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We report on a nanomechanical engineering method to monitor matter growth in real time via e-beam electromechanical coupling. This method relies on the exceptional mass sensing capabilities of nanomechanical resonators. Focused electron beam-induced deposition (FEBID) is employed to selectively grow platinum particles at the free end of singly clamped nanotube cantilevers. The electron beam has two functions: it allows both to grow material on the nanotube and to track in real time the deposited mass by probing the noise-driven mechanical resonance of the nanotube. On the one hand, this detection method is highly effective as it can resolve mass deposition with a resolution in the zeptogram range; on the other hand, this method is simple to use and readily available to a wide range of potential users because it can be operated in existing commercial FEBID systems without making any modification. The presented method allows one to engineer hybrid nanomechanical resonators with precisely tailored functionalities. It also appears as a new tool for studying the growth dynamics of ultrathin nanostructures, opening new opportunities for investigating so far out-of-reach physics of FEBID and related methods., This work is supported by the ERC advanced Grant 692876, ERC PoC Grant 862149, the Foundation Cellex, the CERCA Programme, AGAUR, Severo Ochoa (SEV-2015−0522), the Grants FIS2015-69831-P, RTI2018-097953-B-I00, MAT2017- 82970-C2-1-R, and MAT2017-82970-C2-2-R of MINECO, the Fondo Europeo de Desarrollo Regional (FEDER), and the project E13_17R from Aragon Regional Government (Construyendo Europa desde Aragon). P.V. acknowledges support from the ERC starting Grant 758794 “Q-ROOT”.

By combining X-ray absorption fine structure and X-ray diffraction measurements with density functional and molecular dynamics simulations, we study the structure of a set of AgxBi1&minus, xS2 nanoparticles, a materials system of considerable current interest for photovoltaics. An apparent contradiction between the evidence provided by X-ray absorption and diffraction measurements is solved by means of the simulations. We find that disorder in the cation sublattice induces strong local distortions, leading to the appearance of short Ag&ndash, S bonds, the overall lattice symmetry remaining close to hexagonal.

The use of photothermal sensitizers to facilitate the sintering of polymer powders is rapidly becoming a pivotal additive manufacturing technology, impacting multiple sectors of industry. However, conventional carbon-based sensitizers can only produce black or gray objects. To create white or colorful prints with this method, visibly transparent equivalents are needed. Here, we address this problem by designing resonant photothermal sensitizers made of plasmonic nanoparticles that strongly absorb in the near-infrared, while only minimally interacting with visible light. Gold nanorods were coated with silica before being mixed with polyamide powders to create stable colorful nanocomposite powders. At resonance, these composites showed greatly improved light-to-heat conversion compared with equivalent composites using the industry standard carbon black as a sensitizer and could be sintered using low-power light sources. Furthermore, they appear much whiter and can produce brightly colored 3D objects when mixed with dyes. Our results open a new route to utilize plasmonic nanoparticles to produce colorful and functional 3D-printed objects.

Lead halide perovskite nanocrystals (NCs) are currently emerging as one of the most interesting solution-processed semiconductors because they possess high photoluminescence quantum yield (PLQY) and color tunability through anion exchange reactions or quantum confinement. Here, we show efficient solar cells based on mixed-halide (CsPbBrI2) NCs obtained via anion exchange reactions under ambient conditions. We performed anion exchange reactions in concentrated NC solutions with I–, thus inducing a PL red shift up to 676 nm and obtaining a high PLQY in film (65%). Solar cell devices operating in the wavelength range 350–660 nm were fabricated in air with two different deposition methods. The solar cells display a photoconversion efficiency of 5.3% and open circuit voltage (Voc) up to 1.31 V, among the highest reported for perovskite-based solar cells with band gap below 2 eV, clearly demonstrating the potential of this material.

Colloidal quantum dot (CQD) light-emitting diodes (LEDs) deliver a compelling performance in the visible, yet infrared CQD LEDs underperform their visible-emitting counterparts, largely due to their low photoluminescence quantum efficiency. Here we employ a ternary blend of CQD thin film that comprises a binary host matrix that serves to electronically passivate as well as to cater for an efficient and balanced carrier supply to the emitting quantum dot species. In doing so, we report infrared PbS CQD LEDs with an external quantum efficiency of ~7.9% and a power conversion efficiency of ~9.3%, thanks to their very low density of trap states, on the order of 1014 cm−3, and very high photoluminescence quantum efficiency in electrically conductive quantum dot solids of more than 60%. When these blend devices operate as solar cells they deliver an open circuit voltage that approaches their radiative limit thanks to the synergistic effect of the reduced trap-state density and the density of state modification in the nanocomposite. PbS quantum dot ternary blends enable the realization of high-efficiency colloidal quantum dot infrared light-emitting diodes

AgBiS2 nanocrystals offer a route to solution-processed environmentally friendly solar cells.

