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

Luminescent solar concentrators (LSCs) can serve as large-area sunlight collectors for photovoltaic devices. An important LSC characteristic is a concentration factor (C), 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.

Spectro-electrochemical Probing of Intrinsic and Extrinsic Processes in Exciton Recombination in I-III-VI Nanocrystals.

Ternary CuInS nanocrystals (CIS NCs) are attracting attention as nontoxic alternatives to heavy metal-based chalcogenides for many technologically relevant applications. The photophysical processes underlying their emission mechanism are, however, still under debate. Here we address this problem by applying, for the first time, spectro-electrochemical methods to core-only CIS and core/shell CIS/ZnS NCs.

Thickness-Controlled Quasi-Two-Dimensional Colloidal PbSe Nanoplatelets.

We demonstrate controlled synthesis of discrete two-dimensional (2D) PbSe nanoplatelets (NPLs), with measurable photoluminescence, via oriented attachment directed by quantum dot (QD) surface chemistry. Halide passivation is critical to the growth of these (100) face-dominated NPLs, as corroborated by density functional theory studies. PbCl moieties attached to the (111) and (110) of small nanocrystals form interparticle bridges, aligning the QDs and leading to attachment.

Single-Particle Ratiometric Pressure Sensing Based on "Double-Sensor" Colloidal Nanocrystals.

Ratiometric pressure sensitive paints (r-PSPs) are all-optical probes for monitoring oxygen flows in the vicinity of complex or miniaturized surfaces. They typically consist of a porous binder embedding mixtures of a reference and a sensor chromophore exhibiting oxygen-insensitive and oxygen-responsive luminescence, respectively. Here, we demonstrate the first example of an r-PSP based on a single two-color emitter that removes limitations of r-PSPs based on chromophore mixtures such as different temperature dependencies of the two chromophores, cross-readout between the reference and sensor signals and phase segregation.

Auger Up-Conversion of Low-Intensity Infrared Light in Engineered Quantum Dots.

One source of efficiency losses in photovoltaic cells is their transparency toward solar photons with energies below the band gap of the absorbing layer. This loss can be reduced using a process of up-conversion whereby two or more sub-band-gap photons generate a single above-gap exciton. Traditional approaches to up-conversion, such as nonlinear two-photon absorption (2PA) or triplet fusion, suffer from low efficiency at solar light intensities, a narrow absorption bandwidth, nonoptimal absorption energies, and difficulties for implementing in practical devices.

Shape-Controlled Narrow-Gap SnTe Nanostructures: From Nanocubes to Nanorods and Nanowires.

The rational design and synthesis of narrow-gap colloidal semiconductor nanocrystals (NCs) is an important step toward the next generation of solution-processable photovoltaics, photodetectors, and thermoelectric devices. SnTe NCs are particularly attractive as a Pb-free alternative to NCs of narrow-gap lead chalcogenides. Previous synthetic efforts on SnTe NCs have focused on spherical nanoparticles.

Carrier multiplication detected through transient photocurrent in device-grade films of lead selenide quantum dots.

In carrier multiplication, the absorption of a single photon results in two or more electron-hole pairs. Quantum dots are promising materials for implementing carrier multiplication principles in real-life technologies. So far, however, most of research in this area has focused on optical studies of solution samples with yet to be proven relevance to practical devices.

Highly efficient large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dots.

Luminescent solar concentrators serving as semitransparent photovoltaic windows could become an important element in net zero energy consumption buildings of the future. Colloidal quantum dots are promising materials for luminescent solar concentrators as they can be engineered to provide the large Stokes shift necessary for suppressing reabsorption losses in large-area devices. Existing Stokes-shift-engineered quantum dots allow for only partial coverage of the solar spectrum, which limits their light-harvesting ability and leads to colouring of the luminescent solar concentrators, complicating their use in architecture.

Synthetic Conditions for High-Accuracy Size Control of PbS Quantum Dots.

Decreasing the variability in quantum dot (QD) syntheses is desirable for better uniformity of samples for use in QD-based studies and applications. Here we report a highly reproducible linear relationship between the concentration of ligand (in this case oleic acid, OA) and the lowest energy exciton peak position (nm) of the resulting PbS QDs for various hot-injection temperatures. Thus, for a given injection temperature, the size of the PbS QD product is purely controlled by the amount of OA.

Preparation of Cd/Pb Chalcogenide Heterostructured Janus Particles via Controllable Cation Exchange.

