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We report how the direction of quantum dot (QD) lasing can be engineered by exploiting high-symmetry points in plasmonic nanoparticle (NP) lattices. The nanolaser architecture consists of CdSe-CdS core-shell QD layers conformally coated on two-dimensional square arrays of Ag NPs. Using waveguide-surface lattice resonances (W-SLRs) near the Δ point in the Brillouin zone as optical feedback, we achieved lasing from the gain in CdS shells at off-normal emission angles. Changing the periodicity of the plasmonic lattices enables other high-symmetry points (Γ or M) of the lattice to overlap with the QD shell emission, which facilitates tuning of the lasing direction. We also increased the thickness of the QD layer to introduce higher-order W-SLR modes with additional avoided crossings in the band structure, which expands the selection of cavity modes for any desired lasing emission angle.

III-V colloidal quantum dots (CQDs) are promising semiconducting materials for optoelectronic applications; however, their strong covalent character requires a distinct approach to surface management compared with widely investigated II-VI and IV-VI CQDs-dots, which by contrast are characterized by an ionic nature. Here we show stoichiometric reconstruction in InAs CQDs by ligand exchange. In particular, we find that indium-carboxylate ligands, which passivate as-synthesized InAs CQDs and are responsible for In-rich surfaces, can be replaced by anionic ligands such as thiols. This enables the production of inks consisting of balanced-stoichiomety CQDs; this is distinct from what is observed in II-VI and IV-VI CQDs, in which thiols replace carboxylates. The approach enables the implementation of InAs CQD solids as the active layer in photodiode detectors that exhibit an external quantum efficiency of 36% at 930 nm and a photoresponse time of 65 ns, which is 4 times shorter than that of reference PbS CQD devices.

Increasing the power conversion efficiency (PCE) of colloidal quantum dot (CQD) solar cells has relied on improving the passivation of CQD surfaces, enhancing CQD coupling and charge transport, and advancing device architecture. The presence of hydroxyl groups on the nanoparticle surface, as well as dimers-fusion between CQDs-has been found to be the major source of trap states, detrimental to optoelectronic properties and device performance. Here, we introduce a CQD reconstruction step that decreases surface hydroxyl groups and dimers simultaneously. We explored the dynamic interaction of charge carriers between band-edge states and trap states in CQDs using time-resolved spectroscopy, showing that trap to ground-state recombination occurs mainly from surface defects in coupled CQD solids passivated using simple metal halides. Using CQD reconstruction, we demonstrate a 60% reduction in trap density and a 25% improvement in charge diffusion length. These translate into a PCE of 12.5% compared to 10.9% for control CQDs.

Stokes shift, an energy difference between the excitonic absorption and emission, is a property of colloidal quantum dots (CQDs) typically ascribed to splitting between dark and bright excitons. In some materials, e.g., PbS, CuInS2, and CdHgTe, a Stokes shift of up to 200 meV is observed, substantially larger than the estimates of dark–bright state splitting or vibronic relaxations. The shift origin remains highly debated because contradictory signatures of both surface and bulk character were reported for the Stokes-shifted electronic state. Here, we show that the energy transfer among CQDs in a polydispersed ensemble in solution suffices to explain the excess Stokes shift. This energy transfer is primarily due to CQD aggregation and can be substantially eliminated by extreme dilution, higher-viscosity solvent, or better-dispersed colloids. Our findings highlight that ensemble polydispersity remains the primary source of the Stokes shift in CQDs in solution, propagating into the Stokes shift in films and...

Colloidal quantum dot solids combine convenient solution-processing with quantum size effect tuning, offering avenues to high-efficiency multijunction cells based on a single materials synthesis and processing platform. The highest-performing colloidal quantum dot rectifying devices reported to date have relied on a junction between a quantum-tuned absorber and a bulk material (e.g., TiO(2)); however, quantum tuning of the absorber then requires complete redesign of the bulk acceptor, compromising the benefits of facile quantum tuning. Here we report rectifying junctions constructed entirely using inherently band-aligned quantum-tuned materials. Realizing these quantum junction diodes relied upon the creation of an n-type quantum dot solid having a clean bandgap. We combine stable, chemically compatible, high-performance n-type and p-type materials to create the first quantum junction solar cells. We present a family of photovoltaic devices having widely tuned bandgaps of 0.6-1.6 eV that excel where conventional quantum-to-bulk devices fail to perform. Devices having optimal single-junction bandgaps exhibit certified AM1.5 solar power conversion efficiencies of 5.4%. Control over doping in quantum solids, and the successful integration of these materials to form stable quantum junctions, offers a powerful new degree of freedom to colloidal quantum dot optoelectronics.

