the Ga(In)As middle cell. However, the design can in general beapplied to a wide range of multi-junction solar cell types. We discuss the principles ofoperation of the design, as well as possible methods of its fabrication and integration intomulti-junction solar cells.

Journal article

Mellor AV, Hylton NP, Hauser H, Thomas T, Lee K, Al-Saleh Y, Giannini V, Braun A, Loo J, Vercruysse D, Van Dorpe P, Blasi B, Maier SA, Ekins-Daukes Net al., 2016, Nanoparticle scattering for multi-junction solar cells: the trade-off between absorption enhancement and transmission loss, IEEE Journal of Photovoltaics, Vol: 6, Pages: 1678-1687, ISSN: 2156-3381

This paper contains a combined experimental andsimulation study of the effect of Al and AlInP nanoparticles onthe performance of multi-junction solar cells. In particular, weinvestigate oblique photon scattering by the nanoparticle arraysas a means of improving thinned subcells or those with lowdiffusion lengths, either inherently or due to radiation damage.Experimental results show the feasibility of integratingnanoparticle arrays into the ARCs of commercialInGaP/InGaAs/Ge solar cells, and computational results showthat nanoparticle arrays can improve the internal quantumefficiency via optical path length enhancement. However, adesign that improves the external quantum efficiency of a stateof-the-artcell has not been found, despite the large parameterspace studied. We show a clear trade-off between obliquescattering and transmission loss, and present design principlesand insights into how improvements can be made.

Journal article

richards RD, Harun F, Cheong JS, Mellor AV, Hylton, wilson T, Thomas T, Ekins-Daukes, David JPRet al., 2016, GaAsBi: An Alternative to InGaAs Based Multiple Quantum Well Photovoltaics, 43rd IEEE Photovoltaic Specialists Conference

Conference paper

Hylton N, Hinrichsen TF, Vaquero-Stainer AR, Yoshida M, Pusch A, Hopkinson M, Hess O, Phillips CC, Ekins-Daukes NJet al., 2016, Photoluminescence upconversion at GaAs/InGaP2 interfaces driven by a sequential two-photon absorption mechanism, Physical Review B, Vol: 93, ISSN: 2469-9950

This paper reports on the results of an investigation into the nature of photoluminescence upconversion at GaAs/InGaP2 interfaces. Using a dual-beam excitation experiment, we demonstrate that the upconversion in our sample proceeds via a sequential two-photon optical absorption mechanism. Measurements of photoluminescence and upconversion photoluminescence revealed evidence of the spatial localization of carriers in the InGaP2 material, arising from partial ordering of the InGaP2. We also observed the excitation of a two-dimensional electron gas at the GaAs/InGaP2 heterojunction that manifests as a high-energy shoulder in the GaAs photoluminescence spectrum. Furthermore, the results of upconversion photoluminescence excitation spectroscopy demonstrate that the photon energy onset of upconversion luminescence coincides with the energy of the two-dimensional electron gas at the GaAs/InGaP2 interface, suggesting that charge accumulation at the interface can play a crucial role in the upconversion process.

Journal article

Pusch A, Yoshida M, Hylton NP, Mellor A, Phillips CC, Hess O, Ekins-Daukes NJet al., 2016, Limiting efficiencies for intermediate band solar cells with partial absorptivity: the case for a quantum ratchet, Progress in Photovoltaics, Vol: 24, Pages: 656-662, ISSN: 1099-159X

The intermediate band solar cell (IBSC) concept aims to improve upon the Shockley–Queisser limit for single bandgap solar cells by also making use of below bandgap photons through sequential absorption processes via an intermediate band (IB). Current proposals for IBSCs suffer from low absorptivity values for transitions into and out of the IB. We therefore devise and evaluate a general, implementation‐independent thermodynamic model for an absorptivity‐constrained limiting efficiency of an IBSC to study the impact of absorptivity limitations on IBSCs. We find that, due to radiative recombination via the IB, conventional IBSCs cannot surpass the Shockley–Queisser limit at an illumination of one Sun unless the absorptivity from the valence band to the IB and the IB to the conduction band exceeds ≈36%. In contrast, the introduction of a quantum ratchet into the IBSC to suppress radiative recombination can enhance the efficiency of an IBSC beyond the Shockley–Queisser limit for any value of the IB absorptivity. Thus, the quantum ratchet could be the vital next step to engineer IBSCs that are more efficient than conventional single‐gap solar cells.