We developed a strategy for producing quasi-spherical nanocrystals of anisotropic heterostructures of Cd/Pb chalcogenides. The nanostructures are fabricated via a controlled cation exchange reaction where the Cd(2+) cation is exchanged for the Pb(2+) cation. The cation exchange reaction is thermally activated and can be controlled by adjusting the reaction temperature or time.

Soft contact transplanted nanocrystal quantum dots for light-emitting diodes: effect of surface energy on device performance.

To realize the full-color displays using colloidal nanocrystal quantum dot (QD)-based light emitting diodes (QLEDs), the emissive QD layer should be patterned to red (R), green (G), and blue (B) subpixels on a micrometer scale by the solution process. Here, we introduced a soft contact QD-transplanting technique onto the vacuum-deposited small molecules without pressure to pattern the QD layer without any damage to the prior organic layers. We examined the patternability of QDs by studying the surface properties of various organic layers systematically.

Tuning carrier mobilities and polarity of charge transport in films of CuInSe(x)S(2-x) quantum dots.

CuInSe(x)S(2-x) quantum dot field-effect transistors show p-type, n-type, and ambipolar behaviors with carrier mobilities up to 0.03 cm(2) V(-1) s(-1). Although some design rules from studies of cadmium and lead containing quantum dots can be applied, remarkable differences are observed including a strong gating effect in as-synthesized nanocyrstals with long ligands.

Charge generation in PbS quantum dot solar cells characterized by temperature-dependent steady-state photoluminescence.

Charge-carrier generation and transport within PbS quantum dot (QD) solar cells is investigated by measuring the temperature-dependent steady-state photoluminescence (PL) concurrently during in situ current-voltage characterization. We first compare the temperature-dependent PL quenching for PbS QD films where the PbS QDs retain their original oleate ligand to that of PbS QDs treated with 1,2-ethanedithiol (EDT), producing a conductive QD layer, either on top of glass or on a ZnO nanocrystal film. We then measure and analyze the temperature-dependent PL in a completed QD-PV architecture with the structure Al/MoO3/EDT-PbS/ZnO/ITO/glass, collecting the PL and the current simultaneously.

Design and synthesis of heterostructured quantum dots with dual emission in the visible and infrared.

The unique optical properties exhibited by visible emitting core/shell quantum dots with especially thick shells are the focus of widespread study, but have yet to be realized in infrared (IR)-active nanostructures. We apply an effective-mass model to identify PbSe/CdSe core/shell quantum dots as a promising system for achieving this goal. We then synthesize colloidal PbSe/CdSe quantum dots with shell thicknesses of up to 4 nm that exhibit unusually slow hole intraband relaxation from shell to core states, as evidenced by the emergence of dual emission, i.

PbSe quantum dot solar cells with more than 6% efficiency fabricated in ambient atmosphere.

Colloidal quantum dots (QDs) are promising candidates for the next generation of photovoltaic (PV) technologies. Much of the progress in QD PVs is based on using PbS QDs, partly because they are stable under ambient conditions. There is considerable interest in extending this work to PbSe QDs, which have shown an enhanced photocurrent due to multiple exciton generation (MEG).

Enhanced carrier multiplication in engineered quasi-type-II quantum dots.

One process limiting the performance of solar cells is rapid cooling (thermalization) of hot carriers generated by higher-energy solar photons. In principle, the thermalization losses can be reduced by converting the kinetic energy of energetic carriers into additional electron-hole pairs via carrier multiplication (CM). While being inefficient in bulk semiconductors this process is enhanced in quantum dots, although not sufficiently high to considerably boost the power output of practical devices.

Photocharging Artifacts in Measurements of Electron Transfer in Quantum-Dot-Sensitized Mesoporous Titania Films.

Transient absorption and time-resolved photoluminescence measurements of high-performance mesoporous TiO2 photoanodes sensitized with CuInSexS2-x quantum dots reveal the importance of hole scavenging in the characterization of photoinduced electron transfer. The apparent characteristic time of this process strongly depends on the local environment of the quantum dot/TiO2 junction due to accumulation of long-lived positive charges in the quantum dots. The presence of long-lived photoexcited holes introduces artifacts due to fast positive-trion Auger decay (60 ps time constant), which can dominate electron dynamics and thus mask true electron transfer.

An integrated approach to realizing high-performance liquid-junction quantum dot sensitized solar cells.