Colloidal-quantum-dot (CQD) optoelectronics offer a compelling combination of solution processing and spectral tunability through quantum size effects. So far, CQD solar cells have relied on the use of organic ligands to passivate the surface of the semiconductor nanoparticles. Although inorganic metal chalcogenide ligands have led to record electronic transport parameters in CQD films, no photovoltaic device has been reported based on such compounds. Here we establish an atomic ligand strategy that makes use of monovalent halide anions to enhance electronic transport and successfully passivate surface defects in PbS CQD films. Both time-resolved infrared spectroscopy and transient device characterization indicate that the scheme leads to a shallower trap state distribution than the best organic ligands. Solar cells fabricated following this strategy show up to 6% solar AM1.5G power-conversion efficiency. The CQD films are deposited at room temperature and under ambient atmosphere, rendering the process amenable to low-cost, roll-by-roll fabrication.

The influence of quantum confinement on the one- and two-photon absorption spectra (1PA and 2PA) of PbS and PbSe semiconductor quantum dots (QDs) is investigated. The results show 2PA peaks at energies where only 1PA transitions are predicted and 1PA peaks where only 2PA transitions are predicted by the often used isotropic k x p four-band envelope function formalism. The first experimentally identified two-photon absorption peak coincides with the energy of the first one photon allowed transition. This first two-photon peak cannot be explained by band anisotropy, verifying that the inversion symmetry of the wave functions is broken and relaxation of the parity selection rules has to be taken into account to explain optical transitions in lead-salt QDs. Thus, while the band anisotropy of the bulk semiconductor plays a role in the absorption spectra, especially for the more anisotropic PbSe QDs, a complete model of the absorption spectra, for both 1PA and 2PA, must also include symmetry breaking of the quantum confined wave functions. These studies clarify the controversy of the origin of spectral features in lead-salt QDs.

Solution-processed semiconductors are compatible with a range of substrates, which enables their direct integration with organic circuits, microfluidics, optical circuitry and commercial microelectronics. Ultrasensitive photodetectors based on solution-process colloidal quantum dots operating in both the visible and infrared have been demonstrated, but these devices have poor response times (on the scale of seconds) to changes in illumination, and rapid-response devices based on a photodiode architecture suffer from low sensitivity. Here, we show that the temporal response of these devices is determined by two components--electron drift, which is a fast process, and electron diffusion, which is a slow process. By building devices that exclude the diffusion component, we are able to demonstrate a >1,000-fold improvement in the sensitivity-bandwidth product of tuneable colloidal-quantum-dot photodiodes operating in the visible and infrared.

Solution-processed nanocrystal optoelectronic devices offer large-area coverage, low cost, and compatibility with a wide range of substrates. Recently, photodetectors and photovoltaics based on spin-coated nanoparticle films have shown tremendous progress in performance. However, high-performance devices reported to date have employed either Pb or Cd, raising concerns regarding environmental impact and regulatory acceptance. Herein we report a high-performance solution-processed photodetector based instead on Bi2S3 nanocrystals. The devices exhibit photoconductive gain on the order of 10 combined with temporal response on the 10 ms time scale. The resultant solution-processed Bi2S3 nanorod photoconductive photodetectors are of interest in visible and near-infrared (NIR) wavelength applications requiring video-frame-rate temporal response.

Photoconductive photodetectors fabricated using simple solution-processing have recently been shown to exhibit high gains (1000) and outstanding sensitivities ( D*10(13) Jones). One ostensible disadvantage of exploiting photoconductive gain is that the temporal response is limited by the release of carriers from trap states. Here we show that it is possible to introduce specific chemical species onto the surfaces of colloidal quantum dots to produce only a single, desired trap state having a carefully selected lifetime. In this way we demonstrate a device that exhibits an attractive photoconductive gain (10) combined with a response time ( approximately 25 ms) useful in imaging. We achieve this by preserving a single surface species, lead sulfite, while eliminating lead sulfate and lead carboxylate. In doing so we preserve the outstanding sensitivity of these devices, achieving a specific detectivity of 10(12) Jones in the visible, while generating a temporal response suited to imaging applications.

