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Arthur J Nozik - One of the best experts on this subject based on the ideXlab platform.
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the role of chromophore coupling in singlet fission
Accounts of Chemical Research, 2013Co-Authors: Justin C Johnson, Arthur J Nozik, Josef MichlAbstract:Certain organic materials can generate more than one electron-hole pair per Absorbed Photon, a property that could revolutionize the prospects for solar energy. This process, called singlet fission, is one possible “exciton multiplication” scheme that could be useful in a variety of photovoltaic device designs from dye-sensitized solar cells to solar cell bilayers to bulk heterojunctions. For such applications to be possible, however, singlet fission must occur with near perfect efficiency in compounds that also have other requisite properties such as strong visible light absorption and photostability. Many recent investigations of singlet fission have focused on crystalline polyacenes, which have been known for some time to undergo singlet fission. While these materials have promise, limitations in stability, cost, and performance may hinder practical application of polyacene solar cells, while their complex photophysics may limit our fundamental understanding of singlet fission in crystalline polyacenes...
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effect of solar concentration on the thermodynamic power conversion efficiency of quantum dot solar cells exhibiting multiple exciton generation
Journal of Physical Chemistry Letters, 2012Co-Authors: M C Hanna, Matthew C Beard, Arthur J NozikAbstract:Thermodynamic calculations show that all solar cells can convert solar Photons into electricity or fuel with higher theoretical power conversion efficiencies under concentrated sunlight. For conventional (viz, present day) single-junction solar cells that produce at most one electron–hole pair per Absorbed Photon, the theoretical increase in efficiency is relatively small (absolute values of 38% at 500× vs 33% at 1×). However, when solar concentration is combined with multiple exciton generation (MEG) in semiconductor quantum dots, the increase in theoretical power conversion efficiency is greatly enhanced. For the ideal MEG case, where the threshold for exciton multiplication is twice the bandgap, Eg, the maximum thermodynamic efficiency increases to 75% at 500×, but the optimum Eg shifts to smaller values. If Eg is fixed at the 1-sun optimal level, then the maximum theoretical efficiency still increases markedly, becoming 62% at 500× for the staircase MEG characteristic (defined as producing N electron–...
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comparing multiple exciton generation in quantum dots to impact ionization in bulk semiconductors implications for enhancement of solar energy conversion
Nano Letters, 2010Co-Authors: Matthew C Beard, Aaron G Midgett, M C Hanna, Joseph M Luther, Barbara K Hughes, Arthur J NozikAbstract:Multiple exciton generation (MEG) in quantum dots (QDs) and impact ionization (II) in bulk semiconductors are processes that describe producing more than one electron−hole pair per Absorbed Photon. We derive expressions for the proper way to compare MEG in QDs with II in bulk semiconductors and argue that there are important differences in the photophysics between bulk semiconductors and QDs. Our analysis demonstrates that the fundamental unit of energy required to produce each electron−hole pair in a given QD is the band gap energy. We find that the efficiency of the multiplication process increases by at least 2 in PbSe QDs compared to bulk PbSe, while the competition between cooling and multiplication favors multiplication by a factor of 3 in QDs. We also demonstrate that power conversion efficiencies in QD solar cells exhibiting MEG can greatly exceed conversion efficiencies of their bulk counterparts, especially if the MEG threshold energy can be reduced toward twice the QD band gap energy, which req...
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comparing multiple exciton generation in quantum dots to impact ionization in bulk semiconductors implications for enhancement of solar energy conversion
Nano Letters, 2010Co-Authors: Matthew C Beard, Aaron G Midgett, M C Hanna, Joseph M Luther, Barbara K Hughes, Arthur J NozikAbstract:Multiple exciton generation (MEG) in quantum dots (QDs) and impact ionization (II) in bulk semiconductors are processes that describe producing more than one electron-hole pair per Absorbed Photon. We derive expressions for the proper way to compare MEG in QDs with II in bulk semiconductors and argue that there are important differences in the photophysics between bulk semiconductors and QDs. Our analysis demonstrates that the fundamental unit of energy required to produce each electron-hole pair in a given QD is the band gap energy. We find that the efficiency of the multiplication process increases by at least 2 in PbSe QDs compared to bulk PbSe, while the competition between cooling and multiplication favors multiplication by a factor of 3 in QDs. We also demonstrate that power conversion efficiencies in QD solar cells exhibiting MEG can greatly exceed conversion efficiencies of their bulk counterparts, especially if the MEG threshold energy can be reduced toward twice the QD band gap energy, which requires a further increase in the MEG efficiency. Finally, we discuss the research challenges associated with achieving the maximum benefit of MEG in solar energy conversion since we show the threshold and efficiency are mathematically related.
