Performance Limits of Luminescent Solar Concentrators Tested with Seed/Quantum-Well Quantum Dots in a Selective-Reflector-Based Optical Cavity

Song, Hyung-Jun,Jeong, Byeong Guk,Lim, Jaehoon,Lee, Doh C.,Bae, Wan Ki,Klimov, Victor I. American Chemical Society 2018 NANO LETTERS Vol.18 No.1

<P>Luminescent solar concentrators (LSCs) can serve as large-area sunlight collectors for photovoltaic devices. An important LSC characteristic is a concentration factor (<I>C</I>), which is defined as the ratio of the output and the input photon flux densities. This parameter can be also thought of as an effective enlargement factor of a solar cell active area. On the basis of thermodynamic considerations, the <I>C</I>-factor can reach extremely high values that exceed those accessible with traditional concentrating optics. In reality, however, the best reported values of <I>C</I> are around 30. Here we demonstrate that using a new type of high-emissivity quantum dots (QDs) incorporated into a specially designed cavity, we are able to achieve the <I>C</I> of ∼62 for spectrally integrated emission and ∼120 for the red portion of the photoluminescence spectrum. The key feature of these QDs is a seed/quantum-well/thick-shell design, which allows for obtaining a high emission quantum yield (>95%) simultaneously with a large LSC quality factor (<I>Q</I><SUB>LSC</SUB> of ∼100) defined as the ratio of absorption coefficients at the wavelengths of incident and reemitted light. By incorporating the QDs into a specially designed cavity equipped with a top selective reflector (a Bragg mirror or a thin silver film), we are able to effectively recycle reemitted light achieving light trapping coefficients of ∼85%. The observed performance of these devices is in remarkable agreement with analytical modeling, which allows us to project that the applied approach should allow one to boost the spectrally integrated concentration factors to more than 100 by further improving light trapping and/or increasing <I>Q</I><SUB>LSC</SUB>.</P> [FIG OMISSION]</BR>

Prospects of Nanoscience with Nanocrystals

Kovalenko, Maksym V.,Manna, Liberato,Cabot, Andreu,Hens, Zeger,Talapin, Dmitri V.,Kagan, Cherie R.,Klimov, Victor I.,Rogach, Andrey L.,Reiss, Peter,Milliron, Delia J.,Guyot-Sionnnest, Philippe,Konstan American Chemical Society 2015 ACS NANO Vol.9 No.2

<P>Colloidal nanocrystals (NCs, <I>i.e.</I>, crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today’s strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing <I>in situ</I> characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2015/ancac3.2015.9.issue-2/nn506223h/production/images/medium/nn-2014-06223h_0020.gif'></P>

Effect of the Core/Shell Interface on Auger Recombination Evaluated by Single-Quantum-Dot Spectroscopy

Park, Young-Shin,Bae, Wan Ki,Padilha, Lazaro A.,Pietryga, Jeffrey M.,Klimov, Victor I. American Chemical Society 2014 NANO LETTERS Vol.14 No.2

<P>Previous single-particle spectroscopic studies of colloidal quantum dots have indicated a significant spread in biexciton lifetimes across an ensemble of nominally identical nanocrystals. It has been speculated that in addition to dot-to-dot variation in physical dimensions, this spread is contributed to by variations in the structure of the quantum dot interface, which controls the shape of the confinement potential. Here, we directly evaluate the effect of the composition of the core–shell interface on single- and multiexciton dynamics via side-by-side measurements of individual core–shell CdSe/CdS nanocrystals with a sharp versus smooth (graded) interface. To realize the latter type of structures we incorporate a CdSe<SUB><I>x</I></SUB>S<SUB>1–<I>x</I></SUB> alloy layer of controlled composition and thickness between the CdSe core and the CdS shell. We observe that while having essentially no effect on single-exciton decay, the interfacial alloy layer leads to a systematic increase in biexciton lifetimes, which correlates with the increase in the biexciton emission efficiency, as inferred from two-photon correlation measurements. These observations provide direct experimental evidence that in addition to the size of the quantum dot, its interfacial properties also significantly affect the rate of Auger recombination, which governs biexciton decay. These findings help rationalize previous observations of a significant heterogeneity in the biexciton lifetimes across similarly sized quantum dots and should facilitate the development of “Auger-recombination-free” colloidal nanostructures for a range of applications from lasers and light-emitting diodes to photodetectors and solar cells.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2014/nalefd.2014.14.issue-2/nl403289w/production/images/medium/nl-2013-03289w_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl403289w'>ACS Electronic Supporting Info</A></P>

