Megumi Yoshida | Imperial College London (original) (raw)

Papers by Megumi Yoshida

Limiting efficiencies for intermediate band solar cells with partial absorptivity: the case for a quantum ratchet

Progress in Photovoltaics: Research and Applications, 2016

Limiting efficiencies for intermediate band solar cells with partial absorptivity: the case for a quantum ratchet

Progress in Photovoltaics: Research and Applications, 2016

Applied Physics Reviews

Extensive literature and publications on intermediate band solar cells (IBSCs) are reviewed. A de... more Extensive literature and publications on intermediate band solar cells (IBSCs) are reviewed. A detailed discussion is given on the thermodynamics of solar energy conversion in IBSCs, the device physics, and the carrier dynamics processes with a particular emphasis on the two-step inter-subband absorption/recombination processes that are of paramount importance in a successful implementation high-efficiency IBSC. The experimental solar cell performance is further discussed, which has been recently demonstrated by using highly mismatched alloys and high-density quantum dot arrays and superlattice. IBSCs having widely different structures, materials, and spectral responses are also covered, as is the optimization of device parameters to achieve maximum performance.

Two-photon triplet-triplet annihilation upconversion for photovoltaics

2011 37th IEEE Photovoltaic Specialists Conference, 2011

ABSTRACT Upconversion is a promising technique for significantly enhancing the efficiency of phot... more ABSTRACT Upconversion is a promising technique for significantly enhancing the efficiency of photovoltaic cells. Molecular systems provide an environment in which long lived triplet states can be exploited to achieve high upconversion efficiencies under solar illumination. We report on the investigation of bi-molecular triplet-triplet annihilation upconversion (TTA-UC) in a Palladium (II) tetrakisquinoxalino porphyrin (PQ4Pd)/rubrene solution. These molecules were studied in solution using UV/VIS spectroscopy to determine their stability in air over a period of weeks. Transient absorption spectroscopy (TAS) was used to directly measure the lifetime of triplet states within these mixtures and hence determine the photoinduced kinetics of the system. The lifetime of porphyrin triplets was reduced from 92.4 μs in pristine PQ4Pd to 2.4 μs in the presence of rubrene. From this change, the rate constant associated with triplet energy transfer (kTET) was calculated as 3.38 × 108M−1s−1. Additionally, a reduction in the absorption of 530 nm light (the ground state rubrene absorption peak) was observed, while the mixture was pumped at the absorption peak of the porphyrin (670 nm). This change became apparent nearly 6 μs after the laser pulse, showing energy transfer from the porphyrin to the rubrene, and allowing further insight into the kinetics of the mechanism.

Kinetic insight into bimolecular upconversion: experiment and simulation

RSC Advances, 2014

ABSTRACT We demonstrate a transient rate model for photochemical upconversion that links the inte... more ABSTRACT We demonstrate a transient rate model for photochemical upconversion that links the internal energy transfer and triplet–triplet annihilation processes to spectroscopically measurable quantities, such as delayed fluorescence and bleaching. We confirm that our model is able to reproduce published delayed fluorescence measurements extremely well. We then use transient absorption spectroscopy to directly observe the dynamics of triplet populations through clear observation of delayed bleaching of the emitter species, providing direct evidence of triplet energy transfer from sensitiser to emitter molecules. This more complex experiment is also well reproduced by our model.

Progress Toward Realizing an Intermediate Band Solar Cell—Sequential Absorption of Photons in a Quantum Well Solar Cell

IEEE J. Photovoltaics, 2014

ABSTRACT In order to realize an intermediate band solar cell, which promises high photovoltaic en... more ABSTRACT In order to realize an intermediate band solar cell, which promises high photovoltaic energy conversion efficiency, achieving higher photocurrent while maintaining the cell voltage is essential. We report on a transient photocurrent due to the sequential absorption of photons in a single quantum well by continuously pumping to stimulate interband transitions (from a valence band to an intermediate band) and showing an intersubband transition (from an intermediate band to a conduction band) with a pulsed infrared laser. We demonstrate the extent to which multiple-photon absorption can be achieved in quantum well devices and propose that a quantum well is a suitable candidate for an intermediate band solar cell. From the combination of this and other sequential absorption results, it is clear that enhancing the short lifetime of a carrier in the intermediate band is the next step toward achieving a working intermediate band solar cell. In light of this, we enhance our previous suggestion, the photon ratchet intermediate band solar cell, as a means of increasing the electron lifetime.