Over the last several years tremendous progress has been made in incorporating colloidal quantum dot (CQD) solids as photoactive components in optoelectronic devices. A large part of this progress is associated with significant advancements made in controlling the electronic doping of CQD solids. Today, a variety of strategies exists towards that purpose; this Minireview aims to survey the major published works in this subject. Additional attention is given to the many challenges associated with the task of doping CQDs, as well as to the realization of optoelectronic functionalities and applications upon successful light and heavy electronic doping of CQD solids.

Developing low-cost photovoltaic absorbers that can harvest the short-wave infrared (SWIR) part of the solar spectrum, which remains unharnessed by current Si-based and perovskite photovoltaic technologies, is a prerequisite for making high-efficiency, low-cost tandem solar cells. Here, infrared PbS colloidal quantum dot (CQD) solar cells employing a hybrid inorganic-organic ligand exchange process that results in an external quantum efficiency of 80% at 1.35 µm are reported, leading to a short-circuit current density of 34 mA cm-2 and a power conversion efficiency (PCE) up to 7.9%, which is a current record for SWIR CQD solar cells. When this cell is placed at the back of an MAPbI3 perovskite film, it delivers an extra 3.3% PCE by harnessing light beyond 750 nm.

Two-terminal tandem cell architectures are believed to be an effective way to further improve the power conversion efficiency in solution processed photovoltaics. To design an efficient tandem solar cell, two key issues need to be considered. First, subcells with well-matched currents and complementary absorption characteristics are a prerequisite for high efficiency. Second, identifying the appropriate intermediate layer (IML) to connect the subcells is necessary to minimize the optical and electronic losses. PbS colloidal quantum dots (CQDs) are a notable choice for the subcells due to their low cost, solution processability, and remarkable wide range band gap tunability. Single-layer graphene (Gr) has been proposed to be a promising IML due to its high transparency and conductivity. Here, as a proof of concept, we demonstrate a solution-processed, two-terminal PbS CQDs tandem solar cell employing chemical vapor deposited Gr as the IML. In doing so, we report a PbS CQD cell comprising subcells with band...

Perovskite nanocrystal light-emitting diodes (LEDs) employing architecture comprising a ZnO nanoparticles electron-transport layer and a conjugated polymer hole-transport layer have been fabricated. The obtained LEDs demonstrate a maximum external-quantum-efficiency of 6.04%, luminance of 12998 Cd/m2 and stable electroluminescence at 519 nm. Importantly, such high efficiency and bright-ness have been achieved by employing solution processed transport layers, formamidinium lead bromide nanocrystals (CH(NH2)2PbBr3 NCs) synthesized at room-temperature and in air without the use of a Schlenk line, and a procedure based on atomic layer deposition to insolubilize the NC film. The obtained NCs show a photoluminescence quantum yield of 90% that is retained upon film fabrication. The results show that perovskite NC LEDs can achieve high-performance without the use of transport layers deposited through evaporation in ultra-high-vacuum.

The power conversion efficiency of colloidal PbS-quantum-dot (QD)-based solar cells is significantly hampered by lower-than-expected open circuit voltage (VOC). The VOC deficit is considerably higher in QD-based solar cells compared to other types of existing solar cells due to in-gap trap-induced bulk recombination of photogenerated carriers. Here, this study reports a ligand exchange procedure based on a mixture of zinc iodide and 3-mercaptopropyonic acid to reduce the VOC deficit without compromising the high current density. This layer-by-layer solid state ligand exchange treatment enhances the photovoltaic performance from 6.62 to 9.92% with a significant improvement in VOC from 0.58 to 0.66 V. This study further employs optoelectronic characterization, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy to understand the origin of VOC improvement. The mixed-ligand treatment reduces the sub-bandgap traps and significantly reduces bulk recombination in the devices.