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variations in the quantum efficiency of multiple exciton generation for a series of chemically treated pbse nanocrystal films
Nano Letters, 2009Co-Authors: Matthew C Beard, Aaron G Midgett, Randy J Ellingson, Matt Law, Octavi E Semonin, Arthur J NozikAbstract:We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (NC) films. We find that the average number of excitons produced per Absorbed Photon varies between 1.0 and 2.4 (±0.2) at a Photon energy of ∼4Eg for films consisting of 3.7 nm NCs and between 1.1 and 1.6 (±0.1) at hν ∼ 5Eg for films consisting of 7.4 nm NCs. The variations in MEG depend upon the chemical treatment used to electronically couple the NCs in each film. The single and multiexciton lifetimes also change with the chemical treatment: biexciton lifetimes increase with stronger inter-NC electronic coupling and exciton delocalization, while single exciton lifetimes decrease after most treatments relative to the same NCs in solution. Single exciton lifetimes are particularly affected by surface treatments that dope the films n-type, which we tentatively attribute to an Auger recombination process between a single exciton and an electron produced by ionization of the dopant donor. These results imply that ...
Matthew C Beard - One of the best experts on this subject based on the ideXlab platform.
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effect of solar concentration on the thermodynamic power conversion efficiency of quantum dot solar cells exhibiting multiple exciton generation
Journal of Physical Chemistry Letters, 2012Co-Authors: M C Hanna, Matthew C Beard, Arthur J NozikAbstract:Thermodynamic calculations show that all solar cells can convert solar Photons into electricity or fuel with higher theoretical power conversion efficiencies under concentrated sunlight. For conventional (viz, present day) single-junction solar cells that produce at most one electron–hole pair per Absorbed Photon, the theoretical increase in efficiency is relatively small (absolute values of 38% at 500× vs 33% at 1×). However, when solar concentration is combined with multiple exciton generation (MEG) in semiconductor quantum dots, the increase in theoretical power conversion efficiency is greatly enhanced. For the ideal MEG case, where the threshold for exciton multiplication is twice the bandgap, Eg, the maximum thermodynamic efficiency increases to 75% at 500×, but the optimum Eg shifts to smaller values. If Eg is fixed at the 1-sun optimal level, then the maximum theoretical efficiency still increases markedly, becoming 62% at 500× for the staircase MEG characteristic (defined as producing N electron–...
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multiple exciton generation in semiconductor quantum dots
Journal of Physical Chemistry Letters, 2011Co-Authors: Matthew C BeardAbstract:Multiple exciton generation in quantum dots (QDs) has been intensively studied as a way to enhance solar energy conversion by utilizing the excess energy in the Absorbed Photons. Among other useful properties, quantum confinement can both increase Coulomb interactions that drive the MEG process and decrease the electron–phonon coupling that cools hot excitons in bulk semiconductors. However, variations in the reported enhanced quantum yields (QYs) have led to disagreements over the role that quantum confinement plays. The enhanced yield of excitons per Absorbed Photon is deduced from a dynamical signature in the transient absorption or transient photoluminescence and is ascribed to the creation of biexcitons. Extraneous effects such as photocharging are partially responsible for the observed variations. When these extraneous effects are reduced, the MEG efficiency, defined in terms of the number of additional electron–hole pairs produced per additional band gap of Photon excitation, is about two times bet...
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comparing multiple exciton generation in quantum dots to impact ionization in bulk semiconductors implications for enhancement of solar energy conversion
Nano Letters, 2010Co-Authors: Matthew C Beard, Aaron G Midgett, M C Hanna, Joseph M Luther, Barbara K Hughes, Arthur J NozikAbstract:Multiple exciton generation (MEG) in quantum dots (QDs) and impact ionization (II) in bulk semiconductors are processes that describe producing more than one electron−hole pair per Absorbed Photon. We derive expressions for the proper way to compare MEG in QDs with II in bulk semiconductors and argue that there are important differences in the photophysics between bulk semiconductors and QDs. Our analysis demonstrates that the fundamental unit of energy required to produce each electron−hole pair in a given QD is the band gap energy. We find that the efficiency of the multiplication process increases by at least 2 in PbSe QDs compared to bulk PbSe, while the competition between cooling and multiplication favors multiplication by a factor of 3 in QDs. We also demonstrate that power conversion efficiencies in QD solar cells exhibiting MEG can greatly exceed conversion efficiencies of their bulk counterparts, especially if the MEG threshold energy can be reduced toward twice the QD band gap energy, which req...