Journal article

Mellor AV, Hylton NP, Hohn O, Wellens C, Hauser H, Thomas T, Al-Saleh Y, Tucher N, Oliva E, Blasi B, Ekins-Daukes NJ, Maier Set al., 2016, Nanoparticle scattering for multijunction solar cells, Proc. SPIE 9898, Photonics for Solar Energy Systems VI, Publisher: Society of Photo-optical Instrumentation Engineers (SPIE), Pages: 989809-989809, ISSN: 1996-756X

We investigate the integration of Al nanoparticle arrays into the anti-reflection coatings (ARCs) of commercial triple-junction GaInP/ In0.01GaAs /Ge space solar cells, and study their effect on the radiation-hardness. It is postulated that the presence of nanoparticle arrays can improve the radiation-hardness of space solar cells by scattering incident photons obliquely into the device, causing charger carriers to be photogenerated closer to the junction, and hence improving the carrier collection efficiency in the irradiation-damaged subcells. The Al nanoparticle arrays were successfully embedded in the ARCs, over large areas, using nanoimprint lithography: a replication technique with the potential for high throughput and low cost. Irradiation testing showed that the presence of the nanoparticles did not improve the radiation-hardness of the solar cells, so the investigated structure has proven not to be ideal in this context. Nonetheless, this paper reports on the details and results of the nanofabrication to inform about future integration of alternative light-scattering structures into multi-junction solar cells or other optoelectronic devices.

Conference paper

Curtin OJ, Yoshida M, Pusch A, Hylton NP, Ekins-Daukes NJ, Phillips CC, Hess Oet al., 2016, Quantum cascade photon ratchets for intermediate band solar cells, IEEE Journal of Photovoltaics, Vol: 6, Pages: 673-678, ISSN: 2156-3381

We propose an antimonide-based quantum cascade design to demonstrate the ratchet mechanism for incorporation into the recently suggested photon ratchet intermediate-band solar cell. We realize the photon ratchet as a semiconductor heterostructure in which electrons are optically excited into an intermediate band and spatially decoupled from the valence band through a type-II quantum cascade. This process reduces both radiative and nonradiative recombination and can thereby increase the solar cell efficiency over intermediate-band solar cells. Our design method uses an adaptive simulated annealing genetic algorithm to determine the optimum thicknesses of semiconductor layers in the quantum cascade, allowing efficient transport (via phonon emission) of the electrons away from the interband active region.

Journal article

Pusch A, Yoshida M, Hylton NP, Mellor A, Vaquero-Steiner A, Phillips CC, Hess O, Ekins-Daukes NJet al., 2016, The Purpose of a Photon Ratchet in Intermediate Band Solar Cells, 43rd IEEE Photovoltaic Specialists Conference (PVSC), Publisher: IEEE, Pages: 9-12, ISSN: 0160-8371

Conference paper

Thomas T, Wilson T, Fuhrer M, Hylton N, Ekins-Daukes N, Tan KH, Li D, Wicaksono S, Loke WK, Fatt YS, Johnson Aet al., 2015, Use of Double Band Anti-Crossing to Control Optical Absorption of GaAsSbN for Multi-Junction Solar Cells, 31st European Photovoltaic Solar Energy Conference and Exhibition

Conference paper

Thomas T, Mellor A, Hylton NP, Fuehrer M, Alonso-Alvarez D, Braun A, Ekins-Daukes NJ, David JPR, Sweeney SJet al., 2015, Requirements for a GaAsBi 1 eV sub-cell in a GaAs-based multi-junction solar cell, Semiconductor Science and Technology, Vol: 30, ISSN: 1361-6641

Journal article

Mellor A, Hylton NP, Shirley F, Thomas T, Lee K-H, Al-Saleh Y, Braun A, Loo J, Vercruysse D, Van Dorpe P, Maier SA, Ekins-Daukes NJet al., 2015, Nanoparticle scattering for radiation-hard multi-junction space solar cells, IEEE 42nd Photovoltaic Specialist Conference (PVSC), Publisher: IEEE, ISSN: 0160-8371

Conference paper

Massa E, Giannini V, Hylton NP, Ekins-Daukes NJ, Jain S, El Daif O, Maier SAet al., 2014, Diffractive Interference Design Using Front and Rear Surface Metal and Dielectric Nanoparticle Arrays for Photocurrent Enhancement in Thin Crystalline Silicon Solar Cells, ACS PHOTONICS, Vol: 1, Pages: 871-877, ISSN: 2330-4022

Journal article

Alonso-Alvarez D, Thomas T, Fuehrer M, Hylton NP, Ekins-Daukes NJ, Lackner D, Philipps SP, Bett AW, Sodabanlu H, Fujii H, Watanabe K, Sugiyama M, Nasi L, Campanini Met al., 2014, InGaAs/GaAsP strain balanced multi-quantum wires grown on misoriented GaAs substrates for high efficiency solar cells, Applied Physics Letters, Vol: 105, ISSN: 1077-3118