In this paper we report our first experimental results towards the establishment of a novel nanooptomechanical platform consisting of a hybrid, carbon nanotube-based mechanical resonator. We have been able to optically measure and actively control these resonators demonstrating a record force sensitivity at ambient temperature. Finally, we have observed optomechanically-induced dynamical effects and optomechanical enhancement of the force sensitivity in a cavity-free scheme.

Optoelectronic devices based on colloidal quantum dots (CQD) are ideal candidates to benefit from plasmonic light trapping, due to fact that they have thin active layers and favorable material constants for absorbing light in the near-field of plasmonic resonances. We demonstrate a simple technique for designing gratings to couple incident light to surface plasmon polariton modes propagating on the metal–semiconductor interface of homojunction PbS CQD photodiodes, providing targeted photocurrent enhancement for solar harnessing or photodetection applications. As a result, an external quantum efficiency of ∼45% is achieved at the exciton peak for a photodiode with an ultrathin PbS CQD absorber of ∼100 nm.

We have fabricated infrared active hybrid solar cells composed of mesoporous SnO 2 sensitized with PbS nanoparticles and infiltrated with organic hole-transporters, 2,2',7,7'-tetrakis(N,N-di-p-methoxypheny-amine)-9,9'-spirobifluorene(spiro-OMeTAD) or poly(3-hexylthiophene). We observe photo-action to 1100nm, peak quantum-efficiency over 20%, open-circuit voltages up to 0.5V and power conversion efficiencies of over 0.5% under simulated sun light. As compared to solar cells composed of mesoporous TiO 2 sensitized with the same PbS nanoparticles, the SnO 2 based devices generate 4 times the photocurrent density under simulated sun light. © 2011.

We report advances in the growth, characterization and photovoltaic properties of SnS nanocrystals, with controlled < 10 nm size, and their inclusion into a lead chalcogenide solar cell. The SnS/PbS nanocrystalline film heterojunction is shown to display a type II band alignment, in which the direction of flow of the photocurrent depends on the order of the layers and not the relative work functions of the contacts. On placing the SnS layer next to the indium tin oxide (ITO) cathode we observe a dramatic increase in V(oc) to as much as 0.45 V. Our results suggest that SnS nanocrystal films can be used in multi-junction solar cells, that a SnS/PbS heterojunction on its own shows photovoltaic behaviour, and that a SnS layer in an ITO/SnS/PbS/Al device is acting to suppress the flow of an electron injection current.

Using electron paramagnetic resonance we identify point defects in CdS nanocrystals. A spectrum consistent with Cd vacancies in distorted sites is observed, and evidence for S vacancies is presented. Light-induced paramagnetic states attributed to defects are also detected in PbS nanocrystals. Nanocrystal shallow donor electron states are observed in CdS composites with the conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV), and show an increased g-value compared to the bulk, which is consistent with quantum confinement. Dark state MEH-PPV polarons are observed for composites with small (3.6 nm) CdS nanocrystals. © 2009 American Chemical Society.

We present a simple conceptual model describing the absorption enhancement provided by diffraction gratings due to resonant coupling to guided modes in a multi-layered structure. In doing so, we provide insight into why certain guided modes are more strongly excited than others and demonstrate that the spatial overlap of the mode profile with the grating is important. The model is verified by comparison to optical simulations and experimental measurements. We fabricate metal nanoparticle gratings integrated as back contacts in solution-processed PbS colloidal quantum dot photodiodes. The measured photocurrent at the target wavelength is enhanced by 250%, with reference to planar devices, due to resonant coupling to guided modes with strong spatial overlap with the gratings. In comparison, resonant coupling to weakly overlapping modes results in a 25% increase at the same wavelength.

Multiwall carbon nanotubes (MWCNTs) with an unprecedented degree of internal order were synthesised by chemical vapour deposition (CVD) adding a nitrogen-containing compound to the hydrocarbon feedstock. Ferrocene was used as the metal catalyst precursor. The remarkable crystallinity of these nanotubes lies both in the isochirality and in the crystallographic register of their walls, as demonstrated by electron diffraction and high resolution electron microscopy experiments. High resolution transmission electron microscopy analysis shows that the walls of the nanotubes consist of truncated stacked cones, instead of perfect cylinders, with a range of apex angles that appears to be related to the nitrogen concentration in the synthesis process. The structure of armchair, zigzag and chiral nanotubes is modelled and discussed in terms of density of topological defects, providing an interesting comparison with our microscopy experiments. A growth mechanism based on the interplay of base- and tip-growth is proposed to account for our experimental observations.