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comparing multiple exciton generation in quantum dots to impact ionization in bulk semiconductors implications for enhancement of solar energy conversion
Nano Letters, 2010Co-Authors: Matthew C Beard, Aaron G Midgett, M C Hanna, Joseph M Luther, Barbara K Hughes, Arthur J NozikAbstract:Multiple exciton generation (MEG) in quantum dots (QDs) and impact ionization (II) in bulk semiconductors are processes that describe producing more than one electron-hole pair per Absorbed Photon. We derive expressions for the proper way to compare MEG in QDs with II in bulk semiconductors and argue that there are important differences in the photophysics between bulk semiconductors and QDs. Our analysis demonstrates that the fundamental unit of energy required to produce each electron-hole pair in a given QD is the band gap energy. We find that the efficiency of the multiplication process increases by at least 2 in PbSe QDs compared to bulk PbSe, while the competition between cooling and multiplication favors multiplication by a factor of 3 in QDs. We also demonstrate that power conversion efficiencies in QD solar cells exhibiting MEG can greatly exceed conversion efficiencies of their bulk counterparts, especially if the MEG threshold energy can be reduced toward twice the QD band gap energy, which requires a further increase in the MEG efficiency. Finally, we discuss the research challenges associated with achieving the maximum benefit of MEG in solar energy conversion since we show the threshold and efficiency are mathematically related.
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variations in the quantum efficiency of multiple exciton generation for a series of chemically treated pbse nanocrystal films
Nano Letters, 2009Co-Authors: Matthew C Beard, Aaron G Midgett, Randy J Ellingson, Matt Law, Octavi E Semonin, Arthur J NozikAbstract:We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (NC) films. We find that the average number of excitons produced per Absorbed Photon varies between 1.0 and 2.4 (±0.2) at a Photon energy of ∼4Eg for films consisting of 3.7 nm NCs and between 1.1 and 1.6 (±0.1) at hν ∼ 5Eg for films consisting of 7.4 nm NCs. The variations in MEG depend upon the chemical treatment used to electronically couple the NCs in each film. The single and multiexciton lifetimes also change with the chemical treatment: biexciton lifetimes increase with stronger inter-NC electronic coupling and exciton delocalization, while single exciton lifetimes decrease after most treatments relative to the same NCs in solution. Single exciton lifetimes are particularly affected by surface treatments that dope the films n-type, which we tentatively attribute to an Auger recombination process between a single exciton and an electron produced by ionization of the dopant donor. These results imply that ...
Yutaka Moritomo - One of the best experts on this subject based on the ideXlab platform.
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temperature independent carrier formation dynamics in bulk heterojunction
Applied Physics Express, 2015Co-Authors: Kouhei Yonezawa, Takeshi Yasuda, Yutaka MoritomoAbstract:We investigated the effects of temperature on the carrier formation dynamics in a small-molecular blend film, 2,5-di-(2-ethylhexyl)-3,6-bis-(5''-n-hexy-[2,2',5',2'']terthiophen-5-yl)-pyrrolo[3,4-c]pyrrolo-1,4-dione (SMDPPEH)/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM). We spectroscopically determined the absolute numbers of donor () and acceptor () excitons per Absorbed Photon as functions of the delay time (t), in addition to the relative number of donor carries (). We found that the carrier formation dynamics is independent of temperature at 300 and 80 K: the carrier formation time (τrise = 0.4 ps) is much faster than the decay time (τdecay ≈ 2.5 ps) of donor excitons. The temperature independence strongly suggests that only excitons created near the donor–acceptor interface contribute to the carrier formation.
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temperature effects on carrier formation dynamics in organic heterojunction solar cell
Applied Physics Letters, 2015Co-Authors: Kouhei Yonezawa, Takeshi Yasuda, Yutaka MoritomoAbstract:The femto-second time-resolved spectroscopy was performed on the heterojunction (HJ) solar cell which consists of prototypical low-band gap donor (D), poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5- b′] dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl]] (PTB7), and the C70 acceptor (A). We spectroscopically determined the absolute number of donor exciton ( nD*), acceptor exciton ( nA*), and carrier ( nD+) per an Absorbed Photon against the delay time (t). At 300 K, we found that the decay time ( τdecay = 3.5 ps) of A* is much longer than the carrier formation time ( τform = 1.1 ps), indicating that the late A* component does not contribute to the carrier formation process. The elongated τform (=1.5 ps) at 80 K is ascribed to the exciton migration process, not to the exciton dissociation process.