We report a method for producing colloidal crystals heavily loaded with PbS quantum dots (QDs). The approach employed uses capillary forces to load the QDs in the interstitial voids of the colloid crystals and yields highly ordered structures with a high loading of QDs. The infiltration process is qualitatively monitored using confocal fluorescence microscopy and scanning electron microscopy. The optical properties of the resulting composite structure are examined using optical spectroscopy. The shift in the stopband resulting from the infiltration of the colloid crystal shows that the PbS QDs occupy nearly 100 % of the volume of the interstitial space.

We report efficient electroluminescence in the near-infrared from PbS–MEH-PPV (poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene)) large-area, solution-cast nanocomposite devices. We employ multivariate optimization of the structural and materials components that govern the radiative, energy-transfer, and bipolar-injection efficiencies into the devices. As a result, we report an external electroluminescence quantum efficiency of 0.27 %, which corresponds to an internal electroluminescence quantum efficiency of 1.9 %. The very best devices exhibit internal-radiative-efficiency-limited performance and not transport- or capture-limited performance, indicating that further gains in efficiency may be achieved if the internal radiative efficiency of the nanocrystal–polymer composite can be further increased without compromising transfer and device bipolar-injection efficiency.

We investigate the effect of film morphology and phase segregation in polymer/colloidal PbS nanocrystal films on the near-infrared photoluminescence quantum efficiency (PLQE) of the nanocrystals. A direct correlation between the size of closed packed nanocrystal phase separated domains and lower PLQE was established. Films with a near-infrared PLQE of up to 12% have been prepared.

In contrast to traditional semiconductors, conjugated polymers provide ease of processing, low cost, physical flexibility and large area coverage. These active optoelectronic materials produce and harvest light efficiently in the visible spectrum. The same functions are required in the infrared for telecommunications (1,300-1,600 nm), thermal imaging (1,500 nm and beyond), biological imaging (transparent tissue windows at 800 nm and 1,100 nm), thermal photovoltaics (1,900 nm), and solar cells (800-2,000 nm). Photoconductive polymer devices have yet to demonstrate sensitivity beyond approximately 800 nm (refs 2,3). Sensitizing conjugated polymers with infrared-active nanocrystal quantum dots provides a spectrally tunable means of accessing the infrared while maintaining the advantageous properties of polymers. Here we use such a nanocomposite approach in which PbS nanocrystals tuned by the quantum size effect sensitize the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) into the infrared. We achieve, in a solution-processed device and with sensitivity far beyond 800 nm, harvesting of infrared-photogenerated carriers and the demonstration of an infrared photovoltaic effect. We also make use of the wavelength tunability afforded by the nanocrystals to show photocurrent spectra tailored to three different regions of the infrared spectrum.

Colloidal quantum dots are attractive materials for efficient, low-cost and facile implementation of solution-processed optoelectronic devices. Despite impressive mobilities (1–30 cm2 V−1 s−1) reported for new classes of quantum dot solids, it is—surprisingly—the much lower-mobility (10−3–10−2 cm2 V−1 s−1) solids that have produced the best photovoltaic performance. Here we show that it is not mobility, but instead the average spacing among recombination centres that governs the diffusion length of charges in today’s quantum dot solids. In this regime, colloidal quantum dot films do not benefit from further improvements in charge carrier mobility. We develop a device model that accurately predicts the thickness dependence and diffusion length dependence of devices. Direct diffusion length measurements suggest the solid-state ligand exchange procedure as a potential origin of the detrimental recombination centres. We then present a novel avenue for in-solution passivation with tightly bound chlorothiols that retain passivation from solution to film, achieving an 8.5% power conversion efficiency. Colloidal quantum dots are promising materials for efficient low-cost solar cells and optoelectronics, but their performance does not improve with increased carrier mobility. Here, the authors show instead that the spacing between recombination centres controls the diffusion length.

Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase. Here, we quantify the density of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.