Temperature and Magnetic-Field Dependence of Radiative Decay in Colloidal Germanium Quantum Dots

Robel, Istvá,n,Shabaev, Andrew,Lee, Doh C.,Schaller, Richard D.,Pietryga, Jeffrey M.,Crooker, Scott A.,L. Efros, Alexander,Klimov, Victor I. American Chemical Society 2015 Nano letters Vol.15 No.4

<P>We conduct spectroscopic and theoretical studies of photoluminescence (PL) from Ge quantum dots (QDs) fabricated via colloidal synthesis. The dynamics of late-time PL exhibit a pronounced dependence on temperature and applied magnetic field, which can be explained by radiative decay involving two closely spaced, slowly emitting exciton states. In 3.5 nm QDs, these states are separated by ∼1 meV and are characterized by ∼82 μs and ∼18 μs lifetimes. By using a four-band formalism, we calculate the fine structure of the indirect band-edge exciton arising from the electron–hole exchange interaction and the Coulomb interaction of the Γ-point hole with the anisotropic charge density of the <I>L</I>-point electron. The calculations suggest that the observed PL dynamics can be explained by phonon-assisted recombination of excitons thermally distributed between the lower-energy “dark” state with the momentum projection <I>J</I> = ± 2 and a higher energy “bright” state with <I>J</I> = ± 1. A fairly small difference between lifetimes of these states is due to their mixing induced by the exchange term unique to crystals with a highly symmetric cubic lattice such as Ge.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2015/nalefd.2015.15.issue-4/acs.nanolett.5b00344/production/images/medium/nl-2015-003445_0005.gif'></P>

A Reduction Pathway in the Synthesis of PbSe Nanocrystal Quantum Dots

주진,Victor I. Klimov,Jeffre J. M. Pietryga,John A. Mcguire,Sea Ho Jeon,Darrick J. Williams,Hsing Lin Wang 한국고분자학회 2009 한국고분자학회 학술대회 연구논문 초록집 Vol.2009 No.10

Auger Recombination of Biexcitons and Negative and Positive Trions in Individual Quantum Dots

Park, Young-Shin,Bae, Wan Ki,Pietryga, Jeffrey M.,Klimov, Victor I. American Chemical Society 2014 ACS NANO Vol.8 No.7

<P>Charged exciton states commonly occur both in spectroscopic studies of quantum dots (QDs) and during operation of QD-based devices. The extra charge added to the neutral exciton modifies its radiative decay rate and also opens an additional nonradiative pathway associated with an Auger process whereby the recombination energy of an exciton is transferred to the excess charge. Here we conduct single-dot spectroscopic studies of Auger recombination in thick-shell (“giant”) CdSe/CdS QDs with and without an interfacial alloy layer using time-tagged, time-correlated single-photon counting. In photoluminescence (PL) intensity trajectories of some of the dots, we resolve three distinct states of different emissivities (“bright”, “gray”, and “dark”) attributed, respectively, to the neutral exciton and negative and positive trions. Simultaneously acquired PL lifetime trajectories indicate that the positive trion is much shorter lived than the negative trion, which can be explained by a high density of valence band states and a small hole localization radius (defined by the QD core size), factors that favor an Auger process involving intraband excitation of a hole. A comparison of trion and biexciton lifetimes suggests that the biexciton Auger decay can be treated in terms of a superposition of two independent channels associated with positive- and negative-trion pathways. The resulting interdependence between Auger time constants might simplify the studies of multicarrier recombination by allowing one, for example, to infer Auger lifetimes of trions of one sign based on the measurements of biexciton decay and dynamics of the trions of the opposite sign or, alternatively, estimate the biexciton lifetime based on studies of trion dynamics.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2014/ancac3.2014.8.issue-7/nn5023473/production/images/medium/nn-2014-023473_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn5023473'>ACS Electronic Supporting Info</A></P>

Influence of Shell Thickness on the Performance of Light-Emitting Devices Based on Type-I Core/Shell Heterostructure Quantum Dots

임재훈,정병국,박명진,Jeffrey M. Pietryga,김재경,박영신,Victor I. Klimov,이창희,배완기,이도창 한국고분자학회 2014 한국고분자학회 학술대회 연구논문 초록집 Vol.2014 No.10