Quantum wires (QWRs) form naturally when growing strain balanced InGaAs/GaAsP multi-quantum wells (MQW) on GaAs [100] 6° misoriented substrates under the usual growth conditions. The presence of wires instead of wells could have several unexpected consequences for the performance of the MQW solar cells, both positive and negative, that need to be assessed to achieve high conversion efficiencies. In this letter, we study QWR properties from the point of view of their performance as solar cells by means of transmission electron microscopy, time resolved photoluminescence and external quantum efficiency (EQE) using polarised light. We find that these QWRs have longer lifetimes than nominally identical QWs grown on exact [100] GaAs substrates, of up to 1 μs, at any level of illumination. We attribute this effect to an asymmetric carrier escape from the nanostructures leading to a strong 1D-photo-charging, keeping electrons confined along the wire and holes in the barriers. In principle, these extended lifetimes could be exploited to enhance carrier collection and reduce dark current losses. Light absorption by these QWRs is 1.6 times weaker than QWs, as revealed by EQE measurements, which emphasises the need for more layers of nanostructures or the use light trapping techniques. Contrary to what we expected, QWR show very low absorption anisotropy, only 3.5%, which was the main drawback a priori of this nanostructure. We attribute this to a reduced lateral confinement inside the wires. These results encourage further study and optimization of QWRs for high efficiency solar cells.

Journal article

Hylton NP, Li XF, Giannini V, Lee K-H, Ekins-Daukes NJ, Loo J, Vercruysse D, Van Dorpe P, Sodabanlu H, Sugiyama M, Maier SAet al., 2013, Loss mitigation in plasmonic solar cells: aluminium nanoparticles for broadband photocurrent enhancements in GaAs photodiodes, SCIENTIFIC REPORTS, Vol: 3, ISSN: 2045-2322

Journal article

Yang P, Gwilliam RM, Crowell IF, Papachristodoulou N, Halsall MP, Hylton NP, Hulko O, Knights AP, Shah M, Kenyon AJet al., 2013, Size limit on the phosphorous doped silicon nanocrystals for dopant activation, 18th International Conference on Ion Beam Modifications of Materials (IBMM), Publisher: ELSEVIER SCIENCE BV, Pages: 456-458, ISSN: 0168-583X

Conference paper

Crowe IF, Papachristodoulou N, Halsall MP, Hylton NP, Hulko O, Knights AP, Yang P, Gwilliam RM, Shah M, Kenyon AJet al., 2013, Donor ionization in size controlled silicon nanocrystals: The transition from defect passivation to free electron generation, JOURNAL OF APPLIED PHYSICS, Vol: 113, ISSN: 0021-8979

Journal article

Li X, Hylton NP, Giannini V, Lee K-H, Ekins-Daukes NJ, Maier SAet al., 2013, Multi-dimensional modeling of solar cells with electromagnetic and carrier transport calculations, PROGRESS IN PHOTOVOLTAICS, Vol: 21, Pages: 109-120, ISSN: 1062-7995

Journal article

Hylton NP, Li X, Giannini V, Lee K-H, Ekins-Daukes NJ, Loo J, Vercruysse D, Van Dorpe P, Sodabanlu H, Sugiyama M, Maier SAet al., 2013, Al Nanoparticle Arrays for Broadband Absorption Enhancements in GaAs devices, 39th IEEE Photovoltaic Specialists Conference (PVSC), Publisher: IEEE, Pages: 22-24, ISSN: 0160-8371

Conference paper

Li X, Hylton NP, Giannini V, Lee KH, Ekins-Daukes NJ, Maier SAet al., 2012, 3D device simulation of plasmonic solar cells, Information Optoelectronics, Nanofabrication and Testing, IONT 2012

The device-oriented modelling of plasmonic solar cells in frequency and three-dimensional spatial domains is presented. It enables the simulation of both electromagnetic and carrier transport response accurately for a comprehensive solar cell design. © 2012 OSA.

Journal article

Zhu D, McAleese C, Haeberlen M, Kappers MJ, Hylton N, Dawson P, Radtke G, Couillard M, Botton GA, Sahonta S-L, Humphreys CJet al., 2012, High-efficiency InGaN/GaN quantum well structures on large area silicon substrates, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, Vol: 209, Pages: 13-16, ISSN: 1862-6300

Journal article

Li X, 2012, Aluminum Nanoparticles for Efficient Light-trapping in Plasmonic Gallium Arsenide Solar Cells, 2012 ASIA COMMUNICATIONS AND PHOTONICS CONFERENCE (ACP), ISSN: 2162-108X

Journal article

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