More-efficient charge collection and suppressed trap recombination in colloidal quantum dot (CQD) solar cells is achieved by means of a bulk nano-heterojunction (BNH) structure, in which p-type and n-type materials are blended on the nanometer scale. The improved performance of the BNH devices, compared with that of bilayer devices, is displayed in higher photocurrents and higher open-circuit voltages (resulting from a trap passivation mechanism).

Colloidal quantum dots have emerged as a material platform for low-cost high-performance optoelectronics. At the heart of optoelectronic devices lies the formation of a junction, which requires the intimate contact of n-type and p-type semiconductors. Doping in bulk semiconductors has been largely deployed for many decades, yet electronically active doping in quantum dots has remained a challenge and the demonstration of robust functional optoelectronic devices had thus far been elusive. Here we report an optoelectronic device, a quantum dot homojunction solar cell, based on heterovalent cation substitution. We used PbS quantum dots as a reference material, which is a p-type semiconductor, and we employed Bi-doping to transform it into an n-type semiconductor. We then combined the two layers into a homojunction device operating as a solar cell robustly under ambient air conditions with power conversion efficiency of 2.7%., To use colloidal quantum dots in applications such as p-n junction solar cells, doping of the quantum dots is needed. Here, Stavrinadis et al. achieve lead sulphide quantum dot p-n homojunctions by heterovalent cation substitution of lead using bismuth.

High efficiency organic and hybrid solar cells create demand for novel electron acceptor materials that possess appropriate energetic band levels and bandgap for efficient solar energy harnessing. We present hybrid bulk heterojunction devices based on P3HT and bismuth sulfide nanocrystals, a semiconductor based on environmentally friendly compounds, with a power conversion efficiency of 1% and NIR sensitization at 700 nm of 30%, among the highest ever reported for P3HT.

Understanding the nanomorphology of polymer-based solar cells is key to improving device efficiencies. A detailed study of the changes in polymer solar cell morphology at the molecular level and in real time under process conditions is presented. Using low voltage, high resolution electron microscopy (LVHREM) we observe the mechanisms of PCBM-rich domain migration in real time and provide evidence for vertical segregation within the bulk heterojunction and increased crystallinity of both P3HT and PCBM. The results of the study highlights the fact that P3HT:PCBM bulk heterojunction morphology should be viewed within the framework of a four-phase, three-dimensional, nonequilibrium system. © 2010 American Chemical Society.

We present a method to directly align PbS nanocrystals in micron-sized superstructures within a conjugated polymer. First, lead sulfide nanocrystals are directly synthesized in a MEH-PPV suspension via a single pot, surfactant-free method. Post-synthesis precipitation of the composite solution involving mild oxidation of the nanocrystals results in the formation of nanocrystal-polymer and nanocrystal-oxide superstructures. Detailed TEM is used to study the crystallographic nature of these structures and the roles of polymer and lead sulfate. An epitaxial relationship between lead sulfide and lead sulfate at the nanoscale is shown, giving insight into the oxidation rates of the PbS nanocrystals' facets.

A study was carried out to demonstrate that PbS nanorods can be directly assembled in a conducting polymer. The controlled assembly of the nanorods has been reported through oriented attachment of PbS nanocrystals using a post-synthesis treatment of PbS nanocrystal/poly(2-methoxy-5-(2'-ethyl-hexyloxy) -p-phenylene vinylene) (MEH-PPV) composites synthesized by a low-temperature and surfactant-free method. The composite has broad optical absorption and the results of the experimentation have also shown that the nanorods formation is governed by the polarity of alcohol used for precipitation. This formation also appears to arise from dipole-induced oriented attachment of PbS nanocrystals. The results of the study also have found that the extension of the composite absorption beyond the main polymer exciton peak at ca. 520 nm is due to the presence of nanocrystals.

Lead oxide nanocrystals are synthesised by injecting oxygen gas into an air and moisture free complex of Pb oleylamine and oleic acid in octadecene. Using various characterization methods including fabrication and testing of photovoltaic devices we explore the material properties and photovoltaic application of lead oxide nanocrystal films.