Solution-processed semiconductor quantum dot solar cells offer a path towards both reduced fabrication cost and higher efficiency enabled by novel processes such as hot-electron extraction and carrier multiplication. Here we use a new class of low-cost, low-toxicity CuInSexS2-x quantum dots to demonstrate sensitized solar cells with certified efficiencies exceeding 5%. Among other material and device design improvements studied, use of a methanol-based polysulfide electrolyte results in a particularly dramatic enhancement in photocurrent and reduced series resistance.

Carrier Multiplication in Quantum Dots within the Framework of Two Competing Energy Relaxation Mechanisms.

The realization of high-yield, low-threshold carrier multiplication (CM) in semiconductor quantum dots (QDs) is a promising step toward third-generation photovoltaics (PV). Recent studies of QD solar cells have shown that CM can indeed produce greater-than-unity quantum efficiencies in photon-to-charge-carrier conversion, establishing the relevance of this process to practical PV technologies. While being appreciable, the reported CM yields are still not high enough for a significant increase in the power conversion efficiency over traditional bulk materials.

Heavily doped n-type PbSe and PbS nanocrystals using ground-state charge transfer from cobaltocene.

Colloidal nanocrystals (NCs) of lead chalcogenides are a promising class of tunable infrared materials for applications in devices such as photodetectors and solar cells. Such devices typically employ electronic materials in which charge carrier concentrations are manipulated through "doping;" however, persistent electronic doping of these NCs remains a challenge. Here, we demonstrate that heavily doped n-type PbSe and PbS NCs can be realized utilizing ground-state electron transfer from cobaltocene.

Carrier multiplication in semiconductor nanocrystals: influence of size, shape, and composition.

During carrier multiplication (CM), also known as multiexciton generation (MEG), absorption of a single photon produces multiple electron-hole pairs, or excitons. This process can appreciably increase the efficiency of photoconversion, which is especially beneficial in photocatalysis and photovoltaics. This Account reviews recent progress in understanding the CM process in semiconductor nanocrystals (NCs), motivated by the challenge researchers face to quickly identify candidate nanomaterials with enhanced CM.

Stoichiometry control in quantum dots: a viable analog to impurity doping of bulk materials.

A growing body of research indicates that the stoichiometry of compound semiconductor quantum dots (QDs) may offer control over the materials' optoelectronic properties in ways that could be invaluable in electronic devices. Quantum dots have been characterized as having a stoichiometric bulk-like core with a highly reconstructed surface of a more flexible composition, consisting essentially of ligated, weakly bound ions. As such, many efforts toward stoichiometry-based control over material properties have focused on ligand manipulation.

Controlled alloying of the core-shell interface in CdSe/CdS quantum dots for suppression of Auger recombination.

The influence of a CdSexS1-x interfacial alloyed layer on the photophysical properties of core/shell CdSe/CdS nanocrystal quantum dots (QDs) is investigated by comparing reference QDs with a sharp core/shell interface to alloyed structures with an intermediate CdSexS1-x layer at the core/shell interface. To fully realize the structural contrast, we have developed two novel synthetic approaches: a method for fast CdS-shell growth, which results in an abrupt core/shell boundary (no intentional or unintentional alloying), and a method for depositing a CdSexS1-x alloy layer of controlled composition onto the CdSe core prior to the growth of the CdS shell. Both types of QDs possess similar size-dependent single-exciton properties (photoluminescence energy, quantum yield, and decay lifetime).

Measurement of electronic states of PbS nanocrystal quantum dots using scanning tunneling spectroscopy: the role of parity selection rules in optical absorption.

We study the structure of electronic states in individual PbS nanocrystal quantum dots by scanning tunneling spectroscopy (STS) using one-to-two monolayer nanocrystal films treated with 1, 2-ethanedithiols (EDT). Up to six individual valence and conduction band states are resolved for a range of quantum dot sizes. The measured states' energies are in good agreement with calculations using the k · p four-band envelope function formalism.

Super-Poissonian statistics of photon emission from single CdSe-CdS core-shell nanocrystals coupled to metal nanostructures.

We demonstrate that photon antibunching observed for individual nanocrystal quantum dots (NQDs) can be transformed into photon bunching characterized by super-Poissonian statistics when they are coupled to metal nanostructures (MNs). This observation indicates that, while the quantum yield of a biexciton (Q(2X)) is lower than that of a single exciton (Q(1X)) in freestanding NQDs, Q(2X) becomes greater than Q(1X) in NQDs coupled to MNs. This unique phenomenon is attributed to metal-induced quenching with a rate that scales more slowly with exciton multiplicity than the radiative decay rate and dominates over other nonradiative decay channels for both single excitons and biexcitons.