Operating regimes for second generation luminescent solar concentrators

Progress in Photovoltaics: Research and Applications, 2012

ABSTRACT In this paper, we introduce the concept of first and second generation luminescent solar... more ABSTRACT In this paper, we introduce the concept of first and second generation luminescent solar concentrators. Traditional, first generation devices are characterised by their randomly oriented molecules that absorb sunlight and emit luminescence isotropically. By applying detailed balance to the absorbed and emitted photon fluxes we derive the ShockleyQueisser limit for these devices. It is found that they have inherently low efficiency due to optical losses (the well known reabsorption problem) and also that device performance is strongly affected by the areal ratio between the top and edge surfaces. This latter property makes it very difficult to achieve significant cost reductions because as the edge area is reduced (to lessen the amount of expensive photovoltaic material required for conversion), the efficiency of the system diminishes. First generation concentrators have now approached the fundamental limits which we predict here, thus to achieve a stand-alone luminescent concentrator that enables significant cost reductions, second generation approaches are now needed. New, second generation devices are characterised by either directional emitters or photonic filters which enhance the waveguiding mechanism, allowing high efficiency and large sizes to be achieved simultaneously. Here we define the fundamental operating regime in which second generation technology must reach to surpass the limit of first generation devices. Copyright (c) 2011 John Wiley & Sons, Ltd.

Photon ratchet intermediate band solar cells

Applied Physics Letters, 2012

Quantum Cascade Photon Ratchets for Intermediate-Band Solar Cells

IEEE Journal of Photovoltaics, 2016

Limiting efficiencies for intermediate band solar cells with partial absorptivity: the case for a quantum ratchet

Progress in Photovoltaics: Research and Applications, 2016

Limiting efficiencies for intermediate band solar cells with partial absorptivity: the case for a quantum ratchet

Progress in Photovoltaics: Research and Applications, 2016

Applied Physics Reviews

Extensive literature and publications on intermediate band solar cells (IBSCs) are reviewed. A de... more Extensive literature and publications on intermediate band solar cells (IBSCs) are reviewed. A detailed discussion is given on the thermodynamics of solar energy conversion in IBSCs, the device physics, and the carrier dynamics processes with a particular emphasis on the two-step inter-subband absorption/recombination processes that are of paramount importance in a successful implementation high-efficiency IBSC. The experimental solar cell performance is further discussed, which has been recently demonstrated by using highly mismatched alloys and high-density quantum dot arrays and superlattice. IBSCs having widely different structures, materials, and spectral responses are also covered, as is the optimization of device parameters to achieve maximum performance.

Two-photon triplet-triplet annihilation upconversion for photovoltaics

2011 37th IEEE Photovoltaic Specialists Conference, 2011

ABSTRACT Upconversion is a promising technique for significantly enhancing the efficiency of phot... more ABSTRACT Upconversion is a promising technique for significantly enhancing the efficiency of photovoltaic cells. Molecular systems provide an environment in which long lived triplet states can be exploited to achieve high upconversion efficiencies under solar illumination. We report on the investigation of bi-molecular triplet-triplet annihilation upconversion (TTA-UC) in a Palladium (II) tetrakisquinoxalino porphyrin (PQ4Pd)/rubrene solution. These molecules were studied in solution using UV/VIS spectroscopy to determine their stability in air over a period of weeks. Transient absorption spectroscopy (TAS) was used to directly measure the lifetime of triplet states within these mixtures and hence determine the photoinduced kinetics of the system. The lifetime of porphyrin triplets was reduced from 92.4 μs in pristine PQ4Pd to 2.4 μs in the presence of rubrene. From this change, the rate constant associated with triplet energy transfer (kTET) was calculated as 3.38 × 108M−1s−1. Additionally, a reduction in the absorption of 530 nm light (the ground state rubrene absorption peak) was observed, while the mixture was pumped at the absorption peak of the porphyrin (670 nm). This change became apparent nearly 6 μs after the laser pulse, showing energy transfer from the porphyrin to the rubrene, and allowing further insight into the kinetics of the mechanism.