The performance of ZnO/PbS colloidal quantum dot (CQD)-based heterojunction solar cells is hindered by charge carrier recombination at the heterojunction interface. Reducing interfacial recombination can improve charge collection and the photocurrent of the device. Here we report the use of a mixed nanocrystal (MNC) buffer layer comprising zinc oxide nanocrystals and lead sulfide quantum dots at the respective heterojunction interface. Remote trap passivation of the PbS CQDs taking place within this MNC layer reduces interfacial recombination and electron back transfer which in turn improves charge collection efficiency. Upon the addition of the MNC layer, the overall power conversion efficiency increases from 9.11 to 10.16% and Short-circuit current density (JSC) increases from 23.54 to 25.23 mA/cm2. Optoelectronic characterization of the solar cells confirms that the effects underlying device improvement are reduced trap density and improved charge collection efficiency due to the presence of the MNC buffer layer.

To realize the full potential of colloidal quantum dot (CQD) based solar cells, it is important to address the issue of large open-circuit voltage (VOC) deficit which is a major roadblock in reaching higher efficiencies. The origin of the VOC deficit in these solar cells lies primarily in the presence of sub-bandgap trap states of the QDs. Here, we present a synergistic engineering framework to passivate these sub-bandgap states in PbS QDs through chemical surface passivation and remote passivation exploiting ligand and architecture engineering. In particular, we form bulk nanoheterojunctions (BNH) by mixing PbS QDs with ZnO nanocrystals in conjunction with mixed ligand treatments to passivate surface traps. We employ the mixed ligand system of zinc iodide and 3-mercatopropyonic acid to leverage the benefits of both organic and inorganic ligands for surface passivation and improved charge transport. This mixed ligand treatment in BNH architectures leads to record low VOC deficit for PbS QDs of 0.4–0.55 V compared to previously reported 0.6–0.8 V for the range of 1.1–1.35 eV bandgap PbS QDs.

Surface passivation of PbS colloidal quantum dots (QDs) with iodide has been used in highly efficient solar cells. Iodide passivation is typically achieved by ligand exchange processes on QD films. Complementary to this approach, herein we present a non-intrusive solution-based strategy for doping QDs with iodide to further optimize solar cell performance. The doping step is applied in-situ at the end of the synthesis of the QDs. The optimum precursor I/Pb ratio is found to be in the 1.5-3% range at which iodide substitutes S without excessively altering the dots´ surface chemistry. This allows for band engineering and decreasing the density of deep trap states of the QDs which taken together lead to PbS QD solar cells with efficiency in excess of 10%.

For any emerging photovoltaic technology to become commercially relevant, both its power conversion efficiency and photostability are key parameters to be fulfilled. Colloidal quantum dot solar cells are a solution-processed, low-cost technology that has reached an efficiency of about 9% by judiciously controlling the surface of the quantum dots to enable surface passivation and tune energy levels. However, the role of the quantum dot surface on the stability of these solar cells has remained elusive. Here we report on highly efficient and photostable quantum dot solar cells with efficiencies of 9.6% (and independently certificated values of 8.7%). As a result of optimized surface passivation and the suppression of hydroxyl ligands—which are found to be detrimental for both efficiency and photostability—the efficiency remains within 80% of its initial value after 1,000 h of continuous illumination at AM1.5G. Our findings provide insights into the role of the quantum dot surface in both the stability and efficiency of quantum dot solar cells. Controlling the surface of quantum dots has enabled higher efficiency in quantum dot solar cells. Now, the role of surface passivation and suppression of hydroxyl ligands in the performance and photostability of cells with an efficiency of 9.6% is unveiled.

In this study, we employ a thiol-functionalized polymer (P3HT-SH) as a leverage to tailor the nanomorphology and electronic coupling in polymer–nanocrystal composites for hybrid solar cells. The presence of the thiol functional group allows for a highly crystalline semiconducting polymer film at low thiol content and allows for improved nanomorphologies in hybrid organic–inorganic systems when employing non-toxic bismuth sulfide nanocrystals. The exciton dissociation efficiency and carrier dynamics at this hybrid heterojunction are investigated through photoluminescence quenching and transient absorption spectroscopy measurements, revealing a larger degree of polaron formation when P3HT-SH is employed, suggesting an increased electronic interaction between the metal chalcogenide nanocrystals and the thiol-functionalized P3HT. The fabricated photovoltaic devices show 15% higher power conversion efficiencies as a result of the improved nanomorphology and better charge transfer mechanism together with the higher open circuit voltages arising from the deeper energy levels of P3HT-SH.