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dynamics of charge transfer pairs in the cyano bridged co2 fe3 transition metal compound
Physical Review B, 2008Co-Authors: Hayato Kamioka, Yutaka Moritomo, Wataru Kosaka, Shinichi OhkoshiAbstract:Dynamics of the charge-transfer (CT) pairs has been investigated for the cyano-bridged ${\mathrm{Co}}^{2+}\text{\ensuremath{-}}{\mathrm{Fe}}^{3+}$ particles grown in a hydrophilic cavities of Nafion 117 film. We decomposed the differential absorption spectra into the fast and slow components. We ascribed the fast and slow components to the Frank-Condon state and the CT $({\mathrm{Co}}^{3+}\text{\ensuremath{-}}{\mathrm{Fe}}^{2+})$ pairs with lattice relaxation, respectively. We found that the lifetime ${\ensuremath{\tau}}_{\mathrm{CT}}$ of the pairs at $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ increases with the Absorbed Photon density $n$ from ${\ensuremath{\tau}}_{\mathrm{CT}}\ensuremath{\approx}2\phantom{\rule{0.3em}{0ex}}\mathrm{ns}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}\ensuremath{\approx}6\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$, reflecting the attractive interaction between the pairs.
Mischa Bonn - One of the best experts on this subject based on the ideXlab platform.
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photoexcitation cascade and multiple hot carrier generation in graphene
International Quantum Electronics Conference, 2013Co-Authors: Klaasjan Tielrooij, L S Levitov, Justin C W Song, Soren A Jensen, Alba Centeno, Amaia Pesquera, Zurutuza A Elorza, Mischa Bonn, Frank H L KoppensAbstract:For many optoelectronic applications, such as photodetection and light harvesting, it is highly desirable to identify materials in which an Absorbed Photon is efficiently converted to electronic excitations. The unique properties of graphene, such as its gapless band structure, flat absorption spectrum and strong electron-electron interactions, make it a highly promising material for efficient broadband Photon-electron conversion [1]. Indeed, recent theoretical work has anticipated that in graphene multiple electron-hole pairs can be created from a single Absorbed Photon during energy relaxation of the primary photoexcited e-h pair [2]. A photoexcited carrier relaxes initially trough two competing pathways: carrier-carrier scattering and optical phonon emission. In the former process the energy of photoexcited carriers remains in the electron system, being transferred to secondary electrons that gain energy (become hot), whereas in the phonon emission process the energy is lost to the lattice as heat. While recent experiments have shown that photoexcitation of graphene can generate hot carriers [3], it remains unknown how efficient this process is with respect to optical phonon emission.
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carrier multiplication in bulk indium nitride
Applied Physics Letters, 2012Co-Authors: Soren A Jensen, Jan Versluis, Enrique Canovas, J J H Pijpers, I R Sellers, Mischa BonnAbstract:Carrier multiplication (CM) is the process of generating multiple electron-hole pairs from one Absorbed Photon. Narrow-gap InN is a material that has been proposed for achieving efficient CM. We quantify the CM efficiency in bulk InN using terahertz time-domain spectroscopy. While the CM onset occurs at relatively low Photon energies in InN (1.7 ± 0.2 eV), corresponding to 2.7 ± 0.3 times its bandgap, the excitation efficiency above the onset increases linearly with a slope of only ∼13%/Eg. Based on these numbers, the efficiency increase of an InN based photovoltaic device owing to CM is limited to maximum 1% point.
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carrier multiplication in bulk and nanocrystalline semiconductors mechanism efficiency and interest for solar cells
Physical Review B, 2010Co-Authors: C Delerue, J J H Pijpers, Guy Allan, Mischa BonnAbstract:Carrier multiplication (CM), the possibility to generate more than one exciton in a semiconductor quantum dot (QD) after absorption of a single Photon has been intensely debated in recent years. Following on previous theoretical and experimental work, we report here that: (1) although the CM factor (i.e., number of generated Photons per Absorbed Photon) at a given Photon energy is higher in bulk than in QDs of the same material [Pijpers et al., Nature Phys. 5, 811 (2009)], the energy efficiency (the relative fraction of the Photon energy that is transformed into excitons rather than heat) is higher in QDs; (2) for the same $\ensuremath{\sim}1.2\text{ }\text{eV}$ band gap, CM is more efficient in PbSe QDs than in bulk silicon; (3) nonetheless, the efficiency of solar cells based on PbSe QDs is not significantly enhanced by CM compared to a bulk silicon-based device.