Kinetic insight into bimolecular upconversion: experiment and simulation

RSC Advances, 2014

ABSTRACT We demonstrate a transient rate model for photochemical upconversion that links the inte... more ABSTRACT We demonstrate a transient rate model for photochemical upconversion that links the internal energy transfer and triplet–triplet annihilation processes to spectroscopically measurable quantities, such as delayed fluorescence and bleaching. We confirm that our model is able to reproduce published delayed fluorescence measurements extremely well. We then use transient absorption spectroscopy to directly observe the dynamics of triplet populations through clear observation of delayed bleaching of the emitter species, providing direct evidence of triplet energy transfer from sensitiser to emitter molecules. This more complex experiment is also well reproduced by our model.

Progress Toward Realizing an Intermediate Band Solar Cell—Sequential Absorption of Photons in a Quantum Well Solar Cell

IEEE J. Photovoltaics, 2014

ABSTRACT In order to realize an intermediate band solar cell, which promises high photovoltaic en... more ABSTRACT In order to realize an intermediate band solar cell, which promises high photovoltaic energy conversion efficiency, achieving higher photocurrent while maintaining the cell voltage is essential. We report on a transient photocurrent due to the sequential absorption of photons in a single quantum well by continuously pumping to stimulate interband transitions (from a valence band to an intermediate band) and showing an intersubband transition (from an intermediate band to a conduction band) with a pulsed infrared laser. We demonstrate the extent to which multiple-photon absorption can be achieved in quantum well devices and propose that a quantum well is a suitable candidate for an intermediate band solar cell. From the combination of this and other sequential absorption results, it is clear that enhancing the short lifetime of a carrier in the intermediate band is the next step toward achieving a working intermediate band solar cell. In light of this, we enhance our previous suggestion, the photon ratchet intermediate band solar cell, as a means of increasing the electron lifetime.

Operating regimes for second generation luminescent solar concentrators

Progress in Photovoltaics: Research and Applications, 2012

ABSTRACT In this paper, we introduce the concept of first and second generation luminescent solar... more ABSTRACT In this paper, we introduce the concept of first and second generation luminescent solar concentrators. Traditional, first generation devices are characterised by their randomly oriented molecules that absorb sunlight and emit luminescence isotropically. By applying detailed balance to the absorbed and emitted photon fluxes we derive the ShockleyQueisser limit for these devices. It is found that they have inherently low efficiency due to optical losses (the well known reabsorption problem) and also that device performance is strongly affected by the areal ratio between the top and edge surfaces. This latter property makes it very difficult to achieve significant cost reductions because as the edge area is reduced (to lessen the amount of expensive photovoltaic material required for conversion), the efficiency of the system diminishes. First generation concentrators have now approached the fundamental limits which we predict here, thus to achieve a stand-alone luminescent concentrator that enables significant cost reductions, second generation approaches are now needed. New, second generation devices are characterised by either directional emitters or photonic filters which enhance the waveguiding mechanism, allowing high efficiency and large sizes to be achieved simultaneously. Here we define the fundamental operating regime in which second generation technology must reach to surpass the limit of first generation devices. Copyright (c) 2011 John Wiley & Sons, Ltd.

Photon ratchet intermediate band solar cells

Applied Physics Letters, 2012

Quantum Cascade Photon Ratchets for Intermediate-Band Solar Cells

IEEE Journal of Photovoltaics, 2016