Dual-Color Electroluminescence from Dot-in-Bulk Nanocrystals

Brovelli, Sergio,Bae, Wan Ki,Galland, Christophe,Giovanella, Umberto,Meinardi, Francesco,Klimov, Victor I. American Chemical Society 2014 NANO LETTERS Vol.14 No.2

<P>The emission color from colloidal semiconductor nanocrystals (NCs) is usually tuned through control of particle size, while multicolor emission is obtained by mixing NCs of different sizes within an emissive layer. Here, we demonstrate that recently introduced “dot-in-bulk” (DiB) nanocrystals can emit two-color light under both optical excitation and electrical injection. We show that the effective emission color can be controlled by adjusting the relative amplitudes of the core and shell emission bands via the intensity of optical excitation or applied bias in the cases of photoluminescence (PL) and electroluminescence (EL), respectively. To investigate the role of nonradiative carrier losses due to trapping at intragap states, we incorporate DiB NCs into functional light-emitting diodes and study their PL as a function of applied bias below the EL excitation threshold. We show that voltage-dependent changes in core and shell emissions are not due to the applied electric field but rather arise from the transfer of charges between the anode and the NC intragap trap sites. The changes in the occupancy of trap states can be described in terms of the raising (lowering) of the Fermi level for reverse (direct) bias. We find that the applied voltage affects the overall PL intensity primarily via the electron-trapping channel while bias-induced changes in hole-trapping play a less significant role, limited to a weak effect on core emission.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2014/nalefd.2014.14.issue-2/nl403478s/production/images/medium/nl-2013-03478s_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl403478s'>ACS Electronic Supporting Info</A></P>

Spectroscopic and Device Aspects of Nanocrystal Quantum Dots

Pietryga, Jeffrey M.,Park, Young-Shin,Lim, Jaehoon,Fidler, Andrew F.,Bae, Wan Ki,Brovelli, Sergio,Klimov, Victor I. American Chemical Society 2016 Chemical reviews Vol.116 No.18

<P>The field of nanocrystal quantum dots (QDs) is already more than 30 years old, and yet continuing interest in these structures is driven by both the fascinating physics emerging from strong quantum confinement of electronic excitations, as well as a large number of prospective applications that could benefit from the tunable properties and amenability toward solution-based processing of these materials. The focus of this review is on recent advances in nanocrystal research related to applications of QD materials in lasing, light-emitting diodes (LEDs), and solar energy conversion. A specific underlying theme is innovative concepts for tuning the properties of QDs beyond what is possible via traditional size manipulation, particularly through heterostructuring. Examples of such advanced control of nanocrystal functionalities include the following: interface engineering for suppressing Auger recombination in the context of QD LEDs and lasers; Stokes-shift engineering for applications in large-area luminescent solar concentrators; and control of intraband relaxation for enhanced carrier multiplication in advanced QD photovoltaics. We examine the considerable recent progress on these multiple fronts of nanocrystal research, which has resulted in the first commercialized QD technologies. These successes explain the continuing appeal of this field to a broad community of scientists and engineers, which in turn ensures even more exciting results to come from future exploration of this fascinating class of materials.</P>

Colloidal Spherical Quantum Wells with Near-Unity Photoluminescence Quantum Yield and Suppressed Blinking

Jeong, Byeong Guk,Park, Young-Shin,Chang, Jun Hyuk,Cho, Ikjun,Kim, Jai Kyeong,Kim, Heesuk,Char, Kookheon,Cho, Jinhan,Klimov, Victor I.,Park, Philip,Lee, Doh C.,Bae, Wan Ki American Chemical Society 2016 ACS NANO Vol.10 No.10

<P>Thick inorganic shells endow colloidal nanocrystals (NCs) with enhanced photochemical stability and suppression of photoluminescence intermittency (also known as blinking). However, the progress of using thick-shell 100 90 heterostructure NCs in applications has been limited due to the low photoluminescence quantum yield (PL QY <= 60%) at room temperature. Here, we demonstrate thick-shell NCs with CdS/CdSe/CdS seed/spherical quantum well/shell (SQW) geometry that exhibit near-unity PL QY at room temperature and suppression of blinking. In SQW NCs, the lattice mismatch is diminished between the emissive CdSe layer and the surrounding CdS layers as a result of coherent strain, which suppresses the formation of misfit defects and consequently permits, similar to 100% PL QY for SQW NCs with a thick CdS shell (>5 nm). High PL QY of thick-shell SQW NCs is preserved even in concentrated dispersion and in film under thermal stress, which makes them promising candidates for applications in solid-state concentrators.</P>