The highest-performing colloidal quantum dot (CQD) photovoltaics (PV) reported to date have relied on high-temperature (500°C) annealing of electron-accepting TiO2. Room-temperature processing reduces energy payback time and manufacturing cost, enables flexible substrates, and permits tandem solar cells that integrate a small-bandgap back cell atop a low-thermal-budget larger-bandgap front cell. Here we report an electrode strategy that enables a depleted-heterojunction CQD PV device to be fabricated entirely at room temperature. We find that simply replacing the high-temperature-processed TiO2 with a sputtered version of the same material leads to poor performance due to the low mobility of the sputtered oxide. We develop instead a two-layer donor-supply electrode (DSE) in which a highly doped, shallow work function layer supplies a high density of free electrons to an ultrathin TiO2 layer via charge-transfer doping. Using the DSE we build all-room-temperature-processed small-bandgap (1 eV) colloidal quantum dot solar cells having 4% solar power conversion efficiency and high fill factor. These 1 eV bandgap cells are suitable for use as the back junction in tandem solar cells. The DSE concept, combined with control over TiO2 stoichiometry in sputtering, provides a much-needed tunable electrode to pair with quantum-size-effect CQD films.

Colloidal quantum dots offer broad tuning of semiconductor bandstructure via the quantum size effect. Devices involving a sequence of layers comprised of quantum dots selected to have different diameters, and therefore bandgaps, offer the possibility of funneling energy toward an acceptor. Here we report a quantum funnel that efficiently conveys photoelectrons from their point of generation toward an intended electron acceptor. Using this concept we build a solar cell that benefits from enhanced fill factor as a result of this quantum funnel. This concept addresses limitations on transport in soft condensed matter systems and leverages their advantages in large-area optoelectronic devices and systems.

The time dynamics of the photoexcited carriers and carrier-multiplication efficiencies in PbS quantum dots (QDs) are investigated. In particular, we report on the carrier dynamics, including carrier multiplication, as a function of QD size and compare them to the bulk value. We show that the intraband 1P→1S decay becomes faster for smaller QDs, in agreement with the absence of a phonon bottleneck. Furthermore, as the size of the QDs decreases, the energy threshold for carrier multiplication shifts from the bulk value to higher energies. However, the energy threshold shift is smaller than the band-gap shift and, therefore, for the smallest QDs, the threshold approaches 2.35 Eg, which is close to the theoretical energy conservation limit of twice the band gap. We also show that the carrier-multiplication energy efficiency increases with decreasing QD size. By comparing to theoretical models, our results suggest that impact ionization is sufficient to explain carrier multiplication in QDs.

The two-photon absorption, 2PA, cross sections of PbS quantum dots, QDs, are theoretically and experimentally investigated and are shown to be enhanced with increasing quantum confinement. This is in contrast to our previous results for CdSe and CdTe QDs where the reduced density of states dominated and resulted in a decrease in 2PA with a decrease in QD size. Qualitatively this trend can be understood by the highly symmetric distribution of conduction and valence band states in PbS that results in an accumulation of allowed 2PA transitions in certain spectral regions. We also measure the frequency nondegenerate 2PA cross sections that are up to five times larger than for the degenerate case. We use a k·p four-band envelope function formalism to model the increasing trend of the two-photon cross sections due to quantum confinement and also due to resonance enhancement in the nondegenerate case.

Understanding the nonlinear optical processes in semiconductor nanostructures leads to possible applications in areas including laser amplifiers, optical switches, and solar cells. Here we present a study of the frequency degenerate twophoton absorption (2PA) spectrum of a series of PbS and PbSe quantum dots (QDs). The influence of the quantum confinement is analyzed using a four-band model which considers the mixing of valence and conduction bands. In contrast to our observations of CdSe QDs, the present results point to an increase of the 2PA cross-section (normalized by the QD volume) as the quantum dot size is made smaller. This is explained by the symmetry between the valence and conduction bands which allows the density of states to remain high even for small QDs. A study of the ultrafast carrier dynamics of the PbS quantum dots is also presented. Through nondegenerate femtosecond pump-probe experiments we show evidence of multi-exciton generation with quantum yield (number of excitons generated per absorbed photon) up to 170% for excitation with hω > 3 Eg (where Eg is the bandgap energy).

We report multi-carrier dynamics and two-photon absorption in lead-salt quantum dots. Inter and intra-band relaxation as well as Auger recombination is observed, along with breaking of the one and two-photon transition selection rules.

Experimentally observed two-photon transitions of lead-salt quantum dots, not predicted by k·p theory, are explained as symmetry forbidden one-photon transitions. Similarly features in the one-photon absorption spectra are explained as symmetry forbidden two-photon transitions.