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interchain effects in the ultrafast photophysics of a semiconducting polymer thz time domain spectroscopy of thin films and isolated chains in solution
Physical Review B, 2005Co-Authors: Euan Hendry, Mattijs Koeberg, Juleon M Schins, Hankwang Nienhuys, Villy Sundstrom, Laurens D A Siebbeles, Mischa BonnAbstract:We compare the generation and decay dynamics of charges and excitons in a model polymer semiconductor (MEH-PPV) in solution and drop-cast thin films, by recording the sub-ps transient complex conductivity using THz time-domain spectroscopy. The results show that the quantum efficiency of charge generation is two orders of magnitude smaller in solution (~10–5) than in the solid film (~10–3). The proximity of neighboring chains in the films apparently facilitates (hot) exciton dissociation, presumably by allowing the electron and hole to separate on different polymer strands. For both samples, photoexcitation leads to the predominant formation of bound charge pairs (excitons) that can be detected through their polarizability. Surprisingly, the polarizability per Absorbed Photon is a factor of 3 larger in solution than in the film, suggesting that interchain interactions in the film do not result in a substantial delocalization of the exciton wave function.
Alexander Wokaun - One of the best experts on this subject based on the ideXlab platform.
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latioxny thin film model systems for photocatalytic water splitting physicochemical evolution of the solid liquid interface and the role of the crystallographic orientation
arXiv: Materials Science, 2019Co-Authors: Markus Pichler, Fatima Haydous, Helena Tellez, John Druce, Emiliana Fabbri, Mario El Kazzi, M Dobeli, Silviya Ninova, Ulrich Aschauer, Alexander WokaunAbstract:The size of the band gap and the energy position of the band edges make several oxynitride semiconductors promising candidates for efficient hydrogen and oxygen production under solar light illumination. The intense research efforts dedicated to oxynitride materials have unveiled the majority of their most important properties. However, two crucial aspects have received much less attention. One is the critical issue of the compositional/structural surface modifications occurring during operation and how these affect the photoelectrochemical performance. The second concerns the relation between the electrochemical response and the crystallographic surface orientation of the oxynitride semiconductor. These are indeed topics of fundamental importance since it is exactly at the surface where the visible light-driven electrochemical reaction takes place. In contrast to conventional powder samples, thin films represent the best model system for these investigations. This study reviews current state-of-the-art of oxynitride thin film fabrication and characterization before focusing on LaTiO2N selected as representative photocatalyst. We report the investigation of the initial physicochemical evolution of the surface. Then we show that, after stabilization, the Absorbed Photon-to-current conversion efficiency of epitaxial thin films can differ by about 50% for different crystallographic surface orientations and be up to 5 times larger than for polycrystalline samples.
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latioxny thin film model systems for photocatalytic water splitting physicochemical evolution of the solid liquid interface and the role of the crystallographic orientation
Advanced Functional Materials, 2017Co-Authors: Markus Pichler, Fatima Haydous, Helena Tellez, John Druce, Emiliana Fabbri, Mario El Kazzi, M Dobeli, Silviya Ninova, Ulrich Aschauer, Alexander WokaunAbstract:The size of the band gap and the energy position of the band edges make several oxynitride semiconductors promising candidates for efficient hydrogen and oxygen production under solar light illumination. Intense research efforts dedicated to oxynitride materials have unveiled the majority of their most important properties. However, two crucial aspects have received much less attention: One is the critical issue of compositional/structural surface modifications that occur during operation and how these affect photoelectrochemical performance. The second concerns the relation between electrochemical response and the crystallographic surface orientation of the oxynitride semiconductor. These are indeed topics of fundamental importance, since it is exactly at the surface where the visible-light-driven electrochemical reaction takes place. In contrast to conventional powder samples, thin films represent the best model system for these investigations. This study reviews current state-of-the-art oxynitride thin film fabrication and characterization, before focusing on LaTiO2N, selected as a representative photocatalyst. An investigation of the initial physicochemical evolution of the surface is reported. Then, it is shown that after stabilization the Absorbed Photon-to-current conversion efficiency of epitaxial thin films can differ by about 50% for different crystallographic surface orientations, and be up to 5 times larger than for polycrystalline samples.
